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

Abstract

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%)/Cu 3Mg 3Al 2O x , (ii) Au(0.6 wt%)/Mg 3AlO x , (iii) Au(2.9 wt%)/CeO 2, and (iv) a copper-doped analog, Au(0.6 wt%)/CuO–CeO 2 (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/Cu 3Mg 3Al 2O x showed a small increase of conversion with time on stream accompanied by a slight reduction of the gold component. With Au/Mg 3AlO x and Au/CuO–CeO 2, 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/CeO 2 with ongoing reduction. Results obtained from factor analysis indicate that the decrease in activity of the Au/CeO 2 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 Au 0 species. This finding corroborates the importance of recording XANES spectra for structural identification simultaneously with online determination of the catalytic performance.