CERIUM

Overview

Cerium is the most abundant of the rare earths. It is characterized chemically by having two valence states, the +3 cerous and +4 ceric states. The ceric state is the only non-trivalent rare earth ion stable in aqueous solutions. It is, therefore, strongly acidic and a strong oxidizer. The cerous state closely resembles the other trivalent rare earths.
The numerous commercial applications for Cerium include glass and glass polishing, phosphors, ceramics, catalysts and metallurgy:
In glass industry, it is considered to be the most efficient glass polishing agent for precision optical polishing. It is also used to decolorize glass by keeping iron in its Ferrous state. The ability of Cerium-doped glass to block out ultra violet light is utilized in the manufacturing of medical glassware and aerospace windows. It is also used to prevent polymers from darkening in sunlight and to suppress discoloration of television glass. It is applied to optical components to improve performance.
In phosphors, the role of Cerium is not as the emitting atom, but as a "sensitizer."
Cerium is also used in a variety of ceramics, including dental compositions and as a phase stabilizer in zirconia-based products.
Ceria plays several catalytic roles. In catalytic converters it acts as a stabilizer for the high surface area Alumina, as a promoter of the water-gas shift reaction, as an Oxygen storage component and as an enhancer of the NOX reduction capability of Rhodium. Cerium is added to the dominant catalyst for the production of styrene from methylbenzene to improve styrene formation. It is used in FCC catalysts containing zeolites to provide both catalytic reactivity in the reactor and thermal stability in the regenerator.
In steel manufacturing, it is used to remove free Oxygen and Sulfur by forming stable Oxysulfides and by tying up undesirable trace elements, such as Lead and Antimony.

Description

Cerium is a rare earth metal and the most abundant member of the lanthanide series discovered by Jons J. Berzelius and W. von Hisinger in 1803 in Sweden. Berzelius and Hisinger discovered the new element in a rare reddish-brown mineral now known as cerite, a cerium–lanthanide silicate. Although they could not isolate the pure metal, they found that cerium had two oxidation states: trivalent state (Ce3+, cerous, usually orangered) and the tetravalent state (Ce4+, ceric, usually colorless). Cerium is the only material known to have a solid-state critical point.

Chemical Properties

grey metal ingots (in mineral oil)

Chemical Properties

Elemental cerium has a face-centered cubic crystal structure at 25 °C. Cerium is the most abundant element of the rare-earth group and is 28th in ranking of the naturally occurring elements in the earth’s crust. The element is a silver-gray metal which oxidizes readily at room temperature, particularly in moist air, to form the oxide CeO2, which is of a pale yellowish-green color. Above 300 °C, the element may ignite and burn with a bright red glow. Of the nineteen isotopes of cerium, only four occur in nature, ¹³⁶Ce, ¹³⁸Ce, ¹⁴⁰Ce, and ¹⁴²Ce. The thermal neutron-absorption cross section of the element is low. The element has a low toxicity rating.

Physical properties

Cerium is a grayish/iron-colored, very reactive metallic element that is attacked by bothacids and alkalies. Pure cerium will ignite if scratched with a knife, but it can be combinedsafely with many other elements and materials. It is relatively soft and both malleable andductile.
Its melting point is 798°C, its boiling point is 3,443°C, and its density is 6.770g/cm³.

Isotopes

There are 44 isotopes of cerium, four of which are considered stable. Ce-140accounts for most of the cerium (88.450%) found in the Earth’s crust, and Ce-138makes up just 0.251% of the element in the crust. There are two isotopes with half-liveslong enough to be considered stable: Ce-136 (0.185%), with a half-life of 0.7×10⁺¹⁴years, and Ce-142 (11.14%), with a half-life of 5×10⁺¹⁶ years. All the other isotopes areradioactive with half-lives ranging from 150 nanoseconds to 137.641 days. All are madeartificially.

Origin of Name

Named for the asteroid Ceres, which was discovered two years before the element.

