Transient Techniques: Temporal Analysis of Products and Steady State Isotopic Transient Kinetic Analysis

Efstathiou, Angelos M.; Gleaves, John; Yablonsky, Gregory S.

doi:10.1002/9783527645329.ch22

Cited by 12 publications

(42 citation statements)

References 94 publications

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“…The Ar-reference responses are shown as dotted lines with the same color coding. In order to determine the residence time of oxygen, τ O , the normalized experimental data were fitted to a standard F O ( t ) = 1 – exp(− t /τ O ) decay. , The results show that with the rising temperature the average residence time of oxygen is significantly increased in the sequence τ O (400°C) = 3 ± 1 s, τ O (500°C) = 6 ± 1 s, τ O (600°C) = 17 ± 2 s (see also inset in Figure ).…”

Section: Resultsmentioning

confidence: 66%

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

et al. 2017

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Involvement of suprafacial and intrafacial oxygen species in catalytic combustion of methane over the (100) faceted cobalt spinel was systematically examined as a function of temperature and CH4 conversion (X CH4 ). The clear-cut Co3O4 nanocubes of uniform size were synthesized using a hydrothermal method and characterized with XRD, RS, HR-TEM, XRF, TPSR (CH4/16/18O2), and SSITKA (CH4/16/18O2) techniques. The experimental results were corroborated by first-principles thermodynamic and DFT+U molecular modeling, providing a rational framework for a detailed understanding of the origin of a different redox comportment of the catalyst with the varying temperature and its mechanistic implications. Three temperature/conversion stages of the methane oxidation reaction were distinguished, depending on involvement of the adsorbed or lattice oxygen and the redox state of the catalyst. A stoichiometric (100) surface region (300 °C < T < 450 °C, X CH4 < 25%) is featured by the dominant suprafacial (Langmuir–Hinshelwood) mechanism of methane oxidation. A region of slightly defected surface (450 °C < T < 650 °C, 25% < X CH4 < 80%), in which oxygen vacancies produced upon CO2 and H2O release are virtually refilled by dioxygen, is characterized by coexistence of the suprafacial (Langmuir–Hinshelwood) and intrafacial (Mars–van Krevelen) mechanistic steps. In a nonstoichiometric surface region (T > 650 °C, X CH4 > 80%), the oxygen vacancies are only partially refilled, the catalyst is significantly reduced, and methane is combusted according to the Mars–van Krevelen scheme. Molecular modeling revealed that the suprafacial Co–Oads adoxygen species are more active (ΔE a = 0.83 eV) than the intrafacial Co–Osurf surface sites (ΔE a = 1.11 eV) in the CH4 oxidation. The (100) surface state diagrams for the three distinguished conversion regions were constructed to elucidate the catalyst thermodynamic behavior under those conditions. It was shown that the activity of cobalt spinel is maintained by redox autotuning of the catalyst and dynamic adjustment of uneven participation of the suprafacial and intrafacial oxygen species in methane oxidation to the actual reaction conditions. These factors have important structural and mechanistic consequences for the catalytic CH4 combustion on cobalt spinel and related systems, controlling not only the sustainable versus the stoichiometric turnovers but also for the prevalence or coexistence of the Langmuir–Hinshelwood and the Mars–van Krevelen mechanisms with the reaction progress.

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Section: Resultsmentioning

confidence: 66%

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

et al. 2017

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“…The effluent wet gas stream from the reactor was passed through a condenser (Peltier system of low volume), the exit of which (dry gas) was directed to the mass spectrometer for on line monitoring of the normal and isotope-containing (D, 13 C) reactants and products (e.g., H 2 , HD, D 2 , 12 CO, 13 CO, 12 CO 2 , and 13 CO 2 ) . More details on the SSITKA–MS experiments and the microreactor used were previously reported. ,,,,, It was estimated that the response time, τ (s), of the system used in SSITKA–MS studies (switching valve → microreactor → condenser → mass spectrometer) was about 5 s on the basis of the transient response curve of Ar to the switch He → 1 vol % Ar/He. , The mass of the catalyst was adjusted in every SSITKA–MS experiment so as to keep the CO conversion below 15%. The total mass of the catalyst bed was 0.5 g (catalyst diluted with SiO 2 ).…”

Section: Methodsmentioning

confidence: 99%

“…Fundamental understanding of the WGS reaction at the molecular level is certainly an important tool toward the design of suitably functional catalytic materials for activity, selectivity, and stability optimization under industrial WGS reaction conditions. To achieve this goal, mechanistic studies of the WGS reaction employing in situ coupled spectroscopic and kinetic measurements under reaction conditions (operando studies) become important. , …”

