CH4 Reaction Research Articles

Communication: An accurate full 15 dimensional permutationally invariant potential energy surface for the OH + CH4 → H2O + CH3 reaction.

A globally accurate full-dimensional potential energy surface (PES) for the OH + CH4 → H2O + CH3 reaction is developed using the permutation invariant polynomial-neural network approach based on ∼135,000 points at the level of correlated coupled cluster singles, doubles, and perturbative triples level with the augmented correlation consistent polarized valence triple-zeta basis set. The total root mean square fitting error is only 3.9 meV or 0.09 kcal/mol. This PES is shown to reproduce energies, geometries, and harmonic frequencies of stationary points along the reaction path. Kinetic and dynamical calculations on the PES indicated a good agreement with the available experimental data.

Quantum dynamical study of heavy-light-heavy reactions: application to the (Cl + CH4 → HCl + CH3) reaction

A time dependent quantum-mechanical QM study is performed on the Cl + CH4 → HCl + CH3 reaction, using a pseudotriatomic ab initio based surface. Probabilities present some clear peaks versus t, which we assign to transition state resonances where the light H atom oscillates between heavy Cl and CH3 groups. For ground-state reactants, the reactivity is of quantum origin. The reaction occurs through an abstraction mechanism, following both direct and an indirect mechanisms. The calculations show the participation of a short-lived collision complex in the microscopic reaction mechanism. This theoretical result and other oscillating properties found here could, however, be related to the existence of resonance for the production of HCl, as suggested by experimentalists.

Rate Coefficient for the (4)Heμ + CH4 Reaction at 500 K: Comparison between Theory and Experiment.

The rate constant for the H atom abstraction reaction from methane by the muonic helium atom, Heμ + CH4 → HeμH + CH3, is reported at 500 K and compared with theory, providing an important test of both the potential energy surface (PES) and reaction rate theory for the prototypical polyatomic CH5 reaction system. The theory used to characterize this reaction includes both variational transition-state (CVT/μOMT) theory (VTST) and ring polymer molecular dynamics (RPMD) calculations on a recently developed PES, which are compared as well with earlier calculations on different PESs for the H, D, and Mu + CH4 reactions, the latter, in particular, providing for a variation in atomic mass by a factor of 36. Though rigorous quantum calculations have been carried out for the H + CH4 reaction, these have not yet been extended to the isotopologues of this reaction (in contrast to H3), so it is important to provide tests of less rigorous theories in comparison with kinetic isotope effects measured by experiment. In this regard, the agreement between the VTST and RPMD calculations and experiment for the rate constant of the Heμ + CH4 reaction at 500 K is excellent, within 10% in both cases, which overlaps with experimental error.

An experimental and theoretical study on the kinetic isotope effect of C2H6 and C2D6 reaction with OH

We report experimental and theoretical results for the deuterated kinetic isotope effect (DKIE) of the reaction of OH with ethane (C2H6) and deuterated ethane (C2D6). The reactions were investigated behind reflected shock waves over 800–1350K by monitoring OH radicals near 306.69nm using laser absorption. In addition, high level CCSD(T)/cc-pV(T,Q)Z//MP2/cc-pVTZ quantum chemical and statistical rate theory calculations were performed which agreed very well with the experimental findings. The results reported herein provide the first experimental evidence that DKIE asymptotes to a value of 1.4 at high temperatures.

Time-Dependent Wave Packet Study of the H2 + CH3 → H + CH4 Reaction.

The initial state selected time-dependent wave packet method has been developed to study the H2 + CH3 → H + CH4 reaction, by employing the seven- and eight-dimensional models proposed by Palma and Clary in which the nonreacting CH3 moiety is restricted in C3v symmetry. Total reaction probabilities and integral cross sections were calculated for the ground and a number of vibrationally excited initial states to investigate the effects of vibrational excitations of both reagents on the reaction. The eight-dimensional calculations showed that the CH stretching excitation does not have any important effect on the reaction and the seven-dimensional model with the CH bond length fixed works very well for the reaction. The excitation of H2 vibrations could enhance the reaction but is less effective than the translation in the low energy region. In contrast, the first umbrella excitation is very effective on reducing the reaction threshold. The calculated rate constants are found to be in good agreement with available experimental measurements and other theoretical results.

