O2 Mixtures Over Gold Atoms

Article pubs.acs.org/JPCC

Propene Epoxidation with H2/H2O/O2 Mixtures Over Gold Atoms Supported on Defective Graphene: A Theoretical Study Angeles Pulido, Mercedes Boronat,* and Avelino Corma Instituto de Tecnología Química, Universidad Politécnica de Valencia-Consejo Superior de Investigaciones Científicas, Av. de los Naranjos s/n, E-46022 Valencia, Spain ABSTRACT: The mechanism of propene epoxidation by H2/ H2O/O2 mixtures catalyzed by gold atoms supported on defective graphene has been investigated by means of periodic density functional theory (DFT) calculations. The fact of being isolated and yet strongly bound to the support gold atoms makes formation of gold hydroperoxide H−Au−OOH intermediates from H2 and O2 barrierless and the subsequent reaction with propene yielding propene oxide (PO) and H2O thermodynamically and kinetically favored. The main advantage of the Au/graphene material explored in this work is not related to catalyst activity but to the selectivity toward PO that is greatly enhanced due to suppression of the main competing routes. Thus, neither propene hydrogenation yielding propane nor O2 dissociation finally resulting in acrolein formation and/or propene combustion can compete with the desired epoxidation. The present results suggest a promising route to prepare active and highly selective gold-based catalysts for propene epoxidation

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INTRODUCTION Propene oxide (PO) is a valuable precursor in the chemical industry of polymers,1 and large efforts have been devoted in the last years to develop an environmentally friendly and costefficient synthetic route that replaces the production processes based on chlorohydrin and peroxides.2−6 An interesting alternative route for the production of PO is based on the silver-catalyzed direct epoxidation of propene with molecular oxygen, in a similar way as ethene oxide is nowadays industrially produced.7−11 However, selectivity toward PO during silvercatalyzed epoxidation with air is usually lower than 40% at 10% conversion, which makes industrial implementation not viable.5,6,12,13 A different approach is being developed that consists of direct epoxidation of propene with H2O2 or peroxide groups formed from O2 and H2 on gold nanoparticles supported on Ti-containing materials.14−16 Selectivities toward PO larger than 80% at ∼8% conversion have been reported for gold clusters deposited on three-dimensional mesoporous titanosilicate (Ti-SiO2)17 or supported on titanosilicalite (TS1).15,18−21 Propene epoxidation with H2O2 is efficiently catalyzed by TS-1, and it is accepted that Ti-hydroperoxide (Ti-OOH) species are the epoxidizing agents.22−25 Ti-OOH species were observed by in situ UV/vis spectroscopy during propene epoxidation with H2/O2 over Au/Ti-SiO226 and alkaline-treated Au/TS-127 catalysts, and it was proposed that hydroperoxide (−OOH) groups are formed on gold clusters by reaction of O2 with H2 or H2O and then transferred to neighboring Ti sites to produce the Ti-OOH species responsible for propene epoxidation. Actually, H2O2 formation has been reported on gas-phase gold clusters28 and on gold clusters supported on Al2O3, Fe2O3, SiO2, TiO2, and C,29−33 and formation of © 2012 American Chemical Society

hydroperoxide Au-OOH species from H2 and O2 on Au/TiO2 has been identified by inelastic neutron scattering studies.34 Kinetic35 and theoretical36−38 studies have shown that propene epoxidation could also take place on gold clusters without participation of Ti sites, by direct reaction of propene with AuOOH species. It is therefore of interest to investigate the reactivity toward propene of Au-OOH species generated on gold atoms or clusters supported on materials that do not contain Ti. Recently, we have shown by means of DFT calculations that gold atoms and subnanometer gold clusters can be strongly bonded to defective graphene, preserving their morphology, electronic properties, and ability to activate oxygen.39 We present now a theoretical study of the mechanism of propene epoxidation by H2/H2O/O2 mixtures over gold atoms supported on defective graphene. The reaction pathways for formation of Au-OOH species and the subsequent epoxidation of propene have been investigated, and the influence of cluster size and support is discussed.

