Theoretical Study of the CO Oxidation Mediated by Au3+, Au3, and

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Theoretical Study of the CO Oxidation Mediated by Au3+, Au3, and Au3-: Mechanism and Charge State Effect of Gold on Its Catalytic Activity Fang Wang, Dongju Zhang,* Xiaohong Xu, and Yi Ding* Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, People’s Republic of China ReceiVed: April 13, 2009; ReVised Manuscript ReceiVed: July 16, 2009

By carrying out density functional theory calculations, we studied the CO oxidation promoted by cationic, neutral, and anionic Au trimers, which represent the prototypes of Au-cluster-based catalysts with different charge states. The reaction is explored along three possible pathways: one involves the reaction of the initial complexes between Au trimers and O2 with CO; another is related to O2 interacting with the complexes between Au trimers and CO; and the third refers to a self-promoting mechanism; that is, the second CO oxidation is promoted by a preadsorbed CO molecule. The theoretical results show that all three species may promote the reaction, as indicated by calculated low energy barriers and high exothermicities, supporting the fact that cationic, neutral, and anionic Au species were all observed to present catalytic activity toward CO oxidation. Along the reaction coordinates for all of the reactions, Au-carbonate species are not found to be the necessary intermediates, although they are calculated to be energetically very stable. In contrast, by performing atom-centered density matrix propagation molecular dynamics simulations, the formation of such highly stable species is attributed to the effective collision between Au-oxides and CO2 with the carbon atom of CO2 directly attacking the O atom in the oxides. The present results enrich our understanding of the catalytic oxidation of CO by Au-cluster-based catalysts. 1. Introduction Au nanoparticles have attracted sustained experimental1–10 and theoretical interest11–17 since the pioneering discovery by Haruta et al.18 that Au can exhibit surprisingly high catalytic activity when it is highly dispersed on certain catalyst supports. Pure and supported Au clusters and nanoparticles have now become one of the most promising catalysts for many important chemical reactions at low temperature, such as CO oxidation,2–4 propylene epoxidation,19 water-gas shift,20,21 and hydrogenation of unsaturated hydrocarbons.22 Recently, the interaction between Au clusters and phenyl-coinage has been studied by Liu et al.,17 which provides insight into the mechanism of important heterogeneous catalytic processes in the organometallic chemistry field. Among this research, CO oxidation on Au-based catalysts is of particular interest and has been the subject of extensive studies.23–27 It is important not only for the abatement of toxic pollutant CO but also for many practical applications, such as developing CO sensors and improving the efficiency of CO2 lasers.23 In the past decade, many experimental and theoretical studies for the CO oxidation have been devoted to unveiling the origin of the catalytic activity of Au nanoparticles. The effects of different factors, including size and shape,28,29 oxidation state,27,30 and preparation method31 of Au particles, as well as the heterojunction between the deposited Au particles and their support,32,33 on the activity of Au particles have been investigated, and different models have been proposed. However, it is noteworthy that there is no consensus on the crucial factors that govern the catalytic activity of Au nanoparticles and that the microscopic mechanism of CO oxidation is still under * Corresponding author. E-mail: [email protected] (D.Z.); yding@ sdu.edu.cn (Y.D.).

debate.34 In particular, two basic issues, how the charge state effect of Au nanoparticles influences the catalytic activity, and whether the carbonate species is formed during CO oxidation, and, if so, how it is formed during the reaction, remain controversial. Davis35 speculated that cationic Au may be the key to use this noble metal in areas usually reserved for Pt and Pd, while Sanchez26 and Cox36 proposed that negatively charged Au clusters possess the catalytic activity. In contrast, Weiher37 and Wu38 claimed that neutral Au species is catalytically active. In addition, some authors attributed the catalytic activity to the coexisted Au0 and Aun+.32,39–41 Concerning Au-carbonate species, Hartua1 and Ha¨kkinen11 proposed that it is an intermediate (precursor) for forming CO2, while Schubert42 and Liu43 claimed that such species does not exist. These discrepancies for the two basic issues clearly indicate that our knowledge about the charge state effect of Au nanoparticles on the catalytic activity remains uncertain, and the relevant mechanistic details in the catalytic process are not clearly established yet. In fact, an explicit and complete elucidation on all elementary steps involved in the reaction is not experimentally possible. So, it is highly desired to obtain a complementary source of information by performing quantum-chemical calculations. In this work, we focus our attention on the two basic issues. By performing density functional theory (DFT) calculations, we unravel the mechanistic details of the CO oxidation promoted by neutral, cationic, and anionic Au trimers, which represent the simplest prototypes of Au clusters with different charge states. On the basis of the calculated results, we expect to provide a better understanding about how the additional positive and negative charges affect the reactivity of Au-cluster-based catalysts, and whether the carbonate species is formed during CO oxidation, and, if so, how it is formed during the reaction.

