Oxidation of ammonia to nitrogen oxides

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

Some metallic catalysts in the form of gauzes are employed in certain processes, e.g. the oxidation of ammonia to nitrogen oxides over platinum, and for these heat transfer data have been published by London et al. [41,42]. 

An extraordinary type of packed bed is utilized in the oxidation of ammonia to nitrogen oxide, in connection with the nitric acid production the oxidation reaction takes place at a high temperature (890°C) on a metal net, on which the active catalyst (Pt metal) is dispersed. This reactor type, which can be treated mathematically as a packed bed reactor, is called a gauze reactor. An illustration of a gauze reactor is given in Figure 5.3 [3]. 

Selectivity control by competitive adsorption also explains how the platinum catalyst works in the Ostwald process, the oxidation of ammonia to nitrogen oxide, used to produce nitrates from ammonia. The low reactivity of platinum causes the rate of ammonia dissociation on platinum to be low. The dissociation of N-H bonds of ammonia is the reverse of the ammonia synthesis discussed in the previous section. Whereas the rate of dissociative NH3 adsorption is relatively small, the initial sticking coefficient for dissociative oxygen adsorption is higher, about 0.01. 

Industrial. Nitric acid is itself the starting material for ammonium nitrate, nitroglycerin [55-63-0] trinitrotoluene [118-96-7]., nitroceUulose [9004-70-0] and other nitrogen compounds used in the manufacture of explosives (see Explosives and propellants). Nitric acid is made by oxidation of ammonia to nitrogen dioxide [10102-44-0] which is subsequently absorbed by water. 

In cone, solutions the silver catalysed oxidation of ammonia to nitrogen may be very violent. 

The redox mechanism applies not only to allylic oxidation of olefins and to the oxidation of aromatic hydrocarbons, but also to the oxidation of methanol and sulphur dioxide, as well as the oxidation of ammonia to nitrogen. Only in the case of ethylene oxidation and oxyhydration of olefins do catalysts act according to another mechanism. The latter processes seem to be always low temperature reactions, occurring below 300° C, whereas redox mechanisms are possible above this temperature (e.g. 400—500°C). 

The general rate equation for the oxidation of ammonia to nitrogen is of the redox type (161—163). 

With inorganic compounds, there can also be a selectivity problem, as illustrated by the oxidation of ammonia to nitrogen. Deep oxidation leads to nitrogen oxides. With sulphur dioxide, no selectivity problem rises. 

Ammonia is sometimes used as a reducing agent for the selective reduction of NOE in emissions from industrial installations, but unreacted ammonia creates a secondary air pollution problem because it is itself hazardous. Consequently, selective catalytic oxidation (SCO) is required to convert traces of ammonia to nitrogen downstream of the reactor 134... 

Some dense inorganic membranes made of metals and metal oxides are oxygen specific. Notable ones include silver, zirconia stabilized by yttria or calcia, lead oxide, perovskite-type oxides and some mixed oxides such as yttria stabilized titania-zirconia. Their usage as a membrane reactor is profiled in Table 8.4 for a number of reactions decomposition of carbon dioxide to form carbon monoxide and oxygen, oxidation of ammonia to nitrogen and nitrous oxide, oxidation of methane to syngas and oxidative coupling of methane to form C2 hydrocarbons, and oxidation of other hydrocarbons such as ethylene, methanol, ethanol, propylene and butene. 

A side reaction that lowers the product yield is the oxidation of ammonia to nitrogen and water vapor ... 

The selective catalytic oxidation of ammonia to nitrogen (SCO process) has been proposed for the abatement of ammonia from waste gases. Among different formulations, catalysts based on Cu0-Ti02 offer interesting properties [1]. However, selectivity to nitrogen can be decreased by the parallel formation of N2O and NO. The behaviour of metal oxide catalysts upon these reactions can be influenced by water vapour that is always present in the reacting mixture [2]. 

The study of the oxidation of ammonia to nitrogen and water at low temperature has become more and more important in recent years due to agricultural and industrial waste streams. Noble metals (Pt, Ir) are suitable for the selective, low temperature oxidation of ammonia to nitrogen and water. Alumina supported platinum catalysts are promising in the conversion of gaseous ammonia to N2 and H2O. They possess a high activity and selectivity in N2 formation.I l However, these catalysts are susceptible to rapid deactivation due to irreversible adsorption of reaction intermediates such as NHx species. 

Qi G, Gatt J E, Yang R T (2004) Selective catalytic oxidation (SCO) of ammonia to nitrogen over Fe-exchanged zeolites prepared by sublimation of FeCla. J. Catal. 226 120-128. 

O dation of ammonia to nitrogen oxides over man nese dio de. Milner ... 

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