Theoretical Investigation of the Formation of Hydrogen Peroxide from

Jul 14, 2007 - The direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 over gold nanoparticles has been achieved in the past decade; however, ...
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J. Phys. Chem. C 2007, 111, 11590-11597

Theoretical Investigation of the Formation of Hydrogen Peroxide from H2 and O2 over Anionic Gold Clusters Aun- (n ) 1-4) Fang Wang, Dongju Zhang,* Hui Sun, and Yi Ding* Key Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, P.R. China ReceiVed: April 1, 2007; In Final Form: May 9, 2007

The direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 over gold nanoparticles has been achieved in the past decade; however, our understanding for the mechanism is still far from complete. We present herein a density functional theory investigation of the formation mechanism of H2O2 from H2 and O2 on anionic gold clusters Aun- (n ) 1-4). In all cases, the reaction proceeds via two elementary steps: initial H2 dissociation to form an OOH-containing intermediate and subsequent isomerization of this intermediate into a productlike intermediate. Energetically, the reactions over Au2- and Au4- are significantly less demanding than those over Au- and Au3-. In particular, Au- is relatively less active in the hydrogenation of O2 because the barrier of the rate-determining step is as high as 40.60 kcal mol-1. The barriers for both the odd- and even-numbered sequences slightly decrease with cluster size. The present results represent a prototype for the direct synthesis of H2O2 from H2 and O2 mediated by gold nanoparticles.

1. Introduction The unique properties of nanostructured gold in heterogeneous catalysis have attracted great attention1-5 since the breakthrough discovery by Haruta et al.6,7 that highly dispersed gold nanoparticles can effectively catalyze the oxidation of carbon monoxide at low temperatures. In recent years, research on Au catalysis has covered many aspects, such as H2 energy carriers,8 environmental purification,9,10 chemical synthesis,11-13 and others. Using supported gold nanoparticles as catalysts, many important reactions have been achieved so far, including the combustion of hydrocarbons,3,14-16 the selective oxidation of propene,12,17,18 the selective hydrogenation of acetylene,19 the water-gas shift,20 the reduction of nitrogen oxides,9,21,22 and the oxidation of glucose.23 Hydrogen peroxide (H2O2) is a highly selective green oxidant that is extensively used as a bleaching agent, disinfectant, and chemical feedstock.24 In the past two decades, great attention has been paid to its direct synthesis from H2 and O2 with Pdand Au-based catalysts.25-29 However, Pd-based catalysts work at elevated pressure with a mixture of H2 and O2 in the explosive range, which is extremely dangerous for industrial applications. Moreover, H2 pre-adsorbed on a Pd-based catalyst reduces the rate of oxygen adsorption.25-27 Olivera et al.28 predicted that gold would be a better catalyst for the direct synthesis of H2O2 than Pd, Pt, and Ag. Subsequent experiments did reveal that highly dispersed Au nanoparticles are very selective and active for the synthesis reaction.11,29 Furthermore, H2O2 is also a crucial intermediate in the production of propene oxide, which is an important commercial chemical.30 Research into the oxidation of propylene over nanostructured Au catalysts has thus attracted great attention since Haruta and co-workers17 first reported the direct epoxidation of propylene over Au nanoparticles in the presence of H2 and O2. The effects of different factors, such as catalyst preparation and pretreatment methods, support materials, * To whom correspondence should be addressed. E-mail: zhangdj@ sdu.edu.cn (D.Z.), [email protected] (Y.D.).

