Platinum Nanoclusters
Stabilized on γ-Alumina
by Chlorine Used As a Capping Surface Ligand: A Density Functional
Theory Study
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Abstract
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<sub><i>n</i></sub> 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<sub><i>n</i></sub> cluster
with the available Al<sub>III</sub> site, associated with strong H–Pt<sub><i>n</i></sub>–Cl interaction. In particular, this
trend leads to a local energy minimum, as a function of cluster size,
for the Pt<sub>3</sub> 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