Catalytic NO Oxidation Pathways and Redox Cycles on Dispersed Oxides of Rhodium and Cobalt

The elementary steps and site requirements for the oxidation of NO on Rh and Co and the oxidation state of the catalysts were probed by isotopic tracers, chemisorption methods, and kinetic measurements of the effects of the pressures of NO, O 2, and NO 2 on turnover rates. On both catalysts, NO oxidation rates were first order in NO and O 2 and were inversely proportional to NO 2 pressure, as observed on Pt and PdO. These data implied that O 2 activation on an isolated vacancy on the catalyst surfaces that were saturated with oxygen (O) was the kinetically relevant step. Quasi-equilibrated NO-NO 2 interconversion steps established the coverage of and O and the chemical potential of oxygen during the catalysis. These chemical potentials set the oxidation state of Rh and Co clusters and were described by an O 2 virtual pressure, which was determined from the formalism of non-equilibrium thermodynamics. RhO 2 and Co 3O 4 were the phases that were present during NO oxidation, which had several consequences for catalysis. Turnover rates increased with increasing cluster size because the vacancies that were needed for O 2 activation were more abundant on large oxide clusters, which delocalized electrons better than small clusters. NO oxidation turnover rates on RhO 2 and Co 3O 4 were higher than expected from the oxygen-binding energy on Rh and Co metal surfaces and from the reduction potentials of Rh 3+ and Co 2+. These NO oxidation rates were consistent with the rates on Pt and PdO when one-electron-reduction processes, which were accessible for Rh 4+ and Co 3+ but not for Pt 2+ and Pd 2+, were used to describe the reactivity of RhO 2 and Co 3O 4. One-electron redox cycles caused the 16O 2- 18O 2 exchange rates to be higher than the NO oxidation rates, in contrast with their analogous values on Pt and PdO, although O 2 activation on the vacancies limited NO oxidation and O 2 exchange on all of the catalysts. One-electron redox cycles allowed electron sharing between metal cations and a facile route to form vacancies on RhO 2 and Co 3O 4. This interpretation of the data highlighted the role of vacancies in kinetically relevant O 2-activation steps to explain the higher reactivity of larger metal and oxide clusters and to provide a common framework to describe NO oxidation and the active species on catalysts of practical interest. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.