IR active and inactive CO adsorbed on a Pt electrode in acidic solution observed by reflection absorption spectroscopy

Abstract

Monolayer and submonolayers of CO adsorbed in the double-layer region on a Pt electrode were investigated in 1M HClO 4 by using in situ polarization-modulated IR reflection absorption spectroscopy (IRRAS) and an electrochemical oxidation. As for the monolayer of CO, it was concluded that linear CO accounted tor only 10% of all the adsorbed CO, and the remainder was RAS-inactive CO (reflection absorption spectroscopically inactive CO). As for the submonolayers of CO, the average number of adsorption sites per CO molecule changed from 1.7 to 1.2 with an increase in the total coverage from 0.1 to 0.9. The bridged CO could not be detected for these coverages by the IRRAS method. The band intensity of linear CO was not proportional to the total coverage. Therefore, it was concluded that the RAS-inactive CO were present, and these were one-site occupying and two-site occupying species. The adsorbed species predominating was the two-site occupying RAS-inactive CO for low coverage; and for high coverage the one-site occupying RAS-inactive CO. The RAS-inactive CO are described by a model in which CO is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface.