Journal
SURFACE SCIENCE
Volume 465, Issue 1-2, Pages 103-114Publisher
ELSEVIER
DOI: 10.1016/S0039-6028(00)00674-9
Keywords
catalysis; electrochemical methods; epitaxy; palladium; platinum; solid-liquid interfaces; surface chemical reaction; surface structure, morphology, roughness, and topography; X-ray scattering, diffraction, and reflection
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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.
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