over Zeolite Catalysts - ACS Publications - American Chemical Society

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Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts Runduo Zhang, Ning Liu, Zhigang Lei, and Biaohua Chen* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China ABSTRACT: In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NOx), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as well as mechanism studies based on spectroscopic and kinetic approaches and molecular simulations. Finally, concluding remarks and perspectives are given. In the present work, emphasis is placed on the structure−performance relationship with an aim to design an ideal zeolite-based catalyst for the effective elimination of harmful N-containing compounds.

CONTENTS 1. Introduction 2. General Characteristics of Zeolite Catalysts 2.1. Physical Properties 2.1.1. Shape Selectivity 2.1.2. Specific Surface Area 2.2. Chemical Properties 2.2.1. Si/Al Ratio 2.2.2. Acidity 2.2.3. Redox Properties 3. Selective Catalytic Oxidation (SCO) of Nitriles (HCN/CH3CN/C2H3CN) and Ammonia (NH3) 3.1. Catalytic Performance 3.1.1. SCO of Nitriles 3.1.2. SCO of NH3 3.2. Reaction Mechanisms 3.2.1. SCO Mechanism for Nitriles 3.2.2. NH3-SCO Mechanism 3.3. Kinetic Studies 4. N2O Direct Decomposition 4.1. Active Center Structure 4.1.1. Influence of the Preparation Method 4.1.2. Distribution of Framework Al 4.2. Activity Comparisons 4.2.1. Different Active Components 4.2.2. Different Zeolite Topologies 4.3. Reaction Mechanism 4.3.1. Spectroscopic Approach 4.3.2. DFT Simulations 4.4. Microkinetic Analysis 5. Catalytic Reduction of NO

5.1. Selective Catalytic Reduction (SCR) with Different Reducing Agents 5.1.1. NH3-SCR 5.1.2. Hydrocarbon SCR (HC-SCR) 5.1.3. H2-SCR 5.2. NO Direct Decomposition 5.3. DFT Simulations of the Mechanisms 5.3.1. NO-SCR Mechanism 5.3.2. NO Direct Decomposition Mechanism 6. Concluding Remarks and Perspectives Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION With the rapid development of social and economic communities, increasing attention has begun to be paid to the subject of environmental protection, especially the purification of toxic air pollutants. Currently, in many countries, large-scale actions aimed at air pollution control have been enacted by means of certain laws (e.g., the Clean Air Act and the Pollution Prevention Act in China) and regulations (e.g., exhaust emission limits for passenger cars in the United States). Among the various gaseous pollutants, N-containing exhaust gases, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), Received: August 11, 2015 Published: February 18, 2016

© 2016 American Chemical Society

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DOI: 10.1021/acs.chemrev.5b00474 Chem. Rev. 2016, 116, 3658−3721

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nitrous oxide (N2O), and nitric oxides (NOx), can pose a great threat to human health and ecological environments. (1) Nitrile gases (HCN, CH3CN, and C2H3CN), with a N valence of −5, are commonly classified as very toxic volatile organic compounds (VOCs) because of their hazardous properties; these gases predominantly originate from the production of carbon fibers and acrylonitrile. Levels of HCN in the gas phase on the order of parts per million are lethal to human beings, and CH3CN and C2H3CN are also defined as compounds that are harmful upon inhalation through the respiratory system or contact with the skin. (2) NH3, with a N valence of −3, is produced as a byproduct in many chemical processes, such as biomass or coal gasification. In addition, it is an effective reductant for the selective catalytic reduction (SCR) of NO, which is plagued by an NH3 slip problem. (3) N2O, with a N chemical state of +1, is one type of greenhouse gas, with a global warming potential (GWP) of 310. Moreover, it participates in the destruction of the stratospheric ozone layer. Large amounts of N2O are released due to anthropogenic activities, especially from the production of adipic acid and nitric acid, resulting in an annual growth rate of N2O concentration in the atmosphere of 0.2−0.3%. (4) Nitric oxides (NOx) [NO (+2) and NO2 (+4)] are ubiquitous byproducts of high-temperature combustion and are primarily generated by the burning of fossil fuel for automobiles or stationary power plants. In addition to their contributions to photochemical smog, acid rain, and ozone depletion, they are now suspected to be the main factor causing haze weather. As noted, the valences of atomic nitrogen in the N-containing compounds mentioned above vary from −5 to +4. However, only N2, with a N valence of 0, is environmentally friendly. The effective transformation of these harmful compounds into N2 has therefore attracted considerable scientific attention; catalytic methods using zeolite catalysts are particularly attractive for this purpose because of the high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and excellent thermal stabilities of such porous materials. The catalytic abatement processes of these N-containing waste gases are profiled in Figure 1, wherein the interconversions among these pollutants are also included. The related reactions are further formulated by eqs 1.1−1.16 to illustrate those processes occurring in Figure 1 in detail.

4Cx − 1HyCN + (4x + y + 1)O2 → 2y H2 O + 4xCO2 + 2N2O

(1.2)

4Cx − 1HyCN + (4x + y + 2)O2 → 2y H2 O + 4xCO2 + 4NO

(SCO to NO)

(1.3)

4Cx − 1Hy + 3CN + (4x + y)O2 → 2y H2 O + 4xCO2 + 4NH3

(SCO to NH3)

(1.4)

(2) NH3-SCO: 4NH3 + 3O2 → 6H 2O + 2N2

(SCO to N2)

(1.5)

2NH3 + 2O2 → 3H 2O + N2O

(SCO to N2O)

(1.6)

4NH3 + 5O2 → 6H 2O + 4NO

(SCO to NO)

(1.7)

(3) N2O-DD: 2N2O → 2N2 + O2

2N2O → 2NO + N2

(DD to N2)

(1.8)

(DD to NO)

(1.9)

(4) NO-SCR: 4NO + 4NH3 + O2 → 4N2 + 6H 2O

(standard SCR) (1.10)

NO + NO2 + 2NH3 → 2N2 + 3H 2O

(fast SCR) (1.11)

6NO2 + 8NH3 → 7N2 + 12H 2O

(slow SCR to N2) (1.12a)

2NO2 + 2NH3 → N2 + N2O + 3H 2O (slow SCR to N2O)

(1.12b)

2NO + 2H 2 → N2 + 2H 2O

(H 2‐SCR)

(1.13)

8NO + 4Cx Hy + (y + 4x − 4)O2 → 4N2 + 4xCO2 + 2y H2 O

(1) Cx−1HyCN-SCO:

(hydrocarbon SCR)

(1.14)

(8x + 2y − 4z)NO + 4Cx HyOz → (4x + y − 2z)N2

4Cx − 1HyCN + (4x + y)O2 → 2y H2 O + 4xCO2 + 2N2 (SCO to N2)

(SCO to N2O)

+ 4xCO2 + 2y H2 O

(1.1)

(Cx HyOz SCR)

(1.15)

(5) NO-DD: 2NO → N2 + O2

(1.16)

Over the past two decades, numerous review papers have been published covering various aspects of the subject of N2O/NOx elimination catalyzed by metal-doped zeolites. (1) Early in 1995, Chemical Reviews published a review of the subject of “NO catalytic reduction” with an emphasis on ZSM-5 zeolites as well as the further modification of ZSM-5 by either ion exchange (Fe3+ or Ga3+) or isomorphic substitution (Al3+).1 (2) A review addressing the influence of various hydrocarbons on the selectively catalytic reduction (SCR) of nitrogen oxides over zeolite catalysts was subsequently written by Traa et al.2 (3) Recently, an overview of low-temperature NO reduction over Mn-based oxides and Cu- and Fe-zeolites was presented in Catalysis Today. 3 (4) The first overall review of the

Figure 1. Catalytic abatement of the diverse N-containing exhaust gases: HL, hydrolysis; SCO, selectively catalytic oxidation; DD, direct decomposition; SCR, selectively catalytic reduction. 3659


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is present and fundamentally affects the corresponding performance. For instance, NH3 chemisorption has been confirmed to be an important step in the SCR of NO via the ammonia process. A suitable acidity is undoubtedly crucial for ideal SCR performance. It is therefore important to clarify the effect of the Si/Al ratio. (4) Metallic components invariably act as active centers in all possible forms. Although a debate persists concerning the status of active centers on metal-doped zeolites, such as cations, oxo cations, dimers, and oligonuclear clusters, their chemical natures and redox abilities are thought to strongly influence the transformation behaviors of harmful N-containing compounds (pollutant conversion and N2 selectivity). In general, it is one purpose of the present review to provide a valuable summary of new insights into the roles of the topology and acidity of the zeolite, the nature and redox properties of the active centers, the shape selectivity, the reaction mechanism, and the related kinetics, with the goal of facilitating a better understanding of heterogeneous catalytic purification for Ncontaining exhaust gases. By virtue of the rapid development of computational technology, it has become possible to describe catalytic reactions at the molecular or atomic level on the basis of quantum chemistry. One of the most important theoretical techniques for this purpose is density functional theory (DFT) calculation, which enables multiple approaches to gaining a deeper understanding of reaction mechanisms, including calculating the energy barriers, conducting microkinetic analyses, and clarifying the reaction pathways. Currently, DFT calculations have become a prevalent method for theoretically studying heterogeneous catalysis over zeolite catalysts. However, no overall review of the recent achievements in DFT studies of the purification of N-containing exhaust has previously been presented. Hence, the relevant progress is addressed in the present review. In this review we are devoted to providing an easy-to-read and comprehensive summary of the catalytic abatement of Ncontaining pollutants using zeolite catalysts and to addressing the issue from the perspectives of both experimental approaches and computational chemistry. An effort has been made to achieve the best possible matching between the zeolite topology and the active centers, which is of great importance for defining a set of principles or guidelines for designing highly efficient catalysts. The contents are arranged under the following headings: Introduction, General Characteristics of Zeolite Catalysts, Selective Catalytic Oxidation of Nitriles and Ammonia, N2O Direct Decomposition, Catalytic Reduction of NO, and Concluding Remarks and Perspectives. The sources and toxicities of the various N-containing pollutants are introduced in section 1. Several crucial properties of zeolite catalysts, including their shape selectivity, specific surface area, acidity, and redox ability, are systematically addressed in section 2. The catalytic combustion behaviors of nitriles and ammonia as well as studies of the corresponding mechanisms and kinetics are discussed in section 3. Thereafter, the applications of zeolite catalysts for N2O decomposition are discussed in section 4, with an emphasis on studies of reaction mechanisms ranging from spectroscopic approaches to molecular simulations. Afterward, the catalytic reduction of NO, including selective catalytic reduction of NO by various reductants and NO direct decomposition, is summarized in section 5. Finally, concluding remarks and perspectives are offered in section 6. The relationship between catalyst structure and catalytic performance

decomposition of nitrous oxide using zeolite catalysts was compiled by Kapteijn and Moulijn et al.4 As noted, only one or two types of N-containing waste gases (NOx or N2O) have been considered in previous reviews. As a deeper understanding of the air pollution system has developed, it has begun to be recognized that nitrile gases (volatile organic compounds (VOCs) with CN− functional groups), NH3, N2O, and NOx constitute a large family of N-containing waste gases, and under certain conditions, these gases can be readily interconverted. An overall review is therefore necessary to provide a detailed and comprehensive description of the relevant transformation paths and catalytic behaviors for the effective removal of these pollutants. Moreover, past reviews have described only the features of individual catalysts. However, the structure−performance correlation and the principle on which to design an extraordinary depollution catalyst are also of vital interest and will be clarified in the present review on the following new aspects of this topic. (1) The zeolite framework, which contains a variety of pore apertures, often influences the internal diffusion of the guest molecules. Most heterogeneous reactions that proceed over zeolite catalysts rely on the fact that the micropores of these catalysts have dimensions that are comparable to the sizes of the adsorbed molecules. For a traditional zeolite catalyst (e.g., ZSM5 with pore openings 5.1 × 5.5 Å in size), the gas molecules considered here (such as NOx, N2O, and NH3, with dynamic diameters of 3.17, 3.83, and 2.90 Å, respectively) are too small to give rise to any “shape selectivity”. However, the newly reported SSZ-13 zeolite, with supersmall micropores (3.8 × 3.8 Å), demonstrates outstanding activity for NO-SCR elimination, indicating that “quasi shape selectivity” can be achieved using this catalyst. (2) The pore structures of microporous zeolite and mesoporous silica can affect the dispersion and chemical states of the loaded active species. Interestingly, zeolites with supersmall micropores (such as SSZ-13, UZM-12, and SUZ-4) as well as silica-based supports with mesopores (such as SBA-15, SBA-16, and KIT-6) generally exhibit much higher surface areas than typical microporous zeolites (such as ZSM-5, Y, and Beta) do, which is beneficial for achieving better dispersion of the active components. It is therefore meaningful to compare a series of microporous zeolites and mesoporous silica supports reported in the literature, with diverse pore sizes and specific surface areas, and to analyze their depollution activities. Moreover, the Si-only microporous (Si-Beta)5 and mesoporous (SBA-15, SBA-16, KIT-6)6−8 materials are also taken into account in the present work. Especially the mesoporous silica, possessing a superhigh surface area, an ordered pore arrangement, adjustable pore sizes from 3 to 30 nm, and high hydrothermal and thermal stability,6 are esteemed as one kind of ideal support for the active components. After modification by the transition metals, excellent catalytic behaviors could be achieved, wherein the nature and dispersion of the loaded metal species determined the final activity and selectivity of the supported catalysts.7 Moreover, it was recently found that incorporation of atomic Al into the framework of these Si-only mesoporous zeolites could generate acidic sites and remarkably improve the related catalytic performances.8 (3) The framework Al plays a crucial role in determining the physicochemical properties of zeolites. These skeleton aluminums favorably associate with ion-exchanged cations, which ordinarily serve as active sites. Furthermore, it is well-known that the acidity strongly depends on the amount of framework Al that 3660


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Table 1. Structural Parameters of the Zeolites with Different Topologiesa

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Table 1. continued

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Table 1. continued

Collected from the International Zeolite Association (IZA). Asterisks represent the dimensions of the zeolite pore. The symbol “↔” represents parallel. The symbol “⊥” represents mutually perpendicular. MR = membered ring.

a

2. GENERAL CHARACTERISTICS OF ZEOLITE CATALYSTS Zeolites are water-containing aluminosilicates of natural or synthetic origin with a highly ordered crystal structure, generally formulated as MIMII0.5[(AlO2)x·(SiO2)y·(H2O)z], where MI and MII are preferentially alkali and alkaline-earth metals. They consist of SiO4− and AlO4− tetrahedra, which are interlinked by

is discussed on the basis of a combination of experimental and theoretical investigations, with the ultimate goal of designing an ideal zeolite-based catalyst for improved purification of harmful N-containing compounds. 3663


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common oxygen atoms to form a three-dimensional network. The tetrahedra are the smallest structural units by which zeolites can be classified. Linking these primary building units together leads to 23 possible secondary building blocks (polygons). Zeolites are predominantly distinguished on the basis of the geometries of the cages (α, β, γ, and super) and channels (straight and sinusoidal) formed by the rigid tetrahedral frameworks. To date, 229 types of zeolite framework structures have been recognized by the Structure Commission of the International Zeolite Association.9 Moreover, Table 1 lists the steric topologies and structural parameters of a number of conventional zeolites.

As is well-known, the shape selectivity of zeolites is crucial for the production of petrochemicals. However, for the catalytic removal of gaseous pollutants, the kinetic diameters of the relevant molecules [N2O (3.83 Å), NO (3.17 Å), NO2 (>3.83 Å), NH3 (2.90 Å), CO (3.76 Å), C3H6 (4.68 Å), CH4 (3.80 Å), HCN (3.35 Å), and SO2 (4.11 Å)] are in the range of 2−5 Å and thus are too small to be subject to any constraining effects imposed by the pore apertures of traditional zeolites {ZSM-5 with 10-MR pores of 5.3 × 5.6 Å diameter [010] or Y with 12MR pores of 7.4 × 7.4 Å diameter [111]}. In the past, it was thus believed that no shape selectivity could be achieved in reactions for air pollution control using zeolites as the catalysts. Recently, however, a new advancement has been achieved in the selective catalytic reduction (SCR) of NO. A novel zeolite named SSZ-13, of the CHA topology with supersmall pores (∼3.8 Å for 8-MR pores; although the active sites are located at double 6-MR units, the reactant molecules must pass through these 8-MR pores of 3.8 Å diameter to arrive at these reactive sites), exhibits outstanding SCR performance (∼100% NO conversion) and excellent N2 yields (>95%) across a wide temperature range of 150−400 °C after copper ion exchange.12,13 This remarkable performance is certainly attributable to the special topology of the SSZ-13 zeolite, which has a significant influence on the zeolitic shape selectivity, specific surface area, and distribution of the active centers. An attempt has been made to correlate the deNOx behavior with the unique topology of SSZ-13 on the basis of the “shape selectivity” concept.8 The supersmall pores (∼3.80 Å, 8-MR) of SSZ-13, which serve as the exits of the reaction channels, are smaller than the dimensions of the undesired byproducts [NO2 (>3.83 Å) and N2O (3.83 Å)]; however, they are passable by the target product [N2 (3.64 Å)]. Accordingly, the pore-size effect may provide an explanation for the lesser amounts of harmful NO2 and N2O byproducts and the high N2 yields achieved over Cu-SSZ-13. He et al.14 have also noted a strange SCR behavior observed in the case of Cu-SSZ-13 prepared through one-pot synthesis. Although the “fast SCR mechanism”, the first step of which being a rapid oxidation of partial NO resulting in a NO + NO2 mixture, is typical in effect for the catalytic reduction of NO by NH3 at low temperatures, it has been proposed that the “standard SCR mechanism” is more relevant for Cu-SSZ-13 because of the lesser NO2 generation caused by a transition-state constraint imposed by the small window of the CHA cage. Additionally, it is widely accepted that both C3H6 and SO2 often cause deactivation during the catalytic elimination of NO by Cu-zeolites. Although C3H6 can participate in the SCR reaction as a reductant, it has the disadvantage of giving rise to carbonaceous deposition on the reactive centers, especially at low temperatures.15−17 Moreover, a SO2 poisoning effect is observed, primarily because of its reaction with Cu active sites to form CuSO3 or CuSO4 species at T < 200 °C.18 Fortunately, the pore openings (3.8 Å) of Cu-SSZ-13 are smaller than the kinetic diameters of both C3H6 (4.68 Å) and SO2 (4.11 Å).19 As a result, the pore-size effect can greatly inhibit the diffusion of C3H6 and SO2 into the main channels of SSZ-13, thereby preserving a majority of the active sites. In other words, the shape selectivity exhibited by SSZ-13 due to its supersmall pores is confirmed to be responsible for its high resistance to C3H6 and SO2 poisoning. 2.1.2. Specific Surface Area. Zeolite catalysts possess high specific surface areas (300−1000 m 2 g −1 ), which are fundamentally related to their excellent activities. It is already known that, for a microporous zeolite catalyst, its Brönsted acid sites can be exchanged with metal cations, which further behave

2.1. Physical Properties

Both the physical and chemical properties of a catalyst should be known to understand the relationship between the material’s structure and its catalytic performances, including activity, selectivity, and stability. The major terms used to describe the physical properties of a zeolite catalyst are as follows: morphology, related to the steric array and topology; porosity, indicating the pore volume; and texture, referring to the pore size, pore shape, and pore size distribution. Several remarkable properties of zeolites, such as their shape selectivity and high specific surface area, are fundamentally related to their structural features. 2.1.1. Shape Selectivity. The term “shape-selective catalysis” was first proposed by Weisz and Frilette in 1960,10 who suggested that zeolites can be selective with respect to both shape and size in different catalytic molecular arrangements. The accessibility of the pores for the molecules of interest is subject to definite geometric or steric restrictions. The shape selectivity, which is based on the interaction of the reactants with the welldefined pore system, has become one of the most important characteristics of zeolite catalysts. The shape selectivity of zeolites commonly manifests in three variants: reactant selectivity, product selectivity, and transition-state shape selectivity, as described in detail below.11 (1) Reactant shape selectivity (RSS) is achieved on the basis of size exclusion at or near the pore entrance. Only reactants with suitable kinetic diameters can reach the internal active sites. (2) Product shape selectivity (PSS) refers to a situation in which the pore size of a zeolite provides the ability to effectively distinguish the products exiting from the pores depending on the dimensions of the product molecules. (3) Transition-state shape selectivity (TSS) refers to reactions in which the pore geometry exerts a steric constraint on the transition state. The pore structure hinders the formation of undesirable transition states or reaction intermediates. Zeolites can be roughly divided into three groups according to their pore sizes: microporous (50 nm) zeolites. The use of zeolites in the former two groups for the elimination of N-containing waste gases has been widely reported because of their excellent catalytic behaviors, especially those of microporous zeolites. Moreover, as knowledge of zeolite catalysts has increased, microporous zeolites have been further classified into three types on the basis of their pore sizes: small-micropore [CHA, LEV, OFF, ERI, and AFX with 8-membered ring (MR) pores], medium-micropore (MFI, FER, AFO, and MTT with 10-MR pores), and large-micropore (BEA, MOR, FAU, DFO, and VET with 12-MR pores) zeolites. The detailed pore sizes, spatial structures, and other structural parameters of these materials are summarized in Table 1. 3664


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13 and Cu-SAPO-34), with their relatively high specific surface areas and pore volumes, demonstrate the best SCR performances (greater than 80% NOx conversion across a broad temperature range of 200−400 °C). (ii) Much lower deNOx activity is observed for metal (Fe, Cu, and Co)-modified medium-microporous (MFI, FER, and MOR) and largemicroporous (BEA) zeolites than for small-microporous CuCHA; an NOx conversion of >80% is achieved for these materials only at high temperatures of 300−500 °C. (iii) Although the mesoporous catalysts of SBA-15 and MCM-41 possess extremely high surface areas (∼550 m2 g−1 for SBA-15 and ∼1000 m2 g−1 for MCM-41), the lowest deNOx activities are observed for these types of catalysts, primarily because of the absence of framework Al. The addition of Al into the frameworks of SBA-15 and MCM-41 could promote the related deNOx activity to some extent (nos. 33−42 for SBA-15 and nos. 43−47 for MCM-41).

as reactive centers. The larger specific surface area of a zeolite catalyst undoubtedly creates more opportunities for the Brönsted acid sites to be exposed to cations for exchange, resulting in abundant active sites and improved catalytic activity. Furthermore, the specific surface area strongly affects the catalytic activity by influencing the dispersion of the active components loaded onto the zeolite substrate.20 Zhang et al.21 investigated the catalytic reduction of N2O by NH3 over various Fe-zeolites with similar Si/Al ratios of ∼12 (MOR, FAU, BEA, and MFI) but different specific surface areas. Much higher ionexchange degrees were observed for Fe-FAU (3.38 wt %, 512 m2 g−1) and Fe-MOR (2.68 wt %, 478 m2 g−1) than for Fe-MFI (0.30 wt %, 423 m2 g−1), and this result was primarily attributed to the higher specific surface areas of the former materials. Further investigations [H2 temperature-programmed reduction (TPR), ultraviolet−visible (UV−vis) light spectroscopy, and O2 temperature-programmed desorption (TPD)] suggested that isolated Fe3+ ions were the predominant species in all of these Fe-zeolites. Recently, small-microporous zeolites (Cu-CHA and Cu-ERI) have been observed to exhibit much higher activities for NOx abatement in NH3-SCR compared with medium-microporous (Cu-MFI and Cu-FER) and large-microporous (Cu-BEA and Cu-MOR) zeolites.12,22 The extremely high specific surface areas and large pore volumes constitute two of the important parameters for the extraordinary deNOx (SCR) activities of these supersmall microporous zeolite catalysts: respectively 526 m2 g−1 and 0.26 cm3 g−1 for Cu-SSZ-1312 and 321 m2 g−1 and 0.18 cm3 g−1 for Cu-ZSM-5.16 As noted above, the remarkable specific surface area and pore volume of Cu-SSZ-13 are actually related to the supersmall pore size of this material, being in the range of ∼3.8 Å. Mesoporous catalysts, such as SBA-15, SBA-16, MCM-41, and KIT-6, also possess remarkably high specific surface areas (∼650 m2 g−1) and pore volumes (∼0.8 cm3 g−1); however, the lack of framework Al, which can generate Brönsted acids and act as potential active sites for the loaded metallic species, seriously hinders their broad application. To overcome this drawback, attempts have been made to introduce atomic Al into the framework of SBA-15 through isomorphic substitution. In one of our previous studies,23 after the introduction of atomic Al into the SBA-15, the deNOx activity of Cu-SBA-15 was obviously improved, resulting in a NO conversion of approximately 80% over Cu/Al-SBA-15 at a temperature as low as 350 °C under an atmosphere of 3000 ppm NO, 3000 ppm C3H6, and 1% O2 at a gas hourly space velocity (GHSV) of 60000 h−1. The specific surface area is an important parameter for zeolite catalysts because it strongly influences their catalytic performance. However, it should also be noted that the specific surface area is never the sole determining factor of catalytic behavior. Other factors, such as the zeolite topology, the Si/Al ratio, the nature of the loaded components and their loading amounts, and the preparation method, also contribute to the final activity. In light of these considerations, a database of the properties of zeolite catalysts during the NO-SCR process has been established (see Table 2), including various zeolitic parameters, the NO reduction efficiency, the N2 yield, and the operating temperature range, to facilitate a better understanding of the influences of the physical characteristics of zeolite catalysts on their catalytic activity. From a careful study of Table 2, the following conclusions regarding the zeolite surface area can be deduced: (i) Coppermodified supersmall microporous zeolites (Cu-CHA: Cu-SSZ-

2.2. Chemical Properties

2.2.1. Si/Al Ratio. Zeolites acting as efficient catalysts have been widely investigated by the academic community. Many of them have been successfully applied in industry. The chemical properties of zeolite catalysts strongly depend on the framework Al they contain. The correlations between the molar Si/Al ratio and the chemical properties of these catalysts (hydrothermal stability, acidity, and ion-exchange capacity) are discussed in this section. 2.2.1.1. Correlation with Hydrothermal Stability. Hydrothermal stability plays a decisive role in practical applications of zeolite catalysts, especially for the SCR of NO with NH3 and the selective catalytic oxidation (SCO) of NH3 and nitriles, wherein the presence of water vapor in the effluent gases might cause the framework to collapse due to unavoidable dealumination at high temperatures.45−47 Chung et al.48 investigated the effect of the Si/Al ratio on the hydrothermal stability of MOR and MFI catalysts for NO reduction by hydrocarbons. It was observed that the Si/Al ratio was the most critical characteristic determining the hydrothermal stability of the zeolite catalysts. A higher Si/Al ratio commonly results in greater hydrothermal stability. The hydrothermal stability of zeolites can be further improved through optimization of the Si/Al ratio. To improve the hydrothermal stability of zeolites, various types of pretreatments have been applied, such as thermal, hydrothermal, and mineral acid treatments, which decrease the Al content in a zeolite catalyst, leading to an increase in the Si/Al ratio. However, there are several factors that influence the zeolite dealumination process, such as the zeolite topology and the dealumination method used. As previously reported, Beta (BEA), mordenite (MOR), and ZSM-5 (MFI) zeolites all exhibit distinct degrees of dealumination,49 which decrease in the order BEA > MOR > MFI,49 revealing that MOR and MFI possess less Al flexibility than does BEA. Palella et al.50 compared the hydrothermal stabilities of Cu-SAPO-34 (CHA) and Cu-ZSM-5 during N2O direct dissociation. It was found that Cu-SAPO-34 demonstrated higher hydrothermal stability (surviving under steaming at 600 °C for 80 h) compared with Cu-ZSM-5 (surviving under steaming at 550 °C for 60 h). A spectroscopic Fourier transform infrared (FT-IR) investigation of the adsorbed CO further suggested that, upon steaming, the concentration of Cu+ cations was not significantly affected for Cu-SAPO-34, whereas a dramatic decrease was observed for Cu-ZSM-5, leading to an obvious loss of activity. In light of these findings, the zeolite structure was concluded to play a major role in the deal3665


Cu-SSZ-13

Cu-SAPO-34 Cu-SSZ-13 monolith Cu-SAPO-34 monolith Cu-ZSM-5

Cu-ZSM-5

Cu-ZSM-5

Cu-ZSM-5

Cu-ZSM-5

Cu-ZSM-5

Fe-ZSM-5

Fe-ZSM-5

Fe-ZSM-5

Fe-ZSM-5

Co-ZSM-5

Cr-ZSM-5

Fe-Cu-ZSM-5

3

4 5

8

9

10

11

12

13

14

15

16

17

18

19

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Fe-Mordenite

Fe-Mordenite

In-Mordenite

20

21

22

7

6

Cu-SSZ-13

2

catalyst

Cu-SSZ-13

1

no.

MOR

MOR

MOR

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

MFI

CHA

CHA CHA

CHA

CHA

CHA

topology

3.8 3.6 1.5 1.4 1.0

Cu2+ ions

Cu2+ ions

Cu2+ ions

Cu2+ ions Cu2+ ions

Cu2+ ions

6.3 11.2 4.5 1.9 2.3 6.3

Fe3+ ions

Co2+ ions

Cr3+ ions

Fe3+ ions, Cu2+ ions Fe3+ ions

Fe2+ ions

In /InO 8.0

2.1

Fe3+ ions

+

1.9

Fe3+ ions

+

3.6

Fe3+ ions

20

240

13

30

25

90

25

20

28

14

100

4.7

1.2

Cu2+ ions

100

Cu2+ ions

4.9

Cu2+ ions

100

58

3.4

Cu2+ ions

100

1.0

8.3

8.3

8.3

Si/Al ratio

Cu2+ ions

1.4

ions

Cu

10.3

active species

2+

metal content (wt %)

351

413

420

300

330

295

326

399

324

350

357

334

298

350

374

385

289

600 266

528

526

478

specific surface area (m2 g−1)

0.21

0.13

0.12

0.11

0.19

0.19

0.26

0.26

0.23

pore vol (cm3 g−1)

WIE

WIE

WIE

IMc

WIE

WIE

WIE

WIE

WIE

CVDb

WIE

WIE

WIE

WIE

WIE

WIE

WIE/washcoat

WIE WIE/washcoat

WIE

WIE

WIEa

preparation method reaction conditions

500 ppm NO, 500 ppm NH3, 500 ppm C3H6 (or not), 5% O2, balance of N2, GHSV = 1.6 × 105 h−1 0.1% NO, 0.12% NH3, 8% O2, 10% CO2, 8% H2O, balance of N2 1000 ppm NO, 1000 ppm CH4, 2% O2, balance of He, GHSV = 1.5 × 104 h−1

500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O, balance of N2, GHSV = 4.0 × 105 h−1 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O, balance of N2, GHSV = 4.0 × 105 h−1 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O, balance of N2, GHSV = 4.0 × 105 h−1 500 ppm NH3, 500 ppm NO, 5% O2, balance of N2 350 ppm NO, 350 ppm NH3, 14% O2, 5% CO2, 10% H2O, balance of N2, GHSV = 6.0 × 104 h−1 350 ppm NO, 350 ppm NH3, 14% O2, 5% CO2, 10% H2O, balance of N2, GHSV = 6.0 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 1000 ppm C3H8, 2% O2, balance of He, total flow rate 100 mL min−1 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O, balance of N2, GHSV = 1.0 × 105 h−1 380 ppm NOx (NO + NO2), NO2/NOx = 10%, 400 ppm NH3, 8% O2, 10% CO2, 8% H2O, balance of N2, GHSV = 3.1 × 104 h−1 1000 ppm NO, 1000 ppm NH3, 5% O2, 6% H2O, balance of He, GHSV = 1.9 × 105 h−1 500 ppm NO, 500 ppm NH3, 500 ppm C3H6(or not), 5% O2, balance of N2, GHSV = 1.6 × 105 h−1 1000 ppm NO, 1200 ppm NH3, 8% O2, 10% CO2, 8% H2O, balance of N2 1000 ppm NO, 1000 ppm NH3, 10% O2, balance of N2, GHSV = 1.5 × 105 h−1 2000 ppm NO, 2000 ppm NH3, 3% O2, balance of N2, GHSV = 3.32 × 105 h−1 1000 ppm NO, 1000 ppm NH3, 3% O2, 6% H2O, balance of N2, GHSV = 4.5 × 104 h−1

320

216

250

175

320

165

220

350

245

275

150

380

225

180

200

250

210

200 250

180

175

190

T50e (°C)

350

325

190

215

280

320

400

175

450

275

325

300

350

220

220

225

T90f (°C)

Table 2. Summary of the Physiochemical Properties and Catalytic Behavior of Zeolite Catalysts for NO Selective Catalytic Reduction (SCR)

300 (98)

300 (95)

300 (74)

300 (84)

340 (90)

340 (97)

N2 yield T (° C) (yield max, %)

400−500 (>90%)

350−500 (100%)

200−400 (100%)

300−500 (>90%)

300−500 (>80%)

150−450 (>80%)

300−500 (>60%) 200−550 (>80%)

225−400 (>80%) 200−550 (>85%) 200−550 (>85%)

temperature window (° C)

36

32

31

35

34

33

32

31

30

29

28

27

26

26

26

26

25

24 25

14

14

14

ref

Chemical Reviews Review


3667

Co-Beta

Co-Beta

VOx/SBA-15

TiOx/SBA-15

Pt-SBA-15

Cu-SBA-15

Cu/Al-SBA-15

Fe-SBA-15

Fe/Al-SBA-15

Cr-SBA-15

Cr/Al-SBA-15

30

31

32

33

34

35

36

37

38

39

40

41

Cu-MCM-41

Fe-Beta

29

45

Fe-Beta

28

Pt/Al-MCM-41

Fe-Beta

27

44

Cu-Beta

26

Pt-MCM-41

Cu-Beta

25

43

Cu-Beta

24

Ag-SBA-15

Cu-Beta

23

42

catalyst

Fe-Ferrierite

no.

