engleski

A new kinetic model for solid-state reactions

Solid-state transformations can be described using either bulk-based or reaction-order based models. Bulk-based models, such as the Avrami- Erofeyev, are particularly suitable for the analysis of phase change, as they depict the propagation of the new phase (either by nucleation, geometrical contraction or diffusion). Reaction-order based models, such as Finke’s two- and four-parameter models provide some mechanistic insight at the molecular level. Since the reactants and the products form different phases, most chemical reactions which occur in the solid state are coupled with phase transitions. The models mentioned above might be able to fit the experimental kinetic curves (on account of having enough parameters and flexibility), but the data gained from such fits does not always have a clear physical interpretation. Specifically – does the obtained rate constant and activation energy describe the chemical or the phase transformation? Mnyukh suggests that there are least two activation energies, one describing the nuclei and other the interface formation and growth. In order to solve this problem, we propose a new two-component kinetic model, which combines reaction-order based (chemical) and bulk-based (phase) kinetics. As a training set for the model we use the dimerization of aromatic (di)nitroso compounds, which afford a variety of kinetic curves (see picture). Also, the monomer topochemistry is dependent on the used preparation method (sublimation or photolysis), which means that the chemical and the phase transformations can somewhat be decoupled. The new model successfully describes these reactions and predicts activation energies in accord with previous experimental and theoretical results.

Kinetic model, cooperativity, nitroso compounds, solid-state chemistry

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