Platinum Metals Rev., 1969, 13, (1), 29
Mechanism of the Catalytic Oxidation of Ammonia
The oxidation of ammonia to nitric oxide catalysed by platinum and its alloys forms an essential step in the manufacture of nitric acid, but, because of the high temperature and high space velocity at which the process operates academic investigation of the reaction has only been rarely attempted, and our detailed knowledge of the elementary steps which constitute the total process is very slender. Doubt has often been expressed as to whether the reaction is or is not entirely heterogeneous, and free radical reactions in the gas phase have often been thought to take place, particularly in the formation of nitrogen.
A recent short paper by C. W. Nutt and S. Kapur of the Department of Chemical Engineering, University of Birmingham (Nature, 1968, 220, 697) throws considerable light on the reaction mechanism. These authors examined the reaction in a mass spectrometer by causing an ammonia/oxygen mixture to contact a heated platinum filament: the reaction products passed under collision-free conditions through molecular beam apertures and thence through an ion source. The ionised products were detected in the usual way.
This technique ensures that only those species actually formed on the catalyst surface are sensed; because of the low background pressure, secondary products which might result from gas-phase collisions do not obtrude.
The reaction was examined with filament temperatures between about 400 and 1600°K. The results are very clear: both nitric oxide and nitrogen are formed on the catalyst surface and cannot arise from secondary processes. The reaction starts at about 500°K, the nitrogen yield is maximum at 700°K and nitric oxide at about 850°K. It is suggested that the rate decreases above this temperature because the residence time of the adsorbed species becomes progressively shorter.