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 Hupd and OHad 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 ϑHupd, from ∼42 kJ/mol at ϑHupd = 0 ML to ∼24 kJ/mol at ϑHupd = 0.66 ML. The heat of adsorption of OHad is more sensitive to the nature of the anion in the supporting electrolyte. This is presumably due to coadsorption of the anion and OHad in electrolytes other than the simple alkali bases. From the isosteric heat of adsorption of OHad in alkaline solution (ca. ∼200 kJ/mol) and the enthalpy of formation of OH• we estimated the Pt(111)−OHad bond energy of 136 kJ/mol. This value is much smaller than the Pt−Oad 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 COad and OHad, 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.