Terbium(III,IV) oxide

Terbium(III,IV) oxide, occasionally called tetraterbium heptaoxide, has the formula Tb₄O₇, though some texts refer to it as TbO_1.75. There is some debate as to whether it is a discrete compound, or simply one phase in an interstitial oxide system. Tb₄O₇ is one of the main commercial terbium compounds, and the only such product containing at least some Tb(IV) (terbium in the +4 oxidation state), along with the more stable Tb(III). It is produced by heating the metal oxalate, and it is used in the preparation of other terbium compounds. Terbium forms three other major oxides: Tb₂O₃, TbO₂, and Tb₆O₁₁.

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Synthesis

Tb₄O₇ is most often produced by ignition of the oxalate at or the sulfate in air.^[1] The oxalate (at 1000 °C) is generally preferred, since the sulfate requires a higher temperature, and it produces an almost black product contaminated with Tb₆O₁₁ or other oxygen-rich oxides.

Chemical properties

Terbium(III,IV) oxide loses O₂ when heated at high temperatures; at more moderate temperatures (ca. 350 °C) it reversibly loses oxygen, as shown by exchange with ¹⁸O₂. This property, also seen in Pr₆O₁₁ and V₂O₅, allows it to work like V₂O₅ as a redox catalyst in reactions involving oxygen. It was found as early as 1916 that hot Tb₄O₇ catalyses the reaction of coal gas (CO + H₂) with air, leading to incandescence and often the mixture catches fire.^[2]

Tb₄O₇ reacts with atomic oxygen to produce TbO₂, but a more convenient preparation of TbO₂ is by disproportionation of Tb₄O₇. This is performed by refluxing with an excess of an equal mixture of concentrated acetic acid and hydrochloric acids for 30 minutes.^[3]

Tb₄O₇(s) + 6 HCl(aq) → 2 TbO₂(s) + 2 TbCl₃(aq) + 3 H₂O(l)

Tb₄O₇ reacts with other hot concentrated acids to produce terbium(III) salts, for example sulfuric acid gives terbium(III) sulfate.

References

1. ^  Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
2. ^  J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, pp 692-696, Longmans, Green & Co., London, 1967.
3. ^ Gmelin SE C 1 39, p 397.
4. ^  D. W. Bissel, C. James, J. Am. Chem. Soc 38, 873 (1916).
5. ^  F. T. Edelmann, P. Poremba, in: Synthetic Methods of Organometallic and Inorganic Chemistry, (W. A. Herrmann, ed.), Vol. 6, Georg Thieme Verlag, Stuttgart, 1997. ISBN 3-13-103071-2.