Mechanistic Study of Selective Catalytic Reduction of NO with NH3 on

Article pubs.acs.org/JPCC

Mechanistic Study of Selective Catalytic Reduction of NO with NH3 on W‑Doped CeO2 Catalysts: Unraveling the Catalytic Cycle and the Role of Oxygen Vacancy Bing Liu,† Jian Liu,† Sicong Ma,† Zhen Zhao,*,† Yu Chen,*,‡ Xue-Qing Gong,§ Weiyu Song,† Aijun Duan,‡ and Guiyuan Jiang‡ †

State Key Laboratory of Heavy Oil Processing, College of Science, and ‡State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of PetroleumBeijing, Beijing 102249, P. R. China § Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: The reaction mechanism of selective catalytic reduction (SCR) of NO with NH3 on W-doped CeO2 catalysts was systematically investigated using density functional theory calculations corrected by on-site Coulomb interactions (DFT +U). A complete catalytic cycle was proposed, which consists of four steps, namely (i) Lewis acid site reaction, (ii) Brønsted acid site reaction, (iii) oxygen vacancy reaction, and (iv) catalyst regeneration. The calculated key intermediates in these four steps are in good agreement with previous experimental results, which indicates that our suggested catalytic cycle is rational. The catalytic nature of W-doped CeO2 catalysts for NH3-SCR reaction was discussed by analyzing the role of oxygen vacancy, the synergistic effect between surface acidity and reducibility, and the difference from NH3-SCR reaction on V2O5-based catalysts. Our results show that the oxygen vacancy on the surface which creates two Ce3+ cations plays a critical catalytic role in the NH3SCR reaction, where adsorbed N2O22− species can be readily formed and then acts as a precursor for SCR reaction, opening a unique reaction pathway. The formation of adsorbed NO2 species on W-doped CeO2 facilitates the SCR reaction via Langmuir− Hinshelwood mechanism with a relative low energy barrier. This study provides atomic-scale insights into the catalytic cycle and the important role of oxygen vacancy in NH3-SCR reaction on W-doped CeO2 catalysts, which is of significance for the design of highly active ceria-based SCR catalysts.

1. INTRODUCTION Nitrogen oxides (NOx) emitted from stationary sources and automobile combustion have been regarded as a major cause of air pollution, which can result in the photochemical smog, ozone depletion, acid rain, and greenhouse effect.1−3 The selective catalytic reduction of NOx with NH3 (NH3-SCR) has been proved to be an efficient technology for eliminating NOx emissions from diesel exhaust and coal-fired flue gas, and the general NH3-SCR reaction is 4NO + 4NH3 + O2 = 4N2 + 6H2O.4−6 Currently, V2O5−WO3/TiO2 and V2O5−MoO3/ TiO2 are the most widely used commercial catalysts for NH3SCR reaction in the industry, which exhibit excellent catalytic activity in a temperature window of 300−400 °C.7,8 However, there are still some disadvantages in these two catalyst systems, including the toxicity of vanadium species, the narrow operation temperature window, and the high activity for oxidation of SO2 to SO3 which can cause corrosion of downstream equipment.9,10 As a result, many researchers have attempted to develop novel non-vanadium-based catalysts with merits of high efficiency, low cost, and environmental protection.11 © XXXX American Chemical Society

Ceria (CeO2) has been widely used in heterogeneous catalysis because of its excellent reducibility and remarkable oxygen storage capability.12,13 In recent years, ceria-based NH3SCR catalysts have attracted considerable attention.5,7,14−18 A variety of ceria-based NH3-SCR catalysts, such as CeO2/ TiO2,14 CeO2−MoO3/TiO2,15 MnOx−CeO2,16 and CeO2− WO3,17,18 have been reported and used as model catalysts for NH3-SCR reaction. All these catalysts show relatively high catalytic activity with a broad temperature window and exhibit better resistance to SO2 poisoning. Tungsten (W) has been recognized as an effective additive which can enhance the catalytic activity of SCR catalysts.10 Chen et al. proposed that the addition of W could facilitate the formation of active adsorbed NOx species and enhance the catalytic activity of CeO2/TiO2 catalysts for NH3-SCR.10 Shan et al. found that the introduction of W species increases the amount of active sites and acid sites which is beneficial to the low temperature SCR Received: November 19, 2015 Revised: January 20, 2016

A

DOI: 10.1021/acs.jpcc.5b11355 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C reaction on Ce−Ti mixed oxide catalysts.19 Chen et al. proposed that the cooperation of W promotes the formation of Ce3+ and results in the better SCR activity of CeO2−WO3/ TiO2 catalysts.17 However, to the best of our knowledge, some fundamental issues in the process of NH3-SCR on W-CeO2 catalysts, such as (i) the nature of active sites and active species, (ii) the synergistic effect between ceria and tungsten, and (iii) the underlying reaction mechanism, still remain obscure. Furthermore, an in-depth understanding of the reaction mechanism is necessary to design and develop highly active catalysts.20,21 The reaction mechanism of NH3-SCR has been the subject of considerable debate, and there is still no consensus so far. Several different reaction mechanisms have been proposed. Chen et al. studied the reaction mechanism of NH3-SCR on CeO2−WO3/TiO2 catalysts using the experimental method, and they proposed that this reaction process mainly follows the Eley−Rideal (E−R) mechanism.17 In contrast, Peng et al. proposed that the NH3-SCR reaction mainly follows the Langmuir−Hinshelwood (L−H) mechanism on Mn-doped CeO2−WO3 catalysts.8 Kijlstra et al. suggested that the adsorbed NH3 species can react with both gaseous NO (E−R mechanism) and adsorbed nitrite species (L−H mechanism) on MnO x/Al2 O 3 catalysts.22,23 Yao et al. performed theoretical calculations to investigate the NH3SCR mechanism on V2O5(001) surface and found that the E− R mechanism is dominant.24 However, up to now, a systematic computational investigation of the mechanism of NH3-SCR reaction on ceria-based catalysts has not been reported yet, resulting in a lack of atomic-scale understanding of the reaction mechanism on ceria-based catalysts. In this work, we carried out systematic density functional theory calculations corrected by on-site Coulomb interactions (DFT+U) to investigate the reaction mechanism of selective catalytic reduction of NO with NH3 (NH3-SCR) on W-doped CeO2 catalysts. We aim to provide atomic-scale insights into the underlying reaction mechanism of NH3-SCR reaction on ceria-based catalysts and identify the nature of active sites of cerium−tungsten catalysts for NH3-SCR reaction. A complete catalytic cycle was proposed, which consists of four steps, namely (i) Lewis acid site reaction, (ii) Brønsted acid site reaction, (iii) oxygen vacancy reaction, and (iv) catalyst regeneration. The calculated key intermediates in these four steps are in good agreement with previous experimental results observed in the NH3-SCR reaction on ceria-based catalysts, which indicates that this suggested catalytic cycle is highly rational. The catalytic nature of W-doped CeO2 catalysts for NH3-SCR reaction was discussed by analyzing the role of oxygen vacancy, the synergistic effect between surface acidity and reducibility, and the difference from NH3-SCR reaction on V2O5-based catalysts.

Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional was used in our calculations.31 The Gaussian smearing method with a width of 0.05 eV was used to speed up the convergence of electronic structure optimization. The Brillouin zone was sampled at the 1 × 1 × 1 k-point mesh. Optimized structures were obtained by minimizing the forces on each ion until they were less than 0.05 eV/Å. The adsorption energy is defined as Eads = E(adsorbate + surface) − E(adsorbate) − E(surface)

where E(adsorbate + surface) is the total energy of the interacting adsorbate + surface system, and E(adsorbate) and E(substrate) are the energies of free adsorbate in gas phase and bare surface, respectively. Therefore, a negative value corresponds to an exothermic adsorption, and a more negative value shows a stronger adsorption. Transition states (TSs) for all the elementary reactions were located using the climbing-image nudged elastic band (CINEB) method32,33 and were further confirmed by the existence of only one imaginary frequency. The energy barrier (Ebar) of each elementary reaction was determined by calculating the energy difference between the corresponding transition state and initial state. All the reaction energies and barriers in this work did not include zero-point energy corrections. 2.2. Computational Models. It has been demonstrated that the CeO2(111) surface is the most stable among the three low index surfaces of ceria, namely CeO2(111), (110), and (100),34,35 indicating that the (111) surface is the most common facet exposed at ceria oxide. Accordingly, we used a slab model of CeO2(111) surface in this work, consisting of 9 atomic layers (three trilayers) with a 3 × 3 surface supercell. This model contains 81 atoms and has a vacuum gap of 12 Å. We have also used this CeO2(111) model to investigate CO oxidation in our previous studies.36,37 During geometry optimization, the atoms in the bottom three atomic layers were fixed at their bulk-truncated positions, and the rest of the atoms were allowed to relax. Chen et al. studied the CeO2− WO3 catalysts, and their XRD results show that the tungsten atoms is incorporated into the lattice structure of CeO2, resulting in a replacement of Ce atom by W atom and leading to a corresponding decrease in the surface area.38 Accordingly, in this work, the W-doped CeO2 was modeled by substituting one W atom for one Ce atom in the surface layer. The optimized structures of pure CeO2 and W-doped CeO2 are shown in Figure 1a,b.

3. RESULTS 3.1. Effects of W Doping. As shown in Figure 1b, W doping leads to notable structure distortion compared with

2. COMPUTATIONAL DETAILS 2.1. Computational Methods. All the density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).25,26 To accurately treat the highly localized Ce 4f-orbitals, we conducted spin-polarized DFT+U calculations with a value of Ueff = 5.0 eV applied to the Ce 4f state.27 This Ueff value has been verified to be able to accurately describe the electronic structure of reduced ceria system in previous studies.28,29 The projector-augmented wave (PAW) method was used to represent the core−valence interaction.30 The plane wave energy cutoff was set to 400 eV. The generalized gradient approximation (GGA) with the

Figure 1. (a) Calculated structure of pure CeO2. (b) Calculated structure of W-doped CeO2. (c) Oxygen vacancy formation on Wdoped CeO2. B


Article

The Journal of Physical Chemistry C

Figure 2. NH3 adsorption at (a) W dopant, (b) Ce atom adjacent to W dopant, (c) Ce atom far from W dopant, (d) Ce atom of pure CeO2, (e) Bacid site adjacent to W dopant (W−O−H), (f) B-acid site far from W dopant (Ce−O−H), and (g) B-acid site of pure CeO2.

Surface B-acid site of W-doped CeO2 in this work was modeled by adding a hydrogen atom at the surface oxygen. We considered two different B-acid sites: (i) B-acid site adjacent to W dopant (W−O−H) and (ii) B-acid site far from W dopant (Ce−O−H). The optimized structures of NH3 adsorption at these two B-acid sites as well as the corresponding adsorption energies are presented in Figure 2e,f. Our results show that the B-acid site adjacent to W dopant (W−O−H) has stronger NH3 adsorption ability with an adsorption energy of −0.48 eV. In comparison, we also calculated the NH3 adsorption at the Bacid site of pure CeO2 as shown in Figure 2g. The adsorption energy in this case is −0.35 eV, which is lower than that in Figure 2e, indicating that W doping also increases the acidity strength of B-acid. Therefore, it can be concluded from Figure 2 that the introduction of W dopant promotes the NH3 adsorption at both L-acid site and B-acid site, which is in agreement with Chen et al.’s experimental result which shows that NH3 could be more easily adsorbed at low temperatures with the W modification.17 Besides, the results in Figure 2 also indicate that Ce site neighboring W dopant acts as the most favorable L-acid site for NH3 adsorption and W−O−H species acts as the most favorable B-acid site for NH3 adsorption. 3.3. NO Adsorption and Oxidation. Yao et al. studied the NH3-SCR reaction of NO on V2O5 catalysts and proposed that the Eley−Rideal (ER) mechanism is dominant,24 in which gaseous NO reacts with adsorbed NH2 species. However, in the case of ceria-based catalysts, previous experimental results confirmed the existence of adsorbed NO2 species formed by NO oxidation in the NH3-SCR reaction process on ceria-based catalysts due to the excellent reducibility of CeO2, and this adsorbed NO2 species can react with adsorbed NH3,18,42,43 indicating that the Langmuir−Hinshelwood (LH) mechanism should be dominant. In this section, we examined the NO adsorption and oxidation on W-doped CeO2. As shown in Figure 3, NO is adsorbed on the surface with an adsorption energy of −0.33 eV and then reacts with an adjacent lattice O, leading to the formation of adsorbed NO2 species. This process is exothermic by −0.47 eV. These results indicate that the adsorbed NO2 species can be easily formed, which is in line with previous experimental results.8,18,43 Accordingly, the LH mechanism should play a more important role in the NH3-SCR

pure CeO2. The W−O bond distance in W-doped CeO2 is 1.86 Å, which is much shorter than the original Ce−O bond distance (2.36 Å) in pure CeO2. Consequently, the oxygen atoms neighboring the W dopant get closer to the W dopant, and their distances to the neighboring Ce atoms increase, weakening the Ce−O bonds. Previous Raman results also show that the interaction between ceria and tungsten weakens the Ce−O bond and leads to structure defects,39 which is in agreement with our theoretical results. The structure distortion resulting from doping has also been reported in other metaldoped CeO2 systems.40,41 The oxygen vacancy formation energy can be used as a predictor for the oxidation ability of surface oxygen of CeO2. A lower oxygen vacancy formation energy indicates a higher oxidation ability. The oxygen vacancy on W-CeO2(111) surface is generated by removing a surface oxygen atom adjacent to the W dopant, as shown in Figure 1c. The oxygen vacancy formation energy of W-CeO2(111) is calculated to be 1.62 eV, which is lower than that of pure CeO2(111) (2.39 eV). Therefore, the introduction of W dopant is beneficial to the formation of oxygen vacancy and thus improves the activity of surface oxygen, which is in agreement with previous experimental result.19 3.2. NH3 Adsorption. It has been concluded that the NH3 adsorption is one of the most critical steps in the NH3-SCR reaction.18,42 In this section, we studied the NH3 adsorption at both L-acid sites and B-acid sites of W-CeO2 catalysts, aiming to find the most favorable adsorption site. Three different Lacid sites on the surface were considered: (i) W dopant, (ii) Ce atom adjacent to W dopant, and (iii) Ce atom far from W dopant. The optimized structures of NH3 adsorption at these three L-acid sites as well as the corresponding adsorption energies are presented in Figure 2a−c. It can be concluded that NH3 prefers to be adsorbed at the Ce atom adjacent to W dopant with an adsorption energy of −0.71 eV. In comparison, we also calculated the NH3 adsorption at the L-acid site (Ce site) of pure CeO2 as shown in Figure 2d. The adsorption energy of NH3 at the L-acid site of pure CeO2 is −0.52 eV, which is lower than that in Figure 2b, indicating that W doping promotes the NH3 adsorption. This promotion effect could be mainly attributed to the structure distortion caused by W doping as discussed in section 3.1. C


