Pressure dependent mechanism for H/O/C(1) chemistry (original) (raw)
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Theoretical studies on the C2H+O2 reaction: mechanism for HCO+CO, HCCO+O and CH+CO2 formation
Chemical Physics Letters, 1998
The reaction of the ethynyl radical with molecular oxygen has been examined using density functional theory. Two major reaction routes are open to the chemically activated HCCOO adduct 1: dissociation to HCCOq O 16 and formation of the thermodynamically most stable products HCO q CO 12: HCCOO 1 ™ dioxirenyl 2 ™ oxyrenyloxy 3 ™ oxo-ketene 5 Ž. Ž. Ž. ™ HCO q CO 12 ™ H q 2CO 15. The CCSD T r6-311qqG d,p rr B3LYPr6-311qqG d,p energies of the respective rate controlling transition states, 1 r r r r r16 and 1 r r r r r2, indicate that the route leading to H q CO q CO should w x dominate. Several other C ,H,O isomers and other, minor pathways have also been characterised. The present study 2 2 reveals this reaction to be a capture-limited association-elimination reaction with a high and pressure-independent rate coefficient.
Journal of Physical Chemistry A, 2015
The reaction C 2 H 5 + O 2 (+ M) → C 2 H 5 O 2 (+ M) was studied at 298 K at pressures of the bath gas M = Ar between 100 and 1000 bar. The transition from the falloff curve of an energy transfer mechanism to a high pressure range with contributions from the radical complex mechanism was observed. Further experiments were done between 188 and 298 K in the bath gas M = He at pressures in the range 0.7−2.0 Torr. The available data are analyzed in terms of unimolecular rate theory. An improved analytical representation of the temperature and pressure dependence of the rate constant is given for conditions where the chemical activation process C 2 H 5 + O 2 (+ M) → C 2 H 4 + HO 2 (+ M) is only of minor importance.
ACS Omega, 2020
The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius' equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol −1 , while similar fittings provided, respectively, 1.96 and 3.22 kcal mol −1 , considering now forward rate constants from a temperature range of 298−373 K. This last activation energy determination (3.22 kcal mol −1) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol −1 , also obtained from traditional fittings in the range 298−373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol −1). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 10 4 atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures.
Ab initio chemical kinetics for the HCCO + OH reaction
The mechanism for the reaction of HCCO and OH has been investigated at different high-levels of theory. The reaction was found to occur on singlet and triplet potential energy surfaces with multiple accessible paths. Rate constants predicted by variational RRKM/ME calculations show that the reaction on both surfaces occurs primarily by barrierless OH attack at both C atoms producing excited intermediates which fragment to produce predominantly CO and 1,3 HCOH with k S = 3.12 Â 10 À8 T À0.59 exp[À73.0/T] and k T = 6.29 Â 10 À11 T 0.13 exp[108/T] cm 3 molecule À1 s À1 at T = 300–2000 K, independent of pressure at P < 76 000 Torr.
Third European Combustion …
Emerging combustion technologies, operating at moderate and controlled temperatures, such as solid-oxide fuel cells (SOFC) and flameless oxidation, are particularly attractive due to their potential for improved efficiency and reduced emissions. Additional gains in efficiency are also expected by operating in elevated pressures. A thorough understanding of combustion chemistry under realistic operating conditions is a prerequisite for the accurate quantification of heat release and pollutant emissions. In the present work, a comprehensive detailed kinetic mechanism has been developed for the oxidation of C 1-C 2 hydrocarbons under high pressure and intermediate temperature conditions. The mechanism has been extensively validated against experimental data from perfectly stirred reactors in the temperature range of 800-1400 K, at pressures up to 10 atm and for a wide range of fuels and stoichiometries. Extensive reaction path and sensitivity analyses have been performed in order to analyze major reactions pathways and to identify areas of the mechanism where further development is necessary.
