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.

A novel size dependence in the adsorption reaction of multiple O2 molecules onto anionic silver clusters Agn- (n = 1-5) is revealed by gas-phase reaction studies in an rf-ion trap. Ab initio theoretical modeling based on DFT method provides insight into the reaction mechanism and finds cooperative electronic and structural effects to be responsible for the size selective reactivity of Agn- clusters toward one or more O2. In particular, Agn- clusters with odd n have paired electrons and therefore bind one O2 only weakly, but they are simultaneously activated to adsorb a strongly bound second oxygen molecule. For the clusters Ag3O4- and Ag5O4-, this cooperative effect results in a superoxo-like, doubly bound O2 subunit with potentially high activity in catalytic silver cluster oxidation processes.

Reactions of free silver anions Agn- (n = 1 - 13) with O2, CO, and their mixtures are investigated in a temperature controlled radio frequency ion trap setup. Cluster anions Agn- (n = 1 - 11) readily react with molecular oxygen to yield AgnOm- (m = 2, 4, or 6) oxide products. In contrast, no reaction of the silver cluster anions with carbon monoxide is detected. However, if silver cluster anions are exposed to the mixture of O2 and CO, new reaction products and a pronounced, discontinuous size dependence in the reaction behavior is observed. In particular, coadsorption complexes Agn(CO)O2- are detected for cluster sizes with n = 4 and 6 and, the most striking observation, in the case of the larger odd atom number clusters Ag7-, Ag9-, and Ag11-, the oxide product concentration decreases while a reappearance of the bare metal cluster signal is observed. This leads to the conclusion that carbon monoxide reacts with the activated oxygen on these silver clusters and indicates the prevalence of a catalytic reaction cycle.