Relationship with NO Oxidation Activity

As mentioned in Sect. 8.2.3, the oxidation of NO would be a crucial step in the standard SCR reaction. In fact, there are many reports in the literature which suggest that the SCR activity is correlated with the NO oxidation conversion when the same type of active metal is compared [12-17]. On the other hand, some literature indicates that there is no correlation between them especially when comparing different active species, such as Fe and Cu [11, 18-20]. In this section, the reason for this inconsistency has been discussed. 

However, when the comparison is made between different kinds of active metals, the SCR conversion does not always correlate with the extent of NO oxidation. For instance, Metkar et al. [11, 19]. have reported that Fe/ZSM-5 showed a higher NO oxidation conversion than Cu/CHA, while Cu/CHA 

Regarding standard SCR reaction, on the other hand, the NO2 desorption does not seem to be the rate-determining step because produced NO2 should react with NH3 immediately, which is elucidated and discussed in more detail in the other sections. Therefore, it is difficult to make a correlation between SCR activity and 

NO oxidation when different active metals are compared. Nevertheless, the author believes that the SCR activity is correlated with NO oxidation ability in the presence of NH3 , simply because the fast SCR (NO/NO2/NH3 system) rate is extremely faster than the standard SCR rate. The point is that NO oxidation conversion (NO2 formation rate) under NH3 absent condition does not necessarily correspond to NO oxidation ability under NH3 present condition, i.e., standard SCR condition. Thus, the author presumes that although NO2 is hard to be desorbed from Cu sites, Cu would have a greater potential to oxidize NO to NO2 under NH3 present condition. However, quantifying this ability is very difficult because the production rate of NO2 adspecies can hardly be measured in the presence of NH3. Thus, an advanced technique which makes it possible to verify the assumption is desired. 

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