The high performance of ceria (CeO ₂ ) as an oxygen buffer and active support for noble metals in catalysis relies on an efficient supply of lattice oxygen at reaction sites governed by oxygen vacancy formation. We used high-resolution scanning tunneling microscopy and density functional calculations to unravel the local structure of surface and subsurface oxygen vacancies on the (111) surface. Electrons left behind by released oxygen localize on cerium ions. Clusters of more than two vacancies exclusively expose these reduced cerium ions, primarily by including subsurface vacancies, which therefore play a crucial role in the process of vacancy cluster formation. These results have implications for our understanding of oxidation processes on reducible rare-earth oxides.

Addressing a Burning Issue: Complete combustion of methane is required in order to avoid the unproductive emission of this greenhouse gas into the atmosphere. Palladium catalysts can help to promote complete combustion, but high-temperature operating conditions also promote aggregation of catalyst particles (“sintering”) that lowers their surface area and overall activity. Cargnello et al. (p. 713 ; see the Perspective by Farrauto ) report that cerium oxide–coated Pd catalyst particles could be fully dispersed on an alumina surface prepared with a hydrophobic coating. This treatment resisted Pd sintering up to temperatures of 800°C, and also enabled complete combustion of methane to occur at temperatures as low as 400°C.