Termolecular Reaction Research Articles

Statistical adiabatic channel model for termolecular reactions.

In this work, we present a statistical adiabatic channel model for termolecular reactions, A + B + C → Products. Our approach relies on hyperspherical coordinates, where the adiabatic channels are readily defined in the hyper-radius after averaging the hyperangular degrees of freedom. In this way, we find a general expression for termolecular rate constants. We focus on ion-neutral association reactions to test our approach's accuracy and predictive power, finding a good agreement between theory and experiment, especially in those reactions' temperature dependence.

Ab Initio Chemical Kinetics for Oxidation of CH3OH by N2O4: Elucidation of the Mechanism for Major Product Formation and Its Relevancy to Tropospheric Chemistry.

Next to CH4, CH3OH is the most abundant C1 organics in the troposphere. The redox reaction of CH3OH with N2O4 had been shown experimentally to produce CH3ONO, instead of CH3ONO2. The mechanism for the reaction remains unknown to date. We have investigated the reaction by ab initio MO calculations at the UCCSD(T)/6-311+G(3df,2p)//UB3LYP/6-311+G(3df,2p) level. The result indicates that the reaction takes place primarily by the isomerization of N2O4 to ONONO2 through a very loose transition state within the N2O4-CH3OH collision complex with a 14.3 kcal/mol barrier, followed by the rapid attack of ONONO2 at CH3OH producing CH3ONO and HNO3. The predicted mechanism for the redox reaction compares closely with the hydrolysis of N2O4. The computed rate constant, k1 = 1.43 × 10-8 T1.96 exp (-9092/T) (200-2000 K) cm3molecule-1s-1, for the formation of CH3ONO and HNO3 agrees reasonably with available low-temperature kinetic data and is found to be similar to that of the isoelectronic N2O4 + CH3NH2 reaction. We have also estimated the kinetics for the termolecular reaction, 2 NO2 + CH3OH, and compared it with the direct bimolecular process; the latter was found to be 4.4 × 105 times faster under the troposphere condition. On the basis of the known pollution levels of NO2, N2O4, and CH3OH, both processes were estimated to be of negligible importance to tropospheric chemistry, however.

Mechanisms of glycine formation in cold interstellar media: a theoretical study.

The possibility of the formation of glycine (Gly) from fundamental gas molecules in cold interstellar media was studied using quantum chemical methods, transition state theory and microcanonical molecular dynamics simulations with surface hopping dynamics (NVE-MDSH). This theoretical study emphasized five photochemical pathways in the lowest singlet-excited (S 1) state, thermochemical processes after non-radiative S 1→S 0 relaxations, and photo-to-thermal energy conversion in the NVE ensemble. The optimized reaction pathways suggested that to generate a reactive singlet dihydroxy carbene (HOCOH) intermediate, photochemical pathways involving the H2O…CO van der Waals and H2O-OC hydrogen bond precursors (Ch (1)_Step (1)) possess considerably lower energy barriers than the S 0 state pathways. The Gibbs free energy barriers (∆G ǂ ) calculated after the non-radiative S 1 →S 0 relaxations indicated higher spontaneous temperatures (T s) for the formation of the HOCOH intermediate (Ch (1)_Step (1)) than for Gly formation (Ch (1)_Step (2) and Ch (4)). Although the termolecular reaction in Ch (4) possesses a low energy barrier, and is thermodynamically favourable, the high exothermic S 1 →S 0 relaxation energy leads to the separation of the weakly associated H2O…CH2NH…CO complex into single molecules. The NVE-MDSH results also confirmed that the molecular processes after the S 1 →S 0 relaxations are thermally selective, and because the non-radiative S 1 →S 0 relaxation temperatures are exceedingly higher than T s, the formation of Gly on consecutive reaction pathways is non-synergistic with low yields and several side products. Based on the theoretical results, photo-to-thermal control strategies to promote desirable photochemical products are proposed. They could be used as guidelines for future theoretical and experimental research on photochemical reactions.

Py3BR: A software for computing atomic three-body recombination rates.

