Spectroelectrochemical and Density Functional Theory Study of Squaric Acid Adsorption and Oxidation at Gold Thin Film and Single Crystal Electrodes

The adsorption and oxidation of squaric acid (H2C4O4, H2SQ) at gold single crystal and thin-film electrodes with preferential (111) orientation were studied spectroelectrochemically in perchloric acid solutions. The existence of reversible adsorption–desorption processes in the double-layer region is reflected by structure-sensitive voltammetric features. Infrared reflection absorption spectroscopy experiments carried out with Au(111) and Au(100) single-crystal surfaces in 10 mM H2SQ solutions show potential-dependent adsorbate bands at ca. 1780–1785 and 1511–1577 cm–1 for potentials below 1.00 V RHE. The increasing sensitivity of the attenuated total reflection (ATR)–surface-enhanced infrared reflection absorption (SEIRA) experiments allows the detection of similar features for much lower H2SQ concentrations. According to density functional theory (DFT) calculations, these bands can be assigned to adsorbed squarate anions which are bonded to the gold surfaces in a bidentate configuration through two oxygen atoms, with the molecular plane perpendicular to the metal surface. For 10 mM H2SQ solutions, additional bands are detected in the ATR–SEIRA spectra at ca. 1630 cm–1 both in water and deuterium oxide solutions. Even if this frequency fits with one of the vibrational modes of adsorbed bisquarate, DFT calculations provide an alternative explanation for this potential-dependent feature that could be ascribed to collective vibrational modes of adsorbed squarate appearing at high adsorbate coverage. The existence of in-phase and out-of-phase contributions under these conditions would also explain the broadening and/or splitting of the observed bands. DFT calculations also show that squaric acid molecules adsorb very weakly at the gold surfaces and can be discarded as the origin of the observed infrared bands. The external reflection infrared spectra obtained for gold single-crystal electrodes in the H2SQ oxidation region show bands for dissolved carbon dioxide molecules as the main product. Bands for adsorbed bicarbonate anions formed from carbon dioxide are detected in the ATR–SEIRA spectra