Table 2. continued

2.1 4.5 5.8 1.3 1.9 3.6 6.3 0.7 1.10

1 0.39 0.41 1.1 0.44 0.55 0.53

Cu2+ ions

Cu2+ ions

Cu2+ ions

Cu2+ ions

Fe3+ ions

Fe3+ ions

Fe2+ ions

Co2+ ions

Co2+ ions

VOx

TiOx

Pt

Cu2+ ions

Cu2+ ions

Fe3+ ions

Fe3+ ions

Cr2+ ions

Cr2+ ions

BEA

P6mm

P6mm

P6mm

P6mm

P6mm

P6mm

P6mm

P6mm

P6mm

0.94 3.2

Pt

Pt

Cu2+ ions

P6mm

P6mm

P6mm

1

Ag2+ ions 2.1

5.3

P6mm

BEA

BEA

BEA

BEA

BEA

BEA

BEA

BEA

3.9

6.3

Fe2+ ions

FER

active species

topology

metal content (wt %)

50

20

53

22

27

22

25

50

50

50

50

18

Si/Al ratio

905

538

1131

730

575

568

513

614

441

593

702

612

630

686

493

554

493

510

427

459

463

493

243

specific surface area (m2 g−1)

1.35

0.64

1.07

0.99

0.69

0.61

0.80

0.85

0.60

0.84

0.89

0.72

0.81

0.23

pore vol (cm3 g−1)

WIE

IM

IM

IM

IM

IM

IM

IM

IM

IM

IM

IM

IM

WIE

WIE

WIE

WIE

WIE

IM

WIE

WIE

WIE

WIE

preparation method reaction conditions 1000 ppm NO, 1200 ppm NH3, 8% O2, 10% CO2, 8% H2O, balance of N2 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 750 ppm NO, 750 ppm NH3, 9.5% O2, balance of Ar, GHSV = 9 × 104 h−1 500 ppm NO, 500 ppm NH3, 500 ppm C3H6 with or without 5% O2, balance of N2, GHSV = 1.6 × 105 h−1 1000 ppm NO, 1000 ppm CH3CH2OH, 2% O2, GHSV = 1 × 104 h−1 1000 ppm NO, 1200 ppm NH3, 8% O2, 10% CO2, 8% H2O, balance of N2 1000 ppm NO, 1000 ppm CH3CH2OH, 2% O2, GHSV = 1 × 104 h−1 1000 ppm NO, 1000 ppm C3H8, 5% O2, GHSV = 1.5 × 104 h−1 2500 ppm NO, 2500 ppm NH3, 2.5% O2, balance of He, total flow rate 40 mL min−1 2500 ppm NO, 2500 ppm NH3, 2.5% O2, balance of He, total flow rate 40 mL min−1 1000 ppm NO, 4000 ppm C3H6, 10% O2, balance of He, GHSV = 3 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 3000 ppm NO, 3000 ppm C3H6, 1% O2, balance of He, GHSV = 6 × 104 h−1 500 ppm NO, 2500 ppm C2H5OH, 10% O2 in air, GHSV = 2.2 × 104 h−1 1000 ppm NO, 4000 ppm C3H6, 10% O2, balance of He, GHSV = 3 × 104 h−1 1000 ppm NO, 5000 ppm H2, 6.7% O2, balance of He, GHSV = 8 × 104 h−1 500 ppm NO, 500 ppm acetone, air, GHSV = 8.5 × 103 h−1 330

90

150

500

500

420

320

400

120

360

375

320

200

230

200

260

200

240

225

250

T50e (°C)

550

550

600

400

150

430

410

250

275

325

275

T90f (°C)

275 (47)

140 °C (85)

430 (73)

250 (40)

350 (65)

250 (55)

380 (80)

340 (77)

340 (84)

420 (94)


150−450 (>60%) 150−160 (>60%) 300−450 (>80%)

150−600 (∼ 90%)

300−500 (>90%)

250−500 (>90%) 280−520 (>70%) 275−440 (>70%) 250−395 (>70%) 350−450 (>70%) 300−500 (100%)


43

42

40

41

23

23

23

23

23

23

40

39

39

38

37

32

37

31

26

26

26

26

32

ref



44

43

275 (47)

300−450 (>85%) 275 (47) 400 280 WIE 1.22 952 20 P6mm Rh/Al-MCM-41 47

umination process. In fact, by considering the results of the above investigations in combination with the corresponding zeolitic structure parameters (see Table 1), we find a hint that the pore aperture of the zeolite probably constitutes one of the main factors influencing its hydrothermal stability. A smaller pore size is always associated with a better hydrothermal stability (CHA > MFI > MOR > BEA). This is likely because relatively small pores hinder the diffusion of the extraframework Al (AlxOy) species generated during dealumination, ultimately resulting in less flexibility of the framework Al. 2.2.1.2. Correlation with Acidity and Ion-Exchange Capacity. Because each framework Al center has one less nuclear charge than does a framework Si center, each Al center has a formal negative charge and therefore requires additional cations for neutralization. Brönsted acid sites are thus formed through the neutralization of the Al centers by protons (AlO4−H+), wherein the compensating protons are connected to the framework oxygen atoms, exhibiting a strong acidity. Hence, the number of zeolite acid sites increases with an increasing amount of framework Al (or a decreasing Si/Al ratio). The classification of zeolites according to their Si/Al ratios, which are associated with their acid/base properties and stabilities, is summarized in Table 3. Moreover, the protons of

a Wet ion exchange method. bChemical vapor deposition. cImpregnation method. dIsomorphic substitution. eTemperature corresponding to 50% nitrile gas conversion. fTemperature corresponding to 90% nitrile gas conversion.

reaction conditions

ISd 1.09 1.9 Cu2+ ions, Al3+ ions 46

P6mm

catalyst

Cu/Al-MCM-41

no.

topology

active species

100

1041

preparation method pore vol (cm3 g−1) Si/Al ratio

specific surface area (m2 g−1) metal content (wt %)

Table 2. continued

Review

500 ppm NO, 500 ppm acetone, air, GHSV = 8.5 × 103 h−1 1000 ppm NO, 1000 ppm C3H6, 2% O2, balance of N2, total flow rate 250 mL min−1

T50e (°C)

T90f (°C)



ref

Chemical Reviews

Table 3. Acid/Base Properties and Stabilities of Zeolites with Different Si/Al Ratiosa Si/Al ratio

zeolite

low (1−1.5)

FAU (A, X)

medium (2−5)

ERI (erionite) CHA (SSZ-13, SAPO-34) FAU (Y) CLI (chinoptilolite) MFI (ZSM-5) MOR (modenite) BEA (Beta) FER (ferrierite)

high (>10)

a

acid/base properties relatively low stability of lattice low stability in acids high stability in bases high concentration of acid groups of medium strength

relatively high stability of lattice high stability in acids low stability in bases low concentration of acid groups of high strength

Reprinted with permission from ref 51. Copyright 2006 Wiley-VCH.

Brönsted acid sites can be exchanged with metal cations to form active centers. Therefore, the ion-exchange capacity of a zeolite is related to its Al3+ content. A lower Si/Al ratio results in a higher ion-exchange capacity, which is likely beneficial for the catalytic abatement of N-containing exhausts. The correlation between the acidity and catalytic behavior of a zeolite catalyst for Ncontaining exhaust abatement is described in detail in section 2.2.2. 2.2.2. Acidity. Zeolites are porous crystalline aluminosilicates that are widely used in heterogeneous catalysis. One property of these materials that is essential to a broad range of catalytic applications, especially in the petrochemical industry, is their strong acidity. There are three categories of Al that are present in zeolite: the framework Al atoms, which are responsible for the Brönsted acidity, the partially coordinated Al atoms (structural defects) of the Lewis acid sites, and the extraframework Al species, which exist in monomeric, polymeric, neutral, and cationic states. Among these, the framework Al 3668


Chemical Reviews

Review

catalytic behavior of this material. Similar results have also been reported by Qu et al.,62 who studied the role of Al2O3, SiO2, NaY, and TiO2 supports on the behaviors of Ag-based catalysts during NH3-SCO. The surface acidity was hypothesized to remarkably enhance the N2 selectivity. In addition, the role of Al in the structure and reactivity of the active centers of Fe-ZSM-5 was investigated by Berlier et al.63 on the basis of IR spectroscopy using NO as a probe molecule. It was revealed that the framework Al promoted the dispersion of extraframework iron species. Isolated iron ions with one or two Al atoms in the immediate vicinity were highly active in this selective oxidation. An understanding of the influence of Al on the formation and stabilization of extraframework iron species could shed further light on the superior activity of Fe-ZSM-5. 2.2.2.2. N2O Decomposition. The incorporation of transition-metal ions into zeolites yields bifunctional catalysts, wherein active centers and acid sites can coexist. As reported by Kaucký et al.,64 the presence of Al Lewis acid sites, generated by the partial dehydroxylation of Brönsted acid sites undergoing high-temperature treatment, in the vicinity of active Fe ions improves N2O decomposition activity by accelerating the rate of recombination of neighboring oxygen atoms. This is attributed to the fact that the electron acceptor of a neighboring Lewis acid site affects the electronic properties of Fe cations, thus enhancing the recombination of oxygen atoms over Fe sites. Boron et al.65 prepared Fe-BEA, Fe-MFI, and Fe-MOR catalysts with 1 wt % Fe through a combined ion-exchange and impregnation procedure. Pseudotetrahedral Fe(III) species were observed in the Fe-BEA by means of UV−vis,46 Fe Mössbauer, and X-ray photoelectron (XPS) spectroscopy characterizations. By contrast, both pseudotetrahedral and octahedral Fe(III) species were observed for the Fe-MFI and Fe-MOR zeolites. Measurements of N2O decomposition activity suggested that Fe-MFI and Fe-MOR demonstrated much higher catalytic activities than did Fe-BEA, which was attributed to the presence of welldispersed octahedral Fe(III) by virtue of the much stronger acid sites compared with those identified in Fe-BEA. Since the successful synthesis of a template-free, Al-rich BEA* zeolite with an Si/Al ratio of ∼4.5 was achieved by Xie et al.66 in 2008, this type of zeolite has been frequently used because of its excellent catalytic characteristics. Sazama et al.67 studied the acidity and redox activity of template-free, Al-rich H-BEA* and Fe-BEA* zeolites (Si/Al = 4.6) for the decomposition of N2O and the NH3-SCR of NOx. It was found that the Al-rich H-BEA* displayed a high concentration of Brönsted and Lewis acid sites. One advantage of the high concentration of framework Al atoms in Al-rich BEA* lies in the increased number of counter species of Fe ions exhibiting exceptional redox properties, which is reflected in the enhanced reaction rates for N2O decomposition and the SCR of NOx into molecular nitrogen. Figure 2 clearly illustrates the structures of conventional H-BEA (Si/Al = 15) and Al-rich (Si/Al = 5.4) H-BEA*, with 4 and 10 Al atoms, respectively, per unit cell; the low-Si/Al model (the Al-rich BEA* model) exhibits a higher content of exchangeable Brönsted acid sites than does the high-Si/Al model (the typical Si-rich BEA model). 2.2.2.3. NH3-SCR. The use of Fe- and Cu-modified zeolite catalysts for NH3-SCR has been extensively reported, and the structural and electronic properties of the active centers of the metallic species have been characterized in these studies.68−70 However, the Brönsted acid sites of Fe- and Cu-zeolites are also useful in the SCR process. According to previous reports71,72 regarding the standard SCR process over Fe-ZSM-5, NO

located at the Brönsted acid sites is most important to the performance of zeolite catalysts. It affects various properties of the zeolite, such as the density of negative framework charges, the cation-exchange capacity, the density and strength of the zeolitic acidity, the thermal stability, the hydrophilic and hydrophobic surface properties, and the unit cell dimensions.52,53 Moreover, the stabilization of the counter metallic ions as mono- or polyvalent cations as well as their metal oxo species on the Brönsted acid sites causes the metal-doped zeolites to act as highly efficient and selective catalysts for environmental pollution control. Spectroscopic studies using various probe molecules have been found to be an effective means of clarifying the nature and number of acid sites in heterogeneous catalysts. Two major types of probes have been developed: (i) probe molecules such as NH3, pyridine, or amines chemically bond with the protons of hydroxyl groups, which can be utilized to evaluate the concentration of acid sites in zeolites, whereas (ii) aromatics, olefins, CO, and H2S can be used to acquire information regarding the strength of protonic sites and the accessibility of the Brönsted acid sites to the probe molecules. Niwa et al.54,55 investigated the acidity of zeolites by means of the temperatureprogrammed desorption (TPD) of ammonia, wherein the number and strength of the acid sites in the zeolites were measured, with the intent of determining the underlying principle giving rise to zeolitic acidity. The following conclusions were drawn: (i) the number of Brönsted acid sites was equal to that of tetrahedrally coordinated aluminum atoms in the zeolite framework, and (ii) the strength of the acidity was independent of the number of acid sites and was instead determined by the zeolite topology. For example, zeolite crystals are ordered in terms of acidic strength as follows: MOR > MFI > BEA > FAU.54 Zhang et al.56 investigated the acidity of ultrastable Y (USY), mordenite, and ZSM-12 zeolites at various Si/Al ratios on the basis of NH3-TPD and FT-IR spectroscopy. At a Si/Al ratio of 0.5) result in the formation of cobalt oxides, which exist as the major species in overexchanged zeolites. By contrast, at lower Co loadings (Co/ Al < 0.3), Co is predominantly present as monatomic Co2+ cations located at the ion-exchange positions. The redox properties of Co-modified zeolite catalysts can be investigated by means of H2-TPR, through which two H2 reduction regimes, related to the reduction of the different cobalt species, have been distinguished. The behavior in the high-temperature regime (600−900 °C) is commonly attributed to the reduction of the monatomic Co2+ cations at the ion-exchange sites,108,109whereas that in the low-temperature regime (220−500 °C) is related to the reduction of cobalt oxide species. Although the H2 reduction peak for Co2+ cations is located in the high-temperature region, Co2+ cations have been confirmed to be the active sites for NOSCR and N2O decomposition.107 2.2.3.2. Zeolitic Topology. In addition to the fundamental modifications to the redox properties of metallic zeolite catalysts that are induced by the introduced metals, the zeolite topologies can also modify the redox abilities of these catalysts by affecting the dispersion of the loaded metals. The effect of the zeolite crystal structure on metal dispersion is closely associated with the geometrical positions at which the exchanged or doped metals are implanted as well as the microporosity of the zeolite, which is, in turn, correlated with the specific surface area, as suggested by Yokoyama et al.110 for MFI zeolites and by Li et al.98 for FER zeolites. Similarly, kinetic studies have also suggested that the transition-metal ions exhibit diverse reducibilities depending on the location of the metal ions in the zeolite skeleton.111 Kwak et al.112 investigated the redox abilities of Cu-modified SSZ-13, ZSM-5, Beta, and Y catalysts on the basis of H2-TPR. One intense H2-TPR peak at 230 °C associated with a broad shoulder at ∼300 °C was observed for Cu-SSZ-13, which was related to the reduction of Cu2+ cations to Cu+ cations at different exchange sites (site IV and site I, respectively). Similarly, two H2 reduction peaks at 195 and 310 °C were discerned for Cu-Y, which were attributed to the reduction of Cu2+ to Cu+ inside supercages and sodalite cages, respectively. For Cu-Beta and Cu-ZSM-5, two types of reduction peaks corresponding to Cu2+ → Cu+ and Cu+ → Cu0 reductions were clearly observed. It was thus recognized that the zeolite topology has a remarkable effect on the distribution of the loaded metal species, which further influences the corresponding redox properties of the catalyst. In light of the findings discussed above, it can be concluded that there are two main factors affecting the redox abilities of zeolite catalysts: the nature of the loaded metal species and the zeolite topology. The latter factor affecting the local microenvironment exerts a considerable influence on the chemical status of the loaded metal species, which indirectly affects the final redox ability of the zeolite catalyst. Herein, a H2-TPR database concerning the redox properties of zeolites modified with transition metals (Fe, Cu, and Co) is

undesired byproducts of NOx can potentially be generated during the catalytic oxidation of NH3 and nitrile gases, leading to secondary pollution. In light of these concerns, increasing attention has been paid to transition-metal-modified zeolites because of their satisfactory catalytic activity and much lower price compared with those of noble-metal-modified catalysts. Investigations of transition-metal-modified zeolite catalysts, especially Fe-, Cu-, and Co-modified zeolites, for the elimination of N-containing waste gases have become predominant. The nature of the introduced metals significantly influences the redox properties of the resulting zeolite catalysts, which are relevant to their catalytic behaviors. 2.2.3.1.1. Cu-Zeolites. Copper-modified zeolites are a subject of continual interest from the catalytic community, primarily because of their high activity; particularly notable are Cu-ZSM-5 (MFI), Cu-Beta (BEA), and the recently developed Cu-SSZ-13 and Cu-SAPO-34 with the CHA structural topology for the selective catalytic reduction of NOx and other reactions of environmental interest.11,12,14,16,68−70,76,77,81 The copper species in these modified zeolites predominantly exist in two types of states, depending on the level of copper loading: Cu2+/Cu+ cations dominate at lower loading amounts, whereas CuO appears at higher loading amounts.11 A stepwise reduction process has been identified for Cu2+ cations, consisting of Cu2+ → Cu+ and Cu+ → Cu0, during the H2-TPR process, whereas a one-step reduction of CuO → Cu0 occurs for CuO, for which the H2 reduction peak commonly overlaps with that of Cu2+ → Cu+.11 Pereda-Ayo et al.17 noted the effects of the different copper species on the SCR activity of Cu-zeolite catalysts. The CuO generated at high Cu loadings can catalyze the oxidation of NO into NO2, which is favorable for low-temperature deNOx activity because the NH3 + NO + NO2 (fast SCR) reaction is known to be faster than the NH3 + NO reaction (standard SCR) at low temperatures. By contrast, isolated Cu2+ ions are responsible for the outstanding NOx conversion observed at high temperatures. Bulanek et al.82 have suggested that both the concentration and the distribution of framework Al play crucial roles in determining the redox properties of the loaded Cu species. 2.2.3.1.2. Fe-Zeolites. In Fe-exchanged zeolites, various types of iron species can form, including Fe or Fe−oxo cations, Fe− oxo oligomers, and Fe2O3 oxide particles, depending on the total amount of metal loading. The Fe or Fe−oxo cations that play a compensatory role at Brönsted acid sites are commonly understood to serve as active centers for the catalytic abatement of N-containing waste gases. As reported by Lond and Yang,83,84 the iron species in Fe-exchanged MFI and MOR zeolites predominantly exist in a cationic form and serve as reactive sites for NO-SCR with NH3 (NH3-SCR). Moreover, several groups have recently come to the common conclusion that isolated Fe cation sites can act as active sites for N2O dissociation85−87 and NO-SCR.88 Oligomeric and clustered species have been found to limit SCR activity through the oxidation of the corresponding reductant.88,89 For all of these reactions, the redox properties of the loaded metal species are of considerable importance. Therefore, there have been many studies devoted to the detailed characterization of Fe-modified zeolite catalysts.90−92 Previous H2-TPR investigations93−100 have demonstrated that (i) H2 consumption at temperatures of up to 480 °C corresponds to the reduction of Fe(III) to Fe(II) (isolated cations, oligonuclear and nanoparticle), (ii) the TPR peaks observed in the range of 480−730 °C correspond to the reduction of FeO (from nanoparticles) to Fe0, and (iii) H2 consumption above 730 °C 3671


Chemical Reviews

Review

Table 4. H2-TPR Database for Fe-Zeolite Catalysts H2 reduction temperature (°C) Fe-zeolite

preparation method

Fe3+ → Fe2+

a

Fe-USY Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 Fe-ZSM-5 (15%) Fe-ZSM-5 (25%) Fe-ZSM-5 (35%) Fe-SBA-15 Fe-Beta a

Fe2O3 → Fe3O4

WIE WIE CVDb WIE SSIEc IMd WIE CVD IM

Fe3O4 → Fe0

480 430 410 380 360

FeO → Fe0

Fe2+ → Fe0

ref

477−727

>727

94 95 96

360 380

>830 >830 >830

520, 620

323 318 328

540 518 475

IM IM

97

98

419 395

>800 >730

99 100

Wet ion exchange method. bChemical vapor deposition method. cSolid-state ion exchange method. dImpregnation.

Table 5. H2-TPR Database for Cu-Zeolite Catalysts H2 reduction temperature (°C) Cu-zeolite

a

Cu → Cu+

preparation method

Cu-ZSM-5 Cu-SSZ-13

WIE WIE

Cu-Y

WIE

Cu-Beta Cu-ZSM-5 Cu-ZSM-5 Cu-SSZ-13 Cu-Beta Cu-ZSM-11 (Si/Al = 15) Cu-ZSM-11 (Si/Al = 25) Cu-ZSM-11 (Si/Al = 36) Cu-Beta Cu-SAPO-34 (0.98%) Cu-SAPO-34 (1.42%) Cu-SAPO-34 (1.89%) Cu-SAPO-34 (2.89%) Cu-ZSM-5 Cu-SBA-15 Cu-Y Cu-MCM-41

WIE WIE WIE WIE WIE WIE

a

CuO → Cu0

2+

207 230 300 (shoulder) 195 (Cu2+ inside supercage) 310 (Cu2+ inside sodalite cage) 200 240 248 266 220 270

WIE WIE

211 256 261 256 260 270

IMb IM IM modified hydrothermal

315

240 279 311 246

303 300 319 310 228

259 171

Cu+ → Cu0

348

390 400 356 431 279 410 450 470 335, 422 358 360 383 362 400 288 >800 483

ref 112

116 11

117

65 118

119 99 120 121

b

Wet ion exchange method. Impregnation.

300 °C. As shown in Table 6, which lists the H2-TPR data for Co-zeolites, Co species exist on the surfaces of these catalysts in the forms of CoO+ oxo/Co2+ cations and CoxOy (Co3O4, CoO, and CoOx clusters). The CoO+ oxo cations display the best H2 redox ability, with a TPR peak centered at ∼250 °C; meanwhile, the Co2+ cations exhibit the worst redox ability (Co2+ → Co0), associated with a reduction temperature of higher than 700 °C. The CoxOy species can be reduced in a temperature range of 250−700 °C (Co3O4 → CoO, CoO → Co0, and CoOx → Co0). Although Co2+ cations are barely reduced by H2, they serve as the active sites for related heterogeneous catalytic processes such as N2O decomposition. In addition to that, as for the metal-modified zeolite catalysts, metallic cations acting as active sites play the dominating roles during the catalytic abatement of N-containing waste gases.113,114 The redox abilities of these cations essentially

presented in Tables 4−6. The loaded metal species in the zeolite catalysts predominantly exist in two types of chemical states: metal/oxometal cations and metal oxides (MxOy). With an increasing reduction temperature, the Fe species can be successively reduced in the following sequence: Fe2O3 → Fe3O4, Fe3+ → Fe2+ (metal cations), Fe3O4 → Fe0, FeO → Fe0, and Fe2+ → Fe0 (metal cations) (see Table 4). As noted here, the reduction of Fe2+ cations to Fe0 requires a reduction temperature higher than at least 700 °C. Therefore, the Fe3+ → Fe2+ reduction constitutes the main process responsible for the catalytic activity of Fe-modified catalysts. For Cu-modified zeolite catalysts, the loaded Cu species exist in the chemical states of Cu2+ cations and CuO (see Table 5). Two-step reduction processes are observed for the loaded Cu2+ cations: Cu2+ → Cu+ (100−300 °C) and Cu+ → Cu0 (300−500 °C). The reduction of CuO occurs at temperatures of approximately 250− 3672


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Table 6. H2-TPR Database for Co-Zeolite Catalysts H2 reduction temperature (°C) Co-zeolite

a

preparation method d

Co-SBA-15 Co-ZSM-5 Co-ZSM-5 Co-USY Co-ZSM-5 Co-ZSM-5 Co-ZSM-5 Co-ZSM-5 Co-ZSM-5 Co-Y

IM IM WIEa WIE WIE IM SSIEb CVDc WIE WIE

Co-Y Co−Al-Beta

IM WIE

Co3O4 → CoO

CoO+ oxo cations 322 360 300 320 235 250 220 230

CoO → Co0

CoOx

Co2+

457 437 550 530 390 385

325 300 (inside supercage) 570 (inside sodalite cage) 660 (hexagonal prisms) 342, 372

740 700 707 695 707 650

507

ref 99 122 123 108

124 125

126

Wet ion exchange method. bChemical vapor deposition method. cSolid-state ion exchange method. dImpregnation.

influenced the final catalytic behaviors of the zeolite catalysts. As noted from Tables 4−6, Cu2+ cations can be more readily reduced (Cu2+ → Cu+) in comparison with Fe3+ ions (Fe3+ → Fe2+). A similar trend is also observed during the further reductions of Cu+ → Cu0 and Fe2+ → Fe0. This finding implied that the Cu-zeolite exhibited superior redox characteristics with respect to Fe-zeolite. Thereafter, the diverse redox abilities of Cu- and Fe-zeolite catalysts resulted in their distinct catalytic behaviors. This phenomenon was essentially significant in the NH3-SCR reaction. As reported,15 the Cu-zeolite catalysts of CuZSM-5, Cu-SSZ-13, Cu-SAPO-18, and Cu-SAPO-34 showed much higher deNOx activities than Fe-zeolites (Fe-ZSM-5, FeSSZ-13, Fe-SAPO-18, and Fe-SAPO-34) did in the temperature range of 150−350 °C, which was mainly related to much better redox features of the former. However, good redox abilities of Cu-zeolites also caused an excessive consumption of the reducing agent of NH3 in the high-temperature range (T > 350 °C), which led to a severe decline of deNOx activity. In contrast, the SCR activities of Fe-zeolites were gradually developed along with increasing temperatures from 350 to 550 °C. On the basis of the distinct redox abilities, Cu-zeolites show ideal low-temperature SCR activities and poor high-temperature N2 selectivities, whereas the opposite occurs for the Fe-zeolites during the NH3-SCR process. Table 6 depicts the H2-TPR data of Co-zeolites. It is verified that Co2+ cations were able to be reduced merely in the hightemperature range (T = 700−900 °C). Nevertheless, one should not make a conclusion that Co-zeolites possess poor redox abilities. For example, in our previous work on N2O direct dissociation over Fe-, Co-, and Cu-BEA zeolites, it was verified that Co-BEA exhibited the highest N2O decomposition efficiency, which was correlated with its excellent redox ability.115 In fact, the Co-zeolite was scarcely investigated in the NH3-SCR reaction system in comparison with Cu- and Fezeolites, probably owing to its strong oxidation ability resulting in relatively worse SCR performances.

containing volatile organic compounds (VOCs) and NH3 yielded from chemical industry has become more and more urgent. Traditional incineration is not suitable for the elimination of these gases because it requires a high temperature (∼850 °C),127 causing secondary generation of NOx pollutants. In contrast, selectively catalytic oxidation (SCO) is deemed as an efficient method to catalyze the nitrile gases as well as NH3 into harmless products of CO2, H2O, and N2, wherein atomic N, with valences of −5 (nitrile gases) and −3 (NH3), is selectively oxidized into N2, with a valence of 0. 3.1. Catalytic Performance

3.1.1. SCO of Nitriles. 3.1.1.1. HCN. Hydrogen cyanide (HCN) is one poisonous liquid that is readily evaporated into the air due to its extremely low boiling point of 26 °C. It can be released from various sources, such as vehicle exhaust emissions,128 petrochemical processing,129 coal gasification,130 and NO-SCR processes with hydrocarbon reductants.131 However, only a few publications have focused on the catalytic purification of HCN in the gas phase. In 1952, Marsh el al.132 found that Al2O3 could be utilized for HCN hydrolysis in an oxygen-free atmosphere. Nanba et al.133 reported that H-FER zeolite was reactive for HCN hydrolysis in the presence of water and oxygen, whereas NH3 and CO were simultaneously generated. In addition to the hydrolysis reaction pathway, the catalytic oxidation for HCN abatement was also reported. Miyadera134 pointed out that HCN could be efficiently removed by using it as a reductant in the SCR of NOx reaction over CuSO4/TiO2 catalyst. Peden and co-workers135 gave a detailed investigation of HCN oxidation over Pt/Al2O3 catalyst, the product analysis of which indicated that harmful NO and NO2 were generated in addition to the targeted N2, CO2, and H2O during the reaction. According to the literature reports,134−136 noble-metal catalysts are undesirable for HCN oxidation because of the poor activity of Rh/TiO2 at low temperatures and the prior conversion to NOx for Pd/TiO2 and Pt/TiO2. Recently, Szanyi et al.137 compared the activity of HCN oxidation by NO2 over Na- and Ba-Y zeolites. The HCN adsorption strength achieved over Ba-Y was higher than that over Na-Y, and HCN more easily reacted with NO2 on the surface of Ba-Y at 200 °C, forming N2, CO, CO2, HNCO, NO, NO2, and C2N2. Thereafter, Krocher and Elsener136 conducted a comprehensive investigation on the hydrolysis and oxidation of

3. SELECTIVE CATALYTIC OXIDATION (SCO) OF NITRILES (HCN/CH3CN/C2H3CN) AND AMMONIA (NH3) With the increasing environmental concerns of public and rigid environmental restrictions issued by governments, removal of N3673


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Cu2+ ions appeared as the metal loading dropped to 2.9 wt % or below. Three types of isolated Cu2+ ions (square-planar, squarepyramidal, and distorted square-pyramidal) were distinguished by ESR. However, only square-planar Cu2+ was verified to be responsible for the high nitrile conversion as well as N2 selectivity of Cu-ZSM-5. In addition, Nanba et al.142 also investigated a series of silver catalysts (Ag supported on ZSM-5, Al2O3, TiO2, MgO, ZrO2, TiO2, and SiO2) for C2H3CN catalytic abatement, with the C2H3CN conversion increasing in the order Ag-ZSM-5 < Ag/Al2O3 < Ag/TiO2 < Ag/SiO2 < Ag/MgO < Ag/ ZrO2. Although Ag/ZrO2 and Ag/MgO showed relatively higher activities for C2H3CN elimination, substantial amounts of NOx were formed over them, as depicted in Figure 3. Ag/TiO2 and

gaseous HCN over heterogeneous catalysts, including metal oxide, zeolite, and noble-metal-supported catalyst. According to their works, TiO2, Fe-ZSM-5, and Al2O3 were classified as hydrolysis catalysts, with HCN elimination following a hydrolysis mechanism. On the contrary, Pd- and Pt-based catalysts and Cu-ZSM-5 belonged to the oxidization catalysts, associated with an oxidation mechanism. Among the hydrolysis catalysts, anatase TiO2 showed the highest HCN hydrolysis activity, approximately twice as active with respect to Al2O3. For the oxidization catalysts, Cu-ZSM-5 exhibited not only a satisfactory HCN conversion (similar to those of Pd- and Ptcontaining catalysts) but also excellent N2 selectivity, being proposed as the most promising zeolite catalyst for HCN removal. 3.1.1.2. CH3CN. Acetonitrile (CH3CN) mainly comes from the tail gas of industrial acrylonitrile plants in significant amounts, and its removal has attracted attention due to its potential hazard. CH3CN can be readily decomposed into the highly toxic HCN in the body once it directly contacts humans. Zhuang et al.138 reported photooxidation for CH3CN abatement over TiO2 catalyst. Two kinetic steps were proposed: (i) the CH3 moiety of the adsorbed CH3CN was initially photooxidized, liberating a CN− radical species, and (ii) the CN− radical species subsequently attacked the Ti−O bonds on the TiO2 surface to produce Ti-NCO species, which could be further converted into the final products of CO2 and N2. Mesoporous SBA-15 is regarded as an ideal support for active components due to its superhigh surface area, ordered pore arrangement, adjustable pore sizes from 3 to 30 nm, and high hydrothermal and thermal stability.5,23 A series of M-SBA-15 [M = 3d transition metals (Cu, Co, Fe, V, Mn) and noble metals (Pd, Ag, Pt)] catalysts were prepared via impregnation and were applied for the acetonitrile selective catalytic combustion in our previous work.99 CH3CN conversion of the investigated catalysts follows a trend of Pt-SBA-15 > Pd-SBA-15 > Cu-SBA-15 > Co-SBA-15 > Fe-SBA-15 > V-SBA-15 > Ag-SBA-15 > Mn-SBA-15 > SBA-15. In addition to the desired main product of N2, harmful byproducts (NO, NO2, N2O, NH3, and CO) were observed during this catalytic combustion. The corresponding activities and selectivities were verified to be associated with both the redox properties and chemical natures of the loaded metals. Among the prepared M-SBA-15 samples, Cu-SBA-15 exhibited a nearly complete CH3CN conversion together with N2 selectivity of 80% at T > 350 °C, indicating that it is a promising material for CH3CN catalytic removal. 3.1.1.3. C2H3CN. As a N-containing VOC, acrylonitrile (C2H3CN) has also attracted special concern because of its high carcinogenicity.139 Its half-life in the atmosphere is estimated to be 2−3 days, which is longer than those of formaldehyde or benzene. The group of Nanba has performed a large amount of research on the SCO of C2H3CN.119,140−142 A screening of catalysts, including various metal components (Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Ga, Pd, Ag, and Pt) supported on several metal oxide supports (Al2O3, SiO2, TiO2, ZrO2, and MgO) and ZSM-5, was conducted,119 wherein Cu-ZSM-5 possessing 100% C2H3CN conversion at above 350 °C associated with ∼80% N2 selectivity was believed to be suitable for C2H3CN abatement. Nanba and co-workers140 further investigated the active sites of Cu-ZSM-5 through X-ray diffraction (XRD), H2-TPR, and electron spin resonance (ESR) measurements. It was suggested that large amounts of bulk CuO and highly dispersed CuO existed in Cu-ZSM-5, with Cu loading amounts of 6.4 and 3.3 wt %, respectively, whereas

Figure 3. Product selectivity in C2H3CN decomposition at 400 °C over Ag catalysts. The mass of each catalyst was 0.1 g, and the feed gas composition was ca. 200 ppm C2H3CN and 5% O2 in He, with a total flow rate of 160 mL min−1. Reprinted with permission from ref 142. Copyright 2008 Elsevier.

