Cu-SSZ-13 has been characterized by different spectroscopic techniques and compared with Cu-ZSM-5 and Cu-β with similar Si/Al and Cu/Al ratios and prepared by the same ion exchange procedure. On vacuum activated samples, low temperature FTIR spectroscopy allowed us to appreciate a high concentration of reduced copper centres, i.e. isolated Cu⁺ ions located in different environments, able to form Cu⁺(N₂), Cu⁺(CO)_n (n = 1, 2, 3), and Cu⁺(NO)_n (n = 1, 2) upon interaction with N₂, CO and NO probe molecules, respectively. Low temperature FTIR, DRUV-Vis and EPR analysis on O₂ activated samples revealed the presence of different Cu²⁺ species. New data and discussion are devoted to (i) [Cu–OH]⁺ species likely balanced by one framework Al atom; (ii) mono(μ-oxo)dicopper [Cu₂(μ-O)]²⁺ dimers observed in Cu-ZSM-5 and Cu-β, but not in Cu-SSZ-13. UV-Vis-NIR spectra of O₂ activated samples reveal an intense and finely structured d–d quadruplet, unique to Cu-SSZ-13, which is persistent under SCR conditions. This differs from the 22 700 cm⁻¹ band of the mono(μ-oxo)dicopper species of the O₂ activated Cu-ZSM-5, which disappears under SCR conditions. The EPR signal intensity sets Cu-β apart from the others.

Cu-CHA combines high activity for the selective catalytic reduction (SCR) reaction with better hydrothermal stability and selectivity compared to other copper-substituted zeolites. At the same time Cu-CHA offers an opportunity for unraveling the coordination environment of the copper centers since the zeolite framework is very simple with only one crystallographically independent tetrahedral site (T-site). In this study the results of an X-band electron paramagnetic resonance (EPR) investigation of ion-exchanged Cu-CHA zeolite with a Si/Al ratio of 14 ± 1 is presented. Different dehydration treatments and rehydration experiments are performed in situ while monitoring with EPR. The results are compared with recent literature evidence from temperature-programmed reduction, X-ray methods, IR spectroscopic methods, and UV–visible spectroscopy. On the basis of these findings quantitative information is obtained for the different copper positions in dehydrated Cu-CHA. The well-defined copper sites in the six-membered ring of the CHA structure are found to be EPR active, to give two distinct sets of signals in an approximate 1:1 ratio, and to add up to 19 ± 2% of the total copper in the material. The long-standing question of the EPR silent monomeric Cu²⁺ in copper-substituted zeolites is suggested to be copper species with an approximate trigonal coordination sphere appearing during the dehydration. After complete dehydration at 250 °C the majority of the EPR silent Cu²⁺ is suggested to exist as Cu²⁺–OH^– coordinated to two framework oxygen atoms located in the microenvironment of an isolated Al T-site.