Mode specific dynamics of the H2 + CH3 → H + CH4 reaction studied using quasi-classical trajectory and eight-dimensional quantum dynamics methods

An eight-dimensional quantum dynamical model is proposed and applied to the title reaction. The reaction probabilities and integral cross sections have been determined for both the ground and excited vibrational states of the two reactants. The results indicate that the H2 stretching and CH3 umbrella modes, along with the translational energy, strongly promote the reactivity, while the CH3 symmetric stretching mode has a negligible effect. The observed mode specificity is confirmed by full-dimensional quasi-classical trajectory calculations. The mode specificity can be interpreted by the recently proposed sudden vector projection model, which attributes the enhancement effects of the reactant modes to their strong couplings with the reaction coordinate at the transition state.

Reactions of Th(+) + H2, D2, and HD Studied by Guided Ion Beam Tandem Mass Spectrometry and Quantum Chemical Calculations.

Kinetic energy dependent reactions of Th(+) with H2, D2, and HD were studied using a guided ion beam tandem mass spectrometer. Formation of ThH(+) and ThD(+) is endothermic in all cases with similar thresholds. Branching ratio results for the reaction with HD indicate that Th(+) reacts via a statistical mechanism, similar to Hf(+). The kinetic energy dependent cross sections for formation of ThH(+) and ThD(+) were evaluated to determine a 0 K bond dissociation energy (BDE) of D0(Th(+)-H) = 2.45 ± 0.07 eV. This value is in good agreement with a previous result obtained from analysis of the Th(+) + CH4 reaction. D0(Th(+)-H) is observed to be larger than its transition metal congeners, TiH(+), ZrH(+), and HfH(+), believed to be a result of lanthanide contraction. The reactions with H2 were also explored using quantum chemical calculations that include a semiempirical estimation and explicit calculation of spin-orbit contributions. These calculations agree nicely and indicate that ThH(+) most likely has a (3)Δ1 ground level with a low-lying (1)Σ(+) excited state. Theory also provides the reaction potential energy surfaces and BDEs that are in reasonable agreement with experiment.

Crossed Molecular Beams and Quasiclassical Trajectory Surface Hopping Studies of the Multichannel Nonadiabatic O((3)P) + Ethylene Reaction at High Collision Energy.

The combustion relevant O((3)P) + C2H4 reaction stands out as a prototypical multichannel nonadiabatic reaction involving both triplet and singlet potential energy surfaces (PESs), which are strongly coupled. Crossed molecular beam (CMB) scattering experiments with universal soft electron ionization mass spectrometric detection have been used to characterize the dynamics of this reaction at the relatively high collision energy Ec of 13.7 kcal/mol, attained by crossing the reactant beams at an angle of 135°. This work is a full report of the data at the highest Ec investigated for this reaction. From laboratory product angular and velocity distribution measurements, angular and translational energy distributions in the center-of-mass system have been obtained for the five observed exothermic competing reaction channels leading to H + CH2CHO, H + CH3CO, CH3 + HCO, CH2 + H2CO, and H2 + CH2CO. The product branching ratios (BRs) have been derived. The elucidation of the reaction dynamics is assisted by synergic full-dimensional quasiclassical trajectory surface-hopping calculations of the reactive differential cross sections on coupled ab initio triplet/singlet PESs. This joint experimental/theoretical study extends and complements our previous combined CMB and theoretical work at the lower collision energy of 8.4 kcal/mol. The theoretically derived BRs and extent of intersystem crossing (ISC) are compared with experimental results. In particular, the predictions of the QCT results for the three main channels (those leading to vinoxy + H, methyl + HCO and methylene + H2CO formation) are compared directly with the experimental data in the laboratory frame. Good overall agreement is noted between theory and experiment, although some small, yet significant shortcomings of the theoretical differential cross section are noted. Both experiment and theory find almost an equal contribution from the triplet and singlet surfaces to the reaction, with a clear tendency of the degree of ISC to decrease with increasing Ec and with theory slightly overestimating the extent of ISC.

