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Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet Ceres. Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight. It is found in a number of minerals, the most important being monazite and bastnasite. Commercial applications of cerium are numerous. They include catalysts, additives to fuel to reduce emissions and to glass and enamels to change their color. Cerium oxide is an important component of glass polishing powders and phosphors used in screens and fluorescent lamps.
Cerium is a silvery metal, belonging to the lanthanide group. It resembles iron in color and luster, but is soft, and both malleable and ductile. Cerium has the longest liquid range of any non-radioactive element: 2648 C° (795 °C to 3443 °C) or 4766 F° (1463 °F to 6229 °F). (Thorium has a longer liquid range, but is radioactive)
Phase diagram of cerium
Cerium is especially interesting because of its variable electronic structure. The energy of the inner 4f level is nearly the same as that of the outer or valence electrons, and only small energy is required to change the relative occupancy of these electronic levels. This gives rise to dual valency states. For example, a volume change of about 10% occurs when cerium is subjected to high pressures or low temperatures. It appears that the valence changes from about 3 to 4 when it is cooled or compressed. The low temperature behavior of cerium is complex. Four allotropic modifications are thought to exist: cerium at room temperature and at atmospheric pressure is known as γ cerium. Upon cooling to –16 °C, γ cerium changes to ß cerium. The remaining γ cerium starts to change to α cerium when cooled to –172 °C, and the transformation is complete at –269 °C. α Cerium has a density of 8.16; δ cerium exists above 726 °C. At atmospheric pressure, liquid cerium is more dense than its solid form at the melting point.
Cerium metal tarnishes slowly in air and burns readily at 150 °C to form cerium(IV) oxide:
Ce + O2 → CeO2
Cerium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form cerium hydroxide:
2 Ce (s) + 6 H2O (l) → 2 Ce(OH)3 (aq) + 3 H2 (g)
Cerium metal reacts with all the halogens:
2 Ce (s) + 3 F2 (g) → 2 CeF3 (s) [white]
Cerium dissolves readily in dilute sulfuric acid to form solutions containing the colorless Ce(III) ions, which exist as a [Ce(OH2)9]3+ complexes:
2 Ce (s) + 3 H2SO4 (aq) → 2 Ce3+ (aq) + 3 SO2−4 (aq) + 3 H2 (g)
Cerium(IV) (ceric) salts are orange red or yellowish, whereas cerium(III) (cerous) salts are usually white or colorless. Both oxidation states absorb ultraviolet light strongly. Cerium(III) can be used to make glasses that are colorless, yet absorb ultraviolet light almost completely. Cerium can be readily detected in rare earth mixtures by a very sensitive qualitative test: addition of ammonia and hydrogen peroxide to an aqueous solution of lanthanides produces a characteristic dark brown color if cerium is present.
Cerium exhibits three oxidation states, +2, +3 and +4. The +2 state is rare and is observed in CeH2, CeI2 and CeS. The most common compound of cerium is cerium(IV) oxide (CeO2), which is used as "Jeweller's rouge" as well as in the walls of some self-cleaning ovens. Two common oxidizing agents used in titrations are ammonium cerium(IV) sulfate (ceric ammonium sulfate, (NH4)2Ce(SO4)3) and ammonium cerium(IV) nitrate (ceric ammonium nitrate or CAN, (NH4)2Ce(NO3)6). Cerium also forms a chloride, CeCl3 or cerium(III) chloride, used to facilitate reactions at carbonyl groups in organic chemistry. Other compounds include cerium(III) carbonate (Ce2(CO3)3), cerium(III) fluoride (CeF3), cerium(III) oxide (Ce2O3), as well as cerium(IV) sulfate (ceric sulfate, Ce(SO4)2) and cerium(III) triflate (Ce(OSO2CF3)3).
The two oxidation states of cerium differ enormously in basicity: cerium(III) is a strong base, comparable to the other trivalent lanthanides, but cerium(IV) is weak. This difference has always allowed cerium to be by far the most readily isolated and purified of all the lanthanides, otherwise a notoriously difficult group of elements to separate. A wide range of procedures have been devised over the years to exploit the difference. Among the better ones:
1. Leaching the mixed hydroxides with dilute nitric acid: the trivalent lanthanides dissolve in cerium-free condition, and tetravalent cerium remains in the insoluble residue as a concentrate to be further purified by other means. A variation on this uses hydrochloric acid and the calcined oxides from bastnasite, but the separation is less sharp.
