Gasification of Carbonaceous Deposits: A

Review pubs.acs.org/IECR

Formation and Oxidation/Gasification of Carbonaceous Deposits: A Review Shilpa Mahamulkar,†,# Kehua Yin,‡,# Pradeep K. Agrawal,*,† Robert J. Davis,*,‡ Christopher W. Jones,*,† Andrzej Malek,*,§ and Hirokazu Shibata*,∥ †

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Department of Chemical Engineering, University of Virginia, Charlottesville, Virginia 22904, United States § Hydrocarbons R&D, The Dow Chemical Company, Midland, Michigan 48674, United States ∥ Hydrocarbons R&D, Dow Chemicals Benelux, NL 4530 AA, Terneuzen, The Netherlands ‡

S Supporting Information *

ABSTRACT: A wide variety of hydrocarbon processes, catalytic or noncatalytic, involve the formation of carbon deposits, either on catalysts or on reactor (or engine/exhaust) surfaces. Therefore, researchers have developed a large array of catalysts to aid the combustion of these deposits. Recently, the mechanism of catalytic carbon oxidation and/or gasification has been the focus of research in an attempt to design better catalysts for carbon removal. With this approach, understanding the mechanism of formation of different types of carbon deposits is desired. Efforts undertaken for studying oxidation or gasification of various forms of carbon deposits are discussed in this review, along with the techniques used to study the mechanism of oxidation/gasification. The kinetics of catalyzed and noncatalytic carbon oxidation are described in detail. The effect of reactive gases such as NOx, water vapor, CO2, and SO2 on the gasification behavior of carbon deposits is also discussed. Reaction rates of oxidation/gasification of carbon under different operating conditions have been calculated, allowing for a comprehensive overview of carbon removal reactivity. to the formation of soot.12 The health hazards of soot, such as chronic respiratory disorders, make it important to minimize its emission by capturing it and subsequently oxidizing it.1,2 One of the major reasons for the deactivation of heterogeneous catalysts in chemical processes is the deposition of carbonaceous materials on the active sites or in the pores of the catalytic materials.3 The structure of these deposits is dependent on the catalyst used, the operating conditions, and the chemical reaction itself.13 For processes such as fluidized catalytic cracking (FCC), hydroprocessing,14 hydrogenation,15 and Fischer−Tropsch synthesis (FTS),16 a significant amount of coke deposition occurs, rendering the catalyst inactive. Industrial production must be terminated to allow for the oxidation of carbon deposits and recovery of catalytic activity. Replacement of the catalysts or shutdown of the process can have significant financial consequences. The lifetime of typical catalysts can range from a few seconds (FCC) to a few years (steam reforming reactions), and extension of the lifetime is almost always beneficial.17 Although coking is often inevitable,

1. INTRODUCTION Carbonaceous deposits are produced during incomplete hydrocarbon combustion (soot in diesel engines),1,2 catalytic conversion of hydrocarbons (coke deposited on catalysts),3−6 and thermal decomposition of hydrocarbons (coke formed in steam crackers).7−9 These deposits cause numerous problems in chemical plants and diesel engines. To this end, significant research has been focused on the development of technologies that aid the oxidation of such deposits and/or limit the coke deposition. Two temperature ranges exist in which carbon deposition commonly occurs, namely, a low-temperature regime (below 600 °C) and a high-temperature regime (800−1000 °C). Soot formation in diesel engines and coke deposition on catalysts happen at the lower temperatures.1 However, coke formation during steam cracking reactions and soot formation in diffusion flame occur at very high temperatures.10 Under ideal conditions, CO2 and H2O are the products of complete combustion of hydrocarbons in diesel engines. However, thermodynamic equilibrium is not achieved in engines, because of the limited amount of time available for combustion and the huge temperature variation in the combustion chamber.2,11 Moreover, collisions among hydrocarbon fragments are common in fuel-rich atmospheres, leading © 2016 American Chemical Society

Received: Revised: Accepted: Published: 9760

June 14, 2016 August 15, 2016 August 16, 2016 September 9, 2016 DOI: 10.1021/acs.iecr.6b02220 Ind. Eng. Chem. Res. 2016, 55, 9760−9818

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Industrial & Engineering Chemistry Research

been extensively covered in the literature. Reviews on this topic have been cited in the paper in the appropriate sections.

its consequences can be minimized. Hence, significant efforts have been directed toward understanding the catalytic coking phenomenon and finding ways to minimize its impact. Another major process contributing to carbon deposits is steam cracking, which is a widely used process for the industrial production of olefins. This pyrolysis process involves heating hydrocarbons to very high temperatures (800−1000 °C) in the presence of diluting steam.10,18,19 The cracking units are typically composed of alloys that contain nickel, iron, and chromium, which are capable of withstanding very high temperatures.19 The residence time for these reactions is ∼0.5−1 s. At these high temperatures, radical reactions play an important role in the formation of a wide range of hydrocarbons. One of the major byproducts of the steam cracking process is the formation of carbonaceous deposits (coke) on the walls of the reactor. The amount and type of coke formed is dependent on the operating conditions, the nature of the feed, and the nature of the reactor surface. Steam dilution of the feed has been observed to reduce coke formation.20 Coke formation affects the production efficiency in many ways, as mentioned below:8,10

2. REACTION MECHANISMS OF CARBON DEPOSITION For scientific analysis of carbon oxidation studies, it is important to examine the mechanisms by which the carbon deposits are formed. As mentioned previously, the deposition of carbonaceous compounds is dependent on numerous factors and, hence, can happen by a variety of pathways. Broadly, the mechanisms have been classified in this section based on the temperature range in which carbon deposition occurs. 2.1. Coke Deposited during Steam Cracking. Steam cracking involves thermal decomposition of hydrocarbons at high temperatures (800−1000 °C) to yield valuable products such as olefins. At these high temperatures, radical reactions play an important role in the formation of a wide range of hydrocarbons. The major reaction for cracking of ethane involves dehydrogenation as shown in eq 1: H3C−CH3 → H 2CCH 2 + H 2 (1)

• Increases pressure drop in the reactor21 • Reduces the reactor volume • Increases the temperature of the reactor tube by affecting heat transfer from the tube to the gas • Leads to expensive shutdowns • Causes carburization of steel • Influences the flow in the reactor • Decreases the production capacity22

Steam cracking reactions are highly endothermic and many of the elementary radical reactions have high activation energies. To reduce the production of undesired reactions, such as the formation of large hydrocarbons with structures similar to that of polyaromatics, the products are typically immediately cooled in a transfer line heat exchanger (TLE). The end-product is a complex mixture of hydrocarbons. This mixture is separated by distillation and absorption processes to obtain valuable products such as olefins. Paraffins from natural gas, as well as naphtha and gas oil from petroleum refineries, are the major feedstocks for the production of olefins. The major byproduct of steam cracking is the deposition of coke on the reactor surfaces. Coke deposition in steam cracking reactions is a combination of the several mechanisms, namely, radical reactions in the gas phase, droplet condensation, and coke formed on metallic particles. 2.1.1. Catalytic Coking Mechanism. In a steam cracker, catalytic coke formation takes place in the presence of catalytic sites, which can be the metallic walls of the reactor, more likely at grain boundaries.10,24 Since most industrial cracking units are made of alloys of nickel, iron, and chromium, many types of metallic sites are present on the reactor surface. These sites can catalyze coke formation at temperatures as low as 500 °C. Although the catalytic coking rate increases significantly with increasing reaction temperatures, the coking rate is also highly dependent on the reactor surface used. Because of the presence of metallic sites, coking rates are higher in reactors made of materials like stainless steels or Incoloy (superalloys made of iron−nickel−chromium having good corrosion resistance and stability at high temperatures), compared to more inert oxide materials such as quartz.8−10,25 Figure 124 shows a schematic of the catalytic coking process taking place on a reactor wall. Hydrocarbons adsorb on the metallic surface and form coke. This coke further dissolves and diffuses through the metallic particle. With time, carbon deposits at the end of the metal particle, raising the metal from the reactor surface. This mechanism is most important after the decoking step when the reactor wall is clean, devoid of any coke exposing the metallic sites on the surface for reaction.26 2.1.2. Radical Coking Mechanism. At very high temperatures in the steam cracker, radicals are formed in the gas phase, producing pyrolytic coke, which deposits on the reactor

Coke formed in the cracking reactor is typically combusted by passing a mixture of steam and air at very high temperatures through the reactor in regeneration mode. The plant typically requires a shutdown for decoking every 20−60 days, depending on the type of feed used.23 These shutdowns are usually expensive, and efforts are directed toward increasing the run length of steam crackers. In most research papers on carbon oxidation, only combustion of soot formed in diesel engines is discussed. Relatively fewer papers discuss coke formation during steam cracking and other hydrocarbon reactions. The physical structure and chemical nature of the carbon deposits are dependent on several factors, namely, the operating temperature, pressure, residence time in the reactor, and hydrocarbon feed composition. The combustion behavior of carbon is dependent on the operating conditions of the reactor in which it was formed. Although the formation of carbonaceous deposits is often inevitable in most processes, the deposition can be slowed if the mode of coke formation is well understood. Understanding the physical and chemical properties, as well as the mechanism of oxidation of these deposits, is critically important for future advances in hydrocarbon processing. Because of the varied operating conditions used in these different processes, the literature on oxidation/ gasification of carbon deposits is quite scattered; hence, this contribution seeks to provide a unified review of the fundamental oxidation/gasification reactivities of the carbon deposits (including metal-catalyzed carbon, acid-catalyzed carbon, and coke formed during pyrolytic hydrocarbon reactions) under varying reaction conditions. This review does not intend to provide information on the existing technologies for abatement of carbon deposits, since it has 9761

DOI: 10.1021/acs.iecr.6b02220 Ind. Eng. Chem. Res. 2016, 55, 9760−9818

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furnace. A schematic of the droplet condensation mechanism is shown in Figure 3. At elevated temperatures, molecules

Figure 1. Schematic describing the formation of catalytic coke. (Reproduced with permission from ref 24. Copyright 2002, The Society of Chemical Engineers, Japan.) Figure 3. Schematic describing the droplet condensation mechanism. (Reproduced with permission from ref 24. Copyright 2002, The Society of Chemical Engineers, Japan.)

surface.10,24 These endothermic reactions require significant energy input to form the radical intermediates and, hence, only occur at high temperatures. Figure 2 shows a pathway for the formation of coke via the radical mechanism.24 Coke radicals in the gas phase react with the hydrogen present in unsaturated hydrocarbons. Decomposition of the aliphatic chain gives rise to new radicals. Further dehydrogenation of such molecules results in an increase in the aromatic nature of the coke and also the regeneration of radicals. Coke formed via this mechanism has very low hydrogen content, because of the dehydrogenation of molecules during the coking reactions. The coke formed is typically very hard (graphitic), because of the cross-linking of aromatic molecules under the reaction conditions. The rate of coke formation is highest for acetylene and lowest for paraffins and can be ranked in the following order: acetylene > olefins > aromatics > paraffins. 2.1.3. Droplet Condensation Coke. Aromatics with one or two rings act as coke precursors and condense by dehydrogenation to form tarlike particles after striking the reactor surface.27 This condensation happens downstream from the cracking reactor, where the temperature is lower than that in the reactor

undergo dehydrogenation, aided by radicals. This mechanism helps in further growth of the coke layer. Some molecules undergo hydrogen abstraction in the gas phase itself and are deposited on the reactor surface. The coke formed in the transfer line exchanger results from the condensation of polyaromatics.9 Since the mechanism involves condensation of aromatic molecules at lower temperature, the coking rate is independent of the chemical characteristics of the reactor surface.9 Figure 4 depicts the rate of coke formation on a stainless steel surface, as a function of time. The rate is initially high and decreases with time.23 Coke formation occurs via both catalytic and radical mechanisms in the region where the coking rate changes with time. The deposition of coke on the metal surface decreases the rate of catalytic coke formation until all of the sites are blocked. At this point, the rate of coke formation becomes approximately constant and only radical coke is formed.

Figure 2. Radical mechanism for coke formation. (Reproduced with permission from ref 24. Copyright 2002, The Society of Chemical Engineers, Japan.) 9762


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and alkali and alkaline-earths. In a pilot plant setup for ethane cracking at 65% conversion, the coating showed no pressure drop or temperature drop. The uncoated reactor showed a pressure drop of 27 psi and temperature difference that was 26 °C higher than that observed at the start of the operation. Another furnace coil coating technology, called catalystassisted manufacture of olefins (CAMOL), by BASF, aims to have a coke-free environment in cracking furnaces.35−37 This technology proposed to increase the run length of the reactor and has been undergoing commercial trials since 2006. These coatings have the ability to be used in high-temperature furnaces above 1130 °C, and they can potentially reduce the amount of steam required for dilution to keep coke formation to an optimum. The coatings act as a barrier between the metal sites from the reactor and the hydrocarbon gases and usually exist as an oxide layer Hβ > HZSM-5.74 Silicoaluminophosphates (SAPO) have been extensively used for methanol to olefin reactions. For instance, SAPO-34 is a type of SAPO with a 3-D structure that is composed of channels 3.1 Å in diameter having cavities at the intersection of the channels. During methanol-to-olefins (MTO) reactions, deactivation of this catalyst proceeds in a way where coke grows on the acidic sites and develops mostly in the cavities.75 This behavior is unlike that seen for HZSM-5, where the access to the interior channels is often blocked. The coke formed in cavities has been shown to be harder and thus, more difficult to oxidize, compared to the coke formed in the channels. Paraffins constituted the major component of the coke species formed on HZSM-5 with minor polycyclic aromatics. Similar results were found by Epelde et al. when they compared the coke deposition mechanism on HZSM-5 with that of SAPO-34, during the transformation of ethylene to propylene or 1butene.76 They presented two steps for coke formation, namely, oligomerization (to form alkylated aromatic structures) and condensation of polyaromatics. The first step was found to be similar on both catalysts, but the second step was dependent on the type of catalyst structure. As mentioned earlier, work has suggested that the shape of the pores in HZSM-5 allowed passage of inert gas, purging the coke precursors from the interior of the catalyst and, hence, reducing the coke deposition. For Cu−Ni bifunctional catalysts on different supports in ethanol steam reforming reactions, less coke deposition was found on a SiO2 support.77 Supports containing Al (γ-Al2O3, ZSM-5) caused more coke deposition, because of their higher acidity. The catalyst on the γ-Al2O3 support showed more deactivation than ZSM-5, as the zeolite had channels that presumably helped in sweeping of coke precursors. Nickel was found useful for activity toward hydrogen production, but copper was found to minimize coke formation. During the conversion of acetone/n-butanol mixtures to hydrocarbons, the coke produced was found to be deposited in the channels of the zeolitic structure of HZSM-5, below a coke content of 5 wt %.78 For higher carbon contents, the coke started depositing on the external surfaces as well. Barbier et al. showed that the amount of coke formed on an acidic silica− alumina was much higher than on a nonacidic support having the same BET surface area.66 This is consistent with the wellknown tendency of acidic sites to catalyze coke formation. The amount of coke formed has been very well correlated with the acid sites of the zeolites during the MTG process. 71 Deactivation of HZSM-5 during the transformation of bio-oil and methanol to hydrocarbons was found to be dependent on the concentration of bio-oil oxygenates in the reaction.79 These oxygenates react further to form heavy polyaromatic structures, leading to coke. This problem was resolved by either increasing the reaction temperature, which increased the conversion of bio-oil, increasing the SiO2/Al2O3 ratio, which reduced the acidity, and/or reducing the bio-oil concentration in the feed.

RHCHCHR′H + H+A− → RHCH 2−C+HR′H + A− (2)

RHCH 2−C+HR′H ↔ RCH 2 + H 2C+R′H

(3)

H 2C+R′H ↔ R′CH 2 + H+A−

(4)

These carbenium ions can further react with either olefins or aromatic compounds to form disordered carbonaceous deposits (eq 5).82 RCH 2−C+R′H + RCHCHR′ ↔ RCH2−CHR′−RCH−C+HR′ (5)

Dehydrogenation and isomerization reactions can also happen simultaneously on these carbon compounds to form additional coke precursors. In the cracking of vacuum gas oil (VGO) and raw bio oil (RBO), the amount of coke formed was more on RBO than on VGO, although the coke on RBO was more hydrogenated. An industrial catalyst with 15% HY zeolite was used. This suggests different modes of coke formation using different feedstocks.83 One pathway involved heavy hydrocarbons forming polyaromatic species, while the other was an oxygenate pathway forming lighter coke with more aliphatic species. RBO formed more-soluble coke while VGO formed insoluble species. When RBO and VGO were mixed and fed into the system, less insoluble coke was formed, compared to VGO alone. This was attributed to the attenuation effect from ∼46% water coming from RBO. The authors claimed coke that had formed on VGO was due to the cracking of heavy hydrocarbons, and the hydrides from hydrocarbon transfer to oxygenates in the mixed systems thereby mitigated coke-forming pathways. The minimization of coke deposition on catalysts has also been investigated by a few researchers. Similar to changing the acidity of the catalyst to attenuate coke deposition, the operating conditions of the reaction can also be optimized to achieve the same effect. Discussion on this topic is not in the scope of this review, since it has been covered previously in a recent review.84 Thus, the content and type of coke deposits formed on acidic catalysts is dependent not only on the strength of the acidic 9766


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Figure 7. TEM images of UB soot (soot collected from a diesel particulate filter installed on an urban bus) collected from a diesel engine, showing a hollow interior. (Reproduced with permission from ref 92. Copyright 2007, Elsevier, Amsterdam, The Netherlands.)

Soot produced in engines is highly porous. The internal surface area is comparable to the external surface area, and the micropore area is responsible for the majority of the internal surface area.87,88 Zerda et al. studied the structure of combustion engine deposits from fuels with 22−44 vol % aromatics with gas sorption techniques.87 They found that the majority of the internal surface area came from micropores with widths of ∼0.5 nm. Gas sorption techniques can also be used to study the coking behavior in zeolites. Radwan et al. characterized coked USY zeolites and the liberated coke from catalytic cracking of benzene by N2 adsorption.89 For cracking reactions in H2, the coked zeolites had a slightly lower surface area than the fresh zeolites, while the liberated coke had a low surface area. For cracking reactions in He, the surface area of the coked zeolites decreased significantly, compared with the fresh sample, while the liberated coke had a very large surface area. Thus, they concluded that the coke deposition occurred in the channels of the zeolites during cracking in a helium environment, but was on the external surface when the reactions occurred in a H2 environment. 3.2. Transmission Electron Microscopy (TEM). Transmission electron microscopy (TEM) has been widely used to obtain the particle size and size distribution of soot. Moreover, high-resolution transmission electron microscopy (HRTEM) can be used to reveal the microstructure of soot particles before or during their oxidation. HRTEM studies of soot from a C2H2/O2 flame have shown that the soot has two different parts.90,91 The inner core contains spherical nuclei with bending structure, while the outer shell has a lot of graphitic carbon, which is more difficult to oxidize. The HRTEM images directly show the layered planar graphitic structure in the soot particles, which are 1−5 nm in size. The layers have different curvature and orient concentrically around the inner core. Such a structure supports the soot formation mechanisms described earlier. The inner core can undergo a densification process

sites but also on the shape of the catalyst and reaction conditions, such as those of the hydrocarbon used in the feed.

3. CHARACTERIZATION OF CARBONACEOUS DEPOSITS The composition and the structure of carbonaceous deposits such as soot have been discussed in an earlier review.1 The rate of oxidation of carbon deposits is related to the carbon structure. This section gives an account of the techniques that have been used recently for characterizing various carbon deposits. 3.1. Surface Area and Pore Structure. Surface area and pore structure are important properties of carbonaceous materials, because they can affect the oxidation behavior of such materials. The soot formed during combustion may adsorb volatile organic molecules, which can be released to the environment later, thus posing a threat to human health. The reactivity of coke during combustion in air by itself or on a catalyst surface also is dependent on the surface area and pore structure of the coke. There are three types of pores, as categorized by the International Union of Pure Applied Chemistry (IUPAC), micropores (having a pore width of Hβ > HZSM-5. 3.9. Gas Chromatography−Mass Spectroscopy (GCMS). Gas chromatography−mass spectroscopy (GC-MS) has been used for the characterization of soluble coke components. Guisnet et al. used GC-MS analysis to characterize coke deposited on zeolites during n-heptane cracking.13 Coke was extracted in methylene dichloride. The amount of soluble coke was measured, as a percentage of the total coke, and high amounts were determined to be soluble for HZSM-5 and UHSY zeolites. Low amounts were soluble on H-mordenite and H-erionite. Mass spectroscopy measured a high number of carbon atoms per molecule (>20) on UHSY and H-mordenite catalysts for coke contents of 1−2 wt % and a low number (∼15) of carbon atoms per molecule on HZSM-5 and Herionite. The number of soluble coke molecules decreased as the coke content increased, which implied that aromatic compounds increased with coke content. During the dehydrogenation of alkanes to olefins on Pt−Sn/Al2O3 catalyst, Afonso et al. also used soluble portions of coke in toluene, nhexane, and chloroform for analysis by GC-MS.133 The integration of peak was done to calculate percentage of aromatics, alkanes, and olefins. The maximum amount of aromatics and the least amount of alkanes were found in the toluene extract. Recently, Castanõ et al. used dichloromethane as a solvent for solubilizing coke and used GC-MS for analysis of the extracted coke species.132 In the transformation of bio-oil to hydrocarbons on Ni-HZSM-5, the amount of oxygenates, oxoaromatics, aliphatics, and aromatics was analyzed by GC-MS.131 They found that increasing the content of bio-oil in the feed increased the insoluble coke fraction, along with the amount of oxo-aromatic and oxygenate species. 3.10. Solid-State 13C Nuclear Magnetic Resonance (13C NMR). 13C nuclear magnetic resonance (13C NMR) has been applied in the characterization of coke to provide information on the chemical nature of the coke components.63 Traditional 13 C NMR is suitable for the soluble coke, while solid-state 13C NMR is used to characterize the insoluble coke. In this section, only solid-state 13C NMR will be discussed. Coke formed on zeolites and supported metal catalysts are studied using solid-state 13C NMR.137,138 Derouane et al. were the pioneers to use this technique to investigate the carbonaceous deposits in zeolites.139−141 They found that the nature of the carbonaceous deposits was dependent on the type of zeolite, the type of reactant, and the reaction conditions. The solid-state 13C NMR allowed a distinction between the hydrocarbon molecules trapped in the pores and the strongly chemisorbed surface alkoxide species. Valle et al. used 13C NMR spectroscopy to determine the type of carbon compounds present in the coke formed during transformation of bio-oil with methanol into hydrocarbons.131 Peaks at 22 ppm were assigned to aliphatics, while peaks at 129 ppm were assigned to aromatics. In the absence of bio-oil in the feed, aliphatics and aromatics were observed, with more aliphatics than when bio-oil is present. This result supports the formation of coke by a hydrocarbon pool mechanism where methyl9771


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Figure 10. (a−c) TEM images showing movement of carbon black toward CeO2; (d−f) TEM images showing stability of carbon black in Al2O3. (Reproduced with permission from ref 155. Copyright 2008, Elsevier, Amsterdam, The Netherlands.)

been undertaken to study the mechanism of carbon oxidation, which are discussed in this section. 4.1. In Situ Transmission Electron Microscopy (TEM). In situ TEM has been used to investigate catalysts under reaction conditions, which provides direct observations of structural changes of catalysts during reaction.149,150 This is especially beneficial for carbon oxidation reactions, because the interface between the carbon and catalyst is thought to be important.151,152 Baker et al. studied graphite oxidation by various metals and metal oxides with in situ TEM. 153,154 Catalysts that preferentially oxidized the basal-plane carbon were referred to as “pitting catalysts”, while catalysts that facilitated the oxidation of edges or steps of carbon were termed “channeling catalysts”. The movements of catalyst particles in the channels were monitored in TEM images. The particles were observed to be always on the end of the channels, in contact with the carbon materials. The activation energy determined from the channel propagation rate under different temperatures was similar to that measured by bulk techniques. Simonsen et al. used in situ TEM to observe different motions and morphology changes of carbon particles during CeO2-catalyzed soot oxidation, as well as the noncatalyzed soot oxidation by Al2O3.155 The time-lapsed series of TEM images showed that agglomerates of soot particles move toward the CeO2 and were reacted at the soot/CeO2 interface, while the soot particles were stable in the soot−Al2O3 mixture (see Figure 10). They concluded that the reaction centers for the catalytic carbon oxidation reactions were near the soot/CeO2

molybdenum carbide, molybdenum-associated coke, and the aromatic-type coke on acid sites. They suggested that the aromatic-type coke led to deactivation of the catalyst. Xu et al. quantitatively identified the surface and bulk carbonaceous species on a FePtK/SiO2 used for FTS via the combination of TPH and Mö ssbauer spectroscopy.148 The carbonaceous species, which are listed here in order of decreasing reactivity, include adsorbed atomic carbon, lightly polymerized hydrocarbon, carbidic carbon in iron carbide, disordered graphitic surface carbon, and moderately ordered graphitic surface carbon. A wide variety of techniques has been summarized above for characterizing coke deposits. The choice of techniques is dependent on the type of information needed and the cost. However, Raman spectroscopy recently has experienced great advances, and it is highly recommended to use this technique for characterizing the chemical nature of carbon. It provides not only qualitative information but also quantitative information. To evaluate the physical nature of the coke, both TEM and SEM techniques provide unique information. Taken together, these two types of techniques can help provide a tremendous amount of information.

