Reduction of Aqueous Na(2)WO(4) by NaBH(4) at Ambient Temperatures To Obtain Lower Valent Tungsten Oxides.

A systematic investigation of the reduction of aqueous Na(2)WO(4) with aqueous NaBH(4) 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 Na(x)()WO(3) and the binary oxides WO(2) and W(24)O(68). The nature of the product formed is influenced both by the (i) reducing power of NaBH(4), 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 NaBH(4) 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.