CH4 Reaction Research Articles

Vibrational State-Resolved Differential Cross Sections of the F + CH4 → HF + CH3 Reaction at the Collision Energies of 3.1-13.8 kcal/mol.

We report a high-resolution crossed molecular beam experiment investigating the reaction dynamics of the F + CH4 → HF + CH3 reaction across a broad range of collision energies (3.1-13.8 kcal/mol). By using time-sliced velocity map imaging within the crossed molecular beam apparatus, we obtain correlated angular distributions and branching ratios for various product pairs (CH3(ν), HF(ν')). The resolved reactive rainbow-like features that display a distinct bulge in the angular distribution in different channels reveal the formation of vibrationally ground and excited states of the methyl radical accompanied by different rovibrationally excited HF products. These findings suggest distinct reaction dynamics for channels leading to vibrationally excited methyl radicals compared to those forming ground-state methyl radicals.

Modulating the local electron density at built-in interface iron single sites in Fe-CN/MoO3 heterostructure for enhanced CO2 reduction to CH4 and Photo-Fenton reaction

The catalytic efficiency of heterogeneous photocatalytic CO2 reduction and photo-Fenton H2O2 activationisclosely related to the local electron density of reaction center atoms. However, electron-hole recombination from random charge transfer significantly restricts the targeted electron delivery to the active center. Herein, Fe-C3N4/MoO3 heterojunction with interfacial coordination of atomically dispersed Fe-N4 sites with the O interface of MoO3 was synthesized by simple hydrothermal method. Based on the experimental results and density functional theory calculation (DFT), the heterojunction structure fosters accelerated interfacial electron transfer due to directional interfacial electric field (IEF) between Fe-CN and MoO heterogeneous interfaces, and the interfacial bond between Fe-N4 sites and O at the built-in interface regulates the local electron density of Fe-N4 active center. DFT further reveals that the interfacial electron flow and concentrated electron density at Fe-N4 sites result from the coordination between Fe-N4 and MoO3 interfaces. This directs electron flow towards the Fe center, significantly enhancing CO2 adsorption and H2O2 conversion efficiency. PDOS analysis shows that the dyz and dz2 orbitals of the isolated Fe atom in Fe-CN overlap with the pz orbital of the O atom in MoO3, playing a pivotal role in CO2 adsorption. Consequently, the Fe-CN/MoO3 heterojunction demonstrated highly efficient photocatalytic CO2 reduction to CH4, coupled with benzyl alcohol oxidation and photo-Fenton tetracycline degradation. These findings offer a promising multifunctional catalyst strategy for the development of energy conversion and environmental remediation.

LA-ICP-Q-MS/MS Re[sbnd]Os geochronology: A comparison of N2O and CH4 reaction gases

The ReOs isotope system can directly date sulphide formation, making it useful for ore deposit studies. Recent work has demonstrated that laser ablation inductively coupled tandem mass spectrometry (LA-ICP-MS/MS) can be used to effectively separate 187Re from 187Os during analysis by reacting Os with CH4 gas, allowing for in situ ReOs age determination of molybdenite. However, the age calculation often requires a significant interference correction of 187ReCH2+ on 187OsCH2+. Here, we demonstrate that N2O gas is a viable alternative reaction gas for in situ ReOs geochronology. In this new method, Os forms a tetra-oxide complex, while Re shows very low reactivity with the N2O reaction gas (∼0.15 %), reducing the isobaric interference correction requirement relative to the CH4 method. We compare results from both reaction gases across a range of molybdenite samples of different ages (spanning ca. 1700–300 Ma), demonstrating that the N2O method achieves greater precision and accuracy for young molybdenite samples (ca. 300 Ma) compared to CH4. In addition, we demonstrate that in situ ReOs is capable of dating high-Re pyrite (i.e., >10 ppm).

