Rhodium (Rh, atomic number 45) is a rare and valuable noble metal and a member of the platinum group. It is typically found alloyed with similar metals in the Earth’s crust. This silvery-white transition metal is prized for a unique combination of physical properties, including a very high melting point (approximately 1964°C) and remarkable resistance to corrosion, remaining chemically inert to most acids. It also possesses high reflectivity, making it one of the most lustrous precious metals. These characteristics dictate its use in modern technology requiring durability under extreme conditions.
Essential Role in Catalytic Converters
The largest application for rhodium, consuming the majority of the world’s supply, is in the automotive industry’s three-way catalytic converters. This device manages three harmful exhaust pollutants: unburned hydrocarbons, carbon monoxide, and nitrogen oxides (NOx). Rhodium is specifically responsible for the reduction phase, a function not efficiently performed by other platinum group metals in the converter.
Its function involves stripping oxygen atoms from nitrogen oxide molecules, converting toxic NOx into harmless nitrogen gas (N2) and oxygen gas (O2). Rhodium is typically dispersed alongside platinum and palladium on a ceramic honeycomb structure. The metal maintains its catalytic activity and stability at the high operating temperatures of an engine exhaust system.
Enhancing Aesthetics and Durability in Jewelry
Rhodium plays a role in the jewelry industry, where it is used as a plating material to enhance appearance and durability, a process often called rhodination. Since solid rhodium is naturally brittle and difficult to shape, it is applied as an extremely thin surface layer on items made from other metals. The plating is commonly applied to white gold, which often has a faint yellowish tint from its alloy composition, giving it a pure, brilliant white finish.
This electroplated layer also provides a protective barrier for sterling silver, which is highly prone to tarnishing when exposed to air and moisture. Rhodium’s inertness prevents the formation of silver sulfide, the black compound that causes tarnish. Plating thickness is carefully controlled, usually ranging from 0.75 to 1.0 microns, to ensure durability without cracking. If the layer is too thick, the rhodium’s natural brittleness can cause it to fracture, while a layer that is too thin will quickly expose the underlying metal, requiring the piece to be re-plated over time.
Specialized Uses in Industry and Science
Beyond its two major consumer applications, rhodium is a component in several specialized industrial and scientific tools that require its unique properties.
Chemical Catalysis
It is a powerful catalyst in the chemical manufacturing sector, facilitating complex organic reactions. Rhodium compounds are used in the carbonylation of methanol, central to the production of acetic acid, and in hydroformylation, which converts alkenes into aldehydes.
High-Temperature Applications
Rhodium is frequently alloyed with platinum to create high-temperature thermocouples, devices used for accurate temperature measurement in extreme environments. These alloys are stable up to 1800°C and are used in industries like glass manufacturing and high-temperature metal processing.
Optics and Electronics
Its high reflectivity and corrosion resistance also make it suitable for specialized optical mirrors and coatings used in high-power laser systems. Furthermore, due to its stable, low electrical resistance, rhodium is used in certain electrical contacts requiring reliability in harsh conditions.