Perceptions on the treatment of apparent isotope effects during the analyses of reaction rate and mechanism

Gao, Xin; Yu, Xi-Yang; Chang, Chun‐Ran

doi:10.1039/d2cp00825d

Cited by 14 publications

(9 citation statements)

References 54 publications

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“…Ähnliche Ergebnisse erhielten wir auch in dynamischen Messungen mittels der steady-state isotopic transient kinetic analysis (SSITKA) Methode, in denen die CO 2 Bildung mittels IR Spektrome-trie bestimmt wurde (siehe Abbildung S4 und S5, Hintergrundinformationen). Aufgrund der relativ geringen Unterschiede in den Molekulargewichten sind diese Werte charakteristisch für einen primären inversen KIE, [18,19] was darauf hindeutet, dass der ratenlimitierende Schritt die Bildung oder den Bruch einer Bindung zu dem entsprechenden O-Atom entweder im ratenbestimmenden Schritt oder in einem nachfolgendenden, langsamen und produktbestimmenden Schritt beinhaltet. Höchstwahrscheinlich bezieht sich dies auf die Aktivierung der starken O=O Bindung (O 2 Dissoziation) oder die Abreaktion einer (aktivierten) O-Spezies durch Reaktion mit CO. Dabei nehmen wir an, dass alle vorhandenen Cu Atome aktiv sind und sich nicht gegenseitig beeinflussen, was aufgrund der offenen Struktur und der gleichen Entfernung zwischen benachbarten Cu Atomen plausibel erscheint.…”

Section: Ergebnisse Und Diskussionunclassified

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

et al. 2023

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In der Heterogenen Katalyse ist die Aufklärung des Reaktionsmechanismus essentiell für die systematische Optimierung der Katalysatoren, aber aufgrund der unbekannten Natur der aktiven Zentren herausfordernd. Für einen molekular‐definierten, auf einer metallorganischen Gerüstverbindung (UiO‐66) geträgerten Kupfer‐Einzelatom‐Katalysators konnten wir ein detailliertes mechanistisches Bild der CO Oxidation erarbeiten. Basierend auf einer Kombination von in situ/operando Spektroskopien, kinetischen Messungen inklusive kinetischer Isotopeneffekte sowie Dichtefunktional‐Theorie basierten Rechnungen konnten wir den aktiven Platz, Reaktionsintermediate und Übergangszustände des dominierenden Reaktionszyklus sowie Veränderungen in den Cu Oxidations‐ und Spinzuständen während der Reaktion identifizieren. Die Reaktion verläuft über die reaktive Dissoziation von adsorbiertem O2, durch Reaktion von O2,ad mit koadsorbiertem COad, als ratenlimitierendem Schritt. Das verbleibende O Atom, das das Cu Zentrum mit einem benachbarten Zr4+ Ion verbindet, wird in einem zweiten aktivierten Schritt abreagiert.

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Section: Ergebnisse Und Diskussionunclassified

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

et al. 2023

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“…To show if the HOOH formation is a rate-limiting step along the path to the final product PO, we have carried out the H 2 /D 2 exchange experiment over the 0.1 wt % Au/TS-1-B (100) catalyst. The observed H 2 /D 2 KIE ( k H / k D ) for PO formation (Figure S12) is approximately determined to be 1.4, which is smaller than the typical primary KIE (the normal value for H 2 vs D 2 is 2–5). − Therefore, the relatively small KIE originating from the zero-point energy difference between isotopic isomers indicates that the HOOH formation on Au sites including H–H bond cleavage (step 1.3) may be not the sole rate-determining step (RDS) but is the kinetically relevant step. , In addition, the surface HOOH species can migrate via gas phase to the Ti sites in proximity to Au or to the remote Ti sites (step 1.6 and 2.1). Both the HOOH desorption from Au sites and adsorption on Ti sites have relatively low activation energies, which cannot be rate-limiting.…”

Section: Resultsmentioning

confidence: 93%

Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H₂ and O₂ on Au–Ti Catalysts

Zhang

Tang

et al. 2023

ACS Catal.

