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

Abstract

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.