Metavanadates polymeric

The polymeric metavanadate anions have been described above. Upon warming or ageing solutions that contain the decavanadate ion, it is possible to precipitate salts of the dark red trivanadate anion, MV3Os (M = NH4, K, Rb, Cs). The formula is sometimes written as M2V60i6, and the salts called hexavanadates in the literature. Potassium and cesium trivanadate contain infinite buckled layers of distorted VOs and VOe polyhedra.18 Another layer structure is found in yellow K3VsOi4 which also is deposited from aqueous solution. The [V5O14]3 - anion is composed of layers of comer-shared V04 tetrahedra and VO5 square pyramids.19... 

Evidence for polymeric species of the metavanadate type in submonolayer vanadia on titania catalysts is obtained by the appearance of a Vy=o stretching mode at 870 cm [68]. The stretching mode of V=0 for isolated vanadyls is found at 1030 cm ... 

Lietti and Forzatti [69] have shown by means of transient techniques such as TPD, TPSR, TPR and SSR (steady-state reaction experiments) that isolated vanadyls and polymeric metavanadate species are present on the surface of vanadia on titania catalysts with V2O5 loadings of up to 3.56 wt.-%. Polyvanadate species are more reactive than isolated vanadyls due to the presence of more weakly bonded oxygen atoms. 

The experimental results confirm that lattice oxygen participates in the reaction. Water is formed at two sites the first part comes from the reaction of gaseous nitric oxide and adsorbed ammonia via an Eley-Rideal mechanism the other part comes from a surface dehydration process. Two adjacent V =0 groups were supposed to be the active site. From the work of Lietti and Forzatti [69] it appeared that the oxygen atoms of monomeric species are less reactive than those of polymeric metavanadate species. 

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