Photoelectron spectroscopy of transition-metal clusters: Correlation of valence electronic structure to reactivity

The photoelectron spectra and the reactivity of H₂ with Fe, Co, and Ni clusters have been studied as a function of cluster size for clusters ranging from 5 to 26 atoms in size. It is shown that the variation of the reactivity correlates with the variation of the difference between the ionization potential and the electron affinity of the clusters. This finding is rationalized by using a simple model in which the probability of H₂ chemisorption on the cluster is proportional to the magnitude of an entrance channel barrier caused by Pauli repulsion between H₂ and the cluster. It is argued that the magnitude of this barrier is proportional to the promotion energy of an electron from the frontier orbital of the cluster to the lowest unoccupied d state of the cluster.