Search Results (99)

To accurately describe the combustion process of ammonia/diesel dual-fuel, this paper develops a simplified kinetic mechanism for ammonia/diesel dual-fuel based on the decoupling method and a modular approach, comprising 212 components and 620 elementary reactions. The diesel component is represented by four components: n-heptane, n-hexadecane, isohexadecane, and α-methylnaphthalene. The mechanism was validated using shock tube experimental data. The results indicate that the developed mechanism can accurately predict key parameters such as ignition delay time and laminar flame speed under different ammonia-blending ratios, showing good agreement with experimental values. Single-component ignition delay prediction error ≤ 6%; laminar flame speed deviation error ≤ 2%; CFD validation metrics (e.g., peak cylinder pressure error within 1.35%) Furthermore, the mechanism was coupled with 3D CFD software to validate the cylinder pressure and heat release rates, using a six-cylinder, heavy-duty diesel engine with a bore of 114 mm, a stroke of 145 mm, a displacement of 8.9 L, and a compression ratio of 16.6 as the study subject. Based on the validation of the model and the feasibility of the mechanism, further studies were conducted on combustion and emission patterns under different load conditions and ammonia substitution rates. The results indicate that at low ammonia substitution rates, as the load decreases, the combustion rate slows down and thermal efficiency declines, while the indicated thermal efficiency first decreases and then increases; load primarily influences ignition and combustion processes by altering the thermodynamic state within the cylinder. At ammonia substitution rates of 20–60%, the heat release rate exhibits a “bimodal” pattern under different load conditions. NO, NO₂, and N₂O emissions first increase and then decrease with increasing ammonia substitution rate, peaking in the 40–60% range; CO₂ emissions gradually decrease as the ammonia substitution rate increases. Full article

Achieving high selectivity in the hydrogenation of CO₂ to light olefins remains challenging because of the complex reaction network and the difficulty of regulating key intermediates. Motivated by green-hydrogen-enabled power-to-chemicals pathways, we combine density functional theory (DFT) with first-principles microkinetic simulation (FPMS) to construct a quantitatively predictive reaction-energy landscape and elucidate structure–selectivity relationships. A comprehensive reaction network is established through energy-surface fitting, and steady-state rate constants are solved to capture the microkinetic competition between elementary steps. By introducing electronic density-of-states (DOS) modulation as a design variable, we directly correlate surface structural parameters with rate-controlling steps, thereby enabling targeted regulation of C–C coupling and hydrogen transfer processes. The calculated barrier for CO₂ adsorption to COOH* is 1.35 eV, while the transition state barrier for C–C coupling is 1.50 eV, corresponding to a reaction rate of 9.7 × 10³ s⁻¹; the olefin desorption rate reaches 1.7 × 10⁷ s⁻¹. Crucially, shifting the d-band center from −2.35 eV to −1.60 eV increases the C₂–C₄ olefin selectivity from 42.6% to 68.3%, establishing an actionable electronic structure lever for catalyst optimization. These results reveal the intrinsic mechanism by which surface electronic and geometric regulation governs intermediate stabilization and rate control, providing a verifiable, mechanism-based design principle for efficient CO₂-to-olefin catalysts aligned with green hydrogen deployment. Full article

To meet the global climate change challenge and the International Maritime Organization’s (IMO) greenhouse gas emission reduction strategy, and promote the shipping industry’s transition to clean energy, this study focuses on the 6S35 2-stroke marine low-speed engine to explore hydrogen fuel combustion and emissions in the cylinder. A detailed chemical reaction kinetics model is constructed on the CONVERGE platform, coupling 42 components and 168 elementary reactions, integrating the SAGE combustion model with the extended Zeldovich NOx mechanism for refined numerical simulation of hydrogen combustion. Model validation shows the cylinder pressure peak simulation error is within 5%. Research results indicate hydrogen fuel has significant premixed combustion characteristics with a violent and concentrated heat release. Under simulation, the cylinder explosion pressure reaches about 28 MPa, and the max combustion temperature nears 3000 K, far exceeding traditional diesel engines. In terms of emissions, hydrogen’s carbon-free characteristic keeps CO₂ and CO emissions at extremely low levels (concentrations of approximately 0.02 and 0.085, respectively); whereas NOx emissions exhibit strong “high temperature dependence” and “expansion cooling effect,” with peak concentrations approaching 0.00042. This numerical model can effectively predict the combustion performance of hydrogen fuel, potentially providing a reference for optimizing fuel injection strategies and combustion chamber design to achieve efficient and clean combustion, and offering a theoretical basis for the development and commercial application of marine hydrogen fuel engines. Full article

