Vanadium(V) oxide



Vanadium(V) oxide
IUPAC name Vanadium(V) oxide
Other names Vanadium pentoxide,
vanadic anhydride,
divanadium pentoxide
Identifiers
CAS number 1314-62-1
Properties
Molecular formula V2O5
Molar mass 181.88 g/mol
Appearance Orange-yellow solid
Density 3.357 g/cm³, solid
Melting point

690 °C (963 K)

Boiling point

1750 °C (2020 K)

Solubility in water 0.8 g/100 mL (20 °C)
Hazards
EU classification Toxic (T)
R-phrases R20, R22, R37, R48
R23, R51, R53
S-phrases S61
Related Compounds
Other anions Vanadium oxytrichloride
Other cations Niobium(V) oxide
Chromium trioxide
Related V compounds Vanadium(IV) oxide
Vanadium(V) fluoride
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

Vanadium(V) oxide (vanadia) is the dissolves slightly in water due to hydrolysis.

Vanadium(V) indicates that atoms in the compound are in the -2 oxidation state.

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 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.

VOCl3:

V2O5(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 halogen, e.g.,

V2O5(Cl2

Solid V2O5 is reduced by 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 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 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(NH3 + H2O

Uses

Sulfuric acid production

The most important[citation needed] use of vanadium(V) oxide is in the manufacture of 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 NOx 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

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 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 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, 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.
 
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