Transition metal oxides vanadium oxide

Theory of physical and chemical behavior of transition metal oxides vanadium and molybdenum oxides... 

In transition metal oxides, multiple oxidation states and the propensity for covalent bonding affect both the bulk and surface structures. The change of oxidation state of the anions typically gives these materials a range of possible compositions and is also the basis of much of the catalysis in which they are involved. One example for which there are published calculations is vanadium pentoxide, V2O5. Vanadia-based compounds are used as catalysts for the oxidation of hydrocarbons and other organic molecules under mild conditions and also find application as NOx reduction catalysts by selective reduction with NH3. 

MetaUic conduction occurs in transition-metal oxides such as ReO, vanadium(II) oxide [12035-98-2] VO, titanium(II) oxide [12137-20-17,... 

A thin layer deposited between the electrode and the charge transport material can be used to modify the injection process. Some of these arc (relatively poor) conductors and should be viewed as electrode materials in their own right, for example the polymers polyaniline (PAni) [81-83] and polyethylenedioxythiophene (PEDT or PEDOT) [83, 841 heavily doped with anions to be intrinsically conducting. They have work functions of approximately 5.0 cV [75] and therefore are used as anode materials, typically on top of 1TO, which is present to provide lateral conductivity. Thin layers of transition metal oxide on ITO have also been shown [74J to have better injection properties than ITO itself. Again these materials (oxides of ruthenium, molybdenum or vanadium) have high work functions, but because of their low conductivity cannot be used alone as the electrode. 

As mentioned in the previous section, transition metals in high oxidation states exhibit behavior that is similar to that of some nonmetals. Vanadium(V) does this with the formation of oxyhalides having the formulas VOX3 and V02X. 

Other transition-metal oxidants can convert alkenes to epoxides. The most useful procedures involve /-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium, molybdenum, or titanium compounds. The most reliable substrates for oxidation are allylic alcohols. The hydroxyl group of the alcohol plays both an activating and a stereodirecting role in these reactions. /-Butyl hydroperoxide and a catalytic amount of VO(acac)2 convert allylic alcohols to the corresponding epoxides in good yields.44 The reaction proceeds through a complex in which the allylic alcohol is coordinated to... 

The effect of chemisorption temperature on the ammonia uptake capacity of 6.5 wt% V20c/Ti02 is shown in Fig. 1. Ammonia chemisorption capacities increase with temperature upto 150°C and then decrease with further Increase up to 400°C. It is worth noting that there is considerable NH uptake even at 400°C. These results are in accordance with the reported literature. A number of studies have been reported on the acidic character of supported transition-metal oxides (22,34-38). Ammonia on V20g can be either adsorbed in the form of NH species on Bronsted acid sites or coordlnatively bonded to vanadium ions on Lewis acid sites (39,40). The latter species were observed up to 250°C,... 

The group 5-7 supported transition metal oxides (of vanadium, niobium, tantalum, chromium, molybdenum, tungsten, and rhenium) are characterized by terminal oxo bonds (M =0) and bridging oxygen atoms binding the supported oxide to the cation of the support (M -0-MSUpport). The TOF values for ODH of butane or ethane on supported vanadia were found to depend strongly on the specific oxide support, varying by a factor of ca. 50 (titania > ceria > zirconia > niobia > alumina > silica). 

Salts of other transition metals including vanadium, cerium, chromium and manganese have been used for a-oxygenation, although rarely applied in synthesis. Manganese triacetate has been used for the efficient a -oxidation of enones (Section 2.3.2.2.1.i), but appears not to have been used for the a-hydrox-ylation of saturated ketones des]Hte its known ability to form the corresponding a-keto radicals. Similarly the use of Lewis acid assisted enolization in the oxidative process appears to have been limited to the LTA-mediated examples. 

In addition to the antimony fluorides, silver, mercury, thallium, aluminium, zinc, zirconium, chromium and other fluorides [7] such as mercury(II) fluoride, vanadium pentafluoride [24] and various transition metal oxide fluorides [25] have been used in exchange processes, although much less widely. 

Among the transition metal oxides, mention should also be made of the vanadium oxides, the most studied as positive electrode materials being V2O5, V Oii, and LiV30g, which insert lithium in the potential domain of 3 V vs. Li/Li+. For the vanadium oxides, too, doping and suitable structure have been shown to improve their electrochemical performance, and recently a sol-gel process has yielded high-capacity (500-600 Ah kg ) materials delivering 500 Wh kg at 4 mA cm" [137]. 

Perhaps the most important area of biochemistry in which ESR is used is the study of metalloproteins. Transition metals in certain oxidation and spin states have unpaired electrons, are paramagnetic, and in many cases are amenable to ESR spectroscopy. The most commonly found transition metals in biological systems are iron, copper, molybdenum, cobalt, and manganese. The remainder, including metals such as vanadium and... 

Aluminum is the third most abundant element in the earth s crust (after oxygen and silicon), accounting for 8.2% of the total mass. It occurs most commonly in association with silicon in the aluminosilicates of feldspars and micas and in clays, the products of weathering of these rocks. The most important ore for aluminum production is bauxite, a hydrated aluminum oxide that contains 50% to 60% AI2O3 1% to 20% FeiOs 1% to 10% silica minor concentrations of titanium, zirconium, vanadium, and other transition-metal oxides and the balance (20% to 30%) water. Bauxite is purified via the Bayer process, which takes advantage of the fact that the amphoteric oxide alumina is soluble in strong bases but iron(III) oxide is not. Crude bauxite is dissolved in sodium hydroxide... 

There is considerable overlap in the boiling ranges of the diesel, jet, kerosene, and light fuel oils represented by the distillate fuel oil class. All, if desulfurized, have the capability of burning relatively cleanly with little more than a trace of ash as a residue. The ash consists of transition metal oxides (mainly vanadium). 

A redox mechanism (Mars-van Krevelen) is generally accepted for the ammoxidation reaction of methyl aromatic compounds, thus most catalysts applied contain transition metal oxides (e. g. vanadium, molybdenum) readily enabling changes in valence states. 

As this review is intended to illustrate, the interplay between metal and oxygen leads to a richness of reactivity that is reflected in the surface structure of oxides. Much of this richness can be rationalised as varying proportions of ionic and covalent character in the metal-oxygen bonding, and is manifest in a variety of non-stoichiometric surfeces. We therefore focus on the prototypical transition metal oxide smface rutile Ti(>2 (1 1 0). This is contrasted with computational results for one of the most widely-studied p-block oxide surfaces - corundum Al2O3-(0 0 0 1) - and we refer also to computational surface studies on oxides of ruthenium, iron, vanadium, tin and silver, as well as ternary oxides. 

Many industrially important selective oxidation reactions are catalyzed by transition metal oxides. The activity of such catalysts is related to the reducibility of the transition metal ion, which enables the bulk oxide lattice to participate actively in the redox processes present in the Mars van Krevelen mechanism. Unfortunately, NMR spectroscopic investigations are severely limited by the occurrence of paramagnetic oxidation states. As a general rule, NMR signals from atoms bearing unpaired electron spins cannot be detected by conventional methtxls, and the spectra of atoms nearby are often severely broadened. For this reason, most of the work published in this area has dealt with diamagnetic vanadium(V) oxide-based catalysts. 

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