The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase.

The thermodynamic state of H2 adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H2 reacting with D2 O to HDO, HD, and D2 , and by DFT-based ab initio molecular dynamics simulations of H2 adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H2 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  m o l H 2 - 1 while inducing an additional entropy loss of 50-70 J  m o l H 2 - 1  K-1 . Upon dissociative adsorption of H2 , 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.