Rationalizing the “Anomalous” Electrochemical Stark Shift of CO at Pt(111) Through Vibrational Spectroscopy and Density-Functional Theory Calculations

We employ infrared reflection absorption spectroscopy (IRRAS) and first-principles density-functional theory (DFT) to revisit the reported “anomalous” negative Stark shift of the CO stretch frequency at Pt(111) in aqueous electrolyte. Our comprehensive IRRAS measurements confirm the existence of a potential region with negative Stark shift around 0.5 V vs. the reversible hydrogen electrode, but only at a sufficiently high CO concentration in the electrolyte. As these are exactly the same conditions for the occurrence of a phase transition from a (2×2)-3CO to a (√19×√19)R23.4°-13CO adsorbate structure, we explicitly compute the Stark shift for these two phases using DFT. Neither phase exhibits a negative Stark shift, but the absolute stretch frequencies of the atop CO in the two structures are slightly shifted with respect to each other. Remeasuring IRRAS with high resolution indeed reveals a doublet character of the absorption band in the potential region corresponding to the negative Stark shift. Separate fits of the two components then yield positive Stark shifts in quantitative agreement with the calculated values. The “anomalous” negative Stark shift simply arises from effectively fitting one component to a doublet spectral feature in a potential range with phase coexistence at the surface.