4. ANALYTICAL TECHNIQUES EMPLOYED TO DETERMINE CARBON OXIDATION MECHANISMS The development of in situ techniques has made it possible to understand changes in the chemical structure of coke or soot during the oxidation/gasification reaction. Recent efforts have 9772


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Industrial & Engineering Chemistry Research interface. However, a key question that remained was how far the reaction centers can be away from the interface while still oxidizing the carbon effectively. The overlap of particles in TEM images made this assessment problematic. Similar movements of carbon particles toward catalysts were also observed on a Cu/BaO/La2O3 catalyst,156 a Cs2CO3/nepheline catalyst,157 and a yttria-stabilized zirconia (YSZ) catalyst.158 In contrast, the mobility of Ag was observed with in situ TEM on a Ag/SiO2 catalyst157 and a Ag nanoparticle catalyst159 for soot oxidation. In both cases, the silver particles moved toward the soot, maintaining the contact between the catalyst and soot, and eventually aggregating to form large silver particles, and deactivating the catalysts. However, the formation of agglomerated large silver particles did not happen on a Ag/ CeO2 catalyst, which is likely due to the strong interaction between supported silver nanoparticles and the CeO 2 support.156 The use of in situ TEM in soot oxidation studies has confirmed the importance of physical contacts between the catalyst and carbon by directly visualizing the reaction at the interface. The results indicate that the carbon oxidation occurs at the catalyst/carbon interface, irrespective of the catalyst used. The contact area, combined with the disappearance rate of the carbon, can be used to estimate the intrinsic reaction rate of carbon oxidation.155,157 In the future, a three-dimensional in situ TEM technique is desired to better distinguish the carbon/ catalyst and carbon/catalyst/gas interfaces, which can enable more-detailed investigations of the reaction mechanism of catalyzed carbon oxidation. 4.2. In Situ Fourier Transform Infrared (FTIR) Spectroscopy. Infrared spectroscopy provides information about the functional groups, adsorbates, and surface complexes on catalysts. In situ infrared spectroscopy has been used widely to determine the mechanism of soot oxidation by monitoring the intensity of the bands associated with various functional groups as the reaction progresses. Because of the high opacity of typical soot samples, IR spectroscopy can be a difficult technique for observing soot oxidation.160,161 However, diluent substances such as KBr can be used to achieve high-resolution spectra.161 Muckenhuber et al. used diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy, in conjunction with temperature-programmed desorption−mass spectrometry (TPD-MS), to study the mechanism of soot oxidation with NO2 as the oxidizing gas.160 Experiments were carried out at room temperature and at 400 °C. The formation of an intermediate acidic functional group was observed, which was unstable and decomposed at ∼140 °C. The NO2 molecule was bound to the surface of carbon either through the N atom or the O atom. Characteristic peaks at 1610 and 1230 cm−1 for oxygen bonding and 1330 cm−1, 1485 cm−1, 1565 cm−1, and 1305 cm−1 for nitrogen bonding were observed. The assignment of the peaks for the soot−NO2 reaction is given in Table 3. Zhang et al. observed the presence of a ketene (CCO) (2162 cm−1) and isocyanate (NCO−) (2196 cm−1) species in the catalytic oxidation of soot by NOx and O2 using K/MgAlO catalysts.161 Two parallel reactions between NO2 and soot were proposed, which happened in addition to oxidation of carbon by molecular oxygen, where CC* is a free carbon site on soot. NO2 + CC* → CCO + NO

Table 3. Assignments of Peaks for Functional Groups Resulting from C−NO2 Reactionsa

a

functional group

wavenumber (cm−1)

CO, esters CO, carboxylic acid anhydride NO, in CO−NO asymmetric NO2 in R−N−NO2 or R−NO2 asymmetric NO2 in R*−NO2 symmetric NO2 in R*−NO2 asymmetric NO2 in R−N−NO2 or R−NO2 C−O in C−ONO

1810 1785 1620 1565 1485 1330 1305 1230

Data taken from ref 160.

NO2 + CC* → NCO− + CO2

(9)

The isocyanate ion further decomposed in the presence of NO2 and O2 to form N2 and CO2 via the following reactions: O2 + NCO− → N2 + CO2 −

NO2 + NCO → N2 + CO2

(10) (11)

A similar reaction mechanism was also proposed by Zhang et al. for soot combustion on K/MgAlO mixed oxides.161 Clearly, IR spectroscopy has been useful in determining the functional groups present on carbon and, hence, deducing the reaction mechanism of carbon oxidation. In the future, this technique could be extended to investigate the gasification performance of catalysts in the presence of steam, hydrogen, or CO2. Castanõ et al.132 used FTIR spectroscopy to study the evolution of species during coke combustion on Y5 (HY ultrastable zeolite, SiO2/Al2O3-5) catalyst. The peaks belonging to aliphatic groups decreased in intensity from 100 °C and disappeared below 450 °C, while the aromatic compounds did not react until 250 °C, while continuing at higher temperatures. Nitrogen-containing bonds reacted at temperatures similar to those of the aliphatic bonds. 4.3. In Situ Raman Spectroscopy. In situ Raman spectroscopy can reveal the structural order of carbon structures formed during decoking, and the structural order can be related to the oxidation behavior of the carbons. The reaction mechanism for soot oxidation on V4/ZrO2 and V4/TiO2 catalysts (where 4 is 100 times the molar ratio of vanadium to the support) was studied using UV Raman spectroscopy and visible Raman spectroscopy.162 At room temperature, broad G- and D-peaks associated with the soot were observed. As the oxidation temperature increased, two new bands were formed at 1545 and 1618 cm−1, corresponding to aromatic species and carboxyl groups, respectively, for soot on the ZrO2 catalysts. The intensity of these bands was found to increase with reaction temperature. The formation of surface oxygen complexes was found to be easier in the presence of NO in the oxidizing gas, leading to increased activity for soot oxidation. TiO2 catalysts caused the formation of an extra peak at 1675 cm−1 attributed to vibrations of NO2. Above 300 °C, the bands at 1545, 1618, and 1675 cm−1 disappeared, indicating completion of the oxidation reaction. Raman spectra of pure soot samples during oxidation were recorded after specific intervals of time at 250 °C by Ivleva et al.163 For soot formed by graphite spark discharge, the G- and D2-peak position shifted to higher wavenumbers with increasing oxidation time. This suggested a decrease in the graphitic nature of the soot sample as the oxidation proceeded. The full width at half-maximum (fwhm) also decreased with

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Figure 11. Changes in the full width at half-maximum (fwhm) of (a) the D1-band and (b) the relative intensity of the D3-band, for soot samples subjected to different oxidation treatments. (Reproduced with permission from ref 164. Copyright 2009, American Chemical Society, Washington, DC.)

with 0.5% H2/He. Some of the surface Ce4+ in Ag/CeO2 was reduced to Ce3+. The in situ UV-vis examination of carbon on Ag/CeO2 at 600 °C in flowing helium showed that Ce4+ was reduced by carbon, which confirmed the redox mechanism on this catalyst. The Ce4+ reduction rate by H2 and the Ce3+ reoxidation rate by O2 could be estimated from the spectra. The silver nanoparticles on ceria significantly decreased the Ce4+ reduction and Ce3+ reoxidation activation energies. Corro et al. characterized silver, gold, and copper catalysts supported on fumed SiO2, both before and after reactions, with UV-vis spectroscopy.167 They found that Ag and Au remained in their metallic states after reaction, and Cu formed Cu2O during reaction, which caused the deactivation of the supported copper catalysts. Venkataswamy et al. observed a red shift of a band that was attributed to charge transfer from O2− to Ce4+, which confirmed oxygen vacancy formation after the doping of Mn and Fe to CeO2.168 From these results, it is clear that UV-vis spectroscopy is a useful technique to study coke oxidation catalysts involving changes of oxidation state. 4.5. Miscellaneous Techniques. In addition to the abovementioned techniques to study the carbon oxidation mechanism, there are several other techniques used in the literature. In situ XRD measurements were carried out by Machida et al. to observe the effect of lattice oxygen of CeO2 on soot oxidation in nitrogen flow.169 The temperature was increased during the oxidation process. At 350 °C, when the oxidation process began, the ceria peaks shifted to the lower diffraction angles. These shifts were attributed to the formation of oxygen vacancies as ceria reduced from Ce4+ to Ce3+.169 This observation led to the supposition that lattice oxygen was indeed involved in the oxidation of soot. Machida et al. used electron spin resonance (ESR) to confirm the presence of active oxygen species in the form of superoxide species formed from lattice oxygen and not from gaseous oxygen. These active oxygen species were said to be responsible for soot oxidation.169 Two mechanisms for oxidation of soot were suggested: (i) soot reacting with adsorbed superoxides at the three phase boundary and (ii) soot reacting with active oxygen species at the ceria/soot interface. Oxygen isotope exchange experiments were used to ascertain the importance of lattice oxygen in the oxidation of soot.170 Labeled oxygen was used as the oxidizing gas while unlabeled

increasing oxidation time, constituting the greater structural order and chemical homogeneity of the soot, as shown in Figure 11a.163,164 Figure 11b shows the decrease in the relative intensity of the D3-band that is attributed to the decrease in the amount of amorphous carbon. Seong et al.107 considered a combination of different firstorder Raman peaks in the form of Lorentzian or Gaussian curves to correlate the soot oxidative reactivity with the Raman parameters. The combination of four bandsthe D1-, D3-, D4-, and G-bands, with D3 as a Gaussian and the others as Lorentzianswas found to give the most consistent results. The relative intensity of D1 (ID1/IG) and the fwhm of the D1peak were observed to increase with increasing reactivity of the soot. The fwhm of the D1-peak was associated with a distribution of crystallite sizes. The D1-peak was attributed to the disordered nature of the graphitic species and increasing disorder correlated to higher reactivity of the soot. Knauer et al. found that the fwhm of the D1-peak remained constant up to 20% conversion of soot, after which it decreased slowly until it was 65% of its initial value, indicating an increase in the structural order of soot after TPO.164 Shamsi et al. used Raman spectroscopy to show that the crystallite sizes of the coke deposited on 0.61 wt % Pt-alumina were independent of the chemical process and the hydrocarbon feed.145 The G- and D-peaks were deconvoluted using Lorentzian curves and the relative intensity of the D-peak was inversely related to the crystallite sizes of the produced graphite. Low- and high-temperature coke did not show significant differences in their Raman spectra in this work. Thus, in situ Raman spectroscopy can help reveal significant chemical information about carbon species and help in investigating the mechanism of carbon oxidation. 4.4. UV-vis Spectroscopy. UV-vis spectroscopy can be used to study the electronic states of metals in soot oxidation catalysts, thus revealing the changes of oxidation states, which is helpful in understanding the catalytic reaction mechanisms. Shimizu et al. used diffuse reflectance UV-vis spectroscopy to study the change of the cerium oxidation state in soot oxidation reactions.165 The band at ∼600−750 nm corresponds to Ce3+to-Ce4+ charge-transfer interactions, which can be used to measure the reduction of ceria.166 The catalysts were preoxidized with 10% O2/He at 250 °C and then reduced 9774


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measure the kinetics of carbon oxidation. The loss of carbon in the sample with time can be fit to determine the rate constant and the reaction order. Rate constant values can further be used to give activation energies for the oxidation, assuming an Arrhenius dependence of the rate constants on temperature. Other methods, such as Redhead,174 Ozawa,175 integral,176 and differential177 methods, have also been used to estimate the activation energies and will be briefly described below. TPO studies involve heating the carbon−catalyst mixture at a specific heating rate to high temperatures in the presence of oxygen or other oxidizing gas. This study gives information on the temperature whereby the maximum rate of oxidation occurs, the ignition temperature of the sample and the temperature at which 50% of the carbon present in the sample is oxidized (denoted as T50). In such experiments, it is important to maintain a constant heating rate for accurate measurements. The activation energy obtained with this method might be a function of the degree of carbon conversion.178 Mass-transfer limitations have been observed to affect results obtained from the TGA. Bonnefoy et al. tried to eliminate these effects by using very small amounts of sample for analysis.179 These effects can be significant, especially at higher temperatures. Neeft et al. reported no effect of mass transfer on their kinetic studies as both the micropores and macropores of diesel soot particles were too small for these effects to be significant.180 They recommend the use of small diluted samples for gravimetric analyses. To assess the effect of mass transfer on the kinetic results, the mass used should be changed by a factor of 10.181 5.1.3. Shock Tubes. A shock tube is usually a metallic tube with a rectangular or circular cross-section. It consists of a diaphragm that separates gases at different pressures. The instantaneous rupture of the diaphragm causes a shock wave to pass through the tube, creating high temperatures. It is a pressure-driven device used to measure the oxidation rates of soot formed in combustion flames at very high temperatures. Soot is introduced in the shock tube chamber as a suspension, whereby it is then heated to almost-isothermal conditions. Park et al. used a laser-light transmission method to measure the disappearance of soot in the reflected shock wave region of the tube.182 Roth et al. used shock tubes with the help of scanning pulsed IR-diode laser absorption experiments to measure concentrations of CO formed in the reaction zone.183,184 5.1.4. High-Throughput Techniques. High-throughput experimentation, which involves a combinatorial method of high-throughput automated synthesis and advanced screening technologies, has been applied to accelerate the discovery of heterogeneous catalysts and materials.185 Since effective carbon oxidation catalysts often consist of two or more components, high-throughput experimentation can be a rapid and useful method to explore the vast array of compositions of carbon oxidation catalysts. Olong et al. used a combinatorial approach to discover catalysts for soot oxidation reactions.186,187 They used highthroughput synthesis by a commercial pipetting robot and the screening technique for relative heats of reaction by emissivitycorrected IR thermography. Iojoiu et al. investigated the various parameters that affected the light-off temperature of soot oxidation reaction: contact time, partial pressure of oxygen, degree of contact between catalyst and soot, and catalyst/soot mass ratio.188 They concluded that the key parameters for active soot oxidation catalysts were large amounts of surface-

lattice oxygen was maintained in the ceria lattice. When soot oxidation was performed, unlabeled CO2 was detected in the effluent gases, suggesting the importance of lattice oxygen in soot oxidation. A temporal analysis of products (TAP) reactor has been used to study the mechanism of oxidation of soot by ceria.170 In this analysis, the sample was placed under ultrahigh vacuum and small gas pulses were fed to the reactor (1017 molecules) using high-speed gas valves.171 The exhaust gases were monitored using mass spectrometers. The results revealed the significance of lattice oxygen in the oxidation of soot in the presence of ceria.172 A host of techniques have been discussed above for investigating mechanisms of carbon oxidation or gasification. The most widely used technique is in situ Raman spectroscopy, because it can give definitive knowledge of the type of carbon species undergoing change as the reaction progresses, as well as quantitative information. Other techniques, such as the use of isotopes and ESR, can provide valuable information but, at the same time, can be very expensive. To decide which techniques to employ for investigation, researchers need to weigh the options, depending on the type of information needed.

5. OXIDATION OF CARBONACEOUS DEPOSITS This section will focus on various experiments undertaken to study the oxidation of the array of carbon deposits discussed in previous sections. 5.1. Systems Used for Measuring Oxidation Kinetics. Researchers have used many systems to measure soot or carbon oxidation kinetics. Some of these systems are discussed in the following paragraphs. 5.1.1. Fixed-Bed Reactor. A fixed-bed reactor is normally a straight tube placed in a furnace. Quartz tubes are preferred over stainless steel tubes, because quartz does not catalyze carbon oxidation. Both TPO experiments and isothermal reactions can be performed in a fixed-bed reactor, which are similar to the thermogravimetric analysis (TGA) experiments, which are discussed below. Typically, a bed comprised of a mixture of soot and catalyst is placed in the reactor tube, with a thermocouple located in the center of the bed. For accurate assessments of intrinsic kinetics, reaction conditions should be carefully chosen and verified to be free of mass- and heat-transfer limitations. Conditions varied to achieve this can include the total mass of the mixture, catalyst/carbon mass ratio, catalyst bed height/diameter ratio, gas flow rate, heating rate in TPO experiments, and temperature in isothermal reactions. SiC can be used to dilute the mixture to enhance the heat transfer. Mass-transfer limitations by external and internal diffusion can be tested by the calculation of the Mears and Weisz−Prater criteria. Wagloehner et al. developed a kinetic model for Fe2O3catalyzed soot oxidation in a fixed-bed reactor.173 They concluded that there was no significant gradient of dioxygen concentration and temperature in the bed, because of the continuous flowing gas under the specified reaction conditions. 5.1.2. Thermogravimetric Analysis (TGA). A variety of experiments can be performed to study the catalytic activity of different materials using TGA to monitor the oxidation of carbon. The two most commonly used methods are isothermal oxidation and temperature-programmed reactions. Isothermal studies involve heating the carbon−catalyst mixture to a desired temperature in an inert flow and then switching to the reactive gas. These studies can be used to 9775


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Industrial & Engineering Chemistry Research active oxygen species and the number of contacts between soot and catalysts. They found nine new formulations that showed better activity than a reference catalyst: high surface area (HAS) ceria.189 These results demonstrated that high-throughput methods can be used for the fast screening of new soot oxidation catalysts. 5.1.5. Differential Mobility Analyzer. The methods mentioned above use carbon deposits collected from a flame environment. Higgins et al. have developed a method using online aerosol techniques to measure soot oxidation rates in situ.190 They have pointed out the advantages of using this method, such as eliminating the effect of aging on oxidation activity of soot particles, and the use of fresh soot samples of specific size. The oxidation rate of soot is determined by measuring the particle size of soot before and after reaction with air, using a differential mobility analyzer. 5.2. Methods Used for Determining the Activation Energy of Carbon Oxidation. The well-known Arrhenius law states the relationship between the rate constant and the reaction temperature. The equation can be expressed as follows: ⎛ E ⎞ ⎟ k = k 0 exp⎜ − ⎝ RT ⎠

−

d(rate) =0 dT

⎛ E ⎞ n dC ⎟C = k 0 exp⎜ − ⎝ dt RT ⎠

(16)

For a first-order reaction, the final equation becomes ⎛ ⎞ αE E ⎟ k 0 exp⎜⎜ − ⎟ = RT 2 RT p⎠ ⎝ p

(17)

Thus, knowing the values of the heating rate, peak temperature, and pre-exponential factor, the activation energy of a reaction can be calculated using the Redhead analysis. However, prior knowledge of the reaction order is necessary before calculation using this method. Depending on the reaction parameters available, reactivity of the carbon, and the time available for performing experiments, the type of method used to calculate the activation energy can be varied. For instance, if the reaction order is known beforehand, the Redhead method could be chosen; otherwise, either the Arrhenius method or the Ozawa method could be useful. Before using the Ozawa method, it should be established that, at peak oxidation temperature, the conversion is independent of the heating rate. Nonadherence of this precondition may lead to erroneous results. 5.3. Kinetics of Carbon Oxidation/Gasification. 5.3.1. Noncatalytic Radical Carbon Oxidation Using Oxygen as the Oxidizing Gas. To study the catalytic activity of materials in the oxidation of carbon, it is important to understand the kinetics and nature of the noncatalytic carbon oxidation as well. Screening of the catalysts can be done by comparing oxidation rates in the presence and absence of the catalyst. Catalysts with the highest difference in oxidation temperature, compared with the noncatalytic oxidation and/or activation energy for carbon oxidation, are identified as those with the highest activity. This section discusses the oxidative behavior of carbon deposits formed by radical reactions in the absence of any catalyst. Higgins et al. studied the in situ oxidation of diesel and flame soot in the presence of air.190 They concluded that the overall kinetics of oxidation of these two types of soot were not very different, which is also supported by data obtained by Neeft et al.180 The oxidation rates of flame soot and diesel soot were the same at a temperature of 760 °C, below which the rate of oxidation of diesel soot was observed to be higher than that for flame soot. This was related to the presence of catalytically active metal particles observed in soot particles originating from diesel fuel or lubricating oil. Mechanistically, both the soot samples are formed by a radical mechanism, the difference being the temperature at which they are formed. The reaction order of flame soot oxidation in molecular oxygen was found to be one and that for diesel soot oxidation was slightly less than one.180 The reaction order in carbon for flame soot was observed to be 0.7, according to the power law model. The authors reported that the reaction order was dependent on the carbon conversion; however, no correlation was determined. The activation energy for flame soot oxidation was determined to be 164 kJ mol−1. Dernaika et al.174 found the peak oxidation temperature of diesel soot to be 565 °C, with an activation energy of 164 kJ mol−1, similar to that of flame soot.