The three-body recombination reaction, or ternary association, is a termolecular reaction leading to a molecule after a three-body encounter that plays a vital role in many relevant scenarios in chemical physics. Here, we introduce the Python 3-Body Recombination program, which is dedicated to the computation of atomic three-body recombination rate coefficients. The software is based on a classical trajectory approach in hyperspherical coordinates after mapping the three-body problem as a single particle in a higher-dimensional space. This theoretical approach is fully general and applicable to any ion-atom-atom or atom-atom-atom three-body process. The predictive power of the methodology has been tested in several different experimental scenarios, reaching a good description of every system. The code structure is presented alongside examples and tests to ensure the software's capacity. In addition, the performance of the software after parallelization is shown.

Quasiclassical study of a termolecular reaction: A more detailed description of the HO2 collisional stabilization process

We present detailed studies of the collisional stabilization of the HO2∗ radical in a mixture of hydrogen atoms and oxygen molecules using molecular dynamic studies and accurate potential energy surfaces. Following previous work on this process’s global temperature and pressure dependencies, we analyze each collider’s role and estimate specific rate constants and their temperature dependence.

Revisit flame chemistry of propene at elevated pressures: Insight into pressure effects on chemical structure and laminar flame propagation

This work reports an experimental and kinetic modeling investigation on flame chemistry of propene at elevated pressures. The chemical structures of premixed propene/O2/Ar flames at 2–5 atm and the equivalence ratio of 1.5 including reactants, major products and a series of intermediates were measured using the flame sampling molecular beam mass spectrometry. The maximum mole fractions of H, CH3 and C2H6 decrease with increasing pressure, while that of CH4 follows the opposite trend. Laminar burning velocities (LBVs) of propene/air mixtures at equivalence ratios of 0.7–1.5 and initial pressures up to 10 atm were measured using the spherically propagating flame method. Remarkable pressure effects can be observed, especially under the lean and rich conditions. A kinetic model for propene combustion was developed based on our recent model for ethylene combustion and recently reported chemically termolecular reactions and HCO prompt dissociation reactions. Key reactions in the propene flames were revealed using the modeling analysis, especially for those playing significant roles in the pressure effects. Two chain termination reactions including the self-combination of CH3 and the recombination between CH3 and H are found to be crucial for the observed pressure effects in maximum mole fractions of H, CH3, CH4 and C2H6. The HCO prompt dissociation can significantly promote the laminar flame propagation of propene/air mixtures due to the substantial formation of CH2O through CH2+O2 and CH3+O pathways, while the chemically termolecular reactions exhibit weak inhibition effects at equivalence ratios smaller than 1.2. The reason for the stronger pressure effects of LBVs under the lean and rich conditions is that the excess O2 and abundantly produced CH3 enhances the roles of H+O2(+M)=HO2(+M) and CH4(+M)=CH3+H(+M) in chain termination, respectively.

Influence of nonthermal termolecular reactions on the optimization and uncertainty quantification of a hydrogen combustion model

Nonthermal termolecular reactions were shown to have significant impacts on combustion predictions of H2, CH4, and other fuels. But they are not considered in earlier model development efforts, many of which involve tuning or optimization. While adding new chemistry to optimized models is not recommended, we need to quantify its influence. The impact on model optimization has been examined on the H2 sub-mechanism of the Foundational Fuel Chemistry Model Version 1.0. The current work compared the optimization using the model as is, and with termolecular chemistry fully implemented, with new termolecular reactions and corrections to the stabilization reactions. The models optimized with and without termolecular chemistry have comparable performance and both could reconcile all the experimental targets considered. The two models show similar predictions under most target conditions, which can be explained by the compensation of kinetic parameters within their uncertainty ranges. Extrapolation of the model to conditions outside the target list shows larger discrepancy. While models with proper inclusion of nonthermal reactions should in general give more reliable predictions, it is expected that the difference can be minimized with a proper target list and optimized models can be used with attention on conditions very sensitive to termolecular chemistry. The conclusions provided guidance on model optimization with termolecular chemistry in consideration.

