Cerium

The Not-so-rare Earth, with Down-to-earth uses

Although cerium is one of the so-called rare earth metals (aka the lanthanoids) it is the most abundant of them and is as common as zinc.

It was discovered as the ore cerite (cerium silicate) which was found by Axel Cronstedt at a mine in Vestmanland, Sweden. He thought it was a tungsten ore but Wilhelm Hisinger and Jöns Jacob Berzelius proved in 1804 that it was a new element, which they named after the recently discovered asteroid, Ceres.

Attempts to get a sample of the pure metal at the time were doomed to failure - even heating cerium chloride with potassium failed to produce it - and it was not until 1875 that two American chemists, William Hillebrand and Thomas Norton, first obtained a pure specimen, which they got by passing an electric current through molten cerium chloride.

In the century following its discovery, cerium found use in medical treatment and in gas mantles. In 1854, medic Professor Sir James Simpson of the University of Edinburgh, reported that cerium nitrate suppressed vomiting and especially that associated with morning sickness. Others prescribed it as a treatment for sea-sickness and as a cough suppressant for those with tuberculosis.

Over-the-counter cerium medicaments were on sale as Novonaurin and Cerocol tablets well into the last century. Eventually cerium fell out of favour with doctors, and looked likely never to return until, in 1995, a weak solution of cerium nitrate was found to be an effective first treatment for bathing the skin of people suffering extensive third degree burns. This is now standard procedure in some specialist burns units.

Cerium was needed for the production of incandescent gas mantles. Early gas lighting relied on the presence of traces of benzene which burned with a bright flame, but when this was extracted because it could be sold more profitably to the chemical industry, then an alternative way of producing light from gas had to be found. Gas mantles were devised and these shine brightly when heated. They were made of thorium oxide plus 1% cerium oxide, the latter being added to produce a more softer glow, as well as acting as a catalyst ensuring more complete combustion of the gas.

Today cerium comes from rare earth ores, and cerium oxide, CeO₂, is the most important outlet for cerium. It remains behind when the other metals are dissolved out using hydrochloric acid. The insoluble residue is known as cerium concentrate and production amounts to about 63,000 tonnes a year, but this is likely to increase this century as more uses are found.

Cerium oxide is important for specialist glass. Adding 2% protects glass against damage from radiation, X-ray, and cathode rays, and it will also filter out UV light. Most damage caused by UV is due to wavelengths below 400 nm, but these are strongly absorbed by cerium. Optical glass for lenses also includes cerium, along with lanthanum and yttrium, to provide a high refraction index and low colour dispersion. Cerium oxide concentrate, as a slurry in water, is used in place of ferric oxide rouge for polishing optical glass, giving a better finish more quickly to lenses and screens.

Cerium oxide is also important as a catalyst. It is used by the chemical industry for the production of alcohols, phenols, and ketones. It is also is part of the catalyst of catalytic converters used to clean up vehicle exhausts. It enhances the performance of the other metals that are present, and it is thought to do this by acting to store and release oxygen atoms. These convert CO to CO₂ as well as oxidising unburnt hydrocarbons to CO₂. Cerium is also used to reduce the emission of micron sized particulate emissions from diesel engines.

So-called self-cleaning ovens have CeO₂ incorporated into the walls and there it catalyses the oxidation of cooking resides, which are also mainly carbon based.

Until recently, cadmium red was the preferred pigment to colour containers, toys, household wares, traffic cones, and crates, but cadmium is now considered environmentally undesirable and its pigments are being phased out in favour of cerium sulfide, Ce₂S₃. This gives a rich red colour, stable up to 350°C, and it is completely non-toxic.

Cerium is used in many other ways, such as in flat screen TVs, low-energy light bulbs, magnetic-optic CD disks, and it is part of the core material in carbon-arc electrodes for film studio lights and for flood-lighting large venues.