Catalysts rhodium-platinum oxide

The catalyst exerts some influence on the bonds broken in hydrogenolysis of saturated cyclopropanes (775), but in vinyl and alkylidene cyclopropanes the effect is pronounced. Platinum or palladium are used frequently. In one case, Nishimura s [124a) catalyst, rhodium-platinum oxide (7 3), worked well where platinum oxide failed (.75). An impressive example of the marked influence of catalyst is the hydrogenation of the spirooctane 42, which,... 

The following commercial powder catalysts (all from Degussa) were used in the screening runs rhodium-platinum oxide (45,65% Rh, 19,8% Pt), rhodium oxide (58,2% Rh), platinum oxide (80,6% Pt), palladium-on-carbon (5% Pd) and platinum-on-carbon (5% Pt). 

However, the same author obtained tetrahydrofurfuryl alcohol in nearly quantitative yield, with a rhodium-platinum oxide catalyst in ethanol, and also in glacial acetic acid, i.e., without hydrogeno-lysis.207 Nickel skeleton catalysts also yield mainly alcohols as hydrogenolysis products at 175° in a continuous system ... 

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). 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

The well-known Adams platinum oxide can be prepared conveniently by the procedure of Adams et al. (2). Platinum oxides prepared in this way usually contain some traces of sodium, which in certain reactions can have an adverse effect. The sodium can be removed by washing with dilute acid (53). The Nishimuri catalyst (30% Pt, 70% Rh oxides) can be prepared by the same procedure as for platinum oxide or with variations from platinum and rhodium salts (64,65,66). This catalyst has much merit. It is usually most useful when hydrogenolysis is to be avoided (67,85,86). 

Nowadays, rhodium or ruthenium are often the preferred catalysts. Rhodium can be used under mild conditions, whereas ruthenium needs elevated pressures. If pressure is available, it might as well be used even with rhodium, for increased pressure makes more efficient use of the catalyst, as well as decreases whatever hydrogenolysis might occur at lower pressure. Rhodium 7,8,12 20,21,38,39,45,65,66,68,69,75) and ruthenium 18,26 8,52,68,69,72,74) are especially advantageous in reductions of sensitive phenols and phenyl ethers that undergo extensive hydrogenolysis over catalysts such as platinum oxide. 

Platinum oxide has been used with success (6,15,23,52,80), but nowadays supported rhodium is apt to be the catalyst of choice, (2,24 J5,63,72,75,SS,116). Excellent results have been reported by Freifelder (32) using RuO without solvent at 100-200 "C and 1500 psig. 

Hydrogenation of 3-pyridinecarboxylic acids is apt to be accompanied by extensive decarboxylation (2S), but this unwanted reaction can be prevented by carrying out the reaction in the presence of one equivalent of base (33,79). Ruthenium (33), rhodium (29), platinum oxide (2S,59), and palladium (30) have all proved effective catalysts for reduction of pyridinecarboxylic acids to the saturated acid. 

Direct hydroxylation of benzene to phenol could be achieved using zeolite catalysts containing rhodium, platinum, palladium, or irridium. The oxidizing agent is nitrous oxide, which is unavoidable a byproduct from the oxidation of KA oil (see KA oil, this chapter) to adipic acid using nitric acid as the oxidant. 

A mixture of catalysts is needed to catalyze the variety of reactions that must be carried out in the converter. The most important catalyst is platinum metal, but palladium and rhodium are used as well, as are transition metal oxides such as CuO and Cr2 O3. ... 

By far the most important use of the platinum metals is for catalysis. The largest single use is in automobile catalytic converters. Platinum is the principal catalyst, but catalytic converters also contain rhodium and palladium. These elements also catalyze a wide variety of reactions in the chemical and petroleum industry. For example, platinum metal is the catalyst for ammonia oxidation in the production of nitric acid, as described in Pt gauze, 1200 K... 

Cant, N.W., Angove, D.E. and Chambers, D.C. (1998) Nitrous oxide formation during the reaction of simulated exhaust streams over rhodium, platinum and palladium catalysts, Appl. Catal. B 17, 63. 

The bluish white, hard, yet ductile, metal is inert to all acids and highly non-abrasive. Used for heavy-duty parts in electrical contacts and spinning jets. Reflectors are prepared from the mirror-smooth surfaces (e.g. head mirrors in medicine). Thin coatings provide a corrosion-resistant protective layer, for example, for jewelry, watches, and spectacle frames. The metal is a constituent of three-way catalysts. Rhodium complexes are used with great success in carbonylations (reactions with CO) and oxidations (nitric acid) in industry. Platinum-rhodium alloys are suitable thermocouples. 

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. 

A very active elemental rhodium is obtained by reduction of rhodium chloride with sodium borohydride [27]. Supported rhodium catalysts, usually 5% on carbon or alumina, are especially suited for hydrogenation of aromatic systems [iTj. A mixture of rhodium oxide and platinum oxide was also used for this purpose and proved better than platinum oxide alone [i5, 39]. Unsaturated halides containing vinylic halogens are reduced at the double bond without hydrogenolysis of the halogen [40]. 

However, reaction at 750°C to 900°C in presence of platinum or platinum-rhodium catalyst produces nitric oxide and water ... 

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