Formation of NO⁺ and its possible roles during the selective catalytic reduction of NOx with NH₃ on Cu-CHA catalysts

The catalytic activities of small-pore Cu-CHA and large-pore Cu-BEA catalysts for the selective catalytic reduction of NO with NH₃ were measured at a very high flow rate. Cu-CHA clearly exhibited much higher intrinsic SCR activity and lower N₂O selectivity. In situ DRIFT spectra were recorded during the adsorption and desorption following NO and (NO + O₂) exposure to fully oxidized samples in a flow cell. The results are in agreement with what we have reported previously based on in situ transmission IR studies of partially reduced samples. Both suggest that different SCR reaction pathways might exist on these two catalysts and that NO⁺ could be an important reaction intermediate for Cu-CHA. Detailed IR studies with various isotopically labeled gas mixtures of (NO + O₂), (¹⁵NO + O₂), (NO + ¹⁸O₂) and (¹⁵N¹⁸O + O₂) were conducted to understand the origin of the surface adsorption complexes on Cu-CHA. Formation of NO⁺ was not the consequence of a simple charge transfer reaction, NO + Cu²⁺ = NO⁺ + Cu⁺. Instead, O₂ was found to be essential in changing the oxidation state of N from +2 to +3 although it did not participate in new N–O bond formation. The majority of the adsorbed NO⁺ maintained its isotopic origin of the feed gas.