Probing the electrooxidation of weakly adsorbed formic acid on Pt(1 0 0)

• Concurrent vibrational sum frequency spectroscopy and in situ cyclic voltammetry.
• Weakly adsorbed formic acid on Pt(1 0 0) detected for the first time.
• Formic acid is a reaction precursor in the direct pathway of formic acid electrooxidation.

Understanding the electrooxidation of formic acid on noble metal surfaces is both important for the insight it promises into a formic acid fuel cell and as a model for the electrooxidation of hydrocarbons more generally. As a result, this reaction has been studied for decades. Taken together this body of previous work suggests that on most noble metals the oxidation precedes both through an indirect, via adsorbed CO, and direct pathway via an intermediate that is controversial. In an attempt to rationalize large differences in electrooxidative activity as a function of metal, crystal surface, pH and the presence of other anions, a variety of intermediates in the direct pathway have been proposed. While differing in detail virtually all of the proposed mechanisms invoke a species that only weakly interacts with the metal surface and cannot be detected via conventional spectroscopic techniques. Here we overcome this challenge by characterizing the electrooxidation of formic acid on the Pt(1 0 0) surface using the surface selective nonlinear optical technique, vibrationally resonant sum frequency spectroscopy. We detect a weakly adsorbed form of formic acid and find by correlating its potential dependent spectral amplitude and oxidation peaks apparent in a simultaneously collected in situ cyclic voltammogram, that it is a reaction precursor in the direct oxidation of formic acid on this surface.

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