Preparation of (Co)-Mo-based hydrodesulphurization catalysts:: characterizations of deposited species on lanthanum modified γ-alumina

The preparation of La-doped hydrodesulphurization (HDS) catalysts is investigated by deposition of Mo and Co from solutions containing their salts on a gamma alumina previously modified by impregnation with lanthanum nitrate solutions and further calcined at 773 K. Whatever the pH of the impregnating solution, the n(Mo)/n(La) atomic ratio in the oxide precursor always governs the nature of the adsorbed oxomolybdenum entities and a model of Mo distribution on the surface of the support is proposed. The Mo surface distribution is mainly attributed to the selective adsorption of monomeric MoO42- species on sites associated with La whereas poly-oxomolybdates adsorb on the alumina sites. Presence of the promotor (Co) does not modify the Mo distribution. In addition, it is shown that the presence of La in the catalyst formulation induces a decrease of the thiophene HDS conversion due to the preferential adsorption on the lanthanum hydroxyl groups of the oxomolybdate species present in the impregnating solution as isolated monomeric entities which are less easily sulphided, when deposited on the support, than the poly-oxomolybdates. (C) 1998 Elsevier Science B.V. All rights reserved.