Experimental Activation Energy Research Articles

Structure, density and viscosity of water

We analyzed the possibilities of the use of the cluster model of water to assess its viscosity. The Nemethy-Scheraga model was used in our study. In a simplified version, this model implies the presence of water cluster that are linked by hydrogen bonds as well as individual molecules (monomolecules) interacting only by van der Waals forces. The paper gives an estimation of average cluster size. Based on the experimental temperature dependences of viscosity and density, the content of monomolecules in water was approximately determined. In the first case, the ratio of the viscosity of water to monomolecules was estimated from the inverse Arrhenius temperature dependence of viscosity by considering experimental activation energy ~18.6 kJ mol–1 (0÷300C) and energy of dispersion interactions ~7.4 kJ mol–1. Then, the volumetric content of monomolecules was estimated by using the inverse Betchelor's formula, which relates the viscosity of the suspension (clusters) and dispersion medium (monomolecules) to their ratio. On the other hand, a similar estimation was performed based on the density of water, clusters that were considered similar to ice floes, and the estimated density of monomolecules. Both estimates showed that the volumetric content of water not bound into clusters does not exceed 9%. It was concluded that the structure of water most likely corresponds to the clathrate model, according to which some of the H2O molecules move into the middle of ice-like clusters, and vacancies are stabilized by H3O+–OH– pairs.

Dynamic Stark Effect in Two-Dimensional Spectroscopy Revealing Modulation of Ultrafast Charge Separation in Bacterial Reaction Centers by an Inherent Electric Field.

Despite extensive study, mysteries remain regarding the highly efficient ultrafast charge separation processes in photosynthetic reaction centers (RCs). In this work, transient Stark signals were found to be present in ultrafast two-dimensional electronic spectra recorded for purple bacterial RCs at 77 K. These arose from the electric field that is inherent to the intradimer charge-transfer intermediate of the bacteriochlorophyll pair (P), PA+PB-. By comparing three mutated RCs, a correlation was found between the efficient formation of PA+PB- and a fast charge separation rate. Importantly, the energy level of P* was changed due to the Stark shift, influencing the driving force for P* → P+BA- electron transfer and hence its rate. Furthermore, the orientation and amplitude of the inherent electric field varied in different ways upon different mutation, leading to contrasting changes in the rates. This mechanism of modulation provides a solution to a long-lasting inconsistency between experimental observations and activation energy theory.

“Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

The oxidative coupling of methane to ethylene using gaseous disulfur (2CH4 + S2 → C2H4 + 2H2S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH4 to C2H4 over an Fe3O4-derived FeS2 catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O2 (2CH4 + O2 → C2H4 + 2H2O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH2 coupling over dimeric S-S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS2 by-product forms predominantly via CH4 oxidation, rather than from C2 products, through a series of C-H activation and S-addition steps at adsorbed sulfur sites on the FeS2 surface. The experimental rates for methane conversion are first order in both CH4 and S2, consistent with the involvement of two S sites in the rate-determining methane C-H activation step, with a CD4/CH4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4 oxidative coupling with O2 The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

Preparation, characterization and electrical behaviors of greenish single-chain polymeric molecule-via intramolecular ball type cobalt phthalocyanines/ graphite oxide composites

The greenish single-chain polymeric molecule-ball type CoPc (SCPM-ball type CoPc) was prepared via the intramolecular formation of cobalt phthalocyanine from polystyrene-co-poly[4,4′-((((4-vinylbenzyl)azanediyl)bis(ethane-2,1diyl))bis(oxy))diphthalonitrile] [P(PNDEAMSt12%-co-St)] (linear copolymer precursor) under diluted conditions at 150 °C. The structure of SCPM-ball type CoPc was confirmed by FT-IR, MALDI-TOF-MS, UV and SEM. The conversion dependence of the experimental activation energy was readily determined by an isoconversional method. So, the Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) showed that the linear copolymer precursor's activation energies were as average 128.7 kJ/mol and 124.0 kJ/mol and those of SCPM-ball type CoPc were average 161.9 kJ/mol and 153.8 kJ/mol, respectively. Conductivity mechanism of composites was suggested as Correlated Barrier Hoping (CBH). The linear copolymer precursor/graphite oxide (GO) composite has a significant performance for dielectric constant of 696.4 at 130 °C, but it was with a comparatively greater dielectric loss. Those of SCPM-ball type CoPc/14wt% GO increased virtually the same relative ratio.

