Hydrogen Oxidation Reaction on Pt in Acidic Media:  Adsorption Isotherm and Activation Free Energies

We present a method for quantifying the kinetic current of the hydrogen oxidation reaction (HOR) on Pt in acidic media using the adsorption and activation free energies as the intrinsic kinetic parameters. Our approach involves deriving the HOR intermediate's adsorption isotherm based on the same dual-pathway reaction model as for the kinetic equation. Coupled with density functional theory calculations of the coverage- and site-dependent hydrogen dissociation free energies, we analyzed the adsorption isotherms for H on atop (H_Atop) sites and in hollow/bridge (H_H/B) sites obtained respectively from infrared spectroscopic and electrochemical measurements. Although the hydrogen dissociative adsorption energy is not site-sensitive on Pt, H_Atop has an order of magnitude lower coverage and higher activity than H_H/B. We attributed this distinction to their different entropies; H_H/B is less accessible to water than H_Atop, and the formation of a hydrogen bond is essential in the oxidative generation of H₃O⁺. Using volcano plots, we illustrated possible ways that the exchange current may vary with the adsorption free energy and discussed the factors that contributed to the exceptionally high activity of Pt for the HOR.