Hydrogen chemisorption on gas-phase transition-metal clusters

Abstract

Studies of the reactions of ‘naked’ gas-phase transition-metal clusters with hydrogen have revealed a rich diversity of dramatic, non-monotonic size-selective reaction rates, as well as unusual levels of steady-state saturation coverage as a function not only of the number of constituent atoms, but also of the metal, the charge on the cluster and the degree of ‘dressing’, or coverage. The rate of reaction in a global sense seems to be governed by the ability in the cluster/di-hydrogen system of the attractive short-range partial electron-transfer interactions between the frontier orbitals of the metal cluster and the di-hydrogen to overcome the long-range Pauli repulsion between the hydrogen and cluster valence orbitals, effects which are very sensitive to the electronic structure of the clusters as a function of size. Surprisingly, certain clusters are highly unreactive and as such are presumed to possess a tightly bound (symmetric) structure which cannot reconstruct to accept surface hydrogen addition. Hence the concept of ‘molecular surfaces’ which are electronically ‘hard’, but structurally ‘soft’. Our results also demonstrate that some metal clusters can dissociatively chemisorb an amazing number of hydrogen molecules. (H/N)_max ratios of 3, 5, 5 and 8 have been observed for Pd, Ni and Pt, and Rh clusters, respectively. The implications of some of these results are quite significant for catalysis and organometallic synthesis.