Silica‐Decoration Boosts Ni Catalysis for (De)hydrogenation: Step‐Abundant Nanostructures Stabilized by Silica

AbstractNi nanoparticles supported on SiO2 were decorated with additional SiOx species by impregnation with tetraethoxysilane (Si(OEt)4). The combination of electron microscopy, infrared analysis, and density functional theory (DFT) calculations revealed that the SiOx‐decoration made the surface morphology of Ni nanoparticles highly rough and step‐abundant due to the significant stabilization of low‐coordination‐number sites. The SiOx‐decorated Ni exhibited 50 times higher catalytic activity for the dehydrogenation of methylcyclohexane to toluene than pure Ni. DFT calculations revealed that the Ni(211) step sites had a more favorable C−H activation than Ni(111) terrace sites and well reproduced the experimental apparent activation energies.

Machine learning meets mechanistic modelling for accurate prediction of experimental activation energies.

Accurate prediction of chemical reactions in solution is challenging for current state-of-the-art approaches based on transition state modelling with density functional theory. Models based on machine learning have emerged as a promising alternative to address these problems, but these models currently lack the precision to give crucial information on the magnitude of barrier heights, influence of solvents and catalysts and extent of regio- and chemoselectivity. Here, we construct hybrid models which combine the traditional transition state modelling and machine learning to accurately predict reaction barriers. We train a Gaussian Process Regression model to reproduce high-quality experimental kinetic data for the nucleophilic aromatic substitution reaction and use it to predict barriers with a mean absolute error of 0.77 kcal mol−1 for an external test set. The model was further validated on regio- and chemoselectivity prediction on patent reaction data and achieved a competitive top-1 accuracy of 86%, despite not being trained explicitly for this task. Importantly, the model gives error bars for its predictions that can be used for risk assessment by the end user. Hybrid models emerge as the preferred alternative for accurate reaction prediction in the very common low-data situation where only 100–150 rate constants are available for a reaction class. With recent advances in deep learning for quickly predicting barriers and transition state geometries from density functional theory, we envision that hybrid models will soon become a standard alternative to complement current machine learning approaches based on ground-state physical organic descriptors or structural information such as molecular graphs or fingerprints.

Mechanism of synergistic effects and kinetics analysis in catalytic co-pyrolysis of water hyacinth and HDPE

Co-pyrolysis of water hyacinth (Eichhornia crassipes) and HDPE is a commercially effective method to mitigate the pressure of environmental pollution and the problem of invasive alien species. In this paper, the thermal behavior and kinetic evaluation of the two raw materials with or without catalysts have been investigated. The mechanism of catalytic pyrolysis was also investigated by analyzing the free radicals generated during catalytic pyrolysis. The results showed that the heating rates had a negligible effect on the pyrolysis process. The addition of HDPE could reduce the formation of char and be conducive to the complete reaction with the higher value of the comprehensive pyrolysis index (CPI). The sample containing 75% water hyacinth exhibited the strongest synergistic effect in co-pyrolysis, with the lowest value of the difference between experimental and theoretical weight loss (ΔW) and activation energy among the co-pyrolysis samples. However, the sample with 25% water hyacinth showed an inhibiting effect due to the positive value of ΔW in the main decomposition temperature. Whereas the sample containing HZSM-5 had the lowest ΔW and activation energy compared to the blends without catalysts indicating that the presence of HZSM-5 was conducive to the synergistic effect in co-pyrolysis. The promotion was then demonstrated to be due to the abundance of free radicals generated in the catalytic process. This paper aims to provide some suggestions on the solution for the alleviation of environmental and energy crisis.

