CH4 Reaction Research Articles

A kinetic study of the N(2D) + C2H4 reaction at low temperature.

Electronically excited nitrogen atoms N(2D) are important species in the photochemistry of N2 based planetary atmospheres such as Titan. Despite this, few N(2D) reactions have been studied over the appropriate low temperature range. During the present work, rate constants were measured for the N(2D) + ethene (C2H4) reaction using a supersonic flow reactor at temperatures between 50 K and 296 K. Here, a chemical reaction was used to generate N(2D) atoms, which were detected directly by laser induced fluorescence in the vacuum ultraviolet wavelength region. The measured rate constants displayed very little variation as a function of temperature, with substantially larger values than those obtained in previous work. Indeed, considering an average temperature of 170 K for the atmosphere of Titan leads to a rate constant that is almost seven times larger than the currently recommended value. In parallel, electronic structure calculations were performed to provide insight into the reactive process. While earlier theoretical work at a lower level predicted the presence of a barrier for the N(2D) + C2H4 reaction, the present calculations demonstrate that two of the five doublet potential energy surfaces correlating with reagents are likely to be attractive, presenting no barriers for the perpendicular approach of the N atom to the C[double bond, length as m-dash]C bond of ethene. The measured rate constants and new product channels taken from recent dynamical investigations of this process are included in a 1D coupled ion-neutral model of Titan's atmosphere. These simulations indicate that the modeled abundances of numerous nitrogen bearing compounds are noticeably affected by these changes.

An accurate potential energy surface and ring polymer molecular dynamics study of the Cl + CH4 → HCl + CH3 reaction.

The reaction Cl + CH4 → HCl + CH3, a prototypical bimolecular reaction, has been established as an important proving ground for studying chemical reaction kinetics and dynamics of polyatomic molecules. In this work, a globally accurate full-dimensional potential energy surface (PES) for this reaction is developed by using the permutationally invariant polynomial neural network (PIP-NN) approach based on 74 000 points calculated at the level of the explicitly correlated unrestricted coupled cluster single, double, and perturbative triple level with the augmented correlation corrected valence triple-zeta basis set (UCCSD(T)-F12a/AVTZ). For points in the entrance channel, spin-orbit corrections stemming from Cl(2P) are determined at the level of complete active space self-consistent field (CASSCF) with the AVTZ basis set. With this PES, thermal rate coefficients and kinetic isotope effects are computed for reactions Cl + CH4 → HCl + CH3 and Cl + CD4 → DCl + CD3 using the ring polymer molecular dynamics (RPMD) method, which can provide reliable estimations for thermal rate coefficients effectively. Generally, the agreement with the scattered experimental results is reasonably satisfactory.

Active stereo-control of the Cl + CH4(ν3 = 1) reaction: a three-dimensional perspective.

The transition state in Cl + CH4 is of Cl-H-C collinear geometry. As the reactant CH4 is vibrationally excited by a linearly polarized infrared (IR) light to the antisymmetric-stretching state of ν3 = 1, all four C-H bonds are collectively excited and any one of the H-atoms can be reactive. Yet, a strong alignment of the excited CH4(ν3 = 1), as evidenced from the striking stereo-specificity in the Cl + CH4 reaction, was clearly revealed in a previous, exploratory study. Reported here is the full account of that investigation at two collisional energies of Ec = 4.8 and 2.7 kcal mol-1, using a crossed molecular-beam, product-imaging approach. By active control of the polarization direction of an IR laser under judiciously chosen beam-geometries, a complete set of polarization-dependent differential cross sections is disentangled from the CH3(00) product images. To our surprise, the quantitative results appear nearly identical to those obtained for the isotope-substituted reaction of Cl + CHD3(ν1 = 1) → HCl(ν) + CD3(00). A detailed discussion is presented to elucidate the underlying physics for such an intriguing similarity in stereo-reactivity between a spherical-top and a symmetric-top reactant.

VTST and RPMD kinetics study of the nine-body X + C2H6 (X ≡ H, Cl, F) reactions based on analytical potential energy surfaces.

