ERI and SSZ-13 were subjected to post-synthetic treatments (depending on the zeolite topology) to create micro-/mesoporous materials. The results in terms of NH₃-SCR-DeNO_x show that the applied treatments improved the catalytic activity of the Cu-containing ERI-based materials; however, the NO conversion did not vary for the different materials treated with NaOH or NaOH/HNO₃. For the micro-/mesoporous Cu-containing SSZ-13, a lower NO conversion in NH₃-SCR-DeNO_x was observed. Thus, our findings challenge the current paradigm of enhanced activity of micro-/mesoporous catalysts in NH₃-SCR-DeNO_x. The modification of the supports results in the presence of different amounts and kinds of copper species (especially isolated Cu²⁺ and aggregated Cu species) in the case of ERI- and SSZ-13-based samples. The present copper species further differentiate the formation of reactive reaction intermediates. Our studies show that besides the μ-η²,η²-peroxo dicopper(II) complexes (verified by in situ DR UV-Vis spectroscopy), copper nitrates (evidenced by in situ FT-IR spectroscopy) also act as reactive intermediates in these catalytic systems.