Hydrogen oxidation kinetics on model Pd/C electrodes: Electrochemical impedance spectroscopy and rotating disk electrode study

This work reports on the kinetics of the hydrogen oxidation reaction (HOR) on model Pd nanoparticles supported on a low surface area carbon substrate. Two Pd/C samples, with the average particle size 2.6 and 4.0 nm were used. The structure of the catalysts was characterized with the ex situ (electron microscopy) and in situ (electrochemical) methods. We utilized the electrochemical impedance spectroscopy (EIS) and the rotating disk electrode (RDE) voltammetry to study the kinetics of the HOR on Pd/C. The relevance of these techniques for elucidating the kinetics and the mechanism of the HOR on Pd/C was explored. The experimental results suggest that the catalytic activity of Pd in the HOR is more than 2 orders of magnitude lower than that of Pt, and does not depend on the particle size in the range from 2.6 to 4.0 nm. Computational modeling of the experimental steady-state (RDE) and non-steady-state (EIS) data shows that the reaction kinetics can be adequately described within Heyrovsky-Volmer mechanism, with the rate constants nu(OH) = (8.8 +/- 1.5) x 10(-10) mol cm(-2) s(-1) and nu(OV) = (1.0 +/- 0.3) x 10(-8) mol cm(-2) s(-1). The model suggests that underpotentially deposited hydrogen H-UPD is unlikely to be the active intermediate H-ad of the HOR. It is concluded that the surface coverage of H-ad deviates from that of H-UPD with increasing overpotential, and the lateral interactions within H-ad adlayer are weak. (C) 2010 Elsevier Ltd. All rights reserved.