Thermal rate constants of nine-atom hydrogen abstraction reactions, X + C2H6 → HX + C2H5 (X ≡ H, Cl, F) with qualitatively different reaction paths, have been investigated using two kinetics approaches - variational transition state theory with multidimensional tunnelling (VTST/MT) and ring polymer molecular dynamics (RPMD) - and full dimensional analytical potential energy surfaces. For the H + C2H6 reaction, which proceeds through a noticeable barrier height of 11.62 kcal mol-1, kinetics approaches showed excellent agreement between them (with differences less than 30%) and with the experiment (with differences less than 60%) in the wide temperature range of 200-2000 K. For X = Cl and F, however, the situation is very different. The barrier height is either low or very low, 2.44 and 0.23 kcal mol-1, respectively, and the presence of van der Waals complexes in the entrance channel leads to a very flat topography and, consequently, imposes theoretical challenges. For the Cl(2P) reaction, VTST/MT underestimates the experimental rate constants (with differences less than 86%), and RPMD demonstrates better agreement (with differences less than 47%), although the temperature dependence is opposite to the experiment at low temperatures. Finally, for the F(2P) reaction, available experimental information shows discrepancies, both in the absolute values of the rate constants and also in the temperature dependence. Unfortunately, kinetics theories did not resolve this discrepancy. Different possible causes of these theory/experiment discrepancies were analyzed.

Kinetics and dynamics study of the OH + C2H6 → H2O + C2H5 reaction based on an analytical global potential energy surface.

To describe the gas-phase hydrogen abstraction reaction between the hydroxyl radical and the ethane molecule, an analytical full-dimensional potential energy surface was developed within the Born-Oppenheimer approximation. This reactive process is a ten-body system with 24 degrees of freedom, which represents a theoretical challenge. The new surface, named PES-2020, presents low barrier, 3.76 kcal mol-1, high exothermicity, -16.20 kcal mol-1, and intermediate complexes in the entrance and exit channels. To test the quality and accuracy of the analytical surface several stringent tests were performed and, in general, PES-2020 reasonably simulates the theoretical information used as input, it is a continuous and smooth potential, without spurious minima, it presents great versatility and a reasonable description of this ten-body reaction. Based on this surface, an exhaustive kinetics and dynamics study was performed with a double objective: to analyze the capacity of the new surface to simulate the experimental evidence, and to help understand the mechanism of reaction and the role of the ethyl group in the reaction. In the kinetics study, three approaches were used: variational transition-state theory with multidimensional tunnelling (VTST/MT), ring polymer molecular dynamics (RPMD) and quasi-classical trajectory (QCT) results, in the temperature range 200-2000 K. There is general agreement between the three approaches and they reasonably simulate the experimental behaviour, which gives confidence to the fitness of the new surface to describe the system. In the dynamics study, QCT calculations were performed at 298 K for a direct comparison with the only experimental result reported. We found that ethyl fragment presents a noticeable internal energy (∼20%) and so cannot be considered as a spectator. The water product vibrational energy is reasonably reproduced, though when a level-by-level distribution is analyzed the agreement is only qualitative.

Weak-field ligands enable inert early transition metal oxides to convert methane to methanol: the case of ZrO.

Zirconium monoxide, ZrO, was studied by multi-reference configuration interaction (MRCI) and coupled cluster methods using large basis sets in conjunction with effective core potentials. Complete potential energy curves were constructed and bonding patterns are proposed for several electronic states. Numerical results include accurate equilibrium bond lengths, harmonic vibrational frequencies, anharmonicities, excitation energies, dipole moments, and binding energies for both ground and excited states. The application of a ZrO unit as the catalytic center for methane activation is explored through the reaction ZrO + CH4→ Zr + CH3OH. Optimal density functional structures combined with single-point MRCI energy calculations are obtained for the complete reaction pathway. It is found that the lower energy singlet and triplet multiplicities (oxo states) favor the [2+2] mechanism and the higher energy quintets (oxyl states) favor the radical mechanism, which is overall more efficient in producing methanol. We finally suggest proper ligands that stabilize the oxyl states. These include halogens or other weak-field ligands, which finally convert the inert early transition metal oxide units to efficient methane-to-methanol catalysts.

