Vanadium(V) oxide is the inorganic compound with the formula V2O5. Commonly
known as vanadium pentoxide, it is a brown/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 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, V2O5·3H2O, is also known under the name of navajoite.
Chemical properties Upon heating, it reversibly loses
oxygen, successively forming V2O4, V2O3, VO and the metal.
= Acid-base reactions = Unlike most metal oxides, it dissolves
slightly in water to give a pale yellow, acidic solution. When this compound is
formed by V2O5 it is an amphoteric oxide. 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. 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.
Upon treatment with thionyl chloride, it converts to vanadium oxychloride, 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 ion52+). 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 colourful pathway:
The ions are, of course, all hydrated to varying degrees.
Preparation Technical grade V2O5 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 to produce
sodium metavanadate, NaVO3. This material is then acidified to pH 2–3
using H2SO4 to yield a precipitate of "red cake". 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. 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.
= 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
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 = 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.
Phthalic anhydride is produced similarly by V2O5-catalysed oxidation of
ortho-xylene or naphthalene at 350–400 °C. The equation is for the xylene
oxidation: C6H4(CH3)2 + 3 O2 → C6H4(CO)2O + 3 H2O
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. = Other applications =
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.
Vanadium redox batteries are 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 hazard is
with inhalation of the dust, where the LD50 ranges from 4–11 mg/kg for a 14-day
exposure. Vanadate, formed by hydrolysis of V2O5 at high pH, appears to inhibit
enzymes that process phosphate. However the mode of action remains elusive.
References 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 .
Vaidhyanathan, B.; Balaji, K.; Rao, K. J., "Microwave-Assisted Solid-State
Synthesis of Oxide Ion Conducting Stabilized Bismuth Vanadate Phases",
Chem. Mater. 10: 3400–4, doi:10.1021/cm980092f .
External links How Vanadium Oxide Is Used In Energy
Storage International Chemical Safety Card 0596
"NIOSH Pocket Guide to Chemical Hazards #0653". National Institute for
Occupational Safety and Health. "NIOSH Pocket Guide to Chemical Hazards
#0654". National Institute for Occupational Safety and Health.
Vanadium Pentoxide and other Inorganic Vanadium Compounds
IPCS Environmental Health Criteria 81: Vanadium
IPCS Health and Safety Guide 042: Vanadium and some vanadium salts