Theoretical study of CO oxidation on Au nanoparticles supported by MgO(100)

We present a density-functional-theory (DFT) study of the reactivity towards CO oxidation of Au nanoparticles supported by MgO(100). We model two geometrical aspects of the Au particles, the low index facets of the Au particles, and the Au-MgO interface boundary. The precise structure of the interface boundary depends on the size of the Au particles, and different models with either small or large Au-MgO contact angles are introduced. For all Au systems, we find that the CO oxidation reaction proceeds via CO adsorption, trapping of ${\mathrm{O}}_{2},$ leading to the formation of a metastable $\mathrm{CO}\ensuremath{\cdot}{\mathrm{O}}_{2}$ reaction intermediate, which dissociates into ${\mathrm{CO}}_{2}$ and adsorbed atomic oxygen. The atomic oxygen reacts directly with gas phase CO. No separate ${\mathrm{O}}_{2}$ molecular or dissociative adsorption is found to be favorable. Important differences were found in the reactivity of the various Au-MgO interface boundaries. This is explained in terms of two properties: the Au-Au coordination determining the local reactivity of the Au atoms and the presence of the MgO support that, besides providing excess electrons to the Au clusters, forms ionic bonds to the peroxo part of the $\mathrm{CO}\ensuremath{\cdot}{\mathrm{O}}_{2}$ reaction intermediate. We find that the type of interface boundary likely to be predominant for medium-sized nanoparticles provides the optimal degree of low-coordinated Au atoms in the neighborhood of the MgO support. Our DFT study therefore provides a rational for why the reactivity per site may reach a maximum at a critical particle size as has been observed experimentally for similar systems.