Characterization of sulphided molybdenum-containing hydroprocessing catalysts by oxygen and hydrogen chemisorption

Abstract

Low-temperature oxygen (–78 °C) and hydrogen (300 °C) chemisorption have been applied to characterize a series of sulphided Mo/Al and Co–Mo/Al hydroprocessing catalysts containing up to 12% Mo. Three commercial Co–Mo/Al catalysts have also been studied. Attention has been focussed mainly on the merit of oxygen chemisorption as a surface-specific probe for characterizing hydroprocessing catalysts. From the results it appears that while oxygen chemisorbed at –78 °C can titrate the number of coordinately unsaturated sites (CUS) irrespective of the nature of the support they are on, it cannot distinguish between two CUS with different intrinsic hydrodesulphurization (HDS) activity (arising either because of carrier–catalyst interaction or because of the influence of a promoter) or between a hydrogenation (HYD) and a HDS site. Hydrogen-chemisorption results are also found to throw some light on the structural growth of MoS₂ crystallites on the support. The results strongly indicate that hydrogen first dissociates on the CUS on the edge planes of MoS₂ prior to its migration to the basal planes where it remains as SH groups. There is also evidence that the van der Waals gap between two MoS₂‘sandwiches’ cannot accommodate hydrogen.