Ag/SiO2 exhibited much higher N2 selectivities (>80% at T = 400 °C) with respect to the others. XRD, UV−vis, and H2-TPR were employed to provide further information about the chemical states of the loaded Ag species over these different zeolitic supports. It was noted that Ag species over ZSM-5 and Al2O3 mainly existed in the form of silver oxides, whereas mostly metallic Ag was observed over ZrO2 and MgO. Both metallic and oxidized species were found on the TiO2 and SiO2 supports. Therefore, the high activity, as well as N2 selectivity, of Ag/TiO2 and Ag/SiO2 was due to the coexistence of oxidized and metallic Ag, wherein silver oxides promoted the hydrolysis of C2H3CN to NH3, and metallic Ag was necessary for NH3 selective oxidation into N2 in a successive step. Recently, our group prepared various metal-doped mesoporous catalyts (Cu-, Co-, Fe-, and Pt-SBA-15, Cu-SBA-16, and Cu-KIT-6) for C2H3CN-SCO.143 On the basis of the characterizations of N2 adsorption/desorption, XRD, X-ray fluorescence (XRF), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and H2-TPR, it was found that (i) the metallic cations were the main species over Cu-, Co-, and FeSBA-15, (ii) CuO was largely formed over Cu-KIT-6 and CuSBA-16, and (iii) the metallic Pt particles constituted the major species on Pt-SBA-15. The activity measurements suggested that Cu-SBA-15 exhibiting a nearly complete C2H3CN conversion associated with the N2 selectivity of approximately 64% at T > 400 °C was the best catalyst. NH3 was observed as the major 3674


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°C, respectively. However, only a slight decrease of NH3 conversion was observed for Fe-ZSM-5: as high as 94% NH3 conversion was obtained at 500 °C. In light of the above investigations, Fe-ZSM-5 was proposed to be a promising candidate for NH3-SCO. Furthermore, iron-exchanged zeolite catalysts with diverse topologies (MFI, MOR, HEU, FAU, BEA, FER, and CHA) were prepared and evaluated for NH3 combustion in the presence of excess oxygen.146 Among these catalysts, Fe-MFI, Fe-MOR, and Fe-HEU showed excellent SCO performances: greater than 97% NH3 conversion associated with nearly 100% N2 selectivity was achieved. It was revealed that (i) Fe3+ cations served as the active sites for NH3-SCO and the variable valence of Fe cations (Fe3+ and Fe2+) was responsible for the ideal SCO behavior of the Feexchanged zeolites, (ii) higher iron loading, a lower Si/Al ratio, and a lower NH3 concentration were favorable for higher activities of NH3 → N2 oxidation, and (iii) for the investigated Fe-exchanged zeolite catalysts, there was a good correlation between the activity for NH3-SCR and the selectivity of NH3SCO toward N2. Namely, better SCR activity accompanied higher SCO selectivity for N2. As well-known, the preparation method could essentially influence the chemical states of the loaded metals over zeolites, which resulted in diverse catalytic behaviors of the prepared zeolite catalysts. Qi et al.154 utilized chemical vapor deposition (CVD) to prepare various Fe-exchanged zeolites (Fe-MFI, FeMOR, Fe-FER, Fe-BEA, Fe-FAU) for NH3-SCO investigation. This preparation method efficiently increased the iron loading, which made the prepared Fe-exchanged zeolites highly active for the SCO of NH3 to nitrogen. The influence of the CVD temperature was also taken into account, which suggested that the high temperature of 700 °C was optimal for the preparation of highly active Fe-zeolite catalysts. At 700 °C, the Fe2+ ions were found to be mainly located at the γ-site of ZSM-5, which was more stable than the α- and β-sites and was proposed to be the active site. Among different Fe-zeolite catalysts prepared by CVD at 700 °C, the activity performance decreased in the sequence Fe-MFI > Fe-MOR > Fe-FER > Fe-BEA > Fe-FAU. Fe-MFI showed the best SCO performances, wherein over 99% NH3 conversion and nearly 100% N2 selectivity were obtained at 400 °C under a high space velocity of 2.3 × 105 h−1, which is related to the special zeolitic topology (pore size and pore structure). Zeolites with narrow channel & pore structures favor ammonia oxidation to nitrogen by oxygen.146 Akah et al.155 compared a series of Fe-ZSM-5 catalysts prepared by means of incipient wetness impregnation, ion exchange, and hydrothermal synthesis for NH3-SCO. Among them, Fe-ZSM-5 prepared by incipient wetness impregnation was the most active, showing 100% NH3 conversion and N2 selectivity at 450 °C. The zeolitic activity was found to depend on the amount of Fe loading and the nature of Fe. In addition, the incipient wetness method was thought to be convenient and much cheaper for practical application. Apart from iron-modified zeolites, copper-modified zeolites were also used for NH3-SCO. Gang et al.152,156 investigated the low-temperature NH3 selective oxidation toward N2 on the basis of the copper-containing catalysts (Cu-Y and Cu/Al2O3). The activity measurement suggested that Cu/Al2O3 was comparable to Cu-Y at high temperatures (>300 °C), achieving 100% NH3 conversion and >95% N2 selectivity. However, in the lowtemperature range, the Cu-Y zeolite catalyst was superior. Further characterizations by TPD, TPR, UV−vis, and TEM indicated that three types of copper species existed in Cu-Y,

product for Fe-SBA-15, whereas NOx was largely formed over Co- and Pt-SBA-15. The different catalytic behaviors of the Cu-, Co-, Fe-, and Pt-SBA-15 samples were correlated with the special physiochemical properties of the loaded metal species. In comparison with the other mesoporous copper catalysts (CuKIT-6, Cu-SBA-16), the higher SCO activity of Cu-SBA-15 was attributed to its larger pore aperture facilitating copper dispersion with Cu2+ cations formed and acting as active centers. 3.1.2. SCO of NH3. Ammonia has negative effects on human health and the biological environment. Therefore, the elimination of NH3 from waste gas streams is becoming increasingly urgent. Several types of materials have been proposed as reactive for the SCO of NH3 toward N2, such as Fe, Cu, Pt, Rh, and Pd ion-exchanged ZSM-5,144−146 copper, nickel, iron, and manganese oxides supported on γ-Al2O3,147 Fe2O3, V2O5, CuO, and Cu−Mn supported on TiO2,148 and manganese oxide−silica aerogels.149Although noble metals, such as Pt, Pd, Rh, and Ag, are active for NH3-SCO at low temperatures, the poor N2 selectivity, being a main drawback, is not satisfactory for the environmental requirements.150 Therefore, more attention is focused on transition metals to improve N2 selectivity during NH3 catalytic abatement.151,152 As far as we know, there is only one review focusing on NH3SCO, previously represented in Chemical Reviews in 1976,153 wherein a large number of catalysts were compared, including metal, metal oxide, and zeolite catalysts. The related NH3 oxidation activity and N2 selectivity, as well as the reaction mechanism, were also introduced. However, as for the zeolite catalyst, only the Y-type zeolite was mentioned in this review. With a rapid development of the synthesis and application of the zeolite catalyst, it makes sense to give an overall review concerning the recent achievements of zeolite catalysis for NH3-SCO. NH3, serving as a reductant, is continuously injected into the NH3-SCR system. To increase the deNOx efficiency, the use of an excess amount of NH3 is desirable. However, this would commonly result in NH3 slip, which causes secondary pollution. Therefore, NH3-SCO should be applied after the SCR system as a successive catalyst bed to oxidize the residual ammonia into N2. As for the NH3-SCO technology, N2 selectivity constitutes an important issue, other than NH3 conversion, to evaluate the catalytic performances of the applied catalysts. Long and Yang145 investigated a series of transition-metal (Cr, Mn, Fe, Co, Ni, Cu) ion-exchanged ZSM-5 for NH3 oxidation. The results showed that the catalytic performances (NH3 conversion and N2 selectivity) were improved in a trend of Co-ZSM-5 ≈ NiZSM-5 < Mn-ZSM-5 < H-ZSM-5 < Pd-ZSM-5 < Cr-ZSM-5 < Cu-ZSM-5 < Fe-ZSM-5 at a high gas hourly space velocity (GHSV = 2.3 × 105 h−1). Among them, Fe-, Cu-, and Crmodified ZSM-5 were active for NH3-SCO and could achieve nearly 100% NH3 conversion to N2 at 450 °C. The high activity of Fe-, Cu-, and Cr-ZSM-5 was assigned to the variable valences of metal cations in these catalysts, being greatly beneficial for oxygen adsorption and activation during NH3-SCO. Relatively lower activities were observed for Mn-, Co-, and Ni-ZSM-5, which was attributed to their stabilized valences not being favorable for oxygen activation. Additionally, on the basis of the fact that the waste streams usually contain water vapor and small amounts of SO2, the effects of H2O and SO2 on the catalytic performances of Cr-, Fe-, and Cu-ZSM-5 were further investigated. The addition of 500 ppm SO2 and 4% H2O significantly suppressed the NH3 elimination for Cr- and CuZSM-5, resulting in only 50% and 65% NH3 conversions at 450 3675


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Recently, Colombo et al.159 prepared one kind of dual-layer monolith catalyst, wherein a commercial Pt/Al2O3 layer (PGM) and an SCR Fe-zeolite layer (on top of the PGM layer) were washcoated in sequence on the cordierite support. NH3 oxidation measurement suggested that no significant difference between the dual-layer catalyst and mechanically mixed catalyst were observed in terms of NH3 conversion (achieving 100% conversion at T = 225−550 °C). However, a much higher N2 yield was observed for the double-layer configuration. A similar finding was observed by Shrestha et al.,160 who conducted a systematic study of NH3 oxidation over the dual-layer monolith catalyst with a combination of Fe-ZSM-5 and Pt/Al2O3. This dual-layer catalyst was evidenced to possess much higher N2 selectivity than that of the mixed catalysts at high temperatures. Meanwhile, it was proposed that the diffusion resistance provided by the Fe-ZSM-5 layer inhibited the excessive depletion of ammonia by a complete oxidation process.

including Cu2+ ions, CuAl2O4, and [Cu−O−Cu]2+ species, wherein the [Cu−O−Cu]2+ species were verified as the active centers of Cu-Y. Lenihan and Curtin157 compared the NH3-SCO behaviors of Cu/Al2O3 and Cu-Beta catalysts, which were, respectively, prepared by the conventional impregnation and wet ionexchange methods. The activity measurement suggested that Cu-Beta displayed much higher SCO activity (T50 = 250 °C, temperature corresponding to 50% NH3 conversion) than that of Cu/Al2O3 (T50 = 300 °C), which was attributed to the different Cu species generated on Cu/Al2O3 (CuO) and CuBeta (Cu2+ ions), wherein the Cu2+ ions acting as the active centers are more active than CuO during NH3-SCO. The influence of H2O was also demonstrated for these two types of samples. The activity was restored by the removal of water from the feed stream for Cu/Al2O3; however, exposure of Cu-Beta to water irreversibly decreased its catalytic activity due to transformation of Cu2+ ions into CuO species. In addition to that, it has been recently reported that the coupling of an SCR zeolite catalyst with a precious-metal one as a dual-layer architecture enables minimal NH3 slip from the NH3−urea SCR systems for vehicle deNOx.158 It relies on the implementation of these bifunctional catalysts, wherein the lower layer is commonly composed of platinum group metal (PGM) based catalysts to oxidize ammonia and the upper layer is composed of SCR catalysts. The NOx generated over the PGM layer due to NH3 unselective oxidation has a chance to further react with NH3 existing over the SCR catalyst layer to yield N2 and greatly inhibit the NH3 slip. The scheme of the dual-layer catalyst is shown in Figure 4.

3.2. Reaction Mechanisms

3.2.1. SCO Mechanism for Nitriles. 3.2.1.1. Hydrolysis Mechanism. For the metal-modified zeolite catalysts, the SCO of nitrile gases can undergo different reaction pathways associated with distinct products (NH3, N2O, NOx, N2) according to the chemical natures of the loaded metals. This section gives a brief summary of the recently proposed mechanisms for nitrile elimination. 3.2.1.1.1. HCN. In 2009, Kröcher and Elsener136 screened heterogeneous catalysts for the purification of gaseous HCN, among which the zeolite catalyst of Fe-ZSM-5 was noted to decompose HCN through a hydrolysis mechanism, wherein NH3 was largely generated. As shown in Figure 5, HCN can be initially hydrolyzed into methanamide in the presence of water vapor. The further hydrolysis of methanamide occurs to generate ammonium formate, which finally decomposes into ammonia and formic acid. The formic acid itself thermolyzes to water and CO. Cant et al.161 also proposed that the generated methanamide could also directly decompose into NH3 and CO in the absence of steam over the metal-exchanged zeolites at temperatures above 250 °C. 3.2.1.1.2. CH3CN. On the basis of DFT study, Barbosa and van Santen162 performed a detailed simulation of CH3CN hydrolysis over Zn(II) ion-exchanged zeolites, generating acetamide. A 4T cluster (Si/Al = 1) of FAU was constructed to model the zinc-

Figure 4. Scheme of the dual-layer catalyst concept. Reprinted with permission from ref 158. Copyright 2012 Elsevier.

Figure 5. Possible reaction pathways of the heterogeneous decomposition of HCN in the gas phase: (red arrows) oxidation; (blue arrows) hydrolysis; (black arrows) thermolysis. Reprinted with permission from ref 136. Copyright 2009 Elsevier. 3676


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Figure 6. Hydration mechanism: (a) water molecule activation and (b) acetonitrile activation. Reprinted with permission from ref 162. Copyright 2001 Elsevier.

pathways, paths C, D, and G were the most favorable due to the smaller number of steps and the relatively low activation energy. In the isomerization route, two types of reaction pathways were proposed (see Figure 7), and each pathway contained two different steps: OH bond rotation and proton transfer from the OH group to the nitrogen atom. The models investigated in this part were chosen from mechanisms C and G in Figure 7. The activation energies of the C−OH bond rotation, as well as tautomerization (proton transfer), were 5.2 and 21.9 kcal·mol−1 for configuration A and 11.4 and 29.7 kcal·mol−1 for configuration B in Figure 7. Therefore, it is implied that configuration A is probably the right configuration for the

loaded zeolite. The reaction mechanisms consisted of three parts: (i) hydration, (ii) isomerization, and (iii) product desorption. In the first part, Zn2+ first reacted with H2O to generate ZnOH+ species and zeolitic protons. Subsequently, a nucleophilic attack of water molecules occurred during the hydration reaction, which became the rate-limiting step over the whole catalytic cycle. Different reaction pathways starting with two reactants of H2O and CH3CN were analyzed (Figure 6, mechanisms A−G) and were divided into two groups: water activation (Figure 6a, mechanisms A−D) and acetonitrile activation (Figure 6b, mechanisms E−G). Among these reaction 3677


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Figure 7. Isomerization and automerization mechanism: water ancillary effect of (a) configuration A and (b) configuration B. Reprinted with permission from ref 162. Copyright 2001 Elsevier.

temperature range of 150−300 °C suggested the generation of NH3 (3388 cm−1), CO2 (2364 cm−1), acetic acid [1738 cm−1, ν(CO)], and acetamide (1571, 1658 cm−1) species. The CH3CN hydrolysis mechanism over Fe-SBA-15 is accordingly proposed in Scheme 1. As noted, the adsorbed CH3CN is

isomerization route. In light of the above simulations, H2O, which is known to play an important role in many elementary reaction steps, acted as the proton donor in the hydration and tautomerization steps, facilitating the desorption of the reaction products. In our previous work,99 the CH3CN-SCO mechanism over a series of metal-modified mesoporous-SBA-15-supported catalysts was investigated on the basis of in situ DRIFTS. As shown in Figure 8, two extensive bands at 2260 and 2296 cm−1

Scheme 1. CH3CN-SCO Mechanism over Fe-SBA-15b

a b

σ represents the active site of the supported metal catalyst. Reprinted with permission from ref 99. Copyright 2014 Elsevier.

hydrolyzed into acetamide (eq 3.1), which could generate NH3 and CH3COOH through the further hydrolysis (eq 3.2). Finally, NH3 and CH3COOH are oxidized into N2 and CO2, as well as the other NOx side products (eqs 3.3−3.7). 3.2.1.1.3. C2H3CN. The acrylonitrile (C2H3CN) SCO mechanism for Fe-SBA-15 was investigated on the basis of DRIFTS in our recent work.143 A similar hydrolysis mechanism could be observed (see Scheme 2): (i) the C2H3CN molecule was hydrolyzed into the acylamino species (−CONH2) (eqs 3.8 and 3.9), (ii) a large amount of NH3 intermediate was generated through the continuous hydrolyzation of the −CONH2 species (eq 3.10), (iii) the N2 target product and other NOx byproducts were produced following the NH3-SCO mechanism (eq 3.13), and (iv) the generated ethenyl group in eq 3.8 was oxidized into

Figure 8. DRIFTS spectra of adsorbates produced from the flow of CH3CN (1 vol %) + O2 (5 vol %) + He (94 vol %) for 10 min over FeSBA-15 at (a) 150 °C, (b) 200 °C, (c) 250 °C, (d) 300 °C, and (e) 350 °C. The total flow rate is 20 mL min−1 (STP). Reprinted with permission from ref 99. Copyright 2014 Elsevier.

(including the shoulder at 2323 cm−1) were observed at 150 °C after exposure to the flow of CH3CN (1 vol %) + O2 (5 vol %) + He (94 vol %) for 10 min over Fe-SBA-15, which were related to the ν(CN) of the chemisorbed CH3CN. This suggested that the CH3CN was adsorbed on Fe-SBA-15 through its N end. Meanwhile, the DRIFTS of adsorbates produced from the flow of CH3CN + O2 over Fe-SBA-15 within the 3678


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Scheme 2. C2H3CN-SCO Mechanism over Fe-SBA-15b

a b

Scheme 3. CH3CN-SCO Mechanism over Cu-SBA-15b


carboxylic acids in eq 3.11, which were further oxidized into the final products of CO2 and H2O. 3.2.1.2. Oxidation Mechanism. 3.2.1.2.1. HCN. The intermediate isocyanate (−NCO) is a predominant characteristic of the oxidation mechanisms for the SCO of nitrile gases. Krocher and Elsener136 pointed out that the HCN-SCO over Cu-ZSM-5 followed an oxidation mechanism. As shown in Figure 5, HCN is initially oxidized into HNCO in the presence of O2. Two types of reaction pathways for the HNCO intermediate occur: (i) hydrolysis to generate unstable carbonic acid, which is successively decomposed into CO2 and NH3, where the generated NH3 is then oxidized into the desired N2 product and other N-containing byproducts (N2O and NOx), and (ii) direct selective oxidation of HCNO to produce N2 along with the hostile N2O and NOx. Apart from O2, NO2 was also utilized as an oxidant for HCN-SCO as reported by Szanyi et al.,137 who investigated the SCO mechanisms for Na-Y and Ba-Y zeolites on the basis of in situ DRIFTS and TPD/TPR spectroscopies. It is suggested that the initial step of HCN oxidation by NO2 was the abstraction of atomic hydrogen from HCN, generating ionic CN− and NC− species. The formation of N2 proceeded through two routes: (i) direct interactions of these ionic species (CN− and NC−) with NO+ to generate N2 and (ii) oxidation of cyanide species of CN− and NC− into isocyanates (NCO), which thereafter reacted with NOx to yield the final products of N2, N2O, and COx. As noted, the adsorbed CN− and NCO− ionic species formed in the HCN + NO2 reaction were believed to be the crucial surface intermediates. 3.2.1.2.2. CH3CN. The SCO of CH3CN over Cu-, Co-, and PtSBA-15 was verified to belong to the oxidation mechanism on the basis of in situ DRIFTS studies according to our past work.99 The related mechanisms are illustrated in Scheme 3. The adsorbed CH3CN (σCH3CN) generated through eq 3.14 is dissociated into σCH and σCN (adsorbed species) (eq 3.15). Thereafter, various N-containing intermediates are generated through the further oxidation of σCN, which is correlated with the redox abilities of the loaded metals on M-SBA-15. As for CuSBA-15, with suitable redox properties, σCN is mainly oxidized into the surface isocyanate (NCO) species (eq 3.16), which display IR spectra at approximately 2200−2210 cm−1. The final products of N2 and CO2 are formed through oxidation of NCO through eq 3.17. However, NO and N2O constitute the major products for Co- and Pt-SBA-15, respectively, being related to their strong oxidation ability (eqs 4.17 and 4.18). Accordingly, the SCO mechanisms of CH3CN over Cu-, Co-, Mn-, and noblemetal (Pd, Ag, and Pt)-modified SBA-15 samples could be classified into three types according to the generated N-

a b


containing products: N2 formation mechanism for Cu-, Mn-, and Ag-SBA-15, N2O formation mechanism for Pd- and Pt-SBA-15 in the low-temperature range of 200−400 °C, and NO formation mechanism for Co-, Pd-, and Pt-SBA-15 [at high temperature (T > 400 °C) for Pd- and Pt-SBA-15]. 3.2.1.2.3. C2H3CN. Nanba et al.141 investigated the acrylonitrile (C2H3CN) oxidation mechanism over Cu-ZSM-5, wherein oxygen was thought to be necessary for C2H3CN dissociation, and excess oxygen relative to C2H3CN was reported to be crucial for N2 formation. HCN, HCNO, NH3, and NOx behaved as intermediates and were observed along with the exposure time between Cu-ZSM-5 and C2H3CN during in situ DRIFTS investigation. Moreover, C2H3CN was mostly converted to isocyanate (−NCO), which could be further transformed into the adsorbed NH3 through the hydrolysis pathway. The detailed mechanism for Cu-ZSM-5 is proposed in Figure 9. As noted, C2H3CN was initialized by cyanide oxidation to form gaseous HCN, NOx, and HCN, as well as surface −NCO and nitrate species. The isocyanate species was hydrolyzed to

Figure 9. Proposed mechanism of C2H3CN decomposition to form N2. Reprinted with permission from ref 141. Copyright 2007 Elsevier. 3679


Chemical Reviews

Review

mechanism of CH3CN-SCO. The C2H3CN dissociated into σCN (eq 3.23) on the active sites in the low-temperature range (50−200 °C). Then σCN was oxidized into the surface isocyanate (σNCO) species (eq 3.24). As the temperature increased, the ethenyl group was gradually oxidized to form acetaldehyde (CH3CHO) or acetic acid (CH3COOH) (eq 3.25). Subsequently, CxHyOz (CH3CHO/CH3COOH) were further oxidized into CO2 at elevated temperatures (eq 3.26). This σNCO was readily oxidized into N2, CO2, and a small amount of side products (NOx, N2O) (eq 3.27). 3.2.1.3. Summary of Nitrile SCO. Table 7 gives a summarization of the nitrile gas (HCN, CH3CN, C2H3CN) SCO occurring over various metal-modified zeolite catalysts as well as Si-based mesoporous support catalysts, including information on the zeolitic topology, mesoporous support, preparation method, specific surface area, activity (nitrile conversion), main product (N2 selectivity), reaction conditions, and detailed reaction mechanisms. The SCO mechanisms of nitrile gases over the catalyst samples are classified into two categories: oxidation and hydrolysis mechanisms, which are closely correlated with the physiochemical properties of the loaded metal species on the catalysts. For example, Cu-, Mn-, Co-, Ag-, Pd-, and Pt-modified catalysts prefer the oxidation mechanism, whereas Fe-, V-, and Zn-modified catalysts follow the hydrolysis mechanism.99 The characteristic intermediates generated during these two types of mechanisms are, respectively, isocyanate (NCO), with an IR band of 2198 cm−1, and amide species, with IR bands of 1571 and 1658 cm−1. N2, NOx, and N2O are generated in the oxidation mechanism, whereas NH3 is formed in the hydrolysis mechanism. The oxidation mechanism can be further divided into three groups, depending on the generated N-containing products: N2 formation mechanism (Cu-, Mn-, and Ag-modified catalysts), N2O formation mechanism (Pd-, and Pt-modified catalysts), and NOx formation mechanism (Co, Pd- and Pt-modified catalysts). For the noble-metal (Pd, Pt)-modified catalysts, although higher catalytic activities are achieved, the lower N2 selectivity makes these catalysts not suitable for nitrile gas abatement. N2O was formed in the low-temperature range (200−400 °C), and NOx constituted the main N-containing products in the hightemperature range (T > 400 °C). In summary, the Cu-modified catalysts exhibited superior activity and better N2 selectivity with respect to the other metal-modified zeolite catalysts and are the most promising candidates for catalytic purification of nitrile gases. 3.2.2. NH3-SCO Mechanism. According to the literature reports, two kinds of reaction pathways were proposed for NH3SCO to yield N2. One is a direct route by the recombination of NH2 species to NH2−NH2 and the following oxidation of NH2− NH2 into N2. This route is not considered in the present review because it mainly occurs over the metal oxide catalysts (V2O5, V2O5/TiO2, V2O5−WO3/TiO2, and CuO/TiO2).164 Another NH3-SCO mechanism consists of two steps, including oxidation of NH3 to NOx and the subsequent reduction of NOx to N2 by NH3, which has been established over the zeolite catalysts on the basis of FT-IR.154,165,166 As revealed by the IR spectra of the reaction between ammonia and oxygen (1000 ppm NH3 + 2% O2/He) over Fe-ZSM-5 (previously treated by O2/He at 400 °C for 0.5 h),154 the intensity of the NH4+ species decreased, with the generation of a new band at 1870 cm−1 assigned to the adsorbed NO species, along with increasing temperature. N2 was formed through the further NO reduction with NH3, the mechanism of which is similar to that of SCR. As noted, for this

NH3, which generated N2 through two pathways: reaction with the surface nitrate species and selective catalytic oxidation. A similar finding was proposed by Poignant et al.,163 who suggested that the transformation of C2H3CN over Cu-ZSM-5 followed the reaction processes of adsorption species → Cu− CN → Cu−NC → Cu−NCO → Cu−NH3. The adsorbed NH3 thereafter reacts with NO to form N2. The C2H3CN-SCO mechanism over Cu-SBA-15 was also investigated on the basis of in situ DRIFTS in our recent work.143 As shown in Figure 10, achieved after exposure to 0.3%

Figure 10. DRIFTS of adsorbates produced from the flow of C2H3CN (0.3 vol %) + O2 (8 vol %) + He (91.7 vol %) for 25 min over Cu-SBA15: (a) 50 °C (b) 150 °C, (c) 200 °C, (d) 300 °C, and (e) 350 °C. The total flow rate was 40 mL min−1 (STP). Reprinted with permission from ref 143. Copyright 2015 Elsevier.

C2H3CN + 8% O2 in He at various temperatures over Cu-SBA15 (50−350 °C), the bands appearing at 2237 and 2286 cm−1 (at T = 50 °C) were attributed to the vibration of ν(CN) in C2H3CN, and the bands at 2994, 3040, and 3078 cm−1 were ascribed to the ethenyl ν(CH) band frequency. Increasing the temperature resulted in a gradual decrease of the band intensity of ν(CN) at 2237 and 2286 cm−1, with only one band at 2286 cm−1 being left at T > 300 °C. Another band at 2198 cm−1 was generated at T > 300 °C, which is related to the isocyanate (−NCO) species. The N2 product and NOx byproducts were generated through the oxidation of NCO species. The transformation of the −C2H3 group to CO2 was known to pass through two successive steps (−C2H3 → CxHyOz → CO2) via a CxHyOz intermediate. The oxidation mechanism of C2H3CN-SCO over Cu-SBA-15 is summarized in five steps, as listed in Scheme 4, which is similar to the corresponding Scheme 4. C2H3CN-SCO Mechanism over Cu-SBA-15b

a b

σ represents the active site of the supported metal catalyst. Reprinted with permission from ref 143. Copyright 2015 Elsevier. 3680


3681

a

IM

IM

P6mm

P6mm

P6mm

MFI

Fe-SBA-15

Fe-SBA-15

V-SBA-15

Fe-ZSM-5

503

614

614

402

402

632

588

588

460

579

575

550

T50 = 225 °C,c T90 = 275 °Cd T50 = 280 °C, T90 = 300 °C T50 = 320 °C, T90 = 340 °C T50 = 320 °C, T90 = 340 °C T50 = 320 °C, T90 = 340 °C T50 = 320 °C, T90 = 340 °C T50 = 560 °C, T90 = 700 °C T50 = 430 °C, T90 = 550 °C T50 = 340 °C, T90 = 380 °C T50 = 320 °C, T90 = 340 °C T50 = 280 °C, T90 = 300 °C T50 = 220 °C, T90 = 250 °C T50 = 260 °C, T90 = 270 °C T50 = 360 °C, T90 = 500 °C T50 = 375 °C, T90 = 500 °C T50 = 380 °C, T90 = 700 °C T50 = 260 °C, T90 = 400 °C

activity

NH3

NH3

NH3

N2O, NO N2O, NO N2O, NO NH3

NO

NO

N2

N2

N2

N2

N2

N2

N2

N2

main products 50 ppm HCN + 5% H2O + 10% O2 balanced by N2, GHSV = 2 × 105 h−1 200 ppm C2H3CN + 5% O2 balanced by He, W/F = 3.75× 10−2 g s mL−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.7 × 104 h−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.7 × 104 h−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.75 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.7 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.7 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 0.3% C2H3CN + 8% O2 balanced by He, GHSV = 3.7 × 104 h−1 1% CH3CN + 5% O2 balanced by He, GHSV = 2.0 × 104 h−1 50 ppm HCN + 5% H2O + 10% O2 balanced by N2, GHSV = 5.2 × 104 h−1

reaction conditions

99 99

● ●

● ● ●

● ● ●

99 143 99 136

● ● ● ●

99

99

99

99

99

●

●

143

●

99

143

●

●

143

●

ref

140

hydrolysis mechanism

●

NOx formation mechanism

136

N2O formation mechanism

●

N2 formation mechanism

Wet ion-exchange method. bImpregnation method. cTemperature corresponding to 50% nitrile gas conversion. dTemperature corresponding to 90% nitrile gas conversion.

commercial catalyst

IM

P6mm

Co-SBA-15

P6mm

IM

P6mm

Co-SBA-15

Pt-SBA-15

IM

P6mm

Ag-SBA-15

IM

IM

P6mm

Mn-SBA-15

P6mm

IM

P6mm

Cu-SBA-15

Pt-SBA-15

IM

Ia3d

Cu-KIT-6

IM

IM

Im3m

Cu-SBA-16

P6mm

IM

P6mm

Cu-SBA-15

Pd-SBA-15

575

IMb

MFI

Cu-ZSM-5

543

293

commercial catalyst WIEa

MFI

specific surface area (m2 g−1)

Cu-ZSM-5

preparation method

topology

sample

oxidation mechanism

Table 7. Mechanism Summarization during Nitrile SCO over Various Metal-Modified Zeolite Catalysts and Various Mesoporous Catalysts



Chemical Reviews

Review

the adsorption mode of the first NH3 molecule was similar to that in HNO formation. The other NH3 was adsorbed on the HZSM-5 through an interaction between the H of the NH3 and the framework O through a hydrogen bond (see Figure 11, ST C). The DFT energy calculation indicated that the initial reaction of oxygen with adsorbed ammonia (part i) exhibited the highest energy barrier and was believed to be the ratedetermining step. Several pathways were proposed for the decay of HNO (part ii) and NH2OH (part iii), which are energetically similar and accessible. In addition to that, a database (see Table 8) similar to that of Table 7 was set up for the NH3-SCO reaction system over various metal-modified zeolite catalysts. It was seen that FeZSM-5 exhibits not only high NH3 conversion but also excellent N2 selectivity, being one kind of promising catalyst candidate. Although Pt-, Pd-, and Rh-ZSM-5 showed much higher activity for NH3 elimination than Fe-ZSM-5, the high cost hinders their wide applications. As noted, NH3-SCO of Pd- and Rh-ZSM-5 displayed high priority for the product N2, whereas NH3-SCO over Pt-ZSM-5 preferred to generate large amounts of N2O. CoZSM-5 was found to exhibit the worst catalytic behavior for NH3-SCO, which displayed low NH3 conversion and N2 selectivity (mainly forming NO). In addition to that, similar NO conversions were achieved over Fe-MFI, Fe-FER, and FeCHA. However, lower N2 selectivity was observed for the latter two zeolites (forming NO). This is probably attributed to the different chemical states of the loaded metal species, which can be greatly influenced by the zeolitic topology.

kind of mechanism, NH3 molecules can be first adsorbed on the Brönsted acid (H+) and Lewis acid (transition-metal ions) sites to form NH4+ ions (1705 and 1470 cm−1) and coordinated NH3 (1630 and 1280 cm−1), respectively.154 In addition to NH4+ and NH3 species, some NH2− species were also detected, which was related to the hydrogen abstraction for some ammonia species. Because the Brönsted acid sites act as active centers for the SCO of NH3, Bruggemann and Keil167 investigated the NH3SCO mechanism over H-ZSM-5 zeolite by employing the DFT calculation, which was represented by a 5T model (see Figure 11) for the purpose of clarifying the exact role of Brönsted acid

Figure 11. T5 cluster model of the acid site in H-ZSM5 (ST A), geometry of the minima of adsorbed ammonium (ST B), and geometry of the minima of two adsorbed ammonia molecules (ST C). Reprinted from ref 167. Copyright 2009 American Chemical Society.

