| Thiosulfate is a very important chemical substance, applying in diverse areas, including chemical synthesis, biochemistry, waste water treatment, mineral separation, paper manufacturing, and photographic industry, etc. Thiosulfate can be oxidized by chemical or electrochemical method to sulfate,among which a large number of intermediate with different oxidation states can be produced, therefore many kinds of complex reaction mechanisms is to be frequently observed and investigated.This dissertation is concerned on complex kinetics and reaction mechanism caused by thiosulfate electrocatalysis on gold electrode. The electrocatalytic oxidation of thiosulfate was investigated in the buffer solution of p H 6.0. The contents mentioned primarily indicate the complicated dynamic phenomena, for instance, highly periodic oscillations and series of mixed-mode oscillation states, which appeares under both galvanostatic and potentiostatic regimes. The system displays supercritical Hopf, period-doubling, homoclinic, chaotic and bursting bifurcations.Especially the period-doubling of mixed-mode oscillation distributes along two unstable regions when adjusting the relevant parameters. Oscillation periods develop from 1 to a dozen or even dozens, whose types and sequences of oscillations are abundance and relatively intact, rarely published in former literatures.Experiments are carried on the factors, which could influence the electrochemical oscillation of the system from different aspects, such as the reference electrode, buffer solution, area of the working electrode, concentration of thiosulfate, scan rate, the external force and p H.. The external force affects the oscillation by changing the surface diffuse condition, accordingly influence the adsorption and deposition structure of working electrode. The dependence of the electrochemical oscillation on the p H was investigated by synchronous detecting the current and p H. The experiment results demonstrate that the voltage range, where the electrochemical oscillation occurred is synchronized with the p H oscillation, indicates that the oscillations in the first oscillation area are associated with the H+ concentration. p H of the solution changed from 6.2 to 11.3 when scanning, which is relatively wide.After presenting the dynamics under both potentiostatic and galvanostatic regimes, the author catalogue and characterize the system in terms of its electrical analysis by ohmic drop compensation(ODP), electrochemical impedance spectroscopy(EIS), and chemical analysis by capillary electrophoresis(CE) high performance liquid chromatography(HPLC),attenuated total reflection infrared spectra(ATR-IR) and surface enhanced Raman spectroscopy(SERS) aspects.The experiments with the methods of adding external ohmic resistance, internal solution ohmic resistance fixed compensation, and internal solution ohmic resistance dynamic compensation, confirmed that the oscillation is neither chemical oscillation nor strictly potentiostatic oscillators. EIS depicts the hidden N-shaped negative differential resistance oscillatory character for both oscillatory regions. EIS, linear sweep curves of potential and current density, as well as the potentiostatic and gavonastatic dynamics, indicated that the system displayed HN-NDR oscillations.Valence of sulfur changes magnificently, varying from-2 to +6, whose electrochemical oxidation can produce complex dynamic phenomena. In order to further determine the negative feedback forming two complex oscillation areas of potential and current in gold- thiosulfate system, CE and HPLC combined with in-situ ATR-IR and SERS were utilized to find out that the system produced sulfur compounds as the applied voltage was gradually increased, including S3O62-, S4O62-, S5O62-, S6O62-, S7O62-, SO42-, and S. S3O62- and S4O62-, indicates that competitive adsorption between gold-sulfide and gold-oxide(or OH adsorption) were occurred, meanwhile, the reduction of gold-oxide(or OH adsorption) exhibits the two negative feed backs of HN-NDR oscillations. Sn O62-(n = 5-7) was obtained from the reaction between Sn-1O62- and S2O32-. The solid products generated on working electrode surface are main S8.In fact, mixed-mode oscillation sequences obtained here evidences much more robustness and intricate dynamics than other comparable systems. Besides the sophisticated and presently unknown chemicals of the overall process, there are uncertain underlying the feedback loops, whose complexity can be attributed to the occurrence of two well-defined consecutive HN-NDR regions. The diversity in dynamics observed here widens the dynamic complexity in complex structures anticipated for HN-NDR electrochemical systems. From the surface chemistry perspective, this is, for the best of our knowledge, the first report on the oscillatory dynamics during an electro-oxidation reaction on gold surface. The presented results illustrated the complexity associated to processes such as the degradation of sulfur-containing compounds and also oriented synthesis of sulfur oxides. |
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