“…The choice of CO as a probe molecule for Cu ions is implied from the existence of a characteristic carbonyl frequency shift, which depends strictly on the copper coordination environment, as well as the oxidation state [ 14 , 15 , 16 ]. In the case of Cu 2+ cations, the CO-IR analysis is insufficient, thus complementary techniques such as Electron Paramagnetic Resonance (EPR) [ 17 , 18 , 19 ], X-ray photoelectron spectroscopy (XPS) [ 20 , 21 , 22 ] and temperature-programmed reduction (TPR) [ 20 , 23 , 24 , 25 , 26 ], as well as IR studies of NO sorption, are needed to receive a detailed characterization of the oxidation state and properties of Cu in zeolites under various red-ox conditions, e.g., [ 27 , 28 , 29 ].…”