CH4 Reaction Research Articles

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.

Vibronically and spin-orbit coupled diabatic potentials for X(P) + CH4 → HX + CH3 reactions: General theory and application for X(P) = F(2P).

An approach to systematically construct vibronically and spin-orbit coupled diabatic potential energy surfaces (PESs) for X(P) + CH4 → HX + CH3 reactions is proposed. Permutational symmetry and permutational invariants of the S4 group and its S3 and S2 × S2 subgroups are used to construct a diabatic model which properly describes the reaction starting from reactants to products. As a first example, the approach is applied to the construction of diabatic potentials for the F(2P) + CH4 → HF + CH3 reaction. The description of the entrance channel relies on a set of vibronically and spin-orbit coupled diabatic PESs previously developed by Westermann et al. [Angew. Chem., Int. Ed. 53, 1122 (2014)]. The same set of diabatic electronic states is also used in the transition state region and all four exit channels. There the lowest adiabatic PES derived from the diabatic model reproduces the CSBB-PES of Czakó et al. [J. Chem. Phys. 130, 084301 (2009)]. Interesting aspects of the newly developed diabatic potential matrix and the corresponding adiabatic PESs are discussed.

Hydrogen generation by steam reforming of tar model compounds using lanthanum modified Ni/sepiolite catalysts

The paper describes the preparation of Ni/xLa-SEP catalysts selected low-cost clay minerals as support that possess high catalytic performance for steam reforming of phenol-ethanol (SRPE) to sustainably produce hydrogen. The catalysts were characterized using inductively coupled plasma optical emission spectrometry (ICP-OES), N2 adsorption-desorption, X-ray diffraction (XRD), H2 pulse chemisorption, high-resolution transmission electron microscopy (HRTEM), hydrogen temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the addition of La reduced the active metal size and increased the active metal surface of the Ni/SEP catalyst. The Ni/10La-SEP catalyst exhibited the smallest active metal size (5 ± 0.5 nm) and highest active metal surface area (12.7 m2/gcat.). As expected, Ni/10La-SEP presented the highest carbon conversion and H2 yield (92.1 and 87.6%, respectively) during SRPE reactions. Moreover, the La addition enhanced the steam reforming (SR) of CH4 and water-gas shift (WGS) reactions and successfully suppressed the sintering and coking of the Ni/SEP catalyst.

Dissection of the multichannel reaction of acetylene with atomic oxygen: from the global potential energy surface to rate coefficients and branching dynamics.

The O(3P) + C2H2 reaction is the first step in acetylene oxidation. The accurate kinetic data and the understanding of the reaction dynamics is of great importance. To this end, a full-dimensional global potential energy surface (PES) for the ground triplet state of the O(3P) + C2H2 reaction is constructed based on approximately 85 000 ab initio points calculated at the level of explicitly correlated unrestricted coupled cluster single, double, and perturbative triple excitations with the explicitly correlated polarized valence triple zeta basis set (UCCSD(T)-F12b/VTZ-F12). The PES is fit using the permutation invariant polynomial-neural network (PIP-NN) approach with a total root mean square error of 0.21 kcal mol-1. The key topographic features of the PES, including multiple potential wells and saddle points along different reaction pathways, are well represented by this fit PES. The kinetics and dynamics of the O(3P) + C2H2 reaction are investigated using the quasi-classical trajectory (QCT) method. The calculated rate coefficients are in good agreement with experimental data over a wide temperature range, especially when the temperature is lower than 1500 K. The product branch ratio has also been determined, which indicates the H + HCCO channel as the dominant reaction pathway at 298-3000 K, accounting for 80-90% of the overall rate coefficient, in agreement with experimental observations. The dynamics of the reaction is analyzed in detail.

Symmetry-breaking effects on time-dependent dynamics: correct differential cross sections and other properties in H+ + C2H4 at ELab = 30 eV.