Bimetallic MOF@CdS nanorod composite for highly efficient piezo-photocatalytic CO2 methanation under visible light

CO2 methanation is leading progress in both dwindling the emitted greenhouse gas and taking advantage of CO2 conversion to a worthwhile fuel. Various types of catalysts have gained researchers' attention. On the other hand, those catalysts chiefly suffer from being uneconomical, owning laborious processes, and having low efficiency. Particularly in the photocatalytic process, electron-hole recombination, charge separation efficiency, and the photocorrosion are the most remarkable obstacles in the path of gaining high efficiency. To conquer the aforementioned hindrances, Cu/Zr-MOF@CdS had been designed in order to not only do elevate CH4 selectivity but also increase CO2 conversion by altering the electron transfer mechanism. Doping Cu in Zr-MOF structure restrains C-C coupling and ameliorates the viability of protonation of *CO to *HCO during methane production. CdS and Zr-MOF both grant piezoelectricity trait to the catalyst in a way that by merging it with the photocatalytic process the mechanism of process converted from type (II) scheme to Z-scheme, culminating in thwarting recombination and increase of charge separation efficiency. The photocatalytic process achieved 23.6 μmol. g−1. h−1 CH4 reaction rate and 80 % CO2 conversion, hereafter applying the piezo-photocatalytic process, these two factors reached 52.2 μmol. g−1. h−1 and 99 %, respectively. This work unveils the viable reaction routes along with their several quotas in piezo-photocatalytic CO2 methanation process by scrutinizing the intricate mechanisms via in-situ analyses.

Formation of N–bearing complex organic molecules in molecular clouds: Ketenimine, acetonitrile, acetaldimine, and vinylamine via the UV photolysis of C2H2 ice

Context. The solid-state C2H2 chemistry in interstellar H2O-rich ice has been proposed to explain astronomically observed complex organic molecules (COMs), including ketene (CH2CO), acetaldehyde (CH3CHO), and ethanol (CH3CH2OH), toward early star-forming regions. This formation mechanism is supported by recent laboratory studies and theoretical calculations for the reactions of C2H2+OH/H. However, the analog reaction of C2H2+NH2 forming N-bearing species has been suggested to have a relatively low rate constant that is orders of magnitude lower than the value of C2H2+OH. Aims. This work extends our previous laboratory studies on O-bearing COM formation to investigate the interactions between C2H2 and NH3 ice triggered by cosmic ray-induced secondary UV photons under molecular cloud conditions. Methods. Experiments were performed in an ultra-high vacuum chamber to investigate the UV photolysis of the C2H2:NH3 ice mixture at 10 K. The ongoing chemistry was monitored in situ by Fourier-transform infrared spectroscopy as a function of photon fluence. The IR spectral identification of the newly formed N-bearing products was further secured by a quadrupole mass spectrometer during the temperature-programmed desorption experiment. Results. The studied ice chemistry of C2 H2 with NH2 radicals and H atoms resulting from the UV photodissociation of NH3 leads to the formation of several N-bearing COMs, including vinylamine (CH2CHNH2), acetaldimine (CH3CHNH), acetonitrile (CH3CN), ketenimine (CH2CNH), and tentatively ethylamine (CH3CH2NH2). The experimental results show an immediate and abundant CH2CHNH2 yield as the first-generation product, which is further converted into other chemical derivatives. The effective destruction and formation cross-section values of parent species and COMs were derived, and we discuss the chemical links among these molecules and their astronomical relevance.

Theoretical Study of the Thermal Rate Coefficients of the H3+ + C2H4 Reaction: Dynamics Study on a Full-Dimensional Potential Energy Surface.

Ion-molecular reactions play a significant role in molecular evolution within the interstellar medium. In this study, the entrance channel reaction, H3+ + C2H4 → H2 + C2H5+, was investigated using classical molecular dynamic (classical MD) and ring polymer molecular dynamic (RPMD) simulation techniques. We developed an analytical potential energy surface function with a permutationally invariant polynomial basis, specifically employing the monomial symmetrized approach. Our dynamic simulations reproduced the rate coefficient of 300 K for H3+ + C2H4 → H2 + C2H5+, aligning reasonably well with the values in the kinetic database commonly utilized in astrochemistry. The thermal rate coefficients obtained using both the classical MD and RPMD techniques exhibited an increase from 100 K to 300 K as the temperature rose. Additionally, we analyzed the excess energy distribution of the C2H5+ fragment with respect to temperature to investigate the indirect reaction pathway of C2H5+ → H2 + C2H3+. This result suggests that the indirect reaction pathway of C2H5+ → H2 + C2H3+ holds minor significance, although the distribution highly depends on the collisional temperature.

Dynamics of the HCl + C2H5 Multichannel Reaction on a Full-Dimensional Ab Initio Potential Energy Surface.

