New Insights on Formic Acid Electrooxidation at a Polycrystalline Au Film Electrode: Simultaneous in Situ ATR-Ftirs and Online Dems Measurements
Authors: Jusys Zenonas , Behm Rolf Jürgen 
Published: February 2020

Formic acid oxidation to CO2 on a gold electrode was proposed recently to occur via dimers of adjacent bridge-bonded adsorbed formates [1]. A systematic theoretical study of different pathways for formic acid electro-oxidation at a Au(111) electrode [2] indicates that the reaction path depends on the potential and formate coverage (via single formate, direct path, complex of formate and formic acid, formate dimers), which is difficult to address experimentally.
In the present work, formic acid oxidation on a polycrystalline Au thin film electrode was studied using simultaneous in situ ATR-FTIRS, online DEMS and electrochemical measurements [3] in both potentiodynamic and potentiostatic modes in a thin-layer flow cell configuration under well-defined mass transport conditions, and compared to the mass transport controlled bulk CO and oxalic acid oxidation. In addition, oxalic acid oxidation was studied as a model reaction for in situ sensing of adsorbed oxalates [4]. These may be plausible intermediates in the formic acid oxidation, resulting from the dimerization of neighboring adsorbed formate species. Simultaneous in situ ATR-FTIRS and online DEMS measurements were employed to address the relation between the oxidation current, the CO2 formation rate, and the appearance of the adsorbed formate during formic acid oxidation. The kinetic H/D isotope effect (kH/kD) in formic acid oxidation was studied by DEMS as a function of Au electrode potential to discriminate between adsorption of formate (C-H bond remains intact, kH/kD = 1), or oxidation to CO2 (C-H bond rupture, kH/kD > 1).
A thin (~50 nm) Au film was electrolessly deposited on the top plane of a hemi-cylindrical Si prism, which was subsequently annealed in a butane flame in a N2 stream to improve the stability of the Au film. The IR spectra were acquired using p-polarized IR radiation at 1 s-1 time resolution and 4 cm-1 spectral resolution using a Cary 680 FTIR spectrometer (Agilent Technologies). A quadrupole mass spectrometer QMS 422 (Pfeiffer Vacuum) was used for online monitoring of CO2 at m/z = 44. Potentials were controlled and currents acquired by an AFRDE 5 (Pine Instruments) potentiostat.
The onset of CO2 formation during formic acid oxidation during the potentiodynamic scan is significantly delayed compared to the onset of the Faradaic current, while in bulk CO oxidation and in oxalic electrooxidation they increase in parallel. In contrast, the bridge-bonded adsorbed formate species appear instantaneously with the current increase, indicating that at low potentials the Faradaic current results from adsorption of formate species under these conditions. At higher potentials (>0.9 V), the coverage of bridge-bonded formate still increases, while the overall Faradaic current saturates. A steep increase in formic acid oxidation rate at potentials positive of 1.25 V correlates with the appearance of a band at ca. 1250 cm-1, which was detected also in oxalic acid oxidation and assigned to adsorbed oxalate [4]. Accordingly, we postulate that at these potentials adsorbed oxalates are formed by dimerization of adsorbed formates. Although we cannot decide whether these species subsequently decompose to CO2, it is clear that at 1.25 V a new reaction pathway is activated.
The kH/kD, found from the Faradaic currents in HCOOH and DCOOH potentiodynamic oxidation is close to one at the onset of formic acid oxidation, which indicates the C-H bond remains intact, as could be expected for the selective formic acid oxidation to adsorbed formate, in agreement with a low current efficiency for CO2 formation under these conditions. In contrast, the kH/kD, as found from the corresponding CO2 formation currents, is close to 5 at the onset of CO2 formation, approaching ca. 4 at potentials, where the current efficiency for CO2 formation is 100%, indicating that C-H bond splitting is the rate determining step of CO2 formation.
Overall, the present findings demonstrate that at very low potentials formic acid oxidation starts by adsorption of formate species, followed by CO2 formation at potentials > 0.4 V. First spectroscopic evidence was provided for the dimerization of adsorbed formates to form adsorbed oxalate at high potentials. The present work shows the role of the adsorbed formate in formic acid oxidation to depend on the reaction conditions [2].
Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2)
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