CO Oxidation Mechanism on Tungsten Nanoparticle - American

Article pubs.acs.org/JPCC

CO Oxidation Mechanism on Tungsten Nanoparticle Meng Hsiung Weng and Shin Pon Ju* Department of Mechanical and Electro-Mechanical Engineering, Center for Nanoscience and Nanotechnology, National Sun Yat-sen University, Kaohsiung, Taiwan 804 S Supporting Information *

ABSTRACT: CO oxidations on the surface of tungsten nanoparticle W10 and on W(111) surface were investigated by density functional theory (DFT) calculations. The molecular structures and surface−adsorbate interaction energies of CO and O2 on the W10 and W(111) surfaces were predicted. Three CO oxidation reactions of CO + O2 → CO2 + O, CO + O + O → CO2 + O, and CO + O → CO2 were considered in Eley− Rideal (ER) and Langmuir−Hinshelwood (LH) reaction mechanisms. The nudged elastic band (NEB) method was applied to locate transition states and minimum energy pathways (MEP) of CO oxidation on the W10 and W(111) surfaces. All reaction barriers were predicted, implying the CO oxidations on both the W10 nanoparticle and W(111) surfaces prefer the ER mechanism. The electronic density of states (DOS) was calculated to understand the interaction between adsorbates and surfaces for the CO oxidation process. In this study, we have demonstrated that the catalytic ability of W10 nanoparticles is superior to that of the W(111) surface for CO oxidation.

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INTRODUCTION The oxidation of carbon monoxide (CO) to carbon dioxide (CO2) has attracted great interest in recent years because the reaction has many industrial applications such as in catalytic conversion of automobile exhaust and in fuel cells.1,2 The oxidation of the CO molecule on transition metals usually follows two reaction pathways, either the Eley−Rideal (ER) mechanism or the Langmuir−Hinshelwood (LH) mechanism. In the ER mechanism, the CO molecule in the gas phase directly reacts with activated oxygen (O2). In the LH mechanism, the coadsorbed CO and O2 react to form an intermediate. Many studies have been performed for CO oxidation on different transition metals by experimental and theoretical studies.3−22 Among catalysts for CO oxidation, platinum (Pt) metal and Pt-group metals (Ru, Rh and Ir) are the most commonly used formulations due to their high catalytic activity. Alavi et al.3 explored CO oxidation on Pt(111) and identified the reaction path of CO oxidation on Pt(111) via the LH mechanism with an energy barrier of 1.0 eV. The oxidation of CO on Ru(0001) was reported to follow the ER mechanism with an energy barrier of 1.2 eV by Stampfl and Scheffler.10 However, Pt metal and Pt-group metals are both rare and very expensive. Moreover, it is easy for Pt catalysts to become contaminated by CO strong adsorption by reducing their catalytic effectivity. Therefore, it is necessary to find alternative materials to replace Pt-group metals in catalytic and electrocatalytic applications. Tungsten (W) and W-based metals have been demonstrated that possess good catalytic activities for gas molecules, such as CO, NH3, and CH4,23−25 and are applied in different areas. For © 2012 American Chemical Society

example, W carbides have been used as electrocatalysts, cocatalysts, catalyst supports, and electrolytes in different types of fuel cells, such as proton exchange membrane fuel cells (PEMFCs). WC and W2C were also demonstrated to possess a Pt-like characteristic in a variety of catalytic reactions in studies by Levy and Boudart.23 Up to now, a series of single crystal studies of W material have been conducted to investigate the interaction of O,26−32 O2,33 CO,34−36 and CO234 on W material surface. Those studies can provide useful information because the mechanism steps for the CO oxidation reaction on transition metals generally include adsorption of reactants (CO or O2), dissociation of O2, surface diffusion, and desorption of CO2. Chen et al.35 explored the adsorption and dissociation mechanisms of the CO molecule on the W(111) surface by DFT calculation. They reported that the activation energy of CO dissociation is about 0.8 eV, which is also smaller than that of a CO molecule desorbed from the surface. The mechanisms of adsorption and dissociation of CO and CO2 on W(111) surface have been reported using DFT calculation by Chen et al. in 2008.34 They analyzed the molecular structures, vibration frequencies, and binding energies of W(111)/CO2, W(111)/ CO, W(111)/C, and W(111)/O systems. From results of these studies, O2 adsorbed in a top (T) site (located at the top of a W atom) by side-on configuration is the most stable configuration, whereas the O atom prefers to adsorb on the bridge site (located between two three-coordinated W atoms). In addition, Received: May 24, 2012 Revised: July 13, 2012 Published: July 13, 2012 18803