We present a computational procedure that introduces low degrees of symmetry breaking into a restricted Hartree-Fock (RHF) state in order to induce higher symmetry breaking during the state's subsequent dynamics. The symmetries herein considered are those of electronic HF states as classified by Fukutome; those symmetries affect bond dissociations and internal rotations among other phenomena. Therefore, this investigation extends a part of Fukutome's time-independent analysis of symmetry breaking to the time-dependent (dynamical) regime. The procedure is formulated in the framework of the simplest-level electron nuclear dynamics, a time-dependent, variational, on-the-fly and non-adiabatic method that employs classical dynamics for the nuclei and a Thouless single-determinantal state for the electrons. We test this procedure on the H+ + C2H4 reaction at 30 eV due to its conspicuous display of symmetry-breaking effects; this reaction is relevant in astrophysics and proton cancer therapy. Fukutome's axial spin density wave (ASDW) HF state is used to represent the symmetry-broken initial states. Through a Thouless parameter, small degrees of symmetry breaking are introduced into the initial ASDW states in a controlled manner. After starting the dynamics from those states, higher degrees of symmetry breaking emerge or not as determined by the direct-dynamics equations without external interventions. Simulations starting from symmetry-conforming states preserve symmetry features during dynamics, whereas simulations starting from symmetry-broken states display an upsurge of symmetry breaking when the reactants collide. Initial symmetry breaking increases the total integral cross sections of collision-induced fragmentations and of target-to-proton 1-electron-transfer reactions and decreases the scattering angle function and primary rainbow angle of the outgoing projectile. Remarkably, symmetry-breaking simulations reproduce the correct relative order and values of the experimental 0- and 1-electron-transfer differential cross sections, whereas symmetry-conforming simulations predict incorrect order and values. Our calculated scattering angle functions and differential cross sections also exhibit expected primary and secondary rainbow angle features that experiments fail to detect. A detailed discussion on the description of symmetry-breaking processes with the ASDW and Thouless states is included to provide a rigorous theoretical basis for this investigation.

The hydrogen abstraction reaction H + C2H6→ H2(v,j) + C2H5. Part II. Theoretical kinetics and dynamics study.

Two important issues motivated the present study: the role of the tunnelling contribution at low temperatures and the role of the alkyl fragment in the dynamics. Using a recently developed full-dimensional analytical potential energy surface (PES), named PES-2018 (Part I), kinetics and dynamics studies were performed. The kinetics study was performed using the variational transition-state theory with multidimensional tunnelling over the temperature range of 200-2000 K. At high temperatures, T≥ 400 K, the calculated thermal rate constants reproduce the experimental evidence, with differences of 25%, with respect to experimental measures, while at low temperatures, T≤ 300 K, the tunnelling effect plays an important role although, unfortunately, no experimental information is available for comparison. We found that the tunnelling contribution is strongly dependent on the theoretical approach used to calculate it, with differences of a factor of about ∼30. For the dynamics, quasi-classical trajectory calculations were performed at different collision energies in the range of 10-50 kcal mol-1, taking into account the zero-point energy violation problem in the final analysis. Excitation function increased with collision energy, reproducing the experimental values, and the H2(v,j) product showed cold vibrational and rotational distributions, thus again simulating experiments. We found that the ethyl radical product presents small internal energy, similar to the methyl radical product in the H + CH4 reaction, indicating that a priori the size of the alkyl radical does not play an important role in the dynamics.

Detailed benchmark ab initio mapping of the potential energy surfaces of the X + C2H6 [X = F, Cl, Br, I] reactions.

We investigate three reaction pathways of the X + C2H6 [X = F, Cl, Br, I] reactions: H-abstraction, methyl-substitution, and H-substitution, with the latter two proceeding via either Walden-inversion or front-side-attack mechanisms. We report classical and adiabatic relative energies of unprecedented accuracy for the corresponding stationary points of the reaction potential energy surfaces (PESs) by augmenting the CCSD(T)-F12b/aug-cc-pVQZ energies by core-correlation, post-CCSD(T) and spin-orbit corrections. Taking these correction terms into account turns out to be essential to reach subchemical, i.e. <0.5 kcal mol-1, accuracy. Our new benchmark 0 K reaction enthalpies show excellent agreement with experimental data. Spin-orbit coupling in these open-shell systems is also monitored throughout the reaction paths and found to be non-negligible even in some transition-state geometries. Barrier heights corresponding to the different channels of the title reactions appear in the same order with increasing energy: H-abstraction, Walden-inversion methyl-substitution, Walden-inversion H-substitution, front-side-attack H-substitution and front-side-attack methyl-substitution, except for X = I where the latter two come in reverse order. Similarly, product channels follow the energy order of the corresponding barrier heights in all four cases. We find strongly reactant-like transition-state structures for the exothermic F + C2H6 reaction paths, while more and more product-like transition states are observed along with increasing endothermicity as going from Cl to I. Several entrance and exit channel minima are also identified for the studied reactions with significant spin-orbit effects for the formers.

