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 on an epitaxial Pd layer on Pt(111) in 0.05 M H2SO4 in a temperature range of 278-333 K. The morphology and stability of the Pd film during the electrochemical reactions was investigated by means of in situ surface X-ray scattering (SXS). The SXS results show that Pd is deposited onto Pt(111) as a uniform epitaxial metallic layer having the Pt lattice constant, that is, pseudomorphic growth. Due to the strong interaction between the Pd film and adsorbed hydrogen (H-upd), it appears that some of the H-upd on Pt(111)-Pd may be in the subsurface state. This change in the energetics of the H-upd state plays a dominant role in the kinetics of the HER/HOP. The kinetics of the HER/HOR were studied on Pt(111) and Pt(111)-Pd by utilizing the rotating disk electrode method. We find significant differences in the electrochemical properties between these two systems, the rate of reaction being much faster 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 overpotentials on Pt(111)-Pd is one which follows application of the Langmuir (ideal) adsorption isotherm for the reaction intermediate (H-opd) and the Volmer-Heyrowsky sequence, the Heyrowsky step being the rds. (C) 2000 Elsevier Science B.V. All rights reserved.