Au particle size, and loading amount, on the catalytic activity have been studied,19,31,32 and a possible mechanism has been proposed.19,32 Compared to the abundant experimental studies, the formation mechanism of H2O2 over Au nanoparticles is still not wellunderstood, although many researchers have proposed that the reaction could involve a hydroperoxy-like species (OOH).19,32 For example, Goodman et al.4 provided evidence for the presence of an OOH-containing intermediate using inelastic neutron scattering. Through density functional theory (DFT) calculations, Thomson and co-workers33,34 recently confirmed the possible existence of an OOH species during the reaction. Despite these efforts, our understanding of the formation mechanism of H2O2 is still far from complete. In this article, we present a systematic theoretical study on H2O2 formation from H2 and O2 over anionic gold clusters Aun- (n ) 1-4) to better understand the catalytic properties of nanosized Au. Because Au is the most electronegative metal, Au clusters can easily attract electrons from substrates to become negatively charged. In this sense, anionic gold clusters should be appropriate target systems for modeling this reaction. The main issues to be considered are the detailed mechanism along the potential energy surface, the quantum size effect for the reactivity, and the odd-even effect of Aun- activities. 2. Computational Details The calculations presented in this work were carried out with the Gaussian 03 suite of programs.35 The structures of the reactants, products, intermediates, and transition states were optimized within the framework of DFT without any symmetry constraints. We choose Becke’s 1988 exchange functional36 with Perdew and Wang’s 1991 correlation functional (BPW91)37,38 and the Los Alamos LANL2DZ39,40 effective core pseudopotentials (ECPs) and valence double-ζ basis sets for gold, as well as 6-311G(d,p) basis sets for hydrogen and oxygen. All geometries were fully optimized without any symmetry constraints by performing unrestricted DFT calculations. The

10.1021/jp072546e CCC: $37.00 © 2007 American Chemical Society Published on Web 07/14/2007

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TABLE 1: Calculated and Experimental Electron Affinities (EAs), Ionization Energies (IEs), and Bond Lengths (R) of Aun (n ) 1-4), AuO, AuO-, AuO+, and O2a theory species Au Au2 Au3 Au4 AuO AuOAuO+ O2

EA

IE

experiment R

52.07 48.15 85.67 65.42 52.49

EA b

IE

R

213.54c

219.12 51.82 218.98 2.552 46.51b 211.23c 169.26 2.607 78.41-91.09e 167.65 ( 3.46c 190.43 59.96 ( 2.31e 198.07f 231.30 1.902 54.75 ( 0.16e 1.896 1.917 9.13 283.94 1.221 10.35e 277.79h

2.47d 1.912g 1.899g 1.207i

a Energies are in kcal mol-1, and lengths are in Å. b Reference 44. Reference 45. d Reference 46. e Reference 47. f Reference 48. g Reference 49. h Reference 50. i Reference 51.

c

synchronous transit-guided quasi-Newton (STQN) method41 was used for locating transition structures. Frequency analyses were carried out for all species to identify the natures (minima or first-order saddle points) of the stationary points and to calculate the zero-point vibrational energies (ZPEs). To identify the minimum-energy paths, intrinsic reaction coordinate (IRC)42 calculations were conducted in both directions (forward and reverse) from the transition states to the corresponding local minima. Natural population analysis (NPA) was performed to show the nature of the bonding between the anionic gold clusters and molecular oxygen. A previous study by Legge et al.43 showed that the BPW91/ LANL2DZ combination is sufficiently accurate for describing noble-metal systems. For this reason, we carried out the present DFT calculations using the BPW91 functional with the ECPs and basis sets described above. To support our choice of functional, pseudopotential, and basis sets, we provide benchmark calculations of the electron affinities (EAs), ionization energies (IEs), and bond lengths (R) for Aun- (n ) 1-4), AuO, and O2, as well as the bond lengths for AuO- and AuO+. As shown in Table 1, all calculated data are in fairly good agreement with the corresponding experimental results,44-51 indicating the accuracy and reliability of the computational method employed. 3. Results and Discussion 3.1. Reactants, Products, and Catalysts. Figure 1 shows the optimized geometries of the reactants (H2 and O2), product (H2O2), and anionic Au clusters (Aun-, n ) 1-4) with their respective bond lengths. It was found that the structural parameters for H2, O2, and H2O2 are in excellent agreement with the experimental values,51-53 indicating the good performance of the BPW91/6-311G(d,p) method. For the anionic Au clusters, our calculations predict that the ground states are all low-spin states, which agrees well with the previous predictions.54 The Au-Au bond lengths are in a range of 2.6142.777 Å, which are shorter than the bulk value. For Au3-, the linear configuration was found to be more stable (by 131.42 kcal mol-1) than the triangular configuration, which is also consistent with previous reports.32,55,56 Three possible geometries for Au4-, namely, the linear, rhombus, and Y-shaped configurations, were taken into account. Consistent with earlier calculations,54,57 the Y-shaped geometry was found to be energetically most favorable, being 4.84 and 19.60 kcal mol-1 more stable than the linear and rhombus structures, respectively. 3.2. Initial Complexes of Aun- (n ) 1-4) with O2 and H2. Previous theoretical and experimental studies have addressed the adsorption of O2 on cationic, neutral, and anionic gold

