Parallel Reaction Mechanism Research Articles

Electro-Catalytic Nitrogen Reduction by Metal Oxynitride Thin Films: Effect of Metal Oxophilicity, and Role of Lattice Nitrogen

The electrocatalytic reduction of molecular nitrogen to ammonia—the nitrogen reduction reaction (NRR)—is of broad interest as an environmentally- and energy-friendly alternative to the Haber-Bosch process for agricultural and emerging energy applications. Herein, we present our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we demonstrate that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars-van Krevelen mechanism and that the reduction of lattice nitrogen and N2 to NH3 occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice, and (b) that NRR and the hydrogen evolution reaction (HER) do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride. Additionally, these results highlight the importance of both O-ligation of the V or Co center for the metal-binding of dinitrogen, and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state for effective N≡N bond activation.1-3 Acknowledgment: This work was supported by the National Science Foundation through grants DMR-2112864 (to JAK, TRC, and FD) and partial support by CHE-1953547 (to TRC). Additional NSF support for the UNT CASCaM HPC cluster via Grant CHE-1531468 is also gratefully acknowledged. Ganesan, A. Osonkie, P. Chukwunenye, T. Cundari, F. D'Souza, J. Kelber. Electrochemical Reduction of N2 to ammonia by vanadium oxide thin films at neutral pH, J. Electrochem. Soc., 2021, 168, 026504.Osonkie, A. Ganesan, P. Chukwunenye, F. Anwar, K. Balogun, M. Gharee, I. Rashed, T. R. Cundari, F. D’Souza, and J. A. Kelber, Electro-catalytic reduction of nitrogen to ammonia: The roles of lattice O and N in reduction at vanadium oxynitride surfaces, ACS Adv. Mater & Interface, 2022, 14, 531-542.Chukwunenye, A. Ganesan, M. Gharaee, K. Balogun, F. Anwar, Q. Adesope, T. R. Cundari, F. D’Souza, and J. A. Kelber, ‘Electrocatalytic Selectivity for Nitrogen Reduction vs. Hydrogen Evolution: A Comparison of Vanadium and Cobalt Oxynitrides at Different pH Values.’ J. Mater. Chem. A, 2022, 10, 21401-21415.

Extracting information on reaction intermediates from single-molecule data resulting from parallel reaction mechanisms.

Enzymes can increase the rate of a chemical reaction by stabilizing one or more reaction intermediates. Therefore, a full understanding of how a given enzyme works requires awareness of the various reaction intermediates involved in the reaction mechanism. Single-molecule studies of enzyme activity have become an increasingly common tool used to gain insight into enzyme reaction mechanisms. At the single-molecule level, product formation by any enzyme is a stochastic process. Accordingly, there is a great deal of variation in the amount of time it takes for each molecule of product to be generated, but the shape of the probability distribution for event waiting times contains information about the underlying mechanism. With adequate statistics, it is possible to determine the number of reaction intermediates for a process and their average lifetime from the experimental dwell-time distribution, even though the intermediates cannot be directly observed. However, when product can be formed via more than one pathway, the dwell-time distribution will be a mixture of the distributions for the different pathways. We use simulated data and blinded fitting to demonstrate that it is possible to extract useful information about parallel reaction pathways from mixed data distributions. By considering the fractional error of each parameter recovered, we assess what fitting methods are most successful, and determine what conditions adversely impact accuracy. We further explore the impact of heterogeneity among the lifetimes of intermediates, naturally occurring variation among enzymes, and measurement error on our ability to accurately characterize two parallel reaction mechanisms producing the same product.

A novel approach to inulin depolymerization: A Monte Carlo based model

• Monte Carlo model for inulin depolymerization. • End-biting/internal random cleavage mechanism was adopted. • The detailed molecular weight distribution can be simulated. • The model can distinguish between two scission mechanisms. • The obtained kinetic parameters are in line with literature. Modelling the depolymerization of natural and synthetic polymers is still a challenge in the field of chemical reaction engineering, as the classical approaches based on complex consecutive/parallel reaction mechanisms show limited capabilities in predicting phenomena that nature would do statistically. A Kinetic Monte Carlo model is proposed, which assumes that polymers can be cleaved following two parallel reaction mechanisms: (i) end-biting (i.e., unzipping), (ii) internal random cleavage. The model was studied via a systematic parametric investigation, and it showed high flexibility in predicting detailed product distribution and concentration profiles of monomers as a function of reaction time. The model was applied to a industrially interesting case: inulin hydrolysis, giving mostly fructose as reaction product. A very good fit was obtained and the model was capable to explain the often-overlooked details observed in the experimental results, e.g., the sigmoidal kinetics in the early stages of the hydrolysis and its dependence on experimental conditions.

