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Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.


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Journal ArticleDOI
TL;DR: In this paper, a transient kinetic model was developed for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst.
Abstract: A transient kinetic model was developed for the CO oxidation by O2 over a Pt/Rh/CeO2/γ-Al2O3 three-way catalyst. The experiments which were modelled consisted of periodically switching between a feed stream containing 0.5 mol% CO in helium and a feed stream containing 0.5 mol% O2 in helium, with a frequency from 0.1 to 0.25 Hz, in the temperature range 393–433 K. These temperatures are representative for cold start conditions. The transient experiments yield information about the reaction mechanism. A transient kinetic model based on elementary reaction steps was developed which describes the experimental data in the above mentioned range of experimental conditions adequately. The kinetic model consists of two monofunctional and one bifunctional contribution. The first monofunctional reaction path comprises competitive adsorption of CO and O2 on the noble metal surface followed by a surface reaction. The second monofunctional reaction path consists of CO adsorption on an oxygen atom adsorbed on the noble metal surface, followed by a reaction to CO2. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal surface and oxygen from ceria at the noble metal/ceria interface. Also, reversible adsorption of carbon dioxide on the support is taken into account. The kinetic parameters, i.e. preexponential factors and activation energies for the different elementary reaction steps, and the oxygen storage capacity were estimated using multi-response non-linear regression analysis of the oxygen, carbon monoxide and carbon dioxide outlet concentrations.

112 citations

Journal ArticleDOI
TL;DR: Significantly, experimental and computational studies question the predictability of primary EIEs in these systems based on the notion that deuterium prefers to occupy the highest frequency oscillator, and the applicability of these rules to the interactions of H-H and C-H bonds with a transition metal center is evaluated.
Abstract: Deuterium kinetic isotope effects (KIEs) serve as versatile tools to infer details about reaction mechanisms and the nature of transition states, while equilibrium isotope effects (EIEs) associated with the site preferences of hydrogen and deuterium enable researchers to study aspects of molecular structure. Researchers typically interpret primary deuterium isotope effects based on two simple guidelines: (i) the KIE for an elementary reaction is normal (kH/kD > 1) and (ii) the EIE is dictated by deuterium preferring to be located in the site corresponding to the highest frequency oscillator. In this Account, we evaluate the applicability of these rules to the interactions of H−H and C−H bonds with a transition metal center. Significantly, experimental and computational studies question the predictability of primary EIEs in these systems based on the notion that deuterium prefers to occupy the highest frequency oscillator. In particular, the EIEs for (i) formation of σ-complexes by coordination of H−H and ...

111 citations

Journal ArticleDOI
TL;DR: In this paper, a simple method for calculating rate constants of addition and recombination reactions, based on unimolecular quantum-RRK theory, was proposed, which can be used to predict rate constants and reaction branching with remarkable accuracy.
Abstract: Bimolecular QRRK (Quantum Rice-Ramsperger-Kassel) analysis is a simple method for calculating rate constants of addition and recombination reactions, based on unimolecular quantum-RRK theory. Input parameters are readily derived, and rate constants and reaction branching can be predicted with remarkable accuracy. Such predictive power makes the method especially useful in developing mechanisms of elementary reactions. Furthermore, from the bimolecular QRRK equations, limiting forms of the rate constants in the limits of low and high pressure are developed. Addition/stabilization is pressure-dependent at low pressure but pressure-independent at high pressure, as is conventionally understood for simple decomposition, its reverse. In distinct contrast, addition with chemically activated decomposition has the opposite behavior: pressure independence at low pressure and pressure dependence [as (pressure)−1] at high pressure. The method is tested against data and illustrated by calculations for O + CO → CO2; for H + O2 → HO2 or O + OH; for H + C2H4 → C2H5 or C2H3 + H2; and for H + C2H3 → C2H4 or H2 + C2H2.

110 citations

Journal ArticleDOI
TL;DR: In this article, a new reaction mechanism of diamond (100) growth from methyl radicals is proposed, which offers a plausible explanation for the experimentally observed fast and smooth growth of diamond surfaces.
Abstract: Chemical reactions of methyl radicals on (100) diamond surfaces have been investigated theoretically. Quantum-mechanical calculations at the PM3 semiempirical level were performed on a series of small- and large-size clusters to explore possible reaction steps responsible for diamond growth at conditions typical of chemical vapor deposition. Among a variety of possible chemisorption sites considered, surface dimer radicals not only were the most favorable on kinetic grounds but appeared to be the only type capable of sustaining the subsequent incorporation of adsorbed methyl groups into the diamond lattice. Surface migration of H atoms, radical sites, and chemisorbed CH[sub 2] groups proved to be important for diamond growth. A new reaction mechanism of diamond (100) growth from methyl radicals is proposed which offers a plausible explanation for the experimentally observed fast and smooth growth of diamond surfaces. The mechanism consists of two principal features, conversion of dimer sites into bridge sites and surface migration of bridge sites toward continuous bridge chains; it does not require any particular order of dimer formation but establishes the governing role of surface diffusion. 67 refs., 9 figs., 3 tabs.

110 citations

Journal ArticleDOI
TL;DR: The ring polymer molecular dynamics method is used to study the Azzouz-Borgis model for proton transfer between phenol and trimethylamine in liquid methyl chloride and results are discussed in light of the wide body of earlier theoretical work on the model and the considerable range of previously reported values for its proton and deuteron transfer rate coefficients.
Abstract: We have used the ring polymer molecular dynamics method to study the Azzouz–Borgis model for proton transfer between phenol (AH) and trimethylamine (B) in liquid methyl chloride. When the A–H distance is used as the reaction coordinate, the ring polymer trajectories are found to exhibit multiple recrossings of the transition state dividing surface and to give a rate coefficient that is smaller than the quantum transition state theory value by an order of magnitude. This is to be expected on kinematic grounds for a heavy-light-heavy reaction when the light atom transfer coordinate is used as the reaction coordinate, and it clearly precludes the use of transition state theory with this reaction coordinate. As has been shown previously for this problem, a solvent polarization coordinate defined in terms of the expectation value of the proton transfer distance in the ground adiabatic quantum state provides a better reaction coordinate with less recrossing. These results are discussed in light of the wide body of earlier theoretical work on the Azzouz–Borgis model and the considerable range of previously reported values for its proton and deuteron transfer rate coefficients.

109 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202321
202229
202185
202088
201971
201871