Vanadium

Vanadium is a chemical element with symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The element is found only in chemically combined form in nature, but once isolated artificially, the formation of an oxide layer stabilizes the free metal somewhat against further oxidation.

Andrés Manuel del Río discovered compounds of vanadium in 1801 in Mexico by analyzing a new lead-bearing mineral he called "brown lead," and presumed its qualities were due to the presence of a new element, which he named erythronium (Greek for "red") since, upon heating, most of its salts turned from their initial color to red. Four years later, however, he was (erroneously) convinced by other scientists that erythronium was identical to chromium. Chlorides of vanadium were generated in 1830 by Nils Gabriel Sefström who thereby proved that a new element was involved, which he named "vanadium" after the Scandinavian goddess of beauty and fertility, Vanadís (Freyja). Both names were attributed to the wide range of colors found in vanadium compounds. Del Rio's lead mineral was later renamed vanadinite for its vanadium content. In 1867 Henry Enfield Roscoe obtained the pure element.

Vanadium occurs naturally in about 65 different minerals and in fossil fuel deposits. It is produced in China and Russia from steel smelter slag; other countries produce it either from the flue dust of heavy oil, or as a byproduct of uranium mining. It is mainly used to produce specialty steel alloys such as high speed tool steels. The most important industrial vanadium compound, vanadium pentoxide, is used as a catalyst for the production of sulfuric acid.

Large amounts of vanadium ions are found in a few organisms, possibly as a toxin. The oxide and some other salts of vanadium have moderate toxicity. Particularly in the ocean, vanadium is used by some life forms as an active center of enzymes, such as the vanadium bromoperoxidase of some ocean algae. Vanadium is probably a micronutrient in mammals, including humans, but its precise role in this regard is unknown.

History

Vanadium was discovered by Andrés Manuel del Río, a Spanish-Mexican mineralogist, in 1801. Del Río extracted the element from a sample of Mexican "brown lead" ore, later named vanadinite. He found that its salts exhibit a wide variety of colors, and as a result he named the element panchromium (Greek: παγχρώμιο "all colors"). Later, Del Río renamed the element erythronium (Greek: ερυθρός "red") as most of its salts turned red upon heating. In 1805, the French chemist Hippolyte Victor Collet-Descotils, backed by del Río's friend Baron Alexander von Humboldt, incorrectly declared that del Río's new element was only an impure sample of chromium. Del Río accepted Collet-Descotils' statement and retracted his claim.

In 1831, the Swedish chemist Nils Gabriel Sefström rediscovered the element in a new oxide he found while working with iron ores. Later that same year, Friedrich Wöhler confirmed del Río's earlier work. Sefström chose a name beginning with V, which had not been assigned to any element yet. He called the element vanadium after Old Norse Vanadís (another name for the Norse Vanr goddess Freyja, whose facets include connections to beauty and fertility), because of the many beautifully colored chemical compounds it produces. In 1831, the geologist George William Featherstonhaugh suggested that vanadium should be renamed "rionium" after del Río, but this suggestion was not followed.

The isolation of vanadium metal proved difficult. In 1831, Berzelius reported the production of the metal, but Henry Enfield Roscoe showed that Berzelius had in fact produced the nitride, vanadium nitride (VN). Roscoe eventually produced the metal in 1867 by reduction of vanadium(II) chloride, VCl₂, with hydrogen. In 1927, pure vanadium was produced by reducing vanadium pentoxide with calcium. The first large-scale industrial use of vanadium in steels was found in the chassis of the Ford Model T, inspired by French race cars. Vanadium steel allowed for reduced weight while simultaneously increasing tensile strength.

German chemist Martin Henze discovered vanadium in the blood cells (or coelomic cells) of Ascidiacea (sea squirts) in 1911.

Characteristics

Vanadium is a medium hard, ductile, steel-blue metal. Some sources describe vanadium as "soft", perhaps because it is ductile, malleable and not brittle. Vanadium is harder than most metals and steels (see Hardnesses of the elements (data page) and iron). It has good resistance to corrosion and it is stable against alkalis, sulfuric and hydrochloric acids. It is oxidized in air at about 933 K (660 °C, 1220 °F), although an oxide layer forms even at room temperature.

