Exafs investigations of Fe-, Ni-, Co- and Cu- MoS₂/γ-Al₂O₃ hydrodesulphurization catalyst systems

Abstract

In-situ EXAFS studies have been carried out on several transition metal (T)-MoS₂ (T = Fe, Co, Ni or Cu) catalysts supported on γ-Al₂O₃. While Mo is present in small crystallites of MoS₂ in all the systems studied, the local sulphidic environment around the transition metal atom varies significantly with the catalytic activity. Short T-S distances (compared to the bulk sulphides) are found in the case of the Ni and Co catalysts due to the formation of the active 'Ni(Co)-Mo-S' state. In the case of Fe, which is not a good promoter, the Fe-S distance in the catalyst is only slightly shorter than in the bulk sulphides. No such short distance is found in the Cu-MoS₂/Al₂O₃ system since Cu acts as a poison; instead only bulk sulphides are formed. Effects of the method of preparation, order of impregnation, metal loading and other factors have been examined to arrive at the conditions favourable for the formation of the active Ni(Co)-Mo-S state.