Niobium pentoxide is the
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
with the
formula
In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwe ...
Nb2 O5. A colorless, insoluble, and fairly unreactive solid, it is the most widespread precursor for other compounds and materials containing niobium. It is predominantly used in alloying, with other specialized applications in
capacitor
A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passivity (engineering), passive electronic component with two termi ...
s, optical glasses, and the production of
lithium niobate
Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
.
[Francois Cardarelli (2008) ''Materials Handbook'' Springer London ]
Structure
It has many polymorphic forms all based largely on octahedrally coordinated niobium atoms.
[Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ] The polymorphs are identified with a variety of prefixes.
The form most commonly encountered is
monoclinic
In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...
H-
Nb2 O5, which has a complex structure with a unit cell containing 28 niobium atoms and 70 oxygen, where 27 of the niobium atoms are octahedrally coordinated and one tetrahedrally. There is an uncharacterised solid hydrate, , the so-called niobic acid (previously called columbic acid), which can be prepared by hydrolysis of a basic solution of niobium pentachloride or Nb
2O
5 dissolved in HF.
[D.A. Bayot and M.M. Devillers, ''Precursors routes for the preparation of Nb based multimetallic oxides'' in Progress in Solid State Chemistry Research, Arte M. Newman, Ronald W. Buckley, (2007),Nova Publishers, ]
Molten niobium pentoxide has lower mean coordination numbers than the crystalline forms, with a structure comprising mostly NbO
5 and NbO
6 polyhedra.
Production
Hydrolysis
Nb
2O
5 is prepared by hydrolysis of alkali-metal niobates, alkoxides or fluoride using base. Such ostensibly simple procedures afford hydrated oxides that can then be
calcined
Calcination refers to thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), gener ...
. Pure Nb
2O
5 can also be prepared by hydrolysis of
NbCl5:
:2 NbCl
5 + 5 H
2O → Nb
2O
5 + 10 HCl
A method of production via
sol-gel techniques has been reported hydrolysing niobium alkoxides in the presence of acetic acid, followed by calcination of the gels to produce the
orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
form,
T-Nb
2O
5.
Oxidation
Given that Nb
2O
5 is the most common and robust compound of niobium, many methods, both practical and esoteric, exist for its formation. The oxide for example, arises when niobium metal is oxidised in air.
The oxidation of
niobium dioxide
Niobium dioxide, is the chemical compound with the formula NbO2. It is a bluish-black non-stoichiometric solid with a composition range of NbO1.94-NbO2.09. It can be prepared by reducing Nb2O5 with H2 at 800–1350 °C. An alternative method i ...
, NbO
2 in air forms the polymorph, L-Nb
2O
5.
Nano-sized niobium pentoxide particles have been synthesized by LiH reduction of NbCl
5, followed by aerial oxidation as part of a synthesis of nano structured niobates.
Reactions
Nb
2O
5 is attacked by HF and dissolves in fused alkali.
Reduction to the metal
The conversion of Nb
2O
5 is the main route for the industrial production of niobium metal. In the 1980s, about 15,000,000 kg of Nb
2O
5 were consumed annually for reduction to the metal.
The main method is reduction of this oxide with
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
:
:3 Nb
2O
5 + 10 Al → 6 Nb + 5 Al
2O
3
An alternative but less practiced route involves carbothermal reduction, which proceeds via reduction with carbon and forms the basis of the two stage Balke process:
:Nb
2O
5 + 7 C → 2 NbC + 5 CO (heated under vacuum at 1800 °C)
:5 NbC + Nb
2O
5 → 7 Nb + 5 CO
Conversion to halides
Many methods are known for conversion of Nb
2O
5 to the halides. The main problem is incomplete reaction to give the oxyhalides. In the laboratory, the conversion can be effected with thionyl chloride:
:Nb
2O
5 + 5 SOCl
2 → 2 NbCl
5 + 5 SO
2
Nb
2O
5 reacts with
CCl4 to give
niobium oxychloride
Niobium oxychloride is the inorganic compound with the formula NbOCl3. It is a white, crystalline, diamagnetic solid. It is often found as an impurity in samples of niobium pentachloride, a common reagent in niobium chemistry.
Structure
In the s ...
NbOCl
3.
Conversion to niobates
Treating Nb
2O
5 with aqueous
NaOH
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali t ...
at 200 °C can give crystalline sodium niobate, NaNbO
3 whereas the reaction with
KOH may yield soluble Lindqvist-type hexaniobates, .
Lithium niobate
Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
s such as LiNbO
3 and Li
3NbO
4 can be prepared by reaction
lithium carbonate
Lithium carbonate is an inorganic compound, the lithium salt of carbonate with the formula . This white salt is widely used in the processing of metal oxides. It is listed on the World Health Organization's List of Essential Medicines because it c ...
and Nb
2O
5.
[
]
Conversion to reduced niobium oxides
High temperature reduction with H2 gives NbO2:
:Nb2O5 + H2 → 2 NbO2 + H2O
Niobium monooxide arises from a comproportionation
Comproportionation or synproportionation is a chemical reaction where two reactants containing the same element but with different oxidation numbers, form a compound having an intermediate oxidation number. It is the opposite of disproportionation. ...
using an arc-furnace:
:Nb2O5 + 3Nb → 5 NbO
The burgundy-coloured niobium(III) oxide, one of the first superconducting oxides, can be prepared again by an comproportionation:
:Li3NbO4 + 2 NbO → 3 LiNbO2
Uses
Niobium pentoxide is used mainly in the production of niobium metal,[ but specialized applications exist in the production of optical glasses and ]lithium niobate
Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
.
Thin films of Nb2O5 form the dielectric layers in niobium electrolytic capacitor
A niobium electrolytic capacitor (historically also ''Columbium capacitor'') is an electrolytic capacitor whose anode (+) is made of passivated niobium metal or niobium monoxide, on which an insulating niobium pentoxide layer acts as a dielectr ...
s.
Nb2 O5 have been considered for use as an anode in a lithium ion battery, given that their ordered crystalline structure allows charging speeds of 225 mAh g−1 at 200 mA g−1 across 400 cycles, at a Coulombic efficiency of 99.93%.
External links
Basic Niobium Information and Research Data
References
{{Oxides
Niobium(V) compounds
Transition metal oxides