Hydrogen Adsorption over Transition Metals in Water

The adsorption free energy of atomic hydrogen on Pt(111), Pd(111), Ni(111), Ru(0001), Cu(111), and Rh(111) in liquid water was computed by using a quantum mechanical/molecular mechanical free-energy perturbation scheme. The Pt(111) computations indicate that the solvent effect on H adsorption on atop sites (+0.20 eV) is almost twice that on fcc (+0.12 eV), showing that it is less likely to find adsorbed hydrogen in the atop position in the presence of water than in the gas phase. The solvent effect for the fcc site, which is the most favorable site for adsorbed H on Pt(111), agrees qualitatively with experimental work by Lercher et al., who reported an effect of +0.2 eV. Overall, an endergonic solvent effect for hydrogen adsorption is observed for all metals, indicating a lower hydrogen coverage relative to free site coverage at metal-water interfaces compared to metal-gas interfaces, even when hydrogen transport effects through the fluid phase are negligible, a result with important implications for (de)hydrogenation catalysis.