CH4 Reaction Research Articles

DFT mechanistic study of reactions of С6H6 and 1,3,5-Ad3C6H3 with CBr3. The first example of hydride transfer from aromatic C[sbnd]H bond to electrophile

The DFT B3LYP/6-31G* calculations were carried out for the reactions of C6H6 and Ad3C6H3 (Ad=1,3,5-adamantyl) with superelectrophile CBr3+ as a model of superelectrophilic catalyst CBr3+ Al2Br7−. The reaction of C6H6 with CBr3+ proceeds via the classical scheme of electrophilic reactions of aromatic C−H bond to form initially the barrier-free σ-complex C6H6CBr3+. This mechanism was confirmed by the aug-cc-pVDZ basis set calculations. The reaction of Ad3C6H3 with CBr3+ occurs via a quite novel mechanism involving aryl cation formation followed by hydride abstraction of the Ar+ from the 2-Ad group and the rearrangement of the 2-Ad+ cation into the 4-phenyl-4-protoadamantyl cation. The hydride transfer from both arenes was shown to be more favorable than H radical transfer by more than 40 and 55kcalmol−1 in the case of C6H6 and Ad3C6H3, respectively.

Quantum Tunneling Rates of Gas-Phase Reactions from On-the-Fly Instanton Calculations.

The instanton method obtains approximate tunneling rates from the minimum-action path (known as the instanton) linking reactants to the products at a given temperature. An efficient way to find the instanton is to search for saddle-points on the ring-polymer potential surface, which is obtained by expressing the quantum Boltzmann operator as a discrete path-integral. Here we report a practical implementation of this ring-polymer form of instanton theory into the Molpro electronic-structure package, which allows the rates to be computed on-the-fly, without the need for a fitted analytic potential-energy surface. As a test case, we compute tunneling rates for the benchmark H + CH4 reaction, showing how the efficiency of the instanton method allows the user systematically to converge the tunneling rate with respect to the level of electronic-structure theory.

CO2 reforming of CH4 over a highly active and stable Ni[sbnd]Mg[sbnd]Al catalyst

Although the catalytic systems containing the NiOMgO solid solution possess excellent catalytic performance in CO2 reforming of CH4 reaction, a serious drawback was that the higher than 10 wt% Ni loadings would increase the burden on the environment and finance. A NiMgAl catalyst with 3 wt% Ni loadings derived from a hydrotalcite precursor was prepared in order to study the CO2 reforming of CH4 reaction. For comparative purpose, a NiOMgO catalyst and a Ni/γ-Al2O3 catalyst containing NiAl2O4, were also synthesized. Analysis showed that the NiMgAl catalysts had high catalytic activity due to the improved reducibility which was caused by the presence of Al3+ ions in the NiOMgO cubic lattice. Moreover, the NiMgAl catalyst showed ultra-stability without carbon formation for 320 h period due to the small Ni particles and the suitable Ni-support interaction which effectively prevented the sintering of Ni particles.

Thermal Rate Constants for the O(3P) + CH4 → OH + CH3 Reaction: The Effects of Quantum Tunneling and Potential Energy Barrier Shape.

The rate constants and kinetic isotope effects for the O(3P) + CH4 reaction have been investigated with the quantum instanton method in full dimensionality. The calculated rate constants are in good agreement with the experimental values above 400 K, below which the measured values are scattered. Compared to other theoretical approaches, the quantum instanton method predicts the largest quantum tunneling effect, so it gives the largest rate constants at low temperatures. The calculated kinetic isotope effects are always much larger than 1 and increase with decreasing temperature, due to the zero-point energy and quantum tunneling. Our calculations on different potential energy surfaces demonstrate that the potential energy barrier shape dominates the magnitude of quantum tunneling and has a great effect on the kinetic isotope effect.

