Gas-phase Model Research Articles

Uncertainty Quantification of Linear Scaling, Machine Learning, and Density Functional Theory Derived Thermodynamics for the Catalytic Partial Oxidation of Methane on Rhodium.

Accurate and complete microkinetic models (MKMs) are powerful for anticipating the behavior of complex chemical systems at different operating conditions. In heterogeneous catalysis, they can be further used for the rapid development and screening of new catalysts. Density functional theory (DFT) is often used to calculate the parameters used in MKMs with relatively high fidelity. However, given the high cost of DFT calculations for adsorbates in heterogeneous catalysis, linear scaling relations (LSRs) and machine learning (ML) models were developed to give rapid estimates of the parameters in MKM. Regardless of the method, few studies have attempted to quantify the uncertainty in catalytic MKMs, as the uncertainties are often orders of magnitude larger than those for gas phase models. This study explores uncertainty quantification and Bayesian Parameter Estimation for thermodynamic parameters calculated by DFT, LSRs, and GemNet-OC, a ML model developed under the Open Catalyst Project. A model for catalytic partial oxidation of methane (CPOX) on Rhodium was chosen as a case study, in which the model's thermodynamic parameters and their associated uncertainties were determined using DFT, LSR, and GemNet-OC. Markov Chain Monte Carlo coupled with Ensemble Slice Sampling was used to sample the highest probability density (HPD) region of the posterior and determine the maximum of the a posteriori (MAP) for each thermodynamic parameter included. The optimized microkinetic models for each of the three estimation methods had quite similar mechanisms and agreed well with the experimental data for gas phase mole fractions. Exploration of the HPD region of the posterior further revealed that adsorbed hydroxide and oxygen likely bind on facets other than Rhodium 111. The demonstrated workflow addresses the issue of inaccuracies arising from the integration of data from multiple sources by considering both experimental and computational uncertainties, and further reveals information about the active site that would not have been discovered without considering the posterior.

Dynamic evaporation characteristics of liquefied natural gas droplets

To delve into the intricate evaporation and dispersion mechanisms of dense droplets formed in the vicinity of liquefied natural gas (LNG) accidental releases, it is imperative to first examine the evaporation dynamics of individual moving LNG droplets. This paper presents a visual experimental setup designed to scrutinize the temporal evolution of diameter and displacement of single free-falling LNG droplets. Additionally, eight typical drag force models used for droplet motion state calculations were assessed. The optimal drag force models were selected to accurately predict the displacement of LNG droplets in the wide range of 100 < Re < 10 000. Moreover, eight typical gas phase models applied to predict heat and mass transfer were evaluated, revealing that none accurately capture the dynamic evaporation of free-falling LNG droplets. Subsequently, a new gas phase model suitable for predicting LNG droplet evaporation behavior is proposed. Furthermore, the periodic oscillation behavior of LNG droplet shape during the falling process is uncovered. The oscillation amplitude and dominant frequency of droplets are quantitatively investigated using the aspect ratio of droplets. Finally, an in-house program is developed to comprehensively analyze the evaporation characteristics of LNG droplets under different initial droplet diameters, velocities, and ambient temperatures. Based on gray relational analysis, the relative importance of three impacting factors on the evaporation coefficient is ranked.

On Some Origins of Tautomeric Preferences in Neutral Creatinine in Vacuo: Search for Analogies and Differences in Cyclic Azoles and Azines

In order to look for the origins of tautomeric preferences in neutral creatinine in vacuo, we examined prototropic conversions for model azoles, namely mono-hydroxy and mono-amino imidazoles, and also for their selected 1-methyl derivatives. All possible isomeric forms of creatinine and model compounds, resulting from intramolecular proton transfer (prototropy), conformational isomerism about –OH, and configurational isomerism about =NH, were studied in the gas phase (model of non-polar environment) by means of quantum-chemical methods. Because the bond-length alternation is a consequence of the resonance phenomenon, it was measured for all DFT-optimized structures by means of the harmonic oscillator model of electron delocalization (HOMED) index. Important HOMED analogies were discussed for investigated azoles and compared with those for previously studied cyclic azines, including pyrimidine nucleic acid bases. The internal effects were taken into account, and the stabilities of the investigated tautomers-rotamers were analyzed. Significant conclusions on the favored factors that can dictate the tautomeric preferences in creatinine were derived.

