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

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 Au III (CH 3 ) 2 (C 5 H 7 O 2 ) on MgO followed by treatment in He or H 2 for 2 h at 573 K and 1 atm. The XANES and TPR results identify Au 3+ in supported gold complexes and show that treatment in H 2 caused reduction at 480 K to metallic gold, which could be reoxidized to Au 3+ at about 490 K by treatment in O 2, 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 XANES and TPR/TPO used in combination can effectively determine the oxidation states of metals on supports, even when the metals are present in various oxidation states.