An EXAFS study of the influence of fluorine on the structure of sulfided W/Al2O3 and NiW/Al2O3 catalysts

The effect of fluorine on the structure of sulfilded W/Al2O3 and NiW/Al2O3 catalysts was studied by means of EXAFS on the W L-III-edge and Ni K-edge. In the WO3/Al2O3 catalyst, prepared from ammonia metatungstate, the tungsten atoms are still mainly coordinated by oxygen after sulfidation at 400 degreesC for 1 h, as shown by the high W-O and low W-S coordination numbers. Fluorine substantially decreases the W-O coordination number and increases the W-S and W-W coordination numbers, demonstrating that it facilitates the sulfidation of tungsten. In an ATT/Al2O3 catalyst, prepared from ammonia tetrathiotungstate (ATT), no W-O coordination is present after sulfidation at 400 degreesC, indicating that tungsten is fully sulfided. Nevertheless, fluorine still improves the formation Of WS2 in the ATT/Al2O3 catalyst, as shown by the increased W-S and W-W coordination numbers corresponding to WS2. The nickel-promoted catalysts were prepared by impregnating the corresponding tungsten-only catalysts with a nickel nitrate solution. The nickel in the WO3/Al2O3 catalysts can only be partially sulfided at 400 degreesC, and fluorine hardly affects the degree of sulfidation of nickel. The omission of the calcination step during the preparation of alumina-supported tungsten catalysts strongly facilitates the sulfidation of the tungsten oxidic species. The nickel in the ATT/Al2O3 catalyst is already fully sulfided at 150 degreesC to a nickel sulfide with a prevailing Ni-S distance of 2.20 Angstrom. At high temperature, after the formation Of WS2, the nickel sulfide formed at low temperature combines with WS2 and forms the so-called NiWS phase. In the NiWS phase, sulfur atoms coordinate to nickel at a longer distance (2.35-2.43 Angstrom) than in the nickel sulfide formed at low temperature. Fluorine increases the formation of this long distance Ni-S coordination.