A temperature-programmed reduction study of sulfided CoMo/Al 2O 3 hydrodesulfurization catalysts

Abstract

The reduction of sulfided hydrodesulfurisation catalysts has been studied using temperature-programmed reduction of sulfides (TPR-S). Hydrogenation of stoichiometric sulfur of bulk Co 9S 8 and MoS 2 occurs at much higher temperatures than HDS operating temperatures and is thermodynamically limited under the TPR-S reduction conditions. Al 2O 3-supported Co and Mo catalysts contain sulfided species which are reduced at lower temperatures than bulk Co 9S 8 and MoS 2. In Co/Al 2O 3 catalysts with a high Co content Co 9S 8 is found, while a small amount of a CoS surface species is also present. High temperatures of calcination cause Co to migrate into the support, and this Co species remains oxidic during the sulfiding treatment. On Mo/Al 2O 3 catalysts it is found that around 600 K a sulfided Mo monolayer species is reduced. At higher temperature, reduction is observed of bulk-like MoS 2. In CoMo/Al 2O 3 catalysts the reduction of Mo occurs at nearly the same temperature as that of Mo/Al 2O 3. When the Co loading is low, no separate Co sulfide phase is detected, whereas catalysts with a high Co content contain Co 9S 8. A high temperature of calcination leads to the formation of nonsulfidable Co species. For all supported catalysts the TPR-S patterns indicate that some S is hydrogenated around 400 K. This S is chemisorbed on coordinatively unsaturated sites of Mo and Co sulfides when the H2S pressure is sufficiently high, and it is hydrogenated at a lower H 2S/H 2 ratio. On Co/Al 2O 3 catalysts this S species is hydrogenated at a higher temperature than that of the corresponding species in Mo/Al 2O 3. On CoMo/Al 2O 3 a similar S species is found; it is hydrogenated at a lower temperature than that found for either Co/Al 2O 3 or Mo/Al 2O 3. This is an indication of the formation of a disperse CoMo species. The temperature of hydrogenation of the chemisorbed S species depends on the metal loading and the temperature of calcination. For all CoMo/Al 2O 3 catalysts, it is found that a low temperature of hydrogenation of S corresponds with a high HDS activity. It is concluded that the capacity to hydrogenate S, which is abstracted from S-containing compounds during the HDS reaction, is a key parameter for the overall HDS activity of CoMO/Al 2O 3 catalysts.