3.3. Kinetic Studies

Four reaction paths for NH3 oxidation are thermodynamically possible (eqs 3.28−3.31). Equation 3.28 shows one type of method to solve the environmental problem due to NH3 emission.

Scheme 5. Schematic of the Investigated Reaction Network of the SCO of Ammoniaa

a

NH3 + 0.75O2 → 0.5N2 + 1.5H 2O

(3.28)

NH3 + O2 → 0.5N2O + 1.5H 2O

(3.29)

NH3 + 1.25O2 → NO + 1.5H 2O

(3.30)

NH3 + 1.75O2 → 1.5NO2 + 1.5H 2O

(3.31)

Heterogeneous kinetic analysis commonly includes seven steps: adsorption/desorption, internal/external diffusion, and surface reaction. Hahn et al.171 performed a global kinetic modeling of NH3 oxidation over Fe-BEA zeolite, which involved the adsorption/desorption of NH3 and the following reaction with O2. On the basis of the fact of that neither the NOx nor the N2O formed during NH3-SCO was above the detection limit (10 ppm), the kinetic models were proposed on the basis of eq 3.28, assuming all NH3 converted into N2 and H2O. Equation 3.28 can be further divided into a two-stage reaction scheme (eqs 3.32 and 3.33), wherein S represents the active site of the zeolite catalyst. The first stage reflected the adsorption and desorption of NH3 on the catalyst (eq 3.32), whereas the second one implied a surface reaction of adsorbed NH3 with gaseous O2 to form N2 and H2O (eq 3.33). The related kinetic models were constructed in eqs 3.34−3.36, in which E1 is the activation energy of adsorption, α represents the dependency of the activation energy of NH3 desorption (E2) on NH3 coverage (θNH3) due to repulsive interactions of the surface species, E3 is the apparent activation energy, and m1 is the reaction order of adsorbed NH3 and gaseous O2.

Reprinted from ref 167. Copyright 2009 American Chemical Society.

sites during NH3-SCO (see Scheme 5). The simulation results suggested that the corresponding mechanism could be classified into three parts: (i) direct reaction of NH3 with O2, forming HNO or NH2OH intermediates, and the subsequent decay of (ii) nitroxyl (HNO) and (iii) hydroxylamine (NH2OH). In part i, the direct reaction of NH3 with O2 to form HNO proceeds through the adsorption of one NH3 molecule via the 2H mode, wherein the atomic H of NH3 interacts with the framework oxygen and the Brönsted acid (H+), forming NH4+ with a relatively strong adsorption energy of −18.9 kcal mol−1 (see Figure 11, ST B). For the formation of NH2OH, two NH3 molecules were simultaneously adsorbed on H-ZSM-5, wherein 3682


Chemical Reviews

Review

Table 8. Mechanism Summarization during NH3-SCO over Various Metal-Modified Zeolite Catalysts main product sample

topology

preparation method

specific surface area (m2 g−1)

a

Pt-ZSM-5

MFI

WIE

Pd-ZSM-5

MFI

WIE

Rh-ZSM-5

MFI

WIE

Pt/Fe-ZSM-5

MFI

IM

Co-ZSM-5

MFI

WIE

Fe-ZSM-5

MFI

IMb

Fe-ZSM-5

MFI

WIE

Fe-mordenite

MOR

WIE

Fe-clinoptilolite

HEU

WIE

Fe-Y

FAU

WIE

Fe-ZSM-35

FER

WIE

Fe-SAPO-34

CHA

WIE

Fe-ZSM-5 + Pt/Al2O3 Pd-Y

MFI

WIE + IM

FAU

WIE

activity

reaction conditions

T50 = 180 °C, T90 = 250 °Cd T50 = 200 °C, T90 = 250 °C T50 = 230 °C, T90 = 300 °C T50 = 160 °C, T90 = 200 °C c

448

347

937

T50 = 350 °C, T90 = 400 °C T50 = 300 °C, T90 = 400 °C T50 = 280 °C, T90 = 450 °C T50 = 250 °C, T90 = 400 °C T50 = 350 °C, T90 = 400 °C T50 = 350 °C, T90 = 450 °C T50 = 350 °C, T90 = 400 °C T50 = 250 °C, T90 = 350 °C T50 = 150 °C, T90 = 175 °C

4% O2, 1000 ppm NH3, He balance, F = 100 mL min−1 4% O2, 1000 ppm NH3, He balance, F = 100 mL min−1 4% O2, 1000 ppm NH3, He balance, F = 100 mL min−1 2% O2, 1000 ppm NH3, He balance, W/F = 0.0001.2 × 10−4 g min mL−1 4% O2, 1000 ppm NH3, He balance, F = 100 mL min−1 2% O2, 1000 ppm NH3, He balance, GHSV = 3.8 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 2% O2, 1000 ppm NH3, He balance, GHSV = 2.3 × 105 h−1 5% O2, 500 ppm NH3, GHSV = 6.6× 104 h−1 2.5% O2, 0.5 vol % NH3, He balance, GHSV = 1.54 × 104 h−1

N2

NOx

N2O

ref

●

144

●

144

●

144

●

168 ●

144

●

165

●

146

●

146

●

146

●

146

●

●

146

●

●

146

●

169

●

170

a

Wet ion-exchange method. bImpregnation method. cTemperature corresponding to 50% nitrile gas conversion. dTemperature corresponding to 90% nitrile gas conversion. r1

NH3(g) + S ⇔ H3N−S r2

Cu-ZSM-5 zeolite. This model was verified to accurately predict ammonia storage, desorption, and oxidation. The calculated energy barrier for NH3-SCO was 23.2 kcal mol−1 (see Table 9, no. 3), which was much lower than that reported by Olsson,173 who constructed NH3-SCO kinetics on the basis of a one-site model with a calculated energy barrier of 38.8 kcal mol−1 (see Table 9, no. 2). This finding suggests that the two adsorption sites were more favorable for NH3-SCO compared to the single adsorption site. Metkar et al.174 performed a kinetic comparison between Cu-SAPO-34 and Fe-ZSM-5 for NH3-SCO, which suggested that Fe-ZSM-5 possessed a lower energy barrier (42.7 kcal mol−1) with respect to that of Cu-SAPO-34 (21.7 kcal mol−1), as listed in Table 9, no. 7. Additionally, the kinetic analysis of NH3-SCO over Cu-SAPO-34 in the presence or in the absence of H2O was conducted. It was suggested that the presence of H2O could reduce the NH3-SCO activity, with the activation energy barrier being increased from 13.1 to 17.7 kcal mol−1 (see Table 9, no. 8). The inhibition effect of H2O was thought to be mainly related to the increment of acidity of NH3 due to the formation of hydrogen bonds, which greatly increased the strength of the adsorbed NH3 species and resulted in hard oxidation of NH3.175

(3.32)

r3

H3N−S + 0.75O2 (g) → 0.5N2(g) + 1.5H 2O(g) + S (3.33)

⎛ E ⎞ r1 = A1 exp⎜ − 1 ⎟C NH3(1 − θNH3) ⎝ RT ⎠

(3.34)

⎛ E2 − α NH3θNH3 ⎞ ⎟C NH3(1 − θNH3) r2 = A 2 exp⎜ − RT ⎠ ⎝

(3.35)

⎛ E ⎞ m1 m2 r3 = A3 exp⎜ − 3 ⎟θNH C ⎝ RT ⎠ 3 O2

(3.36)

The calculation results suggested that (i) a good correlation between the modeled and experimental effective rates of NH3 upon a variation of O2 and NH3 was obtained, (ii) the kinetic model shows a higher dependency of the NH3 oxidation rate on θNH3 (m1 = 2) than that on the O2 concentration (m2 = 0.5), which confirms that the adsorbed NH3 did not react directly with O2 to yield N2 and H2O, and (iii) the rate of NH3 adsorption (r1 of eq 3.32) is 3 orders of magnitude faster than that of NH3 oxidation (r3 of eq 3.33), indicating that there is sufficient availability of the adsorbed NH3 during the reaction and the surface reaction is the rate-determining step. The detailed kinetic parameters are listed in Table 9, no. 1. Other literature reports concerning the NH3-SCO kinetics for the zeolite catalysts are also collected in Table 9. Sjovall et al.172 proposed a two-active-site kinetic model (adsorption of NH3 and O2 at the S1b and S1a sites, respectively) for NH3-SCO over

4. N2O DIRECT DECOMPOSITION N2O being one kind of greenhouse gas contributes to global warming and ozone layer depletion. Because of these potentially hazardous effects on the environment, the control of N2O emission has been claimed in Kyoto, 1997 (The United Nations Framework Protocol on Climate Change). The chemical productions of nitric acid and adipic acid constituted two major stationary sources for N2O emissions. In the adipic acid 3683


3684

Cu-SAPO-34

Fe-zeolite, commercial

9

Fe-ZSM-5

Cu-CHA

8

7

H-BEA

6

Fe-BEA

Cu-BEA

5

Cu-ZSM-5

3

Fe-ZSM-5

Cu-ZSM-5

2

4

Fe-BEA

zeolite

1

no.

r3

reaction

4NH3−S + 3O2 → 2N2 + 6H 2O + 4S

4NH3 + 3O2 → 2N2 + 6H 2O r = kCO2θNH3

r2 = k 2f XO2θNH3

r = kC NH3CO2

4NH3 + 3O2 → 2N2 + 6H 2O

2NH3−S + 1.5O2 → N2 + 3H 2O + 2S

⎛ E ⎞ r3 = A3 exp⎜− 3 ⎟θS−NH3[O2 ]β1 ⎝ RT ⎠

r = − kaC NH3 (first‐order with respect to NH3)

4NH3−S + 3O2 → 2N2 + 6H 2O + 4S

4NH3 + 3O2 → 2N2 + 6H 2O

(see footnote b)

r11 = k11θO−s1aθNH3−S1b

2NH3−S1b + 3O−S1a → N2 + 2H 2O−S1b + H 2O−S1a + 2S1a

r11

r2 = k 2CO2θNH3−S

⎛ E ⎞ m1 m2 r3 = A3 exp⎜− 3 ⎟θNH C ⎝ RT ⎠ 3 O2

⎛ E2 − α NH θNH ⎞ 3 3 ⎟C NH3(1 − θNH3) r2 = A 2 exp⎜− RT ⎝ ⎠

⎛ E ⎞ r1 = A1 exp⎜− 1 ⎟C NH3(1 − θNH3) ⎝ RT ⎠

constructed reaction rate model

2NH3−S + 1.5O2 → N2 + 3H 2O + 2S

r2

H3N−S + 0.75O2 → 0.5N2 + 1.5H 2O + S

(see footnote a)

r2

NH3(g) + S⇔H3N−S

r1

Table 9. Kinetic Models and Parameters for NH3 Selective Oxidation to N2 and H2O over Zeolite Catalysts

mol m−3 s−1

19.8 3.4 × 1015 39.7 5.6 × 1016 42.7 7.2 × 1016 21.7

E K E A2f E2f A2f E2f

m3 mol−1 s−1

17.7 (H2O) 8.4 × 1011

E A

kcal mol−1

13.1

E

kcal mol−1

kcal mol mol m−3 s−1 kcal mol−1

−1

kcal mol−1 m2 s−1 m3washcoat kcal mol−1

8.76 × 10 K

m2 s−1 m3washcoat

kcal mol−1

kmol m−3 s−1

6

23.7

E3

1.0 × 10 1.7 × 102 21.0

ka (350 °C) ka (300 °C) E

3.0 × 106

−1 3

s s−1 kcal mol−1

s−1 2.6 × 103

ka (400 °C)

kcal mol−1

mol s−1 kg−1zeolite

kcal mol−1

m3 mol−1 s−1

kcal mol−1

mol0.5 m−0.5 s−1

kcal mol−1 kcal mol−1

mol m−2 s−1

23.2

A3

unit

kcal mol−1

m s−1

E

4.1 × 105

38.8

E k11

1.2 × 1011

47.5 2.0 0.5

E3 m1 m2 A

1.9 × 106

33.5 13.2

E2 αNH3 A3

4.97 × 107

0

0.87

value

A2

E1

A1

param

178

175

174

177

176

100

172

173

171

ref



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29.9

As for nitric acid production by a high-temperature oxidation of ammonia over Pt−Rh wire gauzes, the tail gases contain nonselectively formed traces of N2O. Several N2O-exhausttreatment approaches have been developed, which include thermal decomposition, direct catalytic decomposition, and selective catalytic reduction (SCR) using NH3, CH4, and H2 as reductants. Among them, the direct catalytic decomposition was widely accepted as the simplest and most effective method for N2O abatement (see eq 4.2).

unit

E

param

value

kcal mol−1

ref

industry, for synthesizing each molecule of adipic acid one molecule of N2O is produced, yielding high concentration levels of N2O (30−50 vol %), wherein HNO3 is utilized as the oxidant to oxidize phenol into adipic acid (see eq 4.1).

2N2O → 2N2 + O2

As reported, the companies DuPont and UOP (ElimiNox) both have developed a N2O direct decomposition process to abate N2O in flue gases of adipic acid plants, based on a binary metal oxide catalyst.179 Air Products also developed one kind of Co-ZSM-5 zeolite for N2O direct decomposition, which was deemed a good candidate as an industrial catalyst.181 The typical flowsheet for N2O treatment in the chemical industry is shown in Figure 12. Before entering into the deN2O system, the flue gas of

S represents the active site of the zeolite catalysts. bS1a and S1b represent two kinds of active sites.

constructed reaction rate model

Figure 12. Flow scheme of the flue-gas treatment for the adipic acid plant. Reprinted with permission from ref 4. Copyright 1996 Elsevier.

the adipic acid plant is first introduced into a deNOx system to eliminate NOx impurity gases. Due to the fact that N2O dissociation is an exothermic reaction, a heat exchanger is placed before the reactor to preheat the fresh flow. After the heat exchanging unit, the cooled gases are recycled back to dilute the fresh gas (with a N2O content of 30−50 vol %), resulting in the inlet N2O concentration being below 12%. Although the N2O decomposition system with only a few operating units is not so complicated, the development of highly efficient catalysts is never an easy task. The reported catalysts include noble metals (Pt, Au), metal oxides (CaO, Fe2O3, CuO), mixed oxides (CoO in MgO, NiO in MgO), spinels (MAl2O4; M = Co, Cu, Ni, Mg, Zn), perovskites (LaBO3; B = Co, Ni, Cr, Mn, Fe), and the zeolite catalysts (Fe-, Cu-, and Co-modified MFI, BEA, MOR, FER, and FAU). Among them, the zeolite catalysts were widely investigated due to their high activities, thermal stabilities, and strong resistances to other impurity gases (NO, CO, O2).4 In this section, an overview of the recent new achievement of zeolite catalysts for N2O direct decomposition is given, wherein both the experimental and theoretical (based on DFT) studies are involved, including the active center structure, activity comparison, reaction mechanism, and microkinetic analysis.

a

reaction zeolite no.

Table 9. continued

(4.2)

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4.1. Active Center Structure

greatly facilitates the ion-exchange process by shortening the diffusion length. Finally, it is concluded that the individual metal ions present in the cationic sites behave as the active sites with lower degrees of metal loading (metal/Al < 0.1). The higher degrees of metal loading cause the formation of di- and polynuclear Fe(III)−oxo species, which can also perform as active sites. However, the excess metal loading leads to the appearance of metal oxide particles, which not only possess low activity but also induce blockage of zeolite pores and further decrease the catalytic activity. The most active sites for N2O decomposition were suggested to be the isolated metal ions64,87,191−194 or binuclear metal oxo species195−197 in the cationic sites. 4.1.1.2. Other Preparation Methods. In addition to WIE, other preparation methods were also reported to be applied to gain catalysts for N2O dissociation. Lobree and Bell93 prepared the Fe-ZSM-5 catalyst via the SSIE method, similar to one previously reported by Chen et al.95 They determined that Fe3+ cations could be exchanged with the protons of Brönsted acids. This occurs for individual cations, such as Fe(OH)2+/Fe(OH)+/ FeO+ at Fe/Al < 0.56, whereas small FeOx particles could be formed on the surface of catalysts with higher Fe/Al ratios. Two preparation methods, SSIE and CVD, were employed for FeZSM-5 preparation.198 The mononuclear Fe, such as [Fe(OH)2]+ or [FeO]+ in a distorted tetrahedral coordination, was formed at a lower degree of metal loading, whereas the binuclear oxocations, such as [OH−Fe−O−Fe−OH]2+, and Fe2O3 oxide particles emerged in the overexchanged samples. On the basis of the activity test, it is revealed that the binuclear sites of [OH− Fe−O−Fe−OH]2+ were much more active for N2O decomposition than the mononuclear ones. Park et al.199 compared two types of Fe-ZSM-5 prepared by the WIE and SSIE methods for N2O direct decomposition. The mononuclear Fe species constituted the major active species for both of these zeolites at Fe/Al < 0.2, leading to similar N2O conversions. However, distinct reactivity could be achieved at Fe/Al > 0.2. The decrease of deN2O activity was observed for FeZSM-5 prepared by WIE, being mainly related to the generation of inactive FexOy species. This finding is in agreement with our previous work for N2O direct decomposition with Fe-BEA prepared by WIE,200 suggesting that no further increase of deN2O activity could be observed at Fe contents >1 wt %, due to the appearance of inert iron oxides. For Fe-ZSM-5 prepared by SSIE with higher degrees of Fe loading (Fe/Al > 0.2), the oxygen-bridged binuclear Fe was formed, acting as the active site and possessing high activity for N2O decomposition. This type of binuclear Fe site was further verified by Xia et al.201 on the basis of in situ visible Raman spectra showing a peak at 743 cm−1. Boron et al.5 prepared a series of CoxSi-BEA samples by twostep postsynthesis with various Co contents (1.0, 4.0, and 9.0 wt % Co). The proposed method allowed control of the introduction of cobalt into the zeolite matrix to form a catalyst with specific Co sites. On the basis of the characterization by UV−vis and XPS, two types of Co(II) were present: Co(II) incorporated into the framework of the zeolite as pseudotetrahedral Co(II) species (2 wt % Co). The catalytic activity of the CoxSi-BEA samples depended on the amount and nature of the Co species. The higher number of Lewis acidic sites present in Co4.0Si-BEA and Co9.0Si-BEA were responsible for their higher activities with respect to Co1.0SiBEA.

Modification of zeolites by metal ions (especially by transitionmetal ions) brings about special properties for the related heterogeneous catalysis, which usually results in improved catalytic activities. As far as the metal-modified zeolites are concerned, the nature and distribution of the loaded metal components are key issues relevant to the catalytic behaviors. However, due to the complexities of zeolite matrixes, the academic investigation of the structures of active centers has become one of the hottest issues for the chemical community over the past 10−15 years.181−186 Generally, there are two main factors greatly influencing the structure of the active sites of zeolite catalysts, the preparation method and zeolite topology, as described below. 4.1.1. Influence of the Preparation Method. 4.1.1.1. Wet Ion Exchange. Preparation methods commonly include wet ion exchange (WIE), solid-state ion exchange (SSIE), chemical vapor deposition (CVD), and isomorphous substitution (IS). Among them, the WIE method is simpler and more feasible; in this method, the metal ions are exchanged with protons located at the Brönsted acid sites, which form the active centers. Since the pioneering works of Panov et al.182 and Uddin et al.,187 it has been well established that iron-based zeolites are among the most active catalysts for the decomposition of N2O into O2 and N2. As reported,186 the Fe-zeolite catalysts prepared by the WIE method possessed not only the higher N2O decomposition activity but also the better hydrothermal stability with respect to those catalysts prepared by SSIE and CVD, whereas metal cations on the extraframework are deemed as active centers. However, during the preparation of Fe-, Co-, and Cu-zeolites by WIE, a problem arises, in that the metal ions tend to hydrolyze at pH > 4, resulting in the formation of metal oxide aggregates. Therefore, the pH value of the suspension is one of the key factors in preparing catalysts with high deN2O activity on the basis of the WIE method. In addition to the pH value of the suspension, the extent of ion exchange is another important factor for the WIE preparation method, which leads to the diverse oxidation states and stereochemical conformations of the loaded metal species. Campa et al.188 studied the catalytic activity dependence on the degree of ion exchange for cobalt or copper in Na-MOR, HMOR, and Na-MFI zeolites for N2O decomposition. It was known that the turnover frequency (TOF) of the Co atom was independent of the cobalt content for the sample having a Coexchange percentage of less than 61%; however, it became significantly lower for more extensively exchanged sample. This finding suggested that the Co2+ ions, serving as the active centers for deN2O, were the dominant species at the lower extent of Co exchange, whereas the higher Co loading resulted in the formation of CoOx species which possessed relatively worse deN2O activity. Mauvezin et al.189 investigated the influence of the degree of ion exchange on the surface species of Fe-BEA. Iron can be introduced as isolated Fe3+ cations up to a theoretical exchange degree of 100%; nevertheless, for the overexchanged samples, small amorphous Fe2O3 aggregates were formed after calcination in air. This is because the compensated Fe3+ cations at the neighbor sites could be transformed into binuclear oxocations [(OH)FeOFe(OH)]2+ by binding bridging oxygen. To enhance the ion-exchange process, Melian-Cabrera et al.190 reported one type of alkaline leaching method which resulted in a significant dismission of zeolite agglomerates and the formation of an additional mesopore network. This network 3686


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Figure 13. Schematic diagram for the formation of various iron species in Fe(III)-USY. Reprinted with permission from ref 94. Copyright 2008 Elsevier.

4.1.1.3. Calcination Effect on Active Centers. Recently, it was reported that high-temperature pretreatment under an inert atmosphere could improve the N2O dissociation activity of the modified zeolite catalyst, which is usually deemed as an activation procedure. Sun et al.202 studied the effect of hightemperature treatment on the active sites of Fe-ZSM-5, which was prepared by the SSIE method, for N2O direct decomposition. High-temperature calcination was conducted at 900 °C under a He atmosphere, which induced profound changes in the distributions of the iron species. Most notably, a substantial fraction of oligonuclear and cationic Fe3+ species converted to Fe2+ species stabilized by extraframework Al (Fe−O−Al species), resulting in enhanced catalytic activity. A similar finding was reported by Pirngruber et al.,203 who noted that the N2O decomposition efficiency strongly depended on the extent of autoreduction of the catalysts during their high-temperature pretreatments by the He stream. 4.1.1.4. Influence of the Zeolitic Topology. The zeolite topology has a significant influence on the distribution of loaded metals, which further affects the related activity of the zeolite catalyst. Smeets et al.204 investigated several copper-modified zeolites with different topologies (MFI, MOR, FER) and Cu/Al ratios for N2O direct decomposition. Two types of active sites were observed: the bis(μ-oxo) dicopper cores in ZSM-5 zeolites and the EPR-silent Cu species (not identified) in highly loaded MOR and FER zeolites (Cu/Al > 0.29). It was found that the former, bis(μ-oxo) dicopper, was rather active for N2O dissociation, which essentially favors the recombination of dissociated oxygen atoms from N2O, being thought of as the rate-limiting step for N2O direct decomposition. The channel sizes of the zeolites and sites available for exchange play key roles in determining the optimal Fe loading over the zeolite. Li et al.94 prepared several Fe-zeolites with MFI and FAU structural topologies (with identical Si/Al ratios of 12) via the WIE method for N2O direct decomposition. Various Fe(III) species, including isolated Fe(III) species (mononuclear and binuclear oxygen bridged), oligonuclear Fe(III)xOy clusters, and Fe2O3 nanoparticles, were identified in these Fe-zeolites. A schematic formation diagram of these Fe(III) species during the preparation process are further profiled in Figure 13.

On the basis of the structural analyses and activity measurements, the prepared Fe-FAU exhibited much higher activity than that of Fe-MFI, which was ascribed to the large channel dimension of FAU facilitating the diffusion of iron cations. In the case of Fe2+ exchange, hydrated ferrous ion [Fe(H2O)62+] (having a kinetic diameter of 0.7 nm) was the only ion species in aqueous solution at room temperature at a pH of ∼3. Therefore, it could readily diffuse into the Y (FAU) channels with a size of 0.74 × 0.74 nm [111], achieving a high degree of ion exchange. However, for ZSM-5 (MFI) with a channel size of 0.51 × 0.55 nm [100] ↔ 0.53 × 0.56 nm [010], it was obviously difficult for Fe(H2O)62+ to diffuse into the ZSM-5 channels, therefore leading to an extremely low degree of exchange. 4.1.2. Distribution of Framework Al. Apart from the zeolite topology, it was recently proposed that the distribution of the framework Al of a zeolite could also influence the location, structure, redox ability, and catalytic properties of the doped metal ions. It is preferential for Co(II), Cu(II), and Fe(II) ions to be balanced by Al pairs [Al−O−(Si−O)2−Al in one ring], whereas Co−oxo and Cu−oxo or Cu(I) species are adjacent to the isolated Al atoms [Al−O−(Si−O)n>2−Al in two rings].205 Thus, it creates the possibility to tailor the structural and catalytic properties of the metallozeolite catalyst by controlling the Al distribution during the synthesis process. Simultaneously, it was realized that the distribution of Al atoms in a zeolite framework was never random, but controlled by statistical rules for the Sirich zeolites (Si/Al > 12), as noted for the FER, MFI, BEA, and MWW topologies.206−211 The framework Al atoms in Si-rich zeolites predominantly occur in Al−Si−Si−Al sequences (Al pair) situated in a 6-MR or as single Al atoms located in diverse rings,205 mostly in the absence of the closest Al atoms in an Al− Si−Al arrangement, as shown in Figure 14.212 After introduction of iron ions into ZSM-5, different Fe species could be formed according to the dispersion of the framework Al, as illustrated in Figure 15.205 It was reported that there are three predominant Fe species in Fe-ZSM-5: (a) Fe(II) ions, (b) binuclear [Fe(II)−O−Fe(II)]2+ or [Fe(II)−μ-O2− Fe(II)]2+, and (c) Fe(III)−oxo species of low nuclearity. The Al pair in the 6-MR of the zeolite skeleton could stabilize the divalence of the Fe(II) ions and [Fe(II)−O−Fe(II)] 2+ complexes even in an oxidative atmosphere (see Figure 15A), 3687


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4.2. Activity Comparisons

As stated above, transition-metal-ion-exchanged zeolites (TMIZ), especially the Fe-, Co-, and Cu-exchanged zeolites, exhibiting unique and fascinating properties, were widely applied for the catalytic decomposition of N2O.58,92,104,191,195,196,203,213−217 The nature of the transition-metal-ion (TMI) species was investigated in detail, and the isolated cations or oxocations of TMIs with low nuclearities were thought to be highly active species. Both the type of zeolite and the nature of the TMI are important for N2O catalytic dissociation. The zeolite framework commonly acts as a ligand for the cationic metals, whereas the final location of metallic cations essentially depends on the topology of the zeolite and the chemical nature of the TMI. The structure− activity relationship is crucial for understanding the catalytic performance of the zeolites and is also helpful for the design of highly efficient zeolite catalysts. 4.2.1. Different Active Components. It is commonly accepted that catalytic activity is determined by a combination of the metal ions, zeolite matrix, and preparation method. Metalloaded zeolites are effective catalysts for direct N2O decomposition into N2 and O2. A number of metals (i.e., Fe, Co, Ni, Cu, Mn, Ce, Ru, Pd, and Pt) deposited in various zeolite structures (MFI, BEA, MOR, FAU, and FER) have been reported as active components for this reaction.4 Li and Armor103 determined the following activity order of metals: Rh ≈ Ru > Pd > Cu > Co > Fe > Pt > Ni > Mn. Abu-Zied et al.214 prepared a series of transitionmetal-modified ZSM-5 zeolites on the basis of the SSIE method for N2O direct decomposition. An exchange degree of more than 60% was achieved for the prepared samples. According to the activity measurement, the prepared catalysts could be divided into two groups: (a) the first group exhibits higher activity than that of H-ZSM-5, including Co-, Cu-, Fe-, Pd-, Ag-, Ce-, and LaZSM-5 (Cu ≈ Fe > Co > Pd > Ag ≈ Ce > La); (b) the second group, involving Ni-, Y-, Mn-, Zn-, and Cd-ZSM-5 (Y > Ni ≈ Mn > Cd > Zn), possesses relatively lower activity with respect to HZSM-5. The higher activities of the first group of catalysts compared to the second group of catalysts corresponded with their good abilities to undergo reduction under heating treatment. Because Cu-, Fe-, and Co-ZSM-5 showed superior activities among the tested catalysts, the preparation parameters of the Si/Al ratio, exchange level, calcination temperature, and milling time during the SSIE process were thereafter investigated to evaluate their influences on the related deN2O activity. It was noted that the activity of such a series of catalysts was more sensitive to the calcination temperature as well as the milling time. A comparative study on the direct decomposition of nitrous oxide over Fe-, Co-, and Cu-BEA zeolites (prepared by WIE) was conducted in our previous work on the basis of both the experimental and theoretical (DFT) methods.115 The metal cations were verified to be the main species on Fe-, Co-, and CuBEA on the basis of H2-TPR, UV−vis, and XPS. Accordingly, the 5T Fe-, Co-, and Cu-BEA models were constructed and further applied for the N2O direct decomposition simulations. The calculation results revealed that Co-BEA possessed the lowest energy barrier for O2 desorption (the rate-determining step). This calculation is in good agreement with the experimental result revealed by the turnover frequency (TOF), suggesting that the activity follows a trend of Co-BEA > Fe-BEA > Cu-BEA. As noted here, because the amounts of metal loading were different from each other and cannot be readily controlled, especially for the WIE method, the TOF was employed to evaluate the activity performances of Co-, Fe-, and Cu-BEA zeolite samples.

