Theoretical investigations of the structures and properties of molybdenum-based sulfide catalysts

The development of new hydrotreating catalysts to meet the need for producing cleaner transportation fuels requires a better understanding of the structures and properties of active sites of industrially relevant catalysts. Molecular modeling and simulation has made important contributions towards achieving this goal. This review summarizes theoretical results and conclusions regarding the structures and properties of molybdenum-based hydrotreating catalysts through a logical discussion of surface structures, hydrogen activation, and adsorption and reaction of organosulfur molecules. Ab initio calculations of molybdenum-based sulfide catalysts have been used to predict the equilibrium sulfur coverage on the edge planes of promoted and unpromoted MoS(2) catalysts, identify the energetically favorable locations of promoter (Ni, Co) atoms. consider the mechanism of hydrogen dissociation and adsorption, and characterize the bonding of organosulfur molecules by evaluating the adsorption energies as a function of catalyst composition. We summarize published computational results from the previous decade, and in the process highlight the need to consider the appropriate model when performing calculations. In this review, we conclude the single slab MoS(2) model containing two rows of molybdenum atoms is adequate for discussing general energetic trends when sulfur coverage changes on the edge surface. however more substantive models are required to obtain accurate energetic and structural data. The stable structures at reaction conditions also have to be considered when discussing the locations of promoter atoms, the dissociation of hydrogen, and the adsorption of molecules on the edge surface. Additional challenges and future applications to increase the fundamental understanding of molybdenum-based sulfide catalysts are discussed. (C) 2003 Elsevier B.V. All rights reserved.