Isotopic Exchange Study on the Kinetics of Fe Carburization and the Mechanism of the Fischer–Tropsch Reaction

The kinetics of the transformation of metallic Fe to the active Fe carbide phase at the start of the Fischer–Tropsch (FT) reaction were studied. The diffusion rates of C atoms going in or out of the lattice were determined using 13C-labeled synthesis gas in combination with measurements of the transient 12C and 13C contents in the carbide by temperature-programmed hydrogenation. In the initial 20 min, C diffuses rapidly into the lattice occupying thermodynamically very stable interstitial sites. The FT reaction starts already during these early stages of carburization. When reaching steady state, the diffusion rates of C in and out of the lattice converge and the FT reaction continues via two parallel reaction mechanisms. It appears that the two outer layers of the Fe carbide are involved in hydrocarbon formation via a slow Mars–Van Krevelen-like reaction contributing to ∼10% of the total activity, while the remainder of the activity stems from a fast Langmuir–Hinshelwood reaction occurring over a minor part of the catalyst surface.

Mechanisms of Si Nanoparticle Formation by Molten Salt Magnesiothermic Reduction of Silica for Lithium‐ion Battery Anodes

AbstractMolten salt methods enable the synthesis of Si nanostructures by moderating the thermal energy evolved in highly exothermic magnesiothermic reduction reactions (MRR) of silica. Due to their cost‐effectiveness and scalability, these techniques are well suited for producing nanoscale Si for a number of applications, including energy storage. To control the microstructure morphology and particle size, it is necessary to understand the formation mechanism of the Si produced. By evaluating the time‐resolved phase evolution, when NaCl moderates the thermal energy generated by MRR of SiO2, we elucidate 3 parallel interfacial reaction mechanisms yielding Si nanoparticles – via Mg vapor, Mg‐rich eutectic liquid, and Mg ions dissolved in molten NaCl. These individual Si nanoparticles offer a striking contrast to the typical by‐product of MRR of SiO2 with and without NaCl, which yields a 3‐dimensional (3‐D) porous network of sintered Si nanoparticles. Lithium‐ion battery half‐cells with electrodes composed of individual Si nanoparticles showed a greater first‐cycle irreversible discharge capacity and faster capacity loss over the first 5 cycles at a current density of 200 mA g−1 compared to half‐cells with electrodes of a porous 3‐D Si network–indicative due to thicker solid electrolyte interphase (SEI) formation on individual particles. At a higher current rate of 400 mA g−1, once SEI formation and activation of Si are established, both cells exhibit a similar capacity retention rate over 100 cycles.

5‐(Chloromethyl)furfural production from glucose: A pioneer kinetic model development exploring the mechanism

AbstractConversion of glucose to 5‐(chloromethyl)furfural (CMF) is one of the well‐known high yield unit processes in lignocellulosic biomass valorization. A kinetic modeling study was not reported for the reaction, owing to the complexity in quantification of CMF. Herein we have successfully developed a rapid, sensitive, and specific HPLC method (reverse phase) to quantify the generated CMF (range: 10–650 μg/mL) in a dichloroethane solvent. The Box–Behnken design of experiment method employed for the statistical optimization. A kinetic model was developed based on the homogeneous first‐order kinetic model, and the results are in good agreement with the experiment data. The formation of CMF, 5‐(hydroxymethyl)furfural, formic acid, levulinic acid, and humins from glucose and HCl were modeled using a serial parallel reaction mechanism. The apparent activation energy (Ea) for glucose decomposition and CMF formation is 99 and 31 kJ/mol.