dx.doi.org/10.1021/jp305059x | J. Phys. Chem. C 2012, 116, 18803−18815

The Journal of Physical Chemistry C

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Figure 1. (a) Optimized geometries of the W10 nanoparticle and (b) schematic diagram of a W(111) surface: (left) side view; (right) top view. The labels T, D, B, and S represent top, deep, bridge, and shallow sites. Note that the bridge site is between two surface W atoms.

Monkhorst−Pack grid.43 The calculations were carried out by using the (1 × 1 × 1) and (3 × 4 × 1) Monkhorst−Pack mesh k-points for the W10 nanoparticle and W(111), respectively. A 400 eV cutoff energy, which allows convergence to 1 × 10−4 eV in total energy, was used. Parts a and b of Figure 1 show the structures of the W10 nanoparticle and W(111) surface, respectively. We performed the calculations for adsorption of CO on p(3×2) and p(3×3) lateral cells of the W(111) surface, which corresponds to the coverage of 1/6 monolayer (ML) and 1/9 ML. These studies show that the coverage effect on the calculated stability of this specie is negligible (smaller than 0.1 eV). Therefore, we chose the computationally less expensive p(3×2) model of the W(111) surface as the simulation model. The p(3×2) lateral cell of the W(111) surface is modeled as periodically repeated slabs with 6 layers, as shown in Figure 1(b). The bottom three atomic layers were kept frozen and set to the experimentally estimated bulk parameters, and the remaining layers were fully relaxed during the calculations. The lateral cell has dimensions of a = 13.5 Å, b = 7.80 Å, and c = 17.47 Å, which includes a vacuum region of thickness ca. 13 Å upon the W(111) substrate, which guarantees no interactions between the upper and lower slab of the W(111) substrate. In this study, coadsorption energies were calculated according to the following equation:

the CO molecule also prefers to coordinate to a bridge site by the lying-down coordination where the C and O atoms are bound preferentially at the bridge and top sites of metal surface, respectively. The W(111) surface is more structurally open than the closepacked surfaces of W(100) and W(110). Therefore, the W(111) surface can accommodate more adsorbates per unit surface and has a much higher sticking probability. Because heterogeneous catalysis is sensitive to the surface structure, in this work we will focus on CO oxidation on the W(111) surface. On the other hand, the shape and size effects of metal materials have demonstrated that there is a strong influence on the catalytic activity and electronic properties of catalysts37,38 In this study, we chose the W10 nanoparticle (diameter smaller than 1.0 nm) as our target to investigate CO oxidation mechanism compared to that on W(111) surface. We predicted the lowest-energy structure of W10 nanoparticle by theoretical methods (Supporting Information). The structural and coadsorption properties of CO and O2 on W(111) and W10 nanoparticle surfaces were studied in this work. Finally, the reaction barriers and electronic properties for CO oxidation on the two surfaces were investigated. We believe that this study is vital for an understanding of the factors affecting CO oxidation when catalyzed by nanometer-scale tungsten.

