Supported gold catalysts for the decomposition of VOC: Total oxidation of propene in low concentration as model reaction

Supported gold catalysts prepared by deposition–precipitation with urea were studied in the reaction of oxidation of propene in low concentration in a large excess of oxygen, so as to mimic the conditions of catalytic decomposition of a volatile organic compound of hydrocarbon-type (1200 ppm C3H6, 9% O2 in He). Several parameters were investigated: the nature of the oxide support (alumina, titania, ceria), the gold loading, the conditions of catalyst activation (oxygen or hydrogen). Titania and alumina alone did not show any conversion in C3H6 oxidation up to 500 °C, but when gold was added (1 wt%), active catalysts were obtained with a higher activity for titania than for alumina. Ceria was the only support showing activity, and gold on ceria (1 wt%) led to the most active catalyst. For the Au/CeO2 system, activation under H2 at 300 °C leads to more active catalysts than activation in O2/He at 500 °C, especially for gold loadings lower than 1 wt%. XPS and CO oxidation performed at RT showed that gold on CeO2 was fully reduced to Au0 after activation under H2 whatever the gold loading. In contrast, after calcination, most of the gold remained under the initial AuIII state for the low loaded samples (≤1 wt%) whereas part of it was reduced for the 4 wt% Au/CeO2. Thus, ceria seems to be able to stabilise gold as AuIII up to a limited loading. Change in the gold oxidation state was detected for the calcined Au/CeO2 (1 wt%) during C3H6 oxidation performed at increasing temperature, using CO oxidation and DRIFTS combined to CO adsorption. Indeed, gold, initially AuIII, starts reducing at 100 °C to form metallic gold Au0, which was the active species for the reaction. Above 300 °C, when 100% conversion was achieved, reoxidation of metallic gold species was observed.