3 research outputs found

    Probing the Activity of Different Oxygen Species in the CO Oxidation over RuO<sub>2</sub>(110) by Combining Transient Reflection–Absorption Infrared Spectroscopy with Kinetic Monte Carlo Simulations

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    Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to gain a wealth of molecular information on the kinetics of catalytic surface reactions as exemplified by the CO oxidation reaction over RuO<sub>2</sub>(110). This approach surpasses the common problem that in the steady-state reactions, the prevailing species detectable by in operando surface-sensitive spectroscopy are frequently spectator species, thereby obscuring the reactive surface species. Our experiment is sensitive to the relative activity of different oxygen species by saturating the surface with loosely bound oxygen, leaving only single vacancies where CO can adsorb and recombine with oxygen. With in situ reflection–absorption infrared spectroscopy (RAIRS) in combination with ab initio based KMC simulations, we follow the time evolution toward steady state (transient experiment). In this way, we are able to resolve a long-standing controversy about the active oxygen species in the CO oxidation over RuO<sub>2</sub>(110), evidencing that both surface O species (O<sub>br</sub> and O<sub>ot</sub>) are equally active, although their adsorption energies differ by more than 150 kJ/mol

    Oxidation-Induced Dispersion of Gold on Ru(0001): A Scanning Tunneling Microscopy Study

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    With scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) we studied the redox properties of Au islands supported on Ru(0001) as a function of the island thickness. Both the size and the height of Au islands on Ru(0001) can be controlled by the density of the oxygen precoverage on ruthenium and the sample temperature during the deposition of gold. The oxidation of the Au islands at 300 K was accomplished by exposing atomic oxygen produced from a thermal gas cracker. Regardless of the lateral size of the three monolayer (ML) thick Au islands, the oxidation leads to a fragmentation into a number of small particles (3–5 nm) whose arrangement reflects the shape of the former intact Au islands. This oxygen-induced dispersion of Au on Ru(0001) is explained by a shoveling process. Quite in contrast, no fragmentation of the 4–5 ML thick Au islands into smaller entities is observed. Rather, the entire Au island transforms into one big particle. From Au 4f core level spectroscopy we provide evidence that the nanoparticles consist of Au oxide and metallic Au. The Au oxide/Au particles can be reduced by thermal annealing to 670 K under vacuum or by chemical reduction via CO exposure at 670 K, forming again extended Au islands. However, reduction of Au oxide/Au metal particles by CO exposure at room temperature retains the high dispersion of the prior formed nanoparticles

    Versatile Model System for Studying Processes Ranging from Heterogeneous to Photocatalysis: Epitaxial RuO<sub>2</sub>(110) on TiO<sub>2</sub>(110)

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    The binary model system RuO<sub>2</sub>/TiO<sub>2</sub>(110) can be prepared with single crystallinity and excellent control of the morphology of the RuO<sub>2</sub>(110) nanoislands. The interface of RuO<sub>2</sub>/TiO<sub>2</sub>(110) is structurally well-defined since RuO<sub>2</sub> grows with the same lattice constants as TiO<sub>2</sub>(110). The actual growth of RuO<sub>2</sub> on TiO<sub>2</sub>(110) single crystals starts from square-shaped 3–4 ML thick RuO<sub>2</sub> islands with narrow size and thickness distributions. After TiO<sub>2</sub>(110) is completely covered by RuO<sub>2</sub>, the further growth proceeds via a step flow mechanism, forming very large and flat RuO<sub>2</sub>(110) terraces with well-defined thickness. Both the flat RuO<sub>2</sub>(110) films and RuO<sub>2</sub>(110) nanoislands are very reactive toward CO oxidation, and the RuO<sub>2</sub>(110) nanoislands are robust in the redox reactions, i.e., easily recovering their morphology after reoxidation from the reduced state. The RuO<sub>2</sub>/TiO<sub>2</sub>(110) heterojunction forms a Schottky barrier of 1.4 eV which is important for photocatalysis
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