Preferential CO oxidation (PROX) on LaCoO₃–based catalysts: Effect of cobalt oxidation state on selectivity

Abstract

The perovskite LaCoO₃ (LCO) was used as catalyst for preferential oxidation of CO (PROX). LCO was synthesized via the modified Pechini method and characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and CO– and H₂– temperature programmed reduction (TPR). Different reductive and oxidative pretreatments were applied to systematically vary the Co oxidation state in order to examine its effect on catalytic performance and to single out active site requirements. Upon reduction at increasing temperature, LaCoO₃ transformed to brownmillerite-type La₂Co₂O₅, exsolved Co⁰ nanoparticles supported on La₂O₃ and, upon reoxidation, to Co₃O₄/La₂O₃. The Co oxidation state of the various catalysts correlated with their CO₂ selectivity: LCO containing only Co³⁺ exhibited 100 % CO₂ selectivity in a wide temperature window, whereas La₂Co₂O₅, Co/La₂O₃ and Co₃O₄/La₂O₃ had markedly lower selectivity. It is suggested that Co³⁺ is crucial and that the strong resistivity of LaCoO₃ towards reduction is responsible for the high and stable CO₂ selectivity over a temperature range of 100 °C–220 °C. Higher oxygen concentration further broadens the PROX window.