A hybrid kinetic and optimization approach for biomass pyrolysis: The hybrid scheme of the isoconversional methods, DAEM, and a parallel-reaction mechanism

Biomass is a renewable source and is considered as an alternative to fossil fuels. Bioenergy can be released from biomass combustion while various bioproducts can be produced through gasification and pyrolysis. Among these three technologies, pyrolysis is more attractive, because multiple types of bioproducts including biogas, bio-oil, and biochar can be generated during the process. On the other hand, due to the convolution of biomass pyrolysis, the design of pyrolysis reactors is very challenging. In this work, a new hybrid kinetic and optimization approach was proposed to facilitate the design and process optimization of pyrolysis reactors. In the hybrid approach, the isoconversional methods were integrated with a parallel-reaction mechanism to provide more detailed description on thermal decomposition of biomass. Duckweed samples were analyzed in a TGA analyzer at three heating rates (10, 20, and 30 °C/min). The data were studied and compared with our proposed hybrid model. In the proposed approach, the isoconversional methods including the Friedman, FWO, and KAS were first applied to gain the values of kinetic parameters. A kinetic model using a parallel reaction mechanism was then developed to predict the thermal decomposition of duckweed. Furthermore, a nonlinear dynamic optimization model was established and coupled with the kinetic model to gain the optimal kinetic parameters for each parallel reaction and achieve good model-fitting. The same hybrid scheme was also employed and coupled with a three pseudo-components DAEM (distributed activation energy model) to predict duckweed pyrolysis. The model calculations of the hybrid models were all well-matched with experimental data that exhibited great potentials of our proposed hybrid scheme for providing detailed description of biomass pyrolysis.

On the Non-Faradaic Hydrogen Gas Evolution from Electrolytic Reactions at the Interface of a Cathodic Atmospheric-Pressure Microplasma and Liquid Water Surface

The electrolysis of water is an important reaction in many aqueous electrochemical systems, including those where an atmospheric-pressure microplasma jet is formed at a liquid surface. Here, we quantitatively study the hydrogen gas evolved from this plasma electrolytic system. Unlike conventional water electrolysis with a metal cathode in contact with the solution, more hydrogen gas is produced than expected based on the quantity of electricity passed, and the apparent faradaic efficiency exceeds 100%. By varying the solution temperature and carrying out kinetic analysis, we show two parallel reaction mechanisms exist , specifically faradaic liquid-phase reactions involving solvated electron-mediated reduction of hydronium ions and non-faradaic gas-phase reactions involving electron impact dissociation of water vapor, that lead to the distinct behavior.

Comparative kinetics of carbon dioxide (CO2) absorption into EAE, 1DMA2P and their blends in aqueous solution using the stopped-flow technique

The amine blends as the potential carbon dioxide (CO2) capture absorbents have been received much attention in recent years, but this amines interaction mechanism remains unclear. In the present work, the kinetics of CO2 absorption into aqueous solutions of a secondary amine (Ethylethanolamine, EAE), a tertiary amine (1-Dimethylamino-2-propanol, 1DMA2P) and their blend were investigated by using a stopped-flow setup. The kinetic data of aqueous EAE solution can be interpreted well by the zwitterion mechanism, and the reaction rate of 1DMA2P can be fitted well using the base-catalyzed hydration mechanism. Experimental results showed that the pseudo first-order rate constants (k0) of EAE/1DMA2P is much higher than the calculated values of individual EAE and 1DMA2P using the parallel reaction mechanism. It has been accepted that the tertiary amine (i.e. 1DMA2P) can attract the CO2 from the zwitterion to release the free EAE molecules or the 1DMA2P as a base join the reaction of the CO2 and EAE. Then, the reaction pathway was proposed and the kinetic model with acceptable AARD% values of 6.642 was developed. Therefore, it can be concluded that the addition of EAE into 1DMA2P with low energy consumption can importantly enhance the absorption performance to solve the contradiction between low absorption heats and high CO2 absorption rates.

