Kinetics and Catalysis最新文献

The rate constant of the reaction of 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3H)-one (1) with peroxyl radicals was measured in a model system of radical chain oxidation of 1,4-dioxane. Oxidation of 1,4-dioxane with atmospheric oxygen was carried out at a temperature of 333 K. Oxygen uptake was monitored using a universal differential manometric setup. It was found that 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3H)-one slowed down the rate of 1,4-dioxane oxidation. The effective rate constant of the interaction of the peroxyl radical of 1,4-dioxane with 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3H)-one was found: fk₇ = (6.2 ± 0.1) × 10⁵ L mol^–1 s^–1. The stoichiometric inhibition coefficient f = 1.3 ± 0.1 was calculated from the slope of the dependence of the induction period length on the concentration of 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3H)-one. The experimental data indicated that 6-amino-5-hydroxy-2,3-dimethylpyrimidin-4(3H)-one has antioxidant properties. It was assumed that the hydroxyl group –OH located in the 5-position of the ring was the site attacked by the peroxyl radical.

Propylene polymerization titanium–magnesium catalysts in the presence of ethers are synthesized. It is shown that aliphatic ethers and 1.1-diethers are participated in the formation of magnesium chloride although they are not contained in catalysts. Aromatic ethers (except methyl phenyl ether) are ineffective in the MgCl₂ formation. Subsequent adding of the phthalate (DBP/Mg = 0.05 mol) lead to the synthesis of the phtalate titanium–magnesium catalyst which in the case of aliphatic ethers and 1.1-diethers have catalytic properties higher than the titanium–magnesium catalyst for comparison (DBP/Mg = 0.05 mol) and close ones (for some ethers) to the standard titanium–magnesium catalysts (DBP/Mg = 0.2 mol). Comparative analysis of the composition of phthalate titanium–magnesium catalysts prepared using ethers or ketones and X-ray data indicate a close size of MgCl₂ crystallites in both cases. The composition of titanium–magnesium catalysts synthesized in the presence of ethers is consistent with the distribution of dimeric TiCl₄ complexes alternating with chlorine vacancies (…□TiTi□TiTi…) on the lateral cuts 104.

Platinum catalysts with the additives of vanadium and tin were synthesized and studied in the reaction of methylcyclohexane dehydrogenation to toluene. Experimental data were obtained for the process of methylcyclohexane dehydrogenation, which are consistent with the synergistic effect of vanadium addition to a platinum–alumina catalyst in hydrogenation and dehydrogenation reactions, as predicted by quantum-chemical calculations.

The X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR) spectroscopy were used to study the features of anchoring of the [Ir₂(COD)₂Cl₂] complex on the surface of modified silica gel L–SiO₂ (where L is NC₅H₅–CH₂–CH₂–, N(CH₃)₂–CH₂–CH₂–CH₂–, NH₂–C₃H₆–) depending on the nature of the linker and the conditions of preparation of the systems. The catalytic activity was tested in reactions of gas-phase selective hydrogenation of propylene with parahydrogen (p-H₂). According to the XPS data, a single-site iridium catalyst is prepared in all cases. Analysis of the XPS spectra indicates the possibility of anchoring the complex through one of the iridium atoms while preserving the dimer. It was shown that at different durations of interaction of the iridium complex solution with modified NH₂–C₃H₆–silica gel approximately the same amount of the complex is anchored, but the nature of the complex coordination changes. For the sample obtained by long-term interaction of the complex solution with the modified support (24 h), a high increase in the NMR signal was observed at 60°C, while in the case of the sample prepared by short-term interaction (1 h), the signal increased with a rise in temperature to 80°C.

A nickel-based catalyst supported on alumina–zirconia–ceria oxides was investigated to evaluate its performance in the dry reforming of glycerol. The reaction was carried out at 700°C and atmospheric pressure, with a glycerol/CO₂ molar ratio of 1. The catalyst demonstrated stable performance for 7 hours onstream, achieving glycerol and CO₂ conversions of 60% and 47%, respectively, with corresponding H₂ and CO yields of 48% and 58%. Thermogravimetric analysis (TGA) confirmed carbon deposition; however, this did not lead to significant catalyst deactivation. These results highlight the potential of the synthesized catalyst for glycerol conversion for the production of syngas and hydrogen from renewable feedstock.

The catalytic peroxidase-like properties of silver nanoparticles (Ag NPs) immobilized into a polymethacrylate matrix (PMM) were studied. The Ag NPs were prepared by thermal reduction of silver cations preimmobilized in a PMM. The morphology of the nanocomposite was studied using scanning electron microscopy (SEM), and the average size of the synthesized individual spherical nanoparticles was 18 ± 5 nm. It was demonstrated that silver nanoparticles immobilized in a polymethacrylate matrix (PMM–Ag⁰) exhibited pronounced peroxidase-like activity in the oxidation reaction of a chromogenic substrate, indigo carmine, under the action of H₂O₂. The Michaelis–Menten model was used to estimate the kinetic parameters of the reaction. The Michaelis constants (K_m) of 0.1 and 1.0 mM for indigo carmine and H₂O₂, respectively, indicated a strong affinity of the substrates to silver nanoparticles in PMM.

