The precious metal rhodium is one of the platinum group elements alongside platinum, palladium, iridium, ruthenium and osmium. As one of the rarest and most expensive elements, it has a wide range of applications with many irreplaceable uses. At the same time, however, rhodium suffers the highest losses of all precious metals in application and processing, so that the ever decreasing ranges of this element will lead to considerable shortages in the foreseeable future and confront us with existential questions: What are the inimitable properties of rhodium? How can losses during use and recycling be reduced? Why is there no alternative to rhodium in its applications?
Rhodium, precious metal and platinum group element
Fig. 1: Relationships within the platinum group metals, PGMRhodium, Rh, is a chemical element with atomic number 45 and a natural, isotope-free atomic weight of 103. This pure metal is a silver-white, hard, extremely low-reactivity transition element that can be found in the 9th subgroup of the periodic table between cobalt and iridium and in the period between ruthenium and palladium (Fig. 1). Together with the other platinum group elements (PGE), it has a high melting point of almost 2,000 °C and the high and sometimes selective catalytic activity that is characteristic of precious metals.
Compact rhodium cannot be dissolved even with "aqua regia" (HCl/HNO3 = 3:1). Even high alloys, such as Pt-Rh30, resist acid attacks. There is no alternative but to alloy such alloys or even compact rhodium ingots with palladium in order to get the rhodium into solution in this way.
However, potassium disulphate melts (K2S2O7) attack the rhodium in the same way as common salt melts with a chlorine atmosphere or cyanide and soda melts.
Due to the poor solubility of compact metal ingots or semi-finished products, rhodium is usually sold as a sponge or powder. Chemically precipitated ultrafine powders are also available on the market, although it must be ensured that these black precious metal dusts are baked at 600 °C. Otherwise they will still contain interatomic particles. Otherwise, as they still contain interatomic hydrogen, especially from precipitation with boranate, NaBH4, explosions cannot be ruled out.
Due to its lower density, rhodium is one of the lighter PGEs. In its compounds it can be found in the oxidation states 0, +1, +2, +3, +4, and even compounds with the valences +5 and +6 have been synthesized, whereby the oxidation state +3 predominates in inorganic compounds and the oxidation state +1 in organic preparations. Further properties of the metal can be found in the box.
The metal got its name from its red chlorine compounds after the Greek "rhodeos", which means "rose-red" [1, 2, 3, 4].
Occurrence and prices
Rhodium only occurs in its pure form in alloys with other elements in the earth's crust in a statistical distribution of 10-7 % (1 ppb). This makes it one of the rarer elements even among the PGE. Significant geo-accumulations can be found in South Africa and Russia. In 2004, the Republic of South Africa alone isolated 18.5 tons, or 82%, of the total 22.6 tons of rhodium produced. In 2010, global demand was already 27.2 tons, of which 22.5 tons (77%) alone went into the production of automotive catalytic converters. However, in the same year, 7.3 tons of rhodium were also recovered from car catalytic converters worldwide.
The strongly fluctuating kilo prices for rhodium reflect the "tug-of-war" between supply and demand: while the market was still demanding 64,000 US dollars per kilo in 2000, two years later it was only 27,000 US dollars. This was followed by a moderate increase to 32,000 US dollars in 2004, before rising to 147,000 US dollars in 2006 and even 210,000 US dollars in 2008. Investors who did not get out in time were left behind two years later at a market price of 80,000 US dollars [5].
In the years that followed, the ups and downs on the metal exchanges hardly changed [6], if one looks at the development over the past five years (Fig. 2).
Fig. 2: Rhodium price on the stock exchanges
Rhodium and its compounds
In its inorganic compounds, rhodium mainly occurs in the +3 oxidation state. In addition to the numerous diamagnetic octahedral rhodium(III) complexes, square-planar rhodium(I) complexes with organic ligands are important. High valence levels +IV to +VI are found in the fluorides [RhF6]- and RhF6. The oxidation state 0 is also found in polynuclear rhodium carbonyls [7].
