Influence of the preparation conditions on the surface properties of HDS catalysts

Abstract

This work presents results on the nature of the precursor structures of active CoMo/Al 2O 3 catalysts. Catalysts with the same chemical composition prepared by different procedures and calcined at two temperatures are studied. The catalysts are characterized by gravimetrically determined reducibility in H 2, oxygen chemisorption at low temperature (LTOC), diffuse reflectance spectroscopy, infrared spectroscopy using NO as probe molecule, and thiophene hydrodesulphurization (HDS) activity measurements. The presence of ammonia and a low concentration of molybdate ions in the solution (with longer time of impregnation) favour molybdena dispersion. By using beads of alumina, molybdena profiles appeared, and they markedly depend on the subsequent incorporation of the promoter in aqueous media. Upon increasing the calcination temperature, from 670 K to 820 K, no significant variations in the HDS activity were observed, however an increase in the concentration of the tetrahedrally coordinated Co 2+ ions at the interface (with a decrease of the amount of octahedrally coordinated Co which chemisorbs NO) was found. All results indicate that the optimization of the oxidic surface species (dispersion degree of molybdena and interacting Co-Mo phase) in these hydrodesulphurization catalysts may be achieved by varying preparation conditions, i.e. low concentration of Mo with long time of impregnation or presence of ammonia in solution.