Mechanistic Studies by Transient Response Methods

Assuming that nitrates have been formed by NO2 interaction with the catalyst, as shown in the previous paragraph, we proceed here to examine their reactivity with ether one of the other two Fast SCR reactants, namely NO and NH3. 

The reactivity of surface nitrates with NO is demonstrated by a transient experiment where we first adsorbed NO2 (1,000 ppm, concentration step change, in a stream of water (3 % v/v) and Helium) on the Cu-zeolite at 2(X) °C, then we added 1,000 ppm of NO to the water/Helium feed stream, and eventually performed a T-ramp where the catalyst temperature was linearly increased from 200 to 550 °C at 20 °C/min. 

The results of the full experiment are displayed in Fig. 9.5. In the first stage, saturation of the catalyst by NO2 is apparent, with the associated NO evolution indicating formation of surface nitrates according to (9.1)-(9.3). 

a preliminary conclusion here is that surface nitrates formed via NO2 disproportionation, though thermally stable, are readily reduced at very low temperature by NO, if available, forming NO2, while their direct reduction by NH3 proceeds only at higher temperatures. 

we discuss how such a reduction step contributes to the overall SCR pathways leading from NOx -I- NH3 to N2. 

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