Isoconversional methods as single-step kinetic approximation

Abstract

Isoconversional methods are based on the single-step kinetic approximation that may call in question their applicability to multistep processes. It is stressed that in isoconversional methods this approximation is applied only to a specific set of conditions, which are a constant conversion and a limited interval of the isoconversional temperatures related to it. The accuracy of the single-step approximation is tested on a process involving two competing reactions. It is demonstrated that substituting the kinetic parameters determined by an isoconversional method in the single-step rate equation yields the rate values that approximate accurately the rate of the multistep process. It is also demonstrated that the variation in the activation energy determined by an isoconversional method, i.e., under the single-step approximation, approximates accurately the variation predicted theoretically by applying the isoconversional derivative to the process rate, i.e., without using the single-step approximation. This confirms that the variable activation energy evaluated by an isoconversional method has a meaning of an effective parameter linked to and determined by the activation energies of the individual steps. For either the activation energy or the process rate the relative deviations caused by the approximation do not exceed a few percent. These results hold for the conditions of the simulations considered as practically realistic, viz., the isoconversional temperature range of 50 K and twofold difference in the activation energies of the steps.