Vanadium solid-state reaction

Salts of hexafluorovanadate can be obtained by high temperature techniques or from solutions containing hydrofluoric acid. For instance, X-ray patterns and DTA were used for the characterization of 17 double fluorides obtained by solid state reactions in the systems VF3 + MF (M = Li, Na, K, Rb, Cs, Tl) and seven double fluorides in the systems VF3 + MF2 (M = Ca, Sr, Ba, Pb),296 and the lattice constants297,29s and magnetic properties298 of A2B[VF6] (A, B = Cs, Rb, Tl, K, Na, Li) were also reported. The structure of the high temperature j0 phase of Li3VF6 has been determined and compared with the cryolite-type stable a form. The vanadium atoms have an octahedral coordination.299... 

An ESR study by Yabrov et al. [355] revealed that, at least at low V205 content (0.05—5 wt. %), vanadium forms a solid solution of V4+ and V3+ in Ti02. The samples investigated were sealed in the reactor after steady state operation of the o-xylene oxidation at 350°C. The V4+ solid solution, which is considered the active phase, is not formed by the catalyst pretreatment at high temperature, but requires the interaction of the reaction mixture as was shown by the analysis of fresh catalysts. Solid state reactions between V2Os and Ti02 were also studied by Cole et al. [89]. 

We shall summarize here fundamental results which point to newly discovered mechanisms which permit a control of ageing processes in catalysts. These mechanisms involve the acdon of surface mobile species, so-called spillover. The spillover species can stabilize catalysts against harmful solid-state reactions, in particular prevent reduction to less selective phases. Such reactions occur very frequently in selective oxidation catalysts, and constitute a major cause of deactivation. A typical example is constituted by vanadium phosphate catalysts used in the selective oxidation of butane to maleic ahydride. A few years ago, for example, many such catalysts lost a large part of their selectivity in a few months this selectivity dropped from the modest initial molar value of 55-60% to 45% or less. 

Most of the supported oxides referred to above are reduced under the reducing conditions mentioned however, contrasting behavior has been observed for ceria-supported vanadia. Ce4+ is reduced to Ce3+ while vanadium remains fully oxidized it is assumed that, as a consequence of the strong interaction between vanadia surface species and the ceria support, reduction of ceria occurs at the interface (Martinez-Huerta et al., 2004). A TPR/TPO-Raman investigation by Martinez-Huerta et al. showed that the solid-state reaction between surface vanadia and the ceria support to form CeV04 is facilitated in a reducing atmosphere at approximately 200 °C (Martinez-Huerta et al., 2007). 

Deactivation of SCR catalysts also occurs by solid-state reactions between the reagents or poisons and the catalytically active surface. The active metal oxides are thus reduced (or over oxidized) to inactive oxidation states. As an example, if, as often presumed, is the active form of vanadium, then the formation of vanadium pentoxide (as a separate phase) would result in a loss of activity. Alternately, vanadium may be reduced to vanadium +3 or less which, again, may be inactive. 

Centi, G., Giamello, E., Pinelli, D., et al. (1991). Surface Structure and Reactivity of Vanadium-titanium Oxide Catalysts Prepared by Solid-state Reaction. 1. Formation of a Vanadium(IV) Interacting Layer, J. Catal, 130, pp. 220-237. 

In similar experiments, Ignatovych et al. [205] modified H-ZSM-5 and characterized the oxidation state, coordination and dispersion of vanadium in the products of the solid-state reaction by UV-Vis diffuse reflectance spectroscopy as well as photoluminescence spectroscopy. It was shown that the reaction conditions, i.e., calcination of, e.g., V2O5/NH4-ZSM-5 in (i) a flow of O2 at 973 K or (ii) N2 at 773 K, affected the properties of the products in the case of (i) isolated and tetrahedral were predominant, whereas in case (ii) mainly polyvana-... 

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