CH3 Radicals Research Articles

Ab Initio Investigation of CH4 Dehydrogenation on a (CeO2)10 Cluster

We report a theoretical investigation of the activation and dehydrogenation of CH4 on a (CeO2)10 cluster based on density functional theory calculations combined with the unity bond index-quadratic exponential potential approach. As expected, we identified a physisorption interaction mechanism between the CH4 and (CeO2)10 systems. Along the dehydrogenation, the interaction CHn–(CeO2)10 occurs by chemisorption via the formation of C–O and H–O bonds. Due to adsorption effects, the oxidation state of Ce atoms changes from Ce4+ to Ce3+, which results in the elongation of the Ce–O bonds due to the larger radius of the Ce3+ species, and hence, it affects the formation of the CO and CH3OH species. Our combined approach showed that the first dehydrogenation step involves a transition-state formation induced by the molecular hybridization before the CH3 radical formation. Finally, our results indicate trends for new compound formation, once the energy barrier involved in the first step of dehydrogenation is overcome.

Experimental and Numerical Study on the Effect of NO2 on n-Butanol/Biodiesel Dual-Fuel Combustion in a Compression Ignition Engine.

Nitrogen dioxide (NO2) is an active species of exhaust gas recirculation gas, and it has a significant impact on the autoignition and combustion processes of fuels. This study presented a comprehensive investigation of the effect of NO2 on the combustion characteristics of the n-butanol/biodiesel dual fuel. Experiments were conducted on a single-cylinder engine with 0, 100, 200, and 400 v/v ppm NO2 addition at two fuel injection ratios. The findings of the experiments indicated that adding NO2 resulted in an earlier start of heat release and an increase in peak in-cylinder pressure as compared to experiments where no NO2 was added. The evolutions of n-butanol, biodiesel, and OH radicals were evaluated using the computational fluid dynamics software coupled with the n-butanol–biodiesel–NO2 mechanism. The results revealed that when 400 v/v ppm NO2 was added, the consumption of n-butanol and biodiesel occurred earlier, and the formation of OH radicals was approximately an order of magnitude higher before the biodiesel was injected. Furthermore, reaction rate and flux analyses were performed to understand the effect of NO2 addition on the reaction process. When NO2 was added, 35% of the HO2 radicals reacted with NO which converted from NO2 via the reaction NO + HO2 ⇌ NO2 + OH, promoting the formation of OH radicals in the reaction system. The addition of NO2 can also enhance the consumption of CH3 radicals via the reaction CH3 + HO2 ⇌ CH3O + OH.

Chemical structure effects on coal pyrolyzates and reactions by using large-scale reactive molecular dynamics

The relationship between the coal chemical structure and its thermal reactivity is vital to understand coal pyrolysis behaviors. In order to explore chemical structure effects on pyrolysis process, five large-scale coal models of different ranks were constructed and simulated with ReaxFF MD simulations by a combined approach of high performance computing and cheminformatics based reaction analysis in this work. The qualitative temperature mapping results between ReaxFF MD simulations and thermogravimetry experiments were obtained for the first time through the detected covalent bond breaking, which suggests a promising scheme to map the simulation results to the real world. Importantly, the typical structures in coal can be used as the indicators to predict pyrolysis stages, weight loss profiles and major pyrolyzate distributions from the atomistic level. The starting temperature of coal thermal decomposition is anchored by the parameters of falO in 13C NMR representing alkyl ether amounts; and the largest phenol tar generation links closely to the faP NMR parameter; meanwhile methoxy groups determine the initial generation of CH3 radicals and CH4 at relatively low temperature. Additionally, the dynamic profiles of Car-Car bonds and the second increasing trend for CH3 radicals have strong relationship with recombination reactions. With the reasonable coal structures, the large-scale ReaxFF MD simulation alone can complement experimental observation comprehensively to understand the complex coal thermal chemistry and used as a preliminary screening approach to select the coal type or rank for industrial utilization.

Room-Temperature Photooxidation of CH4 to CH3OH with Nearly 100% Selectivity over Hetero-ZnO/Fe2O3 Porous Nanosheets.

