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

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.