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Theoretical Study of the Decomposition and Hydrogenation of H2O2 on Pd and Au@Pd Surfaces: Understanding toward High Selectivity of H2O2 Synthesis Jun Li,† Aleksandar Staykov,† Tatsumi Ishihara,‡ and Kazunari Yoshizawa*,† †

Institute for Materials Chemistry and Engineering, International Research Center for Molecular Systems, and ‡Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan

bS Supporting Information ABSTRACT: Three possible pathways for the conversion of hydrogen peroxide to water on Pd and Au@Pd catalysts are investigated with periodic density functional theory calculations: (1) the decomposition of H2 O 2 (H2O2 f H2O þ O), including the dissociation of H2O2 to two OH groups (H2O2 f 2OH) and the disproportionation of two OH groups to water and oxygen (OH þ OH f H2O þ O); (2) the hydrogenation of the OH group to water (OH þ H f H2O); and (3) the direct hydrogenation of H2O2 to water (H2O2 þ 2H f 2H2O). The results show that the decomposition of H2O2 and the hydrogenation of OH groups are two available channels for the formation of water, and the former plays a main role. A key step in the overall process is the dissociation of H2O2, which is facile and irreversible. The direct hydrogenation of H2O2 to water has a very high activation barrier and is unlikely to occur. The competitions between the dissociation of H2O2 and the release of H2O2 on Pd and Au@Pd surfaces are analyzed. The high selectivity of H2O2 synthesis cannot be explained simply by the relatively increased barrier for H2O2 dissociation on the Au@Pd surface. Actually, the less active Au atoms on the Au@Pd surface weaken the interaction of the metal surface with H2O2, and thus suppress the dissociation of H2O2, and, on the other hand, facilitate the release of H2O2. The opposite effects of Au atoms on the dissociation and release of H2O2 move the balance to the release side, which is responsible for the high H2O2 selectivity of the Au@Pd catalysts. The effects of the unreacted H atoms are also considered. It is found that the H atoms coadsorbed on Pd and Au@Pd surfaces can decrease the interaction between the metal surfaces and H2O2 as well and, consequently, facilitate the release of H2O2 and suppress the dissociation of H2O2.

1. INTRODUCTION Hydrogen peroxide (H2O2) is an environmentally friendly oxidizing agent that is widely used in many industrial fields, such as waste treatment, pulp/paper bleaching, chemical synthesis, and so on.1,2 The byproduct of reactions involving H2O2 is only water, and thus, it is regarded as one of the “greenest” among the available chemical oxidants. Currently, H2O2 is industrially manufactured by the sequential hydrogenation and oxidation of anthraquinones.3 This process consumes considerable energy and generates hazardous waste; besides, it cannot be used for the novel one-pot process that involves the direct synthesis of H2O2 in small amounts and its direct application in a desired oxidation reaction. Thus, the development of new, cleaner, and smallerscale processes for the production of H2O2 is of environmental and commercial importance. The direct synthesis of H2O2 from H2 and O2 is attracting much interest as a promising alternative for the production of H2O2. It is reported that palladium catalysts are highly efficient for the direct synthesis of H2O2.4
14 Interestingly, the addition of gold into palladium enhances the catalyst activity and especially the selectivity; so recently, much attention is paid to the palladium
gold (Au@Pd) catalysts. Hutchings and co-workers prepared a series of Au@Pd catalysts that lead to a significant increase in the rate of H2O2 synthesis as well as the concentration of H2O2 formed in r 2011 American Chemical Society