Effects of non-thermal termolecular reactions on wedge-induced oblique detonation waves

The shock-induced combustion ramjet (Shcramjet) based on oblique detonation waves (ODWs) is among the promising choices for hypersonic propulsion systems. An understanding of the ignition, propagation, and stability of ODWs is critical to harnessing their propulsive potential. In such high speed reacting flows, there is a high probability of occurrence of non-thermal reactions due to the presence of non-trivial amounts of highly reactive radicals including H, O and OH apart from O2 as demonstrated recently [M. P. Burke, S. J. Klippenstein, Nat. Chem. 9 (2017) 1078–1082, Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. The present work focuses on examining the initiation, propagation and structure of oblique detonation waves in stoichiometric H2-air mixtures through numerical simulations with and without non-thermal reactivity on a two-dimensional adaptive grid. Non-thermal reactions were included in the macroscopic kinetic model as chemically termolecular reactions facilitated by the H + OH radical-radical recombination and the H + O2 radical-molecule association reactions. Since, the non-thermal reactions result in a corresponding decrease in the reaction fluxes of the incipient recombination/association reactions, an additional simulation was performed by applying corrections to the respective incipient recombination/association rate constants using the methodology demonstrated by Tao et al. [Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. Results show that, under ODWE relevant conditions, non-thermal reactivity fundamentally alters the induction length, intensity as well as the structure of the ODW. Specifically, it is found that non-thermal reactivity leads to a noticeable reduction in initiation length and a simultaneous increase in instantaneous peak heat release rate and the degree of unsteadiness of the ODW. Statistical analysis of key thermo-chemical variables is performed to elucidate the important species as well as reactions responsible for the observed variations.

Effects of non-thermal termolecular reactions on detonation development in hydrogen (H[formula omitted])/methane (CH[formula omitted]) - air mixtures

The binary fuel blend of H2/CH4 is one of the most promising hydrogen-enriched hydrocarbon fuels in spark-ignition (SI) engines. Yet, the undesirable phenomenon of super-knock, which can severely and instantaneously damage an SI engine, limits its widespread adoption. Moreover, there is still a lack of consensus on the precise mechanism by which this phenomenon occurs i.e. via flame acceleration or spontaneous ignition, despite numerous previous investigations. At the same time, recent studies [M. P. Burke, S. J. Klippenstein, Nat. Chem. 9 (2017) 1078 - 1082, Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522] have demonstrated a high probability of occurrence of non-thermal reactions in premixed flames of such H2/CH4 fuel blends with air due to the presence of non-trivial amounts of highly reactive radicals including H, O and OH apart from O2. The present study focuses on the evolution of an initial deflagration front to a detonation wave in H2/CH4-air mixtures under SI engine relevant conditions through fully resolved, constant volume 1D simulations with and without non-thermal reactivity. Non-thermal reactions were included in the macroscopic kinetics model as chemically termolecular reactions facilitated by the H + CH3 and H + OH radical-radical recombination and the H + O2 radical-molecule association reactions. The nonthermal reactions result in a corresponding decrease in the reaction fluxes of the incipient recombination/association reactions. Therefore, an additional set of simulations were performed by applying corrections to the respective incipient recombination/association rate constants using the methodology demonstrated by Tao et al. [Y. Tao, A. W. Jasper, Y. Georgievskii, S. J. Klippenstein, R. Sivaramakrishnan, Proc. Combust. Inst. 38 (2021) 515–522]. Compared to the baseline case, the onset of spontaneous ignition in the end-gas region was observed to be delayed in the presence of non-thermal termolecular reactions. Concurrently, the developing detonation was observed to be significantly stronger. In contrast, applying corrections to the recombination/association rate constants resulted in a completely different behavior. Specifically, detonation was observed to occur due to self acceleration of the primary flame in the absence of spontaneous ignition in the end-gas region. Sensitivity analysis was performed to quantify the effects of non-thermal reactions on the duration of heat release rate and thereby the mechanism of detonation formation. In addition, chemical explosive mode analysis (CEMA) was performed to identify the dominant species/reactions responsible for the observed results.

