ABSTRACT
ABSTRACT Kinetics of the hydrogen evolution/oxidation reaction (HE/OR) and the oxygen reduction reaction (ORR) on well-defined Pt(hkl) surfaces in aqueous solutions are reviewed. Surface x-ray scattering (SXS) was used to determine the structure in situ at Pt(hkl)-solution interfaces as a function of the applied potential. SXS measurements show that both Pt(111) and Pt(100) prepared by flame-annealing method are not reconstructed at any potential. Depending on the particular flame-annealing procedure, a reconstructed Pt(110)-(1 × 2) phase or an unreconstructed Pt(110)-(1 × 1) surface can be created. On all single-crystal surfaces, the lattice spacing between the first and the second layer was expanded relative to the bulk spacing, a phenomenon generally referred to as relaxation. The relaxation was found to be a function of both the crystallographic orientation and the electrode potential and is related to the adsorption of Hupd and/or anions. The rotating ring disk technique was used to establish the kinetics of the HE/OR and the ORR on Pt(hkl) single-crystal surfaces. An analyses of the structure sensitivity of the kinetics of the HE/OR and the ORR is presented based on the premise that the differences in the activities of the three low index faces of Pt arise mainly from the structure-sensitive adsorption of Hupd and/or anions, such as hydroxyl and (bi)sulfate. Finally, reaction mechanisms (reaction pathways) are proposed for the ORR and the HE/OR on Pt(hkl) single-crystal surfaces.
