Gas-phase kinetics and catalytic reactions of small silver and gold clusters

Recent work on the chemical reactivity of mass-selected small silver and gold, as well as binary silver-gold cluster ions in a temperature controlled radio frequency ion trap arrangement is reviewed. Reactions with molecular oxygen, carbon monoxide, and mixtures of both reactant gases are investigated, in order to reveal the possible role of these small noble metal cluster ions to act as gas-phase catalysts in the carbon monoxide combustion reaction. The obtained gas-phase reaction kinetics enable the determination of the reaction mechanisms as well as the energetics along the reaction pathway. A strong dependence of the chemical reactivity on cluster size, composition and charge state is found and correlations between electronic structure and reactivity are discussed in terms of simple frontier orbital pictures. Special emphasis is put on the importance of cooperative adsorption effects on the small noble metal clusters. Through comparison of the kinetic data with first principles quantum chemical simulations, a comprehensive picture of the molecular details of the reaction behavior emerges. In particular, the experiments provide evidence that selected cluster ions act as active gas-phase catalysts for the oxidation of CO to CO2 by molecular oxygen. In the case of Au-2(-). the combination of reaction kinetics measurements with ab initio calculations even revealed the full and detailed reaction cycle of the catalytic gas-phase oxidation of CO. (c) 2004 Elsevier B.V. All rights reserved.