Two-dimensional effects on the in situ infrared spectra of CO adsorbed at palladium-covered Pt(111) electrode surfaces

The in situ infrared spectra obtained for CO-saturated palladium layers deposited on Pt(111) and several vicinal surfaces are analyzed in this paper by correlating the observed C-O stretching bands with the cyclic voltammogram recorded for the corresponding surface before CO dosing. In the case of the Pt(111) electrode, results are reported in the whole palladium coverage range from (sub)monolayers to multilayers. For palladium coverages up to the monolayer, the C-O stretching spectra are consistent with previous results suggesting the formation of wide epitaxial islands upon palladium deposition. Increasing the palladium coverage above the monolayer on the Pt(111) electrode gives rise to a splitting of the C-O stretching band in the region between 1900 and 1950 cm(-1). This splitting is also observed when a palladium monolayer is deposited on surfaces vicinal to the Pt(111) surface irrespective of the (111) or (100) orientation of the step sites in the corresponding regular terrace-step structure. Decreasing the terrace width favors the high-frequency feature, which could be related to CO adsorbed on terrace sites near the step edge. When comparing this effect of the size of the two-dimensional palladium domains on the frequency of the C-O stretching band with the spectra obtained for the palladium-covered Pt(111) surface, it can be suggested that palladium films does not grow layer-by-layer once the first layer is completed.