Potential-induced COad island formation on a platinum thin-film electrode in the H-upd potential region

In situ electrochemical IR spectroscopy data, obtained on a Pt thin-film electrode in 0.5 M H2SO4 solution in an attenuated total reflection configuration (ATR-FTIRS), show an anomalous, reversible blue-shift of the IR peak frequency of linearly bound adsorbed CO with decreasing potential in the H-upd potential range, together with an increase in band intensity. This phenomenon, which is observed only at coverages <= 0.3 monolayers and becomes more pronounced with decreasing COad coverage, is explained by a potential-induced island formation in the CO adlayer at low potentials. The phase transition is driven either indirectly by coadsorption of H-upd or directly by a potential-induced change in the COad-COad interaction energies.