The 2D or 3D morphology of sub-nanometer Cu 5 and Cu 8 clusters changes the mechanism of CO oxidation.

The mechanism of the CO oxidation reaction catalysed by planar Cu 5 , three dimensional (3D) Cu 5 , and 3D Cu 8 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 Cu 5 and Cu 8 clusters, O 2 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 Cu 5 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 CO 2 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.