comparison with the pure Pd or Au catalyst.15
19 These Au@Pd catalysts were supported on Al2O3, TiO2, SiO2, Fe2O3, or carbon.20
22 They reported very recently that a high yield of H2O2 with H2 selectivity greater than 95% was achieved on a nitricacid-pretreated Au@Pd catalyst supported on carbon.23 Other authors also reported the promotional effect of Au in the Au@Pd catalysts for the direct synthesis of H2O2 from H2 and O2 over various supports.24
28 However, the origin of the high performance of the Au@Pd catalysts is not very clear. Some studies indicated that the catalytic performance of the Au@Pd bimetals is greatly influenced by the particle size and the surface composition.19,24,25 Considering that it is difficult to control the particle size and the surface composition in the case of using a supported metal catalyst, Ishihara and co-workers prepared a Au@Pd nanocolloid with an almost uniform particle size as a catalyst for the direct H2O2 synthesis.27 They reported that the formation rate of H2O2 reaches a maximum at 30 mol % Au (a proportion of gold used to prepare the colloid), and the selectivity toward H2O2 showed a sharp rise at 25 mol % Au. The further X-ray photoelectron spectroscopy (XPS) evidence Received: July 28, 2010 Revised: March 12, 2011 Published: March 28, 2011 7392

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revealed that the prepared Au@Pd colloid with 25 mol % Au has the highest surface Au concentration (15 mol %). They suggested that less active Au on the Au@Pd colloid surface suppresses the decomposition of H2O2 with H2. In our previous work, we have investigated the formation of H2O2 from H2 and O2 on the Pd(111) and Au@Pd(111) surfaces based on periodic density functional theory (DFT) calculations.29 We proposed a two-step reaction mechanism, that is, O2 in a superoxo precursor state on the metal surface is hydrogenated in two steps with two H atoms situated over neighboring 3-fold positions. H atoms are derived from H2 dissociation on the surface with a low activation barrier. O2 dissociation leading to the nonselective formation of water was also studied. We found that, although the presence of Au on the Au@Pd surface increases the activation barrier for H2O2 formation to some extent, it significantly suppresses the dissociation of O2 on this surface, which could play some role in the improvement of the H2O2 selectivity. We know that, after H2O2 is formed, it may be converted into water via the following reactions: (1) the decomposition of H2O2 (eq 1), including two steps, namely, the dissociation of H2O2 to two OH groups (eq 2) and the disproportionation of two OH groups to water and oxygen (eq 3); (2) the hydrogenation of the OH group to water (eq 4); and (3) the direct hydrogenation of H2O2 to water (eq 5). H 2 O2 f H 2 O þ O

ð1Þ

H2 O2 f OH þ OH

ð2Þ

OH þ OH f H2 O þ O

ð3Þ

OH þ H f H2 O

ð4Þ

H2 O2 þ H2 f 2H2 O

ð5Þ

Usually, the catalysts that favor the formation of H2O2 are favorable for these side reactions as well. Recently Hwang and co-workers studied the role of Pd ensembles introduced into the Au surface in the direct synthesis of H2O2 from H2 and O2 with periodic DFT calculations.30 Their results also demonstrated that the formation of H2O2 is affected by the arrangement of surface Pd and Au atoms and the Pd@Au bimetals suppress the O
O bond cleavage. Their explanation furthers the understanding of the role of Au in the direct H2O2 synthesis on the Au@Pd catalysts. However, theoretical investigation of the conversion of H2O2 to water on the Au@Pd catalysts with Au impurities introduced into the Pd surface is still lacking. In this work, we investigated these three possible pathways for the conversion of H2O2 to water on Pd(111) and Au@Pd(111) surfaces with periodic DFT calculations. We found that the main channel leading to the formation of water is the decomposition of H2O2 and the hydrogenation of OH groups, and in the overall process, the dissociation of H2O2 is a key step. We then compared the competitions between the dissociation of H2O2 and the release of H2O2. We noticed that the Au atoms on the Au@Pd(111) surface can suppress the dissociation of H2O2 and, at the same time, can facilitate the release of H2O2 from it. The opposite effects of Au atoms on the dissociation and the release of H2O2 move the balance to the release side, which results in the high H2O2 selectivity of the Au@Pd catalysts. Moreover, we considered the effects of the unreacted H atoms and discovered

that the H atoms on the Pd and Au@Pd surfaces can also facilitate the H2O2 release and suppress the H2O2 dissociation.

