The Low Temperature Interaction of NO + O₂ with a Commercial Cu-CHA Catalyst: A Chemical Trapping Study

We investigate the interaction of NO + O₂ with a Cu-CHA SCR catalyst in the low temperature region. To this purpose, a well validated experimental protocol, including a combination of transient response methods, ex situ FTIR spectroscopy and chemical trapping techniques, is applied. In particular, the combination of a Cu-CHA catalyst powder and a BaO/Al₂O₃ NOx trap in a physical mixture is used in an attempt to capture and stabilize reactive and possibly elusive NOx intermediates, which might go otherwise undetected. As a result, direct evidence for the formation of nitrites in the oxidative activation of NO over Cu-CHA is obtained. Identification of trapped nitrites intermediates has been confirmed in several ways, including: (i) their thermal decomposition to an equimolar mix of NO and NO₂ during temperature programmed desorption, (ii) N₂ formation during their reaction with NH₃ at low temperature, (iii) ex situ IR analysis of the BaO/Al₂O₃ phase after discharge and separation from the mechanical mixture. On such a basis, a low temperature redox mechanism for NO oxidation and Standard SCR reaction over Cu-zeolite catalysts is proposed and discussed.