Article


marked with red color in Scheme 1. The active sites in the first two steps are L-acid site and B-acid site, respectively. Oxygen vacancy acts as the active site in both the third and fourth steps. After these four steps, a catalytic cycle is completed and the catalyst is regenerated to the original state. Our proposed reaction scheme is consistent with the stoichiometry of NH3SCR reaction, which is 4NH3 + 4NO + O2 = 4N2 + 6H2O.4−6 3.4.1. Step I: L-Acid Site Reaction. Previous experimental studies have shown that the surface acidities of catalysts play a critical role in the NH3-SCR reaction.11,18 Both the L-acid site and B-acid site can act as the active site for the adsorption and activation of NH3.8,42 In this section, we investigated the NH3SCR reaction occurring at the L-acid site to initiate the catalytic cycle. The B-acid site can be generated at the end of this reaction step. The calculated structures of the intermediates and the transition states in this step are shown in Figure 4, and the corresponding energy profiles are depicted in Figure 5. As shown in section 3.2, NH3 prefers to be adsorbed at the Ce site adjacent to W dopant with an adsorption energy of −0.71 eV (IM1 in Figure 4). Starting from this adsorption structure of NH3 (IM1), we calculated the NH3-SCR reaction process following the Langmuir−Hinshelwood (LH) mechanism. NO is adsorbed on the surface (IM2) and then reacts with an adjacent lattice O with a low energy barrier of 0.34 eV (TS1), leading to the formation of an adsorbed NO2 species (IM3). This process is exothermic by −0.46 eV. Subsequently, one H atom of the adsorbed NH3 migrates to the adsorbed NO 2 species, and the N atom of the adsorbed NH 3 simultaneously binds to the N atom of the adsorbed NO2, which eventually leads to the formation of NH2NO2H intermediate (IM4). This process is endothermic by 0.98 eV and has an energy barrier of 1.36 eV (TS2). Then the NH2NO2H intermediate (IM4) undergoes dehydration by transferring one H atom to its O−H group with an energy barrier of 0.39 eV (TS3), resulting in the formation of NHNO species and H2O (IM5). This process is highly exothermic by −1.43 eV. The desorption of H2O requires a desorption energy of 0.38 eV (IM6). The formation of N2 (IM8) from the

Figure 3. NO adsorption and oxidation on W-doped CeO2.

reaction of NO on W-CeO2 catalysts. In the next section, we will investigate the reaction mechanism in detail. 3.4. Catalytic Cycle and Reaction Mechanism. The reaction mechanism in this study consists of four steps as shown in Scheme 1. The active sites in these four steps are Scheme 1. Our Proposed Catalytic Cycle of NH3-SCR Reaction on W-Doped CeO2 Catalystsa

a

The active sites in these four steps are marked with red color. Ov represents the oxygen vacancy on the surface.

Figure 4. Calculated structures of the intermediates and the transition states in step I. D


Article


Figure 5. Calculated energy profiles in step I.

Figure 6. Calculated structures of the intermediates and the transition states in step II. E


Article


Figure 7. Calculated energy profiles in step II.

energy barrier of 0.02 eV (TS7), with two H atoms simultaneously migrating to the surface of W-CeO2. Subsequently, NHNOH species (IM14) is formed via structure transformation of NH2NO with an energy barrier of 1.46 eV (TS8). Dehydration reaction then proceeds with an energy barrier of 0.91 eV (TS9), in which one H atom on the surface of W-CeO2 reacts with the O−H group of NHNOH species, leading to the formation of NHN species and H2O (IM15). The desorption of H2O from the IM15 state to the gas phase requires a desorption energy of 0.33 eV (IM16). The H atom of NHN species can be transferred to the surface of W-CeO2 with a low energy barrier of 0.08 eV (TS10), leading to the formation of N2 (IM17). This process is highly exothermic by −2.93 eV. The N2 can be readily desorbed into gas phase (IM18). In the following process, adsorbed H2O species (IM19) is formed via H transfer on the surface of W-CeO2. Subsequent desorption of H2O takes place with a desorption energy of 0.89 eV, creating an oxygen vacancy on the surface of CeO2 (IM20). In the NH4NO2 pathway, after the formation of IM11 state, the adsorbed NO is oxidized by the neighboring surface oxygen atom to form the adsorbed NO2 species, and simultaneously this NO2 species interacts with the adsorbed NH4 species by hydrogen bond to form a NH4NO2 intermediate (IM12*). This process is exothermic by −0.60 eV and has a low energy barrier of 0.14 eV (TS6*). Subsequently, NH2NO2H species (IM13*) is formed through H transfer with an energy barrier of 1.73 eV (TS7*). This process is endothermic by 1.32 eV. Dehydration reaction then proceeds with an energy barrier of 0.97 eV (TS8*), in which one H atom of the NH2NO2H species is transferred to its own O−H group, leading to the formation of NHNO species and adsorbed H2O (IM14*). The desorption of H2O from the IM14* state to the gas phase requires a desorption energy of 0.20 eV (IM15*). Next, the H atom of the NHNO species migrates to the surface oxygen atom with a low energy barrier of 0.09 eV (TS9*), leading to the formation of N2 (IM16*). This process is highly exothermic by −2.43 eV. The N2 can be readily desorbed into gas phase (IM17*). In the

