Concept of Variable Activation Energy and Its Validity in Nonisothermal Kinetics

Abstract

The concept of variable activation energy in solid-state kinetics under nonisothermal conditions has been suffering from doubt and controversy. Rate equations of nonisothermal kinetics of solid decomposition, which involve the factors of thermodynamics conditions, pressure of gaseous product, structure parameters of solid, and/or extent of conversion, are derived from the models of the interface reaction, the diffusion of gaseous product, and the nuclei growth of the solid product, respectively. The definition of the validity function in the rate equations represents the influence of the factors on the reaction rate. A function of variable activation energy depending on the validity function is also developed. The changing trend and degree of activation energy are extrapolated from the function of variable activation energy and based on the data of nonisothermal thermal decomposition of calcium carbonate. It is shown that the concept of variable activation energy is meaningfully applicable to solid-state reactions under nonisothermal conditions.

Thermal conversion mechanism and thermodynamics of mixed oil in coal‐based needle coke production

“…The equal conversion method is the most commonly used method for calculating nonisothermal kinetics, which refers to the method of kinetic analysis of multiple thermal analysis curves measured at different heating rates. The theoretical basis of the equal conversion method is that the reaction rate is only a function of temperature, whereas the reaction degree remains constant …”

Time- and Temperature-Varying Activation Energies: Isobutane Selective Oxidation to Methacrolein over Phosphomolybdic Acid and Copper(II) Phosphomolybdates

“…During the 4 th run, two upward steps are apparent. The first step reaches a maximum from 357 • C and drifts up until 403 • C, while the second maximum is from 417 • C. Significant variations in the profiles between the 3 rd and 4 th runs for H 3 PMo 12 O 40 and Cu 1.5 PMo 12 O 40 demonstrate that the catalysts deactivate during proceeding temperature-programmed runs; due, either to restructuring caused by elevated temperatures, or the reduction of accessible oxygen active sites. (Figure 5).…”

“…This compensation effect highlights the challenge in determining the true activation energies and prefactors. Rate coefficients for selective oxidation can be simplified to Equation (12) where the rate coefficient ratio of activated complex to surface diffusion away from active sites control the temperature-dependence [21].…”

Magnesium hydride for energy storage applications: The kinetics of dehydrogenation under different working conditions

“…According to the microreversibility principle, in a reversible reaction the mechanism in one direction is exactly the reverse of the mechanism in the other direction. This principle has been employed by different authors to study the influence of product gas pressure on reversible reactions, leading to the following expression for the reaction rate [26,30,[34][35][36][37]:…”

“…Moreover, in many cases the kinetic analysis has been carried out by previously assuming a given kinetic model for fitting the experimental data, but it is known, firstly, that a same set of experimental data can be simultaneously fitted by a number of kinetic models and, secondly, that the activation energy is strongly dependent on the kinetic model previously assumed [29][30][31][32][33]. A proper kinetic analysis of reversible thermal decomposition reactions under a given constant pressure of the gas self-generated in the reaction would imply to consider the thermodynamic of the process by taking into account the microreversibility principle [26,30,[34][35]. On the other hand, it would be advisable to associate the microreversibility principle with methods of kinetic analysis that would allow determining both the kinetic parameters and the kinetic model obeyed by the reaction without any previous assumption on the kinetic model.…”