Rate Coefficients for OH + NO (+N2) in the Fall-off Regime and the Impact of Water Vapor

The termolecular, association reaction between OH and NO is a source of nitrous acid (HONO), an important atmospheric trace gas. Rate coefficients for the title reaction as recommended by evaluation panels differ substantially at the temperatures and pressures that prevail in the Earth’s boundary layer where the reaction is in the fall-off regime between low- and high-pressure limiting rate coefficients. Using pulsed laser methods for generation and detection of OH, we have reinvestigated the kinetics of the title reaction at pressures of 22–743 Torr (1 Torr = 1.333 hPa) and temperatures (273, 298, and 333 K) in pure N2 and in N2–H2O bath gases. In situ optical absorption measurements were used to rule out any bias due to NO2 or HONO impurities. Our rate coefficients (k1) in N2 bath gas are parametrized in terms of low-pressure (k0) and high-pressure (k∞) rate coefficients and a fall-off parameter (FC) with k1,0N2 = 7.24 × 10–31 (T/300 K)−2.17 cm6 molecule–2 s–1, k1,∞ = 3.3 × 10–12 (T/300 K)−0.3 cm3 molecule–1 s–1, and FC = 0.53. Used with the “Troe” expression for termolecular reactions, these parameters accurately reproduce the current data in the fall-off regime and also capture literature rate coefficients at extrapolated temperatures. The presence of water vapor was found to enhance the rate coefficients of the title reaction significantly. The low-pressure limiting rate coefficient in H2O bath gas is a factor 5–6 larger than in N2, at room temperature (k1,0H2O = 4.55 × 10–30 (T/300 K)−4.85 cm6 molecule–2 s–1) indicating that H2O is much more efficient in quenching the association complex HONO* through collisional energy transfer. Based on measurements in N2–H2O mixtures, a parametrization of k1 including both N2 and H2O as third-body quenchers was derived. Neglecting the effect of H2O results, e.g., in an underestimation of k1 by >10% in the tropical boundary layer.

Kinetics of OH + SO<sub>2</sub> + M: temperature-dependent rate coefficients in the fall-off regime and the influence of water vapour

Abstract. The OH-initiated oxidation of SO2 is the dominant, first step in the transformation of this atmospherically important trace gas to particulate sulfate, and accurate rate coefficients for the title reaction under all atmospheric conditions (pressures, temperatures, and humidity) are required to assess its role in, for example, new particle formation. Prior to this study, no temperature-dependent data were available in the fall-off regime for atmospherically relevant bath gases. We thus address an important omission in the kinetic database for this reaction and highlight significant discrepancies in recommended parameterizations. In this work, generation of OH via pulsed laser photolysis at 248 and 351 nm was coupled to its detection by laser-induced fluorescence to obtain rate coefficients (k1) for the title reaction at pressures of 14–742 Torr (1 Torr = 1.333 hPa) and temperatures of 220–333 K in N2 bath gas. In situ SO2 concentrations, central to accurate kinetic measurements under pseudo-first-order conditions, were measured by optical absorption. Under the conditions of the present study, the termolecular reaction between OH and SO2 is in the fall-off regime, and we parameterized the rate coefficients in N2 in terms of low- (k1,0) and high-pressure (k1,∞) limiting rate coefficients and a broadening factor (FC) to obtain k1,0N2=3.03×10-31 (T/300 K)−4.10 cm6 molecule−2 s−1, k1,∞=2.00×10-12 cm3 molecule−1 s−1, and FC=0.58. The effects of water vapour on the title reaction were explored through measurements in N2–H2O mixtures at 273, 298, and 333 K using the same experimental methods. The rate coefficients are significantly enhanced by the presence of water vapour with k1,0H2O=1.65×10-30 (T/300 K)−4.90 cm6 molecule−2 s−1, which indicates that H2O is a factor >5 more efficient in quenching the HOSO2* association complex than N2. A model-based comparison of our rate coefficients and parameterization with previous literature measurements and recommendations of evaluation panels are presented and discussed. The use of the new parameterization instead of the IUPAC or NASA evaluations, particularly after including H2O as a third-body quencher, leads to a significant (10 %–30 %) reduction in the lifetime of SO2 in some parts of the atmosphere and increases the H2SO4/SO2 ratio concomitantly.

