State of gold on an Au/Al2O3 catalyst subjected to different pre-treatments:: An FTIR study

An Au/Al2O3 sample was prepared by deposition-precipitation and characterized by different techniques (XRD, TEM, FTIR spectroscopy of adsorbed CO and catalytic test). IR spectroscopy of adsorbed CO on a sample evacuated at ambient temperature reveals no gold sites. The results were explained by the fact that gold on this sample was present as Au3+ cations and these sites (as well as the Al3+ sites) were blocked by water. When the sample was activated at 473 K, CO adsorption at 100 K produced: (i) carbonyl bands associated with the support and (ii) a band at 2167 cm(-1), stable towards evacuation and assigned to Au+-CO species. Evacuation of the sample at 573 K led to auto-reduction of the Au+ sites. Subsequent CO adsorption resulted in formation of two types of Au-0-CO species (bands at 2112 and 2105 cm(-1)). Only one kind of Au-0 sites (respective carbonyl band at 2105 cm(-1)) were detected with samples activated at 673 K. Re-oxidation of the sample at 673 K in O-2 atmosphere resulted in oxidation of all accessible metallic gold and appearance of (i) a small amount of isolated Au+ sites (respective carbonyl band at 2175 cm-1) and (ii) another kind of sites (respective carbonyl band at 2139 cm(-1), disappearing during evacuation) associated with Au+ cations on the surface of the metal particles. Both kinds of cations were stable at 573 K but gold cations were auto-reduced at 673 K. Comparison of the FTIR results with catalytic studies allowed some conclusions on the nature of the active sites in the CO oxidation reaction. (c) 2005 Elsevier B.V. All rights reserved.