Catalyst rhodium alloys

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. 

Japanese chemists succeeded in obtaining good yields of methane by reaction of H2 with a mixture of carbon monoxide and carbon dioxide, at temperatures as low as 270 °C, by use of a special mixed catalyst containing nickel as the most important metallic constituent. Why is nickel used In the same vein, why is platinum or platinum-rhodium alloy (but not nickel) used in catalytic converters for automobile exhausts (See also Section 17.4.)... 

The first step in the process is the heterogeneous, highly exothermic, gas-phase catalytic reaction of ammonia with oxygen (Reaction 2). The primary oxidation of ammonia to nitric acid (over a catalyst gauze of 9 l platinum/rhodium alloy) proceeds rapidly at process temperatures between 900-970°C. 

Ammonia reacts with air on platinum/rhodium alloy catalysts in the oxidation section of nitric acid plants. Nitric oxide and water are formed in this step according to Eq. (9.6). 

Some of the materials that have been examined as catalysts include Pure Platinum, Platinum-Iridium Alloys, Various Compositions of Platinum-Rhodium Alloys, Platinum-Palladium Alloys, Platinum-Ruthenium Alloys, Platinum-Rhenium Alloys, Platinum-Tungsten Alloys, FejOj-M CVI Oj (Braun Oxide), CoO-Bi20j, CoO with AI2O3, Thorium, Cerium, Zinc and Cadmium. 

Ammonia reacts with air on platinum/ rhodium alloy catalysts in the oxidation section of nitric acid plants. 

The world s supply of rhodium is in approximate balance with demand with erratic releases onto the world market from Russia being counterbalanced by national and industrial stockpiles. These fluctuations in availability are reflected in the spot price, which fell from US 64 at the millennium to US 17g by 2001. The current price in 2004 is US 26 g. Of the 2002 world production of 19.0 tonnes and recovered scrap from automobile catalysts of 3.1 tonnes, over 80% was used as rhodium alloy catalysts for automobile emission reduction. The rhodium component is vital in controlling NO emissions and looks set to increase in order to meet higher emission control standards. 

A platinum-rhodium alloy is used as a catalyst at 1100°C. Approximately equal amounts of ammonia and methane with 75 vol% air are introduced to the preheated reactor. The catalyst has several layers of wire gauze with a special mesh size (approximately 100 mesh). The Degussa process, on the other hand, reacts ammonia with methane in the absence of air using a platinum aluminum-ruthenium alloy as a catalyst at approximately 1200°C. The reaction produces hydrogen cyanide and hydrogen, and the yield is over 90%. The reaction is endothermic and requires 251 kJ/mol. 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

The influence of simultaneous thermal and chemical cycling on commercial three-way catalysts has been examined after ageing in a specifically designed automated laboratory bench. For all cycles tested, reproducing repeated fiiel cutoff procedures between two temperatures (850°C-850°C, cycle 1 850°C-950°C, cycle 2 850°C-1050°C, cycle 3), X-ray diffraction evidences the formation of platinum/rhodium alloys only when the atmosphere cycle comprises a reducing step. Evaluation of the rhodium concentration in alloyed phases suggests that some rhodium remains unalloyed in catalysts. 

Chemical-process Platinum-rhodium alloy is used as a long service life catalyst in the industrial... 

Bimetallic catalysts, mostly combinations of platinum with other metals such as tin [331] and rhodium [324], are promising candidates for the preferential oxidation reaction. Platinum/tin oxide catalysts showed significant reaction rates even at 0 °C [214]. Similar with platinum/rhodium catalysts, an alloy is formed from both metals, which changes the properties of both source metals [214]. Other additive metals, which may improve the activity and selectivity of platinum catalysts, may well be ruthenium and cobalt [329]. 

Rhodium is a hard and wear resistant PGM. Advanced optical equipment is plated with thin layers of rhodium in order to get good corrosion and wear protection. Rhodium-alloyed platinum is used in perforated crucibles for making glass fibers. Organic rhodium compounds have been found to be effective catalysts (homogenous) in industrial hydrogenation and in pharmaceutical production of, for instance, the drug L-Dopa for the treatment of Parkinson s disease [32.8]. 

One reason for recent increases in the prices of metals such as platinum, rhodium and palladium is that they are used in catalytic converters in vehicles and as catalysts in some industrial processes. For example, a platinum—rhodium alloy can be used in the Contact process (Chapter 7). 

A platinum/rhodium alloy is employed as catalyst for the reaction at a ratio of 9 1 as in nitric acid manufacture. Platinum alone is not stable at reaction conditions. When it is alloyed with rhodium, the catalyst life can range from 4000 to as much as 10,000 hours. The catalyst bed usually is made of layers of 40 mesh gauze made of alloy wire 0.003 inch in diameter. The spent catalyst is mostly recoverable. 

Platinum/rhodium alloy gauze is used as a catalyst in the selective oxidation of ammonia during nitric acid production and in the production of hydrogen cyanide. The wire in the gauze is only a few thousandths of an inch in diameter woven at 80 wires per inch. Several layers of gauze, up to about 8 ft in diameter, are used. 

The oxidation of ammonia, however, was less economic at high pressure than at atmospheric pressure because the burner temperatures had to be increased in order to achieve the same selectivity. This led to shorter catalyst life as the metal gauzes deteriorated more rapidly and the loss of platinum became uneconomic. The 90% platinum/10% rhodium alloy introduced by DuPont solved this problem by reducing platinum loss by 50% and also improving selectivity. DuPont also found that the loss of platinum was proportional to the oxygen content of the gas mixture. It was realized that the smface of the catalyst etched as it was activated and the wires were covered by tinsel. 

In 1996, consumption in the western world was 14.2 tonnes of rhodium and 3.8 tonnes of iridium. Unquestionably the main uses of rhodium (over 90%) are now catalytic, e.g. for the control of exhaust emissions in the car (automobile) industry and, in the form of phosphine complexes, in hydrogenation and hydroformylation reactions where it is frequently more efficient than the more commonly used cobalt catalysts. Iridium is used in the coating of anodes in chloralkali plant and as a catalyst in the production of acetic acid. It also finds small-scale applications in specialist hard alloys. 

The points for Ag and Pd-Ag alloys lie on the same straight line, a compensation effect, but the pure Pd point lies above the Pd-Ag line. In fact, the point for pure Pd lies on the line for Pd-Rh alloys, whereas the other pure metal in this series, i.e., rhodium is anomalous, falling well below the Pd-Rh line. Examination of the many compensation effect plots given in Bond s Catalysis by Metals (155) shows that often one or other of the pure metals in a series of catalysts consisting of two metals and their alloys falls off the plot. Examples include CO oxidation and formic acid decomposition over Pd-Au catalysts, parahydrogen conversion (Pt-Cu) and the hydrogenation of acetylene (Cu-Ni, Co-Ni), ethylene (Pt-Cu), and benzene (Cu-Ni). In some cases, where alloy catalysts containing only a small addition of the second component have been studied, then such catalysts are also found to be anomalous, like the pure metal which they approximate in composition. 

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