CO2 Electrochemical Reduction to Hydrocarbon Fuels on Copper Nanoparticles Supported on Nanostructured Carbons

Recently [1,2], we reported activity and selectivity of Cu nanoparticles for electroreduction of CO2 to hydrocarbon fuels (CH4 and C2H4) on three nanostructured supports: reduced graphene oxide (rGO), single walled carbon nanotubes (SWNT) and onion-like carbon (OLC) [3]. Cu/OLC catalyst was found to be superior to the other two catalysts in terms of stability, activity and selectivity towards C2H4generation. We propose that the unusual Cu/OLC catalyst design offers superior activity and stability. The nanostructured catalyst envelops a shell of OLC around Cu nanoparticles; this layering acts as a filter that protects the catalyst surface from undesirable adsorbed species. Herein, we examine the effect of nanostructured support on catalysts’ activity and selectivity in more detail. In addition, we determine whether supported Cu nanoparticles are durable enough to withstand potential excursions to open circuit potential, which are commonly encountered during CO2electrolysis. Cu nanoparticles of 15-25 nm diameters on different carbon supports were synthesized by the reduction of CuCl2 by NaBH4. This procedure was carried out in suspensions of carbon supports in aqueous ethylene glycol solutions (20% v/v). X-Ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-Ray photoelectron spectroscopy (XPS) and pair-distribution function (PDF) were used for ex-situanalysis of nanoparticles. The catalytic activities of supported Cu nanoparticles and their selectivities towards hydrocarbon generation were evaluated using (1) a sealed rotating disk electrode (RDE) setup connected to a gas chromatograph (GC) and (2) a scanning flow cell (SFC) coupled to differential electrochemical mass-spectrometer (DEMS) [4]. Durability of supported Cu nanoparticles was evaluated using SFC connected to inductively coupled plasma mass spectrometer (ICP-MS) setup [5]. Fig. 1 shows partial currents of H2 (A), CH4 (B), C2H4 (C),CH3OH (D) and CO2electroreduction reaction total currents (E) measured on Cu/OLC, Cu/SWNT and Vulcan Carbon-supported Cu (Cu/VC) catalysts in DEMS-SFC configuration. Cu/VC catalyst is shown as a reference. Structural HRTEM and PDF (Fig. 2) analysis shows long-range ordering of the Cu-OLC catalyst and confirms high crystallinity of synthesized Cu nanoparticles. This presentation will provide analysis of the effect of support on catalytic activity, selectivity and durability of Cu nanoparticles as applied to electrochemical CO2reduction.

H+CH4 → H2 + CH3 initial state-selected reaction probabilities on different potential energy surfaces

Initial state-selected reaction probabilities for the H+CH4→H2+CH3 reaction on a recently developed potential energy surface which employs neutral network fitting based on permutational invariant polynomials are reported. The quantum dynamics calculations use the quantum transition state concept and the multi-layer multi-configurational time-dependent Hartree approach and study the reaction process in full-dimensionality for vanishing total angular momentum. A detailed comparison with previous results obtained on other high-level potential energy surfaces is given. The connection between the level of quantum state resolution and the sensitivity of the results on differences in the potential energy surfaces is highlighted. Employing a decomposition of the total reactivity into contributions of the different vibrational states of the activated complex, it is found that differences between the potential energy surfaces are mainly related to the umbrella motion of the methyl group.

Experimental determination of the activation energies of [formula omitted], [formula omitted] and [formula omitted] reactions on [formula omitted

In the present work we experimentally determine the activation energies of CH4,SO2 and O2 reactions on Cr2O3/γ-Al2O3. To our knowledge there is no previous determination of these parameters, so fundamental information is provided to determine the velocity laws of these reactions and understand their kinetic behavior.

Temperature- and pressure-dependent rate coefficients for the HACA pathways from benzene to naphthalene

