M A T T E R A T E X T R E M E S
S C I E N T I F I C H I G H L I G H T S
2 6 H I G H L I G H T S 2 0 2 2 I
Revealing the influence of zeolite composition on oxygen-bridged diamino dicopper(II) complexes in Cu-CHA deNOx catalysts
Ammonia-assisted Selective Catalytic Reduction (NH3-SCR) of Cu-exchanged chabazite is a widespread deNOx technology in the automotive sector. Exploiting in-situ X-ray absorption spectroscopy powered by wavelet transforms and machine learning-assisted extended X-ray absorption fine structure fitting, the influence of the zeolite composition on mobile Cu- complexes formed in these catalysts under NH3-SCR- relevant conditions is revealed.
Cu-exchanged chabazite (Cu-CHA) represents the catalyst of choice for NOx abatement in diesel vehicles via NH3- SCR, involving the reaction of NO and NH3 in the presence of O2 to form N2 and H2O. At 200°C, NH3-SCR proceeds over NH3-mobilised Cu-ions, electrostatically tethered to their framework exchange sites . For characterisation purposes, the reaction is often decoupled in reduction and oxidation half-cycles. In the reduction half-cycle, exposure to a NO/NH3 mixture leads to [CuI(NH3)2]+ complexes. In the oxidation half-cycle, a pair of [CuI(NH3)2]+ interacts with an O2 molecule to form [Cu2(NH3)4O2]2+ complexes, described as a µ-h2,h2-peroxo diamino dicopper(II) .
The volumetric Cu density was identified as the key descriptor of the oxidation half cycle efficiency, due to the limited mobility of [CuI(NH3)2]+ complexes up to ca. 9 Å from their anchoring point in the zeolite . Thus, a lower Cu density implies a larger fraction of unreacted [CuI(NH3)2]+. Despite the quasi-homogenous conditions, density/ distribution of framework Al atoms and Brønsted acid sites may influence the mobility of [CuI(NH3)2]+ complexes and the stability of Cu pairs. The implications for NH3-SCR performance through such zeolite-driven modification in [Cu2(NH3)4O2]2+ properties are still unexplored.
Using in-situ X-ray absorption spectroscopy (XAS) at beamline BM23, three Cu-CHA catalysts with different composition, namely 0.1_5, 0.5_15 and 0.6_26 ( Cu/Al_Si/Al labels), and controlled variations in Cu density were investigated to assess the impact of the zeolite composition on NH3-mobilised Cu-complexes formed during the reduction/oxidation half-cycles.
Figures 16a to c compare Cu K-edge X-ray absorption near-edge structure (XANES) spectra of the three catalysts after pre-treatment, reduction in NO/NH3 and oxidation in O2 at 200°C. Consistently with previous reports, exposure to NO/NH3 leads to [CuI(NH3)2]+ complexes, with indistinguishable spectra for all the catalysts. During the oxidation step, the characteristic XANES features of
Fig 16: In-situ Cu K-edge XANES for (a) 0.1_5, (b) 0.5_15 and (c) 0.6_29 catalysts after pre- treatment, reduction, and oxidation. Pie charts illustrate CuI/CuII percentages evaluated by XANES LCF at the end of the oxidation step. d) NOx conversion in the 150-500°C range and correlation between residual CuI fraction and TOF at 200°C (inset).