Chemical Evolution of CO2 Ices under Processing by Ionizing Radiation: Characterization of Nonobserved Species and Chemical Equilibrium Phase with the Employment of PROCODA Code

Abstract Astrophysical ices are being exposed to ionizing radiation in space environments, which trigger new reactions and desorption processes. In the lab, such processing by radiation has revealed the appearance of several new species and complements the study of the chemical evolution of icy astrophysical scenarios. Here, we develop a computational methodology that helps to clarify the chemical evolution of ices investigated experimentally under photolysis/radiolysis processes until reaching chemical equilibrium (CE). Briefly, the code (named PROCODA) solves a system of coupled differential equations and describes the evolution of the molecular abundances with the irradiation time for ices under processing by radiation. Two experimental ice samples containing pure CO2 and irradiated by two ionizing agents (cosmic rays and ultraviolet photons) were considered prototype systems. Here, we considered 11 different chemical species within the ice (four observed: CO2, CO, O3, and CO3; seven nonobserved or unknown: O, O2, C, C2, C2O, C2O2, and C2O3), 100 reaction routes (e.g., direct dissociation reactions, bimolecular and termolecular reactions) and radiation-induced desorption processes. The best-fit models provide the reaction rates, several desorption parameters, as well as the characterization of the CE phase. At CE, the percentage of nonobserved species in the UV model was almost triple the one calculated in the CR model (which also includes a lot of O and C atoms). The determined values can be employed in future astrochemical models to map chemical evolution embedded species in astrophysical regions under the presence of an ionizing radiation field.

Quasiclassical study of a termolecular reaction: Application to the HO2 collisional stabilization process

We present a multiprocess reaction dynamics program to study the termination reaction H + O2 + M → HO2 + M, one main uncertainty source in hydrogen combustion studies. We simulate the behavior of a mixture of hydrogen atoms and oxygen molecules at different conditions of temperature and pressure, using classical mechanics and accurate Potential Energy Surfaces. In this simulation we treat all the reaction channels, including the stabilization step, in the same dynamical procedure. The contribution of the collisional effects on all the reactions is also accounted for. Following the fate of the excited HO2∗ radical, we analyze the collisional stabilization and estimate the kinetic parameters of the involved reactions. A Lindemann-Hinshelwood type reaction scheme is shown to be able to describe the kinetics of this reaction as a function of pressure and temperature. This way, we propose a procedure to directly study termolecular reactions.

Kinetic modeling of Polyamine-based Water-Lean solvents for CO2 capture: Reverse temperature dependence of the overall mass transfer coefficient

Amine-based water-lean solvents (WLS) are promising energy-saving alternatives to the aqueous amine solutions for CO2 capture. Herein, we determined the apparent kinetic parameters of WLS containing 3,3′-iminobis(N,N-dimethylpropylamine) (60 wt%), water (10–20 wt%), and a non-amine chemical (balance). For CO2 absorption rate modeling, the termolecular reaction model and reaction rate model of tertiary amines were incorporated into the equilibrium model proposed by previous work, and the absorption rate of CO2 was estimated using a wetted wall column-based apparatus. The average CO2 absorption rates of the studied 10 wt%–H2O WLS at 313 and 333 K were approximately 5.0e-03 and 4.0e-03 mol/m2∙kPa∙s, respectively, which were much higher than that of a conventional aqueous amine solvent for CO2 capture. In addition, we observed that the overall mass transfer coefficient decreased with increasing temperature in low content water of WLS. This is due to the change of physical properties (Henry's law constant and diffusion coefficient) of WLS with decreasing water concentration. In this work, the cause of the reverse temperature dependence of WLS was investigated and verified through experiments and modeling.

