Unexpected ion exchange between NH4+ and CuII ions in Cu-CHA zeolites upon formation of the [CuII2(NH3)4O2]2+ peroxo complex

Journal article, 2026

In this work, we have followed by in situ IR spectroscopy the stepwise formation of the [Cu^II2(NH3)4O2]²⁺ peroxo complex in Cu-CHA zeolites with different Cu loading and Si/Al ratios. Unexpected changes were observed while oxidizing the [Cu^I(NH3)2]⁺ complexes formed by reaction of Cu-CHA with an NO/NH3 mixture. Namely, this process, causes a decrease in the intensity of physisorbed NH3 (bending mode at 1620 cm⁻¹) and of its protonated form, NH4⁺ (bending mode at 1434 cm⁻¹). These changes are correlated with the increase of a band at 900 cm⁻¹, which is coherent with the growth of framework coordinated Z2Cu^II sites, where Z represent a negative charge on the framework, as confirmed by DFT calculations. The process can be described as an ion exchange between NH4⁺ and Cu^II ions during the oxidation, with formation of H2O and removal of protons. More Cu^II ions are replacing NH4⁺ at high Cu loading and low Si/Al ratio, in agreement with the current understanding of the effect of these parameters on ions diffusion, which in turns affect the efficiency of the oxidation and reduction half cycles of Cu-CHA in the low temperature NH3-SCR reaction.