Proposed Reaction Steps Research Articles

Oxygen Reduction Reaction Mechanism Study Via the Mean-Field Microkinetic Modeling and Uncertainty Quantification of Model Parameters

Electrocatalytic oxygen reduction reaction (ORR) finds important applications as the cathodic reaction in hydrogen fuel cells or metal-air batteries. However, its slow kinetics even on the most active catalysts requires an understanding of the reaction mechanism for the cost-effective catalyst search.The first insight into the reaction mechanism can be obtained via the rotation ring disk electrode experiment (RRDE), which can be used to extract information about the reaction kinetics. A substantial amount of experimental work has been carried out in this field with various reaction mechanisms proposed. In many cases, they show a similar level of agreement with the experimental data, because ORR is a complex reaction, while the only intermediate observed experimentally is H2O2 (HO2 - in alkali). More accurate results may be obtained within the mean-field microkinetic modeling approach (MF-MKM). It is built upon the Butler-Volmer formalism for the charge-transfer steps, the law of mass action for the chemical steps, mass transfer equations for the reaction intermediates in the electrolyte, and various interactions between the adsorbed species (Langmuir, Frumkin, and Temkin isotherms).However, for the complex reactions, even the simplest numerical models have numerous parameters, such as forward/backward rate constants of the proposed reaction steps. They are usually adjusted manually, or fitted to the experimental data using the local optimization methods. The main drawback of both these approaches is that one should have prior knowledge of the approximate model parameters, i.e. guess the reaction pathway. While being successful for relatively simple models, it can be hardly done for the more complicated cases. Additionally, the uncertainty in the estimated model parameters is introduced from the errors in experimental data.In this work, we describe the quantitative framework for the reaction mechanism selection based on model parameters uncertainty quantification via the Tree-Structured Parzen estimator algorithm and accuracy of fitting of available experimental data by the suggested reaction mechanism within the MF-MKM formalism. So it provides the elucidation of the reaction mechanism which can be considered based on the available experimental data and should be crucial to study the reaction mechanism of ORR and other sufficiently complex reactions.

The multistep decomposition of boric acid

AbstractDue to its high potential for thermal energy storage systems (Huber, Setoodeh Jahromy, Jordan, et al, Energies. 2019;12:17) the decomposition of boric acid is of particular interest in the field of applied research. The complexity of the reaction mechanism, with its multiple partial‐overlapping reaction steps, hitherto prevented a clear identification and analysis of each stoichiometric reaction step. So far, various research teams performed different kinetic analyses of boric acid, which led to various reaction mechanisms and stoichiometric reaction steps with yet inconclusive results for process modeling. Thus, a deeper examination of the process was desirable, to validate whether a proposed reaction is reasonable or not. For this purpose, experimental data were used for a deconvolution of the reaction sequence, using the Fraser‐Suzuki function, which clearly revealed the respective single reactions. The results of the deconvolution were compared with the proposed reaction steps in consideration of the stoichiometric ratio and thereby illustrated that the decomposition of polycrystalline boric acid more likely consists of three reaction steps. In contrary to the two‐step mechanism, the three‐step mechanism showed a very good correlation (r > 99%). Based on these outcomes, kinetic analyses were performed for each reaction step, by means of the nonparametric kinetics 2 (NPK2) method with subsequent determination of kinetic parameters. Additionally, for a deeper insight into the reaction, analyzing techniques like X‐ray diffraction (XRD), scanning electron microscopy (SEM) and simultaneous thermal analysis (STA) were applied.

Prediction of Organic Reaction Outcomes Using Machine Learning.

Computer assistance in synthesis design has existed for over 40 years, yet retrosynthesis planning software has struggled to achieve widespread adoption. One critical challenge in developing high-quality pathway suggestions is that proposed reaction steps often fail when attempted in the laboratory, despite initially seeming viable. The true measure of success for any synthesis program is whether the predicted outcome matches what is observed experimentally. We report a model framework for anticipating reaction outcomes that combines the traditional use of reaction templates with the flexibility in pattern recognition afforded by neural networks. Using 15 000 experimental reaction records from granted United States patents, a model is trained to select the major (recorded) product by ranking a self-generated list of candidates where one candidate is known to be the major product. Candidate reactions are represented using a unique edit-based representation that emphasizes the fundamental transformation from reactants to products, rather than the constituent molecules’ overall structures. In a 5-fold cross-validation, the trained model assigns the major product rank 1 in 71.8% of cases, rank ≤3 in 86.7% of cases, and rank ≤5 in 90.8% of cases.

