Vanadium(V) oxide

Vanadium(V) oxide (vanadia) is the chemical compound with the formula V₂O₅. Commonly known as vanadium pentoxide, this orange solid is the most important^{[citation needed]} compound of vanadium. Upon heating it reversibly loses oxygen. Related to this ability, V₂O₅ catalyses several useful aerobic oxidation reactions, the largest scale of which underpins the production of sulfuric acid from sulfur dioxide. It is a poisonous orange solid which, because of its high oxidation state, is both an amphoteric oxide and an oxidising agent. Unlike most metal oxides, it dissolves slightly in water due to hydrolysis.

Vanadium(V) indicates that vanadium is in the +5 oxidation state. The oxygen atoms in the compound are in the -2 oxidation state.

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Chemical properties

Acid-base reactions

V₂O₅ is an amphoteric oxide. Thus it reacts with strong non-reducing acids to form solutions containing the pale yellow salts containing dioxovanadium(V) centers:

V₂O₅ + 2 HNO₃ → 2 "VO₂(NO₃)" + H₂O

It also reacts with strong alkali to form polyoxovanadates, which have a complex structure that depends on pH.^[1] If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium metavanadate, Na₃VO₄. If acid is slowly added to a solution of Na₃VO₄, the colour gradually deepens through orange to red before brown hydrated V₂O₅ precipitates around pH 2. These solutions contain mainly the ions HVO₄²⁻ and V₂O₇⁴⁻ between pH 9 and 13, but below pH 9 more exotic species such as V₄O₁₂⁴⁻ and HV₁₀O₂₈⁵⁻ predominate.

Thionyl chloride converts it to VOCl₃:

V₂O₅(s) + 3 SOCl₂(l) → 2 VOCl₃(l) + 3 SO₂(g)

Redox reactions

V₂O₅ is easily reduced in acidic media to the stable vanadium(IV) species, the blue vanadyl ion (VO(H₂O)₅²⁺). This conversion illustrates the redox properties of V₂O₅. For example, hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,

V₂O₅(s) + 6 HCl + 7 H₂O → 2 [VO(H₂O)₅]²⁺ + 4 Cl⁻ + Cl₂

Solid V₂O₅ is reduced by oxalic acid, CO, and SO₂ to give vanadium(IV) oxide, VO₂ as a deep-blue solid. Further reduction using hydrogen or excess CO can lead to complex mixtures of oxides such as V₄O₇ and V₅O₉ before black V₂O₃ is reached. Vanadates or vanadyl(V) compounds in acid solution are reduced by zinc amalgam through the interestingly colorful pathway -

colorless "VO₃^-" → yellow "VO₂⁺" → blue "VO²⁺" → green "V³⁺" → purple "V²⁺" The ions are of course hydrated to varying degrees.

Preparation

Technical grade V₂O₅ is produced as a black powder used for the production of vanadium metal and ferrovanadium.^[1] A vanadium ore or vanadium-rich residue is treated with sodium carbonate to produce sodium metavanadate, NaVO₃. This material is then acidified to pH 2-3 using H₂SO₄ to yield a precipitate of "red cake" (see above). The red cake is then melted at 690 °C to produce the crude V₂O₅.

Vanadium(V) oxide is also the main product when vanadium metal is heated with excess oxygen, but this product is contaminated with other lower oxides. A more satisfactory laboratory preparation involves the decomposition of ammonium metavanadate at around 200 °C:

2 NH₄VO₃ → V₂O₅(s) + 2 NH₃ + H₂O

Uses

Sulfuric acid production

The most important^{[citation needed]} use of vanadium(V) oxide is in the manufacture of sulfuric acid, an important industrial chemical with an annual production of 145 million tonnes in 1986.^[1] Vanadium(V) serves the crucial purpose of catalysing the mildly exothermic oxidation of sulfur dioxide to sulfur trioxide by air in the contact process:

2 SO₂ + O₂ → 2 SO₃

The discovery of this simple reaction, for which V₂O₅ is the most effective catalyst, allowed sulfuric acid to become the cheap commodity chemical it is today. The reaction is performed between 400 and 620 °C; below 400 °C the V₂O₅ is inactive as a catalyst, and above 620 °C; it begins to break down. Since it is known that V₂O₅ can be reduced to VO₂ by SO₂, one likely catalytic cycle is as follows:

SO₂ + V₂O₅(s) → SO₃(g) + 2 VO₂(s) followed by

2 VO₂(s) +¹/₂ O₂(g) → V₂O₅

Paradoxically, it is also used as catalyst in the selective catalytic reduction of NO_x emissions in some power plants. Due to its effectiveness in converting sulfur dioxide into sulfur trioxide, and thereby sulfuric acid, special care must be taken with the operating temperatures and placement of a power plant's SCR unit when firing sulfur-containing fuels.

Other oxidations

Maleic anhydride is another important industrial material, used for the manufacture of polyester resins and alkyd resins.^[2] Vanadium(V) oxide can catalyse its production from a variety of organic starting materials such as n-butane, furfural and benzene, the last of which is the usual commercial method. In a related process, phthalic anhydride, used for making plasticisers for PVC manufacture, may be produced by V₂O₅ catalysed oxidation of ortho-xylene or naphthalene at 350-400°C.

Other applications

In terms of quantity, the major use for vanadium(V) oxide is in the production of ferrovanadium (see above). The oxide is heated with scrap iron and aluminium, producing the iron-vanadium alloy along with alumina as a by-product. In 2005 a shortage of V₂O₅ caused a price rise to around $40/kg, which in turn caused a rise in the price of ferrovanadium.

Due to its high thermal coefficient of resistance, vanadium(V) oxide finds use as a detector material in bolometers and microbolometer arrays for thermal imaging.

Possible new uses include the preparation of bismuth vanadate ceramics for use in solid oxide fuel cells.^[3]

Biological activity

   Despite being highly toxic in humans, vanadium occurs in some organisms, notably the Ascidiacea (sea squirts). These organisms contain the protein vanabins, the role of which is unclear. Vanadate (VO₄³⁻), formed when V₂O₅ by hydrolysis of V₂O₅ at high pH, appears to inhibit enzymes that process phosphate (PO₄³⁻). However the exact mode of action remains elusive.^[1]

References

1. ^ ^{a b c d} N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. ^ Basic Organic Chemistry: Part 5, Industrial Products, J.M. Tedder, A. Nechvatal, A.H. Tubb (editors), John Wiley & Sons, Chichester, UK (1975).
3. ^ B. Vaidhyanathan, K. Balaji, K. J. Rao (1998). "Microwave-Assisted Solid-State Synthesis of Oxide Ion Conducting Stabilized Bismuth Vanadate Phases". Chem. Mater. 10: 3400. doi:10.1021/cm980092f.