Occurrence

Cerium is the 25th most abundant element on Earth. It is also the most abundant rareearthmetal in the lanthanide series. Its major ores are monazite and bastnasite. Cerium isfound in the Earth’s crust in 46 ppm, which is about 0.0046% of the Earth’s crust. Ceriumis mixed with other elements in its ores, making it difficult to find, isolate, and identify. Itsexistence was unknown until about 1803.
Monazite sands contain most of the rare-earths. The sands of the beaches of Florida andparts of California contain monazite. Monazite is also found in South Africa, India, andBrazil. Bastnasite is found in southern California and New Mexico.

Characteristics

As a pure metal, cerium is unstable and will decompose rapidly in moist air. It also decomposesin hot water to form hydrogen. Its oxide compounds and halides are stable and have anumber of uses.
Cerium is separated from other rare-earth elements by an ion-exchange process in whichit reacts with fluoride. This compound is then reduced with calcium metal (3Ca +2CeF₃ →2Ce + 3CaF₃). Cerium can also be produced by the electrolysis of molten cerium salts. Themetal ion collects at the cathode, and the chlorine or fluorine gases of the salt compound atthe anode.

History

Cerium was discovered in 1803 by Klaproth and by Berzelius and Hisinger; metal prepared by Hillebrand and Norton in 1875. Cerium is the most abundant of the metals of the so-called rare earths. It is found in a number of minerals including allanite (also known as orthite), monazite, bastnasite, cerite, and samarskite. Monazite and bastnasite are presently the two most important sources of cerium. Large deposits of monazite found on the beaches of Travancore, India, in river sands in Brazil, and deposits of allanite in the western United States, and bastnasite in Southern California will supply cerium, thorium, and the other rare-earth metals for many years to come. Metallic cerium is prepared by metallothermic reduction techniques, such as by reducing cerous fluoride with calcium, or by electrolysis of molten cerous chloride or other cerous halides. The metallothermic technique is used to produce highpurity 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 amounts of energy are 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 β 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 is an iron-gray lustrous metal. It is malleable, and oxidizes very readily at room temperature, especially in moist air. Except for europium, cerium is the most reactive of the “rare-earth” metals. It slowly decomposes in cold water, and rapidly in hot water. Alkali solutions and dilute and concentrated acids attack the metal rapidly. The pure metal is likely to ignite if scratched with a knife. Ceric salts are orange red or yellowish; cerous salts are usually white. Cerium is a component of misch metal, which is extensively used in the manufacture of pyrophoric alloys for cigarette lighters, etc. Natural cerium is stable and contains four isotopes. Thirtytwo other radioactive isotopes and isomers are known. While cerium is not radioactive, the impure commercial grade may contain traces of thorium, which is radioactive. The oxide is an important constituent of incandescent gas mantles and it is emerging as a hydrocarbon catalyst in “self-cleaning” ovens. In this application it can be incorporated into oven walls to prevent the collection of cooking residues. As ceric sulfate it finds extensive use as a volumetric oxidizing agent in quantitative analysis. Cerium compounds are used in the manufacture of glass, both as a component and as a decolorizer. The oxide is finding increased use as a glass polishing agent instead of rouge, for it is much faster than rouge in polishing glass surfaces. Cerium compounds are finding use in automobile exhaust catalysts. Cerium is also finding use in making permanent magnets. Cerium, with other rare earths, is used in carbon-arc lighting, especially in the motion picture industry. It is also finding use as an important catalyst in petroleum refining and in metallurgical and nuclear applications. In small lots, cerium costs about $5/g (99.9%).

Uses

Cerium metal finds wide application in mischmetal, which is a rareearth metal comprised of 50% Ce, 25% La, 18% Nd, 5% Pr, and 2% other rare earths. This alloy is used in shell linings for military projectiles, as an alloying agent for improving the malleability of ductile iron, and in lighter “flints” where the alloy is compounded with a 30% iron alloy. The pyrophoric and incendiary nature of cerium are evident when ceriumbase alloys are machined. Mischmetal also improves the creep resistance of magnesium alloys, the resistance to oxidation of nickel alloys, the hardness of copper alloys, and the strength of aluminum alloys. Both cerium metal and mischmetal are used as getters to remove traces of oxygen in vacuum tubes and equipment. When alloyed with cobalt, cerium is gaining importance as a magnet material. CeCo5, as a permanent magnet material, has properties that exceed those of the alnicos and ferrites. Mixed rare-earth oxides and fluorides containing up to 50% cerium are used as cores for carbon arcs which, for illuminating purposes, have much greater intensity and color balance. The mixed oxides with cerium also are used as catalysts (petroleum cracking and chemical oxidation reactions) and in a variety of waterproofing agents, fungicides, and polishing materials.

