Nature of the active sites for the total oxidation of toluene by CuOCeO2/Al2O3

Abstract The binary metal oxide, CuO CeO 2 /γ-Al 2 O 3 , has been compared with the single oxide components CuO/γ-Al 2 O 3 and CeO 2 /γ-Al 2 O 3 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 CuO CeO 2 /γ-Al 2 O 3 catalyst compared to CuO/γ-Al 2 O 3 is attributed to the formation of a Ce 1− x Cu x O 2− x solid solution with a crystallite size of 6 nm. Within this phase, oxidation of toluene occurs at Cu 2+ sites and reduction of oxygen at Ce 3+ sites. Similar to Wacker chemistry, two redox couples, Ce 4+ /Ce 3+ and Cu 2+ /Cu 1+ , are operational. Apart from the solid solution, a copper oxide phase with a crystallite size of 100 nm shows significantly lower catalytic activity. X-ray absorption near-edge structure (XANES) experiments at the copper and cerium edge indicate that Ce 4+ is reduced at lower temperature than Cu 2+ . Upon re-oxidation with CO 2 or H 2 O, Ce 3+ is partly re-oxidized, while Cu 0 is not. This explains an activity increase in the CuO CeO 2 /γ-Al 2 O 3 in the presence of H 2 O or CO 2 . CuO/γ-Al 2 O 3 shows loss of activity in the presence of H 2 O as site blocking is not compensated by an increase in the re-oxidation rate.

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