Spatiotemporal Self-Organization And Mechanism Analysis Of Oscillatory Sulfur Deposition In Electro-Catalytic Oxidation Of Sodium Sulfide

The electro-catalytic oxidation of sodium sulfide possesses complex spatiotemporal dynamic behaviors. Electrochemical methods are adopted to systematic study the dynamics of electro-catalytic oxidation of sodium sulfide on platinum disk and the oscillatory mechanism in N-NDR region is analyzed. Meanwhile, the sulfur deposition and dissolution on the disk is simultaneously observed with CCD camera and rich patterns are observed, including front, synchronization of sulfur deposition and dissolution, spirals, pulses, alternation of pulses and synchronization and alternative structure between twinkling-spots and synchronization in high potential region. The electro-catalytic oxidation of sodium sulfide has both N-NDR and HN-NDR oscillatory behaviors. Within the current oscillations, the formation of elemental sulfur on the electrode surface correspondingly displays periodic deposition and dissolution. Different sulfur deposited patterns can be formed under different conditions.Firstly, the effects of species concentration on the oscillation and sulfur deposition patterns are investigated. It is found that sulfide ion as the negative feedback species can consume the platinum oxides on the electrode surface and the oxidative product, elemental sulfur, can occupy the active site and thus inhibit the formation of platinum oxides. As the sulfide ion concentration increase, the dynamic behavior of the positive slope region on the dynamic curve changes gradually from stable state to HN-NDR oscillations and the sulfur deposition pattern transforms from front to synchronous sulfur deposition and dissolution. While in the negative slope branch area, sulfide transfer-limited process acts as the negative feedback consuming the platinum oxides and thus the dynamic behavior changes from small amplitude current density oscillations to large ones and the potential range, period and amplitude of the oscillations increase with sulfide ion concentration. Local pulses accordingly transform into synchronous sulfur deposition and dissolution.However, the hydroxide ion is directly involved in the formation of platinum oxides and consequently it acts as the positive feedback species promoting the reduction of current density. The HN-NDR and N-NDR oscillations are both suppressed with increasing hydroxide ion concentration on the entire dynamic curve. HN-NDR oscillations turn into stable state and the onset potential of sulfide oxidation shifts negatively. N-NDR oscillatory range, amplitude and period reduce apparently. Patterns in N-NDR oscillatory region change from synchronous sulfur deposition and dissolution into local pulses and the pulse width decreases with increasing hydroxide ion concentration.Chloride ion as a strong metal adsorbate gives rise to another NDR in the sulfide electrochemical system. It causes the promotion of HN-NDR oscillations and pattern transition from front to synchronization on the positive slope branch. The N-NDR oscillations on the negative slope branch increase in the amplitude and oscillatory potential range shifts to higher values with increasing chloride ion concentration. Nevertheless the pulse width in the N-NDR oscillatory region decreases with increasing chloride ion concentration. Since another positive feedback is introduced by the chloride ion, with the interaction of multiple feedbacks this system presents the coexistence of different time-scale oscillations, which as a result induces the coexistence of large wavelength synchronization and small wavelength pulses.Secondly, the effect of external resistance on the oscillatory behaviors and sulfur deposition patterns is investigated and it is detected that the external resistance can not only promote the HN-NDR oscillations on the positive slope branch, but also suppress the N-NDR oscillations with decay form until the stable state on the negative slope branch. Meanwhile, due to the external resistance the system can concurrently display multiple HN-NDR and N-NDR oscillations. The coupling of these two oscillatory dynamics probably causes the complex oscillations and alternative pattern between local pulses and synchronization. During the HN-NDR oscillations in transpassive region, the occurrence of complex oscillations in high potential region is possibly connected to the oxidation of sulfide with high valent platinum oxides and even the generated oxygen, thus complicated multiple feedbacks coupled together induce the complex patterns.Finally, based on the experimental results, the mechanistic model of electro-catalytic oxidation of sodium sulfide is proposed. The autocatalytic process of electrode potential promotes the formation of platinum oxide, which gives rise to the NDR of the system. Hydroxide and sulfide ions act as positive and negative feedback species promoting and suppressing the formation of platinum oxides, respectively. Sulfur competing adsorption and sulfide transfer-limited process are the different negative feedbacks. The coupling of positive and negative feedbacks induces the HN-NDR and N-NDR oscillations in the system. According to the reaction mechanism, and considering the charge balance of the circuit, the mass balance and the potential dependence of the sulfur adsorption, a dimensionless three-variable model is built to qualitatively simulate and explain the effect of sulfide, hydroxide ions and external resistance on the dynamic behavior of the sulfide system. The simulated sulfur depositing patterns are consistent with the experiment and as a result it proves the rationality of the mechanism.