NHNO species (IM6) needs to overcome two energy barriers (TS4 and TS5) by transferring H atom to the surface O atom. Finally, the N2 is desorbed into the gas phase with a desorption energy of 0.04 eV, and B-acid site is formed on the surface of W-CeO2 (IM9). The Eley−Rideal (ER) mechanism has been proposed in previous theoretical study of NH3-SCR reaction on V2O5 catalysts,24,44 in which adsorbed NH2 species is formed by the cleavage of one N−H bond of NH3 and then reacts with gaseous NO. We also calculated the ER mechanism in this work. However, we found that the first step of the ER mechanism (N−H bond cleavage) is highly endothermic by 1.57 eV as shown in IM2* in Figure 5, which indicates that the ER mechanism is unfavorable compared with the LH mechanism. Zhang et al.’s experimental results also show that ceria can readily adsorb NH3 and NO simultaneously, indicating that the LH mechanism is dominant.43 Shan et al. also proposed that the NO oxidation to NO2 enhances the SCR activity of CeWTiOx catalysts at low temperatures, which is in agreement with our results.19 3.4.2. Step II: B-Acid Site Reaction. The B-acid site is generated at the end of step I. Subsequently, the NH3-SCR reaction can occur at the B-acid site. We considered two different reaction pathways at the B-acid site: NH3NHO pathway and NH4NO2 pathway. The calculated structures of the intermediates and the transition states in these two reaction pathways are shown in Figure 6, and the corresponding energy profiles are depicted in Figure 7. In the NH3NHO pathway, NH3 binds to the B-acid site to form adsorbed NH4 species (IM10 in Figure 6) with an adsorption energy of −0.48 eV. In the following process, NO is adsorbed on the surface (IM11). Then the adsorbed NO reacts with the NH4 species leading to the formation of NH3NHO intermediate (IM12). This process is endothermic by 0.78 eV and has an energy barrier of 1.30 eV (TS6). Yuan et al. also found the formation of NH3NHO intermediate in the NH3SCR reaction on V2O5 catalysts.44 The formation of NH2NO species (IM13) from the NH3NHO intermediate has an low F


Article


Figure 8. Calculated structures of the intermediates and the transition states in step III.

Figure 9. Calculated energy profile in step III.

G


Article


Figure 10. Calculated structures of the intermediates and the transition states in step IV.

Figure 11. Calculated energy profile in step IV.

molecules are absorbed on two neighboring Ce3+ atoms using DFT calculation method.46 Starting from the adsorption structure of cis-N2O2 species (IM22), we investigated the detailed NH3-SCR reaction process. The calculated structures of the intermediates and the transition states in this reaction process are shown in Figure 8, and the corresponding energy profile is depicted in Figure 9. NH3 is adsorbed at the surface Ce site with an adsorption energy of −0.56 eV (IM23). Subsequently, one H atom of the adsorbed NH3 migrates to the adsorbed N2O2 species, leading to the formation of NH2 species and N2O2H species (IM24). This process is endothermic by 1.37 eV and has an energy barrier of 1.72 eV (TS12). The formation of N2 from the N2O2H species by breaking the two N−O bonds is highly exothermic by −2.02 eV without energy barrier (IM25), indicating that this process is spontaneous. After the breaking of two N−O bonds of N2O2H species, one surface OH group is formed and the other O atom replenishes the oxygen vacancy. The desorption of N2 is exothermic by −0.10 eV (IM26). In the following process, one H atom of NH2 species migrates to the neighboring OH group with an energy barrier of 0.90 eV (TS13), leading to the formation of NH species and H2O (IM27). The desorption of H2O requires a desorption energy of 0.53 eV (IM28). Subsequent dissociation of NH species takes place by transferring the H atom to a surface O atom of CeO2 with an energy barrier of 0.65 eV (TS14), forming a surface OH group and an adsorbed N atom (IM29). This process is endothermic by 0.40 eV.

following process, adsorbed H2O species (IM19) is formed via H transfer on the surface of W-CeO2. Subsequent desorption of H2O takes place with a desorption energy of 0.89 eV, creating an oxygen vacancy on the surface of W-CeO2 (IM20). 3.4.3. Step III: Oxygen Vacancy Reaction. Previous experimental results have confirmed the existence of cis-N2O2 species in the NH3-SCR reaction on ceria-based catalysts.8,42,43 Zhang et al. proposed that the cis-N2O2 species reacts with NH3 more favorably than other nitrates species using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method.43 However, the detailed reaction pathway in which the cis-N2O2 species is involved still remains obscure. In this section, we aim at illustrating the NH3-SCR reaction pathway involving the adsorbed cis-N2O2 species. We investigated the adsorption of two NO molecules on the oxygen vacancy which is generated at the end of step II. Our results show that the adsorption energy of the first NO molecule is −0.66 eV (IM21 in Figure 8), and the adsorbed cis-N2O2 species (IM22) is spontaneously formed when the second NO molecule is adsorbed on the vacancy, which is in agreement with previous experimental studies.8,42,43 Ding et al. studied the NO reduction reaction with CO on Pd-doped CeO2(111) using DFT calculation and also found that the adsorbed N2O2 intermediate is formed on the oxygen vacancy by the combination of two adsorbed NO molecules without energy barrier.45 Zhang et al. also found the formation of N2O2 species on reduced CeO2(110) surface without energy barrier when two NO H


Article


4. DISCUSSION In this section, the catalytic nature of W-doped CeO2 catalysts for NH3-SCR reaction is discussed by analyzing the role of oxygen vacancy, the synergistic effect between surface acidity and reducibility, and the difference from NH3-SCR reaction on V2O5-based catalysts. We also compare our theoretical results with previous experimental results, verifying that the proposed catalytic cycle in this work is highly rational. 4.1. Role of Oxygen Vacancy. Our results indicate that the cis-N2O2 species is formed on the surface oxygen vacancy, which is highly exothermic by −2.20 eV. In order to deeply understand the role of oxygen vacancy and the chemical nature that controls the formation of cis-N2O2 species, we performed the spin charge density calculations during the formation of cisN2O2 species. The spin charge density is defined as the charge difference between the spin-up and spin-down electrons. As shown in Figure 12a, W dopant leads to the occurrence of two

In the following process, another NH3 molecule binds to the N atom with an adsorption energy of −1.41 eV, leading to the formation of N2H3 species (IM30). The N2H3 species then dissociates by transferring one H atom to a nearby surface O atom with a low energy barrier of 0.05 eV (TS15), forming a surface OH group and N2H2 species (IM31). The N2H2 species is not stable and further dissociates with an energy barrier of 0.02 eV (TS16), leading to the formation of another surface OH group and N2H species (IM32). Both of these two dissociation processes have lower energy barriers and form more stable intermediates. Subsequently, the N2H intermediate moves to a more stable position (IM33). Next, the H atom of the N2H intermediate migrates to a neighboring surface OH group, leading to the formation of adsorbed H2O and N2 (IM34). This process is exothermic by −1.54 eV and has an energy barrier of 0.60 eV (TS17). Finally, N2 and H2O are desorbed into the gas phase, resulting in the occurrence of one oxygen vacancy and two OH groups on the surface of W-CeO2 (IM36). Zhang et al. studied NO reduction reaction by CO on isolated Rh1Co3 bimetallic sites and found that the formation of N2O2 intermediate on Rh1Co3 sites plays an important role,47 which is consistent with our results. Besides, our results show that W−O vac −Ce site can adsorb two NO molecules simultaneously to form W−N2O2−Ce (IM22 in Figure 8), and Ce site can adsorb NH3 to form Ce−NH3 (IM23 in Figure 8), which indicate that W and Ce sites can work together in adsorbing and activating NO and NH3, which is also consistent with the synergetic mechanism proposed in Zhang et al.’s work.47 Some experimental studies also found the formation of transN2O2 species in the SCR reaction on ceria-based catalysts.43,48,49 Accordingly, we also studied the SCR reaction involving trans-N2O2 species. This reaction process is similar to that of cis-N2O2 species. The calculated structures of the intermediates and the transition states are shown in Figure S1 in the Supporting Information. The energy profile is depicted in Figure S2. 3.4.4. Step IV: Catalyst Regeneration. Starting from the IM36 state, we investigated the catalyst regeneration process by O2 molecule. The calculated structures of the intermediates and the transition states in this reaction process are shown in Figure 10, and the corresponding energy profile is depicted in Figure 11. A gaseous O2 molecule is adsorbed on the surface oxygen vacancy with an adsorption energy of −1.72 eV (IM37 in Figure 10). The bond distance of the adsorbed O2 species is calculated to be 1.45 Å. Subsequent H transfer (IM38) from one OH group to the adsorbed O2 species takes place with an energy barrier of 0.40 eV (TS18). The second H transfer from the other OH group to the adsorbed O2 species needs to overcome an energy barrier of 0.21 eV (TS19), leading to the breaking of O−O bond and the formation of adsorbed H2O species (IM39). This process is highly exothermic by −1.82 eV. Finally, the adsorbed H2O is desorbed into the gas phase, and the catalyst is regenerated to the original state (IM40), completing the catalytic cycle. Xu et al. studied the NH3-SCR reaction of NO on MnO x /CeO 2 catalysts using the experimental method and also proposed that the gaseous O2 reoxidizes the reduced catalyst surface to regenerate the active site and close the catalytic cycle, which is in agreement with our theoretical results.48

