ABSTRACT
Abstract Mössbauer emission spectroscopy and X-ray absorption spectroscopy were applied for the identification of ’Co-sulfide’ species formed during sulfidation of supported Co and Co-Mo hydrotreating catalysts. In the sulfided catalysts a ’Co-sulfide’ species is formed which is characterized by a doublet with a varying Q.S. value, depending on e.g. the presence of Mo, sulfidation temperature and Co-loading. Combining the variation in Q.S. values observed by MES and the coordination parameters deduced from EXAFS, the ’Co-sulfide’ species turns out to be a particle characterized by its size and ordering. In Co/C highly dispersed Co^Sg-like species are formed in which no ordering can be discerned beyond 4 A, while in uncalcined Co/Al20 3 these ’Co-sulfide’ particles are substantially larger. In CoMo catalysts the highly dispersed ’Co-sulfide’ species are smaller
than in the Co/C catalysts and are most likely located at the edge of the MoS2 crystallites that function as a secondary support. By varying the Co/Mo ratio the number of Co atoms associated with a ’Co-sulfide’ particle is varied continuously over a large range, with the lower and upper limit being the structure proposed in the ’Co-Mo-S’ and contact synergy model, respectively. In the case of commercially applied Co/Mo ratios ’Co-sulfide’ particles will be involved rather than the atomically dispersed Co of the ’Co-Mo-S’ structure.
