Density-Functional Analysis of Hydrogen on Pt(111): Electric Field, Solvent, and Coverage Effects

Density-functional theory is utilized to study the energetics and vibrational properties of hydrogen on the Pt(111) surface in order to clarify the adsorption state of hydrogen on a Pt electrode in an electrochemical condition. Particular attention is paid to the Pt−H stretching frequency (νPt−H) of hydrogen on the atop site, which is often referred to as overpotentially deposited hydrogen and considered to be the reaction intermediate of the hydrogen evolution reaction. We investigate the origin of the large potential dependence of νPt−H observed in the electrochemical experiments by taking into account the effects of electric field, solvent, and hydrogen coverage to simulate water/metal electrode interfaces realistically. The electric field effect on νPt−H without water solvent, the Stark tuning rate, is less than 20 cm−1·V−1. Although it is increased by a factor of 2.5 by taking into account the solvent effect, the electric field effect alone cannot account for the experimentally observed large potential-dependent frequency shift. It is found that the coverage effect on νPt−H is significant indicating that the electric field, solvent, and hydrogen coverage effects should be taken into account to explain fully the experimentally observed large frequency dependence on the electrode potential. The large hydrogen coverage effect on the vibration frequency shift is attributed to the shift of the d-band center due to the hybridization between the hydrogen s state and the substrate d-band.