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MODELS AND METHODS From the optimized structure of graphite, with unit cell composition C4, space group P63/mmc, and cell parameters ap = 2.45 Å, cp= 6.64 Å, and γ = 120°, a fixed-volume supercell containing one graphene sheet was obtained with unit cell composition C128, cell parameter a = 19.60 Å, and a distance of 20 Å between periodic images. A graphene sheet with a single Received: June 5, 2012 Revised: August 3, 2012 Published: August 24, 2012 19355

dx.doi.org/10.1021/jp3055125 | J. Phys. Chem. C 2012, 116, 19355−19362

The Journal of Physical Chemistry C

Article

vacancy defect, denoted as the G1V model, was created by removing one carbon atom from the C128 unit cell. Finally, an isolated gold atom was placed on top of the vacancy defect, generating the Au1/G1V model (see Figure 1), described in

Figure 1. Properties of an isolated gold atom supported on the single vacancy graphene sheet, Au1/G1V: (a) geometry around the gold atom where Au−C bond distances are shown in Å, (b) Bader’s partial charges for the gold and carbon atoms around the C vacancy, and (c) partial charge population of the highest occupied band (yellow cloud) viewed along the [001] direction. Charge density surface represented for a constant value of 0.005 e Å−3 and (d) unpaired spin density surface (yellow cloud) viewed along the [001]. Spin density represented for a constant value of 0.005 e Å−3.

detail in ref 39. O2, H2, and C3H6 were adsorbed on the periodic Au1/G1V model, and their reactivity was investigated by means of density functional theory (DFT) calculations, using the PW91 functional40,41 as implemented in the VASP code.42 A plane wave basis set with a kinetic energy cutoff of 400 eV was employed in the calculations, and the effect of the core electrons was taken into account by means of the projector-augmented-wave (PAW) method by Blöchl43 as adopted by Kresse and Joubert.44 Spin-polarized calculations were performed restricted to the γ-point of the Brillouin zone. Electronic PW91 energies were further corrected by adding the intermolecular dispersion energies evaluated using the D3 method45,46 over the periodic PW91 optimized geometries. In the geometry optimizations, the positions of all atoms in the model were allowed to fully relax until atomic forces were smaller than 0.01 eV Å−1. Optimization of transition state structures was performed with the DIMER method using only first derivatives,47 and stationary points were characterized by frequency analysis. The vibrational frequencies were evaluated in a subspace of the Hessian matrix containing the degrees of freedom of the nongraphene atoms, with the second derivatives calculated numerically with ±0.01 displacements. Partial charge distributions were estimated by means of the theory of atoms in molecules (AIM) of Bader, using the algorithm developed by Henkelman.48,49

Figure 2. Reaction pathways for propene epoxidation by H2/H2O/O2 mixtures over the Au1/G1V model (a) and selected structures and energies (in kJ mol−1) at the GGA-D3 level on these pathways (b).

As previously described,39 a single gold atom supported on top of a vacancy defect in a graphene sheet (Au1/G1V model) is directly bonded to three undercoordinated carbon atoms, with an average Au−C distance of 2.08 Å (see Figure 1). The carbon atoms around the vacancy defect are negatively charged, and the gold atom bears a positive charge of 0.317. The charge density of the highest occupied band, where the unpaired spin density is held, is localized on the gold atom, protruding from the graphene sheet, which should allow for a good orbital overlap with adsorbed molecules. Adsorption of the three reactants, C3H6, O2, and H2, was initially investigated as a first step in the reaction mechanism. Each of the three molecules was placed above the gold atom supported on a single-vacancy graphene sheet, and after geometry optimization, complexes 1, 2, and 3, respectively, were obtained (see Figures 2 and 3). The interaction energies

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RESULTS AND DISCUSSION The mechanism of propene epoxidation by H2/H2O/O2 mixtures over the Au 1 /G 1V material was theoretically investigated, and the reaction pathways considered are schematically shown in Figure 2. The optimized geometries of all intermediate and transition state structures identified along these reaction pathways are depicted in Figure 3; Bader charges are given in Figure 4; and calculated activation and reaction energies for all elementary steps considered are summarized in Table 1. 19356



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Figure 3. Structures involved in the reaction pathways shown in Figure 2. Optimized distances at the periodic PW91 level are given in Å. For the sake of clarity, only carbon atoms around the gold atom are shown. Hydrogen, carbon, oxygen, and gold atoms are depicted in white, gray, red, and yellow, respectively.

