Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the

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Formation of HO on Au and Au Pd Clusters: Understanding the Structure Effect on Atomic Level Anna V. Beletskaya, Daria A. Pichugina, Alexander F. Shestakov, and Nikolay E. Kuz'menko J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp4040437 • Publication Date (Web): 16 Jul 2013 Downloaded from http://pubs.acs.org on July 21, 2013

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The Journal of Physical Chemistry

Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the Structure Effect on Atomic Level

Anna V. Beletskaya1, Daria A. Pichugina1,2*, Alexander F. Shestakov2, Nikolay E. Kuz’menko1 1

Department of Chemistry, Lomonosov Moscow State University, Leninskie gory, 1 str. 3, 119991, Moscow, Russian Federation 2

Institute Problems of Chemical Physics RAS, Semenova pr. 1, 142400, Moscow reg., Russian Federation *Telephone: 0079394765. Fax: 0079328846. E-mail: [email protected].

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Abstract Supported gold nanoparticles are promising catalysts for production of H2O2 from O2 and H2. Size, structure and palladium doping effects play the key role in activity and selectivity of a gold catalyst. We performed a study of the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation on the atomic level, using the DFT/PBE approach with relativistic all electron basis set. The top, edge and facet atoms of the tetrahedral Au20 cluster as well as a palladium atom of Au19Pd located on the top, edge and facet of a tetrahedron have been considered as active sites of steps involved in H2O2 synthesis. The thermodynamic and kinetic data including Gibbs free energies and the activation Gibbs free energies were calculated for the steps determining the formation of H2O2 (H(s)+OOH(s)=H2O2(s), H2O2(s)=H2O2(g)) and for one step decreasing the selectivity (H2O2(s)=OH(s)+OH(s)). Gold tends to have low activity and high selectivity in H2O2 synthesis regardless of the structure of active site. Low coordinated palladium atoms promote H2O2 formation as well as its dissociation. Pd on a facet of a cluster facilitates H2O2 production with high activity and selectivity.

Keywords: Gold, Nanocatalysis, Selectivity, Hydrogen Peroxide, DFT


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Introduction Hydrogen peroxide is one of the most important green oxidants because the bi– product is water. At the present time H2O2 is produced by sequential hydrogenation and oxidation of anthraquinones which has a significant drawback from the view of green chemistry.1 Alternative synthesis involving a direct hydrogenation of oxygen has attracted interest as it is an environmentally friendly process. There are some problems devoted to direct H2O2 synthesis. Gas phase mixture of oxygen and hydrogen is kinetically stable.2 Using a catalyst it is possible to perform the reaction between hydrogen and oxygen, but the process does not always results in the production of H2O2 in sufficiently high yields.1,3–7 For instance, the traditional palladium–based catalysts have high activity in direct synthesis of H2O2 but low selectivity due to H2O formation as an unwanted bi–product. To make this method profitable for implementation in industry, one must identify the active sites responsible for the formation of H2O2 and H2O. Despite extensive research, precise information concerning the catalytic sites governing H2O2 synthesis has not been achieved yet. The development of new active and selective catalysts is still in progress. Heterogeneous gold catalysts are known to be active in oxidation and hydrogenation reactions such as low temperature CO oxidation, propene oxidation to propene oxide, oxidation of alcohols with molecular oxygen, selective hydrogenation of acetylene.8–15 It has inspired scientists to test catalytic activity of gold nanoparticles in the direct synthesis of hydrogen peroxide.16–18 Surprisingly, Au/Al2O3 produces H2O2 with higher yield than Pd/Al2O3 catalysts.16 The most striking finding is that it is possible to increase the yield of H2O2 using a bimetallic AuPd/Al2O3 catalyst. The increase in activity or selectivity of a bimetallic catalyst compared with the individual metal is called synergistic effect.19 Intensive experimental and theoretical researches focusing on the origin of unusual properties of AuPd catalysts demonstrate that the new active sites on AuPd catalysts are formed.20 The modification of the surface properties is associated with ensemble and/or ligand effects responsible for electronic structure change and the formation of isolated Pd ensembles.21–26 According to the model proposed by Gao and Goodman,25 Ham et al.27 one palladium atom surrounded by gold atoms is likely to be active site in direct H2O2 synthesis. To obtain precise information concerning the catalytic sites governing H2O2 formation, information about features of the reaction mechanism is helpful. An ambiguous and complicated mechanism is hidden behind the seemingly simplistic of the chemical equation (H2 + O2 = H2O2). Key reaction steps leading to H2O2 and unwanted H2O formation are presented on Figure 1. The reaction starts with the molecular or dissociative adsorption of oxygen or hydrogen on catalyst surface and the formation of active O2(s), O(s), H2(s), and H(s) ACS Paragon Plus Environment


