Higher Temperatures The NO2-SCR Reaction

Contrary to NO (Fig. 9.5), ammonia was unable to reduce directly nitrates up to 200 °C over V-based and metal-promoted zeolites catalysts (Fig. 9.6) but what happens at higher temperatures  

The run in Fig. 9.8, showing the reactivity of nitrates with ammonia, has been compared with a Temperature Programmed Reaction run over the same catalyst, where the feed included gaseous ammonia and NO2, and during which we also observed consumption of ammonia, and production of N2 and of N2O. Most 

The reactivity of nitrates has to do not only with the selective (to N2) and desired Fast and NO2 SCR reactions, but also with the unselective reactions responsible for the formation of undesired by-products. 

At low temperatures, below 200 °C, ammonia is not able to reduce nitrates, but it can however react with them to form ammonimn nitrate [18, 24], 

In our early study over a V-based catalyst [24], reaction (9.10) was identified by the stoichiometry of NO2 -I- NH3 conversion, and of N2 formation. Ammonium nitrate, formed and deposited onto the catalyst, could not of course be directly detected, but its formation was confirmed both by the lack in the N-balance, by dedicated IR analyses performed on the catalyst downloaded from the reactor after the experiment, and by subsequent TPD mns which showed formation of N2O (see Fig. 9.9), in line with the well-known thermal decomposition of ammonium nitrate, 

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