Rhodium (Rh)

Rhodium does one job that nothing else does properly, and the entire economics of the element follow from it. In a petrol engine's exhaust, rhodium reduces nitrogen oxides to nitrogen while oxygen is still present in the gas stream — a chemically awkward demand, since reduction and oxidation are supposed to be opposites, and it is why the device is called a three-way catalyst. Platinum and palladium handle the oxidation of carbon monoxide and unburnt fuel. The nitrogen oxides are rhodium's, and after fifty years of substitution research they still are.

Rose-red salts in the residue

William Hyde Wollaston was working through crude platinum ore from South America in 1803 and 1804, dissolving it in aqua regia and precipitating the platinum out. What was left in solution, after palladium had also been removed, gave a beautiful deep red when treated with certain reagents. He obtained the metal in 1804 and named it from Greek rhodon, a rose, for the colour of its salts rather than of the metal, which is a bright, slightly bluish white.

Rhodium's discovery is far less contentious than palladium's, which Wollaston pulled out of the very same ore at the same time. He published rhodium normally, in the Philosophical Transactions, and nobody disputed it.

Twenty-nine thousand dollars an ounce

Rhodium is not mined. It occurs as a minor component of platinum group metal ores, chiefly in the Bushveld Complex of South Africa, which supplies around 80% of world output, with Russia and a little from Zimbabwe and North America making up the rest. Annual production is on the order of twenty tonnes, and no producer can raise it in response to price, because the rhodium content of the ore is fixed by geology and the mine is really operating for platinum and palladium.

Demand, meanwhile, is set by emissions law. Every tightening of nitrogen oxide limits raises the rhodium loading per vehicle, and the loading cannot be reduced without failing the test. A market with almost no supply elasticity and legally mandated demand is a market that can go anywhere, and in March 2021 it went to about $29,000 per troy ounce — roughly fifteen times the price of gold at the time, and the highest price ever paid for any traded metal.

The consequence on the street was a wave of catalytic converter thefts across North America and Europe, concentrated on hybrid vehicles, whose converters run cooler and are loaded more heavily to compensate. A part worth a few hundred dollars as scrap steel was suddenly worth thousands for the few grams of rhodium in its washcoat. Prices have since fallen back substantially as substitution and thrifting took effect and as electric vehicles began to erode the underlying demand.

L-DOPA, and the first asymmetric catalyst in a factory

Rhodium's other great contribution is to synthesis, and it is a subtler one. William Knowles at Monsanto, working in the late 1960s, took Geoffrey Wilkinson's soluble rhodium hydrogenation catalyst and replaced its triphenylphosphine ligands with a chiral phosphine. The modified catalyst added hydrogen preferentially to one face of a prochiral alkene, producing a large excess of one enantiomer.

Applied to the manufacture of L-DOPA for Parkinson's disease, this became the first industrial process to use a chiral catalyst to make a single-handed drug, and it did so in 1974, decades before regulators began requiring single enantiomers as a matter of course. Knowles shared the 2001 Nobel Prize in Chemistry for it. The whole field of asymmetric catalysis, now central to pharmaceutical manufacture, begins with a rhodium complex.

Rhodium runs two other large-scale industrial reactions. In low-pressure hydroformylation it converts propene and synthesis gas to butyraldehyde far more selectively and at far gentler conditions than the cobalt catalysts it replaced, and that chemistry underlies a great deal of the world's plasticiser and detergent alcohol production. And in the Monsanto acetic acid process, a rhodium iodide complex carbonylates methanol; for decades this was how most of the world's acetic acid was made, until an iridium-based successor took a share of it.

A wire that counts neutrons

Rhodium is mononuclidic — every atom of it is rhodium-103 — and that uniformity is what makes a particular reactor instrument possible.

A self-powered neutron detector is a thin rhodium wire inside an insulated sheath, pushed into the core of an operating reactor. Neutrons convert rhodium-103 into rhodium-104, which beta decays within minutes; the emitted electrons leave the wire and the resulting current is measured directly, with no external power supply and no gas, high voltage or amplifier in the core. The signal is proportional to the local neutron flux.

Rhodium works for this because its capture cross-section is high, its product's half-life is short enough to track power changes but long enough to give a steady reading, and there is no isotopic mixture to complicate the response. Strings of rhodium detectors map the power distribution inside pressurised water reactors and CANDUs, and the wire is slowly consumed by the very neutrons it measures, so the detectors are replaced on a schedule.

The whitest surface anyone plates

Rhodium is exceptionally reflective across the visible spectrum, does not tarnish, and is very hard. A few tenths of a micrometre of it electroplated over another metal gives a surface that looks brilliantly white and stays that way.

This is why almost all white gold jewellery sold is rhodium plated. White gold alloys are in truth a pale yellow or grey; the bright white that customers expect is a rhodium skin, and it wears through after some years of use, which is why rings periodically need replating. Sterling silver items are sometimes rhodium plated to stop them tarnishing altogether. The same properties put rhodium on searchlight and optical instrument mirrors, and on some electrical contacts where low, stable contact resistance matters more than cost.

Rhodium plating is also the reason for one of the odder objects in the record books: certain presentation discs awarded for record sales have been plated in rhodium specifically because it was more expensive than gold or platinum.