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species (steps 2c, 2d, 2i and 3d). Steps 1a–1d, 2b–2h are described by Eley–Rideal28 or Langmuir–Hinshelwood29 mechanisms. According to the first one, O2 from the gas phase reacts directly with adsorbed H(s) or H2(s) forming OH(s), OOH(s), and vice-versa, the gas phase H2 molecule interacts with adsorbed O2(s), O(s), OOH(s), OH(s) species. LangmuirHinshelwood mechanism implies interaction of coadsorbed molecules (O2(s), H2(s), O(s), H(s), OOH(s)) with each other. Following both mechanisms, OOH(s) can be formed (steps 1e, 2b, 2e, 2f). The formation of OOH(s) on Au/TiO2 has been detected using inelastic neutron scattering and confirmed by theoretical calculations.30,31 OOH(s) is suggested to be an oxidizing agent in propene epoxidation and CO oxidation catalyzed by gold nanoparticles using an O2 and H2 stream.32–34 According to the majority of quantum chemical data OOH hydrogenation is the rate determining step in H2O2 synthesis. 31,35–37 It allows one to assume that OOH(s) is the main precursor for the H2O2 synthesis (step 1). According to physical chemistry formalism, the negative value of Gibbs energy changes and low activation energies in steps 1a, 1e, 2a–2f, 3e favors the formation of H2O2(s), and the small value of desorption energy of H2O2(s) in step 2a provides the formation of the final product, H2O2(g). Besides the desired steps leading to H2O2, one should pay attention to the steps leading to the production of water (steps 3a–3d, 2g–2h, 1b–1d). It is obvious that the formation of H2O requires breaking of the O–O bond in at least one of the intermediates (O2(s), OOH(s) and H2O2(s)). Once the О–О bond is broken on a surface, it cannot be formed again. Thus to increase the selectivity, steps 3a–3d should have high activation energies, and the desorption energy of H2O2 should be lower than the activation energy of H2O2(s) dissociation (step 3a).


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Figure 1. Key steps leading to H2O2 and unwanted H2O (red lines) formation. Steps 1a–1e indicate the process in which adsorbed hydrogen reacts with OOH(s), OH(s), O2(s), O(s), or hydrogen of OH group transfers to other OH(s); 2a–2i are adsorption/desorption steps; 3a–3d are steps of O–O bond cleavage.

A quantum chemical simulation based on DFT was widely used for the calculation of energy changes and the activation energies of all steps of H2O2 formation.27,31,35–50 The application of quantum chemical methods for simulation of the process on the surfaces of a real catalyst is not a regular problem. Active sites of catalysts are usually simulated using clusters or model surface. There are some works devoted to the computational study of H2O2 synthesis on Aun (n=2–5, 12–14, 25, 29, 55) clusters,31,35,36,38–41 Au(111), Au(211)31,43 and mixed gold-palladium surfaces.46–50 It is considered that O–O bond cleavage is possible on small gold clusters.51–54 Thus, small gold clusters potentially have low selectivity in H2O2 formation in contrast to the regular extended Au(111) surface.31,40,43 Dependence of the ACS Paragon Plus Environment


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selectivity on the model of a catalyst suggests an idea that H2O2 formation is sensitive to catalyst structure. Structure and size effects are believed to be important in H2O2 synthesis.55–57 Special active sites may form as a result of interaction of support and gold nanoparticles. Carbon is found to be the optimum support for AuPd catalysts in H2O2 synthesis.55,58 Carbon support does not influence significantly on electronic properties of gold nanoparticles.59 This fact opens up the possibility to perform the simulation of structural effects on the atomic level without direct account of support. The tetrahedral Au20 cluster is an appropriate and popular model for studying the structural effects. It has extraordinary stability due to closed shell electronic structure. The cluster has atoms with a different coordination number (c.n.) located on the cluster’s top (c.n=3), edge (c.n.=6), and facet (c.n.=9).60 The Au20 facet as the fragment of a Au(111) surface is suitable for simulation of a defect free surface. The edge and top of Au20 are appropriate to simulate structure defects. To model the influence of palladium on structural sensitivity and on catalytic activity and selectivity of an H2O2 formation, it is possible to consider Au19Pd clusters containing one Pd atom located on the top, edge and facet sites of the tetrahedron, taking into consideration that a Pd atom surrounded by gold is an active site in H2O2 synthesis. 25,27 The article presents the investigation of the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation using the DFT and cluster approach. We have focused our attention on H2O2 formation from OOH (step 1a), its desorption (step 2a) and H2O2 dissociation (step 3a). As an example of selectivity decrease step 3a has been considered. Gibbs energy changes were calculated in all steps; activation energies of step 1a and 3a were determined.

