Rhodium

Rhodium (Rh, atomic number 45) is a Group 9 FCC platinum-group metal (PGM) that is monoisotopic (¹⁰³Rh, 100%), among the rarest elements in Earth's crust (~0.001 ppb), and — by price — historically the most expensive of all commonly traded metals, reaching over $29,000/troy oz in 2021 before retracting. Rh has a melting point of 1,964 °C, density of 12.41 g/cm³, exceptionally high thermal conductivity for a PGM (150 W/m·K), and outstanding chemical inertness — it resists attack by all common acids including aqua regia at room temperature, is unattacked by O₂ and N₂ at room temperature, and forms only thin surface oxides (Rh₂O₃) below ~600 °C. Rh is produced exclusively as a byproduct of Ni-Cu-Pt mining, primarily from the Bushveld Complex (South Africa, ~80% of world supply), with annual production of only ~20–30 tonnes — roughly 1/30th that of platinum — making supply inelastic and price highly volatile. Rh has an 4d⁸5s¹ electron configuration and the most common oxidation state is +3 (Rh³⁺ in RhCl₃, Rh₂O₃, Wilkinson's catalyst), though +1, +2, +4, +5, and +6 are all accessible in coordination chemistry.

Rh is the critical component in automotive three-way catalytic converters (TWCs) specifically for NOₓ reduction — the only practically viable heterogeneous catalyst for converting NOₓ to N₂ under the cycled rich/lean exhaust conditions of gasoline engines, consuming ~85% of annual Rh supply (~17–25 tonnes/year). In a TWC washcoat, Rh (~0.1–0.3 g/converter) loaded on CeO₂-ZrO₂ mixed oxide catalyzes 2NO + 2CO → N₂ + 2CO₂ and NO reduction by H₂ and hydrocarbons; the oxygen storage capacity of CeO₂ buffer enables simultaneous CO/HC oxidation and NOₓ reduction over the stoichiometric window (λ ≈ 1). Rh loading in TWCs has been reduced ~10-fold since the 1980s through improved nanoparticle dispersion and washcoat engineering; further reduction is constrained by Rh sintering above ~800 °C and by sulfur poisoning from fuel sulfur. Wilkinson's catalyst (RhCl(PPh₃)₃, prepared from RhCl₃·3H₂O), discovered by Geoffrey Wilkinson (Nobel Prize 1973), was the first practically useful homogeneous hydrogenation catalyst — it hydrogenates alkenes under mild conditions (1 atm H₂, 25 °C) with high selectivity and catalyzed the development of the entire field of homogeneous organometallic catalysis.

Rhodium electroplating produces the hardest, most reflective, and most chemically durable decorative metal coating available — Rh-plated jewelry, white-gold alloys, and silverware have a brilliant white luster (~78% reflectivity at 500 nm) and resist tarnish, scratch, and skin discoloration far better than the underlying Pt or Au alloy. Rh plating from sulfate or phosphate electrolytes deposits layers of 0.1–5 µm hardness ~800 HV — significantly harder than Au or Pt. Rhodium's high thermal conductivity (150 W/m·K) and chemical stability also make it the preferred material for Pt-Rh thermocouples (Type S, R, B) and for Rh-coated X-ray tube targets used in mammography — the Rh K-edge (23.2 keV) produces characteristic X-rays in the 20–23 keV range optimal for imaging breast tissue of intermediate density, where Mo targets produce suboptimal contrast.