Kinetics and Catalysis最新文献

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) at microscope probe electron energies (E₀) of 5 and 20 keV in a range of magnification factors from ×50 to ×100 k were used to characterize the morphology, microstructure and chemical compositions of surface and subsurface regions of etching structures in the industrial Pt−Pd−Rh−Ru gauze with concentrations of 81, 15, 3.5, and 0.5 wt %, respectively, which was used in the oxidation of NH₃ with air at a pressure of 3.6 bar and a temperature of 1133 K. After NH₃ oxidation on the Pt−Pd−Rh−Ru gauze, we observed the microgranular etching structure with grains about 10 µm in size, oxide particles with a diameter of ~80 nm on grain boundaries, and crystal facets 25−50 nm in height on the grain surfaces in the region of weak etching. The oxide particles consisted of 25−50 nm cores with the MgRh₂O₄ spinel composition and 25−50 nm thick SiO₂ shells. These particles were formed during the diffusion of Rh atoms along grain boundaries from the alloy bulk to the surface, the formation of Rh₂O₃ particles, and their reactions with MgO_х and SiO_х impurities from the gas flow of reagents. These processes led to the formation of etch pits, increased the intensity of NH₃ oxidation and temperature, and accelerated the evaporation of volatile oxides PtO₂(gas), RhO₂(gas), etc., into the gas phase, which resulted in the catalyst etching. In the region of strong etching, the etching layer of porous crystal cauliflower agglomerates with different shapes and a size of ~10 µm was observed. The agglomerates had a close phase composition and a low concentration of defects up to a depth of ~500 nm. During the NH₃ oxidation on Pt–Pd–Rh–Ru gauzes, etching structures were formed upon evaporation of volatile oxides PtO₂(gas), RhO₂(gas), etc., on hot regions of the surface and their subsequent condensation on cold fragments of the catalyst.

The kinetics of tetrahydrofuran oxidation in aqueous solution initiated by 2,2'-azobis(2-amidinopropane) dihydrochloride were studied. The reaction rate was monitored by oxygen uptake using the manometric method. The initiation rate constant of water-soluble 2,2'-azobis(2-amidinopropane) dihydrochloride was determined within the range 303–323 K: log k_i = (11.9 ± 0.6) – (104.8 ± 4.3)/θ [s^–1], θ = 2.303RT kJ/mol. The rate of tetrahydrofuran oxidation (w) was found to depend linearly on its concentration. A quadratic dependence of w on the initiator concentration was observed. The values of k₂(2k₆)^–0.5 (oxidation parameter) for tetrahydrofuran oxidation were obtained. In Arrhenius coordinates, this parameter is expressed as log (k₂(2k₆)^–0.5) = (2.8 ± 0.1) – (36.1 ± 0.7)/θ [L^0.5 mol^–0.5 s^–0.5], θ = 2.303RT kJ/mol. The rate constants for the reactions of tetrahydrofuran peroxyl radicals with α-tocopherol and with 2,2,5,7,8-pentamethyl-6-chromanol in aqueous medium were determined as (2.0 ± 0.3) × 10⁵ L mol^–1 s^–1 (303–318 K) and (2.8 ± 0.5) × 10⁵ L mol^–1 s^–1 (309 K), respectively.

A novel catalytic system based on 2,2'-bipyridine binuclear cobalt(II) complexes containing bridging phosphinic acid ligands (Ph₂P(O)OH and MesP(H)(O)OH) is presented for tandem oligomerization of ethylene and subsequent Friedel–Crafts alkylation of toluene. It was found that incorporation of sterically less hindered {μ-O₂P(H)Mes}^– ligand significantly enhances the catalytic activity up to 1007 kg ({text{mol}}_{{{text{Co}}}}^{{ - 1}},,{{{text{h}}}^{{ - 1}}}), doubling the value compared to the analogous complex with {μ-O₂PPh₂}^– ligand. The reaction products were characterized by GC–MS analysis and include butenes, mono- and dibutyltoluenes along with minor amounts of ethyltoluenes, consistent with the mechanism involving ethylene oligomerization followed by toluene alkylation.

Ammonia is a promising fuel, the implementation of which could reduce carbon emissions into the atmosphere. However, to ensure the appropriate combustion characteristics of ammonia, additional technologies are required, since pure ammonia has poor combustion properties. One of them is to use a pilot fuel with high reactivity, for example, acetylene. Successful implementation of NH₃/C₂H₂ mixtures requires a detailed understanding of the chemical processes taking place during their oxidation and combustion. At present, there are very few fundamental works focused on such systems. In this work, the experimental data on the oxidation of a stoichiometric NH₃/C₂H₂/O₂/Ar mixture in a jet-stirred reactor at atmospheric pressure, as well as on the chemical structure of stoichiometric NH₃/C₂H₂/O₂/Ar flames at 1–5 atm, are presented. The work also includes numerical modeling and analysis of NH₃ and C₂H₂ consumption pathways at high and low temperatures, as well as at atmospheric and high pressure. Analysis of the consumption pathways shows that the first steps of oxidation of the fuel mixture components include reactions with the main radicals (H, O, and OH), while the contribution of cross reactions becomes noticeable only at later stages of ammonia and acetylene conversion. The main mechanism of ammonia–acetylene interaction during their oxidation and combustion is the coupling of their individual conversion processes, which involve reactions with H, O, and OH radicals. These radicals are common for both ammonia and acetylene oxidation.