Formerly used commercially was a method whereby a solution of cerium(IV) in nitric acid would be added to dilute sulfuric acid. This caused cerium(IV) to largely precipitate as a basic salt, leaving trivalent lanthanide in solution. However, the finely divided precipitate was difficult to filter from the highly corrosive medium. Using the classical methods of rare-earth separation, there was a considerable advantage to a strategy of removing cerium from the mixture at the beginning. Cerium typically comprised 45% of the cerite or monazite rare earths, and removing it early greatly reduced the bulk of what needed to be further processed (or the cost of reagents to be associated with such processing). However, not all cerium purification methods relied on basicity. Ceric ammonium nitrate [ammonium hexanitratocerate(IV)] crystallization from nitric acid was one purification method. Cerium(IV) nitrate (hexanitratoceric acid) was more readily extractable into certain solvents (e.g. tri-n-butyl phosphate) than the trivalent lanthanides. However, modern practice in China seems to be to do purification of cerium by counter-current solvent extraction, in its trivalent form, just like the other lanthanides.
Cerium(IV) is a strong oxidant under acidic conditions, but stable under alkaline conditions, when it is cerium(III) that becomes a strong reductant, easily oxidized by atmospheric oxygen (O2). This ease of oxidation under alkaline conditions leads to the occasional geochemical parting of the ways between cerium and the trivalent light lanthanides under supergene weathering conditions, leading variously to the "negative cerium anomaly" or to the formation of the mineral cerianite. Air-oxidation of alkaline cerium(III) is the most economical way to get to cerium(IV), which can then be handled in acid solution.
Naturally-occurring cerium is composed of 4 stable isotopes; 136Ce, 138Ce, 140Ce, and 142Ce with 140Ce being the most abundant (88.48% natural abundance). 136Ce and 142Ce are predicted to be double beta active but no signs of activity were ever observed (for 142Ce, the lower limit on half-life is 5×1016 years). 26 radioisotopes have been characterized with the most long-lived being 144Ce with a half-life of 284.893 days, 139Ce with a half-life of 137.640 days, and 141Ce with a half-life of 32.501 days. All of the remaining radioactive isotopes have half-lives that are less than 4 days and the majority of these have half-lives that are less than 10 minutes. This element also has 2 meta states.
The known isotopes of cerium range in atomic weight from 123 u (123Ce) to 152 u (152Ce).
Cerium 144 is a high-yield product of nuclear fission; the ORNL Fission Product Pilot Plant separated substantial quantities of cerium-144 from reactor waste, and it was used in the Aircraft Nuclear Propulsion and SNAP programs.
When the rare earths were first discovered, since they were strong bases like the oxides of calcium or magnesium, they were thought to be divalent. Thus, "ceric" cerium was thought to be trivalent, and the oxidation state ratio was therefore thought to be 1.5. Berzelius was annoyed to keep on getting the correct ratio 1.33. He was after all one of the finest analytical chemists in Europe. In the late 1950s, the Lindsay Chemical Division of American Potash and Chemical Corporation of West Chicago, Illinois, then the largest producer of rare earths in the world, was offering cerium compounds in two purity ranges, "commercial" at 94-97% purity, and "purified", at a reported 99.9+% purity. In their October 1, 1958 price list, one-pound quantities of the oxides were priced at $3.30 or $8.10 respectively for the two purities; the per-pound price for 50-pound quantities were respectively $1.95 or $4.95 for the two grades. Cerium salts were proportionately cheaper, reflecting their lower net content of oxide.
Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight. It is found in a number of minerals including allanite (also known as orthite)—(Ca, Ce, La, Y)2(Al, Fe)3(SiO4)3(OH), monazite (Ce, La, Th, Nd, Y)PO4, bastnasite (Ce, La, Y)CO3F, hydroxylbastnasite (Ce, La, Nd)CO3(OH, F), rhabdophane (Ce, La, Nd)PO4-H2O, zircon (ZrSiO4), and synchysite Ca(Ce, La, Nd, Y)(CO3)2F. Monazite and bastnasite are presently the two most important sources of cerium. Large deposits of monazite, allanite, and bastnasite will supply cerium, thorium, and other rare-earth metals for many years to come.
The mineral mixtures are crushed, ground and treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3-4. Thorium precipitates out of solution as hydroxide and is removed. After that the solution is treated with ammonium oxalate to convert rare earths in to their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose salts are insoluble in HNO3. Metallic cerium is prepared by metallothermic reduction techniques, such as by reducing cerium fluoride or chloride with calcium, or by electrolysis of molten cerous chloride or other cerous halides. The metallothermic technique is used to produce high-purity cerium.
A major technological application for Cerium(III) oxide is a catalytic converter for the reduction of CO emissions in the exhaust gases from motor vehicles. In particular, cerium oxide is added into Diesel fuels. Another important use of the cerium oxide is a hydrocarbon catalyst in self cleaning ovens, incorporated into oven walls and as a petroleum cracking catalyst in petroleum refining. 
Cerium(IV) oxide is considered one of the most efficient agents for precision polishing of optical components. Cerium compounds are also used in the manufacture of glass, both as a component and as a decolorizer. For example, cerium(IV) oxide in combination with titanium(IV) oxide gives a golden yellow color to glass; it also allows for selective absorption of ultraviolet light in glass. Cerium oxide has high refractive index and is added to enamel to make it more opaque.
Cerium(IV) oxide is used in incandescent gas mantles, such as the Welsbach mantle, where it was combined with thorium, lanthanum, magnesium or yttrium oxides. Doped with other rare earth oxides, it has been investigated as a solid electrolyte in intermediate temperature solid oxide fuel cells: The cerium(IV) oxide-cerium(III) oxide cycle or CeO2/Ce2O3 cycle is a two step thermochemical process based on cerium(IV) oxide and cerium(III) oxide for hydrogen production.
The photostability of pigments can be enhanced by addition of cerium. It provides pigments with light fastness and prevents clear polymers from darkening in sunlight. Television glass plates are subject to electron bombardment, which tends to darken them by creation of F-center color centers. This effect is suppressed by addition of cerium oxide. Cerium is also an essential component of phosphors used in TV screens and fluorescent lamps.
A traditional use of cerium was in the pyrophoric mischmetal alloy used for light flints. Because of the high affinity of cerium to sulfur and oxygen, it is used in various aluminium alloys, and iron alloys. In steels, cerium degasifies and can help reduce sulfides and oxides, and it is a precipitation hardening agent in stainless steel. Adding cerium to cast irons opposes graphitization and produces a malleable iron. Addition of 3-4% of cerium to magnesium alloys, along with 0.2 to 0.6% zirconium, helps refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings.
Cerium alloys are used in permanent magnets and in tungsten electrodes for gas tungsten arc welding. Cerium is used in carbon-arc lighting, especially in the motion picture industry. Cerium oxalate is an anti-emetic drug. Cerium(IV) sulfate is used extensively as a volumetric oxidizing agent in quantitative analysis. Ceric ammonium nitrate is a useful one-electron oxidant in organic chemistry, used to oxidatively etch electronic components, and as a primary standard for quantitative analysis.
Cerium, like all rare-earth metals, is of low to moderate toxicity. Cerium is a strong reducing agent and ignites spontaneously in air at 65 to 80 °C. Fumes from cerium fires are toxic. Water should not be used to stop cerium fires, as cerium reacts with water to produce hydrogen gas. Workers exposed to cerium have experienced itching, sensitivity to heat, and skin lesions. Animals injected with large doses of cerium have died due to cardiovascular collapse. Cerium(IV) oxide is a powerful oxidizing agent at high temperatures and will react with combustible organic materials. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. Cerium serves no known biological function.
1. ^ Magnetic susceptibility of the elements and inorganic compounds, in Handbook of Chemistry and Physics 81st edition, CRC press.
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