Thermodynamic studies of anion adsorption at the Pt(111) electrode surface from glycolic acid solutions

Abstract

Kinetic glycolic acid (GA) oxidation and thermodynamic glycolate adsorption have been studied on Pt single crystal electrodes. The voltammetric profiles of Pt(111), Pt(100) and Pt(110) in 0.1 M GA are shown, and the effect of the inclusion of steps on the Pt(111) surface has been studied by cyclic voltammetry. For Pt(111) electrode, different concentrations and sweep rates have been applied, revealing that both adsorption and oxidation processes take place. By establishing the appropriate conditions, a complete thermodynamic analysis has been performed by using the electrode potential and the charge as independent variables. Total charge density curves, surface pressure at total charge density and at constant electrode potential were determined to calculate the Gibbs excess and the charge number at constant electrode potential for glycolate adsorption on Pt(111). Maximum glycolate coverage on the surface reaches a value of ∼ 6.0 ×10 14 ions/cm 2 . Spectroscopic results show the formation of CO 2 during the oxidation of glycolic acid, indicating that the cleavage of the C-C bond occurs during the oxidation process.

pKa as a Predictive Descriptor for Electrochemical Anion Adsorption

“…In the presence of the acetic acid derivatives, a new peak appears just positive of the hydrogen desorption peak, corresponding to the adsorption of these acetate derivatives with concomitant electron transfer. The voltammograms measured with acetate, glycolate, and trifluoro-acetate match those measured previously by Fukuda et al, [26] Arán-Ais et al, [27] and Pastor et al, [3a] respectively. Replicate measurements in each electrolyte are given in Figure S1.…”

“…Using the voltammetry shown in Figure 1, we calculate adsorption isotherms for each anion by subtracting part of the blank voltammogram (to remove contributions from hydrogen adsorption and double‐layer charging) and then integrating the current with respect to potential from just below the onset of the anion adsorption peak (0.3 V) and dividing by scan rate. To calculate the coverage of the anion, we divide this total charge transferred in the anion adsorption peak by the charge expected if one electron is transferred per surface Pt atom (240 μC/cm2 for Pt(111)), assuming that each anion transfers a complete electron upon adsorption [27] . Figure 6 shows these isotherms, where it is clear that while propionate, acetate, and glycolate show only small differences in final coverage, trifluoroacetate adsorbs to a significantly lower coverage; it is not clear whether the final increase in coverage near 0.75 V corresponds to additional trifluoroacetate adsorption or to hydroxyl adsorption, resulting in two separate peaks in the voltammetry.…”

“…To calculate the coverage of the anion, we divide this total charge transferred in the anion adsorption peak by the charge expected if one electron is transferred per surface Pt atom (240 μC/cm2 for Pt(111)), assuming that each anion transfers a complete electron upon adsorption. [27] Figure 6 shows these isotherms, where it is clear that while propionate, acetate, and glycolate show only small differences in final coverage, trifluoroacetate adsorbs…”

On the oxidation mechanism of C1-C2 organic molecules on platinum. A comparative analysis

“…Alternatively, the C-C bond can be destabilized when the methyl group of the EtOH molecule is partially oxidized [51][52][53][54]. In this sense, the oxidation of ethylene glycol (EG) in acid media occurs via the spontaneous and extensive C-C bond breaking in contact with Pt to produce adsorbed CO as major surface residue [47,51,55].…”