| Summary: | The experimental and theoretical research on the hydrogen under-potential deposition (UPD) on Rh and Pt electrodes has led to determination of the Gibbs free energy ([delta]G[subscript ads]), the standard entropy ([delta]S[omicron][subscript ads]) and the standard enthalpy ([delta]H[omicron][subscript ads]) of adsorption based on a general adsorption isotherm rederived in the course of this work. The research has resulted in development of theoretical methodology allowing to evaluate, for the first time, the bond energy between the metal substrate, M, and the under-potential deposited H (H[subscript UPD]), E[subscript M]-[subscript H][subscript UPD] (here M = Pt or Rh). The impact of the specifically adsorbed anions on [delta]G[subscript ads], [delta]S[omicron][subscript ads] and [delta]H[omicron][subscript ads] has also been studied. Theoretical treatment of the experimental data based on the general electrochemical adsorption isotherm has allowed elucidation of [delta]G[subscript ads] as a function of the temperature, T, and the H surface coverage, [thêta][subscript H][subscript UPD]. The experimentally evaluated value of E[subscript M]-[subscript H][subscript UPD] is close to that of the bond energy between M and the chemisorbed H (H[subscript chem]). Proximity of the magnitude of E[subscript M]-[subscript H][subscript UPD] to that of E[subscript M]-[subscript H][subscript chem] (here M = Pt or Rh) points to the similar binding mechanism of H under the conditions involving presence of the electrified solid/liquid interface. Closeness of E[subscript M]-[subscript H][subscript UPD] to E[subscript M]-[subscript H][subscript chem] might also point to the same adsorption site of the two distinct H surface species, thus indicating that H[subscript UPD] alike H[subscript chem] is strongly embedded in the surface lattice of the Rh and Pt substrates. Temperature-dependent research on the UPD H and anion adsorption on Pt(111) and Pt(100) in 0.5 M aqueous H[subscript 2]SO[subscript 4] solution by cyclic-voltammetry, cv, conducted for the first time ever, indicates that the overall adsorption/desorption charge density is affected by temperature variation. An analysis of the [delta]G[subscript ads] (H [subscript UPD]) versus [thêta][subscript H][subscript UPD] plots reveal that the UPD H follows the Frumkin isotherm and the energy of lateral repulsions, [oméga], and the respective dimensionless parameter g are coverage independent. The value of E[subscript Pt(111)]-[subscript H][subscript UPD] is close to the surface bond energy between the chemisorbed H, H[subscript chem], and Pt(111), E[subscript Pt(111)]-[subscript H][subscript chem]. The influence of submonolayers of chemisorbed sulfur, S [subscript chem], on the UPD H on Pt(poly) and Pt(111) electrodes was studied. The research has demonstrated that a monolayer of S [subscript chem] on Pt can be formed by its immersion in aqueous Na[subscript 2]S solution. The S[subscript chem] can be gradually removed through oxidative desorption and its coverage can be controlled with the precision of some 1-2% of a mono layer. Theoretical treatment of the experimental results indicates that a submonolayer of S[subscript chem] having the nominal coverage of 0.10 increases the Gibbs free energy, the entropy and the enthalpy of adsorption for H[subscript UPD]. The bond energy between Pt and H[subscript UPD] decreases in presence of S[subscript chem] and the effect is brought about by local electron withdrawing effects that propagate through the underlying metal which acts as a mediator."--Résumé abrégé par UMI
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