Oxidation states vanadium

Because of the intensity of Mzp peaks, which is much lower than that of the nearby Ois peak, the vanadium oxidation state could be reliably ascertained only for ZV(a) and ZV(i) specimens with a V loading > 1% (2.5 atom nm 2). The binding energy value of the Vzpjyg component, obtained by curve fitting of the region Ois-V2p,... 

Exposure of s.o. samples to NH3-NO at 623 K, caused a slight reduction of V to V v, whereas exposure to NO-NH3-O2, did not affect the vanadium oxidation state. Exposure of reduced samples (CO at 623 K) to NH3-NO caused slight oxidation, whereas exposure to NO-NH3-O2 oxidized all V . ... 

Lithium intercalation in VeOis has been studied by Stallworth et al. ° Variable-temperature Li NMR indicated considerable mobility for Li+ in the intercalated materials. The Li NMR data were compared with ESR spectra and near-edge X-ray absorption fine structure (NEXAFS) data on the same materials, and a correlation between vanadium oxidation state (from NEXAFS data) and NMR shift was observed. The authors explained the shifts in terms of different coupling mechanisms between the and shifts. The shifts were, however, extracted from static NMR experiments, and it is possible that some of the different local environments, typically revealed in a MAS spectrum, were not seen in this study. 

The potentially serious aspects of vanadium pollution, the function of biologically occurring enzyme systems, the role of vanadium on the function of numerous enzymes, and the associated role in the insulin-mimetic vanadium compounds are inextricably linked. The key to our understanding all such functionality relies on understanding the basic chemistry that underlies it. This chemistry is determined to a significant extent by the V(IV) and V(V) oxidation states but clearly is not restricted to these states. Indeed, the redox interplay between the vanadium oxidation states can be a critical aspect of the biological functionality of vanadium, particularly in enzymes such as the vanadium-dependent nitrogenases, where redox reactions are the basis of the enzyme functionality. 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

Bruckner and Kondratenko (2006) used a similar approach to characterize VOx/Ti02 catalysts. In a separate TPR experiment carried out with a quartz reactor equipped with a UV-vis fiber optical probe, the relationship between the "absorbance" at 800 nm and the degree of reduction as determined from H2 consumption via mass spectrometry was established. The absorbance at 800 nm increased with increasing reduction of the vanadium, but not linearly. During the catalytic reaction experiment, the absorbance at 800 nm was then used to determine the average valence of vanadium. Because contributions of reduced titanium species in the analyzed spectral range could not be excluded, only a lower limit of the vanadium oxidation state could be determined, which was 4.86 at 523 K and C3H8/02 = 1 1. 

Stizza et al. (73,274) have investigated amorphous vanadium phosphates, which are also of interest in relation to a XAS study of the butane-maleic anhydride (V, P)0 catalysts (99a). From the V K edge useful information is obtained about the distortions in the vanadium coordination sphere [molecular cage effect on the pre-edge intensity (312)] and on the vanadium oxidation state. Notably, V4+ is silent to most spectroscopic methods. A mixed V4+-V5+ valence state can be measured from the energy shift of the sharp core exciton at the absorption threshold of the Is level of vanadium due to Is -f 3d derived molecular orbitals localized within the first coordination shell of vanadium ions. 

The most active and selective catalysts consist mainly of vanadyl pyrophosphate, which during operation has a vanadium oxidation state of close to -1-4. 

The final oxidation state of the vanadium in the activated catalysts varies between -1-4.00 and -1-4.40, depending on the amount of V " present, and there has been extensive discussion as to whether V " and V " " phases are important in the reaction mechanism. Ebner and Thompson (104) postulated that the V phases that are formed during the activation period are unimportant and do not contribute to the oxidation mechanism. They found that after several hundred hours on stream, the V " orthophosphate phases are reduced to (VO)2P207, giving an active catalyst with a final vanadium oxidation state of +4.00 to +4.04. The concentrations of O2 and butane in the reactant determine the time needed to equilibrate the catalyst. On the basis of this study, the authors suggested that other researchers (who foimd V -containing phases in the active catalyst) had not performed the activation process fully, or had an unfavorable redox potential in the gas stream. 

Research by Ruiz et al. (128) into catalysts with high and low P/V ratios led to the h)/pothesis that the active catalyst is made up of vanadyl P3TTophosphate in conjunction with an amorphous phase with a high vanadium oxidation state near - -5. 

