TiO₂-Al₂O₃ supports with different incorporation methods of titania were synthesized via three methods: impregnation (TA-I), co-precipitation (TA-CP), and co-precipitation–hydrothermal treatment (TA-HT). And the NiMoP catalysts prepared on the corresponding supports were evaluated for hydrodesulfurization (HDS) reactions. The results demonstrated that the Ti atoms in TA-I are attached to alumina through hydroxyl groups, while the Ti atoms in TA-CP and TA-HT can be dispersed in the alumina skeleton. Variations in the incorporation modes of TiO₂ affect the support properties, consequently influencing the nature of the active metal on the supports. The Ti atoms dispersed in the Al₂O₃ skeleton allow an increase in the basic hydroxyl groups. Meanwhile, TiO₂ in TA-CP and TA-HT can absorb hydrogen molecules and be partially reduced. Furthermore, metal species supported on the TA-CP and TA-HT are more easily reduced and better dispersed. For the NiMoP catalysts prepared with TA-CP and TA-HT, the Ti element promotes the sulfidation degree of Mo, besides shortening the average (Ni)MoS₂ slab. The catalysts prepared with TA-CP exhibited superior activity for 4,6-DMDBT hydrodesulfurization. This can be ascribed not only to the relatively high sulfidation degree of Mo and proportion of the NiMoS active phase but also to the well-dispersed (Ni)MoS₂ slabs. Moreover, the Ti⁴⁺ ions dispersed in the Al₂O₃ skeleton can be partially reduced to act as electron donors, enhancing the metallic character of the S layers in MoS₂, which facilitates the improvement of the hydrogenation desulfurization activity.

The CoMo/Al2O3 catalysts with varying Co/(Co+Mo) ratios were employed to probe sulfur compound transformation in two decant oils with distinct molecular structures, using Fourier transform ion cyclotron resonance mass spectrometry for molecular characterization. Results show that increasing the Co/(Co+Mo) ratio enhanced both the fraction of Mo atoms at edge and corner sites. S-edge/Mo-edge ratio in the CoMoS phase reached its maximum at 0.10 of Co/(Co+Mo) atomic ratio. Both sulfur removal and aromatic saturation increase with Co loading, but the catalyst with a Co/(Co+Mo) atomic ratio of 0.10 prioritizes selectivity. Thiophenic sulfur compounds with double bond equivalent (DBE) of 9 are the most refractory species in both oils. In DO-JL (decant oil from Jinling ), these sulfur compounds with DBE of 9 show increasing conversion with carbon number due to easily removable naphthenic-benzothiophene structures with long side chains. At low Co loadings, S-edge predominance favors direct desulfurization, leading to high selectivity for sulfur removal relative to aromatic hydrogenation. With increasing Co loading, the greater exposure of Mo-edges enhances hydrogenation, which may contribute to overall HDS conversion but also boosts aromatic saturation, thereby compromising sulfur removal selectivity. To maximize both the efficiency and selectivity of sulfur removal from decant oils, future catalyst design should focus on optimizing the distribution of these active edges through promoter incorporation.