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

The surface electrochemistry of adsorbed hydrogen (Hupd) 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 (Hupd), it appears that some of the Hupd on Pt(111)Pd may be in the subsurface state. This change in the energetics of the Hupd state plays a dominant role in the kinetics of the HER/HOR. 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 (Hopd) and the Volmer–Heyrowsky sequence, the Heyrowsky step being the rds.