An EXAFS Study of the Influence of Fluorine on the Structure of Sulfided W/Al2O3 and NiW/Al2O3 Catalysts
journal contributionposted on 06.02.2002, 00:00 by Viviane Schwartz, Mingyong Sun, Roel Prins
The effect of fluorine on the structure of sulfided W/Al2O3 and NiW/Al2O3 catalysts was studied by means of EXAFS on the W LIII-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 °C 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 °C, 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 °C, 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 °C to a nickel sulfide with a prevailing Ni−S distance of 2.20 Å. 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 Å) than in the nickel sulfide formed at low temperature. Fluorine increases the formation of this long distance Ni−S coordination.