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(CH3)(2)(C5H7O2) 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 Au3+ in supported gold complexes and show that treatment in H-2 caused reduction at 480 K to metallic gold, which could be reoxidized to Au3+ at about 490 K by treatment in O-2, as shown by TPO and XANES. Treatment of the sample containing supported Au(Ill) complexes with CO led to the formation of partially reduced gold (An(+), 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.