clusters, with an emphasis on the molecular adsorption and dissociative adsorption of O2.58,59 It was found that the dissociation barrier for O2 over gold clusters is so high that dissociative adsorption takes place only under special experimental conditions.58 Stolcic et al.59 observed that molecular oxygen bound to Aun- clusters. Here, we calculated various complexes between Aun- and O2 molecules (geometry-spin combinations), denoted as AunO2- (n ) 1-4). We found that the ground states for the complexes with n ) 1 and 3 are triplets, whereas the most stable complexes for n ) 2 and 4 are on doublet potential energy surfaces (PESs). As shown in Figure 2, the singlet complexes AuO2- and Au3O2- are less stable by 15.07 and 26.55 kcal mol-1 than their triplet counterparts, and the quadruplet complexes Au2O2- and Au4O2- lie above their corresponding doublet counterparts by 21.57 and 17.54 kcal mol-1, respectively. This indicates that the initial interaction between the anionic gold clusters and an O2 molecule involves strong spin-orbit coupling effects for the clusters with odd numbers of electrons (Au2- and Au4-).60 From Figure 2, it can be seen that the adsorbed O2 in AunO2(n ) 1-4) is bound bend-on to the gold anionic clusters Aun(n ) 1-4) and that all of the configurations are planar except Au4O2-. The binding energies (Eb) of O2 to Aun-, defined as the energy necessary to dissociate AunO2- into ground-state Aun- and O2 (a similar definition applies to the calculations of the binding energies of H2 to Aun- to be discussed below), were calculated to be 7.61, 22.89, 5.07, and 18.91 kcal mol-1 for the ground-state complexes AunO2- with n ) 1-4, respectively, which present an obvious odd-even oscillating behavior, i.e., the Eb values of O2 on clusters with even numbers of electrons (n ) 1 and 3) are smaller than those on clusters with odd numbers of electrons (n ) 2 and 4). This fact can be understood by considering the electronic structures of Aun-. There is a single electron in the ground state of each Au2- and Au4-, which is partly transferred into the antibonding π orbital of O2 as the O2 molecule is attached to the anions, leading to stronger interactions between the anions and O2. Whereas Au- and Au3have a closed-shell, paired structure, they show a weak adsorption of O2.55,61 Computed atomic charges and O-O distances for the molecular complexes also show an odd-even oscillation behavior. As shown in Table 2, the charges transferred from Au2- and Au4- to O2 (0.296 and 0.488 e) are larger than those transferred from Au- and Au3- to O2 (0.254 and 0.201 e), resulting in larger O-O distances in ground-state Au2O2- and Au4O2- (1.307 and 1.297 Å) than in ground-state AuO2- and Au3O2- (1.256 and 1.258 Å) (Figure 2). Clearly, the O-O distances in all four most stable complexes are significantly elongated compared to that in a free O2 molecule (1.221 Å), indicating that the O2 attached to Aun- has been activated. The optimized geometries of the complexes of Aun- (n ) 1-4) with H2, denoted as AunH2- (n ) 1-4), are also shown in Figure 2. The ground states of the complexes with n ) 1 and 3 are singlets, whereas those of the complexes with n ) 2 and 4 are doublets. Various possible adsorption geometries (bend-on, side-on, and end-on) of H2 on Aun- were taken into account in the present calculations. The calculated results show that all of the complexes are planar. For the complexes with n ) 1-3, the most stable geometries are those in which the hydrogen molecules are physically adsorbed on Aun- (n ) 1-3) in the end-on form, whereas for the complex with n ) 4, two different geometries with an energetic difference of only 0.10 kcal mol-1 were located, as illustrated in Figure 2. It should be noted that the calculated Eb values are negative, except that for

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Figure 1. Optimized geometries for H2, O2, H2O2, and Aun- (n ) 2-4). The distances are in angstroms. The numbers in parentheses are experimental values for H2, O2, and H2O2 and theoretical results from ref 51 for Aun-. The electronic states of Aun- are given in square brackets. The larger and smaller white balls represent Au and H atoms, respectively, and the black balls represent O atoms.