adsorbed O2 that becomes negatively charged by −0.762 e. Finally, the interaction of H2 with the supported Au atom is different in nature, and after adsorption, only a small charge transfer of 0.08 e from H2 to the Au atom is observed. The optimized Au−H distances in structure 3 are ∼1.79 Å, and the H−H bond is noticeably activated, with the H−H bond length increasing by ∼0.1 Å with respect to the gas-phase value. Coadsorption of reactants on the supported Au atom was also investigated, and it was found that, once propene is coordinated to the Au atom forming structure 1, neither O2 nor H2 is able to interact with this complex. However, coadsorption of O2 and H2 is possible and results in formation of a gold hydroperoxide Au-OOH intermediate. Two different reaction paths were investigated for formation of the gold hydroperoxide species. The first one involves the addition of molecular H2 to structure 2, leading to formation of an intermediate species 4 in which the H2 molecule is weakly interacting with activated O2. H2 adsorption to form complex 4 releases 31 kJ mol−1, and dissociation of the H−H bond to produce the hydroperoxide H−Au−OOH species (structure 6 in Figures 2 and 3) via

calculated for C3H6 and O2 adsorption on the supported Au atom are similar, −156 and −164 kJ mol−1, respectively, suggesting that both molecules will compete for the Au atoms, while H2 adsorption is much weaker and only releases −69 kJ mol−1. Propene interacts with the supported Au atom through its CC double bond, resulting in the formation of two Au−C bonds with optimized bond lengths of 2.173 and 2.253 Å and in an increase of the C−C distance from 1.335 Å in the gas phase to 1.414 Å in structure 1. This interaction does not imply any important transfer of electron density between propene and the Au1/G1V material, with the Au atom bearing a positive charge of 0.421 e (see Figure 4), similar to that found in the isolated Au1/ G1V model. As depicted in Figure 2, the two oxygen atoms in structure 2 are directly bonded to the gold atom, and the optimized O−O distance (1.431 Å) is considerably larger than the value obtained for gas-phase O2 at the same level of theory, 1.236 Å. The calculated Bader charges shown in Figure 4 indicate that this noticeable activation of the O−O bond is due to electron density transfer from the Au1/G1V material to 19357



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Figure 4. Bader charges on selected atoms of the structures involved in the reaction pathways shown in Figure 2. For the sake of clarity, only carbon atoms around the gold atom are shown. Hydrogen, carbon, oxygen, and gold atoms are depicted in white, gray, red, and yellow, respectively.

transition state 5 is exothermic by 49 kJ mol−1 and involves an activation barrier of only 54 kJ mol−1 (see Table 1). Analysis of the Bader charges in Figure 4 shows that the Au atom is positively charged by ∼0.8 e during the whole process and that H2 dissociation is heterolytic. One of the H atoms becomes negatively charged and forms a gold hydride, while the other one becomes a proton attached to O. The two oxygen atoms in hydroperoxide structure 6 are not equivalent as they are in oxospecies 2. While the O atom bonded to H bears a negative charge of −1.016 e, the O atom directly attached to Au is ∼0.5 e less negatively charged, suggesting a more electrophilic nature and therefore a higher tendency to interact with electron-rich CC bonds. The second pathway considered starts with H2 adsorbed on the supported Au atom, forming structure 3, and involves its dissociation via transition state 7 into two H atoms of hydride nature strongly attached to the Au atom (structure 8 in Figures 2 and 3). The activation barrier for H2 dissociation is low, 27 kJ mol−1, and the process is exothermic by −33 kJ mol−1. In a second and barrierless step, subsequent adsorption of molecular O2 over intermediate 8 leads to insertion of the O−O group into one of the Au−H bonds, resulting in formation of the

hydroperoxide H−Au−OOH species 6 and releasing −142 kJ mol−1. A competitive reaction involving addition of molecular O2 to adsorbed H2 (structure 3) was also investigated and resulted in spontaneous formation of a double hydroxylated Au atom (structure 9 in Figures 2 and 3) which is 291 kJ mol−1 more stable than the desired hydroperoxide intermediate 6. The Au atom in species 9 bears a positive charge of 0.972 e, and the system is stabilized by a H bond between one of the protons and the O atom of the other hydroxyl group, that is, negatively charged by −1.527 e. Despite the higher stability of structure 9 with respect to the desired hydroperoxide 6, its formation implies the attack of O2 to a H2 molecule already adsorbed on the Au site (3), but as described above, initial adsorption of molecular O2 is energetically preferred. To check the stability of the hydroperoxide species 6, recombination of the hydride −H and hydroperoxide −OOH fragments in this intermediate resulting in formation and desorption on H2O2 was investigated at this point. The process was found to be endothermic by 87 kJ mol−1 while involving a high activation barrier of 114 kJ mol−1. To further check the stability of the hydroperoxide intermediate, a direct pathway for conversion of H−Au−OOH species 6 into the double 19358



