Quantifying Adsorption of Organic Molecules on Platinum in Aqueous Phase by Hydrogen Site Blocking and in Situ X-ray Absorption Spectroscopy

The adsorption equilibrium constants of phenol, benzaldehyde, cyclohexanol, and benzyl alcohol on Pt in aqueous phase have been determined via cyclic voltammetry (CV) measuring the capacity to adsorb hydrogen in the presence of organic molecules. The enthalpies of adsorbed phenol on Pt in aqueous phase estimated by this technique were approximately −41 and −21 kJ/mol relative to aqueous phenol for (110)/(100)-like sites and (111) facets, respectively. The enthalpy of phenol aqueous adsorption on (111) (−21 kJ/mol) can be compared to the gas-phase enthalpy of adsorption of phenol on single-crystal Pt(111) from previous work (−200 kJ/mol) to understand the individual factors influencing adsorption in the liquid phase. The results show that adsorbates with very strong intrinsic bonding at a gas/solid interface have markedly reduced adsorption enthalpy due to strong solvation of both the Pt surface and the organic. Using the CV method, the decrease of the heat of adsorption in the sequence benzaldehyde > benzyl alcohol > phenol > cyclohexanol points to a strong interaction of the aromatic ring with the metal surface. Combining CV and X-ray absorption spectroscopy, we show that the inhibition of hydrogen adsorption by organic molecules is a suitable method of determining the coverage of organic reactants and that these molecules have defined Pt–C bonds in the adsorbed state.