No abstract

A prototype of an electrochemical oscillator is represented by three nonlinear differential equations. Through numerical computation it is possible to study the system by calculating the time series and the respective Lyapunov exponents which describes whether the system is periodic or chaotic.

A descoberta da reação de Belousov-Zhabotinsky é considerada o marco inicial dos estudos sistemáticos das reações químicas oscilantes. Tais reações formam espontaneamente padrões auto-organizados quando mantidas afastadas do equilíbrio e são capazes de mimetizar diversos sistemas biológicos encontrados na natureza.

O planejamento fatorial é um método multivariado que possibilita com um pequeno número de experimentos extrair o máximo de informação de um sistema. Utilizamos essa metodologia a fim de quantificar o efeito da concentração de ácido malônico e bromato além da temperatura no período oscilatório da reação de Belousov-Zhabotinsky.

The spontaneous formation of self-organized spatiotemporal patterns under far from thermodynamic equilibrium conditions is a characteristic behavior in reaction-transport systems. Indeed, this spatial structuration can be understood as a collective behavior of a large number of individual elements in the system. Consequently the pattern emerges as a result of the interaction between the local dynamic of these subunits and the spatial coupling. Multistable, excitable and oscillatory nonlinear dynamics are typical examples of complex temporal patterns usually associated to the spatial structuration. In this doctoral thesis, two work fronts are presented using the nonlinear chemical dynamics in the elucidation of reaction mechanisms under far from thermodynamic equilibrium regime: (a) the investigation of the chemical nature and effect of the drift in the transient time-series in electrochemical oscillators. The analysis of the temporal evolution of the bifurcation parameter was based on an empiric method of stabilization, being the slow accumulation of oxygenated species the main responsible for the drift; (b) the decoupling of the parallel electrochemical routes for CO 2 production by a combination of experiments, modeling and numerical simulations during the oscillatory electro-oxidation of methanol on polycrystalline platinum. The effect of perchlorate and sulfate anions in the parallel reactions was investigated by the global production of CO 2 and HCOOCH 3. Remarkably, sulfate anions inhibited more strongly the catalytic activity from direct pathway in contrast to the small alteration in the indirect pathway. In parallel to the two work fronts, an experimental setup was built in order to obtain a spatiotemporal evolution of a electrochemical reaction with a multichannel data acquisition system. A description of the confection process of the cell and the multichannel working electrode, data treatment and some preliminary experimental results are included as an additional chapter. The main idea of this thesis converges in the obtainment of chemical kinetic information which is not observed in conditions close to the thermodynamic equilibrium. This interpretation might be used as an alternative methodology in the study of electrocatalysis in complex chemical reactions.