Effect of the Co-cation on Cu Speciation in Cu-Exchanged Mordenite and ZSM-5 Catalysts for the Oxidation of Methane to Methanol

ACS Catalysis, Journal Year: 2021, Volume and Issue: 11(9), P. 4973 - 4987

Published: April 8, 2021

Copper-exchanged zeolites are promising catalysts for the direct methane-to-methanol reaction, but design of improved has been hampered by limited understanding active site structures. Here, we show that identity co-cation (H vs Na) in Cu-MOR and Cu-ZSM-5 significantly affects Cu speciation resulting reactivity cyclic reaction. The combination results with spectroscopy density functional theory (DFT) calculations suggests prevailing structure depends on co-cation. H-form contains a high concentration mono-μ-oxo dicopper(II) species, which selective methanol formation, whereas presence Na appears to shift distribution toward species greater oxygen content (attributed μ-1,2-peroxo species), promote overoxidation methane carbon oxides. Results from DFT indicate preferentially forms 8MR side pockets MOR, is favored 12MR main channels MOR some Al pair configurations. Competition between ion exchange sites displacement into channels, thus affecting catalyst selectivity. These findings suggest choice can be used control transition-metal ion-exchanged zeolites.

Language: Английский

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Thermodynamics Perspective on the Stepwise Conversion of Methane to Methanol over Cu-Exchanged SSZ-13

ACS Catalysis, Journal Year: 2021, Volume and Issue: 11(13), P. 7719 - 7734

Published: June 11, 2021

Transition-metal exchanged zeolites are known to convert methane methanol with high selectivity, in a stepwise process, involving exposure oxidants, followed by methane, and finally water vapor. However, comprehensive theoretical study on the nature of possible active sites their respective changes during this process is still lacking. Here, we use combination density functional theory calculations its generalized-gradient approximation (DFT-GGA) post-DFT methods identify thermodynamically preferred Cu-exchanged zeolite SSZ-13 conversion methanol. We develop thermodynamic model for an extensive set sites, that is, Cu monomers, dimers, trimers, which anchored different ring structures supported series local Al distributions. Subsequently, phase diagrams constructed used favored at each step find O2, hydroxylated dimers—Cu2O2H2 and, depending configuration, Cu2OH—are preferred. Upon site-bound molecules formed. With subsequent increase vapor pressure, preference monoatomic release observed. Furthermore, compare our predicted results experimental measurements published literature close agreement terms coordination number bond distances some considered. expect insights obtained here can be improve understanding reaction mechanism optimize

Language: Английский

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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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Relationships among the Catalytic Performance, Redox Activity, and Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Investigated Using In Situ XAFS and UV–Vis Spectroscopies

ACS Catalysis, Journal Year: 2022, Volume and Issue: 12(4), P. 2454 - 2462

Published: Feb. 3, 2022

Catalytic reaction systems for the direct conversion of methane to methanol have been previously developed using Cu zeolites. Among these materials, Cu-CHA has reported show relatively high catalytic performance during CH4–O2–H2O mixtures, although this activity varies with composition. In present study, four catalysts having different compositions and levels were prepared, redox properties local structures specimens analyzed in situ X-ray absorption fine structure UV–vis diffuse reflectance spectroscopies conjunction a mixture. The relationships between rates materials activities (as reflected turnover frequency (TOF) CH4 oxidation) assessed, analysis showed that reduction rate was highly correlated activity. data also suggested Cu2+ is associated activation C–H bonds CH4, which rate-determining step overall reaction. effects on selectivity CH3OH, TOF value, studied. Those samples proportions ions coordinated six-membered rings (Z2Cu) CHA framework exhibited higher than [CuOH]+ eight-membered (ZCuOH), values former not those latter because slower species. catalyses can be attributed difference Cu2Ox active formed Z2Cu-rich ZCuOH-rich samples.

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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