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SIMULATION MODELS All calculations were performed by Vienna ab initio simulation package (VASP) with spin polarization.39 To investigate the adsorption properties of the molecule on the metal surface, we used the projector-augmented wave method (PAW)40 in conjunction with the revised Perdew−Burke−Ernzerhof (rPBE)41 density functional rather than the Perdew−Wang (PW91)42 density functional. This is because the PW91 method predicts larger adsorption energies of small molecules on metal surfaces when compared with the rPBE and experimental data.44 The Brillouin zone was sampled with the

ΔEcoads = E[total] − (E[substrate] + E[CO] + E[O2(or O)])

(1)

In the aforementioned equation, the E[total], E[substrate], ECO, and EO2 (or O) correspond to the electronic energies of adsorbed species on the W substrate, the bare W substrate, gaseous CO, and gaseous O2 (or O), respectively. Here E is the electronic energy calculated at 0K in vacuum. Therefore, negative coadsorption energy indicates a stable adsorption. 18804



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The nudged elastic band (NEB) method45,46 was applied to locate transition states, and minimum energy pathways (MEP) were constructed accordingly. The NEB method is an efficient method for finding the MEP between the given initial and final state of a transition state. At least 16 images were used to locate each calculated transition state (TS).

are listed in Tables 1 and 2. For CO + O2 adsorption on the W10, as shown in Figure 3(c), the CO-top-W(5),O2-top-W(6) configuration with a coadsorption energy of −5.15 eV is energetically the most stable among the calculated adsorptions. In CO-top-W(5),O2-top-W(6), CO adsorbs at the top site on the W(5) atom of the nanoparticle, and the bond length of W−C and C−O is 2.044 Å and 1.196 Å, respectively. In addition, O2 adsorbs at the top site on the W(6) atom of the particle by sideon coordination and an O−O peroxide bond length of 1.497 Å. The adsorbed O2 bond length is about 21% longer than that of molecular oxygen in the gas phase because of the transfer of charge from the W10 surface to oxygen. This charge transfer causes the increase of density in the antibonding πg* orbital of oxygen and then increases the bond length of O−O. In addition, coadsorption energies of CO-top-W(8),O2-top-W(6) and CO-top-W(8),O2-top-W(6)-2 configurations are calculated to be −5.08 and −5.01 eV, which are less stable than that of CO-top-W(5),O2-top-W(6). For CO-top-W(8),O2-top-W(6), CO adsorbs at the top site on the W8 atom, and the bond length of W−C and C−O is 2.013 and 1.175 Å, respectively. In addition, O2 adsorbs on the top site of the W6 atom by side-on coordination and an O−O peroxide bond length of 1.492 Å. From our calculations, the vibration frequency and Bader charge summary of adsorbed O2 for CO-top-W(8),O2-top-W(6) configuration are 859 cm−1 and −1.41 e, respectively. In COtop-W(8),O2-top-W(6)-2, CO adsorbs at the same site with COtop-W(8),O2-top-W(6), and the bond length of W−C and C−O is 2.008 and 1.176 Å, respectively. In addition, O2 adsorbs on the top site of the W atom with a different orientation (with a rotation of 90 degrees) from CO-top-W(8),O2-top-W(6). The result of the Bader charge summary for the adsorbed O2 molecule is −1.39 e and the O−O bond is 1.487 Å. For CO2 + O adsorption on the W10, the CO2-top-W(8)-2,O-top-W(6) configuration with coadsorption energy of -8.25 eV is the favored structure. In the CO2-top-W(8)-2,O-top-W(6) configuration, the CO2 molecule and O atom adsorb at the top of the W8 and W6 atoms of the nanoparticle, respectively. The bond lengths of W−C and C−O are 2.10 and 1.204/1.355 Å, respectively. For CO + O adsorption on the W10, CO-topW(6),O-bridge-W(6)‑(8) is found to be the most stable configuration. In CO-top-W(6),O-bridge-W(6)‑(8), CO adsorbs at the top site on W(6) atom of the nanoparticle, and the O atom adsorbs at the bridge site between W(6) and W(8) atoms. Regarding the coadsorptions of CO + O2, CO2 + O, and CO + O on the W(111) surface, as shown in Figure 4, for CO + O2, the coadsorption energy of the CO-bridge(lying‑down);O2top(side‑on) configuration is found to be the largest by CO adsorbing on the bridge site of the W(111) surface by the lyingon configuration. The CO + O2 coadsorption energies on W(111) were also calculated to be -4.55 eV, still lower than that of the most stable coadsorption configuration (CO-topW ( 5 ) ,O 2 -top-W ( 6 ) ) on the W 1 0 surface. In CObridge(lying‑down);O2-top(side‑on), the CO adsorbs at the bridge site by the lying-down configuration, and the bond length of W−C and C−O is 1.987 and 1.234 Å, respectively. In addition, O2 adsorbs at the top site of the W atom by side-on configuration, and an O−O peroxide bond length of 1.482 Å, about 20% longer than that of the O2 molecule in the gas phase. The vibration frequency and Bader charge summary of adsorbed O2 for CO-bridge(lying‑down);O2-top(side‑on) configuration are found to be 982 cm−1 and −1.40 e, respectively. It should be mentioned that previous studies12,52−54 show that the carbonate (CO3) and peroxo-type (OOCO) intermediates

