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

Abstract

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.