Rhodium

Rhodium is one of the rarest and most expensive metals on Earth. The small amount mined each year, just a few tonnes, makes it extraordinarily valuable, and its primary use is as the secret ingredient in catalytic converters that clean the exhaust gases from every petrol and diesel car.

• Atomic Number4545 protons, 45 electrons
• Atomic Mass102.9055 u45× heavier than hydrogen
• State at Room TempSolidSolid
• Density12.4 g/cm³
• Melting / Boiling1963.8°C / 3694.8°C
• Discovered1803

What is Rhodium?

Rhodium is a transition metal in Group 9 of the periodic table, sitting below cobalt. With 45 protons, it is one of the platinum group metals: hard, shiny, highly corrosion-resistant and an excellent catalyst. Its most common oxidation state is +3. Rhodium does not tarnish in air and is the most reflective metal known, making it ideal as a reflective coating.

Rhodium gets its name from the ancient Greek word rhodon meaning rose, because of the rose-pink colour of its chloride salts in solution. It was discovered by the English chemist William Hyde Wollaston in 1803 while he was processing platinum ore, along with palladium (which he also discovered in the same year). The symbol Rh comes from the name.

Fact Rhodium is often the most expensive of all metals, far more costly than gold or platinum. Its price fluctuates dramatically based on car production rates. In 2021, rhodium reached over £25,000 per troy ounce, more than 15 times the price of gold. The reason: essentially every petrol and diesel car needs rhodium, and there is simply very little of it.

Where you find Rhodium

In space

Rhodium is produced in stars and is present in the Sun in trace amounts.

On Earth

Rhodium is one of the rarest stable elements in the Earth's crust, just 0.0002 parts per million.

• Platinum group metal ores. Rhodium occurs in native platinum ores and is recovered as a minor by-product of platinum and palladium mining. South Africa produces approx. 80% of global supply.
• Nickel-copper ores. Some rhodium is recovered from the complex nickel-copper sulfide deposits of Norilsk in Russia and Sudbury in Canada.

How we use Rhodium

• Catalytic converters.. About 80% of all rhodium is used in catalytic converters, where it catalyses the reduction of nitrogen oxides (NOx) to harmless nitrogen gas. Without rhodium, modern cars could not meet air quality standards.
• Industrial catalysis.. Rhodium catalysts are used to make acetic acid (for plastics and textiles) and in hydroformylation reactions to make aldehydes from alkenes.
• Reflective coatings.. Rhodium's extreme reflectivity is used in mirror coatings for professional searchlights, projectors and reflectors. Jewellery is rhodium-plated to give silver and white gold a more durable, brighter finish.

Did you know? Platinum-group metal catalytic converters, containing platinum, palladium and rhodium, have dramatically improved air quality in cities since their introduction in the 1970s. A modern three-way catalytic converter converts 99% of the carbon monoxide, hydrocarbons and nitrogen oxides in exhaust into carbon dioxide, water vapour and nitrogen.

How it was discovered

Rhodium was discovered in 1803 by William Hyde Wollaston, who was analysing platinum ore and found an unusual residue after dissolving it in aqua regia. He separated two new metals from this residue: palladium (named for the asteroid Pallas) and rhodium. Wollaston's careful analytical chemistry in this period was among the most productive in history.

Deeper dive: rhodium chemistry and applications

The three-way catalytic converter works because different catalytic metals handle different exhaust pollutants. Platinum and palladium oxidise carbon monoxide (CO) and unburned hydrocarbons to CO₂ and water. Rhodium reduces nitrogen oxides (NOx, mainly NO and NO₂) back to harmless nitrogen gas. This reduction reaction is the hardest part: breaking the strong nitrogen-oxygen bond requires a specific catalyst. Rhodium is uniquely effective because it adsorbs NO molecules, weakens the N-O bond, and allows N atoms to recombine as N₂. No other element does this job as well at the temperatures inside an exhaust system.

Moving to 46 protons on the periodic table brings us to Palladium.