International Journal of Chemical Kinetics最新文献

This study investigates the thermal degradation kinetics of mesoporous triazine-based polymers, namely triazine-amine and triazine-ether polymers. The synthesis, physicochemical characterization, and catalytic applications of these polymers were discussed in our previous report. Herein, the thermal stability parameters, including kinetic triplets and thermodynamic parameters, were determined using thermogravimetric analysis (TGA) and non-isothermal mathematical approximations such as Coats-Redfern, Broido, and Horowitz–Metzger methods. Triazine-ether polymers exhibit thermal stability within the range of 200°C–300°C, while triazine-amine polymer demonstrates superior thermal stability, reaching up to 450°C. According to the Coats-Redfern method, the degradation follows reaction orders of 0.5 ≤ n ≤ 1. The activation energy of triazine-amine polymer is notably high, particularly at the third degradation stage (e.g., 89.0 kJ/mol by the Broido method), attributed to its high nitrogen content. Conversely, the higher carbon content of triazine-ether polymers reduces their activation energy to approximately 30 kJ/mol at all stages and thus, facilitates the degradation process. Thermodynamically, the degradation process is favorable yet non-spontaneous, with intermediate states of the polymers exhibiting higher entropy, indicative of their enhanced degradation capability.

The current research highlighted the usage of serpentine clay to remove Nile red dye from an aqueous solution. At first serpentine clay minerals were analyzed by various analytical techniques like Fourier transform infrared spectroscopy (FTIR), X ray diffraction (XRD), and thermal gravimetric analysis (TGA) analysis. From the characterization results it was found that the clay was determined to be a separate group. Sorption studies investigated the impacts of adsorbent dosage, initial pH, initial dye concentration, and temperature on Nile red color elimination. From the test results it was found that the capacity of adsorption was seen to increase from 32.4 mg/g to a high value of 43.8 mg/g by raising the pH value from 2 to 6. Adsorption on serpentine clay decreased from 234.7 to 33.2 mg/g due to an increase in the adsorbent dosage. The removal capacity of Nile red dye increased from 12.2% to 88.5% with the rise in the adsorbent dosage. This rise in the Nile red dye removal may be observed due to the increase in the area as well as the pore volume of the surface. Experimental study was carried out to study the effect of initial concentration of adsorbate on adsorption at a pH of 6, adsorbent dosage of 3 g/L, and at a temperature of 28°C. The removal efficiency of the Nile red dye was reduced from 96.7% to 42.6%. To determine the temperature effect on the removal of Nile red dye by the clay, the initial pH value was set to 6, and the temperature was set at 28, 38, 48, and 58°C. Without reaching the equilibrium conditions, at a time of 30 min, the removal efficiency of dye rises from 60% to 81% due to the temperature rise. The experimental findings indicated that the adsorption of the dye on the clay followed the “Langmuir adsorption” isotherm rather than the Freundlich adsorption isotherm. Adsorption on clay minerals follows the pseudo-second-order adsorption kinetics compared to pseudo-first-order adsorption kinetics.

As part of a series of studies of hydrogen-atom transfer or tautomerization reactions of imidic acid-amide species, $�H$─$�O$─$�C$═$�N$─ $��\rightleftharpoons �O�$═$�C$─$�\text{NH}$─, we report the rate constants for a set of 16 four-membered cyclic compounds at low, 50–300 K, and high, 500–1500 K, temperatures. The compounds are labeled according to the two ring groups X and Y, which can be $�{\text{CH}}_2$, NH, CH, N, O, or C(O) and which are at some remove from the reactive site. These rate constants are for both the direct reaction and for that mediated by an additional water molecule, which facilitates the hydrogen transfer reaction. In the latter case, we show that the rate of reaction from a pre-reaction complex is rapid at temperatures down to 50 K and dominated by quantum mechanical effects as evaluated by small-curvature and quantized-reaction-states tunneling. In addition, we present thermochemical data such as enthalpies of formation, entropies, isobaric heat capacities, and enthalpy functions for these largely unknown species, which span a range of compounds from $�\beta$-propiolactone to 1,3-diazetidine-2,4-dione.