AuH2-, which has a small positive value (1.03 kcal mol-1), indicating that the initial Coulomb interaction between Aunand H2 is repulsive, which is disadvantageous for complex formation. This case is opposite to the adsorption of O2 discussed above, but it is in line with earlier reports that reactions between anionic gold clusters and molecular H2 were not observed.62,63 3.3. Formation of H2O2 over Aun- (n ) 1-4). First, we consider the electronic states of the discussed systems. Considering the spin combinations between ground-state O2 and Aun- (n ) 1-4), it seems that the reactions between them should be considered on the singlet and triplet surfaces for n ) 1 and 3 and on the doublet and quartet surfaces for n ) 2 and 4. As seen above, however, the ground states of the primary adducts Au2O2- and Au4O2- are doublets, even though the initial interactions of Au2- and Au4- with O2 have to occur on quartet surfaces. This fact indicates that intersystem crossings between the quartet and doublet surfaces must take place at the entrances of the reaction channels to lead the systems to the energetically most favorable surfaces via spin-orbit interactions. In this sense, we discuss the reactions only on the doublet surfaces for Au2and Au4-. In contrast, the energetically most favorable primary adducts AuO2- and Au3O2- have two unpaired electrons. However, the products occur on singlet surfaces, indicating that intersystem crossings to the singlet surfaces must occur after the primary adducts. Therefore, our calculations have to involve both the singlet and triplet surfaces for Au- and Au3- reactions to inspect the efficient switch from one surface to another that is brought about by spin-orbit coupling in such systems. As mentioned above, we obtained stable geometries of the primary complexes of Aun- with O2 or H2. The following processes are the attack of H2 or O2 on the complexes to form the three-molecule intermediates and launch the reactions. Figures 3 and 4 and Figures S1-S2 in the Supporting Information show the optimized geometries of the intermediates (IMs) and transition states (TSs) involved in the H2O2 formation reactions from H2 and O2 over Ann- (n ) 1-4). Figures 5 and 6 and Figures S3-S4 in the Supporting Information show the relevant PES profiles along the reaction coordinates. 3.3.1. Reaction oVer Au-. As can be seen in Figure 3, 1IM1 and 3IM2 (superscripts denote spin multiplicities) are two threemolecule intermediates located on the singlet and triplet surfaces, respectively. The former seems to be an adduct of AuO2- with H2, and the latter is formally a complex of AuH2- with O2. We also tried to locate the triplet counterpart of 1IM1; however,

the initially guessed structure always converged to 3IM2. As shown in Figure 5, 1IM1 originates from either the singlet surface or the triplet surface. On the reaction entrance, the split between the two surfaces is 38.24 kcal mol-1, which is actually the energy difference between 1O2 and 3O2. This value is in good agreement with the experimental value of 37.50 kcal mol-1.64 Similarly, on the triplet surface, the only three-molecule intermediate obtained in the present work is 3IM2, which originates from either the interaction of AuH2- with triplet O2 or the interaction of triplet AuO2- with H2. 3IM2 is 5.61 kcal mol-1 more stable than 1IM1. OOH-containing species, 1IM3 and 3IM4, have been identified as minima on the singlet and triplet surfaces of [Au, H2, O2]-. They are related to 1IM1 and 3IM2 via 1TS1-3 and 3TS 2-4, respectively, where both H-H and O-O bonds have been elongated, indicating that the H-H single bond is breaking and the O-O double bond is changing into a single bond (see structural parameters in Figure 3). The imaginary frequencies of 1TS1-3 and 3TS2-4 are 594i and 843i cm-1, respectively, and the normal modes correspond to the formation of Au-H and O-H bonds and the breaking of a H-H bond. Note that the energy difference from 3IM2 to 3TS2-4 is only 2.39 kcal mol-1, that is, adding the ZPE correction reverses the order of 3IM4 and 3TS2-4, so that 3TS2-4 is actually lower in energy by 1.29 kcal mol-1. This suggests that 3IM4 might not exist on the triplet surface. From Figure 5, it is also noted that 1IM3 is energetically more favorable than all stable points located on the singlet and triplet surfaces before it is formed, implying that an intersystem crossing from the triplet surface to the singlet surface has to take place in the vicinity of 1IM3. In Figure 5, we denote the possible crossings with circles. An effective approach to estimating the spin-orbit coupling term between two states is to perform multiconfiguration CI calculations. However, such calculations in Gaussian series codes are limited to elements H through Cl and are not available for the present Au-containing systems. Alternatively, to support our claim, we analyzed the nature of 3IM4 and 3TS2-4 by calculating the atomic spin densities, which are expressed as percent spin densities on an atomic center as compared to the sum of all atoms in the molecule. NPA spin density results show that the Au and O1 atoms in 3IM4 and 3TS2-4 (Figure 3) exhibit the highest percentages of total spin density: 31.2% and 33.5%, respectively, in 3IM4 and 29.0% and 33.8%, respectively, in 3TS2-4. These data indicate that 3IM4 and 3TS2-4 can be best described as biradicals with unpaired electrons concentrated on Au and

