Determination of equilibrium isotope effect at Pd/alkaline solution (regular and heavy water) interfaces by the phase-shift method and its comparison with other Pt-group metals

Abstract

The Frumkin isotherm provides information about the atomic behaviors of hydrogen (H) and deuterium (D) on the electrocatalyst surface. However, it cannot be obtained easily by the conventional electrochemical methods, owing to the similar electrochemical behaviors of the under- and over-potential deposition of H and D (UPD H, OPD H, UPD D, and OPD D) and the numerous overlapping factors at the electrocatalyst interface. In this study, the Frumkin isotherms for the discharge reactions of the OPD H and OPD D in the Volmer steps, isotopic shift of these isotherms, and equilibrium isotope effect (EIE) at the interfaces of Pd/alkaline (regular and heavy water: H2O and D2O) solutions are precisely determined by the phase-shift method. The interaction parameter (ɡ) and equilibrium constant (K) are 6.6 and 9.7 × 10−6 exp (−6.6θ) mol−1, respectively, for OPD H and 8.7 and 1.1 × 10−6 exp (−8.7θ) mol−1, respectively, for OPD D, where θ is the fractional coverage (0 ≤ θ ≤ 1). In the range of θ (0 ≤ θ ≤ 1), the standard Gibbs energy (ΔGθ0) of OPD D is 5.4–10.6 kJ mol−1 higher than that of OPD H. The EIE values increase from 8.8 to 72.1 with increasing θ and are extraordinarily high compared to those at other Pt-group metal interfaces. The EIE values at the interfaces of Pt-group metal/alkaline (H2O and D2O) solutions decrease in the following order: Pd >> Pt–Ir alloy > Pt > Ru > Ir and Rh. We expect that the Frumkin isotherms of OPD H and OPD D and the related EIE established in this study will be a useful and effective tool for studying the isotope effects at various electrocatalyst interfaces.