Theoretical Revisit of the Direct Synthesis of H2O2 on Pd and Au@ Pd

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Theoretical Revisit of the Direct Synthesis of H2O2 on Pd and Au@Pd Surfaces: A Comprehensive Mechanistic Study Jun Li,† Tatsumi Ishihara,‡ and Kazunari Yoshizawa*,† †

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

bS Supporting Information ABSTRACT: The direct synthesis of H2O2 from H2 and O2 on Pd(111) and Au@Pd(111) surfaces is studied with periodic density functional theory calculations. Ten possible reactions and processes involved in the H2O2 synthesis steps are considered: For O2, (1) O2* + H* f OOH*, (2) O2* f 2O*, and (3) O2* f O2; for OOH, (4) OOH* + H* f H2O2*, (5) OOH* + H* f H2O* + O*, (6) OOH* + H* f 2OH*, (7) OOH* f O* + OH*, and (8) OOH* f OOH; for H2O2, (9) H2O2* f 2OH* and (10) H2O2* f H2O2, where the asterisks indicate these species to be surface species. All side reactions involve OO bond dissociation. On the Pd(111) surface with H atoms coadsorbed, O2 dissociation is suppressed; OOH dissociation is more favorable than all OOH hydrogenation reactions; three OOH hydrogenation reactions have comparable activation barriers; the barrier for H2O2 dissociation is also comparable to that for H2O2 desorption. However, on the H atoms coadsorbed Au@Pd(111) surface, the main reactions for H2O2 production exceed all side reactions. The competition between the main reactions and the side reactions is actually the competition between the OO bond and the OM bond, where M is Pd in the case of the Pd(111) surface and Au in the case of the Au@Pd(111) surface. The OPd bond is usually stronger than the OO bonds in the OOH intermediate and H2O2; however, the OAu bond is weaker than the OO bonds. Consequently, the final product H2O2 is easily produced and released from the Au@Pd(111) surface, and the side reactions involving OO bond dissociation are suppressed. The role of the metal surface in the direct synthesis of H2O2 from H2 and O2 is to provide H atoms as the feedstock for the hydrogenation of O2.

1. INTRODUCTION Hydrogen peroxide (H2O2) is an environmentally friendly oxidant that is widely used in many fields such as waste treatment, pulp/paper bleaching, and chemical synthesis.1,2 The development of green processes for H2O2 production is of environmental and commercial importance. Palladiumgold (PdAu) bimetal catalysts show high activity and selectivity for the direct synthesis of H2O2 from H2 and O2. Since Hutchings and coworkers discovered the synergetic effect of PdAu bimetal for the direct synthesis of H2O2 in 2002,3 significant efforts2,4 have been made to study in detail effects of catalyst compositions,57 supports,813 catalyst preparation and pretreatment methods,1418 reaction conditions,1922 and so on. In contrast to extensive experimental studies on the direct synthesis of H2O2 from H2 and O2, the mechanistic thinking on this subject is insufficient. Current theoretical investigations focused on exploring the pathways for the formation and decomposition of H2O2 and did not yet provide a comprehensive understanding toward the effects of all the factors mentioned above. In 2008, we examined the formation of H2O2 from H2 and O2 on Pd(111) and Au@Pd(111) surfaces using periodic density functional theory (DFT) calculations.23 We proposed a two-step reaction r 2011 American Chemical Society

mechanism for the formation of H2O2, namely, O2 in a superoxo precursor state on the Pd(111) or Au@Pd(111) surface is hydrogenated in two steps with H atoms situated on neighboring 3-fold positions. Recently, we researched the pathways for the conversion of H2O2 into water on Pd(111) and Au@Pd(111) surfaces.24 We found that the decomposition of H2O2 is the main channel for the nonselective formation of water and the hydrogenation of H2O2 is energetically unfavorable. In addition, we noticed that the dissociation of the OO bond in H2O2 is very facile. The barrier for H2O2 dissociation on the Pd(111) surface is merely 0.5 kcal/mol. We then considered the barrier for OOH dissociation on the Pd(111) surface and determined it to be 6.3 kcal/mol, which is lower than the barrier (14.6 kcal/mol) for OOH hydrogenation to H2O2. Therefore, we decided to reexamine the mechanism for the synthesis of H2O2 from H2 and O2. All possible reactions and processes involved in each H2O2 synthesis step are summarized below. For the reactant O2, the hydrogenation of O2 gives the OOH group (eq 1) and the Received: August 23, 2011 Revised: October 13, 2011 Published: October 14, 2011 25359

dx.doi.org/10.1021/jp208118e | J. Phys. Chem. C 2011, 115, 25359–25367

The Journal of Physical Chemistry C

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dissociation of O2 gives two surface O atoms (eq 2). Additionally, the adsorbed O2 may be desorbed from the metal surface (eq 3). ð1Þ O2 þ H f OOH O2 f O þ O

ð2Þ

O2 f O 2

ð3Þ

Here, the asterisks indicate these species to be surface species. For the intermediate OOH, the hydrogenation of OOH may result in the formation of H2O2 (eq 4); at the same time, the hydrogenation of OOH may yield H2O and an O atom (eq 5) or two OH groups (eq 6). The dissociation of OOH brings an O atom and an OH group (eq 7). In addition, the adsorbed OOH species may be desorbed (eq 8). OOH þ H f H2 O2 ð4Þ OOH þ H f H2 O þ O

