Surface characterization of the deactivation of alumina catalysts used for COS synthesis

One of the possible routes of producing COS, an important material in organic synthese, is the CO2 + CS2 ⇄ 2COS reaction, preferably over gamma-alumina catalysts. Under long-term loading, however, a remarkable loss of the catalytic activity occurs. The nature of the ageing process was studied by a multi-technique apporach on high purity [I] and on a commerical grade [II] gamma-alumina, which latter contained Na, Ca, and S as surface impurities. On catalyst [I] weak chemisorption of CO2, CS2 and COS was observed by the volumetric method, as characterized by the isosteric heats of adsorption. Catalyst [II] chemisorbed CO2 and COS but not CS2 and showed much less catalytic activity. Gravimetric measurements have shown that adsorption of CS2 and COS leads to the surface deposition of side products. XPS studies on new, loaded and aged samples have revealed that the ageing of the catalyst is primarily caused by the deposition of sulphur and carbon with carbon in excess, presumably in CS, CS and CC environements.