X-ray absorption spectroscopy and CO oxidation activity of Au/Al2O3 treated with NaCN

Abstract

Gold nanoparticles were supported on Al2O3 by a deposition–precipitation method followed by heating in He. The resulting catalyst was then treated with NaCN solutions to remove most of the gold. X-ray absorption spectroscopy at the Au LIII edge was used to monitor the oxidation state and atomic structure of Au throughout the procedure. The gold or a Au/Al2O3 sample after deposition–precipitation was in a +3 oxidation state and was subsequently reduced to metal after heating to 623 K in He. Treatment of a reduced sample with either 1 or 2 wt. % NaCN in water removed 79% or 86% of the Au, respectively. Although the remaining Au was cationic, it was also reduced to metal after heating to 623 K in He. The rate of CO oxidation per gold atom, the apparent activation energy, and the orders of reaction were unaffected by the NaCN treatments. These results show that alumina does not stabilize cationic Au species and that metallic Au appears to be important for the CO oxidation reaction. The presence of water or H2 increased the rate of CO oxidation over all of the Au/Al2O3 catalysts. However, the promotional effect of co-fed water was greater over the NaCN treated samples.