Platinum Nanoclusters Stabilized on γ-Alumina by Chlorine Used As a Capping Surface Ligand: A Density Functional Theory Study

Controlling the size of metallic nanoclusters supported on an oxide support such as γ-alumina represents a challenging but important task in the case of noble metals such as platinum. By using density functional theory (DFT), we investigate the thermodynamic, structural and electronic properties of small nanometer-sized Pt_<i>n</i> clusters (<i>n</i> ≤ 13) interacting with four relevant γ-alumina surfaces exhibiting various hydroxylation and chlorination states. The presence of chlorine on the (110) surface of γ-alumina implies a thermodynamic stabilization of small platinum clusters. This stabilization originates from the simultaneous migrations of chlorine atoms and protons from the support toward the Pt clusters. The migration of H and Cl from the support induces a stronger interaction of the Pt_<i>n</i> cluster with the available Al_III site, associated with strong H–Pt_<i>n</i>–Cl interaction. In particular, this trend leads to a local energy minimum, as a function of cluster size, for the Pt₃ cluster. This atomic-scale stabilization of subnanometer clusters is thus proposed to be at the origin of the formation of highly dispersed platinum particles and to prevent their sintering into supranano ones. A detailed energetic and electronic analysis is provided to rationalize this effect of chlorine. A rational interpretation of experimental data is finally given