Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts. Such interactions involve electron transfer across the metal/support interface. In this work we quantify this charge transfer on a well-defined platinum/ceria catalyst at particle sizes relevant for heterogeneous catalysis. Combining synchrotron-radiation photoelectron spectroscopy, scanning tunnelling microscopy and density functional calculations we show that the charge transfer per Pt atom is largest for Pt particles of around 50 atoms. Here, approximately one electron is transferred per ten Pt atoms from the nanoparticle to the support. For larger particles, the charge transfer reaches its intrinsic limit set by the support. For smaller particles, charge transfer is partially suppressed by nucleation at defects. These mechanistic and quantitative insights into charge transfer will help to make better use of particle size effects and electronic metal-support interactions in metal/oxide nanomaterials.

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

Smallest metallic (blue) and ionic (grey) Pt objects on ceria are shaped and controlled by surface oxygen content.

Reactions of reduced cerium oxide CeO x with water are fundamental processes omnipresent in ceria-based catalysis. Using thin epitaxial films of ordered CeO x , we investigate the influence of oxygen vacancy concentration and coordination on the oxidation of CeO x by water. Upon changing the CeO x stoichiometry from CeO2 to Ce2O3, we observe a transition from a slow surface reaction to a productive H2-evolving CeO x oxidation with reaction yields exceeding the surface capacity and indicating the participation of bulk OH species. Both the experiments and the ab initio calculations associate the effective oxidation of highly reduced CeO x by water to the next-nearest-neighbor oxygen vacancies present in the bixbyite c-Ce2O3 phase. Next-nearest-neighbor oxygen vacancies allow for the effective incorporation of water in the bulk via formation of OH– groups. Our study illustrates that the coordination of oxygen vacancies in CeO x represents an important parameter to be considered in understanding and improving the reactivity of ceria-based catalysts.