First-principles study of intrinsic point defects and Xe impurities in uranium monocarbide

Based on density functional theory (DFT) calculations, we perform an extensive investigation of intrinsic point defects and Xe impurities in uranium monocarbide (UC). The DFT calculations involve both the conventional generalized gradient approximation (GGA) and the GGA+U approach with the Hubbard parametric term (U), using up to 5×5×5 supercells. GGA calculations for the formation energy of intrinsic defects demonstrate the significant effect of using larger supercells than in previous studies. Results confirm that the ⟨111⟩ and ⟨100⟩ dumbbell interstitials are the most stable interstitial configurations for U and C, respectively. The interstitial mechanisms are favored for self-diffusion of both uranium and carbon and diffusion of Xe under equilibrium conditions. Calculations also reveal that the Xe substitutional defect at the C lattice site tends to adopt an off-site configuration, which can be interpreted as a Xe interstitial–C vacancy complex. We also utilize GGA+U to assess the impact of effective U parameter (Ueff) on the results. Moreover, we introduce a method to estimate the carbon chemical potential by fitting the phase diagram composition data and propose a selection Ueff=1.25 eV based on the experimental Xe diffusion activation energy. With this approach, GGA+U calculations reproduce the available experimental data for the formation energy of the carbon Frenkel pair and can explain the stepwise recovery of intrinsic properties and burst Xe release behavior in UC observed in annealing experiments.

Adsorption and Reaction of Trimethyl and Triethyl Phosphite on Fe3O4 by Density Functional Theory

First-principle density functional theory (DFT) calculations are used to calculate the most stable structures and heats of adsorption of trimethyl and triethyl phosphites on an Fe3O4 surface in order to understand their behavior as lubricant additives. Previous surface science studies of phosphate and phosphite esters on iron oxide surface have shown that they react by sequentially desorbing the corresponding alcohol and aldehyde in equimolar amounts. This implies that the reactions are limited by the rate of alkoxide removal, followed by a rapid reaction to form the alcohol and aldehyde. It is found that the trialkyl phosphites, dialkyl phosphite, and monoalkyl phosphite all adsorb with the phosphorus atoms bound to surface Fe3+ ions, with the alkoxy groups close to parallel to the surface. The heat of adsorption increases as the alkoxide groups are removed. The postulate that the rate-limiting step in the decomposition of the phosphate esters is the sequential removal of the alkoxide group is tested by plotting the experimental activation energy obtained from temperature-programed desorption experiments versus the corresponding heats of adsorption calculated by DFT. A linear dependence shows that the reaction obeys the so-called Evans–Polanyi relation, thereby confirming the above postulate. The slope of the plot is close to unity and thus implies that the structure of the transition state of the reaction resembles the product.

Mechanisms of Solid–Gas Reactions: Reduction of Air Pollutants on Carbons

Ozone is a strong oxidizer and sulfur dioxide is a precursor to acid rain, both are air pollutants that can damage the respiratory tissues of animals and plants making them hazardous to the environment. They are isoelectronic valence O = X = O (X = S, O) molecules and their mechanism of reduction on carbons is similar. The solid–gas kinetics were studied in a flow system with a tubular reactor under differential and steady state conditions. Initial rates and product distribution were used to determine the stoichiometry of the reaction. The reduction of XO2 on carbons proceeds through a common primary mechanism with oxidized and reduced intermediates. The reactivity of the intermediates that were inserted on carbons (graphite, activated carbon, graphene oxide) modified by SO2 is selective with respect to aminolysis and thiolysis. A theoretical study of the chemisorption of SO2 on dehydrogenated pyrene as graphite active site model showed that at 900 °C the chemisorption occurs mainly on the diradical zigzag edge. Computational quantum chemistry calculations were carried out for the reduction of SO2 on graphite to produce elemental sulfur and CO2 using tetradehydrogenated-benzo[α]anthracene (TBA) as model. The mechanisms of the decarboxylation and sulfur transport steps were postulated. Ozonation of graphite showed that the 1,2,3-trioxolane decomposes to an oxyrene, eliminating O2. Both reactions, the SO2 and O3 with graphite, have the same experimental free energy of activation for the decarboxylation reaction. The results show that for SO2 the desulfurization has a much lower energetic demand than the decarboxylation route raising the important possibility of using the reaction of reduction of SO2 on carbons to reduce the acid rain, producing elemental sulfur as the main product, without increasing the greenhouse effect due to the formation of CO2.

