Metal particles adsorbed on an oxide support are the archetypal heterogeneous catalyst. As interest increases in developing well-defined discrete metal particles on supports, it becomes important to characterize the metal support interaction and in particular its dependence on reaction environment. Gates et al. have demonstrated the preparation and characterization of discrete Re3 clusters on gamma-alumina and have reported that these clusters both are active for hydrogenation catalysis and retain their integrity after reaction(1). In this work we use density functional theory (DFT) simulations to characterize the structure, bonding, and equilibrium composition of discrete Re3 clusters on two limiting models of an á-alumina surface, a hydrogen-free “dry” model and a fully hydroxylated “wet” model. Supercell DFT calculations show that a Re3 cluster binds strongly on the dry surface in registry with three-fold surface oxygen sites, and that this cluster is stable to fragmentation. Binding is driven by charge transfer to undercoordinated surface Al atoms. Molecular adsorption on the wet surface is relatively weak, but spillover of hydrogen from support to cluster greatly enhances bonding. Both results are consistent with Re-surface binding mediated by individual Re-O surface bonds. Based on these results, cluster models are used to probe the ability of supported Re clusters to adsorb hydrogen. An equilibrium coverage of six hydrogens per Re3 cluster are expected under conditions representative of hydrogenation catalysis.

Reference:

(1) Fung, A. S.; Tooley, P. A.; Kelley, M. J.; Koningsberger, D. C.; Gates, B. C. Journal Of Physical Chemistry 1991, 95, 225.