Basic Reaction Steps in the Sulfidation of Crystalline Tungsten Oxides

Abstract

The sulfidation of crystalline m-WO3 and WO3·H2O and the thermal decomposition of (NH4)2WO2S2 to an {WOS2} oxysulfide and (NH4)2WS4 to {WS3} 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-WO3 and WO3·H2O. The sulfidation reaction starts at low temperatures with a reduction of the crystalline oxides by reaction with H2S without incorporation of sulfur. Only after this reduction does an exchange of terminal O2- ligands of the oxides for S2- take place. In subsequent W−S redox reactions S22- ligands form, which leads to oxysulfidic intermediates. The {WOS2} 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 (NH4)2WS4 show that the sulfidation of m-WO3 and WO3·H2O does not proceed via {WS3}-type intermediates.