Isotopes

Naturally occurring vanadium is composed of one stable isotope ⁵¹V and one radioactive isotope ⁵⁰V. The latter has a half-life of 1.5×10¹⁷ years and a natural abundance 0.25%. ⁵¹V has a nuclear spin of 7/2 which is useful for NMR spectroscopy. 24 artificial radioisotopes have been characterized, ranging in mass number from 40 to 65. The most stable of these isotopes are ⁴⁹V with a half-life of 330 days, and ⁴⁸V with a half-life of 16.0 days. The remaining radioactive isotopes have half-lives shorter than an hour, most of which are below 10 seconds. At least 4 isotopes have metastable excited states. Electron capture is the main decay mode for isotopes lighter than the ⁵¹V. For the heavier ones, the most common mode is beta decay. The electron capture reactions lead to the formation of element 22 (titanium) isotopes, while for beta decay, it leads to element 24 (chromium) isotopes.

Compounds

The chemistry of vanadium is noteworthy for the accessibility of the four adjacent oxidation states 2-5. In aqueous solution, vanadium forms metal aquo complexes the colours are lilac [V(H₂O)₆]²⁺, green [V(H₂O)₆]³⁺, blue [VO(H₂O)₅]²⁺, yellow VO₃^âˆ'. Vanadium(II) compounds are reducing agents, and vanadium(V) compounds are oxidizing agents. Vanadium(IV) compounds often exist as vanadyl derivatives which contain the VO²⁺ center.

Ammonium vanadate(V) (NH₄VO₃) can be successively reduced with elemental zinc to obtain the different colors of vanadium in these four oxidation states. Lower oxidation states occur in compounds such as V(CO)₆, [V(CO)
6]âˆ'
and substituted derivatives.

The most commercially important compound is vanadium pentoxide. It is used as a catalyst for the production of sulfuric acid. This compound oxidizes sulfur dioxide (SO
2) to the trioxide (SO
3). In this redox reaction, sulfur is oxidized from +4 to +6, and vanadium is reduced from +5 to +4:

V₂O₅ + SO₂ â†' 2 VO₂ + SO₃

The catalyst is regenerated by oxidation with air:

2 VO₂ + O₂ â†' V₂O₅

Similar oxidations are used in the production of maleic anhydride, phthalic anhydride, and several other bulk organic compounds.

Oxyanions

In aqueous solution, vanadium(V) forms an extensive family of oxyanions. The interrelationships within this family are described by the predominance diagram, shows at least 11 species, depending on pH and concentration. The tetrahedral orthovanadate ion, VO3âˆ'
4, is the principal species present at pH 12-14. Analogies exist between orthovanadate and orthophosphate owing to the similarity in size and charge of phosphorus(V) and vanadium(V). Orthovanadate VO3âˆ'
4 is used in protein crystallography to study the biochemistry of phosphate. The tetrathiovanadate [VS₄]^3âˆ' is analogous to the orthovanadate ion.

At lower pH's, the monomer [HVO₄]^2âˆ' and dimer [V₂O₇]^âˆ' are formed, with the monomer predominant at vanadium concentration of less than ca. 10^âˆ'2M (pV > 2; pV is equal to minus the logarithm of the total vanadium concentration/M). The formation of the divanadate ion is analogous to the formation of the dichromate ion. As the pH is reduced, further protonation and condensation to polyvanadates occur: at pH 4-6 [H₂VO₄]^âˆ' is predominant at pV greater than ca. 4, while at higher concentrations trimers and tetramers are formed. Between pH 2-4 decavanadate predominates, its formation from orthovanadate is represented by this condensation reaction:

10 [VO₄]^3âˆ' + 24 H⁺ â†' [V₁₀O₂₈]^6âˆ' + 12 H₂O

In decavanadate, each V(V) center is surrounded by six oxide ligands. Vanadic acid, H₃VO₄ exists only a very low concentrations because protonation of the tetrahedral species [H₂VO₄]^âˆ' results in the preferential formation of the octahedral [VO₂(H₂O)₄]⁺ species. In strongly acidic solutions, pH<2. [VO₂(H₂O)₄]⁺ is the predominant species, while the oxide V₂O₅ precipitates from solution at high concentrations. The oxide is formally the inorganic anhydride of vanadic acid. The structures of many vanadate compounds have been characterized by X-ray crystallography.

