Dynamic Binuclear Cu^II Sites in the Reduction Half-Cycle of Low-Temperature NH₃–SCR over Cu-CHA Catalysts

ACS Catalysis, Journal Year: 2022, Volume and Issue: 12(9), P. 5263 - 5274

Published: April 18, 2022

As the state-of-the-art catalyst for selective catalytic reduction (SCR) of NOx from lean-burn engines, Cu-exchanged chabazite zeolite (Cu-CHA) has been a spotlight in environmental catalysis because its preeminence DeNOx performance and hydrothermal stability. The microscopic cycling active Cu cations between CuII CuI response to dynamic, macroscopic reaction conditions dominates SCR over Cu-CHA zeolites. In such cycling, are solvated by gas-phase reactants, e.g., NH3, under low-temperature (LT) conditions, conferring peculiar mobility Cu-NH3 complexes making them act as mobilized entities during LT-SCR turnovers. Such motions provide LT-SCR─a typical heterogeneous process─with homogeneous features Cu-CHA, but, differently conventional catalysis, tethered electrostatic interactions conjugate Al centers. These affect distinctly redox chemistry on resulting in, example, involvement two CuI-diamines activating O2 reoxidizing (oxidation half-cycle, OHC). kinetically relevant half-cycle (RHC) that reduces is far less understood particularly within context linked homo- catalysis. Here, we focus LT-RHC summarize observations series recent, dedicated works our group, benchmarking these findings against those closely literature. We thus attempt reconcile rationalize results informed independent, multitechnique evidence further progress mechanistic insights into especially dynamic interconversion mono- binuclear sites.

Language: Английский

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Kinetic Model for the Reduction of Cu^II Sites by NO + NH₃ and Reoxidation of NH₃-Solvated Cu^I Sites by O₂ and NO in Cu-SSZ-13

ACS Catalysis, Journal Year: 2022, Volume and Issue: 12(11), P. 6418 - 6433

Published: May 16, 2022

In this work, a kinetic model is developed for the reduction of CuII sites by NO + NH3 and reoxidation NH3-solvated CuI O2 in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy spatially resolved capillary inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify rate parameters associated with RHC (reduction half-cycle), proposed occur via two distinct pathways involving adsorbed gas-phase NH3. The resulting validated using spatiotemporal N2 covering wide range temperatures (200–450 °C) space velocities (53 000–640 000 h–1). N2O formation observed modeled RHC, quantitative validation under standard selective catalytic (SCR) conditions. Experimentally measured enthalpic entropic changes adsorption on (ZCu(NH3)2) complexes [Kamasamudram, K. Catal. Today 2010, 151(3–4), 212−222], along activation energies estimated computationally intercage diffusion ZCu(NH3)2 [Paolucci, C. Science 2017, 357(6 354), 898−903], incorporated into mean field low-temperature oxidation half-cycle (OHC). Significant release isothermal sites, attributed desorption ligands from dimers (Z2Cu2(NH3)4O2). Reduction these dimeric leads consumption one NO/CuII, contradicting expected stoichiometry. Inclusion global Arrhenius titration Z2Cu2(NH3)4O2 provides accurate representations SCR reduced oxidized catalysts, predicting between 150 250 °C as function hydrothermal aging. Deactivation at high pressures, superoxo amino (ZCu(NH3)3OO*) [Negri, J. Am. Chem. Soc. 2020, 142(37), 15884−15896]. redox presented here foundational description active site SCR, combining recent kinetic, spectroscopic, computational findings mechanism over

Language: Английский

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Selective Catalytic Reduction of NO_x with NH₃ over Cu/SSZ-13: Elucidating Dynamics of Cu Active Sites with In Situ UV–Vis Spectroscopy and DFT Calculations

The Journal of Physical Chemistry C, Journal Year: 2022, Volume and Issue: 126(20), P. 8720 - 8733

Published: May 12, 2022

Cu/SSZ-13 is the current state-of-the-art catalyst for selective catalytic reduction of NOx with NH3 (NH3-SCR) in diesel after-treatment systems. Recent investigations under situ or operando conditions yielded rich molecular level understanding about dynamic transformations Cu sites during reactions. However, aiming at distinction two SCR-active species this catalyst, that is, Z2CuII and ZCuIIOH, are still scarce. Herein, we apply UV–vis spectroscopic studies combined theoretical calculations to investigate SCR-relevant low temperatures. We demonstrate presence O2, isolated ZCuIIOH readily transform into a double O-bridged dicopper Cu–Cu distance 3.37 Å, whereas cannot undergo such transformation. In addition, displays stronger activity than both by NO oxidation bidentate nitrates. Despite these differences, exhibit similar features NH3-SCR conditions. These findings spectroscopy powerful tool be used provide information on mechanism rational design catalysts.

Language: Английский

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Rate Controlling in Low-Temperature Standard NH₃-SCR: Implications from Operando EPR Spectroscopy and Reaction Kinetics

Journal of the American Chemical Society, Journal Year: 2022, Volume and Issue: 144(22), P. 9734 - 9746

Published: May 23, 2022

A series of seven Cu/SSZ-13 catalysts with Si/Al = 6.7 are used to elucidate key rate-controlling factors during low-temperature standard ammonia-selective catalytic reduction (NH3-SCR), via a combination SCR kinetics and operando electron paramagnetic resonance (EPR) spectroscopy. Strong Cu-loading-dependent kinetics, Cu atomic efficiency increasing nearly by an order magnitude, is found when per chabazite cage occupancy for ion increases from ∼0.04 ∼0.3. This due mainly the release intercage transfer constraints that facilitates redox chemistry, as evidenced detailed Arrhenius analysis. Operando EPR spectroscopy studies reveal strong connectivity between Cu-ion dynamics based on which it concluded under steady-state SCR, kinetically most relevant species those highest mobility. Transient binuclear mechanistically species, but their splitting cohabitation indispensable kinetics.

Language: Английский

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Revealing the Formation and Reactivity of Cage-Confined Cu Pairs in Catalytic NO_x Reduction over Cu-SSZ-13 Zeolites by In Situ UV–Vis Spectroscopy and Time-Dependent DFT Calculation

Environmental Science & Technology, Journal Year: 2023, Volume and Issue: 57(33), P. 12465 - 12475

Published: Aug. 9, 2023

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH3-SCR) toxic NOx pollutants from heavy-duty vehicles, remains debate and needs to be clarified further improvement NH3-SCR performance. In this study, we established experimental protocols follow the dynamic redox cycling (i.e., CuII ↔ CuI) Cu sites in during catalysis by situ ultraviolet-visible spectroscopy infrared spectroscopy. Further integrating spectroscopic observations with time-dependent density functional theory calculations allows us identify two cage-confined transient states, namely, O2-bridged dimers μ-η2:η2-peroxodiamino dicopper) proximately paired, chemically nonbonded CuI(NH3)2 sites, confirm pair as precursor dimer. Comparative experiments reveal particularly high reactivity pairs NO-to-N2 at low temperatures. Our study demonstrates direct evidence formation paired CuI under zeolite confinement provides new insights into monomeric-to-dimeric transformation completing cycle over Cu-SSZ-13.

Language: Английский

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