2. COMPUTATIONAL METHODS All calculations were performed with the CASTEP suite of programs.31 Spin-polarized Perdew
Burke
Ernzerhof functional within the generalized gradient approximation (GGAPBE32) and ultrasoft pseudopotentials33 were employed. The geometry optimizations and the transition-state searches were carried out with medium accuracy except for the options specifically mentioned below. An energy cutoff of 340 eV was applied for the plane-wave expansion of the electronic eigenfunctions. For the Brillouin zone integration, a (3  3  1) Monkhorst
Pack mesh of k points is applied. The transition state was determined using the linear and quadratic synchronous transit (LST/QST) complete search method. The spin state was not optimized during the SCF procedure in order to avoid high spin states of the metal surface. The Pd(111) surface was modeled by a supercell slab that consists of a 3  3 surface unit cell with four atomic layers. The slab was separated in the vertical direction by a vacuum space with a height of 10 Å. The top two layers of the four-layer slab were fully relaxed until residual forces on all of atoms became smaller than 0.05 eV/Å, whereas the bottom two layers were fixed at the corresponding bulk positions. The lattice constant for bulk Pd was predicted to be 3.95 Å, which is identical to that from previous DFT-GGA calculations30 and also in good agreement with the experimental value of 3.89 Å.34 Ishihara and co-workers indicated that the ratio of the Au concentration in the surface to that in the bulk of the Au@Pd bimetals is nearly 1:1 when the proportions of Pd used to prepare the colloid are above 75%, which leads to high activity and selectivity of the catalyst.27 The slab model representing the Au@Pd surface is obtained when one Au atom was put in the (111) surface layer of the pure Pd slab to replace a Pd atom; at the same time, another Au atom was placed in the second top layer at the previous Pd site, assuring the singlet ground state of the model. In nine slab models derived in this method, the one with the lowest energy after geometry optimization was finally selected. The content of Au in the surface of this model is 11.1 mol %, near to the experimentally measured value (15 mol %) that lead to the highest H2O2 selectivity reported by Ishihara and co-workers.27 3. RESULTS AND DISCUSSION After H2O2 is produced on Pd or Au@Pd surfaces, it would be released from the metal surface into the gas phase. On the other hand, the adsorbed H2O2 would be converted into water. As mentioned above, there are three possible pathways for the conversion of H2O2 to water: one is the decomposition of H2O2, another is the hydrogenation of the OH group formed in the decomposition process, and the third is the direct hydrogenation of H2O2. With the periodic DFT calculations, we found that the main channel leading to the formation of water is the decomposition of H2O2 and the hydrogenation of OH groups. In the overall process, a key step is the dissociation of H2O2. To understand the role of Au in the enhancement of the H2O2 selectivity on the Au@Pd catalysts, we also studied the dissociation of H2O2 on the Au@Pd surface. Thus, in section 3.1, we present the decomposition of H2O2 on the Pd(111) surface that includes the dissociation of H2O2 to two OH groups and the disproportionation of two OH groups to water and oxygen. In 7393

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Figure 1. Dissociation of H2O2 on the Pd(111) surface. The relative energies are given in kcal/mol and the distances in Å. The edging atoms of the surface layer are shown, and the Pd atoms of the bottom two layers are omitted for clarity.

