Using a variety of dedicated surface sensitive techniques, we studied the interaction of hydrogen with bare and adsorbate modified RuO 2 (110) surfaces on the atomic scale. Hydrogen interacts strongly with the undercoordinated O atoms, thereby forming hydroxyl groups and passivating available oxygen species on the oxide surface, for instance, for the catalytic CO oxidation reaction. Temperature programmed reaction and desorption elucidate the complex reaction behavior of hydrogen with O precovered RuO 2 (110), including the hydrogen transfer reaction between the different kinds of undercoordinated surface oxygen atoms. Hydroxyl, water species, and hydrogen transfer are identified with high-resolution O1s core level spectroscopy by comparison with density functional theory (DFT) calculated O1s core level shifts. DFT calculations provide adsorption energies, atomic geometries, as well as diffusion barriers of H atoms on the RuO 2 (110) surface.

Selective substitution: In the oxidation of HCl with oxygen to give Cl2 and water, RuO2(110) serves as a stable, active model catalyst for the Sumitomo process (see picture; Ru in red and blue). The stability of the catalyst is related to the selective replacement of undercoordinated bridging surface O atoms (Obr) by Cl atoms (Clbr). The chlorination of RuO2(110) is self‐limiting, in that chlorine incorporation terminates when all bridging O atoms are replaced.

It is shown that both the materials and the pressure gaps can be bridged for ruthenium in heterogeneous oxidation catalysis using the oxidation of carbon monoxide as a model reaction. Polycrystalline catalysts, such as supported Ru catalysts and micrometer-sized Ru powder, were compared to single-crystalline ultrathin RuO 2 films serving as model catalysts. The microscopic reaction steps on RuO 2 were identified by a combined experimental and theoretical approach applying density functional theory. Steady-state CO oxidation and transient kinetic experiments such as temperature-programmed desorption were performed with polycrystalline catalysts and single-crystal surfaces and analyzed on the basis of a microkinetic model. Infrared spectroscopy turned out to be a valuable tool allowing us to identify adsorption sites and adsorbed species under reaction conditions both for practical catalysts and for the model catalyst over a wide temperature and pressure range. The close interplay of the experimental and theoretical surface science approach with the kinetic and spectroscopic research on catalysts applied in plug-flow reactors provides a synergistic strategy for improving the performance of Ru-based catalysts. The most active and stable state was identified with an ultrathin RuO 2 shell coating a metallic Ru core. The microscopic processes causing the structural deactivation of Ru-based catalysts while oxidizing CO have been identified.

RuO2(110) exposes two kinds of active surface species (acidic and basic centers) that govern the interaction of the gas phase in contact with the catalyst's surface. Here we will elucidate the cooperative interplay of these two active surface sites for a simple model reaction, namely the water formation over RuO2 catalysts when supplying hydrogen and oxygen from the gas phase. The bridging O atoms harvest the hydrogen from the gas phase, while the on-top O atoms pick up those adsorbed hydrogen atoms from the bridging O atoms to form water. This mechanism of hydrogen transfer is mediated by a strong hydrogen bond. Hydrogen transfer is expected to play a vital role for the whole class of catalyzed hydrogenation and dehydrogenation reactions of hydrocarbons over RuO2.

High-resolution core-level shift spectroscopy and temperature-programmed reaction experiments together with density functional theory calculations reveal that the oxidation of HCl with oxygen producing Cl 2 and water proceeds on the chlorine-stabilized RuO 2 (110) surface via a one-dimensional Langmuir-Hinshelwood mechanism. The recombination of two adjacent chlorine atoms on the catalyst's surface constitutes the ratedetermining step in this novel Deacon-like process.

The reduction mechanism of the RuO(2)(110) surface by molecular hydrogen exposure is unraveled to an unprecedented level by a combination of temperature programmed reaction, scanning tunneling microscopy, high-resolution core level shift spectroscopy, and density functional theory calculations. We demonstrate that even at room temperature hydrogen exposure to the RuO(2)(110) surface leads to the formation of water. In a two-step process, hydrogen saturates first the bridging oxygen atoms to form (O(br)-H) species and subsequently part of these O(br)-H groups move to the undercoordinated Ru atoms where they form adsorbed water. This latter process is driven by thermodynamics leaving vacancies in the bridging O rows.

The science and technology of catalysis are of central practical importance. About 80 % of all industrial chemicals are manufactured by utilizing (heterogeneous) catalysis. Besides activity and selectivity, catalyst deactivation during use is a key issue in practical catalysis. "The importance of understanding and being able to predict loss of activity during catalyst usage must not be under-estimated" [1] since replacement of a catalyst means high operational costs. Industrially used catalysts are, however, far too complex to allow for a microscopic understanding of why a catalyst deactivates. This knowledge calls rather for the use of model catalysts (such as single-crystalline surfaces) and their investigations under well-controlled ultrahigh vacuum conditions. The trade off for this so-called surface-science approach [2] is the introduction of a pressure and a materials gap by which catalytic properties determined under well defined conditions may not be extrapolated to those at realistic reaction conditions.[3]For a ruthenium-based catalyst, activity loss was reported for the CO oxidation reaction. In particular, under oxidizing reaction conditions the activity of supported ruthenium catalysts declines substantially. [4,5] This finding has been quite puzzling as recent investigations clearly indicate that RuO 2 is much more active than ruthenium in the oxidation of CO.[6] Since the pressure and materials gap for the CO oxidation over ruthenium are considered to be bridged [7] we can utilize the surface-science approach to clarify the microscopic processes determining the structural deactivation of ruthenium-based catalysts and how this atomic-scale knowledge is used to optimize the performance of practical ruthenium catalysts.We concentrate herein mainly on polycrystalline RuO 2 powder which is calcined at 573 K, resulting in a specific surface area of 0.9 m 2 g À1 . Complementary data of supported ruthenium catalysts are provided in the Supporting Information. The mean diameter of the particles in the RuO 2 powder is about 1 mm. Therefore RuO 2 powder represents a natural link between single-crystal ruthenium model catalysts and ruthenium catalysts supported on SiO 2 or MgO with an active surface area of 10 m 2 g À1 .[8] Applied partial pressures of CO and O 2 are in the range of 5-35 mbar.Figure 1 a displays the conversion of CO over oxidized RuO 2 polycrystalline powder as a function of time on stream, using a stepwise temperature variation and a CO/O 2 feed ratio of 1:2. During the first temperature cycle each temperature jump is accompanied by a rapid increase of the CO conversion followed by a transient decrease of the CO conversion whose steady state is not reached within 1 h. This deactivation process occurs faster at higher temperatures, whereas the extent of deactivation increases with higher concentrations of O 2 (see Supporting Information). For all investigated CO/O 2 feed stocks it is found that the conversion

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