With iridium black

FIGURE 8.8 (a) MEA 28 and 29 current density measured at 40°C, and (b) comparison of the current density for various MEA made with iridium black as the anode catalyst using a potential of —1.7V. 

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

Iridium black is only slightly soluble in aqua regia. When fused with alkalies and alkaline nitrates or NuTT. the metal is converted to an aetd-soluhle form. The metal at red heal reacts to a small extent with O., S. and P. At elevated temperature, die metal is attacked by CL and F-When fused with NaCl and treated with Cli, the water-soluble sodium hexachloroiridaletIVi. NadrCI,.. is formed. 

Iridium Black.204 Iridium(III) hydroxide is reduced in water at 90°C and 8 MPa H2 for 40 min, in the same way as in the preparation of osmium black. The iridium(III) hydroxide is prepared by adding an aqueous lithium hydroxide solution dropwise to an 1% aqueous solution of water-soluble iridium(III) chloride, IrCl3 3H20, at 90-95°C until the pH of the solution becomes 7.5-7.8 under stirring. By keeping the solution at the same temperature under stirring, the precipitate of iridium(III) hydroxide is separated out from its colloidal solution. The precipitate is collected, washed repeatedly with hot water, and then dried in vacuo. 

Indium Black consists of an indefinite mixture of finely divided iridium and its oxides,12 and is obtained by reducing iridium compounds. For example, the sesquioxide dissolved in alkaline solutions yields a deposit of iridium black on boiling with alcohol, the latter being oxidised to formic acid and aldehyde. 

Hydrogenation catalyst. Iridium black is a highly selective hydrogenation catalyst. Yamamoto and Sham used this catalyst to effect the remarkably stereoselective (>98%) hydrogenation of 1 to 2. Mixtures of stereoisomers are obtained when 1 is hydrogenated with palladium on charcoal or Raney nickel. 

Halogen Electrodes.—The determination of the standard potentials of the halogens is simple in principle it involves measurement of the potential of a platinum electrode, coated with a thin layer of platinum or iridium black, dipping in a solution of the halogen acid or a halide, and surrounded by the free halogen. The uncertainty due to liquid junction can be avoided by employing the appropriate silver-silver halide or mercury-mercurous halide electrode as reference electrode. In practice, however, difficulties arise because of the possibility of the reactions... 

Iridium black effects the stereoselective ( 98%) complete hydrogenation of 4 to 5, whereas mixtures of isomers are obtained when 4 is hydrogenated with Pd-on-carbon or finely divided Ni ... 

Hydrogenation of the tricyclic diene 1 over palladium on carbon yields the product of 1,4-addition with a tetrasubstituted double bond 2, which is extremely resistant to further hydrogenation. In contrast, hydrogenation with Adams catalyst affords 9-pupukeanone (3)5. Iridium black as catalyst gives the latter product with excellent diastereoselectivity (yield not reported d.r. >98 2)6. 

This metal is found associated with iridium, constituting a small part of the ore of platinum, which is insoluble in aqua regia. When this residue is acted on at a red-heat, by nitre, both the metals are oxidised, and the mass being distilled with sulphuric acid, yields osmic acid. Os 0, which condenses in fusible crystals. From the acid, the metal may be obtained by the action of reducing agents, as a black powder, which becomes metallic by friction. 

The notorious black powder was, evidently, a natural alloy of osmium with iridium, the so-called osmiridium. Iridium is known to be chemically stable and in the compact form does not dissolve even in aqua regia. On the contrary, osmium is readily soluble in aqua regia in general among platinum metals osmium has the most atypical chemical... 

The most recent version of the cell (Ho et al., 1994) consists of two coaxial 75% platinum-25% iridium electrodes coated with platinum black. As can be seen in Figure 4.1, one electrode is external platinum-iridium lined high-pressure vessel (6.35 cm long and 1 cm volume) and the other, a thin platinum wire, insulated by a non-porous sintered AI2O3 or Al203/Zr02 tube (for alkaline media), welded to a platinum-iridium cylinder located in the center. 

For iridium the position is reversed. This time it is the black dioxide, Ir02, with the rutile structure (p. 961), which is the only definitely established oxide. It is obtained by heating the metal in oxygen or by dehydrating the precipitate produced when alkali is added to an aqueous solution of [IrCl6] . Contamination either by unreacted metal or by alkali is, however, difficult to avoid. The other oxide, Ir203, is said to be... 

The most stable carbonyls of rhodium and iridium are respectively red and yellow solids of the form [M4(CO)i2] which are obtained by heating MCI3 with copper metal under about 200 atm of CO. The black cobalt analogue is more simply obtained by heating [Co2(CO)g] in an inert atmosphere... 

Iridium and its parmer osmium were discovered in 1803 by the Enghsh chemist Smithson Tennant (1761-1815). In essence, he employed the same technique to separate these elements from platinum ores that is used today to pmify iridium. He dissolved the minerals with aqua regia, which left a black residue that looked much like graphite. After analyzing this shiny black residue, he identified two new elements—Ir and Os. Tennant was responsible for naming iridium after the Latin word iris because of the element s rainbow of colors. 

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