RRKM-Master Equation calculations have been performed to evaluate temperature- and pressure-dependent rate coefficients for acetylene addition reactions to the C6H5, C6H4C2H, C6H5C2H2, and C6H4C2H3 radicals. These calculations indicate a strong pressure dependence for the role of various Hydrogen-Abstraction-C2H2-Addition (HACA) sequences for the formation of naphthalene from benzene. At atmospheric and lower pressures the C8H7 radicals, C6H4C2H3 and C6H5C2H2, cannot be stabilized above 1650K. As a result, both the Bittner–Howard HACA route, in which a second acetylene molecule adds to C6H5C2H2, and the modified Frenklach route, where a second C2H2 adds to the aromatic ring of C6H4C2H3 obtained by internal hydrogen abstraction, are unrealistic under low pressure flame conditions. At the higher pressures of some practical combustion devices (e.g., 100atm) these routes may be operative. Naphthalene is predicted to be the main product of the C6H5C2H2+C2H2 and C6H4C2H3+C2H2 reactions in the entire 500–2500K temperature range independent of pressure (ignoring the issues related to the instability of C8H7 species). Frenklach's original HACA route, where the second C2H2 molecule adds to the aromatic ring activated by intermolecular H abstraction from C8H6, involves the C6H4C2H+C2H2 reaction, which is shown to predominantly form dehydrogenated species with a naphthalene core (naphthyl radicals or naphthynes) at T <2000K and diethynylbenzene at higher temperatures. The temperature and pressure dependence of rate coefficients for the various reaction channels has been analyzed and the results clearly demonstrate the importance of pressure for the reaction outcome. Thus, one must use caution when using low-pressure flame studies to validate PAH mechanisms for use in broader ranges of pressure.

Temperature dependent product yields for the spin forbidden singlet channel of the C(3P) + C2H2 reaction

The atomic hydrogen formation channels of the C+C2H2 reaction have been investigated using a continuous supersonic flow reactor over the 52–296K temperature range. H-atoms were detected directly at 121.567nm by vacuum ultraviolet laser induced fluorescence. Absolute H-atom yields were determined by comparison with the H-atom signal generated by the C+C2H4 reaction. The product yields agree with earlier crossed beam experiments employing universal detection methods. Incorporating these branching ratios in a gas-grain model of dense interstellar clouds increases the c-C3H abundance. This reaction is a minor source of C3-containing molecules in the present simulations.

Theoretical kinetics of O + C2H4

The reaction of atomic oxygen with ethylene is a fundamental oxidation step in combustion and is prototypical of reactions in which oxygen adds to double bonds. For 3O+C2H4 and for this class of reactions generally, decomposition of the initial adduct via spin-allowed reaction channels on the triplet surface competes with intersystem crossing (ISC) and a set of spin-forbidden reaction channels on the ground-state singlet surface. The two surfaces share some bimolecular products but feature different intermediates, pathways, and transition states. The overall product branching is therefore a sensitive function of the ISC rate. The 3O+C2H4 reaction has been extensively studied, but previous experimental work has not provided detailed branching information at elevated temperatures, while previous theoretical studies have employed empirical treatments of ISC. Here we predict the kinetics of 3O+C2H4 using an ab initio transition state theory based master equation (AITSTME) approach that includes an a priori description of ISC. Specifically, the ISC rate is calculated using Landau–Zener statistical theory, consideration of the four lowest-energy electronic states, and a direct classical trajectory study of the product branching immediately after ISC. The present theoretical results are largely in good agreement with existing low-temperature experimental kinetics and molecular beam studies. Good agreement is also found with past theoretical work, with the notable exception of the predicted product branching at elevated temperatures. Above ∼1000K, we predict CH2CHO+H and CH2+CH2O as the major products, which differs from the room temperature preference for CH3+HCO (which is assumed to remain at higher temperatures in some models) and from the prediction of a previous detailed master equation study.

Theoretical Study of the Reactions of Methane and Ethane with Electronically Excited N2(A(3)Σu(+)).

