Analysis of O(2) adsorption on binary-alloy clusters of gold: energetics and correlations.

We report a B3LYP density-functional theory (DFT) analysis of O(2) adsorption on 27 Au(n)M(m) (m, n = 0-3 and m + n = 2 or 3; M = Cu, Ag, Pd, Pt, and Na) clusters. The LANL2DZ pseudopotential and corresponding double-zeta basis set was used for heavy atoms, while a 6-311+G(3df) basis set was used for Na and O. We employed basis-set superposition error (BSSE) corrections in the electronic adsorption energies at 0 K (deltaE(ads)) and also calculated adsorption thermodynamics at standard conditions (298.15 K and 1 atm), i.e., internal energy of adsorption (deltaU(ads)) and Gibbs free energy of adsorption (deltaG(ads)). Natural Bond Orbital (NBO) analysis showed that all the clusters donated electron density to adsorbed O(2) and we successfully predicted intuitive linear correlations between the NBO charge on adsorbed O(2), O-O bond length, and O-O stretching frequency. Although there was no clear trend in the O(2) binding energy (BE = -deltaE(ads)) on pure and alloy dimers, we found the following interesting trend for trimers: BE (MAu(2)) < BE (M(3)) < or = BE (M(2)Au). The alloy trimers containing only one Au atom are most reactive toward O(2) while those with two Au atoms are least reactive. These trends are discussed in the context of the ensemble effect and coulomb interactions. We found an approximate linear correlation between the O(2) BE and charge transfer to O(2) for all 27 clusters. The clusters having strongly electropositive Na atoms (e.g., Na(3) and Na(2)Au) donated almost one full electron to adsorbed O(2), and the BE is maximum on these clusters. Although O(2) dissociation is likely in such cases, we have restricted this study to trends in the adsorption of molecular O(2) only. We also found an approximate linear correlation between the charge transfer and BE versus energy difference between the bare-cluster HOMO and O(2) LUMOs, which we speculate to be a fundamental descriptor of the reactivity of small clusters toward O(2). Part of the scatter in these correlations is attributed to the differences in the O(2) binding orientations on different clusters (geometric effect). Relatively higher bare-cluster HOMO energy eases the charge transfer to adsorbed O(2) and enhances the reactivity toward O(2). The Frontier Orbital Picture (FOP) is not always useful in predicting the most favorable O(2) binding site on clusters. It successfully predicted the cluster-O(2) ground-state configurations for 10 clusters, but failed for the others. Finally, the energetics of fragmentation suggest that the bare and O(2)-covered clusters reported here are stable.

CO adsorption on pure and binary-alloy gold clusters: a quantum chemical study.

We performed density-functional theory analysis of nondissociative CO adsorption on 22 binary Au-alloy (Au(n)M(m)) clusters: n=0-3, m=0-3, and m+n=2 (dimers) or 3 (trimers), M=Cu/Ag/Pd/Pt. We report basis-set superposition error corrections to adsorption energies and include both internal energy of adsorption (DeltaU(ads)) and Gibbs free energy of adsorption (DeltaG(ads)) at standard conditions (298.15 K and 1 atm). We found onefold (atop) CO binding on all the clusters except Pd2 (twofold/bridged), Pt2 (twofold/bridged), and Pd3 (threefold). In agreement with the experimental results, we found that CO adsorption is thermodynamically favorable on pure Au/Cu clusters but not on pure Ag clusters and also observed the following adsorption affinity trend: Pd>Pt>Au>Cu>Ag. For alloy dimers we found the following patterns: Au2>M Au>M2 (M=Ag/Cu) and M2>M Au>Au2 (M=Pd/Pt). Alloying Ag/Cu dimers with (more reactive) Au enhanced adsorption and the opposite effect was observed for PdPt dimers. The Ag-Au, Cu-Au, and Pd-Au trimers followed the trends observed on dimers: Au3>M Au2>M2Au>M3 (M=Ag/Cu) and Pd3>Pd2Au>PdAu2>Au3. Interestingly, Pt-Au trimers reacted differently and alloying with Au systematically increased the adsorption affinity: PtAu2>Pt2Au>Pt3>Au3. A strikingly different behavior of Pt is also manifested by the triplet spin state and onefold (atop) binding in Pt3-CO which is in contradiction with the singlet spin state and threefold binding in Pd3-CO. We found a linear correlation between CO binding energy (BE) and elongation of the CO bond. For Ag-Au and Cu-Au clusters, the increase in CO BE (and elongation of the C-O bond which is probably due to the back donation) is accompanied by the decrease in the cluster-CO distance suggesting that the donation (from 5sigma highest occupied molecular orbital in CO to cluster lowest unoccupied molecular orbital) mechanism also contributes to the BE. For Pd-Au clusters, the cluster-CO distance (and CO bond length) increases with increase in the BE, suggesting that the donation mechanism may not be important for those clusters. No clear trend was observed for Pt-Au clusters.