ð5Þ

OOH þ H f OH þ OH

ð6Þ

OOH f O þ OH

ð7Þ

OOH f OOH

ð8Þ

For the product H2O2, it may be dissociated into two OH groups (eq 9) or desorbed from the metal surface (eq 10). H2 O2 f OH þ OH ð9Þ H2 O2 f H2 O2

ð10Þ

Except for the desorption processes, all side reactions involve OO bond dissociation. These side reactions eventually result in the nonselective formation of H2O and greatly influence the selectivity toward H2O2. Understanding how these side reactions are suppressed on the Au@Pd(111) surface is important for the mechanistic explanation to the direct synthesis of H2O2 from H2 and O2. Ham et al. computationally studied three competing couples involved in the formation of H2O2 on Pd(111) and Pd@Au(111) surfaces: (1) O2 hydrogenation (eq 1) versus O2 dissociation (eq 2); (2) OOH hydrogenation (eq 4) versus OOH dissociation (eq 7); (3) H2O2 dissociation (eq 9) versus H2O2 desorption (eq 10).25 They observed that Pd monomer on Aubased catalyst can suppress the OO bond dissociation. More recently, Todorovic and Meyer further investigated these three competing couples occurring on Pd(111), Pt(111), and Pd0.11Au0.89(111) surfaces with periodic DFT calculations.26 They suggested that, as the surface changed from Pd to Pt and to Au, the step that governs the nonselective formation of H2O shifts from O2 dissociation to OOH dissociation and to H2O2 dissociation. However, the situation on the Au@Pd(111) surface has not been considered yet. In many studies, Pd-based PdAu bimetal catalysts also show high activity and selectivity for the direct synthesis of H2O2.57,10 Nomura et al. reported that in their work the formation rate of H2O2 reaches a maximum at 30 mol % Au (a proportion of gold used to prepare the catalyst), and the selectivity toward H2O2 showed a sharp rise at 25 mol % Au (the actual surface Au concentration is 15 mol %).6 A difficulty in studying the direct synthesis of H2O2 on the Au@Pd(111) surface is determining the adsorption configuration of O2 on

this surface. The number of Au atoms on the surface is much less than that of Pd atoms and O2 adsorption at Au sites is much less stable than that at Pd sites; thus, the majority of O2 will be located at Pd sites. If the Au site is not included in the reactions, what role does Au on the Au@Pd(111) surface play? We now find that the case is not so simple. Because H2 is preferentially adsorbed and dissociated at Pd sites and H atom migration from Pd sites to Au sites is thermodynamically unfavorable, the Pd sites will be occupied by H atoms. Au sites on the Au@Pd(111) surface are like islands in the sea of H atoms. Accordingly, the adsorption of O2 at the Au site becomes favored. In this work, our aim is to demonstrate a more comprehensive and matter-of-fact picture for the direct synthesis of H2O2 from H2 and O2 on Pd(111) and Au@Pd(111) surfaces. At the beginning, we studied the dissociative adsorption of O2 and H2 on the Pd(111) surface. H2 dissociation at Pd sites is very facile nearly without activation barrier. Next, we researched all possible reactions involved in each H2O2 synthesis step on the Pd(111) surface, assuming that the concentrations of surface O2 and H are at the stoichiometric ratio of 1:2. In this case, we found that actually only a part of O2 will be hydrogenated and the majority of O2 is in adsorptiondesorption equilibrium. Accordingly, the real situation would be that O2 on the Pd(111) surface is surrounded by more H atoms, for example, with a ratio of 1:4 (O2:H). Under this circumstance, O2 dissociation will be suppressed, but OOH dissociation is energetically more favorable than all OOH hydrogenation reactions. Three OOH hydrogenation reactions (eqs 46) have comparable activation barriers. The barriers for H2O2 dissociation and H2O2 desorption are comparable as well. For the case of H2O2 synthesis on the Au@Pd(111) surface, we first determined that O2 is preferentially adsorbed at the top-bridge-top site on one Pd atom and one Au atom, no matter whether the Au impurity on the Au@Pd(111) surface exists as an isolated atom or ensemble. With this configuration as the precursor state, the main reactions leading to the selective formation of H2O2 are more favorable than all side reactions. Pd(111) and Au@Pd(111) surfaces show different efficiency for the synthesis of H2O2 from H2 and O2 because on the Pd(111) surface the competition between the main reactions and the side reactions is actually the competition between the OO bond and the OPd bond. However, on the Au@Pd(111) surface, the competition is between the OO bond and the OAu bond. Furthermore, we noticed that O2 in the triplet dioxygen state is even more reactive for the hydrogenation than O2 in the doublet superoxo state, which implies that preactivation of O2 by the metal surface is unnecessary. The role of the metal surface in the direct synthesis of H2O2 from H2 and O2 is probably to supply H atoms by cleaving the H2 molecule. This work will promote the understanding toward the mechanism for the direct synthesis of H2O2 from H2 and O2 on Pd-based PdAu bimetal catalysts.