10.1021/jp903392w CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

CO Oxidation Mediated by Au3+, Au3, and Au3-

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TABLE 1: Comparison for Calculated and Experimental Electron Affinities (EAs) and Ionization Potentials (IPs) for Au, Au3, AuO, and O2a calculated EA Au Au3 O2 AuO

2.31 3.84 0.55 2.46

IP 9.58 7.30 12.43 10.17

experiment EA b

2.31 3.40-3.95d 0.45d 2.37d

IP 9.26 ( 0.10c 7.27 ( 0.15c 12.05e

a Energies are in eV. b Reference 59. c Reference 60. d Reference 61. e Reference 62.

2. Computational Details All calculations have been conducted by using Gaussian 03 code.44 We chose the Perdew-Wang’s 91 exchange and correlation functional (commonly referred to as PW91)45–47 in view of its excellent performance for describing Au clusters.48 Considering the strong relativistic effect of Au, the Los Alamos LANL2DZ effective core pseudopotentials (ECP) and valence double-ζ basis sets49,50 were used for Au. The standard 6-311+G(d) basis set was used for oxygen and carbon. No symmetric constraints were imposed during geometrical optimizations. The synchronous transit-guided quasi-Newton method51 was adopted for locating the transition states. The nature (minima or first-order saddle points) of optimized structures is identified by the subsequent frequency calculations, which also provide zero-point vibrational energy (ZPE) corrections. Intrinsic reaction coordinates (IRC)52 calculations have been performed to verify that each saddle point links two desired minima. The natural bond orbital (NBO) analysis is performed to show the bonding nature between Au trimers and the O2 or CO molecule. Stability tests of wave functions for all identified stationary points have been carried out to ensure that the lowest energy solutions in the SCF procedures are found. All calculations are carried out by resolving unrestricted Kohn-Sham equations. To obtain the right Kohn-Sham wave functions for a few singlet species, including Au3O2+, Au3O2-, Au3O+, Au3O-, OCAu3O+, and OCAu3O-, which essentially are of the multiconfiguration nature, the symmetry-broken Kohn-Sham scheme is used. To ascertain the possible formation of Au-carbonate species, we have carried out atom-centered density matrix propagation (ADMP)53–55 molecular dynamics (MD) calculations. The stationary points on the potential energy surfaces (PESs) from quantum-chemical calculations were used as the starting point for the MD simulations. Because of the employment of ECP for Au, the fictitious electron mass of -0.1 amu was utilized to make equal weighting for all electrons, and the time step of 0.05 fs was chosen. A thermostat was applied to maintain a constant temperature of 298 K.56,57 The initial nuclear kinetic energy was set to 0.30 kcal mol-1, which is much lower than the HOMO-LUMO gaps of the stationary points, to ensure the reaction proceeding along the ground-state PES. The calculated trajectory was visualized via GaussView.58 3. Results and Discussion 3.1. Calibration. To support our choice for the combination of the functional, ECP, and basis sets described above, we provide benchmark calculations of the electron affinities (EAs) and ionization potentials (IPs) for Au, Au3, molecular O2, and AuO, and of the geometrical parameters for O2, CO, and CO2. As shown in Table 1, the calculated EA and IP values for Au, Au3, O2, and AuO are comparable to the corresponding

Figure 1. Optimized geometries for O2, CO, CO2, and the complexes of Au3+, Au3, and Au3- with O2/CO. The symbols and values in square brackets denote the electronic states and binding energies (in kcal mol-1) of the complexes, and the values in parentheses for CO, O2, and CO2 are experimental bond lengths (in Å). The golden, gray, and red balls denote Au, C, and O atoms, respectively.

