TPS, XPS, QEXAFS, and XANES Investigation of the Sulfidation of NiW/Al2O3–F Catalysts

The sulfidation behavior of alumina-supported Ni–W catalysts was investigated by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), quick extended X-ray absorption fine structure (QEXAFS), and X-ray absorption near-edge structure spectroscopy (XANES). Either ammonium tetrathiotungstate or ammonium metatungstate was used as the precursor of tungsten, and nickel nitrate was the source of nickel. The effect of fluorination of the alumina support on the sulfidation behavior of tungsten and nickel on these two series of catalysts was studied as well. The sulfidation of the catalysts prepared from ammonium metatungstate passes through W(VI) oxysulfide intermediates. Fluorination of the alumina support aids the sulfidation of tungsten and nickel at low temperature and promotes the transformation of the W(VI) oxysulfide intermediates to WS2. After sulfidation at 400°C and atmospheric pressure for 4 h, about 50% of tungsten and 60% of nickel in the catalysts prepared from ammonium metatungstate were sulfided. EXAFS showed that ammonium tetrathiotungstate supported on alumina decomposes to oxidic tungsten during the second impregnation with nickel nitrate. Nevertherless, sulfidation of the catalysts prepared from ammonium tetrathiotungstate is much easier. It also passes through W(VI) oxysulfide intermediates, and fluorination aids the formation WS2. In the sulfided catalysts prepared from ammonium tetrathiotungstate and nickel nitrate, 100% of tungsten and nickel is in the sulfided state, but a small amount of tungsten is in a {WS3} state, with fully sulfided W(VI), rather than in the WS2 state. The fluorine-containing catalyst contains a larger fraction of WS2 than the fluorine-free catalyst.