Comprehensive Structural Characterization of the Bacterial Homospermidine Synthase-an Essential Enzyme of the Polyamine Metabolism.

The highly conserved bacterial homospermidine synthase (HSS) is a key enzyme of the polyamine metabolism of many proteobacteria including pathogenic strains such as Legionella pneumophila and Pseudomonas aeruginosa; The unique usage of NAD(H) as a prosthetic group is a common feature of bacterial HSS, eukaryotic HSS and deoxyhypusine synthase (DHS). The structure of the bacterial enzyme does not possess a lysine residue in the active center and thus does not form an enzyme-substrate Schiff base intermediate as observed for the DHS. In contrast to the DHS the active site is not formed by the interface of two subunits but resides within one subunit of the bacterial HSS. Crystal structures of Blastochloris viridis HSS (BvHSS) reveal two distinct substrate binding sites, one of which is highly specific for putrescine. BvHSS features a side pocket in the direct vicinity of the active site formed by conserved amino acids and a potential substrate discrimination, guiding, and sensing mechanism. The proposed reaction steps for the catalysis of BvHSS emphasize cation-π interaction through a conserved Trp residue as a key stabilizer of high energetic transition states.

Computational Insights into the Mechanism of Porphobilinogen Synthase

Porphobilinogen synthase (PBGS) is a key enzyme in heme biosynthesis that catalyzes the formation of porphobilinogen (PBG) from two 5-aminolevulinic acid (5-ALA) molecules via formation of intersubstrate C-N and C-C bonds. The active site consists of several invariant residues, including two lysyl residues (Lys210 and Lys263; yeast numbering) that bind the two substrate moieties as Schiff bases. Based on experimental studies, various reaction mechanisms have been proposed for this enzyme that generally can be classified according to whether the intersubstrate C-C or C-N bond is formed first. However, the detailed catalytic mechanism of PBGS remains unclear. In the present study, we have employed density functional theory methods in combination with chemical models of the two key lysyl residues and two substrate moieties in order to investigate various proposed reaction steps and gain insight into the mechanism of PBGS. Importantly, it is found that mechanisms in which the intersubstrate C-N bond is formed first have a rate-limiting barrier (17.5 kcal/mol) that is lower than those in which the intersubstrate C-C bond is formed first (22.8 kcal/mol).

Mechanisms for Formation of Diazocinones, Pyridazines, and Pyrazolines from Tetrazines—Oxyanion-Accelerated Pericyclic Cascades?

A computational approach is utilized to study the diazocinone- and pyridazine-forming cascade reactions resulting from the reaction of 1,2,4,5-tetrazines with cyclic enolates. Many of the proposed reaction steps can be formulated as oxyanion-accelerated pericyclic processes. In examining these, a unique stepwise version of a formal (4 + 2) cycloaddition/(4 + 2) cycloreversion was discovered. For the key ring-opening step in these cascades, theoretical evidence for two distinct processes is reported. Of these two possibilities, an allowed six-electron electrocyclic ring-opening is predicted to be highly favored both kinetically and thermodynamically. Evidence for an unexpected oxyanion-accelerated 1,2-sigmatropic shift was also found for certain systems, leading to the theoretical prediction that seven- and eight-membered ring-fused pyrazoline systems could be formed experimentally under conditions similar to those for diazocinone and pyridazine formation.

A combined DRIFTS and MS study on reaction mechanism of NO reduction by CO over NiO/CeO2 catalyst

Previously, we reported the prominent catalytic activity of ceria-supported nickel oxide catalyst for the reduction of NO by CO [Y. Wang, A.M. Zhu, Y.Z. Zhang, C.T. Au, X.F. Yang, C. Shi, Appl. Catal. B 81 (2008) 141–149]. In the present study, the reaction mechanism of NO and CO over the NiO/CeO2 catalyst has been examined in two kinds of reaction modes: (i) NO reaction with CO pre-treated catalyst and (ii) CO reaction with NO pre-treated catalyst, by employing in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupled with mass spectroscopy (MS) techniques. It was found that the generation of surface NCO complexes and N2 (g) occurs only in mode (i), which gives obvious evidence of NO dissociation over CO pre-reduced NiO/CeO2 catalyst. The result is similar to that obtained in the case of NO and CO co-adsorption, but different from that of mode (ii). The overall results indicate that CO reduction of surface oxygen should be the first and crucial step, and the dissociation of NO on the CO-reduced surface is a pathway for N2 generation. The other pathway for N2 generation is the interaction of NCO complexes with NO. Based on these understandings, we proposed reaction steps for the catalytic reduction of NO by CO over the NiO/CeO2 catalyst.

