Vanadium(V) oxide

Upon heating a mixture of vanadium(V) oxide and vanadium(III) oxide, comproportionation occurs to give vanadium(IV) oxide, as a deep-blue solid:[9]

V₂O₅ + V₂O₃ → 4 VO₂

The reduction can also be effected by oxalic acid, carbon monoxide, and sulfur dioxide. Further reduction using hydrogen or excess CO can lead to complex mixtures of oxides such as V₄O₇ and V₅O₉ before black V₂O₃ is reached.

V₂O₅ is an amphoteric oxide. Unlike most transition metal oxides, it dissolves slightly in water to give a pale yellow, acidic solution. Thus V₂O₅ 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.[10] If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium orthovanadate, 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 pH 13, but below pH 9 more exotic species such as V₄O₁₂⁴⁻ and HV₁₀O₂₈⁵⁻ (decavanadate) predominate.

Upon treatment with thionyl chloride, it converts to the volatile liquid vanadium oxychloride, VOCl₃:[11]

V₂O₅ + 3 SOCl₂ → 2 VOCl₃ + 3 SO₂

Hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,

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

Vanadates or vanadyl compounds in acid solution are reduced by zinc amalgam through the colourful pathway:

VO₂⁺yellow → VO²⁺blue → V³⁺green → V²⁺purple[12]

The ions are all hydrated to varying degrees.

In terms of quantity, the dominant use for vanadium(V) oxide is in the production of ferrovanadium (see above). The oxide is heated with scrap iron and ferrosilicon, with lime added to form a calcium silicate slag. Aluminium may also be used, producing the iron-vanadium alloy along with alumina as a byproduct.

Another important use of vanadium(V) oxide is in the manufacture of sulfuric acid, an important industrial chemical with an annual worldwide production of 165 million tonnes in 2001, with an approximate value of US$8 billion. Vanadium(V) oxide 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 → SO₃ + 2VO₂

followed by

2VO₂ +½O₂ → V₂O₅

It is also used as catalyst in the selective catalytic reduction (SCR) of NO_x emissions in some power plants and diesel engines. 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.

Proposed early steps in the vanadium-catalyzed oxidation of naphthalene to phthalic anhydride, with V₂O₅ represented as a molecule vs its true extended structure[14]

Maleic anhydride is produced by the V₂O₅-catalysed oxidation of butane with air:

C₄H₁₀ + 4 O₂ → C₂H₂(CO)₂O + 8 H₂O

Maleic anhydride is used for the production of polyester resins and alkyd resins.[15]

Phthalic anhydride is produced similarly by V₂O₅-catalysed oxidation of ortho-xylene or naphthalene at 350–400 °C. The equation for the vanadium oxide-catalysed oxidation of o-xylene to phthalic anhydride:

C₆H₄(CH₃)₂ + 3 O₂ → C₆H₄(CO)₂O + 3 H₂O

The equation for the vanadium oxide-catalysed oxidation of naphthalene to phthalic anhydride:[16]

C₁₀H₈ + 4½ O₂ → C₆H₄(CO)₂O + 2CO₂ + 2H₂O

Phthalic anhydride is a precursor to plasticisers, used for conferring pliability to polymers.

A variety of other industrial compounds are produced similarly, including adipic acid, acrylic acid, oxalic acid, and anthraquinone.[8]

Due to its high coefficient of thermal resistance, vanadium(V) oxide finds use as a detector material in bolometers and microbolometer arrays for thermal imaging. It also finds application as an ethanol sensor in ppm levels (up to 0.1 ppm).

Vanadium redox batteries are a type of flow battery used for energy storage, including large power facilities such as wind farms.[17] Vanadium oxide is also used as a cathode in lithium ion batteries.[18]