An eight-dimensional quantum dynamics study of the Cl + CH4→ HCl + CH3 reaction.

In this work, the later-barrier reaction Cl + CH4 → HCl + CH3 is investigated with an eight-dimensional quantum dynamics method [R. Liu et al., J. Chem. Phys. 137, 174113 (2012)] on the ab initio potential energy surface of Czakó and Bowman [J. Chem. Phys. 136, 044307 (2012)]. The reaction probabilities with CH4 initially in its ground and vibrationally excited states are calculated with a time-dependent wavepacket method. The theoretical integral cross sections (ICSs) are extensively compared with the available experimental measurements. For the ground state reaction, the theoretical ICSs excellently agree with the experimental ones. The good agreements are also achieved for ratios between ICSs of excited reactions. For ICS ratios between various states, the theoretical values are also consistent with the experimental observations. The rate constants over 200-2000 K are calculated and the non-Arrhenius effect has been observed which is coincident with the previous experimental observations and theoretical calculations.

Removal of oxygen from (bio-)methane via catalytic oxidation of CH4—Reaction kinetics for very low O2:CH4 ratios

For evaluation of design specifications for catalytic oxygen removal units, examinations on reaction kinetics of catalytic oxidation of methane at very low O2:CH4 ratios have been performed in the temperature range between 200<T<350°C and pressures below 10bar, applying a commercial noble metal (Pt) Al2O3-based catalyst. On the basis of the experimental results, a mass based rate equation is proposed, taking into account the influence of both, reactants and products of the reaction on the conversion rate of oxygen. This equation is valid for oxygen inlet pressures of as low as 1mbar and has further been tested in a balance of methane of up to pCH4=10bar (abs.).For comparison of conversion rates of oxygen with various other reductives, such as ethane, propane and butane, equivalent experiments were performed and kinetic parameters of corresponding power law approaches are given. Finally, the catalytic activity of further noble metals and some transition metal catalysts was tested and compared to the platinum reference catalyst.

Communication: Rate coefficients of the H + CH4 → H2 + CH3 reaction from ring polymer molecular dynamics on a highly accurate potential energy surface.

The ring polymer molecular dynamics (RPMD) calculations are performed to calculate rate constants for the title reaction on the recently constructed potential energy surface based on permutation invariant polynomial (PIP) neural-network (NN) fitting [J. Li et al., J. Chem. Phys. 142, 204302 (2015)]. By inspecting convergence, 16 beads are used in computing free-energy barriers at 300 K ≤ T ≤ 1000 K, while different numbers of beads are used for transmission coefficients. The present RPMD rates are in excellent agreement with quantum rates computed on the same potential energy surface, as well as with the experimental measurements, demonstrating further that the RPMD is capable of producing accurate rates for polyatomic chemical reactions even at rather low temperatures.

Exploring the stereodynamics and microscopic mechanism of the O(3P) + CH4, CD4 → OH + CH3, OD + CD3 combustion reactions

The dynamics of the O(3P)+CH4(X1A1)→OH(X2∏)+CH3(X2A″2) reaction and its isotopic variant with CD4 was studied at several collision energies (Ecol) of experimental interest, using the quasiclassical trajectory (QCT) method on the ground potential energy surface (GHMS PES). This ab initio analytical PES, previously developed using the OX(CX3) pseudo-triatomic approximation, allowed us to perform an exhaustive dynamics study of these low reactive systems. Around 300million of trajectories were calculated to simulate the conditions of the low and intermediate Ecol experiments. The results describe in detail scalar and vector properties and the reactive microscopic mechanism, obtaining a generally good agreement with the experiments.