Computational details Structures optimization and energy calculation were performed using spin-polarized Perdew−Burke−Ernzerhof (PBE) functional61 of density functional theory (DFT). The Priroda code62 was used for an all–electron calculation in scalar-relativistic approximation, which is based on the full four–component one–electron Dirac equation63 with spin–orbit effects separated out. The energy-optimized extended Gaussian basis set of triple–polarized quality of the large component, and the corresponding kinetically balanced basis for the small component was used: Au [30s29p20d14f/8s756d2f], Pd [26s23p16d5f/7s6p4d1f], O [10s7p3d/3s2p1d], and H [6s2p/2s1p].64 Firstly, to clarify whether the method is suitable for the investigation we have calculated the characteristics of small AuO, AuH, PdO and PdH molecules. Equilibrium distances (Re), ACS Paragon Plus Environment

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dissociation energy (De), and harmonic vibration frequency (we) were calculated and compared with the available experimental data60,65–68 (Table 1). The calculated values of Re of all testing molecules agree with the measured one with high accuracy, the difference between the calculated and measured data is approximately 0.01 Å. The DFT/PBE approach employed in this work tends to slightly overestimate dissociation energies of AuO and PdO. Slight deviations between calculated and measured De values of AuO and PdO are the result of imprecise calculations of the energy of the oxygen ground state. The calculated we are in line with experimental values. Thus, scalar–relativistic DFT/PBE has acceptable accuracy for the calculation of AuH, AuO, PdH and PdO. Table 1. Comparison of the calculated properties of AuH, AuO, PdH, PdO, and Au20 with measured dataa–f Molecule Parameter

Experimental

Calculated

data

data

Re, A

1.524a

1.537

De, eV

3.36a

3.24

we, cm–1

2305a

2270

Re, A

1.85b

1.871

De, eV

2.41b

2.70

we, cm–1

625b

590

Re, A

1.534c

1.534

De, eV

2.42d

2.5

we, cm–1

2083e

2034

PdO

De, eV

2.87a

3.04

Au20

HOMO–LUMO, eV 1.77f

1.74

AuH

AuO

PdH

Reference65, bReference66, cReference67, dReference68, frequency of Pd–D bond from Reference67, fReference60. a

e

Estimation using the vibration

A tetrahedral Au20 cluster has been chosen as a model of a gold catalyst. The coordinates of Au20 have been taken from literature.69 The structure of Au20 was fully optimized without symmetry constraints. The optimized structure, Au–Au distances and IR spectra of Au20 agree well with the data.70,71 The calculated HOMO–LUMO gap was also in line with experimental the one.60 The Au19Pd cluster has been considered as a model of the bimetallic particle. Three models of Au19Pd were generated by the substitution of one gold atom located on the top (Au19Pd_1), edge (Au19Pd_2) and facet (Au19Pd_3) of the Au20 clusters by the palladium atom. The structures of three Au19Pd clusters in doublet state were fully optimized ACS Paragon Plus Environment


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(Figure 2). Spin contamination error (Ŝ2) was calculated for all Au19Pd. To examine the electronic properties of the Au20 and Au19Pd clusters we have calculated spin density and atomic charges using the Mulliken population analysis and projected local density of d–states (PLDOS) on the top, edge and facet atoms of Au20 and on Pd atoms of Au19Pd clusters. PLDOS have been performed within the plane–wave DFT approach using CASTEP package.72 The Au20 and Au19Pd clusters have been calculated by placing them in the center of a large enough cubic supercell with a vacuum space of 2.5 nm in all directions to avoid interactions between replicated images in the neighboring cells. Spin–polarized PBE61 generalized gradient approximation and ultrasoft pseudopotentials73 were employed. The kinetic energy cutoff of 420 eV was applied for the plane–wave basis set. Only the Γ−point for the reciprocal space has been considered in our work. To enhance the convergence, calculations have been performed applying Gaussian smearing technique with a 0.1 eV width. We have considered the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation using physical chemistry formalism. We have simulated three main stages determining H2O2 synthesis: (1a) migration of H atom to OOH forming H2O2, (2a) H2O2 desorption, and (3a) H2O2 decomposition on two OH. (1a) H(s) + OOH(s) → H2O2(s) (2a) H2O2(s) → H2O2(g). (3a) H2O2(s) → OH(s) + OH(s) These reactions have been considered on three different sites, that are gold or palladium atoms located on the top, edge, and facet of the tetrahedral Au20 or Au19Pd. Energies and structures of all reactants have been calculated in the stationary points of the potential energy surface (PES). The types of stationary points have been determined from the analysis of the Hessian matrix; the second derivatives have been calculated analytically. To avoid errors in the calculation of frequencies, we compared calculated value of vibration frequencies of Au20−OOH, Au20−H2O2 and frequencies of OOH and H2O2 adsorbed onto Au/TiO2.30 The calculated υ(OOH) is 1060 cm−1 that is a little lower than the measured one (1065 cm−1). The same high accuracy was obtained for H2O2, the calculated value υ(H2O2) is 864 cm−1 and the measured value υ(H2O2) is 900 cm−1. Thus, scalar–relativistic DFT with PBE functional is reliable method for the simulation of adsorption–desorption processes. Gibbs free energies (∆rG0) at 298 K and 1 atm have been calculated for steps 1a, 2a, 3a. We included the entropic contribution since it was found to be important for the examination of catalytic activity of gold nanoclusters in the H2O2 formation.36,42 The ∆rG0 have been calculated as the Gibbs energy (G0) difference between the product and reactant of each step. The Gibbs energy of each reactant has been calculated as the sum of the total energy ACS Paragon Plus Environment

