Catalytic activity of small MgO-supported Au clusters towards CO oxidation: A density functional study

In order to explain the experimentally found catalytic characteristics of Au₁₋₋₄/MgO(100) we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and O₂ molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in Au₁₋₋₄/MgO+CO(O₂). Experimentally Au1,2/MgO were found to be inactive under a mixed atmosphere. We show that O₂ strongly binds to Au₁/MgO that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving Au₂^-, is energetically possible for Au₂/MgO, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of Au3,4/MgO could be explained by their ability to coadsorb CO and O₂, hence indicating the occurrence of a Langmuir-Hinshelwood-type reaction mechanism for these clusters.