component more critical. [ 2 ] Nevertheless, both studies revealed that ceria favors the production of methanol and olefi ns, owing to a combination of factors, including (i) sites that facilitate the adsorption of reactants (H 2 , CO 2 , and C 3 H 4 ), (ii) chemical steps that lead to the scission of adsorbate bonds (H H, C O, and C C), (iii) a hydrogenrich reducing environment, and (iv) high concentration (pressure) of reactants. Hence, we are motivated by these results to probe further the reducibility and active structure of ceria using the key reducing reactant H 2 in this process, targeting an understanding of the mechanism and the identifi cation of the essential surface states at the source of breaking the H H bond, a key step common to all hydrogenation reactions.The reactivity of ceria is intimately tied to the presence of oxygen vacancies, [ 3 ] and the nature of their formation and ordering remains a challenging research problem typically dealt with using density functional theory (DFT) [ 4 ] or static experimental measurements (e.g., by scanning probe microscopy). [ 5,6 ] Our experimental approach, utilizing state-of-the-art in situ low-energy electron microscopy and microdiffraction (LEEM/µLEED) coupled to spatially and chemically resolved X-ray spectroscopy, specifi cally elucidates the dynamic structural-chemical changes during the hydrogenation process. This approach represents a key experimental step toward correlating and understanding of catalyst structure and its function. Results and DiscussionFor our investigation, we choose a model catalyst composed of crystalline ceria in the form of microparticles supported on Ru(0001) ( Figure 1 a), a system we have characterized extensively including its growth, chemical reactivity, and surface/ interfacial structure. [ 7 , 8b ] This model catalyst is a polymorph, in between that of a single crystal and a thin fi lm surface, both of which are typically used to model the chemical behavior of ceria. [ 9 ] The advantage of such a system is that it provides a structurally well-defi ned surface and subsurface of the predominant and energetically most stable CeO 2 (111) orientation suitable for in situ surface science methods. When combined with time-resolved microscopy, this approach also enables the identifi cation of potential cooperative effects between the oxide and the metal support. Figure 1 a shows a typical LEEM image of the The interaction of molecular hydrogen with ceria is of important relevance for heterogeneous catalysis related to green chemistry and renewable energy. Here, the complex structural transformations of a well-defi ned cerium oxide model catalyst are followed in situ and in real time when exposed to a reactive H 2 environment. By using electron spectromicroscopy and diffraction with chemical and structural sensitivities, it is demonstrated that the transition from CeO 2 to crystalline Ce 2 O 3 occurs through a mixture of transient, coexisting phases on the nanoscale. The fi ndings establish a clear relationship between structure ...

Cerium oxide is often applied in today's catalysts due to its remarkable oxygen storage capacity. The changes in stoichiometry during reaction are linked to structural modifications, which in turn affect its catalytic activity. We present a real-time in situ study of the structural transformations of cerium oxide particles on ruthenium(0001) at high temperatures of 700 °C in ultra-high vacuum. Our results demonstrate that the reduction from CeO to cubic CeO proceeds via ordered intermediary phases. The final reduction step from cubic to hexagonal CeO is accompanied by a lattice expansion, the formation of two new surface terminations, a partial dissolution of the cerium oxide particles, and a massive mass transport of cerium from the particles to the substrate. The conclusions allow for new insights into the structure, stability, and dynamics of cerium oxide nanoparticles in strongly reducing environments.

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The growth, morphology, structure, and stoichiometry of ultrathin praseodymium oxide layers on Ru(0001) were studied using low-energy electron microscopy and diffraction, photoemission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. At a growth temperature of 760 °C, the oxide is shown to form hexagonally close-packed (A-type) PrO(0001) islands that are up to 3 nm high. Depending on the local substrate step density, the islands either adopt a triangular shape on sufficiently large terraces or acquire a trapezoidal shape with the long base aligned along the substrate steps.

Electron scattering by oxygen monolayers on the Ru(0 0 0 1) surface is studied both experimentally and theoretically. Sharp transmission resonances at low energies are revealed and established to originate from critical points of a special kind in the complex band structure of the substrate. Electron reflection from the clean and oxidized Ru(0 0 0 1) is measured for kinetic energies up to 40 eV at normal incidence for oxygen coverages of 1/4, 1/2, 3/4, and one monolayer. The reflection spectra R(E) are analyzed using a Bloch-waves based ab initio scattering theory. In addition to the substrate-induced resonances the reconstructed (2 × 1) and (2 × 2) surfaces show surface resonances due to pre-emergent secondary diffraction beams. The R(E) spectra are shown to give unambiguous evidence of the hcp stacking of the oxygen layer.