Cerium Under the Lens Eric J

in your element Cerium under the lens Eric J. Schelter ponders on cerium’s rather puzzling redox reactivity, and the varied practical applications that have emerged from it. erium is one of the seventeen rare- of cerium oxides and comprise a strongly stabilized open-shell earth metals (scandium, yttrium and the widespread use singlet ground state4,5. the lanthanides La–Lu) but, despite of CAN as a one- O O This deceptively simple compound C M M the group’s name, is fairly abundant — only electron oxidant represents a stimulating case where Ce slightly less so than copper in the Earth’s in both organic O O the very human concept of a formal O O crust. Element 58 is much more present and inorganic M oxidation state fails to capture the in modern life than it may seem. Aqueous chemistry, the essential essence of a molecule. The slurries consisting of ceria (CeO2) are used coordination and simultaneous local/non-local character of for the chemical–mechanical polishing of organometallic the 4f-electron in cerocene is reminiscent © INGRAM PUBLISHING/ ALAMY surfaces, including microelectronic device chemistry of cerium(iv) of f-element superconductors’ behaviour6, wafers, electronic displays, eye-glass lenses is not particularly developed. and investigations on cerium compounds can and other optical materials. Through This lack of studies is likely to be provide insight on how local behaviour gives chemical attack of basic sites on surfaces, linked to the fact that it is unexpectedly rise to exotic materials properties. ceria provides greater polishing rates than difficult to oxidize a cerium(iii) Alongside all this fertile academic simple mechanical techniques. coordination compound and isolate a inquiry are also excellent practical The use of cerium in many applications cerium(iv) product in good yield. One motivations for studying cerium comes from the interconversion between possible reason is the slow rates of these chemistry. Cerium is found naturally in 4f 1–Ce(iii) and 4f 0–Ce(iv) oxidation states oxidation reactions, which arise from bastnaesite and monazite ores, together — a unique behaviour among the rare-earth the steric hindrance necessary to prepare with other light rare-earth elements metals. Cerium(iii/iv) redox chemistry discrete complexes comprising a single such as neodymium. Neodymium makes oxides useful in heterogeneous cerium cation. In our group, we recently presents a high value owing to catalysis through the storage and release tackled this issue by attempting to control its parentage of Nd2Fe14B — a of oxygen. The non-stoichiometric cerium the metal coordination sphere through hard magnetic material that finds 2 oxide system CeO2–x has unusually high heterobimetallic complexes (generalized diverse uses, such as in wind-power ion mobilities owing in part to octahedral structure pictured). Surrounding the cerium generators. In a typical rare-earth-metal oxygen vacancies in its lattice1. atom with an interlocked, flexible structure separations process, cerium, in as much as Cerium oxides also serve in the of lithium cations and aryloxide ligands kept three-fold excess of neodymium by mass, is promotion of the industrially important it accessible and enabled its quick and easy removed and discarded as a byproduct. This water-gas-shift reaction, and in solid-oxide conversion to cerium(iv), lending support to is an excellent opportunity to search for new fuel cells. Hydrocarbon fuels encounter the idea that cerium oxidation reactions are applications and add value to cerium ‘waste’ element 58 at both the beginning and the under kinetic control. — even more so when the tight economic end of their useful life: a zeolite (faujasite) Another fun aspect of element 58 margins of rare-earth-metals mining are impregnated with cerium and lanthanum engendered by its redox activity is the taken into account. ❐ serves as a petroleum-cracking catalyst in somewhat unconventional and controversial refining. And harmful fuel exhaust gases are electronic structures of its compounds, ERIC J. SCHELTER is at the P. Roy and converted into N2, CO2 and H2O using ceria such as cerocene (Ce(C8H8)2), an 8-fold Diana T. Vagelos Laboratories, Department and precious metals in automotive three-way symmetric, eclipsed sandwich complex. of Chemistry, University of Pennsylvania, catalytic converters. Owing to their uptake of The most accurate picture of the valence at 231 South 34th Street, Philadelphia, reactive oxygen species, ceria nanoparticles its cerium cation has remained somewhat Pennsylvania 19104, USA. are also being explored in medicinal ambiguous3. Energy decomposition analysis e-mail: [email protected] applications as antioxidant therapeutics. suggests a strong ionic interaction between For the synthetic chemist, cerium is the cerium centre and each cyclooctatetraene References most familiar as a potent oxidizing agent ring, and X-ray absorption spectroscopy 1. Trovarelli, A. (ed.) Catalysis by Ceria and Related Materials Ch. 2, 16 (Catalytic Science Series Vol. 2, Imperial College Press, 2002). in the ubiquitous ceric ammonium nitrate indicates that cerocene has a ground state 2. Robinson, J. R., Carroll, P. J., Walsh, P. J. & Schelter, E. J. Angew. 4 (CAN) — a drastic ‘nuclear option’ for that is strongly multiconfigurational — so Chem. Int. Ed. 51, 10159–10163 (2012). oxidation reactions. In contrast to the utility much so that the compound is now described 3. Edelstein, N. M. et al. J. Am. Chem. Soc. 118, 13115–13116 (1996). as ‘intermediate valent’. It is trapped between 4. Booth, C. H., Walter, M. D., Lukens, W. W. & Andersen, R. A. Phys. Rev. Lett. 95, 267202 (2005). configurations of Ce(iii) and Ce(iv) character 5. Kerridge, A. & Kaltsoyannis, N. C. R. Chim. 13, 853–859 (2010). that are quantum-mechanically admixed and 6. Hegger, H. Phys. Rev. Lett. 84, 4986–4989 (2000). Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Re 348 NATURE CHEMISTRY | VOL 5 | APRIL 2013 | www.nature.com/naturechemistry © 2013 Macmillan Publishers Limited. All rights reserved.

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