Kinetics of the Hydrogen Abstraction PAH + •OH → PAH Radical + H2O Reaction Class: An Application of the Reaction Class Transition State Theory (RC-TST) and Structure-Activity Relationship (SAR).

A reaction class transition state theory (RC-TST) augmented with structure-activity relationship (SAR) methodology is applied to predict high-pressure limit thermal rate constants for hydrogen abstraction by •OH radical from polycyclic aromatic hydrocarbons (PAHs) reaction class in the temperature range of 300-3000 K. The rate constants for the reference reaction of C6H6 + •OH → C6H5 + H2O is calculated by the canonical variational transition state theory (CVT) with small curvature tunneling (SCT). Only the reaction energy is needed to predict RC-TST rates for other processes within the family, the parameters needed were obtained from M06-2X/cc-pVTZ data for a training set of 34 reactions. The systematic error of the resulting RC-TST rates is smaller than 50% in comparison with explicit rate calculations, which facilitates application of the proposed methodology to the automated reaction mechanism generators (ARMGs) schemes.

Kinetic Modeling of the Metal/Support Interaction for CH4 Reaction over Oxidized Pd/Al2O3

This paper deals with the kinetics of CH4 dissociation on model Pd/Al2O3 catalysts. The adsorption and conversion of CH4 over oxidized catalysts, fresh or aged, were studied with a temporal analysis of products (TAP) reactor. Single CH4 pulse TAP experiments were performed on a stabilized surface. The experiments are discussed in the light of a selected mechanism involving CH4 decomposition into CO2 taking into account an interaction between the metallic active sites and the support. TAP experiments over the oxidized catalysts confirm the involvement of the metal/support interface, with a spill-over effect. Optimized kinetics parameters validate this interaction of OH species adsorbed on Pd sites with alumina, showing a relatively good agreement between experimental and calculated outlet gas composition. Both fresh and aged catalysts follow the chosen mechanism with the same kinetic constants at 400 °C only an alteration of the catalytic surface areas explain the loss of activity.

Simulation of experimental imaging results for four isotopic variants of the OH + CH4 reaction with a simple and relatively accurate theoretical approach

In 2005, Kopin Liu and co-workers used crossed molecular beam and velocity map imaging technics in order to measure pair-correlated product speed distributions for the OH + CH4 reaction and seven isotopic variants. We recently reproduced half of them with good accuracy by means of the classical trajectory method in a quantum spirit, which takes into account the semiclassical quantization of product vibrational motions. The second half are successfully reproduced here by using the same approach. The present results constitute one more proof of the ability of the classical trajectory method to provide satisfying predictions for polyatomic processes provided that a few quantum constraints are included in the calculations.

Effect of hydrogen additive on methane decomposition to hydrogen and carbon over activated carbon catalyst

The effect of H2 addition on CH4 decomposition over activated carbon (AC) catalyst was investigated. The results show that the addition of H2 to CH4 changes both methane conversion over AC and the properties of carbon deposits produced from methane decomposition. The initial methane conversion declines from 6.6% to 3.3% with the increasing H2 flowrate from 0 to 25 mL/min, while the methane conversion in steady stage increases first and then decreases with the flowrate of H2, and when the H2 flowrate is 10 mL/min, i.e. quarter flowrate of methane, the methane conversion over AC in steady stage is four times more than that without hydrogen addition. It seems that the activity and stability of catalyst are improved by the introduction of H2 to CH4 and the catalyst deactivation is restrained. Filamentous carbon is obtained when H2 is introduced into CH4 reaction gas compared with the agglomerate carbon without H2 addition. The formation of filamentous carbon on the surface of AC and slower decrease rate of surface area and pores volume may cause the stable activity of AC during methane decomposition.