experimental findings.59–62 In particular, the theoretical EA value for AuO (2.46 eV) is in good agreement with the experimental value (2.37 eV), indicating that the PW91-LANL2DZ-6311+G(d) level gives a satisfactory description for the interaction of Au and oxygen. The optimized geometrical parameters for O2, CO, and CO2 are shown in Figure 1, where the corresponding experimental values63–65 are also given for comparison. It is found that the theoretical results are in excellent agreement with experiment values. Additionally, the bond energies of Au dimer and O2 have also been tested at the PW91LANL2DZ-6-311+G(d) level, and the calculated values (2.19 eV for Au2 and 5.95 eV for O2) are in fair agreement with experimental findings (2.29 eV for Au2 and 5.12 eV for O2).43 These facts indicate the acceptable accuracy and reliability of the level of theory used. Another reason we prefer the functional PW91 for our calculations over the most popular functional B3LYP in the quantum-chemical field today is that we want to provide an application prototype of other functionals, which perform well for an Au-containing system. 3.2. Complexes of Au Trimers with the O2 and CO Molecules. The initial step of the reaction involves the formation of complexes in which either the CO or the O2 binds to Au trimers. Theses complexes are hereafter denoted as Au3O2+, Au3O2, Au3O2-, Au3CO+, Au3CO, and Au3CO-. The optimized ground-state geometries are shown in Figure 1, and those for the corresponding excited electronic states are not given for simplification. It should be noted that the interactions of the O atom of CO with Au trimers, which are referred as the upsidedown adsorption of CO on Au trimers, have also been considered. The calculated results indicate that these nonnormal adsorption geometries are much less stable than the corresponding normal adsorptions with the C atom of CO adsorbed on Au trimers. Concretely, the calculated binding energy for the upside-

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TABLE 2: Computed Atomic Charges (q) of O2 and CO Moieties in 3Au3O2+, 2Au3O2, 3Au3O2- and 1Au3CO+, 2 Au3CO, 1Au3CO-, Respectively, by Natural Population Analysis species q

3

Au3O2+ 0.036

2

Au3O2

–0.412

3

Au3O2–0.324

1

Au3CO+ 0.120

2

Au3CO

–0.054

1

Au3CO–0.265

down adsorption is 9.91 kcal mol-1 on Au3+ and 3.61 kcal mol-1 on Au3, which are less stable by 32.84 and 35.74 kcal mol-1 than those for the normal adsorptions, respectively. In the case of Au3-, we cannot even local a minimum with the upsidedown adsorption of CO, and only the normal adsorbed geometry is obtained. Thus, we consider that the normal adsorption of CO is dominant and the upside-down adsorption of CO was not taken into account. Because of the triplet ground state of O2 having two unpaired electrons in the degenerated 2π antibonding orbitals, the spin-orbital coupling may occur as it interacts with Au trimers. We have investigated the complexes between Au trimers and O2 with two possible spin states, that is, the singlet and triplet states for both Au3O2+ and Au3O2-, and doublet and quartet structures for Au3O2. The ground states of Au3O2+ and Au3O2are found to be in their triplet states, while it is doublet for Au3O2. As compared to a free O2 molecule with a bond length of 1.220 Å, we find that the O-O distance (1.223 Å) in 3Au3O2+ is slightly elongated, while those in 2Au3O2 and 3Au3O2remarkably increase to 1.292 and 1.263 Å. This result can be understood by analyzing the electronic interaction between Au trimers and O2. As shown in Table 2, in Au3O2+, the amount of electron transfer from O2 to Au3+ is only 0.036e, leading to the slightly elongated O-O bond length. In contrast, 0.412e of Au3 in Au3O2 and 0.324e of Au3- in Au3O2- are transferred to the O2 molecule, resulting in the evident increase of O-O distance. In addition, comparing the charge transfer in the three complexes, we find that the amount of charge transfer in Au3O2+ is the smallest. This can be attributed to the larger ionization potential of O2 (12.43 eV) as compared to those of Au3 and Au3-, which make the charge transfer from O2 to Au3+ in Au3O2+ more difficult than that in Au3O2 from Au3 to O2 and that in Au3O2- from Au3- to O2. This fact implies that the interaction between small cationic gold clusters and O2 is weak, which is also observed in recent publications.66,67 The calculated binding energy (Eb) values of O2 over Au3+, Au3, and Au3- are 9.94, 18.18, and 8.79 kcal mol-1, respectively, indicating the bond between Au and O in the neutral complex Au3O2 is stronger than those in both the cationic and the anionic complexes, which is consistent with the relative order of stabilities of complexes Au3O2+, Au3O2, and Au3O2- reported by Ding et al.68 This fact can be understood by analyzing the charge transfer in the complexes between the Au trimers and O2. For Au3O2, the charge transfer makes both the Au3 and the O2 moieties carry on opposite charges (positive for Au3 moiety and negative for the O2 moiety), resulting in an electrostatic energy gain and hence stabilizing the Au3O2 complex. In contrast, in the cases of Au3O2+ and Au3O2-, the charge transfer leads to the two moieties of the complexes carrying on charges with same signs, making these two complexes less favorable in energy due to the electrostatic repulsions between charges on the two moieties. NBO analysis shows that the O-O bond order is 1.5 in Au3O2+, 1.3 in Au3O2, and 1.4 in Au3O2-, resembling that in superoxo. Thus, all three complexes can be described as superoxo-like species, as proposed in the literature.69