Important binary rhodium compounds are the oxides: the gray rhodium(III) oxide, Rh2O3, and the black rhodium(IV) oxide, RhO2. The relative densities of 8.2 and 7.2 g/cm3 form a straight line with the metal density of 12.4 g/cm3 in relation to their molecular weights standardized to one rhodium atom (Fig. 3).
While Rh2O3 is accessible by air oxidation, the production of dioxide requires the use of ozone. When rhodium dioxide is heated to 850 °C, rhodium(VI) oxide, RhO3, is formed, which decomposes back into the elements rhodium and oxygen at 1050 °C [8]. After extrapolation from the densities of the other oxides, the trioxide would have a density of around 6.0 g/cm3.
The densities of the rhodium(III) halides also follow a straight line with increasing molecular weights (Fig. 3).
The red, rhombic crystalline rhodium(III) fluoride with a density of 5.38 g/cm3 is formed by fluorination at 500 °C. RhF4 can be obtained by reaction with bromine trifluoride, while the very reactive black rhodium(VI) fluoride, RhF6, can be isolated from the gas phase by fluorination.
Red-brown rhodium(III) chloride, RhCl3, is produced by chlorination of rhodium powder at 700 °C. The melting point is 450 °C. The compound is not soluble in water. The chloride must therefore be melted with common salt to form Na3[RhCl6]. A soluble rhodium chloride, [RhCl3(H2O)3], is formed when rhodium powder is dissolved in hydrochloric acid/chlorine and then evaporated [9]. The soluble chloride is the starting compound for numerous subsequent preparations (Fig. 4).
Fig. 3: Densities of some rhodium compounds
Fig. 4: Some reactions of the soluble rhodium chloride
Rhodium(III) bromide, RhBr3, can be obtained by reacting rhodium with bromine at 450 °C or with a mixture of bromine and hydrobromic acid [10]. It crystallizes in reddish-brown, thin platelets and is also insoluble in water. The density is 5.56 g/cm3 and the decomposition temperature is 800 °C.
The black rhodium(III) iodide, RhI3, precipitates from hexachlorohodate(III) solutions with iodide as a sparingly soluble precipitate. It has a density of 6.4 g/cm3. Toxicologists attest that the compound may cause genetic defects [11].
Rhodium ammine complexes, such as [Rh(NH3)6]X3 or the yellow [Rh(NH3)5Cl]Cl2, which is suitable for separating rhodium from iridium, are also of some importance.
When rhodium(III) chloride hydrate is heated in alcohol, square-planar rhodium(I) compounds are formed in the presence of π-acceptor ligands, such as the dimeric, chlorine-bridged carbonyl chloride [Rh(CO)2Cl]2, which crystallizes in red needles, and the tris(triphenylphosphine)chloro complex [RhCl(P(C6H5)3)3]. Rhodium complexes such as the "Wilkinson catalyst" can be used as accelerators in homogeneous hydrogenation and in oxo synthesis.
The "Wilkinson complex" can be synthesized by substituting triphenylphosphane with rhodium(III) chloride in boiling ethanol:
[RhCl3(H2O)3] + 4 PPh3 → [RhCl(PPh3)3] + Ph3P=O+ 2 HCl + 2 H2O
The homogeneous catalyst is suitable for hydrogenations, hydroformylations, hydrosilylations and isomerizations. It was developed and named by the British chemist and Nobel Prize winner Geoffrey Wilkinson (1921-1996). The molecular structure and properties are shown in Figure 5. An example of a hydrogenation cycle with the "Wilkinson catalyst" is shown in Figure 6.