The Pourbaix diagram for vanadium in water, which shows the redox potentials between various vanadium species in different oxidation states is also complex.

Vanadium(V) also forms various peroxo complexes, most notably in the active site of the vanadium-containing bromoperoxidase enzymes. The species VO(O)₂(H₂O)₄⁺ is stable in acidic solutions. In alkaline solutions species with 2, 3 and 4 peroxide groups are known; the last forms violet salts with the formula M₃V(O₂)₄ nH₂O (M = Li, Na, etc.), in which the vanadium has an 8-coordinate dodecahedral structure.

Halide derivatives

Twelve binary halides, compounds with the formula VX_n, are known. VI₄, VCl₅, VBr₅, and VI₅ do not exist or are extremely unstable. In combination with other reagents, VCl₄ is used as a catalyst for polymerization of dienes. Like all binary halides, those of vanadium are Lewis acidic, especially those of V(IV) and V(V). Many of the halides form octahedral complexes with the formula VX_nL_6âˆ'n (X = halide; L = other ligand).

Many vanadium oxyhalides (formula VO_mX_n) are known. The oxytrichloride and oxytrifluoride, VOF₃) and VOCl₃) are the most widely studied. Akin to POCl₃, they are volatile, adopt tetrahedral structures in the gas phase, and are Lewis acidic.

Coordination compounds

Complexes of vanadium(II) and (III) are relatively exchange inert and reducing. Those of V(IV) and V(V) are oxidants. Vanadium ion is rather large and some complexes achieve coordination numbers greater than 6, as is the case in [V(CN)₇]^4âˆ'. The coordination chemistry of V⁴⁺ is dominated by the vanadyl center, VO²⁺, which binds four other ligands strongly and one weakly (the one trans to the vanadyl center). An example is vanadyl acetylacetonate (V(O)(O₂C₅H₇)₂). In this complex, the vanadium is 5-coordinate, square pyramidal, meaning that a sixth ligand, such as pyridine, may be attached, though the association constant of this process is small. Many 5-coordinate vanadyl complexes have a trigonal bypyramidal geometry, such as VOCl₂(NMe₃)₂. The coordination chemistry of V⁵⁺ is dominated by the polyoxovanadates, such as decavanadate.

Organometallic compounds

Organometallic chemistry of vanadium is well developed, although they are mainly only academic significance. Vanadocene dichloride is a versatile starting reagent and even finds some applications in organic chemistry. Vanadium carbonyl, V(CO)₆, is a rare example of a paramagnetic metal carbonyl. Reduction yields V(CO)âˆ'
6 (isoelectronic with Cr(CO)₆), which may be further reduced with sodium in liquid ammonia to yield V(CO)3âˆ'
5 (isoelectronic with Fe(CO)₅).

Occurrence

Metallic vanadium is not found in nature, but vanadium compounds occur naturally in about 65 different minerals. Economically significant examples include patronite (VS₄), vanadinite (Pb₅(VO₄)₃Cl), and carnotite (K₂(UO₂)₂(VO₄)₂·3H₂O). Much of the world's vanadium production is sourced from vanadium-bearing magnetite found in ultramafic gabbro bodies. Vanadium is mined mostly in South Africa, north-western China, and eastern Russia. In 2010 these three countries mined more than 98% of the 56,000 tonnes of produced vanadium.

Vanadium is also present in bauxite and in fossil fuel deposits such as crude oil, coal, oil shale and tar sands. In crude oil, concentrations up to 1200 ppm have been reported. When such oil products are burned, the traces of vanadium may initiate corrosion in motors and boilers. An estimated 110,000 tonnes of vanadium per year are released into the atmosphere by burning fossil fuels. Vanadium has also been detected spectroscopically in light from the Sun and some other stars.

Production

Most vanadium is used as an alloy called ferrovanadium as an additive to improve steels. Ferrovanadium is produced directly by reducing a mixture of vanadium oxide, iron oxides and iron in an electric furnace. The vanadium ends up in pig iron produced from vanadium bearing magnetite. Depending on the ore used, the slag contains up to 25% of vanadium.