(12)

(13)

where Φ is the heating rate, B a constant that is dependent on the reaction, E the activation energy, and Tx% the temperature at which X% conversion of carbon occurs.175 The activation energy is calculated from the slope by plotting log Φ vs (1/ Tx%). The assumptions of this method involve (i) constant conversion at the oxidation peak, (ii) its independence, relative to the heating rate, and (iii) the theory that the temperature dependence of the rate constant is defined by the Arrhenius equation. Atribak et al.191 used the Ozawa plot to estimate the activation energy of noncatalytic soot oxidation and found it to be 143 kJ mol−1. The Redhead method has been recently used for estimating activation energy of carbon oxidation accurately. The reaction rate for oxidation of soot can be written as rate = −

(15)

The rate of reaction is maximum when

where k is the rate constant, k0 the pre-exponential factor, E the activation energy, T the temperature, and R the gas constant. The common differential method of analysis, which involves a plot of ln k vs 1/T, would yield a slope of −E/(RT), which can be used to calculate the activation energy. The Ozawa method for determining activation energy is a nonisothermal isoconversion method. The principle of this method is that the reaction rate at a constant conversion is a function of temperature alone. The equation used relates the heating rate used for oxidation experiments to the oxidation temperature at a particular conversion value: ⎛ E ⎞ log Φ = B − 0.4567⎜ ⎟ ⎝ RTx% ⎠

⎛ E ⎞ n k dC ⎟C = 0 exp⎜ − ⎝ RT ⎠ dT α

(14)

where C is the concentration of the reactive species and n is the reaction order, with respect to this species. If the temperature can be written as a function of the heating rate, T = T0 + αt, where t is time, then eq 14 can be written in the following form: 9776


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Industrial & Engineering Chemistry Research Table 4. Kinetics Parameters Reported in the Literature for the Noncatalytic Oxidation of Carbon reaction order type of carbon

method

reaction conditions

amorphous carbon carbon black flame soot Printex-U Printex-U Printex-U diesel soot diesel soot Printex-U diesel soot flame soot diesel soot Printex-U diesel soot Printex-U Degussa AG soot from gas oil burner amorphous carbon

Ozawa Ozawa TEM 10% Ozawa 50% Ozawa 50% Ozawa Redhead Redhead Differential Differential Differential Differential Differential Differential 50% Ozawa Horowitz Arrhenius Ozawa

21% O2 15% O2 21% O2 21% O2 21% O2 21% O2 21% O2, 5 K/min 21% O2, 2 K/min 21% O2 21% O2 21% O2 3−25% O2 3−25% O2 21% O2 21% O2 21% O2 21% O2 21% O2

in carbon

0.65−0.8 0.56 0.7

0.26

dm = ωA dt

ω=

ρ (r0 − rt ) t

Ea (kJ mol−1) 159 163 148 159 143 151 164 158 168

1 0.61 0.71

137 132 160 156 164.1 129.6 159

reaction rate (× 10−7 gcarbon s−1)

ref

138.6 (575 °C) 227.5 (600 °C)

203 204 205 206 206 172 174 174 180 180 180 177 177 207 208 209 210 211

28.62 (500 °C) 136.59 (550 °C) 265 (575 °C)

0.34 0.64 0.77 6.6

(455 (494 (514 (520

°C) °C) °C) °C)

the size of the individual particles was taken into account. The size of the soot particles was measured using light scattering methods. The authors claimed that neglecting oxidation of soot under fuel-rich flame conditions underestimated the oxidation behavior. Puri et al. used laser-induced fluorescence to measure the concentration of OH radicals during soot oxidation199 and the concentration was reduced significantly in the presence of soot particles. Evidently, a higher concentration of soot led to an increased collision efficiency for OH radicals. CO is more reactive than soot; however, the presence of large concentration of soot suppresses CO oxidation. In laminar flames, oxygen diffusion allows for the oxidation of soot particles, CO, or hydrogen. Feugier studied a fuel-rich flame system and calculated the activation energy for soot oxidation to be 138 kJ mol−1.200 Fragmentation of soot particles is another mechanism by which oxidation can occur, in addition to surface reactions with OH or O2. In this mechanism, the oxidizing species can penetrate the soot particles, causing a breakdown of big particles into smaller ones. Neoh et al. showed that, in lean flames, after a burnout of 80% of the soot, the breakdown of the soot particles occurs.201 This phenomenon has been attributed to the internal burning of particles by O2. This breakdown is absent in rich flames where the major oxidant is OH radicals. OH is more reactive than O2 and causes less internal burning. Fragmentation of particles can happen two ways: via the breakdown of aggregates or of a single particle.202 In the case of aggregates, the fragmentation occurs at the contact points of the primary particles. For single particles, the fragmentation is dependent on the internal structure of the particle. The rate of fragmentation can be related to the oxidation rate, since oxygen causes the fragmentation. This model was used to accurately predict the burnout of particles in nonpremixed flames.202 Table 4, as well as Table S1 in the Supporting Information, list the activation energies and reaction orders, with respect to oxygen and carbon during the oxidation of various types of carbon materials. The activation energy values vary over a range of 130−170 kJ mol−1. An activation energy of 100 kJ mol−1 has been associated with the dissociative chemisorption of oxygen on carbon sites.172 The reaction order in oxygen for low-

López-Suárez et al. showed that the noncatalytic oxidation of soot yielded mainly CO, indicating incomplete oxidation of the carbon.192 The selectivity toward CO2 did not change when NOx/O2 was used instead of air for oxidation. However, the onset temperature for the noncatalytic soot oxidation decreased in the presence of NOx/O2. For surface reactions between oxygen and carbon, a shrinking core model has been used for analysis of the oxidation data. The model is given by −

in oxygen

(18)

(19)

where A is the external surface area of the particle, ω the rate of mass loss (g/(s cm2)), m the mass burned in time t, r0 the diameter of the particle at time t = 0, and rt the diameter at time t. The density of most carbons is in the range of 1.8−2.0 g/ cm3.193 In this model, the rate of combustion is proportional to the surface area of the carbon.194 The total surface area of a particle shrinks slower than the volume, leading to an increase in the specific surface area. The reaction order, with respect to carbon, for this model, is 0.67.177,195 Burning of carbon follows a shrinking core model at high temperatures (>800 °C), whereas at low temperatures, the oxidation differs according to the reactivity of the carbon species. Darcy et al. found a reaction order of 0.5 under 5%−20% O2 in the temperature range of 500−560 °C in the conversion range of 15%−90%, and claim it to be close to the value of the shrinking core model.196 According to a few researchers, OH radicals instead of molecular oxygen, are responsible for soot oxidation. Fenimore et al. found that the oxidation rate of flame soot was independent of the partial pressure of oxygen.197 They claimed that the oxidation of soot involved OH radicals, rather than molecular oxygen. Neoh et al. published a similar result and proposed that oxidation by OH radicals was of primary importance and O2 was of secondary importance.198 The collision efficiency of soot with OH radicals was calculated to be 0.28 if the particles were assumed to be spherical and 0.13 if 9777


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likely the most important. Much attention has been paid to the contact conditions in evaluating catalyst performance. However, the importance of catalyst/carbon mass ratios is often neglected, thus leading to questionable conclusions. 5.3.2.1.1. Carbon−Catalyst Contact Conditions. Because of the nature of the solid−solid−gas reactions that occur for catalyzed carbon oxidation with an oxidizing gas, the contact between the carbon and the catalyst is very important. During the early studies of diesel soot oxidation, Neeft et al. defined two types of contact between the carbon and the catalyst: tight contact and loose contact.212 The loose contact mixture was obtained by mixing with a spatula, while the tight contact mixture was achieved by mixing the catalyst and carbon in a mechanical mill for an extended time. A loose contact mixture can also be obtained through other methods, such as filtration with a soot aerosol, shaking in a sample bottle, and dipping in a soot dispersion.152 The catalysts that are in tight contact with carbon have higher oxidation activity than those in loose contact conditions.213 In fact, the activity ranking of an array of catalysts may be different under tight contact and loose contact conditions. Shimokawa et al. compared the carbon oxidation activity over TiO2- and CeO2-supported silver and potassium catalysts.214 Supported silver catalysts were more active under tight contact conditions, and supported potassium catalysts were more active under loose contact conditions. Thus, the carbon oxidation activity of different catalysts should be compared under the same contact conditions. The contact of soot with diesel particulate filters under practical conditions resembles loose contact conditions.151,152 However, to achieve reproducible results and compare the carbon oxidation activity of different catalysts, the experiments should be performed with samples under tight contact conditions. Therefore, most researchers have used tight contact samples for the evaluation of catalysts. Clearly, to improve the catalytic oxidation activity, good contact of the carbon and catalyst is desired. One method to achieve good contact of the carbon and catalyst is to use a liquid-phase catalyst at reaction temperature, such as a eutectic salt mixture with a low melting point. Jelles et al. observed a significant increase in carbon oxidation activity over CsVO3− MoO3 and Cs2MoO4−V2O5 molten salts at 347 °C or higher and attributed it to the melting of these salts, which resulted in the wetting of the soot by a liquid-phase catalyst.215 These two salts also showed high stability at 752 °C in air. Despite their high activity for carbon oxidation and high stability at elevated temperature, their practical applications for diesel soot oxidation are limited due to the liquid phase transformation at reaction temperatures, making the deposition of molten salts on proper supports desired. CsVO3−MoO3 and Cs2MoO4− V2O5 molten salts were deposited on α-alumina, γ-alumina, cordierite, diatomaceous earth, silica, silicon carbide, and silicon nitride.216,217 These catalysts showed similar carbon oxidation activity as their corresponding molten salts under loose contact conditions and the highest soot oxidation rate obtained was similar to that of the best catalytic fuel additives (a combination of cerium and platinum). Considering both the reactivity and stability, Van Setten et al. concluded that an ideal support should meet three requirements: (i) the molten salts should have an affinity for the support to wet it; (ii) the wetted support should enforce a stable liquid distribution; and (iii) the supports should stabilize the molten salts in such a way that, even after prolonged heating, the molten salts should remain

temperature oxidation of soot has been widely accepted to be nearly one.180 However, some researchers do report a lower value of the reaction order in oxygen.177 The reaction order in carbon has scattered values in literature due to the different sources of carbon and varied reaction conditions used by different researchers. 5.3.2. Catalytic Oxidation of Radical Carbon (Soot) Using Oxygen as an Oxidizing Gas. Although there is an abundance of literature on catalyzed carbon oxidation, most research reports qualitative observations rather than quantitative ones. Most results are obtained from TPO reactions, in which specific temperatures of oxidation are reported and compared. The most common characteristic temperatures used are T50 (the temperature at which 50% of the carbon is oxidized) and Tmax (the peak temperature of the TPO curve). The wide variety of reaction conditions, including variations of carbon type, catalyst/carbon mass ratio, heating rate, and oxygen concentration, make comparing catalyst performance difficult. The oxidation temperature of noncatalytic carbon oxidation is often reported to be different in the literature, even when the same carbon and similar reaction conditions are applied. The noncatalytic oxidation temperature, T50, in different reports varies between 586 °C and 651 °C. Therefore, one may surmise that the relative decrease of the oxidation temperature of the catalyzed reaction, compared with the noncatalytic reaction, may better represent catalyst activities than the absolute catalyzed carbon oxidation temperature. In this review, the type of temperature will be reported, along with its decrease with the use of catalysts, in parentheses. Despite the relatively few efforts reported in the literature to obtain quantitative reaction rates from isothermal reactions, one can calculate reaction rates from TPO results reported in the literature when enough information is provided. In this review, to make comparisons easier, the rates were calculated at 300, 350, 400, and 450 °C. These rates are based on the total surface area of the catalysts, which may be used for simple reaction modeling. The comparison of reaction rates is complex for catalyzed carbon oxidation reactions. The most important parameter to use as a reference for quantitative comparisons is likely the noncatalytic reaction temperature. A small shift in the TPO curves can cause a large change in the calculated reaction rates. Ideally, these changes are associated with variations in the use of a well-characterized catalyst, and not other factors. Fortunately, the parameter that can likely cause the biggest variations, the nature of the carbon, is not as significant a problem as one might predict, as most researchers have used Printex-U as a model carbon, which enables the comparisons made here. When considering other carbons, a useful reference point is the noncatalytic oxidation temperature. If it is the same in different reports, the comparison of reaction rates between the reports might be deemed reasonable. In this section, two important parameters in catalyzed carbon oxidation, the contact between the catalyst and carbon and the catalyst/carbon mass ratio, are discussed first, followed by a review of the different types of catalysts used for catalytic carbon oxidation reactions. Ceria-based catalysts are summarized first, including CeO2 and modified (by various elements) CeO2, followed by discussion of non-ceria-based catalysts, including single component catalysts, multicomponent catalysts, and perovskite-type catalysts. 5.3.2.1. Experimental Conditions. Among the various experimental conditions, the contact conditions between carbon and catalyst and the catalyst/carbon mass ratios are 9778


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carbon oxidation activity for different oxide catalysts, as shown in Table 5.224 The general trend is that a higher catalyst/carbon

accessible for carbon (e.g., low-porosity materials are required).216 Efforts have also been made to study the contact between carbon and catalysts during the oxidation reaction. Bassou et al. evaluated ceria/soot contacts through temperature-programmed experiments (TPEs) and successive adsorption of O2.218 In a typical experiment, the mixture of ceria and carbon was first heated in helium from 27 °C to 827 °C, with the products CO and CO2 tracked. The mixture was then cooled to 23 °C in helium, followed by a switch from He to O2 for ceria to adsorb O2. The consumption of O2 was quantified and used to represent the amount of oxygen transferred from ceria to carbon. Successive TPE/O2 adsorption cycles were repeated on the same sample to mimic the progressive oxidation of carbon. They found that the amount of oxygen transferred remained constant and concluded that the contact area remained constant during the reaction. These results are consistent with conclusions from the ceria-catalyzed carbon oxidation studied with in situ TEM by Simonsen et al.155 Besides tight and loose contact conditions, mixtures with full contact conditions were obtained by impregnating carbon with the aqueous alkali metal (sodium, potassium, cesium) and alkaline-earth metal (magnesium, calcium, barium) acetate solutions.219,220 These mixtures were heated at 500 °C in helium to turn the salts to oxides, followed by carbon oxidation tests. The intrinsic reactivity of the alkaline and alkaline-earth metal oxides was thus determined under full contact conditions, showing higher activity than the tight contact mixtures. There was a good correlation between the electronegativity of these elements and their carbon oxidation activity: the lower the electronegativity, the higher the activity. Although the contact conditions can significantly affect the carbon oxidation activity, the direct contact between soot and catalyst may not be necessary as long as the catalyst has high intrinsic activity and the materials between them allow the diffusion of active oxygen species. Yamazaki et al. separated the catalyst layer and carbon particles with an ash layer of either alumina or calcium sulfate.221 CeO2 did not show any catalytic activity while the Ag/CeO2 (CeO2 supported Ag catalyst) and CeO2−Ag (silver nanoparticles surrounded by aggregated of CeO2 particles) catalysts showed remote carbon oxidation activity, even when the ash layer thickness was >50 μm. Based on results from 18O/16O isotopic exchange reaction and ESR techniques, they proposed a reaction mechanism in which the superoxide ion (O2−) species generated on the catalyst surface migrated to the ash layer and then to the carbon particles, where they oxidized the carbon. While diesel soot oxidation reactions have been extensively studied, the interest in catalytic coating materials for steam crackers is growing, whereby there is intimate contact between the catalyst and carbon through in situ coke formation. Obeid et al. deposited carbon on YSZ powder by propylene cracking, which showed higher oxidation activity than the tight contact mixture, confirming the importance of contact area for catalytic carbon oxidation reactions.222 Mahamulkar et al. deposited coke directly on catalytic powders by ethylene pyrolysis in a specialized TGA, where the coke oxidation properties could be modified by thermally aging the coke.223 The in situ coke− catalyst contact was close to the tight contact conditions. 5.3.2.1.2. Catalyst/Carbon Mass Ratio and Ef fects of Catalyst Surface Area. The catalyst/carbon mass ratio of samples is a critical parameter for carbon oxidation studies. Neeft explored the influence of catalyst/carbon mass ratio on

Table 5. Influence of Catalyst/Printex-U Ratio on the Combustion Temperature for Co3O4, CuO, MnO2, and V2O5 Catalystsa Combustion Temp, Tmax (°C) catalyst/Printex-U/SiC ratio

Co3O4

CuO

MnO2

V2O5

8/2/50 4/2/54 2/2/56 1/2/57

375 403 437 471/561b

490 497 495 566

403 457 525 562

404 411 429 456

a Adapted from the thesis work of J. P. A. Neeft.224 bDouble peak; the first peak is smaller than the second peak.

mass ratio resulted in a lower combustion temperature. However, the relationship varied for different catalysts: the combustion temperatures were almost the same with a catalyst/ carbon ratio of >1 for CuO; the combustion temperature continued to decrease when the catalyst/carbon ratio increased for other oxides. Neeft et al. further increased the catalyst/soot ratio up to 50 for a V2O5 catalyst, showing that the combustion temperature was independent of the ratio when the ratio was >4. Peralta et al. also investigated the effects of catalyst/carbon mass ratio over a K/CeO2 catalyst in carbon oxidation with TPO experiments and found that the oxidation temperature did not change for catalyst/carbon mass ratios of >10, while the oxidation temperature increased for a catalyst/carbon ratio of 5.225 At a low catalyst/carbon ratio, it is likely that only a fraction of the carbon particles are in contact with the catalyst particles. A higher ratio can increase the number of carbon particles that are in contact with catalysts, leading to higher overall carbon oxidation rates and lower observed carbon oxidation temperature. Carbon is then saturated with catalysts at a certain catalyst/ carbon mass ratio, and the oxidation temperature does not decrease with further increases in catalyst amount. In the coke oxidation literature, a variety of catalyst/carbon mass ratios was used, with the most common values ranging from 4 to 20. One fixed ratio was normally used for all of the catalysts in a certain study and it was not tested whether the catalyst/carbon mass ratio was high enough. Therefore, the catalyst activities may be compared under different conditions: carbon may not be saturated by the catalyst for some samples but is saturated by the catalyst in other samples, leading to inaccurate conclusions. Carbon oxidation activity is highly dependent on the contact area of catalyst and carbon. Therefore, the total catalyst/carbon surface area ratio may be a better parameter than the catalyst/ carbon mass ratio. Catalysts with different surface area are likely to saturate soot particles at different catalyst/carbon mass ratios, which are the cases that have been discussed above. One may think about an extreme case of a catalyst with low surface area and a carbon with high surface area, in which coke may not be saturated with the catalyst, even when the catalyst/carbon mass ratio is high. Therefore, the carbon oxidation activities obtained from these samples may be misleading. The total catalyst/carbon surface area ratio is a function of catalyst/carbon mass ratio and the surface area of catalyst and carbon. The effects of catalyst surface area were studied by many researchers. It has been shown that the oxidation temperature decreases as the surface area of catalysts increases.206,226−229 However, the effects of catalyst surface 9779


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5.3.2.2. Ceria-Based Materials for Carbon Oxidation. 5.3.2.2.1. Cerium Oxide. Cerium-based oxides are used in the three-way catalytic converter for the elimination of CO, hydrocarbons, and NOx in engine exhaust gas, because of their excellent oxygen storage capacity.234 These oxides have also gained much attention for diesel soot oxidation. Early studies used cerium salts as fuel additives in diesel engines. Cerium oxides were formed in the combustion process and entered the soot particles, which significantly lowered the ignition temperature of the soot and increased the oxidation rates by 20-fold.235,236 However, the exhaust of cerium nanoparticles poses potential environmental, health, and ecological effects, which must be taken into consideration, disfavoring the use of these species as fuel additives.237 Bueno-López et al. studied the CeO2-catalyzed soot oxidation reaction with labeled oxygen in an advanced TAP reactor.170 They found that the oxygen from CeO2 reacted with soot, and the direct reaction of the gas-phase oxygen with soot did not occur under the conditions used. They defined such oxygen as “active oxygen”, which was induced from the chemisorption of gas-phase dioxygen on CeO2. Such “active oxygen” may be oxygen superoxide ions (O2−), which were detected by ESR169 and FTIR experiments.238,239 However, these measurements were not performed under in situ soot oxidation reactions. DFT calculations have also confirmed the formation of superoxide and peroxide ions on ceria-based catalysts.240,241 Machida et al. concluded that active oxygen could be formed by either the lattice oxygen at the CeO2/soot interface or gas-phase oxygen adsorbed at the boundary of soot + reduced CeO2 + the gas phase, with the former being much more important.169 The active oxygen was not only localized at the CeO2/soot interface, but also transferred to the soot surface by spillover through surface diffusion.242 However, the distance of spillover for active oxygen species was still not clear. The active oxygen species then reacted with soot to form surface oxygen species, followed by their decomposition to form CO and CO2. Figure 12 shows a schematic depiction of the proposed active oxygen mechanism for CeO2-catalyzed carbon oxidation. Shape-dependent activity of ceria has been observed in soot oxidation.226,243Aneggi et al. synthesized ceria nanocubes displaying (100) surfaces, ceria nanorods with a mixture of (100), (110) and (111) surfaces exposed, and conventional

area may not be as significant as described in those studies. First, a large increase of oxidation temperature was observed over catalysts with lower surface area (50 nm). One interesting type of catalyst for carbon oxidation are three-dimensionally ordered macroporous materials. Threedimensionally ordered macroporous LaFeO 3 230 and Ce1−xZrxO2 mixed oxides231 showed higher carbon oxidation activity than their corresponding materials with conventional porosity. Such ordered materials had a uniform macropore structure with a diameter of ∼250 nm, which was large enough for carbon particles to enter. Therefore, the total catalyst/ carbon surface area ratio and the accessible surface area of a catalyst must be examined carefully in evaluating the effects of catalyst surface area. Catalysts with good activities for carbon oxidation almost always consist of a mixture of several metal elements. Discovery of new catalyst compositions is normally carried out by adding an element to a benchmark catalyst. After doping the catalysts with other elements, the changes in both the surface area and the composition complicate the evaluation of the catalysts, which might lead to questionable conclusions in some cases. Atribak et al. found that Ce0.69Zr0.31O2 (surface area = 17−19 m2 g−1) decreased the T50 value by ∼120 °C, relative to CeO2 (surface area = 2 m2 g−1).232 Reddy et al. showed that the T50 value was only 8 °C lower with Ce0.5Zr0.5O2 (surface area = 84 m2 g−1) was used, compared with CeO2 (surface area = 41 m2 g−1).233 For the above two studies, air was used as the reactant gas and the catalyst/soot mass ratio was 4. Although the cerium−zirconium mixed oxides that have been studied above had different compositions, this alone cannot explain the significant differences in the catalyst activity. It is likely that carbon was not saturated by CeO2 (surface area = 2 m2 g−1) in the study of Atribak et al., resulting in a much higher oxidation temperature than for the mixed oxide. In the study of Reddy et al., carbon was likely saturated by both CeO2 and Ce0.5Zr0.5O2, suggesting that the addition of zirconium does not significantly improve the activity for carbon oxidation. Thus, catalysts with different compositions should be compared with a high catalyst/carbon mass ratio to ensure that carbon is saturated with catalysts, especially for the low surface area catalysts. The choice of catalyst/carbon surface area ratio is also dependent on the purpose of the study. If reaction rates must be obtained, low catalyst/carbon ratios should be used to ensure that all of the catalysts participate in the reaction; if TPO experiments are performed to obtain the oxidation temperature, then the samples should be prepared with high catalyst/carbon ratios, so that the lowest oxidation temperature can be achieved.