The huge challenge for CH4 photooxidation into CH3OH lies in the activation of the inert C-H bond and the inhibition of CH3OH overoxidation. Herein, we design two-dimensional in-plane Z-scheme heterostructures composed of two different metal oxides, with efforts to polarize the symmetrical CH4 molecules and strengthen the O-H bond in CH3OH. As a prototype, we first fabricate ZnO/Fe2O3 porous nanosheets, where high-resolution transmission electron microscopy and in situ X-ray photoelectron spectroscopy affirm their in-plane Z-scheme heterostructure. In situ Fourier transform infrared spectra and in situ electron paramagnetic resonance spectra demonstrate their higher amount of ·CH3 radicals relative to the pristine ZnO porous nanosheets, in which density functional theory calculations validate that the high local charge accumulation on Fe sites lowers the CH4 adsorption energy from 0.14 to 0.06 eV. Moreover, the charge-accumulated Fe sites strengthen the polarity of the O-H bond in CH3OH through transferring electrons to the O atoms, confirmed by the increased barrier from 0.30 to 2.63 eV for *CH3O formation, which inhibits the homolytic O-H bond cleavage and thus suppresses CH3OH overoxidation. Accordingly, the CH3OH selectivity over ZnO/Fe2O3 porous nanosheets reaches up to nearly 100% with an activity of 178.3 μmol-1 gcat-1, outperforming previously reported photocatalysts without adding any oxidants under room temperature and ambient pressure.

Density Functional Theory Study on the Insertion Reaction Mechanism of Methylidyne with Ethanol

CH and C2H5OH are important species in the combustion system. The reaction mechanism between CH and C2H5OH was elucidated at the CCSD(T)//BHandHLYP/6-311+G(3df, 2p) level of theory. The results of quantum chemistry calculation showed that the CH free radical preferentially inserted the O-H and C-H bonds of CH2 and CH3 groups in the C2H5OH molecule. Meanwhile, the insertion of the CH radical into C-O and C-C bonds are much more difficult due to the high energy barriers. The main products of the reaction are: CH3CHCH2 + OH, CH3C(OH)CH2 + H, CH3CHO + CH3, CH3COCH3 + H, C2H4 + CH3O, CH2O + CH3CH2 and C2H4 + CH2OH. The geometries of the species and the heats of reaction at 0 K are in good agreement with the available experimental results. These results can provide guidance for future experimental work.

Modelling the Radical Chemistry on Ice Surfaces: An Integrated Quantum Chemical and Experimental Approach

Heterogeneous radical processes on ice surfaces play a vital role in the formation of building blocks of the biologically relevant molecules in space. Therefore, quantitative mechanistic details of the radical binding and radical reactions on ices are crucial in rationalizing the chemical evolution in the Universe. The radical chemistry on ice surfaces was explored at low temperatures by combining quantum chemical calculations and laboratory experiments. A range of binding energies was observed for OH, HCO, CH3, and CH3O radicals binding on ices. Computed reaction paths of the radical reactions on ices, OCS + H and PH3 + D, explained the experimentally observed products. In both radical reactions, quantum tunnelling plays a key role in achieving the reactions at low temperatures. Our findings give quantitative insights into radical chemistry on ice surfaces in interstellar space and the planetary atmospheres.

Dry reforming of methane in a nanosecond repetitively pulsed discharge: chemical kinetics modeling

Nanosecond pulsed discharge plasma shows a high degree of non-equilibrium, and exhibits relatively high conversions in the dry reforming of methane. To further improve the application, a good insight of the underlying mechanisms is desired. We developed a chemical kinetics model to explore the underlying plasma chemistry in nanosecond pulsed discharge. We compared the calculated conversions and product selectivities with experimental results, and found reasonable agreement in a wide range of specific energy input. Hence, the chemical kinetics model is able to provide insight in the underlying plasma chemistry. The modeling results predict that the most important dissociation reaction of CO2 and CH4 is electron impact dissociation. C2H2 is the most abundant hydrocarbon product, and it is mainly formed upon reaction of two CH2 radicals. Furthermore, the vibrational excitation levels of CO2 contribute for 85% to the total dissociation of CO2.

Simultaneously Accelerating Carrier Transfer and Enhancing O2/CH4 Activation via Tailoring the Oxygen-Vacancy-Rich Surface Layer for Cocatalyst-Free Selective Photocatalytic CH4 Conversion.

