Underpotential deposition of hydrogen on MoPt4 intermetallic phase in acid solution: temperature dependence

Potentiodynamic studies of the underpotential deposition of hydrogen (Hupd) on MoPt4 electrode in 0.5 mol dm−3 HClO4 aqueous solution in the range of temperature from 275 to 312 K are made and thermodynamic state functions for the hydrogen adsorption are determined. Theoretical treatment of the experimental results is derived from Hupd electrochemical adsorption isotherm. It is determined that ΔGadsθ (Hupd), as a function of temperature, varies with the surface coverage from ∼−20 kJ mol−1 (θ=0). The increase of ΔGadsθ (Hupd) with the surface coverage indicates the repulsive interactions between Hupd adatoms. From the temperature dependence of the Gibbs energy of adsorption, the enthalpy and entropy of adsorption are calculated. The values of these functions are determined to be ΔHads(θ=0)θ=4.5 kJ mol−1 and ΔSads(θ=0)θ=82 J mol−1 K−1. The value of ΔHadsθ allows determinations of the bond energy between electrode surface and Hupd, which is found to be EM–H=213 kJ mol−1 for θ=0. The lateral repulsion interactions are the reasons why M–Hupd bond energy decreases significantly with the increase of coverage so the saturation coverage is much less than 1 in the UPD potential region.