Vanadium crystal structure

Vanadium also promotes dehydrogenation reactions, but less than nickel. Vanadium s contribution to hydrogen yield is 20% to 50% of nickel s contribution, but vanadium is a more severe poison. Unlike nickel, vanadium does not stay on the surface of the catalyst. Instead, it migrates to the inner (zeolite) part of the catalyst and destroys the zeolite crystal structure. Catalyst surface area and activity are permanently lost. 

There are several theories about the chemistry of vanadium poisoning. The most prominent involves conversion of VjOj to vanadic acid (H-iVO ) under regenerator conditions. Vanadic acid, through hydrolysis, extracts the tetrahedral alumina in the zeolite crystal structure, causing it to collapse. 

This dement is important mainly because of its use as an additive to iron in the manufacture of steel. A few percent of vanadium stabilizes a high-temperature crystal structure of iron so that it persists at room temperature. This form is tougher, stronger, and more resistant to corrosion than ordinary iron. Automobile springs, for example, are often made of vanadium steel. 

A colorless mineral known as corundum (composed of aluminum oxide) is colorless. A red variety of corundum known as ruby, a precious stone, owes its color to impurities of chromium within the crystal structure of corundum. Blue and violet varieties of corundum are classified as sapphires, the blue being the result of iron and titanium impurities, and the violet of vanadium impurities within the corundum crystal structure. Another colorless mineral is beryl (composed of beryllium aluminum silicate) but blue aquamarine, green emerald, and pink morganite, are precious varieties of beryl including different impurities aquamarine includes iron, emerald chromium and vanadium, and morganite manganese. 

Open M4 units are also quite rare. In the selenobromides M4Se16Br2 (M=Nb, Ta) there are tetranuclear units M4(p-Se2)r,, which result from condensation of three M2( r-Se2)2 clusters. The M-M distances within them are about 3.1 A.52 Single crystal structure determination of V5S8 revealed the presence of rhombic vanadium clusters in the structure with hinge V-V bond of 2.88 A, peripheral bonds of 3.04 A and strong intercluster bonding (V-V 2.92 A).53... 

Simple Binary and Related Compounds.—Oxides. The information presently available concerning the crystal structures and properties of vanadium oxides has been tabulated. 

Nitriloacetic acid reacts with vanadium-(m). -(iv), or -(v) oxides to form oxovanadatranetrionic acid (38). The crystal structure of [VO(CjQHgNO)2] (CjgHgNOH = 2-methyl-8-quinolinol) has shown that the vanadium atom has a five-co-ordinate trigonal-bipyramidal environment, with three oxygen atoms in the equatorial plane and the two nitrogen atoms of the bidentate ligand in the apical positions. The optimum conditions for the formation of a 1 2 2 vanadium(iv)-morin-4-aminoantipyrine system have been described as pH 5, a tenfold excess of morin (39), and a 3300-fold excess of 4-amino-antipyrine. ... 

Since little is known about crystal structures of the low-temperature mked oxides of niobia, it is difficult to explain from a theoretical point of view the good results obtained for doping with vanadium or chromium. It appears that the vanadium and chromium are present in the catalyst in a unique environment, which makes... 

R = R = Cy R = 1-adamantyl, R = CfiH 5. Te2-3,5 5 ). The vanadium analogue of the latter [V N(l-Ad)(C6H3Me2-3,5) 3] " 4 has also been characterized. Furthermore, a more efficient route to [Cr N(SiMe3)2 3] and a new crystal structure determination has been described. Three-coordinate metal amides have been treated in a general review that covers three-coordinate transition metal species with hard ligands. The electronic structure and bonding in tricoordinate amido complexes of transition metals have also been detailed... 

Another example of a complicated polynuclear complex is an octanuclear basic benzoate containing four vanadium(IU) and four zinc(II) atoms [VZn0(02CPh)3(THF)]4-2THF.27D The crystal structure shows that the distances between these four vanadium atoms are just below 3 A and therefore some V—V bonding may occur. 

Assuming the same crystal structure, explain why the density of vanadium (6.11 g-cm-3) is significantly less than that of chromium (7.19 g-cm-3). Both vanadium and chromium crystallize in a body-centered cubic lattice. 

Point defects in the form of cation vacancies () were introduced by Aykan et al. (93-95) into molybdates, tungstates, and vanadates with scheelite-type crystal structures. The authors studied the catalytic properties of more than 30 scheelite-structure phases represented by the formula A1 x< xM04 (M = molybdenum, tungsten, and/or vanadium and A may include Li, Na, K, Ag, Ca, Sr, Ba, Cd, Pb, Bi, and/or arare earth element in quantities appropriate to achieve charge balance for the normal oxidation states). It was found that the defects can be introduced... 

The ubiquitous hemoprotein chloroperoxidase (CPO) (1) continues to be of great mechanistic and practical interest following its isolation more than 40 years ago from Caldariomyces fumago (2138). The CPO gene from this filamentous fungus has been isolated and sequenced (2139), an active recombinant CPO has been produced (2140), and the crystal structure of this CPO has been determined (2141, 2142). The fungus Curvularia inaequalis contains a vanadium CPO, which has been characterized (primary and X-ray structure) (Fig. 4.1) (2143-2147), as has the vanadium haloperoxidase from Corallina officinalis (2324). This enzyme has also been studied by density functional theory lending support to the proposed mechanism of action (Scheme 4.1) (2325). A related vanadium CPO, which shares 68% primary structural identity with the Curvularia inaequalis CPO, is produced... 

Macedo-Ribeiro S, Hemrika W, Renirie R, Wever R, Messerschmidt A (1999) X-Ray Crystal Structures of Active Site Mutants of the Vanadium-Containing Chloroperoxidase from the Fungus Curvularia inaequalis. J Biol Inorg Chem 4 209... 

Isupov MN, Dalby AR, Brindley AA, Izumi Y, Tanabe T, Murshudov GN, Littlechild JA (2000) Crystal Structure of Dodecameric Vanadium-dependent Bromoperoxidase from the Red Algae Corallina officinalis. J Mol Biol 299 1035... 

The next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

Figure 5.7 a Simplified catalytic cycle for vanadium chloroperoxidase, isolated from the fungus Curvularia inaequalis based on b crystal structures ofthe native enzyme and the peroxo intermediate [30,31], Thanks to Dr. Teunie van Herk for the enzyme structure image. 

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