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

Abstract

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 °C by in situ spectroscopic techniques. The catalyst (Si/Al=15; Cu/Al=0.5) was first exposed to NO/O2 (mimicking the SCR oxidative half‐cycle), mainly resulting in the formation of chelating bidentate framework‐interacting CuII‐nitrates. These intermediates were gradually detached from the framework in the presence of NO/NH3 (or NH3 alone), forming mixed‐ligand mobile [CuII(NH3)3(NO3)]+ complexes, with infrared bands at 1624 (δNH3), 1430 and 1325 cm−1 (monodentate nitrate νNO2asym and νNO2sym, respectively). X‐ray absorption and Diffuse Reflectance UV‐Vis spectroscopies showed that during this transformation the CuII/CuI reduction, observed under similar conditions at 200 °C, hardly occurred. DFT calculations confirmed the stability of nitrate ligands in the monodentate conformation in [CuII(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 CuII 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.

Selective Catalytic Reduction of NO with NH3 on Cu−SSZ‐13: Deciphering the Low and High‐temperature Rate‐limiting Steps by Transient XAS Experiments

“…The aqueous copper nitrate reference is labelled as Cu II (NO x ) y due to the negligible difference between the XANES spectra of Cu II (NO 3 ) 2 and Cu II (NO 2 ) x (H 2 O) 6‐x . Cu I (NH 3 ) 2 and Cu II (NH 3 ) 4 are thought to be related directly to solvated linear Cu I[13] and square planar Cu II[41] and may not be limited to ligands composing solely of NH 3 . In the recent work of Negri et al the square planar Cu II species was demonstrated to be composed of mixed ligands .…”

“…[8,40] Cu I (NH 3 ) 2 and Cu II (NH 3 ) 4 are thought to be related directly to solvated linear Cu I [13] and square planar Cu II [41] and may not be limited to ligands composing solely of NH 3 . [19,42] In the recent work of Negri et al the square planar Cu II species was demonstrated to be composed of mixed ligands. [42] Herein these species will be described as solvated linear Cu I and square planar Cu II species, respectively.…”

“…[19,42] In the recent work of Negri et al the square planar Cu II species was demonstrated to be composed of mixed ligands. [42] Herein these species will be described as solvated linear Cu I and square planar Cu II species, respectively.…”

Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNOx by modulation excitation IR spectroscopy

“…Cu 2+ (NH 3 ) 3 (NO 3 )). 4,45,46 It is clear, however, that the identication of intermediate states as achieved here can be very useful in guiding the design of the next generation of new and related technologies for deNO x applications, perhaps using cationic components that could be more active or at least more environmentally benign or at risk than copper.…”

Meet the Women of Catalysis

“…The aim is to define structure‐reactivity correlations for a rational design of materials, often with the support of modeling and in collaboration with industry. Important fields of study are partial oxidation reactions, including the selective catalytic reduction of NO x (SCR) and the direct methane to methanol conversion …”