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(E) and the temperature correction to the Gibbs free energy. The temperature corrections have been calculated based on the statistical thermodynamics formalism using the ideal gas approximation. The value of ∆rG0 is negative for the thermodynamically favorable process. Calculated energy changes (∆E, kJ/mol) for steps 1a, 2a, 3a and activation energies (Ea, kJ/mol) for steps 1a, 3a are presented in Supporting Information. For steps 1a, 3a the activation Gibbs free energies (∆G‡) at 298 K and 1 atm have been calculated. For this purpose, the structure of transition states (TS) have been determined by the procedure.74 The initial structure of TS has been presupposed on the basis of two corresponding structures of intermediates related to PES minima. The exact structure of TS has been determined according to the procedure of eigenvector following with the Berny algorithm.75 For TS that contain only one imaginary frequency the calculations of the intrinsic reaction coordinate through this mode have been performed. Based on the total energy of TS and the temperature corrections to the Gibbs free energy of TS, activation Gibbs energies were calculated as Gibbs energy differences of transition states and reactants. The results are presented in the energy diagrams in which ∆rG0 and ∆G‡ of reactants are given with respect to G0 of HAu20OOH or HAu19PdOOH. Results and discussions We consider the electronic and structural properties of the model Au20 and Au19Pd clusters in detail. The optimized structures of Au20 and Au19Pd clusters are presented on Figure 2. Due to the appropriate atomic radius, Pd is embedded to gold cluster without sufficient distortion of the cluster’s structure. The Au19Pd stability depends on the structural environment of palladium atom. According to the energy calculation, Au19Pd_3 containing palladium on the facet is the most stable. The energies of Au19Pd_1 and Au19Pd_2 are higher than energy of Au19Pd_3 by 13 and 48 kJ/mol, respectively. Spin contamination errors (Ŝ2) of all Au19Pd are not higher than 0.36 %. Redistribution of the electron density and the formation of charged sites are expected in the Au−Pd system.76 The gold atoms have negative charges and palladium atoms have positive charges in Pd−rich Au−Pd particles. In the case of Au−rich clusters, negative charge locates on Pd atoms and low coordinated gold atoms. Surprisingly, the charge distribution in bimetallic clusters depends not only on the composition of a cluster, but also on the type of atom coordination. Using Mulliken population analysis we have demonstrated that Pd atom has negative charge in Au19Pd_1 (−0.34e) and Au19Pd_2 (−0.16e) and it has a positive charge when it is located on the facet of Au19Pd_3 (0.32e). Negatively charged atoms are potential adsorption sites of electrophilic agents, i.e. an oxygen molecule. Spin density is more reliable for identification of potential active sites. Calculated values of spin density are ACS Paragon Plus Environment


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0.15, 0.38 and 0.45 on Au19Pd_1, Au19Pd_2 and Au19Pd_3, respectively. Thus bonding of electrophilic molecules, i.e. oxygen, to palladium atom is possible in all considered bimetallic particles, as maximum spin density is located on palladium atoms.

Figure 2. Optimized structures of Au20 and Au19Pd clusters. Au19Pd_1, Au19Pd_2 and Au19Pd_3 contain Pd on the top, edge and facet of a cluster, respectively. The d–band is used to measure the local surface reactivity: an upshift of the d–band center toward the Fermi level and d–bandwidth narrowing leads to enhancement in the surface chemical reactivity.46 Figure 3 illustrates the density of d−states projected on the palladium atoms in Au19Pd clusters and the top, edge and facet atoms of Au20 cluster. The d−band center for the palladium atom in Au19Pd_1, Au19Pd_2 and Au19Pd_3 are closer to Fermi level compared to the top, edge and facet atoms of Au20, respectively. The lower the coordination number of the palladium atom, the closer the d−band center is to Fermi level. The delocalization of d−electrons of palladium increase from Au19Pd_1 to Au19Pd_3. Thus substitution of gold atoms by palladium is believed to enhance the surface chemical reactivity. The lower coordination number of substituting atom the more significant enhancement is expected.


