Preparing small, highly dispersed Ni2P particles is important for improving the hydrogenation ability of Ni2P. Here, Ni2P nanoparticles (approximately 4.3 nm) on mesoporous zeolite ZSM-5 (Ni2P/MZSM-5-CA) were prepared using citric acid (CA) as an assistant agent. The formation mechanism of small Ni2P particles when CA was added was investigated by combining UV-vis diffuse reflectance spectroscopy, Fourier transform infrared spectroscopy, and temperature-programmed reduction with a transmission electron microscope and CO chemisorption. The results indicated that the formed CA-Ni complex with high viscosity favors the Ni precursor dispersed on the dried catalyst. After calcination, the released Ni species strongly interacted with surface acidic hydroxyl groups on MZSM-5, leading to the formation of Ni2P particles with small sizes and good dispersion under a reducing atmosphere. The reaction rate constants and TOFs over Ni2P/MZSM-5-CA (16.2 x 10(-2) mu mol g(-1) s(-1) and 9.7 x 10(-4) s(-1)) are much higher than over Ni2P/MZSM-5 (8.2 x 10(-2) mu mol g(-1) s(-1) and 83 x 10(-4) s(-1)) in 4,6-dimethyldibenzothiophene hydrodesulfurization. In addition, Ni2P/MZSM-5-CA catalyst shows higher activity than Ni2P catalyst without CA in phenanthrene hydrogenation. (C) 2016 Elsevier Inc. All rights reserved.