2. COMPUTATIONAL METHODS All calculations were performed with the CASTEP suite of program.27 The method applied here is the same as those in our previous work.24 The spin-polarized PerdewBurkeErnzerhof functional within the generalized gradient approximation (GGAPBE28) and ultrasoft pseudopotentials29 were employed. An energy cutoff of 340 eV was used for the plane wave expansion of the electronic eigenfunctions. The Brillouin zone was sampled with a (3 3 1) MonkhorstPack mesh of k points. 25360

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Other computational options were set at medium accuracy level. The transition state was determined by using the linear and quadratic synchronous transit (LST/QST) complete search method. 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 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 first principle DFT calculations25 and also in good agreement with an experimental value of 3.89 Å.30 Two slab models were used to represent the Au@Pd(111) surface. One was obtained via that one Au atom was put in the 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. In nine slab models derived in this method, the one with the lowest energy after geometry optimization was finally selected. The concentration of Au in the surface of this model is 11.1 mol %, near the experimentally measured value (15 mol %) that lead to the highest H2O2 formation rate and selectivity reported by Nomura et al.6 The other was obtained by substituting four Pd atoms in the surface layer of the pure Pd slab with four Au atoms and then optimizing the derived model. This slab model is for representing the Pd ensemble on the Au@Pd(111) surface. The adsorption energy (Ead) of a species, X, on the surface of the slab model was defined in eq 11 Ead ðXÞ ¼ EðslabÞ þ EðXÞ EðX=slabÞ

ð11Þ

where E(X/slab) is the energy of optimized structure for X adsorption on the slab model. The larger (more positive) the value is, the stronger the adsorption of the species concerned in the slab model.

3. RESULTS AND DISCUSSION The adsorption of O2 and H2 on the Pd(111) surface precedes the reaction of O2 with H2 to produce H2O2. Beyond the adsorption, the Pd(111) surface is active enough for the dissociation of O2 and H2. Thus, first of all, we investigated the dissociative adsorption of O2 and H2 on the Pd(111) surface. O2 dissociation has a moderate activation barrier, but H2 dissociation to yield H atoms takes place nearly without activation barrier. We then considered the hydrogenation of O2. A possible case for this reaction is that the concentrations of O2 and H on the Pd(111) surface are at the stoichiometric ratio of 1:2. As described above, in each H2O2 synthesis step, there are several undesired side reactions. In this case, we found that actually a minor part of O2 is hydrogenated and the majority of O2 is in adsorptiondesorption equilibrium. In view of the fact that H2 dissociation is very facile, the real situation on the Pd(111) surface would be that O2 is surrounded by H atoms. Here, we assumed the ratio of surface O2 to H to be 1:4. We examined all possible reactions involved in each H2O2 synthesis step under this circumstance and observed that OOH hydrogenation to H2O2 cannot compete with OOH dissociation. Next, we turned to the study on the adsorption and the subsequent hydrogenation of O2 on the Au@Pd(111) surface. We determined that O2 is preferentially adsorbed at the top-bridge-top site on one Pd atom and one Au atom, no matter whether the Au impurity on

Figure 1. O2 dissociation on the Pd(111) surface. The distances are given in Å.

the Au@Pd(111) surface exists as an isolated atom or an ensemble. On the Au@Pd(111) surface, the main reactions leading to the selective formation of H2O2 are more favorable than all side reactions. On the basis of these observations, we analyzed different H2O2 synthesie mechanisms on Pd(111) and Au@Pd(111) surfaces and discussed a realistic role of the metal surfaces in the direct synthesis of H2O2 from H2 and O2. 3.1. Dissociative Adsorption of O2 and H2 on the Pd(111) Surface. Making it clear how O2 and H2 are adsorbed and dissociated on the Pd(111) surface is the basis of the whole study on the mechanism for the direct synthesis of H2O2 on Pd(111) and Au@pd(111) surfaces. The adsorbed O2 species is the precursor state for the subsequent hydrogenation reactions. O2 dissociation is an undesired side reaction that should be suppressed. However, H2 dissociation provides H atoms as the feedstock for the hydrogenation of O2. Honkala and Laasonena theoretically researched several trajectories for O2 adsorption on the Pd(111) surface and reported that, in the most stable adsorption configuration, O2 is on the top-bridge-top symmetric position and in the superoxo state.31 As mentioned above, the Pd(111) surface is so active that it can undertake the dissociation of O2. Hwang and co-workers previously reported that the activation energy for O2 dissociation on the Pd(111) surface is 11.8 kcal/mol.25 Todorovic and Meyer determined this barrier to be 17.8 kcal/mol.26 In this work, we considered two paths for O2 dissociation on the Pd(111) surface, as depicted in Figure 1. Through path 1, O2 is dissociated into two O atoms that are located on neighboring 3-fold positions, and via path 2, O2 is dissociated into two O atoms located on opposite 3-fold positions. Path 1 is exothermic by 44.4 kcal/mol with an activation energy of 22.3 kcal/mol, and path 2 is exothermic by 43.9 kcal/mol with an activation energy of 17.4 kcal/mol. In the optimized structure for O2 adsorption on the Pd(111) surface, O2 is situated 2.0 Å above the surface with an adsorption energy of 18.9 kcal/mol. The distance of the OO bond is 1.35 Å and the total spin of O2 is 0.40 e. These results suggest that O2 is in the superoxo state on the surface. For the dissociative adsorption of H2 on the Pd(111) surface, our results show that H2 will be preferentially adsorbed on the top of the Pd atom. The adsorption energy for H2 at the topbridge-top site is 4.8 kcal/mol, while the adsorption energy for H2 at the top site is 7.6 kcal/mol. H2 dissociation at the top site of the Pd(111) surface is shown in Figure 2. The dissociation of H2 on the Pd(111) surface is exothermic by 24.4 kcal/mol with an activation energy as low as 1.1 kcal/mol. The resulting H atoms 25361



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Figure 2. H2 dissociation on the Pd(111) surface. The distances are given in Å.