Figure 2. Disproportionation of OH groups to water and oxygen on the Pd(111) surface. The relative energies are given in kcal/mol and the distances in Å.

sections3.2 and 3.3, we describe the hydrogenation of the OH group and the hydrogenation of H2O2 to water, respectively. We then summarize these three pathways for the conversion of H2O2 to water on the Pd(111) surface and investigate the effect of the unreacted H atoms on the dissociation of H2O2. In section 3.4, the dissociation of H2O2 on the clean Au@Pd(111) surface and with H atoms coadsorbed is reported. At last, the comparisons between the dissociation of H2O2 and its competing process, the release of H2O2 from the Pd(111) surface and the Au@Pd(111) surface, are discussed. 3.1. Decomposition of H2O2 on the Pd(111) Surface. As described in our previous work,29 H2O2 is produced from O2 in two hydrogenation steps at a top-bridge-top site on the Pd(111) surface. The first step of the H2O2 formation is the addition of O2 in a superoxo precursor state35 with an H atom located over a neighboring 3-fold position, which has an activation energy of 12.2 kcal/mol. The second H atom is added to the OOH intermediate to produce H2O2 with an activation energy of 15.9 kcal/mol. The overall reaction is exothermic by 9.4 kcal/mol. The formed H2O2 is situated at 2.65 Å high above the Pd(111) surface, as shown in Figure 1. The O
O bond distance (1.49 Å) of H2O2 is a little longer than that (1.47 Å) of the free H2O2 molecule. The O
O bond of H2O2 can be completely cleaved by the cooperation of two surface Pd atoms, leading this system from a singlet state to a triplet state. The elongated O 3 3 3 O distance in the transition state is 1.65 Å. After H2O2 is dissociated, a hydrogen bond is formed between two OH groups and the H 3 3 3 O distance is 2.00 Å. The OH groups are bonded to the Pd atoms and negatively charged as a result of electron transfer from the metal surface. This step is strongly exothermic by 41.0 kcal/mol and has an activation energy as low as 0.5 kcal/mol. Hwang and co-workers reported that this step is exothermic by 48.2 kcal/mol and has an activation energy of 0.2 kcal/mol using similar methods and model.30 In the next step, the OH groups on the Pd surface will react with each other to form water and oxygen, as shown in Figure 2.

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Figure 3. Hydrogenation of the OH group to water on the Pd(111) surface. The relative energies are given in kcal/mol and the distances in Å.

Figure 4. Hydrogenation of H2O2 to water on the Pd(111) surface. The relative energies are given in kcal/mol and the distances in Å.

The OH groups are bound with a hydrogen bond, whose distance is just 2.00 Å. One OH group will donate a proton, and the other accepts this proton to form water. In the transition state, the distances between the proton and two oxygen atoms are 1.27 and 1.35 Å, respectively. The derived oxygen is bonded with the Pd atom with a distance of 1.90 Å and is negatively charged by the metal surface. The H2O molecule has a long O
H bond (1.10 Å), which indicates that it is strongly attracted by the oxygen. This step is endothermic by 7.5 kcal/mol with an activation energy of 9.3 kcal/mol. Although this is an endothermic process, the subsequent hydrogenation of oxygen to water would make it facile. The overall decomposition reaction, including the dissociation of H2O2 to two OH groups and the disproportionation of the OH groups to water and oxygen, is exothermic by 33.5 kcal/mol. 3.2. Hydrogenation of OH Groups on the Pd(111) Surface. Besides the disproportionation of OH groups to water, the OH groups may be attacked by the surface H atoms to form water. The H atoms can be formed from the dissociation of H2 on the Pd surface with an activation energy of 1.1 kcal/mol. In the optimized reactant structure, the distance between the O atom of the OH group and the H atom over an fcc site is 3.22 Å, as shown in Figure 3. The H atom will be driven off the surface toward the OH group, leading to the formation of H2O and the coupling of two unpaired electrons. In the transition state, the H atom is located over a bridge site, being 1.61 Å away from the O atom of the OH group and 1.73 Å away from the nearest surface Pd atom. The formed H2O molecule is 2.37 Å high above the Pd(111) surface. This step is exothermic by 11.5 kcal/mol, and the activation energy is 15.1 kcal/mol. In comparison with the disproportionation of OH groups to water, the hydrogenation of OH groups to water has a higher activation energy. This result is in good agreement with the experimental fact that the disproportionation of OH groups can proceed at 215 K and the hydrogenation of OH groups is open around 300 K.36 3.3. Direct Hydrogenation of H2O2 on the Pd(111) Surface. We have presented above two possible pathways for the conversion of H2O2 to water on the Pd(111) surface. Now let us 7394

