Transient Redox Behavior of a NH3-SCR Cu-CHA SCR Catalyst: Effect of O2 Feed Content Variation

Notwithstanding the high efficiency of the modern NH3-SCR technology in reducing NOx from Diesel engines exhausts, their removal is still a concern in view of the forthcoming application of more stringent environmental guidelines. Actually, a large fraction of the total NOx emitted by the engine derives from transient engine operations, especially at low temperature. In this context, herein we systematically investigate the effect of oxygen feed content variation under Standard SCR reaction conditions over a reference Cu-CHA catalyst. Transient Response Methods (TRM) have been applied to mimic lean to rich and rich to lean transients. Steady state experiments reveal that the higher the O-2 feed content, the higher the NOx conversion and N2O formation, with N2O turning out to be very sensitive to oxygen concentration changes. Additionally, through TRM runs, it is possible to obtain quantitative information on the average fraction of oxidized Cu sites (Cu-II/Cu-tot) at the Standard SCR steady state. Particularly, the NOx conversion and the Cu-II fraction increase both with temperature and with O-2 feed content. Hence, the higher the oxidized copper fraction, the higher the N2O formation in the Standard SCR reaction.