Ковалентное допирование нитрида углерода карбазолом и бензохалькодиазолами: моделирование электронных свойств в альтернативных приближениях

The results of theoretical consideration of the electronic properties are presented for compounds based on carbon nitride fragments consisting of three heptazine rings (melon) covalently bonded with heterocyclic substitutes (electron acceptors like 2,1,3-benzochalcodiazoles and electron do-nors like carbazole). The simulation has been performed at two alternative levels: a molecular gas-phase model and a model of one-dimensional polymer with periodic boundary conditions. These levels have allowed comparing the energy change of frontier orbitals and the band gap of a chain polymer for the same compounds. It has been found that, on the one hand, the selenium-containing heterocycles reduce the energy change of frontier orbitals significantly more than other covalently bonded dopants of the considered series; on the other hand, the doping of melon with mere carba-zole minimizes the band gap. At efficient doping, which minimizes the energy change of frontier orbitals the highest occupied orbital is localized on the electron-donor fragment of the molecule, and the lowest unoccupied orbital is localized on the electron-acceptor fragment of the molecule. It has also been shown that the doped melons form complexes with benzyl alcohol due to appear-ance of non-covalent bonds between them. Wherein, the lower strength of such bonds in complex-es with melon, substituted with the electron-acceptor selenium-containing fragments, suggests more efficient oxidation of an alcohol in such systems. The simulation results appropriately corre-spond to the available experimental data of the band gap of melon doped with electron acceptor molecules. The fast gas-phase calculations are suitable for qualitative estimation of dependences of the energy change of frontier orbitals upon their type and the amount of covalently bonded do-pant, whereas the more expensive calculations of polymer structures allow a correct estimation of the band gap values of carbon nitride polymers with various dopants.

Finite-rate and equilibrium study of graphite ablation under arc-jet conditions

Arc-jet facilities play a primary role in recreating aerothermal conditions experienced by atmospheric entry vehicles and are widely used to test the performance of thermal protection materials. In this work, we utilize a developed coupled framework between an overset flow solver CHAMPS NBS-Cart, and a material solver KATS-MR to study the ablation of graphite under arc-jet conditions. We implement a 12-species gas phase model to accurately represent the air-carbon mixture, including argon species present in the flow. The gas phase is modeled with a two-temperature thermo-chemical non-equilibrium model without considering electronic and ionization effects. The gas-surface interactions are modeled with a newly developed air-carbon ablation model accounting for oxidation, nitridation, and recombination reactions. In addition, the model is augmented with carbon sublimation reactions experienced at high heating conditions. The chemical state at the surface is tightly coupled with the flow solver, resulting in the improved accuracy and effectiveness of the simulation. The coupled approach is applied to study two experimental test cases conducted at the IHF arc-jet facility at NASA Ames. The predicted results are validated against measured recession, surface, and in-depth temperatures and compared to the prediction of the uncoupled, equilibrium-based approach. Finally, the accuracy of the prediction is explored with respect to the environmental properties, such as the diffusion coefficient, and material thermal conductivity.

Droplet heating and evaporation: A new approach to the modeling of the processes

A new model for mono-component droplet heating/evaporation is developed, tested, and applied to the analysis of in-house experimental data. The new model links the previously developed liquid phase model, using the analytical solution to the heat transfer equation at each time step, and the gas phase model, using the solution to the equations of the conservation of mass, momentum, and energy leading to an explicit expression for the Nusselt number and implicit expression for evaporation rate of the droplet. The latter expressions are used as boundary conditions for the liquid phase model. The new model is verified using a comparison between its predictions of the droplet temperatures and radii for very large liquid thermal conductivity [1000 W/(m K)] and those of the model, using the assumption that the thermal conductivity of liquid is infinitely large. The closeness between the predictions of these models supports the reliability of both. The model is validated using the experimental data obtained at the Heat and Mass Transfer laboratory of Tomsk Polytechnical University with regard to the heating/evaporation of droplets. The deviations between the measured and predicted droplet radii and temperatures in most cases are shown to be within experimental error margins.

