Rhodium

Rhodium is a hard, silvery-white metal that gleams with an almost mirror-like luster and refuses to corrode even when heated in air. It belongs to the platinum group and shares their nobility, their scarcity, and their catalytic power — but it surpasses them all in one critical task: converting the toxic nitrogen oxides that pour from internal combustion engines into harmless nitrogen and oxygen. Without rhodium, modern three-way catalytic converters could not meet emissions standards, and cities around the world would choke under far heavier blankets of smog. The element is so rare that it has occasionally traded at prices exceeding gold by a factor of ten or more, making it among the most expensive naturally occurring elements on Earth. Its name traces back to the deep-pink solutions that form when it dissolves in certain acids — a color borrowed from the Greek word for rose.

The dominant use of rhodium, accounting for roughly 80 percent of annual demand, is in automotive catalytic converters. In the three-way converter fitted to gasoline-powered vehicles, a thin wash-coat of rhodium dispersed on a ceramic honeycomb reduces nitrogen oxides (NOx) by splitting them into nitrogen and oxygen, while platinum and palladium simultaneously oxidize carbon monoxide and unburned hydrocarbons. The combination cuts all three regulated pollutants by more than 99 percent. Rhodium also serves as an electroplating material in jewelry: a flash of rhodium plating over white gold or silver increases scratch resistance and produces a bright, highly reflective finish. In the chemical industry, rhodium-triphenylphosphine complexes (Wilkinson's catalyst) enable the hydroformylation of alkenes to produce aldehydes and alcohols, and rhodium catalysts convert methanol to acetic acid via the Monsanto process. Aircraft spark plug electrodes and high-temperature thermocouple wires also rely on rhodium alloys.

William Hyde Wollaston, a British chemist and physicist of remarkable skill, discovered rhodium in 1803 while systematically analyzing crude platinum ore dissolved in aqua regia. After precipitating platinum and palladium from the solution, he treated the remaining liquid with sodium hydroxide and obtained a red powder he identified as a new compound. Reducing that powder with hydrogen yielded a dark gray metal — a new element. Wollaston named it rhodium after the Greek rhodon, rose, for the characteristic pink color of its salts. He announced the discovery in 1804 alongside his simultaneous discovery of palladium, making him the only chemist ever credited with finding two elements in a single investigation. Wollaston kept the details of his platinum-group separation secret for years, operating a profitable private refinery; it was not until 1828 that he published his full methods. Rhodium remained a laboratory curiosity until the mid-twentieth century, when catalytic converter research transformed it into an industrially critical material.

Rhodium is one of the scarcest elements in Earth's crust, present at a concentration of roughly 0.0002 parts per million — about twice as rare as iridium and far rarer than gold. Like its platinum-group cousins, it is a siderophile element that concentrated in the Earth's iron-nickel core during planetary differentiation, leaving only trace amounts in crustal rocks. The primary commercial sources are the sulfide ore deposits of the Bushveld Igneous Complex in South Africa, which alone supplies more than 80 percent of world production, and the Norilsk-Talnakh deposits in Russia. Rhodium occurs in these ores dispersed within platinum-group mineral phases and as inclusions in base-metal sulfides such as pentlandite. It is recovered as a byproduct of nickel and copper refining. Annual global production is only around 30 tonnes, making supply highly inelastic and prices extremely volatile.

Rhodium chemistry is dominated by the +3 oxidation state, though +1 and +5 states are also well-characterized. Rhodium(III) chloride (RhCl3) is the most common starting material in rhodium chemistry; it forms a hydrated red solid that serves as the precursor for most catalysts and coordination compounds. Wilkinson's catalyst, chlorotris(triphenylphosphine)rhodium(I), was the first homogeneous catalyst shown to hydrogenate alkenes under mild conditions and earned Geoffrey Wilkinson a share of the 1973 Nobel Prize in Chemistry. Rhodium carbonyl complexes drive the hydroformylation (oxo) process used industrially to produce millions of tonnes of aldehydes per year. Rhodium(III) acetylacetonate is a common precursor for thin-film deposition. Rhodium sulfate solutions are used in electroplating baths to deposit the thin reflective coatings prized in jewelry and optical components. The element does not form a stable oxide above 600 degrees Celsius, which contributes to its resistance to tarnish.

• In early 2021, rhodium briefly traded at around $29,000 per troy ounce, making it more expensive than any other precious metal and roughly 16 times the price of gold at the time.
• Wollaston discovered both rhodium and palladium in 1803, processing them from the same batch of platinum ore — the only time in history a single scientist found two elements simultaneously.
• The pink color that gave rhodium its name comes from octahedral Rh(III) complexes, which absorb in the blue-green range and transmit red and orange light.
• Because rhodium catalytic converters are so effective, a single converter typically reduces nitrogen oxide emissions by more than 99 percent over its operational life, preventing the formation of acid rain and smog precursors.
• Rhodium is so resistant to chemical attack that it cannot be dissolved in any single acid, including hydrochloric, sulfuric, or nitric; only aqua regia at elevated temperatures will attack it.