Fig. 5: Chloridotris(triphenylphosphine)rhodium(I); "Wilkinson catalyst " 
Fig. 6: Catalytic hydrogenation with the "Wilkinson catalyst " As early as the 1950s, G. Wilkinson was also working on sandwich compounds, of which a rhodium compound could be produced (Fig. 7). Rhodocene [Rh(C5H5)2], more precisely referred to as bis(η5-cyclopentadienyl)rhodium(II), is an organometallic compound from the metallocene series. In the molecule, a rhodium atom lies between two cyclopentadienyl rings in a sandwich complex. Biochemists have found applications for rhodocene derivatives as radiopharmaceuticals for the treatment of small areas of cancer. Other applications of rhodocene derivatives include molecular electronics and research into the mechanisms of catalysts [12].
Fig. 7: Molecular structure and properties of bis(η5-cyclopentadienyl)rhodium(II), "rhodocene "
Fig. 8: Structure and properties of rhodium(III) acetylacetonate
Like many metals, rhodium also forms an acetylacetonate complex, [Rh(acac)3] (Fig. 8). Although these chelates dissolve only slightly in water, they dissolve all the better in organic solvents. They serve as starting materials for chemical reactions and as homogeneous catalysts. If we look at the densities of the metal acyl chelates of various trivalent metal ions, the aluminum and rhodium complexes fall outside the straight line relationship between density and molecular weight (Fig. 9).
Fig. 9: Densities of M(III)acetylacetonates
Fig. 10: Structure and properties of rhodium(II) acetate dihydrate
Rhodium(II) acetate, [Rh2(AcO)4(H2O)2], stands out due to its special binary molecular structure (Fig. 10). The chelate compound serves as a catalyst in organic chemistry, for example. Diazo compounds with a neighboring carbonyl group can be used to produce carbenes, from which cyclopropanes can be obtained. Rhodium carbenes can also be used to obtain ylides and for insertion reactions (insertion of a molecular fragment into a chemical bond) [2, 13].
Finally, rhodium(III) sulphate, Rh2(SO4)3, should also be mentioned. It is used from the oxide or from metal powder as a crystalline compound or as a diluted solution for the galvanic coating of surfaces.
Industrial use of rhodium and its compounds
For many years, the manufacturers of car exhaust catalytic converters have required the largest proportion of rhodium at almost 80 %, or around 27 tons per year. In a mixture with platinum and palladium, rhodium is primarily responsible for the thermal reaction of nitrogen oxides to nitrogen. Despite numerous efforts, rhodium cannot yet be replaced by any other element, simplified according to the following equation:
2 NO2 + 4 CO → N2 + 4 CO2
Automotive catalytic converters come in different sizes and shapes for installation in the various types of cars that run on unleaded gasoline fuels. In Europe, manufacturers work with ceramic or stainless steel honeycomb bodies whose channels are lined with a suspension of rare earth oxides and PGE, approx. 3 g PGE per kilogram of catalytic converter honeycomb body (Fig. 11). As early as the 1990s, up to 0.1 ppm PGE was found in highway road sweepings [14], whereby the geogenic background value of uncontaminated soils is around 1 µg/kg soil.
Fig. 11: Functional diagram of the three-way autocatalyst
As a Pt/Rh(10) catalyst, rhodium also supports the oxidation of ammonia to nitric acid at 800-900 °C using the Ostwald process in the form of braided, woven or knitted nets (Fig. 12). The annual demand for rhodium for this process fluctuates around two tons per year. A further two tons of rhodium per year are used in the production of high-temperature thermometers and crucibles for the glass industry. The demand in the electrical and surface treatment industry is several hundred kilograms per year.
Fig. 12: Platinum-rhodium(10) catalyst network for ammonia oxidation for nitric acid production
All of these products and production residues with rhodium, often distributed in different matrices, sooner or later end up in precious metal processing, which - as is difficult to understand - was subjected to the legal waste regime in the 1990s. The precious metal industry in this country is losing its importance with its eyes wide open. No "sustainability seal" for increasingly scarce industrial metals will help [15].