With the increasing global demand for environmental protection and sustainable energy utilization, methanol, as a clean and renewable fuel, has become a research focus in the field of marine engines. However, its application in compression ignition engines faces bottlenecks such as low combustion efficiency and poor stability. Taking the L23/30H marine diesel engine as the research object, this paper establishes a combustion simulation model for a methanol/diesel dual-fuel direct-injection engine. The reliability of the model is ensured through grid independence verification and model calibration, and a coupled chemical reaction kinetic mechanism containing 126 species and 711 elementary reactions is constructed. A systematic study is conducted on the effects of injection strategies, including fuel operating modes, spray development patterns, injection intervals, and injection timing, on combustion characteristics. The results show that under the optimized injection strategy (vertical cross spray + synchronous injection) proposed in this study and operating conditions with a high methanol substitution ratio, the combustion efficiency, dynamic performance, and soot emission control effect of the dual-fuel mode are superior to those of the pure diesel mode. Simulation results show that the combined strategy of vertical cross injection and synchronous injection can significantly increase the indicated thermal efficiency (ITE) by 3.2%, reduce the brake specific fuel consumption (BSFC) by approximately 4.5%, advance the peak heat release by 2 °CA, and remarkably improve the combustion efficiency, while earlier injection timing is beneficial to air–fuel mixing. Further comparison of combustion and emission characteristics under different boundary conditions such as methanol energy ratios and injection pressures reveals that increasing methanol injection pressure, compression ratio, and initial pressure can improve combustion uniformity and reduce soot emissions, but NO_x emissions increase, which requires the coordination of after-treatment technologies. Through the comprehensive optimization of multiple parameters, efficient and clean combustion under a high methanol substitution rate is achieved. This paper provides theoretical support and practical guidance for the technological development of marine methanol dual-fuel engines. In the future, industrial applications can be promoted by combining actual engine tests and after-treatment technologies. Full article

Reaction mechanisms and rate constants of the acylation reaction of the hepatitis C virus (HCV) NS3/NS4A serine protease with the NS4B/5A natural substrate were studied using SCC-DFTB/MM (self-consistent charge density functional tight binding/molecular mechanics) and EA-VTST/MT (ensemble-averaged variational transition state theory/multidimensional tunneling) methods, considering the isotope effect (H/D). This reaction is crucial in the HCV life cycle. The reaction follows an essentially concerted mechanism. Although two elementary steps are involved, no intermediate step has been found between them. Thus, the proposed general two-step serine protease acylation mechanism, which includes a tetrahedral intermediate, does not occur here. This finding aligns with our studies on another natural substrate (NS5A/5B), indicating a greater variety in mechanism than previously expected. Tunneling and recrossing play an intermediate role; the activation free energy barriers are in good agreement with the experimental value, and the kinetic isotope effect (k(H)/k(D)) is somewhat larger than one (1.3). The rate constant value is not reproduced due to the exponential dependence of the rate constant on the activation free energy. Full article

This paper investigates a controversial phenomenon: the supposed generation of thrust from a symmetric system consisting of two contra-rotating gyroscopes whose spin axes form equal and opposite polar angles with respect to the axis connecting their supports. An elementary mechanical model demonstrates that, for this configuration of gyroscopes, an internal moment arises within the system. This torque, although internally generated, is well known for playing a significant role in satellite attitude control using control moment gyroscopes (CMGs). The mechanical analysis considers the system of gyroscopes mounted on a platform or cart, which is supported at its front and rear ends. In this context, it was found that the resulting dynamic interaction causes unequal reaction forces at the support points, which do not obey the length-ratio rule predicted by static analysis. Such behavior can lead to misinterpretation of the net external thrust, despite the system being closed and momentum-conserving. In this context, the present paper clearly shows that no net force is allowed to develop. Full article

The kinetics of the depolymerization of natural rubber (NR) to hydroxyl-terminated natural rubber (HTNR) by hydrogen peroxide (H₂O₂) in the presence of a Fenton catalyst within an acidic milieu and under ultraviolet radiation has been rigorously examined utilizing infrared spectroscopy to determine the alterations in molar mass and the functional characteristics. The kinetic model was analyzed in accordance with the elementary reaction, encompassing the following mechanisms: the interaction between hydroxyl radicals and NR, producing radical NR and hydroxylated NR; the reaction wherein radical NR and hydroxyl radicals yield hydroxylated NR; and the subsequent reaction of hydroxylated NR with hydroxyl radicals producing lower radical NR, hydroxylated terminated NR, radical NR, and hydroxylated NR. The conversion of the NR polymer and the total hydroxyl content were discerned at the absorption bands of the CH₂-CH₂ and OH groups located at 850 cm⁻¹ and 3400 cm⁻¹, respectively. The absorption peak at 1850 cm⁻¹ attributed to CH₃ was employed as the reference group for calibration. The influence of the temperature on the depolymerization process conformed to the Arrhenius equation, characterized by activation energies of 750 K and 1200 K. The impact of the H₂O₂/Fenton ratio on the depolymerization process follows a power law with power coefficients of 1.97 and 1.82. Full article