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Table 1. Calculated Energy Changes (ΔE) and Activation Energies (Ea) for Each Step Involved in the Conversion of H2O2 to Water on the Pd Surface (Units Are in kcal/mol) H2O2 f OH þ OH OH þ OH f H2O þ O

ΔE

Ea


41.0

0.5

7.5

9.3

OH þ H f H2O


11.5

15.1

H2O2 þ H f H2O þ OH


49.6

30.8

consider the third possible pathway, namely, the hydrogenation of H2O2 to water. Two-step hydrogenation of H2O2 will lead to two H2O molecules. The optimized structures of the reactant, the transition state, and the product, as well as the relative energies, in the first hydrogenation step of H2O2 on the Pd(111) surface are given in Figure 4. As mentioned above, H2O2 is situated in a top-bridge-top position over the Pd(111) surface. In the optimized reactant structure, the H atom is located over an fcc site, being 2.61 Å away from the O atom of H2O2. Next, the H atom would rise up from the surface and approach the H2O2 molecule, leading to the cleavage of the O
O bond of H2O2 and the formation of a H2O molecule and an OH group. In this process, the reactant and the product are in the doublet state and the transition state is in the quartet state. In the transition state, the H atom is situated over a bridge site, being 1.93 Å away from the O atom and 2.21 Å away from the nearest surface Pd atom, and the O 3 3 3 O distance is elongated to 2.16 Å. In the hydrogenation product, the OH group is bonded with a Pd atom, and the formed H2O molecule is situated at 2.37 Å high above the Pd(111) surface. This reaction is strongly exothermic by 49.6 kcal/mol and has a high activation energy of 30.8 kcal/mol. It involves such a high activation barrier because H2O2 is situated in a high position over the surface, and as a result, much more energy is required to lift the H atom up from the metal surface. Compared with the formation of water from the decomposition and further hydrogenation of H2O2, the direct hydrogenation of H2O2 to water is energetically very unfavorable and thus is unlikely to occur. It is helpful to review how H2O2 is converted into water on the Pd(111) surface. The calculated energy changes and activation energies for each reaction are collected in Table 1. In the three possible pathways, the hydrogenation of H2O2 is energetically unfavorable and is unlikely to occur. The other two pathways both start with the dissociation of H2O2. Although, in the following steps, the disproportionation of OH groups to water is more favorable than the hydrogenation of OH groups to water, the experiments showed that the latter can still proceed at higher temperatures. Thus, the decomposition of H2O2 and the hydrogenation of OH groups are two available channels for the conversion of H2O2 to water, and the former plays a main role. It should be emphasized that, in the overall process, the dissociation of H2O2 is a highly exothermic process. Once H2O2 is dissociated into two OH groups on the Pd(111) surface, the coupling of these two negative OH groups back to H2O2 has an activation energy of 41.5 kcal/mol and, hence, is almost impossible to take place. Therefore, the key for increasing the selectivity toward H2O2 is to inhibit the dissociation of H2O2. As described above, the calculated activation energy for the dissociation of H2O2 on the Pd(111) surface is only 0.5 kcal/mol. We think this is because the used surface model is a clean surface, and thus, the activity of the Pd surface for H2O2 dissociation is

Chart 1

Figure 5. Dissociation of H2O2 on the Pd(111) surface with H atoms coadsorbed. The relative energies are given in kcal/mol and the distances in Å.

Figure 6. Dissociation of H2O2 on the clean Au@Pd(111) surface. The relative energies are given in kcal/mol and the distances in Å.

