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 Brønsted 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 CuII sites were reduced according to a 1:1:1:1 molar ratio with NO and NH3 conversion and N2 formation. The ammonia coordinated to Cu sites (Lewis ammonia) was preferentially consumed prior to that stored on the Brønsted acid sites. The catalyst was effectively re-oxidized by O2 in He at 150 °C even when the Cu-coordinated ammonia was depleted. A redox kinetic model assuming NO activation by CuII to a gaseous mobile intermediate (HONO) which reacts first with Lewis-NH3 and then with Brønsted-NH3 was successfully fitted to our transient data assuming the CuII reduction rate to be second order in the CuII sites. This suggests a possible role of CuII dimeric complexes in the RHC of Standard SCR.