Figure 12. Scheme describing the active oxygen mechanism for CeO2catalyzed carbon oxidation. (Reproduced with permission from ref 172. Copyright 2014, Elsevier, Amsterdam, The Netherlands.) 9780


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Industrial & Engineering Chemistry Research Table 6. Reaction Rates and Conditions for Soot Oxidation over Cerium Oxidea Reaction Rate (× 10−6 gsoot mcatal−2 s−1) catalyst

method

catalyst surface area (m2 g−1)

CeO2 CeO2 CeO2 CeO2 CeO2 CeO2−nanocubes CeO2−nanocubes CeO2−nanocubes CeO2−nanocubes CeO2−nanorods CeO2−nanorods CeO2−nanorods CeO2−nanorods CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2 CeO2

isotherm isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO isotherm TPO

93 42 42 13 13 27 27 14 14 33 33 9 9 53 53 44 44 35 35 26 26 17 17 5.6 5.6

heating rate (°C min−1)

10 10 10 10 10 10 10 10 10 10 10 10

ΔT50b (°C) 225 187 187 163 163 220 220 195 195 223 223 205 205 254 254 245 245 238 238 226 226 203 203 159 159

350 °C

400 °C

450 °C

0.48 1.2 0.92 1.6 1.8 1.4 5.0 2.1 4.0 1.6 5.0 1.8 6.2 2.1 1.2 2.4 1.3 2.9 1.1 3.6 0.97

4.8

0.57

2.7

1.0

3.5

4.8 8.7 15

ref 243 243 243 243 243 243 243 243 243 243 243 243 243 226 226 226 226 226 226 226 226 226 226 226 226

Other reaction conditions: Carbon (Printex-U), catalyst/soot mass ratio = 20, reactant gas air. bΔT50 = difference between T50 values for noncatalytic and catalytic oxidation. a

high temperature, but was surface-insensitive at low temperature. The reaction rates for soot oxidation over different cerium oxides are summarized in Table 6. There are several things one can learn from the collected results. First, the reaction rates under isothermal conditions are very close to the rates calculated from the TPO experiments, which are always within 3-fold of each other. This demonstrates that the methods used to calculate rates from TPO curves are reasonable and should be able to give a good estimate of reaction rates. Second, most of the cerium oxide catalysts decreased the T50 value by ∼200 °C, relative to noncatalytic oxidation, which confirms their high activity for soot oxidation. Third, the order of catalyst activity based on the reaction rates may be different from that based on the changes in T50. For example, the change in T50 is smaller with a decrease in surface area for the catalysts in entries 1−5 in Table 6. However, the reaction rates are in the opposite order: the higher the surface area, the lower the rate. This may simply be caused by the oversupply of catalyst area in these samples, given that the catalyst/soot mass ratio was 20. Therefore, it is more accurate to compare catalysts having similar surface areas. Fourth, both of the nanocube and nanorod cerium oxides showed higher activity than conventional cerium oxides, while their relative activity order was dependent on their surface area. 5.3.2.2.2. Modif ied Cerium Oxide. Although ceria is active for the soot oxidation reaction, it suffers from sintering under high temperatures and, thus, a loss of activity. Therefore, modification of ceria with other metal ions has been explored to improve its thermal stability, enhance the reducibility of Ce4+/ Ce3+, and improve the bulk oxygen mobility. These elements

polycrystalline ceria with the majority of the surface displaying (111) faces. The nanoshaped ceria showed higher soot oxidation activity than the polycrystalline ceria. The authors concluded that the activity order of the surface was as follows: (100) > (110) > (111).243 Such surface sensitivity may be caused by the different stability of active oxygen species on each face. Keating et al. concluded that peroxide defects on the (100) and (110) surface were more stable than oxygen vacancies under an oxidizing environment through DFT+U calculations.241 Surface transformations can occur through high temperature calcination.226,239 Aneggi et al. found that more reactive (100) and (110) surfaces were exposed, while less reactive (111) surfaces decreased after thermal aging.226 However, because of a decrease of the surface area, the overall activity of CeO2 decreased, although the specific rate increased. Shen et al. prepared CeO2 with a modified precipitation method, in which the metal salt solution was treated by HNO3 solution.244 The resulting CeO2 showed higher soot oxidation activity and became even more active after thermal aging, which can be related to the preferential exposure of more reactive (100) planes. Such surface sensitivity may also be dependent on the reaction conditions employed. Piumetti et al. compared a set of ceria-based catalysts with different textural properties (ceria nanocubes, ceria nanorods, ceria nanocubes/ZSM-5, mesoporous ceria, and a comparative mesoporous ceria prepared by solution combustion synthesis).245 The ceria nanocubes showed the highest total soot oxidation activity, while the high-surface-area ceria catalysts had lower onset oxidation temperatures. Thus, they concluded that the soot oxidation reaction over ceria catalysts was surface-sensitive at 9781


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(ΔT50 = 30 °C). More oxygen vacancies and lattice defects existed in the lanthanum-doped solid solution, which were detected by Raman and XPS experiments. The authors claimed that the introduction of lanthanum provoked the formation of active oxygen on the surface.266 Similarly, doping with hafnium (ΔT50 = 177 °C) was also more effective than doping with zirconium (ΔT50 = 64 °C).233 Such effects are due to the aliovalency of La and Hf, which leads to more oxygen vacancies. Krishna et al. studied four rare-earth (10 wt % La, Pr, Sm, Y) modified-ceria catalysts for soot oxidation.206 There was an increase in activity for La-doped (ΔT50 = 160 °C), Pr-doped (ΔT50 = 140 °C), and Sm-doped (ΔT50 = 100 °C) mixed oxides, while there were no effects derived from yttrium doping (ΔT50 = 50 °C), compared with undoped cerium oxide (ΔT50 = 50 °C) under tight contact conditions. They claimed that the increased activity was due to the increased mesopore/ micropore volume and the stabilization of the external surface area. The role of the catalyst was to enhance the “active oxygen” transfer to the carbon surface, but not to change the rate-determining step, which was suggested to be the chemisorption of the spillover oxygen on the carbon active sites to form the surface oxygen species (SOC). In another study of yttrium-doped cerium oxide, Atribak et al. showed that the soot oxidation activity increased under tight contact conditions, compared to cerium oxide (ΔT50 = 40 °C), but no improvement was observed under loose contact conditions.267 The most active composition was Ce0.99Y0.01O2 (ΔT50 = 75 °C), and the activity decreased with an increased yttrium content. This was attributed to the surface segregation of yttrium, which blocked the active cerium sites for soot oxidation. The addition of Zr to cerium yttrium mixed oxide was beneficial toward soot oxidation activity, and the most active catalyst composition was Ce0.84 Zr0.15Y0.01O2 (ΔT50 = 143 °C). The deformation of the ceria lattice caused by Zr doping favored the incorporation of yttrium, compared with ceria. In addition, zirconium promoted the surface enrichment of cerium, which helped to hinder the surface accumulation of yttrium. Malecka et al. prepared nanosized CeO2 and CeLnOx solid solutions (Ln = Pr, Tb, Lu, Ce/Ln atomic ratios 5/1) and studied their structural evolution after thermal treatment under O2 and H2.254 All three solid solutions showed improved thermal resistance in O2, but improved thermal resistance was only observed on CeLuOx in H2, compared with CeO2. Although the soot oxidation activity did not increase, the solid solutions maintained their activity during successive catalytic cycles. Aneggi et al. prepared four cerium−zirconium mixed oxides doped with rare-earth metals (La, Pr, Sm, Tb).249 However, there was no improvement of catalytic soot oxidation activity, compared with cerium−zirconium mixed oxides. A manganese-doped ceria solid solution improved the soot oxidation activity under both tight and loose contact conditions, which was attributed to increased oxygen vacancies and improved oxygen chemisorption.257 Ce0.9Mn0.1O2 had Tmax values of 365 and 500 °C for tight and loose contact conditions, respectively, while CeO2 had Tmax values of 390 and 555 °C, respectively. Highly dispersed MnOx species were formed when the Mn molar percentage was 25%. Mn2+ and Mn3+ were the two Mn species existing in the solid solution. Shan et al. observed that the solution pH during precipitation of MnOx− CeO2 affected the materials’ soot oxidation activity.248 The best catalyst was prepared at pH 4. The high soot oxidation activity was attributed to the ability to activate oxygen over the MnOx− CeO2. The addition of Ba to the MnOx−CeO2 mixed oxides

include lanthanum,206,246,247 zirconium,162,228,232,233 hafnium,233 cobalt,204 manganese,248 praseodymium,249,250 samarium,249 terbium,249 neodymium,251 iron,252 gadolinium,253 and lutetium.254 Among those parameters, surface reducibility is thought to be the most important. Mixed oxides can be prepared by various methods: coprecipitation,228 sol− gel,255−257 thermal decomposition of mixed salts,232 solid combustion,258 formation of inverse micromulsion,258 and with poly(methyl methacrylate) (PMMA) colloidal crystal templates.259 The carbon oxidation reaction mechanism of carbon oxidation is thought to remain the same after the doping of rare-earth metals, while the reaction mechanism may change after modifications with alkali metals, transition metals, and noble metals. These mechanisms are discussed in a recent review by Liu et al.260 It is well-established that the doping of cerium oxide with Zr4+ leads to the formation of solid solutions, which increases the thermal stability and reducibility and enhances the oxygen storage capacity of the oxide.261 Aneggi et al. investigated the roles of lattice/surface oxygen in cerium−zirconium oxide catalysts for soot oxidation. They found that there was an inverse correlation of total available surface oxygen and T50. Furthermore, the oxygen storage capacity was only important in the absence of gaseous oxygen.228 Hurtado et al. found that the cerium−zirconium mixed oxides prepared from (NH4)2Ce(NO3)6 showed higher activity than those made from Ce(NO3)6·6H2O.262 The cubic structure of the cerium− zirconium mixed oxides gradually turned into a tetragonal structure with an increased amount of zirconium. The ceria-rich mixed oxides showed higher catalytic activity than the corresponding zirconium-rich mixed oxides. The optimal molar fraction of Ce in the mixed oxides was between 0.7 and 0.8.228,231,262 The morphology of cerium−zirconium mixed oxides was found to affect the soot oxidation activity.263 Hierarchically porous cerium−zirconium mixed oxides were synthesized with a biotemplate method: pine sawdust was impregnated with a solution of cerium and zirconium nitrates and the dried sample was calcined at 600 °C to give a mixed oxide with predictable porosity. The resulting mixed oxides showed higher soot oxidation activity than the corresponding ones made from a coprecipitation method, which was attributed to higher amount of mobile lattice oxygen, as characterized by the lower temperature peak in temperature-programmed reduction in gaseous hydrogen (H2-TPR) and the higher oxygen storage capacity. La-doped CeO2 showed higher soot oxidation activity than CeO2 (ΔT50 = 25 °C) under tight contact conditions, with the best catalyst having 5 wt % La (ΔT50 = 200 °C). The promotion effect was related to the increase of surface area and the enhancement of the sample’s redox properties.247,264 However, as discussed in the previous section, it is difficult for catalysts with low surface area to saturate the carbon. This was likely the case in this study, because of the low catalyst surface area of CeO2 (∼2 m2 g−1); therefore, the dramatic increase in activity after lanthanum doping may be related to both physical and chemical changes in the catalyst. The lanthanum on the surface of CeO2 could also improve the soot oxidation activity by introduction of a carbonate pathway.265 Katta et al. compared zirconium- and lanthanum-doped ceria solid solutions for soot oxidation and found that lanthanumdoped ceria solid solutions (Ce/La = 2/1, ΔT50 = 165 °C) gave much higher activity than the zirconium-doped ceria solid solutions (Ce/Zr = 1, ΔT50 = 110 °C) and undoped ceria 9782


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with ceria and cerium−zirconium solid solutions.168 The observed activity order was as follows: manganese-doped ceria > iron-doped ceria > zirconium-doped ceria > ceria. The high activity of the manganese- and iron-doped ceria catalysts was related to the interactions between Ce−O and M−O (M = Mn, Fe). However, iron-doped ceria deactivated severely after thermal treatment, because of the segregation of Fe2O3.249 Zhang et al. proposed that the active sites in irondoped CeO2 were Fe−O−Ce species.278 They quantified the number of active sites using isothermal anaerobic titration with soot as the probe species, which enabled the calculation of the turnover frequency (TOF) for the comparison of the catalysts’ intrinsic activity. The catalyst with a Fe/(Ce + Fe) atomic ratio of 0.1 had a lower T10 temperature than other samples, while the TOF for the samples with a Fe/(Ce + Fe) ratio within 0.05−0.20 were similar. The active site density was also determined for chromium-doped CeO2 catalysts.279 Li et al. concluded that the active site was Ce−O−Ce, because cerium− chromium solid solutions did not form. The T10 and TOF values for a series of iron- and chromium-doped ceria catalysts are listed in Table 7. Iron-doped ceria catalysts showed similar ignition temperatures to those of the chromium-doped catalysts, but had higher TOF values.

increased the hydrothermal stability, which was due to the formation of BaMnO3 perovskite nanoparticles at the surface.268 Escribano et al. studied a Ce0.75Zr0.25O2 supported Mn catalyst for soot oxidation.269 The manganese species on the surface were a highly dispersed monolayer of α-Mn2O3 and carboxylic ions were identified as intermediate species during catalysis by FT-IR spectroscopy. Cobalt was not effective in forming a solid solution with ceria, but the Co3O4−CeO2 mixed oxides showed higher soot activity than the individual oxides (for CeO2, ΔT50 = 216 °C; for Co2O3, ΔT50 = 220 °C).270 The best catalyst composition was Co0.93Ce0.07O2 (ΔT50 = 271 °C). Zou et al. detected the active oxygen species (superoxide and peroxide) and carbon− oxygen intermediates (carbonyl and formate species) with in situ Raman spectroscopy during Co3O4−CeO2 catalyzed soot oxidation reactions. Harrison et al. prepared ceria-supported cobalt oxide catalysts via coprecipitation of an aqueous solution of Co2+ and Ce3+, and impregnation of a ceria gel with cobalt(II) nitrate or cobalt(II) acetate precursors. The cobalt species in the catalysts were found to be Co3O4. The material prepared by impregnation showed the highest soot oxidation activity, which could be related to its smallest Co3O4 particle size. The authors concluded that the reduction of cobalt associated with the oxygen spillover on CeO2 was the reason for the high activity observed.271 Cu-doped ceria did not form a solid solution, but the copper oxide was highly dispersed on the ceria surface. The strong interaction between dispersed copper oxide and CeO2 may enhance the rapid release of the lattice oxygen of ceria, leading to higher soot oxidation activity.257,272 As noted above, the morphology of ceria can affect the soot oxidation activity. Nakagawa et al. synthesized rod- and ellipsoid-shaped ceria with the assistance of amine surfactants as templates and used the resulting materials as supports for copper.273 The observed better soot oxidation activity than the conventional ceriasupported Cu catalysts was attributed to the enhanced surface reducibility and the improved soot contact on the shaped catalysts. Reddy et al. also found that copper could promote the soot oxidation activity of CoO/CeO2−ZrO2 and NiO/CeO2− ZrO2 catalysts.274 Rao et al. studied soot oxidation catalysts with copper supported on three ceria-based mixed oxides: CeO2−Al2O3, CeO2−ZrO2, and CeO2−SiO2.275 Various copper species were formed on the surface, such as highly dispersed CuO nanoparticles, isolated Cu2+ ions, and large particles CuO. The soot oxidation activity followed the order CuO/CeO2−ZrO2 (T50 = 338 °C) > CuO/CeO2−Al2O3 (T50 = 386 °C) > CuO/CeO2−SiO2 (T50 = 409 °C). The generation of oxygen vacancies with higher incorporation of Cu into the cerium−zirconium mixed oxide was responsible for the high activity of such samples. Muroyama studied the effects of different metal dopants in ceria for soot oxidation reaction.276 The addition of rare-earth metals (La, Nd) enhanced the activity slightly, while the addition of transition metals (Mn, Fe, Cu) significantly increased the activity. The best catalyst was associated with Cu doping (ΔTmax = 295 °C) and was attributed to the large amount of surface active oxygen species. Cousin et al. found that a Cu−V/CeO2 catalyst (ΔTmax = 199− 224 °C, CO2 selectivity (SCO2) of 94.8%−99.4%) showed both enhanced soot oxidation activity and CO2 selectivity, compared with V/CeO2 (ΔTmax = 181 °C, SCO2 = 87.4%).277 Venkataswamy et al. synthesized manganese- and iron-doped ceria solid solutions and compared their soot oxidation activity

Table 7. Ignition Temperature and Turnover Frequency for Iron- and Chromium-Doped CeO2 in Soot Oxidationa

a

catalyst

ignition temperature, T10 (°C)

turnover frequency, TOF (× 10−3 s−1)

blank CeO2 1% Fe/99% CeO2 5% Fe/95% CeO2 10% Fe/90% CeO2 20% Fe/80% CeO2 1% Cr/99% CeO2 3% Cr/99% CeO2 5% Cr/99% CeO2 Cr2O3

475 343 335 334 328 340 330 342 355 377

N.A. 2.78 3.41 3.81 3.85 3.90 2.90 2.91 2.04 1.64

Data taken from refs 278 and 279.

Gd can also form solid solutions with ceria. The doping of gadolinium (Ce/Gd = 4, ΔT50 = 179 °C) induced more oxygen vacancies and increased the soot oxidation activity, compared with undoped cerium oxide (ΔT50 = 48 °C).280 Durgasri et al. deposited the Ce−Gd mixed oxides on three supports (Al2O3, SiO2, and TiO2) and investigated the effect of the supports on the soot oxidation activity.253 The supported Ce−Gd mixed oxide catalysts showed higher activity than cerium oxide (ΔT50 = 49 °C), with the reducible support, TiO2 (ΔT50 = 188 °C), exhibiting the highest activity. The authors claimed that the better performance of the TiO2 supported catalysts was due to the strong metal oxide−support interactions. Liu et al. studied ceria-supported vanadium catalysts for soot oxidation reactions while focusing on the effects of vanadium loading.281 Isolated monovanadate at low vanadium loading gradually turned into polyvanadates, and eventually CeVO4 crystallites formed with increasing vanadium loading. The polyvanadate species were the most active for soot oxidation reactions.281 However, ceria-supported vanadium catalysts did not have higher activity than the supports themselves, including bulk and nanometer-sized cerium oxides.281 9783


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Industrial & Engineering Chemistry Research Table 8. Reaction Rates and Conditions for Soot Oxidation Reactions over Modified Cerium Oxide Catalysts: Part 1 Reaction Rate (× 10−6 gsoot mcatal−2 s−1)

Tmax (°C)

catalyst

surface area (m2 g−1)

carbon

catalyst/ carbon mass ratio


O2 (vol %)

catalytic

noncatalytic

ΔTmax (°C)

300 °C 350 °C

400 °C

450 °C

Ce0.9Cu0.1O2 Ce0.9Mn0.1O2 CeO2 8 wt % K/ MgO CeO2 8 wt % K/ CeO2 39 wt % Ag− 61 wt % CeO2 39 wt % Ag/ CeO2 39 wt % Ag + 61 wt % CeO2 39 wt % Ag/ Al2O3 CeO2 CeO2

68 72 62 13

Printex-U Printex-U Printex-U Printex-U

10 10 10 9

10 10 10 5

10 10 10 5

368 356 390 378

600 600 600 627

232 244 210 249

0.75

28 6.7

Printex-U Printex-U

9 9

5 5

5 5

426 375

627 627

201 252

1.7

15

Printex-V

19

20

10

315

668

353

30

Printex-V

19

20

10

381

668

287

50

Printex-V

19

20

10

350

668

318

13

Printex-V

19

20

10

481

668

187

3.0

9.8

290

78 45

19 20

20 10

10 10

469 393

668 668

199 275

0.60 0.93

2.0 3.5

290 169

10 wt % Ag/ CeO2 CeO2 1% Fe/CeO2 5% Fe/CeO2 10% Fe/CeO2 20% Fe/CeO2 Fe2O3 1% Cr/CeO2 3% Cr/CeO2 5% Cr/CeO2 Cr2O3

45

Printex-V carbon black (Mitsubishi) carbon black (Mitsubishi) Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U

20

10

10

345

668

323

4.3

169

9 9 9 9 9 9 9 9 9 9

5 5 5 5 5 5 5 5 5 5

10 10 10 10 10 10 10 10 10 10

395 387 383 373 392 459 377 390 400 455

600 600 600 600 600 600 600 600 600 600

205 213 217 227 208 141 223 210 200 145

1.2 0.87 1.0 1.6 0.98 0.40 1.2 0.68 0.55 0.82

299 299 299 299 299 299 279 279 279 279

51 63 64 56 53 18 68 63 48 12

8.8 4.2

ref 257 257 257 298

5.8 3.0

298 298

14

15 0.75

290 4.5 1.7

-

290 4.5

2.1

3.1 4.1

290

material showed high activity compared with the poor activity of CeO2/Al2O3 (Tmax = 520 °C).288 Neyertz et al. compared the soot oxidation activity of Ce0.65Zr0.35O2 and CeO2 catalysts supported on cordierite and studied the effect of potassium addition.289 They concluded that the activity was dependent on both the number of surface cerium sites and the homogeneity of the mixed oxides. Noble metals have higher oxygen adsorption and activation ability than many other species; therefore, noble-metal species supported on ceria materials show higher soot oxidation activity than noble-metal-free materials. Bueno-López studied ceria and 10 wt % lanthanum-doped ceria-supported platinum catalysts for soot oxidation under loose contact conditions.246 They found that the role of platinum was to increase the uptake and activation of oxygen. Shimizu et al. found that ceria-supported silver catalysts (Tmax = 270 °C) had higher activity than bare ceria (Tmax = 360 °C) and this catalyst also had good durability.165 The strong interaction between silver and ceria induced the formation of highly active surface oxygen species at the interface, thus increasing the soot oxidation activity. A unique CeO2−Ag catalyst (ΔTmax = 318 °C) with a “rice-ball” morphology was prepared and studied for soot oxidation by Yamazaki et al.290,291 The moderate to large silver particles (ca. 30−40 nm) were surrounded by a CeO2 layer ∼20 nm thick. The high formation and migration rates of the active oxygen

The doping of alkali metals in ceria catalysts increased the soot oxidation activity by improving the contact of the catalyst and soot,282 especially for the mixture under loose contact conditions.283 The sulfur poisoning resistance of cerium oxide and manganese− cerium mixed oxide was improved after the addition of potassium.282 Liang et al. doped different alkali metals (15 mol %) onto Ce0.7Zr0.3O2 and found that the promotional effect ranked in the following order: K (Tmax = 375 °C) > Na (Tmax = 450 °C) > Li (Tmax = 462 °C).284 Gross et al. studied ceria (ΔTmax = 60 °C) and ceria-supported KNO3 catalysts (ΔTmax = 195 °C) and concluded that the reaction of KNO3 with C may form adsorbed NO2 species, as confirmed by FTIR spectroscopy.285 Peralta et al. studied the stability of a series of Ba,K/CeO2 catalysts with high-temperature treatments and the presence of water or sulfur dioxide during catalyst synthesis. They found that this type of catalyst showed high thermal stability and water resistance at low temperature, but deactivated rapidly in high concentrations of SO2.286 The effects of potassium halides were studied by Zhang et al. during soot oxidation.287 The addition of all four potassium halides led to higher soot oxidation activity, with the promotional order of KF ≈ KCl > KBr > KI, based on the ignition temperature of the soot. Zhang et al. used a water-immiscible solvent to deposit a K/CeO2 catalyst onto a porous Al2O3 filter substrate by onetime coating. The resulting K-CeO2/Al2O3 (Tmax = 400 °C) 9784


