Application of In-situ Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy for the Understanding of Complex Reaction Mechanism and Kinetics: Formic Acid Oxidation on a Pt Film Electrode at Elevated Temperatures

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Abstract
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 COad 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 °C). Linearly and multiply bonded COad and bridge-bonded formate are the only formic acid related stable reaction intermediates detected. With increasing temperature, the steady-state IR signal of COad increases, while that of formate decreases. Reaction rates for COad formation via formic acid dehydration and for COad 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 COad during adsorption/reaction transients, using an IR intensity−COad coverage calibration. At potentials up to 0.75 V and temperatures from 25 to 80 °C, the “indirect” CO pathway contributes less than 5% (at potentials ≤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 COad formation and COad oxidation compared with the effective activation energy of the total reaction, derived from the Faradaic currents, support this conclusion.

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