Determination of the Energy of the Metal−Underpotential-Deposited Hydrogen Bond for Rhodium Electrodes

Investigation of the underpotential deposition of hydrogen (UPD H) on Rh electrodes in 0.05, 0.10, and 0.50 M aqueous solutions of H2SO4 in the 273−343 K temperature range by cyclic voltammetry (CV) demonstrates that upon temperature increase the CV profiles shift toward less-positive values. The CV hydrogen adsorption/desorption diagrams are symmetric with respect to the potential axis, indicating that the UPD H is a reversible process. Theoretical treatment of the experimental data based on an electrochemical adsorption isotherm allows determination of the Gibbs free energy of adsorption, ΔGads°(HUPD), as a function of temperature and the H surface coverage; it varies between −8 and −18 kJ mol-1. Temperature dependence of ΔGads°(HUPD) for a constant surface coverage of the underpotential-deposited H (HUPD) allows determination of the standard entropy of adsorption, ΔSads°(HUPD), which is found to be between −15 and −125 J mol-1 K-1. Subsequently, ΔHads°(HUPD) is determined to be between −15 and −52 kJ mol-1. An analysis of the values of ΔHads°(HUPD) and ΔSads°(HUPD) leads to the conclusion that the UPD H is an enthalpy-driven process. Knowledge of ΔHads°(HUPD) leads to determination of the bond energy between Rh and HUPD, ERh-HUPD, which is between 230 and 270 kJ mol-1 depending on the HUPD surface coverage (θHUPD). The value of ERh-HUPD is close to that of the bond energy between Rh and the H chemisorbed from the gas phase (Hchem), ERh-Hchem, which equals 255 kJ mol-1. Proximity of the magnitude of ERh-HUPD to that of ERh-Hchem points to a similar binding mechanism of H under the conditions involving the presence of the electrified solid/liquid interface. Closeness of ERh-HUPD to ERh-Hchem also points to the same adsorption site of HUPD and Hchem indicating that they are strongly embedded in the surface lattice of the Rh substrate. Finally, proximity of ERh-HUPD to ERh-Hchem indicates that HUPD and Hchem are equivalent surface species.