Effect of the activation temperature on the state of gold supported on titania: An FT-IR spectroscopic study
Résumé
The effect of the pre-treatment conditions on the evolution of gold species on an Au/TiO2 sample prepared by deposition–precipitation with urea has been followed by FT-IR spectroscopy of adsorbed CO. Gold is in the form of Au3+ species on the as-prepared sample and is not affected by evacuation at ambient temperature. However, the Au3+ species are readily reduced by CO at ambient temperature giving rise immediately to Au+ cations (the corresponding carbonyls displaying a band at 2168 cm−1), which are then slowly converted into metallic gold (carbonyl band at around 2106 cm−1). The Au3+ species are reduced to metal gold (respective carbonyl bands in the 2135–2100 cm−1 region) when a fresh sample is heated at T ≥ 473 K. Water was found to affect CO adsorption on metal gold sites only slightly. Oxidation of Au0/TiO2 by oxygen failed to produce cationic gold species. However, metallic gold was oxidized by heating the sample in a NO + O2 mixture even at 573 K.