Rapid monitoring of the nature and interconversion of supported catalyst phases and of their influence upon performance: CO oxidation to CO2 by ?-Al2O3 supported Rh catalysts

Newton, M.A., Dent, A.J., Diaz-Moreno, S., Fiddy, S.G., Jyoti, B. and Evans, J. (2006) Rapid monitoring of the nature and interconversion of supported catalyst phases and of their influence upon performance: CO oxidation to CO2 by ?-Al2O3 supported Rh catalysts. Chemistry - A European Journal, 12 (7), 1975-1985. (doi:10.1002/chem.200500644).

Abstract

Spatially and temporally resolved energy dispersive EXAFS (EDE) has been utilised in situ to study supported Rh nanoparticles during CO oxidation by O2 under plug-flow conditions. Three distinct phases of Rh supported upon Al2O3 were identified using EDE at the Rh K edge during CO oxidation. Their presence and interconversion are related to the efficiency of the catalysts in oxidising CO to CO2. A metallic phase is only found at higher temperatures (>450K) and CO fractions (CO/O2 > 1); an oxidic phase resembling Rh2O3 dominates the active catalyst under oxygen-rich conditions. Below ~573K, and in CO rich environments, high proportions of isolated RhI(CO)2 species are found to co-exist with metallic Rh nanoparticles. Alongside these discrete situations a large proportion of the active phase space comprises small Rh cores surrounded by layers of active oxide. Confinement of Rh to nanoscale domains induces structural lability that influences catalytic behaviour. For CO oxidation over Rh/Al2O3 there are two redox phase equilibria alongside the chemistry of CO and O adsorbed upon extended Rh surfaces.

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