Synthesis and characterization of mesostructured tungsten nitride by using tungstic acid as the precursor

Mesostructured tungsten nitride was firstly prepared from tungstic acid via the temperature programmed reaction with ammonia. The N-2 adsorption isotherm of as-synthesized tungsten nitride was of type IV with a type H-3 hysteresis loop. BJH pore size distribution was of bimodal distribution (2.5 and 3.5 nm), among which the latter was the main channel of tungsten nitride. The surface area of as-synthesized tungsten nitride was up to 89 m(2)g(-1). XRD pattern showed that the crystal phase of the product was beta-W2N. The effect of synthesis parameters on the surface area of tungsten nitride was investigated extensively. The nitridation mechanism was investigated by in-situ XRD and N-2 adsorption analysis. It was found that H2WO4 was initially transformed into WO3 by eliminating the axial water molecules, and WO3 retained the layered and porous structure of H2WO4. Below 773 K, WO3 was just partially reduced to W20O58 and W20O40. Above 773 K, beta-W2N phase could be detected. It indicated that during nitridation, WO3 was gradually reduced and then the homogeneous substitution of oxygen vacancies in the reduced oxides with nitrogen atoms occurred. Based on the experimental results, a reduction-nitridation mechanism was firstly proposed.