Structural Characterization of Tungsten Phosphide (WP) Hydrotreating Catalysts by X-ray Absorption Spectroscopy and Nuclear Magnetic Resonance Spectroscopy

This paper presents a detailed discussion of the structure of tungsten phosphide and its relationship to simpler crystalline forms. The structure (space group Pbnm) is based on distorted hexagonal prisms with the phosphorus atoms forming P−P chains. Measurements by 31P magic angle spinning nuclear magnetic resonance show that there is only one type of phosphorus with a Knight shift of 255 ppm from 85% H3PO4, which is located in a highly anisotropic environment. Numerical simulations of extended X-ray absorption fine structure data at the W L3-edge give good agreement with crystallographic data. The W−P distance in WP was found to be 0.246 nm compared to the value of 0.247 nm obtained by X-ray diffraction. The catalytic activity of tungsten phosphide in the hydrotreating of a simulated petroleum feed is reported and compared to those of other tungsten compounds on an areal basis. Bulk (WP) and supported tungsten phosphide (WP/SiO2) are found to be more active in HDS and HDN than tungsten carbide (WC), tungsten nitride (W2N), tungsten sulfide (WS2), and more active in HDN than a commercial Ni−Mo−S/Al2O3 catalyst. The enhanced activity may be related to a considerably reduced white line area in the X-ray absorption edge of WP and WP/SiO2 compared to W metal, which makes their electronic structure resemble those of the highly active elements, Os and Ir, to the right in the periodic table.