Rhodium

Rhodium is a rare and precious platinum group metal with atomic number 45 and the chemical symbol Rh. Discovered in 1803 by William Hyde Wollaston, the transition metal is hard, silvery-white, and chemical inert (1). Although found in platinum and nickel ores, Rhodium is exceedingly rare. It is not only the rarest platinum metal, but the rarest of all nonradioactive metals as well. This scarcity combined with its usefulness as a catalyst and alloy have made Rhodium a valuable commodity, as evidenced by its current (6/2018) $2000+/oz price (4).

Rhodium

Rhodium has only one naturally occurring isotope, 103^Rh, which is stable (2).Radioisotopes created in the lab have half-lives up to 3.3 days. The element has an unusual valence electron configuration (d8 s1) compared to the other group 9 elements which all have a d7 s2 configuration (3). Rhodium is most often found with a +3 oxidation state, as in RhCl3 and Rh2O3, but other oxidation states are seen as well, from Rh(0) to Rh(6) compounds.

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Many rhodium-halogen compounds are found, but the best known may be Wilkenson's catalyst, chlorotris(triphenylphosphene)rhodium(I), which is used for the hydrogenation of alkenes (1). Characterized by its durability and high reflectance, the hard silvery metal is less dense than platinum, but with a higher melting point of 1970 degrees C (3). Rhodium remains stable under highly acidic conditions, as evidenced by its complete insolubility in nitric acid. Even aqua regia can only slightly dissolve the metal (4).

WIlkinson's Catalyst, Benjah-bmm27. Wikipedia

Wollaston's detection of rhodium in 1803 followed close on the heels of his discovery of palladium, both the result of his experimentation with platinum ores from South America (1). He first used aqua regia to dissolve the crude ore and precipitate platinum (5). Subsequent precipitations with ammonium chloride, zinc and sodium chloride were then required to separate the rhodium from palladium. The rose-red precipitate prompted Wollaston's choice of nomenclature, as rhodon is Greek for rose.

Rhodium is found in ores mixed with palladium, platinum, gold and other metals. The main source is South Africa; other supplies include river sands in Russia's Ural Mountains, and the Sudbury mining area in Ontario, Canada (2). Only about 25 tons of the metal are extracted per year. The scarcity of rhodium contributes to high volatility in its price, which attained a high of $10,000 per ounce in 2008. An alternative source for rhodium could be found in used nuclear fuel, as rhodium is a fission product of uranium-235 (1). Unfortunately, the expense of its extraction coupled to the long storage times required have kept this method from being applied (6).

The first important uses for rhodium involved electroplating the metal as a coating for corrosion resistance (1). When the three-way catalytic converter was introduced, rhodium came into high demand for its role in catalyzing the reduction of nitrogen oxides in exhaust fumes to nitrogen and oxygen [2 NOx -> x O2 + N2] (7). Catalytic converters accounted for 81% of world rhodium consumption in 2007 and continue to hold close to this figure (8). Rhodium has a very high recycling rate, due to its scarcity, with recovered supplies averaging between 25-35% of mined supplies per year.

Paul Kiesow. Wikimedia Commons

Many other uses have been found for rhodium based catalysts as well. Rhodium catalysts can be used to reduce benzene to cyclohexane, or to add hydrosylane groups to double bonds in silicone rubber (3). In jewelry, rhodium is often electroplated on gold or platinum to give a white, reflective surface, or to strengthen silver against tarnish from hydrogen sulfide in the air. Because of its ow electrical resistance, and high corrosion resistance, rhodium is often used as an electrical contact (5).

1. Rhodium. www.wikipedia.com/rhodium
2. Bentor, Yion. ChemicalElement.com Rhodium. Apr. 25, 2011
3. The Element Rhodium. http://education.jlab.org/itselemental/ele045.html
4. Rhodium Element Facts. http://www.chemicool.com/elements/rhodium.html
5. Rhodium: The Essentials. http://www.webelementals.com/rhodium
6. Kolarik, Zdenek; Renard, Edouard V. (2003). "Recovery of Valuable Fission Platinoids from Spent Nuclear Fuel. Platinum Metals Review47 (2): 74-87.
7. W Amatayakul. Life cycle assessment of a catalytic converter for passenger cars. Master's thesis. Gothenburg (Sweden): Chalmers University of Technology, March 1999.
8. George, Michael W. "Commodity report:Platinum-group Metals". USGS. Retrieved 2008-09-16.