We report a full-dimensional ab initio analytical potential energy surface (PES), which accurately describes the HCl + C2H5 multichannel reaction. The new PES is developed by iteratively adding selected configurations along HCl + C2H5 quasi-classical trajectories (QCTs), thereby improving our previous Cl(2P3/2) + C2H6 PES using the Robosurfer program package. QCT simulations for the H'Cl + C2H5 reaction reveal hydrogen-abstraction, chlorine-abstraction, and hydrogen-exchange channels leading to Cl + C2H5H', H' + C2H5Cl, and HCl + C2H4H', respectively. Hydrogen abstraction dominates in the collision energy (Ecoll) range of 1-80 kcal/mol and proceeds with indirect isotropic scattering at low Ecoll and forward-scattered direct stripping at high Ecoll. Chlorine abstraction opens around 40 kcal/mol collision energy and becomes competitive with hydrogen abstraction at Ecoll = 80 kcal/mol. A restricted opening of the cone of acceptance in the Cl-abstraction reaction is found to result in the preference for a backward-scattering direct-rebound mechanism at all energies studied. Initial attack-angle distributions show mainly side-on collision preference of C2H5 for both abstraction reactions, and in the case of the HCl reactant, H/Cl-side preference for the H/Cl abstraction. For hydrogen abstraction, the collision energy transfer into the product translational and internal energy is almost equally significant, whereas in the case of chlorine abstraction, most of the available energy goes into the internal degrees of freedom. Hydrogen exchange is a minor channel with nearly constant reactivity in the Ecoll range of 10-80 kcal/mol.

Enhancing Methane Gas Sensing through Defect Engineering in Ag-Ru Co-doped ZnO Nanorods.

Detection of leaks of flammable methane (CH4) gas in a timely manner can mitigate health, safety, and environmental risks. Zinc oxide (ZnO), a polar semiconductor with controllable surface defects, is a promising material for gas sensing. In this study, Ag-Ru co-doped into self-assembled ZnO nanorod arrays (ZnO NRs) was prepared by a one-step hydrothermal method. The Ag-Ru co-doped sample shows a good hydrophobic property as a result of its particular microstructure, which results in high humidity resistance. In addition, oxygen vacancy density significantly increased after Ag-Ru co-doping. Density functional theory (DFT) calculations revealed an exceptionally high charge density accumulated at the Ru sites and the formation of a localized strong electric field, which provides additional energy for the CH4 reaction with •O2- at the surface at room temperature. Optimized AgRu0.025-ZnO demonstrated an outstanding CH4 sensing performance, with a limit of detection (LOD) as low as 2.24 ppm under free-heat and free-light conditions. These findings suggest that introducing defects into the ZnO lattice, such as oxygen vacancies and localized ions, offers a promising approach to improving the gas sensing performance.

Facilitating the dry reforming of methane with interfacial synergistic catalysis in an Ir@CeO2−x catalyst

The dry reforming of methane provides an attractive route to convert greenhouse gases (CH4 and CO2) into valuable syngas, so as to resolve the carbon cycle and environmental issues. However, the development of high-performance catalysts remains a huge challenge. Herein, we report a 0.6% Ir/CeO2−x catalyst with a metal-support interface structure which exhibits high CH4 (~72%) and CO2 (~82%) conversion and a CH4 reaction rate of ~973 μmolCH4 gcat−1 s−1 which is stable over 100 h at 700 °C. The performance of the catalyst is close to the state-of-the-art in this area of research. A combination of in situ spectroscopic characterization and theoretical calculations highlight the importance of the interfacial structure as an intrinsic active center to facilitate the CH4 dissociation (the rate-determining step) and the CH2* oxidation to CH2O* without coke formation, which accounts for the long-term stability. The catalyst in this work has a potential application prospect in the field of high-value utilization of carbon resources.

Role of the Vibrational and Translational Energies in the CN(v)+C2H6(ν1, ν2, ν5 and ν9) Reactions. A Theoretical QCT Study.

