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) demonstrate s that upon temperature increase the CV profiles shift toward less-pos itive values. The CV hydrogen adsorption/desorption diagrams are symme tric with respect to the potential axis, indicating that the UPD H is a reversible process. Theoretical treatment of the experimental data b ased on an electrochemical adsorption isotherm allows determination of the Gibbs free energy of adsorption, Delta G(ads)degrees(H-UPD), as a function of temperature and the H surface coverage; it varies between -8 and -18 kJ mol(-1). Temperature dependence of Delta G(ads)degrees( H-UPD) for a constant surface coverage of the underpotential-deposited H (H-UPD) allows determination of the standard entropy of adsorption, Delta S(ads)degrees(H-UPD), which is found to be between -15 and -125 J mol(-1) K-1. Subsequently, Delta H(ads)degrees(H-UPD) is determined to be between -15 and -52 kT mol(-1). An analysis of the values of De lta H(ads)degrees(H-UPD) and Delta S(ads)degrees(H-UPD) leads to the c onclusion that the UPD H is an enthalpy-driven process. Knowledge of D elta H(ads)degrees(H-UPD) leads to determination of the bond energy be tween Rh and H-UPD, E(Rh-HUPD) Which is between 230 and 270 kJ mol(-1) depending on the H-UPD surface coverage (theta(HUPD)). The value of E (Rh-HUPD) is close to that of the bond energy between Rh and the H che misorbed from the gas phase (H-chem), E(Rh-Hchem), which equals 255 kJ mol(-1). Proximity of the magnitude of E(Rh-HUPD) to that of E(Rh-Hch em) points to a similar binding mechanism of H under the conditions in volving the presence of the electrified solid/liquid interface. Closen ess of E(Rh-HUPD) to E(Rh-Hchem) also points to the same adsorption si te of H-UPD and H-chem indicating that they are strongly embedded in t he surface lattice of the Rh substrate. Finally, proximity of E(Rh-HUP D) to E(Rh-Hchem) indicates that H-UPD and H-chem are equivalent surfa ce species.