An XAFS study of the different influence of chelating ligands on the HDN and HDS of gamma-Al2O3-supported NiMo catalysts

gamma -Al2O3-supported NiMo catalysts prepared in the presence of the chela ting ligands nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic ac id (EDTA) were tested in the hydrodenitrogenation (HDN) of o-toluidine and in the hydrodesulfurization (HDS) of thiophene. For the molar ratio NTA:Ni = 1 a maximum was observed in the HDN activity, but a minimum activity was observed in the HDS and in the hydrogenation of cyclohexene. The highest HD S activities were obtained for the calcined catalyst prepared in the absenc e of ligands and for the catalyst prepared with EDTA. The Mo K-edge Quick E XAFS and normal EXAFS data demonstrated that in the catalysts with the high est HDN activity MoS2 is formed at the lowest temperature and the MoS2 crys tallites have the highest structural order. This suggests that o-toluidine can adsorb better on MoS2 crystallites with more regular edges. The increas e in HDS activity was ascribed to a change in the mechanism of sulfidation of Ni, induced by the chelating ligands, and to a subsequently higher dispe rsion of Ni. Mo-oxysulfides could not be distinguished by Quick EXAFS durin g sulfidation of Mo. The Quick EXAFS data indicate that a MoS3-like phase p recedes MoS2 during the sulfidation process. The chelating ligands screen t he Ni cations and prevent their interactions with the environment. In the a bsence of chelating ligands, Ni interacts with the support and changes its surface properties. In this way the Mo-support interactions and the sulfida tion of Mo are affected. The influence of the chelating ligands on the sulf idation of Mo suggests that the sulfidation process of Mo is closely depend ent on the sulfidation of Ni. (C) 2001 Academic Press.