Adsorption of small Au(n) (n = 1-5) and Au-Pd clusters inside the TS-1 and S-1 pores.

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.

A quantum chemical study of comparison of various propylene epoxidation mechanisms using H2O2 and TS-1 Catalyst.

We present a comparison of the following prominent propylene epoxidation mechanisms using H2O2/TS-1 at a consistent density functional theory (DFT) method: (1) the Sinclair and Catlow mechanism on tripodal site through Ti-OOH species, (2) the Vayssilov and van Santen mechanism on tetrapodal site without Ti-OOH formation, (3) the Munakata et al. mechanism involving peroxy (Ti-O-O-Si) species, (4) the defect site mechanism with a partial silanol nest, and (5) the defect site mechanism with a full silanol nest. We have reproduced the previously published (ethylene epoxidation) pathways (1-3) for propylene epoxidation using larger and SiH3-terminated cluster models of the T-6 crystallographic site of TS-1. Mechanism 5 is a new mechanism reported here for the first time. The use of a consistent level of theory for all the pathways allows for the first time a meaningful comparison of the energetics representing the aforementioned pathways. We have rigorously identified the important reaction intermediates and transition states and carried out a detailed thermochemical analysis at 298.15 K and 1 atm. On the basis of the Gibbs free energy of activation, the Sinclair and Catlow mechanism (Delta G(act) = 7.9 kcal/mol) is the energetically most favorable mechanism, which is, however, likely to operate on the external surface of TS-1 due to the tripodal nature of the Ti site in their model. The newly reported defect site mechanism (with a full silanol nest) is a competitive propylene epoxidation mechanism. There are two main steps: (1) hydroperoxy formation (Delta G(act) = 8.9 kcal/mol) and (2) propylene epoxidation (Delta G(act) = 4.6 kcal/mol). This mechanism is likely to represent the chemistry occurring inside the TS-1 pores in the liquid-phase epoxidation (H2O2/TS-1) process and could operate in direct gas-phase epoxidation (H2/O2/Au/TS-1) as well. If only the propylene epoxidation step is considered, then the Munakata peroxo intermediate (Si-O-O-Ti) is the most reactive intermediate, which can epoxidize propylene with a negligible activation barrier. However, formation of the Munakata intermediate is a very activated step (Delta G(act) = 19.8 kcal/mol). We also explain the trends in the activation barriers in different mechanisms using geometric and electronic features such as orientation of adsorbed H2O2 and propylene, hydrogen bonding, O1-Ti bond distance in the Ti-O1-O2-H intermediate, and O1-O2 stretching in the transition state. Implications of different Ti site models are also discussed in light of the nature of external/internal and nondefect/defect sites of TS-1.

Partial oxidation of propylene to propylene oxide over a neutral gold trimer in the gas phase: a density functional theory study.

