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 a nd OHad on Pt(lll) as a function of pH and nature of the anion of the suppo rting electrolyte. It is shown that the heat (enthalpy) of adsorption of hy drogen on Pt(lll) at the electrochemical interface is essentially independe nt of either the pH of the electrolyte or the nature of the supporting anio n. The heat of adsorption has a linear decrease with Theta(Hupd,) from simi lar to 42 kJ/mol at Theta(Hupd) = 0 ML to similar to 24 kJ/mol at Theta(Hup d) = 0.66 ML. The heat of adsorption of OHad is more sensitive to the natur e of the anion in the supporting electrolyte. This is presumably due to coa dsorption of the anion and OHad in electrolytes other than the simple alkal i bases. From the isosteric heat of adsorption of OHad in alkaline solution (ca. similar to 200 kJ/mol) and the enthalpy of formation of OH. we estima ted the Pt(111)-OHad bond energy of 136 kJ/mol. This value is much. smaller than the Pt-O-ad bond energy at a gas-solid interface (similar to 350 kJ/m ol). In basic solution the electrooxidation of CO proceeds at low overpoten tials (<0.2 V) between the adsorbed states of COad and OHad, the latter for ming at low overpotentials selectively at defect sites. In acid solution, h owever; these sites are not active because they are blocked by specific ads orption of anions of the supporting electrolyte.