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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 RuO 2  powder was calcined prior reaction to ensure the formation of completely oxidized RuO 2 . This pre-calcined RuO 2  is initially inactive in CO oxidation regardless of the CO/O 2  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 CO 2  yield occur under O 2 -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 RuO 2 . Furthermore, the state of the catalyst after activation in O 2 -rich feed conditions differs dramatically from the state after activation in CO-rich feed conditions. Firstly, the catalyst activated in an O 2 -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 RuO 2  crystals become roughened upon activation in the CO-rich feed, they become facetted under O 2  rich activation conditions. Therefore, we conclude that at least two different active surface states on the bulk RuO 2  catalyst exist.

Oscillatory Behavior in the CO-oxidation over Bulk Ruthenium Dioxide — the Effect of the CO/O 2 Ratio