Figure 3. Three configurations for the coadsorption of O2 and H on the Pd(111) surface with a stoichiometric ratio of O2:H = 1:2.

Figure 4. Energy diagram for the synthesis of H2O2 on the Pd(111) surface (case 1), where the asterisks indicate these species to be surface species. The main reactions are indicated by red color. The relative energies are given in kcal/mol.

Scheme 1

are located on two neighboring 3-fold hcp positions. Todorovic and Meyer suggested that this reaction is exothermic by 21.0 kcal/mol without a barrier.26 Accordingly, the dissociation of H2 on the Pd(111) surface takes place nearly with no cost of activation energy. This is very important for the subsequent O2 hydrogenation reactions. 3.2. Synthesis of H2O2 on the Pd(111) Surface (Case 1: O2: H = 1:2). We now know that the adsorbed O2 is at the top-bridgetop site on the Pd(111) surface and H2 dissociation on the Pd(111) surface is very facile. With this information, we carried out a study on the hydrogenation of O2 to H2O2. A possible case for this reaction is that the concentrations of O2 and H on the Pd(111) surface are at the stoichiometric ratio of 1:2. Figure 3 shows three coadsorption configurations for this case, which are energetically almost all degenerate. Among them, configuration I is the most favorable one for the formation of H2O2. Thus, we may take it as the starting point of the pathways for H2O2 synthesis from H2 and O2. In this case, H2O2 is produced via two hydrogenation steps (eqs 1 and 4). Besides, the hydrogenation of OOH may yield two OH groups (eq 6). Moreover, O2 may be dissociated (eq 2); OOH and H2O2 may be dissociated as well (eqs 7 and 9). All these reactions are schematically shown in Scheme 1. Additionally, O2, OOH, and H2O2 desorption also may take place. They are not presented in Scheme 1. The energy diagram for these reactions and processes is depicted in Figure 4. As described above, for the reactant O2, the hydrogenation of O2 will give the OOH group (eq 1) and the dissociation of O2 will give two surface O atoms (eq 2). In addition, O2 may be

desorbed from the metal surface (eq 3). The hydrogenation of O2 to OOH is endothermic by 3.8 kcal/mol with an activation energy of 21.2 kcal/mol. Because the active sites for O2 dissociation via path 1 are occupied by H atoms, the dissociation of O2 will occur via path 2, being exothermic by 42.2 kcal/mol with an activation energy of 27.5 kcal/mol. O2 desorption is endothermic by 10.9 kcal/mol. The hydrogenation of O2 is energetically more favorable than the dissociation of O2. It should be noted that O2 desorption is more facile than O2 hydrogenation. As a result, only a part of O2 will be hydrogenated to OOH and the majority of O2 is actually in adsorptiondesorption equilibrium. The barrier for O2 dissociation in this case is much higher than that (17.4 kcal/mol) on the clean Pd(111) surface, indicating that the coadsorbed H atoms can suppress the dissociation of O2. Some previous work mistakenly compared the barrier for O2 hydrogenation on the H atoms coadsorbed Pd(111) surface with the barrier for O2 dissociation on the clean Pd(111) surface. The adsorbed O2 species in this case has a total spin of 0.88 e, indicating that it is in the doublet superoxo state. For the intermediate OOH, the hydrogenation of OOH can produce H2O2 (eq 4); at the same time, the hydrogenation of OOH also may yield two OH groups (eq 6). In addition, the dissociation (eq 7) and the desorption (eq 8) of OOH may happen as well. The hydrogenation of OOH to H2O2 is endothermic by 1.7 kcal/mol with an activation energy of 14.6 kcal/mol. The hydrogenation of OOH to two OH groups is exothermic by 39.3 kcal/mol with an activation energy of 8.1 kcal/mol. OOH dissociation is exothermic by 44.2 kcal/mol and has an activation energy of 6.3 kcal/mol. OOH desorption is endothermic by 24.1 kcal/mol. The dissociation of OOH and the hydrogenation of OOH to two OH groups are both more favorable than the hydrogenation of OOH to H2O2. OOH hydrogenation to two OH groups exceeds OOH hydrogenation to H2O2 because the OOH group is linked to the Pd(111) surface with a strong OPd bond. This OPd bond is even stronger than the OO bond; as a result, the hydrogenation of the OOH group does result in the dissociation of the OO bond, not the dissociation of the OPd bond. In the OOH dissociation process, a new OPd bond will be formed to compensate for the 25362



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Scheme 2

Figure 5. Optimized structures for the species involved in the synthesis of H2O2 on the Pd(111) surface (case 1). The distances are given in Å. The Pd atoms of the bottom two layers are omitted for clarity.