Rhodium extraction and processing
As rhodium occurs almost exclusively in a mixture with the other PGE in a few places in the world and could only be collected in low concentrations in sulphide non-ferrous metal ores, numerous metallurgical enrichment processes are necessary before a PGE concentrate can be separated from the individual metals.
For decades, rhodium has suffered the fate of being the last metal to be separated from all PGE. Compounds such as rhodium(III) pentammine chloro-dichloride, [Rh(NH3)5Cl]Cl2, which is poorly soluble in aqueous systems, or ammonium hexachlorohodate(III), (NH4)3[RhCl6], which is relatively poorly soluble but remains in the chloride system, come into question (Fig. 13).
Fig. 13: Formula and properties of ammonium hexachloridorhodate(III)
Fig. 14: Solubility of ammonium hexachloridorhodate(III) in hydrochloric acid-ammonium chloride solutions of different concentrations
If compact rhodium ingots are used for processing, the rhodium must be alloyed with palladium, for example, to a Rh content of less than 20% by weight in order to make it soluble in hydrochloric acid and chlorine gas.
The usually low proportions of rhodium in the mixture with the other PGE and the relatively good solubility of ammonium hexachlorohodate(III) as well as the fact that it remains in the trivalent oxidation state of the metal allow the remaining PGE to be separated largely uncontaminated by varying the temperature and oxidation with chlorine. The final precipitation of the hexachlororhodate(III) takes place in concentrated hydrochloric acid and salting out with large quantities of ammonium chloride (Fig. 14).
Rhodium losses during use and reprocessing
In industries where rhodium is used as a catalyst on ceramics, on activated carbon, on metal surfaces or in other applications, technicians are often surprised at the high precious metal losses, which are sometimes less than 50% by weight over the entire application and processing cycle. This is often the case with rhodium in particular.
This is because millions of cubic meters of reaction gases often pass by the catalysts during the application phase, usually at high temperatures. Losses are inevitable, depending on many parameters. It will not be possible to recover rhodium economically from highway dust in the near future. The same applies to the exhaust gases from other catalytic converters. In the Ostwald process, attempts are being made to recover some of the platinum and rhodium that is lost using precious metal catcher nets made from palladium-gold alloys. However, very different yields are also recorded in this process.
The distribution of the rhodium in the many processing steps via lead, copper, bismuth or other collector metals or sulphides is easy to see, especially as it often runs through the separation process steps in a ratio of 1:100 with the other PGE or even precious metals, even if it is only a few ppm each time.
Final mother liquors from the (NH4)3[RhCl6] precipitation are precipitated with hydrazine, H2N-NH2, boranate, NaBH4, zinc or iron to form the metal powder. This is also often not quantitatively successful with rhodium.
This is the reason why users lose 10 to 30 % of their rhodium in the process and why precious metal refineries can only offer general recovery rates of 60 to 90 % by weight.
On the other hand, there is great potential for optimization when it comes to the "loss-free" use and processing of rhodium.
New recycling processes
Experience in dealing with extremely poorly soluble organic ammonium compounds has been available for many years. Back in the 1960s, Fritz Seel derived the thermodynamic explanation for the low solubility of ion pairs [16].
When an extremely poorly soluble rhodium compound was found in the Hanau precious metal refinery of Degussa AG at the end of the 1980s by precipitating hexachlororhodate(III) with diethylenetriamine, or diene for short, in a strongly hydrochloric acid solution, in whose mother liquor only four ppm of rhodium could be detected, the rhodium losses and the long accumulation times changed rapidly. The ion pair accumulates in wine-red, crystalline precipitates of high purity.
The operating process for quantitative dienH3[RhCl6] precipitation (Fig. 15) became widespread within the Group and also in other separating plants. A patent specification was therefore submitted. Unfortunately, the official application to the European Patent Office was delayed for six years due to those responsible for metal research feeling snubbed. This came to an end in 1994 when competitor Johnson Matthey patented this rhodium precipitation with "dien" worldwide in one fell swoop.