Study of gas-phase reactions within the modified Marcus model. II. CH3OH + CH3 → CH2OH + CH4

In the framework of the Marcus theoretical model, using the intramolecular reorganization (IMR) method, the kinetics of the CH3OH + CH3 → CH2OH + CH4 reaction was studied over the temperature range of 10–550 K. The electronic activation energy (Ea) was calculated at the UCCSD(T)/6-31+G**//B3LYP/6–31+G** level. The calculation of the reaction rate constant, k, defined as the integral over the distance Q(C⋯C), was carried out on the assumption that for a hydrogen atom in the activated complex (AC) two extreme types of vibrations are possible: isolated vibration in the reagent well (oscillatory model 1, in which all frequencies are identified as real) and vibration in the C⋯H⋯C structure (oscillatory model 2 corresponding to TST). In both cases, the temperature dependence of the rate constant includes nonlinear (80–550 K) and linear (10–80 K) sections. It is shown that the change in the dependence character is due to the presence of a minimum in the Q - Ea plot. Kinetic data were used to analyse the “temperature plateau” phenomenon, observed for the CH3OH + CH3 reaction in the solid phase. The course of the temperature dependence of this reaction at 10–150 K can be reproduced assuming that the energy of the reagents in the solid phase is close to the average for the effective interval Q of the enthalpy of activation of the gas-phase reaction, ΔHm∗, in a temperature range of 10–40 K. The value of the deuterium KIE for model 1 is ∼104, which is about an order of magnitude higher than the experimental value.

The C2H2 + NO2 reaction: Implications for high pressure oxidation of C2H2/NOx mixtures

Abstract The reaction of C2H2 with NO2 has been studied theoretically. It is a complex overall reaction with multiple wells and multiple product channels. The calculated rate constant for the preferred channel, formation of a CHOCHON adduct, is compatible with the only experimental determination. The CHOCHON adduct is assumed to dissociate rapidly to form the triplet carbene CHCHO and NO. An experimental and kinetic modeling study of the interaction between C2H2, O2 and NOx was performed under flow reactor conditions in the intermediate temperature range (600–900 K), high pressure (50–60 bar), and for stoichiometries ranging from reducing to strongly oxidizing. The results show that presence of NOx serves both to sensitize and inhibit oxidation of C2H2. Calculations with a detailed chemical kinetic model, partly established in the present work, confirm that C2H2 + NO2 is the major initiation step, as well as the major sensitizing reaction. This reaction converts NO2 to NO, which is then partly converted to HCN by reaction with C2H3 and CHCHOH. The latter reactions are both chain terminating and serve as the major inhibiting steps.

Excited state photodissociation dynamics of 2-, 3-, 4-hydroxyacetophenone: Theoretical study

Photodissociation of 2-, 3-, and 4-hydroxyacetophenone (2-, 3-, and 4-HAP) has been studied by analyzing excited-state potential energy surfaces (PES). For that, ab initio CASSCF(12,12)/6–31++G(d,p) calculations of low-lying excited states have been performed. Molecular beam study of photodissociation of 2-, 3-, and 4-HAP under laser excitation at 193 nm revealed that there are three possible dissociation channels of HAP: separation of H atom, CH3 and COCH3 fragments. In 2-HAP isomer the H separation channel is quenched. In this study we explain the quenching mechanism of the H separation channel and suggest a new model of CH3 and COCH3 fragment elimination reactions. Calculations of the excited-state PES reveal that the most of H, CH3 and COCH3 fragments arise after relaxation into the second, optically “dark” 2A″ state, while some of the reaction products result from the 1A″ state dissociation. Dissociation of 2-HAP through H separation in the 2A″ state is deemed impossible, which explains the absence of deprotonated fragments in the 2-HAP mass spectrum. Breaking of intramolecular hydrogen bond in 2-HAP needs much more energy than the CH3 detachment, which makes relaxation through the S1 − S0 conical intersection unfavorable.

Fragmentation of C2H2 2+ in collisions with 100 eV electrons

The fragmentation of C2H2 2+ dications following 100 eV electron impact double ionization of acetylene was studied experimentally using a cold target recoil-ion momentum spectroscopy. A weak extraction field was adopted to increase the experimental resolution. Meanwhile, as a consequence of the large momentum generated by the Coulomb repulsive force, the particles which have similar charge to mass ratio such as C+ and CH+ cannot be distinguished directly from the time of flight. In order to obtain a comprehensive understanding of the experimental results, a Monte Carlo (M-C) method was used to simulate the time of flight under experimental conditions. With the help of the M-C method, four reaction channels, the Coulomb explosion of the doubly charged parent ions C2H2 2+ into the fragment ions CH+ + CH+,C+ + CH+2, C+ + CH+, C+ + C+, were simulated and the kinetic energy release (KER) distributions were deduced. In particular, the structure of fragmentation channel to C+ + CH+ represents a typical feature of the absence of hydrogen migration, which was not observed in previous studies. The KER in the fragmentation reaction of C2H2 2+ has been used to obtain precursor state energy for the formation of product ion pairs. In general, good agreement was achieved between our KER results and available experimental data in previous works.

