Rhodium
Also: Rh
Rhodium (Rh) is a silvery-white platinum-group precious metal used mainly as a catalyst in vehicle exhaust systems and for surface finishing; it ranks among the rarest and most expensive metals on Earth.
Rhodium (chemical symbol Rh, atomic number 45) belongs to the platinum-group metals and was discovered in 1803 by the British chemist William Hyde Wollaston. Its name derives from the Greek word rhodon (rose), a reference to the rose-coloured solution that rhodium salts form in hydrochloric acid. The metal is silvery-white, exceptionally corrosion resistant and barely attacked even by aqua regia.
Occurrence and extraction
Rhodium is one of the rarest stable elements in the Earth's crust. It occurs almost exclusively as a by-product of platinum and palladium mining. More than 80% of annual world output comes from the Bushveld Complex mines in South Africa, with further quantities supplied by Russian nickel ore deposits and Zimbabwean platinum mines. Global annual production rarely exceeds 30 tonnes; by comparison, gold is produced at a rate of roughly 3,500 tonnes a year.
Extraction is demanding: raw platinum concentrate passes through several hydrometallurgical separation steps in a refinery before pure rhodium of 99.9% fineness is isolated.
Industrial importance
By far the most important field of use is the three-way catalytic converter in the automotive sector. Rhodium reduces nitrogen oxides (NOx) to nitrogen and oxygen, and in this role no other metal can fully replace it. Around 80-85% of world rhodium demand goes into this sector.
| Application area | Approx. share of consumption |
|---|---|
| Vehicle catalysts (NOx reduction) | 80-85% |
| Chemical industry (acetic acid synthesis) | 5-8% |
| Glass production (spinnerets) | 3-5% |
| Jewellery / surface finishing (rhodium plating) | 2-3% |
| Other (electronics, laboratory) | under 2% |
In the jewellery trade rhodium is electroplated as a wafer-thin protective layer onto white and yellow gold as well as silver (rhodium plating). The layer gives jewellery a brilliant white lustre and protects against tarnishing.
Pricing and volatility
The rhodium price is not traded on a regulated exchange such as COMEX; instead it is set by a handful of large traders and refiners in an OTC market (over the counter). This factor, combined with extremely tight supply, produces volatility that far exceeds that of the other precious metals.
The historical price record shows this vividly:
- Around 2008 rhodium briefly exceeded USD 10,000 per troy ounce, then fell below USD 1,000.
- In spring 2021 rhodium reached an all-time high of nearly USD 30,000 per troy ounce, before correcting back below USD 5,000 by 2023.
Current historical price charts and daily EUR/USD exchange rates are essential for valuing rhodium positions, since rhodium is quoted internationally in US dollars.
Rhodium as an investment
Rhodium is not a classic investment precious metal like gold or platinum. There are no exchange-traded standard products (ETFs, ETCs) and hardly any standardised bars. The market is extremely illiquid: small shifts in automotive demand or production outages in South Africa can multiply or halve the price within a few weeks.
Points relevant to private investors in Ireland (not a substitute for tax or investment advice):
- Rhodium bought physically in Ireland is subject to the standard rate of VAT, currently 23%; it does not qualify as investment gold and enjoys no VAT exemption.
- A gain realised on selling rhodium may be liable to Capital Gains Tax at 33%, after the annual personal exemption of EUR 1,270, with no holding-period relief (unlike Germany, where a one-year speculative period applies). Professional advice should be sought for individual decisions.
Physical properties at a glance
Density: 12.41 g/cm3 (platinum: 21.45 | gold: 19.32)
Melting point: 1,964 C (higher than platinum and palladium)
Reflectivity: ~80% (silver: ~95%, platinum: ~70%)
Mohs hardness: 6.0 (gold: 2.5 - much harder)
Its high hardness makes rhodium more durable than any other platinum-group metal and is a chief reason for its use as a protective coating in rhodium plating.
In a nutshell
Rhodium is the rarest and most price-volatile platinum-group precious metal, whose industrial role in the vehicle catalyst almost entirely determines its demand and therefore its price. Anyone watching the platinum price or palladium price should always keep an eye on rhodium as a closely linked PGM.