Gas phase chemistry of neutral metal clusters: Distribution, reactivity and catalysis

Recent work on gas phase distribution, reactivity, and catalysis of neutral metal, metal oxide/carbide/sulfide clusters, investigated by single photon ionization coupled with time-of-flight mass spectrometry, is reviewed. Oxidation-reduction and bond activation reactions catalyzed by neutral metal and metal compound clusters are investigated, in order to understand the catalytic process at a molecular level, and reveal possible full catalytic cycles for related condensed phase reactivity and processes. Density functional theory calculations for these systems enable exploration of the geometric and electronic structures of clusters and determination of reaction intermediates and transition states, as well as reaction mechanisms, by comparing the results of theoretical calculations and experimental observations. Reactivity of metal oxide clusters with small gas phase molecules (CO, SO2, and hydrocarbons), which is associated with the oxygen-rich or -deficient nature of the cluster, is discussed in terms of bond enthalpy, cluster spin state, and unpaired spin density of the clusters. For bond activation catalytic reactions on metal and metal compound clusters, we describe how adsorption of reactant molecules on active metal sites of clusters, association energy between reactant molecules and clusters, and activation energy (barriers) on the reaction potential energy surface can be constructed and compared to generate a reaction mechanism. Experimental and theoretical studies shown in this review also provide unique insights into how the application of gas phase neutral metal cluster chemistry is instructive in the understanding of important fundamental aspects of practical catalysis in the condensed phase. (c) 2012 Elsevier B.V. All rights reserved.