Reaction pathways of methanol over Pt/zeolite catalysts: effect of different active sites

<div> <div> <a>A crucial consideration for supported heterogeneous catalysts is the non-uniformity of the active sites, particularly for Supported Organometallic Catalysts (SOMCs). Standard spectroscopic techniques, such as X-ray absorption spectroscopy (XAS), reflect the nature of the most populated sites, which are often intrinsically structurally distinct from the most catalytically active sites. With computational models, often only a few representative structures are used to depict catalytic active sites on a surface, even though there are numerous observable factors of surface heterogeneity that contribute to the kinetically favorable active species. A previously reported study on the mechanism of a surface organovanadium(III) catalyst [(SiO)VIII(Mes)(THF)] for styrene hydrogenation yielded two possible mechanisms: heterolytic cleavage and redox cycling. These two mechanistic scenarios are challenging to differentiate experimentally based on the kinetic readouts of the catalyst are identical. To showcase the importance of modeling surface heterogeneity and its effect on catalytic activity, density functional theory (DFT) computational models of a series of potential active sites of [(SiO)VIII(Mes)(THF)] for the reaction pathways are applied in combination with kinetic Monte Carlo (kMC) simulations. Computed results were t then compared to the previously reported experimental kinetic study</a><a>.: 1) DFT free energy reaction pathways indicated the likely active site and pathway for styrene hydrogenation; a heterolytic cleavage pathway requiring a bare tripodal vanadium site. 2) From the kMC simulations, a mixture of the different bond lengths from the support oxygen to the metal center was required to qualitatively describe the experimentally observed kinetic aspects of a supported organovanadium(III) catalyst for olefin hydrogenation. </a>This work underscores the importance of modeling surface heterogeneity in computational catalysis.</div></div>

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New Insight into the In Situ SO2 Poisoning Mechanism over Cu-SSZ-13 for the Selective Catalytic Reduction of NOx with NH3

Catalysts ◽

10.3390/catal10121391 ◽

2020 ◽

Vol 10 (12) ◽

pp. 1391

Author(s):

Yu Qiu ◽

Chi Fan ◽

Changcheng Sun ◽

Hongchang Zhu ◽

Wentian Yi ◽

...

Keyword(s):

Selective Catalytic Reduction ◽

Active Sites ◽

Catalytic Reduction ◽

Zeolite Catalysts ◽

So2 Poisoning ◽

Negative Effect ◽

Temperature Programmed ◽

Diffuse Reflectance Infrared ◽

Sulfate Species

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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Nanocarbon-Based Catalytic Ozonation for Aqueous Oxidation: Engineering Defects for Active Sites and Tunable Reaction Pathways

ACS Catalysis ◽

10.1021/acscatal.0c04232 ◽

2020 ◽

Vol 10 (22) ◽

pp. 13383-13414

Author(s):

Yuxian Wang ◽

Xiaoguang Duan ◽

Yongbing Xie ◽

Hongqi Sun ◽

Shaobin Wang

Keyword(s):

Active Sites ◽

Catalytic Ozonation ◽

Reaction Pathways ◽

Aqueous Oxidation

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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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Computational Investigation of the Role of Active Site Heterogeneity for a Supported Organovanadium(III) Hydrogenation Catalyst

10.26434/chemrxiv.13694446.v2 ◽

2021 ◽

Author(s):

Prajay Patel ◽

Robert Wells ◽

David Kaphan ◽

Massimiliano Delferro ◽

Rex T. Skodje ◽

...

Keyword(s):

Active Site ◽

Active Sites ◽

Density Functional ◽

Computational Models ◽

Surface Heterogeneity ◽

Reaction Pathways ◽

Spectroscopic Techniques ◽

Organometallic Catalysts ◽

Heterolytic Cleavage ◽

Styrene Hydrogenation

<div> <div> <a>A crucial consideration for supported heterogeneous catalysts is the non-uniformity of the active sites, particularly for Supported Organometallic Catalysts (SOMCs). Standard spectroscopic techniques, such as X-ray absorption spectroscopy (XAS), reflect the nature of the most populated sites, which are often intrinsically structurally distinct from the most catalytically active sites. With computational models, often only a few representative structures are used to depict catalytic active sites on a surface, even though there are numerous observable factors of surface heterogeneity that contribute to the kinetically favorable active species. A previously reported study on the mechanism of a surface organovanadium(III) catalyst [(SiO)VIII(Mes)(THF)] for styrene hydrogenation yielded two possible mechanisms: heterolytic cleavage and redox cycling. These two mechanistic scenarios are challenging to differentiate experimentally based on the kinetic readouts of the catalyst are identical. To showcase the importance of modeling surface heterogeneity and its effect on catalytic activity, density functional theory (DFT) computational models of a series of potential active sites of [(SiO)VIII(Mes)(THF)] for the reaction pathways are applied in combination with kinetic Monte Carlo (kMC) simulations. Computed results were t then compared to the previously reported experimental kinetic study</a><a>.: 1) DFT free energy reaction pathways indicated the likely active site and pathway for styrene hydrogenation; a heterolytic cleavage pathway requiring a bare tripodal vanadium site. 2) From the kMC simulations, a mixture of the different bond lengths from the support oxygen to the metal center was required to qualitatively describe the experimentally observed kinetic aspects of a supported organovanadium(III) catalyst for olefin hydrogenation. </a>This work underscores the importance of modeling surface heterogeneity in computational catalysis.</div></div>

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