TEMPERATURE-DEPENDENT HYDROGEN ELECTROCHEMISTRY ON PLATINUM LOW-INDEXSINGLE-CRYSTAL SURFACES IN ACID-SOLUTIONS

The hydrogen evolution (HER) and the hydrogen oxidation reaction (HOR) were studied on platinum single crystals in a sulfuric acid solution over the temperature range 274-333 K. We found, for the first rime, th at at a fixed temperature (274 K) the exchange current densities (i(o) ) increase in the order (111) much less than (100) < (110), with the i (o) on the (110) surface being 3 times that on the (111) surface. We a lso found that each crystal face has an unique, temperature-dependent Tafel slope for the HOR, and that the activation energies for the HER and the HOR decrease in the sequence Delta H-111(#) > Delta H-100(#) > Delta H-110(#), the same sequence as the order these differences in a ctivation energy with crystal face are attributed to structure-sensiti ve heats of adsorption of the active intermediate, H-ad, whose physica l state is unclear. We analyzed the kinetic data with a model for the coupling of this unknown state, Had, with the well-known adsorbed stat e of hydrogen, H-upd, whose adsorption energy is strongly structure-se nsitive. We concluded that on Pt(110), the reaction follows the Tafel- Volmer mechanism with the Tafel (recombination) step rate determining. On Pt(100), the reaction follows the Heyrovsky-Volmer sequence, with the Heyrovsky (ion-atom) reaction step being the rate-determining step . The reaction mechanism on Pt(111) could not, however, be resolved by analyzing the kinetic parameters. The relatively low activity and hig h activation energy for the (111) surface is attributed to strong repu lsive interaction between H-ad adatoms on this surface.