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Figure 3. Density of d−states projected on palladium atom in Au19Pd_1 (blue solid line), Au19Pd_2 (green solid line), Au19Pd_3 (red solid line) and on top (blue dashed line), edge (green dashed line) and facet (red dashed line) atoms of Au20. The results of the pathway simulation of steps (1a), (2a), and (3a) including the optimized structures of corresponding intermediates and transition states are presented in the diagrams (Figure 4). Let’s first consider the activity of the top atom of Au20 (Figure 4A). The step 1a starts at IM1_a, in which OOH(s) species is bounded with the gold atom at the top through an unhydrogenated oxygen atom, and H(s) adsorbed in a bridge position bonding with the top and edge gold atoms. The hydrogen atom migrates to OOH(s) through TS1_a, that leads to IM2_a formation. Step 1a is favored from a thermodynamic point of view, due to the exothermicity of the process and suppressed from the kinetic aspect because of the high energy barrier of the hydrogen atom migration (106 kJ/mol). The high activation energy of step 1a is probably due to geometric features, as the H atom needs to overcome a long distance to be connected with OOH(s). Two competitive processes are possible after H2O2(s) formation: desorption or dissociation of H2O2. Adsorbed H2O2(s) bends forward to the edge of a cluster in TS of dissociation (TS2_a). The O−O bond distance in TS2_a is increased by 0.25 Å compared with IM2_a. Subsequent O−O bond elongation results in H2O2 dissociation on two OH(s). The OH(s) species are located nearly perpendicularly to each other. Such an orientation obviously favors H atom migration from one OH to another resulting in undesired H2O that lower the selectivity. The energy barrier of H2O2 dissociation is 36 kJ/mol and ∆rG0 is 75 kJ/mol. In spite of H2O2, dissociation is allowed from both a thermodynamic and kinetic point of view, H2O2 desorption is more likely to occur as desorption energy is only 3 kJ/mol. Because of the low coordination of the top of Au20, we compared our results with the activation Gibbs free energies of step (1a) and desorption Gibbs energies calculated on Au3, and Au5 clusters.36 ∆G‡ of interaction between hydrogen atom and OOH catalyzed by Au3 and Au5 is 91 and 97 kJ/mol, respectively. That is in a good agreement with ∆G‡ of H2O2 ACS Paragon Plus Environment


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formation on the top of Au20 (∆G‡(Au20)=106 kJ/mol). Desorption of H2O2 from Au3 and Au5 clusters has negative value of Gibbs free energy (∆rG0(Au3)= −17 kJ/mol, ∆rG0(Au5)= −26 kJ/mol), while the desorption of H2O2 from the top of the gold atom of Au20 is approximate to zero. The comparison of the results with other theoretical predictions confirmed our conclusion that the activation energy of step 1a on the gold atoms with low coordination is high and desorption of H2O2 is possible. Figure 4B illustrates energy change in the steps (1a), (2a), (3a) proceeding on the edge of the Au20. In contrast to the previous case, formation of IM1_b results in Au20 distortion. The OOH(s) is located on the bridge position bonding with two gold atoms through the hydrogenfree oxygen atom. The hydrogen atom migration to OOH(s) occurs with low activation energy (37 kJ/mol) and results in the recovery of the Au20 structure in TS1_b. The hydrogenation of OOH(s) leads to its rotation toward the Au−Au bond on the edge of the cluster that results in the nearly parallel position of H2O2 toward the edge of the cluster in IM2_b as each oxygen atom of H2O2 is bounded with one gold atom. The O−O bond elongation leads to H2O2 dissociation. This process is accompanied with significant cluster distortion and with Au−Au bond cleavage. The mutual orientation of OH in IM3_b is similar to its orientation on the top of a cluster (IM3_a). The hydrogen atom of one OH is oriented toward the oxygen atom of the other OH. As in the previous case H2O2 dissociation is allowed as the activation energy is 39 kJ/mol, and the energy release is 113 kJ/mol. But, H2O2 desorption is more preferable than step 3a (∆rG0 =−10 kJ/mol). The calculated values of energy changes are in good agreement with the data obtained for the cluster Au12 as a model of a cluster’s corner.40 The calculated activation energy without entropic contribution of step (1a) on Au12 (45 kJ/mol) coincides with our data (31 kJ/mol). We slightly overestimated the activation energy (Ea) of stage (3a) compared to the Au12 cluster. It has a lower value than on the edge of Au20 (35 kJ/mol) and Au12 (17 kJ/mol). Figure 4C illustrates the steps (1a), (2a), (3a) proceeding on the facet of the Au20. In IM1_c intermediate OOH(s) is bounded with the central atom of a facet pulling it from the facet plane. H(s) also locates on the cluster facet on the hollow position under three gold atoms shearing one gold atom with OOH. The hydrogen atom migrates with low activation energy (19 kJ/mol) to the bridge position under two gold atoms forming the structure of TS1. The OOH orientation in TS1_c is like in IM1_c, but 0.11 Å is further from the cluster. Further H migration leads to H2O2 formation and recovery of the Au20 structure. In contrast to the previous case, the hydrogen atoms of H2O2 are oriented toward the cluster. The oxygen atoms in TS2_c move closer to the cluster so the hydrogen atoms orientation toward the cluster is changing. In contrast to the previous cases, H2O2 dissociation on the facet of the ACS Paragon Plus Environment

