Catalytic Activity, Surface Redox Properties, and Structural Evolution during the Thermal Processing of Chromium-Promoted Ceria Oxidation Catalysts

Chromium-promoted ceria catalyst materials have been prepared by three routes: impregnation of ceria gel using an aqueous solution of CrO 3 (Cr(VI)/CeO 2 -imp), impregnation of ceria gel using an aqueous solution of chromium(III) nitrate (Cr(III)/CeO 2 -imp), and coprecipitation from an aqueous solution containing both chromium(III) nitrate and cerium(III) nitrate (Cr(III)/CeO 2 -cop). Optimum catalytic performance toward the oxidation of carbon monoxide and propane is achieved after thermal activation in air in the temperature range 573−673 K. After such treatment the catalytic activity is similar, irrespective of whether the source of the chromium promoter is in oxidation state of 3+ or 6+. Thermal treatment at more elevated temperatures results in a progressive lowering of activity, although 100% conversion of propane is still achieved even after calcination at 1273 K. Examination of the nature of the materials by powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), Fourier transform Raman (FT-Raman), electron paramagnetic resonance (EPR), and X-ray near-edge absorption (XANES) spectroscopies show that the nature of the catalysts formed by thermal processing of the initial materials formed by all three routes in the temperature range 573−673 K is essentially the same. The active catalyst appears to be a composite oxide material comprising microparticulate mixed-valence chromium oxide, possibly Cr 5 O 12, and Cr(III) oxide-like clusters dispersed on particulate ceria. Thermal processing at ≥873 K results in an increase of particle size and phase separation of Cr 2 O 3 . In situ EPR studies demonstrate that the mixed-valence component of the catalyst undergoes facile reduction upon exposure to carbon monoxide at 473−573 K and is reoxidized upon exposure to oxygen at the same temperature.