Thermodynamic Analysis of the Temperature Dependence of OH Adsorption on Pt(111) and Pt(100) Electrodes in Acidic Media in the Absence of Specific Anion Adsorption

The effect of temperature on the voltammetric OH adsorption on Pt(111) and Pt(100) electrodes in perchloric acid media has been studied. From a thermodynamic analysis based on a generalized adsorption isotherm, ΔG°, ΔH°, and ΔS° values for the adsorption of OH have been determined. On Pt(111), the adsorption enthalpy ranges between −265 and −235 kJ mol^-1, becoming less exothermic as the OH coverage increases. These values are in reasonable agreement with experimental data and calculated values for the same reaction in gas phase. The adsorption entropy for OH adsorption on Pt(111) ranges from −200 J mol^-1 K^-1 (low coverage) to −110 J mol^-1 K^-1 (high coverage). On the other hand, the enthalpy and entropy of hydroxyl adsorption on Pt(100) are less sensitive to coverage variations, with values ca. ΔH° = −280 kJ mol^-1 and ΔS° = −180 J mol^-1 K^-1. The different dependence of ΔS° with coverage on both electrode surfaces stresses the important effect of the substrate symmetry on the mobility of adsorbed OH species within the water network directly attached to the metal surface.