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cluster leads directly to H2O without formation of OH overcoming the high activation barrier (79 kJ/mol). Taking into account H2O2 desorption is almost a thermo neutral process, H2O2 desorption is more preferable than dissociation on the Au20 facet which can be considered as a fragment of Au(111). The activation energy (Ea) of step (1a) calculated on the Au(111) surface is 11 kJ/mol43 and is in good agreement with our value (15 kJ/mol). We slightly overestimated the activation energy of stage (3a) from 49 to 81 kJ/mol, and the desorption energy from 15 to 36 kJ/mol.43 The results obtained for the facet atom on Au20 are in qualitative agreement with the data on the Au(111) surface, the small numerical discrepancies are due to the difference in the electronic properties of the cluster and surface. In summary, we can conclude that gold atoms with low coordination sites demonstrate low activity in H2O2 synthesis because of the high energy barrier of the hydrogen atom migration. Gold atoms located on the edges or facets of the cluster have low activation energies of step (1a), but the formation of a pre-reactant (HAu20OOH) from molecular oxygen and hydrogen requires the additional energies (31 kJ/mol for IM1_b and 64 kJ/mol for IM1_c). As an advantage, the small desorption energy of H2O2, from all sites of Au20, facilitates the high selectivity of the process. These suggestions correlate with recent experiments.58 It was estimated that gold nanoparticles supported on carbon have little activity in the synthesis of H2O2 and in H2O2 decomposition.58



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Figure 4. Energy diagrams of H2O2 formation and dissociation on different sites of Au20 and Au19Pd: A) on the top Au, B) on the edge Au, C) on the facet Au, D) on the top Pd, E) on the ACS Paragon Plus Environment

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edge Pd, F) on the facet Pd. The values of Gibbs Energy (∆rG0) and activation Gibbs free energies (∆G‡) are in kJ/mol. Energies of steps (1a), (2a), (3a) on Au19Pd_1 and corresponding structures are presented in Figure 4D. The structures of the reactants of all steps on Au19Pd are differ a little bit from that on Au20, but the energies of the main steps change dramatically. The type of OOH(s) orientation on Au19Pd_1 differs from that on the top of Au20. OOH is closer to the hydrogen atom in the case of Au19Pd_1 that correlates with the lower energy barrier (54 kJ/mol) of H2O2 formation on Au19Pd_1 compared to Au20. OOH shifts toward the hydrogen atom and captures it during the formation of H2O2. That is in contrast with H2O2 synthesis on the top of Au20, where the OOH and hydrogen is in the close to each other. The substitution of the low coordinated gold atom by palladium favors the formation of H2O2 on Au19Pd_1, the energy barrier of H2O2 synthesis is nearly two times lower than in the case of Au20. The structures of H2O2 adsorbed on Au19Pd_1 as well as the structures of transition state of dissociation and the product of dissociation are similar to corresponding structures in the case of Au20. The palladium atom facilitates H2O2 dissociation as well, because the energy barrier of H2O2 dissociation becomes lower than the energy required for desorption. Thus, the low coordinated palladium atom in the mixed gold−palladium particle facilitates H2O2 formation and decomposition. How does palladium influence the efficiency of a catalyst when it is located on the edge of a cluster? In contrast to IM1_b, related to the H2O2 formation on the edge of Au20, OOH(s) is bonded with one atom of the Au19Pd_2 cluster in IM1_e and the formation of this intermediate doesn’t lead to cluster distortion. The structures of IM2_e and TS2_e are less symmetric compared to IM2_b and TS2_b because Pd–O bond length is shorter than Au−O. The energy barrier of the H conjunction to OOH on Au19Pd_2 is nearly two times lower than in the case of the Au20 edge and three times lower than in the case of Au19Pd_1. The orientation of hydroxyls in IM3_e is like in IM3_b, but the existence of the palladium atom on the cluster edge prevents it from cluster distortion during H2O2 dissociation. The substitution of the gold atom by palladium lowers the selectivity of H2O2 synthesis as the energy barrier of H2O2 dissociation becomes lower than the energy needed for H2O2 desorption. Let’s proceed to the analysis of H2O2 formation on the facet of Au19Pd. The palladium atom on the facet of the cluster improves the structural stability of the cluster, as the cluster structure in IM1_f is not distorted in opposite to IM1_c. IM1_c and IM1_f structures also differ in hydrogen atom orientation. The hydrogen atom is located on the bridge position under the two gold atoms on the cluster’s edge. The barrier of the H atom migration to OOH is a 9 kJ/mol higher than in the case of the Au20 facet, but the energy of IM1_f is equal to the total ACS Paragon Plus Environment


