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Structure and Nature of the Active Sites in CoMo Hydrotreating Catalysts: An EXAFS Study of the Reaction with Selenophene

By D.C. Koningsberger, R.G. Leliveld, M. de Boer, A.J. van Dillen and John W. Geus

Abstract

The genesis of sulfur vacancies on sulfided (Co)Mo/Al2O3 catalysts was studied with EXAFS at the reaction temperature (673 K) in a H2 and H2/thiophene gas atmosphere. For Mo no significant changes in the sulfur coordination were observed for either the Co-promoted or the unpromoted sample. The experiments indicated that in Co-promoted Mo/Al2O3 vacancies are primarily formed on the Co atom. The sulfur coordination of the cobalt changed from 6.0 to 5.2 upon reduction with H2, followed by a slight increase to 5.4 by treatment with H2/thiophene. The reaction of selenophene (a structural analogue of thiophene) in hydrogen with sulfided (Co)Mo/Al2O3 catalysts was also investigated with EXAFS. It was possible to monitor the incorporation of Se in the metal sulfide phase. Characterization of the promoted catalyst after HD-Se at 473 K showed that Se was exclusively coordinated to the Co atoms. In contrast, data obtained after HD-Se at 673 K revealed that the Se atoms were located in the position of the bridging sulfur atoms between Co and Mo. Accordingly, a structural model is proposed that involves two types of active sites for hydrodesulfurization on the sulfided Co-promoted Mo catalyst. The first type consists of a sulfur vacancy that is only associated with the promoter atoms, which is created at low temperatures. At higher temperatures, a second type of site is produced by removal of sulfur atoms that are bonded to both Co and Mo atoms

Topics: Scheikunde
Year: 1997
OAI identifier: oai:dspace.library.uu.nl:1874/5947
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