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Figure 2. Optimized geometries of the initial complex of Aun- (n ) 1-4) with O2 and H2. The symbols and values in parentheses indicate the electronic states and binding energies (in kcal mol-1), respectively. The larger and smaller white balls represent Au and H atoms, respectively, and the black balls represent O atoms.

TABLE 2: Calculated Atomic Charge Using the NPA Analysis for AunO2- (n ) 1-4) species

Aun moiety

O2 moiety

AuO2Au2O2Au3O2Au4O2-

-0.746 -0.704 -0.799 -0.512

-0.254 -0.296 -0.201 -0.488

O1. As indicated by their higher relative energies (Figure 5), such biradicals are transient species on the triplet surface and would couple quickly to form the thermodynamically stable singlet product 1IM3. In this sense, the triplet-singlet transition

seems realizable even though the difference between the singlet and triplet structures is large. After 1IM3, the reaction proceeds only on the singlet surface. The next step in understanding the PES is to connect 1IM3 to the productlike intermediate 1IM4, which is the global minimum on the PES of [Au, H2, O2]-. 1TS3-4 is characterized as a transition structure connecting 1IM3 and 1IM4. Its imaginary frequency is 358i cm-1, and the transition vector corresponds to the expected components of the reaction coordinates, i.e., the formation of the second O-H bond and the breaking of the Au-H bond. The relative energy of 1TS3-4 is 8.33 kcal mol-1 above the total energy of the separate reactants, and the barrier

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Figure 3. Optimized structures of the intermediates and transition states involved in the reaction of H2O2 formation from H2 and O2 over Au-. The superscripts denote the spin multiplicities. The larger and smaller white balls represent Au and H atoms, respectively, and the black balls represent O atoms.

Figure 4. Optimized structures of intermediates and transition states involved in the formation of H2O2 over Au2-. The larger and smaller white balls represent Au and H atoms, respectively, and the black balls represent O atoms.

Figure 5. Potential energy profiles for the H2O2 formation reaction from H2 and O2 over Au-. The total energy of isolated H2, O2, and Au- is taken as the zero point of energy. Circles indicate possible PES crossings. 1IM3

1IM4

mol-1,

from to is calculated to be 40.60 kcal indicating that the formation of the second O-H bond is very energetically demanding and that the OOH-containing species

Figure 6. Potential energy profiles for the H2O2 formation reaction from H2 and O2 over Au2-. The total energy of the isolated reactants is taken as the zero point of energy. 1IM3

is a metastable intermediate that could possibly be observed in experiments. Finally, the direct dissociation of 1IM4 leads to the formation of H2O2 and release of the catalyst Au-. This process requires 20.11 kcal mol-1, and the overall reaction

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Figure 7. Optimized structures of intermediates and transition states involved in the formation of H2O2 over Au2- according to the Wells et al. mechanism. The larger and smaller white balls represent Au and H atoms, respectively, and the black balls represent O atoms.