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energy of the reactants, proving the possibility of H2O2 synthesis. The palladium atom significantly lowers the energy barrier of H2O2 dissociation. But, H2O2 desorption is more preferable than its dissociation, as the energy barrier of the O–O bond cleavage in H2O2 is higher than the energy necessary for H2O2 desorption. The activation energies (Ea) of step (1a) on Au19Pd decreases with the increase of coordination number of palladium active site from 60 kJ/mol to 24 kJ/mol for Au19Pd_1 and Au19Pd_3, respectively (see Supporting Information). The calculated Ea of step (1a) are comparable

with

the

activation

energies

on

Au41Pd@Pd13

and

AuPd/Pd(111).46

Au41Pd@Pd13 is a 55-atom icosahedral cluster with 13-atom Pd core and an Au shell containing a Pd monomer. In this model palladium has low coordination number. The activation energy of step (1a) calculated on Au41Pd@Pd13 equals to 60 kJ/mol as for Au19Pd_1. The second model AuPd/Pd(111) refers to a Pd monomer located on gold monolayer on Pd(111) slab. Palladium atom in AuPd/Pd(111) is surrounded by six gold atoms as in Au19Pd_3. The activation energy (1a) calculated on AuPd/Pd(111) is 36 kJ/mol that is a little bit higher than on Au19Pd_3. Step (3a) has a low activation energy for all Au19Pd (15–29 kJ/mol), this fact agrees with the calculated values of the activation energy of step (3a) on Au41Pd@Pd13 (16 kJ/mol) and AuPd/Pd(111) (30 kJ/mol).46 The calculated activation energy of H2O2 dissociation on Au@Pd(111) surface is in the same range, 19 kJ/mol.49 So, activation energy of H2O2 dissociation on mixed gold-palladium particles depends slightly on the structure of active site. The substitution of the gold atom by palladium dramatically changes the energy of steps (1a), (2a), (3a). The comparison of the Gibbs energy of these steps on Au20 and Au19Pd clusters are summarized in Figure 5. Red lines represent the energy of IM1 intermediates, calculated with respect to the total energy of the reactants (O2, H2, Au20 or Au19Pd). Blue and crimson lines relate to the values of the energy barriers of H2O2 formation and dissociation, respectively. Green lines correspond to H2O2 desorption energies. Step of formation of IM1 intermediates on the palladium atom of Au19Pd, in contrast to Au20, becomes energetically favorable regardless of the coordination number of the reaction site. This fact, and the low activation energy of the H atom migration to OOH, contributes to the higher activity of mixed gold–palladium catalysts compared to monometallic gold catalysts. In contrast to Au20 that has low activity in H2O2 decomposition, low coordinated palladium atoms of Au19Pd decrease the selectivity of H2O2 synthesis. Only fcc fragments containing Pd atoms of high coordination number are likely to be responsible for high activity and high selectivity of H2O2 synthesis on mixed gold–palladium systems. Substitution of gold by palladium leads to more sufficient H2O2 synthesis because of more energetically favorable formation of IM1 ACS Paragon Plus Environment

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intermediates. It correlates with the fact that the d–band center of Au19Pd is closer to the Fermi Level than the d–band center of the Au20. The closer d–band center to the Fermi level the more exothermic IM1 formation from the isolated clusters, H2 and O2. The top atom of Au20 and the palladium atom of Au19Pd demonstrate comparable PLDOS plots and ∆rG0 of IM1 formation. The lower activation energy of Au19Pd_3 in step (1a) is due to the geometric factor, as Au19Pd provides a close position between the migrating hydrogen atom and OOH in IM1. The higher activity of mixed gold–palladium catalyst in H2O2 synthesis and its decomposition compared to the gold monometallic catalyst is in agreement with the experimental results.58 The higher activity of low coordinated palladium atoms, which is evident from PLDOS plots, results in stronger binding of H2O2 with the cluster that favors H2O2 dissociation. It is worth noting that the palladium atom in Au19Pd_3 is positively charged in contrast to palladium in Au19Pd_1 and Au19Pd_2, demonstrating the most sufficient synthesis of H2O2 among all considered mixed particles This is in agreement with experimental evidence that positively charged palladium facilitates active and selective H2O2 synthesis.77



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Figure 5. Comparison of Gibbs free energies in steps (1a), (2a) (3a) on Au20 and Au19Pd in kJ/mol.


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Substitution of gold atom by palladium decreases sensitivity of steps (1a) and (3a) to the structure of the reaction site. The difference between the maximum and minimum value of the activation energy of the H(s) atom migration to OOH(s) on Au20 is 87 kJ/mol. The presence of palladium atom reduces this value to 33 kJ/mol. The variation between energy barriers of H2O2 dissociation on the different sites of Au20 is 43 kJ/mol, and only 10 kJ/mol on Au19Pd clusters containing a palladium atom on different sites. As H2O2 desorption energy depends on the structure of the reaction site, selectivity of gold–palladium clusters is also determined by structure of the reaction site. The Pd atom surrounded by gold atoms, assumed to be an active site in H2O2 synthesis, should be of high coordination number to be less active in the reaction, lowering the selectivity. Our results support the idea that the sites responsible for H2O2 synthesis and reactions lowering selectivity are different.77 The unwanted reaction, H2O2=OH+OH, can be switched off by synthesizing gold–palladium nanoparticles containing fcc fragments. The larger the particles size, the more (111) fragments particles may contain; they are expected to be more selective in H2O2 synthesis. Conclusions The gold atomic clusters demonstrate low activity in hydrogen peroxide formation H+OOH=H2O2 that has been shown on the Au20 cluster. They are also of low activity in the process lowering the selectivity H2O2=OH+OH. Substitution of gold atoms by palladium in Au20 leads to increase in the activity of a catalyst in the considered reaction of H2O2 formation, but low coordinated palladium atoms are also responsible lowering the selectivity because of water formation. The compromise between activity and selectivity suggests that a Pd atom surrounded by gold atoms facilitates H2O2 synthesis when the palladium atom belongs to fcc like motives. Palladium lowers the sensitivity of a H2O2 formation and dissociation to catalyst structure. Acknowledgments This research was supported by the Russian Federation for Fundamental Research through the Projects 12-03-31011, 13-03-00320, 11-01-00280 and by the Council for Grants of the Russian Federation President (State Program of Support of Young Candidates of Science) through

