Niobium pentoxide

Structure

It has many polymorphic forms all based largely on octahedrally coordinated niobium atoms. The polymorphs are identified with a variety of prefixes. The form most commonly encountered is monoclinic H-Nb₂O₅ 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, Nb₂O₅.nH₂O, the so-called niobic acid (previously called columbic acid), which can be prepared by hydrolysis of a basic solution of niobium pentachloride or Nb₂O₅ dissolved in HF.

Production

Nb₂O₅ is prepared by hydrolysis of alkali-metal niobates and alkoxides and the fluorides using base. Such ostensibly simple procedures afford hydrated oxides that are calcined.

Other routes

Given that Nb₂O₅ 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, NbO₂ in air forms the polymorph, L-Nb₂O₅. Pure Nb₂O₅ can be prepared by hydrolysis of NbCl₅:

2 NbCl₅ + 5 H₂O → Nb₂O₅ + 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 polymorphic form, T-Nb₂O₅.

Nano-sized niobium pentoxide particles have been synthesised by LiH reduction of NbCl₅, followed by aerial oxidation as part of a synthesis of nano structured niobates.

Reactions

Nb₂O₅ is attacked by HF and dissolves in fused alkali.

Reduction to the metal

The conversion of Nb₂O₅ is the main route for the industrial production of niobium metal. In the 1980s, about 15,000,000 kg of Nb₂O₅ were consumed annually for reduction to the metal. The main method is reduction of this oxide with aluminium:

3 Nb₂O₅ + 10 Al → 6 Nb + 5 Al₂O₃

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:

Nb₂O₅ + 7 C → 2 NbC + 5 CO (heated under vacuum at 1800 °C)5 NbC + Nb₂O₅ → 7 Nb + 5 CO

Conversion to halides

Many methods are known for conversion of Nb₂O₅ to the halides. The main problem is incomplete reaction to give the oxyhalides. In the laboratory, the conversion can be effected with thionyl chloride:

Nb₂O₅ + 5 SOCl₂ → 2 NbCl₅ + 5 SO₂

Nb₂O₅ reacts with CCl₄ to give niobium oxychloride NbOCl₃.

Conversion to niobates

Treating Nb₂O₅ with aqueous NaOH at 200 °C can give crystalline sodium niobate, NaNbO₃ whereas the reaction with KOH may yield soluble Lindqvist-type hexaniobates, Nb
6O8−
19. Lithium niobates such as LiNbO₃ and Li₃NbO₄ can be prepared by reaction lithium carbonate and Nb₂O₅.

Conversion to reduced niobium oxides

High temperature reduction with H₂ gives NbO₂:

Nb₂O₅ + H₂ → 2 NbO₂ + H₂O

Niobium monooxide arises from a comproportionation 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 for lithium niobate and as a component of optical glass.