Figure 14. Illustration of the mutual location of Al atoms in the ZSM-5 framework and Al pairs located in the α-, β-, and γ-type framework rings balancing corresponding Co(II) ions. Reprinted with permission from ref 212. Copyright 2015 Elsevier.

Figure 15. Schematic illustration of the main Fe species in the vicinity of (A) Al pairs (Al−O−(Si−O)n=2−Al sequences in one 6-MR ring) and (B) single distant Al atoms (Al−O−(Si−O)n>2−Al). Reprinted with permission from ref 205. Copyright 2014 Elsevier.

which constitute the active sites for N2O dissociation. On the contrary, the prevailing concentration of single Al sites resulted in a high population of Fe(III)−oxo species (see Figure 15B), which facilitate the formation of highly active oxygen species in an O2-containing atmosphere by associating with partially compensated positive charges by the zeolite framework. Li et al.213 performed a DFT study of the stability and reactivity of active sites for the direct oxidation of benzene to phenol over Fe-ZSM-5 utilizing N2O as the oxidant. It was further proposed that the mononuclear Fe2+ cations could only be stabilized in the 6-MR with a symmetric distribution of framework Al atoms in the same ring. This site is denoted as the δ site, which is at the intersection of straight and zigzag channels (see Figure 16). If these requirements are not satisfied, the introduced iron would be present as isolated [FeO]+, oxygen/ hydroxy-bridged binuclear Fe2+, and Fe3+ complexes.

Figure 16. Structure of ZSM-5 zeolite (a) and the relative location of selected cation sites in it (b). Reprinted with permission from ref 213. Copyright 2011 Elsevier. 3688


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Therefore, the activity sequence derived from our work was different from those reported before: Cu > Co > Fe126 and Cu ≈ Fe > Co.214 Additionally, two series of BEA zeolites, respectively prepared by conventional thermal synthesis and mesoporous construction methods, were applied for N2O direct decomposition after Cu doping.218 Cu-BEA-meso exhibited a much higher reaction rate than that of the usual Cu-BEA, attributed to the hierarchical micro-mesoporous structure of the BEA-meso sample. Similarly, the hierarchical porous structure of Fe-BEA-meso resulted in an improved accessibility of ion-exchange positions in comparison with the conventional Fe-BEA, which allowed the introduced Fe exclusively in the form of monomeric Fe3+ cations and possessed considerably high activity for N2O elimination.219 4.2.2. Different Zeolite Topologies. The metal-modified pentasyl-type zeolites of MFI, BEA, and FER were widely utilized for N2O direct decomposition due to their excellent catalytic activities and stabilities. Among them, Fe-MFI was first reported to be an efficient zeolite catalyst for N2O direct decomposition and thereafter was extensively investigated.220−222 Subsequently, it was determined that Fe-BEA and Fe-FER exhibited higher activities than that of Fe-MFI. Large numbers of studies focused on the zeolitic structure effects on the distribution of active species and thereby the catalytic activity. Øygarden et al.223 investigated various commercially available H-zeolites (MFI, FER, BEA, MOR, FAU) for N2O direct decomposition to illustrate the role of the zeolite framework. The results suggested that H-BEA was the most effective material in the reaction, followed by H-FER and H-MFI zeolites. Guzman-Vargas et al.224 further compared N2O direct dissociation activities over Fe-BEA, Fe-MFI, and Fe-FER, which were prepared by the classical WIE method. It was found that the nature of the zeolite topology had a great influence on the reactivity of the loaded Fe species. A large amount of “oxo species” (αO), being readily reducible at low temperature, could be formed upon the interaction of an Fe(II) active site with N2O for the Fe-FER zeolite, which was proposed to be the reason for the highest catalytic activity observed for Fe-FER. Kaucky et al.64 further investigated the effect of the Fe-zeolite (BEA and FER) structure and Al Lewis sites on N2O direct decomposition, for which the iron species were mainly located at the cationic sites, with total loading amounts of 0.6 and 0.55 wt %, respectively. It was determined that the higher N2O dissociation activity of FeFER was attributed to the much shorter distance of the neighboring β-type Fe−Fe active sites (6.1 Å for Fe-FER vs 7.5 Å for Fe-BEA), which significantly facilitated the N2O bond fracture to form N2 and the adsorbed oxygen. Jisa and coworkers further performed a detailed investigation on the role of the zeolite structure and the iron state of Fe-FER, Fe-BEA, and Fe-MFI on N2O direct decomposition.191,225 It was observed that the iron was mostly distributed in the cationic positions, with small amounts of less well-established oxide species over the Fe-based zeolites. Although a comparable content of Fe(II) in the cationic positions was observed for these three samples, the catalytic activity of Fe-FER obviously exceeded those of Fe-BEA and Fe-MFI, which was proposed to be mainly attributed to the special topology of FER. Two adjacent β-sites of Fe-FER with the Fe−Fe distance being 7−7.5 Å were comparable to the length of the N2O molecule, as shown in Figure 17. Such an arrangement of two Fe(II) cations allows the formation of collaborating Fe(II) cations in Fe−Fe pairs, which facilitates the activation of the N2O molecule.

Figure 17. Periodic DFT-optimized structure of Fe-FER including N2O interacting with two Fe cations occupying adjacent β cationic positions. Reprinted with permission from ref 191. Copyright 2009 Elsevier.

The microporous zeolite catalysts of MFI, BEA, and FER have shown ideal catalytic activity for N2O direct decomposition. However, the efforts to pursue a new type of efficient zeolite catalyst have continued. Recently, Lee et al.226 utilized the isomorphously substituted method to prepare three types of medium-microporous zeolites of 10-MRs (Fe-TNU-9, Fe-TNU10, and Fe-IM-5) for N2O direct decomposition. Both atomic Fe and Al were present in the zeolite framework. The activity sequence followed the order Fe-TNU-10 < Fe-IM-5 < Fe-TNU9 on the basis of the TOF investigation. Moreover, it was indicated that the effect of the zeolite topology on the decomposition activity was much stronger than those of the Fe/Al ratio and extraframework Al content because the zeolite structure has a great influence on the nature and distribution of the extraframework irons. Abu-Zied227 prepared a series of Cu-X zeolite catalysts by the WIE method using a Cu(CH3COO)2 solution as the copper source. The preparation method had a remarkable influence on the structure and the texture of the obtained catalysts. N2O activity was known to be closely related to the crystallinity loss as well as to the presence of reduced copper sites formed during calcination. Xie et al.117 conducted a comparison of Cu-ZSM-11 and Cu-ZSM-5 with the same Si/Al ratio for N2O dissociation, for the purpose of illustrating the impact of the zeolitic structure (ZSM-11 vs ZSM-5). Both ZSM11 and ZSM-5 belong to the pentasil family. Nevertheless, ZSM11 has only straight pore channels, whereas ZSM-5 has both straight and sinusoidal channels. This demonstrated that CuZSM-11 was apparently more active than Cu-ZSM-5, likely due to the enhanced reducibility of the active Cu+ species and better accessibility to the active sites in Cu-ZSM-11. 4.3. Reaction Mechanism

For N 2O direct decomposition, two types of reaction mechanisms were previously proposed: the Eley−Rideal (E− R) mechanism228 and the Langmuir−Hinshelwood (L−H) mechanism (see Scheme 6),229 which is generally determined by the chemical microstructures of the active sites. Generally speaking, the E−R reaction mechanism prevails at the active sites (Mn+-Z) of the isolated metal cations, wherein two N2O molecules are decomposed into N2 and adsorbed O on the same site. Subsequently, the adsorbed O atoms are recombined into an O2 molecule. After the desorption of O2, this active site is regenerated. Among the above reaction steps, the last step of O2 desorption has commonly been reported as the rate-determining step.230 In comparison, the L−H 3689


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Si−O−[Cu2+O] or Si−O−[Cu2+(OH)]. The oxygen recombination readily occurred to form O2 when two such sites were located in close proximity, such as the opposite corners of a 4MR, having tetrahedral Al at T9 sites of ZSM-5. This mechanism incorporated specific aspects of both the copper ions and the zeolite structures to explain the ideal activity of Cu-ZSM-5. Rivallan et al.92 comparatively studied the adsorption and reactivity of nitrogen oxides (N2O, NO, N2O) for the Fe-ZSM-5 zeolite. Two families of mononuclear Fe(II) centers (FeA and FeB) were distinguished by FT-IR spectroscopy using N2O as the probe molecule, with IR bands appearing at 2248 and 2282 cm−1, respectively. The diversity of these two iron sites was ascribed to the different numbers of Si−O−Si and Si−O−Al ligands present in the coordination sphere of Fe(II). The FeA site being less coordinated to the MFI zeolitic framework was found to be more active and associated with the classical α-sites (the sites responsible for αO generation during N2O catalytic dissociation). The Brönsted acid site could also adsorb N2O through a hydrogen bond displaying an IR band at 2226 cm−1. More importantly, it is understood that the N2O decomposition mechanism is more complex than expected, because nitrogen oxides could be formed as byproducts during this catalytic process, with the majority of the NO2 species adsorbed on the iron sites. The proposed NOx formation pathway is listed in Scheme 7 (eqs 4.9 and 4.10), where σ represents the active

Scheme 6. N2O Direct Decomposition Mechanism: E−R and L−H Reaction Mechanisms

mechanism predominately occurs at the multinuclear active sites. The adsorption and dissociation of N2O take place at two neighboring active sites. The deposited oxygen atoms can migrate over the diverse active sites and finally form molecular O2. The transfer of oxygen from one active site (σ) followed by recombination with another oxidized site is the rate-determining step for the L−H mechanism.216,229 The distance between deposited O atoms strongly influences O transportation as well as O2 desorption. For example, the bis(μ-oxo)dicopper core in Cu-ZSM-5, with an O−O distance of 2.3 Å, was noted to be greatly favorable for the recombination of deposited O atoms.231 An appropriate distance existing for the neighboring active sites was also reported by Smeets et al.,204 wherein the average Cu− Cu distance of 4.1 Å appeared to be a threshold value, distinguishing the high and low activities for Cu-modified MFI, MOR, FER, and BEA zeolites. In Fe-ZSM-5,216,232 both the fast and slow O2 desorption pathways were observed during N2O direct decomposition. The fast one is assigned to a recombination of two oxygen atoms deposited in close vicinity, whereas the slow one is associated with a migration of oxygen atoms over the catalyst surface. Although the mechanism for N2O decomposition is relatively clear, the detailed reaction pathways could still be different for the zeolite catalysts, being correlated with the topology of the zeolite as well as the chemical structure of the active site. 4.3.1. Spectroscopic Approach. The spectroscopic approach, especially for in situ DRIFTS, can provide a deep insight into the intermediates generated from interactions between the reactant molecules and the zeolite active sites, which allows a better understanding of the related reaction mechanism. Concerning the N2O direct decomposition mechanism, αO being first proposed by Panov et al.233 was a particular state of surface oxygen generated upon the contact between Fe-ZSM-5 and a N2O molecule. In our previous DRIFTS study,200 NO was utilized as a probe molecule to test the possible NO2 generation on the Fe-BEA zeolite, which was prepared by the WIE method and pretreated by N2O. In addition to the bands of chemisorbed NO being clearly observed, two bands at 1596 and 1630 cm−1, correlated with the stretch of −NO2 species, were also discerned. This finding experimentally verified that, after N2O preadsorption, αO could be yielded, which further interacted with the NO probe molecule to form NO2 species. By means of DRIFTS, Fanning and Vannice234 investigated the decomposition mechanism occurring over Cu-ZSM-5. It was noticed that the gas-phase N2O adsorbed via the O end on a Cu+ ion at a Si−O−Cu site, which was maintained in a highly dispersed state. This adsorbed N2O could be irreversibly decomposed into gaseous N2 and αO. During this process, the Cu+ ion was oxidized to Cu2+, which could be stabilized as either

Scheme 7. Proposed NOx Formation Mechanism during N2O Direct Decomposition over Fe-ZSM-5

center. Similar findings were also reported in other works conducted for Fe- and Cu-ZSM-5,189,198,233 suggesting that the N−NO bond splitting could occur to generate a small amount of NO during N2O direct decomposition. Thereafter, the generated NO could further react with adsorbed oxygen, forming NO2 (N2O4). Because N2O and NO exit simultaneously in the exhaust, Bulushev et al.235 used NO as the probe molecule to investigate the intermediate yielded after N2O introduction in the NOcontaining flow during the DRIFTS study. It was observed that NO was strongly adsorbed on the active sites of Fe-ZSM-5, showing IR bands at 1860−1900 cm−1. After the introduction of N2O, a new band at 1628 cm−1, related to the NO2, emerged. This finding suggests that NO does not block the active sites for an oxygen subtraction from N2O. On the contrary, it facilitates the dissociation of N2O by interacting with the deposited O atom from the N2O molecule, forming NO2. A similar finding was also mentioned by Xia et al.,201 who noted a promotional effect of NO on N2O decomposition over the binuclear Fe sites in Fe-ZSM-5 on the basis of the transient-state and steady-state kinetics as well as in situ IR and Raman spectroscopic studies. It was demonstrated that the addition of NO could not only increase the desorption rate of O2 but also lower the related apparent activation energy. Moreover, the in situ IR study provided a clue that NO could reactivate the Fe-ZSM-5 catalyst, which had been deactivated by steam, through removal of the hydroxyl groups bound to the Fe active sites. As shown in Figure 18, the intensities of the bands at 3628 and 3672 cm−1, owing to the hydroxyl groups adsorbed on Fe-ZSM-5, first reach their maximum and then decline over the course of the reaction. New 3690


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dual 10-MR (25T) models, with [FeO]+ as the active center, for N2O decomposition. It was determined that the longer range interactions with the zeolite framework hardly affect the energies of the mechanistic steps. On the basis of the DFT calculation, Guesmi et al.237 investigated the elementary reaction mechanisms for N2O dissociation over the binuclear active site of [FeII(μ-O)(μ-OH)FeII]+ inserted inside two different zeolite (Z) clusters of Z− and Z−OH supported on Fe-ZSM-5. Similar energetic barriers were found for these two zeolite clusters. In addition, in our previous work, two different sizes of Fe-BEA models (5T and 24T, containing double 12-MRs) were constructed for mechanism simulation of N2O direct dissociation.238 The calculation results also suggested that the model size did not significantly influence the energy barrier for N2O direct dissociation. Therefore, the zeolite catalyst represented by the small-sized models (5T−10T) can be safely utilized for the DFT simulations, especially for the mechanism simulation achieving reliable energy barriers. 4.3.2.2. Single Active Site Simulation. 4.3.2.2.1. DFT Simulation for the ZSM-5 Zeolite Catalyst. For the catalytic purification of N-containing waste gases, metal cations compensated at the Brönsted acid sites are commonly accepted as the active centers. In light of that, Yakovlev et al.239 investigated N2O direct decomposition catalyzed by 5coordinated transition-metal (M = Fe, Co, and Rh) ions, modeled as M(OH3)(H2O)2 on the basis of DFT. The calculation results suggested that Co and Rh active sites were more active than Fe, but the activities of the former two were quite similar. Fellah and Onal240 simulated N2O direct decomposition behaviors for Fe- and Co-ZSM-5 samples using a relaxed model of [(SiH3)4AlO4M] (M = Fe, Co), as shown in Figure 19a. Meanwhile, a two-layer-Fe-ZSM-5 model was also

Figure 18. In situ DRIFT spectra of N2O decomposition on Fe-ZSM-5 in the presence of 1 vol % NO at 633 K at certain time intervals: (a) 0 min, (b) 2 min, (c) 3 min, (d) 4 min, (e) 5 min, (f) 10 min, (g) 15 min, (h) 30 min. Reprinted with permission from ref 201. Copyright 2010 Elsevier.

bands at 1600 and 1630 cm−1 generated at 4 min were respectively assigned to NO2 chemisorbed on Fe sites and gaseous NO2. 4.3.2. DFT Simulations. In the past two decades, significant advances have been made in acquiring a deeper mechanistic understanding of heterogeneously catalytic processes, including diffusion, adsorption, chemical reaction, and desorption on active sites. In comparison with traditional experimental methods, theoretical simulations can provide molecular or atomic level details of catalytic phenomena that cannot be viewed directly during experiments, which makes theoretical simulations an indispensable tool for studying heterogeneous catalysis. The first principle calculation, especially involving quantum chemistry based on density functional theory (DFT), is an efficient theoretical method for microscale catalytic investigations, such as active site identification, charge-transfer analysis, mechanism study, and microkinetic analysis. DFT simulation has become an important method, which can compensate for the spectroscopic observations, especially for those that are unavailable through the traditional experiments. Due to the fact that the reaction system of N2O direct decomposition is relatively simpler with respect to those removals of the other N-containing waste gases, the related DFT simulations for zeolite catalysts have been extensively reported. On the basis of the DFT study, the structures of the zeolite framework and active center can be well represented, which not only favors better understanding of the reaction mechanism but also facilitates gaining deeper insight into the structure−activity relationship. During the zeolitic DFT simulations, the following aspects should be highly emphasized to obtain reliable conclusions: the size of the zeolite model and the chemical state of the active center (single or binuclear active site). 4.3.2.1. Size of the Zeolite Cluster. To determine the effect of the zeolitic model size on the DFT energy calculation accuracy, Lund et al.236 compared two types of Fe-ZSM-5 models, 5T and

Figure 19. Optimized geometries of (a) the M-ZSM-5 cluster (M = Fe, Co) and (b) the Fe-ZSM-5 channel cluster. The high layer (DFT region) is represented by a ball−bond view, and the low layer (molecular mechanics region) is represented by a tube view. Reprinted with permission from ref 240. Copyright 2008 Elsevier.

constructed on the basis of the ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) method to evaluate the channel effect of MFI during N2O dissociation (see Figure 19b). Generally, the N2O direct dissociation over the single active center was commonly fulfilled with the following five steps (see Scheme 8). Equations 4.11 and 4.13 involve the adsorption of a gaseous N2O molecule on an active site. Equations 4.12 and 4.14 describe the dissociation of the adsorbed N2O to produce N2 and an adsorbed O atom. Equation 4.15 represents the desorption of molecular O2. The sum of these five steps constitutes a complete catalytic cycle. The DFT calculation suggested that (i) the O2 desorption was the 3691


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Scheme 8. N2O Direct Dissociation over Zeolite Catalysta

is probably mainly due to the transformation of an Fe active site from [FeO]+ to [Fe(OH2)]+, which exhibited low N2O dissociation activity. This calculation result was also consistent with the experimental studies, suggesting that the apparent activation barriers determined for this temperature range varied from 28.4 kcal mol−1 (1 ppb H2O) to 54.8 kcal mol−1 (100 ppb H2O). Additionally, in view of a number of experimental studies reporting that a parts per million level addition of nitric oxide (NO) could enhance the N2O decomposition rate, Heyden et al.242 further investigated the N2O decomposition mechanism in the presence of NO on the basis of DFT calculation for a 5T FeMFI model of [(SiH3)4AlO4(FeO)]. The evolutions of the [FeO]+ site forming various types of Fe species during DFT simulations are profiled in Figure 21, including iron nitrite, nitrate, and hydroxo zeolite clusters.

a

M represents the active center. Z is the zeolite framework. Reprinted with permission from ref 240. Copyright 2008 Elsevier.

rate-limiting step for both clusters of Fe- and Co-MFI, wherein Co-MFI (64.95 kcal mol−1), possessing a relatively lower energy barrier, was verified to be more active than Fe-MFI (67.3 kcal mol−1), and (ii) a slight increase of the energy barrier during the first N2O dissociation was observed for the channel model (12.63 kcal mol−1) with respect to the 5T cluster model (4.41 kcal mol−1). Due to the fact that water vapor commonly exists in the exhaust, Heyden et al.241 comprehensively investigated the N2O direct decomposition mechanism over dehydrated ([FeO]+) and hydrated ([Fe(OH)2]+) mononuclear iron sites in Fe-ZSM-5 zeolite, which is represented by the simple 5T model shown in Figure 20.

Figure 20. Iron oxo/hydroxo zeolite cluster of the most abundant surface species: left, active site for N2O decomposition; middle, possible αO site; right, deactivated iron hydroxo site. Reprinted from ref 241. Copyright 2005 American Chemical Society.

Figure 21. Iron nitrite, nitrate, and hydroxo zeolite clusters. The structures are potential-energy minima on the potential-energy surface with a spin multiplicity of MS = 5. Atomic distances are indicated in angstroms. Reprinted from ref 242. Copyright 2005 American Chemical Society.

In total, 46 different surface species with diverse spin states (spin multiplicity, MS = 4 or 6) and 63 elementary reactions were taken into account. It was suggested that the N2O dissociation mechanism over [FeO]+ and [Fe(OH)2]+ sites followed the E−R mechanism. Two reaction pathways were found during the first N2O dissociation over the [FeO]+ site: (i) one pathway involves the formation of an [FeO2]+ site, in which two O atoms form either a superoxide (O2−) or a peroxide (O22−) anion attached to an Fe2+ or Fe3+ cation; (ii) another accompanies the formation of dioxo [OFeO]+ species. The energy barriers leading to the [FeO2]+ and [OFeO]+ sites are close to each other, with values of 29.0 and 27.5 kcal mol−1, respectively. The generated [FeO2]+ and [OFeO]+ sites can also catalyze N2O dissociation into N2 and adsorbed O with respective energy barriers of 24.0 and 42.9 kcal mol−1. In addition, it was observed that H2O adsorption on the [FeO]+ site was barrierless to form the hydrated Fe site of [Fe(OH)2]+. The DFT energy calculation suggested that the further dissociation of N2O into N2 and adsorbed O over this hydrated Fe site had to overcome extremely high energy barriers: 54.4 (for the first N2O molecule) and 52.2 (for the second N2O molecule) kcal mol−1, respectively. This finding provides a clue that the strong inhibiting effect of H2O in the temperature range of 325−425 °C

A mechanism network consisting of over 100 elementary reaction steps was thereafter established. It was indicated that most active iron sites ([FeO]+) could be poisoned by a small concentration of water in the gas phase in the absence of NO and at low temperatures (T < 4252 °C), forming [Fe(OH2)]+. However, the addition of NO could convert this poisoned site into a novel active iron center of [FeOH]+ over a low activation barrier of 13.3 kcal mol−1, which was significantly lower than the activation barrier for the desorption of H2O from Z−[Fe(OH)2]+. The generated [FeOH]+ active sites were capable of promoting the dissociation of N2O into a surface oxygen atom and gas-phase N2. The energy barriers for N2O dissociation into N2 and adsorbed O, as well as O2 desorption, over such novel iron sites were, respectively, 24.0, 46.5, and 7.6 kcal mol−1. As the temperature increased (T > 425 °C), the H2O could easily desorb from inactive Fe sites, resulting in recovery of various Fe active sites of Z−[FeO]+, Z−[FeO2]+, and Z−[OFeO]+. The further exposure of Z−[FeO]+, Z−[FeO2]+, and Z−[OFeO]+ sites to NO could lead to the formation of three new stable nitrite and n it r at e speci es , Z − [FeONO] + , Z − [FeO 2 N] + , a nd Z−[FeO2NO]+. The reaction routes for N2O dissociation occurring over such sites were further simulated. According to the simulation result, these nitrite and nitrate species were 3692


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believed to play a marginal role in N2O decomposition over FeZSM-5. Large amounts of NO did not increase the overall activity for N2O decomposition by forming iron nitrite and nitrate species, and the iron nitrite and nitrate species were likely as active as the Z−[FeO]+ and Z−[FeOH]+ species for N2O decomposition. On these iron nitrite and nitrate species, the energy barriers of the second N2O dissociation into N2 and adsorbed O was calculated to be 44.9, 45.2, and 41.1 kcal mol−1, respectively, which constituted the rate-determining step for the NO-assisted N2O decomposition on iron nitrite and nitrate species. Therefore, the promotion effect of NO during N2O direct dissociation could be concluded to be mainly due to the transformation of inactive Fe sites into active ones at low temperatures (T < 425 °C). 4.3.2.2.2. DFT Simulation for BEA Zeolite Catalyst. For N2O direct decomposition, a large number of studies have concentrated on the metal-modified MFI zeolites due to their excellent catalytic behaviors. It was recently found that Fe-BEA exhibited a superior activity for the direct decomposition and selective catalytic reduction (SCR) of N2O with respect to FeMFI.224,243,244 However, the related DFT simulations, aiming at gaining an insight into the reaction mechanism over the metalmodified BEA zeolite catalysts, were rarely reported. In light of that, the 5T-TMI Fe-, Co-, and Cu-BEA models were constructed in our previous work for the mechanism simulations,115,200,245 wherein the monofunctional metal cation behaved as the active center (see Figure 22).

Scheme 9. N2O Direct Decomposition Mechanism over (a) Co-BEA during DFT Calculations and (b) M-BEA (M = Fe, Cu) during DFT Calculations and (c) Reaction Steps of the Mechanism with NO Formationf

Figure 22. Optimized geometries of the 5T-M-BEA models (M = Fe, Co, Cu) (atomic distance values are in units of angstroms). Reprinted with permission from ref 115. Copyright 2012 Elsevier.

a Co-BEA is noted as Co-Z. bThe transition state is noted as TS. cThe intermediate is noted as IM. dM-BEA (M = Fe, Cu) is noted as M-Z. e Fe−O−Z−N2O (N2O adsorption on αO through the N end). f Reprinted with permission from ref 115. Copyright 2012 Elsevier.

On the basis of the constructed models, two types of mechanisms were thereafter proposed, including an O2 formation mechanism [Scheme 9a,b for M-BEA (M = Fe, Co, Cu)] and a NOx formation mechanism (Scheme 9c for Fe-BEA). The main difference between these two mechanisms lies in the adsorption mode of the second N2O molecule: O-end adsorption is involved for the O2 formation mechanism (part 2 of Scheme 9a,b), and N-end adsorption on αO takes place for the NOx formation mechanism (part 1 of Scheme 9c). For the O2 formation mechanism, a somewhat different pathway was found for Co-BEA during the first N2O dissociation as compared to those of Fe- and Cu-BEA. An additional intermediate (IM) was generated over Co-BEA, resulting in two energy barriers. The DFT energy calculation implied that Co-BEA possessing the lowest energy barrier for O2 desorption (58.3 kcal mol−1), which is the rate-determining step for the whole catalytic cycle,

exhibited the highest catalytic activity. The lowest activity was observed for Cu-BEA displaying the highest energy barrier (78.4 kcal mol−1) during the O2 desorption step. For the NOx formation mechanism, two types of transition states were observed, with energy barriers of 26.9 kcal mol−1 (eq 4.37 of Scheme 9c) and 2.5 kcal mol−1 (eq 4.39 of Scheme 9c), respectively. This finding suggested that eq 4.37 was the ratedetermining step for the NOx formation mechanism. Additionally, comparing the energy barriers of the second N2O dissociation over Fe-BEA during the O2 formation and NOx formation mechanisms, it was determined that the energy barriers were comparable to each other, respectively being 35.8 and 26.9 kcal mol−1. This finding gives us a clue that N2O was 3693


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able to be adsorbed upon Fe−O−Z in two ways (O end on Fe and N end on αO), and both of them could occur under the reaction conditions with the respective formation of O2 or NO. Similar 5T models of Fe- and Cu-BEA of [Si(OH)3]4AlO4M (M = Fe, FeO, Cu) were also constructed to investigate the N2O dissociation mechanism in the presence of CO in our previous work.246 Two types of reaction mechanisms were thereafter proposed, respectively denoted as the redox mechanism and the associative mechanism (Scheme 10, part A and part B, Scheme 10. Reaction Steps of the Redox Mechanism (Part A) and Associative Mechanism (Part B) over Fe- and Cu-BEAa

Figure 23. Cluster model Z−[HO−Fe−O−Fe−OH]2+Z− and its position in the ZSM-5 straight channel. Reprinted from ref 247. Copyright 2001 American Chemical Society.

the NO formation mechanism were thereafter proposed over [HO−Fe−O−Fe−OH]2+ and [Fe−O−Fe]2+ sites. The detailed pathways for N2O dissociation into O2 and N2 (O2 formation mechanism) upon binuclear [Fe−O−Fe]2+ and [HO−Fe−O− Fe−OH]2+ sites were quite similar to the L−H reaction mechanism. The N2O molecule could be adsorbed on the active Fe site, which was further dissociated into N2 and an adsorbed O atom. Migration of the O atom from one iron site to another resulted in the formation of O2. The DFT energy calculation suggested that this O migration was the rate-determining step for both of these active sites with similar energy barriers of 17.7 ([Fe−O−Fe]2+) and 17.9 ([HO−Fe−O−Fe−OH]2+) kcal mol−1, respectively. However, the energy barrier of the O2 desorption step for the [HO−Fe−O−Fe−OH]2+ active site (5.5 kcal mol−1) was lower than that of the [Fe−O−Fe]2+ site (13.9 kcal mol−1), revealing that the [HO−Fe−O−Fe−OH]2+ site exhibited relatively higher activity than that of the [Fe−O− Fe]2+ site during N2O decomposition. In addition, the NO formation mechanism was further simulated over the [HO−Fe− O−Fe−OH]2+ active site. It was revealed that NO could be generated over the binuclear active site of [HO−Fe−O−Fe− OH]2+, wherein the second N2O molecule was adsorbed on the “αO” (subtracted from the first N2O molecule) through the N end. It is understood that reaction pathways for NO formation over the binuclear active site are similar to those occurring over the single active site, as reported in our previous work.200 NO desorption with an energy barrier of 34.6 kcal mol−1 constitutes the rate-determining step for the NO formation mechanism. Similar active site models of Fe-ZSM-5 (Z−[HO−Fe−O− Fe−OH]2+Z−and Z−[Fe−O−Fe]2+Z−) were also constructed by Hansen et al.,86 and further applied for the N2O decomposition mechanism simulations. Fe was respectively located at the T9 and T12 sites of the MFI zeolite matrix. However, the reaction pathways over the [HO−Fe−O−Fe−OH]2+ site were totally different from those proposed by Yakovlev et al.247 Two types of catalytic cycles could occur over the [HO−Fe−O−Fe−OH]2+ site. The first cycle revealed that the bridging atomic O of the active [HO−Fe−O−Fe−OH]2+ site could participate in the N2O dissociation process through a recombination with the O atom subtracted from the N2O molecule and subsequent formation of the adsorbed O2 molecule ([HO−Fe−O2−Fe− OH]2+). After O2 desorption, the bridging oxygen could be compensated by the atomic O of the second N2O molecule, finally completing the catalytic cycle. The DFT energy calculation suggested that the first N2O molecule dissociation into adsorbed O and N2, with an energy barrier of 40.2 kcal mol−1, was the rate-determining step for this catalytic cycle. Another cycle suggested that the atomic H of the hydroxy group

a M represents the active centers of Fe+, Cu+, [FeO]+, and [CuO]+. Reprinted with permission from ref 246. Copyright 2013 Elsevier.

respectively). The calculation results suggested that (i) the CO2 desorption step was the rate-determining step for both the redox and associative mechanisms, (ii) the redox mechanism was preferred for both the Fe- and Cu-BEA samples, due to much lower energy barriers with respect to the associative mechanism, (iii) the energy barrier on the [Cu]+ site was higher than that on the [Fe]+ site, indicating that Fe-BEA was superior to Cu-BEA for the reduction of N2O by CO, and (iv) for the [Fe]+ and [FeO]+ active sites, the former exhibited much higher activity for N2O dissociation in the presence of CO. 4.3.2.3. Binuclear Active Site Simulation. As stated above, large amounts of research have focused on the single active site (individual cation or oxo particles) simulations based on DFT, which exist under the conditions of a low amount of metal loading and a high Si/Al ratio. However, the introduced metals in the zeolite catalysts might appear in many other chemical states, for example, the binuclear structures, owing to the diverse zeolite topologies, various Al framework distributions, and excess amounts of metal loading. Nevertheless, due to their complicated structures, the related DFT simulations were less reported in comparison with those of the single active site. Yakovlev et al.247 utilized DFT to investigate the binuclear Fe active site in Fe-MFI for N2O direct decomposition. The constructed model is depicted in Figure 23, wherein the framework Al atoms are respectively located at the T12 and T9 sites of the MIF structure and are separated by two silicon− oxygen tetrahedrons. It was reported that the binuclear active site was predominant in the form of the [HO−Fe−O−Fe− OH]2+ structure in the temperature range of 200−500 °C and in the presence of water vapor. It would be completely dehydroxylated into [Fe−O−Fe]2+ at T > 500 °C, and these active sites could be fully hydroxylated (and therefore not be available for the N2O dissociation) at T < 200 °C. Three types of cycled O2 formation mechanisms (cycles A−C) associated with 3694