Vanadium metal is obtained via a multistep process that begins with the roasting of crushed ore with NaCl or Na₂CO₃ at about 850 °C to give sodium metavanadate (NaVO₃). An aqueous extract of this solid is acidified to give "red cake", a polyvanadate salt, which is reduced with calcium metal. As an alternative for small-scale production, vanadium pentoxide is reduced with hydrogen or magnesium. Many other methods are also in use, in all of which vanadium is produced as a byproduct of other processes. Purification of vanadium is possible by the crystal bar process developed by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925. It involves the formation of the metal iodide, in this example vanadium(III) iodide, and the subsequent decomposition to yield pure metal.

2 V + 3 I₂ 2 VI₃

Applications

Alloys

Approximately 85% of vanadium produced is used as ferrovanadium or as a steel additive. The considerable increase of strength in steel containing small amounts of vanadium was discovered in the beginning of the 20th century. Vanadium forms stable nitrides and carbides, resulting in a significant increase in the strength of the steel. From that time on vanadium steel was used for applications in axles, bicycle frames, crankshafts, gears, and other critical components. There are two groups of vanadium containing steel alloy groups. Vanadium high-carbon steel alloys contain 0.15% to 0.25% vanadium and high speed tool steels (HSS) have a vanadium content of 1% to 5%. For high speed tool steels, a hardness above HRC 60 can be achieved. HSS steel is used in surgical instruments and tools. Some powder metallurgic alloys can contain up to 18% percent vanadium. The high content of vanadium carbides in those alloys increases the wear resistivity significantly. One application for those alloys are tools and knives.

Vanadium stabilizes the beta form of titanium and increases the strength and temperature stability of titanium. Mixed with aluminium in titanium alloys it is used in jet engines, high-speed airframes and dental implants. One of the common alloys is Titanium 6AL-4V, a titanium alloy with 6% aluminium and 4% vanadium.

Other uses

Vanadium is compatible with iron and titanium, therefore vanadium foil is used in cladding titanium to steel. The moderate thermal neutron-capture cross-section and the short half-life of the isotopes produced by neutron capture makes vanadium a suitable material for the inner structure of a fusion reactor. Several vanadium alloys show superconducting behavior. The first A15 phase superconductor was a vanadium compound, V₃Si, which was discovered in 1952. Vanadium-gallium tape is used in superconducting magnets (17.5 teslas or 175,000 gauss). The structure of the superconducting A15 phase of V₃Ga is similar to that of the more common Nb₃Sn and Nb₃Ti.

The most common oxide of vanadium, vanadium pentoxide V₂O₅, is used as a catalyst in manufacturing sulfuric acid by the contact process and as an oxidizer in maleic anhydride production. Vanadium pentoxide is also used in making ceramics. Another oxide of vanadium, vanadium dioxide VO₂, is used in the production of glass coatings, which blocks infrared radiation (and not visible light) at a specific temperature. Vanadium oxide can be used to induce color centers in corundum to create simulated alexandrite jewelry, although alexandrite in nature is a chrysoberyl. The possibility to use vanadium redox couples in both half-cells, thereby eliminating the problem of cross contamination by diffusion of ions across the membrane is the advantage of vanadium redox rechargeable batteries. Vanadate can be used for protecting steel against rust and corrosion by electrochemical conversion coating. Lithium vanadium oxide has been proposed for use as a high energy density anode for lithium ion batteries, at 745 Wh/L when paired with a lithium cobalt oxide cathode. It has been proposed by some researchers that a small amount, 40 to 270 ppm, of vanadium in Wootz steel and Damascus steel, significantly improves the strength of the material, although it is unclear what the source of the vanadium was. Lithium vanadium phosphate has been proposed for a new battery as well,and is very commercially applicable because phosphates are inexpensive and vanadium makes the battery very energy dense.

Biological role

Vanadium plays a very limited role in human biology. It is more important in marine environments than terrestrial ones.