Selective Catalytic Reduction of NO by Hydrocarbons on a Stepped Pt Surface: Influence of SO2 and O2

The catalytic reduction of NO by hydrocarbons (C2H4 and its derivatives) in the presence of both S18O2 and O2 on Pt(332) was investigated. S18O2–C2H4 interactions are much stronger than NO–C2H4 interactions. Coadsorbed S18O2 molecules consume most of the H atoms from C2H4 dissociation before NO dissociation, a major cause for suppressing the NO–C2H4 reaction. The S18O2 molecules also reduce the reactivity of Pt(332) toward NO–C2H4 reaction by passivating the metal surface and by blocking the step sites for NO adsorption. NO–C2H4 reactions are also suppressed by coadsorbed O(ads) atoms due to the stronger Pt–O interaction and facile H–O reaction. However, when the coverage of O atoms is appropriate, the O atoms effectively lifts the suppressive effect of S18O2, a result of the facile O–S18O2 reaction, which scavenges the S18O2 related species and consequently gives the Pt(332) surface a more reactive pathway toward the NO dissociation (compared to that covered with sulfur oxides).

Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products.

The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214-1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 10(1)(T/K)(3.15) exp(5270 K/T) cm(3) mol(-1) s(-1) ±30%, which is valid over 1000-2500 K.

Adsorption and Reaction of C₂H₄ and O₂ on a Nanosized Gold Cluster: A Computational Study.

We have investigated the adsorption and reaction mechanisms of C2H4 and O2 catalyzed by a Au38 nanoparticle based on periodic density-functional theory (DFT) calculations. The configurations of the adsorption of C2H4/Au38, O2/Au38, and O/Au38 as well as the coadsorption of C2H4-O2/Au38 were predicted. The calculation results show that C2H4, O2, and O are preferably bound at top (T), bridge (B), and hexagonal (h) sites with adsorption energies of -0.66, -0.99, and -3.93 eV, respectively. The detailed reaction mechanisms for ethylene epoxidation on the Au38 nanoparticle has been illustrated using the nudged elastic band (NEB) method. The oxidation process takes place via the Langmuir-Hinshelwood (LH) mechanism to generate ethylene oxide and acetaldehyde. The overall reaction of C2H4 + O2 + Au38 → ethylene oxide + O/Au38 is exothermic by 2.20-2.40 eV whereas those are 3.03-3.08 eV for the production of acetaldehyde and O/Au38. The nature of the interaction between the adsorbate and gold nanocluster has been analyzed by the detailed electronic local density of states (LDOS) to understand the high catalytic activity of the gold nanoclusters.

A new exploration of the addition reaction of the silylenoid H2SiLiF with ethylene.

The addition reactions of the simplest silylenoid H2SiLiF with ethylene were studied theoretically. The geometries of the stationary points along the potential energy surfaces were optimized using DFT B3LYP method with the 6-311+G(d,p) basis set, and the single point energies were calculated at QCISD/6-311++G(d,p) level of theory. The theoretical calculations demonstrated that the addition reaction of H2SiLiF and C2H4 can occur through two different pathways. One is path A via a three-membered ring transition state, the other is path B, while through a four-membered ring transition state. The calculated energy barriers of path A and path B are 58.90 and 248.08 kJ∙mol(-1), respectively. Therefore, pathway A is more favorable than pathway B. The solvent effect on the addition reactions were investigated using the PCM model, and the calculated results indicated that in the THF solvent, the addition reaction is much easier than that in vacuum. The present work provided a new pathway to synthesize silicon heterocyclic compounds.

Kinetics for Steam and CO2 Reforming of Methane Over Ni/La/Al2O3 Catalyst.

Kinetic studies of mixed (steam and dry) reforming of methane on Ni/La/Al2O3 and Ni/La-Co (1, 3 wt%)/Al2O3 catalysts were performed in an atmospheric fixed-bed reactor. Kinetic parameters for the mixed reforming over these catalysts were obtained under reaction conditions free from heat and mass transfer limitations. Variables for the mixed reforming were the reaction temperature and partial pressure of reactants. The fitting of the experimental data for the rate of methane conversion, rCH4, using the power law rate equation rCH4 = k(PrCH4)α(PCO2)β(PH2O)γ showed that the reaction orders α, β, and γ are steady and obtained values equal to α = 1, β = 0, and γ = 0. In other words, among CH4, CO2, H2O, and H2, only CH4 reaction orders were not zero and they were affected by the promoters. The apparent activation energy on catalysts Ni/La/Al2O3, Ni/La-Co (1)/Al2O3 and Ni/La-Co (3)/Al2O3 is 85.2, 93.8, and 99.4 kJ/mol, respectively. The addition of Co to Ni/La/Al2O3 was increased the apparent activation energy of the mixed reforming reaction. And the Ni/La-Co (3 wt%)/Al2O3 catalyst showed the highest reforming activity and apparent activation energy. The Co promoters can increase the apparent activation energy of mixed reforming of methane.

