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.

Platinum is the most versatile element in catalysis, but it is rare and its high price limits large-scale applications, for example in fuel-cell technology. Still, conventional catalysts use only a small fraction of the Pt content, that is, those atoms located at the catalyst's surface. To maximize the noble-metal efficiency, the precious metal should be atomically dispersed and exclusively located within the outermost surface layer of the material. Such atomically dispersed Pt surface species can indeed be prepared with exceptionally high stability. Using DFT calculations we identify a specific structural element, a ceria "nanopocket", which binds Pt(2+) so strongly that it withstands sintering and bulk diffusion. On model catalysts we experimentally confirm the theoretically predicted stability, and on real Pt-CeO2 nanocomposites showing high Pt efficiency in fuel-cell catalysis we also identify these anchoring sites.

Changes of stoichiometry in reducible oxides are inevitably accompanied by changes of the oxide structure. We study the relationship between the stoichiometry and the structure in thin epitaxial films of reduced ceria, CeO x , 1.5 ≤ x ≤ 2, prepared via an interface reaction between a thin ceria film on Cu(111) and a Ce metal deposit. We show that the transition between the limiting stoichiometries CeO 2 and Ce 2 O 3 is realized by equilibration of mobile oxygen vacancies near the surface of the film, while the fluorite lattice of cerium atoms remains unchanged during the process. We identify two surface reconstructions representing distinct oxygen vacancy ordering during the transition, a (√7 × √7)R19.1°reconstruction representing a bulk termination of the ι-Ce 7 O 12 and a (3 × 3) reconstruction representing a bulk termination of CeO 1.67 . Due to the special property to yield ordered phases of reduced ceria the interface reaction between Ce and thin film ceria represents a unique tool for oxygen vacancy engineering. The perspective applications include advanced model catalyst studies with both the concentration and the coordination of oxygen vacancies precisely under control. ■ INTRODUCTIONReducible oxides play an important role in heterogeneous catalysis. 1−7 Due to their ability to store or release oxygen, reducible oxides usually act as an oxygen supply or a reducing agent during catalytic reactions. 8,9 Reactions over reducible oxides are typically accompanied by changes in the oxide stoichiometry that are often realized on complex phase diagrams 10−17 and may influence the catalytic activity through changes in local coordination, surface termination, and longrange ordering in the oxide. 18−21 Model studies isolating the changes of the oxide stoichiometry are of the utmost importance for understanding the role of stoichiometry in the reaction mechanisms over reducible oxides and for improving and developing new catalysts.The reactivity of cerium oxide-based catalysts is greatly influenced by the presence of oxygen vacancies in ceria. 22,23 The ability to adjust the concentration and the distribution of oxygen vacancies allows for the control over the reactivity and the selectivity of ceria-based catalysts. 24,25 For this reason, having experimental access to ordered phases of cerium oxide with different concentration and coordination of oxygen vacancies greatly enhances the possibilities of model catalytic studies. Several phases of ordered reduced ceria have been prepared in the past in the form of powder or single-crystal samples, 26−28 but only recently ordered reduced phases of ceria have been realized in the form of thin films on single crystalline supports. The thin film of the ι-Ce 7 O 12 phase on hexPr 2 O 3 (0001)/Si(111) substrate was obtained by Wilkens et al. via heating of the CeO 2 layer in vacuum. 29 A thin film of the c-Ce 2 O 3 phase on Cu(111) was obtained by our group via an alternative method of reducing the CeO 2 layer in an interface reaction with metallic Ce. 30 The thin film of the ...

Thin films of reduced ceria supported on metals are often applied as substrates in model studies of the chemical reactivity of ceria based catalysts. Of special interest are the properties of oxygen vacancies in ceria. However, thin films of ceria prepared by established methods become increasingly disordered as the concentration of vacancies increases. Here, we propose an alternative method for preparing ordered reduced ceria films based on the physical vapor deposition and interfacial reaction of Ce with CeO2 films. The method yields bulk-truncated layers of cubic c-Ce2O3. Compared to CeO2 these layers contain 25% of perfectly ordered vacancies in the surface and subsurface allowing well-defined measurements of the properties of ceria in the limit of extreme reduction. Experimentally, c-Ce2O3(111) layers are easily identified by a characteristic 4 × 4 surface reconstruction with respect to CeO2(111). In addition, c-Ce2O3 layers represent an experimental realization of a normally unstable polymorph of Ce2O3. During interfacial reaction, c-Ce2O3 nucleates on the interface between CeO2 buffer and Ce overlayer and is further stabilized most likely by the tetragonal distortion of the ceria layers on Cu. The characteristic kinetics of the metal-oxide interfacial reactions may represent a vehicle for making other metastable oxide structures experimentally available.

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.