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.

Water dissociation is crucial in many catalytic reactions on oxide-supported transition-metal catalysts. Supported by experimental and density-functional theory results, the effect of the support on OH bond cleavage activity is elucidated for nickel/ceria systems. Ambient-pressure O 1s photoemission spectra at low Ni loadings on CeO2 (111) reveal a substantially larger amount of OH groups as compared to the bare support. Computed activation energy barriers for water dissociation show an enhanced reactivity of Ni adatoms on CeO2 (111) compared with pyramidal Ni4 particles with one Ni atom not in contact with the support, and extended Ni(111) surfaces. At the origin of this support effect is the ability of ceria to stabilize oxidized Ni(2+) species by accommodating electrons in localized f-states. The fast dissociation of water on Ni/CeO2 has a dramatic effect on the activity and stability of this system as a catalyst for the water-gas shift and ethanol steam reforming reactions.

The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH → CHOH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.

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.

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO ) surfaces indicate that metal-oxide interactions can play a very important role for the activation of methane and its reforming with CO at relatively low temperatures (600-700 K). Among the systems examined, Co/CeO (111) exhibits the best performance and Cu/CeO (111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO (111) at temperatures as low as 300 K-generating CH and CO species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co /CeO (111), and to only 0.05 eV on Co /CeO (111). At 700 K, under methane dry reforming conditions, CO dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CH formed on the Co /CeO (111) catalyst recombines to yield ethane or ethylene.

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 ...

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