Oxidation reactions on neutral cobalt oxideclusters: experimental and theoretical studies

Abstract

Reactions of neutral cobalt oxide clusters (Co_mO_n, 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 Co_mO_n 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.