Analysis of the Enhanced SCR Chemistry

It is worth noticing that both the Fast SCR and the Enhanced SCR chemistries can be formally represented according to a simple sequential scheme, that we originally proposed for the Fast SCR mechanism only. We showed by transient kinetic experiments [17] that at temperatures as low as 150-170 °C the Fast SCR reaction proceeds via a two-step sequence in which ammonium nitrate is first formed from NO2 and NH3, reaction (9.14), and then reacts with NO, reaction (9.15)  

The sum of (9.14) and (9.15) yields in fact the Fast SCR, reaction (9.6), wherein NO2 is a reactant and NH4NO3 acts as an intermediate. The same sequence, however, also describes the Enhanced SCR chemistry, wherein however NH4NO3 is now a reactant and NO2 becomes an intermediate the stoichiometry of reaction (9.12) is obtained in this case adding (9.15), multiplied by two, to (9.11). 

Clearly, the analogy between the Fast SCR and the Enhanced SCR chemistries has deeper roots than just a combination of stoichiometries. It actually originates from the key mechanistic role played by nitrates adspecies in both reactions. Such nitrates are either formed via NO2 dimerization, disproportionation, and hetero-lytic chemisorption in the Fast SCR chemistry (see Sect. 9.5.3 and Table 9.1), or are formed directly by nitric acid adsorption when feeding aqueous solutions of NH4NO3 or nitric acid in the case of Enhanced SCR. It is well known that ammonium nitrate participates in the dissociation equilibrium (9.16) with nitric acid and ammonia. 

From a conceptual standpoint, injecting nitrates in order to reduce NOx may seem quite paradoxical. Nevertheless, data prove that this is indeed effective for promoting the DeNOx activity at low temperature. 

Acknowledgments The financial support of Daimler AG (Germany) to our investigation of NH3-SCR during many years is gratefully acknowledged. The authors are indebted to Dr. Bernd Krutzsch, Dr. Michel Weibel, and Dr. Volker Schmeisser (Daimler) for many useful suggestions and discussions. 

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