Evaluation of the effect of two-dimensional geometry of pt single-crystal faces on the kinetics of upd of h using impedance spectroscopy

Abstract

Coverage of several of the noble metals by H in the sub-Nernstian potential range is a well known and relatively well characterized process. However, direct kinetic studies of such underpotential deposition (upd) processes are made difficult by their high rate constants. One of the most sensitive techniques available that can allow kinetic studies of the upd of H to be made is a.c. impedance spectroscopy. The principal aims of the present work were to examine the dependence of the kinetics of the charge transfer on various crystal-plane geometries of single-crystal surfaces of Pt through evaluation of the charge-transfer resistance R ct , leading to the deposition of H and how this process depends on the orientation and two-dimensional structure of the surface. Analysis of the a.c. results by means of equivalent circuit fitting allows determination of the double-layer capacitance C dl and the H adsorption pseudocapacitance C p , which is determined by H coverage in the upd range, as well as the kinetics of the charge transfer as a function of the potential. Very good correlation between the results for total capacitance ( C dl , + C p ) determined by cyclic voltammetry and by impedance measurements was obtained for Pt(100), (111), (110) and (311) surfaces in 0.5 mol dm −3 H 2SO 4 solution, with allowance being made, where appropriate, for HSO 4 − co-adsorption. The variation of parameters with lowering of the temperature was also investigated for Pt(100) in 0.5 mol dm −3 H 2SO 4. This allowed determination of the apparent activation energy for this process. The important influence of the geometry of the crystal face on the kinetics of the upd process and on the thermodynamics of the interfacial process ( C dl and C p ) is also discussed.