Basic Reaction Steps in the Sulfidation of Crystalline Tungsten Oxides

The sulfidation of crystalline m -WO 3 and WO 3 ·H 2 O and the thermal decomposition of (NH 4 ) 2 WO 2 S 2 to an {WOS 2 } oxysulfide and (NH 4 ) 2 WS 4 to {WS 3 } were studied by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), thermogravimetrical analysis (TGA), and laser Raman spectroscopy. Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structures of crystalline m -WO 3 and WO 3 ·H 2 O. The sulfidation reaction starts at low temperatures with a reduction of the crystalline oxides by reaction with H 2 S without incorporation of sulfur. Only after this reduction does an exchange of terminal O 2- ligands of the oxides for S 2- take place. In subsequent W−S redox reactions S 2 2- ligands form, which leads to oxysulfidic intermediates. The {WOS 2 } oxysulfide phase serves as a reference of structural features of oxysulfides. The presence of oxygen at bridging and terminal positions as well as of disulfide ligands seems to be a common structural property of oxysulfides. The decomposition experiments with (NH 4 ) 2 WS 4 show that the sulfidation of m -WO 3 and WO 3 ·H 2 O does not proceed via {WS 3 }-type intermediates.