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RESULTS AND DISCUSSION To make certain that our computational approach is appropriate, the lattice constant and W−W bond distance of bulk tungsten were examined, and the results show 3.183 Å and within 2.750−2.754 Å, respectively, which are both consistent with the experimental data of 3.165 Å and 2.741 Å.47 The bond distance of gas-phase O2, CO, and CO2 are also predicted to be 1.237 Å (O−O for O2), 1.150 Å (C−O for CO), and 1.176 Å (C−O for CO2) in a 15 × 15 × 15 Å3 cubic box, which are in good agreement with available experimental data and theoretical results (1.21 Å for O−O of O2;48 1.15 Å for C− O of CO;49 1.16 Å for C−O of CO2).50 Coadsorption of CO + O2, CO2 + O, and CO + O. To map out the mechanism of CO oxidation, we calculated possible coadsorptions of CO + O2, CO2 + O, and CO + O on the W10 and W(111) surfaces, shown in Figures 3 and 4.

Figure 2. (a) HOMO orbital and (b) Fukui function f −k of the W10 nanoparticle.

Because the surfaces of small clusters usually have no welldefined planes, we have predicted the most stable binding sites for O, CO, and CO2 on the W10 surface by two criteriathe orbital roughness50 and Fukui function.51 Metiu et al.50 described the orbital roughness where the strongest binding of the electron donor occurs to be at the site where the lowest unoccupied molecular orbital (LUMO) protrudes farthest; similarly, the strongest binding site of the electron acceptor is where the highest occupied molecular orbital (HOMO) protrudes farthest. Parr et al.51 also stated the atom with the largest value of the Fukui function is, in general, associated with the most reactive site. They found that a site having a larger f −k value is a better electron donor; whereas one having a larger f +k value is a better electron acceptor. When O2 and CO adsorb on the W metal surface, both molecules are electronic acceptors. Therefore, we calculated the highest occupied molecular orbital (HOMO) and Fukui function ( f k−) profiles for the W10 nanoparticle, as shown in Figure 2. The adsorbates were placed at W atoms with large protruding HOMO orbitals and f −k isosurface because those W atoms could have good activity. (The calculated data of Fukui function in W10 nanoparticle are listed in Table S-III of the Supporting Information). The coadsorption energies, geometrical parameters, vibrational frequencies, and Bader charges on W10 and W(111) surfaces 18805



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Figure 3. Optimized geometries of CO + O2, CO2 + O, and CO + O coadsorption on the W10 surface. The bond lengths are given in angstroms (Å).

