The Phase‐Shift Method for the Frumkin Adsorption Isotherms at the Pd /  H 2 SO 4 and KOH Solution Interfaces

Abstract

The phase‐shift method for the Frumkin adsorption isotherms at the Pd/acidic and alkaline (KOH) solution interfaces has been studied using an ac impedance method. The phase shift (φ) depends on the cathode potential (E < 0) and is inversely proportional to the fractional surface coverage (θ) of the adsorbed hydrogen atom . The phase‐shift profile (φ vs. E) at the intermediate frequencies (ca. 130 Hz for the 0.1 M solution and ca. 63 Hz for the 0.1 M KOH solution) can be used as an experimental method to estimate or plot the Frumkin adsorption isotherm (θ vs. E). The rates (r) of change of the free energy of adsorption with θ at the Pd/0.1 M and 0.1 M KOH solution interfaces are 7.4 and 49.6 kJ/mol, respectively. The equilibrium constant (K) and the standard free energy (ΔGθ) of at the Pd/0.1 M solution interface are and 35.9 > ΔGθ > 28.5 kJ/mol, respectively. The equilibrium constant (K) and the standard free energy (ΔGθ) of at the Pd/0.1 M KOH solution interface are and 83.8 > ΔGθ > 34.2 kJ/mol, respectively. The electrode kinetic parameters (r, K, ΔGθ) depend on θ (0 < θ < 1).