Disordered Vanadium Oxides

LiCo02 films obtained by PLD with polycrystalline morphology were successfully used as cathode materials in lithium microbatteries. Typical charge and discharge curves of a Li//LiCo02 cell using pulsed-laser deposited film grown at 

50 mTorr. Films deposited onto Ni foil exhibit the best discharge capacity 

Mechano-chemical technique is an efficient method to obtain high dispersed compounds directly during mechanical activation at room temperature [72]. LiMn204 was synthesized by mechanical activation in plenary mill with steel balls from the mixture Li20 and Mn02 electrochemically activated (EMD) [73]. 

The product prepared by reactive milling for 5 h shows a rather disordered spinel structure as observed by Raman scattering spectroscopy (Fig. 9.21a). For this compound the Raman active Aig mode at 625 cm is shifted to 633 cm and broadened in comparison with that of a A-LiMn204 ceramic. This disordered structure is also at the origin of the electrochemical behavior shown in Fig. 9.21b. The first charge-discharge curve recorded at the C/10 rate shows deviation from the standard electrochemical profile of LiMn204 spinel. We do not observe the 

Denton EP, Rawson H, Stanworth JE (1954) Vanadate glasses. Nature 173 1030-1032 

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From the chemical shifts measured by V NMR spectroscopy for the catalysts CP I and CP II, it appears that the bands observed between - 640 and -710 ppm correspond to vanadium in a tetrahedral environment [12] with vanadium bound to the support [13]. For the catalyst CP III, an octahedral environment is observed. For the highly vanadium loaded catalyst CP IV, the chemical shifts observed, - 300, - 650 and -1242 ppm correspond to those of bulk vanadium oxide, - 280, - 609 and - 1250 ppm [14]. This could correspond to the "disordered vanadium oxide" growing away from the surface as descrited by Bond... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

Finally Kozlowski et al146 conclude from EXAFS measurements that the catalytically active surface phase of vanadium oxide on anatase is not in epitactic registry with the Ti02 but in a state of structural disorder. 

A. Magneli, Structural order and disorder in oxides of transition metals of the titanium, vanadium, and chromium groups, in The Chemistry of Extended Defects in Non-Metallic Solids (L. Eyring and M. O Keeffe, eds.), pp. 148-162, North-Holland, Amsterdam (1970). 

The bulk stmcture of the catalyticaHy active phase is not completely known and is under debate in the Hterature (125,131—133). The central point of controversy is whether (Valone or in combination with other phases is the most catalyticaHy active for the conversion of butane to maleic anhydride. The heart of this issue concerns the role of stmctural disorder in the bulk and how it arises in the catalyst (125,134,135). Most researchers agree that the catalysts with the highest activity and selectivity ate composed mainly of (Vthat exhibits a clustered or distorted platelet morphology (125). It is also generaHy acknowledged that during operation of the catalyst, the bulk oxidation state of the vanadium in the catalyst remains very close to the +4 valence state (125). 

The selective oxidation of n-butane and 1-butene on vanadium phosphate catalysts prepared via different routes was investigated by Cavani and coworkers [77] Precursors prepared in aqueous medium were found to have greater crystallinity than those prepared in organic solvents (the activity and selectivity of which was the same for 1-butene oxidation). However, for butane activation, the crystalline catalyst was considerably less active than the organically prepared catalyst, which had an XRD pattern showing some disorder in the (100) plane. 

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