Inorganic cluster compounds as models for the structure of active sites in promoted hydrodesulphurization catalysts

Abstract

Inorganic clusters of the type [Co(WS₄)₂NO]^2–, [Co(WS₄)(NO)₂]^–(PPh₄ salts), [Co(MoS₄)(NO)₂]^– and [Co(MoO₂S₂)(NO)₂]^–(NEt₄ salts) have been synthesized and characterized by means of EXAFS, i.r. spectroscopy, X-ray diffraction and magnetic susceptibility measurements. By comparing these results with results for Co–Mo/Al₂O₃ hydrodesulphurization catalysts, a better understanding has been achieved of the structural and electronic environment of the active Co-edge promoter atoms (the so-called Co—Mo—S Co atoms) which can bind NO. The ‘adsorption complex’ in the catalyst has a pseudo-tetrahedral structure in which the {Co(NO)₂S₂} portion is diamagnetic and obeys the 18-electron rule. The promotional effect of Co is explained in terms of an increased electron density on the neighbouring S and Mo atoms, and analogies are made to a recent classification of the acid/base properties of the bulk sulphides.