next step, coadsorption of CO and O2 (W10-O2-CO‑‑IM1) occurs on the W10 surface. First, an O2 molecule chemisorbs on a top site of the surface, and then a gas CO adsorbs on a neighbor top site. This coadsorption process is found to be barrierless. The coadsorption configuration also corresponds to the COtop-W(8),O2-top-W(6) with an coadsorption energy of −5.08 eV (Figure 3(a) and Table 1). After coadsorption, the dissociation of adsorbed O2 has taken place and one atomic oxygen starts to approach the adsorbed CO by crossing two energy barriers of 0.42 and 0.32 eV, forming an adsorbed carbon dioxide (OCO) complex, which is 8.16 eV lower than the energy of the reactant. Finally, the adsorbed OCO desorbs from the surface and forms a CO2 in gas via a barrier of 0.82 eV. In another pathway of LH-Path-I, a coadsorption of CO and O2 has been described as the intermediate W10-O2-CO‑‑IM3, which corre-

may exist on gold (Au) and silver (Ag) catalysts, which would be the important precursor states for CO2 formation. However, both stable adsorption configurations of CO3 and OOCO species on the W10 and W(111) surfaces are not visible in this work. Reaction Profile for CO Oxidation on the W 10 Nanoparticle and W(111) Surfaces. On the basis of the adsorption results, we studied a complete reaction mechanism for CO oxidation on the W10 nanoparticle and W(111) surfaces. To find MEP for CO oxidation on the W metal surface, the LH and ER reaction mechanisms were considered. The MEP of CO oxidation on the W10 surface is depicted in Figures 5 and 6. As shown in Figure 5(a), the pathway I of the LH mechanism starts from the individual CO and O2 molecules in the gaseous phase as the reactant W10+O2+CO‑‑R1. In the 18806



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Figure 4. Optimized geometries of CO + O2, CO2 + O, and CO+ O coadsorption on the W(111) surface. The bond lengths are given in angstroms (Å).

Table 1. Coadsorption Energies (eV), Geometrical Parameters (Å), Predicted Vibrational Frequencies (cm−1), and Bader Charges (e) for CO + O2, CO2 + O, and CO + O Coadsorption on the W10 Nanoparticle Bader charge species

Ecoads

CO-top-W(8), O2-top-W(6) CO-top-W(8), O2-top-W(6)-2 CO-top-W(5), O2-top-W(6) CO-near-O2, O2-top(end‑on)-W(6)

−5.08 −5.01 -5.15 −2.70

CO2-top-W(8), O-top-W(6) CO2-top-W(8)-2, O-top-W(6) CO2-top-W(5), O-top-W(6)

−7.52 −8.25 −8.22

CO-top-W(8), O-top-W(6) CO-top-W(6), O-bridge-W(6)‑(8)

−9.05 −9.08

d(O−O) 1.492 1.487 1.497 1.355

d(C−O)

d(W−O)

CO + O2 Coadsorption 1.175 1.964; 1.976 1.176 1.963; 2.006 1.196 1.960; 2.03 1.146 1.845 CO2 + O Coadsorption 1.221;1.268 1.755 1.204;1.355 1.758 1.216;1.405 1.760 CO + O Coadsorption 1.177 1.757 1.172 1.856; 2.062

d(W−C)

ν(O−O)

2.013 2.008 2.044

859 863 860 1146

adsorbed O1;O2 −0.68; −0.67; −0.65; −0.28;

−0.73 −0.72 −0.72 −0.81

sum. −1.41 −1.39 −1.37 −1.09

2.157 2.10 2.190 1.996 2.05

Subsequently, the dissociation reaction of O2 (O2 → O−O) occurs. Then the O atom approaches the CO molecule on the

sponds to the configuration of CO-top-W(8),O2-top-W(6)-2 with an coadsorption energy of −5.01 eV (Figure 3(b) and Table 1). 18807



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Table 2. Coadsorption Energies (eV), Geometrical Parameters (Å), Predicted Vibrational Frequencies (cm−1), and Bader Charges (e) for CO + O2, CO2 + O, and CO + O Coadsorption on the W(111) Surface Bader charge species

