The preparation conditions of a Co-Mo/Al2O3 model sulfide catalyst, in which COWS phases are selectively formed, were studied by a CVD technique using Co(CO)(3)NO as a precursor. The CVD technique used for the preparation of the Co-Mo/Al2O3 model catalyst was applied to reveal the effects of calcination and the addition of a chelating agent (NTA; nitrilotriacetic acid) on the thiophene hydrodesulfurization (HDS) activity of Co-Mo/Al2O3 catalysts prepared by impregnation methods. The catalysts were characterized by means of NO adsorption, TEM, FTIR, and XAFS. Al2O3-supported Co-Mo model catalysts were prepared by decorating presulfided Mo/Al2O3 with Co(CO)(3)NO and subsequent sulfiding. It is demonstrated that the model catalyst is prepared, when the Mo content exceeds a monolayer loading and when Mo/Al2O3 is calcined prior to the sulfidation. The amount of Co forming CoMoS phases was estimated from the correlation between NO/Mo and Co/Mo ratios. TEM observations have suggested that the NTA addition in the preparation of Mo/Al2O3 promotes the lateral growth of MOS2 slabs. The calcination of Mo/Al2O3 increased the dispersion of Mo sulfide phases regardless of NTA addition. It is suggested that the NTA addition to Co-Mo/Al2O3 coimpregnation catalysts reduces detrimental effects of Co on the dispersion of Mo species. Furthermore, the calcination increased the Co coverage of the edge sites of MOS2 particles on simultaneous and double impregnation catalysts. It is suggested that the present preparation technique of the model catalyst predicts the potential maximum activity of a Co-Mo catalyst under study, since the edge sites of MOS2 particles are fully covered by CoMoS phases. In addition, the CVD technique provides a unique characterization method to evaluate the Co coverage on the edge sites of MOS2 particles for supported Co-Mo sulfide catalysts. (C) 2003 Elsevier Science (USA). All rights reserved.