Analysis of methane pyrolysis experiments at high pressure using available reactor models

Most batch reactor methane thermal decomposition (MTD) studies have focused on sub-atmospheric pressures. In this study, MTD experiments were performed between 892 K and 1292 K and 4 atm inside a quasi-isothermal quartz batch reactor at residence times ranging from 0 to 300 s. At a given temperature, elevated pressure resulted in higher methane conversion compared with the sub-atmospheric conditions. A zero-dimensional, closed, isochoric, homogeneous gas-phase model was used to compare numerical predictions from available reaction mechanisms in literature to experimental data. It was found that most models accurately predict conversion at low pressure and high temperature and at high pressure and high temperature, but did not accurately predict methane, hydrogen, and intermediate species mole fractions at 4 atm and either 892 K or 1093 K. A reaction path analysis showed the reactions responsible for the delay in hydrogen formation at elevated pressure are 2CH3(+M)⟺ C2H6, C2H6 ＋ CH3⟺ C2H5 ＋ CH4, C2H6 ＋ H ⟺ C2H5 ＋ H2 and C2H5(+M)⟺ C2H4 ＋ H, where M is the collision partner. A gas-phase model accounting for the solid carbon shows that the observed discrepancy is not due to the unavailability of the carbon formation model but to slower reaction kinetics, especially at lower temperatures. The experimental data presented can be used to obtain a methane pyrolysis reaction mechanism suitable for high pressure conditions.

Frontispiece: Thermally Activated vs. Photochemical Hydrogen Evolution Reactions–A Tale of Three Metals

An in-depth analysis and comparison of hydrogen evolution reactions in gas phase model systems reveal five distinct mechanistic routes to form either atomic or molecular hydrogen. For more information, see the Concept by M. Beyer et al. (DOI: 10.1002/chem.202203590).

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions-A Tale of Three Metals.

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature black-body radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Towards a lumped approach for solid plastic waste gasification: Polystyrene pyrolysis

In a circular economy perspective, solid plastic wastes (SPW) are a valuable source of chemicals, energy vectors and fuels. Chemical recycling through pyrolysis, gasification, and partial oxidation are promising alternatives to incineration and landfill disposal, allowing efficient exploitation of SPW. Modelling the combustion and gasification both at the particle and reactor scale requires first the definition of a suitable condensed phase pyrolysis mechanisms for each constituent. This work proposes a semi-detailed kinetic model for polystyrene (PS) pyrolysis. Following our recent work on polyethylene (PE) and polypropylene (PP), the model is based on the functional group approach firstly developed for polyvinylchloride (PVC) and biomass pyrolysis aiming at a consistent description of a wide palette of SPW components. The proposed approach retains an accurate description of low molecular weight species while representing high molecular weight species through their chemical functionalities only. The resulting liquid-phase model accounts for 34 species and 440 reactions and is attached to this paper together with the complementary gas-phase model. Model validation by a comprehensive comparison with experimental data proves its capability of predicting both mass loss and product distribution profiles with comparable accuracy to more expensive detailed models from the literature. The proposed model, together with other available subsets in the CRECK kinetic framework (i.e., biomass, PVC, PE, PP), offers a powerful tool for investigating key aspects in thermochemical recycling technologies thus supporting optimal reactor design. These aspects include mixture interactions (SPW and SPW/biomass mixtures), secondary cracking, catalytic, and gasification reactions.

C versus O Protonation in Zincate Anions: A Simple Gas-Phase Model for the Surprising Kinetic Stability of Organometallics.

For better understanding the intrinsic reactivity of organozinc reagents, we have examined the protolysis of the isolated zincate ions Et3 Zn- , Et2 Zn(OH)- , and Et2 Zn(OH)2 Li- by 2,2,2-trifluoroethanol in the gas phase. The protonation of the hydroxy groups and the release of water proceed much more efficiently than the protonation of the ethyl groups and the liberation of ethane. Quantum-chemical computations and statistical-rate theory calculations fully reproduce the experimental findings and attribute the lower reactivity of the more basic ethyl moiety to higher intrinsic barriers, which override the thermodynamic preference for its protonation. Thus, our minimalistic gas-phase model provides evidence for the intrinsically low reactivity of organozinc reagents toward proton donors and helps to explain their remarkable kinetic stability against moisture and even protic media.