the

Project

МК-92-2013-3.

The

calculations

supercomputer SKIF MSU “Chebyshev”. Supporting Information


were

performed

using

the


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Cartesian coordinates of the reactants, products, and intermediates of reactions, described in the article. Calculated total energy changes and activation barriers for hydrogen peroxide formation and dissociation on different sites of the Au20 and palladium atom of Au19Pd clusters. This information is available free of charge via the Internet at http://pubs.acs.org.

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(67) Knight Jr., L.B.; Weltner Jr. W. Hyperfine Interaction and Chemical Bonding in the PdH Molecule. J. Mol. Spectrosc. 1971, 40, 317–327. (68) Tolbert, M. A.; Beauchamp, J. L. Homolytic and Heterolytic Bond Dissociation Energies of the Second Row Group 8, 9, and 10 Diatomic Transition–Metal Hydrides: Correlation with Electronic Structure. J. Phys. Chem. 1986, 90, 5015–5022. (69) Zubarev, D. Y.; Boldyrev, A. I. Deciphering Chemical Bonding in Golden Cages. J. Phys. Chem. A. 2009, 113, 866–868. (70) Molina, B.; Soto, J. R.; Calles, A. DFT normal modes of vibration of the Au20 cluster. Rev. Mex. Fis. 2008, 54, 314–318. (71) Gruene, P.; Rayner, D. M.; Redlich, B.; van der Meer, A. F. G.; Lyon, J. T.; Meijer, G.; Fielicke, A. Structures of Neutral Au7, Au19, and Au20 Clusters in the Gas Phase. Science 2008, 321, 674–676. (72) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. First Principles Methods Using CASTEP. Z. Kristallogr. 2005, 220, 567–570. (73) Vanderbilt, D. Soft self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892–7895. (74) Askerka, M.; Pichugina, D.; Kuz’menko, N.; Shestakov, A.: Theoretical Prediction of S–H Bond Rupture in Methanethiol upon Interaction with Gold.J. Phys. Chem. A 2012, 116, 7686– 7693. (75) Schlegel, H. B. Optimization of equilibrium geometries and transition structures. J. Comput. Chem. 1982, 3, 214–218. (76) Zanti, G.; Peeters, D. DFT Study of Bimetallic Palladium–Gold Clusters PdnAum of Low Nuclearities (n + m ≤ 14). J. Phys. Chem. A 2010, 114, 10345–10356. (77) Edwards, J. K.; Pritchard, J.; Piccinini, M.; Shaw, G.; He, Q.; Carley, A. F.; Kiely, C. J.; Hutchings, G. J. The Effect of Heat Treatment on the Performance and Structure of CarbonSupported Au–Pd catalysts for the direct synthesis of Hydrogen peroxide. J. Catal. 2012, 292, 227–238.

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Key steps leading to H2O2 and unwanted H2O (red lines) formation. Steps 1a–1e indicate the process in which adsorbed hydrogen reacts with OOH(s), OH(s), O2(s), O(s), or hydrogen of OH group transfers to other OH(s); 2a–2i are adsorption/desorption steps; 3a–3d are steps of O–O bond cleavage. 154x134mm (300 x 300 DPI)



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Optimized structures of Au20 and Au19Pd clusters. Au19Pd_1, Au19Pd_2 and Au19Pd_3 contain Pd on the top, edge and facet of a cluster, respectively 77x82mm (300 x 300 DPI)


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Density of d−states projected on palladium atom in Au19Pd_1 (blue solid line), Au19Pd_2 (green solid line), Au19Pd_3 (red solid line) and on top (blue dashed line), edge (green dashed line) and facet (red dashed line) atoms of Au20 75x60mm (300 x 300 DPI)



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Figure 4. Energy diagrams of H2O2 formation and dissociation on different sites of Au20 and Au19Pd: A) on the top Au, B) on the edge Au, C) on the facet Au, D) on the top Pd, E) on the edge Pd, F) on the facet Pd. The values of Gibbs Energy (∆rG0) and activation Gibbs free energies (∆G‡) are in kJ/mol 170x240mm (300 x 300 DPI)


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Comparison of Gibbs free energies in steps (1a), (2a) (3a) on Au20 and Au19Pd in kJ/mol. 82x232mm (300 x 300 DPI)



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Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the

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