Reactivity of Metal Clusters with Diatomic Molecules

Doctoral thesis, 1998

The reactivity of transition-metal clusters, in the approximate size range 10-60 atoms, with diatomic molecules has been investigated. A pulsed beam of neutral clusters is produced by a laser-vaporization source and passed through a low-pressure reaction cell containing the reactive gas. The clusters experience a low number of collisions with reactive molecules, and the bare and reacted clusters are detected by photoionization and time-of-flight mass spectrometry. The reaction probability with the first and, in several cases, a second molecule is determined by fitting a pseudo-first-order kinetic model to the abundance of bare and reacted clusters. In general, we find that clusters of the transition metals Ni, Nb, Rh, V and W react readily with CO, NO and O2, exhibiting relatively low size dependence. The only exception is the CO reactivity of Nbn, which exhibits striking size selectivity around Nb10. The reactivity with D2 and N2 is found to be lower, overall, and to vary non-monotonically with cluster size throughout the size range studied, often with coinciding minima and maxima in the reactivity with the two molecules. Cu clusters are found to exhibit relatively low and strongly size-dependent reactivity with CO, NO and O2 and to appear unreactive with N2. Cu clusters have been found to exhibit electronic shell structure, with high ionization potential and low O2 reactivity of clusters with closed electronic shells, according to the spherical-jellium model. We have measured the shift in ionization potential of Cun upon adsorption of O2 and found that the shell structure is destroyed in the oxidized clusters. This is believed to be the result of charge being localized at the adsorbed oxygen, perturbing the spherical symmetry of the closed-shell clusters. By comparison with electronic-structure calculations of the clusters, the size dependence in the CO reactivity of Cun and Nbn is rationalized by variations in charge transfer and hybridization between CO and the cluster, depending on the symmetries of the most diffuse cluster electronic orbitals. Adsorption of CO is favored when the highest occupied and lowest unoccupied molecular orbital of the cluster is of similar symmetry to the lowest unoccupied and highest occupied molecular orbital of CO.