Rhodium black catalysts

The treatment of solutions of platinum metals with aqueous borohydride results in the formation of finely divided black precipitates that are active catalysts for alkene hydrogenations. The platinum black obtained in this way was twice as active as that obtained by the hydrogenation of platinum oxide. The borohydride reduced rhodium black is even more active. While the borohydride reduction of base metals gives the corresponding metal borides, there is little, if any, boron incorporated into these platinum metal blacks. Analysis of the borohydride reduced palladium found that while the palladium boron ratio in the bulk was 10 1, less than 1% of the surface was boron.59 7, 5 small amount of boron, however, can impart a significant difference in catalytic activity to this catalyst as compared with other, more common, palladium catalysts. The most striking difference is the inability of the borohydride reduced palladium to promote the hydrogenolysis of activated C-0 and C-N bonds, a reaction that takes place readily over standard palladium catalysts. 

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. 

Figure 3.26 Exchange labeling of pyridines from deuterium gas catalyzed by rhodium black 3.3.3 Other Catalysts...

Black nickel oxide is used as an oxygen donor in three-way catalysts containing rhodium, platinum, and palladium (143). Three-way catalysts, used in automobiles, oxidize hydrocarbons and CO, and reduce NO The donor quaUty, ie, the abiUty to provide oxygen for the oxidation, results from the capabihty of nickel oxide to chemisorb oxygen (see Exhaust control, automotive). 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

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. 

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. 

CATALYSTS Chlorotrisftriphenylphos-phine)rhodium(I). (S)-a-(R)-2-Di-phenylphosphinoferrocenyl ethyldl-methylamine. Hydridotrisftriisopropyl-phosphine)rhodium(I). Iridium black. 2,3-0-Isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane. Lindlar catalyst. Nickel-Alumina. Palladium catalysts. Raney nickel. Rhodium oxide-Platinum oxide. 

Palladium in the form of Pd black or Pd/C is the most effective catalyst. Although Raney nickel has been used, there is doubt that it serves only as a hydrogen transfer catalyst because it contains a considerable amount of adsorbed hydrogen. Platinum and rhodium have been found to be ineffective. Both alkenes and alkynes have been hydrogenated and syn addition to 1,2-diphenylacetylene has been demonstrated. ... 

The reduction of palladium oxide, rhodium oxide or ruthenium oxide gives the corresponding metal blacks generated by in situ hydrogenation in the reaction mixture. At present the use of these oxides, as well as Adam s catalyst, is not common because of the cost of the materials and the relatively large amounts which are required. These materials have been replaced by the more reactive and less expensive supported metal catalysts described in Chapter 13. 

The catalysts used in the gas diffusion electrodes for oxygen reduction are from the Pt-group, including this metal itself or rhodium compounds. They are for instance incorporated into graphitized carbon or acetylene black. Binding material is a perfluorinated copolymer (Nafion). The conducting material necessary in the gas diffusion setup is represented by steel or nickel mesh, in some cases silver-plated for better withstanding the 32 wt% caustic alkali. 

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. 

Various rhodium catalysts, such as [(l,5-hexadiene)RhCl]2, RhCl3, or Rh black, in methanol solution under H2 reduce and esterify unsaturated carboxylic acids. Even the phenyl ring of cinnamic acid derivatives is reduced at ambient conditions as shown in equation (3). The substrate must contain a C=C double... 

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