The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase

The thermodynamic state of H-2 adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H-2 reacting with D2O to HDO, HD, and D-2, and by DFT-based ab initio molecular dynamics simulations of H-2 adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H-2 on Pt is significantly weakened in the aqueous phase compared to adsorption at gas-solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19-22 kJ molH2-1 while inducing an additional entropy loss of 50-70 J molH2-1 K-1. Upon dissociative adsorption of H-2, the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H-Pt bond strength.