Solar energy-driven direct CH4 conversion to liquid oxygenates provides a promising avenue toward green and sustainable CH4 industry, yet still confronts issues of low selectivity toward single oxygenate and use of noble-metal cocatalysts. Herein, for the first time, we report a defect-engineering strategy that rationally regulates the defective layer over TiO2 for selective aerobic photocatalytic CH4 conversion to HCHO without using noble-metal cocatalysts. (Photo)electrochemical and in situ EPR/Raman spectroscopic measurements reveal that an optimized oxygen-vacancy-rich surface disorder layer with a thickness of 1.37 nm can simultaneously promote the separation and migration of photogenerated charge carriers and enhance the activation of O2 and CH4, respectively, to •OH and •CH3 radicals, thereby synergistically boosting HCHO production in aerobic photocatalytic CH4 conversion. As a result, a HCHO production rate up to 3.16 mmol g-1 h-1 with 81.2% selectivity is achieved, outperforming those of the reported state-of-the-art photocatalytic systems. This work sheds light on the mechanism of O2-participated photocatalytic CH4 conversion on defective metal oxides and expands the application of defect engineering in designing low-cost and efficient photocatalysts.

A novel hybrid machine learning model for predicting rate constants of the reactions between alkane and CH3 radical

The rate constants of H-abstraction reactions of alkanes by free radicals are crucial for optimizing combustion reaction network, improving combustion efficiency and designing aero engines. In this study, three different machine learning (ML) algorithms-Feedforward Neural Network (FNN), eXtreme Gradient Boosting (XGB) and Random Forest -were used to develop ML models for predicting the rate constants of reactions of alkane and CH3 radical. The results showed that the FNN algorithm with 6 descriptors has better overall performance than the other two ML algorithms. A novel hybrid ML model combining FNN with XGB (XGB-FNN) was then developed, by which the prediction accuracy was visibly improved (p-value < 0.05). The average deviation of XGB-FNN model on the prediction set is 42.35%. The rate constants of the primary hydrogen abstraction reactions between CH3 radical and normal alkanes with 5 ∼ 12 carbon atoms were predicted over a temperature range of 300 ∼ 2500 K, which follow well the modified three-parameter Arrhenius equation and/or agree well with available experimental rate constants, indicating that the hybrid XGB-FNN model is robust in predicting the rate constants in the temperature range of combustion.

Understanding indium nitride thin film growth under ALD conditions by atomic scale modelling: From the bulk to the In-rich layer

In recent decades, indium nitride (InN) has been attracting a great deal of attention for its potential applicability in the field of light-emitting diodes (LEDs) and high-frequency electronics. However, the contribution from adsorption- and reaction- related processes at the atomic scale level to the InN growth has not yet been unveiled, limiting the process optimization that is essential to achieve highly crystalline and pure thin films. In this report, we investigate the reaction pathways that are involved in the crystal growth of InN thin film in atomic layer deposition (ALD) techniques from trimethylindium (TMI) and ammonia (NH3) precursors. To accomplish this task, we use a solid-state approach to perform the ab-initio calculations within the Perdew–Burke–Ernzerhof functional (PBE) level of theory. The results clarify the activation role from the N-rich layer to decrease the barrier for the first TMI precursor dissociation from Δ‡H= +227 kJ/mol, in gas phase, to solely +16 kJ/mol, in the surface environment. In either case, the subsequent CH3 release is found to be thermo- and kinetically favored with methylindium (MI) formed at the hcp site and ethane (C2H6) as the byproduct. In the following step, the TMI physisorption at a nearby occupied hcp site promotes the sequential hydrogen removal from the N-rich layer at the minimum energy cost of Δ‡H < +105 kJ/mol with methane (CH4) release. An alternative mechanism involving the production of CH4 is also feasible upon dissociation in gas phase. Furthermore, the high concentration of CH3 radicals, from precursor dissociation, might be the origin of the carbon impurities in this material under the experimental conditions of interest. Finally, the passivation methodology is not found to affect the evaluation of the surface-related processes, whereas the inclusion of spin-polarization is demonstrated to be essential to the proper understanding of the reaction mechanism.

Pyrolysis mechanism study of n-heptane as an endothermic hydrocarbon fuel: A reactive molecular dynamic simulation and density functional theory calculation study

The pyrolysis of endothermic hydrocarbon fuel with high heat sink can reduce the surface temperature of engines and aircraft fuselage. The pyrolysis mechanism of n-heptane as a model compound for endothermic hydrocarbon fuel was investigated by using ReaxFF reactive molecular dynamic simulations and density functional theory calculations. The results showed that the breakage of CC bonds of n-heptane were easier than that of CH bonds, in which the cleavage of C2C3 bond is the easiest to generate C2H5 and C5H11 radicals. Temperature has a significant effect on the pyrolysis process of n-heptane, and the increase of temperature will accelerate the pyrolysis of n-heptane and promote the formation of pyrolysis products. The pre-exponential factor and apparent activation energy of the n-heptane pyrolysis were 5.4456 × 1014 s−1 and 231.3 kJ·mol−1, respectively.

