Abstract: A series of NiMo/γ-Al2O3 catalysts with different metal atomic ratio (n(Ni)/n(Ni+Mo)) was prepared with or without citric acid (CA) as chelating agent. The effect of CA on the hydrodesulfurization activity of NiMo/γ-Al2O3 catalysts was studied. The catalysts were characterized by means of XRD, XPS, HRTEM techniques to inspect active metal dispersion, sulfidation extent, active phase morphology as well as active phase ratio. The results showed that when the metal atomic ratio is below 0.3, CA has a positive effect on the catalytic activity of NiMo/γ-Al2O3 catalysts; the sulfidation degree of Mo species and the ratio of NiMoS active phases are increased, which is in favor of forming active phases with smaller slab length and slightly higher stacking number, indicating a moderate positive influence on the role of Ni. However, when the atomic ratio is above 0.3, CA has little effect on the catalytic activity of NiMo/γ-Al2O3 catalysts. The effect of CA on the role of Ni is small due to the fact that CA almost has no influence on the dispersion of active metals, the sulfidation degree of Mo species, the ratio of NiMoS active phases, and the morphology of active phases.

Key words: NiMo/γ-Al2O3, citric acid, metal atomic ratio, hydrodesulfurization, promoter effect