Conversion of acenaphthalene to phenalene via methylation: A theoretical study

Ab initio calculations of the C13H10 and C13H9 potential energy surfaces related to the reaction of 1-acenaphthyl and methyl radicals and secondary isomerization of C13H9 primary radical products have been performed at the chemically accurate G3(MP2,CC)//B3LYP/6-311G** level of theory to unravel the mechanism of conversion of acenaphthalene to phenalene or phenalenyl radical + H. The computed energetics and molecular parameters were utilized in Rice-Ramsperger-Kassel-Marcus Master Equation (RRKM-ME) calculations of reaction rate constants and relative product yields. The 1-acenaphthyl + CH3 reaction is predicted to proceed by a fast radical-radical recombination mechanism and to predominantly produce collisionally stabilized 1-methylacenaphthalene B1 or a C13H9 benzylic radical A1 + H with exothermicities of 106.8 and 25.2 kcal/mol, respectively. The A1 + H channel is preferable at higher temperatures, whereas the stabilization of 1-methylacenaphthalene is favored at higher pressures. The radical A1 can nearly irreversibly interconvert to phenalenyl radical AP via a 22.7 kcal/mol exothermic isomerization process involving formal insertion of the CH2 group into a CC bond of the five-member ring leading to the expansion of this ring to a six-member ring. The rate constants for the H addition reaction to phenalenyl radical to form phenalene and the reverse H loss from phenalene were also computed and the strength of the weakest CH bond in the CH2 group of phenalene is evaluated as 62.2 kcal/mol. The analysis of the reaction kinetics allowed us to deduce a mechanism for the conversion of 1-acenaphthyl radical to phenalene or phenalenyl radical + H via methylation involving the formation of A1 via a well-skipping channel or stabilization and dissociation of (or H abstraction from) B1 followed by isomerization of A1 to phenalenyl AP, which can add an H atom producing phenalene. Rate constants for the significant elementary reactions are fitted to modified Arrhenius expressions and are proposed for kinetic modeling of the expansion of a five-member ring on a zigzag edge of PAH to a six-member ring by methylation.

First principles study on methane reforming over Ni/TiO2(110) surface in solid oxide fuel cells under dry and wet atmospheres

Understanding the carbon-tolerant mechanisms from a microscopic view is of special importance to develop proper anodes for solid oxide fuel cells. In this work, we employed density-functional theory calculations to study the CH4 reaction mechanism over a Ni/TiO2 nanostructure, which experimentally demonstrated good carbon tolerance. Six potential pathways for methane reforming reactions were studied over the Ni/TiO2(110) surface under both dry and wet atmospheres, and the main concerns were focused on the impact of TiO2 and Ni/TiO2 interface on CO/H2 formation. Our calculations suggest that the reaction between carbon and the interfacial lattice oxygen to form CO* is the dominant pathway for CH4 reforming under both dry and wet atmospheres, and intervention of steam directly to oxidize C* with its dissociated OH* group is less favorable in energy than that to wipe off oxygen vacancy to get ready for next C* oxidation. In all investigated paths, desorption of CO* is one of the most difficult steps. Fortunately, CO* desorption can be greatly promoted by the large heat released from the previous CO* formation process under wet atmosphere. H2O adsorption and dissociation over the TiO2 surface are found to be much easier than those over Ni, yttria stabilized zirconia (YSZ) and CeO2, which should be the key reason for the greatly depressed carbon deposition over Ni-TiO2 particles than traditional YSZ-Ni and CeO2-Ni anode. Our study presents the detailed CO* formation mechanism in CH4 reforming process over the Ni/TiO2 surface, which will benefit future research for exploring new carbon-tolerant solid oxide fuel cell anodes.

Temperature dependence of rate constants for the H(D) + CH4 reaction in gas and aqueous phase: deformed Transition-State Theory study including quantum tunneling and diffusion effects

In recent years, the understanding and control of mechanisms involving radical attacks to hydrocarbons have been object of investigation in several fields, especially in combustion reactions and energy resource technology. The H(D) + CH4 ⟶ CH3 + H2(HD) reactions are known as prototypical reactions of hydrocarbons and have been extensively investigated both experimentally and theoretically in the gas-phase. Here, the reaction rate constants for the hydrogen abstraction of methane by atomic hydrogen (and deuterium) in the gas phase have been validated by employing the deformed Transition-State Theory ($$ d $$-TST): The results motivated the use of Collins-Kimball approaches to provide kinetics data in the aqueous phase. The $$ d $$-TST has been found to be accurate for absolute values and temperature dependence of the reaction rate constant in the gas phase, especially for what concerns the excellent agreement with experimental data for the variant isotopic when compared with previous formulations. For the first time, theoretical rate constants in aqueous solution for the title reaction are presented reproducing the experimental data at 288.15 K.

