Ajay M. Joshi

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.

Chapter 11 – Propylene Epoxidation by O2 + H2 over Au Nanoparticles on Ti-Nanoporous Supports

Publisher Summary Production of propylene oxide in a single step with no side products has been a long-sought industrial target. While a liquid-phase H2O2/TS-1-based route appears to be imminent, due to handling problems and cost associated with H2O2, researchers have also focused on propylene epoxidation using H2 and O2 over Au/Ti catalysts. Au nanoparticles on mesoporous and nanoporous Ti supports are promising due to their remarkable stability and commercially interesting activity, but significant improvements in H2 efficiency are desired for commercialization. This chapter summarizes the advances in propylene epoxidation using H2 and O2 over Au/TS-1 and Au/Ti-mesoporous supports. Implications of several interesting findings, such as Au particle size and support effects, effect of catalyst pretreatments, effect of gas-phase additives and catalyst promoters, reaction kinetics, and mechanistic insights from quantum chemical calculations, are discussed. A benefit of the catalysts discussed is that many of them delay the propylene oxide (PO) degradation until higher temperatures are reached, thus allowing higher temperatures to enhance PO desorption and avoiding fouling by PO oligomerization. The temperature at which the maximum PO rate occurs is, therefore, a characterization parameter that can be important in the search for higher performance.