A STRUCTURAL AND KINETIC STUDY OF COBALT-MOLYBDENUM - GAMMA - ALUMINUM OXIDE HYDRODESULFURIZATION CATALYST USING EXAFS AND XANES.

摘要

In situ Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge (XANES) spectroscopies were combined in a study designed to elucidate the molecular structure of molybdenum in CoMo/(gamma)Al(,2)O(,3) hydrodesulfurization catalysts. This structure was related to catalytic activity using a model thiophene hydrodesulfurization reaction. Oxide CoMo/(gamma)Al(,2)O(,3) was found to consist of tetrahedrally coordinated molybdenum with two or three short Mo=O bonds and one or two oxide ligands with longer Mo-O distances. The catalyst reacts with H(,2)S at 25 C with terminal Mo=O species being replaced by sulfur ligands. As the sulfiding temperature is increased, oxysulfides are replaced with a MoS(,2)-like phase. No evidence was found of oxysulfides at sulfiding temperatures in excess of 200 C. The first shell sulfur and second shell molybdenum coordination numbers were observed to increase linearly with sulfiding temperature, as was the present MoS(,2)-like phase.; The MoS(,2)-like phase is remarkably similar to MoS(,2), but has a Mo-S coordination number of only four. The second shell coordination is also lower, with only three molybdenum neighbors at 3.16 (ANGSTROM). Sulfiding is about 90% complete after two hours at 400 C, with only minor change after 24 hours of sulfiding. This information is consistent with a structural model consisting of small MoS(,2) crystallites of c.a. 10 (ANGSTROM) if the perimeter molybdenums are considered to have lost labile sulfur. The data are equally consistent with a monolayer model consisting of a chain-like structure of MoS(,4) sub-units. A correlation was observed between HDS activity and both Mo-S and Mo-Mo coordination numbers, suggesting that activity is related to the presence of the MoS(,2)-like phase. A one hour inert gas purge of a sulfided catalyst at 400 C removed considerable excess sulfur, but resulted in no change detectable by EXAFS or XANES.; Kinetic EXAFS, XANES and sulfur uptake measurements were made in situ during catalyst sulfiding and HDS. These measurements showed that at high temperature (400 C) the rate of sulfur adsorption onto the catalyst limits the rate of reaction between surface sulfur and molybdenum. The rate of conversion of surface molybdenum sulfides into the MoS(,2)-like phase lags these two processes. At lower temperature (200 C) the surface reaction is slower and no longer rate limited by the adsorption process.