Abstract

This work reports the effect of Co content and catalysts preparation on the surface structure of two catalyst series prepared by either co-impregnation or sequential impregnation and their catalytic activity for the simultaneous hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of gas oil. Structural characteristics and degree of dispersion of the Co and Re phases in the oxidic catalyst precursors were obtained by X-ray diffraction, energy-dispersive X-ray analysis, UV-vis diffuse reflectance spectroscopy (DRS) and electrophoretic migration (EM), and in the sulphided catalysts by X-ray photoelectron spectroscopy (XPS). The UV-vis diffuse reflectance spectra demonstrated that Co in its oxidic state was in tetrahedral coordination (as surface CoAl2O4) and in octahedral coordination (as surface Co3O4), with a relative Co3O4/CoAl2O4 proportion increasing with Co content, and that Re was present mainly as distorted isolated ReO4 - monomeric species on the alumina surface and in small amounts probably as associated ReO4 - type Re2O7 in high Co-content catalysts. Consistently, XPS results of sulphided catalysts revealed that in both series the major part (>67%) of the Co remained in oxided state, while the Re phase was fully sulphided. The dispersion of sulphided Re decreased gradually with Co loading for the co-impregnated catalysts, due to the strong Co/alumina interaction, and drastically with Co incorporation for sequentially impregnation catalysts due to a partial dissolution and re-distribution of Re during Co impregnation. Catalytic activity results revealed synergetic effects in HDS and HDN between Co and Re in both series, with an optimum at an atomic ratio of about 0.40 Co/Re. The magnitude of the synergy was greater for HDS than for HDN, especially at low reaction temperature, and slightly higher for the coimpregnated catalysts reflecting the better Re dispersion. The observed CoRe synergy was thought to be related to the formation of a mixed Co-Re-S phase and to hydrogen spillover from separate Co sulphide to Re sulphide, participating to a different extent depending on the reaction temperature. © 2008 Elsevier B.V. All rights reserved.