Molybdo-vanadates

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

Molybdo-vanadosilicates.—The simple vanado-silicates do not appear to be stable, but a number of molybdo-vanadosilicates have been obtained either by acidifying solutions which contain molybdates, vanadates and silicates, or by addition of vanadium pentoxide to the molybdo-silicates. They form heavy, yellowish-red to brownish-red crystals, which are very easily hydrolysed and are, therefore, not always reproducible. The following isomorphous series has been recognised —... 

Tungsto-vanadosilicates.—These complexes are analogous to the molybdo-vanadosilicates, and are prepared as yellowish-red to red crystals by the action of vanadates on the silico-tungstates, or by the action of hydrofluosilicic acid on the tungsto-vanadates. The following series has been recognised —... 

Vanadium, the first element in Subgroup oA, and neighbour to chromium, is found in association with molybdenum in certain complex compounds. For example, when boiling solutions of molybdo-oxalates react with vanadium pentoxide, crystalline products are obtained which are thought to be substituted vanadates containing the complex anion... 

Spectrophotometric methods are usually preferred for routine analysis of this parameter, most of them relying on the reaction between orthophosphate ions and molybdate in acidic medium in order to form a heteropoly acid. Color formation can be enhanced by adding vanadate to obtain the yellow vanadomolybdate complex (vanadomolybdophosphoric acid method) or by reducing the molybdo-phosphoric acid to yield strongly colored phosphomolybdenum blue species. 

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