Critical Appraisal of Kinetic Calculation Methods Applied to Overlapping Multistep Reactions.

Nikita V Muravyev,Nobuyoshi Koga,Alla N Pivkina

doi:10.3390/molecules24122298

Abstract

Thermal decomposition of solids often includes simultaneous occurrence of the overlapping processes with unequal contributions in a specific physical quantity variation during the overall reaction (e.g., the opposite effects of decomposition and evaporation on the caloric signal). Kinetic analysis for such reactions is not a straightforward, while the applicability of common kinetic calculation methods to the particular complex processes has to be justified. This study focused on the critical analysis of the available kinetic calculation methods applied to the mathematically simulated thermogravimetry (TG) and differential scanning calorimetry (DSC) data. Comparing the calculated kinetic parameters with true kinetic parameters (used to simulate the thermoanalytical curves), some caveats in the application of the Kissinger, isoconversional Friedman, Vyazovkin and Flynn–Wall–Ozawa methods, mathematical and kinetic deconvolution approaches and formal kinetic description were highlighted. The model-fitting approach using simultaneously TG and DSC data was found to be the most useful for the complex processes assumed in the study.

Highlights

Thermally-induced transformations in heterogeneous system usually do not obey the idealized single-step kinetic pattern but are comprised of consecutive or concurrent reaction steps
When the isoconversional methods are applied to the multistep processes, the physical meaning of conversion degree α should be carefully reconsidered: while, in the fundamental kinetic equation, α is defined as the fractional reaction for the single reaction step [25], for the multistep process the α value is determined experimentally as the fraction of the total changes in the physical quantities during the overall process
As for kinetic pairs (Ea, lgA) the correct results were obtained for simple Cases 2 and 4 where the ratio between the rate constants are independent on temperature

Summary

Introduction

Thermally-induced transformations in heterogeneous system usually do not obey the idealized single-step kinetic pattern but are comprised of consecutive or concurrent reaction steps. Thermal decomposition of solids represents a typical example of such complex process [4,5,6,7,8]. Svoboda et al [15] analyzed the simulated process with independent reactions and revealed that the commonly used Kissinger method [16,17] provides the good estimate for the activation energy of the dominant reaction. Vyazovkin et al applied isoconversional methods to the simulated parallel independent [18] and consecutive [19] reactions, and summarized the typical shapes of the activation energy dependency on the conversion degree [20]. Burnham [24] proposed the formal kinetic methods to describe the complex processes, but usually some preliminary insights are necessary to give the close initial guesses for many fitted parameters.

Objectives

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Results