Spatially resolved ATR-FTIRS study of the formation of macroscopic domains and microislands during CO electrooxidation on Pt.

Electrooxidation of CO in CO-saturated sulfuric acid electrolyte solutions with controlled mass transport is investigated with spatially resolved attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy under galvanostatic and potentiostatic conditions. The reaction conditions are chosen such that steady states with intermediate current densities and intermediate average CO coverages are accessible. We demonstrate that under these conditions the reaction never proceeds uniformly on the electrode surface. Instead, macroscopic domains form spontaneously, composed of areas with high CO coverage and areas essentially free of adsorbed CO molecules. The average coverage within the CO-covered domains depends on the electrolyte concentration and the applied potential and can vary between saturation coverage and a few tenths of a monolayer. However, the absence of a red-shift of the CO vibrational band points to a further substructuring of the domains in densely packed CO microislands. These microislands most likely also form in the boundary layer between the CO-rich and CO-free electrode domains. This hierarchical patterning of the electrode surface is attributed to the interplay of autocatalytic reaction steps, spatial coupling through migration or the galvanostatic control of the experiment, and molecular interactions between molecules co-adsorbed on the electrode surface.