Oxidation reactions on neutral cobalt oxide clusters: experimental and theoretical studies

Reactions of neutral cobalt oxide clusters (ComOn, m = 3-9, n = 3-13) with CO, NO, C2H2, and C2H4 in a fast flow reactor are investigated by time of flight mass spectrometry employing 118 nm (10.5 eV) single photon ionization. Strong cluster size dependent behavior is observed for all the oxidation reactions; the Co3O4 cluster has the highest reactivity for reactions with CO and NO. Cluster reactivity is also highly correlated with either one or more following factors: cluster size, Co(III) concentration, the number of the cobalt atoms with high oxidation states, and the presence of an oxygen molecular moiety (an O-O bond) in the ComOn clusters. The experimental cluster observations are in good agreement with condensed phase Co3O4 behavior. Density functional theory calculations at the BPW91/TZVP level are carried out to explore the geometric and electronic structures of the Co3O4 cluster, reaction intermediates, transition states, as well as reaction mechanisms. CO, NO, C2H2, and C2H4 are predicted to be adsorbed on the Co(II) site, and react with one of the parallel bridge oxygen atoms between two Co(III) atoms in the Co3O4 cluster. Oxidation reactions with CO, NO, and C2H2 on the Co3O4 cluster are estimated as thermodynamically favorable and overall barrierless processes at room temperature. The oxidation reaction with C2H4 is predicted to have a very small overall barrier (<0.23 eV). The oxygen bridge between two Co(III) sites in the Co3O4 cluster is responsible for the oxidation reactions with CO, NO, C2H2, and C2H4. Based on the gas phase experimental and theoretical cluster studies, a catalytic cycle for these oxidation reactions on a condensed phase cobalt oxide catalyst is proposed.