Gold supported on Cu–Mg–Al and Cu–Ce mixed oxides: An in situ XANES study on the state of Au during aerobic alcohol oxidation

Liquid-phase in situ XANES experiments were used to elucidate the oxidation state of differently supported Au catalysts during the selective oxidation of 1-phenylethanol with molecular oxygen. The catalysts investigated included (i) Au(0.6 wt%)/Cu3Mg3Al2Ox, (ii) Au(0.6 wt%)/Mg3AlOx, (iii) Au(2.9 wt%)/CeO2, and (iv) a copper-doped analog, Au(0.6 wt%)/CuO–CeO2 (20 wt% CuO). The XANES and EXAFS data were collected mainly in the fluorescence mode. Simultaneous determination of the structure (XANES) and the activity of the catalysts in the continuous-flow microreactor was achieved by following the oxidation state using XANES and the characteristic carbonyl vibration of the desired reaction product acetophenone using an IR spectrometer equipped with a transmission cell. Au/Cu3Mg3Al2Ox showed a small increase of conversion with time on stream accompanied by a slight reduction of the gold component. With Au/Mg3AlOx and Au/CuO–CeO2, a stronger reduction accompanied by increased catalytic activity was observed. The results of these mixed oxide-supported Au catalysts demonstrate that metallic gold is the main active species in catalytic aerobic alcohol oxidation. In contrast, a decrease in the product formation rate was observed for Au/CeO2 with ongoing reduction. Results obtained from factor analysis indicate that the decrease in activity of the Au/CeO2 with time on stream does not correlate with the much faster reduction of the catalyst and thus cannot be attributed to the increasing fraction of Au0 species. This finding corroborates the importance of recording XANES spectra for structural identification simultaneously with online determination of the catalytic performance.