Shock Tube Measurement of the CH3 + C2H6 → CH4 + C2H5 Rate Constant.

The rate constant for the CH3 + C2H6 → CH4 + C2H5 reaction was studied behind reflected shock waves at temperatures between 1369 and 1626 K and pressures from 8.6 to 47.4 atm in mixtures of methane, ethane, and argon. Ethylene time histories were measured using laser absorption of radiation from a carbon dioxide gas laser near 10.532 μm. The resulting rate constant data can be represented by the Arrhenius equation k (T) = 3.90 × 1013 exp(-16670 cal/mol/RT) cm3 mol-1 s-1. We believe this is the first study to extend experimental data for this rate constant to temperatures above 1400 K. The overall 2σ uncertainty of the current data is +18%/-21% resulting primarily from uncertainties associated with the influence of secondary reactions and the fitting of rapidly changing species time histories at the higher temperatures.

Vibronic coupling in the F·CH4 prereactive complex

The F + CH4 → HF + CH3 reaction shows a counter-intuitive mode-selective chemistry and prominent resonances. The prereactive F·CH4 complex formed in the entrance channel is assumed to play an important role in the dynamics of the reaction. The present work investigates the effect of nonadiabatic transitions and the geometric phase on the low-lying quasibound states of the F·CH4 complex. Quantum dynamics calculations employing the multiconfigurational time-dependent Hartree approach and accurately accounting for vibronic as well as spin-orbit coupling are performed. Extending previous work [D. Schäpers and U. Manthe, J. Phys. Chem. A 120, 3186 (2016)], which was restricted to the dynamics on a single adiabatic potential energy surface and found the relative rotation of F and CH4 to proceed almost freely, we found chaotic patterns if vibronic coupling is included. While nonadiabatic transitions strongly affect individual resonances, their effect on averaged quantum state densities and the photodetachment spectrum of F⋅CH4 - is found to be minor.

QCT study of the vibrational and translational role in the H + C2H6(ν1, ν2, ν5, ν7, ν9 and ν10) reactions

Two important issues were analysed in the title reaction: the effects of vibrational excitation, associated with mode selectivity, and the role of translational energy, associated with Polanyi’s rules. Based on a global analytical potential energy surface, PES-2018, recently developed in our group, quasi-classical trajectory (QCT) calculations were performed at total energy of 35 kcal mol−1, either as translation or as a combination of translation and vibration energy. Independent vibrational excitation by one quantum of any of the CH3 stretching modes in ethane leads to similar dynamics pictures of reaction cross sections and H2(v′, j′) rotovibrational and scattering distributions, ruling out mode selectivity. Normal mode analysis showed a cold, non-inverted, H2(v′) product vibrational distribution, while the C2H5(v′) co-product presented many vibrational states, all of them with a low population, practically simulating a classical behaviour. An equivalent amount of energy as translation raises reactivity somewhat less effective than vibrational energy, contrary to that found for the O(3P) + CH4 reaction. Both reactions present “central” barriers, so this opposite behaviour shows the difficulties for a straightforward application of the Polanyi′s rules. The role of vibrational and translational energy on dynamics has been rationalized by the coupling between vibrational modes, which makes analysis of vibrational excitation difficult in polyatomic systems. Finally, the role of the total energy on reactivity and mode selectivity was analysed, concluding that at lower energy, 15 kcal mol−1, translational energy is much more effective than vibrational energy to enhance reactivity, while at intermediate energy, 20 kcal mol−1, the situation is more confusing and strongly dependent on the counting methods used in the QCT calculations. Therefore, very small mode selectivity is found, and translation seems to be more effective in enhancing reactivity than vibration at low collision energies, while this behaviour is reversed as we increase the collision energy, being the turning point around 20 kcal mol−1.

