Ammonium tungstates, decompositions

A new photocatalyst, carbon-coated W18O49 active under visible light, was successfully synthesized by the same process as carbon-coated Ti02 a mixture of para-ammonium tungstate ((NH4),oW,204,5H20) with PVA was prepared in the aqueous solution and then heat-treated at 800°C for 1 h in inert atmosphere [200,201], Its activity was studied through the aqueous phase decomposition of MB, Ph, and dimethylsulfoxide ( 113)280 (DMSO) under both UV and visible light. 

One of the simplest methods of preparation is by decomposition of a thermally unstable compound. The nitrate or chloride is often preferred, sulphates tend to decompose at higher temperatures. Where the presence of residual traces of anion is to be avoided, the metal salts of organic acids are particularly useful. Formates, oxalates, acetates etc, decompose at low temperatures and often reduce the metal at the same time. For the preparation of catalysts from anions, the ammonium salt is frequently used. Metallic salts of complex acids can be used as a source of metal oxide mixtures. Decomposition of the appropriate chromate, tungstate, molybdate or vanadate will produce the mixed oxide. 

Because the decompositions of many ammonium salts evolve NHj and HjO simultaneously (or conseeutively) in a 2 1 molar ratio, it is often convenient to represent the formula of such salts in the form m(NH4)20.n(metal oxide).xH20, where m, n and x are integers (see ammonium chromates, vanadates, tungstates and molybdates below). 

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