Vanadium redox species

A microfluidic fuel cell design based on soluble vanadium redox species was recently introduced (Fig. 4). Vanadium redox fuel cells utilize two different aqueous vanadium redox couples, and VO " "/VO, as fuel... 

Oxidants can be dksolved oxygen in aqueous form [26,39,42,62], air [25,32-35], hydrogen peroxide [45,47], vanadium redox species [18,27,29], potassium permanganate [46,49], and sodium hypochlorite [48]. 

In summary, the control of the different experimental parameters involved in the preparation step is crucial to develop new materials with improved catalytic properties. The optimal conditions found for the preparation of the VAION catalyst are pH= 5.5, V/Al ratio= 0.25 and [V]= 30 10 M. Under these conditions, the catalyst shows the maximum ACN productivity which is related to the existence of an optimum balance between the surface nitrogen species and the vanadium redox capacity [2],... 

As arguably the most weU-known RFB chemistry, VRBs take advantage of the four oxidation states of vanadium within the stability window of water. This enables operation with the same element as an electroactive species as both negative and positive electrolytes and limits concerns about solution crossover and the associated permanent deleterious effects (e.g., capacity fade, irreversible side reactions). Since the initial electrochemical studies of the V(IV)A (V) and the V(II)A (III) redox couples in 1985 [48,49] and the first demonstration of an all-vanadium redox flow cell in 1986 [50] by Skyflas-Kazacos and co-workers, VRBs have been the focus of... 

Ferrigno et al. [18] Vanadium (II) redox species (1 M) Vanadium (V) redox species 1M) Sulfuric add (25%) Bare Bare PDMS/SU8 Carbon on dqx>sit ] Au by E beam evaporator... 

Kjeang et al [29] Vanadium ll) redox species (2 M) Vanadium(V) redox species (2 M) Sulfuric acid (2 M) Bare Bare PDMS Toray carbon paper as electrodes... 

The challenge of acid-alkaline hybrid power sources lies in how to separate the acid and alkaline electrolytes effectively while maintaining ion transport between these two electrolytes. As shown in Table 11.1, some conventional electrochemical power sources do operate in two chambers with two electrolytes separated by a membrane or an ionic interface, such as the Daniell cell and most flow batteries. In these well-known electrochemical cells with two electrolytes, the pEt value of both anolyte and catholyte are almost the same and there usually exists a common species to function as a charge carrier between the anolyte and catholyte. Examples include S04 ion for Daniell battery, and H+ ion for vanadium redox flow battery, in which there is a separator to avoid electrolyte mixing. An ion-exchange... 

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

A vanadium catalyst is essential although the combination of Zn and MejSiCl is capable of reductive dimerization of aldehydes [20]. A reversible redox cycle for the in situ generated low-valent vanadium species mediating the electron transfer is achieved in the presence of Zn as the stoichiometric co-reductant (Scheme 4). 

In the NO-SCR by NH3, we note the highest reduction activity and selectivity on catalyst containing both vanadium and molybdenum than catalysts issued containing Mo or V, only. Furthermore, it should be underlined that a higher efficiency is obtained with ZSM-5 as host structure than samples issued from USY and MOR. Where a higher loss of porous volume were observed. On the basis of characterization data it has been suggested that the observed synergism in the SCR reaction is related to the existence of electronic interaction between the V and Mo species. In particular, it has been proposed that the presence of such electronic interactions modifies the catalysts redox properties, which have been claimed an essential property in the NO-SCR by NH3 reaction. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

Dominant forms of V(V) are H2VOT and F1V04 at pH 7, analogous to those of P(V). However, V(V) species are more labile and relatively easily interconvertable. Vanadium also has a rich redox chemistry which is missing with phosphorus. 

Vanadium occurs in soils predominantly as the +5 vanadate species (VO(OH)3°, V02(0H)2 and V03(0H) ) and under reducing conditions as the +4 vanadyl cation (VO +). Less commonly V + may also form and substitute for Fe in minerals. Interchange between these oxidation states with redox conditions greatly alters the solubility of V in soils. 

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