Oxidation States of Gold in MgO-Supported Complexes and Clusters: Characterization by X-ray Absorption Spectroscopy and Temperature-Programmed Oxidation and Reduction

Abstract

X-ray absorption near edge structure (XANES) spectroscopy, temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO) were used in combination to determine the oxidation states of gold in clusters and complexes supported on MgO. The samples were also characterized by X-ray absorption fine structure (EXAFS) spectroscopy to determine the structure of the supported gold species. The samples were prepared by adsorption of AuIII(CH3)2(C5H7O2) on MgO followed by treatment in He or H2 for 2 h at 573 K and 1 atm. The XANES and TPR results identify Au3+ in supported gold complexes and show that treatment in H2 caused reduction at 480 K to metallic gold, which could be reoxidized to Au3+ at about 490 K by treatment in O2, as shown by TPO and XANES. Treatment of the sample containing supported Au(III) complexes with CO led to the formation of partially reduced gold (Au+, 40 ± 5%) and zerovalent gold (60 ± 5%), as shown by the combination of XANES, TPR, and TPO. The results demonstrate how XAN...