Effect of Liquid Passivation on the Oxidation Resistance and Hydrodesulfurization Activity of the NiMoS_x/Al₂O₃ Catalyst

Toluene and rapeseed oil were, respectively, used to passivate a commercial oxidic NiMo/Al₂O₃ hydrotreating catalyst to prevent the sulfurized catalyst from oxidation and reduce the activity loss. The passivation at 400 and 500 °C by rapeseed oil can form carbon deposits with an amorphous-like graphite structure on the catalyst; however, passivation at 300 °C forms less carbon deposits. The high temperature facilitates carbon deposits with bigger size and more defects. After sulfurization and air exposure for 7 days, the oxidation degrees of Mo and Ni sulfides in the catalyst determined by TG are, respectively, decreased by 82.7 and 59.3% and 83.8 and 79.1% after 400 and 500 °C passivation. The rate constants of dibenzothiophene hydrodesulfurization of the initial sulfurized catalysts without passivation and passivated at 400 and 500 °C are 1.46, 1.49, and 0.83 mmol g^–1 h^–1, respectively. The enhanced activity at 400 °C is ascribed to shorter MoS₂ slabs and the formation of Mo₂C. However, the inert NiAl₂O₄ formed at 500 °C causes significantly decreased activity. After 7 days of air exposure, the rate constants of catalysts without passivation and passivated at 400 °C are 1.15 and 1.40 mmol g^–1 h^–1, respectively. The carbon deposits produced by passivation with rapeseed oil at 400 °C could effectively inhibit oxidation and reduce the activity loss, which is promising for ex situ sulfurized hydrotreating catalysts.