Vanadium(V) oxide (vanadia) is the inorganic compound with the formula V<sub>2</sub>O<sub>5</sub>. Commonly known as vanadium pentoxide, it is a dark yellow solid, although when freshly precipitated from aqueous solution, its colour is deep orange. Because of its high oxidation state, it is both an amphoteric oxide and an oxidizing agent. From the industrial perspective, it is the most important compound of vanadium, being the principal precursor to alloys of vanadium and is a widely used industrial catalyst.
The mineral form of this compound, shcherbinaite, is extremely rare, almost always found among fumaroles. A mineral trihydrate, V<sub>2</sub>O<sub>5</sub>÷3H<sub>2</sub>O, is also known under the name of navajoite.
Upon heating a mixture of vanadium(V) oxide and vanadium(III) oxide, comproportionation occurs to give vanadium(IV) oxide, as a deep-blue solid:
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 V<sub>4</sub>O<sub>7</sub> and V<sub>5</sub>O<sub>9</sub> before black V<sub>2</sub>O<sub>3</sub> is reached.
V<sub>2</sub>O<sub>5</sub> is an amphoteric oxide, and unlike most transition metal oxides, it is slightly water soluble, giving a pale yellow, acidic solution. Thus V<sub>2</sub>O<sub>5</sub> reacts with strong non-reducing acids to form solutions containing the pale yellow salts containing dioxovanadium(V) centers:
It also reacts with strong alkali to form polyoxovanadates, which have a complex structure that depends on pH. If excess aqueous sodium hydroxide is used, the product is a colourless salt, sodium orthovanadate, Na<sub>3</sub>VO<sub>4</sub>. If acid is slowly added to a solution of Na<sub>3</sub>VO<sub>4</sub>, the colour gradually deepens through orange to red before brown hydrated V<sub>2</sub>O<sub>5</sub> precipitates around pH 2. These solutions contain mainly the ions and between pH 9 and pH 13, but below pH 9 more exotic species such as and (decavanadate) predominate.
Upon treatment with thionyl chloride, it converts to the volatile liquid vanadium oxychloride, VOCl<sub>3</sub>:
Hydrochloric acid and hydrobromic acid are oxidised to the corresponding halogen, e.g.,
Vanadates or vanadyl compounds in acid solution are reduced by zinc amalgam through the colourful pathway:
The ions are all hydrated to varying degrees.
Technical grade V<sub>2</sub>O<sub>5</sub> is produced as a black powder used for the production of vanadium metal and ferrovanadium. A vanadium ore or vanadium-rich residue is treated with sodium carbonate and an ammonium salt to produce sodium metavanadate, NaVO<sub>3</sub>. This material is then acidified to pH 2âÂÂ3 using H<sub>2</sub>SO<sub>4</sub> to yield a precipitate of "red cake" (see above). The red cake is then melted at 690 ðC to produce the crude V<sub>2</sub>O<sub>5</sub>.
Vanadium(V) oxide is produced 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 500âÂÂ550 ðC:
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.
Vanadium(V) oxide is used as the oxygen-transfer catalyst in the contact process, the only process now used for the industrial production of sulfuric acid. The catalyst phase is a molten salt formed from vanadium(V) oxide and added alkali metal sulfates acting as co-catalysts. In this melt, the reactive complex [(VO)<sub>2</sub>O(SO<sub>4</sub>)<sub>4</sub>]<sup>4âÂÂ</sup>, which is regarded as the actual catalytic species, is formed. Oxygen and sulfur dioxide coordinate to this complex and react to form sulfur trioxide without a change in the oxidation state of vanadium.
The reaction is carried out at about 420 to 620 ðC; at lower temperatures the catalyst is deactivated by formation of vanadium(IV) compounds, while at higher temperatures it begins to decompose. Industrial conversion is performed in multi-bed contact furnaces, in which the catalyst is arranged in four superposed layers and the process gas is cooled between the beds to maintain the required temperature range.
Maleic anhydride is produced by the V<sub>2</sub>O<sub>5</sub>-catalysed oxidation of butane with air:
Maleic anhydride is used for the production of polyester resins and alkyd resins.
Phthalic anhydride is produced similarly by V<sub>2</sub>O<sub>5</sub>-catalysed oxidation of ortho-xylene or naphthalene at 350âÂÂ400 ðC. The equation for the vanadium oxide-catalysed oxidation of o-xylene to phthalic anhydride:
The equation for the vanadium oxide-catalysed oxidation of naphthalene to phthalic anhydride:
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.
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. Vanadium oxide is also used as a cathode in lithium-ion batteries.
Vanadium pentoxide is often used as a component in glazes where it produces a wide range of colours from greens and yellows to blues and grays.
Vanadium(V) oxide exhibits very modest acute toxicity to humans, with an of about 470 mg/kg. The greater hazard is with inhalation of the dust, where the LD<sub>50</sub> ranges from 4âÂÂ11 mg/kg for a 14-day exposure. Vanadate (), formed by hydrolysis of V<sub>2</sub>O<sub>5</sub> at high pH, appears to inhibit enzymes that process phosphate (). However the mode of action remains elusive.