A very limited role of tropospheric chlorine as a sink of the greenhouse gas methane

Abstract. Unexpectedly large seasonal phase differences between CH4 concentration and its 13C ∕ 12C isotopic ratio and their inter-annual variations observed in southern hemispheric time series have been attributed to the Cl + CH4 reaction, in which 13CH4 is discriminated strongly compared to OH + CH4, and have provided the only (indirect) evidence of a hemispheric-scale presence of oxidative cycle-relevant quantities of tropospheric atomic Cl. Our analysis of concurrent New Zealand and Antarctic time series of CH4 and CO mixing and isotope ratios shows that a corresponding 13C ∕ 12C variability is absent in CO. Using the AC-GCM EMAC model and isotopic mass balancing for comparing the periods of presumably high and low Cl, it is shown that variations in extra-tropical Southern Hemisphere Cl cannot have exceeded 0.9 × 103 atoms cm−3. It is demonstrated that the 13C ∕ 12C ratio of CO is a sensitive indicator for the isotopic composition of reacted CH4 and therefore for its sources. Despite ambiguities about the yield of CO from CH4 oxidation (with this yield being an important factor in the budget of CO) and uncertainties about the isotopic composition of sources of CO (in particular biomass burning), the contribution of Cl to the removal of CH4 in the troposphere is probably much lower than currently assumed.

Direct Stepwise Oxidation of Methane to Methanol over Cu–SiO2

Cu supported on SiO2 can be used to directly convert methane to methanol in a stepwise process with no intrinsic need for a zeolite support. Effects of parameters such as the O2 activation temperature, activation time, CH4 reaction temperature, CH4 partial pressure (pCH4), and Cu wt % on methanol yield were investigated. Increasing the O2 activation temperature in the 200–800 °C range significantly improved the methanol yield, and when carried out at 800 °C, a methanol yield of 11.5 μmol/gcatalyst was obtained after reaction with methane at 200 °C for the sample with 2 wt % Cu. Yield per mole of Cu increased exponentially from 1.0 to 59.1 mmol with decreased Cu wt % from 30 to 1, respectively. The increase in the O2 activation time also strongly influenced the yield which corresponded to the increase in yield by a factor of >2 between 1 and 8 h. Increasing pCH4 from 0.05 to 1 atm resulted in a 5-fold increase in yield after activation at 450 °C; however, it resulted in at least 20% lower yields after acti...

Enhanced activity of TiO2 by concentrating light for photoreduction of CO2 with H2O to CH4

Concentrating light technology is used to improve photoreduction of CO2 to CH4 reaction on semiconductor catalysts such as TiO2. The catalysts were prepared by anodization method and characterized by XRD, SEM, XPS and photoillumiation. The results showed that the CH4 yield rate could be raised from 0.70 μmol g−1 h−1 to 20.67 μmol g−1 h−1 under concentrating light, and it could be further increased to 28.49 μmol g−1 h−1 after pretreatment in the air. It is inferred that the concentrating light increases CO2 conversion not only by heightening light intensity, but also by improving the surface properties of the catalyst.

Efficient removal of benzene in air at atmospheric pressure using a side-on type 172nm Xe2 excimer lamp.

The photochemical removal of benzene was studied in air at atmospheric pressure using a side-on type 172nm Xe2 excimer lamp with a wide irradiation area. After 1.5min photoirradiation, C6H6 (1000ppm) in air was completely converted to HCOOH, CO, and CO2 at a total flow rate of 1000mL/min. The initial decomposition rate of C6H6 was determined to be 1.18min-1. By using a flow system, C6H6 (200ppm) was completely removed at a total flow rate of 250mL/min. The conversion of C6H6 and the energy efficiency in the removal of C6H6 changed in the 31-100% and 0.48-1.2g/kWh range, respectively, depending on the flow rate, the O2 concentration, and the chamber volume. On the basis of kinetic model simulation, dominant reaction pathways were discussed. Results show that the O(3P) + C6H6 reaction plays a significant role in the initial stage of the C6H6 decomposition. Important experimental parameters required for further improvement of the C6H6 removal apparatus using a 172 excimer lamp were discussed based on model calculations.