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

Abstract

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 31 P magic angle spinning nuclear magnetic resonance show that there is only one type of phosphorus with a Knight shift of 255 ppm from 85% H 3 PO 4, which is located in a highly anisotropic environment. Numerical simulations of extended X-ray absorption fine structure data at the W L 3 -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/SiO 2 ) are found to be more active in HDS and HDN than tungsten carbide (WC), tungsten nitride (W 2 N), tungsten sulfide (WS 2 ), and more active in HDN than a commercial Ni−Mo−S/Al 2 O 3 catalyst. The enhanced activity may be related to a considerably reduced white line area in the X-ray absorption edge of WP and WP/SiO 2 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.