Excellent sulfur and water resistance for CO oxidation on Pt single-atom-catalyst supported by defective graphene: The effect of vacancy type

The competitive adsorption of sulfur dioxide and water molecules will degrade the performance for CO oxidation catalyst in some processes such as automobile exhaust cleaning or Claus tail gas treatment. In order to study the effect of carbon vacancy on the sulfur and humidity resistance, the adsorption of CO, O2, SO2 and H2O molecules on Pt single-atom-catalyst supported by graphene with single carbon vacancy (Pt-SG) and double carbon vacancy (Pt-DG) have been comparatively investigated by using density functional theory calculations. It shows that the active site of Pt-SG can be blocked by the SO2 and H2O molecules due to their strong affinity towards Pt-SG. However, the carbon divacancy makes Pt atom less attractive towards SO2 and H2O molecules on Pt-DG compared with that on Pt-SG, where the pre-adsorbed SO2 and H2O molecules will be substituted by the CO molecule with larger adsorption energy on Pt-DG, which finally achieves the sulfur and water resistance for Pt-DG. In addition, an efficient Termolecular Eley-Rideal reaction path with a low energy barrier of 0.49 eV for CO oxidation is found on Pt-DG. The significant downshift of CO-5σ orbital levels plays an important role in the formation of new O-C bonds in OCOOCO intermediate on Pt-DG, where the O2-1π orbital is weakened due to its interaction with CO-5σ and CO-1π orbital, which finally facilitates the break of O-O bond and the production of CO2 molecules along the TER mechanism. Overall, the double carbon vacancy can enhance the selective adsorption for CO molecule other than the SO2 and H2O molecules on Pt atom, making the Pt-DG to be an efficient CO oxidation catalyst even in sulfur and humid environment.

Measurements of the laminar flame speed of premixed, hydrogen-air-argon stagnation flames

As the foundation for detailed hierarchical combustion chemistry models, the accuracy of the hydrogen oxidation mechanism is paramount to achieve truly predictive models. The recent introduction of new chemically termolecular reactions to this system suggests that our understanding of this fundamental chemistry could have initially been plagued with structural inaccuracies that have been carried along in the development of larger thermochemical mechanisms. Consequently, there is a need for additional independent experimental data in various experimental setups to assess the accuracy of model predictions. An atmospheric jet-wall stagnation-flame configuration is used in this work to measure the reactivity of premixed, lean-to-rich hydrogen-air flames diluted with argon. Mixtures with estimated adiabatic flame temperatures of ∼1800K are obtained by argon dilution to maintain the oxygen-nitrogen ratio while allowing for stable, laminar flames over the desired conditions. The accuracy of various thermochemical mechanisms available in the literature is assessed with direct comparisons of the velocity profiles in the stagnation configuration obtained through Particle Tracking Velocimetry to the solution of the quasi–1D flame simulation. Unstretched laminar flame speeds (SLo) are subsequently inferred using a direct comparison technique between the experiments and the simulations at multiple stretch values, similarly to a non-linear extrapolation to unstretched conditions. Significant discrepancies are observed between model predictions and experimental measurements, which supports the current efforts in model development. The inaccuracies in kinetic rates of the hydrogen oxidation model must first be resolved to avoid optimization and validation errors in the hierarchical development of increasingly complex hydrocarbon mechanisms.

Noise-enhanced stability and resonance effect of a termolecular enzymatic reaction

Here, we investigate an enzymatic reaction system subjected to additive and multiplicative noises and a weak periodic signal. By means of numerical simulations, we find that structure of stationary probability distribution function changes from one peak to two peaks by regulating not only the noise intensities (i.e., noise-induced transition), but also system parameters. The mean first passage time as a function of the multiplicative noise intensity exhibits a maximum, showing that the noise can enhance stability of the system. Moreover, by using the two-states theory, analytically expression of signal-to-noise ratio (SNR) of the system in the adiabatic limit is derived. The results indicate that: (1) The SNR as a function of noise intensity exhibits a maximal value, i.e., a stochastic resonance (SR) phenomenon; (2) as the noise intensities remain unchanged, the curve of the SNR with respect to the parameters of the system displays a peak, i.e., a SR-like phenomenon. Because the noise and the parameters reflect, respectively, temperature and rates of the enzymatic reactions, choosing optimal temperature and rates can enhance response of the system. This findings will be beneficial for controlling enzymatic reaction systems.

