ENERGETICS OF THE UNDERPOTENTIAL DEPOSITION OF HYDROGEN ON PLATINUM-ELECTRODES .1. ABSENCE OF COADSORBED SPECIES

Research on the underpotential deposition of hydrogen (UPD H) on Pt el ectrodes by cyclic voltammetry in 0.05, 0.10, and 0.50 M aqueous H2SO4 solutions at temperatures from 273 to 343 K shows that the adsorption -desorption profiles shift toward less positive values upon temperatur e (T) increase. Treatment of the experimental results based on a gener al electrochemical adsorption isotherm allows determination of Delta G (ads)degrees, (H-UPD) as a function of T and the H-UPD surface coverag e, theta(HUPD); Delta G(ads)degrees(H-UPD) varies from -11 to -25 kJ m ol(-1). The relation between Delta G(ads)degrees(H-UPD) and T for thet a(H?(UPD)) = const allows determination of Delta S(ads)degrees(H-UPD), which is from -40 to -90 J mol(-1) K-1. Subsequently, Delta H(ads)deg rees(H-UPD) is determined and varies from -27 to 46 kJ mol(-1). An ana lysis of the values of Delta H(ads)degrees(HUPD) and Delta S(ads)degre es(H-UPD) leads to conclusion that the UPD H between Pt and H-UPD, EPt -HUPD, which is from 245 to 265 kJ mol(-1). The value of EPt-H?(UPD) f alls close to that of the bond energy between Pt and H-chem, EPt-Hchem , which varies from 243 to 247 kJ mol(-1). Proximity of EPt-HUPD to EP t-Hchem indicates that H-UPD, and H-chem have a similar binding mechan ism in presence of the electrified solid/liquid and under gas-phase co nditions; it shows that H-UPD and H-chem might occupy the same adsorpt ion sites, thus indicating that H-UPD, like H-chem, is strongly embedd ed in the surface lattice of the Pt substrate.