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Industrial & Engineering Chemistry Research Table 9. Reaction Rates and Conditions for Soot Oxidation Reactions over Modified Cerium Oxide Catalysts: Part 2

Reaction Rate (× 10−6 gsoot mcatal−2 s−1)

T50 (°C)

catalyst CeO2 CeLaOx (10 wt % LaOx) CePrOx (10 wt % PrOx) CeSmOx (10 wt % SmOx) CeYOx (10 wt % YOx) CeLaOx (10 wt % LaOx) CePrOx (10 wt % PrOx) CeO2 CeO2−NC CeO2−NR 50 wt % CeO2−NC/ ZSM-5 CeO2-mesoporous CeO2 (solution combustion synthesis) Ce0.69Zr0.31O2 Ce0.50Zr0.50O2 Ce0.8Hf0.2O2 CeO2

carbon



O2 (vol %)

catalytic

noncatalytic

ΔT50 (°C)

2 16

Printex-U Printex-U

4 4

15 15

21 21

576 459

618 618

42 159

9.3

28 47

242 242

11

Printex-U

4

15

21

484

618

134

9.2

37

242

5

Printex-U

4

15

21

520

618

98

40

242

2

Printex-U

4

15

21

572

618

46

23

242

48

Printex-U

4

15

21

425

618

193

1.8

11

242

44

Printex-U

4

15

21

422

618

196

2.7

13

242

57 4 4 425


4 19 19 19

15 5 5 5

21 10 10 10

456 400 416 439

618 620 620 620

162 220 204 181

1.2 1.4 1.6 0.018

5.0 20 13 0.042

242 245 245 245

75 69

Printex-U Printex-U

19 19

5 5

10 10

464 476

620 620

156 144

0.028

0.12 0.15

0.45 0.38

245 245

17 84 78 41


4 4 4 4

10 10 10 10

21 21 21 21

455 514 409 522

613 586 586 586

158 72 177 64

7.2 0.35 3.7 0.85

33 0.60

232 233 233 233


350 °C

0.93

400 °C

450 °C

1.8

ref

formed in the catalyst with a zirconium-rich ceria−zirconia support. Improved soot oxidation activity was also observed on Ce0.4Zr0.6O2-supported Ru (0.5 wt %) catalysts (ΔTmax = 256 °C), compared with Ru-free Ce0.4Zr0.6O2 (ΔTmax = 170 °C).296 Deposition of zinc and cerium mixed-oxide-supported Ru catalysts onto cordierite reduced the soot emissions by 80% in a diesel stationary motor.297 Tables 8 and 9, as well as Table S2 in the Supporting Informaiton summarize the reaction rates and conditions for soot oxidation reactions over modified cerium oxide catalysts from literature reports that contain enough information to calculate rates, including ceria-based mixed oxides and catalysts using ceria-based oxides as supports. The characteristic temperature reported is Tmax in Table 8 and T50 in Table 9 and Table S2. The results in the tables are so complex that direct comparisons among different catalysts are difficult. Generally, the amount of catalyst in the soot samples does not affect the decrease of the oxidation temperature as long as there is sufficient catalyst present, relative to soot. The catalyst/ carbon mass ratio used in the literature is normally between 4 and 20, which may often meet this requirement when the catalyst surface area was large. The results in the tables, as well as those discussed above, highlight the superior activity of cerium oxides modified by silver, ruthenium, gold, copper, cobalt, manganese, and potassium. Catalysts containing silver decreased the oxidation temperature (Tmax) by ∼300 °C, especially the 39 wt % Ag−61 wt % CeO2 catalyst with a “rice ball” structure, which decreased Tmax by 353 °C and revealed a reaction rate of 15 × 10−6 gcarbon mcatalyst−2 s−1 at 300 °C. However, these catalysts may not be stable after hightemperature thermal treatments. The transition metals may segregate to form metal oxides, while catalysts may lose

species at the interface led to high soot oxidation activity, regardless of the contact conditions. Using DFT calculations, Preda et al. found that the presence of isolated Ag atoms and clusters with five Ag atoms on the CeO2 surface promoted the oxygen vacancy formation.240 Wei et al. synthesized threedimensionally ordered macroporous Ce0.8Zr0.2O2 supported Au catalysts [Au/(Ce + Zr) = 0.005−0.04, ΔTmax = 238 °C] with Au particle sizes of 2−3 nm.259 The high soot oxidation activity observed was attributed to both better soot−catalyst contact in the uniform macroporous support and strong adsorption and activation of oxygen by Au. Ce1−xRuxO2 (x = 0.03−0.16) solid solutions were prepared using a water-in-oil microemulsion method.292 The doping of Ru significantly enhanced the surface reducibility and thus increased the soot oxidation activity. Ru (ΔTmax = 186 °C) had a higher promotional effect than Rh (ΔTmax = 172 °C) and Pd (ΔTmax = 169 °C) over the undoped cerium oxide (ΔTmax = 135 °C). Phase separations occurred at different temperatures depending on the composition, with Ce0.97Ru0.03O2 being stable even after heating at 800 °C. Aouad et al. found that ceria-supported Ru catalysts lowered the soot combustion temperature under loose contact conditions by 122 °C, compared to that observed with a ceria catalyst, while the decrease of oxidation temperature of the ceria catalyst was only 17 °C.293 The higher activity was attributed to the weak Ru− O−Ce bonds, which increased the oxygen mobility and reducibility of the ceria. It is interesting that the existence of 6000 ppm propylene in air increased the soot oxidation rate 4fold with a 1.5 wt % Ru/CeO2 (ΔTmax = 256 °C) catalyst.294 Homsi et al. found that the soot oxidation activity over a Ru/ Ce1−xZrxO2 catalyst (ΔTmax = 35−135 °C) increased as the cerium content increased, which was attributed to the larger amount of Ru−O−Ce species produced.295 RuOx clusters were 9785


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Industrial & Engineering Chemistry Research Table 10. OSC Values for Different Catalytic Materials Relevant to Soot Oxidation catalyst

calcination temperature (°C)

temperature of OSC measurement (°C)

OSC (μg O2 gsample−1)


ref

Ce0.28Zr0.72O2 Ce0.44Zr0.66O2 Ce0.75Zr0.25O2 CeO2 Ce0.28Zr0.72O2 Ce0.44Zr0.66O2 Ce0.75Zr0.25O2 CeO2 CeO2 Ce0.75Zr0.25O2 Ce0.80Zr0.20O2 Ce0.15Zr0.85O2 Ce0.50Zr0.50O2 Ce0.68Zr0.32O2 Ce0.80Zr0.20O2 CeO2 30 mol % CeO2−Al2O3 30 mol % CeO2−Al2O3

500 500 500 500 700 700 700 700 500 500 500 600 600 600 600 600 600 800

400 400 400 400 400 400 400 400 400 400 400 377 377 377 377 377 600 600

1585 2944 1947 552 1092 1543 1196 68 640 2592 3600 444.8 1067.2 1156.8 1147.2 1057.6 1100 835

90 90 73 53 59 53 49 35 41 84 66 92 105 99 108 97

228 228 228 228 228 228 228 228 266 266 266 304 304 304 304 304 302 302

optimum composition of ceria versus zirconia that exhibits the highest OSC. This optimum has been found to be in the 50−80 mol % Ce range. The collected results are quite scattered over different papers, and it is difficult to find a correlation between the surface area of the material and the OSC values. Mamontov et al. suggested that the small nanocrystalline domains of zirconia were responsible for high OSC values and were more important than the surface area of the samples.308 However, as the calcination temperature increases, the OSC values of the catalytic materials are seen to decrease. A similar trend was also observed for ceria−alumina materials.302 Katta et al. showed that ceria−lanthana mixed oxides act as better catalysts for soot oxidation, compared to ceria−zirconia oxides, because of the their higher OSC.266 Aneggi et al. observed a correlation between the T50 value in inert atmosphere and the OSC for ceria−zirconia materials, as shown in Figure 13.228 However, in the presence of oxygen as the oxidizing gas, no correlation was found between OSC and

potassium, because of its volatility. The addition of elements such as lanthanum and zirconium is important to improve the thermal stability of ceria-based materials and maintain their soot oxidation activity under harsh conditions. 5.3.2.2.3. Oxygen Storage Capacity (OSC) of Ceria-Based Materials. The oxygen storage capacity (OSC) is the amount of oxygen released or stored in materials under fuel lean or rich conditions.300 Ceria materials have been found to show high OSC values, allowing them to be highly effective in soot or carbon oxidation, as noted above.301 The total OSC is defined as the oxygen stored under thermodynamic control. It is typically measured by performing temperature-programmed reduction (TPR) or by reducing the sample at a fixed temperature and then reoxidizing it.300,302 TGA has also been used to measure the total OSC, by measuring the weight loss of the sample under a reducing atmosphere, such as a dilute hydrogen stream.228 The observed weight loss can be attributed to the loss of oxygen, which combines with hydrogen to form water. Reddy et al. and Katta et al. used TGA to calculate the OSC under cyclic heat treatments in flowing nitrogen and dry air.266,303 They found out that the OSC of mixed oxides was higher than that of pure ceria. Ceria−zirconia mixed oxides showed the highest OSC. These experiments may not correlate directly with catalytic activity under working conditions. Hence, the idea of measuring the OSC under transient conditions emerged.304 The dynamic OSC of materials is found by injecting alternating pulses of hydrogen and oxygen in a flow reactor.300 CO pulse experiments can also be performed to estimate the OSC. In these experiments, alternating pulses of CO and O2 are passed over the catalyst.305 Temperature-programmed experiments under cyclic CO and O2 flow have also been used to measure dynamic OSC values.304 Incorporation of zirconia into the ceria lattice has been observed to increase the OSC and the thermal stability of ceria.228,304,306 At high temperatures, the OSC of ceria−zirconia materials decreases significantly due to sintering. To increase the thermal stability of these materials, alumina is sometimes introduced as a diffusion barrier.307 Table 10 gives an account of the OSC values for different catalyst samples studied by various researchers. For every catalyst synthesis, there exists an

Figure 13. Relation between oxygen storage capacity (OSC) and catalytic activity in an inert atmosphere at 400 °C. (Reproduced with permission from ref 228. Copyright 2012, Elsevier, Amsterdam, The Netherlands.) 9786


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were not active under loose contact conditions, because of their poor mobility.219 Mul et al. screened metal chlorides and oxychlorides for soot oxidation.312 Most of the studied chlorides and oxychlorides were not converted to oxides at the reaction temperature for soot oxidation. They observed a correlation between the melting points and the catalytic soot oxidation activity. They found that PdCl2, CuCl2, and CuCl were very active catalysts, reducing the oxidation temperature by 200−275 °C, which was due to their low melting points and, thus, the wetting of soot by the catalyst. The DRIFTS results showed that surface oxygen complexes were formed and that active oxygen species were transferred from the catalyst to the soot surface. Neri et al. studied soot oxidation reactions with various metal oxides and concluded that there were two combustion steps for diesel soot: one was for adsorbed hydrocarbon oxidation and the other was for graphitic carbon oxidation.313 Fe2O3 and γAl2O3 were only active for hydrocarbon oxidation, while V2O5, CuO, and TiO2 also catalyzed graphitic carbon oxidation. They observed a correlation between the strength of the metal− oxygen bond and the hydrocarbon oxidation activity: the stronger the metal−oxygen bond, the lower the hydrocarbon oxidation activity. The activity of graphitic carbon oxidation was higher on the metal oxides with low melting points. Wang et al. observed that the soot oxidation activity ranked the following order: Cr2O3 ≈ CuO > Al2O3 > SiO2 > Fe2O3.314 Neeft et al. studied various metal oxides for soot oxidation and found that the activity order was CuO > Cr2O3 > Fe2O3.213 Some of the metal oxides (Ag2O, MoO3, PbO, Sb2O3) were also very active, which may be caused by the low melting points and thus high mobility of these catalysts. Mul et al. investigated the reaction mechanisms of metaloxide-catalyzed soot oxidation by feeding pulses of 18O2 to a high-vacuum batch reactor.315 They claimed that three mechanisms existed: a redox mechanism for Co3O4 and Fe2O3, an oxygen spillover mechanism for Cr2O3, and a push−pull mechanism for MoO3, V2O5, and K2MoO4. In the redox mechanism, surface oxygen on the catalyst reacted with soot at their interface, which is then refilled by the gas-phase dioxygen. The oxygen spillover mechanism involves the activation of dioxygen on the catalyst surface, which forms adsorbed surface oxygen species, followed by their spillover to the soot surface. The push−pull mechanism is a combination of exchange of surface oxygen with gas-phase oxygen and the spillover mechanism. Wagloehner et al.316 studied the reaction mechanism of carbon oxidation on a Fe2O3 catalyst. Oxygen defects were formed by oxygen transfer from the catalyst to soot, followed by the refilling of the defects with gas-phase dioxygen, through surface oxygen migration or by bulk oxygen. The higher activity of specific Fe2O3 catalysts was observed, with catalysts with moderate crystallinity and surface Lewis acid sites being more active.317 Based on the above reaction mechanism, they developed a kinetic model for the Fe2O3 catalyzed soot oxidation in a packed-bed reactor.173 Wagloehner et al. also compared the soot oxidation activity of different manganese oxide compositions (MnO2, Mn2O3, Mn3O4, and nanosized Mn3O4 from flame spray pyrolysis).318 The soot oxidation activity could be related with the amount of surface oxygen vacancies and the particle sizes. The nanosized Mn3O4 from flame spray pyrolysis exhibited the highest activity and also showed good hydrothermal and thermal stability. The

catalytic activity. They also claimed that the soot combustion activity in the presence of air was dependent both on surface area and composition. To relate the two quantities, the number of surface active oxygens was calculated by a method devised by Madier et al.309 The calculation is based on the assumption that oxygen associated with zirconium does not participate in redox activity, all the surface oxygen on cerium is available for reaction and only one out of four Ce atoms is involved in the redox process. Total available surface oxygen is a fraction of the total surface oxygen and is given by the form total surface oxygen = ⎡⎣(surface area (nm 2/g)) × (mole fraction of Ce) × (surface oxygen (atoms/nm 2)) × 0.25⎤⎦/(Avogadro’s number)

(20)

where surface oxygen is calculated based on the average exposure of the (111), (100), and (110) surfaces of the ceria. In the presence of air, a correlation between the total surface oxygen atoms and soot combustion activity was found, as shown in Figure 14.

Figure 14. Relation between total surface oxygen and soot combustion activity for ceria-based catalysts. (Reproduced with permission from ref 228. Copyright 2012, Elsevier, Amsterdam, The Netherlands.)

Thus, measuring OSC using a multitude of techniques can be a great way to assess the oxidation activity of catalysts in the presence of an inert. In the presence of air, the ability to give away oxygen for oxidation reaction can be calculated using a theoretical quantity of total surface oxygen. 5.3.2.3. Nonceria Materials for Carbon Oxidation. Early studies on carbon oxidation focused on the screening of singlecomponent materials, including metal salts and metal oxides. An understanding of these single-component materials enables the design of better catalysts that combine the properties of different materials. The perovskite-type materials have received much attention for carbon oxidation for over a decade. In this section, three types of catalyst are discussed: single-component catalysts, multicomponent catalysts, and perovskite catalysts. 5.3.2.3.1. Single-Component Catalysts. Alkali metals have been shown to be active catalysts for soot/carbon oxidation reactions. The observed activity order was Cs > K > Na ≫ Li, under tight contact and full contact conditions.213,310,311 BaO showed similar activity as the alkali-metal oxides and the activity order of alkaline-earth metal oxides was Ba ≫ Ca > Mg under full contact conditions. However, alkaline-earth metal oxides 9787


Review

Industrial & Engineering Chemistry Research contribution of bulk oxygen in manganese oxide was more pronounced than for the iron oxide case. 5.3.2.3.2. Multicomponent Catalysts. While no singlecomponent catalyst was good enough to match the state-ofthe-art catalysts for soot oxidation, the combination of multiple elements can provide a route to more-effective catalysts. Because of their high oxygen adsorption and activation activity, supported noble-metal catalysts have shown high soot oxidation activity. The important role of the noble metals is to adsorb and activate gas-phase oxygen, and to transfer such active oxygen species to other active components, such as ceriabased materials or perovskites. The effect of supports (ZrO2, Al2O3, and CeO2) for supported silver (1−10 wt %) catalysts was studied by Aneggi et al. for soot oxidation.319 ZrO2 (ΔT50 = 288 °C) and CeO2 (ΔT50 = 288 °C) supported catalysts showed better activity than the Al2O3 (ΔT50 = 270 °C) supported catalyst. Both Ag2O and Ag existed on the catalyst surface, while the metallic silver was suggested to be the active species. More Ag2O was formed on the CeO2 support, as CeO2 could stabilize Ag2O. After thermal aging at 750 °C for 12 h, there was significant deactivation of the ceria-supported silver catalyst (ΔT50 = 248 °C), while the deactivation for the other two catalysts was negligible. Guilhaume et al. studied the soot oxidation activity over a 3.5 wt % Ag/MnOx (ΔT50 = 279 °C) catalyst with in situ XRD and DTA-TGA measurements.320 There was no structural change during the soot oxidation reaction under oxygen, but the stoichiometric soot oxidation reaction by lattice oxygen in the catalysts resulted in changes of MnOx, which confirmed its redox mechanism. Silver enhanced both the manganese reducibility and gaseous dioxygen activation on the catalyst surface. The catalytically active Ag species on an Ag/ZrO2 catalyst was determined to be Ag nanoparticles of 2−10 nm size.321 Nanba et al. used temperature-programmed reduction by NH3 (NH3-TPR) to perform quantitative analysis of the active oxygen species over Ag/ZrO2 catalysts and suggested that two types of active oxygen species were present during soot oxidation, whereas the forms of active oxygen species were not known.322 In another report, the formation of bimetallic Ru−Co particles could improve the thermal stability of Ru supported on zirconia and the soot oxidation activity was related to the content of ruthenium.208 The promotional effects of potassium were studied on various oxides, such as MgO, 323 CaO−MgO physical mixtures,324 Co3O4,325 Mg−Al hydrotalcite-based mixed oxide,299,326 and MAlO (M = Ni, Co, Cu) hydrotalcite-based mixed oxides.327 The addition of potassium improved the contact between the catalyst and soot and facilitated the decomposition of carbonates. Sun et al. claimed that potassium could enter the Co3O4 lattice (ΔTmax = 250 °C), based on the shifts of Co3O4 XRD peaks, which caused lattice distortion, weakened the Co−O bonds, and induced the formation of Co3+ species.325 The Co3+ species enhanced the oxygen adsorption and activation, resulting in increased active oxygen formation. There was an inverse relationship between the Co3+ content in Co3O4 and the soot oxidation temperature. Ogura et al. investigated the reaction mechanism of soot oxidation over an aluminosilicate-supported potassium carbonate catalyst and found that the carbonate ion acted as an electron pool and electron donor to promote gaseous dioxygen activation.328 Zhang et al. identified two catalytically active sites on a Mg−Al hydrotalcite mixed-oxide-supported potassium catalyst: Mg(Al)−O-K (tightly bound to Mg or Al) and free (isolated) K.299

They proposed an oxygen spillover mechanism for soot oxidation on this catalyst. The active sites adsorbed gas-phase dioxygen and generated surface active oxygen species, which then spilled over to the soot surface and reacted with soot to form a carbon−oxygen complex (ketene species), followed by further reaction with another active oxygen to form CO2. The strong interaction between potassium and aluminum helped to prevent the loss of potassium during reaction. Li et al. investigated the soot oxidation mechanism on potassium (8 wt %)-supported oxides [MgO (ΔTmax = 249 °C), CeO2 (ΔTmax = 252 °C), and ZrO2 (ΔTmax = 257 °C)] and identified the active sites to be free K+ rather than K2CO3.298 Ketene was found to be the reaction intermediate. Despite the high activity of alkali-metal-containing catalysts, they always suffer from deactivation during soot oxidation due to the loss of the alkali metals. To solve this problem, glasses (52 wt % SiO2−35 wt % K2O−13 wt % CaO) have been used as soot oxidation catalysts.329−331 The loss of alkali metals could be mitigated since the alkali-metal ions could diffuse from the bulk to surface, thus maintaining the concentration of alkali-metal ions on the surface and preserving the soot oxidation activity for extended periods. Hleis et al. compared the promotion effects of different alkali metals (M/Zr = 0.14, where M = Li, Na, K, Rb, Cs) over a ZrO2 catalyst during carbon oxidation. The following order was observed: Cs (ΔTmax = 242 °C) > Rb (ΔTmax = 215 °C) > K (ΔTmax = 208 °C) > Na (ΔTmax = 187 °C) > Li (ΔTmax = 116 °C).332 Doggali et al. synthesized mesoporous ZrO2 by using chitosan as a template and used it as a support for a set of transition metals (10 mol % Fe, Co, Ni, Cu, and Mn).333 Co/ ZrO2 (ΔT50 = 118 °C) was the most active soot oxidation catalyst while Ni/ZrO2 (ΔT50 = 37 °C) showed little catalytic activity. An alumina-supported Co−K−Mo catalyst lowered the soot oxidation temperature by 190 °C under loose contact conditions, exhibiting higher activity than the individual components supported on alumina.334 The molecular structure of vanadium species on supported vanadium catalysts affected their soot oxidation activity. Saracco et al. found that pyrovanadates showed much higher soot oxidation activity than metavanadates.335 The structure of vanadium species on the surface was dependent on both the support properties and the loading of vanadium. At low vanadium loading, the vanadium species on the surface were isolated 4-fold-coordinated monovanadates. The monovanadate species gradually turned into polyvanadate species and, finally, V2O5 crystallites with increasing vanadium loading.336−338 The polyvanadate species were more active than the monovanadate species and the V2O5 crystallites. Liu et al. observed the formation of surface oxygen complexes (SOCs) in soot oxidation over both ZrO2- and TiO2-supported vanadium catalysts with in situ UV-Raman spectroscopy.162 These SOC species were carboxyl groups, which were formed by the reaction of soot with the active oxygen spillover from the catalyst. The addition of Au to VOx/TiO2 and VOx/ZrO2 improved the soot oxidation activity over catalysts with low loadings of vanadium (20 °C decrease in oxidation temperature), which was attributed to the high oxygen activation ability of gold nanoparticles and the oxygen transfer from gold species to vanadium species.337 Moreover, the formation of polyvanadates was favored over V2O5 crystallites after the addition of gold at a high loading of vanadium. The doping of alkali metals can improve the soot oxidation activity over vanadium-based catalysts. Neri et al. studied K- and Cs-doped 9788


Review

Industrial & Engineering Chemistry Research Table 11. Reaction Rates and Conditions for Soot Oxidation Reactions over Multicomponent Catalysts

