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.

The oxidation of CO over Ru/MgO and Ru/SiO2 catalysts was used as a simple model reaction to derive turnover frequencies at atmospheric pressure, which were observed to agree with kinetic data obtained under high-vacuum conditions with supported ruthenium catalysts and the RuO2(110) single-crystal surface. Thus, it was possible to bridge both the pressure and the materials gap. However, a partial deactivation was observed initially, which was identified as an activated process, both under net reducing and net oxidizing conditions. Temperature-programmed reduction (TPR) experiments were performed subsequently in the same reactor, to monitor the degree of oxidation, as a function of the reaction temperature and the CO/O2 reactant feed ratio. Using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, the structural changes of the ruthenium catalysts during the oxidation of CO were confirmed, under relevant reaction conditions. Under net reducing conditions, only domains of RuO2 seem to exist on the metallic ruthenium particles, whereas, under net oxidizing conditions, the ruthenium particles were fully oxidized to bulk RuO2 particles, which may expose less-active facets, such as the RuO2(100)−c(2 × 2) surface.

Ruthenium Dioxide (RuO 2 ) / CO Oxidation / Kinetics / Temperature-Programmed Reduction (TPR) / Shell-Core ModelThe oxidation of carbon monoxide was studied at atmospheric pressure in a plug-flow reactor over polycrystalline ruthenium dioxide powder in the temperature range from 363 to 453 K as a function of the pretreatment. Calcining RuO 2 in flowing oxygen resulted in purified bulk RuO 2 , whereas reduction in hydrogen led to bulk Ru metal, which was partially oxidized again in flowing oxygen at increasing temperatures (T ox ) up to 573 K to obtain RuO 2 /Ru shell-core particles with increasing RuO 2 shell thickness. Using the TPR technique subsequent to steady-state CO oxidation to monitor the degree of oxidation, the most active and stable state of the unsupported ruthenium catalysts was identified as an ultra-thin RuO 2 layer covering a metallic Ru core in agreement with the shell-core model established for supported Ru catalysts. Steady-state turnover frequencies (TOFs) obtained with the ultra-thin RuO 2 films are in good agreement with TOFs reported for studies on Ru single crystal surfaces and with supported Ru catalysts. Only for RuO 2 films thicker than 1 nm (T ox ≥ 473 K) and for fully oxidized RuO 2 deactivation was observed, presumably due to the formation of inactive RuO 2 surfaces such as the RuO 2 (100)-c(2 × 2) facet. Moreover, it was demonstrated that the presence of moisture in the reactant feed inhibits the oxidation of CO completely.

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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