Site-Specific Reactivity of Copper Chabazite Zeolites with Nitric Oxide, Ammonia, and Oxygen

Insitu electron paramagnetic resonance (EPR) spectroscopy was applied to dilute copper chabazite (CHA) zeolites under gas flows relevant for the selective catalytic reduction of NO with ammonia (NH3-SCR). Under both reducing and oxidizing conditions, we observed differences in reactivity between the different monomeric copper sites present: Upon reduction of Cu2+ with NO+NH3, the rate is seen to depend on the NH3 coverage. Subsequent oxidation with O-2 resulted in a clean EPR spectrum of only one type of copper site, whereas oxidation with NO+O-2 gave two types of copper sites. The rate of oxidation differed significantly between the reaction with O-2 alone and with NO+O-2 together. Thus, it was revealed that [Cu(NH3)(2)](+) complexes, which are regarded to be only weakly associated with the framework, nevertheless have different reactivity depending on the Al distribution in the proximity. The observed differences in reactivity of the copper sites have implications for the mechanistic understanding of NH3-SCR with Cu zeolites.