THE PHASE-SHIFT METHOD FOR THE FRUMKIN ADSORPTION-ISOTHERMS AT THE PDH2SO4 AND KOH SOLUTION INTERFACES/

The phase-shift method for the Frumkin adsorption isotherms at the Pd/ acidic (H2SO4) and alkaline (KOH) solution interfaces has been studied using an ac impedance method. The phase shift (phi) depends on the ca thode potential (E < 0) and is inversely proportional to the fractiona l surface coverage (theta) of the adsorbed hydrogen atom (H-ads). The phase-shift profile (phi vs. E) at the intermediate frequencies (ca. 1 30 Hz for the 0.1 M H2SO4 solution and ca. 63 Hz for the 0.1 M KOH sol ution) can be used as an experimental method to estimate or plot the F rumkin adsorption isotherm (theta vs. E). The rates (r) of change of t he free energy of adsorption with theta at the Pd/0.1 M H2SO4 and 0.1 M KOH solution interfaces are 7.4 and 49.6 kJ/mol, respectively. The e quilibrium constant (K) and the standard free energy (Delta G(0)) of H -ads at the Pd/0.1 M H2SO4 solution interface are 5.0 x 10(-7) < K < 1 .0 x 10(-5) and 35.9 > Delta G(0) > 28.5 kJ/mol, respectively. The equ ilibrium constant (K) and the standard free energy (Delta G(0)) of H-a ds at the Pd/0.1 M KOH solution interface are 2.1 x 10(-15) < K < 1.0 x 10(-6) and 83.8 > Delta G(0) > 34.2 kJ/mol, respectively. The electr ode kinetic parameters (r, K, Delta G(0)) depend on theta (0 < theta < 1).