Effect of High Pressures on the Formation of Nitric Oxide in Lean, Premixed Flames

Abstract Increasingly stringent regulations are imposed on nitrogen oxides emissions due to their numerous negative impacts on human health and the environment. Accurate, experimentally validated thermochemical models are required for the development of the next generation of combustors. This paper presents a series of experiments performed in lean, premixed, laminar, jet-wall stagnation flames at pressures of 2, 4, 8, and 16 atm. To target postflame temperatures relevant to gas turbine engines, the stoichiometry of the nonpreheated methane–air mixture is adjusted to an equivalence ratio of 0.7. One-dimensional (1D) profiles of temperature and NO mole fraction are measured via laser-induced fluorescence (LIF) thermometry and NO-LIF, respectively, to complement previously published flame speed data (Versailles et al., 2018, “Measurements of the Reactivity of Premixed, Stagnation, Methane-Air Flames at Gas Turbine Relevant Pressures,” ASME. J. Eng. Gas Turbines Power, 141(1), p. 011027). The results reveal that, as the pressure increases, the maximum postflame temperature stays relatively stable, and the concentration of NO produced through the flame front remains constant within uncertainty. Seven thermochemical models, selected for their widespread usage or recent date of publication, are validated against the experimental data. While all mechanisms accurately predict the postflame temperature, thanks to consistent thermodynamic parameters, important disagreements are observed in the NO concentration profiles, which highlights the need to carefully select the models used as design tools. The lack of pressure dependence of NO formation that many models fail to capture is numerically investigated via sensitivity and reaction path analyses applied to the solution of flame simulations. The termolecular reaction H+O2(+M)↔HO2(+M) is shown to hinder the production of atomic oxygen and to consume hydrogen radicals at higher pressures, which inhibits the formation of nitric oxide through the N2O pathway.

Unraveling the Distinct Relationship between the Extent of a Nanoconfined Reaction and the Equilibrium Constant

Equilibrium constant differential equations (ECDEs) are derived for several nanoconfined elemental bimolecular reactions, one termolecular reaction, and nanoconfined adsorption, in the framework of...

Understanding and Representing the Distinct Kinetics Induced by Reactive Collisions of Rovibrationally Excited Ephemeral Complexes across Reactive Collider Mole Fractions and Pressures.

Rovibrationally excited ephemeral complexes AB**, formed from the association of two molecules A + B, are generally considered to undergo collisions only with an inert bath gas M that transfer energy-inducing termolecular association reactions A + B (+M) → AB (+M). Recent studies have demonstrated that reactive collisions of AB** with a third molecule C-inducing chemically termolecular reactions A + B + C → products-can also be significant in combustion and planetary atmospheres. Previous studies on systems with reactive collisions have primarily focused on limited ranges of reactive collider mole fraction, XC, and pressure, P, specific to the chosen application. Yet, it remains to be established how such systems, and the rate constants of their emergent phenomenological reactions, behave over the wide XC and P ranges of potential interest-a gap in the present understanding that has impeded the development of broadly applicable rate laws and general treatment of such systems in kinetic modeling. Here, we present results from master equation calculations for HO2** formed from H + O2 and its reactions with H to advance understanding and explore representations of systems with reactive colliders across wide ranges of XC and P. With regard to understanding, we demonstrate that reactive collisions can both (1) increase the overall rate of conversion of reactants to products and (2) alter the branching ratio among final products. With regard to representations in kinetic models, we find that rate constants of all emergent phenomenological reactions-termolecular association A + B (+M), chemically termolecular A + B + C, and bimolecular AB + C-exhibit a rich XC and P dependence. We also present analyses to explore the existence of a unique phenomenological representation (or lack thereof) and assess ways for the distinct effects of reactive collisions to be represented in kinetic models.