Underpotential deposition of hydrogen on MoPt 4 intermetallic phase in acid solution: Temperature dependence

Abstract

Potentiodynamic studies of the underpotential deposition of hydrogen (H upd ) on MoPt 4 electrode in 0.5 mol dm -3 HClO 4 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 H upd electrochemical adsorption isotherm. It is determined that ΔG ads θ (H upd ), as a function of temperature, varies with the surface coverage from ~-20 kJ mol -1 (θ=0). The increase of ΔG ads θ (H upd ) with the surface coverage indicates the repulsive interactions between H upd 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 ΔH ads(θ=0) θ =4.5 kJ mol -1 and ΔS ads(θ=0) θ =82 J mol -1 K -1 . The value of ΔH ads θ allows determinations of the bond energy between electrode surface and H upd , which is found to be E M-H =213 kJ mol -1 for θ=0. The lateral repulsion interactions are the reasons why M-H upd bond energy decreases significantly with the increase of coverage so the saturation coverage is much less than 1 in the UPD potential region.