Rhodium black, preparation

As a partial conclusion, it can be remarked that an important work was completed some time ago by Soviet chemists. In particular, the effects of ultrasonic activation on the properties of palladium, platinum, and rhodium blacks prepared and/or used under sonication were reviewed, and frequency effects were noted.But no clear-cut conclusions can be drawn from this not easily accessible literature, and catalytic reactions still offer very broad possibilities for new sonochemical investigations, either concerning the preparation of the catalysts, the study of their modified properties, or their synthetic applications at the laboratory or industrial scale. 

Nishimura and Kasai studied the hydrogenation of acetophenone in f-butyl alcohol using carefully prepared ruthenium and rhodium blacks.176 The selectivities for the formation of cyclohexyl methyl ketone and 1-phenylethanol as simultaneous products have been determined by application of the equation in Scheme 11.7. The values of K and/as well as the composition of the final products obtained are summarized in Table 11.13. Three ruthenium blacks—Ru (A), Ru (N), and Ru (B)—were prepared from the ruthenium hydroxide precipitated at pH 5,7, and 7.8, respectively, by adding lithium hydroxide solution to an aqueous solution of ruthenium chloride. It is seen that the selectivity for the saturated ketone (see figures in parentheses) was considerably higher over Ru (B) (43%) than over Ru (A) (25%) and Ru (N) (20%). The selectivity over Ru (N) increased markedly to 65% at 100°C and 5.9-7.8 MPa H2. Over rhodium... 

For the preparation of rhodium black of permanent catalytic activity the presence of sulphur compounds appears to be necessary.3 Convenient methods of obtaining such consist in reducing rhodium sesquisulphide with formic acid and by electrolytic deposition of rhodium black from a solution of rhodium in 60 per cent, sulphuric acid by a current of 0-02 ampere at 180° C . 

Occurrence and History of Rhodium—Preparation—Proportion- Colloidal Rhodium—Rhodium Black—Uses—Atoraio Weight —Alloys. 

The anionic cluster [Ir6(CO)i5] is octahedral and an increasing number of Ir clusters have been reported recently though their preparations are more difficult and yields usually smaller than for rhodium. [Iri4(CO)27] has the highest nuclearity so far and is obtained as black crystals by oxidizing [Ir6(CO)i5] with ferricinium ion (Fig 26.9b). 

Dichloro-tetrapyridino-rhodium Chloride, [Rh py4Cl2]Cl, is prepared by dissolving rhodium zinc in aqua-regia, and, after removal of acid, heating the aqueous solution with pyridine. On cooling, the pyridino-salt is deposited in yellow prisms. It melts when heated, yielding a black oil, and on further heating, metallic rhodium. 

Platinum in a finely divided form is obtained by the in situ reduction of hydrated platinum dioxide (Adams catalyst) finely divided platinum may also be used supported on an inert carrier such as decolourising carbon. Finely divided palladium prepared by reduction of the chloride is usually referred to as palladium black. More active catalysts are obtained however when the palladium is deposited on decolourising carbon, barium or calcium carbonate, or barium sulphate. Finely divided ruthenium and rhodium, usually supported on decolourising carbon or alumina, may with advantage be used in place of platinum or palladium for some hydrogenation reactions. 

As obtained in this way rhodium sesquisulphide is a black, unctuous powder conserving the same crystalline form as the chloride from which it was prepared. It is not acted upon by acids, even aqua regia having no effect. Bromine and alkali sulphides are likewise without action on it. 

Prepared in this manner, rhodium sesquisulphide is a black powder, insoluble in alkali sulphides, as also in nitric and hydrochloric acids, and even aqua regia at 100° C. Moist air is without action on it, as also is bromine. 

When the reaction is followed by HPLC using a p-Bondapak-CN column with 2% acetonitrile in methanol as the eluent, two bands are observed initially a broad band eluting with the solvent front [chlorobenzene and excess methyl 2-pyrrolidone-5(R)-carboxylate] and a second band at 1.6 min when the flow rate is 1.5 mlVmin (rhodium(ll) acetate]. As the reaction progresses, the rhodium(ll) acetate band diminishes and is replaced by several bands with longer retention volumes until one major band, in addition to that for chlorobenzene and ligand, is observed at about 4 min. Only minor impurities elute at intermediate times. A brown-black material, insoluble in all common solvents, is observed in some preparations. The origin of this material is unknown, but its presence decreases product yield by 25%. 

Alkoxycarbonylation of amines. Carbamates can be prepared by reaction of a primary amine with carbon monoxide, oxygen, and an alcohol catalyzed by either 5% rhodium on activated carbon or palladium black and an alkali metal halide, particularly an iodide such as Csl or K1 (equation 1). Essentially no reaction occurs in the absence of the salt. Dialkylureas are intermediates in the reaction, and can be isolated as the major product when less active catalysts such as IrCI, arc used. 

The rhodium/sulfur system is complex and the best defined sulfide is the black Rh2S3. This compound can be prepared by heating rhodium and sulfur and subliming the product at 1100°C. 

Binary rhodium(lV) compounds are confined to the purple red tetrafluoride and the black dioxide. The hydrated dioxide may be prepared by oxidizing rhodium(IIl) compounds, either with chlorine or electrochemically. Attempts to dehydrate this material lead to decomposition. No cationic rhodium(IV) complexes have been characterized unambiguously, but both [RhCle] -and [RhFe] are well established. The alkali metal salts of the hexafluororhodate(lV) ion are all isomorphous with their platinate(fV) analogs. 

Work in this area is, of course, not limited to using Pt black as the electrocatalyst. Virtually any electroactive catalyst, such as gold, rhodium, or palladium, which can be prepared with a particle size small enough to provide the necessary surface area and RF penetration, can be investigated. 

Other methods have been used for the decomposition of carbonyl precursors. For example, an early report on nickel particles preparation through UV irradiation of Ni(CO)4 was published by Tanner et al. Triangular particles of ca. 6 nm mean size were produced in this way. Similarly, the irradiation of a solution of the carbonyl polyoxoanion rhodium complex [Rh(C0)2-P2WisNb3062] in the presence of H2 and cyclohexene involves the formation of a black precipitate containing Rh(0) nanoclusters with a size in the range l.O. Onm. ... 

---