Activity of different metal oxides towards COS hydrolysis. Effect of SO2 and sulfation

Activity measurements for COS hydrolysis were carried out at 503 K for three metal oxides : ZrO2, TiO2 and Al2O3. Taking the surface areas into account, their activity varied in the following order : ZrO2 ⪢ Al2O3 ⪢ TiO2a. COS adsorption, studied by IR spectroscopy, showed that type I OH groups, the most basic, were the catalytic sites and gave rise to hydrogen-thiocarbonate species. Such sites can be detected by CO2 adsorption through the formation of hydrogen-carbonate species which are indeed more abundant on ZrO2 than on TiO2 or Al2O3. The presence of SO2 in the gas feed substantially decreased the catalytic activity whatever the sample. A less pronounced deactivation was observed on TiO2. This effect was reversible and is described as a competitive adsorption between SO2 and COS on the basic hydroxyl sites. Catalyst sulfation, either provoked by sulfate impregnation on the pure oxides or by injection of O2 pulses during the reaction, irreversibly decreased the activity. However, ZrO2 remained the most active. This poisoning effect was found irreversible and explained by the creation of Brönsted acidity.