The intrinsic kinetics of methane steam reforming over a nickel-based catalyst in a micro fluidized bed reaction system

Methane steam reforming (MSR) is studied experimentally and numerically. The intrinsic kinetics of the reaction are determined using a micro fluidized bed with a catalyst containing more than 50 wt % NiO/α-Al2O3. Intrinsic kinetic models are developed for parallel and serial reaction mechanisms, but the parallel mechanism is found to better match the experimental data. The activation energies for CO and CO2 formation are 81.69 kJ/mol and 59.38 kJ/mol, respectively, and the pre-exponential factors are 316.6 mol/(g h kPa0.85) and 0.00263 mol/(g h kPa3.1), respectively. As the reaction temperature increases, the rate of CO formation increases and that of CO2 decreases. At 800 °C, almost all the CH4 is converted to CO and H2, and the methane conversion rate (XCH4), the hydrogen production rate (YH2), and the CO selectivity (SCO) are 92.28%, 3.34, and 0.99, respectively. The effects of the steam-to-carbon ratio (S/C), inlet velocity, and preheating temperature at different reaction temperatures are simulated using the FLUENT software package. As S/C increases, XCH4 and YH2 increase, but SCO decreases. The higher the reaction temperature, the less S/C promotes XCH4 and YH2. When the reaction temperature is 700 °C and the inlet velocity is 0.2 m/s (residence time is 0.5 s), XCH4 is above 95%, and changes in the inlet velocity strongly influence the formation of CO. With increasing preheating temperature, XCH4, YH2, and SCO all increase gradually.

Experimental and numerical simulation of multi-component combustion of typical charring material

The direct combustion of typical charring material, with wood as the main representative, has received extensive attention due to its potential as sustainable source of heat and power generation, and the substantial fraction of fuel load in many building fires. In real fire situations, multi-component condensed phase reactants and gas products are involved in the pyrolysis process and the subsequent combustion process. Interestingly, the numerical simulation of these multi-component reactions is relatively not yet well studied. To address this shortcoming, we consider how the reactants can be dealt with using a three-component parallel reaction mechanism and moisture model embedded into the pyrolysis model, wherein the reaction kinetic parameters are optimized by Shuffled Complex Evolution algorithm. The evolved gas products, measured by the TG-FTIR experiment, can be coupled with the extended EDC multi-component combustion model and soot model using FireFOAM. Most of the thermophysical parameters are measured directly by experiments as the input values of simulation. In this work, the predicted results of mass loss rate and heat release rate are compared with experimental data of cone calorimetry, and the good agreement between them validates the applicability of the current multi-component model. Moreover, the effects of three sub-models (three-component parallel reaction mechanism, multiple evolved gas products and the extended EDC multi-component combustion model) are further analyzed based upon the predicted results.

Kinetic parameters estimation of pinus sylvestris pyrolysis by Kissinger-Kai method coupled with Particle Swarm Optimization and global sensitivity analysis

Pyrolysis of pinewood (pinus sylvestris) was investigated based on thermogravimetric analysis. A new method was put forward to estimate its kinetic parameters by coupling model-free and model-fitting models. Kissinger-Kai method updated from Kissinger method was used as the representative of model-free method. Particle Swarm Optimization heuristic algorithm, as the typical model-fitting method, was coupled with three-component parallel reaction mechanism to search the optimized values, wherein its search ranges of kinetic parameters were referred to the original calculated values by Kissinger-Kai method. Furthermore, to explore the influence of separate kinetic parameter on the final predicted thermogravimetric results, global sensitivity analysis about these parameters was conducted by comparison of Spearman rank correlation coefficient based on Latin Hypercube Sampling and rank transformation. It was found that the top three parameters affecting the predicted results were activation energy of lignin, reaction order of cellulose and pre-exponential factor of lignin.