The article is devoted to the experimental and theoretical study of regular and complex oscillations during ethylene oxidation on the nickel foil. The simplest mathematical model was based on the 14-stage mechanism of reaction including the stages of oxidation and reduction of the Ni catalyst. A precursor-mediated adsorption of CO and C₂H₄ was shown to be the crucial condition for the origin of the oscillatory behavior under reducing conditions. It was demonstrated that for real values of the parameters, the mathematical model can simulate both regular and irregular oscillations, as well as the “mixed-mode” oscillations observed in the experiment. For the first time oscillations with different properties and distinct mechanisms of their occurrence were detected in the same model. It was demonstrated that oscillations occurred as a result of a strong dependence of the reaction rate on the concentration of active sites due to a variation in the concentration of the surface oxide or the surface carbon.

Electrodes based on TiO₂ nanotube arrays for the photoelectrochemical process of water splitting were modified with Cu₂O, a p-type semiconductor (p-Cu₂O). Cyclic voltammetry (CV) was used for the deposition of p-Cu₂O nanoparticles to achieve a more uniform distribution of the particles over the inner and outer surfaces of TiO₂ nanotubes. The measurements of incident photon-to-current conversion efficiency (IPCE) in the range of 365–660 nm demonstrated that the proposed method significantly enhanced photoactivity in the visible light region compared to the potentiostatic deposition method. The IPCE value was 0.18% at a wavelength of 523 nm, which was 7 and 45 times higher than those for the potentiostatically modified and pristine samples, respectively. Under continuous illumination with visible light at a wavelength of 523 nm and a potential of 0.2 V (Ag/AgCl_(sat.)), a transition from Cu₂O to CuO was observed for 5 h, which was accompanied by a decrease in the photocurrent density.

The utilization of lignocellulosic biomass for the production of energy, biofuels, and chemical products has become increasingly viable from technical, economic, and environmental perspectives, particularly when the biomass is sourced from agricultural waste. The conversion of biomass-derived levulinic acid (LA) into biofuel alkyl levulinate (AL) using alcohols as the alkoxy group (R–O) donor and reaction medium represents a promising synthesis route for AL, which is regarded as an environmentally friendly chemical product. In this study, we investigated the synthesis of alkyl levulinate from linear-chain alcohols (R–OH: 1-butanol, 1-octanol, 1-decanol, 1-dodecanol, and 1-tetradecanol) catalyzed by zinc oxide (ZnO) under the following conditions: LA : ROH molar ratio of 1 : 1, 1 wt % ZnO, temperature of 125°C, and reaction duration of 3 h. Our findings indicate that under the studied reaction conditions, the conversion of LA gradually decreases as the alcohol chain lengthens. The ester yields (%) were as follows: butyl levulinate—100, octyl levulinate—80, decyl levulinate—72, dodecyl levulinate—69 and tetradecyl levulinate—64. This trend is directly related to the physicochemical properties of the synthesized levulinates, including boiling point, polarity, and miscibility.

This investigation examined the combustion characteristics of ammonia/methanol blends under varying conditions of hydrogen enrichment, comparing direct hydrogen additon against ammonia cracking. The analysis encompassed key parameters including laminar burning velocity, adiabatic temperature, pool radical (H/O/OH/HO₂) concentrations, ammonia and methanol reaction pathways, and NO_x emissions. The research was conducted under premixed combustion conditions with air as the oxidizer, across a wide range of equivalence ratios (0.6 to 1.2 with a step of 0.1) and hydrogen fractions (from 0 to 60%). A modified one-dimensional model (Premix) integrated with Chemkin II and a detailed kinetic mechanism combining the chemistries of hydrogen, ammonia, methanol, syngas, and methane was employed. The neat laminar premixed flame consisted of 60% ammonia and 40% methanol at 1 atm pressure. Hydrogen was incrementally incorporated to this mixture, either through direct addition or via ammonia cracking, in 10 wt % steps, while maintaining constant equivalence ratios. Particular focus was given to the concentration-dependent effects of these blends on the formation of NO, NO₂, and N₂O. The modified fictitious diluent gas method was utilized to isolate thermal contributions from other effects in enhancement of laminar burning velocities of NH₃/CH₃OH mixtures. The findings revealed that both hydrogen incorporation methods substantially enhanced the combustion intensity of NH₃/CH₃OH mixtures, with direct addition showing superior performance. In the case of ammonia cracking, the effects of H₂ and N₂ on laminar burning velocity became more pronounced with increasing NH₃ cracking. Notably, the H₂-promoting effect consistently outweighed the N₂-inhibiting effect. For a fixed hydrogen percentage (whether from direct addition or ammonia cracking), NO emissions peaked at an equivalence ratio of 0.9 before declining. Furthermore, the relationship between hydrogen content and NO formation exhibited two distinct zones: 0–40 and 40–60% hydrogen. These findings were explained through a comprehensive analysis of radical species dynamics and reaction pathways.