Quasi-classical trajectory (QCT) calculations were conducted on the newly developed full-dimensional potential energy surface, PES-2023, to analyse two critical aspects: the influence of vibrational versus translational energy in promoting reactivity, and the impact of vibrational excitation within similar vibrational modes. The former relates to Polanyi's rules, while the latter concerns mode selectivity. Initially, the investigation revealed that independent vibrational excitation by a single quantum of ethane's symmetric and asymmetric stretching modes (differing by only 15 cm-1) yielded comparable dynamics, reaction cross-sections, HCN(v) vibrational product distributions, and scattering distributions. This observation dismisses any significant mode selectivity. Moreover, an equivalent amount of energy provided as translational energy (at total energies of 9.6 and 20.0 kcal mol-1) gave rise to slightly lower reactivity compared to the same amount of energy provided as vibrational energy. This effect is more evident at low energies, presenting a counterintuitive scenario in an 'early transition state' reaction. These findings challenge the straightforward application of Polanyi's rules in polyatomic systems. Regarding CN(v) vibrational excitation, our calculations reveal that the reaction cross-section remains practically unaffected by this vibrational excitation, suggesting that the CN stretching mode is a spectator mode. The results were rationalized by considering several factors: the strong coupling between different vibrational modes, and between vibrational modes and the reaction coordinate; and a significant vibrational energy redistribution within the ethane reactant before collision. This redistribution creates an unphysical energy flow, resulting in loss of adiabaticity and vibrational memory before the reactants' collision. These theoretical findings require future confirmation through experimental or theoretical quantum mechanical studies, which are currently unavailable.

Isotope Effect and Heavy-Light-Heavy Reactivity Oscillation in the Cl + CHD3/CHT3 Reaction.

Previous experiments and theories have shown the existence of heavy-light-heavy (HLH) reactivity oscillation in the Cl + CH4 reaction and anticipated that similar oscillations should exist in many HLH reactions involving polyatomic reagents. However, the total reaction probabilities for the Cl + CHD3 → HCl + CD3 reaction exhibit only a step-like feature, and the total reaction probabilities for Cl + CHT3 → HCl + CT3 do not show any structure at all. Here, we report seven-dimensional state-to-state quantum dynamics studies for this reaction on the FI-NN PES, and we demonstrate that HLH reactivity oscillations also exist in these two reactions, manifesting as peaks in the reaction probabilities for low product rotational states. These oscillations, however, are obscured in the total reaction probability because of the higher excitation of j ≥ 2 product rotational states. Furthermore, the isotope replacement of nonreactive hydrogen with deuterium and tritium significantly enhances reactivity at collision energies above 0.112 eV, indicating an inverse secondary isotope effect on the probabilities, which is proved to be also caused by HLH mass combination. We also demonstrate that the highly rotational excitation of CHD3 substantially enhances reactivity and the HLH oscillations, similar to HLH triatomic reactions. These observations are completely different from those in the H + CHD3 reaction, which is also a late-barrier reaction. Therefore, the HLH mass combination is very important, which affects not only the reactivity oscillation but also the amplitude and product rotational state distribution and makes the initial rotation excitation play a pivotal role in the reaction.

Single-Atom Alloys Materials for CO2 and CH4 Catalytic Conversion.

The catalytic conversion of greenhouse gases CH4 and CO2 constitutes an effective approach for alleviating the greenhouse effect and generating valuable chemical products. However, the intricate molecular characteristics characterized by high symmetry and bond energies, coupled with the complexity of associated reactions, pose challenges for conventional catalysts to attain high activity, product selectivity, and enduring stability. Single-atom alloys (SAAs) materials, distinguished by their tunable composition and unique electronic structures, confer versatile physicochemical properties and modulable functionalities. In recent years, SAAs materials demonstrate pronounced advantages and expansive prospects in catalytic conversion of CH4 and CO2. This review begins by introducing the challenges entailed in catalytic conversion of CH4 and CO2 and the advantages offered by SAAs. Subsequently, the intricacies of synthesis strategies employed for SAAs are presented and characterization techniques and methodologies are introduced. The subsequent section furnishes a meticulous and inclusive overview of research endeavors concerning SAAs in CO2 catalytic conversion, CH4 conversion, and synergy CH4 and CO2 conversion. The particular emphasis is directed toward scrutinizing the intricate mechanisms underlying the influence of SAAs on reaction activity and product selectivity. Finally, insights are presented on the development and future challenges of SAAs in CH4 and CO2 conversion reactions.