Figure 12. (a) Spin charge density of W-CeO2. (b) Spin charge density after the formation of oxygen vacancy. (c) Spin charge density after the adsorption of NO on the oxygen vacancy. (d) Spin charge density after the adsorption of cis-N2O2 on the oxygen vacancy. Yellow and blue isosurfaces depict spin-up and spin-down electrons, respectively.

localized Ce 4f electrons on the surface, which indicates that two surface Ce3+ cations are formed. This is mainly attributed to the fact that W dopant takes on an oxidation state of 6+. The formation of oxygen vacancy leads to the occurrence of two additional Ce3+ cations (Figure 12b). These results are in agreement with Janik et al.’s previous theoretical results.50 When one NO molecule is adsorbed on the vacancy, one localized electron is transferred to the adsorbed NO, leading to the formation of NO− species (Figure 12c). When the cis-N2O2 species is adsorbed on the vacancy, two localized electrons are transferred to the adsorbed cis-N2O2 species. As a result, negatively charged cis-N2O22− species is formed. The electrostatic attraction between cis-N2O22− and Ce4+ cations largely increases the stability of cis-N2O22− species and contributes to its strong adsorption. We also calculated the spin charge density of trans-N2O2 species as shown in Figure S3. The result also indicates that negatively charged trans-N2O22− species is formed. In our previous study,36 we investigated the CO oxidation on Pd-doped CeO2 catalysts; we found that molecular O2 can be adsorbed on the oxygen vacancy, and I


Article


4.3. Comparison with NH3-SCR Reaction on V2O5Based Catalysts. Yao et al. studied the selective catalytic reduction of NO with NH3 on V2O5(001) surface and proposed that the Eley−Rideal (ER) mechanism is dominant because the interaction of adsorbed NO with adsorbed NH3 is negligible.24 Yuan et al. also proposed that the selective catalytic reduction of NO with NH3 on both L-acid and B-acid of V2O5 follows the ER mechanism.44 In contrast, our results show that the Langmuir−Hinshelwood (LH) mechanism dominates the NH3-SCR reaction on W-CeO2 catalysts. This difference is mainly attributed to the excellent reducibility and oxygen storage capability of CeO2. As shown in Figures 3 and 4, NO can be easily oxidized by the surface oxygen of CeO2 to form the adsorbed NO2 species. Subsequently, this NO2 species reacts with adsorbed NH3 via LH mechanism with a relatively low energy barrier. In the case of V2O5 catalysts, NO cannot be adsorbed and oxidized by V2O5, leading to the reaction between gaseous NO and adsorbed NH3.24,44 In addition, our results indicate that Ce3+ and oxygen vacancy on CeO2 surface are beneficial to the adsorption of NO, which also facilitates the SCR reaction via LH mechanism. Our results in step III show that the oxygen vacancy of CeO2 plays a critical role in the SCR reaction. The oxygen vacancy of CeO2 promotes the formation of N2O22− species which then acts as a precursor for SCR reaction. Previous experimental studies have also confirmed the participation of N2O22− species in the NH3-SCR reaction on ceria-based catalysts.8,42,43 However, in the case of V2O5, no previous studies reported the formation of N2O22− species on V2O5 catalysts. Yao et al. proposed that the oxygen vacancy of V2O5 (001) just acts as the adsorption site for molecular O2.24 This difference can be mainly attributed to the unique catalytic role of the localized 4f electrons of Ce3+ resulting from the oxygen vacancy of CeO2. When two NO molecules are simultaneously adsorbed on the oxygen vacancy, two localized 4f electrons are transferred to these two NO molecules, leading to the formation of activated N2O22− species. 4.4. Comparison with Previous Experimental Results. The mechanism of NH3-SCR reaction on ceria-based catalysts has been the subject of intense debate in recent years. Different types of intermediates have been put forward in previous experimental studies of NH3-SCR on ceria-based catalysts. The proposed key intermediates in previous experimental studies are summarized in Table 1, and we also compared our theoretical results with these experimental results in Table 1. The results in Table 1 clearly indicate that our proposed catalytic cycle includes all of these important intermediates observed in previous experimental studies. The adsorbed NH3 and NH4,8,17,18 NO− species,43,48,53,54 cis- and trans-N2O22−