The role of manganese ore reduction morphology development in setting reduction mechanisms

Manganese ores differ extensively in their mineralogical and bulk chemical composition. Consequently these ores follow different mineralogical morphology development paths as the reduction reaction proceeds in the production furnace. Mineralogical factors such as composition and distribution of initial liquid silicate formed at specific temperatures, as well as iron and manganese metallisation patterns, are significant determining factors in the relative importance of different parallel reduction reaction mechanisms in each ore type. Applied mineralogy techniques were used to identify and analyse matrix liquid silicate phase formation in manganese ore reacted at 1300 °C with Carbon Black reductant. The study includes three South African ores from the Kalahari Manganese Field, the ores represent both the Wessels-type and Mamatwan-type mineralogy. FactSage computational thermodynamic calculations were made to explain liquid silicate phase formation and its evolution with increased reaction time. Matrix silicate phase liquidus temperatures and pseudo-ternary phase diagrams in the MnO-FeO-SiO2-CaO-MgO-Al2O3 composition system were calculated. Matrix liquid silicate formed at the relatively low reaction temperature of 1300 °C within all three ores, although different quantities of liquid silicate formed along differing composition paths set by ore mineralogy.

The accuracy and efficiency of GA and PSO optimization schemes on estimating reaction kinetic parameters of biomass pyrolysis

Reaction kinetic parameters estimation of biomass pyrolysis is a relatively difficult optimization problem due to the complexity of pyrolysis model. Two common heuristic algorithms, Genetic Algorithm (GA) and Particle Swarm Optimization (PSO), are applied to estimate the kinetic parameters of three-component parallel reaction mechanism based on the thermogravimetric experiment in wide heating rates. The accuracy and efficiency of GA and PSO algorithms are compared with each other under the identical optimization conditions. The results indicate the better optimization abilities of PSO with the closer convergence solution to the global optimum and quicker convergence to the solution than GA based on the three-component parallel reaction mechanism of biomass pyrolysis. Especially, the improvement of best fitting value of PSO reaches up to 30% compared with that of GA. Furthermore, 14 estimated kinetic parameters of best fitting value are obtained and the mass loss rate predicted results including three separate components (hemicellulose, cellulose and lignin) are compared with experimental data.

DFT study on the structure and racemization mechanism of 2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl

AbstractTo explore the structure of chiral ligands important in asymmetric catalysis, the density functional theory has been applied to study the ground‐state geometries and isomerization processes of 2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl. The study shows that there are four isomers for 2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl, each of which has R‐ and S‐ enantiomer. The isomerization between R‐conformations is easier than the racemization between R‐ and S‐ enantiomer. A parallel reaction mechanism via anti‐enantiomerization routes was proposed for the racemization of 2‐amino‐2′‐hydroxy‐1,1′‐binaphthyl, which provides a valuable reference for the design, synthesis, and resolution of chiral ligands.

The effect of chemical reaction kinetic parameters on the bench-scale pyrolysis of lignocellulosic biomass

The pyrolysis of lignocellulosic biomass has received extensive attention due to its potential as an alternative and renewable energy source. The chemical reaction kinetic parameters, obtained by micro-scale thermogravimetric experiments and optimized by the Shuffled Complex Evolution method, are one of the key factors to represent the pyrolysis process. The bench-scale Fire Propagation Apparatus experiment with sample size of 0.1 m × 0.1 m is conducted to investigate the scale effect of these parameters during the pyrolysis process in a N2 environment. These optimized parameters are applied to the pyrolysis model based on Gypro considering the three-component parallel reaction mechanism, moisture and volume change to simulate the bench-scale experiment based on FireFOAM coupled with the dynamic mesh technology. Eventually, the predicted results agree well with experimental data, validating the effectiveness of the current parameters. Moreover, the effects of chemical reaction kinetic parameters from different references or models are further analyzed based upon the predicted results.

Reaction kinetics of carbon dioxide absorption in aqueous solutions of piperazine, N-(2-aminoethyl) ethanolamine and their blends

Amine blends have been extensively studied as advanced solvents modification for CO2 absorption. Due to the lack of knowledge on mechanism of amines interaction, reaction in blends was traditionally regarded as parallel reactions between CO2 and different amines. In this work, the kinetics of CO2 reaction with blends of two promising diamines, piperazine (PZ), N-(2-aminoethyl) ethanolamine (AEEA), were studied in a wetted wall column. The reaction rate constants of single amine were derived from kinetic data based on the zwitterion and termolecular mechanism for PZ and AEEA, respectively. It is interesting to find that the overall reaction rate of the blend is 12.0–28.1% lower than the calculated value through parallel reaction mechanism, which indicates the strong interaction between PZ and AEEA. A ‘zwitterion bridge’ reaction pathway was proposed as the microscopic mechanism of the interaction in the blend. Based on the kinetic models and experimental data, the interaction that PZ can promote the reaction in amines blend by transferring CO2 to AEEA through the PZ-CO2 intermediate was revealed. Besides, the interaction was proved to be a fast reaction with the reaction rate of same magnitude compared with the zwitterion hydrolysis reaction. The mechanisms of interaction between the two amines are further investigated through density functional theory (DFT). Results of simulation, in terms of activation energy and molecular geometry deformation, indicate that the PZ prefers a direct reaction with CO2 rather than the reaction with the AEEA-CO2 intermediate.