Kinetics and Mechanism of Nickel Oxide Reduction by Methane

Nickel oxide reduction by methane is of particular importance in catalysis, extractive metallurgy, and clean power generation technologies. Despite extensive investigations of the NiO + CH4 reaction, many questions remain about its kinetics and molecular and structural transformation mechanisms. This work reports the reduction kinetics of bulk polycrystalline NiO by CH4 using a new calorimetric method. The method permits rapid, controllable heating of the NiO/Ni wires and continuous data (electrical power, the electrical resistivity of the wire, and temperature) acquisition with a frequency of 10 kHz. The method also allows arresting and preservation of the structure of the sample by programmed termination of electric power in thin specimens. This approach, coupled with ex situ electron microscopy, allows determination of the reaction rate and tracking of the ongoing structural transformations. The mechanism of NiO reduction by methane is suggested based on direct correlations between reaction kinetics an...

Deactivation reactions on a commercial lean nox-trap - Effect of hydrocarbon nature, concentration and operation temperature

Engine-out emissions during rich phase operation of a lean NOX trap (LNT) catalyst contain a mixture of different hydrocarbons (HCs) depending on engine operation and fuel type. The impact of temperature, as well as type and concentration of HC on reactions and deactivation potential on a commercial LNT catalyst was analyzed by lean-rich cycle measurements and subsequent temperature programmed oxidation. Deactivation during the rich phase is omitted initially by O species adsorbed or stored by CeO2, but formation of amorphous carbon and significant deactivation of noble metal sites occur in the presence of C2H2. Carbon formation and deactivation of Pt sites correlate with the reforming reaction of C2H2, thus emphasizing the significance of this reaction in the rapid build-up of carbonaceous deposits. Coking and deactivation decreases with decreasing C2H2 concentration and reaction temperature that determine the extent of reforming reaction. DFT calculations of adsorption energies on sites of Pt (111) or Pt10/CeO2 verify that abundance of HCs on the Pt surface, relative to other feed components, determines which reactions occur and thus the coking degree. This aligns with the finding that HCs with lower adsorption energy, such as C2H4 or C3H6, are less reactive and significant catalyst deactivation is absent during their presence.

Development of new comprehensive kinetic models for Fischer–Tropsch synthesis process over Fe–Co/γ-Al2O3 nanocatalyst in a fixed-bed reactor

Kinetics and mechanisms of Fischer–Tropsch synthesis were investigated over a %10Fe/%10Co/%80γ-Al2O3 nano catalyst prepared by the impregnation method in a fixed bed micro-reactor. The ranges of operating conditions varied as T = 573.15–643.15 K, P = 1–7 bar, H2/CO feed ratio =1–2, and GHSV=3000 h−1. The kinetic expressions for CH4, olefins, paraffins formation, and water gas shift reaction (WGSR) were developed based on the occupied sites in three steps. According to this theory, eighteen rate expressions for WGSR, CO consumption, and products formation were tested, and finally, the ethylene production and water step gave the best fitted kinetic model. The Levenberg–Marquardt algorithm was used to estimate the activation energy (66.01 kJ/mol) and the kinetic parameters. It was found that the proposed model could perfectly predict the effects of water and ethylene. These models can be applied to determine the optimum conditions and products in the reactor. Characterization of catalysts was carried out using different techniques, including BET and SEM.

Vibronically and spin-orbit coupled diabatic potentials for X(2P) + CH4 → HX + CH3 reactions: Neural network potentials for X = Cl.

Vibronically and spin-orbit (SO) coupled diabatic potentials for the Cl(2P) + CH4 → HCl + CH3 reaction are constructed based on a recently developed approach [T. Lenzen and U. Manthe, J. Chem. Phys. 150, 064102 (2019)]. Diabatic potentials and couplings describing the entrance channel of the reaction are obtained based on ab initio data using a diabatization by an ansatz scheme. A detailed investigation of the electronic structure in the entrance channel using multireference configuration interaction (MRCI), coupled cluster [CCSD/CCSD(T)], and SO-MRCI calculations is presented. Neural networks using permutationally invariant polynomials as inputs are employed to represent the elements of the diabatic potential energy matrix. The same set of diabatic states is also used in the transition state region and all four exit channels. Here, the lowest adiabatic potential energy surface (PES) derived from the diabatic model is chosen to reproduce an adiabatic PES recently developed by Li and Guo. The accuracy of the resulting PES is evaluated, and the properties of the newly developed coupled diabatic potentials are analyzed in detail.

