Go with the flow: A novel spectroelectrochemical flow cell with well‐defined mass transport allows time‐resolved electrochemical and in situ FTIR spectroscopy measurements under continuous electrolyte flow (e.g. during electrolyte exchange). Its potential for mechanistic and kinetic studies was demonstrated in studies on the electrooxidation of formic acid.

We present and discuss the results of an in situ IR study on the mechanism and kinetics of formic acid oxidation on a Pt film/Si electrode, performed in an attenuated total reflection (ATR) flow cell configuration under controlled mass transport conditions, which specifically aimed at elucidating the role of the adsorbed bridge-bonded formates in this reaction. Potentiodynamic measurements show a complex interplay between formation and desorption/oxidation of COad and formate species and the total Faradaic current. The notably faster increase of the Faradaic current compared to the coverage of bridge-bonded formate in transient measurements at constant potential, but with different formic acid concentrations, reveals that adsorbed formate decomposition is not rate-limiting in the dominant reaction pathway. If being reactive intermediate at all, the contribution of formate adsorption/decomposition to the reaction current decreases with increasing formic acid concentration, accounting for at most 15% for 0.2 M DCOOH at 0.7 VRHE. The rapid build-up/removal of the formate adlayer and its similarity with acetate or (bi-)sulfate adsorption/desorption indicate that the formate adlayer coverage is dominated by a fast dynamic adsorption-desorption equilibrium with the electrolyte, and that formate desorption is much faster than its decomposition. The results corroborate the proposal of a triple pathway reaction mechanism including an indirect pathway, a formate pathway, and a dominant direct pathway, as presented previously (Chen, Y. X.; et al. Angew. Chem. Int. Ed. 2006, 45, 981), in which adsorbed formates act as a site-blocking spectator in the dominant pathway rather than as an active intermediate.

The interaction of ethanol and its oxidative C2 derivatives acetaldehyde and acetic acid with a Pt thin film electrode in 0.5 M H 2 SO 4 solution was investigated by in situ Fourier transform infrared spectroscopy in an attenuated total reflection configuration (ATR-FTIRS). Time-resolved spectro-electrochemical measurements were carried out under well-defined mass transport to/from the electrode in a thin-layer flow cell setup, allowing to in situ monitor the electrode|electrolyte interface and the formation/removal of adsorbed species in both potentiodynamic and potentiostatic mode. Spectro-electrochemical transients at constant electrode potentials upon electrolyte exchange were employed to identify adsorbed species and their temporal evolution, followed by subsequent stripping of the resulting adsorbates in the supporting electrolyte. Adsorption transient and stripping measurements performed at different constant potentials lead to the following conclusions. (i) Ethanol does not adsorb on Pt at potential below 0.15 V (RHE), whereas acetaldehyde decomposes to CO ad already at 0.06 V. Acetaldehyde decomposition proceeds via adsorbed acetyl species and the decomposition rate depends on the potential, having its maximum at 0.25 V RHE and being slow at 0.06 V RHE . (ii) At potentials between 0.3 and 0.5 V RHE , both ethanol and acetaldehyde adsorption result in CO ad and adsorbed acetyl species coexisting on the Pt surface. (iii) Stable acetyl species adsorbed in the double layer region are decomposed to CO ad and CH x,ad fragments when scanning the potential into the H upd region. (iv) At potentials where CO ad is oxidized to CO 2 and the ethanol oxidation current is increased, adsorbed acetate is observed. The latter species are in a fast adsorption-desorption equilibrium with acetic acid in the solution.

The potential of in-situ Fourier transform infrared (FTIR) spectroscopy measurements in an attenuated total reflection configuration (ATR-FTIRS) for the evaluation of reaction pathways, elementary reaction steps, and their kinetics is demonstrated for formic acid electrooxidation on a Pt film electrode. Quantitative kinetic information on two elementary steps, formic acid dehydration and CO(ad) oxidation, and on the contributions of the related pathways in the dual path reaction mechanism are derived from IR spectroscopic signals in simultaneous electrochemical and ATR-FTIRS measurements over a wide temperature range (25-80 degrees C). Linearly and multiply bonded CO(ad) and bridge-bonded formate are the only formic acid related stable reaction intermediates detected. With increasing temperature, the steady-state IR signal of CO(ad) increases, while that of formate decreases. Reaction rates for CO(ad) formation via formic acid dehydration and for CO(ad) oxidation as well as the activation energies of these processes were determined at different temperatures, potentials, and surface conditions (with and without preadsorbed CO from formic acid dehydration) from the temporal evolution of the IR intensities of CO(ad) during adsorption/reaction transients, using an IR intensity-CO(ad) coverage calibration. At potentials up to 0.75 V and temperatures from 25 to 80 degrees C, the "indirect" CO pathway contributes less than 5% (at potentials < or =0.6 V significantly below 1%) to the total Faradaic reaction current, making the "direct" pathway by far the dominant one under the present reaction conditions. Much higher activation energies for CO(ad) formation and CO(ad) oxidation compared with the effective activation energy of the total reaction, derived from the Faradaic currents, support this conclusion.

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