非経験的分子軌道法による五酸化バナジウム触媒効果を考慮したベンゼン環開裂過程に関する計算化学的解析

In this study, in order to elucidate benzene decomposition reaction path on V2O5 catalysis, ab initio molecular orbital calculation has been conducted for 5 elemental reaction steps involved in the reaction between benzene and V2O9H8 cluster, proposed by Jaine, et al., i.e. benzene adsorption, hydrogen atom surface transfer, hydrogen gas de-sorption, oxygen gas adsorption and benzene ring decomposition. Molecular structures in stable state, transition state and intermediate molecular structures have been revealed in this proposed reaction steps. The reaction scheme can be described as follows; (1) electron in benzene transfer themselves to the catalysis cluster, whereby hydrogen atom transfer on the catalysis surface proceeds for the subsequent hydrogen molecular gas de-sorption, and (2) benzene ring decomposition takes place via the unique Mars-van Krevelen mechanism on V2O5 catalysis.

Radiolysis of Bicarbonate and Carbonate Aqueous Solutions: Product Analysis and Simulation of Radiolytic Processes

An understanding of the radiation-induced effects in groundwater is essential to evaluate the safe geological disposal of spent fuel. In groundwater, the bicarbonate ion is the predominant and common anion; this work investigated radiation-induced chemical reactions of (bi)carbonate aqueous solutions with steady-state irradiation and pulse radiolysis methods. Aqueous solutions of sodium (bi)carbonate as high as 50 mmol·dm-3 were used. The formation of formate, oxalate, and H2O2 were measured under different conditions. A complete set of reaction steps and reliable kinetic data for the radiolysis of (bi)carbonate aqueous solutions at ionic strength close to the groundwater were proposed. Kinetic calculations were completed based on the proposed reaction steps and the kinetic data obtained in the present work. The results from the calculation are in good agreement with the experimental results. With these proposed reaction steps and kinetic data, computer simulation can be performed to predict the yield of radiolytic products of (bi)carbonate aqueous solutions as a function of irradiation time and used to evaluate the safety of geological disposal options of spent fuel.

Fouling phenomena in multi stage flash (MSF) distillers

Fouling in multi stage flash (MSF) distillers has been occupying researchers for many years. A lot of work has been done and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. In this paper an attempt is made based primarily upon visual and reported observations of fouling in various parts along the flow path of brine solutions in MSF distillers. This analysis is aimed at proposing certain sequence of scale forming reaction steps and to suggest certain experiments that could verify the validity of the proposed reaction mechanism. The proposed reaction steps are shown in alphabetic order to denote the sequence that could prevail inside heat transfer tubes of recovery and heat input (brine heater) sections and water boxes and thereafter in flash chambers including demister pads of MSF distillers. To predict the fate of various species and pH values one must identify reaction steps. The common practice is that: pH values are measured of solutions drawn out of heat exchanger tubes of recovery section or brine heater. This leads to the evolution of carbon dioxide. In view of the above, variation in pH values which could support the proposed mechanism were never addressed. Certain experiments should, therefore, be devised where pH values of recirculating brine inside heat exchanger tubes could be measured on-line while still under pressure. It would then be probable (especially if particular species detection electrodes are installed) to measure variation in pH values due to hydronium ion generation and its consumption. Meanwhile, the presence of hydroxyl ions will be short lived primarily due to Mg(OH) 2 precipitation. The proposed steps support CaCO 3 precipitation ahead of Mg(OH) 2. The abundance of magnesium ions and the extreme low solubility of magnesium hydroxide will rapidly then lead to its formation. However, scale precipitation inside tubes are not only from initial scale formation under pressure inside the tubes but also due to nucleates recirculation from flash chambers back into heat gain exchanger tubes because of brine recycling. Recent analysis of variation in coloration of water boxes came as a strong support to this hypothesis. Reaction steps suggested by other workers will of water boxes came as a strong support to this hypothesis. Reaction steps suggested by other workers will be shown for the sake of comparison. It is worth to note that the end results of various reaction mechanisms are almost the same in the cited ones also the same to the proposed overall reaction.