then two localized 4f electrons are transferred to the adsorbed O2, leading to the formation of activated O22− species. These results clearly show the unique catalytic role of the localized 4f electrons of Ce3+ in tuning the electrons distribution in adsorbates and reacting molecules. The electron transfer from Ce3+ to N2O22− strengthens the N−N bond, which greatly promotes the formation of N2. Accordingly, the N2O22− species acts as a precursor for SCR reaction. As shown in Figure 8, after one H atom of adsorbed NH3 is transferred to the N2O22− species, N2 can be easily formed from the N2O22− species by the cleavage of two N−O bonds. Zhang et al. studied the NO adsorption and reaction on reduced CeO2(110), and they also proposed that oxygen vacancy facilitates the N2O2 formation and the subsequent reduction of N2O2 to N2 can readily proceed.51 These results indicate that oxygen vacancy on ceria-based catalysts plays a critical catalytic role in the NH3-SCR reaction by providing a unique reaction pathway. 4.2. Synergistic Effect between Surface Acidity and Reducibility. It can be concluded from the catalytic cycle that the acidity and reducibility play different roles. As shown in section 3.4, the L-acid site and B-acid site are responsible for the adsorption of NH3, leading to the formation of adsorbed NH3 species and NH4 species. Previous NH3-TPD experimental results have also confirmed the formation of these two species on ceria−tungsten catalysts.8,17,18 Peng et al. found that both Lewis and Brønsted acid sites are involved in the SCR reaction on VOx/CeO2 catalysts, and the decrease of surface acidity caused by alkali poisoning suppresses NH3 adsorption, leading to the deactivation of catalysts,42,52 which indicates the important role of surface acidity. In addition, our results in section 3.2 indicate that the adsorption of NH3 at L-acid site is stronger than that at B-acid site, which is in line with Zhang et al.’s experimental results.43 On the other hand, the reducibility of CeO2 is responsible for NO oxidation and N−H bond cleavage. As shown in step I in section 3.4, NO is oxidized by the lattice oxygen of CeO2 to form the adsorbed NO2 species, which facilitates the NH3-SCR reaction following the LH mechanism with a low energy barrier. Previous experimental studies also show that the adsorbed NO2 species can take part in the SCR reaction,8,10,18,43 which is in agreement with our theoretical reaction mechanism. Our results also indicate that the surface lattice oxygen of CeO2 acts as the active site for the breaking of N−H bond of NH3. Zhang et al. studied the NH3 adsorption on ceria using IR spectroscopy method and found the breaking of N−H bond after the NH3 adsorption.43 Peng et al. found that low reducibility caused by alkali poisoning prohibits NH3 activation and NO oxidation on VOx/CeO2 catalysts,52 indicating the crucial role of surface reducibility in SCR reaction. Therefore, the catalytic nature of W-CeO2 catalysts for NH3SCR reaction can be attributed to the synergistic effect between acidity and reducibility. Both acidity and reducibility in WCeO2 catalysts are necessary to complete the catalytic cycle. The introduction of W dopant not only promotes the reducibility by lowering the oxygen vacancy formation energy but also facilitates the adsorption of NH3 at acid sites. Peng et al. investigated the SCR reaction of NO with NH3 on CeO2− WO3 catalysts using in situ IR and Raman spectroscopy, and they proposed a reaction mechanism consisting of an acid site cycle and a redox cycle,18 which is in good agreement with our theoretical results.

Table 1. Comparison with Previous Experimental Results of NH3-SCR on Ceria-Based Catalysts

J

species

exptl refs

our calcd results

adsorbed NH3 (L-acid) adsorbed NH4 (B-acid) NO− cis-N2O22− trans- N2O22− adsorbed NO2 NH4NO2, nitrite NH3NO−

8, 17, 18 8, 17, 18 43, 48, 53, 54 8, 42, 43 43, 48, 49 18, 42, 43, 55 11, 53, 56, 57 48

IM1 in step I IM10 in step II IM21 in step III IM22 in step III IM22* in step III IM3 in step I IM12* in step II IM12 in step II



■

species,8,42,43,48,49 adsorbed NO2 species,18,42,43,55 NH4NO2 and nitrite species,11,53,56,57 and NH3NO− species,48 which have been reported in previous experimental studies, are demonstrated to play critical roles in our proposed catalytic cycle as shown in section 3.4, indicating that our theoretical results in this work are in good agreement with previous experimental results. In addition, some experimental studies observed the formation of nitrate NO3− species in the NH3-SCR reaction on ceria-based catalysts.8,55,58,59 Liu et al.58 suggested that this NO3− species can react with NO to form the active NO2 species, and then this active NO2 species participates in the NH3-SCR reaction, indicating that the NH3-SCR reaction process of NO3− species is similar to that of active NO2 species as discussed in steps I and II in section 3.4. Therefore, steps I and II in our proposed catalytic cycle can also be used to explain the detailed reaction process of nitrate NO3− species. Accordingly, we believe that the catalytic cycle proposed in this work is rational and can be widely used to understand and describe the mechanism of NH3-SCR reaction on ceria-based catalysts.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11355. Calculated structures of the intermediates and transition states in the NH3-SCR reaction of trans-N2O2 (Figure S1), calculated energy profile for the NH3-SCR reaction of trans-N2O2 (Figure S2), and spin charge density after the adsorption of trans-N2O2 on the oxygen vacancy (Figure S3) (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Tel 86-10-89731586; Fax 8610-69724721 (Z.Z.). *E-mail [email protected]; Tel 86-10-89731072; Fax 86-1069724721 (Y.C.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for financial support from Natural Science Foundation of China (21503273, 21477164, 21376261, 21322307, 21173270, and 21177160), Beijing Natural Science Foundation (2142027), China Scholarship Council (201406440013), and Science Foundation of China University of PetroleumBeijing (No. ZX20150025). Computing time in the National Super Computing Center in Jinan is acknowledged.

5. CONCLUSIONS In this work, the reaction mechanism of selective catalytic reduction of NO with NH3 on W-doped CeO2 catalysts was systematically investigated using DFT+U calculations. The main conclusions can be drawn as follows: (1) A complete catalytic cycle was proposed, which consists of four steps, namely (i) L-acid site reaction, (ii) B-acid site reaction, (iii) oxygen vacancy reaction, and (iv) catalyst regeneration. The calculated key intermediates in these four steps are in good agreement with previous experimental results, which indicates that our proposed catalytic cycle is highly rational and can be widely used to understand and describe the mechanism of NH3-SCR reaction on ceria-based catalysts. (2) The oxygen vacancy on the surface which creates two Ce3+ cations plays a critical catalytic role in the NH3-SCR reaction, opening a unique reaction pathway. Adsorbed N2O22− species can be readily formed on the oxygen vacancy, which then acts as a precursor for SCR reaction. The electron transfer from Ce3+ to N2O22− strengthens the N−N bond, which greatly promotes the formation of N2. (3) The catalytic nature of W-doped CeO2 catalysts for NH3SCR reaction originates from the synergistic effect between surface acidity and reducibility. The acid sites promote the adsorption of NH3 on the surface. The excellent reducibility of CeO2 leads to the formation of adsorbed NO2 species, facilitating the SCR reaction via Langmuir−Hinshelwood mechanism with a lower energy barrier. The surface lattice oxygen of CeO2 also acts as the active site for the breaking of N−H bond of NH3. The introduction of W dopant not only promotes the reducibility by lowering the oxygen vacancy formation energy but also facilitates the adsorption of NO and NH3 on the surface. Our study provides atomic-scale insights into the mechanism and the important role of oxygen vacancy in the NH3-SCR reaction on W-doped CeO2 catalysts, which will be helpful for the design of highly active ceria-based catalysts for SCR reaction.