For the complexes between Au trimers and CO, we found that Au-C-O in Au3CO+ and Au3CO have linear structures, while it presents bent geometry in Au3CO-, as has been reported formerly.70 The ground states of Au3CO+ and Au3CO- are in their singlets, while that of Au3CO is doublet. The natural population analysis manifests that 0.120e is transferred to Au3+ from CO, while CO in Au3CO and Au3CO- receive 0.054e and 0.265e, respectively (as shown in Table 2). The calculated Eb value for Au3CO+ is larger than those for Au3CO and Au3CO-. The Au-C distances in these three complexes are 1.945, 1.913, and 2.014 Å, and the corresponding Au-C bond orders are 0.7, 0.8, and 0.6, respectively. These results indicate that Au3+ possesses the strongest interaction with CO, which coincides with previous reports.70,71 3.3. CO Oxidation Catalyzed by Au3+, Au3, and Au3-. Three different pathways for CO oxidation have been considered in the present work. One involves CO attacking the initial complexes between Au trimers and O2 (denoted as path I), another is related to O2 interacting with the complexes of Au trimers and CO (denoted as path II), and a third refers to the self-promoting mechanism (denoted as path III); that is, the energy gained from a preadsorbed CO molecule drives the oxidation of the second CO. In the flowing sections, these pathways will be denoted as I+, II+, and III+ for Au3+-mediated reaction, I0, II0, and III0 for Au3-mediated reaction, and I-, II-, and III- for Au3--mediated reaction, respectively. To search for the minimum-energy pathway for CO oxidation, we calculated both the singlet and the triplet PESs along each pathway for the reaction promoted by Au3+ and Au3-, and the doublet and quartet PESs for the reaction promoted by Au3. Figures 2-4 only show the energy profiles for CO oxidation on the ground-state surfaces, where all of the geometries of minima and transition states along each path are given to understand the reaction processes clearly. 3.3.1. Oxidation of CO Promoted by Au3+. We first consider the reaction along the path I+. As shown in Figure 2, IM1 is a weakly bound complex between Au3O2+ and CO on the triplet PES. It is more stable by 11.68 kcal mol-1 than the separated reactants (Au3+ + O2 + CO). Along the reaction coordinate, this initial complex is directly converted into a product-like intermediate IM2 via transition state TS1-2. The imaginary frequency of TS1-2 is 495 cm-1, and the corresponding transition vector indicated by vibration analysis as well as the subsequent IRC calculations manifest that the O-O bond is breaking and the O-C bond is forming. The barrier to be surmounted from IM1 to TS1-2 is 23.09 kcal mol-1, which is larger than the released energy by forming IM1. This pathway seems not to be energetically favorable due to the difficulty in breaking the strong O-O double bond at room temperature.72 In the case of path II+, the reaction involves two elementary steps on the triplet surface. The initial complexes of Au3CO+ with O2 are denoted as IM3, which can be considered as a coadsorption complex of CO and O2 on Au3+. Our calculations show that this complex lies 51.74 kcal mol-1 below the separated reactants, which is much more stable than IM1 in path I+. From IM3, the reaction proceeds via TS3-4, and the barrier involved is 48.90 kcal mol-1. Calculated geometrical parameters of TS3-4 indicate that the O-O bond is weakening and the O-C bond is forming. Its imaginary frequency is 545 cm-1, and the corresponding transition vector obviously indicates the elongation of the O-O bond and the shortening of the C-O distance. The IRC calculations indicate that the forward minimum connected by TS3-4 is IM4, where the O-O bond length is elongated to 1.346 Å and the O-C distance is shortened to 1.446



Figure 2. Energy profiles for the oxidation of CO promoted by Au3+ along path I+ (blue line), path II+ (black line), and path III+ (red line). Optimized geometries for the intermediates and transition states involved in the three pathways are also presented. All energies are given in kcal mol-1 relative to the energies of the reactants (Au3+ + O2 + 2CO). The golden, gray, and red balls denote Au, C, and O atoms, respectively.