is exothermic by 20.09 kcal mol-1. As shown in Figure 5, the PES profile along the reaction coordinate clearly demonstrates that the conversion from 1IM3 to 1IM4 seems to be the ratedetermining step, which involves a barrier as high as 40.60 kcal mol-1. This suggests that Au- is relatively less active in the hydrogenation of O2, which is in good agreement with the observed low reactivity of Au- toward O2 in experiments.62 3.3.2. Reaction oVer Au2-. As mentioned above, we discuss here the reaction over Au2- on the doublet surface. The relevant geometries of the intermediates and transition states are very similar to those found in the reaction over Au- (cf. the geometries shown in Figures 3 and 4). The main difference between the two reactions is the relative energies of these stable points on the PESs (cf. Figures 5 and 6). At the entrance of the reaction, the three-molecule intermediate IM5 is first formed, which is a formally complex of Au2O2with H2, as indicated by the geometrical parameters shown in Figure 4. Its formation releases 22.63 kcal mol-1, which is in contrast to the energetically slightly demanding processes for the formation of 1IM1 and 3IM2. Along the reaction coordinate, we located two transition structures, TS5-6 and TS6-7, and the two other intermediates IM6 and IM7 on the PES of [Au2, H2, O2]-, which are analogues of TS1-3/TS3-4 and 1IM3/1IM4, respectively, on the PES of [Au, H2, O2]-. IRC calculations confirm the expected contacts between these intermediates and transition states, as shown in Figure 6. Clearly, the catalytic cycle proceeds via two sequential elementary steps: the first step involves H-H bond breaking to form an intermediate complex between OOH and Au2H-, and the subsequent step is the abstraction by OOH of the H atom in Au2H- to form H2O2 and release Au2-. The barriers of these two processes are 8.98 and 10.81 kcal mol-1, respectively. From the PES profile shown in Figure 6, it is clear that the reaction proceeds below the entrance channel, and the overall reaction is exothermic by 20.09 kcal mol-1. Compared to Au-, Au2- presents a higher reactivity for the direct synthesis of H2O2 from H2 and O2. This suggests that the odd-even effect for anionic gold clusters might exist even at the smallest size. 3.3.3. Reactions oVer Au3- and Au4-. We are now interested in two fundamental questions regarding nanogold catalysis: (1) Is there any correlation between the reactivity and the cluster size? (2) Does the quantum size effect play a dominant role in the reactivity? To better understand Au nanocatalysis, we extended our studies to reactions mediated by anionic Au3- and Au4- clusters. The optimized structures are shown in Figures S1 and S2 of the Supporting Information, and the corresponding PESs are given in Figures S3 and S4. For simplification, we only summarize here the general conclusions drawn from our

calculations, and the relevant details of the structures and PESs are not involved in view of their similarities to the reactions discussed above. We find that the reaction over Au3- is very similar to that on Au-, whereas the behavior of Au4- in the reaction closely resembles that of Au2-. The reaction mechanism discussed above also applies to these two reactions, i.e., the formation of H2O2 consists of two elementary steps: initial H-H bond activation and subsequent H-atom abstraction. The PES crossing takes place after the breaking of H-H bond for the reaction mediated by Au3-, whereas it occurs in the vicinity of the entrance to the reaction catalyzed by Au4-. The reaction over Au3- proceeds above the entrance, whereas that over Au4occurs below the corresponding entrance. Comparing Figure S1 with Figure 3 and Figure S2 and Figure 4, we find that the barriers of the reactions over Au3- and Au4- for two elementary steps are slightly smaller than the corresponding barriers of the reactions over Au- and Au2-, respectively, indicating that the reactivity of anionic gold clusters increases slightly with cluster size and presents an obvious odd-even effect. 3.4. Another Possible Mechanism for H2O2 Formation. Recently, Wells et al.33 reported a theoretical work on H2O2 formation from H2 and O2 over neutral Au3 clusters. They proposed that the catalyzed cycle started from an OOHcontaining species, denoted as HAu3-OOH, whose formation from Au3O2 and H2 was found to be an energetically downhill and an unactivated process. A second H2 was first added to HAu3-OOH and then dissociated via an H-abstraction transition structure to form the product H2O2 and the intermediate HAu3H, a hydride of Au3. We now examine whether the Wells et al. mechanism found for neutral Au3 could also apply to the reactions over anionic clusters. As an example, we studied the H2O2 formation reaction over Au2- according to the Wells mechanism. Figure 7 shows the optimized geometries involved in the reaction. At the entrance of the catalyzed cycle, IM8, an OOH-containing intermediate, interacts with a second H2 molecule to form the complex IM9. This process exhibits an association energy of 0.47 kcal mol-1. Then, IM9 is converted into the productlike intermediate IM10 via the H-abstraction transition state TS9-10 with a barrier of 11.46 kcal mol-1. The direct dissociation of IM10 results in the formation of H2O2 and IM11, a hydride of Au2-. This hydride further reacts with O2 to yields IM8, and hence, a new catalytic cycle is started again. All structures shown in Figure 7 are analogues of those given by Wells et al.,33 indicating that the reactivity of Au2- is similar to that of Au3. We attribute this finding to the similar open-shell electronic structures for Au2- and Au3. The relative energy profiles of the two reactions, however, are slightly