An experimental and modeling study on homogeneous oxidative coupling of methane utilizing N2O as oxidant

Experimental and kinetic modeling studies were performed to investigate the homogeneous oxidative coupling of methane utilizing nitrous oxide as the oxidant (N2O-OCM). The experiments were performed in a jet stirred reactor at atmospheric pressure and temperatures of 773–1173 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to achieve comprehensive and isomer-resolved identification of major products and nitrogenous, carbonyl and hydrocarbon intermediates. To interpret the experiments, a detailed kinetic model was developed by extending the widely used hydrocarbon growth mechanisms. Rate of production and sensitivity analyses were performed to provide a deep insight into the complex reaction network. The results demonstrate that N2O dissociation is the rate-controlling step, and the mild oxidation of N2O effectively reduces the production of CO2 as byproduct and meanwhile improves the selectivity of C2 species. In the presence of N2O, CH3 can actively participates in H-abstraction reactions of C2H6 and C2H4, besides the self-coupling reactions. Due to chain-propagation competition reactions (N2O+H=N2+OH and N2O+H=NH+NO), NH3 and NO exhibit low concentrations. The detailed formation pathways for NH3 and NO were elaborated, and the evolution routes of carbonyl intermediates in the presence of N2O were analyzed. Benzene formation is found mainly through C3 self-recombination and C5 ring reforming reactions. Finally, comparative analysis with O2-OCM reveals that the presence of oxygen promotes the conversion of CH3 to CH2O and deep oxidation of C2H3, explaining the lower C2 yield of O2-OCM. The inherent limitation of the C2 yield of N2O-OCM primarily arises from the reaction of C2H4 and O, as well as the subsequent reaction CH2(S)+N2O=CH2O+N2. The experiment observations confirmed the existence of C2 carbonyl intermediates such as ketene and acetaldehyde. The study highlights the notable role of O2 molecular and O atoms in the homogeneous OCM and offers kinetic support for the OCM reaction process with catalysts.

Resolving discrepancies between theory and experiment for the NCN + H reaction

Flame modeling studies have highlighted the role of branching in the kinetics for the prompt NO switch reaction, NCN + H. In this reaction, there is strong competition between the CH + N2 (R1) and HCN + N (R2) product channels. Increased branching towards reaction R2 promotes the subsequent formation of Fenimore or Prompt NO. Recent direct shock tube studies on this complex reaction conclude that branching to reaction R2 is much more favorable than the predicted branching from theoretical studies. The experimentally predicted prompt NO switch temperature (TS), i.e., the temperature at which k2/(k1+k2) = 0.5, is at 1670 K, in contrast to theory that predicts Ts > 3200 K. The present theory/modeling work was initiated to highlight potential causes for this significant discrepancy between experiment and theory. Our analysis of the simulations reported in the experimental studies indicate that including a fall-off representation for C2H5 radical dissociation (with C2H5 produced from C2H5I dissociation and acting as an H-atom source) has a noticeable influence on the simulations for NCN decay and HCN formation in the shock tube studies. With the inclusion of a theory-based pressure fall-off representation for C2H5 dissociation, we show that this radical persists for much longer timescales even at the high-temperatures in the shock tube studies. This persistence then facilitates the rapid conversion of reactive H-atoms to CH3 radicals at the shock tube conditions via the H + C2H5 → CH3 + CH3 reaction. An updated theoretical analysis for this classic addition-elimination reaction is also provided in this work. The rapid formation of CH3 radicals necessitates the inclusion of additional reactions to adequately simulate the NCN and HCN data. Our simulations conclude that, with the inclusion of these additional reactions, the NCN and HCN profiles can be reasonably well simulated by the most recent theoretical predictions.

Chemical insights into plasma-assisted dry reforming of methane in a nanosecond discharge