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propene hydrogenation will not easily take place on isolated Au atoms supported on graphene. It can then be concluded at this point that there is a thermodynamically and kinetically favorable pathway for formation of a stable hydroperoxide H−Au−OOH species by reaction of O2 adsorbed and activated on a single Au atom with molecular H2. Formation of H2O2 and hydroperoxide Au− OOH species has been theoretically investigated on gold clusters of increasing size, and a similar pathway consisting of H2 reaction with a previously formed AunO2 complex was reported for Au3, Au4+, Au5, and Au5−.50 The most active clusters were the smallest and more cationic ones, namely, Au3 and Au4+, and therefore the excellent results obtained for the Au1/G1V model that combines positive charge with low coordination of Au are not surprising. The next step in propene epoxidation is interaction of propene with the hydroperoxide species 6 leading to formation of complex 10, with a calculated adsorption energy of only −39 kJ mol−1. The optimized bond lengths and angles of the propene molecule and H−Au−OOH species are not significantly modified after adsorption, and the distance between the electrophilic O atom and the CC bond is 3.786 Å. This distance decreases to 2.304 Å in transition state 11 and to an optimized value of 1.461 Å in structure 12, in which a propene oxide (PO) molecule has been formed and is attached to the supported Au atom at an optimized Au−O distance of 2.389 Å. In the PO formation process, the O−O bond is broken, generating a hydroxyl group bonded to Au (at a Au−O distance of 2.088 Å), and there is also a hydride atom with an optimized Au−H bond length of 1.656 Å. The Bader charge distributions in Figure 4 indicate that neither the Au atom nor the hydride are modified in this step. However the negative charge on each of the two oxygen atoms increases by more than 0.5 e, as a result of the oxidation of the CC bond of propene. The calculated activation barrier is 95 kJ mol−1, and the process is exothermic by −233 kJ mol−1. Propene oxide is weakly bonded to the Au1/G1V system and only requires 69 kJ mol−1 to desorb, leaving a hydroxyl and a hydride fragment coadsorbed on the Au atom (structure 13 in Figures 2 and 3). To close the catalytic cycle, the hydroxyl and hydride fragments react through transition state 14 to form a water molecule adsorbed on the Au atom (structure 15 in Figures 2 and 3) that, in a second step, desorbs, regenerating the Au1/G1V active site, this step requiring 65 kJ mol−1. In the water formation step, the H atom that was directly bonded to Au in intermediate 13 changes its nature from hydride to proton, while the net atomic charge on Au decreases from 0.792 e in 13 to 0.441 e in structure 15. The process is almost thermoneutral but involves an activation barrier of 125 kJ mol−1 that makes this step the rate-determining step of the global mechanism. A similar mechanism was previously reported for propene epoxidation over a small gas-phase Au3 cluster, in which water formation with an activation barrier of 110 kJ mol−1 at the PW91 level was also the rate-determining step.37 However, water is far from being an inert reactant or intermediate in PO formation, and propene epoxidation using a mixture of O2 and H2O was reported on subnanometer gold clusters deposited on alumina51 and on gold nanoparticles of ∼3 nm supported on TiO2.16 The reaction path was theoretically investigated on isolated Au38 and Au10 clusters,52 and it was found that the first step in the mechanism is formation of a hydroperoxide Au−OOH intermediate from coadsorbed O2 and H2O that, in a second and rate-determining

Table 1. PW91-D3 and PW91 Calculated Reaction Energies, in kJ mol−1 step

PW91-D3

PW91

Au1/G1V + C3H6 → 1 Au1/G1V + O2 → 2 Au1/G1V + H2 → 3 2 + H2 → 4 4 → TS5 4→6 3 → TS7 3→8 8 + O2→ 6 3 + O2 → 9 6 + C3H6 →10 10 → TS11 10 → 12 12 → 13 + PO 13 → TS14 13 → 15 15 → Au1/G1V + H2O 3 + C3H6 → 16 8 + C3H6 → 17 15 + O2 → 18 18 → TS19 18 → 20 18 → 2 + H2O 20 + C3H6 → 21 21 → TS22 21 → 23 23 → 9 + PO 9 → H2O2 + Au1/G1V 9 + H2 → 24 24 → TS25 24 → 26 26 → Au1/G1V + 2H2O 13 + O2 → 27 13 + C3H6 → 28 15 + C3H6 → 29

