Theoretical Study of the Successive Hydrogenation of Small Platinum Clusters

Abstract

The successive hydrogenation steps, up to saturation of small platinum clusters Pt_n with n between 2 and 13 are studied by the Extended Huckel Method (EHT) with and without spin-orbit coupling. For each hydrogenation step, a large number of possible structures has been calculated in order to determine the stereochemistry of the best stepwise hydrogenated compounds. The resulting structures can be viewed as the progressive build-up of a three-dimensional network of square planar and/or octahedral monometallic complexes. The energy of the first successive steps of dihydrogen adsorption decreases steeply with the hydrogen rate for the smallest clusters. At the contrary, for some of the larger clusters, the first hydrogenation steps are more and more stabilizing before a decrease occurs. The energy of first adsorption depends on the cluster size. The largest value is found with the Pt₃ and Pt₄ species. These results confirm the interpretation proposed by Fraissard et al. for explaining the observations recently made on platinum clusters supported on zeolites.