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

In this work, we have followed by in situ IR spectroscopy the stepwise formation of the [Cu II 2 (NH 3 ) 4 O 2 ] 2+ peroxo complex in Cu-CHA zeolites with different Cu loading and Si/Al ratios. Unexpected changes were observed while oxidizing the [Cu I (NH 3 ) 2 ] + complexes formed by reaction of Cu-CHA with an NO/NH 3 mixture. Namely, this process, causes a decrease in the intensity of physisorbed NH 3 (bending mode at 1620 cm −1 ) and of its protonated form, NH 4 + (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 Z 2 Cu 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 NH 4 + and Cu II ions during the oxidation, with formation of H 2 O and removal of protons. More Cu II ions are replacing NH 4 + 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 NH 3 -SCR reaction.