■

REFERENCES

(1) Maitarad, P.; Zhang, D.; Gao, R.; Shi, L.; Li, H.; Huang, L.; Rungrotmongkol, T.; Zhang, J. Combination of Experimental and Theoretical Investigations of MnOx/Ce0.9Zr0.1O2 Nanorods for Selective Catalytic Reduction of NO with Ammonia. J. Phys. Chem. C 2013, 117, 9999−10006. (2) Zhao, Y.; Duan, L.; Larssen, T.; Hu, L.; Hao, J. Simultaneous Assessment of Deposition Effects of Base Cations, Sulfur, and Nitrogen Using an Extended Critical Load Function for Acidification. Environ. Sci. Technol. 2007, 41, 1815−1820. (3) Shen, Y. S.; Ma, Y. F.; Zhu, S. M. Promotional Effect of Zirconium Additives on Ti0.8Ce0.2O2 for Selective Catalytic Reduction of NO. Catal. Sci. Technol. 2012, 2, 589−599. (4) Yang, S.; Xiong, S.; Liao, Y.; Xiao, X.; Qi, F.; Peng, Y.; Fu, Y.; Shan, W.; Li, J. Mechanism of N2O Formation During the Low Temperature Selective Catalytic Reduction of NO with NH3 over MnFe Spinel. Environ. Sci. Technol. 2014, 48, 10354−10362. (5) Maitarad, P.; Han, J.; Zhang, D.; Shi, L.; Namuangruk, S.; Rungrotmongkol, T. Structure−Activity Relationships of NiO on CeO2 Nanorods for the Selective Catalytic Reduction of NO with NH3: Experimental and DFT Studies. J. Phys. Chem. C 2014, 118, 9612−9620. (6) Nakajima, F.; Hamada, I. The State-of-the-Art Technology of NOx Control. Catal. Today 1996, 29, 109−115. (7) Liu, C.; Chen, L.; Li, J.; Ma, L.; Arandiyan, H.; Du, Y.; Xu, J.; Hao, J. Enhancement of Activity and Sulfur Resistance of CeO2 Supported on TiO2-SiO2 for the Selective Catalytic Reduction of NO by NH3. Environ. Sci. Technol. 2012, 46, 6182−6189. (8) Peng, Y.; Liu, Z.; Niu, X.; Zhou, L.; Fu, C.; Zhang, H.; Li, J.; Han, W. Manganese Doped CeO2-WO3 Catalysts for the Selective Catalytic Reduction of NOx with NH3: An Experimental and Theoretical Study. Catal. Commun. 2012, 19, 127−131. (9) Moliner, M.; Franch, C.; Palomares, E.; Grill, M.; Corma, A. CuSSZ-39, an Active and Hydrothermally Stable Catalyst for the Selective Catalytic Reduction of NOx. Chem. Commun. 2012, 48, 8264−8266. K


Article


(29) Yang, Z.; Luo, G.; Lu, Z.; Hermansson, K. Oxygen Vacancy Formation Energy in Pd-doped Ceria: A DFT+U Study. J. Chem. Phys. 2007, 127, 074704. (30) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (32) Henkelman, G.; Jonsson, H. Improved Tangent Estimate in the Nudged Elastic Band Method for Finding Minimum Energy Paths and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985. (33) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901−9904. (34) Nolan, M.; Grigoleit, S.; Sayle, D. C.; Parker, S. C.; Watson, G. W. Density Functional Theory Studies of the Structure and Electronic Structure of Pure and Defective Low Index Surfaces of Ceria. Surf. Sci. 2005, 576, 217−229. (35) Cen, W.; Liu, Y.; Wu, Z.; Wang, H.; Weng, X. A Theoretic Insight into the Catalytic Activity Promotion of CeO2 Surfaces by Mn Doping. Phys. Chem. Chem. Phys. 2012, 14, 5769−5777. (36) Liu, J.; Liu, B.; Fang, Y.; Zhao, Z.; Wei, Y.; Gong, X.-Q.; Xu, C.; Duan, A.; Jiang, G. Preparation, Characterization and Origin of Highly Active and Thermally Stable Pd-Ce0.8Zr0.2O2 Catalysts via SolEvaporation Induced Self-Assembly Method. Environ. Sci. Technol. 2014, 48, 12403−12410. (37) Liu, B.; Liu, J.; Li, T.; Zhao, Z.; Gong, X.-Q.; Chen, Y.; Duan, A.; Jiang, G.; Wei, Y. Interfacial Effects of CeO2-Supported Pd Nanorod in Catalytic CO Oxidation: A Theoretical Study. J. Phys. Chem. C 2015, 119, 12923−12934. (38) Chen, L.; Li, J.; Ablikim, W.; Wang, J.; Chang, H.; Ma, L.; Xu, J.; Ge, M.; Arandiyan, H. CeO2−WO3 Mixed Oxides for the Selective Catalytic Reduction of NOx by NH3 Over a Wide Temperature Range. Catal. Lett. 2011, 141, 1859−1864. (39) Chen, L.; Weng, D.; Si, Z.; Wu, X. Synergistic Effect between Ceria and Tungsten Oxide on WO3−CeO2−TiO2 Catalysts for NH3SCR Reaction. Prog. Nat. Sci. 2012, 22, 265−272. (40) Mayernick, A. D.; Janik, M. J. Methane Activation and Oxygen Vacancy Formation over CeO2 and Zr, Pd Substituted CeO2 Surfaces. J. Phys. Chem. C 2008, 112, 14955−14964. (41) Nolan, M. Enhanced Oxygen Vacancy Formation in Ceria (111) and (110) Surfaces Doped with Divalent Cations. J. Mater. Chem. 2011, 21, 9160−9168. (42) Peng, Y.; Wang, C.; Li, J. Structure-Activity Relationship of VOx/CeO2 Nanorod for NO Removal with Ammonia. Appl. Catal., B 2014, 144, 538−546. (43) Zhang, L.; Pierce, J.; Leung, V. L.; Wang, D.; Epling, W. S. Characterization of Ceria’s Interaction with NOx and NH3. J. Phys. Chem. C 2013, 117, 8282−8289. (44) Yuan, R.-M.; Fu, G.; Xu, X.; Wan, H.-L. Brønsted-NH4+ Mechanism versus Nitrite Mechanism: New Insight into the Selective Catalytic Reduction of NO by NH3. Phys. Chem. Chem. Phys. 2011, 13, 453−460. (45) Ding, W.-C.; Gu, X.-K.; Su, H.-Y.; Li, W.-X. Single Pd Atom Embedded in CeO2(111) for NO Reduction with CO: A FirstPrinciples Study. J. Phys. Chem. C 2014, 118, 12216−12223. (46) Zhang, J.; Gong, X.; Lu, G. DFT + U Study of the CO + NOx Reaction on a CeO2(110)-Supported Au Nanoparticle. Chin. J. Catal. 2014, 35, 1305−1317. (47) Zhang, S.; Nguyen, L.; Liang, J.-X.; Shan, J.; Liu, J.; Frenkel, A. I.; Patlolla, A.; Huang, W.; Li, J.; Tao, F. Catalysis on Singly Dispersed Bimetallic Sites. Nat. Commun. 2015, 6, 7938. (48) Xu, L.; Li, X.-S.; Crocker, M.; Zhang, Z.-S.; Zhu, A.-M.; Shi, C. A Study of the Mechanism of Low-Temperature SCR of NO with NH3 on MnOx/CeO2. J. Mol. Catal. A: Chem. 2013, 378, 82−90. (49) Philipp, S.; Drochner, A.; Kunert, J.; Vogel, H.; Theis, J.; Lox, E. S. Investigation of NO Adsorption and NO/O2 Co-Adsorption on NOx-Storage-Components by DRIFT-Spectroscopy. Top. Catal. 2004, 30/31, 235−238.

