Reactivity of niobium clusters with nitrogen and deuterium

Abstract

Absolute rate coefficients are reported for reactions of Nbn clusters (n=2–20) with D2 and N2 at 280, 300, and 370 K. Most clusters are highly reactive but there are conspicuous exceptions at n=8, 10, and 16 for both D2 and N2. The origin of this trend in reactivity with cluster size and the reason why D2 and N2 show similar trends are discussed. Density functional theory (DFT) electronic structure calculations have been used to investigate the details of the reactions for the smallest clusters Nb2 and Nb8 with H2 and N2. The steric and electronic requirements for dissociation of H2 and N2 are described in terms of frontier orbital interactions. The main conclusion from the DFT calculations is that complete dissociation of H2 or N2 requires charge transfer by transit of an avoided crossing between neutral and ionic potentials. This idea is extended to larger clusters by using a simple charge transfer model that predicts an inverse correlation between reactivity and an appropriately defined effective ionization potential. Such a correlation is observed and indicates that the effective ionization potential is the dominant influence on reactivity.