A longstanding project of the chemical kinetics community is to predict reaction rates and the behavior of reacting systems, even for systems where there are no experimental data. Many important reacting systems (atmosphere, combustion, pyrolysis, partial oxidations) involve a large number of reactions occurring simultaneously, and reaction intermediates that have never been observed, making this goal even more challenging. Improvements in our ability to compute rate coefficients and other important parameters accurately from first principles, and improvements in automated kinetic modeling software, have partially overcome many challenges. Indeed, in some cases quite complicated kinetic models have been constructed which accurately predicted the results of independent experiments. However, the process of constructing the models, and deciding which reactions to measure or compute ab initio, relies on accurate estimates (and indeed most of the numerical rate parameters in most large kinetic models are estimates.) Machine-learned models trained on large datasets can improve the accuracy of these estimates, and allow a better integration of quantum chemistry and experimental data. The need for continued development of shared (perhaps open-source) software and databases, and some directions for improvement, are highlighted. As we model more complicated systems, many of the weaknesses of the traditional ways of doing chemical kinetic modeling, and of testing kinetic models, have been exposed, identifying several challenges for future research by the community.

In the realm of combustion kinetic modeling, the norm involves employing thousands of reactions to delineate the chemical conversion of hundreds of species. Notably, theoretically predicted rate coefficients and branching ratios, derived through the RRKM/master equation (ME) model, play an increasing role in kinetic modeling. Thus minimizing the uncertainty of theoretical prediction across wide working conditions is crucial to refine a kinetic model. The present study takes ethyl (C₂H₅) + oxygen (O₂) reaction system to show that combined forward and reverse uncertainty analysis can be used to further constrain calculated rate coefficients and branching ratios, which were already calculated by high-level quantum chemistry methods. Forward global uncertainty analysis with the artificial neural network-high dimensional model representation (ANN-HDMR) method is employed to select key parameters affecting total rate coefficients of C₂H₅ + O₂ and branching ratios of C₂H₅ + O₂ = C₂H₄ + HO₂ (C1). Reverse uncertainty analysis with Bayesian method was then applied to refine the key input parameters based on experimental data at working conditions selected by sensitivity entropy. Although the target RRKM/ME model system was built on high level theoretical calculations, the combined forward and reverse uncertainty analyses are still able to reduce uncertainties of predicted total rate coefficients of C₂H₅ + O₂ and branching ratios for C1 across a wide range of working conditions. Specifically, the uncertainties of total rate coefficient and C1 branching ratio have been reduced from 1.46 and 1.52 to 1.30 and 1.36 at 298 K and 1 Torr. The analysis process proposed in the present work effectively extrapolates the constraint ability of accurate measured data at one condition to wide working conditions based on the RRKM/ME model.

For net-zero carbon emissions, hydrogen/ammonia blends have drawn considerable attention for the application in industrial combustion devices. Various chemical mechanisms have been developed to describe the oxidation and combustion of hydrogen/ammonia mixtures at certain conditions. A comprehensive evaluation and comparison of the performance of these mechanisms is thus of high interest, especially in terms of their application for particular computational studies. Thus, this work aims to compare the existing chemical mechanisms in terms of their performance for the combustion of hydrogen/ammonia mixtures over a wide range of experimental conditions. In addition to previous literature studies, the model performance is evaluated by using two different methods for the assessment of prediction accuracy. Besides the conventional measure of point-wise differences between model and data, the curve-matching method is also applied, which quantifies the dependence of model response on physical conditions additionally, by comparing the similarity between the curve shapes of the predicted and measured results. Extensive experimental data are taken into account in the model evaluation, including 136 datasets obtained from various facilities in the past 10 years. Nineteen mechanisms are compared, which were published in recent five years. It is revealed that these models give strongly different numerical results for combustion targets, such as laminar burning velocities, ignition delay times, and species concentrations. The chemical mechanisms of Zhang et al. (2021), Han et al. (2023), Mei et al. (2019), Li et al. (2019), and Stagni et al. (2020) show relatively satisfactory performance over the entire investigated domain. Moreover, it is found that the estimated prediction accuracy of chemical mechanisms is highly sensitive to model evaluation methods.

Corrosion protection of steel bars in alkaline concrete environments poses a common challenge in marine engineering. One approach to mitigate steel bar corrosion is the addition of corrosion inhibitors to the concrete. In alkaline environments, the passivation of rebars occurs through anodic passivation coupled with the cathodic oxygen reduction reaction (ORR). The catalysis of ORR can expedite anode passivation. To investigate the corrosion inhibition of steel bars in alkaline environments, meso-tetra(4-carboxyphenyl)porphine (TCPP), known for its ORR catalytic properties, is selected. TCPP forms adsorption films on the surface of steel bars, facilitating the formation of passivation films. TCPP primarily adsorbs onto active sites on the surface of the passivation film, where lattice iron ions have leached. The adsorbed TCPP accelerates the formation of the passivation film through ORR catalysis, inhibiting the development of passivation film defects and enhancing the integrity and protection of the passivation film. The most significant effect is observed when the concentration of TCPP is 0.5 mmol/L. The physical adsorption of TCPP is primarily determined by the negative charge centers, namely the carboxyl group O and the pyrrole N. However, due to steric hindrance caused by the unrestricted rotation of the carboxyl benzene, the pyrrole N does not play a dominant role in chemical adsorption. Instead, the active site for chemical adsorption is the carboxyl group O. The adsorption process significantly reduces the diffusion coefficient of TCPP molecules, providing a robust and stable adsorption binding. Phthalocyanine molecules without carboxyl benzene groups adopt a planar structure, allowing them to form stable adsorption configurations on the iron surface through flat adsorption. This observation provides guidance for the design of novel metal phthalocyanine molecules. Specifically, the development of metal phthalocyanine molecules with modifying groups that are coplanar with the phthalocyanine ring and possess restricted rotation can achieve flat adsorption, improve coverage rate, and enhance adsorption configuration stability.

