Thin oxide films
Ruthenium model catalysts in the form of thin ruthenium oxide films grown on Ru(0001) were studied in the CO oxidation reaction at near-atmospheric pressures. The surfaces were prepared under vacuum conditions prior to the reactivity measurements carried out in a circulating flow reactor using gas chromatography. The films possessing oxygen in amounts equivalent to 1–4 monolayers (MLE) on Ru(0001) as determined by electron spectroscopy, exposed both the oxidic (RuO2(110)-like) and O/Ru(0001) surfaces. In addition, one-dimensional oxide structures were observed by scanning tunneling microscopy, which are tentatively assigned to the intermediate state for a crystalline ruthenium oxide thin film that covered the entire surface at higher oxygen coverages. At low temperatures studied (400–470 K), the reaction sets in only in the presence of oxidic structures, i.e. when the oxygen coverage, on average, exceeds 1 MLE. The reaction rate slightly increases with increasing the nominal film thickness up to 7 MLE, reflecting primarily the lateral growth of oxide phases. The disordered oxide films showed even higher reactivity. The results suggest that surface ordering and oxide film thickness are not critical for the superior catalytic activity of ruthenium oxides in this reaction.