Theoretical spectroscopic study of acetyl (CH 3CO), vinoxy (CH 2CHO), and 1-methylvinoxy (CH 3COCH 2) radicals. Barrierless formation processes of acetone in the gas phase.

Background: Acetone is present in the earth´s atmosphere and extra-terrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided. Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH 3, CH 3CO, and CH 3COCH 2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH 3COCH 2 and CH 3CO, as well as the CH 2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Results: The internal rotation barriers were determined to be V 3=143.7 cm -1 (CH 3CO), V 2=3838.7 cm -1 (CH 2CHO) and V 3=161.4 cm -1 and V 2=2727.5 cm -1 (CH 3COCH 2).The splitting of the ground vibrational state due to the torsional barrier have been computed to be 2.997 cm -1, 0.0 cm -1, and 0.320 cm -1, for CH 3CO, CH 2CHO, and CH 3COCH 2, respectively. Conclusions: Two addition reactions, H+CH 3COCH 2 and CH 3+CH 3CO, could be considered barrierless formation processes of acetone after considering all the possible formation routes, starting from 58 selected reactants, which are fragments of the molecule. The spectroscopic study of the radicals involved in the formation processes present non-rigidity. The interconversion of their equilibrium geometries has important spectroscopic effects on CH 3CO and CH 3COCH 2, but is negligible for CH 2CHO.

Reaction of Methylidyne with Ethane: The C–C Insertion Is Unimportant

High-accuracy coupled-cluster calculations in combination with the E,J-resolved master-equation analysis are used to study the reaction mechanism and kinetics of methylidyne with ethane. This reaction plays an important role in the combustion of hydrocarbon fuels and in interstellar chemistry. Two distinct mechanisms, the C-C and the C-H insertions of CH in C2H6, are characterized. The C-C insertion pathway is identified to have a large barrier of 34.5 kcal mol-1 and hence plays no significant role in kinetics. The C-H insertion pathway is found to have no barrier, leading to a highly vibrationally excited n-C3H7 radical, which rapidly dissociates (within 50 ps) to yield CH3 + C2H4 and H + C3H6 in a roughly 7:3 ratio. These findings are in good agreement with an experimental result that indicates that about 20% of the reaction goes to H + C3H6. The reaction of the electronically excited quartet state of the CH radical with C2H6 is examined for the first time and found to proceed as a direct H-abstraction via a small barrier of 0.4 kcal mol-1 to yield triplet CH2 and C2H5. The reaction on the quartet state surface is negligibly slow at low temperatures characteristic of interstellar environments but becomes important at high combustion temperatures.

The dynamics of CO production from the photolysis of acetone across the whole S1 ← S0 absorption spectrum: Roaming and triple fragmentation pathways.

The unimolecular photodissociation dynamics of acetone spanning the entire S1 ← S0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of λ > 305.8 nm, all photoproducts are formed on the S0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window λ = 311.2-305.8 nm. From λ = 305.8to262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C-C bond cleavage on the lowest excited triplet state, T1. At higher energy (λ < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 ± 4 kJ mol-1, which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248nm. We see no evidence of the return of roaming, or any other S0 reaction, in this higher energy region of the first electronic absorption band.

Liquefaction performances of the mixture of paper fibre and LDPE in subcritical water

The waste Tetra pack impose a great pressure on environment. In this paper, we use hydrothermal liquefaction technique to convert it into valuable crude oil. The paper fibre and LDPE were studied in supercritical water at 420 °C, 25 MPa and residence time of 30 min. Effect of paper fibre: LDPE on the crude oil yield, components distribution and high heat value were investigated in depth. Results showed that maximum crude oil yield of 27.33 wt% was obtained at paper fibre to LDPE of 8:2, minimum O/C of 0.094 was obtained at paper fibre to LDPE of 5:5, which reflect that HTL of paper fibre and LDPE had a synergetic effect. And the highest high heat value of the crude oil reach 37.7 MJ/kg at the ratio of 5:5• The CH radicals produced from LDPE chain can promote the hydrogenation reaction that then decrease the production of oxygen-containing groups, and also the broken of C-C bond in lignin will be accelerate, which increased the diesel (C16-C47) compound.