Å. It should be noted that the energy of IM4 is 0.38 kcal mol-1 higher than TS3-4 after the zero-point energy correction. Along the reaction coordinate, IM4 is converted into IM5, a complex of Au3O+ with CO2, which is 59.04 kcal mol-1 below the separate reactants and serves as the direct precursor for the formation of CO2. The saddle point connecting IM4 and IM5 is TS4-5, of which the geometry and transition vector connected to the imaginary frequency of 1155 cm-1 are consistent with the notion of the O-O bond breaking and the O-C bond forming. The dissociation of IM5 into Au3O+ and CO2, indicating the accomplishment of the whole reaction, requires an energy of 15.88 kcal mol-1. From Figure 2, it is clear that the rate-determining step along path II+ is the formation of TS4-5, which is energetically 10.85 kcal mol-1 higher than the separate reactants, implying the CO oxidation along this pathway may also be not viable at room temperature. Considering the stronger interaction of gold clusters with CO as compared to O2, we explored another reaction channel (path III+) with the participation of two CO molecules. This path is similar to path I+ except that a CO is preadsorbed on Au3+. However, with the aid of a CO molecule, the reaction is found to cross into the singlet surface. As illustrated in Figure 2, the formation of initial intermediate IM6 releases the energy of 33.35 kcal mol-1. The following step includes a barely hindered formation of IM7, where the involved barrier is only 1.52 kcal mol-1. The subsequent cleavage of CO2 molecule, which requires 12.72 kcal mol-1 energetically, leads to the formation of OCAu3O+, which can continue to react with another CO

molecule, and the relevant mechanism has been reported in detail by An et al.16 The overall reaction is exothermic by 58.61 kcal mol-1. According to the profile shown in Figure 1, path III+ is the energetically favorable pathway for the CO oxidation mediated by Au3+. 3.3.2. CO Oxidation Promoted by Au3. Similarly, three paths for the CO oxidation promoted by neutral Au3 cluster have been characterized in detail. Figure 3 shows the calculated PES profiles with the optimized geometries of the stationary points. Along path I0, the CO initially binds to Au3O2 to form a trimolecular intermediate, IM8, which is 18.74 kcal mol-1 more stable than the separate reactants. The transition state involved along this path is TS8-9, where the transition vector related to the imaginary frequency of 522 cm-1 is consistent with the notions of the O-O bond breaking and C-O bond forming, giving rise to the formation of product-like intermediate IM9. The barrier along this path is calculated to be 12.04 kcal mol-1. Being different from the reaction mediated by Au3+ along path I+, TS8-9 lies below the reactants by 6.70 kcal mol-1, implying that the reaction along this path is possible. As far as path II0 is concerned, the reaction starts with the formation of IM10, which yields from the interaction between Au3CO and O2. The calculated results indicate that the reaction involves two elementary steps connected by TS10-11 and TS11-12. As shown in Figure 3, the relative energies of the intermediates and transition states involved in path II0 are all lower than that in path I0. Moreover, the energy barriers of two elementary steps are only 6.40 and 5.12 kcal mol-1, and the released energy by

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Figure 3. Energy profiles for the oxidation of CO promoted by Au3 along path I0 (blue line), path II0 (black line), and path III0 (red line). Optimized geometries for the intermediates and transition states involved in the three pathways are also presented. All energies are given in kcal mol-1 relative to the energies of the reactants (Au3 + O2 + 2CO). The golden, gray, and red balls denote Au, C, and O atoms, respectively.

