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

Abstract

In order to explain the experimentally found catalytic characteristics of ${\text{Au}}_{1--4}/\text{MgO}(100)$ we have performed a comprehensive density functional study of these systems and their ability to (co)adsorb CO and ${\text{O}}_{2}$ molecules. Starting from the carefully determined ground-state structures we have analyzed binding mechanisms, the influence of spin-orbit coupling, and charge redistributions in ${\text{Au}}_{1--4}/\text{MgO}+\text{CO}({\text{O}}_{2})$. Experimentally ${\text{Au}}_{1,2}/\text{MgO}$ were found to be inactive under a mixed atmosphere. We show that ${\text{O}}_{2}$ strongly binds to ${\text{Au}}_{1}/\text{MgO}$ that prevents coadsorption. Although a catalytic reaction cycle towards CO oxidation, analogous to the gas phase reaction involving $\text{Au}_{2}{}^{\ensuremath{-}}$, is energetically possible for ${\text{Au}}_{2}/\text{MgO}$, the cluster will get blocked by a strongly bound CO. On the other hand, the catalytic activity of ${\text{Au}}_{3,4}/\text{MgO}$ could be explained by their ability to coadsorb CO and ${\text{O}}_{2}$, hence indicating the occurrence of a Langmuir-Hinshelwood-type reaction mechanism for these clusters.