Uses

The compound cerium oxide (either Ce₂O₃ or CeO₂) is used to coat the inside of ovensbecause it was discovered that food cannot stick to oven walls that are coated with ceriumoxide. Cerium compounds are used as electrodes in high-intensity lamps and film projectorsused by the motion picture industry. Cerium is also used in the manufacturing andpolishing of high-refraction lenses for cameras and telescopes and in the manufacture ofincandescent lantern mantles. It additionally acts as a chemical reagent, a misch metal, anda chemical catalyst. Cerium halides are an important component of the textile and photographicindustries, as an additive to other metals, and in automobile catalytic converters.Cerium is also used as an alloy to make special steel for jet engines, solid-state instruments,and rocket propellants.

Uses

In metallurgy as stabilizers in alloys and as an alternative to thorium oxide in welding electrodes. In glass as polishing agent, decolorizer to stabilize impurities, to render glass opaque to near uv radiation, to resist discoloration from strong light or high energy electron bombardment (as in television screens). In ceramics as an opacifying and strengthening agent. Catalysts to impart high cracking activity for crude oil processing, in automotive exhaust control devices, as combustion additive, polymerization initiator, paint drier, polymer stabilizer. As phosphor in fluorescent lamps, cathode ray tubes and thorium dioxide gas mantles.

Definition

A rare-earth element of the lanthanide group of the periodic table. Four stable isotopes.

Production Methods

Cerium is obtained from its ores by chemical processing and separation. The process involves separation of cerium from other rare-earth metals present in the ore. The ore is crushed, ground, and treated with acid. The extract solution is buffered to pH 3-4 and the element is precipitated selectively as Ce⁴⁺ salt. Cerium also may be separated from other metals by an ionexchange process.
Also, the metal may be obtained by high temperature reduction of cerium(III) chloride with calcium:
2CeCl₃ + 3Ca → 2Ce + 3CaCl₂

Definition

Symbol Ce. A silvery metallicelement belonging to the lanthanoids;a.n. 58; r.a.m. 140.12; r.d.6.77 (20°C); m.p. 799°C; b.p. 3426°C.It occurs in allanite, bastnasite,cerite, and monazite. Four isotopesoccur naturally: cerium–136, –138,–140, and –142; fifteen radioisotopeshave been identified. Cerium is usedin mischmetal, a rare-earth metalcontaining 25% cerium, for use inlighter flints. The oxide is used inthe glass industry. It was discoveredby Martin Klaproth (1743–1817) in1803.

General Description

Cerium is a gray colored, ductile solid. This form of cerium is slabs, ingots or rods. When heated to high temperatures CERIUM will burn readily and may be difficult to extinguish. CERIUM is used to make signaling devices.

Air & Water Reactions

Finely divided metal powder is pyrophoric [Bretherick 1979 p. 170-171]. CERIUM will react vigorously if exposed to water or moist air and will generate flammable and/or toxic fumes.

Reactivity Profile

CERIUM is a strong reducing agent. Resembles aluminum in its chemical properties. [Lewis]. Reactivity is enhanced by a state of high physical subdivision, as in TURNINGS OR GRITTY POWDER. Attacked by dilute and concentrated mineral acids and alkalis with the generation of flammable gases. Readily oxidized by moist air at room temperature. Reacts with zinc with explosively violence. Gives very exothermic reactions with antimony or bismuth. Reacts violently with phosphorus at 400-500°C [Mellor 8, Supp. 3:347 1971].

