Surface electrochemistry on an epitaxial palladium film on Pt(111): surface microstructure and hydrogen electrode kinetics

The surface electrochemistry of adsorbed hydrogen (H-upd) and the kinetics of the hydrogen evolution/oxidation reactions (HER/HOR) have been studied o n an epitaxial Pd layer on Pt(111) in 0.05 M H2SO4 in a temperature range o f 278-333 K. The morphology and stability of the Pd film during the electro chemical reactions was investigated by means of in situ surface X-ray scatt ering (SXS). The SXS results show that Pd is deposited onto Pt(111) as a un iform epitaxial metallic layer having the Pt lattice constant, that is, pse udomorphic growth. Due to the strong interaction between the Pd film and ad sorbed hydrogen (H-upd), it appears that some of the H-upd on Pt(111)-Pd ma y be in the subsurface state. This change in the energetics of the H-upd st ate plays a dominant role in the kinetics of the HER/HOP. The kinetics of t he HER/HOR were studied on Pt(111) and Pt(111)-Pd by utilizing the rotating disk electrode method. We find significant differences in the electrochemi cal properties between these two systems, the rate of reaction being much f aster on the Pt(111)-Pd electrode. We propose that the physical model that appears to rationalize the results for the HER/HOR at low anodic overpotent ials on Pt(111)-Pd is one which follows application of the Langmuir (ideal) adsorption isotherm for the reaction intermediate (H-opd) and the Volmer-H eyrowsky sequence, the Heyrowsky step being the rds. (C) 2000 Elsevier Scie nce B.V. All rights reserved.