Reduction of aqueous Na2WO4 by NaBH4 at ambient temperatures to obtain lower valent tungsten oxides

A systematic investigation of the reduction of aqueous Na2WO4 with aqueous NaBH4 at ambient temperatures reveals the formation of. several lower valent tungsten oxides such as the tetragonal (x < 0.38) and cubic (x > 0.43) tungsten bronzes NaxWO3 and the binary oxides WO2 and W24O68. The nature of the product formed is influenced both by the (i) reducing power of NaBH4, which is controlled by the volume and concentration of the borohydride and the reaction pH, and (ii) the degree of condensation of the tungstate ions, which is controlled by the reaction pH. Although the reducing power of NaBH4 increases with decreasing pH, an increasing degree of condensation of the tungstate tends to lower the degree of reduction in many instances. The as-prepared samples are amorphous as revealed by X-ray diffraction and crystallize around 450 degrees C as revealed by differential scanning calorimerty. The tungsten bronzes undergo interesting crystal-chemical changes with the temperature of heating.