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

gamma -Al2O3-supported NiMo catalysts prepared in the presence of the chelating ligands nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (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 HDS activities were obtained for the calcined catalyst prepared in the absence of ligands and for the catalyst prepared with EDTA. The Mo K-edge Quick EXAFS and normal EXAFS data demonstrated that in the catalysts with the highest HDN activity MoS2 is formed at the lowest temperature and the MoS2 crystallites have the highest structural order. This suggests that o-toluidine can adsorb better on MoS2 crystallites with more regular edges. The increase 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 dispersion of Ni. Mo-oxysulfides could not be distinguished by Quick EXAFS during sulfidation of Mo. The Quick EXAFS data indicate that a MoS3-like phase precedes MoS2 during the sulfidation process. The chelating ligands screen the Ni cations and prevent their interactions with the environment. In the absence of chelating ligands, Ni interacts with the support and changes its surface properties. In this way the Mo-support interactions and the sulfidation of Mo are affected. The influence of the chelating ligands on the sulfidation of Mo suggests that the sulfidation process of Mo is closely dependent on the sulfidation of Ni. (C) 2001 Academic Press.