Nonlinear Phenomena during the Oxidation and Bromination of Pyrocatechol
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Résumé
This chapter reviewed recent studies on the nonlinear dynamics in the bromatepyrocatechol reaction (Harati & Wang, 2008a and 2008b), which showed that spontaneous oscillations could be obtained under broad range of reaction conditions. However, when the concentration of bromate, the oxidant in this chemical oscillator, is fixed, the concentration of pyrocatechol within which the system could exhibit spontaneous oscillations is quite narrow. This accounts for the reason why earlier attempt of finding spontaneous oscillations in the bromate-pyrocatechol system had failed. As illustrated by phase diagrams in the concentration space, it is critical to keep the ratio of bromate/pyrocatechol within a proper range. From the viewpoint of nonlinear dynamics, bromate is a parameter which has a positive impact on the nonlinear feedback loop, where increasing bromate concentration enhances the autocatalytic cycle (i.e. nonlinear feedback). On the other hand, pyrocatechol involves in the production of bromide ions, a reagent which inhibits the autocatalytic process, where an increase of pyrocatechol concentration accelerates the production of bromide ions through reacting with such reagents as bromine molecules. The requirement of having a proper ratio of bromate/pyrocatechol reflects the need of having a balanced interaction between the activation cycle and inhibition process for the onset of oscillatory behaviour in this chemical system. If the above conclusion is rational, one can expect that the role that pyrocatechol reacts with bromine dioxide radicals to accomplish the autocatalytic cycle is less important than its involvement in bromide production in this uncatalyzed bromate oscillator, and therefore when a reagent such as metal catalyst is used to replace pyrocatechol to react with bromine dioxide radicals for completing the autocatalytic cycle, oscillations are still expected to be achievable. This is indeed the case. Experiments have shown spontaneous oscillations when cerium, ferroin or manganese ions were introduced into the bromate-pyrocatechol system. Numerical simulations performed in this research show that the observed oscillatory phenomena could be qualitatively reproduced with a generic model proposed for noncatalyzed bromate oscillators. The simulation further indicates that while either two reaction steps K12 and K13 or K12 and K14 together with ten steps K1 K10 in Scheme I in the OKN mechanism are sufficient to qualitatively reproduce oscillations, three steps K12, K13, and K14 with ten steps K1 K10 are more realistic for representing the chemistry involving the oscillatory reactions, and also for reproducing oscillatory behaviors observed experimentally. The ratio of the rate constants for the equilibrium reaction K13 was a key reference to eliminate reaction step K11 from the original model. Although the present model still needs to be improved to reproduce the experimental results quantitatively, it has
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| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,000 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,001 | 0,000 |
| Intégrité de la recherche | 0,000 | 0,001 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
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