Ruthenium and Iridium Dioxides

Ruthenium dioxide is blue-black, and crystals formed in the growth region of the apparatus described above are tabular and about 3-4 mm in length. Iridium dioxide is somewhat darker (almost black), the normal crystal habit 

Weight percent oxygen calcd. for RuOj 24.05. Found 24.24. Calcd. for IrOj 14.27. Found 14.40. 

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A range of metal catalysts have also been studied in aqueous solution for the transformation of carbon dioxide, including rhodium, ruthenium and iridium bipyridine or phenanthroline complexes.One of the most effective systems is the iridium complex shown in Figure 3.14. The ligand design concept used in this study is very clever. The catalytic activity of the complex and its solubility in aqueous solution can be tuned by the pH of the solution.Under acidic... 

The DSA-type anodes are inert , coated anodes made of a valve metal (titanium, niobium, or tantalum) base coated with an electrochemically active coating. The active coating is made either of noble metals or of mixed metal oxides. Noble metals in active coatings are usually platinum or platinum alloys. Mixed metal-oxide coatings contain active oxides and inert oxides the active components are usually ruthenium dioxide (R.UO2) and iridium dioxide (IrC>2) and the inert components are mostly titanium dioxide (TiC>2) and other oxides such as tantalum... 

Harmer, M. A., Hill, H. A. O., The Direct Electrochemistry of Horse Heart Cytochrome C, Fer-redoxin and Rubredoxin at Ruthenium Dioxide and Iridium Dioxide Electrodes , J. Electroanal. Chem. 170 (1984) 369-375. 

Platinum anodes have a limited operational range of oxidation potentials and thus attention has focused on Sn02-coated titanium materials. The tin oxide material, when doped with Sb (approximately 5%) to impart the appropriate electrical conductivity, has oxygen overpotentials some 600 mV greater than those of platinum. Tin oxide gives higher oxidation efficiencies to those of platinum, lead dioxide, ruthenium and iridium oxide (DSA) electrodes and is reported to be stable to corrosion during anodic oxidation. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

Direct electrooxidation is theoretically possible at low potentials, before oxygen evolution, but the reaction rate usually has low kinetics that depends on the electro-catalytic activity of the anode. High electrochemical rates have been observed using noble metals such as Pt and Pd, and metal-oxide anodes such as iridium dioxide, ruthenium-titanium dioxide, and iridium-titanium dioxide (Foti et al. 1997). 

The electrocatalytic behavior of olefins was studied by Zanta et al. (2000) at thermally prepared ruthenium-titanium- and iridium-titanium-dioxide-coated anodes. The aliphatic olefins were shown to be inactive in the region before oxygen evolution, while aromatic ones showed one or two oxidation peaks, and the catalytic activity seemed to be the same for both substrates. However, as for platinum anodes, voltammetric studies and FTIR analyses have also shown the formation of a polymeric film that blocks the surface of the electrode and decreases its activity. 

Zanta, C. L. P. S., Andrade, A. R. d. and Boodts, J. F. C. (2000) Electrochemical behaviour of olefins Oxidation at ruthenium-titanium dioxide and iridium-titanium dioxide coated electrodes. J. Appl. Electrochem., 467 474. 

Porous anode 1, used in SPE electrolysis (fig.1) consists of mixtxjre of ruthenium dioxide (75%) and iridium oxide bound to the graphite layer. Thickness of such layer depends on the amount of aphitc, covering the anode surface unit. In particular, if this amount is 40 g/m. correspondent thickness reaches 100 0jn [12]. Current feeding to anode is held with the help of point collector 6 (the metal net can sei e as this collector). Anode space is separated from the cathode one with the help of membrane SPE (2). Platinum black, serving as an anode 1, is 100 jim v/ide. Cathode point collector 3 is connected with graphite plate 5, that maintain direct contact with cathode. 

Owing to its catalytic activity, ruthenium dioxide (RuO,) is used extensively in industrial anodes for the chlor-alkali industry and the production of perchlorates. These ruthenium-dioxide-based anodes consist of a thin catalytic layer coated onto a titanium base metal. Iridium-dioxide-based anodes are used for the production of persulfates, in electroplating, and in hydrometallurgy for evolving oxygen. These composites anodes, because of their corrosion resistance in chloride-containing media or concentrated acids and their ability to decrease the overpotential of chlorine and oxygen evolution, are called by the trade name dimensionally stable anodes (acronyms DSA ). Other uses are in fuel cells electrodes electrocatalysts. 

PHOTOCATALYSIS REDUCTION OF CARBON DIOXIDE AND WATER-GAS-SHIFT REACTION PHOTOCATALYZED BY 2 -BIPYRIDINE OR 1,10-PHENANTHROLINE COBALT(H), RUTHENIUM(H), RHENIUM(I) AND IRIDIUM(ni) COMPLEXES ... 

Water and carbon dioxide do not absorb light above 200 nm and their monoelectronic reduction requires an energy too high to be performed by classical transition metal complexes. It is therefore necessary to use a photosensitizer and organometallic complexes which are able to transfer more than one electron (e.g. cobalt(I), ruthenium(O), or rhenium(-I) or iridium(I) complexes). In principle, these species could be oxidized to a higher oxidation state, by reaction with water or carbon dioxide. However this poses certain problems (i) the compatibility of redox potentials between the photosensitizer and the catalyst (ii) finding mediators and... 

Platinum Platinum-coated titanium is the most important anode material for impressed-current cathodic protection in seawater. In electrolysis cells, platinum is attacked if the current waveform varies, if oxygen and chlorine are evolved simultaneously, or if some organic substances are present Nevertheless, platinised titanium is employed in tinplate production in Japan s. Although ruthenium dioxide is the most usual coating for dimensionally stable anodes, platinum/iridium, also deposited by thermal decomposition of a metallo-organic paint, is used in sodium chlorate manufacture. Platinum/ruthenium, applied by an immersion process, is recommended for the cathodes of membrane electrolysis cells. ... 

The electrolytic cells shown in Figures 2—7 represent both monopolar and bipolar types. The Chemetics chlorate cell (Fig. 2) contains bipolar anode/cathode assemblies. The cathodes are Stahrmet, a registered trademark of Chemetics International Co., and the anodes are titanium [7440-32-6]y Ti, coated either with ruthenium dioxide [12036-10-1]y Ru02, or platinum [7440-06-4], Pt—iridium [7439-88-5]y Ir (see Metal anodes). Anodes and cathodes are joined to carrier plates of explosion-bonded titanium and Stahrmet, respectively. Several individual cells electrically connected in series are associated with... 

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