An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst

The reactivity of Lewis and Bronsted ammonia in the reduction half-cycle (RHC) of the NH3-SCR low temperature redox mechanism was studied over a model Cu - CHA catalyst by transient kinetic tests involving reductive NO pulses. The Cu-II sites were reduced according to a 1:1:1:1 molar ratio with NO and NH3 conversion and N-2 formation. The ammonia coordinated to Cu sites (Lewis ammonia) was preferentially consumed prior to that stored on the Bronsted acid sites. The catalyst was effectively re-oxidized by O-2 in He at 150 degrees C even when the Cu-coordinated ammonia was depleted. A redox kinetic model assuming NO activation by Cu-II to a gaseous mobile intermediate (HONO) which reacts first with Lewis-NH3 and then with Bronsted-NH 3 was successfully fitted to our transient data assuming the Cu-II reduction rate to be second order in the Cu-II sites. This suggests a possible role of Cu(II)( )dimeric complexes in the RHC of Standard SCR.