Spatiotemporal Self-organization Of The Electrocatalytic Oxidation Of Sodium Sulfide On Platinum Electrode In Transpassvie Region

In this thesis,cyclic voltammetry and potentiostatic method are applied to investigate the dynamics of the electrocatalytic oxidation of sodium sulfide on platinum in transpassive region,and the effects of external resistance,concentration of sodium sulfide and chloridion are discussed.Meanwhile,rich patterns formed on disk are observed with a CCD camera.High performance liquid chromatography is adopted to detect intermediates occurred in reaction process and mechanism is discussed.Patterns are analyzed by Karhunen-Loève decomposition.Firstly,the effects of external resistance and concentration of soudium sulfide are discussed.The electric double layer potential as an autocatalytic variable to promote the formation of platinum oxide and produce negative differential resistance(NDR).That external resistances promote the increasing of electric double layer potential thus enhances the positive feedback of the system.Sulfide ions consume the platinum oxides as the negative feedback.When the concentration of sodium sulfide is 1.50 mol/L and the external resistance is 300 ?,there is a hystereic cycle between positive and negative feedbacks.Therefore,it presents instability characteristics and hidden N-shape negative differential resistance(HN-NDR)oscillations with small and large amplitude are observed in transassive region.Large amplitude oscillations correspond to synchronous deposition and dissolution.Chloridion has a significant effect on instability characteristics of the system.On one hand,chloridion as a strong chemical adsorbate of metal electrode gives rise to another positive feedback.On the other hand,high concentration of chloridion can dissolute platinum oxides and cause the negative feedback.When the concentration of chloridion is low,the positive feedback induced with adsorption of chloridion is relatively weak,which has little influence in oscillations in transpassive region.With increasing the concentration of chloridion,chemical adsorption enhanced and coupled with negative feedback yielding strengthen oscillations.When the concentration of chloridion is too high,chloridion dissoluting platinum oxides as the negative feedback,causing positive feedback grandually disappear,thus inhibiting oscillations to vanish.Except for synchronous deposition/dissolution,spiral,twinkling,front waves and stripe migration patterns are monitored in small amplitude oscillations of transpassive region.Potentiostatic method are applied to investigate the evolution of oscillations in transpassive region.It is found that oscillating potential shifts toward negative and gets wider.The small amplitude oscillations correspond to breathing patterns.The coexisting of small and large amplitude oscillations correspond to the alternative of breathing patterns and synchronous deposition/dissolution.Karhunen-Loève decomposition analysis shows that the twinkling patterns have multiple modes each with low percentage of energy,while the alternate patterns are mainly characterized by one dominant mode.