Expression of the Si Etch Rate in a CF 4 Plasma with Four Internal Process Variables

The rate equation for Si etching in a plasma where a polymer film consisting of the C‒C bond does not grow on the substrate was derived based on a simple kinetic model. The etch rate was expressed as a function of four internal process variables: the sheath bias voltage, the ion current density, and the partial pressures of atomic fluorine and the polymer precursors. Etching experiments were made in a transformer coupled plasma etcher over a wide range of process conditions measuring the internal variables in situ. The rate constants of the proposed reaction steps, estimated by fitting the etch rate equation to the experimental results, had reasonable values when compared with the values reported in the literature. The derived etch‐rate equation allowed correlation between the reaction probability of F atoms with Si and the fractions of the surface species: Si(c), , and . © 1999 The Electrochemical Society. All rights reserved.

Criteria for the design of monoalkylphosphine precursors for InP metalorganic vapour phase epitaxy

We have studied the reaction between n-butylphosphine and sec-butylphosphine with trimethylindium in the dihydrogen atmosphere of an InP metalorganic chemical vapour deposition (MOCVD) reactor using Fourier transform infrared (FTIR) spectroscopy, since these precursors are structurally related to tertiarybutyl phosphine, an established InP MOCVD precursor. It is found that at room temperature under conditions designed to amplify precursor interactions an undesirable pre-reaction occurs between sec-butylphosphine and TMIn resulting in a clear, viscous, involatile liquid. For n-butylphosphine a similar reaction is observed but to a much lesser extent than that for the cause of the sec-butylphosphine. We have further characterised the structure of the involatile product formed in the case of sec-butylphosphine using FTIR spectroscopy and have assigned this structure to that of a polymer of the type (Me-In-P-sBu) n . We have made a comparison of these findings with data previously reported from our laboratories for the precursors tertiarybutylphosphine, cyclohexylphosphine, cyclopentylphosphine, benzylphosphine and cyanoethylphosphine. With the exception of tertiarybutylphosphine all of the precursors studied give rise to an undesirable pre-reaction with TMIn resulting in an involatile product. This is believed to be a quite general phenomenon for this system with the polymer having the generic structure (Me-In-P-R) n . The uniqueness of tertiarybutylphosphine has been examined in the light of these data using a series of proposed reaction steps and simple molecular modelling and it is suggested that the key to the lack of pre-reaction in the case of tertiarybutylphosphine lies in a combination of its base strength and structure (steric effects) in relation to the other phosphines studied.

Halide promotion of the formation and carbonylation of μ3-imido ligands. Relevance to the halide promotion of nitroaromatic carbonylation catalysis

Abstract The individual reaction steps that are believed to occur during the catalytic carbonylation of nitroaromatics to form isocyanates, carbamates, amines, and ureas are reviewed with focus on stoichiometric organometallic reactions that model the proposed reaction steps. Especially emphasized are recent studies that show how halides dramatically promote both the formation and carbonylation of imido ligands. These results provide insight into the role that halides play in the known halide promoted Ru 3 (CO) 12 catalytic system for the formation of methyl N -phenylcarbamate from nitrobenzene, CO, and methanol [S. Cenini et al., J. Chem. Soc., Chem. Commun. 1984, 1286].

Computational modeling of the mechanisms of the free radical‐chain reaction of alkanes with oxygen. The oxidation of isobutane, n‐butane, and isopentane

AbstractA general computational method for obtaining complete solutions of time‐dependent kinetic equations has been developed and applied to free radical‐initiated reactions of alkanes with oxygen. The method has been applied to the low‐temperature, peroxide‐initiated oxidations of isobutane and isopentane. Using available independently measured and estimated values for the rate constants and activation parameters for each of the 20 proposed reaction steps for the oxidation of isobutane, the rates and products have been calculated for both the liquid phase and gas phase in the range of 100°–155°C. The calculated rates and products of oxidation agree with published experimental values. The oxidation of isopentane was examined by a 32‐reaction model. The rate constants were estimated using values for the appropriate rate steps in the oxidation of n‐butane and isobutane. The calculation of the oxidation rate and products agree with our experiments.