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The 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.

Structure-Activity Correlations for the Oxidation of CO over Polycrystalline RuO 2 Powder Derived from Steady-State and Transient Kinetic Experiments