Identification and Quantification of Copper Sites in Zeolites by Electron Paramagnetic Resonance Spectroscopy

Recent quantitative electron paramagnetic resonance spectroscopy (EPR) data on different copper species present in copper exchanged CHA zeolites are presented and put into context with the literature on other copper zeolites. Results presented herein were obtained using ex situ and in situ EPR on copper ion exchanged into a CHA zeolite with Si/Al = 14 ± 1 to obtain Cu/Al = 0.46 ± 0.02. The results shed light on the identity of different copper species present after activation in air. Since the EPR signal is quantifiable, the content of the different EPR active species has been elucidated and Cu²⁺ in 2Al positions in the 6-membered rings (6mr) of the CHA structure has been characterized. Some copper species are found not to give an EPR signal at ambient or high temperatures. Fortunately, treatments with different gasses under in situ conditions are able to trigger an EPR signal and thus reveal information about the reactivity and the quantity of some of the otherwise EPR silent species. In this way the [Cu–OH]⁺ species in copper substituted low-Al zeolites has been indirectly observed and quantified. EPR active Cu²⁺ species have been followed under reduction and oxidation with gas mixtures relevant for the selective catalytic reduction of NO with NH₃ (NH₃-SCR) revealing that all Cu²⁺ in 6mr are easily reduced and oxidized at 200 °C. Furthermore, a stable [Cu–NO₃]+ species is identified in Cu-CHA after exposure to NO and O₂, but is not stable in 2Al 6mr sites of the CHA structure under the applied conditions.