Ecoads

CO-bridge(lying‑down); O2-top(side‑on) CO-shallow ; O2-top(side‑on) CO-deep; O2(side‑on) CO-near-O2; O2(side‑on) CO-near-O2; O2(end‑on)

−4.55 −4.21 −3.26 −2.76 −1.95

CO2-bridge; O-top CO2-shallow; O-top OCO-shallow; O-top

−6.51 −7.19 −6.82

CO-bridge(lying‑down); O-top

−8.61

d(O−O) 1.482 1.478 1.488 1.494 1.315

d(W−O)

d(C−O)

CO + O2 Coadsorption 1.962; 1.955 1.234 1.926; 1.956 1.183 1.962; 1.955 1.207 1.967; 1.969 1.143 1.933 1.114 CO2 + O Coadsorption 1.742 1.212;1.449 1.734 1.291;1.329 1.757 1.177;1.179 CO + O Coadsorption 1.748 1.249

d(W−C)

ν(O−O)

adsorbed O1;O2

sum.

1.987 2.030 2.068

982 863

−0.72; −0.68 −0.67; −0.74

−1.40 −1.41

850 1207

−0.71; −0.72 −0.21; −0.79

−1.43 −1.00

2.188 2.157 3.414 1.985

intermediate W10-O2+CO‑‑IM1. To form the final product, CO2(gas), the CO(gas) directly extracts one O atom of adsorbed O2(ads) with end-on configuration via the transition state (TS)ERI‑1 with an energy barrier of 0.37 eV. In addition, note that the adsorbed O2 with side-on configuration has not been shown in Figure 6(a) because the O2 bond is always broken when the gas CO is close to the adsorbed O2 in our calculations. It is possible that adsorption of O2 on W10 with side-on configuration is strong, implying the weakening of the O−O bond. Another possible pathway for CO2 oxidation is considered by the attachment of CO to an atomic O on the surface directly. This reaction can be described as pathway II of the ER reaction (O2(gas) + CO(gas) → O2(ads) + CO(gas) → O(ads) + O(ads) + CO(gas) → O(ads) + CO2(gas)), as shown in Figure 6(b). After dissociation of adsorbed O2, the CO(gas) directly extracts one O atom from the top site of the W atom by passing over an energy barrier of 0.62 eV in the presence of a neighboring O atom at bridge sites. In the next reaction mechanism, we consider a path where the CO molecule reacts with an O atom at the top site in the absence of chemisorbed O atoms at bridge sites. In Figure 6(c), the calculated reaction barrier is 0.99 eV for the formation of CO2 because the bonding of O on W metal is too strong to react with CO. This result also indicates that a much higher energy is required for the formation of the CO2 in the absence of a neighboring O atom when compared to that in the presence of a neighboring O atom. However, the bonding of O is demonstrated to become weaker with increasing O coverage. Because dissociation of O2 molecule to O−O atoms easily occurs on W metals, the gas CO2 will be easily produced by the ER mechanism at high O coverage. A similar pathway analysis as W10 was also repeated for the W(111) surface. The potential energy diagrams of CO oxidation on the W(111) surface for both LH and ER paths are given in Figures 7 and 8, respectively. For pathway I of the LH mechanism, as shown in Figure 7(a), the coadsorption of CO and O2 is represented as W(111)-CO-O2‑IM2 corresponding to the configuration of CO-bridge(lying‑down);O2-top(side‑on) with adsorption energy of −4.55 eV. In the next step, the adsorbed O2 crosses the transition state (TS)LHI‑1 to form two adsorbed O atoms with an energy barrier of 0.32 eV. This calculated activation energy is also slightly higher than that of O2 dissociation on W(111) in the absence of a CO molecule30 due to the existence of an adsorbed CO giving a lateral force. Proceeding to the next step, the OCO complex is formed by