C−H bond activation at lattice O atoms on oxides mediates some of the most important chemical transformations of small organic molecules. The relations between molecular and catalyst properties and C−H activation energies are discerned in this study for the diverse C−H bonds prevalent in C 1 −C 4 hydrocarbons and oxygenates using lattice O atoms with a broad range of H atom abstraction properties. These activation energies determine, in turn, attainable selectivities and yields of desired oxidation products, which differ from reactants in their C−H bond strength. Brønsted-Evans−Polanyi (BEP) linear scaling relations predict that C− H activation energies depend solely and linearly on the C−H bond dissociation energies (BDE) in molecules and on the H-atom addition energies (HAE) of the lattice oxygen abstractors. These relations omit critical interactions between organic radicals and surface OH groups that form at transition states that mediate the H atom transfer, which depend on both molecular and catalyst properties; they also neglect deviations from linear relations caused by the lateness of transition states. Thus, HAE and BDE values, properties that are specific to a catalyst and a molecule in isolation, represent incomplete descriptors of reactivity and selectivity in oxidation catalysis. These effects are included here through crossing potential formalisms that account for the lateness in transition states in estimates of activation energies from HAE and BDE and by estimates of molecule-dependent but catalyst-independent parameters that account for diradical interactions that differ markedly for allylic and nonallylic C−H bonds. The systematic ensemble-averaging of activation energies for all C−H bonds in a given molecule show how strong abstractors and high temperatures decrease an otherwise ubiquitous preference for activating the weakest C−H bonds in molecules, thus allowing higher yields of products with C−H bonds weaker than in reactants than predicted from linear scaling relations based on molecule and abstractor properties. Such conclusions contradict the prevailing guidance to improve such yields by softer oxidants and lower temperatures, a self-contradictory strategy, given the lower reactivity of such weaker H-abstractors. The diradical-type interactions, not previously considered as essential reactivity descriptors in catalytic oxidations, may expand the narrow yield limits imposed by linear free energy relations by guiding the design of solids with surfaces that preferentially destabilize allylic radicals relative to those formed from saturated reactants at C−H activation transition states.
Atmospheric Chemistry and Physics, 2018
Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for the reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving estimation of atmospheric lifetimes or oxidation rates for VOCs. A structure-activity relationship (SAR) method is presented for the reactions of OH with aromatic organic compounds, with the reactions of aliphatic organic compounds considered in the preceding companion paper. The SAR is optimized using a preferred set of data including reactions of OH with 67 monocyclic aromatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The SAR can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the reactions of the product radicals under tropospheric conditions are also summarized, specifically the rapid reaction sequences initiated by their reactions with O 2 .
Molecular models of active sites of C1 and C2 hydrocarbon activation
Catalysis Today, 1995
A short review of the quantum chemical approach to the problem of alkane activation is presented. The results of ab initio calculations of oxidative addition of methane molecules to the transition metal (TM) atoms and complexes are discussed, as well as some questions of methane dissociation on TM surfaces. Both homolytic and heterolytic mechanisms of methane activation on oxide systems are considered.
Physical Chemistry Chemical Physics, 2021
The kinetics of the reaction between resonance-stabilized (CH 3) 2 CCHCH 2 radical (R) and O 2 has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH 3) 2 CCHCH 2 + O 2 reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O 2-ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O 2 with (CH 3) 2 CCHCH 2 radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO (t) and ROO (nt) adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O 2 reaction at high temperatures (T 4 500 K) and pressureindependent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) Â 10 À15 cm 3 molecule À1 s À1 , were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at m/z = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO 2 (yield 4 91%) and (2R/S)-3-methyl-1,2-epoxybut-3-ene + OH (yield o 8%). After modest adjustments to ROO (t) and ROO (nt) well-depths (B0.7 kcal mol À1 each) and barrier height for the transition state associated with the kinetically most dominant channel, R + O 2isoprene + HO 2 (B2.2 kcal mol À1), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.
Journal of Physical Chemistry A, 2013
This work presents an ab-initio and chemical kinetic study of the reaction mechanisms of hydrogen atom abstraction by the HȮ 2 radical on five ketones: dimethyl, ethyl methyl, n-propyl methyl, isopropyl methyl and iso-butyl methyl ketones. The Møller-Plesset method using the 6-311G(d,p) basis set has been used in the geometry optimization and the frequency calculation for all the species involved in the reactions, as well as the hindrance potential description for reactants and transition states. Intrinsic reaction coordinate calculations were carried out to validate all the connections between transition states and local minima. Energies are reported at the CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level of theory. The CCSD(T)/cc-pVXZ method (X = D, T, Q) was used for the reaction mechanism of dimethyl ketone + HȮ 2 radical in order to benchmark the computationally less expensive method of CCSD(T)/cc-pVTZ//MP2/6-311G(d,p). High-pressure limit rate constants have been calculated for all the reaction channels by conventional transition state theory with asymmetric Eckart tunneling corrections and 1-D hindered rotor approximations in the temperature range 500-2000 K.