forming IM10 is enough to overcome the barriers. As compared to the activation energy involved along path I0, 12.04 kcal mol-1, path II0 is obviously more favorable in energy. For the reaction along path III0, the preadsorbed CO molecule on the Au3 clusters drives the reaction between CO and O2 with a moderate barrier height of 13.56 kcal mol-1 to occur. The whole pathway is exoergic by 109.99 kcal mol-1. As illustrated in Figure 3, the PES along path III0 is lower than paths I0 and II0, although the barrier involved along this path is 1.52 and 7.16 kcal mol-1 higher than those along paths I0 and II0, respectively, indicating this reaction channel is also viable. 3.3.3. CO Oxidation Mediated by Au3-. The CO oxidation promoted by Au3- is also explored along the three paths. The relevant results are shown in Figure 4. It is found that most of the stationary points are geometrically similar to those in Figures 2 and 3 except for the configuration of Au3- unit in these species. The mechanistic details discussed above also apply to the CO oxidation mediated by Au3-. So, we just discuss the main conclusions obtained from the present calculations for simplification. All of the PESs occur below the separate reactants except path I-. Along path I-, the reaction involves one elementary step with the energy barrier of 15.85 kcal mol-1. For path II-, the barrier to be surmounted is 2.76 kcal mol-1 smaller than that involved in path I-. In the case of path III-, we find that the barrier involved is only 6.71 kcal mol-1, which

is smaller by 6.38 kcal mol-1 than that involved in path II-. So, we conjecture that the Au3--mediated reaction may prefer path III- to paths I- and II-. From the above results, it is clear that the charge state has a substantial effect on the elementary mechanism of CO oxidation. The Au3+-mediated reaction proceeds according to path III+, and the Au3--mediated reaction prefers paths II- and III-, while all three pathways are viable for the Au3-mediated reactions. Furthermore, we conjecture that Au3+ may be the most active species due to the lowest barrier involved in the reaction and that Au3- and Au3 are also catalytically active species because the barriers involved in the reactions are not so large that they can be surmounted by the released energies by forming the initial complexes. Therefore, CO oxidation can be promoted by all three Au species, demonstrating that the charge state of Au plays a less important role for its catalytic activity, which is in agreement with the reports in the literature73–75 that the charges gained from the supports are not the key point for the activity of Au nanoparticles. This can explain the experimental findings that all of positively charged,35 neutral,36 and negatively charged26,36 Au nanoparticles as well as coexisted Au0 and Aun+32,39–41 are able to present effective catalytic activity toward CO oxidation. The observed discrepancy of the catalytic activity of Au particles with different charge states is possibly due to different experimental conditions.



Figure 4. Energy profiles for the oxidation of CO promoted by Au3- along path I- (blue line), path II- (black line), and path III- (red line). Optimized geometries for the intermediates and transition states involved in the three pathways are also presented. All energies are given in kcal mol-1 relative to the energies of the reactants (Au3- + O2 + 2CO). The golden, gray, and red balls denote Au, C, and O atoms, respectively.

One should keep in mind that our study has used free Au clusters as model catalysts without considering support effects due to complicated Au/support interface interactions. However, we emphasize that the conclusions drawn from the present calculations may be essential for understanding the mechanisms involved in the realistic and complicated catalytic systems, although the geometry and chemistry of gas-phase clusters may differ from supported Au nanoparticles. Of course, it is also important to extend this work to supported Au clusters to further shed an insight on the support effect on the catalytic mechanism and activity of Au clusters with different charge states. It can be expected that the supports would act as not only stabilizers of Au nanoparticles but also increase the surface effective charge of ionic Au nanoparticles by their electron-accepting or electrondonating ability. However, precise quantitative predications of the influence of suitable supports for the catalytic activity of Au clusters require a systematic investigation, which is beyond the scope of this work. 3.3.4. Au-Carbonate Species. It should be noted that the catalytic CO oxidation by Au2- has been studied theoretically and experimentally by Ha¨kkinen et al.4,11 Using temperaturedependent rf-ion trap mass spectrometry, they observed a “metastable” carbonate intermediate Au2CO3-. By performing DFT calculations, they proposed the mechanism of the catalytic cycle for the oxidation reaction. Our present results for the Au trimer-mediated reactions give a consistent picture with the early study as a whole. However, the mechanistic detail is somewhat different from the previous reports. In particular, we find that the carbonate species are not necessary intermediates for forming CO2, although they are very stable in energy. In this sense, how is the observed carbonate species understood? To ascertain the possible formation of Au-carbonate species, herein, we have carried out ADMP molecular dynamics trajectory calculations. As a representative example, Figure 5 shows a representative

Figure 5. Calculated ADMP trajectory with selected geometries, starting from TS10 with 0.05 fs time step and 0.30 kcal mol-1 initial kinetic energy.

trajectory starting from TS10, the transition state involved along the path III for the Au3--mediated reaction. As seen from the selected geometries shown in Figure 5, the CO2 is first formed to reach a local minimum at point D at 100 fs with a C-O distance of 3.088 Å, and then the newly formed CO2 leaves away from the OCAu3O- species during 100-250 fs to reach in turn structures at points E and F. After that, the CO2 is drawn back again to reach the structure at point G at 350 fs by overcoming a small barrier. The potential energy fluctuation of the system is small during the CO2 leaving and returning; that is, there is a small potential energy plateau from 100 to 350 fs. After 375 fs, the carbonate species is gradually formed, as indicated by the structure at point H. At 400 fs, the system is trapped into a potential well to reach the structures at point I.