top site, passing over the transition state (TS)LHI‑4 with an energy barrier of 0.89 eV to produce an adsorbed OCO complex. The overall LH reaction (O2(gas)+ CO(gas) → O2(ads) + CO(gas) → O2(ads) + CO(ads) → O(ads) + OCO(ads) → O(ads) + CO2(gas)) is thus calculated to be exothermic by 7.76 eV. It is found that the calculated energies of all reaction intermediates and transition states (TSLH1 and TSLH2) along the aforementioned pathway are all below that of reactant W10+O2+CO‑‑R1, suggesting that these reactions can take place easily. To gain a more detailed understanding of the CO oxidation capability of W 10 , we turn to another consideration: a CO2 gas can be produced via a pathway II of the LH mechanism (O(gas)+ CO(gas) → O(ads)+ CO(gas) → O(ads) + CO(ads) → OCO(ads) → CO2(gas)), as shown in Figure 5(b). This pathway is initiated from the configuration of a gas CO and gas O on the nanoparticle surface as the reactant W10+O+CO‑‑R1. The configuration for coadsorption of CO and O has been described as the intermediate W10-O-CO‑‑IM2, which corresponds to the CO-top-W(6);O-bridge-W(6)‑(8) configuration with a coadsorption energy of −9.08 eV (Figure 3(h) and Table 1). After coadsorption, CO oxidation proceeds to form OCO by an association of the O atom and CO molecule via transition state (TS)LHII‑1 (with a activation barrier of 1.53 eV). In addition, the coadsorption configuration of an O atom and CO molecule adsorbed on different top sites of surface is also considered as W10-O-CO‑‑IM5, which corresponds to the CO-top-W(8),O-top-W(6) configuration with a coadsorption energy of −9.05 eV (Figure 3(i) and Table 1). Then the OCO complex is formed by an association of the O atom and CO molecule via transition state (TS)LHII‑3 (with a activation barrier of 1.09 eV). Finally, we found that the energy gain in the CO2 formation via this mechanism is about 1.95 eV, implying that it is difficult for the CO2 molecule to desorb from the W10 surface because of its large adsorption energy. In addition, it should be noted that we do not show the LH mechanism of the most energetically stable configurations COtop-W(5),O2-top-W(6) because according to our findings, there are many diffusion barriers which need to be overcome because the distance between adsorbed O2 and CO is farther than that of other intermediates. Therefore, those diffusion barriers raise the difficulty of CO oxidation. For the ER mechanism, pathway I (Figure 6(a)) is considered via the reaction: O2(gas) + CO(gas) → O2(ads) + CO(gas) → O(ads) + CO2(gas). In the reaction, the end-on adsorption of O2 on the W10 surface is considered as the 18808



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Figure 5. Schematic potential energy profiles of the LH reaction path for (a) CO + O2 and (b) CO + O reactions on the W10 nanoparticle.

shown in Figure 7(b). In this pathway, two energy barriers (1.11 and 1.90 eV) are needed to produce a second gas molecule CO2. These barrier results also imply that the adsorbed CO2 may not desorb from the W(111) surface at room temperature. For the ER mechanism, pathway I for the W(111) surface (Figure 8(a)) can be described as O2(gas) + CO(gas) → O2(ads) + CO(gas) → O(ads)+ C O2(gas). To compare this with pathway I for the W10 nanoparticle, the side-on adsorption and the end-on adsorption of O2 on the W(111) surface are also considered as

the association of a single oxygen atom and the adsorbed CO molecule. The activation energy of this process is 0.46 eV, slightly higher than that of the most possible reaction channel of 0.42 eV on the W10 surface in the same pathway (Figure 5(a)). Finally, the adsorbed OCO complex desorbs from surface to gas phase via a barrier of 0.86 eV and is exothermic by 6.60 eV. From the results above, we found that it is slightly easier for the CO oxidation on the W10 surface to occur via the LH mechanism than on W(111). After the formation of the first gas CO2, an LH-like mechanism is also considered as 18809



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Figure 6. Schematic potential energy profiles of the ER reaction path for (a) CO + O2, (b) CO + O + O, and (c) CO + O reactions on the W10 nanoparticle.

