Effect of fluorine addition on the formation of active species and hydrotreating activity of NiWS/Al2O3 catalysts

Fluorine-added Ni/Al2O3, W/Al2O3 and NiW/Al2O3 catalysts were prepared and their surface properties investigated using temperature-programmed reduction, diffuse reflectance spectroscopy and temperature-programmed sulfidation. The hydrodesulfurization (HDS) of thiophene was used as a model reaction to examine the activity of fluorinated NiW/Al2O3. Fluorination led to an increase in the formation of polymeric tungstates in W/Al2O3 and the surface nickel spinel species in the case of Ni/Al2O3. Fluorination promoted the sulfidation of the NiW/Al2O3 catalyst and the interaction of nickel with tungsten to produce the Ni-W-O species, a precursor of active catalytic sites. The activity of NiW/Al2O3 was increased by fluorination up to a fluorine content of 4 wt.%, and then decreased at higher fluorine contents. Two opposite effects of fluorination determine the characteristic changes in the activity of NiW/Al2O3: the enhanced sulfidation of the catalyst and incorporation of nickel with tungsten, both of which promote the activity, and a decrease in the dispersion of active species, which lowers the activity. The activity results obtained in this study are different from those reported previously [Appl. Catal. A 144 (1996) 343], suggesting that the extent of catalyst modification by fluorination is strongly dependent on the conditions of catalyst preparation. (C) 2002 Elsevier Science B.V. All rights reserved.