−155.8 −164.2 −68.5 −30.8 53.7 −48.9 27.3 −33.1 −142.3 −466.9 −39.4 95.2 −232.8 68.8 124.7 2.4 64.7 −36.2 −72.8 −160.5 134.6 −1.5 55.1 −20.9 63.4 −248.3 78.5 378.2 2.0 350.7 −84.4 −119.5 −17.4 −41.2 −38.5

−132.2 −157.4 −65.4 −8.6 37.6 −64.5 28.5 −29.7 −135.3 −461.9 −14.5 101.7 −230.1 37.1 123.9 4.4 57.7 −11.3 −36.9 −154.6 134.1 −1.1 50.5 −8.5 73.8 −231.2 43.0 366.7 11.7 354.6 −81.0 −94.8 −4.9 −13.5 −10.8

hydroxylated HO−Au−OH species 9 was explored, but despite the high exothermicity of the process, it was not possible to locate a transition state for direct interconversion between these two species. This means that, once formed, the hydroperoxide intermediate 6 could desorb as H2O2 and then adsorb again on the Au atom, yielding the highly stable double hydroxylated species 9, but this step would require a barrier of 114 kJ mol−1. It has also been reported that, during propene epoxidation with a H2/O2 mixture over Au NPs supported on TiO2, propene hydrogenation becomes a competitive route with decreasing particle size, and propane is the only product observed when Au particle size becomes smaller than 1.8 nm.14 Therefore, propene interaction with adsorbed H2 (structure 3) and with a Au dihydride intermediate (structure 8) was also investigated as the starting point for competitive hydrogenation routes leading to propane. However, the low values obtained for interaction energies, −36 and −73 kJ mol−1, respectively, together with the optimized geometries of the resulting complexes, structures 16 and 17 in Figures 2 and 3, in which propene is not activated by interaction with the Au atom but only weakly adsorbed above the hydrogen atoms, suggest that 19359



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catalytic cycle over Au38 and Au10 clusters does not require formation of H2O2 but involves formation of H2O and an adsorbed O atom that subsequently would epoxidize another propene molecule. The activation barriers reported for this pathway are 76 and 131 kJ mol−1 over Au38 and Au10 clusters, respectively. However, this route was discarded on the Au1/G1V system due to the high instability of a single O atom adsorbed on the Au atom supported on graphene that makes the process HO−Au−OH → H2O + Au−O endothermic by 502 kJ mol−1. It can therefore be concluded that propene epoxidation by a H2O/O2 mixture cannot occur on a single Au atom supported on graphene, due to the high energy involved in the regeneration of the active site after PO desorption. It is important to remark that the possibility of O2 dissociation into two O atoms attached to Au was considered and finally discarded since geometry optimization always led to structure 2. This behavior is due, on one hand, to the fact that dissociation of molecular O2 is only thermodynamically favored over gold clusters of at least three Au atoms56 and on the other hand to the inertness of the graphene support that does not permit the generation of an active metal−support interface able to dissociate O2. This is a key point from the point of view of selectivity because, on the basis of previous theoretical studies,57−60 O atoms on metal surfaces are expected to preferentially react with H atoms of the propene methyl group, resulting in formation of a highly reactive allyl intermediate that finally causes olefin combustion. Actually, the low selectivity toward PO reported for silver-based catalysts has been attributed to formation of these allylic intermediates.5,6 Thus, avoiding O2 dissociation and therefore the competing routes causing olefin combustion should considerably increase the selectivity to PO.