Thermal behavior and kinetics of municipal solid waste during pyrolysis and combustion process

The thermal behavior of municipal solid waste (MSW) in south China during pyrolysis and combustion processes was investigated via thermogravimetric analysis. In order to investigate the effect of metal oxides in the grate ash, the pyrolysis and combustion experiments were carried out with and without additives (ZnO, Fe2O3, CuO and Al2O3). Different artificial atmospheres (O2: N2 = 1:9, O2: N2 = 3:7, O2: N2 = 5:5, O2: N2 = 7:3) were adopted to investigate the oxygen-enriched combustion characteristics. Results showed that all additives facilitated the decomposition of MSW, while the CuO prohibited the decomposition of biowaste matter. The same effort was applied on the combustion process. All additives had a catalytic effect on ignition and combustion characteristics of MSW, except for biowaste volatile combustion of MSW with CuO. The oxygen-enriched combustion conditions reduced the ignition temperature of MSW and provided more help to the combustion process of MSW. Based on several compositions parallel reaction mechanism, the kinetic parameters were obtained and confirmed that both some additives and high oxygen concentration improved MSW combustion characteristics.

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: I. Kinetic study

The role of surface structure and defects in the oxidative coupling of methane (OCM) was studied over magnesium oxide as a model catalyst. Pure, nano-structured MgO catalysts with varying primary particle size, shape and specific surface area were prepared by sol–gel synthesis, oxidation of metallic magnesium, and hydrothermal post treatments. The initial activity of MgO in the OCM reaction is clearly structure-sensitive. Kinetic studies reveal the occurrence of two parallel reaction mechanisms and a change in the contribution of these pathways to the overall performance of the catalysts with time on stream. The initial performance of freshly calcined MgO is governed by a surface-mediated coupling mechanism involving direct electron transfer between methane and oxygen. The two molecules are weakly adsorbed at structural defects (steps) on the surface of MgO. The proposed mechanism is consistent with high methane conversion, a correlation between methane and oxygen consumption rates, and high C2H4 selectivity after short times on stream. The water formed in the OCM reaction causes sintering of the MgO particles and loss of active sites by degradation of structural defects, which is reflected in decreasing activity of MgO with time on stream. At the same time, gas-phase chemistry becomes more important, which includes the formation of ethane by coupling of methyl radicals formed at the surface and the partial oxidation of C2H6. The mechanistic concepts proposed in this work (Part I) will be substantiated in Part II by spectroscopic characterization of the catalysts (Schwach et al. [1]).

Effect of Carbon Deposition on the Oxidation Rate of Copper/Bentonite in the Chemical Looping Process

The presented work is part of the Industrial Carbon Management Initiative (ICMI) on the development of metal oxide oxygen carriers for use in the chemical looping combustion process. An oxygen carrier, CuO/bentonite (60:40%), was reacted with methane gas and then oxidized in air. The change in weight and reaction gas concentrations were measured using a thermogravimetric analyzer (TGA) equipped with a real-time gas analyzer. The reduction–oxidation cycle was conducted within the temperature range of 750–900 °C for 10 cycles, using 20, 50, and100% CH4 concentrations in N2 for the reduction segment and dry air for the oxidation segment. Several analysis methods were evaluated to fit the oxidation of reduced CuO (i.e., Cu) data over the complete conversion range with suitable rate expressions derived from existing models for oxidation, including the shrinking core model (diffusion and reaction control), first- and second-order reaction rates, parallel and series reaction mechanisms, and Johnson–Mehl–Avrami (...