O(ads)+ O(ads) + CO(gas) → O(ads) + CO2(gas), as shown in Figure 8(b). After dissociation of adsorbed O2, an O atom of the oxygen moves to the bridge site whereas the other O atom stops on a top site. The value of the activation barrier for this dissociation step is 0.26 eV. Then the CO(gas) directly extracts one O atom from the top site, in the presence of an O atom at the bridge site by overcoming an energy barrier of 0.92 eV, a value 0.30 eV larger than that on the W10 surface in pathway II of its ER reaction (Figure 6(b)). In Figure 8(c), we also consider pathway III of the ER mechanism as O (ads) + CO(gas) → CO2(gas), where a gas CO molecule reacts with the remaining O atom at top site in the absence of a neighboring oxygen atom. The CO2(gas) can be formed by CO in gas phase by

initial intermediates, respectively. To form the gas CO2, CO reacts with one O atom of adsorbed O2 in the side-on adsorption orientation by crossing a lower energy barrier, 0.34 eV. This value not only is the lowest among all elementary steps of CO oxidation pathways but also is slightly lower than the energy barrier of 0.37 eV for W10. In the other channel, an O atom of adsorbed O2 with end-on configuration is extracted directly by gas CO to form gas CO2. The value of the activation barrier for this step is 0.63 eV, larger than that of the former case mainly because the strong O2 binding by side-on adsorption weakens the bonding between O−O atoms. In addition, pathway II of the ER reaction is also considered via the reaction pathway: O2(gas) + CO(gas) → O2(ads) + CO(gas) → 18810



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Figure 7. Schematic potential energy profiles of the LH reaction path for (a) CO + O2 and (b) CO + O reactions on the W(111) surface.

CO + O + O → CO2 + O, and CO + O → CO2 are considered as initial configurations for comparison with other transition metals. In the first reaction pathway, CO + O2 → CO2 + O, we found that the reaction barriers of the most probable pathways on W10 and W(111) surfaces (0.37 eV for W10 and 0.34 eV for W(111)) are both lower than that for Au(221) (0.59 eV53) and Au(211) (0.65 eV53). As for the second reaction pathway, CO + O + O → O + CO2, the energy barriers on W10 and W(111) surfaces are found to be 0.62 eV and 0.92 eV, smaller than that found for Ir(100) (1.56 eV11). Furthermore, the energy barrier of the reaction pathway on W10 surface is smaller than that for Pt(111) (0.46 eV55), whereas on the W(111) surface, it is

extracting the adsorbed O atom with a large energy barrier of 1.02 eV. This barrier is slightly larger than that of pathway II of the ER reaction in the presence of a neighbor O atom; therefore, it will be more difficult to produce gas CO2 though pathway III. In addition, this calculated barrier is also larger slightly that for W10 in the same reaction path, shown in Figure 6(c). Comparison of CO Oxidation on the W10 Nanoparticle and W(111) Surfaces with Transition Metals. To thoroughly understand the CO oxidation ability on the W10 nanoparticle and W(111) surfaces, three coadsorption structures for CO oxidation pathways CO + O2 → CO2 + O, 18811



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Figure 8. Schematic potential energy profiles of the ER reaction paths for (a) CO + O2, (b) CO + O + O, and (c) CO + O reactions on the W(111) surface.

(0.29 eV55). Nevertheless, the dissociation of O2 on Au(111) (energy barrier 1.97 eV55) is much more difficult than for either

larger. In addition, we also found the energy barriers of the pathway for both surfaces are larger than that for Au(111) 18812



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W10 or W(111) surfaces (

CO Oxidation Mechanism on Tungsten Nanoparticle - American

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