On the CO-Oxidation over Oxygenated Ruthenium

Abstract

The oxidation of carbon monoxide over polycrystalline ruthenium dioxide (RuO 2 ) powder was studied in a packed-bed reactor and by bulk and surface analytical methods. Activity data were correlated with bulk phases in an in-situ X-ray diffraction (XRD) setup at atmospheric pressure. Ruthenium dioxide was pre-calcined in pure oxygen at 1073 K. At this stage RuO 2 is completely inactive in the oxidation of CO. After a long induction period in the feed at 503 K RuO 2 becomes active with 100% conversion, while in-situ XRD reveals no changes in the RuO 2 diffraction pattern. At this stage selective roughening of apical RuO 2 facets was observed by scanning electron microscopy (SEM). Seldom also single lateral facets are roughened. EDX indicated higher oxygen content in the following order: flat lateral facets > rough lateral facets > rough apical facets. Further, experiments in the packed bed reactor indicated oscillations in the CO 2 formation rate. At even higher temperatures in reducing feed (533-543 K) the sample reduces to ruthenium metal according to XRD. The reduced particles exhibiting lower ignition temperature are very rough with cracks and deep star-shaped holes. An Arrhenius plot of the CO 2 formation rate below the ignition temperature reveals the reduced samples to be significantly more active based on mass unit and shows lower apparent activation energy than the activated oxidized sample. Micro-spot X-ray photoelectron spectroscopy (XPS) and XPS microscopy experiments were carried out on a Ru(0001) single crystal exposed to oxygen at different temperature. Although low energy electron diffraction (LEED) images show a strong 1x1 pattern, the XPS data indicated a wide lateral inhomogeneity with different degree of oxygen dissolved in the subsurface layers. All these and the literature data are discussed in the context of different active states and transport issues, and the metastable nature of a phase mixture under conditions of high catalytic activity.

Oscillatory Behavior in the CO-oxidation over Bulk Ruthenium Dioxide — the Effect of the CO/O2 Ratio

“…In-situ XRD was employed to investigate the bulk phase(s) of the catalyst under reaction conditions. However, as we have already shown in a previous publication [14], the bulk state of the catalyst does not correlate directly with the observed activity. From XRD, pure bulk RuO 2 was observed in both active and inactive states, while active catalysts may show bulk RuO 2 , Ru metal, or mixtures thereof.…”

“…As already mentioned in an earlier paper [14] commercial RuO 2 powders are typically not homogeneous single phase RuO 2 as evidenced by XRD. Furthermore, EDX generally reveals some aggregates of particles with an overall stoichiometry of RuO.…”

“…The catalytic experiments were carried out in an insitu XRD setup described in detail in a previous paper [14]. Briefly, a STOE Theta/theta X-ray diffractometer (CuKα radiation, secondary graphite monochromator, scintillation counter) is equipped with an Anton Paar XRK 900 in-situ reactor chamber.…”

“…firstly the oscillation behavior of the activated RuO 2 in an oxygen-rich feed and secondly the necessary additional induction period after exposing the oxygen-rich feed activated catalyst to a COrich feed [14], we have tried to reproduce these results in the quite different in-situ XRD reactor. Additionally we studied the undiluted catalyst under net-oxidizing and netreducing conditions at the end of this part.…”

Model Studies in Heterogeneous Catalysis

“…Maybe that these new (old) concepts could be used as a guideline to design catalysts and also to understand systems that have been controversially discussed in the literature. [117][118][119][120] One such case is the increased activity of RuA C H T U N G T R E N N U N G (0001) surfaces, claimed to be due to the formation of RuO 2 on the surface. It is, however, possible that the formation of a thin oxide film that exhibits appropriate properties promotes the reaction.…”