Comprehensive quantum chemical analysis with the usage of density functional theory and post-Hartree-Fock approaches were carried out to study the processes in the N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 systems. The energetically favorable reaction pathways have been revealed on the basis of the examination of potential energy surfaces. It has been shown that the reactions N2(A(3)Σu(+)) + CH4 and N2(A(3)Σu(+)) + C2H6 occur with very small or even zero activation barriers and, primarily, lead to the formation of N2H + CH3 and N2H + C2H5 products, respectively. Further, the interaction of these species can give rise the ground state N2(X(1)Σg(+)) and CH4 (or C2H6) products, i.e., quenching of N2(A(3)Σu(+)) by CH4 and C2H6 molecules is the complex two-step process. The possibility of dissociative quenching in the course of the interaction of N2(A(3)Σu(+)) with CH4 and C2H6 molecules has been analyzed on the basis of RRKM theory. It has been revealed that, for the reaction of N2(A(3)Σu(+)) with CH4, the dissociative quenching channel could occur with rather high probability, whereas in the N2(A(3)Σu(+)) + C2H6 reacting system, an analogous process was little probable. Appropriate rate constants for revealed reaction channels have been estimated by using a canonical variational theory and capture approximation. The estimations showed that the rate constant of the N2(A(3)Σu(+)) + C2H6 reaction path is considerably greater than that for the N2(A(3)Σu(+)) + CH4 one.

S-matrix decomposition, natural reaction channels, and the quantum transition state approach to reactive scattering.

A new approach for the quantum-state resolved analysis of polyatomic reactions is introduced. Based on the singular value decomposition of the S-matrix, energy-dependent natural reaction channels and natural reaction probabilities are defined. It is shown that the natural reaction probabilities are equal to the eigenvalues of the reaction probability operator [U. Manthe and W. H. Miller, J. Chem. Phys. 99, 3411 (1993)]. Consequently, the natural reaction channels can be interpreted as uniquely defined pathways through the transition state of the reaction. The analysis can efficiently be combined with reactive scattering calculations based on the propagation of thermal flux eigenstates. In contrast to a decomposition based straightforwardly on thermal flux eigenstates, it does not depend on the choice of the dividing surface separating reactants from products. The new approach is illustrated studying a prototypical example, the H + CH4 → H2 + CH3 reaction. The natural reaction probabilities and the contributions of the different vibrational states of the methyl product to the natural reaction channels are calculated and discussed. The relation between the thermal flux eigenstates and the natural reaction channels is studied in detail.

Full-dimensional quantum dynamics study of the H2 + C2H → H + C2H2 reaction on an ab initio potential energy surface

This work performs a time-dependent wavepacket study of the H2 + C2H → H + C2H2 reaction on a new ab initio potential energy surface (PES). The PES is constructed using neural network method based on 68 478 geometries with energies calculated at UCCSD(T)-F12a/aug-cc-pVTZ level and covers H2 + C2H↔H + C2H2, H + C2H2 → HCCH2, and HCCH2 radial isomerization reaction regions. The reaction dynamics of H2 + C2H → H + C2H2 are investigated using full-dimensional quantum dynamics method. The initial-state selected reaction probabilities are calculated for reactants in eight vibrational states. The calculated results showed that the H2 vibrational excitation predominantly enhances the reactivity while the excitation of bending mode of C2H slightly inhibits the reaction. The excitations of two stretching modes of C2H molecule have negligible effect on the reactivity. The integral cross section is calculated with J-shift approximation and the mode selectivity in this reaction is discussed. The rate constants over 200-2000 K are calculated and agree well with the experimental measured values.

An in-situ synthesis of low-cost mesostructured nickel nanoparticles embedded carbon/silica composite via a solid–liquid grinding route and its application for the carbon dioxide reforming of methane

A facile synthesis route for directly preparing Ni-based mesoporous carbon/silica composite catalysts without further reduction was synthesized successfully. The synthesis procedure was achieved by using the one-step solid–liquid grinding method with a mixture of SiO2, soybean oil, and nickel acetyl acetonate. Calcination was then performed with different temperatures to obtain the Ni-based mesoporous carbon/silica composite catalyst. All experimental steps were conducted without solvent addition. In this catalyst structure, soybean was used as the carbon source and provided a large surface area within the silica skeleton; Ni0 was used as the active metal. The active component Ni0 was formed directly during the soybean oil carbonization process. A series of Ni/SiO2–C–X catalysts (X = 400–1100, X stands for the carbonization temperature) were applied for the CO2 reforming of CH4 reaction. The unreduced Ni-based catalyst showed a better catalytic activity than that of the reduced catalyst under hydrogen atmosphere because the NiO phase was in-situ reduced by the carbonized soybean under nitrogen. The Ni/SiO2–C–500 catalyst showed stable activity and high reaction activity in the continuous CO2 reforming reaction of CH4 at atmospheric pressure.