Figure 7. Dissociation of H2O2 on the Au@Pd(111) surface with H atoms coadsorbed. The relative energies are given in kcal/mol and the distances in Å.

overestimated. Actually, the real Pd surface is saturated with many other adsorbates (unreacted H atoms and some ions, such as Cl
). These adsorbates may attract or localize the electrons of the metal surface and then weaken the interaction between the metal surface and H2O2. As shown in Chart 1, before the hydrogenation, O2 might be surrounded by H atoms (four H atoms shown here), and after H2O2 is formed, there are still two unreacted H atoms on the surface. We investigated the dissociation of H2O2 on the Pd(111) surface with H atoms coadsorbed. The result showed that the coadsorbed H atoms slightly increase the activation energy of H2O2 dissociation to 2.1 kcal/mol (Figure 5). In our previous work, we reported that the activation energy for the dissociation of O2, leading to the formation of H2O as well, on the Pd(111) surface is 20.3 kcal/mol.29 Here, we 7395

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Table 2. Activation Energies for H2O2 Dissociation and Energy Changes for H2O2 Release on Pd(111) and Au@Pd(111) Surfaces (Units Are in kcal/mol) activation energy for H2O2

energy change for H2O2

dissociation

release

clean

H atoms

clean

H atoms

surface

coadsorbed

surface

coadsorbed

Pd(111)

0.5

2.1

4.6

1.8

Au@Pd(111)

4.6

4.8

2.8

0.9

showed that the dissociation of H2O2 on the Pd(111) surface has an activation energy as low as 2.1 kcal/mol. Therefore, the dissociation of H2O2 is a more important side reaction that will significantly affect the selectivity toward H2O2. 3.4. Dissociation of H2O2 on the Au@Pd(111) Surface. We next investigated the dissociation of H2O2 on the Au@Pd(111) surface. The optimized structures of the reactant, the transition state, and the product, as well as their relative energies, in the dissociation of H2O2 on the clean Au@Pd(111) surface and with H atoms coadsorbed are given in Figures 6 and 7. Because the adsorption energy for H2 on the top of Pd is 7.6 kcal/mol and the activation energy for H2 dissociation on the Pd site is 1.1 kcal/mol, whereas the adsorption energy for H2 on the top of Au is 1.1 kcal/mol and the activation energy for H2 dissociation on the Au site is 20.6 kcal/mol, H2 will be preferentially adsorbed and dissociated on the Pd sites. As a result, the Pd sites on the surface will be gradually occupied by H atoms, and finally, the adsorption of O2 proceeds at the Au site.37 In our previous work, we reported the first hydrogenation step of O2 on the Au@Pd(111) surface containing the Au atom at three possible positions, that is, under the O atom of O2 reacting with the H atom, under the second O atom, and beside a reactive H atom.29 The activation energies for these three cases are 41.3, 22.6, and 27.4 kcal/mol, respectively, which are all higher than those on the pure Pd surface. After H2O2 is formed on the Au@Pd(111) surface, it is situated at the top-bridge-top site on one Pd atom and one Au atom. The O atom of H2O2 over the Au atom is 0.23 Å higher above the surface than the O atom over Pd atom, as shown in Figure 6. The O
O bond distance of H2O2 over the Au@Pd(111) surface is 1.48 Å, being a little shorter than that (1.49 Å) of H2O2 over the pure Pd(111) surface. In the transition state of the dissociation reaction, the O 3 3 3 O distance is elongated to 1.70 Å. In the product, two OH groups are interacted with a hydrogen bond and the H 3 3 3 O distance is merely 1.91 Å. Both the product and the transition state are in the triplet state. This reaction is exothermic by 24.8 kcal/mol with an activation energy of 4.6 kcal/mol, indicating that it is also facile and nearly irreversible. When H atoms are coadsorbed on the surface, the dissociation of H2O2 is exothermic by 21.7 kcal/mol and the activation energy is 4.8 kcal/mol, very near to those on the clean Au@Pd(111) surface. As summarized in Table 2, in comparison with the activation energy for the dissociation of H2O2 on the Pd surface, the activation energy for the dissociation of H2O2 on the Au@Pd surface is increased, which means that the Au atoms on the Au@Pd surface can suppress the dissociation of H2O2. This was widely thought as the main reason for the high selectivity of H2O2 synthesis on the Au@Pd surface. However, as mentioned above, the activation energy for H2O2 formation on the Au@Pd