Vanadoenzymes

A number of species of marine algae produce vanadium-containing vanadium bromoperoxidase as well as the closely related chloroperoxidase (which may use a heme or vanadium cofactor) and iodoperoxidases. The bromoperoxidase produces an estimated 1â€"2 million tons of bromoform and 56,000 tons of bromomethane annually. Most naturally occurring organobromine compounds, accounting arise by the action of this enzyme. They catalyse the following reaction (R-H is hydrocarbon substrate):

R-H + Br^âˆ' + H₂O₂ â†' R-Br + H₂O + OH^âˆ'

A vanadium nitrogenase is used by some nitrogen-fixing micro-organisms, such as Azotobacter. In this role vanadium replaces more common molybdenum or iron, and gives the nitrogenase slightly different properties.

Vanadium accumulation in tunicates and ascidians

Vanadium is essential to ascidians and tunicates, where it is stored in the highly acidified vacuoles of certain blood cell types, designated vanadocytes. Vanabins (vanadium binding proteins) have been identified in the cytoplasm of such cells. The concentration of vanadium in these ascidians' blood is up to ten million times higher than the concentration of vanadium in surrounding seawater, which normally contains 1 to 2 µg/l. The function of this vanadium concentration system, and these vanadium-containing proteins, is still unknown but the vanadocytes are later deposited just under the outer surface of the tunic where their presence may deter predation.

Fungi

Several species of macrofungi, namely Amanita muscaria and related species, accumulate vanadium (up to 500 mg/kg in dry weight). Vanadium is present in the coordination complex amavadin, in fungal fruit-bodies. However, the biological importance of the accumulation process is unknown. Toxin functions or peroxidase enzyme functions have been suggested.

Mammals and birds

Deficiencies in vanadium result in reduced growth and impaired reproduction in rats and chickens. Vanadium is a relatively controversial dietary supplement, used primarily for increasing insulin sensitivity and body-building. Whether it works for the latter purpose has not been proven; some evidence suggests that athletes who take it are merely experiencing a placebo effect. Vanadyl sulfate may improve glucose control in people with type 2 diabetes. Decavanadate and oxovanadates appear to play a role in a variety of biochemical processes, such as those relating to oxidative stress.

Safety

All vanadium compounds should be considered toxic. Tetravalent VOSO₄ has been reported to be over 5 times more toxic than trivalent V₂O₃. The Occupational Safety and Health Administration (OSHA) has set an exposure limit of 0.05 mg/m³ for vanadium pentoxide dust and 0.1 mg/m³ for vanadium pentoxide fumes in workplace air for an 8-hour workday, 40-hour work week. The National Institute for Occupational Safety and Health (NIOSH) has recommended that 35 mg/m³ of vanadium be considered immediately dangerous to life and health. This is the exposure level of a chemical that is likely to cause permanent health problems or death.

Vanadium compounds are poorly absorbed through the gastrointestinal system. Inhalation exposures to vanadium and vanadium compounds result primarily in adverse effects on the respiratory system. Quantitative data are, however, insufficient to derive a subchronic or chronic inhalation reference dose. Other effects have been reported after oral or inhalation exposures on blood parameters, on liver, on neurological development in rats, and other organs.

There is little evidence that vanadium or vanadium compounds are reproductive toxins or teratogens. Vanadium pentoxide was reported to be carcinogenic in male rats and male and female mice by inhalation in an NTP study, although the interpretation of the results has recently been disputed. Vanadium has not been classified as to carcinogenicity by the United States Environmental Protection Agency.

Vanadium traces in diesel fuels present a corrosion hazard; it is the main fuel component influencing high temperature corrosion. During combustion, it oxidizes and reacts with sodium and sulfur, yielding vanadate compounds with melting points down to 530 °C, which attack the passivation layer on steel, rendering it susceptible to corrosion. The solid vanadium compounds also cause abrasion of engine components.

See also

• Vanadium redox battery
• Grid energy storage
• Flow battery
• Vanadium carbide
• Vanadium(V) oxide
• Vanadium tetrachloride

References

Further reading

• Slebodnick, Carla et al. (1999). "Modeling the Biological Chemistry of Vanadium: Structural and Reactivity Studies Elucidating Biological Function". In Hill, Hugh A.O. et al. Metal sites in proteins and models: phosphatases, Lewis acids, and vanadium. Springer. ISBN 978-3-540-65553-4. 

External links

• Vanadium at The Periodic Table of Videos (University of Nottingham)
• National Instrument Vanadium Technical Report Vanadium recovery methods
• ATSDR â€" ToxFAQs: Vanadium

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