Structure-Activity Correlations for the Oxidation of CO over Polycrystalline RuO2 Powder Derived from Steady-State and Transient Kinetic Experiments

Abstract

Abstract 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 RuO2 in flowing oxygen resulted in purified bulk RuO2, 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 RuO2/Ru shell-core particles with increasing RuO2 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 RuO2 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 RuO2 films are in good agreement with TOFs reported for studies on Ru single crystal surfaces and with supported Ru catalysts. Only for RuO2 films thicker than 1 nm (T ox ≥ 473 K) and for fully oxidized RuO2 deactivation was observed, presumably due to the formation of inactive RuO2 surfaces such as the RuO2(100)-c(2×2) facet. Moreover, it was demonstrated that the presence of moisture in the reactant feed inhibits the oxidation of CO completely.