We report a B3LYP study of a novel mechanism for propylene epoxidation using H(2) and O(2) on a neutral Au(3) cluster, including full thermodynamics and pre-exponential factors. A side-on O(2) adsorption on Au(3) is followed by dissociative addition of H(2) across one of the Au-O bonds (DeltaE(act) = 2.2 kcal/mol), forming a hydroperoxy intermediate (OOH) and a lone H atom situated on the Au(3) cluster. The more electrophilic O atom (proximal to the Au) of the Au-OOH group attacks the C=C of an approaching propylene to form propylene oxide (PO) with an activation barrier of 19.6 kcal/mol. We predict the PO desorption energy from the Au(3) cluster with residual OH and H to be 11.5 kcal/mol. The catalytic cycle can be closed in two different ways. In the first subpathway, OH and H, hosted by the same terminal Au atom, combine to form water (DeltaE(act) = 26.5 kcal/mol). We attribute rather a high activation barrier of this step to the breaking of the partial bond between the H atom and the central Au atom in the transition state. Upon water desorption (DeltaE(des) = 9.9 kcal/mol), the Au(3) is regenerated (closure). In the second subpathway, H(2) is added across the Au-OH bond to form water and another Au-H bond (DeltaE(act) = 22.6 kcal/mol). Water spontaneously desorbs to form an obtuse angle Au(3) dihydride, with one H atom on the terminal Au atom and the other bridging the same terminal Au atom and the central Au atom. A slightly activated rearrangement to a symmetric triangular Au(3) intermediate with two equivalent Au-H bonds, addition of O(2) into the Au-H bond, and rotation reforms the hydroperoxy intermediate in the main cycle. On the basis of the DeltaG(act), which contains contribution from both pre-exponetial factor and activation energy, we identify the propylene epoxidation step as the actual rate-determining step (RDS) in both the pathways. The activation barrier of the RDS (epoxidation step: DeltaE(act) = 19.6 kcal/mol) is in the same range as that in the published computationally investigated olefin epoxidation mechanisms involving Ti sites (without Au involved) indicating that isolated Au clusters and possibly Au clusters on non-Ti supports can be active for gas-phase partial oxidation, even though cooperative mechanisms involving Au clusters/Ti-based-supports may be favored.

Comparison of the catalytic activity of Au3, Au4+, Au5, and Au5- in the gas-phase reaction of H2 and O2 to form hydrogen peroxide: a density functional theory investigation.

We report a detailed density functional theory (B3LYP) analysis of the gas-phase H2O2 formation from H2 and O2 on Au3, Au4+, Au5, and Au5-. We find that H2, which interacts only weakly with the Au clusters, is dissociatively added across the Au-O bond, upon interaction with AunO2. One H atom is captured by the adsorbed O2 to form the hydroperoxy intermediate (OOH), while the other H atom is captured by the Au atom. Once formed, the hydroperoxy intermediate acts as a precursor for the closed-loop catalytic cycle. An important common feature of all the pathways is that the rate-determining step of the catalytic cycle is the second H2 addition to form H2O2. The H2O2 desorption is followed by O2 addition to AunH2 to form the hydroperoxy intermediate, thus leading to the closure of the cycle. On the basis of the Gibbs free energy of activation, out of these four clusters, Au4+ is most active for the formation of the H2O2. The 0 K electronic energy of activation and the DeltaGact at the standard conditions are 12.6 and 16.6 kcal/mol respectively. The natural bond orbital charge analysis suggests that the Au clusters remain positively charged (oxidic) in almost all the stages of the cycle. This is interesting in the context of the recent experimental evidence for the activity of cationic Au in CO oxidation and water-gas shift catalysts. We have also found preliminary evidence for a correlation between the activation barrier for the first H2 addition and the O2 binding energy on the Au cluster. It suggests that the minimum activation barrier for the first H2 addition is expected for the Au clusters with 7.0-9.0 kcal/mol O2 binding energy, i.e., in the midrange of the expected interaction energy. This represents a balance between more favorable H2 dissociation when the Aun-O2 interaction is weaker and high O2 coverage when the interaction is stronger. On the basis of this work, we predict that the hydroperoxy intermediate formation can be both thermodynamically and kinetically viable only in a narrow range of the O2 binding energy (10.0-12.0 kcal/mol)-a useful estimate for computationally screening Au-cluster-based catalysts. We also show that a competitive channel for the OOH desorption exists. Thus, in propylene epoxidation both OOH radicals and H2O2 can attack the active Ti in/on the Au/TS-1 and generate the Ti-OOH sites, which can convert propylene to propylene oxide.