step, reacts with adsorbed propene yielding PO. The calculated activation barriers for PO formation decrease from 104 kJ mol−1 on the Au38 nanoparticle to 87 kJ mol−1 on the Au10 cluster. Therefore, we considered it pertinent to investigate this pathway on the Au1/G1V model to bring some light on the influence of Au cluster size and on the role of water. Starting from structure 15, with water coordinated to the Au atom, O2 coadsorption is possible and leads to formation of reaction intermediate 18, with the three oxygen atoms bonded to Au at optimized Au−O distances between 2.1 and 2.4 Å. The positive charge on the Au atom increases from 0.441 to 0.919 e, and −161 kJ mol−1 is released in this step. H2O desorption from structure 18 would only require 55 kJ mol−1 and would result in formation of the initial O2 adsorption complex 2, starting the catalytic cycle described above. However, a second pathway exists in which a hydrogen atom is transferred from the H2O molecule to adsorbed O2 through transition state 19, with an activation energy of 135 kJ mol−1. The resulting structure 20 is as stable as the initial complex 18 and contains a hydroperoxyl and a hydroxyl group bonded to a cationic Au atom. The charge distribution in the hydroperoxide fragment in structure 20 is equivalent to that in intermediate 6, where it was coadsorbed with a hydride. Propene adsorbs weakly on the hydroperoxide species 20, leading to formation of complex 21, with a calculated interaction energy of only −21 kJ mol−1. Then, insertion of the electrophilic oxygen atom of the Au− OOH group into the CC bond of propene through transition state 22 results in the formation of PO coadsorbed with two hydroxyl groups on the Au atom (structure 23 in Figures 2 and 3). This step is highly exothermic, releasing −248 kJ mol−1, and involves an activation energy of 63 kJ mol−1. PO desorption requires 63 kJ mol−1 more and leaves two hydroxyl groups strongly attached to the Au atom, structure 9, which is previously described. To close the catalytic cycle, these two hydroxyl groups should recombine and desorb as hydrogen peroxide, regenerating the Au1/G1V active site, but this process was found to be endothermic by 378 kJ mol−1. H2O2 dissociation into two OH groups over isolated Au10 clusters was theoretically investigated at the RPBE level53 and found to be barrierless and energetically favorable, though the calculated reaction energies, between 209 and 237 kJ mol−1 depending on the adsorption position, are not as high as the value obtained in this work over the Au1/G1V system. A second possibility considered was formation of two H2O molecules bonded to Au (structure 26) by reaction of a H2 molecule with intermediate 9 via transition state 25. Although this step is thermodynamically favored, the calculated activation energy value of 351 kJ mol−1 makes it difficult from a kinetic point of view, and therefore it seems that formation of doubly hydroxylated gold 9 would block the active sites, being a sink in the mechanism. The activation barriers for formation of Au−OOH species from a H2O/O2 mixture over Au38 and Au10 clusters, ∼30 kJ mol−1,38 are considerably lower than that obtained over the Au1/G1V model, 135 kJ mol−1. However, in the work by Li and co-workers,38 the O2 molecule coadsorbed with H2O in the reactant structure adopted an end-on conformation with only one O atom bonded to Au, which is known to be much less stable than other adsorption modes.39,54,55 Indeed, an end-on mode can also be formed on the Au1/G1V model, which is 50 kJ mol−1 less stable than intermediate species 1. However, the main difference between the reaction mechanism over the Au1/ G1V system and larger gold clusters is that closure of the

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CONCLUSIONS The mechanism of propene epoxidation by H2/H2O/O2 mixtures over gold atoms supported on defective graphene has been theoretically investigated and the results compared with previous data reported over Aun clusters of different size. Formation of a gold hydroperoxide H−Au−OOH intermediate from H2/O2, and the subsequent insertion of the electrophilic oxygen atom of the hydroperoxide group into the CC bond of propene yielding propene oxide (PO) and H2O, is thermodynamically and kinetically favored and follows a mechanism similar to that previously reported on Au3 and larger clusters, with formation of H2O being the ratedetermining step. However, a H2O/O2 mixture is not catalytically efficient since it results in formation of an extremely stable HO−Au−OH intermediate whose decomposition leads to catalytic cycle closure and regeneration of the Au1/G1V active site is energetically too demanding. The main advantage of the Au1/G1V material explored in this work with respect to other isolated or supported Au clusters previously investigated is not related to catalyst activity but to the selectivity toward PO that is greatly enhanced due to suppression of the main competing routes, i.e., propene hydrogenation and unselective oxidation or combustion. On one hand, despite the fact that H2 dissociation is energetically feasible, the coordinating capability of an isolated gold atom is limited, and propene does not interact with H atoms attached to Au, so that propene hydrogenation is suppressed. On larger clusters, propene adsorption close to H atoms would be possible and would result in formation of 19360



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propane, as reported by Haruta on highly dispersed Au/TiO2 catalysts. On the other hand, dissociation of molecular O2 into two O atoms that would finally cause propene combustion is energetically impeded on the Au1/G1V material. The preferential adsorption and activation of O2 on the isolated Au atoms and subsequent reaction with H2 allows us to form the gold hydroperoxide Au−OOH species that selectively produces the desired epoxide. Thus, on the basis of the present theoretical study, a high selectivity toward PO is expected on Au/ graphene.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Spanish MINECO (MAT2011-28009 and Consolider Ingenio 2010-MULTICAT, CSD2009-00050) and Generalitat Valenciana (PROMETEO/2008/130) for financial support and Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de València for offering computing facilities and technical assistance. A. P. thanks Spanish MINECO (Juan de la Cierva program) for her grant.

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O2 Mixtures Over Gold Atoms

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