Numerical study on the effects of CO2/H2O dilution on the ignition delay time of methane

AbstractIn this study, the effects of CO2/H2O dilution on the ignition delay time (IDT) of methane under different conditions were studied by the method of sensitivity analysis, mole fraction analysis and reaction path analysis. The predictions of six published kinetic models were first compared with experimental data from the literature; the kinetics of the model judged best were then analyzed in greater detail to investigate coupling effects of different ratios of CO2/H2O dilution, temperature, pressure, and equivalence ratios on the IDT of methane. Through this research, it was found that under high temperature conditions (1700–2000 K), the IDT increases with CO2/H2O ratios increasing. At the equivalence ratio of 2.0 and low to medium temperatures (1000–1700 K), the diluent gas at the ratio of CO2/H2O = 0.4/0.6 has the maximum suppression effect on methane ignition, and the inhibitory effect of diluted gas in the IDT can be enhanced by 4.9% compared with other cases. Through the path analysis, it was found that the reaction path under the condition of CO2/H2O = 0.4/0.6 is changed into the O radicals generation reactions and CH3 consuming Reactions (R155, R106, and R158) comparing with the condition of CO2/H2O = 0.8/0.2. By providing insight into the factors affecting the IDT of methane under a wide variety of conditions, the present study extends our understanding of methane combustion and can be used to guide the development of a combined application of EGR and in‐cylinder water injection.

Transition-metal-free, direct C[sbnd]H radical trifluoromethylation of nitroimidazoles with Togni’s reagent

Trifluoromethyl moiety in bioactive compounds can dramatically change both the physicochemical and physiological properties, which could further improve both the pharmacokinetic and pharmacological profiles. It has therefore become indispensable to develop an efficient and mild synthetic methodology for the incorporation of trifluoromethyl group. In present study, a green, mild and practical method which allows for the transition-metal-free, direct CH radical trifluoromethylation of nitroimidazoles with Togni’s regent II as the CF3 radical precursor is described. Diverse nitroimidazoles and their drug molecules were smoothly transformed into trifluoromethylated compounds in moderate to good yields under mild conditions. This new method provides an alternative approach for the prepare of trifluoromethylated compounds, as well as late-stage modification of bioactive nitroimidazoles.

Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.

3D kinetic Monte-Carlo simulations of diamond growth on (1 0 0) surfaces

A 3D Kinetic Monte-Carlo (KMC) model is implemented and used to simulate the growth of (1 0 0)-oriented diamond films. The model considers four processes: adsorption and desorption of CH3 radicals, etching of carbon atoms and migration of adsorbed radicals. The atomic structure of diamond is taken into account including the formation of dimer rows on the surface. The model correctly reproduces the step-flow growth mechanism of diamond (1 0 0) surfaces and the obtained growth rates are close to experimental data. The propagation of the steps shows a clear anisotropy. Steps are usually two atomic layers high but step bunching can be observed in presence of defects. The model thus can be used as a predictive tool to obtain growth rates and to understand the effect of atomic interactions on the film morphology.

Heterogeneous Reactions of Methane with Cl Radicals on Large ArN Clusters.

Heterogeneous chemistry on the surfaces of atmospheric particles has a wide impact on the properties and composition of the Earth's atmosphere. In laboratory studies, clusters can represent proxies to atmospheric aerosols and help to discern the individual steps in reactions on or in aerosols. We investigate the reactivity of Cl and CCl3 radicals with methane on argon clusters using the pickup method. For radical generation, we built a new pyrolysis source partially adapting the design of radical sources that utilize the supersonic expansion into a heated silicon carbide tube. Large ArN, N̅ ≈ 110, clusters were generated in a supersonic expansion, and CH4 molecules were embedded in the clusters via a pickup process followed by the uptake of the radicals produced in the pyrolysis source. The analysis of the mass spectra recorded under different experimental conditions (i.e., with the pyrolysis ON and OFF and with only one or both reactants) allowed us to identify various products of the radical reactions on ArN. We propose a sequence of reactions based on the reaction energetics. It starts with the hydrogen abstraction from CH4 by a Cl radical resulting in HCl and CH3 followed by a halogenation step where CCl4 molecules react with the available CH3 radicals, yielding CH3Cl. By analogy, the CH3Cl enters another hydrogen abstraction by Cl, producing HCl and the CH2Cl radical, which again undergoes a halogenation step with CCl4, generating CH2Cl2. Further reaction of CH2Cl2 with Cl terminates the sequence by the production of HCl and CHCl2.