Cerium is a soft, silvery lanthanide named after the dwarf planet Ceres. It oxidizes readily in air, its filings can be pyrophoric, and it commonly shows +3 and +4 oxidation states.
The ground-state configuration is [Xe] 4f1 5d1 6s2 (often simplified as 4f16s2 with a nearly empty 5d). The presence of one 4f electron enables accessible Ce(III) (4f1) and Ce(IV) (4f0) states, a hallmark of cerium’s redox versatility.
+3 and +4 dominate:
Ce(IV) compounds are strong oxidants; Ce(III) salts are mild reducing agents and common starting materials.
Ceria can reversibly store and release oxygen via the Ce(IV)/Ce(III) couple and oxygen vacancies. A simplified redox exchange is:
\(\mathrm{CeO_2 \rightleftharpoons CeO_{2-\delta} + \tfrac{\delta}{2}\,O_2\,(g)}\)
This oxygen-buffering behavior enhances three-way automotive catalysts, soot oxidation, and fuel reforming.
CAN (\(\mathrm{(NH_4)_2[Ce(NO_3)_6]}\)) is a convenient Ce(IV) oxidant used for single-electron transfers, oxidative deprotections, and functionalization of electron-rich aromatics. A schematic one-electron step is:
\(\mathrm{Ce^{IV} + R \rightarrow Ce^{III} + R^{\bullet+}}\)
Workups often reduce Ce(IV) back to Ce(III), evidenced by a color change (yellow/orange to colorless).
Freshly generated fine particles have high surface area and react rapidly with oxygen, releasing heat. In mischmetal (Ce-rich alloy) for lighter flints, scraping produces sparks as hot cerium fragments oxidize:
\(\mathrm{2\,Ce + O_2 \rightarrow 2\,CeO_2}\)
Across the series, poor shielding by 4f electrons causes a steady decrease in ionic radius (lanthanide contraction). Being near the start, Ce3+ is relatively larger than later Ln3+, affecting coordination numbers, complex stability, and separations.
In strongly acidic aqueous media, the standard potential for \(\mathrm{Ce^{4+}/Ce^{3+}}\) is high (commonly quoted around \(\sim\) +1.6 V), making Ce(IV) a powerful oxidant. Complexation, pH, and ligand environment shift effective potentials and kinetics.
Ceria acts as an oxygen buffer to maintain stoichiometric conditions over transients, improving conversion of CO/HC/NOx. It facilitates CO oxidation and NOx reduction by providing/removing lattice oxygen as exhaust composition fluctuates.
Ore concentrates are chemically cracked (acid/alkali), followed by solvent extraction and ion-exchange to separate REEs. Cerium can be selectively oxidized to Ce(IV) and precipitated (e.g., as CeO2), aiding its separation from trivalent lanthanides.
Ce(III) is a well-known scintillator activator (e.g., Ce:YSO, Ce:LuAG). Allowed 5d \(\rightarrow\) 4f transitions give fast blue/green emission, useful in medical imaging detectors and high-energy physics.
Bulk metal has low acute toxicity, but fine powders are reactive and can irritate skin/eyes; some salts may affect the respiratory tract. Handle powders under ventilation, avoid ignition sources (pyrophoric risk for filings), and store metal to limit oxidation.
Hydrolysis/precipitation and a simple redox step:
\(\mathrm{Ce^{3+} + 3\,OH^- \rightarrow Ce(OH)_3(s)\downarrow}\)
\(\mathrm{Ce^{4+} + e^- \rightarrow Ce^{3+}}\)
Ce(OH)3 can be oxidized in air to hydrated cerium(IV) oxides under basic conditions.