The 2D or 3D morphology of sub-nanometer Cu5 and Cu8 clusters changes the mechanism of CO oxidation

Abstract

The mechanism of the CO oxidation reaction catalysed by planar Cu₅, three dimensional (3D) Cu₅, and 3D Cu₈ clusters is theoretically investigated at the B3PW91/Def2TZVP level. All three clusters are able to catalyse the reaction with similar activation energies for the rate determining step, about 16–18 kcal mol⁻¹, but with remarkable differences in the reaction mechanism depending on cluster morphology. Thus, for 3D Cu₅ and Cu₈ clusters, O₂ dissociation is the first step of the mechanism, followed by two consecutive CO + O reaction steps, the second one being rate determining. In contrast, on planar Cu₅ the reaction starts with the formation of an OOCO intermediate in what constitutes the rate determining step. The O–O bond is broken in a second step, releasing the first CO₂ and leaving one bi-coordinately adsorbed O atom which reacts with CO following an Eley–Rideal mechanism with a low activation energy, in contrast to the higher barriers obtained for this step on 3D clusters.