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

Abstract

CO oxidation over polycrystalline ruthenium dioxide was monitored in an in-situ XRD setup. The evolution of the bulk state of the catalyst was followed by in-situ XRD during reaction, while the surface morphology and chemical state before and after reaction were investigated by HRSEM and EDX. The commercial RuO2 powder was calcined prior reaction to ensure the formation of completely oxidized RuO2. This precalcined RuO2 is initially inactive in CO oxidation regardless of the CO/O2 feed ratio and requires an induction period, the length of which strongly depends whether the catalyst is diluted with boron nitride or not. After this induction period oscillations in the CO2 yield occur under O2-rich conditions only. These oscillations exhibit two time constants for the diluted catalyst, while the low frequency oscillations were not observed in the case of undiluted RuO2. Furthermore, the state of the catalyst after activation in O2-rich feed conditions differs dramatically from the state after activation in CO-rich feed conditions. Firstly, the catalyst activated in an O2-rich atmosphere remains inactive under CO-rich conditions in contrast to the catalyst activated in CO-rich conditions which is afterwards active under all feed ratios examined. Secondly, the surface morphology of the catalyst is quite different. While the apical surfaces of the RuO2 crystals become roughened upon activation in the CO-rich feed, they become facetted under O2 rich activation conditions. Therefore, we conclude that at least two different active surface states on the bulk RuO2 catalyst exist.

Heterogeneous Catalysis

“…This state can be interpreted as a surface oxide and/or a defect state in an oxide film 156b. 264b It clarifies in part the contradictory perspectives334d on the nature of the active phase and on the oscillations of this reaction336 that have been found. Clarification is also provided for the result that in the oxidation of methanol over Ru systems the most active state is a surface oxide that, finally adjusting to stationary conditions,220, 337 forms either from a metal or an oxide precursor.…”

Exploring Pretreatment–Morphology Relationships: Ab Initio Wulff Construction for RuO2 Nanoparticles under Oxidising Conditions

“…The objectives of our study are the different RuO 2 particle shapes obtained after different calcination procedures as well as the different initial activities under net oxidising feed conditions, under which the RuO 2 bulk structure was shown to be intact 15–17. Narkhede et al 15.…”

“…and Rosenthal et al 16. 17 They have been related to different calcination temperatures during catalyst preparation, which yields distinctly different crystal habits 15. 16 Understanding these reactivity differences in terms of the different facets and surface terminations exposed then offers an interesting new perspective on the controversially discussed active phases of Ru catalysts for this reaction.…”

Functional surfaces in heterogeneous catalysis: A short review

“…This gives evidence to the fact that a completely oxygen covered ideal RuO 2 (110) surface is in fact catalytically completely inactive. Indeed recent experiments on the CO oxidation on crystalline RuO 2 reveal a very long induction period of hours until the reaction starts 25, 26. Additionally, the catalyst crystallites change their habit after this induction period; i.e .…”