Oxidation of Al-Co Alloys at High Temperatures.

In this work, the high temperature oxidation behavior of Al71Co29 and Al76Co24 alloys (concentration in at.%) is presented. The alloys were prepared by controlled arc-melting of Co and Al granules in high purity argon. The as-solidified alloys were found to consist of several different phases, including structurally complex m-Al13Co4 and Z-Al3Co phases. The high temperature oxidation behavior of the alloys was studied by simultaneous thermal analysis in flowing synthetic air at 773–1173 K. A protective Al2O3 scale was formed on the sample surface. A parabolic rate law was observed. The rate constants of the alloys have been found between 1.63 × 10−14 and 8.83 × 10−12 g cm−4 s−1. The experimental activation energies of oxidation are 90 and 123 kJ mol−1 for the Al71Co29 and Al76Co24 alloys, respectively. The oxidation mechanism of the Al-Co alloys is discussed and implications towards practical applications of these alloys at high temperatures are provided.

Activation energy barriers for Na migration in Na12A zeolite: The main contribution to ionic current via doubly occupied NaII site?

Abstract The activation barriers for Na jumps between the cationic sites NaI, NaII, and NaIII are calculated using VASP in the presence of anions (Cl−, Br−) or H2O as well as for the empty Na12A zeolite model. The inter-cage cationic NaIII→NaII’→NaIII′ path, where NaIII′ is located in the neighbor α–cage, was modeled via an intermediate state (NaII′) corresponding to two NaII cations in one 8R window. This NaII′ point can also serve as an intermediate for the second type of intra-cage Na drift, i.e., to a NaIII site in the same cage. Reasonable agreement with experimental activation energy was achieved for NaII’→NaIII′, NaII→NaIII, and NaIII→NaIII jumps. The lower activation energy is obtained for the NaII’→NaIII' jump than for NaII→NaIII, which indicates the possible role of the doubly occupied 8R window as an intermediate state in the Na transfer with high frequency and lower activation energy related to ionic current (inter-cage transfer) in Na12A.

The study of decomposition of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide by using Termogravimetry: Dissecting vaporization and decomposition of ILs

The question of thermal stability of ionic liquids (ILs) is often raised in the scope of industrial application of ILs. The most widely used approach for this purpose is Thermo Gravimetric Analysis (TGA) with the determination of decomposition “Tonset” incorrectly called Decomposition Temperature. In the present study with the help of TGA study of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide we have shown the Tonset or specified mass-loss rate cannot be applied as the innitial point of decomposition. We have proposed a new approach allowing dissecting the mass loss caused by vaporization of the sample and its thermal decomposition. The further analysis of the decomposition mass loss by using “isoconversional kinetics” allows to determine kinetic parameters of decomposition such as pre-exponential factor A and activation energy EA. The experimental activation energy EA = (250 ± 15) kJ·mol−1 agreed with the gas phase B3LYP calculated value by Kroon EA = 250 kJ·mol−1.

From Carbodiimides to Carbon Dioxide: Quantification of the Electrophilic Reactivities of Heteroallenes.

