Rhodium sesquioxide

Moore, M. D. and Kaplan, S. (1992) Identification of intrinsic high-level resistance to rare-earth oxides and oxyanions in members of the class Proteobacteria Characterization of tellurite, selenite, and rhodium sesquioxide reduction in Rhodobacter sphaeroides. J. Bacteriol. 174,1505-14. 

Rhodium sesquioxide is obtained by heating rhodium metal to red heat in air. 

Obtained by these methods rhodium trichloride is a red powder, insoluble in water and in acids. It is decomposed by concentrated aqueous potash, the resulting solution behaving like that of rhodium sesquioxides in the alkalies. It dissolves in concentrated solutions of potassium cyanide and of alkali oxalates, yielding complex cyanides and oxalates respectively. 

When heated in dry hydrogen chloride gas, the water of crystallisation is liberated without the formation of rhodium sesquioxide, and the salt is transformed, slowly at 360° C., but rapidly at 440° C., into insoluble rhodium trichloride and potassium chloride. The latter may be dissolved away with hot water. 

Rhodium Sesquioxide, Rh2Oa, results when rhodium is heated in air or oxygen between 600° C. and 1000° C. The rate of oxidation of the metal increases rapidly with the temperature. Above 1150° C. the oxide dissociates, metallic rhodium being obtained. The sesquioxide is greyish black in colour.8... 

Obtained in the above manner rhodium sesquioxide is a yellow powder, and on being heated loses water and oxygen, yielding a black powder, which Claus regarded as the anhydrous sesquioxide, but for which Leidie s 2 analyses suggest the empirical formula Rh304. 

In the freshly precipitated condition rhodium sesquioxide dissolves in concentrated potassium hydroxide solution, from which it is precipitated again on dilution with water. If, however, chlorine is passed through the concentrated alkaline solution, a dark green precipitate of uncertain composition is formed, for v hich Claus suggested the formula Rh203.Rh02 or Rh3Os. 

Rhodium Sulphite, Rha(S03)3.6H>0, is obtained as a white crystalline mass on dissolving freshly precipitated rhodium sesquioxide in sulphurous acid. On concentration the crystals separate out, and are washed with alcohol, in which they are insoluble. Rhodium sulphite is fairly stable, but evolves sulphur dioxide on heating, leaving a residue of sesquioxide. With sulphites of the alkali metals, double salts are formed.2... 

Rhodium Nitrate, Rh(N03)3.2H20, is prepared by dissolving rhodium sesquioxide in nitric acid. On concentration it separates as a deep yellow hygroscopic salt, readily soluble in water, but not -in alcohol. With nitrates of the alkali metals double salts are formed. 

On heating to dull redness a mixture of finely divided rhodium and phosphoric acid, a phosphate is obtained which is soluble in water,3 whilst a basic phosphate results on treating rhodium sesquioxide with phosphoric acid. 

Solid rhodium sesquioxide, Rh203, was first identified in 1927,1148 and has the corundum structure1149 (hexagonally close packed O2- anions distorted by the presence of Rh3+ cations in of the octahedral interstices). Each Rh is surrounded by six oxygens, three at 2.03 A and three at 2.07 A. An orthorhombic phase forms above 750 °C.1150... 

Compounds of rhodium are used as catalysts. A catalyst is a substance used to speed up a chemical reaction without undergoing any change itself. As an example, rhodium sesquioxide (Rh203) is sometimes used as a catalyst for the hydrogenation of oils. Hydrogenation is the process by which liquid oils are converted to semi-solids. 

Table 4.10. Some lattice spacings characteristic of rhodium, ceria and rhodium sesquioxide phases.

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