Rhodium-platinum alloys

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

Gold can be used only in very small portions or very thin coatings because of its cost. Most of the applications for wliich it was used in the past have now been accomplished with tantalum at a much lower cost. A gold/ platinum/rhodium alloy is used in the manufacture of rayon-spinning jets in the production of rayon fibers. This alloy presents the combination of strength, corrosion resistance and abrasion resistance necessary to prevent changes in hole dimensions. 

Table 21.16 Exchange current densities for several noble metals and a platinum-rhodium alloy in the reduction of oxygen from perchloric acid solution ...

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. 

Wire Gauzes. Wire gauzes are commonly used in the oxidation of ammonia and hydrocarbons. A gauze is a series of wire screens stacked one on top of another (Figure 11-12). The wire is typically made out of platinum or a platinum-rhodium alloy. The wire diameter ranges between 0.004 and 0.01 cm. 

Ammonia is oxidized using a mixture of ammonia gas (9-11%) in air which is passed through multiple layers of fine platinum-rhodium alloy gauze (Eq. 11.35). 

The aged solution is forced by a pump through a spinning jet or spinneret. This consists of a cap made of a chemically inert metal, such as tantalum or platinum or platinum-rhodium alloy, which contains a number of small holes usually between 0 05 and O-l mm in diameter (see Fig. 6.1). The spinning mixture is extruded through these holes into a medium which causes it to solidify or coagulate. 

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. 

For a simple example of heterogeneous catalysis, think of the catalytic converter in your car. It s made of a platinum-rhodium alloy that is finely deposited on aluminum oxide (specifically y-alumina) to decompose nitrogen oxides to less toxic nitrogen, as well as promote the combination of O2 with toxic CO and hydrocarbons to form CO2. 

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