Structure and activity of NiCoMo/SiO₂ hydrodesulfurization catalysts

A series of NiCoMo hydrodesulfurization (HDS) catalysts supported on silica was prepared varying the ratio r = Co/(Co + Ni) between 0 and 1, maintaining Mo and (Co + Ni) concentrations constant. Calcination temperatures employed were 500 and 600°C. The promoter (Co and/or Ni) was present in the precursor as a hydrated molybdate phase that suppressed both anhydrous MoO₃ crystallite growth and decrease in surface area resulting from sintering of the support. Both phenomena occurred in the nonpromoted catalyst after calcining at 600°C, probably by a dehydroxylation process resulting from dislinking of polymolybdates from the silica support. A correlation between TPR and dispersion given by XRD was established, confirming that the dispersion of molybdenum on silica was lower than that on the aluminas previously studied. According to the proposed correlation, sulfidation caused an increase in dispersion in all the catalysts. The presence of anhydrous MoO₃ was held responsible for the formation of amorphous MoO₂ observed by TPR after sulfidation. The increase in dispersion after sulfidation was accompanied by a remarkable increase in the initial HDS activity of both nonpromoted and promoted catalysts. However, the nonpromoted sample showed a pronounced initial deactivation during use. Compared with another similar, but supported on alumina, series of catalysts, a minimum instead of a maximum steadystate HDS activity was found for an intermediate value of r.