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

摘要

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%)/Cu3Mg3Al2O_x,(ii)Au(0.6 wt%)/Mg3AlO_x,(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/Cu3Mg3Al2O_x showed a small increase of conversion with time on stream accompanied by a slight reduction of the gold component.With Au/Mg3 ALO_X 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 Au~0 species.This finding corroborates the importance of recording XANES spectra for structural identification simultaneously with online determination of the catalytic performance.