Hydrogen atom transfer (HAT), a concerted charge transfer involving two elementary particles, a proton and an electron, plays a key role in many areas of chemistry and biochemistry. A molecular dynamics study based on density functional theory was performed to investigate the reaction mechanism of hydrogen atom transfer from quercetin anions to the hydroxyl radical in a neutral aqueous media. Intrinsic bond orbital (IBO) analysis of a series of structures obtained from trajectories was performed in simulations in which the reaction occurred, and the electron flow along the reaction coordinate was determined and applied to investigate the reaction mechanism. The reaction in the simulations proceeded rapidly as proton-coupled electron transfer (PCET) or electron transfer–proton transfer (ET-PT) depending on the initial position and solvation of the reactants. Full article

Ammonia synthesis remains a cornerstone of global chemical manufacturing, essential for fertilizer production, energy storage, and emerging carbon capture technologies. This overview examines recent developments in the understanding of elementary reaction mechanisms in heterogeneous catalysis, with emphasis on transition metal thermocatalysts operating under the Haber–Bosch process. Traditionally, the dissociative adsorption of nitrogen (N₂) has been considered the rate-determining step. However, recent studies challenge this view, revealing possible shifts in rate-determining steps and suggesting that alternative mechanistic pathways may be operative. The discussion critiques studies that adhere strictly to the classic dissociative mechanism—often inferred from the reaction order of N₂—while overlooking alternative pathways that could offer more efficient catalytic routes and deeper mechanistic insight into ammonia synthesis. These insights offer a pathway toward more rational catalyst design and improved process efficiency in ammonia synthesis. Full article

Hydrogen (H₂) has become a more important alternative source in the current energy transition process. Beyond its role in clean energy production, it also serves as a key reactant in a wide range of industrial chemical transformations, such as hydrogenation and hydroprocessing. A fundamental step in many of these processes is the dissociation of hydrogen on catalyst surfaces. This short review provides an overview of the fundamental mechanisms involved in hydrogen dissociation over catalysts, with a specific emphasis on heterolytic pathways. Meanwhile, the influence of surface coordination environments on hydrogen activation is discussed, focusing on key factors—Lewis acid–base pairs, lattice oxygen and oxygen vacancies, and metal–support interfaces. With recognizing the significance of understanding the reaction mechanisms, we provide a critical review of experimental techniques, including spectroscopy, temperature-programmed methods, and kinetic analysis, that have been successfully applied or appear promising for probing active sites, reaction dynamics, chemisorbed intermediates, and elementary steps. Our goal is to highlight how these techniques contribute to a mechanistic understanding and to outline future directions, making this review a valuable resource for both new and experienced researchers. Full article

RP-3 is the most widely used aviation kerosene in China. Studying its chemical kinetic model is of great significance for analyzing its combustion characteristics and emissions. This paper proposes a new five-component RP-3 surrogate fuel mixture model. The three low-temperature oxidation reaction pathways of 1,3,5-trimethylbenzene were determined using the Rate of Production (ROP) analysis method, and a simplified mechanism containing 22 species and 69 elementary reactions was constructed. The reaction mechanisms of each component were simplified and coupled with the decoupling method, and the kinetic parameters of the coupled mechanism were corrected with the temperature sensitivity analysis method, obtaining a five-component surrogate fuel mixture mechanism containing 142 species and 502 elementary reactions. The mechanism was well matched with the experimental values in the verification of three parameters, ignition delay time, laminar flame speed, and small-molecule product concentration, within the ranges of 0.1–2 MPa, 500–1300 K, and equivalence ratio of 0.7 to 1.5. Whether under normal pressure or high pressure, the relative error between the simulation data and the experimental data was within ±5%. The mechanism can accurately predict the ignition and oxidation process of RP-3 aviation kerosene. Full article