This paper presents the results of the development and interpretation of machine learning models using big experimental datasets where different ways are implemented to take into account information on the kinetics of reaction using the Suzuki–Miyaura reaction as an example. An original method for interpreting the resulting models was proposed, which consists in the multistep ablation of predictors used in the models, allowing for the identification of the most important reaction parameters required for the successful operation of catalytic systems. The obtained sets of the parameters determining the success of the reaction (rate, selectivity, and yield of the target product; E-factor and other environmental metrics of the reaction, energy efficiency, cost, etc.) as the minimal sets of the predictors of ML models that describe satisfactorily the differential and/or integral quantitative reaction parameters allowed us for the first time to identify the patterns of the functioning mechanism of catalytic systems in the Suzuki–Miyaura reaction, consistent with big experimental kinetic data obtained in a wide range of reaction conditions (nature and amounts of substrates, reagents, catalyst precursors, bases, additives, and solvents; temperature, stirring speed, and type of an atmosphere in the reactor).

The kinetics of the Mizoroki–Heck reaction was studied using a green catalyst consisting of Pd nanoparticles (Pd NPs) deposited on the bacterial cells of Paracoccus yeei (P. yeei) VKM B-3302. The study showed that the kinetics of the Mizoroki–Heck reaction catalyzed by Pd/P. yeei and the commercially available Pd/C, at first glance, did not differ significantly. However, more noticeable changes were found in the selectivity and specific productivity of the catalysts. Thus, the bacterial catalyst demonstrated an increase in selectivity for the target product with increasing process temperature. A probable reason for the concomitant increase in selectivity with temperature is the effect of Pd “release” from inside the bacterial cells as a result of their destruction under the action of the solvent and high temperature.

The interaction of palladium deposited by vacuum evaporation onto the surface of highly oriented pyrolytic graphite (HOPG) with hydrogen was studied using X-ray photoelectron spectroscopy (XPS). Graphite was preliminarily subjected to oxidative treatment including the stages of etching by Ar⁺ ions and exposure to air at room temperature. Before contact with hydrogen, the spectrum of the Pd/O-HOPG sample contained two O 1s lines with binding energies E_b ∼ 530.5 and ∼532.5 eV belonging to two types of oxygen atoms in the functional groups on the surface of the carbon support. After interaction with H₂ at a temperature of 150°C and a pressure of 20 mbar, the line with E_b ≈ 530.5 eV disappeared, probably, due to the reduction of the corresponding functional groups. Under these conditions, the surface oxygen was retained in the oxidized graphite sample without deposited palladium (O-HOPG), which indicated the participation of palladium particles in the reduction of oxygen-containing groups. Apparently, dissociation of hydrogen molecules occurred on the palladium surface, followed by spillover of the resulting H atoms onto the carbon support.

The kinetics of photocatalytic decomposition of rhodamine B dye in aqueous solutions with initial concentrations of 20, 25, 30, 35, and 40 mg/L was studied using a TiO₂–montmorillonite composite photocatalyst with high adsorption capacity and photoactivity. The experimental data were analyzed based on the pseudo-second-order and Langmuir–Hinshelwood (L–H) models. In all cases, the apparent reaction order was less than unity, and the half-life of the dye molecules increased monotonically with increasing dye concentration in the solution. Based on the Langmuir adsorption isotherm obtained for the “dark” period, the equilibrium adsorption constant K_L was determined, which is in good agreement with the adsorption parameter K_a of the L–H model, indicating the adequacy of this model for photocatalysts with high adsorption capacity. The use of equations representing the analytical solution of the L–H equation based on perturbation iteration method algorithms makes it possible to solve the direct kinetic problem for a photocatalytic experiment with high accuracy.

The process of alkaline hydrolysis of the organophosphorus ecotoxicant paraoxon and carboxylic acid esters in micellar solutions of piperidinium carbamate-containing surfactants was studied using spectrophotometry. The influence of concentration and structural factors, as well as the holding time of surfactant solutions, on the catalytic effect of the systems was analyzed. During the hydrolytic decomposition of paraoxon, the highest catalytic activity was exhibited by the surfactant with a carbamate fragment at the quaternary nitrogen atom in the cycle (1-CB(Et)-P-16). The increase in the catalytic response for aged solutions of this surfactant was explained by its partial hydrolysis in a highly alkaline medium and the formation of a mixed system of 1-CB(Et)-P-16 with the product of its decomposition, 1-hexadecyl-1-(2-hydroxyethyl)piperidinium bromide. The formation of the latter was confirmed by ¹H NMR spectroscopy. In neutral solutions, the decomposition of piperidinium carbamate-containing surfactants did not occur for more than three to four months, indicating their hydrolytic stability under these conditions. In micellar catalysis of alkaline hydrolysis of carboxylic acid esters, a favorable effect of an increase in the hydrophobicity of the substrate (from p-nitrophenyl acetate to p-nitrophenyl caprate) and an alkyl substituent in the carbamate fragment of the amphiphile was established.

The process of formation of palladium hydroxo complexes during hydrolysis of palladium chloride in an alkaline medium was investigated by the in situ small-angle X-ray scattering (SAXS) technique. It was shown that the aging process of palladium hydroxo complexes corresponded to the Ostwald ripening mechanism. The time dependent particle shapes and particle size distributions were determined for the palladium hydroxo complexes in solution. It was found that loose particles described by the form factor of Gaussian chains were formed in the solution at the beginning of the aging process. After they reached a certain size, the process of chain folding and transformation into particles with a compact shape took place. Further, compact particles aggregated to form larger secondary aggregates.