Study on the Self-Pressurization Laws of Cryogenic Liquid Krypton Tank for the Electric Propulsion System

To meet the application requirements of zero boil-off storage of cryogenic liquid krypton propellant in the process of completing deep space exploration missions by electric propulsion system, it is important to study the thermodynamic changes in cryogenic liquid krypton tanks under different environments. The gas phase and liquid phase material model of krypton working fluid, the gas-liquid phase change model at the interface layers, and the heat transfer process between the wall and the internal fluid are established by ANSYS-Fluent. The energy source term, gas phase, liquid phase mass source term, and phase change saturation temperature in the liquid krypton cryogenic tank are defined. This paper’s three influencing factors of gravity, initial liquid filling rate, and wall heat leakage are simulated and analysed. The results show that: (1) The self-pressurization rate significantly rises as the wall heat flux rises, and the gas-liquid interface drops less quickly. The pressure and temperature in the tank are also increasing at the same liquid level. (2) In the normal gravity environment, the bigger the filling rate, the lower the corresponding self-pressurization rate. (3) In the process of variable gravity, the gas pressurization rate in the cryogenic liquid krypton tank falls with the decrease of gravity.

Mechanisms controlling the transport and evaporation of human exhaled respiratory droplets containing the severe acute respiratory syndromecoronavirus: a review.

Transmission of the coronavirus disease 2019 is still ongoing despite mass vaccination, lockdowns, and other drastic measures to control the pandemic. This is due partly to our lack of understanding on the multiphase flow mechanics that control droplet transport and viral transmission dynamics. Various models of droplet evaporation have been reported, yet there is still limited knowledge about the influence of physicochemical parameters on the transport of respiratory droplets carrying the severe acute respiratory syndromecoronavirus 2. Here we review the effects of initial droplet size, environmental conditions, virus mutation, and non-volatile components on droplet evaporation and dispersion, and on virus stability. We present experimental and computational methods to analyze droplet transport, and factors controlling transport and evaporation. Methods include thermal manikins, flow techniques, aerosol-generating techniques, nucleic acid-based assays, antibody-based assays, polymerase chain reaction, loop-mediated isothermal amplification, field-effect transistor-based assay, and discrete and gas-phase modeling. Controlling factors include environmental conditions, turbulence, ventilation, ambient temperature, relative humidity, droplet size distribution, non-volatile components, evaporation and mutation. Current results show that medium-sized droplets, e.g., 50µm, are sensitive to relative humidity. Medium-sized droplets experience delayed evaporation at high relative humidity, and increase airborne lifetime and travel distance. By contrast, at low relative humidity, medium-sized droplets quickly shrink to droplet nuclei and follow the cough jet. Virus inactivation within a few hours generally occurs at temperatures above 40°C, and the presence of viral particles in aerosols impedes droplet evaporation.

Room-Temperature Conversion of Methane to Methanediol by [FeO2]+

Inspired by the activities of P-450 enzyme and Rieske oxygenases in nature, in which the high-valent Fe-oxo complexes play a key role for oxidation of alkanes, the oxidation process of methane by the high-valent iron oxide cation [FeO2]+ has been explored by using Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometry complemented by high-level quantum chemical calculations. In contrast to the previously reported [FeO]+/CH4 and [Fe(O)OH]+/CH4 systems, which afford [FeOH]+ as the main product, the generation of Fe+ dominates the reaction of [FeO2]+ with CH4. Theoretical calculations suggest a novel "oxygen rebound" pathway for the liberation of methanediol. In particular, the inevitable valence increase of Fe prior to C-H activation is similar to the cytochrome P-450 mediated processes. To our best knowledge, this study provides the first example of methane activation by the high-valent Fe(V)-oxo species in the gas phase, which may thus bridge the gas-phase model and the condensed-phase biosystems.

Gas phase models of hydride transfer from divalent alkaline earth metals to CO2 and CH2O.