Density functional theory calculations have been performed to explore the detailed mechanism of a ruthenium-catalyzed dehydrogenative annulation between α-carbonyl phosphonium ylide (A) and sulfoxonium ylide (B). The proposed catalytic cycles consist of several elementary steps in succession, namely the C–H activation of ylide A, the insertion of ylide B, reductive elimination, protodemetallation, and an intramolecular Wittig reaction, in which C–H activation is rate-limiting, with a free energy barrier of 31.7 kcal/mol. As A and B are both capable of being a C–H activation substrate and a carbene precursor, there are potentially four competing pathways including homo-coupling reactions. Further calculations demonstrate that A is more reactive in the C–H activation step than B, while the opposite conclusion is true for the ylide insertion step, which can successfully explain the fact that the solely observed product originated from the use of A as the C–H activation substrate and B as the carbene precursor. Molecular electrostatic potential, charge decomposition, and electron density difference analyses were performed to understand the distinct behaviors of the two ylides and the nature of the key ruthenium–carbene intermediate. Full article

In this work, we studied the main decomposition reactions on the ground state of nitromethane (CH₃NO₂) with the CASPT2 approach. The energetics of the main elementary reactions of the title molecule have been analyzed on the basis of Gibbs free energies obtained from standard expressions of statistical thermodynamics. In addition, we describe a mapping method (orthogonalized 3D representation) for the potential energy surfaces (PESs) by defining an orthonormal basis consisting of two $R^n$ orthonormal vectors (n, internal degrees of freedom) that allows us to obtain a set of ordered points in the plane (vector subspace) spanned by such a basis. Geometries and harmonic frequencies of all species and orthogonalized 3D representations of the PESs have been computed with the CASPT2 approach. It is found that all of the analyzed kinetically controlled reactions of nitromethane are endergonic. For such a class of reactions, the dissociation of nitromethane into CH₃ and NO₂ is the process with the lower activation energy barrier (ΔG); that is, the C-N bond cleavage is the most favorable process. In contrast, there exists a dynamically controlled process that evolves through a roaming reaction mechanism and is an exergonic reaction at high temperatures: CH₃NO₂ → [CH₃^…NO₂]* → [CH₃ONO]* → CH₃O + NO. The above assertions are supported by CASPT2 mappings of the potential energy surfaces (PESs) and classical trajectories obtained by “on-the fly” CASSCF molecular dynamics calculations. Full article

Methylcyclohexane (MCH, C₇H₁₄) is a typical component in hydrocarbon fuels and is frequently utilized in surrogate fuel mixtures as a typical representative of alkylated cycloalkanes. However, comprehensive experimental studies on speciation during its combustion remain limited. This research investigates for the first time the chemical structure of laminar premixed flames of lean and stoichiometric mixtures (φ = 0.8 and 1.0) of MCH/O₂/Ar under atmospheric pressure. Using probe-sampling molecular-beam mass spectrometry (MBMS), the spatial distribution of 18 compounds, including reactants, products, and intermediates, in the flame front was measured. The obtained results were compared with numerical simulations based on three established chemical–kinetic models of MCH combustion. The comparative analysis demonstrated that while the models effectively describe the profiles of reactants, primary products and key intermediates, significant discrepancies were observed for various C₂–C₆ compounds. To indicate the roots of the discrepancies, a rate of production (ROP) analysis was performed in each simulation. ROP analyses revealed that the primary cause for the discrepancies could be attributed to the overprediction of the rates of initial stages during MCH decomposition. Particularly, the role of non-elementary reactions was emphasized, indicating the need for refinement of the mechanisms based on new experimental data. Full article

The co-production of CO₂ continues to remain the bane of several hydrogen production technologies, including the steam reforming of methane and the dry reforming of methane processes. Efficient utilization of abundant greenhouse gas in the form of methane provides opportunities for the design of an innovative system that will maximize the use of such a raw material in the most environmentally friendly manner. The study of the mechanism of the pyrolysis of methane reactions over molten metals provides promise for improved hydrogen yield and methane conversion with a greater turnover frequency. Catalyst electronic properties computed via Density Functional Theory using the Quantum Espresso code provided data that were built into a database. Using Bismuth as the base metal, active transition metals including Ni, Cu, Pd, Pt, Ag, and Au of different concentrations of 5, 10, 15, and 25% were placed on 96 atoms of the base metal and relaxed to obtain the optimized geometric structures for the catalytic reaction studies. The kinetics of the individual elementary steps of the pyrolysis reaction at preset temperatures over the bi-metals were calculated using the Car-Parinello (CP) method and Nudge Elastic Band (NEB) computations. The collated data of the various pyrolysis of methane reactions over the different bi-metals was used to train machine learning models for the prediction of reaction outcome, catalytic performance, and efficient operating conditions for the pyrolysis of methane over molten metals. The turnover frequency, which is determined using the transition state energies of the fundamental reaction cycles, will be used to simulate the stability of the catalyst. Full article