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% Cu2+-MOR, the progressive thermal activation causes first the loss of water molecules from the Cu2+ coordination sphere, accompanied by a partial aggregation in Cu2+-O-Cu2+ complexes, and the the Cu2+ → Cu+ reduction with oxygen elimination. The presence of EPR inactive cupric pairs, witnessed by EXAFS, explains the systematic underestimation of the fraction of Cu2+ 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 O2 is dramatically favored by the presence of water. This fact explains the poisoning effect of water in the deNOx activity of Cu-exchanged zeolites. The coordination of NO molecules on the Cu+-MOR system was studied in situ at liquid nitrogen temperature. The deNOx 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.