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on the T9 site Fe ([HO−Fe−O−Fe−OH]2+) could migrate to the bridging O atom, forming [O−Fe−OH−Fe−OH]2+, which resulted in the T9 site Fe behaving as the active center for N2O dissociation. As noted here, the left atomic O of the hydroxy group (O−Fe−O−Fe−OH]2+) could take part in the O2 formation process through recombination with the O subtracted from the N2O molecule. Similar to the first cycle, the dissociation of N2O (42.7 kcal mol−1) was the rate-determining step. In lowtemperature ranges, the hydroxylated iron site of [HO−Fe−O− Fe−OH]2+ dominated on the zeolite surface. As the temperature increased, H2O could be desorbed, associating with the generation of the dehydroxylated iron site ([Fe−O−Fe]2+). Hansen et al.86 suggested that both of the Fe sites (T9 and T12) of [Fe−O−Fe]2+ acted as the active centers for N2O direct dissociation. The subtracted O atom from the N2O molecule could be recombined, forming the O2 molecule. The ratedetermining step for this type of mechanism was reported to be the dissociation of the N2O molecule into the adsorbed O atom and gaseous N2. The energy barriers over these two active Fe sites were 27.5 (T9-Fe) and 25.5 (T12-Fe) kcal mol−1, respectively. Guesmi et al.197 constructed another type of binuclear active site Fe-ZSM-5 model for a N2O dissociation mechanism simulation, as profiled in Figure 24. The active site was

isolated ([Fe−O]+) and the binuclear ([FeII(μ-O)(μ-OH)FeII]+) active sites of Fe-ZSM-5 through an interaction with CO as the probe molecule. According to the calculated entropic parameters, it is known that αO formed over the isolated site possessed much higher oxidation ability than that formed at the binuclear active site. Because no consensus on the importance of mononuclear91,93,215,249 and binuclear95,195,196,198,250,251 Fe sites was made for N2O direct decomposition, Li et al.252 recently compared Fe2+, [FeIIIO]+, and binuclear [FeII(μ-O)FeII]2+ and [FeIII(μ-O)2FeIII]2+ by DFT for N2O dissociation, and the constructed models are shown in Figure 25. The simulation

Figure 25. Constructed models for mononuclear (a) Fe2+ and (b) [FeIIIO]+ and binuclear (c) [FeII(μ-O)FeII]2+ and (d) [FeIII(μO)2FeIII]2+. Reprinted with permission from ref 252. Copyright 2013 Elsevier.

results implied that the binuclear sites were more active than the Fe2+ and [FeIIIO]+ mononuclear active Fe sites. Although the activation of the first N2O molecule was favorable over the mononuclear iron site, the rate of the second N2O dissociation was considerably lower due to the extremely high barriers (>43.0 kcal mol−1). For the binuclear iron center, each iron site can be utilized for N2O activation, and two proximate subtracted O atoms are readily available, resulting in the formation of adsorbed O2 and its subsequent desorption occurring at a relatively higher rate. In conclusion, the binuclear cluster can not only lower the activation barrier for the dissociation of the second adsorbed N2O but also facilitate the migration of surface oxygen atoms through recombination over two neighboring active sites. 4.3.2.5. Summary. In this section, the DFT mechanism simulation of N2O direct decomposition over zeolite catalysts is reviewed, wherein the active site status (single or binuclear) and zeolitic framework structures, as well as the influences of impurity gases of H2O, NO, and CO, are addressed. The energy barriers calculated for the reaction steps of N2O dissociation into N2 and adsorbed O, O2 desorption, and O migration (for binuclear active sties) are collected in Table 10. On the basis of the above statements, the following conclusions can be drawn. Over a mononuclear active site (Fe2+, Cu2+, Co2+, [FeO]+, [Fe(OH)2]+, [FeO2]+), N2O dissociation mainly follows the E− R reaction mechanism, wherein two N2O molecules are adsorbed on the Fe active site, and the O−N2 bond fracture follows. The two adsorbed O atoms form O2, and then O2 desorption regenerates the active site. The O2 desorption115,200 or N2O dissociation into N2 and adsorbed O241,242 was diversely reported as the rate-determining step during N2O direct decomposition over the mononuclear active site, which is mainly correlated with the structures of the active centers. For example, over a metal cation active site (Fe2+, Co2+, or Cu2+),115,200,240 the O2 desorption was reported as the ratedetermining step, whereas for a metal oxo cation active site ([FeO]+, [FeO2]+, or [OFeO]+),241,242 the N2O dissociation into N2 and adsorbed O constituted the rate-determining step for the whole catalytic process. The addition of CO, H2O, and NO into the N2O dissociation system could influence the

Figure 24. (a) Diiron center ([Fe(μ-O)(μ-OH)Fe]+) inserted into part of the zeolite framework. Small gray balls are H, large gray balls are Fe, blue balls are Si, the green ball is Al, and red balls are O. (b) Localization of the Z− cluster inside ZSM-5 zeolite. Reprinted from ref 197. Copyright 2008 American Chemical Society.

represented by a binuclear iron active site of [FeII(μ-O)(μOH)FeII]+, wherein two neighboring Fe sites located at the T9 and T6 sites were bridged by O and OH−. Such an active site was verified to be generated through a reductive dehydroxylation pathway by [HOFeIII(μ-O)(μ-OH)FeOHIII]+. N2O dissociation pathways occurring over such a binuclear active site were similar to the pathway reported by Hansen et al.86 occurring over the Z−[Fe−O−Fe]2+Z− model, including N2O adsorption on each binuclear Fe site, subtraction of atomic O from N2O, forming the Z−[O−FeII(μ-O)(μ-OH)FeII−O]+ intermediate, and O2 formation through the migration of the atomic oxygen from one iron site to another one. However, the O migration step, with an energy barrier of 28.4 kcal mol−1, was found to be the ratedetermining step for this binuclear active site, which was different from the report by Hansen et al.86 suggesting that N2O dissociation was the rate-determining step. 4.3.2.4. Comparison between the Single and Binuclear Active Sites. αO was first proposed by Panov et al.233 during the investigation of N2O direct decomposition over Fe-ZSM-5. It is a particular state of surface oxygen which is formed upon the contact of Fe-ZSM-5 with the N2O molecule. αO is reported to be highly effective for N2O dissociation. In light of that, Guesmi et al.248 compared the oxidative ability of αO formed over the 3695


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no.

1 2 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

[(SiH3)4AlO4Fe] [Si36O80AlFe] [(SiH3)4AlO4Fe] [(SiH3)4AlO4FeO] [(SiH3)4AlO4FeO] [(SiH3)4AlO4FeO2] [(SiH3)4AlO4FeO2] [(SiH3)4AlO4Fe(OH)2] [(SiH3)4AlO4Fe(OH)2] + NO [(SiH3)4AlO4FeONO] [(SiH3)4AlO4FeO2N] [(SiH3)4AlO4FeO2NO] Z−[Fe−O−Fe]2+Z− Z−[HO−Fe−O−Fe−OH]2+Z− Z−[HO−Fe−OH−Fe−(OH)2]2+Z− Z−[HO−Fe−O−Fe−OH]2+Z− Z−[Fe−O−Fe]2+Z− Z−[Fe−O−Fe]2+Z− Z−[Fe−O−OH−Fe]2+Z− [(SiH3)4AlO4Fe] [(SiOH3)2AlO4Fe] [(SiH3)4AlO4Co] [(SiH3)4AlO4Co] [(SiH3)4AlO4Cu] [(SiOH3)2AlO4Fe] + NH3 Fe Fe + NH3 [(SiH3)4AlO4Fe] + CO [(SiH3)4AlO4Fe] + CO [(SiH3)4AlO4FeO] + CO [(SiH3)4AlO4FeO] + CO [(SiH3)4AlO4Cu] + CO [(SiH3)4AlO4CuO] + CO

model Fe cation Fe2+ cation Fe2+ cation [FeO]+ [FeO]+ [FeO2]+ [OFeO]+ [Fe(OH)2]+ [FeOH]+ [FeONO]+ [FeO2N]+ [FeO2NO]+ [Fe−O−Fe]2+ [HO−Fe−O−Fe−OH]2+ [HO−Fe−OH−Fe−(OH)2]2+ [HO−Fe−O−Fe−OH]2+ [Fe−O−Fe]2+ [Fe−O−Fe]2+ [Fe−O−OH−Fe]2+ Fe2+ cation Fe2+ cation Co2+ cation Co2+ cation Cu2+ cation Fe2+ cation Fe+ Fe+ Fe2+ cation Fe2+ cation [FeO]+ [FeO]+ Cu2+ cation Cu2+ cation

2+

active site 4.4 12.6 2.8 27.5 29.0 20.1 16.5 54.4 24.0 24.8 25.4 25.2 −31.5 −12.0 −4.8 40.2 27.5 25.5 11.5 8.9 33.5 6.3 9.1/8.6 25.2 30.2 17.4 15.0 3.7 (redox mechanism) 11.4 (associative mechanism) 33.8 (redox mechanism) 41.4 (associative mechanism) 35.0 (redox mechanism) 39.8 (associative mechanism)

first N2O dissociation ΔE (kcal mol‑1)

Table 10. Calculated Energy Barriers during N2O Dissociation over Zeolite Catalysts Based on DFT

3696

50.9 21.6 21.6 11.4 11.4 16.0 7.6 6.0 3.0 3.0 13.9 5.5 −12.9 9.2 8.1 8.1 25.5 63.2 65.0 57.8 77.8

29.0 42.9 24.0 41.1 41.1 52.2 46.5 44.9 45.2 41.1 −9.1 −0.95 8.6 25.9 27.7 30.1 24.3 35.8 48.6 49.4 45.3

0.36 (redox mechanism)

7.9 (redox mechanism)

5.7 (Redox mechanism)

6.3

O2 desorption ΔE (kcal mol‑1)

58.13

second N2O dissociation ΔE (kcal mol‑1)

6.5 7.3 7.3 28.4

17.7 17.9

O migration ΔE (kcal mol‑1)

240 240 241 241 241 241 241 241 242 242 242 242 247 247 247 86 86 86 197 115 253 240 241 241 253 253 253 246 246 246 246 246 246

ref



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microkinetic analysis suggested that the N2O decomposition is first-order with respect to the N2O concentration and zerothorder with respect to the O2 concentration, which was consistent with the experimental observation.254 A profound negative effect was found during N2O catalytic dissociation in the presence of water vapor with concentrations below 100 ppb. In the absence of water vapor, the apparent activation energy was 25.5 kcal mol−1 and the preexponential factor was calculated to be 4.0 × 103 molN2O/(s molFe PaN2O). However, the activation energy increased to 54.8 kcal mol−1 and the preexponential factor increased by 9 orders of magnitude to 6.8 × 1012 molN2O/(s molFe PaN2O) in the presence of 100 ppb H2O. To verify the validity of the computational simulations, N2O dissociation in the presence of water vapor at a concentration of 23 ppb was further simulated, with the calculated apparent preexponential factor [3.9 × 109 molN2O/(s molFe PaN2O)] being comparable to that measured by Wood et al.215 [9.9 × 108 molN2O/(s molFe PaN2O)]. Regarding the effect of NO, it was suggested that the addition of NO could interact with iron oxide species at low temperatures to form various iron nitrites and nitrates; however, these species did not contribute significantly to N 2 O decomposition. In addition, the presence of NO could transform the H2O-poisoned iron site of [Fe(OH)2]+ into an active iron site of [FeOH]+, with a preexponential factor of 1.25 × 1012 s−1 (T = 327 °C). The kinetic parameters of the preexponential factors during the further N2O dissociation over the [FeOH]+ active site were respectively calculated to be 8.07 × 1011 (the first N2O dissociation), 5.48× 1012 (the second N2O dissociation), and 7.23 × 1013 (the O2 desorption step) s−1 at T = 327 °C. The same group86,217 also conducted a microkinetic analysis on N2O decomposition over binuclear oxygen-bridged iron sites in Fe-ZSM-5 to study the influence of H2O on the distribution of binuclear iron sites between catalytically active ([Fe−O−Fe]2+) and inactive ([HOFe−O−FeOH]2+) forms and on the apparent first-order rate constant. The presence of low concentrations of water vapor in the feed stream (parts per billion to parts per million levels) was verified to affect the values of the apparent activation energy and the preexponential factor. Over the H2Oinfluenced binuclear iron site of [HOFe−O−FeOH]2+, the first N2O dissociation energy barrier and preexponential factor were respectively calculated to be 40.2 kcal mol−1 and 9.73 × 1011 s−1 (T = 327 °C). Over the active site of [Fe−O−Fe]2+, the values were respectively calculated to be 27.5 kcal mol−1 and 9.83 × 1012 s−1 (T = 327 °C). Moreover, the microkinetic models for both mononuclear and binuclear iron sites were used to reproduce a temperature-programmed reduction analysis, as reported for the Fe-ZSM-5 samples with low and high iron contents. The analysis results led to the conclusion that mononuclear iron sites prevail at very low Fe/Al ratios. However, both mononuclear and binuclear iron sites are likely to be simultaneously present at higher Fe/Al ratios. In our previous work, the microkinetic analysis was further employed for N2O direct decomposition over Fe-, Co-, and CuBEA, which was represented by the 5T-BEA models of [(SiH3)4AlO4M] (M = Co, Cu) and with metal cations as the active centers. The kinetic parameters of the energy barrier, preexponential factor, reaction rate constant, and reaction rate of each elementary step (see Scheme 9) were all calculated. The schematic diagram of the microkinetic analysis is profiled in Figure 26.

reaction to different extents. CO favors the dissociation of N2O into N2 in association with the generation of CO2, wherein the desorption of CO2 was found to be the rate-determining step.246 H2O has a negative effect on N2O catalytic dissociation, especially in low-temperature ranges (325−425 °C), leading to the transformation of the active site from [FeO]+ to [Fe(OH2)]+, which possesses low N2O dissociation activity.241 NO was known to play a marginal role in N2O dissociation (see Table 10, nos. 11−13); however, the addition of NO could transform the hydrated active site ([Fe(OH)2]+) into a novel active site ([Fe(OH)]+) for N2O dissociation (see Table 10, nos. 9 and 10).242 For the binuclear active sites, three types of active structures were reported: [Fe−O−Fe]2+, [HO−Fe−O−Fe− OH]2+, and [Fe−O−OH−Fe]2+.86,197,247 The binuclear active Fe site of [HO−Fe−O−Fe−OH]2+ commonly exists in the lowtemperature range (200−450 °C) and in the presence of H2O. The dehydration of [HO−Fe−O−Fe−OH]2+ at high temperatures could generate the [Fe−O−Fe]2+ site. Over these binuclear active sites, N2O dissociation mainly obeys the L−H mechanism, wherein the migration of subtracted O from the N2O molecule constitutes the rate-determining step197,247 (see Table 10, nos. 14, 15, and 20). On the basis of the DFT simulations, the comparison between the mononuclear and binuclear active sites suggests that the binuclear iron site exhibits higher deN2O activity than that of the mononuclear iron site, due to ready formation of O2 at the neighboring iron site. The generation of the binuclear iron site commonly needs a special Al distribution located at the neighboring site. Moreover, the introduction of a higher degree of metal loading is also necessary to create such an active site. However, according to experimental experience, high loading amounts could readily result in an agglomeration of metal species, forming inactive metal oxides. Therefore, the binuclear active site is more difficult to achieve with respect to the mononuclear active site. 4.4. Microkinetic Analysis

On the basis of DFT and harmonic transition-state theory (HTST), it is highly possible to conduct a microkinetic analysis on each elementary reaction step of the proposed reaction mechanism. The reaction rate constant (k) is calculated according to eq 4.52, where kB is the Boltzmann constant, ΔG stands for the standard molar activation Gibbs energy, n is the sum of the molar coefficients of the reactants, and h, T, and R denote Planck’s constant, the temperature, and the gas constant, respectively. Microkinetic analysis can provide detailed information on the kinetic analysis of each elementary step, which greatly favors the understanding and verification of the reaction mechanism. 1−n ⎛ −Δ G θ (P θ) ⎞ kBT ⎛ pθ ⎞ r m ⎜ ⎟ ⎟⎟ k= ⎜ ⎟ exp⎜⎜ h ⎝ RT ⎠ RT ⎝ ⎠

(4.52)

As stated in section 5.3, Heyden et al.241,242 utilized the 5T-FeMFI models with mononuclear iron as the active center to comprehensively simulate the N2O dissociation mechanism over hydrated and dehydrated mononuclear iron sites. To further reveal the effects that water vapor and NO played during the N2O catalytic dissociation, microkinetic analyses were conducted for each proposed elementary reaction step. The microkinetic parameters, including the rate constants (k, k−1) and preexponential factors (A, A−1) of the forward and reverse reactions, and the equilibrium constant (K) obtained from k/k−1, were calculated and summarized, forming a database. The 3697


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microkinetic analysis was not widely applied during the mechanism investigation, which is related to two main reasons: (i) the microkinetic analysis requires that each elementary step should be well simulated to ensure the correctness of the calculated kinetic parameters, and (ii) the microkinetic analysis requires relatively complicated mathematical calculations. However, along with the further development of the DFT calculation method as well as computation technology, it will become a prevalent method for the DFT mechanism simulation.

5. CATALYTIC REDUCTION OF NO For the past 50 years, there has been a growing focus on NOx emission control from stationary and automotive sources, leading to worldwide research on deNOx technologies. Over 40% of NOx emissions are from stationary sources, such as power plants and industrial boilers using fossil fuels, and over 50% come from automotive sources, such as gasoline and diesel engine cars. There are three current commercial catalytic systems, which were developed on the basis of a large number of previous efforts: noble-metal-based three-way catalysts (TWCs) for the purification of gasoline engine emissions and transition-metal zeolite catalysts for the purification of diesel engine emissions from a mobile source and vanadium−titania catalysts for the elimination of power plant emissions from a stationary source. However, due to stricter environmental legislation and the desire to achieve energy savings, there is an increasing desire to develop more efficient deNOx catalysts. For the stationary deNOx technology, especially for that utilized in power plants, it is required that the applied catalyst possess excellent low-temperature activity as well as high N2 selectivity. The automobile deNOx technology requires not only that the applied catalyst exhibit high deNOx efficiency and N2 selectivity but also that it possess a wide operating temperature window due to temperature variations in the exhaust stream. The removal of NOx from the oxygen-rich exhaust of diesel engines is a great challenge for the automobile deNO x technology. The selective catalytic reduction (SCR) of NOx by ammonia under strongly oxidizing conditions was first introduced in 1973.255 Since then, NH3-SCR has been widely utilized for NOx emission control in stationary sources and mobile lean-burn diesel engines. However, many problems have been encountered during these applications,256 such as catalyst toxicity/deactivation, NH3 slip, and reductant consumption. Due to its high toxicity, the use of the vanadium catalyst has been prohibited in the United States and Japan. To overcome these drawbacks, two research directions have been pursued: (i) designing an efficient low-temperature zeolite catalyst for NH3SCR and (ii) developing deNOx technologies using different reductants, such as hydrocarbons, H2, and ethanol, etc. For the first direction, the present work will concentrate on the newly discovered small-micropore zeolite catalyst with a CHA topology, which was frequently reported to be extremely active for NH3-SCR with high hydrothermal stability. The detailed microstructure environment and active center distribution as well as the final structure−activity relationship will be herein reviewed. For the second direction, the present work will provide a review of NO-SCR by diverse reductants, including hydrocarbons and H2, to illustrate the recent achievement of other promising deNOx technologies. Finally, this work will review DFT simulations for deNOx mechanisms achieved over zeolite catalysts, including NH3-SCR and NO direct dissociation mechanism simulations. As noted here, due to the complexity of the reaction system of NO-SCR, which consists of at least three

Figure 26. (a) Microkinetic analysis of the main reaction steps during N2O direct decomposition over M-BEA (M = Fe, Cu). (b) Microkinetic analysis of the main reaction steps during N2O direct decomposition over Co-BEA. K represents the overall equilibrium constant, k is the reaction rate constant of the forward reaction, and k−1 is the reaction rate constant of the reverse reaction. Reprinted with permission from ref 115. Copyright 2012 Elesvier.

The calculation results suggested that O2 desorption was the rate-determining step. The calculated reaction rates for Fe-, Co-, and Cu-BEA were, respectively, 1.74 × 10−24, 5.84 × 10−21, and 7.05 × 10−31 mol m−3 s−1, suggesting that Co-BEA, possessing the highest O2 desorption reaction rate, was the most active catalyst for N2O dissociation. This finding was also in good agreement with the data derived from experimental TOF results. Moreover, the microkinetic analysis of Co-BEA suggested that the generation of an intermediate during the first N2O dissociation had a negative effect on the reaction rate for N2O dissociation. It was found that the second forward reaction rate constant of Co-BEA (k3 in Figure 26b, 5.57 × 105 s−1) was much lower than its reverse reaction rate constant (k3−1 = 6.09 × 107 s−1), resulting in a final decline of the N2O decomposition rate. This finding was supported by both the activity evaluation and N2O-TPD, suggesting that related profiles of Co-BEA declined slower than those of Fe- and Cu-BEA. In light of the microkinetic analysis based on DFT, microkinetic models were further constructed for Fe-, Co-, and Cu-BEA. After the correction by the experimental results, semiempirical microkinetic models for N2O direct decomposition over Fe-, Co-, and Cu-BEA at different GHSV values (3.2 × 104, 4.0 × 104, 6.0 × 104, and 8.0 × 104 h−1) and in different reaction temperature ranges (350−400 °C for Fe-BEA, 340−400 °C for Co-BEA, and 440−480 °C for Cu-BEA) were constructed, which could successfully predict the experimental data in the kinetic regime. In summary, the microkinetic analysis based on DFT facilitates better understanding and further verification of the proposed mechanism. As stated above, the effect of water vapor during N2O dissociation over Fe-ZSM-5 simulated for both the mononuclear and binuclear active centers could be well quantified on the basis of the microkinetic analysis. The special catalytic behavior of the Co-BEA during N2O direct dissociation was well explained through the microkinetic analysis; moreover, semiempirical microkinetic models were thereafter constructed.115 However, it should also be noted that the 3698


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molecules (NO, O2, and reductant), DFT studies on the deNOx mechanism have been less reported, but we strongly believe that SCR mechanism simulations based on DFT will become prevalent in the near future.

systems for diesel-engine passenger vehicles and light- and medium-duty trucks in the United States and Europe,267,268 there has been a new surge of research interest in NH3-SCR over Cu-SSZ-13, concentrating on the basic scientific questions of the structure−activity relationship of Cu-CHA zeolite (SSZ-13, SAPO-34) catalysts. In light of that, the recent achievements, including the chemical states of the active species, the structure− activity relationship, and the reaction mechanisms, will be discussed in detail in this section. 5.1.1.2.1. Chemical State of the Active Site. 5.1.1.2.1.1. CuSSZ-13. As is widely accepted, the SCR performance of the Cumodified zeolites was found to depend on the microstructure of the copper. Several types of copper species, such as isolated Cu2+ and Cu+ ions,269 oxide CuO- or Cu2O-like clusters,270 binuclear [Cu−O−Cu]2+,271 [Cu−μ-O2−Cu]2+,272 and [−Cu−O−Cu− O−] chain structures,273 have been proposed as the active sites for Cu-ZSM-5 catalysts on the basis of spectroscopic observations. CHA-type SSZ-13, which was first synthesized by Zones and Chevron in 1985,274 has a three-dimensional tetrahedral framework constructed of double six-membered rings (d6r) in an AABBCC sequence, forming a “cage” per unit cell as depicted in Figure 27.275

5.1. Selective Catalytic Reduction (SCR) with Different Reducing Agents

5.1.1. NH3-SCR. 5.1.1.1. Traditional Microporous Zeolite Catalyst. Metal-exchanged zeolites represent one type of SCR catalyst that exhibits strong performance across a wide temperature range. The type of loaded metal is important for the corresponding catalytic properties. Fe- and Cu-modified zeolite catalysts have been widely investigated for NH3-SCR due to their higher activities and thermal stabilities with respect to commercial vanadium catalysts.257 Cu-based zeolite catalysts (Cu-MFI, Cu-BEA, Cu-FER) commonly exhibited higher activities in lower temperature ranges (400 °C).72 This is mainly because Cu catalysts are capable of high NOx conversions at T < 400 °C, but the overconsumption of NH3 would suppress the deNOx activity as the temperature increases (T > 400 °C). The low NH3 oxidation abilities of Fe catalysts resulted in their better deNOx activities in the hightemperature range. To combine the benefits of Fe- and Cuzeolite systems, Boron et al.259 recently prepared two series of Fe- and Cu-containing BEA zeolites by employing conventional wet-impregnation methods, resulting in the incorporation of iron and copper into the vacant T atom sites of the zeolite framework. An activity evaluation suggested that the simultaneous presence of the Cu and Fe species in the BEA zeolite significantly improved the low-temperature NO conversion. Although many studies have been performed on conventional zeolite catalysts with medium- and large-micropores, such as BEA, MFI, MOR, and FER,260−262 several challenges arise when using these metal-exchanged zeolites for practical lean-burn or diesel vehicles, most significantly hydrothermal deactivation and the chemisorption of impurities (hydrocarbon, phosphorus, and potassium) on the active sites of the catalyst.80,263 5.1.1.2. Small-Microporous Zeolite Catalyst. The selective catalytic reduction of NOx by NH3 has been the subject of extensive research interest for the past 20 years,264 wherein CuZSM-5, exhibiting remarkable deNOx activity, has been frequently investigated. In spite of the high hydrothermal instability of the copper active sites, which strongly limits its practical applications, Cu-ZSM-5 is still considered a “model system” in research on novel catalysts for NO direct decomposition. Along with the continuous research work on NOx removal by zeolitic catalysts, it was recently reported that extremely high deNOx activities (>95% at 150 °C) and N2 selectivities (>90%) covering a wide temperature window (150− 500 °C) are achieved by introducing Cu into the smallmicroporous zeolites of SSZ-13 and silicoaluminophosphates (SAPO-34) [with chabazite (CHA) structures].12,13,265,266 A relatively high hydrothermal stability and poisoning resistance were also clearly observed for such zeolite catalysts, making them potential candidates for practical applications. The detailed catalytic performances, including the NO conversion and N2 selectivity, of such zeolite catalysts with a CHA structural topology are summarized in the database, as listed in Table 2. The excellent catalytic behavior and hydrothermal stability of Cu-CHA-type zeolites (SSZ-13, SAPO-34) were attributed to the special properties of their spatial structures. With the successful implementation of Cu-SSZ-13 in emission control

Figure 27. Hexagonal unit cell of an SSZ-13 zeolite (dashed lines) illustrating the AABBCCAA stacking sequence and equivalent rhombohedral unit cell (solid lines). The zeolite cage (delimited by twelve 4-MRs, six 8-MRs, and the two highlighted 6-MRs) is depicted as well. Light gray spheres (yellow) are Si atoms, and dark gray spheres (red) are O atoms. Reprinted with permission from ref 275. Copyright 2012 Elsevier.

This small cage consists of 4-, 6-, and 8-MRs of TO4 (Si, Al) tetrahedra. The largest pore is in the 8-MR, with a diameter of 3.8 Å, which permits sorption apertures of a minimum width of approximately 3.8 Å. Due to their unique structural topology with supersmall pores, the chemical state of the active sites is one of the hottest topics in NH3-SCR investigations on CHA-type zeolite catalysts. Fickel and Lobo,265 utilizing the Rietveld refinement of synchrotron-based powder X-ray diffraction (XRD), investigated the location of Cu2+ cations in the zeolite pores. It was suggested that the Cu2+ ions were mostly isolated and exclusively occupied the plane of the 6-MR of SSZ-13. The XRD patterns further suggested that the thermal stability of SSZ13 was significantly improved after copper introduction, compared with the parent zeolite in an acidic form. Gao et al.16 used electron paramagnetic resonance (EPR) and H2-TPR to study the distribution of Cu species for Cu-SSZ-13 prepared by the WIE method. Five different locations for Cu2+ cations 3699


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(A−E) were proposed, as depicted by Figure 28, showing one unit cell of the CHA topology. The following conclusions were

Figure 29. Partial density of states (PDOS) of Cu 3d states in (a) ZCu and (b) Z2Cu. The insets are the local structures of Cu in the 6-MR sites. Cu−O bond lengths are indicated in units of angstroms. The numbers of integrated electrons from PDOS plots are shown. In (b), the dashed and solid lines show spin-up and spin-down states for Cu 3d, respectively, and the dotted vertical line highlights the location of the Fermi level. Reprinted from ref 276. Copyright 2014 American Chemical Society.

Figure 28. Schematic of the SSZ-13 hexagonal unit cell structure and possible Cu2+ locations. Reprinted with permission from ref 16. Copyright 2013 Elsevier.

reached: (i) Only under dehydrated conditions and low Cu loading (ion-exchange level 80%). The second Cu2+ could be located in position B or C if it stays close to a 6-MR, or it may instead be located inside the large cage if it is near an 8-MR (position D or E). Zhang et al.,276 also utilizing the DFT method, determined the location and energy of the Cu ions of Cu-SSZ-13 zeolite. The isolated Cu2+ cations were confirmed to be more favorably located at the 6-MR, which is consistent with the results reported from experimental approaches.70 However, in the presence of various adsorbates with −OH ligands, such as [CuII(OH)+], the extraframework site in the 8-MR was found to be more energetically stable than that of the 6-MR for Cu2+ ions. The effects of the Si/Al and Cu/Al ratios were also taken into account during the DFT calculations. It was revealed that the Cu ions could be 3-fold- or 4-fold-coordinated to the lattice O atoms of Cu-SSZ-13 with different Si/Al ratios, as shown in Figure 29 for the ZCu and Z2Cu models. Examination of the partial density of states (PDOS) further verified that the Cu ions were in the +1 and +2 oxidation states in ZCu and Z2Cu, respectively. Additionally, the IR vibrational frequencies of NO adsorption on the Cu+, Cu2+, and [CuII(OH)]+ sites were calculated by DFT to be located in the ranges of 1770−1808, 1850−1950, and 1870−1915 cm−1, respectively, also consistent with the IR experimental results. This finding verified the correctness of the DFT simulations. McEwen et al.,275 on the basis of the DFT, also investigated the oxidation state and coordination environment of the Cu active sites of Cu-SSZ-13 during NH3-SCR. It was indicated that the 4-fold-coordinated Cu (II) was the dominant Cu species under the “fast SCR” and “slow SCR” conditions, wherein the NO2/NOx ratios were 0.5 and 1, respectively. Under the standard SCR conditions, containing no NO2 in the feed, a

mixture of Cu(I) and Cu(II) oxidation states could be observed. As also reported by Zhang et al.,276 2-fold Cu(I) and 4-fold Cu(II) bound with H2O or OH were found to be the most stable species over a wide range of deNOx conditions. As stated above, the Si/Al ratio could influence the dispersion as well as the detailed chemical status of the introduced Cu active species. Verma et al.,277 employing both experimental and theoretical (DFT) approaches, investigated the chemical states of Cu species doped on SSZ-13. It was verified that at least two types of Cu species existed over this zeolite catalyst, depending on the Cu/Altotal ratio. When Cu/Altotal < 0.2, the Cu ions were predominantly in the state of isolated Cu2+ cations located near the 6-MR, while CuxOy species were formed when Cu/Altotal > 0.2 due to their location in the 8-MR. A similar finding was also reported by Bates et al.,68 who prepared a series of Cu-exchanged SSZ-13 catalysts (Cu/Altotal = 0−0.41) for standard NH3-SCR investigations. The isolated Cu2+ ions, acting as the active centers, were confirmed to be located at the 6-MR of SSZ-13 during NH3-SCR when Cu/Altotal < 0.2, and the standard SCR reaction rate increased linearly up to Cu/Altotal = 0.2 to a maximum of 3.8 × 10−6 mol of NO g of cat.−1 s−1. Because NH3 acting as the actual reducing agent plays a key role in the SCR reaction, an effort to identify the species formed by the NH3 adsorption upon Cu-SSZ-13 and their involvement was made by Moreno-Gonzalez et al.278 on the basis of in situ EPR, solid-state NMR, and DFT calculations. Five different NH3 adsorption modes were observed under various conditions: [Cu(NH3)5]2+, [Cu(Of)2(NH3)2]2+, [Cu(Of)3NH3]2+, [Cu(NH3)2]+, and [CuOf(NH3)]+ (Of representing the framework oxygen). The adsorbed NH3 was demonstrated to be able to reduce Cu2+ ions into Cu+ ions. 5.1.1.2.1.2. Cu-SAPO-34. Silicoaluminophosphate-34 (SAPO-34), being generated from an incorporation of a Si atom into neutral AlPO4, has the same spatial topology as SSZ3700


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unit. The regeneration of the DPF results in the formation of a great deal of heat, which is thereafter transferred to the SCR catalyst. The high-temperature exposure (>650 °C) with the presence of moisture in the feed could possibly damage the zeolite framework structure, leading to the sintering of the active species and ultimate deactivation of the catalyst. In light of that, Wang et al.281 compared the hydrothermal stabilities of Cu-SSZ13 and Cu-SAPO-34 prepared by the solid-state ion exchange (SSIE) method that were hydrothermally aged at 750 and 800 °C, respectively, during NH3-SCR. The activity measurement results showed that both Cu-SSZ-13 and Cu-SAPO-34 could maintain their performances after hydrothermal aging at 750 °C for 16 h. However, as the temperature increases to 800 °C, hydrothermal aging for 16 h causes a significant decline in the SCR activity of Cu-SSZ-13, accompanied by a drastic loss in the crystallinity and number of active Cu sites. On the other hand, no obvious decrease in crystallinity was observed for Cu-SAPO34. Moreover, the low-temperature NO conversion of the hydrothermally treated Cu-SAPO-34 was further increased, which was reported to be related to the migration of the surface Cu species into the pores of SAPO-34, forming additional isolated Cu2+ sites. As is widely accepted, dealumination is the major cause of structural damage and activity loss of the zeolite catalysts during the hydrothermal treatment. However, the small pore aperture of the CHA zeolite (3.8 Å for the 8-MR) is lower than that of Al(OH)3 (dealumination product). Therefore, a relatively higher hydrothermal stability can be achieved for the small-micropore zeolite (SSZ-13, SAPO-34) catalysts with respect to that of the traditional medium-micropore zeolite (ZSM-5) catalysts. Yu et al.175 recently investigated the influence of H2O during NH3-SCR over Cu-SAPO-34 prepared by WIE according to its excellent hydrothermal stability. It was found that the presence of 3% H2O could enhance the NO conversion over the whole temperature range. NH3-TPD and H2-TPR indicated that H2O vapor could improve the number of Brönsted acid sites (Si−OH−Al) as well as the reducibility of the Cu2+ species, both of which were beneficial for the enhancement of the related catalytic activity. The presence of H2O at high temperatures also inhibited the NH3 oxidation as well as the related side-product generation through nonselective NH3 oxidation. The low-temperature stability of the zeolite catalyst is generally important for SCR application, especially during the cold-start stage, where the working temperature has not been reached. However, much less attention was paid to such work. In view of that, Leistner and Olsson282 examined the lowtemperature (T = 70 °C) stability of Cu-SAPO-34 in the presence of water vapor. After a few hours of exposure to a steam-containing feed, the prepared Cu-SAPO-34 samples experienced a severe activity loss from 87% to 66% at T = 200 °C, and after another 9 h to 6%. This deactivation was accompanied by a partial loss of micropore volume, but the SAPO-34 framework remained crystalline. The regeneration of this catalyst was attempted, but it had no impact on the activity. The deactivation was further investigated on the basis of H2TPR, indicating that fewer copper sites were available for the redox cycle. On the basis of that, a transformation of the active copper sites into an inert form was proposed to be a possible reason for the deactivation at low temperatures. Therefore, it was concluded that an irreversible deactivation could occur for the Cu-SAPO-34 zeolite at low temperatures (T < 100 °C) and in the presence of water vapor during the NH3-SCR process.