The reactivity of HMg+, HMgCl, and HMgCl2- in hydride transfer reactions with CO2 and CH2O were studied by means of the reverse reactions-decarboxylation of HCO2MgCln+/0/- and deformylation of CH3OMgCln+/0/- (n = 0-2)-by a combination of quantum chemical computations and mass spectrometry experiments. HCO2Mg+, HCO2MgCl and HCO2MgCl2- display similar energetics for unimolecular carbon dioxide loss; for CH3OMg+, CH3OMgCl and CH3OMgCl2-, formaldehyde loss is more favourable for the cationic species than for the anionic one, with the neutral species found in-between. Despite very similar overall thermochemistry for each of the charge states of the CO2 and CH2O systems, the intermediate reaction barriers are higher for the CO2 eliminations due to a more complex and demanding reaction mechanism. Exothermic ligand exchange is observed for CH3OMg+ reacting with CO2, forming HCO2Mg+ and CH2O. Both the thermochemistry and the presence of intermediate energy barriers slightly disfavour this type of ligand exchange for CH3OMgCl and CH3OMgCl2-. It was also found that CH3OMg+ reacts readily with H2O to eliminate H2, whereas quantum chemical computations predict that the corresponding neutral and anionic species suffer unfavourable reaction thermochemistry. A corresponding reaction for the magnesium formate compounds was not observed. Quantum chemical computations were performed to investigate periodic trends in reactivity. The energetic requirements for decarboxylation of HCO2M+, HCO2MCl and HCO2MCl2- increase in the order M = Be, Mg, Ca, Sr and Ba; the only exception is the cationic Be species for which the reaction is more endothermic than for the corresponding Mg species. For deformylation of CH3OM+, CH3OMCl and CH3OMCl2- the trends are more irregular and less pronounced than for the decarboxylation reactions; however, the Be species was found to have higher reaction energies than the Mg species for all the methoxy-compounds irrespective of charge state. The effect on decarboxylation and deformylation upon replacing Cl with F, Br or I was found to be minimal for all aforementioned species. The consequences of these observations on the reverse reactions, hydride transfer to CO2 and CH2O, are discussed, and the effects of systematic structural changes on reactivity are rationalized on the basis of thermochemical cycles and well-known periodic trends.

The structure of DNA fragments: Quantum-chemical modelling

In this review, we analyze and systematize our computational studies of the nucleic acid duplex formations and thermodynamic stability under the different factors of investigation. The proposed structural models of mini-helix contains N nucleobase pairs (N = 3-5); QM structural data suggest that the helical conformations of mini-helix adopt geometrical parameters comparable to those of natural A- and B-DNA forms under specific conditions as micro hydration and charge compensation. The gas-phase models adopt non regular conformations between the helical form and a ladder form.. The natural helical shape of DNA mini-helix is stabilized by the presence of counterions or by explicit micro-hydration of the major and minor groves. The presence of aqueous solution is shown as a minor factor for the helical shape formation. The studies are performed at the level of density functional theory.

New prebiotic molecules in the interstellar medium from the reaction between vinyl alcohol and CN radicals: unsupervised reaction mechanism discovery, accurate electronic structure calculations and kinetic simulations.

Vinyl alcohol (VyA) and cyanide (CN) radicals are relatively abundant in the interstellar medium (ISM). VyA is the enolic tautomer of acetaldehyde and has two low-lying conformers, characterized by the syn or anti placement of hydroxyl hydrogen with respect to the double bond. In this paper, we present a gas-phase model of the barrierless reactions of both VyA's conformers with CN employing accurate quantum chemical computations in the framework of a master equation approach based on the transition state theory. Our results indicate that both VyA conformers feature a similar reactivity with CN, starting with a barrierless addition to the double bond and followed by different isomerization, dissociation, and/or hydrogen elimination steps. The rate constants computed for temperatures up to 600 K show that several reaction channels are open even under the harsh conditions of the ISM, with the favoured one providing the first feasible formation route of a prebiotic molecule not yet detected in the ISM, namely cyanoacetaldehyde. This finding suggests looking for cyanoacetaldehyde in regions where both VyA and CN have already been detected, like, e.g., Sagittarius B2N or G+0.693-0.027.