Prior to this disclosure, Trifiro (154) proposed that the active catalyst is pure vanadyl pyrophosphate and found that the catalyst was characterized by a slight increase in the vanadium oxidation state after the equilibrium period. The small increase from -1-4.00 to -h4.03 was reproducible and attributed to the formation of isolated V " surface sites. The P/V ratio was proposed to be a key characteristic in the stabilization of V + within the catalyst, as VOPO4 formation becomes very difficult at P/V ratios >2.0. Trifiro had stated that a very high surface P/V ratio is required for an active and selective catalyst, and experimentally he has found surface P/V ratios of 10 1. 

Table 5. The effect of Vanadium Oxidation State on Dehydrogenation...

Recently, Carreon and Guliants reported novel hexagonal, cubic and lamellar VPO phases, which displayed improved thermal stability, desirable chemistries (i.e. the P/V ratios and vanadium oxidation states), and pore structures for the partial oxidation of n-butane [143-145]. These novel VPO phases displayed the selectivities to maleic anhydride up to 40 mol. % at 673K at 10 % n-butane conversion [146]. A conventional organic VPO catalyst containing well-crystallized vanadyl(IV) pyrophosphate, the proposed active and selective phase for n-butane oxidation to maleic anhydride, displayed the selectivities to maleic anhydride 50 mol. % under the same reaction conditions. The low yields observed for mesoporous VPO catalysts confirmed the critical role of the vanadyl pyrophosphate phase (VO)2P207 in catalyzing the oxidation of -butane to maleic anhydride. Therefore, the amorphous nature of the mesoporous VPO... 

Zeolite modified with cerium are good models compounds to study the vanadium oxidation state. 

Basically, vanadium is an efficient mediator of 0x0 transfer reactions accompanied by reduction/oxidation processes. Equation (2.19) in Section 2.3, related to the oxidation of thiolate, is a specific example where 0x0 and non-oxo vanadium are involved in a catalytic cycle. More generally, selected vanadium-mediated 0x0 transfer reactions from or to a substrate, and involving the vanadium oxidation states h-II to h-V, can be summarised as depicted in Scheme 4.6,[ 1 where X can be dimethyl sulfide, iodobenzene, triphenylphosphine and other substrates. 

It is well established [1-3] that vanadyl pyrophosphate (VO)2P207 is an essential component of the most selective VPO catalysts. For example, structural and chemical characterization studies of "reactor equilibrated" VPO catalysts indicate that the predominate crystalline phase is vanadyl pyrophosphate (VO)2P207 [1-3], that the bulk P/V ratio is close to 1.0, and that the average vanadium oxidation state is close to -1-4.0 [3-5]. A number of studies [2,5] have indicated that alkane oxidation primarily involves oxygen adspecies adsorbed at vanadium surface sites, and relatively little bulk lattice oxygen. 

Catalyst Preparation. "Reactor-equilibrated VPO catalysts were prepared by a nonaqueous procedure detailed in previous papers [14], and operated at steady-state conditions (1.5% n-butane, 15 psig reactor inlet pressure and 2000 GHSV) for approximately 3000 hours. Under steady-state conditions the catalyst gave selectivities to MA of approximately 66% at 78% conversion. XRD analysis of the reactor-equilibrated samples showed that they were monophasic (VO)2P20 . Chemical analysis gave a P/V ratio of 1.01 and vanadium oxidation state of 4.02. The samples had a BET surface area of 16.5 m /gm. 

The mean vanadium oxidation state in equilibrated (NH4)2(V0)3(P207)2-based catalysts is 4.12 [6], That means half of the vanadium ions in the amorphous phase should be pentavalent. Taking also into account the stoichiometry of the dehydration reaction the follorving equation can be written formally ... 

Vanadium phosphates (VPO) of different structure are suitable precursors of veiy active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Va H2O is transformed into (V0)2P207 applied as the n-butane oxidation catalyst. Otherwise, if VOHPO4 V2 H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(V0)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(V0)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product ( 4.11 [7]) a second, mixed-valent (V 7v ) vanadium-rich phase must be formed. 

Delaney JS, Sutton SR, NewviUe M, Jones JH, Hanson B, Dym MD, Schreiber H (2000) Synchrotron micro-XANES measrrrements of vanadium oxidation state in glasses as a function of oxygen fugacity ... 

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