Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNO_x Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy

Abstract

Cu-exchanged chabazite is the catalyst of choice for NO x abatement in diesel vehicles aftertreatment systems via ammonia-assisted selective catalytic reduction (NH 3 –SCR). Herein, we exploit in situ X-ray absorption spectroscopy powered by wavelet transform analysis and machine learning-assisted fitting to assess the impact of the zeolite composition on NH 3 -mobilized Cu-complexes formed during the reduction and oxidation half-cycles in NH 3 –SCR at 200 °C. Comparatively analyzing well-characterized Cu-CHA catalysts, we show that the Si/Al ratio of the zeolite host affects the structure of mobile dicopper(II) complexes formed during the oxidation of the [Cu I (NH 3 ) 2 ] + complexes by O 2 . Al-rich zeolites promote a planar coordination motif with longer Cu–Cu interatomic distances, while at higher Si/Al values, a bent motif with shorter internuclear separations is also observed. This is paralleled by a more efficient oxidation at a given volumetric Cu density at lower Si/Al, beneficial for the NO x conversion under NH 3 –SCR conditions at 200 °C.

Influence of ion mobility on the redox and catalytic properties of Cu ions in zeolites

“…76 As an example of TOF values in the NH 3 -SCR reaction we mention the recent work by Martini et al , reporting values between 1.0 × 10 −3 and 1.5 × 10 −2 mol mol Cu −1 s −1 for Cu-CHA catalysts with different chemical composition at 200 °C. 102 While data about the diffusion timescale of protons and molecules such as water and ammonia are available in the literature, 19,21–23,103 similar information related to Cu ions mobility are less straightforward. Rizzotto et al measured by in situ electrical IS the relaxation times for local ion motion in zeolite.…”

Review of the application of Cu-containing SSZ-13 in NH3-SCR-DeNOx and NH3-SCO

“… 51 The structure of mobile dicopper( ii ) complexes vary depending on the n (Si)/ n (Al) ratio of the zeolite host. 182 Finally, the newly formed [Cu II (NH 3 ) 2 ] 2+ –O–[Cu II (NH 3 ) 2 ] 2+ hydrolyzes in the presence of H 2 O, restoring two [Cu I (OH)(NH 3 ) 2 ] + adducts. O 2 dissociation and NO 2 formation occur on complexes in the presence of NO ([Cu I (NH 3 ) 2 ] + –O 2 –[Cu I (NH 3 ) 2 ] + + NO → [Cu II (NH 3 ) 2 ] 2+ –O 2− –[Cu II (NH 3 ) 2 ] 2+ + NO 2 ).…”

“…Such proximity assists the oxidation of NH 3 -solvated Cu + ions to form dimeric Cu intermediates [Cu I (NH 3 ) 2 ] + -O 2 -[Cu I (NH 3 ) 2 ] + 51. The structure of mobile dicopper(II) complexes vary depending on the n(Si)/n(Al) ratio of the zeolite host 182. Finally, the newly formed [Cu II (NH 3 ) 2 ] 2+ -O-[Cu II (NH 3 ) 2 ] 2+ hydrolyzes in the presence of H 2 O, restoring two [Cu I (OH)(NH 3 ) 2 ] + adducts.…”

Recent progress in performance optimization of Cu-SSZ-13 catalyst for selective catalytic reduction of NOx

“…Based on the above discussion, several options have been proposed to improve the SCR activity of Cu-SSZ-13 catalysts. An alternative is to add a second phase (metal oxide) to Cu-SSZ-13 to help improve low-temperature activity except for co-exchanging Cu-SSZ-13 with heteroatoms, in which the improvement in low-temperature SCR performance is limited since the co-cations appear to enhance Cu redox capacity, but decrease Cu loading by competing for the exchange sites (Table 4) (Andana et al, 2021;2022). The metal oxide in Cu-SSZ-13 zeolite-metal oxide composite catalyst carried out the oxidative function to partially convert NO to NO 2 to enable the fast SCR reaction over the catalyst.…”

“…As shown in Figure 4A , the Si/Al ratio is vital for the SCR performance of the Cu-SSZ-13 catalyst ( Kim et al, 2014 ; Martini et al, 2022 ). The high concentration of Si in zeolite ensures the framework’s stability, while the substitution of Si by Al supplies acid sites owing to the charge compensation, which further increases the number of active Cu 2+ ions during the IE process and improves the NH 3 adsorption during SCR reaction ( Gao et al, 2015b ; Fan et al, 2018b ).…”