Modelling of the catalytic initiation of methane coupling under non-oxidative conditions

The experimentally observed interplay between catalytic activation of methane on Fe©SiO2 and gas-phase free radical methane coupling under non-oxidative conditions is analyzed by mechanistic modeling as well as by experiments. For the modeling, an off-the shelf gas-phase model, AramcoMech 3.0, was used unaltered to keep the number of adjustable parameters as low as possible. It was complemented by surface reactions specifically accounting for methane activation to methyl radicals. The model was validated against an independent set of experimental data and exhibited good accordance. The model accurately captured the significant contribution of gas-phase reactions responsible for methane conversion in the post-catalytic zone, indicative of gas-phase autocatalytic methane coupling. The low-activity induction period in gas-phase methane pyrolysis can effectively be overcome by adequate catalytic activation. Results show that the catalytic reaction only influences the activity of the system, with gas-phase reactions dictating the selectivity distribution. Simulations demonstrated that the optimum catalytic conversion roughly amounts to 4 % at 1000 °C and 1 atm. An equivalent effect can be reached by adding ca. 2 % of ethane or ethylene to the feed. Detailed reaction-path analyses were employed to corroborate these phenomena. Gas-phase reactions were found to be very rapid at 1000 °C, hence determining the product selectivity, without impact from either catalyst or C2 hydrocarbon addition. Current, freely available gas-phase models lack the required accuracy for detailed kinetic modeling of the product distribution, showing the requirement for the development of a dedicate non-oxidative methane coupling model.

A Critical Remark on the Applications of Gas-Phase Biofilter (Packed-Bed Bioreactor) Models in Aqueous Systems

The principles of gas-phase biofilter systems, modeling, and operations are quite different from liquid-phase biofilter systems. Because of "biofilter" terminology used in both gas and liquid-phase systems, researchers often mistakenly use gas-phase models in liquid-phase applications for the analysis of data and determining kinetic parameters. For example, recent studies show a well-known gas-phase biofilter model, known as Ottengraf-Van Den Oever zero-order diffusion-limited model, is applied for analysis of experimental data from an aqueous biofilter system which is used for the removal of toxic divalent copper [Cu(II)] and chromium (VI). The objective of this research is to present the limitations and principles of gas-phase biofilter models and to highlight the incorrect use of gas-phase biofilter models in liquid-phase systems that can lead to erroneous results. The outcome of this work will facilitate scientists and engineers in distinguishing two different systems and selecting a more suitable biofilter model for the analysis of experimental data in determining kinetic parameters.

Evaporation of single moving liquid nitrogen droplet: Experimental study and numerical simulation

Liquid nitrogen (LN2) spray is composed of numerous LN2 droplets. Understanding the spray evaporation process requires firstly studying the evaporation mechanism of single moving LN2 droplet. This study focuses on experimental study and numerical simulation for evaporation characteristics of single moving LN2 droplet. A visualized experimental setup was established to investigate the dynamic evaporation characteristics of single moving LN2 droplet. The temporal evolution of LN2 droplet diameter and traveling distance was measured. The mass, energy and momentum equations between the droplet and the surrounding gases, as well as their coupled solving algorithm, are also developed to predict the temporal diameter, temperature and velocity of single moving droplet. Six typical gas-phase evaporation models are integrated into an in-house program so as to evaluate their accuracy and applicability for the droplet with a high evaporation rate. The results show that all of the six models underestimate the evaporation rate of single moving LN2 droplet. It is suggested that the Stefan blowing effect should not be taken into consideration for single moving LN2 droplet. A novel gas-phase model is then proposed for the evaporation of single moving LN2 droplet. The model can accurately predict the evaporation rate of single moving LN2 droplet when the relative velocity of LN2 is greater than 0.5 m/s, and the prediction error is within 1.1%. Furthermore, the model is also used to investigate the effects of key droplet parameters in real LN2 spray, including the initial diameter, the initial velocity, the ambient temperature and the mass fraction of nitrogen in the ambient gases, on the evaporation of single moving droplet.