The present study explores the chemical kinetics of plasma-assisted dry reforming of methane (340 K, 30 Torr) in a flow reactor activated by a nanosecond pulsed dielectric barrier discharge (DBD). Experiments are conducted with the inlet CO2/CH4 mol ratios varying from 0.4 to 2.8 in CH4/CO2/Ar mixture. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) is employed for detailed species measurement of the ions, radicals, and molecules in this system. Eight ions are observed, including CH3+, O+, CH4+, Ar2+, CO+, C2H4+, Ar+, and CO2+. The identification of several intermediate species and products are achieved based on the photoionization efficiency (PIE) spectra, such as methyl radical (CH3), water (H2O), acetylene (C2H2), carbon monoxide (CO), ethylene (C2H4), formaldehyde (CH2O), methanol (CH3OH), ketene (CH2CO), and acetone (CH3COCH3). Species mole fraction profiles as a function of inlet CO2 concentrations are obtained. The present work provides a unique dataset and enriches our understanding of the kinetics of plasma-assisted dry reforming of methane. A kinetic mechanism incorporating plasma reactions and combustion reactions is developed for this system, and its prediction performance of the product selectivity is validated against the experimental data. The main reaction pathways for the consumption of CH4 and CO2 and the formation of products and intermediates are indicated based on the rate of production (ROP) analysis. The ionic reactions are predominant for CH4 consumption, leading to CH4+ and CH5+, while CH4 reactions with electrons and Ar* contribute to CH2 and CH3 formation. CO2 mainly reacts with electrons and Ar*, producing CO, and can also undergo reactions with CH4+ and CH5+, forming HCO2+. The principal formation pathways for H2, CO, CH2O, and CH3OH are discussed. A competition between the discharge pathways and oxygen-containing pathways in CH4/CO2/Ar plasma is proposed.

The Effectiveness of Ni-Based Bimetallic Catalysts Supported by MgO-Modified Alumina in Dry Methane Reforming

Syngas is produced through the carbon dioxide reforming of methane. The traditional nickel-based catalysts are substantially destroyed by carbon deposition. The reforming reaction was conducted in a tubular microreactor at 700 °C using bimetallic Ni catalysts supported over 37% Al2O3 and 63% MgO mixtures. The impregnation process formed the catalysts, which were subsequently examined by N2-physisorption, XRD, H2-TPR, TGA, and Raman spectroscopy. The 2.5Ni+2.5Co/37%Al2O3+63%MgO bimetallic catalyst, which displayed 72% and 76% conversions of CH4 and CO2 over the course of a seven-hour procedure, was discovered to be the most active in DRM. The bimetallic catalyst with the largest weight loss in TGA, 2.5Ni+2.5Fe-MG63, had a loss of 61.3%, a difference of 26% and 21% in the activity performance of CH4 and CO2, respectively, of the tested bimetallic Ni catalysts was recorded. The long-time of 30 h on-stream CH4 and CO2 conversion reactions for 2.5Ni+2.5Co-MG63 and 2.5Ni+2.5Ce-MG63 catalysts showed the catalysts’ high stability. The TPO analysis for the 2.5Ni+2.5Cs-MG63 catalyst showed a peak at 650 °C, attributed to the oxidation of the filamentous carbon, whereas the TPO analysis for the 2.5Ni+2.5Co-MG63 catalyst depicted a peak at 540 °C, ascribed to the presence of amorphous/graphite carbon.

Role of O2 on nitrous oxide fuel blend ethylene auto-ignition sensitivity

An experimental and kinetic modeling study of the influence of O2 addition on the ignition behaviors of Ar diluted nitrous oxide/ethylene blend was performed. Ignition delay time (IDT) measurements were taken by detecting pressure evolution profiles in a heated rapid compression machine (RCM) at temperatures and pressures ranging from 910–1500 K to 35–45 bar. Results show that the addition of O2 significantly promotes the ignition of N2O/C2H4/Ar blend. With the O2 blending ratio increase from 10 % to 100 %, the IDT first decreases and then increases. Subsequently, the IDT data were employed to validate several kinetic models in literature. A reaction model developed in our previous work [Combustion and Flam, 2020, 221: 64-73] can reproduce the measured data and predict the promotion effect of O2 on N2O/C2H4/Ar blend well. Reaction pathway and rate of production (ROP) analyses show that high decomposition temperature of N2O inhibits the ignition of N2O/C2H4/Ar blend, whose ignition is mainly driven by reactions of N2O (+M) = N2 + O (+M) and N2O + H = N2 + OH. The dominant pathway is chain branching process: C2H4 + H/OH→C2H3 (+M)→C2H2 + O→HCCO + H2→CH2CO + H→CH3CO (+M)→CO + CH3. However, the presence of O2 in mixture induces reaction flux changes and decreases the global activation energy for chemical reaction. C2H3 radical is the main intermediate product during the ignition initial stage of C2H4 blends, whose ROP can reflect ignition process of C2H4 blends. For N2O/C2H4/O2/Ar and C2H4/O2/Ar ignition, C2H3 radical pool can be rapidly established via H-abstraction reaction of C2H4. However, the reaction of N2O + H = N2 + OH enhances the ROP of OH radical in N2O/C2H4/O2/Ar blends, resulting in C2H3 radical pool rapid establishment and a faster ignition.

