Studying heterogeneous catalysis by the scanning electrochemical microscope (SECM): The reduction of protons by methyl viologen catalyzed by a platinum surface

The feedback mode of the scanning electrochemical microscope (SECM) was use d for studying a surface-catalyzed electron-transfer reaction. As a model s ystem we examined the reduction of protons by reduced N,N'-dimethyl-4,4'-bi pyridinum, methyl viologen radical cation, MV+., catalyzed by a platinum su rface. The experimental configuration comprises a biased amalgamated Au ult ramicroelectrode (UME) that approaches an unbiased Pt UME substrate of the same radius. MV+. that is electrochemically generated at the amalgamated UM E is oxidized at the Pt surface as a result of reducing protons. This novel configuration permits the simultaneous measurement of the catalytic curren t and the mixed potential (open-circuit potential) attained at the Pt surfa ce. The results are interpreted by an electrochemical model which was previ ously suggested; however, our approach makes it possible to measure, for th e first time, the current-potential relation predicted by the theory. Our r esults indicate that methyl viologen behaves reversibly under all the exper imental conditions, and that the rate constant of hydrogen evolution, that is calculated, is in perfect agreement with previous reports. The significa nt advantages of our approach are discussed as well as its applicability fo r studying other electrochemical systems.