cleavage of the OO bond. Because the OPd bond is much stronger than the OO bond, the dissociation of OOH is so favorable. These two side reactions are irreversible, being very disadvantageous for the selective formation of H2O2. Note that the resulting OH and O species are negatively charged by electron transfer from the metal surface; they are not radicals. Provided that H2O2 is formed in the OOH hydrogenation step, H2O2 needs to be desorbed from the metal surface (eq 10); otherwise, it will be dissociated into two OH groups (eq 9). The dissociation of H2O2 is exothermic by 41.0 kcal/mol, and the activation energy is merely 0.6 kcal/mol. In comparison, the barrier for H2O2 desorption is 4.7 kcal/mol. By surveying the three H2O2 synthesis steps, it can be judged that in this case no H2O2 will be obtained. Geometrical parameters of the optimized structures for all species are collected in Figure 5. 3.3. Synthesis of H2O2 on the Pd(111) Surface (Case 2: O2: H = 1:4). As described above, the majority of O2 on the Pd(111) surface is actually in adsorptiondesorption equilibrium. On the other hand, the dissociation of H2 to H atoms is very facile nearly without activation barrier. We can reasonably consider that the Pd(111) surface will be covered with H atoms in the initial stages of the process. Therefore, a more reasonable situation would be that, on the Pd(111) surface, O2 is surrounded by more H atoms, as shown in Scheme 2. Under this circumstance, we can assume that, for example, the ratio of surface O2 to H is 1:4. From this starting point, we explored the possible reactions involved in the H2O2 synthesis steps and schematically present these reactions in Scheme 2, excluding the desorption processes. The energy diagram for the considered reactions and processes is depicted in Figure 6. When O2 is coadsorbed with four H atoms on the Pd(111) surface, the interaction between O2 and the surface will be weakened. In case 1, the adsorbed O2 has an OO bond distance of 1.32 Å and a total spin of 0.88 e. The adsorption energy is 10.9 kcal/mol.

Figure 6. Energy diagram for the synthesis of H2O2 on the Pd(111) surface (case 2), where the asterisks indicate these species to be surface species. The main reactions are indicated by red color. The relative energies are given in kcal/mol.

However, here the adsorbed O2 has a shorter OO bond distance of 1.29 Å and a higher total spin of 1.40 e. The adsorption energy is greatly decreased to 3.2 kcal/mol. In this case, since the active sites for O2 dissociation via both path 1 and path 2 are occupied by H atoms, the dissociation of O2 is not considered. The hydrogenation of O2 to OOH group (eq 1) is endothermic by 6.0 kcal/mol with an activation energy of 18.8 kcal/mol. It is interesting that this activation barrier is lower than that (21.2 kcal/mol) in case 1, as shown in Table 1, indicating that preactivation of O2 might not be required for the subsequent hydrogenation reactions. Once the OOH intermediate is formed, it may be hydrogenated to H2O2 (eq 4). At the same time, it also may be hydrogenated to H2O and O atom (eq 5) or two OH groups (eq 6). The dissociation (eq 7) and the desorption (eq 8) of OOH may also proceed. OOH hydrogenation to H2O2 is exothermic by 7.7 kcal/mol with an activation energy of 11.1 kcal/mol. OOH hydrogenation to H2O and O is exothermic by 65.6 kcal/mol with an activation energy of 12.8 kcal/mol. OOH hydrogenation to two OH is exothermic by 45.5 kcal/mol with an activation energy of 12.3 kcal/mol. OOH dissociation is exothermic by 53.3 kcal/mol with an activation energy of 7.7 kcal/mol. OOH desorption is endothermic by 12.5 kcal/mol. Among these reactions, the hydrogenation of OOH to H2O2 is still not the most favorable one. The dissociation of OOH has the lowest activation barrier. The barriers for the three hydrogenation reactions are nearly comparable, meaning that in this case 25363



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Table 1. Calculated Activation Energies (in kcal/mol) for the Reactions Involved in the H2O2 Synthesis Steps on the Pd(111) and Au@Pd(111) Surfaces Pd(111)

Au@Pd(111)

surface

surface

case 1

case 2 18.8

19.7

11.1

13.5

12.8

21.7

12.3

22.2

O2 þ H f OOH

ð1Þ

21.2

O2 f O þ O

ð2Þ

27.5

OOH þ H f H2 O2

ð4Þ

14.6

OOH þ H f H2 O þ O

ð5Þ

OOH þ H f OH þ OH

ð6Þ

OOH f O þ OH

ð7Þ

6.3

7.7

H2 O2 f OH þ OH

ð9Þ

0.6

2.0

8.1

16.4/16.0 5.4

the strength of the PdO bond is comparable with the OO bond. However, the OO bond is still too weak to resist the cleavage by the surface Pd atoms; as a result, the dissociation of OOH is facile. Anyway, in comparison with case 1, the barriers for OOH dissociation and the side OOH hydrogenation reaction are increased to some extent, and the barrier for OOH hydrogenation to H2O2 is decreased. Given that H2O2 is formed, H2O2 desorption from this surface (eq 10) is endothermic by 1.8 kcal/mol. On the other hand, the dissociation of H2O2 (eq 9) is exothermic by 37.8 kcal/mol and the activation energy is 2.0 kcal/mol. Compared with those in case 1, the barrier for H2O2 desorption is decreased and the barrier for H2O2 dissociation is increased, resulting in them being comparable in this case. However, on the whole potential energy surface, the formation of H2O2 is still not the most favored. As we know, pure Pd catalyst is also effective for the direct synthesis of H2O2 from H2 and O2.3242 However, the Pd catalyst usually is pretreated with strong acids or the catalytic system contains halide ions. The functions of the pretreatment with strong acids and the addition of halide ions are probably to further weaken the interaction between the reagents (O2, OOH, and H2O2) and the surface, and thus suppress the OO bond dissociation. Geometrical parameters of the optimized structures for all species are collected in Figure 7. 3.4. Adsorption of O2 on the Au@Pd(111) Surface. The investigation of the synthesis of H2O2 on the Pd(111) surface is helpful for understanding the mechanism for H2O2 synthesis on the Au@Pd(111) surface. Before the H2O2 synthesis mechanism on the Au@Pd(111) surface is explored, the stable adsorption configuration of O2 on the Au@Pd(111) surface should be known. Nomura et al.6 reported that they achieved the highest selectivity toward H2O2 and formation rate of H2O2 when the surface Au concentration was 15 mol %. The number of surface Au atoms is less than that of Pd atoms. Moreover, O2 adsorption at the Au site on clean Au@Pd(111) is less favorable than that at the Pd site. Thus, one may suppose that most O2 will be adsorbed at Pd sites and then hydrogenated as it is done on the pure Pd(111) surface. We think that this is not true. The effect of the coadsored H atoms on the Au@Pd(111) surface should be taken into account. As shown in Table 2, the adsorption energy for H2 on the top of Pd is 7.6 kcal/mol and the activation energy for H2