Ring polymer molecular dynamics fast computation of rate coefficients on accurate potential energy surfaces in local configuration space: Application to the abstraction of hydrogen from methane.

To fast and accurately compute rate coefficients of the H/D + CH4 → H2/HD + CH3reactions, we propose a segmented strategy for fitting suitable potential energy surface (PES), on which ring-polymer molecular dynamics (RPMD) simulations are performed. On the basis of recently developed permutation invariant polynomial neural-network approach [J. Li et al., J. Chem. Phys. 142, 204302 (2015)], PESs in local configuration spaces are constructed. In this strategy, global PES is divided into three parts, including asymptotic, intermediate, and interaction parts, along the reaction coordinate. Since less fitting parameters are involved in the local PESs, the computational efficiency for operating the PES routine is largely enhanced by a factor of ∼20, comparing with that for global PES. On interaction part, the RPMD computational time for the transmission coefficient can be further efficiently reduced by cutting off the redundant part of the child trajectories. For H + CH4, good agreements among the present RPMD rates and those from previous simulations as well as experimental results are found. For D + CH4, on the other hand, qualitative agreement between present RPMD and experimental results is predicted.

Quasi-Bound States of the F·CH4 Complex

The F + CH4 → HF + CH3 reaction is an intensively studied prototypical example of a polyatomic reaction showing an early transition state. Prereactive complexes are assumed to play an important role in the dynamics of the reaction. In this work, the long-living resonance states resulting from the formation of a metastable F·CH4 complex are investigated in detail. Full-dimensional quantum dynamics calculations employing the multiconfigurational time-dependent Hartree (MCTDH) approach and a single adiabatic potential energy surface are used to study the low-lying quasi-bound states of the F·CH4 complex for vanishing total (nuclear) angular momentum. The computed dissociation energy of the F·CH4 complex with respect to the reactant asymptote is 170 cm(-1). About 60 resonance states with energies below the reactant asymptote are found. A detailed analysis of the computed wave function of the low-lying states shows an almost free relative rotation of F and CH4 and an approximately separable F-CH4 stretching vibration. The present results are compared with transition state spectroscopy experiments which study the photodetachment spectrum of the CH4F(-) anion.

Fate and reactions of methane during biodegradation in an aquifer contaminated with petroleum hydrocarbons in Northeast China

Biodegradation is well known as the most effective and important process in monitored natural attenuation (MNA). Previous studies at a crude oil spill site in Northeast China have documented the progress of oil biodegradation by analyzing geochemical indicators. However, without evidence about the behavior of methane (CH4), methanogenesis was only speculated to be occurring at the source area of the site. In this study, CH4 content, radiocarbon, and stable carbon isotopes of CH4 were analyzed to confirm the occurrence of methanogenesis during the biodegradation of petroleum hydrocarbons (PHCs) and to infer the fate of CH4. The results show that CH4 concentrations were higher in the pollution source than in the intermediate zone and in the fringes of the pollution plume, and they decreased gradually along the groundwater flow path. The 13C of CH4 and CO2 in the groundwater were in the ranges of the groundwater in other sites. However, in the pollution plume, the 13C of CH4 was more depleted than those in other places and the δ13C of CH4 was increasing in the groundwater flow direction. Confirmed by the presence of δ13C of dissolved inorganic carbon (DIC) and 13C Rayleigh fractionation modeling during CH4 reactions, we found that the PHC methanogenesis mainly occurred in the pollution source, while methane oxidation occurred along the downgradient of the plume. Percentage of modern carbon (pMC) data confirms that approximately 29% of the CO2 found in pollution source groundwater is derived from petroleum sources.

Theoretical and experimental investigations of rate coefficients of O(1D) + CH4 at low temperature.

