Design of active sites in zeolite catalysts using modern semiempirical methods: The case of mordenite

To reveal the nature of SO2 poisoning over Cu-SSZ-13 catalyst under actual exhaust conditions, the catalyst was pretreated at 200 and 500 °C in a flow containing NH3, NO, O2, SO2, and H2O. Brunner−Emmet−Teller (BET), X-ray diffraction(XRD), thermo gravimetric analyzer (TGA), ultraviolet Raman spectroscopy (UV Raman), temperature-programmed reduction with H2 (H2-TPR), temperature-programmed desorption of NO+O2 (NO+O2-TPD), NH3-TPD, in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and an activity test were utilized to monitor the changes of Cu-SSZ-13 before and after in situ SO2 poisoning. According to the characterization results, the types and generated amount of sulfated species were directly related to poisoning temperature. Three sulfate species, including (NH4)2SO4, CuSO4, and Al2(SO4)3, were found to form on CZ-S-200, while only the latter two sulfate species were observed over CZ-S-500. Furthermore, SO2 poisoning had a negative effect on low-temperature selective catalytic reduction (SCR) activity, which was mainly due to the sulfation of active sites, including Z2Cu, ZCuOH, and Si-O(H)-Al. In contrast, SO2 poisoning had a positive effect on high-temperature SCR activity, owing to the inhibition of the NH3 oxidation reaction. The above findings may be a useful guideline to design excellent SO2-resistant Cu-based zeolite catalysts.

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An operando DRIFTS study of the active sites and the active intermediates of the NO-SCR reaction by methane over In,H- and In,Pd,H-zeolite catalysts

Applied Catalysis B Environmental ◽

10.1016/j.apcatb.2010.07.023 ◽

2010 ◽

Vol 100 (1-2) ◽

pp. 133-142 ◽

Cited By ~ 32

Author(s):

Ferenc Lónyi ◽

Hanna E. Solt ◽

József Valyon ◽

Hernán Decolatti ◽

Laura B. Gutierrez ◽

...

Keyword(s):

Active Sites ◽

Zeolite Catalysts ◽

Active Intermediates ◽

No Scr

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Identification of the Mechanism of NO Reduction with Ammonia (SCR) on Zeolite Catalysts

10.26434/chemrxiv.13134770 ◽

2020 ◽

Author(s):

Konstantin Khivantsev ◽

Ja-Hun Kwak ◽

Nicholas R. Jaegers ◽

Miroslaw A. Derewinski ◽

János Szanyi

Keyword(s):

Active Sites ◽

No Reduction ◽

Catalytic Reduction ◽

Molecular Nitrogen ◽

Acid Sites ◽

Zeolite Catalysts ◽

Catalytic Cycle ◽

Rate Limiting Step ◽

Catalytically Active

Cu/Zeolites catalyze selective catalytic reduction of nitric oxide with ammonia. Although the progress has been made in understanding the rate-limiting step of reaction which is reoxidation of Cu(I)(NH3)2 with oxygen to restore the catalytically active Cu(II) site, the exact NO reduction chemistry remained unknown. Herein, we show that nitrosyl ions NO+ in the zeolitic micropores are the true active sites for NO reduction. They react with ammonia even at below/room temperature producing molecular nitrogen through the intermediacy of N2H+ cation. Isotopic experiments confirm our findings. No copper is needed for this reaction to occur. However, when NO+ reacts, “freed up” Bronsted acid site gets occupied by NH3 to form NH4+ – and so the catalytic cycle stops because NO+ does not form on NH4-Zeolites due to their acid sites being already occupied. Therefore, the role of Cu(II) in Cu/Zeolite catalysts is to produce NO+ by the reaction: Cu(II) + NO à Cu(I) + NO+ which we further confirm spectroscopically. The NO+ then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(I) gets re-oxidized the catalytic cycle then can continue. Thus, our findings are critical for understanding complete SCR mechanism.

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