Covalent versus localized nature of 4f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory
We have conducted resonant angle-resolved photoemission spectroscopy of well-defined CeO2(111) and c–Ce2O3(111) model surfaces, revealing distinct f contributions in the valence band of the two compounds. In conjunction with density functional theory calculations, we show that the f contribution in CeO2 is of a covalent nature, arising from hybridization with the O 2p bands. In contrast, c–Ce2O3 exhibits an almost nondispersive f state at 1.3 eV, which is indicative of almost negligible c-f hybridization.
Creating single-atom Pt-ceria catalysts by surface step decoration
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.
Counting electrons on supported nanoparticles
Electronic interactions between metal nanoparticles and oxide supports control the functionality of nanomaterials, for example, the stability, the activity and the selectivity of catalysts[1-5]. 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.
Pt–CeOx thin film catalysts for PEMFC
Platinum is the mostly used element in catalysts for fuel cell technology, but its high price limits large-scale applications. Platinum doped cerium oxide represents an alternative solution due to very low loading, typically few micrograms per 1 cm2, at the proton exchange membrane fuel cell (PEMFC) anode. High efficiency is achieved by using magnetron sputtering deposition of cerium oxide and Pt of 30 nm thick nanoporous films on large surface carbon nanoparticle substrates. Thin film techniques permits to grow the catalyst film characterized by highly dispersed platinum, mostly in ionic Pt2+ state. Such dispersed Pt species show high activity and stability. These new materials may help to substantially reduce the demand for expensive noble-metals in catalytic applications. We measured Pt–CeOx thin film anode catalyst activity in a hydrogen PEMFC and compared it with performance of a standard reference cell. Photoelectron spectroscopy was used to investigate chemical composition of Pt–CeOx induced by the catalyst interaction with hydrogen. Nanostructured character of the catalyst was confirmed by electron microscopy.
Ordered Phases of Reduced Ceria As Epitaxial Films on Cu(111)
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, CeOx, 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 CeO2 and Ce2O3 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-Ce7O12 and a (3 × 3) reconstruction representing a bulk termination of CeO1.67. 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.
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Epitaxial Cubic Ce2O3 Films via Ce–CeO2 Interfacial Reaction
Vitalii Stetsovych, Federico Pagliuca, Filip Dvořák, Tomáš Duchoň, Mykhailo Vorokhta, Marie Aulická, Jan Lachnitt, Stefan Schernich, Iva Matolínová, Kateřina Veltruská, Tomáš Skála, Daniel Mazur, Josef Mysliveček, Jörg Libuda, and Vladimír Matolín
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.