Vanadium(V) oxide

Vanadium(V) oxide
IUPAC name Divanadium pentaoxide
Other names Vanadium pentoxide Vanadic anhydride Divanadium pentoxide
Identifiers
CAS number	1314-62-1 Y
PubChem	14814
ChemSpider	14130 Y
EC number	215-239-8
UN number	2862
KEGG	C19308 N
ChEBI	CHEBI:30045 N
RTECS number	YW2450000
Jmol-3D images	Image 1
SMILES O=[V](=O)O[V](=O)=O
InChI InChI=1S/5O.2V Y Key: GNTDGMZSJNCJKK-UHFFFAOYSA-N Y InChI=1/5O.2V/rO5V2/c1-6(2)5-7(3)4 Key: GNTDGMZSJNCJKK-HHIHJEONAP
Properties[1]
Molecular formula	V2O5
Molar mass	181.8800 g/mol
Appearance	Yellow solid
Density	3.357 g/cm3
Melting point	690 °C, 963 K, 1274 °F
Boiling point	1750 °C, 2023 K, 3182 °F (decomp.)
Solubility in water	0.8 g/L (20 °C)
Structure
Crystal structure	Orthorhombic
Space group	Pmmn, No. 59
Lattice constant	a = 1151 pm, b = 355.9 pm, c = 437.1 pm
Coordination geometry	Distorted trigonal bipyramidal (V)
Hazards
MSDS	ICSC 0596
GHS pictograms
GHS signal word	DANGER
GHS hazard statements	H341, H361, H372, H332, H302, H335, H411
EU Index	023-001-00-8
EU classification	Muta. Cat. 3 Repr. Cat. 3 Toxic (T) Harmful (Xn) Irritant (Xi) Dangerous for the environment (N)
R-phrases	R20/22, R37, R48/23, R51/53, R63, R68
S-phrases	(S1/2), S36/37, S38, S45, S61
NFPA 704	0 3 0
Flash point	Non-flammable
LD50	10 mg/kg
Related compounds
Other anions	Vanadium oxytrichloride
Other cations	Niobium(V) oxide Tantalum(V) oxide
Related vanadium oxides	Vanadium(II) oxide Vanadium(III) oxide Vanadium(IV) oxide
N oxide (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Vanadium(V) oxide (vanadia) is the chemical compound with the formula V₂O₅. Commonly known as vanadium pentoxide, this brown/yellow solid is the most stable and common 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 brown/yellow 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 to give a pale yellow, acidic solution. When this compound is formed by precipitation from aqueous solution, then its color is deep orange instead of brown/yellow.

The mineral form of this compound, shcherbinaite,^[2] is extremely rare, almost always found among fumaroles. A mineral trihydrate, V₂O₅·3H₂O, is also known under the name of navajoite.^[3]

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.^[4] 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⁻ predominate.

Thionyl chloride converts it to VOCl₃:

V₂O₅ + 3 SOCl₂ → 2 VOCl₃ + 3 SO₂

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

Solid V₂O₅ is reduced by oxalic acid, carbon monoxide, and sulfur dioxide 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:

VO2+	→	VO2+	→	V3+	→	V2+
yellow		blue		green		purple

The ions are, of course, all hydrated to varying degrees.

Preparation

The orange, partly hydrated form of V₂O₅

Precipitate of "red cake", which is hydrous V₂O₅

Technical grade V₂O₅ is produced as a black powder used for the production of vanadium metal and ferrovanadium.^[4] 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 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 around 200 °C:

2 NH₄VO₃ → V₂O₅ + 2 NH₃ + H₂O

Uses

Ferrovanadium production

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 by-product.^[4] In 2005 a shortage of V₂O₅ caused its price to rise to $40/kg, which in turn caused a rise in the price of ferrovanadium.^{[citation needed]}

Sulfuric acid production

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 metric tons in 2001, with an approximate value of US$8 billion. 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 → SO₃ + 2VO₂

followed by

2VO₂ +½O₂ → V₂O₅

Paradoxically, it is also used as catalyst in the selective catalytic reduction (SCR) 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.^[5] 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

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. It also finds application as an ethanol sensor in ppm levels (up to 0.1 ppm).

Possible new uses include the preparation of bismuth vanadate ceramics for use in solid oxide fuel cells.^[6] Another new application is in vanadium redox batteries, a type of flow battery used for energy storage, including large power facilities such as wind farms.

Biological activity

Vanadium(V) oxide exhibits modest toxicity to humans, with an LD50 of about 470 mg/kg. The greater problem is with inhalation of the dust, where the LD50 ranges from 4-11mg/kg for a 14 day exposure.^[7] Vanadate (VO₄³⁻), formed by hydrolysis of V₂O₅ at high pH, appears to inhibit enzymes that process phosphate (PO₄³⁻). However the exact mode of action remains elusive.^[4]

References

1. ^ Weast, Robert C., ed (1981). CRC Handbook of Chemistry and Physics (62nd ed.). Boca Raton, FL: CRC Press. p. B-162. ISBN 0-8493-0462-8. .
2. ^ Shcherbinaite, WebMineral.com, http://webmineral.com/data/Shcherbinaite.shtml, retrieved 2009-11-30 .
3. ^ Navajoite, MinDat.org, http://www.mindat.org/show.php?id=2867, retrieved 2009-11-30 .
4. ^ ^{a b c d} Greenwood, Norman N.; Earnshaw, A. (1984). Chemistry of the Elements. Oxford: Pergamon. pp. 1140, 1144. ISBN 0-08-022057-6. .
5. ^ Tedder, J. M.; Nechvatal, A.; Tubb, A. H., eds. (1975), Basic Organic Chemistry: Part 5, Industrial Products, Chichester, UK: John Wiley & Sons .
6. ^ Vaidhyanathan, B.; Balaji, K.; Rao, K. J. (1998), "Microwave-Assisted Solid-State Synthesis of Oxide Ion Conducting Stabilized Bismuth Vanadate Phases", Chem. Mater. 10 (11): 3400–4, doi:10.1021/cm980092f 
7. ^ Günter Bauer, Volker Güther, Hans Hess, Andreas Otto, Oskar Roidl, Heinz Roller, Siegfried Sattelberger "Vanadium and Vanadium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a27_367

Further reading

• "Vanadium Pentoxide", Cobalt in Hard Metals and Cobalt Sulfate, Gallium Arsenide, Indium Phosphide and Vanadium Pentoxide, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans 86, Lyon, France: International Agency for Research on Cancer, 2006, pp. 227–92, ISBN 92-832-1286-X, http://monographs.iarc.fr/ENG/Monographs/vol86/mono86-10.pdf .

External links

• How Vanadium Oxide Is Used In Energy Storage
• International Chemical Safety Card 0596
• NIOSH Pocket Guide to Chemical Hazards 0653
• NIOSH Pocket Guide to Chemical Hazards 0654
• Vanadium Pentoxide and other Inorganic Vanadium Compounds (Concise International Chemical Assessment Document 29)
• IPCS Environmental Health Criteria 81: Vanadium
• IPCS Health and Safety Guide 042: Vanadium and some vanadium salts