A Gaussian Process Based Δ-Machine Learning Approach to Reactive Potential Energy Surfaces.

The Gaussian process (GP) is an efficient nonparametric machine learning (ML) method. A distinct advantage of the GP is its ability to provide an estimate of statistical uncertainties. This is particularly useful in constructing high-dimensional potential energy surfaces (PESs) from ab initio data as it offers an optimal way to add new geometries to reduce the overall error. In this work, GP is employed in the context of Δ-machine learning (Δ-ML), in which a correction PES to an inaccurate low-level PES is constructed using a small number of high-level ab initio calculations. This new method is tested in three prototypical reactive systems, namely, the H + H2 → H + H2, OH + H2 → H2O + H, and H + CH4 → H2 + CH3 reactions. The results show that the GP-based Δ-ML approach is more efficient than its direct application in constructing high-level PESs. We also compare the new method to a previously proposed neural-network-based Δ-ML approach [Liu and Li J. Phys. Chem. Lett. 2022, 13, 4729-4738]. The results indicate that the two Δ-ML methods have comparable efficiencies in constructing accurate PESs.

Aspen Plus model of a downdraft gasifier for lignocellulosic biomass adjusted by Principal Component Analysis

The development and improvement of biomass gasification processes have been accelerated by the use of computational tools for process simulation, exploring their feasibility under different conditions and identifying key parameters that are validated through real experiments, contributing to maturing a gasification technology from laboratory to commercial scale. This work presents an Aspen Plus model of a small-scale downdraft gasifier using biomasses with different elemental and proximate analyses, combining stoichiometric and non-stoichiometric equilibrium approaches. The pyrolysis, oxidation, and reduction processes in the proposed model were separated into distinct steps, with the first one based on Gibbs energy minimization and the latter based on stoichiometric equilibrium models. A representative tar composition, with a production ratio to gas production based on literature, was considered as an input parameter to the model. The obtained data were analyzed using principal component analysis (PCA) to investigate the relations between equivalence ratio (ER), fixed carbon and volatile material ratio FCMV, the CO conversion through methanation (Mconv), carbon conversion via hydrogasification (Hconv), and the mean residual sum of square MRSS. The most significant results obtained from this study include the successful adjustment of the model to represent the gasification of different biomasses, particularly in correcting the H2 and CH4 concentrations. Notably, the adjustment resulted in a substantial reduction of the MRSS deviation from an average of 94.7% to an average of 7.2% concerning reported results in the literature. These findings demonstrate the accuracy of the model in simulating the gasification process and highlight the importance of considering CH4 reactions adjustment for more reliable predictions. The achieved adjustments are valuable insights for advancing gasification technology towards engineering applications.

The CN(X 2Σ+) + C2H6 reaction: Dynamics study based on an analytical full-dimensional potential energy surface.

The hydrogen abstraction reaction of the cyano radical with molecules of ethane presents some interesting points in the chemistry from ultra-cold to combustion environments especially with regard to HCN(v) product vibrational distribution. In order to understand its dynamics, a new analytical full-dimensional potential energy surface was developed, named PES-2023. It uses a combination of valence bond and mechanic molecular terms as the functional form, fitted to high-level abinitio calculations at the explicitly correlated CCSD(T)-F12/aug-cc-pVTZ level on a reduced and selected number of points describing the reactive process. The new surface showed a continuous and smooth behavior, describing reasonably the topology of the reaction: high exothermicity, low barrier, and presence of intermediate complexes in the entrance and exit channels. Using quasi-classical trajectory calculations (QCT) on the new PES-2023, a dynamics study was performed at room temperature with special emphasis on the HCN(v1,v2,v3) product stretching and bending vibrational excitations, and the results were compared with the experimental evidence, which presented discrepancies in the bending excitation. The available energy was mostly deposited as HCN(v) vibrational energy with the vibrational population inverted in the CH stretching mode and not inverted in the CN stretching and bending modes, thus simulating the experimental evidence. Other dynamics properties at room temperature were also analyzed; cold rotational energy distribution was found, associated with a linear and soft transition state, and backward scattering distribution was found, associated with a rebound mechanism.