In less acidic solutions, an ochre-colored inner complex compound [dienRhCl3] is formed (Fig. 16). Although it also dissolves with difficulty, it does not achieve the low solubility of the ion pair.
Fig. 15: Crystalline neutral complex "dienH3[RhCl6] " 
Fig. 16: Crystalline neutral complex [RhCl3dien]But this would only be the beginning of a completely new separation process, in which the most expensive PGE rhodium is not separated and purified at the end of the separation process, but at the beginning.
Figure 17 compares the conventional separation process with new quantitative precipitations using diethylenetriamine, dien, and, for example, piperazine, pip.
Similarly, the precipitation-separation step can also be expanded as an extraction cascade if the organic oligoamines are used derivatized with alkyl residues, for example.
The oligoamines, dien and pip (Fig. 18) of the precipitated complexes, can also be recovered in closed systems, e.g. by potassium or ammonium soaping in two-phase systems:
dienH3[RhCl6] + 3 KOOC-R → K3[RhCl6] + dienH3(OOC-R)3
Water and gasoline are suitable phases for the industrial scale.
Fig. 17: Conventional ammonium separation process and separation of PGM using organic oligoamines
Fig. 18: Structure and properties of the quoted oligoamines diethylenetriamine, "dien", and piperazine, "pip"
INFO
Properties of rhodium
Melting point: 1,964 °C
Boiling point: 3,727 °C
Density: 12.38 g/cm3 (20 °C)
Electrical conductivity: 23.3 -106 A/Vm
Thermal conductivity: 150 W/mK
Oxidation states: 0, +1, +2, +3, +4
MOHS hardness: 6
Structure: face-centered cubic
Flammable solid, pyrophoric. Dust
Normal potential: 0.76 V (Rh3+→Rh)
Electronegativity according to Pauling: 2.28
Literature
[1] Cotton, F. A., G. Wilkinson, P L. Gaus: "Fundamentals of Inorganic Chemistry", VCH (1990) 558-588
[2] https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium
[3] Hofmann, U. and W. Rüdorff: "Inorganische Chemie", Vieweg Verl., Braunschweig (1969)
[4] Römpp Chemie Lexikon, Thieme Verl., Stuttgart
[5] Bertau, M, A. Müller, P. Fröhlich and M. Katzberg: "Industrielle Anorganische Chemie", Wiley-VCH, 4th edition (2013)
[6] http://www.finanzen.net/rohstoffe/rhodium/euro
[7] http://www.spektrum.de/lexikon/chemie/rhodiumverbindungen/7971
[8] Holleman, A. F., E. Wiberg and N. Wiberg: "Lehrbuch der Anorganischen Chemie", de Gruyter, Berlin, 102nd edition (2007) 1702
[9] https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium(III)-chloride
[10] https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium" target="_blank">https://de.wikipedia.org/wiki/Rhodium(III)-bromide
[11] https://de.m.wikipedia.org/wiki/Rhodium(III)-iodide
[12] https://de.wikipedia.org/wiki/Rhodocen
[13] Ye, Tao and M. Anthony McKervey: "Organic Synthesis with a- Diazocarbonyl Compounds" In: Chem. Rev. 94 (1994) 1091-1160
[14] Dirksen, F., F. Zereini, B. Skerstupp and H. Urban, Inst. F. Mineralogie, Uni Ffm.: "PGE-Konzentration in Böden entlang der Autobahnen A 45 und A 3 im Vergleich zu Boden im Einflussbereich der edelmetallverarbeitenden Industrie in Hanau", from "Emission von Platinmetallen", Springer-Verl., Berlin, Heidelberg (1999) p. 161
[15] http://www.focus.de/wissen/technik/tid-15030/rohstoffe-interview-bergbau-mit-persilschein_aid_421697.html
[16] Seel, F.: "Grundlagen der analytischen Chemie", Verlag Chemie, Weinheim, 5th ed. (1970) 338-35