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Once the carbonate species is formed, the potential energy surface of the system becomes very flat, and no further structure change is observed, as shown by the trajectory from 400 to 500 fs. From the ADMP calculations, we suggest that the highly stable carbonate species is formed via an effective collision between Au-oxides and just formed CO2 by the carbon atom of CO2 directly attacking the O atom of Au-oxides. The findings of Ojifinni et al.76 that carbonate was observed in the presence of atomic oxygen and CO2 provide an effective support for our conclusions. 4. Conclusions In summary, we have performed a detailed theoretical study for the CO oxidation promoted by cationic, neutral, and anionic Au trimers. The calculated results manifest that all three Au trimers are able to facilitate the CO oxidation with different mechanisms, and the cationic species seems to perform best via the self-promoting mechanism. This fact indicates that the charge state of Au has a substantial effect on the elementary mechanism, but plays a less important role for its catalytic activity toward CO oxidation. The theoretical results support the experimental observations that all Au species with different charge states show the catalytic activity toward CO oxidation. Furthermore, we find that the highly stable carbonate species is not a necessary intermediate on the minimum energy path during CO oxidation, although it presents highly energetic stability. It may be formed via the effective collision between Au-oxides and CO2 with the carbon atom of CO2 directly attacking the O atom in the oxides. The present study shows us a clear picture about the elementary mechanism of the catalytic CO oxidation by Au trimers with different charge state and enriches our understanding for the charge state effect on the Au-mediated CO oxidation. Acknowledgment. This work was sponsored by the National Science Foundation of China (20773078, 20873076, 20776079), the National 863 (2006AA03Z222), and the 973 Program Projects of China (2007CB936602). Y.D. is a Tai-Shan Scholar supported by the State Education Ministry’s NCET Program (NCET-06-0580). References and Notes (1) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. J. Catal. 1993, 144, 175–192. (2) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647– 1650. (3) Xu, C.; Su, J.; Xu, X.; Liu, P.; Zhao, H.; Tian, F.; Ding, Y. J. Am. Chem. Soc. 2007, 129, 42–43. (4) Socaciu, L. D.; Hagen, J.; Bernhardt, T. M.; Woste, L.; Heiz, U.; Ha¨kkinen, H.; Landman, U. J. Am. Chem. Soc. 2003, 125, 10437–10445. (5) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252–255. (6) Campbell, C. T. Science 2004, 306, 234–235. (7) Comotti, M.; Pina, C. D.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812–5815. (8) Tong, X.; Benz, L.; Kemper, P.; Metiu, H.; Bowers, M. T.; Buratto, S. K. J. Am. Chem. Soc. 2005, 127, 13516–13518. (9) Bongiorno, A.; Landman, U. Phys. ReV. Lett. 2005, 95, 106102. (10) Chen, M. S.; Goodman, D. W. Catal. Today 2006, 111, 22–33. (11) Ha¨kkinen, H.; Landman, U. J. Am. Chem. Soc. 2001, 123, 9704– 9705. (12) Bu¨rgel, C.; Reilly, N. M.; Johnson, G. E.; Mitric´, R.; Kimble, M. L.; Castleman, A. W., Jr.; Bonacˇic´-Koutecky´, V. J. Am. Chem. Soc. 2008, 130, 1694–1698. (13) Wang, F.; Zhang, D. J.; Sun, H.; Ding, Y. J. Phys. Chem. C 2007, 111, 11590–11597. (14) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412–4456. (15) Pyykko¨, P. Inorg. Chim. Acta 2005, 358, 4113–4130. (16) An, W.; Pei, Y.; Zeng, X. C. Nano Lett. 2008, 8, 195–202.

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Theoretical Study of the CO Oxidation Mediated by Au3+, Au3, and

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