The rate coefficients of the barrierless O(1D) + CH4 reaction are determined both theoretically and experimentally at 50-296 K. For the calculations, ring polymer molecular dynamics (RPMD) simulations are performed on the basis of a new neural network potential energy surface (PES) in the reactant asymptotic part. Only the reactant asymptotic part of the PES is constructed because of its barrierless and exothermic properties. Experimentally, the reaction rate coefficients are measured using a supersonic flow reactor. Pulsed laser photolysis of O3 molecules is used as the source of O(1D) atoms, which are detected directly through vacuum ultraviolet laser induced fluorescence at 115 nm. The branching ratio for H atom production is measured by comparing the H atom yields of the O(1D) + CH4 and O(1D) + H2 reactions. At T ≥ 75 K, good agreement between theoretical and experimental rate coefficients is found, while at 50 K, the larger difference is discussed in detail.

气相中Th活化C&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;4&lt;/sub&gt;的自旋禁阻反应机理

用密度泛函理论中的UB3LYP方法，研究了单重态和三重态势能面自旋禁阻反应Th(6d27S2) + C2H4→ThH3-CCH的微观机理。找到了两条反应通道，对其中涉及的两态反应(TSR)进行了分析。进而运用Hammond假设和Yoshizawa等的内禀坐标单点垂直激发计算的方法找出了一系列势能面交叉点[crossing points (CPs)]，并作了相应的讨论。通过计算，讨论了势能面交叉和可能的自旋翻转过程。用Harvey等的方法优化出最低能量交叉(MECP)，进一步讨论了Th与C2H4的反应中不同势能面之间的“系间窜越”(ISC)的可能性。最后，对前线分子轨道MECP2做了简单的讨论，这些理论结果可以作为进一步理论研究和实验的指导。 Using UB3LYP method in density functional theory, the micro mechanism of the singlet and triplet potential energy surfaces of the three spin forbidden reactions Th(6d27S2) + C2H4→ThH3-CCH was investigated. Two reaction channels were found, and the two state reaction (TSR) was analyzed. Then we used the method of single point vertical excitation calculation of intrinsic coordinates Hammond and Yoshizawa’s hypothesis, found a series of potential energy surface crossing point of [crossing points (CPs)], and made the corresponding discussion. By calculation, the crossover of potential energy surface and the spin flip process are discussed. Using Harvey and other methods to optimize the minimum energy crossover (MECP), the possibility of the “inter system channeling” (ISC) between different potential energy surfaces in the reaction of Th and C2H4 is further discussed. In the end, we make a brief discussion on the frontier molecular orbital MECP2, which can be used as a guide for further theoretical research and experiment.

Hydrogen-rich Syngas Production from Ethanol Dry Reforming on La-doped Ni/Al2O3 Catalysts: Effect of Promoter Loading

Ethanol dry reforming has been studied over La-promoted Ni catalysts supported on Al2O3 with different promoter loadings at varying CO2 partial pressure of 20-50kPa. Catalysts were prepared via co-impregnation technique and characterized using BET surface area, X-ray diffraction measurement, temperature-programmed calcination and scanning electron microscopy. Doped and undoped catalysts possessed high surface area of about 86-108 m2 g-1 and La promoter was well-dispersed on support surface. X-ray diffraction measurements indicated the formation of La2O3, NiO and NiAl2O4 phases in line with temperature-programmed calcination results. La-addition enhanced the dispersion of NiO particles and reduced the agglomeration of metal oxides. Both C2H5OH and CO2 conversions improved with increasing CO2 partial pressure rationally due to the growing secondary CO2 reforming of CH4 reaction. The ratio of H2/CO produced from ethanol dry reforming varied from 1.1 to 1.4 favored for usage as feedstocks of Fischer-Tropsch synthesis. The yield of H2 and CO also enhanced with increasing CO2 partial pressure whilst the optimal La loading in terms of C2H5OH conversion was observed at 3%La and catalytic activity increased with promoter addition reasonably owing to the redox properties of La promoter. CO2 reforming of ethanol reaction appeared via ethanol decomposition to CH4 intermediate product, which was subsequently converted to CO and H2 mixture through CH4 dry reforming reaction.