The role of titania in supported Mo, CoMo, NiMo, and NiW hydrodesulfurization catalysts:: analysis of past and new evidences

Previous results and new evidence on the role of titania in supported hydrodesulfurization catalysts are analyzed in order to construct a rational explanation for the different findings in Ti-containing HDS catalysts. Some of the important findings follow. The preparation method of Al-Ti oxide supports is important to catalytic activity. It is not the total amount of Ti but the Ti oxide surface structures that is relevant. TiO2 is an electronic promoter in HDS catalysts. Ti3+ species formed under HDS reaction conditions act as electron donors. These electrons can be easily transferred, through the conduction band of the support, and be injected to the Mo 3d conduction band. This causes a weakening of the Mo-S bonds and helps the creation of more CUS. The addition of TiO2 to the surface of alumina eliminates the most reactive surface hydroxyl groups and avoids the formation of tetrahedral Mo oxide species, causing an increase of well-sulfided active phase and hence in catalytic activity. This effect, although significant, is less important than the electronic promotion of Mo by Ti. TiO2-containing catalysts are well suited for deep HDS because they facilitate the formation of a greater number of CUS in MoS2 (or WS2), favoring the hydrogenation-hydrodesulfurization route of transformation of 4,6-DMDBT. (C) 2004 Elsevier B.V. All rights reserved.