Figure 7. Optimized structures for the species involved in the synthesis of H2O2 on the Pd(111) surface (case 2). The distances are given in Å.

Table 2. Adsorption Energy for H2 as Well as Activation Energy for H2 Dissociation at the Pd Site or Au Site on the Au@Pd(111) Surface adsorption energy for H2 (kcal/mol)

activation energy for H2 dissociation (kcal/mol)

Pd site

7.6

1.1

Au site

1.1

20.6

Figure 8. Energy diagram for H atoms migration from Pd site to Au site. The relative energies are given in kcal/mol.

dissociation at 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 at the Au site is 20.6 kcal/mol. As a result, on the Au@Pd(111) surface, H2 will be preferentially adsorbed and dissociated at Pd sites. The energy diagram for H atoms migration from the Pd site to the Au site is depicted in Figure 8. The migration of an H atom from the Pd site to the Au site is endothermic by 3.8 kcal/mol with an activation barrier of 4.4 kcal/mol. The second H atom migration step is endothermic by 5.5 kcal/mol with an activation barrier of 5.9 kcal/mol. As a result, H atom migration from the Pd site to the Au site is thermodynamically unfavorable. On the contrary, the migration of an H atom from the Au site to the Pd site needs to overcome a 25364



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Figure 9. Possible configurations for O2 adsorption on the Au@Pd(111) surface. The distances are given in Å.

Scheme 3

Figure 10. Energy diagram for the synthesis of H2O2 on the Au@Pd(111) surface, where the asterisks indicate these species to be surface species. The main reactions are indicated by red color. The relative energies are given in kcal/mol.

barrier of only 0.40.6 kcal/mol. Accordingly, one can imagine that the concentration of H atoms at the Au site should be very low. The Au atom on the Au@Pd(111) surface is like an island in the sea of H atoms. We considered four possible configurations for O2 adsorption on the Au@Pd surface: (1) O2 is located at the top-bridge-top site on two Pd atoms that are not adjacent to the Au atom, which is like the adsorption on the pure Pd(111) surface, as shown in Figure 7; (2) O2 is at the top-bridge-top site on two Pd atoms that are adjacent to the Au atom, shown as configuration a in Figure 9; (3) O2 is at the top-bridge-top site on one Pd atom and one Au atom, shown as configuration b; and (4) O2 is at the topbridge-top site on two Au atoms of the Au ensemble, shown as configuration c. The adsorption energies for these four configurations are 3.2, 3.4, 5.8, and 5.3 kcal/mol, respectively. The negative adsorption energy for configuration c indicates that O2 cannot be firmly adsorbed on the Au ensemble. The adsorption energies for the other three configurations are nearly equal, but the one for configuration b is the largest. Consequently, on the Au@Pd(111) surface with H atom coadsorbed, O2 will be preferentially adsorbed at the topbridge-top site on one Pd atom and one Au atom, no matter whether the Au impurity exists as an isolated atom or an ensemble. As for the Au ensemble, O2 is situated at its edge and on one Au atom and one Au atom. 3.5. Synthesis of H2O2 on the Au@Pd(111) Surface. We know the adsorption configuration of O2 on the H atoms coadsorbed Au@Pd(111) surface. With this configuration as the precursor state, we studied the reactions possibly involved in each H2O2 synthesis step on the Au@Pd(111) surface. All explored reactions are schematically presented in Scheme 3. Since the H atom is too far away from the O atom of O2 on the Au top to attack O2, O2 will be hydrogenated with H atoms situated on the neighboring 3-fold position near the O atom of O2 on the Pd top. One hydrogenation step leads to the formation of the OOH group (eq 1). The second hydrogenation step results in the formation of H2O and O atom (eq 5). OOH dissociation (eq 7) and desorption (eq 8) will also take place. Then, OOH will be adsorbed again (eq 80 ) with the unhydrogenated O end linked to