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Au−Ti proximity is thought to be responsible for propylene epoxidation by H 2 and O 2 to PO formation (HOPO). However, we find that the Ti sites intimate with Au accounts for small proportion relative to Ti sites remote from Au, based on the semi-quantification of the number of Au nanoparticles and Ti sites on the uncalcined TS-1-supported Au nanoparticle (Au/TS-1-B) catalysts. Whether those remote Ti sites are functioning in PO generation remains unclear. Herein, the abundant remote Ti sites are proposed to catalyze PO formation by the tandem mechanism with the help from the migration of active oxidant species. We isolated the Au sites and Ti sites by physically mixing uncalcined silicalite-1-supported Au (Au/S-1-B) and TS-1-B as a tandem catalyst with much less Au−Ti proximity sites. Comparable performance for HOPO of the mixed catalyst was observed relative to the Au/TS-1-B catalyst, suggesting that the tandem mechanism is also responsible for PO formation. The H 2 /D 2 exchange experiment, in situ diffuse reflectance infrared Fourier transform spectroscopy, and kinetic modeling were combined to further elucidate the mechanism. Only the kinetic model including both the tandem mechanism on remote Ti sites and the simultaneous mechanism on the Au−Ti proximity can well describe the experimental results, and the tandem mechanisms have a larger contribution to PO formation than the simultaneous mechanism on the Au/TS-1-B catalyst with Au nanoparticles on the external surface. However, for the Au/TS-1 catalyst with intraporous tiny Au clusters, the PO formation proceeds by the simultaneous mechanism. This work unveils the function of remote Ti sites in PO formation. Moreover, the kinetic behaviors are connected with the catalyst structure (micropore volume and Au location), which can provide guidance for enhancing the catalytic performance by designing efficient tandem catalysts for the HOPO process.

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“…The use of D-methanol as a reactant led to higher CT 0 values over all of the catalysts (Figure a). This can be ascribed to the D–H kinetic isotope effect, in which the higher energies of the C–D/O–D bonds with respect to those of the C–H/O–H bonds are anticipated to decrease the rates of all HP reactions, including those leading to coke formation. The H- and D-methanol feeds over SSZ-13 and ZSM-35 demonstrated similar product distribution.…”

Section: Resultsmentioning

confidence: 99%

How Micropore Topology Influences the Structure and Location of Coke in Zeolite Catalysts

Rzepka,

Sheptyakov,

Wang

et al. 2024

ACS Catal.

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Zeolite catalysts exhibit microporous structures, akin to the pockets in naturally occurring enzyme catalysts, which enable the confinement of reaction intermediates, thus facilitating chemical transformations. Nonetheless, the micropores also influence the formation of coke species, which is the main source of catalytic activity loss. Unveiling the relationships between the micropore topology and the internal structure and location of deactivating coke compounds is of high relevance for comprehending the deactivation mechanisms. In this study, we used an approach exploiting powder neutron diffraction to assess the location of coke and determine the dominating structures in the topologically distinct ZSM-5 (MFI topology), ZSM-35 (FER), and SSZ-13 (CHA) zeolite catalysts deactivated in industrially relevant methanol-to-hydrocarbon (MTH) conversion. In ZSM-5 and ZSM-35 catalysts, coke resides along the straight 10-membered ring (MR) channels and exhibits the highest concentration in their intersecting regions with sinusoidal 10 MR and straight 8 MR pores, respectively. In the SSZ-13 catalyst, coke is not only located in cages but also protrudes through their 8 MR windows, suggesting the interconnectivity of coke molecules between the large cavities. Notably, the coke-associated signals in the ZSM-5 and ZSM-35 catalysts show a strong planar arrangement that can be fitted by polycyclic and monocyclic arene structures, respectively. These averaged coke structures are consistent with the composition of coke assessed by gas chromatography−mass spectrometry, 13 C and two-dimensional 1 H double-quantum single-quantum magic-angle spinning nuclear magnetic resonance, and operando diffuse reflectance ultraviolet−visible spectroscopic analysis. The findings evidence that the pore topology directs the confinement and structure of coke, wherein the largest void zones of the micropore space are the most susceptible to coking.

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Perceptions on the treatment of apparent isotope effects during the analyses of reaction rate and mechanism

Abstract: Isotope substitution, a compelling tool of physical chemistry, has been broadly applied in the research field of heterogeneous catalysis. In general, upon the differences in mass-related atomic vibrational frequencies and...

Cited by 14 publications

References 54 publications

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H₂ and O₂ on Au–Ti Catalysts

How Micropore Topology Influences the Structure and Location of Coke in Zeolite Catalysts

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Perceptions on the treatment of apparent isotope effects during the analyses of reaction rate and mechanism

Abstract: Isotope substitution, a compelling tool of physical chemistry, has been broadly applied in the research field of heterogeneous catalysis. In general, upon the differences in mass-related atomic vibrational frequencies and...

Cited by 14 publications

References 54 publications

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

Mechanismus der CO Oxidation an einem molekular‐definierten Kupfer‐Einzelatom‐Katalysator auf einer metallorganischen Gerüstverbindung

Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H2 and O2 on Au–Ti Catalysts

How Micropore Topology Influences the Structure and Location of Coke in Zeolite Catalysts

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Kinetic Insights into the Tandem and Simultaneous Mechanisms of Propylene Epoxidation by H₂ and O₂ on Au–Ti Catalysts