Kinetics of the reactions of isocyanates, isothiocyanates, carbodiimides, carbon disulfide, and carbon dioxide with carbanions or enamines (reference nucleophiles) have been measured photometrically in acetonitrile or DMSO solution at 20 °C. The resulting second-order rate constants and the previously published reactivity parameters N and sN of the reference nucleophiles were substituted into the correlation logk2(20°C) = sN(N + E) to determine the electrophilicity parameters of the heteroallenes: TsNCO (E = -7.69) ≫ PhNCO (E = -15.38) > CS2 (E = -17.70) ≈ PhNCS (E = -18.15) > PhNCNPh (E = -20.14) ≫ CyNCNCy (E ≈ -30). An approximate value could be derived for CO2 (-16 < E < - 11). Quantum chemical calculations were performed at the IEFPCM(DMSO)/B3LYP-D3/6-311+G(d,p) level of theory and compared with experimental Gibbs activation energies. The distortion-interaction model was used to rationalize the different reactivities of O- and S-substituted heteroallenes. Eventually it is demonstrated that the electrophilicity parameters determined in this work can be used as ordering principle for literature-known reactions of heteroallenes.

Stress‐life prediction of 25°C polypropylene materials based on calibration of Zhurkov fatigue life model

AbstractIn this study, to evaluate the chemical and mechanical properties of polypropylene (PP), activation‐energy and tensile tests were performed at room temperature (25°C) on pure PP and PP reinforced with glass fibre (GF). To improve the prediction accuracy of the fatigue life, three models based on the calibration of the Zhurkov model were proposed: a regression model, modified strain‐rate model and lethargy coefficient‐based model. Based on the experimental data analysis and statistical assessment results, we proposed a modified strain‐rate model that satisfies the dependency of the physical parameters and is congruent with the predicted fatigue life data. The experimental data and modified strain‐rate model were compared with the direct cyclic analysis results. The tendency of the frequency factor as a correction parameter in the modified strain‐rate model corresponded to the experimental activation energy and the increasing GF content.

Infinite dilution in doped ceria and high activation energies

Oxygen ion diffusion determines the performance of materials in energy conversion, energy storage and catalysis. For nominally pure cerium oxide, experiments measure high activation enthalpies while calculations predict low activation enthalpies. Moreover, for doped oxides, e.g. doped ceria, experiments show a high activation enthalpy for both pure ceria and for high dopant fractions, leading to a minimum in activation enthalpy for small dopant fractions. While for high dopant fractions the increase in activation enthalpy is correlated with the association of oxygen vacancies and dopant ions, which are both created by doping, the minimum in activation enthalpy is assumed in the literature to be related to the maximum in ionic conductivity at similar dopant fractions. In this study, density functional theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations are combined in order to calculate the ionic conductivity and activation enthalpy in doped oxides. We show that the experimental ionic conductivity and activation energy in nominally pure cerium oxide is dominated by impurities. We resolve the discrepancy between activation enthalpies of nominally pure oxides in experiments as opposed to calculations. This will lead to a more comprehensive understanding of the oxygen ion conductivity and its underlying atomistic mechanisms. Moreover, such a focus will be of great benefit to the future development of sustainable and efficient materials.

Master-eğriler kinetik yönteminin izotermal olmayan selüloz pirolizine uygulanması ve piroliz işleminin termodinamik analizi

Kinetic modeling of thermochemical conversion methods such as pyrolysis is one of the most challenging issues for bio-refineries. It is known that cellulose together with hemicellulose and lignin mainly affect the characteristics of biomass pyrolysis. However, there is still limited knowledge about the thermal behaviors of biopolymers that go into complex multi-phase pyrolysis reactions in the literature. Therefore, cellulose pyrolysis kinetics and thermodynamics were investigated in this study. Kinetic parameters of the pyrolysis process were calculated by a combined method of master-plots and Friedman method. Active pyrolysis of cellulose is found to occur between 263 and 455 °C. Applied Friedman method was perfectly fitted with the experimental data and activation energy of the thermochemical conversion process was found between 150.8 and 190.2 kJ/mol while the mean activation energy was calculated as 164.3 kJ/mol. The comparison of kinetic models used of solid-state thermal decomposition processes indicated that the cellulose pyrolysis mechanism is a diffusion-controlled (D3) degradation process at lower conversions (0&amp;lt;α&amp;lt;0.5) and the process can be explained by reaction-based mechanisms at higher conversion degrees.

