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Platinum Metals Rev., 1981, 25, (2), 56

Cogeneration of Nitric Acid and Electricity

Platinum Electrodes Employed in High-temperature Fuel Cell

One of the most important industrial processes to utilise a platinum group metal catalyst is the production of nitric acid. In the process a mixture of ammonia and air is passed over a rhodium-platinum catalyst gauze at a temperature of 750 to 950°C, first producing nitric oxide and then nitrogen dioxide which is subsequently absorbed in water. The reaction is highly exothermic and in many modern high pressure plants the heat energy from the tail-gas is recovered as it passes through a turbine, thus contributing to the power required to compress the process gas.

For a long time the desirability of obtaining this energy as electricity, rather than as heat, has been appreciated. Now workers at the Massachusetts Institute of Technology have reported on an ammonia high temperature solid electrolyte fuel cell which appears to offer an opportunity for producing nitric acid while recovering the exothermic energy as electricity, (R. D. Farr and C. G. Vayenas, J. Electrochem. Soc., 1980, 127, (7), 1478–83).

Their fuel cell consists of an yttria-stabilised zirconia tube with electrodes of porous platinum, having an estimated thickness of 3 microns, deposited on both surfaces. The electrodes performed well over long periods of time, and scanning electron microscopy showed that extensive sintering and faceting had occurred on the anode, similar to that which is observed during the normal catalytic oxidation of ammonia over platinum. The fuel, ammonia diluted with helium, flows through the tube while the cathode is exposed to air. Nitric oxide is the primary electro-oxidation product, and under optimum operating conditions yields in excess of 60 per cent can be obtained.

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