Direct estimation of surface pressures by hydrogen adsorbates on platinum surfaces in perchloric acid

The surface pressure solely by hydrogen species at polycrystalline and single crystalline platinum is calculated from a modified Gibbs-Lippmann Equation taking the contributions of electrode potential and hydrogen coverage in perchloric acid. Hydrogen surface pressures at reference surface coverages are rather low for Pt(100) and Pt(110), i.e., 20-35 μN cm− 1, but 5-fold larger for Pt(111) due the formation of long range water-hydrogen layers on trigonal sites. Surface tensions also follow the same tendencies. On polycrystalline platinum they are sensibly inferior to those obtained on single crystals but similar in magnitude for both types of hydrogens indicating that the presence of water in weakly bound hydrogen responds to a slight interaction (not bound to platinum atoms).Free energy of adsorption for hydrogen ad-atoms on polycrystalline platinum is reported here at experimentally found reference coverages, i.e. − 29 and − 14 kJ mol− 1 for both kinds of hydrogens in perchloric acid. The interaction/heterogeneity factor of the Frumkin/Temkin isotherm, which are similar (ca. 11-12 kJ mol− 1) for both types of hydrogens, again verifies that water interaction is not straight to platinum atoms. On single crystalline surfaces, the most favorable value of adsorption energy was found on Pt(100) and the lowest on Pt(111). The three single crystals exhibit positive interaction/heterogeneity factors (from 11 to 18 kJ mol− 1) being the greatest on Pt(110) as a consequence of being the most disordered surface.

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