Nature of the active sites for the total oxidation of toluene by CuO[sbnd]CeO2/Al2O3

Abstract

The binary metal oxide, CuOCeO2/γ-Al2O3, has been compared with the single oxide components CuO/γ-Al2O3 and CeO2/γ-Al2O3 for toluene total oxidation. The nature of the active sites was determined by means of several spectroscopic techniques, while the transient response technique TAP (Temporal Analysis of Products) was used to investigate the catalytic performance.The improved performance of the CuOCeO2/γ-Al2O3 catalyst compared to CuO/γ-Al2O3 is attributed to the formation of a Ce1−xCuxO2−x solid solution with a crystallite size of 6nm. Within this phase, oxidation of toluene occurs at Cu2+ sites and reduction of oxygen at Ce3+ sites. Similar to Wacker chemistry, two redox couples, Ce4+/Ce3+ and Cu2+/Cu1+, are operational. Apart from the solid solution, a copper oxide phase with a crystallite size of 100nm shows significantly lower catalytic activity.X-ray absorption near-edge structure (XANES) experiments at the copper and cerium edge indicate that Ce4+ is reduced at lower temperature than Cu2+. Upon re-oxidation with CO2 or H2O, Ce3+ is partly re-oxidized, while Cu0 is not. This explains an activity increase in the CuOCeO2/γ-Al2O3 in the presence of H2O or CO2. CuO/γ-Al2O3 shows loss of activity in the presence of H2O as site blocking is not compensated by an increase in the re-oxidation rate.