Evidence of Mixed‐Ligand Complexes in Cu−CHA by Reaction of Cu Nitrates with NO/NH₃ at Low Temperature

The reactivity with a NO/NH 3 mixture of Cu‐nitrate complexes formed on the surface of a Cu−CHA catalyst active in the Selective Catalytic Reduction of NO x with NH 3 (NH 3 −SCR) was followed at 50 °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/NH 3 (or NH 3 alone), forming mixed‐ligand mobile [Cu II (NH 3 ) 3 (NO 3 )] + complexes, with infrared bands at 1624 (δNH 3 ), 1430 and 1325 cm −1 (monodentate nitrate νNO 2asym and νNO 2sym , 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 °C, hardly occurred. DFT calculations confirmed the stability of nitrate ligands in the monodentate conformation in [Cu II (NH 3 ) 3 (NO 3 )] + 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 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.