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
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
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)
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