Stable anion-cation layers on Cu(111) under reactive conditions

Combined voltammetric and in-situ STM studies were employed to gain information about the structure of a sulfate-modified Cu(111) electrode surface exposed to an acidic electrolyte containing a redox-active porphyrin (meso-tetra(N-methyl-4-pyridyl)-porphine, abbreviated as [H2TMPyP](4+)). A particular focus of this study lies on the characterization of the interfacial structure under reactive conditions, for example, during an ongoing electron transfer reaction. The oxidized form of [H2TMPyP](4+) cannot be stabilized within the narrow potential window of copper. A two-electron, transfer reduction affects the central porphine core even at the anodic limit of the copper potential window close to the onset of the oxidative copper dissolution reaction. This porphyrin-related electron transfer reaction can take place even in the presence of a preadsorbed sulfate/ water coadsorption layer. The latter causes a charge inversion at the metal/anion interface with an excess of negative charges within the sulfate/water layer. Enhanced electrostatic interactions between this sulfate/water coadsorption layer and the cationic porphyrin reactants and reaction products. are discussed as the physical origin for the formation of a paired anion-cation layer at the interface that retains its structural integrity even during the ongoing electron transfer reaction, at least on the time scale of the STM experiment. It is the starting sulfate desorption at more negative potentials that causes the loss of lateral order within this paired anion-cation layer. Highly water-soluble cationic porphyrin species coadsorb/codesorb together with the sulfate anions onto/from the Cu(111) electrode.