HYDROGEN ACTIVATION BY TRANSITION METAL SULFIDES

The activity of the conventional catalysts during hydroprocessing results from the ability of Mo(W)S2 to adsorb and activate hydrogen. The optimum of hydrogen adsorption occurs at the S/Mo(W) ratio of about 1.95. This is consistent with removal of the corner and edge sulfur atoms leading to the formation of the sulfur anion vacancies. The heterolytic dissociation, involving both metal and sulfur ions, is the predominant mode of hydrogen activation. It leads to the formation of metal–H and S–H moieties. The homolytic dissociation, occurring on the sulfur ion pairs, is much less evident. For the supported catalysts, the amount of the adsorbed hydrogen per unit of Mo(W) is significantly greater. This results from the spillover of the active hydrogen from the active phase on the support. The surface hydrogen can migrate back from the support to the active phase. Promoters, such as Ni and Co, increase the rate of adsorption, whereas their effect on the total amount of the adsorbed hydrogen is much less pronounced. Similar effect may be observed by increasing hydrogen pressure. Among the noble metal sulfides, most of the attention has been paid to RuS2. The results obtained by spectroscopic techniques confirm the presence of the Ru–H and S–H species. The maximum of hydrogen activation is approached at about 20% reduction of RuS2. The homolytic and heterolytic hydrogen dissociation occur simultaneously. The extent of the former increases with increase in temperature. The activity of the RuS2 based catalysts increases with the decrease in particle size of RuS2, i.e., increasing the ratio of {111}/{100} surfaces. Suitable zeolite supports can significantly enhance the stability and activity of the RuS2 based catalysts.