Kinetic investigations of structural changes and desorption of metal adsorption layers on single crystal planes

Abstract

At the surface of noble metal electrodes, foreign metal ions from the solution (S) may be bound in a random layer (C), in an ordered structure (B) or in a condensed layer (A). Owing to structural differences, desorption processes include various diffusion steps (d) and the charge transfer (t). Models are discussed in analogy to electrocrystallization. Potentiostatic pulse measurements were carried out with the system Au(111)/Bi³⁺, for which different layers with the structure A, B and C exist. A charge transfer step is rate-determining in all cases.

The exchange current densities of the desorption of layer A(< 1 µA cm^–2), B (1 mA cm^–2) and C (10 mA cm^–2) increase with the distance of next neighbours in the adsorption layer. Hence, the desorption transients of all three layers differ and can easily be distinguished. At low potentials the desorption of A proceeds by a transfer of ions leaving a layer of B, but the intermediate formation of B cannot be proved at high potentials. The anodic dissolution of B starts at the edge of homogeneous islands of B. Atoms diffuse to the free surface and are then desorbed. Layers of B and C coexist not only at an unique equilibrium potential but in a range of less than 6 mV which can be explained by the non-ideal behaviour and the small size of areas with ordered structures of B.