Reaction Rate (× 10−6 gsoot mcatal−2 s−1)

Tmax (°C)

catalyst Mg6Al2O9 2 wt % K/ Mg6Al2O9 5 wt % K/ Mg6Al2O9 8 wt % K/ Mg6Al2O9 15 wt % K/ Mg6Al2O9 NiAl hydrotalcite (Ni/Al = 3) CoAl hydrotalcite (Ni/Al = 3) CuAl hydrotalcite (Ni/Al = 3) 5 wt % K/NiAl HT (Ni/Al = 3) 5 wt % K/CoAl HT (Ni/Al = 3) 5 wt % K/CuAl HT (Ni/Al = 3) 8 wt % K/MgO 8 wt % K/ZrO2


carbon



O2 (vol %)

catalytic

noncatalytic

ΔT (°C)

116 89

Printex-U Printex-U

9 9

5 5

10 10

519 419

600 600

81 181

0.16 0.25

73

Printex-U

9

5

10

381

600

219

1.3

299

47

Printex-U

9

5

10

357

600

243

3.8

299

13

Printex-U

9

5

10

364

600

236

89

carbon black carbon black carbon black carbon black carbon black carbon black Printex-U Printex-U

20

1

5

443

0.005

0.038

327

20

1

5

430

0.016

0.098

327

20

1

5

518

0.011

0.068

20

1

5

371

0.011

0.088

327

20

1

5

357

0.030

0.23

327

20

1

5

403

0.023

0.13

9 9

5 5

5 5

378 370

35 18 58 33 17 13 0.7

Fe−V/Al2O3 catalysts for soot oxidation.339 They suggested that soot oxidation was favored on catalysts with mixed Fe−V− O phase, where there was intimate contacts between iron and vanadium species. They concluded that the Fe(II)/Fe(III) redox couple was involved in the activation of V sites. Cesiumdoped catalysts were more active and stable to thermal and sulfur deactivation than the potassium-doped ones. Zhang et al. found that the addition of cerium to a supported V/K catalyst improved the soot oxidation activity.340 There was strong CO2 absorption on both the V/K and V/K/Ce catalysts, which was due to the formation of potassium bicarbonate and the physical CO2 adsorption. Abdullah et al. studied ZSM-5 supported CuO, V2O5, and Fe2O3 catalysts for soot oxidation under loose contact conditions.341 The V2O5/ ZSM-5 catalyst had the highest activity and the addition of potassium could further improve its activity. However, a potassium silicate phase was formed from the interaction of potassium with the extra framework silica in the zeolite, which led to gradual deactivation. Alumina-supported Cu/V/K was also shown to be active for soot oxidation. The high activity came from the formation of liquid eutectic phases at temperatures between 330 °C and 480 °C, which led to the better contact between soot and catalyst.342 The redox properties of a TiO2-supported Cu−V−K-Cl catalyst were studied with TPR and TPO experiments, showing that the soot oxidation rate was determined by the rate of catalyst redution.210 An et al. studied the effect of alkali doping of CuFe2O4 and CoFeO4 catalysts for soot oxidation.343 The promotion effect on CuFe2O4 followed the order K > Cs ≈ Na > Li, while the order on CoFeO4 was K > Na > Cs ≈ Li. Braun et al. investigated the effect of supports (Al2O3, SiO2, and TiO2) on Mo catalysts for soot oxidation.344,345 Different Mo species existed on different supports: Mo2O72− species on SiO2, MoO4− species on Al2O3, and polymolybdate species on TiO2. They claimed that the dispersed Mo species were more active than the polymolybdates or bulk MoO3.344 Leocadio et al. proposed the same reaction pathway for Al2O3-supported,

627 627

249 257

300 °C

350 °C

400 °C

450 °C

ref

2.3

299 299

14

0.75 17

5.8 158

0.40

299

0.15

327

327 298 298

highly dispersed molybdenum and vanadium catalysts.346 Carbonate species were formed at the interface of molybdenum and vanadium species and the carbon from the interaction of carbon with active oxygen species, followed by the decomposition of the carbonate species to give out CO or CO2. The Mo/Al2O3 catalyst showed higher activity, which was due to the easier decomposition of the carbonate species. López-Suárez et al. studied alumina-supported copper oxide catalysts (ΔTmax = 23−55 °C) with varying copper loadings for soot oxidation under loose contact conditions.192 They attributed the activity to the easily reduced Cu(II), which increased with the loading of copper when the loading was LaFeO3 (ΔTmax = 126 °C) ≈ LaMnO3 (ΔTmax = 126 °C).361 The highest activity associated with the chromite catalyst was attributed to the high concentration of suprafacial (weakly chemisorbed) oxygen species from TPD experiments, which transferred from the catalyst to the soot by spillover. Such oxygen species were defined as α-type oxygen with a desorption temperature in the range of 300−600 °C. Wang et al. found, using in situ Raman spectroscopy, that the active oxygen species on a LaMnO3 perovskite catalyst were O22−, O2n− (1 < n < 2), and O2m− (0 < m < 1).362 Ifrah et al. observed that thermal treatment of LaCrO3 perovskites (ΔTmax = 162 °C) could lead to the formation of La2CrO6 or La2O3 phases.363 The measured order of activity for soot combustion was as follows: LaCrO3 (ΔTmax = 162 °C) ≈ 94% LaCrO3−6% La2O3 (ΔTmax = 162 °C) > La2CrO6 (ΔTmax = 156 °C) > 86% LaCrO3−14% La2CrO6 (ΔTmax = 140 °C) > La2O3 (ΔTmax = 160 °C, not all of the carbon was reacted). They concluded that both the mobility of the surface oxygen species and the presence of active Cr species (Cr3+ and Cr6+) were important for the soot oxidation activity. Xiao et al. compared the soot oxidation activity of bulk, supported, and macroporous perovskite LaFeO3 catalysts under loose contact conditions, and they found that the highest activity on macroporous LaFeO3 was related to its better contact with soot, the rich active oxygen species, and better surface reducibility.364 The partial substitution of perovskite catalysts can improve their soot oxidation activity, with the substitution by alkali metals being most widely studied. The properties of the LaCrO3 perovskites were adjusted via the doping of alkali metals (Li, Na, K, Rb) by Russo et al.203 The small Li ion substitutes the B-site in the perovskite, whereas the other alkali metals are located at the A-site, because of their relatively large size. The Li-substituted perovskite La0.8Cr0.9Li0.1O3 (ΔTmax = 242 °C) had the lowest peak oxidation temperature among an array of substituted catalysts, compared to the LaCrO3 (ΔTmax = 155 °C) catalyst. Other alkali-metal-substituted perovskites, La0.9Rb0.1CrO3 (ΔTmax = 200 °C), La0.9K0.1CrO3 (ΔTmax = 195 °C), and La0.9Na0.1CrO3 (ΔTmax = 195 °C), and the two substoichiometric perovskitesLa0.9CrO3 (ΔTmax = 203 °C) and La0.8CrO3 (ΔTmax = 209 °C)showed similar activity. They related the catalytic activity with the weakly adsorbed oxygen species and suggested that these oxygen species were O−, based on XPS results. The effects of the amount of lithium doping in the chromite catalysts were studied by Fino et al.211 The best catalyst composition found was La0.8Cr0.8Li0.2O3 (ΔTmax = 246 °C). LiCrO2 was formed when more than 20% Cr was substituted by Li. The best catalyst, La0.8Cr0.8Li0.2O3, was also lined over the walls of a flow monolith trap under practical conditions, in which shorter times and lower fuel consumption were observed in complete regeneration. Russo et al. studied LaCoO3 perovskites and their alkali-metal (Na, K, Rb)-substituted derivatives, and the promotional effect of the alkali metal followed: La0.9Rb0.1CoO3 (ΔTmax = 167 °C) > La0.9K0.1CoO3 (ΔTmax = 145 °C) > La0.9Na0.1CoO3 (ΔTmax =

species was very stable during soot oxidation at high temperature, which was attributed to the isoelectronic (having same electronic structure) interaction of Cu+ and Zn2+ at the interface. Querini et al. studied MgO-supported cobalt (12 wt % Co) catalysts (ΔTmax = 40−220 °C) for soot oxidation. They found that calcination at temperatures >400 °C led to the formation of Mg−Co mixed oxides, which were not active for soot oxidation.352 The active cobalt species were CoOx on the surface of MgO, which transferred active oxygen species to the soot. The addition of K (1.5 wt % K, ΔTmax = 250 °C) not only improved the contact between the catalyst and soot, but also enhanced the thermal stability of a CeO2 supported Co catalyst (ΔTmax = 150 °C).353 While most of the active metal oxides rely on the reducibility of the metal ions, Obeid et al. reported a nonreducible oxide, YSZ, for continuously regenerating diesel particulate filters, with the soot oxidation starting at 430 °C with oxygen as the reactant gas under realistic contact conditions.222 They proposed that the reaction had a fuel-cell-type electrochemical mechanism at the nanometric scale and the efficiency was determined by both the YSZ/soot contact and the oxygen partial pressure. However, long-term isothermal tests for 24 h lead to the formation of carbon thin films on some of the YSZ grains, which deactivated the catalyst.354 The doping of praseodymium into Zr−Nb oxides showed higher soot oxidation activity, in which the redox reaction of Pr4+/Pr3+ did not occur both under oxidative and reductive conditions.355 The enhanced activity was instead suggested to be due to increased electronic conductivity. Ishihara et al. screened a praseodymium-oxide-based catalyst with doping of alkali metals, transition metals, and rare-earth metals, and found that Pr4.8Bi1.2O11 was the most active.356 The addition of CeO2 further increased its soot oxidation activity. Harada et al. prepared modified praseodymium-rich Pr−Bi mixed oxides with various dopants, such as alkali metals, transition metals, and rare-earth metals.357 Doping with cerium, iron, and nickel increased the carbon oxidation activity, with cerium showing the lowest ignition temperature. However, the carbon oxidation activity could not be related to the oxygen exchange rate or the lattice oxygen desorption properties that were determined in the absence of carbon. Zhang et al. studied porous alumina-supported La/K, Sm/K, and Y/K catalysts for soot oxidation and observed the existence of both a metal− potassium solid solution and KNO3.358 The activity order was found to be Y/K > La/K = Sm/K. Sui et al. investigated the effect of alkaline-earth metals on the soot oxidation activity of porous alumina-supported K−Sm-based catalysts and concluded that the improved activity was likely due to the formation of a liquid phase of K−Sm−Mg.359 Table 11 summarizes the reaction rates and conditions for carbon oxidation reactions over multicomponent catalysts reported in the literature where enough information was provided to calculate rates. For most of the multicomponent catalysts in the table, their activities are not high, compared with ceria-based catalysts. Catalysts with good activity relied on the addition of alkali metals, which may deactivate during reaction, because of the loss of the alkali metal at high temperature. 5.3.2.3.3. Perovskite Catalysts. Perovskites have a general formula ABO3, where A and B represent two cations. The A cation can be a lanthanide, alkaline, or alkaline-earth cation, while the B cation can be a 3d, 4d, or 5d transition metal. The 9790


carbon amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) amorphous carbon (Cabot) Printex-EX2 Printex-EX2 Printex-EX2 Printex-EX2 Printex-EX2 Printex-EX2 Printex-EX2 carbon black carbon black carbon black carbon black


19 25 21 19 18 18 16 8 18 17 14 13 18 14 17 2 7 29 34 32 34 19 29 27 30 30

catalyst

LaCrO3 LaFeO3 LaMnO3 LaCr0.9O3 LaK0.1Cr0.9O3 LaCrO3 La0.9CrO3 La0.9Rb0.1CrO3 La0.9K0.1CrO3 La0.9Na0.1CrO3 La0.8CrO3 La0.8Li0.1Cr0.9O3 LaCrO3 La0.8Li0.2Cr0.8O3 La0.8Li0.3Cr0.7O3 Co2O3 CeO2 LaCoO3 La0.9Ce0.1CoO3 La0.8Ce0.2CoO3 La0.9Sr0.1CoO3 La0.8Sr0.2CoO3 LaMn0.9Co0.1O3 La0.9Ag0.1Mn0.9Co0.1O3 La0.8Ag0.2Mn0.9Co0.1O3 La0.7Ag0.3Mn0.9Co0.1O3

9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 19 19 19 19 19 19 19 4 4 4 4

catalyst/carbon mass ratio 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 10 10 10 10


Table 12. Reaction Rates and Conditions for Soot Oxidation Reactions over Perovskite Catalysts

21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 10 10 10 10 10 10 10 12 12 12 12

O2 (vol %) 508 524 524 500 433 495 447 448 454 455 441 408 495 404 402 455 505 485 454 435 475 486 459 429 393 370

catalytic 650 650 650 650 650 650 650 650 650 650 650 650 650 650 650 645 645 645 645 645 645 645 650 650 650 650

noncatalytic

Tmax (°C)

142 126 126 150 217 155 203 202 196 195 209 242 155 246 248 190 140 160 191 210 170 159 191 221 257 280

ΔT (°C) 300 °C

9791

1.7 6.0 15

0.15 0.27

2.3 1.8 1.7

0.63

11 10 10 0.77 0.16 0.58 0.95 0.25 0.43 5.2 14 40

3.2 4.7 1.1 4.7 4.5 20

0.73 0.58 0.82 1.6 28

1.0 1.8 20

23 3.3 0.90 1.7

2.3

2.3 11 10 17 28

2.3 1.3 1.8 3.8

350 °C 400 °C 450 °C

Reaction Rate (× 10−6 gsoot mcatal−2 s−1) ref 361 361 361 361 361 203 203 203 203 203 203 203 211 211 211 369 369 369 369 369 369 369 374 374 374 374

Industrial & Engineering Chemistry Research Review


a

isotherm

isotherm

TPO

1.5 wt % Ru/CeO2

1.5 wt % Ru/CeO2

1.5 wt % Ru/CeO2

With 6000 ppm propene.

51 63 64 56 53 68 63 48 12 82

isotherm isotherm isotherm isotherm isotherm isotherm isotherm isotherm isotherm isotherm

CeO2 1% Fe/CeO2 5% Fe/CeO2 10% Fe/CeO2 20% Fe/CeO2 1% Cr/CeO2 3% Cr/CeO2 5% Cr/CeO2 Cr2O3 1.5 wt % Ru/CeO2

82

82

82


method

catalyst Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U carbon black Degussa carbon black Degussa carbon black Degussa carbon black Degussa N330

N330

N330

N330

carbon

9

9

9

9 9 9 9 9 9 9 9 9 9

catalyst/carbon mass ratio

5


Table 13. Reaction Rates and Conditions for Soot Oxidation at Different Temperatures

21

21

21a

10 10 10 10 10 10 10 10 10 21

O2 (vol %)

344

344

344

395 387 383 373 392 377 390 400 455 344

catalytic

600

600

600

600 600 600 600 600 600 600 600 600 600

noncatalytic

Tmax (°C)

256

256

256

205 213 217 227 208 223 210 200 145 256

ΔT (°C)

300

250

200

280 280 280 280 280 280 280 280 280 200

reaction temp (°C)

0.058

0.030

0.028

0.017 0.027 0.032 0.048 0.030 0.035 0.023 0.0183 0.028 0.0067

reaction rate (× 10−6 gsoot mcatal−2 s−1)

ref

294

294

294

278 278 278 278 278 279 279 279 279 294

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Industrial & Engineering Chemistry Research 133 °C).365 The high activity of the La0.9Rb0.1CoO3 (ΔTmax = 140 °C) perovskite was explained by its easy reduction to La0.9Rb0.1CoO2.5, which resulted from lattice distortion after the introduction of the large Rb ion. Fang et al. synthesized K- and Mg-substituted perovskite catalysts, La1−xKxCo1−yMgyO3 (x = 0−0.4, y = 0−0.2), by a citric acid complexation method, with La0.6K0.4Co0.9Mg0.1O3 (ΔTmax = 231 °C) showing the highest soot oxidation activity.366 They claimed that the enhanced activity was caused by increased reducibility of the catalysts by the substitution of K and Mg. Ura et al. found that the SrTiO3 perovskite catalyst with partial substitution of Sr with K (2 mol % K, ΔTig = 120 °C) showed higher soot oxidation activity than the corresponding K/SrTiO3 catalyst (ΔTig = 100 °C).367 They proposed that the effects of K addition on the improved activity could be interpreted in terms of the location of K. Specifically, the K incorporated in the perovskite structure enhanced oxygen vacancy formation while the free K on the surface increased the surface oxygen mobility. Shimokawa et al. prepared La0.8K0.2MnO3 perovskite catalysts with different surface areas and observed a correlation between the catalyst surface areas and soot oxidation activities, i.e., higher catalyst surface area resulted in lower soot oxidation temperatures.264 However, as discussed in the previous section, it is difficult for catalysts with low surface areas to saturate the carbon. Since this was likely the case in this study, the correlation may be convoluted by catalyst−carbon contacting effects. Other important elements for the substitution of perovskite catalysts are alkaline-earth metals, cerium, and transition metals. Doggali et al. substituted the A-site with Ba in a LaCoO3 perovskite and observed improved soot oxidation activity.368 The partial substitution of La with Ce and Sr in LaCoO3 perovskite catalysts showed improved soot oxidation activity and could be ranked in the following order, based on their activity: La0.8Ce0.2CoO3 (ΔTmax = 210 °C) > La0.9Ce0.1CoO3 (ΔTmax = 191 °C) > La0.9Sr0.1CoO3 (ΔTmax = 170 °C)> La0.8Sr0.2CoO3 (ΔTmax = 159 °C) ≈ LaCoO3 (ΔTmax = 160 °C).369 The formation of the perovskite LaCoO3 increased the soot oxidation activity for Co−La mixed oxides.370 The partial replacement of La3+ in LaCoO3 by Co2+ from Co3O4 resulted in the formation of Co4+ species, which enhanced the surface reducibility. The substitution of Sr with Ce in SrCoO3 (ΔT50 = 130 °C) perovskite also improved the soot oxidation activity, compared with SrCoO3 (ΔT50 = 70 °C).371 Wu et al. studied the effects of Ce substitution at the A-site and Co substitution at the B-site over LaMnO3 perovskite catalysts for soot oxidation.209 They found that the substitution of the A-site with Ce improved the activity while the substitution of the B-site with Co inhibited the activity. The substitution of the A-site in PrMnO3 perovskite catalysts with different valence state elements (K, Ba, and Ce) was studied by Doggali et al.372 The substitution with these elements increased the Mn4+ content in the oxide. The K-substituted PrMnO3 perovskite (ΔT50 = 100 °C) showed the highest soot oxidation activity, compared with the Ba-substituted catalyst (ΔT50 = 20 °C) and the Ce-substituted catalyst (ΔT50 = 60 °C). Similar to other soot oxidation catalysts, the addition of noble metals improves the soot oxidation activity over perovskite catalysts. Gold supported on perovskites increased the soot oxidation activity, because of its ability to adsorb and activate oxygen, especially with small Au nanoparticles.230 Au supported on three-dimensionally ordered macroporous (3DOM) LaFeO3 (ΔTmax = 226 °C) revealed better soot oxidation activity than 3DOM LaFeO3 (ΔTmax = 170 °C) and

bulk LaFeO3 (ΔTmax = 99 °C). They observed a correlation between the size of the gold nanoparticle and the soot oxidation activity: the smaller the gold nanoparticle, the higher the soot oxidation activity. However, in another study by Russo et al., the presence of gold on LaMnO3, LaCrO3, LaFeO3, and LaNiO3 had no effect on the soot oxidation, compared to the supports themselves.373 Noble metals can also enter the crystal lattice of perovskites. Pecchi et al. prepared (La1−xAgx)Mn0.9Co0.1O3 (x = 0.0, 0.1, 0.2, 0.3) perovskite catalysts (ΔTmax = 221−280 °C) by the amorphous citrate method and claimed that the higher soot oxidation activity was due to the active surface oxygen species from the interaction of Ag+ and O− in the lattice.374 Higher silver substitution resulted in more surface active oxygen and higher soot oxidation activity. Megarajan et al. compared the effects of surface and bulk silver on PrMnO3+δ perovskite catalysts for soot oxidation.375 Higher activity (ΔTmax = 240 °C) was found on the catalyst with surface silver species, which was caused by the redox couple of Ag/Ag2O. LaRuO3 and La3.5Ru4.0O13 perovskite catalysts were synthesized by various methods and showed high soot oxidation activity.376,377 The Ru species in LaRuO3 were Ru3+, and the species in La3.5Ru4.0O13 were Ru4+. Both of them were stable in the lattice of the perovskite structure, resulting in the high thermal stability of these catalysts. Table 12 summarizes the reaction rates and conditions for carbon oxidation reactions over perovskite catalysts from the literature where enough information was available to calculate rates. The most active perovskite catalysts contained noble metals such as gold, silver, and alkali metals, which is similar to that of highly active modified cerium oxides. The addition of cerium to perovskite catalysts significantly increased soot oxidation activity, which suggests that catalysts combining the properties of ceria-based materials and perovskites may be promising. La0.7Ag0.3Mn0.9Co0.1O3 is the most active catalyst in the table, which decreased the Tmax value by 280 °C and revealed an oxidation rate of 15 × 10−6 gcarbon mcatalyst−2 s−1 at 350 °C. Table 13 and Table S3 in the Supporting Information summarize the reaction rates and conditions for carbon oxidation over catalysts at different temperatures (other than 300, 350, 400, and 450 °C). Again, copper-modified cerium oxides and catalysts with alkali metals were quite active for carbon oxidation reaction. Cerium-supported Ru catalysts also revealed good activity. In summary, a large variety of different catalysts have been used for carbon oxidation reactions. Alkali metals have been shown to be effective promoters of soot oxidation catalysts; however, these compositions generally suffer from loss of alkali metal under high temperature reaction conditions, which may prevent their use in commercial applications. The adsorption and activation of gas phase dioxygen is an important step, and thus, catalysts with noble metals such as gold, silver, and ruthenium normally show high activity due to their excellent ability to activate dioxygen. Although metal oxides and salts were widely studied for soot oxidation, providing basic information on the carbon oxidation activity of different elements, over the past 15 years or so, ceria-based materials and perovskites have received much more attention. From the comparison of reaction rates with different catalysts, it appears that ceria-based oxides are more active than perovskite catalysts when no precious metals or alkali metals are employed as promoters. However, when choosing a catalyst for real applications, such as in a DPF, catalyst stability also must be 9793


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Industrial & Engineering Chemistry Research Table 14. Summary of Activation Energies for Catalyzed Carbon Oxidation Reactions catalyst

method

type of carbon

reactant gas (vol % O2)

contact conditions

activation energy (kJ mol−1)

ref

Cu−K−V TiO2 0.2 wt % La-TiO2 1 wt % La-TiO2 2 wt % La-TiO2 4 wt % K/CeO2 7 wt % K/CeO2 14 wt % K/CeO2 7.7 wt % K/CeO2 7.7 wt % K/ZrO2 7.7 wt % K/Ce0.5Zr0.5O2 7.7 wt % K/CeO2 7.7 wt % K/ZrO2 7.7 wt % K/Ce0.5Zr0.5O2 CeO2 ZrO2 Ce0.5Zr0.5O2 CeO2 ZrO2 Ce0.5Zr0.5O2

Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa Ozawa

diesel soot Printex-U Printex-U Printex-U Printex-U diesel soot diesel soot diesel soot Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U

21 21 21 21 21 6 6 6 10 10 10 10 10 10 10 10 10 10 10 10

tight tight tight tight tight tight tight tight tight tight tight loose loose loose tight tight tight loose loose loose

55 142 142 145 143 109 74 106 118 119 121 122 121 120 142 157 138 158 160 151

379 191 191 191 191 285 285 285 380 380 380 380 380 380 380 380 380 380 380 380

Table 15. Reaction Order of Oxygen in Catalyzed Carbon Oxidation Reactions catalyst

type of carbon

reactant gas (vol % O2)

contact conditions

reaction order in oxygen

ref

La0.1K0.2Cu0.9V0.1O4 CeO2 Pt/CeZrO LaCrO3 La0.9CrO3 La0.8CrO3 La0.8Cr0.9Li0.1O3 La0.9Na0.1CrO3 La0.9Rb0.1CrO3 La0.9K0.1CrO3

amorphous carbon carbon black diesel soot Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U Printex-U

0−8 1−10 5−20 1−5 1−5 1−5 1−5 1−5 1−5 1−5

tight tight loose tight tight tight tight tight tight tight

0.31−0.38 0.18−0.21 0.53 0.70 0.74 0.84 0.75 0.92 0.85 0.81

381 382 196 383 383 383 383 383 383 383

The observed reaction orders of dioxygen in catalyzed carbon oxidation are summarized in Table 15. The order was less than that of the noncatalytic carbon oxidation reaction, which was ∼1. There were significant differences observed for the reaction order of dioxygen for different catalysts, which may be caused by the different oxygen species formed through the chemisorption of gas-phase oxygen vs oxygen derived from the oxide lattice in different catalysts. 5.3.3. Oxidation of Catalytic Coke by Using Oxygen as the Oxidizing Gas. The above section discusses the catalysts for carbon oxidation (more specifically, for diesel soot oxidation). In such an application, the formation of carbon and its oxidation by catalysts occur in different physical locations, which enables the use of model carbon to replace the use of real diesel soot. As opposed to the case with diesel soot, catalytic carbon formation typically occurs on the surface of catalyst particles. The carbon formed on catalysts leads to their deactivation, thus requiring the regeneration of catalysts by the oxidation of the deposited carbon. The oxidation of carbon formed on metal sites and acidic sites are discussed in the following section. 5.3.3.1. Filamentous Carbon. Carbon formation on metallic sites occurs in the form of filaments, as noted earlier, which causes the metals to show catalytic activity toward coke formation for a period of time until the metal sites are covered

taken in account. To evaluate catalyst stability, accelerated deactivation tests can be applied, for example, by heating for 50 h at 850 °C in 10% steam.58 These tests have been described in detail by a few researchers57,378 but are outside the scope of this review. 5.3.2.4. Kinetics of Catalyzed Carbon Oxidation Reaction. Table 14 and Table S4 in the Supporting Information summarize the apparent activation energies for carbon oxidation associated with various catalysts reviewed in this paper. There are discrepancies in the values from various reports, even for catalysts with the same composition. The most accurate method for activation energy determination is from an Arrhenius plot of intrinsic reaction rates. However, it is very challenging to calculate the intrinsic rate in carbon oxidation, because of the unknown number of active sites that actually participate in the reaction. Moreover, the reaction conditions used by different researchers vary, making it quite difficult to accurately compare activation energies. Ceria-based oxides, perovskite catalysts, and other transitionmetal oxides had activity energies of ∼140 kJ mol−1 for carbon oxidation. The addition of alkali metals to these catalysts can significantly decrease the activation energy to ∼120 kJ mol−1 or, in some cases, 625 °C), the reaction order in CO2 was almost 9797


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Industrial & Engineering Chemistry Research Table 17. Kinetic Parameters for CO2 Gasification of Coke/Soot catalyst

carbon

C/metal ratio

reaction rate (μmol (mol Ci)−1 s−1)

temperature (°C)

ref

beryllium magnesium calcium calcium calcium strontium barium lithium sodium potassium rubidium cesium

activated activated activated activated activated activated activated activated activated activated activated activated

51.8 51.8 107 51.8 25.3 51.8 51.8 51.8 51.8 51.8 51.8 51.8

1.4 3.2 16 79 280 44 28 22 33 61 114 200

727 727 727 727 727 727 727 727 727 727 727 727

439 439 439 439 439 439 439 431 431 431 431 431

unity, whereas, at low temperatures (550 °C), it was zero. The addition of CO at low temperatures had an inhibiting effect but above 650 °C, no effect was observed. Nickel supported on alumina had lower activation energy than nickel foils. Takenaka et al. studied the CO2 gasification activity of carbon fibers deposited on Ni-SiO2 catalysts.424 The amount of CO formed, according to the following reaction, was detected at the end of the reactor: CO2 + C(s) → 2CO

Alkali-metal and alkaline-earth catalysts have been widely used and found to be active for CO2 gasification of carbon in both their oxide and carbonate forms. Spiro et al. found that alkali-metal carbonates were better CO2 gasification catalysts than alkaline-earth carbonates.430 The order of efficacy for these catalysts was Ba > Sr > Ca for alkaline-earth catalysts and Cs > K > Na > Li for the alkali-metal series.430,431 The activity increased with increased cesium loading, decreased with increased lithium loading, and showed intermediate behavior for potassium and sodium. For alkali-metal carbonates, the following gasification mechanism was suggested to be similar to that of steam gasification of graphite, by McKee et al.:432

(34)

For comparison, graphite and carbon black were mixed with a nickel catalyst and the gasification reaction was executed. Carbon fibers deposited on the nickel catalyst were found to be more active than the carbon black and graphitic samples at 630 °C. For CO2 gasification, nickel that was dispersed on alumina had faster kinetics than nickel foils in the temperature range of 650−750 °C. This effect is more pronounced for the H2 gasification of carbon. Ohtsuka et al. used iron for gasification of carbon obtained by carbonizing phenol−formaldehyde.425 Fe (4 wt %) was loaded on carbon through impregnation with an aqueous solution of (NH4)3Fe(C2O4)3. They found the presence of magnetite in the catalyst using in situ XRD measurements.425 Magnetite was found to have low gasification activity. Compounds such as Fe3O4 (magnetite), Fe1−xO, Fe3C, and α-Fe were observed during examination by in situ XRD, which might be responsible for the activity. Noting the limitations of the XRD technique to detect only relatively large, crystalline species, the authors noted that other undetected species cannot be ruled out as being catalytic. In another study, iron-loaded carbon black and activated carbon samples showed increased reactivity toward CO2 gasification, as demonstrated by Tanaka et al.426 The average gasification rate of carbon black with CO2 increased by a factor of 222 in the presence of the iron catalyst at 800 °C. This high activity was attributed to the atomically dispersed α-Fe species, as observed by EXAFS. Similar active Fe species were observed by Ohme et al. 427 They also demonstrated that sintered iron and oxidized iron were the deactivating species for CO2 gasification. Miura et al. studied CO2 gasification on demineralized and carbonized coconut shells with a variety of catalysts. At 835 °C, they reported the following order of activity toward gasification: Ni > Na ≈ K > Ca > Fe.428 CO2, when present as a mixture with steam, reduced the catalytic activity of copper catalysts toward the gasification of soot. This effect, which was studied by Gálvez et al., was attributed to the stronger adsorption of CO2 on Cu.429

Na 2CO3 + 2C → 2Na + 3CO

(35)

2Na + CO2 → Na 2O + CO

(36)

Na 2O + CO2 → Na 2CO3

(37)

Cerfontain et al. were able to correlate the gasification activity of the alkali-metal materials to the quantity of CO2 chemisorbed, using CO2/CO mixtures.433 Sams et al. showed that increasing the K/C ratio increased the reactivity until the carbon surface was saturated with catalytically active sites.434 They demonstrated that potassium was vaporized during the thermal treatments. Instantaneous K/C ratios were thus determined by modeling the catalyst loss. An increase in the instantaneous K/C ratios showed a linear increase in the gasification reactivity. Zhang et al. found that the gasification activity of potassium increased with conversion.435 It was suggested that an intercalation compound of potassium with the carbon must be formed, which released potassium during the reaction, thus increasing the activity. The mechanism for potassium release was suggested to be according to the following equation: 2KCx + CO2 → K 2O + CO + 2Cx

(38)

A reaction mechanism for gasification of carbon using Na2O was suggested by Saber et al.436 The mechanism involved three steps for both catalytic oxidation and reduction: oxidation: Na 2O−C + CO2 ↔ (NaO)2 −C + CO

(39)

reduction: (NaO)2 −C + C → Na 2O−C + CO 9798

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Industrial & Engineering Chemistry Research reduction: (NaO)2 −C + CO ↔ Na 2O−C + CO2

storage and reduction (NSR) catalysts containing platinum, barium, and potassium were found to reduce the gasification temperature of soot by 150 °C, compared to noncatalytic gasification.445 Since ceria materials are widely used for the oxidation of carbon, they have also found significant use in NOx-assisted gasification. Synergy between ceria and a transition metal is seen to have improved the activity in the presence of NO. Cerium- and iron-activated soot mixed with a supported Pt catalyst increased the reactivity of the soot at 377 °C.446 The activation energy decreased from 167 kJ mol−1 to 93 kJ mol−1 and from 170 kJ mol−1 to 120 kJ mol−1 for cerium- and ironactivated soot, respectively. Figure 15 shows the proposed

(41)

The use of calcium increased the gasification rate 100-fold, using the disordered carbon, Carbosieve.419 The rate constant was the same as the noncatalytic gasification. Thus, the increased rate was attributed to an increase in the number of active sites associated with Ca. Zhang et al. found that the gasification activity of the carbon decreased with conversion for a calcium catalyst but increased with conversion for a potassium catalyst at 850 °C. This was related to the decrease in the surface area of CaO during the reaction.435 Similar sintering was observed by Cazorla-Amoros et al. through in situ thermogravimetry, isothermal chemisorption, and temperature-programmed desorption studies.437 However, in the CO2 gasification of carbonized phenol formaldehyde using calcium, the reactivity increased as the calcium content increased up to 4 wt %, after which it remained constant. This result was suggested to be related to the ion exchange capacity of the carbon, after which the excess Ca had little contact with carbon, leading to no change in the reactivity.438 In the CO2 gasification of activated carbon using alkali and alkaline metals, magnesium and beryllium did not show any activity.439 Cesium showed the highest gasification activity, as shown in Table 17. Thus, a wide variety of alkali, alkaline, and transition catalysts have been used for CO2 gasification of carbon. Many researchers have investigated the mechanism of the catalyzed reaction, which can help in the future, with regard to designing better catalysts. 5.3.4.2. NOx Gasification. Diesel particulate filters are generally used in diesel engines to trap the particulate matter produced in the engine, such as soot. Continuous regeneration of these filters is necessary to avoid blockage of the filter. Catalysts that help the oxidation of carbon deposits on the filter have been utilized for regeneration at temperatures of 350 °C and above.440 The temperature of the diesel exhaust gases is typically ∼300 °C.441 Hence, there is a need to find catalysts that can reduce the ignition temperature of the carbonaceous deposits. NO2 has been found to be effective in oxidizing carbon deposits at low temperatures. However, NO2 is present in very small amounts in the exhaust gas (below 50 ppm), which causes reduced reaction rates.442 NO, which is present in exhaust gases in considerable quantity, can be oxidized to NO2, and this increased concentration of NO2 can aid in soot gasification at low temperatures. A detailed review of the reaction mechanism of carbon with NOx is discussed by Stanmore et al.443 Hence, this section will primarily focus on the different catalysts that have been used for gasification of carbon in the presence of NOx and their mechanism of action. Transition-metal elements have been widely used for NOx gasification. Liu et al. showed that MoO3 and V2O5 had high catalytic activity toward diesel soot gasification in the presence of only NO2 without water vapor. SiO2, Al2O3, HZSM-5, ZrO2, and SnO2 were used as supports for the catalysts. An inverse relationship between catalyst−support interactions and activity was claimed as the reason for different activities of MoO3 on various supports. In the case of the V2O5 catalyst, the activity was attributed to its ability to convert SO2 to SO3, which acts as a co-catalyst for carbon gasification.441 For CoOx−PbOx catalysts, the incorporation of 0.5 wt % Pt increased the catalytic activity, since platinum helps in the conversion of NO to NO2, which was used for the gasification of soot.444 NOx

Figure 15. Mechanism of gasification of cerium-activated soot mixed with supported platinum in the presence of O2 and NO. (Reproduced with permission from ref 446. Copyright 1999, Elsevier, Amsterdam, the Netherlands.)

mechanism for the platinum-assisted gasification of ceriumactivated soot in the presence of NO and O2. Platinum aids the conversion of NO to NO2, which, along with cerium, helps in the gasification of soot, forming NO. The NO thus formed enters the cycle. The soot−NO reaction rate is negligible below 600 °C. Above 600 °C, O2 is the most active oxidant, whereas below 600 °C, NO2 is the most active oxidant. An activation energy of 134 kJ mol−1 was found by Leistner et al. for soot oxidation in the presence of NO in the temperature range of 650−800 °C.447 A CuO−CeO2 catalyst was found to be effective in the NOx-assisted oxidation of soot.256 The activity of the catalyst was related to the incorporation of CuO into the ceria lattice, which helped in the activation of oxygen and adsorption of NO on the catalyst. Cu−Ce−Al catalysts were suggested to be more effective than Cu−Ce catalysts, because of the presence of γalumina, which reduced the sintering of the active species.448 These catalysts were quite effective under loose contact conditions. The incorporation of cesium into MnOx−CeO2 catalysts increased the oxidation activity for soot samples with NO in the oxidizing mixture. This effect was attributed to the basicity of cesium, which increased the oxidative adsorption of NO.449 However, when a molten salt Cs2SO4·V2O5 catalyst was used for soot oxidation in the presence of NO2, it was found to be of little use in the conversion of NO to NO2. Hence, the activity of the catalyst was associated with only the meager amount of NO2 already present in the exhaust gas.450 Atribak et al. found increased catalytic activity of CeO2 for soot oxidation at 650 °C in the presence of NOx in an oxygen stream.451 Ceria facilitated the oxidation of NOx to NO2, and 9799


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NOx gasification.381 Vanadium had a significant role in the activity of the catalyst. Teraoka et al. reported the use of copper-loaded MFI zeolites in NOx-assisted soot oxidation.457 Catalysts prepared via impregnation and ion exchange showed similar activity. The mechanism of the catalytic oxidation was not discussed. These catalysts were not only active toward soot oxidation but also selective toward the conversion of NO to N2. The addition of water or oxygen to the NO2 feed was found to increase the activity toward oxidation of carbon black. However, water was involved in the reaction only as a catalyst, as the O atoms from water molecules were not found to be part of the products.458 Jeguirim et al. showed that no gasification of carbon black occurred at temperatures below 450 °C in the absence of NO2 and in the absence of a catalyst.459 Shrivastava et al. carried out kinetic experiments in the temperature range of 500−950 °C for soot oxidation in the presence of NO2, and they calculated an activation energy of 47 kJ mol−1, which was significantly lower than that for oxidation in air. The pre-exponential factor was found to be 2.4 × 10−14 (nm K−0.5 s−1 cm3 molecule−1).460 In the temperature range of 200−450 °C, Kandylas et al. obtained an activation energy of 40 kJ mol−1 for the C-NO2 reaction.442 Zouaoui et al. found an activation energy of 39−66 kJ mol−1 for the noncatalytic CNO2 reaction and 67−69 kJ mol−1 for the noncatalytic CNO2−O2 reaction.461 Jung et al. reported an activation energy of 60 kJ mol−1 in the temperature range of 250−500 °C for the C-NO2−O2 reaction.195 Introduction of water vapor in this system reduced the activation energy to 40 kJ mol−1. Table 18, as well as Table S5 in the Supporting Information, lists the kinetic parameters for studies that provided enough information for calculation of reaction rates. Various catalytic and noncatalytic NOx gasification studies using carbon deposits as substrate are listed. Oxides of cerium, zirconium, and titanium showed higher reaction rates than platinum/γ-alumina and Cs2SO4·V2O5 catalysts. 5.3.4.3. Steam Gasification. Steam is widely used in industrial operations and could be of use in the gasification of carbon deposits. However, steam exposure can affect the physical properties of various catalysts and investigation of the stability of catalysts under such conditions is necessary. Steam has been used in the literature as a gasifying agent, in the presence or absence of oxygen. The following section gives a brief overview of the catalysts that are active in the presence of steam, as well as the effect of steam on the gasification of carbon. 5.3.4.3.1. Oxygen-Assisted Steam Gasif ication. The effect of steam addition to oxygen has been investigated by a few researchers, and there seems to be no unanimous relationship between the addition of steam and activity. Neeft et al. found no dependence of water on the noncatalytic oxidation of diesel soot.180 However, the addition of water in the range of 4−10 vol % to oxygen increased the oxidation rate of flame soot while keeping the activation energy constant (168 kJ mol−1). The apparent reaction order in carbon increased with the addition of water. Jeguirim et al. studied the effect of water on noncatalytic carbon black oxidation in the presence of NO2. They found that water vapor increased the oxidation rate, but the mechanism of oxidation remained the same, regardless of the presence or absence of water vapor. The activity of water vapor was attributed to the formation of nitric and nitrous acids, which allowed reaction between carbon and NO2.459 The effect of water vapor on the oxidation rate was found to decrease with increasing temperature. The reactions forming

the NO2 thus formed further oxidized the soot to form CO and CO2. The addition of 700 ppm of NO2 to 10% O2 was demonstrated to be effective in soot oxidation by platinum/ ceria−zirconia materials. Noncatalytic oxidation under NO2 reduced the activation energy (132 kJ mol−1) of oxidation, compared to that in air (160 kJ mol−1). However, there was little effect on the reaction order in carbon. When catalyst was introduced in the system, the activation energy in NO2 decreased even further (77 kJ mol−1) and the reaction order in carbon increased, because of the ability of NO2 to recycle NO.207 Milt et al. obtained an activation energy of 83.6 kJ mol−1 for the oxidation of soot in NO2 with Co, Ba, K/ZrO2 as the catalyst. The reaction order was found to be 0.5, with respect to NO. Catalytic activity of Co was attributed to its redox ability, while that of potassium was claimed to be due to its increased mobility.452 A manganese−cerium (MnCe) mixed oxide (ratio = 1:3) was found to be the best of the catalysts studied for NO-assisted soot oxidation. NO could be stored as various nitrates which release NO2 above 200 °C.453 A few researchers have studied the effect of alkali metals on NOx gasification. Lin et al. used a BaAl2O4 catalyst in the presence of NOx as the oxidizing gas.454 NOx was shown to adsorb on the catalyst, forming active nitrites. Gaseous oxygen converted these nitrites into nitrates, which reacted with the surface carbon complexes formed by reaction of soot with O2, NO, and NO2. The redox reaction between nitrates and carbon complexes gave rise to the observed products CO, CO2, N2, and N2O. In the absence of gaseous oxygen, redox reactions of soot with NO can be accomplished with lattice oxygen, but only at high temperatures. Castoldi et al. used Pt−Ba/Al2O3 and suggested the formation of barium nitrate, leading to the removal of NOx which can oxidize the soot.455 Su et al. used potassium-supported Mg−Al hydrotalcite mixed oxides for soot combustion and proposed a mechanism based on their findings from in situ FTIR.161 Figure 16 shows a schematic of their proposed mechanism.

Figure 16. Reaction pathways for NOx-assisted soot combustion on K/MgAlO. (Reproduced with permission from ref 161. Copyright 2010, American Chemical Society, Washington, DC.)

