Does enhanced oxygen activation always facilitate CO oxidation on gold clusters?

We investigate the catalytic activity of the subnanometer-sized bimetallic Au19Pt cluster for oxidation of CO via first-principles density functional theory calculations. For this purpose we consider two structurally similar and energetically close homotops of the Au19Pt cluster with the Pt atom occupying an edge (Td-E) or a facet (Td-S) site of a 20-atom tetrahedron. Using these homotops as catalysts we calculate the complete reaction paths and the thermodynamic functions corresponding to the oxidation of CO to CO2. It is found that the oxidation of CO on the Td-S isomer occurs through a smaller reaction barrier (0.38 eV) as compared with that on the Td-E isomer (0.70 eV), although the activation of O2 on the latter is much higher than that on the former. Therefore, a clear conclusion is that a higher O2 activation, which is generally believed to be the key factor for CO oxidation, solely cannot determine the catalytic efficiency of the Au-Pt bimetallic clusters. In addition, we find a stronger CO adsorption on the Td-E isomer (2.06 eV) as compared with that on the Td-S isomer (1.68 eV). Although stronger CO adsorption on the Td-E isomer leads to a higher O2 activation; however, high value of CO adsorption energy deteriorates the catalytic activity of the Td-E isomer towards the CO oxidation reaction.