Photoelectron-Photofragment Coincidence Studies on the Dissociation Dynamics of the OH-CH4 Complex.

Photoelectron-photofragment coincidence (PPC) spectroscopy was used to characterize the energetics and dynamics of the OH + CH4 → H2O + CH3 reaction initiated by photodetachment of the OH-(CH4) anion complex. PPC measurements at a photon energy of 3.20 eV yielded stable (OH-CH4 + e-) and dissociative (OH + CH4 (ν1 or ν3, v = 0, 1) + e-) channels. The main channel is dissociation to OH + CH4 + e- with a low kinetic energy release (KER), peaking at 0.04 eV. Interpretation of the experimental results was supported by quantum chemistry and quasiclassical trajectory calculations. The anion potential energy surface was constructed at the correlated coupled cluster singles, doubles, and perturbative triples level with augmented correlation consistent polarized valence triple-ζ basis set, and previously calculated neutral potential energy surfaces were used. Quasiclassical simulation of the dynamics of the OH-CH4 complex was carried out by selecting the momenta and coordinates from the Wigner distribution for the anion, providing the starting point for 4000 trajectories on the neutral potential energy surface. In agreement with the experimental results, most of the trajectories yield slowly recoiling OH + CH4 reactants while some are trapped in the entrance channel van der Waals well.

Counter-propagating wave packets in the quantum transition state approach to reactive scattering.

The quantum transition state concept provides an intuitive and numerically efficient framework for the description of quantum state-resolved reactive scattering and thermal reaction processes. Combining multiconfigurational time-dependent Hartree wave packet dynamics calculations with a flux correlation function based analysis, rigorous full-dimensional calculations of initial state-selected and state-to-state reaction probabilities for six atom reactions are feasible. In these calculations, a set of wave packets is generated in the transition state region, propagated into the asymptotic area, and analyzed. In the present work, an alternative approach which employs counter-propagating sets of wave packets is introduced. Outgoing wave packets started in the transition state region are matched with incoming wave packets generated in the reactant (or product) asymptotic area. Studying the H + CH4 → H2 + CH3 reaction as a prototypical example, one finds that the incoming wave packets can be propagated closely up to the transition state region with minor numerical effort. Employing cross correlation functions of incoming and outgoing wavefunctions, the propagation times required for the outgoing wave packet and thus the numerical costs of the entire calculation can be reduced significantly. Detailed full-dimensional calculations studying initial state-selected reaction probabilities for the H + CH4 → H2 + CH3 reaction are presented to illustrate the new approach. It is found that converged results can be obtained using shorter propagation times of the outgoing wave packets and less single-particle functions.

Automated reaction path search calculations of spin-inversion mechanisms in the 6,4,2Nb + C2H4 reaction

The mechanisms of CH bond activation in C2H4 by 6,4,2Nb metal atom have been systematically investigated using the automated reaction path search calculation with the mixed-spin Hamiltonian model. It is found that the most stable 4NbC2H2 + H2 products are formed from the 6Nb + C2H4 reactants on the sextet, quartet, and double potential energy surfaces via multiple spin-inversion (intersystem crossing) points, which are mostly associated with hydrogen atom transfer. The spin-inversion mechanisms have been further investigated by analyzing the crossing behaviors of the potential energy profiles with different spin multiplicities. We also discuss the importance of multidimensional effects on the spin-inversion dynamics.

Kinetic study of the F2 + C2H4 reaction: Disagreement between theory and experiment resolved?

Abstract Reaction F2 + C2H4 → F + C2H3F (1) has been studied using a discharge flow reactor combined with an electron impact ionization mass spectrometer. The reaction rate constant was determined either from kinetics of the reaction product, F-atom, formation or from the kinetics of F2 consumption in excess of C2H4: k1 = (7.94 ± 2.06) × 10−12 exp(−(3867 ± 120)/T) cm3 molecule−1 s−1 at T = 297–833 K. The reaction activation energy, 7.7 ± 0.3 kcal mol−1, is consistent with current theoretical prediction for the reaction barrier height and seems to solve the long-standing problem of the divergence between theory and experiment.