Effect of Temperature on Surface Processes at the Pt(111) - Liquid Interface: Hydrogen Adsorption, Oxide Formation, and CO Oxidation

The variation of the adsorption pseudocapacitance with temperature is used to obtain the enthalpy, entropy, and free energies of adsorption of H_upd and OH_ad on Pt(111) as a function of pH and nature of the anion of the supporting electrolyte. It is shown that the heat (enthalpy) of adsorption of hydrogen on Pt(111) at the electrochemical interface is essentially independent of either the pH of the electrolyte or the nature of the supporting anion. The heat of adsorption has a linear decrease with Θ_Hupd, from ∼42 kJ/mol at Θ_Hupd = 0 ML to ∼24 kJ/mol at Θ_Hupd = 0.66 ML. The heat of adsorption of OH_ad is more sensitive to the nature of the anion in the supporting electrolyte. This is presumably due to coadsorption of the anion and OH_ad in electrolytes other than the simple alkali bases. From the isosteric heat of adsorption of OH_ad in alkaline solution (ca. ∼200 kJ/mol) and the enthalpy of formation of OH^• we estimated the Pt(111)-OH_ad bond energy of 136 kJ/mol. This value is much smaller than the Pt-O_ad bond energy at a gas-solid interface (∼350 kJ/mol). In basic solution the electrooxidation of CO proceeds at low overpotentials (<0.2 V) between the adsorbed states of CO_ad and OH_ad, the latter forming at low overpotentials selectively at defect sites. In acid solution, however, these sites are not active because they are blocked by specific adsorption of anions of the supporting electrolyte.