Kinetics and Catalysis最新文献

Quantum chemical modeling of CO oxidation on the Cu₁₂S₆(PH₃)₈ and Cu₁₂S₆ clusters was performed in order to establish the general tendencies in the process on metal nanoclusters stabilized by ligands and to find out if the presence of a phosphine ligand is needed in the active site. The Langmuir–Hinshelwood mechanism was studied, which involves sequential oxidation of two CO molecules with oxygen. The calculated activation energies on Cu₁₂S₆(PH₃)₈ are lower than on Cu₁₂S₆ for all oxidation stages; therefore, the PH₃ ligands have a positive effect on the catalytic properties of the copper sulfide cluster in the CO oxidation. A linear correlation was found between the energy of CO adsorption on various copper sulfide clusters and the activation energy of the oxidation stage: the lowest activation energy is observed for the cluster with a CO adsorption energy of 36 kJ/mol.

In this work, a series of nickel-containing samples of carbon xerogels (Ni@CX) were synthesized by introducing nickel acetate into a solution of precursors (resorcinol and formaldehyde), joint polycondensation, and subsequent pyrolysis in an atmosphere of argon. The samples were tested in the selective hydrogenation reaction of 1,3-butadiene. It was shown that catalysts prepared without the addition of complexing agents contained nickel nanoparticles (4–7 nm) stabilized in a carbon matrix. The Ni@CX catalyst made it possible to achieve a selectivity of 93% for butenes at a conversion of 94% and a temperature of 200°C, while the selectivity for the formation of butenes in the presence of an impregnated reference sample was lower than 1%, although it provided a conversion of 100% already at 75°C. It was supposed that the selectivity of catalysts prepared by the joint synthesis increased due to the blockage of nonselective catalytic sites by carbon of the support.

Results of a kinetic study of the pyrolysis of woody biomass (Quercus robur) under conditions of continuous heating to a temperature of 873 K at a constant rate of 1.25, 2.5, 5, and 10 K/min have been discussed. An integral method has been used to describe the reaction mechanism and determine the macrokinetic parameters. It has been found that, from a phenomenological point of view, the averaged woody biomass pyrolysis reaction under test conditions corresponds to a three-dimensional diffusion model (1.25 K/min), a model described by the third-order reaction equation (2.5, 5 K/min), and a one-dimensional diffusion model (10 K/min). In this case, the relative standard deviation of the conversion values calculated using this equation from the test data is <11.2%. The division of the averaged reaction into three stages (first stage is completed at a temperature of 390 K; the second, at 579 K; the third, at the completion of the conversion process) leads to agreement between the calculated degree of degradation of the studied biomass samples and the test values in a range of the degree of degradation of 0–1. Although the relative standard deviation does not exceed 3.5%, the division of the averaged reaction into stages does not exclude discrepancies in the values of the determined macrokinetic parameters. It has been shown that the choice of the model that provides the best description of the wood conversion process depends on heating rate during the tests. Owing to this dependence, the macrokinetic parameter values significantly differ from each other. Taking this fact into account, it can be concluded that the selected models and calculated macrokinetic parameters are of a formal nature and cannot be thought of as physicochemical characteristics that are universal with respect to the subject of research. Discrepancies in calculations of macrokinetic parameters that are caused by the effect of heating rate can be eliminated by studying the kinetics of conversion under isothermal conditions (at a constant temperature).

The paper compares the properties and activity of palladium clusters deposited on aluminum oxides of different phase compositions obtained by different methods. The numbers of palladium atoms available for the adsorption of hydrazine were determined for each catalyst. A correlation was found between the number of such atoms and the rate of hydrogen formation in the decomposition reaction of hydrazine monohydrate. It was concluded that an active palladium catalyst for the decomposition of hydrazine can be obtained using a modified support with a large specific surface area. The specific surface area and its modification determined the surface coverage with the active reagent. Obviously, the study of the role of various properties of supports in the formation of the active phase should be continued.

The formal kinetics of the interaction of acrylonitrile (AN) and cyclopentadiene (CPD) by the Diels–Alder reaction in the presence of industrial zeolites has been studied. The dependence of the reaction rate constants of interacting components on the geometric properties of porous materials was revealed. The established dependence had a negative extremum at a zeolite pore diameter of 4 Å. It was shown that, in the presence of zeolites, the rate constant of the reaction under consideration was lower than that in the reaction carried out without porous materials.

The reasons for the mistakes underlying the neglect and denial of the reaction chains discovered by N.N. Semenov and S. Hinshelwood in the combustion of gases at pressures close to atmospheric are analyzed. A method for unequivocal experimental proof of the chain nature of gas combustion is described. Examples are given proving the chain nature of combustion and explosion in a wide range of pressures and temperatures.

Based on the theory of nonisothermal chain reactions developed in the studies presented in the previous chapters, an experimentally verified explanation is given for the anomalous behavior of pyrolysis and combustion of a technically important product—hydrazine. The revealed kinetic features of N₂H₃ radicals, which led to the unusual properties of the hydrazine-oxygen ignition peninsula, are also explained. The mechanism of easy ignition and explosion has been clarified.

In this chapter, we will discuss the issues of inhibitor consumption outside the ignition region and the advantages of the method for determining rate constants from ignition limits, as well as a method for determining rate constants for reactions of displacement of atoms and from a molecule by an attacking atom.

Contradictions between generally accepted ideas about a one-stage reaction underlying detonation and experimental data, which existed before the appearance of the author’s works are listed. Publications that deny the chain nature of reactions in gas detonation are mentioned. The necessity of a reaction of free atoms and radicals for the realization of explosion and detonation is explained. A test bench with a shock tube is described, on which experiments were carried out to identify the chain nature of reactions in detonation. Results are presented showing that, contrary to generally accepted ideas about the one-stage model of detonation reactions, chemical processes in the detonation of gases are chain processes with all the characteristic properties of this class of reactions. A stationary detonation wave was split into a combustion wave and a shock wave, and the stationary detonation velocity was varied by inhibition. It is noted that the results of identifying combustion, explosion, and detonation are a priority of Russia and Russian Academy of Sciences.

It is shown that the features of chain processes are determined mainly by the fact that most of the internal energy of the initial reagents is converted into the energy of free valences of atoms and radicals. This results in high concentrations of active particles, which react with the initial reagents at higher rates, providing intensive self-heating, which additionally accelerates the process. It is noted that, without considering the chain nature of reactions, it is impossible to explain ignition upon heating, since the high activation energies of reactions of only molecular reagents exclude an increase in the rate during heating. Taking into account the chain nature of combustion reactions made it possible to explain ignition upon heating and develop a theory of the phenomenon. An explanation is given to the most important characteristic of gas combustion: the length of reaction chains, which determines the role of the chain mechanism in combustion, and the dependence of kinetics on temperature. A solution to the problems of the third flammability limit, confirmed by kinetic and spectroscopic experiments, was obtained, as well as the evidence of the presence of HO₂ radicals in the flame, their active role in the heterogeneous development of reaction chains and in combustion in general. The possibility of replacing the solution of diffusion differential equations by effective kinetic parameters when describing combustion has been experimentally proven.