STRUCTURE AND NATURE OF THE ACTIVE-SITES IN COMO HYDROTREATING CATALYSTS - AN EXAFS STUDY OF THE REACTION WITH SELENOPHENE

The genesis of sulfur vacancies on sulfided (Co)Mo/Al2O3 catalysts was studied with EXAFS at the reaction temperature (673 K) in a H-2 and H -2/thiophene gas atmosphere. For Mo no significant changes in the sulf ur coordination were observed for either the Go-promoted or the unprom oted sample. The experiments indicated that in Go-promoted Mo/Al2O3 va cancies are primarily formed on the Co atom. The sulfur coordination o f the cobalt changed from 6.0 to 5.2 upon reduction with H-2, followed by a slight increase to 5.4 by treatment with H-2/thiophene. The reac tion of selenophene (a structural analogue of thiophene) in hydrogen w ith sulfided (Co)Mo/Al2O3 catalysts was also investigated with EXAFS. It was possible to monitor the incorporation of Se in the metal sulfid e phase. Characterization of the promoted catalyst after HD-Se at 473; K showed that Se was exclusively coordinated to the Co atoms. In contr ast, data obtained after HD-Se at 673 K revealed that the Se atoms wer e located in the position of the bridging sulfur atoms between Co and Mo. Accordingly, a structural model is proposed that involves two type s of active sites for hydrodesulfurization on the sulfided Go-promoted Mo catalyst. The first type consists of a sulfur vacancy that is only associated with the promoter atoms, which is created at low temperatu res. At higher temperatures, a second type of site is produced by remo val of sulfur atoms that are bonded to both Co and Mo atoms.