Since the advent of intense cluster sources, physical and chemical properties of isolated metal clusters are an active field of research. In particular, gas phase metal clusters represent ideal model systems to gain molecular level insight into the energetics and kinetics of metal-mediated catalytic reactions. Here we summarize experimental reactivity studies as well as investigations of thermal catalytic reaction cycles on small gas phase metal clusters, mostly in relation to the surprising catalytic activity of nanoscale gold particles. A particular emphasis is put on the importance of conceptual insights gained through the study of gas phase model systems. Based on these concepts future perspectives are formulated in terms of variation and optimization of catalytic materials e.g. by utilization of bimetals and metal oxides. Furthermore, the future potential of bio-inspired catalytic material systems are highlighted and technical developments are discussed.

A detailed analysis of experimentally obtained temperature-dependent gas-phase kinetic data for the oxygen and carbon monoxide adsorption on small anionic gold (Au(n)(-), n = 1-3), silver (Ag(n)(-), n = 1-5), and binary silver-gold (Ag(n)Au(m)(-), n + m = 2, 3) clusters is presented. The Lindemann energy transfer model in conjunction with statistical unimolecular reaction rate theory is employed to determine the bond dissociation energies E(0) of the observed metal cluster complexes with O(2) and CO. The accuracy limits of the obtained binding energies are evaluated by applying different transition-state models. The assumptions involved in the data evaluation procedure as well as possible sources of error are discussed. The thus-derived binding energies of O(2) to pure silver and binary silver-gold cluster anions are generally in excellent agreement with previously reported theoretical values. In marked contrast, the binding energies of O(2) and CO to Au(2)(-) and Au(3)(-) determined via temperature-dependent reaction kinetics are consistently lower than the theoretically predicted values.

Small gas-phase gold cluster cations are essentially inert toward molecular oxygen. Preadsorption of molecular hydrogen, however, is found to cooperatively activate the binding of O(2) to even-size Au(x)(+) (x = 2, 4, 6) clusters. Measured temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, in conjunction with first-principles density-functional theory calculations, reveal promotion and activation of molecular oxygen by preadsorbed hydrogen. These processes lead to the formation of a hydroperoxo intermediate on Au(4)(+) and Au(6)(+) and culminate in the dissociation of O(2) via the release of H(2)O. Langmuir-Hinshelwood reaction mechanisms involving the coadsorption of both of the reactant molecules are discussed for both cluster sizes, and an alternative Eley-Rideal mechanism involving hydrogen molecules adsorbed on a Au(6)(+) cluster reacting with an impinging gaseous oxygen molecule is analyzed. Structural fluctionality of the gold hexamer cation, induced by the adsorption of hydrogen molecules, and resulting in structural isomerization from a ground-state triangular structure to an incomplete hexagonal one, is theoretically predicted. Bonding of H(2) on cationic gold clusters is shown to involve charge transfer to the clusters. This serves to promote the bonding of coadsorbed oxygen through occupation of the antibonding 2pi* orbitals, resulting in excess electronic charge accumulation on the adsorbed molecule and weakening of the O-O bond. The theoretical results for hydrogen saturation coverages and reaction characteristics between the coadsorbed hydrogen and oxygen molecules are found to agree with the experimental findings. The joint investigations provide insights regarding hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.