Hazard

May ignite on heating to 300F (148.9C). Strong reducing agent.

Hazard

Most of the compounds of cerium are toxic if ingested or if the fumes are inhaled. Ceriumwill ignite when heated.

Health Hazard

Oxides from metallic fires are a severe health hazard. Inhalation or contact with substance or decomposition products may cause severe injury or death. Fire may produce irritating, corrosive and/or toxic gases. Runoff from fire control or dilution water may cause pollution.

Fire Hazard

May react violently or explosively on contact with water. Some are transported in flammable liquids. May be ignited by friction, heat, sparks or flames. Some of these materials will burn with intense heat. Dusts or fumes may form explosive mixtures in air. Containers may explode when heated. May re-ignite after fire is extinguished.

Flammability and Explosibility

Flammable

Industrial uses

A chemical element, cerium (Ce) is the mostabundant metallic element of the rare earthgroup in the periodic table. Cerium occursmixed with other rare earths in many minerals,particularly monazite and blastnasite, and isfound among the products of the fission of uranium,thorium, plutonium.
Ceric oxide, CeO2, is the oxide usuallyobtained when cerium salts of volatile acids areheated. CeO2 is an almost white powder that isinsoluble in most acids, although it can be dissolvedin H2SO4 or other acids when a reducingagent is present. The metal is an iron-gray colorand it oxidizes readily in air, forming a graycrust of oxide. Misch metal, an alloy of cerium,is used in the manufacture of lighter flints.Cerium has the interesting property that, at verylow temperatures or when subjected to highpressures, it exhibits a face-centered cubicform, which is diamagnetic and 18% denserthan the common form.

Safety Profile

Cerium resembles aluminum in its pharmacological action as well as in its chemical properties. The insoluble salts such as the oxalates are stated to be nontoxic even in large doses. It is used to prevent vomiting in pregnancy. The average dose is from 0.05 to 0.5 g. The effect on the central nervous system of the rare-earth metals following inhalation may preclude welding operations with these materials to any large extent. Cerium is stated to produce polycythemia but is useless in the treatment of anemia owing to its toxic effects. The salts of cerium increase the blood coagulation rate. See also RARE EARTHS. A strong reducing agent. Moderate fire hazard; ignites spontaneously in air at 150-180'. Moderate explosion hazard in the form of dust when exposed to flame. The metal or its alloys spark with friction. Many alloys are pyrophoric in air. See also IRON DUST. Explosive reaction with zinc. Very exothermic reaction with antimony or bismuth. Ignites when heated in atmospheres of CO2 + N2, Cl2, or Br2. Violent reaction when heated with phosphorus (4OO℃), silicon (1400℃).

Environmental Fate

Cerium resembles aluminum in its biologic and chemical properties. Cerium and cerium compounds have low to moderate toxicity unless the associated anions are toxic. Intratracheally administered nanoparticles tend to accumulate in liver and cause damage there.

Toxicity evaluation

Although cerium is a rare earth element, it is relatively abundant in the earth’s crust. It makes up about 0.0046% of the Earth’s crust by weight and ranks 25th in occurrence at an average distribution of 20–60 ppm. Cerium is a malleable, soft, ductile, iron-gray metal, slightly harder than lead. It is very reactive and readily tarnishes in the air. Cerium oxidizes slowly in cold water and rapidly in hot water. It dissolves in acids. Cerium can burn when heated or scratched with a knife.
Cerium is not expected to exist in elemental form in the environment since it is a reactive metal. Cerium is dumped in the environment in many different places, mainly by petrolproducing industries. It can also enter the environment when household equipment is thrown away. Cerium compounds exist solely in particulate form if release into air and not expected to volatilize. Water-soluble cerium compounds usually have a pKa of 8.5, which indicates that the hydrated Ce3+ ion will remain in solution at environmental pHs of 4–9. The ion is expected to hydrolyze and polymerize at environmental pH and may precipitate out of solution. Thus, cerium will gradually accumulate in soils and water, which eventually leads to increasing concentrations in humans, animals, and soil particles.