13. According to the literature, Cu-SAPO-34 catalysts exhibit outstanding activities and durability for the NH3-SCR process, and their active site status has attracted an especially large amount of attention. In an early study, Prof. Wang et al.69 prepared a series of Cu-SAPO-34 samples by a WIE and precipitation method for NO reduction. Various techniques [XRD, H2-TPR, scanning transmission electron microscopy (STEM), and DRIFTS] were used to identify the location and status of the Cu species in these samples. The results consistently indicated that the Cu species existed predominantly as isolated ions at the exchange sites in the ion-exchanged samples; however, as for the precipitated sample, the CuO on the external surface is the dominant species. Superior NH3-SCR activity was observed for the ion-exchanged samples, suggesting that isolated Cu cations at the exchange sites constituted the active centers. They also prepared a series of Cu-SAPO-34 samples with various Cu loadings (0.7−3.0 wt %) on the basis of the solid-state ion exchange (SSIE) method.279 The chemical statuses of the loaded Cu species were characterized by in situ DRIFTS, XRD, H2TPR, and UV−vis, which suggests that two different Cu species existed on the prepared Cu-SAPO-34 samples: Cu2+ ions and CuxOy clusters (dimeric or oligomeric Cu species). Cu2+ ions were verified to be the active centers for NO abatement, while CuxOy could promote the NH3 oxidation, leading to the observed decrease in the standard SCR process at high temperatures. The acidity−activity correlation for Cu-SAPO34 during NH3-SCR was subsequently studied by the same group,279 wherein a series of Cu-SAPO-34 samples with varying numbers of Brönsted acid sites were prepared by WIE with potassium to investigate the role of the Brönsted acidity. Along with an increase in the potassium loading, the SCR activity of the Cu/K-SAPO-34 catalysts diminished in accordance with the decreasing Brönsted acidity. The reaction rate was further found to be dependent on the NH3 coverage on Brönsted acid sites at 180−280 °C. At elevated temperatures, the acidic sites could act as a source of NH3 for the SCR reaction because the NH4+ was initially adsorbed on the Brönsted acid sites and could then gradually migrate to the copper sites to finally react with the NOx species. Deka et al.,280 utilizing a postsynthesis method (via an aqueous or vapor phase) and a direct synthesis method, prepared several Cu-SAPO-34 samples for NH3-SCR investigations. The isolated mononuclear Cu2+ ions in the vicinity of the 6-MR (part of the d6r subunits of CHA) were verified to be reactive. X-ray absorption fine structure (XAFS) data thereafter suggested that the catalysts prepared via the chemical vapor deposition method possessed Cu in two different chemical environments: isolated Cu2+ cations and CuAlO2-type species. A strong correlation between the number of isolated mononuclear Cu2+ ions in or near the d6r subunit and N2 production was observed during the activity measurement, whereas the presence of the CuAlO2 species appears to promote the formation of undesired N2O. 5.1.1.2.2. Poisoning Resistance. Although the CHA-type zeolite catalyst with supersmall micropores exhibited extraordinary performance for the NH3-SCR reaction, the possible deactivation under real conditions should also be taken into account for the purpose of better understanding its catalytic behavior, including its hydrothermal stability, resistance to hydrocarbon deposition, and chemical aging. 5.1.1.2.2.1. Hydrothermal Stability. In a typical NH3-SCR after-treatment system, a diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) are used to remove the unburned hydrocarbons, CO, and particulate matter upstream of the SCR 3701


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5.1.1.2.2.2. Resistance to Hydrocarbon Poisoning. As for the automobile engine, the presence of hydrocarbons in the exhaust streams also has a poisoning effect during the NH3-SCR reaction, which can block the active sites and decrease the generation of the reactive intermediates needed for NO conversion. Ye et al.15 made a comparison of the activity, propene-poisoning resistance, and hydrothermal stability of copper-exchanged chabazite (Cu-CHA: Cu-SSZ-13, Cu-SAPO18, and Cu-SAPO-34) and Cu-ZSM-5 catalysts during NH3SCR. It was noted that the Cu-modified CHA zeolite, displaying little change in its specific surface area, Cu+/Cu2+ ratio, and surface amount of Cu between the used and fresh samples, exhibited much higher resistance toward propene poisoning than Cu-ZSM-5. This is closely related to the geometry differences between the small-micropore CHA zeolite and the medium-micropore MFI zeolite. The supersmall pores of CHAtype zeolites (3.8 Å) result in hard deposition and further diffusion of the hydrocarbon species (propane, 4.3 Å; n-butane, 4.3 Å; isobutane, 4.9 Å), which efficiently prevents the poisoning caused by hydrocarbon deposition. 5.1.1.2.2.3. Aging. Trace levels of platinum deposited on the SCR catalyst of diesel engines can also be volatilized, usually causing severe deactivation. Additionally, phosphorus, potassium, calcium, magnesium, and zinc are typical impurities derived from antiwear, antioxidant, and corrosion-inhibiting additives in the engine lubrication oil. Therefore, Lezcano-Gonzalez et al.283 conducted investigations on the chemical deactivation of CuSSZ-13 by Zn, Ca, and P during NH3-SCR, wherein a crucial poisoning impact of P was confirmed, while a less pronounced drop in activity was observed following Ca and Zn introductions. The addition of P could result in several deactivation mechanisms, such as site blocking, disruption of the zeolite framework, CuO formation, and the reduction of a number of isolated Cu2+ ions. Ma et al.284 chose Cu-SAPO-34 (prepared by the WIE) as a model catalyst to investigate potassium poisoning for the NH3-SCR process. It was suggested that the prepared CuSAPO-34 sample showed an ideal resistance to potassium poisoning, when the potassium content was below 0.5 wt %. However, NH3-SCR activities at low temperatures greatly declined for the 1.5% K/Cu-SAPO-34 and 2.5% K/Cu-SAPO34 samples. The transformation of Cu2+ ions into CuxOy clusters, as proven by UV−vis and H2-TPR, was reported to be the key factor resulting in an activity loss after K addition. 5.1.1.2.3. Reaction Mechanism. 5.1.1.2.3.1. Standard, Fast, and NO2 SCR Reaction Mechanisms. The NH3-SCR process generally occurs via three types of reaction paths, which depend on the fraction of NO2 attended in reaction as listed in eqs 1.10− 1.12: (i) standard SCR with NO2/NOx = 0 (eq 1.10); (ii) fast SCR with NO2/ NOx = 0.5 (eq 1.11); (iii) NO2 SCR with NO2/ NOx = 1 (eqs 1.12a and 1.12b). As for the standard SCR, many authors suggest that the reaction starts with an oxidation of NO to NO2 on the active sites. Subsequently, NO2 can react with the adsorbed NH3, yielding NH4NO2 or NH4NO3, which can be further decomposed into the final products of N2 and H2O as well as the undesired pollutants of N2O and NOx.77,80 The NO oxidation into NO2 is reported to be the rate-determining step.285 Most researchers believed that the standard SCR could take place over the zeolite catalysts, fulfilling the Langmuir− Hinshelwood mechanism. For example, Klukowski et al.286 investigated the standard SCR reaction conducted over Fe-BEA and proposed a dual-site mechanism, that the adsorption and reaction of NO and NH3 occurred on the neighboring Fe3+ active sites, as shown in Figure 30. Fe3+ was partially reduced to

Figure 30. Scheme of the proposed mechanism of the standard SCR reaction on Fe/H-BEA zeolite. Reprinted with permission from ref 286. Copyright 2009 Elsevier.

Fe2+ during adsorption of NH3, and a reoxidation of the iron site by O2 is assumed to happen. Moreover, it was also found that some cycles of adsorption and desorption of NH3 took place on the BEA substrate, which reveals that the zeolite framework played an important role in NH3 spillover from the zeolitic adsorption site to the active Fe site. A similar finding was also reported by Wang et al.77 during an NH3-SCR investigation over Cu-SAPO-34, and it was further clarified that the Brönsted acid sites of the zeolite substrate behaved as the NH3 reservoir instead of being directly involved in the SCR reaction. However, the standard SCR mechanism is still a subject of debate, which is mainly due to the fact that the generated intermediates are so active that they cannot be readily identified. Ruggeri et al.287 made progress in identifying the existence of an intermediate of nitrite/HONO during a standard SCR mechanism study of Fe-ZSM-5 by means of a novel method. During his research, BaO/Al2O3, known as an LNT (lean NOx trap) catalyst, was mixed with the SCR catalyst of Fe-ZSM-5. A trap experiment was thereafter conducted by exposing this mixed catalyst under an atmosphere of NO and O2 at low temperature (120 °C). As reported, the inclusion of BaO/Al2O3 in a physical mixture with Fe-ZSM-5 resulted in stabilizing nitrite species adsorbed on Fe sites after exposure to NO + O2 and captured upon BaO via gas-phase equilibrium with HONO. Finally, an alternative mechanism for the standard SCR process was also proposed: (i) the nitrite species (NO2−) in equilibrium with gaseous nitrous acid (HONO) could be first generated in the presence of NO and O2; (ii) the formed nitrite/HONO species were subsequently decomposed into gaseous NO2 to a certain degree. However, in the presence of NH3, the nitrite/HONO could quickly react with NH3 to form the final product of N2. Additionally, it was also reported that NO oxidation to NO2 was not the rate-determining step of the standard SCR reaction, whereas the nitrite/HONO served as the most important intermediate. A similar result was also announced by Schwidder et al.,18 who investigated the role of NO2 during standard SCR over the Fe-ZSM-5 zeolites with various iron contents (0.2−5 wt %), that the NO oxidation was significantly lower than its reduction. On the basis of this finding, NO2 generated due to NO oxidation was ruled out as a possible intermediate for the standard SCR reaction. The fast SCR process showing excellent low-temperature activity has attracted a lot of attention, especially for the cold startup stage of a motor vehicle. Unlike the standard SCR mechanism, a general agreement on the fast SCR mechanism has been achieved, which consists of three steps: (i) NO 2 disproportionation to form adsorbed species of nitric acid/ nitrate through interaction with H2O (eq 5.1), (ii) reaction between adsorbed nitric acid/nitrate with gaseous NO to form nitrous acid/nitrite and NO2 (eq 5.2), and (iii) reaction of ammonia with nitrous acid/nitrite adsorbed species to form nitrogen and water via ammonium nitrite decomposition (eq 5.3). Among them, the reduction of surface nitrates (eq 5.2) is 3702


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intermediates and could be thereafter reduced into N2 and H2O by reaction with gaseous NO. The excellent lowtemperature N2 selectivity was observed for both Cu-SSZ-13 and Cu-SAPO-34, which was reported to be mainly related to the fact that only a portion of NO was oxidized into NO2 and the remaining NO would react with adsorbed NH4NO3 to form N2 and H2O. In the high-temperature zone, the gas-phase NO2 becomes more important for the NH3-SCR reaction: the preadsorbed ammonia reacts with gaseous NO2 to form the intermediate before decomposing into the final products of N2 and H2O. Although NH3 could be adsorbed on both Lewis and Brönsted acidic sites, ammonia on the Brönsted acidic sites was believed to be more active for the SCR reaction. 5.1.1.2.3.2. Lewis Acid−Base Pair in the Reaction Mechanism. As stated in section 2.2.2, the zeolite catalyst is commonly composed of SiO4− and AlO4− tetrahedra, wherein the Brönsted acid site can be formed after AlO4− is compensated by H+ and the Lewis acid site can be generated by the partially coordinated AlO4−. As noted, after being ion exchanged with metal cations, the Brönsted acid site can also be converted into a Lewis acid site, which plays an important role in the catalytic reactions, especially for NH3-SCR. For example, the alkaline molecules of NH3 and NO acting as the Lewis base molecules can be adsorbed on the Lewis acid (Mn+, metal cation), forming coordinated NH3 and NO to obtain the acid−base pair. Formation of such a type of Lewis acid−base pair is very crucial for the NH3-SCR, wherein the Lewis acid site generated by the metal cation participated in the adsorption and activation of the adsorbed NH3 and NO reactant molecules. Boron et al.65 employing various experimental approaches [XPS, NH3-TPD, and diffuse reflectance (DR) UV−vis, 57Fe Mössbauer, and FTIR spectroscopies] investigated the influence of the iron chemical state and acidity of zeolites on the catalytic activity of Fe-BEA, Fe-MFI, and Fe-MOR during NH3-SCR. It was reported that the catalytic activity for Fe-zeolite samples in NH3-SCR strongly depended on the speciation of irons introduced into the zeolite structure as well as their acidities. After introduction of iron into the matrix of the zeolite, various Fe species were observed, including pseudotetrahedral and octahedral Fe(III); among them one kind of Fe3+−OH group was reported to be important. Such a type of iron site could be reduced at room temperature by NO, forming Fe2+−NO groups and NO+ in cationic form, and at T > 100 °C, it could be regenerated into Fe3+−OH groups. It can be assumed that the Fe3+−OH on Fe-zeolite samples actually acted as a Lewis acid site, which could adsorb NO and preactivate NO into NO+, being beneficial for the further reaction of NH3-SCR reactions. The structure and locations of the Lewis acid site inside zeolite catalysts as well as the adsorption mode of the introduced molecular ligands can be well represented by DFT, through which deeper insight can be illustrated. Yang et al.289 recently investigated the Lewis acidity of the M4+-doped ZSM-5 zeolite (M = Ti, Zr, Ge, Sn, Pb) as well as the interactions with probe molecules (NH3, CH3CN, H2O, C5H5N) on the basis of DFT, wherein the M-ZSM-5 was represented by a 17T cluster model, as shown in Figure 32. According to the DFT calculation result, the Lewis acidities increased in the order silicate < Ge < Ti < Pb < Sn < Zr. Meanwhile, it was proposed that the Lewis acidity should be defined as the local sites around the M4+ ions. To better understand the initial step of NH3-SCR over MoZSM-5 zeolite, Yan et al.290 on the basis of DFT simulated different adsorption modes for NH3 and NO ligand molecules on the Lewis acid sites of Mo-ZSM-5. The Mo-ZSM-5 was

the rate-determining step in the low-temperature region, which could be further blocked due to the generation of ammonium nitrate to cover the active sites.288 Moreover, a cooperative effect for acid sites of zeolite and active sites of metal species was also noted for the fast SCR process, wherein ammonia could be stored at the zeolite acidic sites and the active metallic cations were involved in the reduction of surface nitrate by NO. 2NO2 + H 2O → HNO3 + HONO

(5.1)

HNO3 + NO → NO2 + HONO

(5.2)

NH3 + HONO → [NH4NO2 ] → N2 + 2H 2O

(5.3)

To elucidate the contributions of the “standard”, “fast”, and “NO2” SCR mechanisms achieved over Fe-zeolite under conditions where they occur simultaneously, Iwasaki and Shinjoh29 conducted a comparative NH3-SCR investigation through varying the NO2/NOx ratios (0−100%) and reaction temperatures (150−400 °C). It was suggested that (i) the NOx conversion rate decreased in the order of fast SCR > standard SCR > NO2 SCR, (ii) the standard SCR and NO2 SCR could merely progress as fast SCR was completed, and (iii) at T < 250 °C N2O represented the main product of NO2 SCR, which was reported to be generated via ammonium nitrate decomposition. An overall SCR scheme as a function of the NO2/NOx ratio as well as reaction temperature was thereafter profiled, as shown in Figure 31. It is seen that a common step in each kind of SCR mechanism is the formation of surface species of adsorbed ammonium nitrate.

Figure 31. Proposed overall SCR scheme as a function of the NO2/NOx ratio and temperature. Reprinted with permission from ref 29. Copyright 2010 Elsevier.

Due to the extraordinary performances of Cu-CHA zeolites for the NH3-SCR process, the related mechanism study also attracted great attention. Ma et al.,25 by means of in situ DRIFTS, temperature-programmed desorption (TPD), and temperature-programmed surface reaction (TPSR), investigated the intrinsic NH3-SCR mechanism for Cu-SSZ-13 and CuSAPO-34 catalysts. It was assumed that the reaction pathways achieved in the low-temperature range were totally different from those achieved in the high-temperature range. At low temperatures, ammonium nitrates formed due to an interaction between surface ammonia and nitrates were the crucial 3703


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FER, and MOR) catalysts for NO-SCR using low-molecularweight hydrocarbons (methane and ethene) for the sake of exploring effective deNOx catalysts. In- and Co-FER catalysts were evidenced to possess significantly high activity and selectivity during CH4-SCR, making them promising candidates for industrial applications. Cu-FER was found to exhibit the highest NO conversion with respect to those of Cu-MFI and CuMOR during HC-SCR by ethene, being correlated with its special matrix. The group of Janas, employing a two-step postsynthesis method, prepared a series of metal-modified BEA (Co-BEA, CuBEA) catalysts for an HC-SCR investigation.296,297 Through this preparation method, the loaded metals could be introduced into the vacant tetrahedral sites of the zeolite catalyst. The activity measurement296 revealed that the NOx catalytic reduction by ethanol occurring over Cu-BEA was enhanced by a high Cu content (with maximum NO conversions of 33%, 45.5%, and 50%, respectively). On the other hand, the N2 selectivity decreased as the Cu loading increased (90%, 97%, and 75%, respectively). The drop in N2 selectivity associated with the increasing Cu was reported to be related to the transformation of tetracoordinated Cu(II) to extralattice octacoordinated Cu(II). Further investigation297 on the SCR of NO by ethanol and propane over Cu-BEA revealed that HC-SCR occurred on isolated mononuclear Cu(II) sites in a D2d-distorted tetrahedral symmetry after atomic Al removal, although the atomic Al negligibly influenced the catalytic behavior for HC-SCR. This finding is totally contrary to that of the traditionally WIEprepared zeolite catalyst, for which the distribution of framework Al was one of the most important issues that influenced its catalytic behavior. As reported,298 the distribution of framework Al determined the location of the Co cations of Co-BEA, which then affected its catalytic performance during HC-SCR by propane. Superior activity was found with the counter Co−oxo species balanced by isolated Al atoms [Al−O−(Si−O)n>2−Al], surviving even in wet (10 wt % H2O) NOx streams. On the other hand, the bare Co(II) ions coordinated by an Al pair [Al−O− (Si−O)2−Al in one ring] were only active in dry NOx. Recently, Palomares and Corma299 introduced two different zeolites named ITQ-27 and ITQ-7 for the investigation of HCSCR (propane) after modification by Co ions based on the WIE method. The activity evaluation suggested that Co-ITQ-27 possessed SCR activity similar to that of Co-BEA, but with a high hydrothermal ability; the activity of Co-ITQ-27 negligibly changed after hydrothermal treatment by steam. A conclusion was thereafter proposed that the zeolite catalyst applied for C3H8-SCR should have a suitable hydrothermal stability and Si/ Al ratio (ranging between 8 and 35). ITQ-27, with a bidirectional spatial structure connected by 12-MR channels, was thought to present these characteristics, making it promising for the NOSCR by propane. It was also reported that the addition of H2 could enhance the low-temperature HC-SCR activity of Ag-ZSM-5 by accelerating the transformation of the exchanged Ag+ ions into Agnδ+ clusters in Ag-ZSM-5.300 H2 can also facilitate the removal of nitrates from the catalyst surface, which is of great importance in HCSCR.301 In light of that, Popovych et al.302 studied the effect of H2 during ethanol-SCR over Ag-containing BEA catalysts of AgxAlBEA and Agx-SiBEA. The promoting effect of H2 on the SCR of NO with ethanol was observed for Agx-AlBEA, whereas it was almost absent on Agx-SiBEA. This finding suggested that the presence of silver nanoclusters (Agnδ+) in close proximity to strong Lewis acidic sites in the zeolite catalysts was crucial for the

Figure 32. Local structures of the (a) Lewis acidity (LA) in the M4+doped zeolite and (b) Lewis acidic site with adsorption of NH3. Reprinted with permission from ref 289. Copyright 2012 Elsevier.

represented by a 20T model of MoSi18Al2O53H30, with [MoO2]2+ acting as the active center (or Lewis acid site), and the framework Si atoms at the T3 and T12 sites were substituted by atomic Al, as shown in Figure 33. The DFT simulation

Figure 33. Optimized structure of (MoO2)2+/HZSM-5. The labels of the ZSM-5 framework atoms correspond to the ZSM-5 crystallographic structure. Reprinted with permission from ref 290. Copyright 2014 Elsevier.

suggested that NH3 could be more favorably adsorbed on Lewis acid sites, forming a Lewis acid-base pair, with respect to adsorption of NO. Furthermore, it was also suggested that NH3 adsorption on the Lewis acid site could result in activation of the N−H bond, which facilitated the further reaction of NH3-SCR. The N−H activation ability was correlated with the electron donations from NH3 to the Lewis acid site: more donations from NH3 are always associated with the higher activity of the H atom of adsorbed NH3. As noted, this finding is consistent with the other literature reports suggesting that the Lewis acid site and NH3 simultaneously participated in the SCR reaction. 5.1.2. Hydrocarbon SCR (HC-SCR). In the 1990s, a critical result was achieved by utilizing hydrocarbon as the reductant during the deNOx process (HC-SCR) based on ion-exchange zeolite catalysts to break through the deadlock in NO removal. HC-SCR was reported to be an alternative technology for eliminating NOx from automobile engine exhaust under lean conditions that are free from the drawbacks of the commercially available deNOx technologies such as urea-SCR and the lean NOx trap (LNT). After the pioneering work of Iwamoto et al.,291 a large number of papers dealing with HC-SCR using metalcontaining zeolites,292 Ag-supported catalysts,293 and precious metals294 were published. The present work will mainly concentrate on the zeolite catalyst applied for HC-SCR. 5.1.2.1. Activity Performance. Kubacka et al.295 investigated diverse metal-ion (Cu2+, Co2+, and In3+)-modified zeolite (MFI, 3704


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zeolite. However, the cyanide (HCN) or cyanogen (C2N2) species were observed during isobutane-SCR over Fe-ZSM-5 (HCN) and Cu-ZSM-5 (C2N2). In such cases, N2 was mainly generated by reactions between HCN or C2N2 and NO2, and the NO oxidation to form NO2 species was reported to be the limiting step. Cu-ZSM-5 was found to be very effective at forming adsorbed cyanide, while Fe-ZSM-5 achieved higher activity for the oxidation of NO to NO2. Shimizu et al.307 studied the reaction mechanism of H2promoted C3H8-SCR over Ag-ZSM-5 zeolite on the basis of spectroscopic technology. The proposed reaction mechanism consisted of five steps: (1) oxidation of NO to NO2, (2) partial oxidation of propane to an oxygenate such as acetate, (3) reaction of the oxygenates with NO2 to form an NCO intermediate via nitromethane (CH3NO2), (4) hydrolysis of NCO intermediates to form NH4+ and CO2, and (5) reaction of NH4+ with NO2 to yield N2 and H2O. The addition of H2 could promote the reaction rates of steps 1 and 2. During these processes, the Ag+ species was reduced to Ag42+ clusters, which could further activate O2 into O2−. The generated O2− acting as an effective oxidant then promoted the reaction rates of steps 1 and 2. Sawabe et al.,308 using the DFT method, studied the promotion effect of H2 during HC-SCR over Ag-ZSM-5 zeolite. H2 was found to have two beneficial impacts on the HC-SCR. One promotion effect is to favor the formation of Ag4 clusters, which can be dispersed over ZSM-5, acting as the active centers. Another role of H2 is to produce HAg4H clusters from the Ag4 clusters. As reported, the intermediate HAg4H-Z could preactivate the O2 molecule to generate more active species of HOO−, which is active in the partial oxidation of hydrocarbons during the HC-SCR reaction. In addition to that, it is worth noticing that, due to the porous structure of the zeolitic catalyst, it facilitates the adsorption of reactant molecules. This phenomenon also exists in HC-SCR and NH3-SCR reactions. Gaudin et al.309 investigated the C3H6SCR over Cu-ZSM-5 zeolite. It was interestingly noted that at low temperatures ( 300 °C. On the basis of the steady-state and transient activity measurements, a reaction mechanism correlated with the adsorption property of this zeolite catalyst was proposed, wherein the molecular O2 was reported to play a special role during the HC-SCR process. As illustrated in Figure 34, the HCs

appearance of this H2 promotion effect. H2 addition could promote ethanol oxidation on silver nanoclusters and the interaction of the formed intermediates with nitrate species adsorbed on strong Lewis acidic sites. As for the heterogeneous zeolitic catalysis, the reductant diffusion inside the complicated zeolite channels also has a significant influence on the related catalytic performance. Many researchers have reported that the zeolite diffusivity can be correlated with the critical diameters of the guest molecule and the pore openings of the zeolite.303 The diffusivity of molecules within the zeolite channels was controlled by this type of geometrical limitation, namely, geometry-limited diffusion. In the case of HC-SCR, various hydrocarbons with different molecular sizes could be utilized as the reductant for HC-SCR. In light of that, Shichi et al.304 studied the influence of the hydrocarbon molecular size on HC-SCR over Cu-ZSM-5 catalyst, wherein two different reductants, n-hexane and 2,2dimethylbutane, were applied. It was found that the catalytic activity did not depend on the zeolite crystal size (calculated from the external surface area) in the case of n-hexane, although for 2,2-dimethylbutane, the observed reaction rate was dependent on the crystal size. This revealed that the HC-SCR activity upon using a larger hydrocarbon as a reductant was restricted by the geometry-limited diffusion. 5.1.2.2. Mechanism Investigation. Recently, Resini et al.305 investigated CH4-SCR over Co-MFI and Co-FER catalysts, wherein Co2+ cations were confirmed to be the main cobalt species. The following steps were verified to occur during CH4SCR (see Scheme 11, eqs 5.4−5.6), wherein the NO oxidation Scheme 11. Schematic Reaction Steps of CH4-SCR over CoMFI and Co-FER Zeolites

(eq 5.4) prevails in the low-temperature range. At elevated temperatures, wherein the NO oxidation step was thermodynamically limited, eqs 5.5 and 5.6 become the prevailing ones. This finding suggests that NO2 was not a necessary intermediate during CH4-SCR, being different from the NH3-SCR process. Equation 5.5 becoming the main reaction step for CH4-SCR occurred via a redox mechanism essentially involving a Co2+ → Co3+ redox couple. Meanwhile, the excessive consumption of CH4 reductant in eq 5.6 could decrease the deNOx activity to a certain degree. Cant et al.306 investigated the reaction mechanism for HCSCR over 3d-TMI (Fe, Co, Cu)-modified ZSM-5 catalysts on the basis of IR spectroscopy. As for the CH4-SCR mechanism over Co-BEA, the initial step was reported to be NO oxidation to generate adsorbed NO2 species, which could successively subtract the hydrogen atom from the hydrocarbon, forming −CH3 and HONO−. The subsequent path involved the formation of nitromethane (CH3NO2) and nitrosomethane (CH3NO) compounds through the interaction of −CH3 with NO. Finally, the CH3NO2 or CH3NO could be further dissociated into CO2 and NH3. The N2 could be generated through the NH3-SCR reaction routes. As noted, the abstraction of hydrogen from the CH4 by adsorbed NO2 was reported to constitute the rate-determining step for CH4-SCR over Co-BEA

Figure 34. Mechanistic scheme proposed for the selective NO reduction by C3H6. Reprinted with permission from ref 309. Copyright 1996 Elsevier. 3705


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and NO could be initially trapped by the zeolite catalyst, forming a CHxNy (or CHxNyOz) intermediate. Only after introduction of O2 can the generated CxHyNz intermediate be decomposed into the final products of N2 and CO2. 5.1.3. H2-SCR. To meet strict emissions regulations, the effective elimination of NOx has become a great challenge for researchers. With respect to the former two SCR technologies, H2-SCR is much greener. It can not only efficiently reduce NOx emissions at relatively low temperatures (90% at T = 450 °C; W/F = 4.0 g s cm−3; 4 vol % NO/He). Thereafter, significant efforts were made to gain a deeper understanding of the NO-DD mechanism for the metalmodified zeolite catalysts.322,323 A series of Cu2+-ion-modified zeolites (X, Y, mordenite, and ZSM-5), with the same copper content, was prepared for activity evaluation.322 The activity was known to follow the order Cu-ZSM-5 > Cu−Y ≈ Cu-mordenite > Cu-X, which indicated that ZSM-5 had some special properties that made it particularly useful in NO decomposition. Further investigation conducted by Iwamoto et al.323 suggested that the overexchanged Cu-ZSM-5 (ion-exchange level >100%) obtained much higher NO conversion and N2 selectivity, with a maximum level of ∼90% NO conversion at 450−500 °C. A further increase of temperature led to a slight decline (about 5%) in the decomposition efficiency, which was attributed to the reversible change of the active site structure. In addition to that, the effect of O2 during NO-DD was studied, which revealed that the O2 addition could decrease the deNOx activity of Cu-ZSM-5 samples; however, an increase of the Cu2+ ion loading could enhance the tolerance to oxygen poisoning. The influence of SO2 on the NO-DD process conducted over Cu-ZSM-5 was also investigated, which implied that SO2 addition could completely poison the Cu-ZSM-5 at T < 650 °C, but the activity could be partially regenerated under higher temperature treatment due to the release of sulfurous species from the zeolite surface. To improve the NO-DD performance of Cu-ZSM-5, Kustova et al.324 introduced a high degree of mesoporosity into ZSM-5 by nucleating the zeolite crystallite inside the carbon material. After crystallization of ZSM-5, the carbon is entirely removed by combustion. It was observed that an obvious improvement of the catalytic activity was achieved over the mesoporous Cu-ZSM-5 with respect to the conventional microporous sample. On the basis of the NH3-TPD results, the activity difference between mesoporous and microprous Cu-ZSM-5 was reported to be related to the distinct accessibility of Cu species in those zeolitic carriers. The electron paramagnetic resonance (EPR) results further indicated that the mesoporosity in the zeolites facilitates the formation of dimeric or oligomeric Cu species, which possessed much higher NO decompostion ability in comparison with monomeric species dominating in the common Cu-ZSM-5 sample. Additionally, Dedecek et al.325 investigated the nature of the active site of Cu-ZSM-5 (prepared by the wet ion-exchange method) during NO-DD by means of 29Si magic angle spinning (MAS) NMR and vis−NIR (NIR = near-infrared) spectroscopies. It was verified that Cu(I) ions coordinated to two or three framework oxygens and located at the channel intersection adjacent to the single Al atoms in the α- or β-site predominantly control the reaction rate, as depicted in Figure 36. In addition to Cu-ZSM-5, Chang et al.326 investigated the NO-DD behavior achieved over Co-ZSM-5 using temperatureprogrammed desorption (TPD) of the isotope-labeled 15N18O. It was reported that the cobalt species located in the framework had the highest activity for NO decomposition (TOF = 2.05 × 10−3 s−1, T = 335 °C), almost an order of magnitude greater than that of Cu-ZSM-5 (TOF = 2.27 × 10−4 s−1, T = 442 °C). The superior activity of Co-ZSM-5 was ascribed to the unique incorporation of Co2+ species into the silica skeleton with MFI topology, which behaved differently from those of Co2+ cations in the countercation position or extraframework CoO. Such a

Scheme 12. Schematic Reaction Steps of H2-SCR over PdMOR Zeolite in the Presence of CO

could react with the adsorbed NO (NOads), forming an intermediate of NCO. The further reaction of NCO and NO could produce the final products of N2 and CO2. As stated above, recent H2-SCR investigations have mainly focused on noble-metal-modified zeolites. Although high activity was observed, their high cost as well as their relatively low N2 selectivity greatly hinders their broader applications. In light of that, Wang et al.318 recently for the first time reported a Znmodified ZSM-5 catalyst (Zn-ZSM-5) that exhibits satisfactory deNOx activity (maximum NOx conversion of 54%, T = 250 °C) and N2 selectivity (∼80%) in excess oxygen. The excellent catalytic performance of Zn-ZSM-5 was reported to be mostly related to the Zn cations, which played a determining role in hydrogen chemisorption on zeolites. Therefore, the development of non-noble-metal-modified zeolite catalysts may constitute another direction for H2-SCR. 5.2. NO Direct Decomposition

Currently, commercial technologies of the deNOx method include three-way catalysis and SCR, using ammonia or urea as the reductants. However, there are some drawbacks for these two approaches. For three-way catalysis, the carbon monoxide (CO) and hydrocarbons (HCs) could be consumed nonselectively in excess oxygen, reducing the efficiency of NO reduction to some degree. The introduction of the NH3 reductant into the catalytic system is another shortcoming of SCR technology because the storage and leakage of the harmful ammonia has to be taken into consideration. NO direct decomposition (DD) into N2 and O2 was once proposed as one of the desirable routes for NO removal,319 as this system is extremely simple and does not require an additional reductant. NO is thermodynamically unstable and tends to decompose according to eq 5.10.256 Nevertheless, the electronic distribution with the orbitals of the NO molecule is spin-forbidden, and NO is thus kinetically stable. 2NO ↔ N2 + O2

ΔGf ° = −86 kJ mol−1

(5.10)

There are two key issues for the practical application of this technology: (i) finding a proper catalyst to decrease the huge energy barrier of NO dissociation (ΔE = 364 kJ mol−1);320 (ii) overcoming the deactivation caused by oxygen poisoning (O2 formed during decomposition essentially competes with NO for the surface adsorption sites).321 Several types of solid materials have been reported as active catalysts for NO-DD, such as zeolites,319,322−326 La2O3,327 Ba/MgO,328 alkali-metal-doped Co3O4,329 perovskite-type oxides,330 and rare-earth-metal oxides.331 Unfortunately, the reported activities of these catalysts are still insufficient for practical use: T90 (temperature corresponding to 90% NO conversion) of these catalysts was commonly located in the high-temperature range of 450−600 °C. Moreover, most of them are readily deactivated by strong chemisorption of superficial oxygens. So far, no catalyst with sufficiently satisfactory activity and stability for commercial applications is known. In this section, a brief summary of the recent achievement for NO-DD over the zeolitic catalysts will be given. 3707


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for NO direct decomposition is also reviewed in this work, as detailed in section 5.3.2. 5.3. DFT Simulations of the Mechanisms

5.3.1. NO-SCR Mechanism. A 5T model of Fe-ZSM-5 with mononuclear [FeO]+ as the active center was constructed for the “fast SCR” (NO + NO2 + 2NH3 → 2N2 + 3H2O) mechanism simulation with NH3 as the reductant by Li,328 as shown in Figure 38.

