Thermal Reduction of Cu²⁺−Mordenite and Re-oxidation upon Interaction with H₂O, O₂, and NO

We present a multitechnique (EPR, XANES, EXAFS, and IR of adsorbed NO) study of the coordination and oxidation chemistry of copper species hosted in mordenite (MOR) zeolite under different conditions. Starting from a 100% Cu 2+ −MOR, the progressive thermal activation causes first the loss of water molecules from the Cu 2+ coordination sphere, accompanied by a partial aggregation in Cu 2+ −O−Cu 2+ complexes, and then the Cu 2+ → Cu + reduction with oxygen elimination. The presence of EPR inactive cupric pairs, witnessed by EXAFS, explains the systematic underestimation of the fraction of Cu 2+ species evaluated by EPR, with respect to that obtained from XANES. The data discussed here confirm the interpretation of the so-called “self-reduction” phenomenon of cupric ions emerging in a previous study performed on Cu−ZSM-5 [ J. Phys. Chem. B 2000, 104, 4064]. The reoxidation of the so obtained Cu + −MOR by O 2 is dramatically favored by the presence of water. This fact explains the poisoning effect of water in the deNO x activity of Cu-exchanged zeolites. The coordination of NO molecules on the Cu + −MOR system was studied in situ at liquid nitrogen temperature. The deNO x chemistry was then switched on by allowing the system to reach room temperature in the NO atmosphere. In all stages of this study, comparison is made with a Cu + −ZSM-5 model system. The differences observed between these two systems are explained in terms of the different structural (cation concentration and environment) characteristics.