Dispersive kinetic models predict variation of the activation energy with extent of conversion observed experimentally in isoconversional data

• Dispersive kinetic models are used to simulate complex activation energy variation.
• The two dispersive kinetic models relate nucleation and denucleation mechanisms.
• The simulated ΔG‡–x plots describe experimentally obtained isoconversional data.
• Heating rate is found to affect the shapes of plots of dispersive conversions.
• Dispersive kinetic models suggest development of more sophisticated thermal methods.

Two dispersive kinetic models (DKMs) are used for the first time to precisely simulate the evolution of the activation energy barrier, ΔG‡, as a function of the extent of conversion, x, of hypothetical conversions with realistic physical parameters. The simulated ΔG‡–x plots closely resemble certain trends reported in the recent experimental literature obtained using so-called isoconversional methods of thermal analysis (TA), thus forging a new link between the experimental results and dispersive kinetics theory. The simulations provide unprecedented mechanistic insight into such data trends. It is easily deduced that the activation energy distributions underpinning the two DKMs are responsible for producing the distinct variations observed in ΔG‡. That is because DKMs utilize the concept of a distribution of activation energies to simultaneously treat the kinetics and dynamics that can be observed in elementary conversions and that classical kinetic models (CKMs), which assume a single activation energy to model just the kinetics in the absence of dynamical effects, cannot properly describe (Skrdla, 2013). While the use of DKMs in TA applications remains quite limited, the two DKMs considered herein have been discussed in detail elsewhere and their application to a host of different conversions/phase transformations has been demonstrated under isothermal conditions (Skrdla, 2009). In the present work, those DKMs are used to simulate non-isothermal ΔG‡–x trends. Through the course of these investigations, it is found that the simulated data sets also indicate that the heating (cooling) rate can have a dramatic impact on kinetic determinations, whereas current isoconversional methods, relying on classical kinetic theory, predict no such effect. The last finding points to a need to develop new thermal methods, based on the theory underpinning DKMs rather than CKMs, to more rigorously model dispersive kinetic processes that exhibit distributed reactivity.

Plots of the activation energy, ΔG‡, evolution as a function of the extent of conversion, x, predicted by two dispersive kinetic models (DKMs), each of which is underpinned by a different distribution of activation energies, g(ΔG‡), are found to simulate data frequently observed experimentally using present-day isoconversional methods—thus, providing a simple physicochemical basis for the seemingly complex kinetic behavior.Figure optionsDownload as PowerPoint slide