Figure 36. Illustration of the cationic sites in the ZSM-5 structure. Reprinted with permission from ref 325. Copyright 2006 Elsevier.

type of Co2+ species could facilitate the interaction of NO and O2, forming an intermediate of N2O3, which was responsible for the enhancement in the NO decomposition rate. Recently, a breakthough was made by Kustova et al.,327 who reported that Cu-ZSM-11 (MEL) and Cu-ZSM-12 (MTW) possessed about 2-fold higher activity in comparison with CuZSM-5 (MFI) for NO-DD, as shown in Figure 37 displaying the

Figure 38. 5T-Fe-ZSM-5 model with [FeO]+ as the active site. Reprinted from ref 328. Copyright 2008 American Chemical Society.

On the basis of the constructed models, three mechanisms were simulated: (i) the reaction of NO + NO2 with NH3 in the gas phase, (ii) the decomposition of NH4NO2, and (iii) the reaction of NO + NO2 with NH3 catalyzed by Z-[FeO]+. For mechanisms i and ii, the activation barriers of the rate-limiting steps were 22.5 and 24.0 kcal mol−1, respectively. The key intermediate was NH2NO, which could be decomposed into the final products of N2 and H2O. For mechanism iii, the detailed reaction steps are listed in Scheme 13, wherein eqs 5.11−5.15 represent the generation of the key intermediate of NH2NO.

Figure 37. Arrhenius plot illustrating the activity difference among CuZSM-5, Cu-ZSM-11, and Cu-ZSM-12 catalysts. The inlet concentration of NO was 1% NO in Ar. The total pressure was 1 atm. Reprinted with permission from ref 327. Copyright 2006 Elsevier.

Scheme 13. Schematic Reaction Steps of NH3-SCR over 5TFe-ZSM-5a

TOF values. The activity difference was believed to be mainly attributed to the distinguishing zeolite topology between CuZSM-5 (containing both straight and sinusoidal pore channels) and Cu-ZSM-11 and Cu-ZSM-12 (containing only the straight pore channels). Both ZSM-11 and ZSM-5 are constructed from the same secondary building unit (SBU 5-1) and have similar pore sizes. However, ZSM-11 only has interconnected straight pores. ZSM-12 (MTW), being built by SBU 5-[1-1], features one-dimensional pores without any interconnection. According to the EPR results, it was suggested that the Cu2+ active species were exclusively located in the straight zeolite pores, being much more favorable for the accessibility of the reactant with respect to that of the sinusoidal pore channels. Therefore, rather higher NO-DD activity was gained over Cu-ZSM-11 and Cu-ZSM-12. The investigation on NO-DD behaviors occurring over the zeolite catalyst is still in progress, which majorly focuses on the structure−activity relationship and mechanism study to overcome its two obstacles (high temperature for initially effective decomposition and O2 inhibition). Especially with the aid of computational chemistry, a deeper understanding can be obtained more readily than ever before. The DFT simulation

a

Z represents the zeolite framework.

As shown in Scheme 13, NH2NO could be formed through the reaction of NO with NH3 over the [FeO]+ site, with an energy barrier as low as 3.0 kcal mol−1. The resulting reduced active site of [FeOH]+ can then be reoxidized by NO2 and NH3 to regenerate the active site by overcoming the energy barrier of 32.0 kcal mol−1 (eq 5.15). As noted, the DFT energy calculation provides an explanation of the much higher activity of the “fast SCR” of NOx over the Fe-modified zeolite catalysts with respect to those of uncatalytic reaction mechanisms i and ii. It was also revealed that excellent SCR catalysts should exhibit higher reactivities in catalyzing the formation of the NH2NO 3708


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correct simulation result. The ONIOM method, based on quantum mechanics/molecular mechanics (QM/MM), has shown considerable promise in simulating larger zeolite clusters. Through the ONIOM method, the following reaction steps were simulated for the NO direct dissociation (see Scheme 14 of

intermediate. Similar works were also conducted by Bruggemann et al.,329 who also investigated the fast SCR by DFT employing the similar 5T-Z-[FeO]+ model of Fe-ZSM-5. It was reported that NH3 would be initially adsorbed on Z-[FeO]+, and proton transfer led to the formation of Z−[NH2FeOH]+. After reaction with NO or NO2, nitrosamine or nitramide together with Z−[FeOH]+ could be formed, constituting the most abundant surface species. The Z−[FeOH]+ further reacted with NH3, yielding H2O and Z−[FeNH2]+. The latter species finally interacted with NO2 to form nitrosamine, and then the active site was simultaneously restored. Cu-ZSM-5 is one of the earliest reported catalysts that showed good NOx selective reduction activities; however, it could be readily deactivated by the presence of impurity gases of H2O and SO2 in the effluent during the SCR process. Unfortunately, few theoretical calculations have been conducted for Cu-ZSM-5 poisoned by H2O and SO2. In light of that, Sierraalta et al.330 performed a theoretical investigation on the interactions among H2O, SO2, O2, NO, and NO molecules on active sites of CuZSM-5, which was simulated by a simple 3T model [H3SiOAl(OH)2OSiH3]. The DFT calculation result showed that the interaction energies of H2O and O2 with copper were lower than those corresponding to the Cu−NO2 and Cu−NO interactions, which indicate that neither H2O nor O2 can easily displace the adsorbed NOx. However, the H2O could displace the adsorbed O2 over the active site. Therefore, it can block some reaction pathways such as nitrate formation. These findings could account for the observed reversibility of the water deactivation. On the other hand, the deactivation effect of SO2 is similar to that of H2O. Nevertheless, in the presence of O2, it can also react with the active center, forming CuSO4, which significantly deactivates the catalyst. 5.3.2. NO Direct Decomposition Mechanism. 5.3.2.1. Mononuclear Active Site. Although experimental investigations on NO direct dissociation have been less reported than those of NO-SCR, the related DFT mechanism simulations were largely investigated due to the relatively simple reaction pathways. Izquierdo et al.,331 using DFT−ONIOM, studied the NO direct decomposition taking place over Cu-ZSM-5, which was modeled by 217 atoms, with one Al atom being situated at the T12 site and compensated by a Cu+ ion as the active site (see Figure 39). Ten atoms, including the active center Cu+, were at the high level, and 207 atoms were at the low level, during the ONIOM calculations. As noted, building a reliable system to model the zeolite active site is very important for achieving a

Scheme 14. Schematic Reaction Steps of NH3-SCR over 5TFe-ZSM-5a

a

Z represents the zeolite framework.

eqs 5.16−5.19). The simulation result showed that a novel copper k 2 mononitrosyl species (Z- 2 Cu-k 2NO) was in equilibrium with the Z-2CuNO and Z-2CuON species (Z represents ZSM-5, and the superscript indicates the spin multiplicity). Such a form of Z-2Cu-k2NO was found to be the intermediary key of the direct catalytic decomposition mechanism of NO by Cu-ZSM-5. On the basis of DFT, Kawakami and Ogura332 proposed a new reaction pathway for NO decomposition occurring over Fe(II)ion-exchanged zeolites (LTA, MFI, and FAU), represented by 192T (LTA), 96T (MFI), and 192T (FAU) models containing a single Fe(II) site (see Figure 40). The NO molecules could be

Figure 40. Optimized QM/MM models of Fe(II)-LTA (a), Fe(II)FAU (b), and Fe(II)-ZSM-5 (c). The QM region is represented by balland-stick models. The red, gray, pink, and purple atoms are O, Si, Al, and Fe(II), respectively. Reprinted with permission from ref 332. Copyright 2015 Elsevier.

selectively adsorbed on the Fe(II) site, forming dinitrosyl species, even in the presence of excess oxygen, due to the much higher adsorption energy of NO. Thereafter, the generated dinitrosyl species could be further decomposed into N2O intermediates, which was deemed to be the rate-determining step for NO direct dissociation. As noted, the energy barrier of this step could be as high as 160 kcal mol−1 if it followed the traditional reaction pathway. However, Kawakami and Ogura332

Figure 39. Cu-ZSM-5 model (Z-Cu). Minimum energy structure: (a) front view and (b) lateral view. Key: high level, ball−stick; low level, tube; O, red; Si, gray; Al, pink; Cu, orange. Reprinted with permission from ref 331. Copyright 2011 Elsevier. 3709


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wherein the Cu+ site was separated by one tetrahedron (1T model of Figure 42a) and two SiO4 tetrahedra (2T model of Figure 42b), respectively.

proposed that if there existed an adjacent proton near the Fe(II) active site, the energy barrier of 2NO → N2O + O could be greatly decreased to 40 kcal mol−1. 5.3.2.2. Binuclear Active Site. Morpurgo et al.333 conducted a DFT simulation for NO direct decomposition over Cu-ZSM-5, the active site of which being respectively represented by two different active site models: (i) a binuclear Cu+ site, wherein the pair of Cu+ ions was located at opposite sides of the 10-MR at the intersection of the linear and sinusoidal channels (see Figure 41a) and (ii) an isolated Cu+ site (see Figure 41b). Such a

Figure 41. Reaction site considered for NO decomposition catalyzed by Cu-ZSM-5. A large portion of the crystal is presented to show the location of the cluster model employed in the calculations. Reprinted with permission from ref 333. Copyright 2012 Elsevier.

Figure 42. 10T cluster model in the intersection of straight and zigzag channels with a pair of Cu atoms along with the aluminum: (a) 1T model separated by one SiO4− ion and (b) 2T model separated by two SiO4− ions. Reprinted with permission from ref 335. Copyright 2014 Elsevier.

binuclear active site was also reported by Moretti et al.,334 who utilized DFT simulation to verify that such a binuclear active site could be generated by the self-reduction of Cu2+ during thermal activation under vacuum or by N2 generated from two adsorbed NO molecules. The NO direct dissociation mechanism over a binuclear active site was found to be similar to that of a mononuclear active site, wherein the N2O as an important intermediate could be generated through the adsorption of NO on the active site. The final products of N2 could be produced through the further dissociation of N2O. Slight differences exist in the NO adsorption mode on the active site. Over the mononuclear active site, the first NO molecule was adsorbed on the active site through the O-down mode and the second NO molecule could be further adsorbed to form an ONNO dinitrosyl species. However, the NO molecules were all adsorbed on the binuclear active site through the N-down mode, which was more favored by 14 kcal mol−1 with respect to the O-end adsorption mode over the single site. The N2O dissociation reaction pathways were similar, including the atomic O transfer from the adsorbed N2O molecule to the active site of [Cu−O]+ or [Cu−O−Cu]2+. The DFT energy calculations suggest that the binuclear Cu model (Figure 41a) exhibited a much higher activation energy barrier in the N2O dissociation reaction step (50−56 kcal mol−1) with respect to that of the mononuclear active site (40−43 kcal mol−1), which was reported to be mainly related to the stability of the generated Cu−O−Cu local structure. According to the above DFT energy calculations, it can be deduced that the mononuclear active site of the zeolite catalyst is much more active than that of the binuclear active site of the zeolite catalyst for NO direct dissociation. Recently, Sajith et al.335 simulated another two types of binuclear Cu+ active site for NO direct decomposition over CuZSM-5 zeolite on the basis of DFT and QM/MM methods,

Through geometry optimization and energy calculation, the 1T model was found to be slightly more stable than the 2T model by 0.8 kcal mol−1. Additionally, it was interesting that the acidic proton located near the Cu active site could significantly decrease the energy barrier for NO direct dissociation. For example, the energy barrier of 2NO → N2O + O (N2O formation through two NO molecules) could be reduced from 56.3 to 31.4 kcal mol−1 for the 1T model and from to 55.3 to 17.3 kcal mol−1 for the 2T model. This finding was consistent with that reported by Kawakami and Ogura,332 who simulated the NO direct dissociation over the mononuclear active site (as stated above). Therefore, the proton of the zeolite catalyst played an important role in the NO direct dissociation. In this section, deNOx technologies, including NO-SCR by NH3/hydrocarbon/H2 and NO direct dissociation occurring over the zeolite catalysts, were reviewed. (1) For NH3-SCR, the present work focuses on the Cumodified small-microporous zeolites of SSZ-13 and SAPO-34 with CHA topology, which were recently investigated due to their excellent catalytic behaviors and are promising candidates for practical applications. Cu2+ cations located at the d6r subunit of SSZ-13 were widely accepted as serving as the active sites for deNOx. Similarly, the isolated mononuclear Cu2+ ions in the vicinity of 6-MR (part of the d6r subunits of CHA) were also reported to act as active sites. Both Cu-SSZ-13 and Cu-SAPO-34 exhibited high hydrothermal stability and chemical aging resistance, which were closely correlated with their supersmall pore sizes and special spatial structures. Moreover, Cu-SAPO-34 was reported to exhibit a much stronger hydrothermal resistance with respect to Cu-SSZ-13. The deNOx mechanism was widely deemed to follow the reaction pathway of the interaction of nitrite (or nitrate) with NH4+ to generate the final products of N2 and H2O, wherein the NO could be oxidized into NOx− over 3710


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the Cu2+ cation sites, and the adsorption of NH3 on a Brönsted acid site could produce NH4+. (2) The HC-SCR and H2-SCR provide alternative deNOx methods for NOx abatement, which can overcome the shortcomings of NH3-SCR, for example, the NH3 slip and the NH3 storage problem. The transition-metal (Cu2+, Co2+, and In3+)-modified zeolites of FER, BEA, and MOR were reported to be active for HC-SCR. Noble-metal (Pt, Pd)-modified zeolites are widely used for H2-SCR, wherein the H2 preactivation of the noble-metal active site to form adsorbed H was believed to be a key step for H2-SCR. However, it should be noted that relevant investigations of these two deNOx methods have been less reported than that of NH3-SCR. Especially for the H2-SCR, the application of the noble-metal-modified zeolite catalysts at high cost hinder their practical applications to some degree. (3) The last part described the DFT simulations for the deNOx mechanism studies, including NH3-SCR and NO direct dissociation. The NH2NO was found to be a key intermediate during NH3-SCR, which could be decomposed into the final products of N2 and H2O. On the other hand, N2O constituted an important intermediate during NO direct dissociation over both the mononuclear and binuclear active sites. As noted, the zeolitic deNOx mechanism simulation based on DFT has so far not been sufficiently investigated. There is still much work to do, such as zeolitic microenvironment simulation, especially for describing the active site structure and structure−activity relationship (on the basis of a theoretical mechanism study). Therefore, it is certain that, in the near future, there will be a large number of studies conducted on the deNOx mechanism simulation with the aid of DFT calculations.

accordingly ascribed to the fact that the framework Al of zeolite offers ion-exchanged sites in favor of the formation of highly dispersed active centers as well as modifies the acidity. Therefore, an incorporation of Al into mesoporous silica is an effective way to further enhance the related activities. Third, the hydrothermal stability is determined by both the Si/Al ratio and structural topology. A higher Si/Al ratio and relatively smaller pore opening result in a better hydrothermal resistance. Fourth, a higher Si/Al ratio usually leads to better redox ability. Moreover, the zeolite topology could obviously affect the distribution of the loaded metal species and influence their redox properties. Selective catalytic oxidation is available for purifying the nitrile gases. The related nitrile conversion and N2 selectivity correspond to the redox properties and the chemical nature of the metal species doped on zeolite. This oxidation process usually involves an oxidation mechanism with an isocyanate intermediate for Cu-, Mn-, Co-, Ag-, Pd-, and Pt-modified zeolites, while a hydrolysis mechanism with an amide intermediate for Fe-, V-, and Zn-exchanged zeolites. Among them, Cu-ZSM-5 exhibits the best N2 selectivity and superior activity for nitrile removal. Fe- and Co-exchanged zeolites (BEA and MFI) are effective for N2O direct decomposition that commonly follows an E−R mechanism involving two N2O molecules adsorbed on metallic active sites together with a subsequent fracture O−N2 bond to generate α-O species, which is highly active for the further dissociation of another N2O molecule. The final O2 desorption is deemed as the rate-determining step. In addition, hightemperature pretreatment of the fresh sample under a He or N2 atmosphere facilitates the formation of low-valence cations (Fe2+, Co2+), significantly improving the decomposition efficiency. As for NOx elimination, SCR technology has already been industrialized using NH3 as a reductant and vanadium/titania as the commercial catalyst. Facing the new requirements for SCR catalysts, including remarkable low-temperature activity, wide operation-temperature window, working under excess oxygen, and avoiding the toxicity of the vanadium catalyst, the zeolite catalysts seem to be the only choice to satisfy all these terms. As stated for NO-SCR by NH3, Cu-based zeolites (Cu-MFI, CuBEA, Cu-FER) exhibit higher activities in the low-temperature region (400 °C). Inspiringly, the extremely high NOx conversion (>95% at 150 °C) and N2 selectivity (>90%) at a wide temperature window of 150−500 °C occurred over Cu-SSZ-13 catalysts. Cu2+ cations located at the d6r subunit are responsible for these outstanding performances. The ideal hydrothermal stability and poisoning resistance also make this catalyst a promising substitute for the present vanadium/titania catalyst. In addition, because of NH3 slip and NH3 storage problems arising during a traditional SCR process, an attempt to introduce reductants other than NH3 has been made. The transition-metal-exchanged zeolites prefer to utilize hydrocarbons as reducing agents, while H2 is effective to reduce NO over the noble-metal-exchanged zeolites, likely attributed to its ability for an easy H2 dissociation into highly reactive H atoms. Theoretical simulation based on DFT is believed to be promising to describe the structure and local environment of the active site, as well as determine the depollution mechanism at the molecular or atomic level. However, due to the complications of some depollution systems, such as NO-SCR involving many molecules (NO, O2, and reductant), causing a huge amount of

6. CONCLUDING REMARKS AND PERSPECTIVES Gaseous nitrogen-containing pollutants, including HCN, CH3CN, C2H3CN, NH3, N2O, NO, and NO2, with diverse N valences, are commonly recognized as harmful and even fatal for humans. Considering the easy interconversion and possible coexistence of these waste gases, this work gives an overall view for their catalytic removals by means of zeolite catalysts, which are well commercialized and widely utilized in industry owing to their satisfactory characteristics, including high surface area, welldefined and fully developed pore structure, and considerable thermal stability. The physicochemical properties, such as structural topology, shape selectivity, specific surface area, Si/Al ratio, acidity, and redox ability, of zeolite catalysts are presently correlated to the catalytic performances for pollutant elimination to reveal the structure−activity relationship as well as present a guideline of designing highly efficient catalysts. First, the “shape selectivity” behavior marginally occurs as for the traditional zeolite catalysts because of their relatively larger pore apertures with respect to the corresponding gas molecules without causing any constraint effect. However, a “quasi shape selectivity” is indeed observed over SSZ-13 with supersmall micropores during the NO-SCR reaction, manifesting as high N2 selectivity associated with less NO2 and N2O generation, better poisoning resistance toward SO2 and hydrocarbons, and good hydrothermal stability due to hard dealumination. Second, high surface areas are ordinarily achieved in the case of small-microporous zeolites (with different Si/Al ratios, e.g., SSZ-13) and mesoporous supported catalysts (pure silica, e.g., SBA-15 and KIT-6). Although a superior surface area is beneficial for a better dispersion of active components, good SCR performances are found merely over the CHA-typed zeolite with supernarrow pores of ∼3.8 Å. This is 3711


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with supersmall apertures, such as OFF (3.6 × 4.9 Å), ERI (3.6 × 5.1 Å), LEV (3.6 × 4.8 Å), and AFX (3.6 × 3.4 Å), should also be taken into account for the NH3-SCR study to obtain a common principle for the design of highly efficient zeolite catalysts. (b) As also mentioned in this review, the “standard SCR” mechanism is still a subject of debate. However, combination with DFT simulations can facilitate the deeper understanding of this issue through constructing zeolitic models and simulating adsorption behaviors as well as clarifying reaction pathways. (c) The last perspective is developing deNOx technologies other than the traditional NH3-SCR, such as H2-SCR and NO direct decomposition. Although some research has already been done on these topics, they still need a lot of effort. (4) Mesoscale simulation of Monte Carlo (MC) and molecular dynamics (MD) is described. Mass transfer is an omnipresent phenomenon in environmental catalysis based on zeolitic catalysts due to the powerful and complicated channel system of zeolite catalysts. The detailed diffusion behaviors of sorbent molecules inside the pores of the zeolite catalysts are difficult to study experimentally and require computational chemistry approaches. Monte Carlo (MC) and molecular dynamics (MD) techniques, coupled with the availability of high-performance computing facilities, have enabled the determination of the adsorption and diffusion characteristics of a variety of molecules with a reasonable degree of accuracy and reliability. MC and MD simulations mainly focus on the fluent dynamics at the mesoscale regime, which serves as an important supplementary theoretical method to facilitate better understanding of the zeolitic structure effect on the adsorption and diffusion of the introduced reactant molecule: (i) MC simulation can provide clues to the superior adsorption site as well as the adsorption energy for the reactant molecule inside the complicated zeolite channels; (ii) MD simulation is able to predict the detailed diffusion pathway of the reactant molecules associated with the calculation of the diffusion energy barrier. However, such simulations for the abatement of N-containing waste gases over zeolite catalysts have rarely been reported. Herein, the importance of utilization of these simulation methods should be emphasized. Along with the rapid progress of the computational technology as well as the calculation methodology, it is believed that in the near future there will be a lot of work concentrating on the application of molecular simulation (MC, MD) for the N-containing exhaust removals by zeolite catalysts, based on which the effect of the zeolitic structural topologies and the properties of the active species can be illustrated from the mesoscale point of view.

calculation, and being more uncertain, related DFT studies on the deNOx mechanism were scarcely conducted. Nevertheless, these mechanism simulations might become prevalent in the future with the development of computational technology. Up to now, great success has been achieved for the utilization of zeolite catalysts in industry for environmental protection. According to new progress in this field, the CHA-type zeolites are highly appreciated due to their high surface areas along with unique catalytic behaviors. Although the mesoporous support catalysts also display quite high surface areas beneficial for dispersion of active components, the poor stability limits their practical applications. Furthermore, the construction of a fine zeolite powder into a honeycomb monolith is necessary for industrial utilization. However, it is always plagued by a cracking problem due to the high thermal sensitivity of the porous zeolite substrate, which must be well solved by adding a binder and carefully controlling the drying speed. In the end, some perspectives for the abatement of Ncontaining waste gases by means of the zeolite catalysts are described because the related systems are still far from well established. (1) First, for the SCO of nitrile gases (HCN, CH3CN, and C2H3CN) as well as NH3, Cu- and Fe-zeolite catalysts were believed to be efficient. The related mechanisms have also been proposed according to the experimental approaches. However, the detailed reaction pathways as well as information on the generated intermediates have up to now been unclear. For instance, the nitrile gas oxidation followed the “N2 formation” mechanism for Cu-zeolite, while the “NH 3 formation” mechanism was obeyed for Fe-zeolite. It is therefore necessary to give some deeper insight into the related reaction mechanism, especially on the basis of DFT simulations. (2) N2O direct dissociation has been comprehensively investigated on the basis of both experimental and theoretical approaches, as also indicated in the present review. This is probably related to the fact that the reaction pathways of N2O dissociation are much simpler than the other reaction systems (e.g., SCR of NO by NH3). As far as we known, the technologies for the monolith catalyst preparation and practical application are needed to be dealt with for the N2O catalytic decomposition nowadays. The macroscale simulation method of computational fluid dynamics (CFD) is efficient in the simulations of fluid dynamics inside the monolith channels of zeolite catalysts. (3) There are three perspectives for the deNOx reaction system. (a) NH3-SCR has become one of the most promising deNOx methods under lean conditions. However, due to some drawbacks for the commercialized V−W−Ti catalysts, more attention is now paid to the development of new superiorly active materials for NOx removal. Fortunately, Cu-SSZ-13 zeolite was recently proposed to exhibit extremely high lowtemperature activity and N2 selectivity. The related structure− activity relationship is also illustrated on the basis of both experimental and theoretical approaches. However, the utilization of the very expensive and toxic template of N,N,Ntrimethyl-1-adamantylammonium hydroxide (TMA-daOH) for Cu-SSZ-13 synthesis leads to a significant increase of the catalyst cost, which seriously prevents industrial realization. Therefore, the research works focusing on developing an economical way to synthesize SSZ-13 by utilizing a cheap template have become important directions for the NH3-SCR technique. In addition, on the basis of the concept of “quasi shape selectivity” illustrated for SSZ-13, investigations of other microporous zeolite catalysts

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +86 10 64433695. Fax: +86 10 94419619. Notes

The authors declare no competing financial interest. Biographies Runduo Zhang was awarded the degree of B.Sc. (1994) from Tianjin University. He received his Ph.D. from the Dalian University of Technology (cooperatively educated by the Pohang University of Science & Technology, South Korea) in 2001. He subsequently worked as an assistant professor in the Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. In 2003, he began his postdoctoral research at Laval University, Canada. He was promoted to 3712


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full professor at the Beijing University of Chemical Technology in March 2008 and Ph.D. supervisor status in May 2008. He was selected to participate in the “Innovative Research Groups” of the National Natural Science Foundation of China (NSFC) in 2008 and the “New Century Outstanding Talent” scheme of the Ministry of Education in 2009 and became a chair scientist of "National High Technology Project" in 2013. His research is focused on air pollution catalytic control as well as preparation and application of microporous/ mesoporous materials (SSZ-13, SAPO-34, ZSM-5, Beta, SBA-15, KIT-6, etc.) and fine perovskite-type oxides. Over 60 publications in international journals such as the Journal of Catalysis, Applied Catalysis A/B, Environmental Science & Technology, and Chemical Communication, as well as 15 patents and 2 English books, have resulted from his work. He was awarded Natural Science Progress Prize (second class) of the Ministry of Education in 2014. Ning Liu was born in Shanxi Province (China) in 1984. He received his bachelor’s degree in 2007 and Ph.D. degree in 2013 from the Beijing University of Chemical Technology under the supervison of Professor Biaohua Chen and thereafter worked as a lecturer. His research focuses on N2O direct decomposition over zeolite-based catalysts and the related mechanism study based on DFT calculations. Zhigang Lei received his B.S. degree in 1995 from the Wuhan Institute of Technology and his Ph.D. degree in 2000 from Tsinghua University. Then he became a postdoctoral researcher at the Beijing University of Chemical Technology (BUCT) working with Professor Chengyue Li. In 2003−2005, he worked as a researcher at the Research Center of Supercritical Fluid Technology (Tohoku University, Sendai, Japan). In 2005−2006, he was awarded the world-famous Humboldt Fellowship and carried out his research as the Chair of Separation Science and Technology (Universität Erlange-Nürnberg, Erlangen, Germany). In 2006, he came back to China. He is now a professor in the State Key Laboratory of Chemical Resource Engineering (BUCT, China). His current research interests include chemical process intensification and predictive molecular thermodynamics. He has contributed to more than 90 papers in international journals and one book entitled “Special Distillation Processes” published by Elsevier B.V. (2005). He has been acting as an associate editor of Chemical Papers since 2014. In 2014, he became a Fellow of the Royal Society of Chemistry (FRSC). Biaohua Chen was born in Jiangxi Province (China) in 1963. He received his Ph.D. degree in 1996 from the China University of Petroleum (Beijing). In 2000, he was a visiting scholar at Washington University in St. Louis and the University of Washington. He has received two National Science and Technology Progress Prizes (second class), two provincial or ministerial level Science and Technology Progress Prizes (first class), and one Natural Science Progress Prize. Now he is a member of the Standing Committee of the Beijing Chemical Industry Association and a member of the Editorial Board of the Journal of Petrochemical Universities (China). His main research interests are environmental and green chemistry. He has contributed to more than 200 papers in international journals.

ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (NSFC) (Grants 214777007 and 21407007), the Fundamental Research Funds for the Central Universities (Grant YS1401), and the National “863” Project (Grant 2013AA065900) for financial support. REFERENCES (1) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. Rev. 1995, 95, 209−225. 3713


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