Product Translational and Vibrational Distributions for the OH/OD + CH4/CD4 Reactions from Quasiclassical Trajectory Calculations. Comparison with Experiment.

For the OH + CH4/CD4 hydrogen abstraction reactions, the methyl radical (CH3 and CD3) product translational distributions and the water (H2O and HOD) product vibrational distributions experimentally reported by Liu's group are reproduced by quasi-classical trajectory (QCT) calculations on an analytical full-dimensional potential energy surface when a quantum spirit is included in the analysis. Our simulations correctly predict: (i) the vibrational excitation of the water product, (ii) the inversion of the water vibrational population, and (iii) the propensity of transfer from reactant kinetic energy to product translational energy. These reactions therefore present a marked isotopic effect. In addition, the water product vibrational distributions for the OH/OD + CH4 reactions agree reasonably well with Butkovskaya and Setser's experiments for a similar alkane reaction. The theory/experiment agreement is better for the HOD than for the H2O product due to the mode coupling in the H2O molecule, which is absent in the HOD stretching modes, which show a more "local" character. In summary, for polyatomic systems with many degrees of freedom (15 in the present reaction), QCT calculations analyzed with a quantum spirit represent a useful alternative to quantum scattering methods.

Reanalysis of Rate Data for the Reaction CH3 + CH3 → C2H6 Using Revised Cross Sections and a Linearized Second-Order Master Equation.

Rate coefficients for the CH3 + CH3 reaction, over the temperature range 300-900 K, have been corrected for errors in the absorption coefficients used in the original publication ( Slagle et al., J. Phys. Chem. 1988 , 92 , 2455 - 2462 ). These corrections necessitated the development of a detailed model of the B̃(2)A1' (3s)-X̃(2)A2″ transition in CH3 and its validation against both low temperature and high temperature experimental absorption cross sections. A master equation (ME) model was developed, using a local linearization of the second-order decay, which allows the use of standard matrix diagonalization methods for the determination of the rate coefficients for CH3 + CH3. The ME model utilized inverse Laplace transformation to link the microcanonical rate constants for dissociation of C2H6 to the limiting high pressure rate coefficient for association, k∞(T); it was used to fit the experimental rate coefficients using the Levenberg-Marquardt algorithm to minimize χ(2) calculated from the differences between experimental and calculated rate coefficients. Parameters for both k∞(T) and for energy transfer ⟨ΔE⟩down(T) were varied and optimized in the fitting procedure. A wide range of experimental data were fitted, covering the temperature range 300-2000 K. A high pressure limit of k∞(T) = 5.76 × 10(-11)(T/298 K)(-0.34) cm(3) molecule(-1) s(-1) was obtained, which agrees well with the best available theoretical expression.

Reaction CH3 + CH3 → C2H6 Studied over the 292-714 K Temperature and 1-100 bar Pressure Ranges.

Reaction of recombination of methyl radicals, CH3 + CH3 → C2H6 (1) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 292-714 K temperature and 1-100 bar pressure ranges (bath gas He), very close to the high-pressure limit. Methyl radicals were produced by photolysis of acetone at 193.3 nm or in the reaction of electronically excited oxygen atoms O((1)D), produced in the photolysis of N2O at 193.3 nm, with CH4, and subsequent reaction of OH with CH4. Temporal profiles of CH3 were recorded via absorption at 216.36 and 216.56 nm using a xenon arc lamp and a spectrograph. The absolute intensity of the photolysis light inside the reactor was determined by an accurate in situ actinometry based on the ozone formation in photolysis of N2O/O2/N2 mixtures. The rate constant of reaction 1 in the high-pressure limit has a negative temperature dependence: k1,inf = (5.66 ± 0.43) × 10(-11)(T/298 K)(-0.37) cm(3) molecule(-1) s(-1) (292-714 K).