Section: Introductionmentioning

confidence: 99%

Mechanistic Studies of the Water–Gas Shift Reaction over Pt/Ce_xZr_1–xO₂ Catalysts: The Effect of Pt Particle Size and Zr Dopant

2012

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A series of y wt % Pt/Ce x Zr1–x O2 catalysts (y = 0.1, 0.5, and 1.0; x = 0.3, 0.5, and 0.7) were synthesized and characterized to investigate the effect of CeO2 doping with Zr4+ and of Pt particle size (Pt/Ce0.5Zr0.5O2) on important mechanistic and kinetic aspects of the water–gas shift (WGS) reaction. These included the concentration (μmol·g–1 or θ (surface coverage based on Pts)) and chemical structure of active reaction intermediates present in the “carbon path” and “hydrogen path” of the WGS reaction in the 200–300 °C range and the prevailing mechanism among “redox” and “associative formate” largely considered in the literature. Toward this goal, steady-state isotopic transient kinetic analysis coupled with in situ DRIFTS and mass spectrometry experiments were performed for the first time using D2O and 13CO isotopic gases. A novel transient isotopic experiment allowed quantification of the initial transient rate of reaction of adsorbed formate (HCOO−) with water and that of adsorbed CO with water under steady-state WGS reaction conditions. On the basis of these results, it was concluded that formate should not be considered as an important intermediate. It was found that on Pt/Ce x Zr1–x O2 catalysts, the WGS reaction mechanism switches from “redox” to a combination of “redox” and “associative formate with −OH group regeneration” mechanisms by increasing the reaction temperature from 200 to 300 °C. The superior WGS activity exhibited by Pt/Ce x Zr1–x O2 (x = 0.3, 0.5, and 0.7) catalysts in comparison with Pt/CeO2 was explained by the fact that the site reactivity of Pt across the metal–support interface was increased as a consequence of the introduction of Zr4+ into the ceria lattice. The concentration of active reaction intermediates was found to strongly depend on reaction temperature, support composition (Ce/Zr ratio), and Pt particle size, parameters that all determine the shape of the light-off CO-conversion curve.

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“…Not all these techniques are used during catalyst characterization due to the specificity and nature of sample environment of each, but usually, a number of the techniques are combined to gather as much data to support the characterization. − However, most of these characterization techniques are typically not conducted under true reaction conditions, and some of the techniques are even the subject of debate concerning the information obtained and whether it can be transferred to real life catalysts under real working conditions. , …”

Section: Traditional Approaches To Catalyst Kinetic and Physical Char...mentioning

confidence: 99%

Evolution and Enabling Capabilities of Spatially Resolved Techniques for the Characterization of Heterogeneously Catalyzed Reactions

et al. 2016

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The development and optimization of catalysts and catalytic processes requires knowledge of reaction kinetics and mechanisms. In traditional catalyst kinetic characterization, the gas composition is known at the inlet, and the exit flow is measured to determine changes in concentration. As such, the progression of the chemistry within the catalyst is not known. Technological advances in electromagnetic and physical probes have made visualizing the evolution of the chemistry within catalyst samples a reality, as part of a methodology commonly known as spatial resolution. Herein, we discuss and evaluate the development of spatially resolved techniques, including the evolutions and achievements of this growing area of catalytic research. The impact of such techniques is discussed in terms of the invasiveness of physical probes on catalytic systems, as well as how experimentally obtained spatial profiles can be used in conjunction with kinetic modeling. Furthermore, some aims and aspirations for further evolution of spatially resolved techniques are considered.

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Transient Techniques: Temporal Analysis of Products and Steady State Isotopic Transient Kinetic Analysis

Cited by 12 publications

References 94 publications

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Mechanistic Studies of the Water–Gas Shift Reaction over Pt/Ce_xZr_1–xO₂ Catalysts: The Effect of Pt Particle Size and Zr Dopant

Evolution and Enabling Capabilities of Spatially Resolved Techniques for the Characterization of Heterogeneously Catalyzed Reactions

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Transient Techniques: Temporal Analysis of Products and Steady State Isotopic Transient Kinetic Analysis

Cited by 12 publications

References 94 publications

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Total Oxidation of Lean Methane over Cobalt Spinel Nanocubes Controlled by the Self-Adjusted Redox State of the Catalyst: Experimental and Theoretical Account for Interplay between the Langmuir–Hinshelwood and Mars–Van Krevelen Mechanisms

Mechanistic Studies of the Water–Gas Shift Reaction over Pt/CexZr1–xO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant

Evolution and Enabling Capabilities of Spatially Resolved Techniques for the Characterization of Heterogeneously Catalyzed Reactions

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Product

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About

Mechanistic Studies of the Water–Gas Shift Reaction over Pt/Ce_xZr_1–xO₂ Catalysts: The Effect of Pt Particle Size and Zr Dopant