Rhodium

Category: transition metal Atomic number: 45 Colour: silvery white Melting point: 1,964°C (3,567°F) Boiling point: 3,695°C (6,683°F) First identified: 1803

The streets of your local town or city would be much more unhealthy places if it weren’t for rhodium, one of the rarest non-radioactive elements. Catalytic converters in cars use a variety of oxidation and reduction processes to convert toxic gases and pollutants into cleaner emissions in exhaust gas. Palladium and platinum are also used for this purpose, but rhodium plays a key part in the conversion of 80 per cent of the nitrogen oxides into harmless nitrogen and oxygen.

It was discovered by William Wollaston in 1803 – he was collaborating with Smithson Tennant on an attempt to purify platinum for sale. After they dissolved platinum in the fierce acid blend known as aqua regia, Smithson Tennant investigated the residue left behind (see here) while Wollaston continued working with the solution of platinum. He removed the platinum and then palladium (see here) by precipitation, which is the creation of a solid from a solution. He was left with sodium rhodium chloride, in the form of beautiful rose-red crystals, which inspired the name rhodium, from the Greek rhodon, for ‘rose-red’. He went on to extract the metal itself from this compound.

Rhodium is used in alloys with platinum for thermocouples, as a coating for optic fibres, and as an electrical contact material. As well as its role in catalytic converters, the chemicals industry uses it as a catalyst in a variety of other ways. You might, for example, encounter it on a regular basis in mint flavouring. Such flavouring was originally derived from mint plants, but the Nobel Prize-winning Japanese chemist Ryoji Noyori devised a high-quality method of menthol production that relies on rhodium catalysis.

Green Chemistry

Ryoji Noyori is an inspirational example of how chemists can impact on the world positively. He is an advocate of green chemistry: the design of sustainable products and processes that minimize the use of hazardous substances. In one recent article, he argued that ‘our ability to devise straightforward and practical chemical syntheses is indispensable to the survival of our species’.