surface is more than 20 kcal/mol, whereas the activation energy for the dissociation of H2O2 on this surface is merely 4.6 kcal/ mol (or 4.8 kcal/mol). As a result, H2O2 will be still dissociated completely if it is not released from the surface. Therefore, the high selectivity of H2O2 synthesis cannot be explained simply by the relatively increased barrier for the dissociation of H2O2 on the Au@Pd surface. We should remember that, after H2O2 is produced, its release and dissociation are two important competing processes. On the clean Pd surface, the energy change for the release of H2O2 is 4.6 kcal/mol, being greater than the activation energy for the dissociation of H2O2, as shown in Table 2. The situation on the clean Au@Pd surface is just contrary. The energy change for the release of H2O2 from the clean Au@Pd surface is 2.8 kcal/mol, less than the activation energy for the dissociation of H2O2. We think this is due to the weaker interaction between the less active Au atom and H2O2. In the structure of H2O2 adsorption on the clean Au@Pd surface, the distance between the Au atom and one O atom of H2O2 is longer by 0.23 Å than that between the Pd atom and the other O atom of H2O2. The weaker interaction of the Au atom with H2O2 results in the shorter O
O bond of H2O2 on the Au@Pd surface, which indicates that H2O2 on the Au@Pd surface is less activated than on the Pd surface. For this reason, the dissociation of H2O2 on the Au@Pd surface is less favorable and the release of H2O2 from the Au@Pd surface is easier. In a word, the less active Au atoms on the Au@Pd catalysts increase the barrier for the dissociation of H2O2 and, on the other hand, decrease the barrier for the release of H2O2. The opposite effects of the Au atoms on the dissociation of H2O2 and the release of H2O2 shift the balance to the release side on the Au@Pd surfaces. This may be a more plausible reason why the facile dissociation of H2O2 does not proceed completely and the origin of the high H2 O2 selectivity in the direct synthesis of H2 O2 on the Au@Pd catalysts. Interestingly, the H atoms coadsorbed on the Pd and Au@Pd surfaces have similar effects on the release of H2 O2 and the dissociation of H2O2 . On the Pd surface, the energy change for the release of H 2O2 is 1.8 kcal/mol and, on the Au@Pd surface, the energy change for the H2O2 release is 0.9 kcal/mol when H atoms are coadsorbed on the surfaces. They are both smaller than the energy changes for the release of H2O2 on the clean Pd and Au@Pd surfaces, which indicates that the interaction between the surfaces and H2O2 is decreased. This is due to that the H atoms attract the electrons of the surfaces and lead to a slight loss of the surface electrons. The weakened interaction between the surfaces and H2O2 makes the H2O2 dissociation less favorable. On the Pd surface with H atoms coadsorbed, the activation energy for the H2O2 dissociation is 2.1 kcal/mol, and on the Au@Pd surface with H atoms coadsorbed, the activation energy for the H2O2 dissociation is 4.8 kcal/mol. Moreover, one can find that, on the Pd surface with H atoms coadsorbed, the energy change for the release of H2O2 is comparable to the activation energy for the dissociation of H 2O2 . However, on the Au@Pd surface with H atoms coadsorbed, the energy change for the H2O2 release is much lower than the activation energy for the H2O2 dissociation. It means that the dissociation of H2 O2 cannot compete with the release of H2 O2 in this case. This result is compatible with Ishihara and co-workers’ report that, on the pure Pd catalyst, the selectivity toward H2 O2 is nearly 50% and the selectivity reaches almost 100% when the surface concentration of Au on the Au@Pd catalyst is 15 mol %.27 7396

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The Journal of Physical Chemistry C Chart 2

