Kinetics and Catalysis最新文献

The formation of atoms and radicals in the flame in concentrations by many orders of magnitude higher than the equilibrium values at combustion temperatures is explained. Results that prove the important role of homogeneous and heterogeneous reactions between atoms and radicals in combustion are presented. The decisive role of previously unknown reactions of heterogeneous chain propagation in combustion is illustrated by experimental results. The reasons for the misconceptions of several foreign and Russian authors who tried to deny chain combustion are explained.

The widely discussed phenomenon of flame propagation without self-heating is considered using as examples the combustion of carbon disulfide and the decomposition of nitrogen trichloride. It is shown that, in the nonthermal propagation of a carbon disulfide flame and in its combustion, a determining role is played by a new type of elemental reaction: the displacement of an atom from a molecule by an attacking atomic reactant. For nitrogen trichloride as an example, an unambiguous quantitative relationship between the flame speed and the nonlinear branching rate constant is shown. In both processes, the proposed mechanisms are confirmed by the identification, using EPR and optical spectroscopy, of atoms and radicals that play the main role in the process. The equations corresponding to the identified mechanisms of the processes are confirmed by experiments.

The rarely considered fundamental difference between the temperature dependences of the reaction rate and the rate constant is emphasized. The small change in the rate of reactions with high activation energies, contrary to existing ideas, is illustrated. It was shown that the main reason for the deviation between the calculated and experimental rates is the neglect of the temperature dependence of the reagent concentrations during the reaction. It has been shown that the main reason for the deviation between the calculated and experimental rate values is the neglect of the temperature dependence of the reactant concentrations in the course the reaction. The concept of the temperature rate constant is introduced: the change in the rate with a unit change in temperature, i.e., the temperature derivative of the rate constant. It is shown that this characteristic determines the competition between the stages of a complex process under nonisothermal conditions. The law of temperature dependence of the chain process was discovered, and its agreement with experiment was verified. The difference between the self-acceleration of a reaction from an increase in temperature and from the multiplication of active particles is explained. An experimental illustration is provided. The difference between the temperature dependences of the reaction rates in a gas heated from outside before and after the onset of ignition is explained. Based on experimental data, the determining role of the hydrogen atom concentrations in the combustion rate at hundredths of atmospheric pressure and at atmospheric pressure is quantitatively demonstrated. This demonstrates the determining role in combustion of the conversion of a significant part of the enthalpy of the initial reagents into the free-valence energy.

In this chapter, we will consider the mechanism of inhibition efficiency, stability of inhibitors, which determines the reliability and duration of ignition prevention, and the mechanism of synergy between the combined action of an inhibitor and inert gases. Special consideration will be given to the effect of inhibitors on flame propagation and detonation.

The theory of ignition induction periods, developed by the author, is presented. Quantitative confirmation of the theory by experiment at different pressures is provided. The same experimental data also confirm the theory of heterogeneous development of reaction chains.

The existence of two regimes of chain combustion is predicted, the phenomenon of explosion with a chain mechanism is explained, and the condition for the transition of combustion to this regime is formulated. The abrupt changes in the kinetic curves during the transition of combustion to this regime are illustrated and explained. The results of an experimental study of the transition from combustion to explosion are presented; it is found out that the ignition peninsulas, which are presented in courses and monographs on chemical kinetics as kinetically homogeneous regions, actually consist of two regions that are different in all reaction characteristics: the combustion region and the explosion region. Experimental evidence is given of the chain nature of combustion in an explosion under conditions of cumulation and control of such an explosion by means of inhibitors.

Until recently, it was attempted to explain the flame propagation without considering the chain nature of combustion, but the contradictions between theory and experiments failed to be explained. The difference in the roles of reaction chains and self-heating was not considered, as well as the role of heterogeneous chain termination. These issues are addressed in this chapter. Greater uncertainty in the quantitative interpretation of gas combustion is due to the limited information on heterogeneous reactions of active particles and the nonstationary state of the surface. Under these limitations, the propagation of flame of the 4H₂ + O₂ mixture was simulated. To reduce the role of the nonstationary state of the surface, conditions for the limiting role of diffusion of H atoms to the walls were chosen. A method for solving differential equations and conditions for replacing the Laplace operator with an averaging rate constant for the chemisorption of atomic hydrogen are described. The flat flame approximation is used. The flame characteristics for their comparison with the results of experiments conducted with the participation of the author were obtained. Experimental results showing the strong dependence of all characteristics of flame propagation on the changing properties of the surface are presented.

Based on many years of research, a theory of gas dynamics of combustion, explosion, and detonation processes was developed. However, the fundamental problems of their chemical and physicochemical characteristics began to be solved only in the last two or three decades. The cardinal problem in gas combustion is the discovery of the physicochemical mechanism of these rapid processes, which occur despite the strong chemical bonds of molecules. A brief citation of some of the basic principles of chemical kinetics, mostly known to readers, is intended to make the book easier to read. The current ideas about the role of activation energy in the temperature dependence of reaction rates are briefly analyzed. Several important patterns of combustion reactions that contradict previously generally accepted ideas are pointed out. The impossibility of combustion of gases in reactions of only valence-saturated molecules is proven on the basis of their very high activation energies. Previously unknown important patterns of combustion considered in the book are presented.

Mo₂C/AC-x converters were prepared using nitric acid pretreated activated carbon (AC) as a carrier, with x representing the carbonization time. The performance of the converter in converting NO₂ to NO was evaluated in a fixed bed reactor. The Mo₂C/AC-x converters were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ physisorption and desorption, H₂ programmed temperature reduction (H₂-TPR) and NO₂ temperature-programmed desorption–mass spectrometry (NO₂ TPD-MS). The NO₂ to NO conversion rate decreases in the following order: Mo₂C/AC-4 > Mo₂C/AC-2 > Mo₂C/AC-6 > Mo₂C/AC-0.5. Short carbonization times, like 0.5 h, led to incomplete carbonization of MoO₂ to β-Mo₂C. Conversely, long carbonization times, like 6 h, resulted in the formation of carbon deposits that can block pores or cover active sites, leading to decreased catalytic performance. Mo₂C/AC-4 has the highest specific surface area and pore volume. The NO₂ conversion rate of Mo₂C/AC-4 reached 98.9% at 150°C, demonstrating direct efficient conversion of NO₂ to NO at a lower temperature.