STRUCTURAL STUDIES OF HYDRODESULFURIZATION CATALYSTS

Hydrodesulfurization (HDS) is a catalytic process used to remove sulfur from S-containing compounds by converting it to hydrogen sulfide in the presence of dihydrogen. Catalysts used in HDS are mainly made of molybdenum and cobalt sulfides supported on gamma-alumina. The objective of this work was to correlate changes in the HDS activity of these catalysts with structural information obtained by X-ray absorption spectroscopy (XAS).; Two series of catalysts were prepared, one by incipient wetness impregnation and the other by bulk impregnation. The catalysts had approximately the same Mo content and varying Co content, and were tested for thiophene HDS in a fixed bed reactor at 573 K and atmospheric pressure. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption edge spectroscopy (XAES) were used at the Mo and Co K-edges. The catalysts were treated in a mixture of hydrogen sulfide/dihydrogen.; For both series of catalysts, HDS activity increased with atomic ratio r = Co/(Co + Mo), reached a maximum and declined with further increase in r. Catalysts prepared by incipient wetness impregnation showed higher HDS activity.; From EXAFS and XAES, it appears that molybdenum has a local environment similar to that of molybdenum disulfide, very dispersed on the surface of the alumina. Cobalt has sulfur atoms as nearest neighbors. No discrete cobalt phase is observed for r less than or equal to .044. The predominant Co phase r equal to 0.72 is nanocobalt octasulfide. These results were rationalized in terms of the "CoMoS" phase proposed by Topsoe and co-workers, with most of the cobalt located on the surface or at the edges of the S-Mo-S layers of the (molybdenum disulfide)-like phase.; The number N(S) of sulfur neighbors of Mo, determined from EXAFS, depends on r like HDS activity, and shows a linear correlation with HDS activity that holds for both series of catalysts. It seems reasonable that as r increases, the sulfur binding energy of Mo increases until the maximum value of N(S) is reached, and then declines. These results are compatible with Pecoraro and Chianelli observation that there is a sulfur binding energy corresponding to a maximum HDS activity.