the metal surface. The subsequent hydrogenation reaction may give H2O2 (eq 4) or two OH groups (eq 6). The dissociation of OOH (eq 7) possibly occurs as well. The yielded H2O2 may be dissociated (eq 9) or desorbed (eq 10). The energy diagram for these reactions and processes is depicted in Figure 10. In the precursor state of O2 on the Au@Pd(111) surface, its total spin is 1.88 e, indicating that it is nearly in the triplet dioxygen state. The hydrogenation of O2 to the OOH group (eq 1) is endothermic by 6.2 kcal/mol with an activation energy of 19.7 kcal/mol. This barrier is close to that (18.8 kcal/mol) for O2 hydrogenation on the Pd(111) surface in case 2 and is lower than that (21.2 kcal/mol) for O2 hydrogenation on the Pd(111) surface in case 1, as shown in Table 1. It means that O2 in the triplet dioxygen state is reactive enough for the hydrogenation, and thus preactivation of O2 by the metal surface is not indispensable. After the OOH intermediate is formed in the first hydrogenation step, the hydrogenated O end is raised by 0.11 Å from the surface, and the other O end is greatly lowered by 0.63 Å. The interaction between the O end and the Pd atom is decreased. This OOH species has a total spin of 0.50 e, indicating that it exists nearly as a radical. The hydrogenation of OOH to H2O and O (eq 5) is exothermic by 51.2 kcal/mol with an activation energy of 21.7 kcal/mol. The dissociation of OOH (eq 7) is exothermic by 35.6 kcal/mol with an activation energy of 16.4 kcal/mol. As shown in Table 1, the barriers for these two reactions are greatly increased, in comparison with those on the Pd(111) surface in case 1 and case 2. In the transition state for OOH hydrogenation to H2O and O and the transition state for OOH dissociation, the OO bond is being cleaved and an OAu bond is being formed. The formation of the OAu bond cannot compensate for the cleavage of the OO bond; as a result, these two reactions involve high activation barriers. The barrier for OOH desorption (eq 8) is 13.8 kcal/mol, being comparable to that (12.5 kcal/mol) for OOH desorption from the Pd(111) surface in case 2. The barrier for OOH desorption is more favorable than the barriers for OOH dissociation and OOH hydrogenation to H2O and O. Consequently, the OOH group in this case will be desorbed rather than dissociated or hydrogenated to H2O and O. 25365



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To sum up these H2O2 synthesis steps, on the Au@Pd(111) surface, all side reactions are suppressed and the main reactions become the most favorable. Accordingly, PdAu bimetal catalysts show higher selectivity for the direct synthesis of H2O2 in comparison with pure Pd catalysts. Because the dissociation of H2O2 is suppressed, the experimentally measured format rate of H2O2 is increased. Geometrical parameters of the optimized structures for all species are collected in Figure 11.

Figure 11. Optimized structures for the species involved in the synthesis of H2O2 on the Au@Pd(111) surface. The distances are given in Å.

Then, the OOH group will be readsorbed on the Au@Pd(111) surface with the unhydrogenated O end linked to the Pd atom (eq 80 ). The distance of the new OPd bond is 2.02 Å and the total spin of this OOH species is 0.08 e, indicating that it is more strongly bound with the surface than the previous OOH species. The energy of this configuration is lower by 2.7 kcal/mol. The hydrogenation of OOH in this configuration can yield H2O2 (eq 4), being exothermic by 5.3 kcal/mol with an activation energy of 13.5 kcal/mol. This barrier is a littler higher than that (11.1 kcal/mol) for OOH hydrogenation of H2O2 on the Pd(111) surface in case 2 and is lower than that (14.6 kcal/mol) in case 1. Another hydrogenation reaction will give two OH groups (eq 6), which is exothermic by 28.2 kcal/mol with an activation energy of 22.2 kcal/mol. The dissociation of OOH (eq 7) is exothermic by 26.9 kcal/mol with an activation energy of 16.0 kcal/mol. The barriers of these two reactions are greatly increased, in contrast to those on the Pd(111) surface in case 1 and case 2. From the transition states for these two reactions, we think that the essence of these two reactions is the competition between the OO bond and the OAu bond. The OAu bond cannot compete with the OO bond; as a result, these two reactions are not favorable. After H2O2 is formed, it will be desorbed from the metal surface (eq 10) or dissociated into two OH groups (eq 9). The dissociation of H2O2 is exothermic by 22.9 kcal/mol with an activation barrier of 5.4 kcal/mol. The barrier for H2O2 desorption is 2.5 kcal/mol, being lower than the barrier for H2O2 dissociation. Compared with the case on the Pd(111) surface, the barrier for H2O2 dissociation is increased; as a result, H2O2 dissociation cannot compete with H2O2 desorption in this case.

4. CONCLUSIONS In this work, we examined the direct synthesis of H2O2 on Pd(111) and Au@Pd(111) surfaces. In each H2O2 synthesis step, there are several competing side reactions. For the reactant O2, the hydrogenation of O2 (eq 1) competes with the dissociation of O2 (eq 2). For the intermediate OOH, the hydrogenation of OOH to H2O2 (eq 4) will compete with the hydrogenation of OOH to H2O and O (eq 5), the hydrogenation of OOH to two OH (eq 6), and the dissociation of OOH (eq 7). For the product H2O2, the desorption of H2O2 (eq 10) needs to compete with the dissociation of H2O2 (eq 9). All side reactions are actually OO bond dissociation processes. The competition between the main reactions and the side reactions is in fact the competition between the OO bond and the OM bond, where M is Pd in the case of the Pd(111) surface and Au in the case of the Au@Pd(111) surface. The OPd bond is usually stronger than the OO bonds in the OOH intermediate and H2O2; as a result, the side reactions exceed the main reactions. To achieve the catalytic activity experimentally, the Pd(111) surface should be pretreated with strong acids or halide ions to weaken the interaction between the metal surface and the reagents. In the case of the Au@Pd(111) surface, the OAu bond cannot compete with the OO bond; consequently, the final product H2O2 is easily produced and released from the Au@Pd(111) surface, and the side reactions involving OO bond dissociation are suppressed. Surface H atoms also play important roles in the synthesis of H2O2. The coadsorbed H atoms decrease the interaction between the metal surface and the reagents, which is helpful for the main reactions. Moreover, H atoms occupy the Pd sites on the Au@Pd(111) surface, making the Au site on the Au@Pd(111) surface like an island in the sea of H atoms and leading the adsorption of O2 at the topbridge-top site on one Pd atom and one Au atom to be favored. This O2 adsorption configuration is very important for the subsequent hydrogenation reactions. In addition, we found that O2 in the triplet dioxygen state is even more reactive for the hydrogenation than O2 in the doublet superoxo state. This suggests that preactivation of O2 by the metal surface is not necessary. On the contrary, if O2 is activated by the metal surface, the side reactions involving OO bond dissociation may become more favorable than the main reactions. Accordingly, the role of the metal surface in the direct synthesis of H2O2 from H2 and O2 would be to provide H atoms as the feedstock for the hydrogenation of O2. ’ ASSOCIATED CONTENT

