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

Abstract

Ruthenium Dioxide (RuO 2 ) / CO Oxidation / Kinetics / Temperature-Programmed Reduction (TPR) / Shell-Core ModelThe 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.

Catalytically active states of Ru(0001) catalyst in CO oxidation reaction

“…It was evidenced by the evolution of the Ru 3d and O 1s spectra, which was terminated at the formation of 'surface oxide'. The CO 2 yield grew steadily showing stable activity at constant temperature until levelling off at around 500 K and declining slowly above 550 K. This result is qualitative agreement with the results with powdered Ru catalyst which demonstrate that an ultra-thin oxide layer covering the metallic Ru core is the active state in CO oxidation at temperatures below 500 K [33].…”

“…The longrange ordering of the RuO 2 (110) surface is not prerequisite for catalytic function but was instrument to unravel and theoretically understand mechanisms of CO oxidation at the atomic level. As noted above, very recent study of the CO oxidation on polycrystalline powdered Ru catalyst in the temperature range 363-453 K showed that under dynamic catalytic conditions the active state are the ultra-thin Ru oxide films, whereas fully oxidized RuO 2 particles, formed at higher temperature, exhibit lower activity [33]. This deactivation is tentatively attributed to roughening and formation of inactive RuO 2 facets.…”

On the CO-Oxidation over Oxygenated Ruthenium

“…The occurrence of this reconstructed RuO 2 (100)-c(2 × 2) surface is nicely supported by our experiments: First, the irregular octagonal cross section habit of the calcined RuO 2 crystals in contrast to the obelisk-like habit shown in the literature [24] points to an equilibration of the (110) and (100) surface energies. This would be the case for a RuO 2 (100)-c(2 × 2) microfacetted (110) surface.…”

“…The same argument holds for the immediate activity of polycrystalline pre-calcined RuO 2 in Narkhede et al [24] because they used a low precalcination temperature of 573 K. Since our very long induction times are not reported in the literature we will discuss them in the context of deactivation.…”

“…Since it is obviously very ambiguous, if not impossible, to grow solely one single surface oxygen-phase on Ru(0001) except for RuO 2 at very high temperatures and oxygen dosages, it is interesting to analyze catalytic experiments with fully oxidized RuO 2 . Several papers dealt with re-oxidized Ru particles as "core-shell" catalysts with a thin RuO 2 layer on a Ru metal core [10,24,25]. These catalysts should exhibit the highest formation rates of CO 2 [10].…”