gamma-Al2O3 supported P-doped nickel molybdenum (NiMo) nitride hydrotreating catalysts with 12 wt.% of Mo, 3 wt.% Ni and 0-2.5 wt.% P were synthesized by means of temperature-programmed reaction, and characterized by BET surface area analysis, elemental analysis, CO uptake titration, DRIFT study of adsorbed CO, temperature-programmed oxidation (TPO), temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) of NH3. The activity of the nitride catalysts was studied in a trickle-bed reactor operated at 8.8 MPa and 385 degrees C in the hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) of heavy gas oil (HGO) derived from Athabasca bitumen. The CO uptakes measurement showed that P addition to NiMo nitride improves the dispersion. DRIFT spectra of CO adsorption evidenced that the number of Mo2+ sites on the surface of NiMo nitride increases with increasing P concentration and reaches optimum with 2.0 wt.% of P. TPD of NH3 indicated an increase of strong acid sites and also strengthening of moderate acid sites in NiMo nitride on P doping. All the nitride catalysts have suffered partial sulfidation during initial period of hydrotreating of HGO. HDN activity of NiMo nitride was promoted greatly by the phosphorus content, reached a maximum with P content of 1.6 wt.%. An increased acidity of P-doped NiMo nitrides accelerated C-N bond breaking and thus enhanced the HDN activity. No significant change was observed in HDS of HGO over NiMo/Al2O3 nitride catalyst on P doping. (c) 2007 Elsevier B.V. All rights reserved.