Ammonia Blocking of Nitrates Reduction

We have shown in Sect. 9.5.2 that the reduction of surface nitrates by NO, the key step in the Fast SCR mechanism over Me-exchanged zeolites and V-based catalysts, is active already at 50 °C. A dedicated study over an Fe-BEA catalyst [20] pointed out, however, that in the presence of ammonia the reaction between NO and nitrates is stopped and proceeds only on raising the temperature up to 140-160 °C, which thus represents an intrinsic lower bound to the Fast SCR activity. Similar results were reported for a BaNa-Y zeolite [5]. 

The NH3 blocking effect is possibly associated with a strong interaction between ammonia and nitrate species when both are present on the catalyst surface. More specifically, at low temperature NH3 could react with nitrates to form ammonium nitrate precursors (or strongly interacting ammonia-nitrate adspecies), e.g.. 

Basic reaction steps in NO/NO2-NH3 SCR chemistry over V-based and metal-promoted zeolite catalysts 

Notably, the inhibiting effect of NH3 on the Fast SCR activity at low temperature is not due to the ammonia competitive chemisorption on the catalytic sites, but occurs because ammonia captures a key intermediate in an unreactive form. In this respect, one way to partially prevent this undesired effect is to modify the equilibrium of ammonium nitrate dissociation, e.g., by increasing the temperature or by decreasing the gas-phase ammonia concentration. Since the blocking effect is related to the acid properties of the formed nitrates, another possibility to moderate its negative impact on the Fast SCR reactivity at low T would be to modify the catalyst acid/base properties in order to favor the interaction of ammonia with the catalyst sites rather than with the nitrates [5]. 

It is worth noticing that the steps discussed above for the Fast SCR chemistry, reaction (9.6) does not involve changes in the formal oxidation state of the catalytic sites. In fact, formation of nitrates and nitrites occurs via disproportionation of NO2 (steps R1 -F R2 in Table 9.1), reduction of nitrates to nitrites is compensated by the simultaneous oxidation of NO to NO2 (step R3), and decomposition of nitrites to N2 by NH3 (step R5) is also red-ox neutral. Accordingly, we can expect that in the presence of NO2, a strong oxidizer, the SCR catalyst sites remain at their highest oxidations states. This is a substantial difference from the case of the Standard SCR reaction (9.5), wherein reduction of NO by NH3 to N2 does involve catalyst reduction, and therefore closure of the catalytic cycle 

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