Other Platinum Group Metals

Ruthenium has been studied for methane or methanol formation in CO2 reduction. The partial current densities reported are below 0.1 mA cm. Rh was employed as an electrode under 30 atm in 0.1 M KHCO3, giving CO and HCOO as the major products in constant current electrolysis at 163 mA cm . Re deposited on Au was employed for CO2 reduction in methanol based electrolyte, giving CO as the product. 

Single crystal electrodes of Ir and Rh have also been studied by anode stripping. Ir and Rh reduce CO2 to adsorbed CO in 

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The platinum-group metals comprise ruthenium (Ru), rhodium (Rh) and palladium (Pd) from the second transition series and osmium (Os), iridium(Ir), and platinum (Pt) from thethird transition series. Little or no C VD investigation of palladium and osmium have been reported and these metalsarenotincludedhere. The properties of the other platinum-group metals are summarized in Table 6.9. 

CVD Reactions. The rhodium halides, like those of the other platinum group metal s, are volatile with a decomposition pointtoo close to the vaporization point to make them usable for CVD transport. The metal is commonly produced by the decomposition of metallo-organic precur-... 

Alam, M. S. Inoue, K. Extraction of rhodium from other platinum group metals with Kelex 100 from chloride media containing tin. Hydrometallurgy 1997, 46, 373-382. 

Mhaske, A. Dhadke, P. Liquid-liquid extraction and separation of rhodium(III) from other platinum group metals with Cyanex 925. Sep. Sci. Technol. 2001, 36, 3253-3265. 

Several platinum compounds, including trimethylplatinum derivatives, are explosively unstable. Some compounds of the other platinum group metals are also of... 

Rhodium is recovered from platinum and other ores by refining and purification processes that start by dissolving the other platinum group metals and related impurities with strong acids that do not affect the rhodium itself Any remaining platinum group elements are removed by oxidation and bathing the mixture in chlorine and ammonia. 

Osmium occurs in nature, always associated with other platinum group metals. It usually is found in lesser abundance than other noble metals. Its most important mineral is osmiridium (or iridosmine), a naturaUy occurring mineral alloyed with iridium. 

The commercial applications of osmium are limited and considerably fewer than other platinum group metals. Its aUoys are very hard and are used to make tips of fountain-pen nibs, phonograph needles, and pivots. The metal also exhibits effective catalytic properties in hydrogenation and other organic reactions. Such catalytic apphcations, however, are limited and osmium fads to replace other noble metals, particularly paUadium and platinum, which are more effective as catalysts and cost less. 

A 1987 review surveys the methods available for the determination of Os and other platinum group metals (13), with a more recent review examining the use of thiourea complexes in the determination of Os (14). In the following text, reviews on more specific topics will be referred to where appropriate. 

Several platinum compounds, including trimethylplatinum derivatives, are explosively unstable. Some compounds of the other platinum group metals are also of limited stability. Individually indexed compounds are Amminedecahydroxydiplatinum, 4588 Amminepentahydroxyplatinum, 4566 Ammonium hexachloroplatinate, 4180 Ammonium tetranitroplatinate(II), 4573 Diacetatoplatinum(II) nitrate, 1507 Diamminedichloroamidotrichloroplatinum(IV), 4173... 

Os is a rare element with an estimated abimdance in the Earth s crust of some 0.005 ppm. It is found with other platinum group metals in ores in South Africa, the Urals, and Canada. 

Besides nanometer-scale thin film materials of Pt, Pd, and Ru studied above, some preliminary studies on nanometer-scale thin film materials of three other platinum group metals, Rh [20,52], Os [53], and Ir [54], have illustrated the AIREs of all these metals in CO adsorption. Based on studies so far, we can conclude that the AIREs are general phenomena of platinum group metals in chemisorptions of CO and other molecules [20]. 

O.A. Shpigun and Yu. E. Pazuktina, Ion chromatographic determination of platinum in the presence of other platinum-group metals and inorganic anions, Zh. Analit. Khim., 42,1285,1987. 

Platinum and palladium dominate the enantioselective catalysis scene as far as alkaloid modifiers are concerned. Of the other platinum group metals, Ir follows Pt in cinchona-modified pyruvate ester hydrogenation,132 the fact that Rh can hydrogenolyse alkaloids adsorbed at its surface133 makes it unlikely to function successfully and Ru tends to be too easily oxidised by adventitious oxygen in these reactions. 

In this respect palladium is superior to all other platinum group metals, but it cannot be used because of its fast dissolution under working conditions. On the other hand, it has been known that certain amorphous alloys have extremely high corrosion resistance (S). These facts led to the studies of amorphous palladium-based alloys as anode materials in the electrolysis of soda. 

The chloride is obtainable also as a water-sol hydrate with variable amounts of water. Prepn of trihydrate Anderson, Basolo, Inorg. Syn. 7, 214 (1963). Similar to the chlorides of other platinum group metals, rhodium chloride readily forms double salts with alkali chlorides. 

Similar considerations are also applicable for the reductive elimination reactions for the other platinum group metals. Our calculations on reductive eUmination reactions have shown an energetic preference of Rh compounds to Ir , Ru and Os compounds. The fact can be rationaUzed taking into account that the only exothermic promotion energy of -37.6 kcal/moR is expected for s d — s d (Rh —>Rh )- In contrast, s d —>sid (Ir —>Ir ) and s d —>d (Ru — Ru ) are endothermic by 25.1 and 9.2 kcal/mol respectively [13]. 

Electrochemistry of other platinum group metals was also roughly characterized before 1960s (see, e g.. Refs. 13-15 for Rh... 

Natural occurrence. Rhodium is one of the rarest element in the Earth s crust with an abundance of 1 pg/kg (i.e., ppb wt.). Rhodium occurs in nature as a native metal along with other platinum-group metals in the native mineral iridosmine or in sulfide ores such as rhodite, sperrylite, and some copper-nickel ores. 

The typical catalyst in fuel cells is platinum (and possibly other platinum group metals). Usually, nanoparticles of these metals are applied on carbon black as a conductive, hydrophilic carrier. In alkaline solution also silver is suitable for ORR. The moderate price of silver permits the application of pure metal as catalyst without a carrier. Additionally, the high conductivity of silver is advantageous. No silver is lost during operation and can be recycled after the ODC service life. However, the high fixed capital may be a problem. 

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