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Free keywords:
Ag; peroxodisulfate; pzc; specific adsorption; stationary potential pattern
Abstract:
In this thesis two aspects of peroxodisulfate reduction at Ag-electrodes are studied: On the one hand, the influence of peroxodisulfate on the double layer composition is investigated. On the other hand, the reduction of S2O82- serves as a model system for studying spatial pattern formation in systems with a negative differential resistance in the polarization curve. In particular the influence of the geometrical position of the reference electrode on pattern formation is examined. The system Ag/S2O82- (basic pH) and the subsystems were studied by using the SP-resonance shift technique (Surface Plasmons). Changes of the SP-resonance with potential are only detectable positive to a threshold potential. This threshold potential of the basic peroxodisulfate system was shifted 0.35V to more cathodic values in comparison to the corresponding basic sulfate system. An XPS-study showed that the SP-resonance changes of both systems are due to specifically adsobed OH- and an oxide like surface species O2-. The pzc ((point of zero charge) calculated from the XPS-data correlates with the threshold potential of the SP-study. An SEM-study of the peroxodisulfate system excluded surface roughening as source of the SP-resonance changes. The shift of the pzc (threshold potential) originates from the competition of the chemical oxidation of adsorbed OH- by S2O82- and the electrochemical reduction of formed OH. Both processes are very fast, whereas the desorption of specifically adsorbed OH- is much slower. For this reason an OH- coverage can build up at more cathodic values compared to the corresponding basic sulfate system, resulting also in a shift of the pzc to more cathodic values. The potential distribution in front of a rotating ring electrode was studied by using a potential microprobe. The reference electrode was placed in the sense of a classical Luggin capillary next to the working electrode. It was shown that such an arrangement will introduce a negative global coupling into the system which destabilizes homogeneous stationary states if the current-potential characteristics exhibits a negative differential resistance. As a consequence of this coupling standing potential waves exists in a certain parameter regime. The addition of an external resistance introduces a positive global coupling which can compensate the negative global coupling. In such a case potential patterns could not be detected any more.
