Rhodium is the scarcest and most price-volatile of the platinum-group metals; global annual production is measured in tonnes and comes almost entirely as a by-product recovered from copper-nickel ores. Its most unassailable position is in automotive three-way catalytic converters: rhodium has a unique ability to reduce nitrogen oxides in exhaust, converting NOx into nitrogen and oxygen over a wide temperature window, while palladium and platinum are better at handling carbon monoxide and hydrocarbons; the three together form the active coating of the modern gasoline-vehicle three-way catalyst. As long as cars are still driven by internal combustion engines and must meet emissions regulations, this demand for rhodium is hard to displace. Besides automobiles, rhodium's high melting point, chemical stability, and good reflectivity make it suitable for high-precision thermocouples (platinum-rhodium thermocouples), which can measure temperatures continuously in furnaces and glass tanks above 1000 °C. It is also electroplated at very low current densities onto electrical contacts, waveguides, and optical mirrors to produce hard layers with stable contact resistance and resistance to sulfidation, used in aerospace connectors, relays, and mirrors. In chemicals, rhodium complexes are the core catalysts for oxo synthesis (hydroformylation) of olefins, which yields aldehyde intermediates on the way to plasticizer alcohols such as 2-ethylhexanol.
Rhodium trichloride (RhCl3, usually sold as the trihydrate) is the most important starting material in rhodium chemistry—a reddish-brown crystal from which nearly all rhodium catalysts and rhodium plating baths are prepared; when supported on alumina it becomes a hydrogenation catalyst. Rhodium sesquioxide (Rh2O3) is a higher oxide of rhodium, a black solid used mainly in the electronics industry for thick-film resistor pastes and electrode materials, and also as a support coating in certain catalytic reactions. Wilkinson's catalyst (tris(triphenylphosphine)rhodium(I) chloride, RhCl(PPh3)3) is one of the most famous homogeneous hydrogenation catalysts in organometallic chemistry. A yellow crystal, it selectively hydrogenates alkenes to alkanes under mild conditions without affecting other functional groups, and is widely used in fine chemicals, pharmaceuticals, and organic-synthesis laboratories.
