Surface Storage of NOx

Several literature reports confirm, in fact, that zeolite catalysts are able to adsorb NO2 in the form of nitrates. NO2 chemisorption can be described by the following two-step mechanism, schematically representing disproportionation and heterolytic chemisorption of NO2 to form surface nitrites and nitrates, step (9.1), followed by NO2 oxidizing the nitrites to nitrates, step (9.2)  

The nitrates storage capacity of the two zeolite systems at 50 °C was determined from Fig. 9.1 taking into account the stoichiometry of reaction (9.3), resulting in about 0.87 and 0.50 mmol/gacuve phase for the Cu- and the Fe-zeolite, respectively. Expectedly, the storage capacity was foimd to decrease with increasing adsorption temperature. 

In a first attempt to confirm the presence of nitrates onto the catalyst surface, and in order to study their thermal stability, TPD runs were performed after saturation of the catalyst samples with NO2. During the temperature ramps evolution of NO2, NO and oxygen were indeed recorded, in line with what was already observed over other zeolite systems [5, 6] and over V205-W03/Ti02 catalysts [7]. 

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