Evidence of mixed‐ligand complexes in Cu‐CHA by reaction of Cu nitrates with NO/NH3 at low temperature

The reactivity with a NO/NH3 mixture of Cu-nitrate complexes formed on the surface of a Cu-CHA catalyst active in the Selective Catalytic Reduction of NOx with NH3 (NH3-SCR) was followed at 50 degrees C by in situ spectroscopic techniques. The catalyst (Si/Al=15; Cu/Al=0.5) was first exposed to NO/O-2 (mimicking the SCR oxidative half-cycle), mainly resulting in the formation of chelating bidentate framework-interacting Cu-II-nitrates. These intermediates were gradually detached from the framework in the presence of NO/NH3 (or NH3 alone), forming mixed-ligand mobile [Cu-II(NH3)(3)(NO3)](+) complexes, with infrared bands at 1624 (delta NH3), 1430 and 1325 cm(-1) (monodentate nitrate nu NO2asym and nu NO2sym, respectively). X-ray absorption and Diffuse Reflectance UV-Vis spectroscopies showed that during this transformation the Cu-II/Cu-I reduction, observed under similar conditions at 200 degrees C, hardly occurred. DFT calculations confirmed the stability of nitrate ligands in the monodentate conformation in [Cu-II(NH3)(3)(NO3)](+) complexes when solvated by ammonia. The resulting structure was successfully used to fit the corresponding experimental EXAFS spectra. The gradual change of ligands in the Cu-II coordination sphere was confirmed by the blue shifts of both d-d and Ligand to Metal Charge Transfer bands in Diffuse Reflectance UV-Vis spectra, with formation of features (27500, 32000 and 38000 cm(-1)) ascribable to the mixed-ligand configuration.