Experimental and theoretical study of the thermal decomposition of ethyl acetate during fast pyrolysis

The thermal decomposition of ethyl acetate was experimentally studied in a newly designed fast pyrolysis set-up. The results were compared to theoretical calculations and literature values in order to proof the experimental concept. The reaction was carried out in a free fall tubular reactor with a residence time of 0.15 s. The identification and quantification of products stream composition was performed online using a GC-TCD/FID. First, an overview of the reaction rate at feed volumes of 0.25, 0.50 and 0.75 mL was obtained at reaction temperature between 400 to 600 °C in intervals of 50 °C. As a result mass transfer limitation for feeds larger than 0.5 mL were identified. For the second approach, a constant feed volume of 0.25 mL and temperatures between 420 to 550 °C were investigated. Using the experimental results, a global kinetic model is proposed for the thermal decomposition of ethyl acetate into ethylene and acetic acid through a first order unimolecular reaction. Also, theoretical calculations were performed at ωB97XD/6-311++G(d,p) level. A concerted mechanism through a six-membered transition state was identified in the reaction path. The theoretical and experimental activation energy values lie within the literature values between 193 and 213 kJ/mol.

Polymeric Electrolyte Comprising a NAFION Membrane Plasticized by Dimethyl Sulfoxide and the Transport Specifics of Alkali Metal Ions in It: Quantum-Chemical Simulation

The first theoretical study of alkali-metal ion transport in a polymeric inorganic electrolyte based on a dimethyl sulfoxide-plasticized Nafion membrane is reported. The structure and intermolecular interactions in XNafion · nDMSO (X = Li, Na, K, Rb, and Cs; n = 8 and 12) ionomers are simulated by the DFT method with hybrid density functionals B3LYP, wB97XD, and PBE taking into account periodic boundary conditions and the projector augmented wave (PAW) method in the VASP and GAUSSIAN program packages. According to the calculations, the barriers rise from 0.2 to 0.4 eV in the series Li–K but lower to 0.3–0.2 eV as the radius increases further in the series Rb, Cs. These results are quantitatively consistent with experimental conductivity activation energy data: 0.26 (Li+), 0.37–0.38 (Na+, K+), 0.27 (Rb+), and 0.20 (Cs+) eV. The conclusion is drawn about the structure and conductivity of the electrolyte depending on the nature of the cations.

Selective hydrogen production at Pt(111) investigated by Quantum Monte Carlo methods for metal catalysis

AbstractThis rapid communication gives the salient points and results of the theoretical investigation of a chemical reaction for efficient selective hydrogen production. The clean fuel produced is a sustainable energy source. Accurate methods based on quantum theory are used because the changing electronic structure is a probe that monitors reactions. The reaction between water and carbon monoxide is used industrially with metal catalysts, usually platinum. There is a considerable economic and environmental challenge underpinning this fundamental investigation where bond dissociation plays an essential role. A bond dissociation process is often the limiting step of reaction rates for industrial catalysis. Most mainstream quantum approaches fail to a greater or lesser degree in the description of this process. The present work advocates a promising alternative: the initial analysis of statistical data generated by the Quantum Monte Carlo (QMC) method demonstrated very stringent statistical accuracy for essential information on hydrogen production via the water‐gas shift reaction with platinum catalyst. The transition state structure is obtained from QMC force constants and illustrated here. It corresponds to water OH‐stretch concerted with Pt‐H bond formation, whilst the OH oxygen atom begins to interact with the CO carbon. The present QMC evaluation of the corresponding activation barrier is low: 17.0 ± 0.2 kcal/mol. It is close to the experimental apparent activation energy of 17.05 kcal/mol. This method is applicable to a wide range of similar systems.