Platinum-rhodium wire

Nitric acid is commercially produced hy oxidizing ammonia with air over a platinum-rhodium wire gauze. The following sequence represents the reactions occurring over the heterogeneous catalyst ... 

The high cost of platinum prevents the use of compensating leads of the same metal in the case of a rare-metal couple but inexpensive lead wires of copper and an alloy of nickel-copper are now available for use with the platinum-platinum 90, rhodium 10 couple. These lead wires do not compensate individually but taken together they compensate to within 5°C. for a variation of 200 C. in the couple-lead wire junctions. Since the compensating lead wires for the rare metal couple do not compensate individually both terminals on the head of the couple should be always as nearly as possible at the same temperature. The copper wire of the compensating leads is connected to the platinum-rhodium wire of the couple and the copper-nickel wire is connected to the platinum wire of the couple, i.e., alloy wire to pure metal in each case. The cold junction is then located at the indicator end of the compensating leads. The temperature of this junction may be controlled if necessary by one of the methods described above. Copper wires are carried from this point to the indicator. 

One of the earliest sources was the Nernst filament. This is a mixture of rare earth oxides, primarily of zirconium, made into rods 20-50 mm in length and 1-2.5 mm in diameter. These are fired at 1800°C and have platinum wire connections at both ends. The resultant output at 1800-2000°C is approximately blackbody. However, while relatively cheap to produce these have a very unpredictable life and are very sensitive to environmental factors including draughts. Refractory coated platinum-rhodium wire has been used as a more robust solution but is very expensive. 

The cobalt oxide catalyst for oxidation of ammonia, worked out in our laboratory, has the form of granules of high mechanical strength, owing to which it may be applied both in stationary and in fluidized beds. The yields of ammonia oxidation to NO measured during laboratory and large laboratory studies of that catalyst exceeded 95%. Optimum temperature of ammonia oxidation process carried out on our catalyst (760-780 ) is lower than that needed for platinum-rhodium wire gauze currently appli in industrial reactors. 

The noble metal thermocouples, Types B, R, and S, are all platinum or platinum-rhodium thermocouples and hence share many of the same characteristics. Metallic vapor diffusion at high temperatures can readily change the platinum wire calibration, hence platinum wires should only be used inside a nonmetallic sheath such as high-purity alumina. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

A platinum-rhodium ahoy 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). 

Pyrolysis experiments were performed using a heated wire grid technique. Reactor details have been reported previously [7, 8]. The technique has been successfully applied in both pyrolysis and gasification studies [8, 10, 11]. In brief, the mesh or grid is housed in a stainless steel chamber known as the grid reactor (Fig. 8.1). The reactor is 227 mm long and has an inner diameter of 15 mm. The grid (9x4 mm) is constructed of interwoven wires (platinum/rhodium 10%)... 

A couple consisting of gold wire and a ware containing 60 per cent, of gold, the remainder being palladium, has approximately six times the thermo-electromotive force of a platinum-rhodium couple. Its utility, however, is curtailed by the liability to undergo disintegration which characterises palladium alloys.4... 

For many catalysts, the major component is the active material. Examples of such unsupported catalysts are the aluminosilicates and zeolites used for cracking petroleum fractions. One of the most widely used unsupported metal catalysts is the precious metal gauze as used, for example, in the oxidation of ammonia to nitric oxide in nitric acid plants. A very fast rate is needed to obtain the necessary selectivity to nitric oxide, so a low metal surface area and a short contact time are used. These gauze s are woven from fine wires (0.075 mm in diameter) of platinum alloy, usually platinum-rhodium. Several layers of these gauze s, which may be up to 3 m in diameter, are used. The methanol oxidation to formaldehyde is another process in which an unsupported metal catalyst is used, but here metallic silver is used in the form of a bed of granules. 

Figure 2 Changes of the surface morphology of a traditional woven platinum/rhodium gauze during use. The surface of fresh catalyst is smooth, but during use dendritic excrescences of alloy grow from the wire surface.

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. 

Heating and Cooling. Heat must be applied to form the molten zones, and this heat much be removed from the adjacent solid 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 wire is useful to ca 1000°C, Kanthal to ca 1300 0, 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 0. 

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. 

In certain reactions involving precious metals like platinum, rhodium, or silver as catalyst, the catalyst is used in the form of wire gauze or filament. Examples of reactions that use wire-gauze... 

Profiles of mean and root-mean-squared (rms) temperatures were measured at 1-millimeter intervals along the stagnation plane. The latter was identified by the zero mean axial velocity as described below, of the opposed flames with thermocouple junctions comprising butt-welded bare platinum with platinum-rhodium (13%) wires of 50-micrometer diameter, with an aspect ratio of approx-... 

Tips are normally made of thin metal wires (ca. 0.25 mm), such as tungsten, platinum, or alloys (such as platinum-iridium and platinum-rhodium). Gold and silver tips can also be useful in some systems. To obtain the desired high resolution of the STM. it is necessary to have a very sharp tip with which to acquire the images. Tips can be prepared electrocheinically by etching the metal in a convenient solution, such as potassium hydroxide for W or basic aqueous cyanide solution for Pt-Ir tips. They can also be... 

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