bS

Supporting Information. 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.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We acknowledge Grants-in-Aid (Nos. 18GS0207 and 22245028) for Scientific Research from Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), the Nanotechnology Support Project of MEXT, the MEXT Project of Integrated Research on Chemical Synthesis, the Kyushu University Global COE Project “Science for Future Molecular Systems”, and CREST of the Japan Science and Technology Cooperation for their support of this work. ’ REFERENCES (1) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Angew. Chem., Int. Ed. 2006, 45, 6962–6984. (2) Samnata, C. Appl. Catal., A 2008, 350, 133–149. (3) Landon, P.; Collier, P. J.; Papworth, A. J.; Kiely, C. J.; Hutchings, G. J. Chem. Commun. 2002, 2058–2059. (4) Edwards, J. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2008, 47, 9192–9198. (5) Han, Y.-F.; Zhong, Z.; Chen, F.; Chen, L.; White, T.; Tay, Q.; Yaakub, S. N.; Wang, Z. J. Phys. Chem. C 2007, 111, 8410–8413. (6) Nomura, Y.; Ishihara, T.; Hata, Y.; Kitawaki, K.; Kaneko, K.; Matsumoto, H. ChemSusChem 2008, 1, 619–621. (7) Menegazzo, F.; Burti, P.; Signoretto, M.; Manzoli, M.; Vankova, S.; Boccuzzi, F.; Pinna, F.; Strukul, G. J. Catal. 2008, 257, 369–381. (8) Edwards, J. K.; Solsona, B.; Landon, P.; Carley, A. F.; Herzing, A.; Watanabe, M.; Kiely, C. J.; Hutchings, G. J. J. Mater. Chem. 2005, 15, 4595–4600. (9) Edwards, J. K.; Thomas, A.; Solsona, B. E.; Landon, P.; Carley, A. F.; Hutchings, G. J. Catal. Today 2007, 122, 397–402. (10) Ishihara, T.; Hata, Y.; Nomura, Y.; Kaneko, K.; Matsumoto, H. Chem. Lett. 2007, 36, 878–879. (11) Edwards, J. K.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Faraday Discuss. 2008, 138, 225–239. (12) Edwards, J. K.; Thomas, A.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Green Chem. 2008, 10, 388–394. (13) Ghedini, E.; Menegazzo, F.; Signoretto, M.; Manzoli, M.; Pinna, F.; Strukul, G. J. Catal. 2010, 273, 266–273. (14) Menegazzo, F.; Signoretto, M.; Manzoli, M.; Boccuzzi, F.; Cruciani, G.; Pinna, F.; Strukul, G. J. Catal. 2009, 268, 122–130. (15) Pritchard, J. C.; He, Q.; Ntainjua, E. N.; Piccinini, M.; Edwards, J. K.; Herzing, A. A.; Carley, A. F.; Moulijn, J. A.; Kiely, C. J.; Hutchings, G. J. Green Chem. 2010, 12, 915–921. (16) Edwards, J. K.; Solsona, B.; N, E. N.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Science 2009, 323, 1037–1041. (17) Edwards, J. K.; N, E. N.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J. Angew. Chem., Int. Ed. 2009, 48, 8512–8515. (18) Ntainjua, N., E.; Piccinini, M.; Pritchard, J. C.; He, Q.; Edwards, J. K.; Carley, A. F.; Moulijn, J. A.; Kiely, C. J.; Hutchings, G. J. ChemCatChem 2009, 1, 479–484. (19) Landon, P.; Collier, P. J.; Chadwick, D.; Papworth, A. J.; Burrows, A.; Kiely, C. J.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2003, 5, 1917–1923. (20) Edwards, J. K.; Solsona, B. E.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. J. Catal. 2005, 236, 69–79. (21) Solsona, B. E.; Edwards, J. K.; Landon, P.; Carley, A. F.; Herzing, A.; Kiely, C. J.; Hutchings, G. J. Chem. Mater. 2006, 18, 2689–2695. (22) Piccinini, M.; Ntainjua, N. E.; Edwards, J. K.; Carley, A. F.; Moulijn, J. A.; Hutchings, G. J. Phys. Chem. Chem. Phys. 2010, 12, 2488–2492.

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Theoretical Revisit of the Direct Synthesis of H2O2 on Pd and Au@ Pd

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