(10) Chen, L.; Li, J.; Ge, M.; Zhu, R. Enhanced Activity of Tungsten Modified CeO2/TiO2 for Selective Catalytic Reduction of NOx with Ammonia. Catal. Today 2010, 153, 77−83. (11) Yao, X.; Zhang, L.; Li, L.; Liu, L.; Cao, Y.; Dong, X.; Gao, F.; Deng, Y.; Tang, C.; Chen, Z.; et al. Investigation of the Structure, Acidity, and Catalytic Performance of CuO/Ti0.95Ce0.05O2 Catalyst for the Selective Catalytic Reduction of NO by NH3 at Low Temperature. Appl. Catal., B 2014, 150−151, 315−329. (12) Paier, J.; Penschke, C.; Sauer, J. Oxygen Defects and Surface Chemistry of Ceria: Quantum Chemical Studies Compared to Experiment. Chem. Rev. 2013, 113, 3949−3985. (13) Zhang, L.; Zou, W.; Ma, K.; Cao, Y.; Xiong, Y.; Wu, S.; Tang, C.; Gao, F.; Dong, L. Sulfated Temperature Effects on the Catalytic Activity of CeO2 in NH3-Selective Catalytic Reduction Conditions. J. Phys. Chem. C 2015, 119, 1155−1163. (14) Gao, X.; Jiang, Y.; Fu, Y.; Zhong, Y.; Luo, Z.; Cen, K. Preparation and Characterization of CeO2/TiO2 Catalysts for Selective Catalytic Reduction of NO with NH3. Catal. Commun. 2010, 11, 465− 469. (15) Liu, Z.; Zhang, S.; Li, J.; Ma, L. Promoting Effect of MoO3 on the NOx Reduction by NH3 over CeO2/TiO2 Catalyst Studied with in Situ DRIFTS. Appl. Catal., B 2014, 144, 90−95. (16) Qi, G.; Yang, R. T. Performance and Kinetics Study for Low Temperature SCR of NO with NH3 over MnOx-CeO2 Catalyst. J. Catal. 2003, 217, 434−441. (17) Chen, L.; Li, J.; Ge, M. DRIFT Study on Cerium-Tungsten/ Titiania Catalyst for Selective Catalytic Reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590−9596. (18) Peng, Y.; Li, K.; Li, J. Identification of the Active Sites on CeO2WO3 Catalysts for SCR of NOx with NH3: An in Situ IR and Raman Spectroscopy Study. Appl. Catal., B 2013, 140−141, 483−492. (19) Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C. A Superior Ce-WTi Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3. Appl. Catal., B 2012, 115−116, 100−106. (20) Kim, H. Y.; Lee, H. M.; Henkelman, G. CO Oxidation Mechanism on CeO2-Supported Au Nanoparticles. J. Am. Chem. Soc. 2012, 134, 1560−1570. (21) Liu, B.; Zhao, Z.; Wang, D.; Liu, J.; Chen, Y.; Li, T.; Duan, A.; Jiang, G. A Theoretical Study on the Mechanism for Thiophene Hydrodesulfurization over Zeolite L-Supported Sulfided Co-Mo Catalysts: Insight into the Hydrodesulfurization over Zeolite-Based Catalysts. Comput. Theor. Chem. 2015, 1052, 47−57. (22) Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A. Mechanism of the Selective Catalytic Reduction of NO with NH3 over MnOx/ Al2O3 I. Adsorption and Desorption of the Single Reaction Components. J. Catal. 1997, 171, 208−218. (23) Kijlstra, W. S.; Brands, D. S.; Smit, H. I.; Poels, E. K.; Bliek, A. Mechanism of the Selective Catalytic Reduction of NO with NH3 over MnOx/Al2O3 II. Reactivity of Adsorbed NH3 and NO Complexes. J. Catal. 1997, 171, 219−230. (24) Yao, H.; Chen, Y.; Zhao, Z.; Wei, Y.; Liu, Z.; Zhai, D.; Liu, B.; Xu, C. Periodic DFT Study on Mechanism of Selective Catalytic Reduction of NO via NH3 and O2 over the V2O5 (001) Surface: Competitive Sites and Pathways. J. Catal. 2013, 305, 67−75. (25) Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (26) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculation for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (27) Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P. Electron-Energy-Loss Spectra and the Structural Stability of Nickel Oxide: An LSDA+U Study. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 1505−1509. (28) Nolan, M.; Parker, S. C.; Watson, G. W. CeO2 Catalysed Conversion of CO, NO2 and NO from First Principles Energetics. Phys. Chem. Chem. Phys. 2006, 8, 216−218. L


Article

The Journal of Physical Chemistry C (50) Krcha, M. D.; Mayernick, A. D.; Janik, M. J. Periodic Trends of Oxygen Vacancy Formation and C−H Bond Activation over Transition Metal-Doped CeO2 (1 1 1) Surfaces. J. Catal. 2012, 293, 103−115. (51) Zhang, J.; Gong, X. -Q; Lu, G. A DFT + U Study of NO Evolution at Reduced CeO2(110). Phys. Chem. Chem. Phys. 2014, 16, 16904−16908. (52) Peng, Y.; Li, J.; Huang, X.; Li, X.; Su, W.; Sun, X.; Wang, D.; Hao, J. Deactivation Mechanism of Potassium on the V2O5/CeO2 Catalysts for SCR Reaction: Acidity, Reducibility and Adsorbed-NOx. Environ. Sci. Technol. 2014, 48, 4515−4520. (53) Qi, G.; Yang, R. T.; Chang, R. MnOx-CeO2 Mixed Oxides Prepared by Coprecipitation for Selective Catalytic Reduction of NO with NH3 at Low Temperatures. Appl. Catal., B 2004, 51, 93−106. (54) Cao, F.; Xiang, J.; Su, S.; Wang, P.; Sun, L.; Hu, S.; Lei, S. The Activity and Characterization of MnOx−CeO2−ZrO2/γ-Al2O3 Catalysts for Low Temperature Selective Catalytic Reduction of NO with NH3. Chem. Eng. J. 2014, 243, 347−354. (55) Zhang, R.; Zhong, Q.; Zhao, W.; Yu, L.; Qu, H. Promotional Effect of Fluorine on the Selective Catalytic Reduction of NO with NH3 over CeO2-TiO2 Catalyst at Low Temperature. Appl. Surf. Sci. 2014, 289, 237−244. (56) Hernández-Giménez, A. M.; Lozano-Castelló, D.; Bueno-López, A. Effect of CO2, H2O and SO2 in the Ceria-Catalyzed Combustion of Soot under Simulated Diesel Exhaust Conditions. Appl. Catal., B 2014, 148−149, 406−414. (57) Liu, L.; Cao, Y.; Sun, W.; Yao, Z.; Liu, B.; Gao, F.; Dong, L. Morphology and Nanosize Effects of Ceria from Different Precursors on the Activity for NO Reduction. Catal. Today 2011, 175, 48−54. (58) Liu, K.; Liu, F.; Xie, L.; Shan, W.; He, H. DRIFTS Study of a Ce-W Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3. Catal. Sci. Technol. 2015, 5, 2290−2299. (59) Chen, L.; Si, Z.; Wu, X.; Weng, D. DRIFT Study of CuO-CeO2TiO2 Mixed Oxides for NOx Reduction with NH3 at Low Temperatures. ACS Appl. Mater. Interfaces 2014, 6, 8134−8145.

M


Mechanistic Study of Selective Catalytic Reduction of NO with NH3 on

Recommend Documents