HYDROGEN ADSORPTION ON PT AND RH ELECTRODES AND BLOCKING OF ADSORPTION SITES BY CHEMISORBED SULFUR

Research on the under-potential deposition of H, UPD H, on Pt and Rh e lectrodes in aqueous H2SO4 solution at temperatures between 273 and 34 3 K by cyclic-voltammetry followed by theoretical treatment leads to d etermination of Delta G(ads)(degrees), Delta S-ads(degrees) and Delta H-ads(degrees) and the bond energy between the metal substrate, M, and H-UPD, EM-HUPD. Knowledge of EM-HUPD results in elucidation of the su rface adsorption site of H-UPD based on thermodynamic deliberation. Th e UPD H on Pt is suppressed completely by a monolayer of chemisorbed S , S-chem, or partially by a submonolayer of S-chem having its surface coverage less than 0.33, theta S<0.33. A submonolayer of S-chem having theta(S)=0.10 affects Delta G(ads)degrees, Delta S-ads(degrees) and D elta H-ads(degrees) the Pt-H bond energy, EPt-HUPD, becomes weaker in presence of the S-chem submonolayer. The ads lateral interactions betw een H-UPD and S-chem are brought about by local electron withdrawing e ffects that propagate through the underlying and S metal which acts as a mediator.