The permanganate–H₂SO₄ redox reaction, useful in oxidative treatments under aqueous conditions, was studied spectrophotometrically in the absence and presence of cetyltrimethylammonium bromide (CTAB). The decolorization reactions were influenced by the [MnO₄⁻], [H₂SO₄], and temperature. Permanganate reduction follows first-, and complex–order kinetics with permanganate, and H₂SO₄ concentrations, respectively. The reduction of permanganate (Mn(VII)) proceeds through a complex formation between MnO₄⁻ and H₂SO₄. The characteristic absorption peaks for MnO₄²⁻ (λ_max = 439 and 606 nm), MnO₄³⁻ (λ_max = 667 nm), and MnO₂ (λ_max = 400–418 nm) were not appeared during the redox reaction. The KMnO₄ degradation efficiency remains unaffected with sodium pyrophosphate and sodium fluoride. The results of this study demonstrated the formation of Mn(II) as the stable product in acidic reaction media. The degradation efficiency increases drastically from 15 to 100% with 2.0 × 10⁻⁴ to 16.0 × 10⁻⁴ mol/L CTAB concentration under sub-, and post-micellar reaction conditions, respectively. The thermodynamic parameters (activation energy = 98.8 and 43.2 kJ/mol), activation of enthalpy (96.3, and 39.0 kJ/mol), activation of entropy (16.2 and −149.5 J/K/mol), free energy of activation (93.1 and 83.5 kJ/mol) were calculated without and with CTAB, respectively. Hence, CTAB can be exploited for its multifunctional applications, and specifically for the catalytic role in the permanganate-assisted redox reactions in future.

Benzyl radical (C₇H₇), one of the resonantly stabilized hydrocarbon radicals, is one of the significant precursors of polycyclic aromatic hydrocarbons in interstellar media and combustion engines. The unimolecular decomposition of benzyl radical is still incompletely understood despite of its importance and relatively small molecular size. The decomposition reactions of benzyl radical were investigated in the present study by using the ab initio transition state theory (TST) and the multi-well master equation theory. Specifically, all reaction pathways on the potential energy surface of C₇H₇ was calculated at the level of QCISD(T)/CBS. For the reactions with multireference characters, the CASPT2(9e,7o)/aug-cc-pVTZ method was used to calculate the vibrational frequencies and energies of structures along the one-dimensional reaction coordinate of the breaking bond. The high-pressure limits of rate constants for all the reactions were obtained by using the TST except those for C₇H₆ + H and C₆H₄ + CH₃ by the variational TST. The pressure-dependent rate constants were obtained by using the multi-well master equation simulations. The calculated rate constants agree well with available experimental and theoretical data in the literature. Moreover, the present results identify the composition of the non-hydrogen-atom production observed in previous experiments, which provide new insights into the reactions of aromatic compounds.

Removing polychlorinated biphenyls (PCBs) from subsurface water, soils, and transformer oil is crucial to save the environment from these pollutant materials. Hydrodechlorination (HDC) of PCBs consists of numerous chemical reactions and the simple kinetic models may not provide details for the process. To gain more awareness of the reaction mechanism, in the proposed approach, the isoconversional methods of the Friedman were investigated paralleling other kinetic models of Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R), pseudo-first-order, and pseudo-second-order methods. The analysis was validated by laboratory results of HDC of contaminated transformer oil in front of Pd/MWCNTs. The most reactivity was observed for biphenyls with a higher number of chlorines. Finding a suitable model, Akaike Information Criteria were applied. It was attained that Friedman model was the most suitable for monitoring of HDC of PCBs in front of catalyst. Besides, E-R reaction was appropriate to elucidate the theoretical interpretations of the adsorption and desorption of reactants and chlorinated benzene.