Catalytic activity of gold nanoclusters supported by cerium oxide: interplay between cluster reactivity, size, and interface charge transfer revealed by DFT calculations

The parameters controlling the catalytic activity of oxide-supported Au atoms and clusters are studied by means of density functional theory calculations. CeO$_2$(111) surfaces containing positively charged Au ions, either as supported Au$^+$ or as substitutional Au$^{3+}$ ions, are shown to activate molecular CO and to catalyze its oxidation to CO$_2$ via participation of lattice O. For the Au$^+$ adatoms, the limiting rate is determined by the adsorbate spillover. The reaction proceeds with CO oxidation via O vacancy formation. These vacancies readily attract the Au$^+$ adatoms, turn them into negatively charged Au$^{\delta-}$ adspecies that prevent further CO adsorption, thus deactivating the catalyst. The reactivity of gold nanoparticles nucleated at O vacancies can be recovered for cluster sizes as small as Au$_2$. Substitutional Au3+ ions dispersed into the ceria lattice can instead sustain a full catalytic cycle maintaining their charge state and activity along the reaction process. The interplay between the reversible Ce$^{4+}$/Ce$^{3+}$ and Au$^{3+}$/Au$^+$ redox couples underpins the high catalytic activity of dispersed Au atoms into the ceria substrate. Ab-initio surface thermodynamics is used to investigate the stability of different solid solutions and to predict more reactive catalysts.