In Situ EPR Study of the Redox Properties of CuO–CeO2 Catalysts for Preferential CO Oxidation (PROX)

Understanding the redox properties of metal oxide based catalysts is a major task in catalysis research. In situ electron paramagnetic resonance (EPR) spectroscopy is capable of monitoring the change of metal ion valences and formation of active sites during redox reactions, allowing for the identification of ongoing redox pathways. Here in situ EPR spectroscopy combined with online gas analysis, supported by ex situ X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), temporal analysis of product (TAP), and mass spectrometry (MS) studies, was utilized to study the redox behavior of CuO–CeO 2 catalysts under PROX conditions (preferential oxidation of carbon monoxide in hydrogen). Two redox mechanisms are revealed: (i) a synergetic mechanism that involves the redox pair Ce 4+ /Ce 3+ during oxidation of Cu 0 /Cu + species to Cu 2+ and (ii) a direct mechanism that bypasses the redox pair Ce 4+ /Ce 3+ . In addition, EPR experiments with isotopically enriched 17 O 2 established the synergetic mechanism as the major redox reaction pathway. The results emphasize the importance of the interactions between Cu and Ce atoms for catalyst performance. With the guidance of these results, an optimized CuO–CeO 2 catalyst could be designed. A rather wide temperature operation window of 11 K (from 377 to 388 K), with 99% conversion efficiency and 99% selectivity, was achieved for the preferential oxidation of CO in a H 2 feed.