11596 J. Phys. Chem. C, Vol. 111, No. 31, 2007 different. For the Au3 reaction, the formation of HAu3-OOH was found to be a barrierless process, whereas in the Au2reaction, a barrier of 8.98 kcal mol-1 was found for the formation of HAu2--OOH. In addition, according to the Wells et al. mechanism, the calculated barrier for the formation of H2O2 is 11.46 kcal mol-1, which is only 0.65 kcal mol-1 higher than that of the pathway described above, indicating that the Wells et al. mechanism might be an alternative pathway for the formation of H2O2. 4. Conclusions In conclusion, a systematic density functional theory study of the formation of H2O2 over anionic gold clusters Aun- (n ) 1-4) has been performed to understand direct synthesis of H2O2 from H2 and O2 catalyzed by gold nanoparticles. The calculated results provide a consistent picture of the H2O2 formation mechanism at the molecular level: the reactions proceed via two elementary steps, i.e., initial H2 dissociation to form an OOH-containing intermediate and subsequent isomerization of this intermediate into a productlike intermediate. The reactivity of anionic gold clusters presents an odd-even effect even at the smallest size, i.e., the even-numbered anionic clusters present higher catalytic activities. In particular, Au- is relatively less active in the hydrogenation of O2 because the barrier of the rate-determining step is as high as 40.60 kcal mol-1. The present results show that quantum size effects appear to play a less important role in the reactivity of anionic Au clusters; in contrast, the additional charge on even-numbered gold clusters seems to be a dominant factor in the high reactivity. Acknowledgment. This work was sponsored by the National Science Foundation of China (Nos. 20473047 and 50601015), the National 863 Program Project (2006AA03Z222) and the 973 Program Project of China (2005CB623601). We gratefully acknowledge Virtual Laboratory for Computational Chemistry of CNIC, Supercomputing Center of CNIC (Chinese Academy of Sciences) and Shandong University High Performance Computational Center for providing computer resources. Y.D. is a Tai-Shan Scholar supported by Shandong Province. Supporting Information Available: Optimized structures and profiles of the potential energy surface involved in the reactions over Au3-and Au4-. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Campbell, C. T. Science 2004, 306, 234-235. (2) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252-255. (3) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; London, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, J. C. Nature 2005, 437, 11321135. (4) Sivadinarayana, C.; Choudhary, T. V.; Daemen, L. L.; Eckert, J.; Goodman, D. W. J. Am. Chem. Soc. 2004, 126, 38-39. (5) Cho, A. Science 2003, 299, 1684-1685. (6) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405-408. (7) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301-309. (8) Andreeva, D. Gold Bull. 2002, 35, 82-88. (9) McClure, S. M.; Kim, T. S.; Stiehl, J. D.; Tanaka, P. L.; Mullins, C. B. J. Phys. Chem. B 2004, 108, 17952-17958. (10) Wallace, W. T.; Whetten, R. L. J. Am. Chem. Soc. 2002, 124, 7499-7505. (11) Okumura, M.; Kitagawa, Y.; Yamagcuhi, K.; Akita, T.; Tsubota, S.; Haruta, M. Chem. Lett. 2003, 32, 822-823. (12) Stangland, E. E.; Taylor, B.; Andres, R. P.; Delgass, W. N. J. Phys. Chem. B 2005, 109, 2321-2330.

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