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 MoS, crystallites on the support. The results strongly indicate that hydrogen first dissociates on the CUS on the edge planes of MoS, 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 MoS, ' sandwiches ' cannot accommodate hydrogen.

The amount of oxygen chemisorbed at -78°C has been measured on the sulphided surface of tungsten catalysts supported on Al2O3 and SiO,; it is shown for the first time that there is a correlation with the activity of the catalyst for hydrodeoxygenation of furan.