Formic Acid Electrooxidation on a Au Electrode Studied by Potential Step and Fast Scan ATR-FTIR Spectroscopy

The role of adsorbed formates in the electro-oxidation of formic acid on Pt electrodes was widely debated [1-3]; on a gold electrode formic acid oxidation was assumed to proceed via dimers of contiguously adsorbed bridge-bonded formates [4]. In the present work, formic acid oxidation on Au thin film electrode was studied by simultaneous in situ ATR-FTIRS and electrochemical measurements in both potentiodynamic and potential step mode with a fast IR spectra acquisition in a thin-layer flow cell configuration under well-defined mass transport conditions [2], to address the impact of adsorbed spectator species (perchlorate) and adsorbed bridge-bonded formate on the formic acid electrooxidation rate. 0.1 M HCOOH and DCOOH were used to i) assess the kinetic H/D isotope effect in formic acid oxidation on Au electrode; ii) identify the adsorbed species and follow their temporal development and the potential dependence.

A thin (~30 nm) Au film was electrolessly deposited onto the top plane of a hemi-cylindrical Si prism, which was subsequently carefully annealed in a butane flame in nitrogen atmosphere to improve the adhesion and thus the stability of the Au film under enforced hydrodynamic conditions and large repetitive perturbations of the working electrode potential (from 0.0 up to 1.65 V vs. RHE). This gold thin film electrode exhibited an unprecedented sensitivity of DR/R ~ 0.03 for the linearly adsorbed CO band in CO saturated supporting electrolyte at 0.0 V (reference spectrum acquired in CO-free electrolyte at 0.0 V). The IR spectra were acquired applying p-polarized IR radiation at 0.025 s^-1 time resolution, 16 cm^-1 spectral resolution and 75 kHz frequency employing a Cary 680 FTIR spectrometer (Agilent Technologies) equipped with a step scan accessory, and operated in a dual scan / quad compute mode. A EG&G 273A (PAR) potentiostat was used for potentiodynamic (100 mV s^-1) and multistep (from 0.0 V to desired oxidation potential) experiments.

A base CV in 1M HClO₄ supporting electrolyte revealed a clean Au film electrode as confirmed by in situ IR spectroscopy measurements, which exhibit only the potential dependent bipolar bands at 3610 / 3455 and 1654 / 1515 cm^-1 related to adsorbed water, and a unipolar band at ca. 1125 cm^-1 of adsorbed perchlorate. Formic acid oxidation starts at about 0.5 V, approaches a plateau from 0.9 to 1.2 V (HCOOH) and from 0.9 to 1.3 V (DCOOH), and then passes through a maximum at around 1.45 V, which is about three times higher for HCOOH than for DCOOH, due to kinetic isotope effects. The corresponding IR spectra show pronounced potential dependent HCOO_ad and DCOO_ad bands at 1326 and 1303 cm^-1, respectively. Plots of the integrated adsorbed formate band intensity vs. the potential are qualitatively similar to those of the perchlorate band in supporting electrolyte. Perchlorate adsorption is suppressed by adsorbed formate, due to competitive adsorption of the more strongly adsorbing anion.

 When stepping the potential from 0.0 V to the respective oxidation potential, the integrated band intensities for both adsorbed bridge bonded formate and perchlorate increased instanta­neously (within 25 ms) and remained constant over the step duration (1 s). A similar behavior is observed for adsorbed perchlorate, a clear spectator species. The integrated intensity of adsorbed formate at constant potentials increases with the adsorption potential up to 1.0 V, similar to that of adsorbed perchlorate in pure supporting electrolyte. The development of adsorbed formate reduces the perchlorate adsorption at higher potentials, compared to pure supporting electrolyte, indicating a competitive adsorption of formate and perchlorate.

Overall, the data reveal an adsorption behavior of the adsorbed bridge-bonded formate species, which is identical to that of other anions that are known to act as spectator species. Although the participation of adsorbed bridge-bonded formate as active intermediate in formic acid oxidation on a Au electrode [4] cannot be ruled out based on the present data, they are at least equally consistent with a model proposing reaction via coadsorbing formate anions and formic acid species [5].   

Acknowledgement. This work was supported by the Deutsche Forschungsgemeinschaft (Research unit FOR 1376, JU 2781/2-2)

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