4. CONCLUSIONS We have investigated three pathways for the conversion of H2O2 to water on Pd and Au@Pd catalysts with DFT methods for periodic Pd(111) and Au@Pd(111) surfaces. These three pathways are (1) the decomposition of H2O2 to water, including two steps, namely, the dissociation of H2O2 to two OH groups and the disproportionation of two OH groups to water and oxygen; (2) the hydrogenation of the OH group to water; and (3) the direct hydrogenation of H2O2 to water. We found that the third one is energetically very unfavorable and is unlikely to occur. Two feasible channels leading to the formation of water are the decomposition of H2O2 and the hydrogenation of OH groups, and the former plays a main role. The key step in the overall process is the dissociation of H2O2, which is facile and irreversible. Therefore, the dissociation of H2O2 must be inhibited for obtaining high selectivity toward H2O2 in the direct H2O2 synthesis. On the Pd surface, the activation energy for H2O2 dissociation is less than the energy change for H2O2 release from it, whereas on the Au@Pd surface, the energy change for H2O2 release is lower than the activation energy for H2O2 dissociation on it. The less active Au atoms on the Au@Pd surface weaken the interaction of the surface with H2O2, and hence suppress the dissociation of H2O2, and, on the other hand, facilitate the release of H2O2. The opposite effects of Au atoms on the dissociation and release of H2O2 shift the balance to the release side, resulting in the enhanced H2O2 selectivity. The unreacted H atoms on the Pd and Au@Pd surfaces have similar effects. The H coadsorbed atoms decrease the interaction between the metal surfaces and H2O2 as well and, consequently, facilitate the release of H2O2 and suppress the dissociation of H2O2. As a result, the dissociation branch is suppressed on the Au@Pd surface, as schematically represented in Chart 2. Our theoretical results provide a new understanding for the origin of the high selectivity toward H2O2 in the direct synthesis of H2O2 from H2 and O2 on the Au@Pd catalysts. ’ ASSOCIATED CONTENT

bS

Supporting Information. The configurations of O2 adsorption on the Au@Pd surface and atomic Cartesian coordinates for the optimized geometries of all investigated structures. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge Grants-in-Aid (Nos. 18066013 and 18GS0207) for Scientific Research from Japan Society for the

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Promotion of Science (JSPS) and the Ministry of Culture, Sports, Science and Technology of Japan (MEXT); the Nanotechnology Support Project of MEXT; and the Joint Project of Chemical Synthesis Core Research Institutions of MEXT for their support of this work. A.S. acknowledges JSPS for a postdoctoral fellowship.

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dx.doi.org/10.1021/jp1070456 |J. Phys. Chem. C 2011, 115, 7392–7398

The Journal of Physical Chemistry C

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(34) Eichler, A.; Mittendorfer, F.; Hafner, J. Phys. Rev. B 2000, 62, 4744. (35) Honkala, K.; Laasonena, K. J. Chem. Phys. 2001, 115, 2297.  (36) Nyberg, C.; Tengstal, C. G. J. Chem. Phys. 1984, 80, 3463. (37) There exit three possible sites for O2 adsorption on the Au@Pd(111) surface: (1) the top-bridge-top site on two Pd atoms that are not adjacent to the Au atom, as shown in Chart 1 and Figure S2a (Supporting Information); (2) on two Pd atoms that are adjacent to the Au atom (Figure S2b, Supporting Information); and (3) on one Pd atom and one Au atom (Figure S2c, Supporting Information). The adsorption energies for O2 at the three sites are 3.2, 3.4, and 5.8 kcal/mol, respectively. The most stable adsorption of O2 on the Au@Pd(111) surface occurs at the top-bridge-top site on one Pd atom and one Au atom because the concentration of H atoms around the Au atom is very low. Detailed results are presented in the Supporting Information.

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dx.doi.org/10.1021/jp1070456 |J. Phys. Chem. C 2011, 115, 7392–7398