In-Situ Infrared Study of the Adsorption and Oxidation of Oxalic Acid at Single-Crystal and Thin-Film Gold Electrodes:  A Combined External Reflection Infrared and ATR−SEIRAS Approach

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR−SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm-1 that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the νs(O−C−O) and ν(C−OH) + δ(C−O−H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK a for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.