Some researchers have tried to use nonconventional catalysts for the gasification of carbon, namely, perovskites and zeolites. These studies have reported some interesting results. Perovskite catalysts of La−K−Mn-O were found to be suitable for NOx-assisted carbon oxidation.456 The activity was observed to increase as the potassium content increased from 0 to 40%. K2Mn4O8, which was produced as a byproduct during the synthesis of the perovskite, was also found to increase the oxidation activity of the activated carbon. Another study with La−K−Cu-V perovskite showed increased catalytic activity for 9800


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increased to 800 °C, the presence of water led to a significant decrease in activity. For Cu−K−V−Cl catalysts, Badini et al. found a similar effect and hypothesized the reason for the effect as an increase in the volatility of chloride compounds in the presence of water vapor at high temperatures.463 However, they found that binary catalysts such as CsVO3 + KCl and KVO3 + KCl showed different behavior, wherein water vapor did not affect the catalytic activity. 5.3.4.3.2. Steam Gasif ication without Oxygen. In the presence of a catalyst, different results have been reported for effect of water vapor on activity and selectivity of catalysts toward carbon gasification. Hermann et al. have suggested a detailed reaction pathway for noncatalytic gasification of carbon using water vapor (Figure 17).464 According to this report, water dissociated at carbon active sites to form hydroxyl groups that then condensed. At ∼500 °C, ether groups were formed on the surface of carbon. At higher temperatures, the water vapor reacted with the ether groups to form hydroxyl groups again, desorbing CO. Gàlvez et al. showed that, in catalytic filters used in diesel engines, the activity of copper catalysts toward soot oxidation was reduced in the presence of steam and CO2.429 They attributed this reduced activity to the stronger chemisorption of CO2 than steam on the catalyst surface. The photocatalytic degradation of soot, studied by Lee et al., showed enhanced catalytic activity in the presence of water vapor on TiO2 catalysts, because of the formation of OH radicals acting as oxidant species.465 Zouaoui et al. demonstrated that the oxygen from water vapor was not consumed during the reaction, suggesting that water acted as a catalyst for the direct reaction between carbon and NO2.461 Jung et al. obtained an activation energy of 60 kJ mol−1 for noncatalytic soot oxidation using O2 and NO2.195 In the presence of H2O, this energy was reduced to 40 kJ mol−1 in the temperature range of 250−500 °C. McKee et al. suggested that, during steam gasification using potassium, the hydroxide of potassium was formed.432 Reduction−oxidation cycles, which involved the reduction of metal and the oxidation of carbon, were suggested to be the pathway for gasification. Sodium and potassium salts showed similar catalytic activity for the gasification of graphite. However, the activity shown by lithium was an order of magnitude higher than that of the potassium and sodium salts. This was hypothesized to be due to the lower melting point of the lithium salt. The suggested reaction mechanism for gasification for all three of these salts was

Table 18. Kinetics of NOx Gasification of Carbon Deposits reaction rate (× 10−8 gcarbon s−1)

ref

1000 ppm of NO + 5% O2 1000 ppm of NO + 5% O2 5% O2 5% O2

0.06 (260 °C)

161

0.18 (310 °C)

161

0 (260 °C) 0 (310 °C)

161 161

0.96 (260 °C)

161

0.78 (310 °C)

161

soot

1000 ppm of NO + 5% O2 1000 ppm of NO + 5% O2 5% O2

0.12 (260 °C)

161

soot

5% O2

0.78 (310 °C)

161

5K/ MgAlO 5K/ MgAlO 5K/ MgAlO 5K/ MgAlO

soot

1.38 (260 °C)

161

2.76 (310 °C)

161

soot


0.48 (260 °C)

161

soot

5% O2

0.318 (310 °C)

161


soot

2.1 (260 °C)

161

3.96 (310 °C)

161

soot


0.78 (260 °C)

161

soot

5% O2

0.486 (310 °C)

161

CeO2 CeO2 CeO2

soot soot soot

0.05% NOx + 5% O2 0.05% NOx + 5% O2 0.05% NOx + 5% O2

1420 (525 °C) 2470 (550 °C) 2740 (575 °C)

451 451 451

ZrO2 ZrO2 ZrO2

soot soot soot

0.05% NOx + 5% O2 0.05% NOx + 5% O2 0.05% NOx + 5% O2

607 (525 °C) 1065(550 °C) 2027 (575 °C)

451 451 451

TiO2 TiO2 TiO2

soot soot soot

0.05% NOx + 5% O2 0.05% NOx + 5% O2 0.05% NOx + 5% O2

542 (525 °C) 1044 (550 °C) 2049 (575 °C)

451 451 451

catalyst

carbon

oxidizing gas

MgAlO

soot

MgAlO

soot

MgAlO MgAlO

soot soot


soot soot

soot

soot

acidic active intermediates involved in soot oxidation in the presence of NO2 and H2O were suggested as follows: 2NO2 + H 2O → HNO3 + HNO2

(42)

3HNO2 → HNO3 + 2NO + H 2O

(43)

Arnal et al.462 observed results similar to those of Neeft et al. for the influence of water vapor on the noncatalytic oxidation of a commercial carbon sample. The dominant reaction in the presence of water vapor was the gasification of carbon. Oxidation in the presence of water vapor resulted in morecomplete oxidation to CO2 than for oxidation using only oxygen. The stability of carbon surface oxygen complexes was higher in the presence of water vapor, causing the formation of CO2 instead of CO. Peralta et al. observed that Ba,K/CeO2 catalyst showed no change in activity toward soot oxidation in the presence of water at 400 °C.286 However, when the temperature was

Na 2CO3(l) + 2C(s) → 2Na(g ) + 3CO(g )

(44)

2Na(g ) + 2H 2O(g ) → 2NaOH(l) + H 2(g )

(45)

2NaOH(l) + CO(g ) → Na 2CO3(l) + H 2(g )

(46)

Freriks et al. showed that dispersed potassium was more active than bulk potassium during the gasification of carbon deposited from poly(fufuryl alcohol) pyrolysis.466 Also, the activity was found to be dependent on the dispersion level of potassium. They suggested the formation of a potassium surface complex as the active species and claimed that the earlier prediction of formation of metallic potassium was not probable. At low temperatures, this complex reacted with water vapor to form potassium carbonate and a primary alcohol. Mouljin et al.467 also showed that no intercalation compounds of potassium were formed during gasification. A renewed 9801


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Figure 17. Reaction pathway for noncatalytic steam gasification of carbon. (Reproduced with permission from ref 464. Copyright 1986, Pergamon Journals, Ltd.)

char, agglomeration occurred only after 80% conversion, leading to rapid gasification activity. Gasification occurred around the catalyst particles. Tomita et al.471 saw pit formation near the edges of the nickel particles, indicating that gasification occurred mainly in the vicinity of the catalyst. The agglomeration of nickel during steam and hydrogasification, leading to deactivation of the nickel catalyst, has also been demonstrated by Colle et al.472 They performed magnetic susceptibility and X-ray analyses to evaluate the magnetic behavior and the crystallite size of the nickel particles. The crystallite size was observed to increase with conversion, which reduced the carbon−catalyst contact area. This resulted in rapid deactivation at 30%−50% conversion. No nickel oxides were detected in these experiments, and the deactivation of the nickel catalyst due to the formation of oxides was neglected. Figueiredo reported that the rate of gasification of carbon by steam with polycrystalline Ni foils and supported Ni catalyst was dependent on the diffusion of carbon through nickel.473 Carbon was deposited on the catalyst by pyrolysis of C3H8 and H2 mixtures. The rate of the steam gasification reaction did not change up to 60%−70% carbon burnoff and was independent of the steam concentration. Supported nickel catalysts showed lower activation energy, compared to the nickel foil. In another study, by Bernardo et al.,423 similar diffusion control dependence was observed. They reported that, during the reaction of n-hexane and hydrogen at 600 °C, coke was deposited on Ni/ Al2O3 catalysts. During gasification of this coke, at high temperatures, diffusion control was evident, while at lower temperatures (700 °C). No explanation was provided for the observed high activation energy for the catalyzed gasification, compared to the noncatalytic gasification. Figure 18 shows a detailed mechanism for the iron-catalyzed gasification of carbon. McKee suggested that the most effective catalysts for steam gasification were carbonates, oxides, and hydroxides.475 He described the mechanisms involved in the gasification of carbon using these species. The significant steps involved redox cycling of the catalyst, followed by reoxidation by the oxidizing gas. Nishiyama476 suggested that the method of loading the catalyst on the carbon substrate was important for the activity. The synthesis method had an effect on the dispersion of the catalyst on the carbon surface. A linear correlation was found between the CO−CO2 ratio and the steam gasification reactivity of different metal catalysts, as shown in Figure 19.476 Calcium showed the highest rate of gasification in steam. The noncatalytic gasification of activated carbon in 50% steam showed a higher concentration of CO in the product stream. When catalytic gasification was carried out, the concentration of CO2 increased with rate, except for the nickel catalyst. In the case of transition metals and metal oxides, transition metals have shown significant catalytic activity toward steam gasification of carbon. Holstein et al.420 proposed a reaction mechanism for the steam gasification of carbon using a platinum catalyst: C−C−C + Pt* ⇆ C−C + Pt*−C

Figure 19. Relationship between gasification activity and CO2−CO composition with a metal/carbon ratio of 0.25 (mol %) at 800 °C. (Reproduced with permission from ref 476. Copyright 1991, Elsevier, Amsterdam, The Netherlands.)

Pt* + H 2O ⇆ Pt*−O + H 2

(51)

Pt*−C + Pt*−O → 2Pt* + CO

(52)

Pt*−O + CO ⇆ Pt* + CO2

(53)

However, transition-metal oxides usually do not show activity toward CO2 and H2O gasification of carbon. This difference was related to the oxygen transfer mechanism.477 In the case of the C−O2 reaction, the active oxygen species can be bulk oxide lattice oxygen, surface lattice oxygen, or activated oxygen on the metal from the oxygen atmosphere. However, for C−H2O and C−CO2 reactions, only oxygen species that are strongly bound to the metal oxide are present, which causes lower activity. The mobility of the catalyst, dispersion, and redox properties are the key factors observed for increasing in the catalytic activity for the gasification of carbon and calcium, potassium, and nickel are among the best steam gasification catalysts. 5.3.4.4. Hydrogasification of Carbon. Hydrogasification is a process in which carbon materials are gasified in the presence of hydrogen. Hydrogen is a product of steam gasification of some carbon sources and can affect the surface species on a catalyst,

(50) 9803


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activities, requiring temperatures of >850 °C. The lowest peak temperatures were observed for gasification in hydrogen and the highest peak temperatures were observed in CO2. A low concentration of steam (5%) was shown by Macák et al. to increase coke deposition on a nickel catalyst during methane−water reactions at 727 °C. However, increasing the steam concentration showed an inhibiting effect on the deposition on carbon.483 This may be due to the gasification activity from the added steam, since carbon deposits can be gasified with hydrogen or steam. These gasification processes generally have different kinetics. Hydrogasification and the formation of filamentous carbon are reversible processes.484 Carbon diffuses through the metal particle during the formation of filaments, while carbon diffusion happens in the reverse direction in the presence of adsorbed hydrogen. The ratedetermining step in hydrogasification was found to be the reaction between hydrogen and the surface carbon. However, steam gasification involves carbon diffusion as the ratedetermining step. 5.3.4.5. Carbon Gasification by SO2. SO2 is present in trace amounts in diesel emissions; however, its effects on catalysts can be catastrophic. Hence, it is critical to study the stability of these catalysts in the presence of sulfur. Dopant addition to ceria materials or in the presence of a support has been observed to increase the resistance to sulfur poisoning. Mn−Ce oxides used for low-temperature soot oxidation deactivated rapidly upon exposure to SO2. They could not be regenerated and showed negligible activity after SO2 exposure.453 This effect was attributed to metal sulfate formation, which reduced the NOx storage capacity, leading to decreased oxidation activity. Mn and Ce impregnated on γalumina were more resistant to SO2 exposure.485 The activity was not completely lost after regeneration of the sulfated catalyst in air. Liu et al. incorporated platinum on the MnCeAl catalyst materials, which drastically improved the sulfur resistance of the catalyst.486 Peralta et al. reported a decline in the activity of Ba,K/CeO2 catalysts during oxidation of soot in the presence of sulfur dioxide at 400 °C.286 This behavior was attributed to the formation of sulfates. Higher concentrations of SO2 also showed rapid deactivation of catalysts.286 Later, Milt et al. showed that the effect of sulfur on the deactivation of catalysts was minimized in the presence of NO2.487 Gorte measured the oxygen storage capacity of 1 wt % palladium on ceria materials for fresh and sulfated catalysts.488 He observed that the OSC of the catalysts was drastically reduced after exposure to 1% SO2 in O2 at 400 °C. Potassium nitrate incorporated into the Cu−CeO2 catalyst system showed partial resistance to sulfur poisoning.489 Potassium nitrate was converted to potassium sulfate upon exposure to SO2, which led to decreased activity for soot oxidation. However, the activity was not completely lost, because KNO3 was not completely sulfated. The addition of molybdenum to ceria in higher loadings (10Mo10Ce, 2Mo10Ce) showed lower activity to carbon black oxidation in air, but improved the sulfur poisoning resistance of ceria.490 In the case of nonceria materials, a cesium-doped FeV catalyst has shown good stability after exposure to SO2 for extended period of time. Aging the catalyst in 1000 ppm of SO2 and air increased the peak oxidation temperature of soot only by 17 °C, from 322 °C to 339 °C.339 V2O5/Cu/Pt also showed increased resistance to sulfur poisoning in a soot oxidation study. A 2% change in oxidation rate was measured before and after SO2 exposure.491 Exposing La2O2CO3 and V2O5 to 100

because of its highly reducing nature. Relatively little is known about hydrogasification catalysis and much less attention has been paid to understand the process, compared to gasification with the other species discussed above. The objective of this section is to summarize the various catalysts that have been used for the hydrogasification of carbon. A wide range of studies exist for coal gasification using hydrogen in the literature. However, carbon species from coal are not within the scope of this review and, hence, will not be discussed. Hydrogasification is usually promoted by transition-metal elements but alkali metals have been found to be less active in the presence of hydrogen. Hydrogasification of carbon deposits in the presence of nickel was found to be second order in hydrogen.473 The reaction was also found to be slower than that with water vapor. Nickel supported on alumina and nickel foils showed the same activation energy in hydrogasification. Bernardo et al. also studied the gasification of carbon using H2 on nickel catalysts.423 This study was an extension of the results of Figueiredo473 in the temperature range of 475−850 °C. They conducted gasification studies using controlled-atmosphere electron microscopy and suggested that hydrogen can dissociate at the metal tip and combine with carbidic carbon diffusing from the carbon filament to form methane. Methane was the only product detected that could have been formed by the suggested mechanism. A negative activation energy was observed at temperatures above 700 °C, which was related to operation under equilibrium conditions. Hydrogasification of coke deposited on supported Ni−Cu/SiO2 catalysts during methane decomposition was carried out by Tavares et al.478 They found that catalysts containing >10% Cu had gasification activity that was one order of magnitude lower than that of the copper-free nickel catalyst. Petroleum coke impregnated with the transition-metal elements iron, nickel, and cobalt showed improved hydrogasification.479 The lowest activation energy was observed for the iron catalyst. The mechanism of hydrogasification on iron was suggested to be a spillover mechanism, wherein the H atoms dissociated on metal surface and spilled over to the carbon surface. They concluded that the dissociation of hydrogen was not possible on elemental iron and the spillover mechanism was largely responsible for hydrogasification on iron. They also eliminated the possibility of electronic interactions between the catalyst and the carbon. In another study, calcium was found to show such electronic interactions, leading to increased activity.480 The addition of 1% Ca to the cobalt, nickel, and iron catalysts increased the hydrogasification activity of pitch coke significantly below 850 °C.480 They hypothesized that calcium mediated the interactions between the catalyst and carbon, being more significant than its ability to activate hydrogen. Filamentous carbon in the form of filaments formed on iron, cobalt, and nickel was hydrogasified and the reactivities of these carbons was compared by Avdeeva et al.481 The order of reactivity in hydrogen was as follows: Co > Fe−Co = Ni. Tamai et al.482 hypothesized that the metal−carbon interaction was more important than the metal−gas interaction for transition metals, since they showed similar reactivity during steam, hydrogen, or carbon dioxide gasification. The carbon was impregnated with metals by mixing the carbon with metal chloride solutions and drying in a rotary evaporator. Nickel, ruthenium, osmium, and rhodium showed gasification at relatively low temperatures (500−600 °C) and had high activity. Iron, cobalt, and palladium showed the lowest 9804


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Industrial & Engineering Chemistry Research Table 19. Catalyst Activity for Carbon Gasification under Different Gasifying Conditions catalyst

carbon

oxidizing gas

activation energy (kJ mol−1)

reaction rate (× 10−4 gcarbon mcatal−2 s−1)

ref

Ni foils (99.7% Ni) Ni foils (99.7% Ni) Ni foils (99.7% Ni)

pyrolysis of n-hexane/H2 pyrolysis of n-hexane/H2 pyrolysis of n-hexane/H2

CO2 (1 atm) CO2 (1 atm) CO2 (1 atm)

163 ± 13 163 ± 13 163 ± 13

0.45 (750 °C) 0.29 (700 °C) 0.19 (650 °C)

423 423 423

Ni foils (99.7% Ni) Ni foils (99.7% Ni) Ni foils (99.7% Ni)



71 ± 9 71 ± 9 71 ± 9

0.063 (600 °C) 0.018 (550 °C) 0.0035 (500 °C)

423 423 423

18% Ni−Al2O3 18% Ni−Al2O3 18% Ni−Al2O3



140 ± 9 140 ± 9 140 ± 9

0.5106 (750 °C) 0.333 (700 °C) 0.134 (650 °C)

423 423 423

18% Ni−Al2O3 18% Ni−Al2O3 18% Ni−Al2O3



66 ± 8 66 ± 8 66 ± 8

0.047 (600 °C) 0.015 (550 °C) 0.0039 (500 °C)

423 423 423

Spheron − (99.95% purity) Spheron − (99.95% purity) char from bituminous coal char from bituminous coal

CO2 (0.008−1 atm) CO2 (0.004−1 atm) H2 (68 atm) H2

280 ± 15 170 ± 10 122 112

3.36 × 10−5 (677−817 °C) 2.017 × 10−5 (527−697 °C)

420 420 494 494

1% Pt/C 5 wt % K(KHCO3)

ppm S/15% O2/N2 at 352 °C for 6 h, showed a very small effect on the catalytic activity for soot oxidation.492 Pt/Al2O3, when impregnated with tungsten oxide, not only improved the oxidation activity of soot but also increased the sulfur poisoning resistance of the catalyst.493 Sulfates are formed on amphoteric alumina, which migrate to the platinum surface and cover the platinum particles. When WOx is introduced in the catalyst, the acidic nature of WOx causes less sulfation and also decreases the desulfation temperature of the catalyst. This leads to improved sulfur poisoning resistance. Thus, sulfur poisoning is a very important issue that must be addressed in designing future catalysts. This section has provided a summary of dopants or catalysts that have shown improvement toward sulfur resistance that can aid in designing better catalysts. 5.3.4.6. Comparison of Different Catalysts with Different Gases Used for Gasification. Table 19, as well as Table S6 in the Supporting Information, give a comprehensive list of reaction rates for the various catalysts used for the gasification of carbon materials in the presence of CO2, H2O, or H2.

very effective technique in recent years. Raman spectroscopy, in combination with other techniques, such as X-ray diffraction (XRD), ultraviolet−visible (UV-vis) spectroscopy, small-angle X-ray scattering (SAXS), and transmission electron microscopy (TEM) can help obtain a comprehensive understanding of the physical and chemical structure of carbon deposits. In situ analytical techniques have helped in the deduction of the mechanisms of carbon oxidation. The mechanism of oxidation/ gasification is dependent on the catalyst used, as well as the gasifying agent. Carbon deposits from different sources, such as diesel soot, thermal cracking, and catalyst coking, appear to have similar oxidation behavior. The amount and type of carbon deposited may change, depending on the reaction temperature, pressure, feed hydrocarbon, and characteristics of the surface present. However, the oxidation kinetics seem to have some similarity, where the reaction order in carbon for most carbon species is 1. There is no consensus for the reaction order in oxygen across the different carbon types. However, the activation energy values for noncatalytic oxidation of most carbon forms fall in the range of 130−170 kJ mol−1. The trend of increasing activation energy with increasing hardness of the carbon remains the same across all carbon sources. The rate of gasification/oxidation is an important parameter to screen catalysts for the oxidation of carbon. The oxidation/gasification of carbon has been conducted with different reactant gases, such as O2, H2O, CO2, and NOx, among others. Catalyzed carbon oxidation with O2 is widely studied in the field of diesel soot abatement. The model soot used in most of the literature is Printex-U carbon black. The type of contact between the carbon and the catalyst is critical in determining the catalytic performance toward catalytic oxidation. The loose contact conditions resemble those in catalytic soot filters, while the samples with tight contact conditions provide for more-reproducible results. The total catalyst/carbon surface area ratio is another very important parameter. Carbon is only saturated by the catalyst when the total catalyst/carbon surface area ratio is sufficiently high. Comparisons of activity for different catalysts should be made

6. CONCLUSIONS The formation of carbon deposits on equipment and catalysts is, to a large degree, unavoidable in hydrocarbon processes and often leads to process interruptions for catalyst regeneration or equipment maintenance. Removal of the carbon deposits is often critical to process or catalyst performance. To aid carbon removal, researchers have sought to gain an understanding of the structure of the carbon, to help shed light on structure− property correlations for carbon removal by oxidation or gasification. Also, the mechanism of oxidation/gasification/ removal of such deposits in the presence of a catalyst is important in designing better catalysts that (i) are resistant to coke formation or (ii) can help improve the catalyst regeneration. In this review, the formation mechanisms of catalytic, radical, and condensation coke were discussed. Catalytic coke can be formed either by metal sites or in the presence of acidic sites. To understand the chemical structure of these deposits, Raman spectroscopy has been found to be 9805


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affects the oxidation/gasification behavior of such deposits. Nickel catalysts are known to be excellent for hydrogen gasification. Calcium has been widely investigated for steam gasification of carbon but is known to sinter at high temperatures. Studies on oxidation of catalytic coke are hard to compare across the literature, because the reactions that are causing coke formation and the reaction conditions differ across various papers. This heterogeneity also leads to different coke species forming in different catalysts/systems, which cannot be directly compared easily. However, in this paper, we sought to summarize the mechanism of oxidation of catalytic coke and the observed kinetics to the extent possible. Combining knowledge about coke/soot deposition mechanisms and soot oxidation/gasification reactivities can help to solve significant practical problems related to minimizing soot emission from diesel engines, catalyst deactivation by coking, and coke deposition in steam crackers. This article catalogues a wide range of catalysts used for oxidation/gasification of carbon with their rates of reaction. This mechanistic and kinetic information may be useful for future researchers in choosing catalysts for their required application.

only when the carbon is saturated by the catalyst in all of the samples. Early studies focused on screening of metal oxides and salts. In the past 15 years or so, much effort has been made to investigate ceria-based oxides and perovskite catalysts. Although their structures and properties of the catalysts are, in some cases, very different, the soot oxidation activity is heavily dependent on a catalyst’s ability to activate gas-phase O2 in the reactions in all cases. The addition of noble metals such as gold, silver, and ruthenium can significantly increase the catalytic carbon oxidation activity. Alkali metals can also enhance their activity, because of their high mobility. However, the loss of alkali metals under harsh reaction conditions can lead to catalyst deactivation, which prevents their use in some hightemperature applications. Cobalt, copper, and manganese are other important promoters. The high activity of ceria-based materials derives from the Ce4+/Ce3+ redox cycle. For these catalysts and many others, the oxygen storage capacity (OSC) is an important parameter that often correlates the catalytic activity of materials toward carbon oxidation in the presence of inert gas, while surface oxygen plays a role in catalysis during oxidation in oxygen. Although there is a wealth of literature reporting carbon oxidation activity, most of it only reports the characteristic oxidation temperature, such as T50 and Tmax, which makes quantitative comparisons of the different catalysts very difficult. Among the few isothermal studies in the literature, the rates are typically calculated with the total amount of catalysts. In the future, efforts should be made to quantify the active sites on the catalysts, so that intrinsic reaction rates can be obtained. To the extent possible, when they are not presented in the original works, reaction rates and kinetic parameters have been calculated using literature data in this review. While choosing a catalyst for practical application, the stability of the catalyst should be considered, with the stability being estimated by accelerated deactivation tests in early research. Understanding the behavior of catalysts in the presence of reactive gases is important, because these gases might be present as part of the feed or generated during a reaction in various hydrocarbon processes. For instance, in diesel engines, NOx species are important components whose effect on soot oxidation cannot be neglected. In the steam cracking reactions, steam is part of the feed and hydrogen is generated during the process. Among the reactive gases used for gasification, NO2 appears to be the most reactive gas. Gasification with hydrogen seems to show the lowest reactivity. The presence of SO2 usually poisons the catalysts, causing a decrease in catalytic activity. V2O5, molybdenum, and palladium have been determined to be helpful in improving the sulfur resistance in some cases. For CO2 gasification, cesium as well as supported nickel catalysts were found to be active for the removal of solid carbon. Note that most studies involving reactions of carbon with CO2, H2, and steam are from the last century and, hence, do not employ the latest techniques that could be used to gain insight into the reaction mechanisms of the oxidation/ gasification of the carbon deposits. Metal-catalyzed carbon has been found to oxidize at lower temperatures than acid-catalyzed carbon, because of the high catalytic activity of the metal. Metal-catalyzed carbon deposits are filamentous in nature when the tip of the filament is the metallic site. Acidic zeolites are known to have carbon deactivation as a major issue in hydrocarbon reactions. In zeolite-catalyzed reactions, the carbon that are deposited are dependent on the shape and structure of the zeolite, which also

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02220. Kinetics parameters reported in the literature for the noncatalytic oxidation of carbon; reaction rates and conditions for soot oxidation reactions over modified cerium oxide catalysts; reaction rates and conditions for soot oxidation at different temperatures; summary of activation energies for catalyzed carbon oxidation reactions; kinetics of NOx gasification of carbon deposits and catalyst activity for carbon gasification under different gasifying conditions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P. K. Agarwal). *E-mail: [email protected] (R. J. Davis). *E-mail: [email protected] (C. W. Jones). *E-mail: [email protected] (A. Malek). *[email protected] (H. Shibata). Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge The Dow Chemical Company for funding this project.

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