Abstract
The underpotential deposition (UPD) of Cd on “real” and “quasi-ideal” silver single crystal surfaces of (111) and (100) crystallographic orientation as well as on polycrystalline silver substrate has been investigated in 0.5 M Na 2SO 4 supporting electrolyte solutions. At high underpotentials 100 < Δ E < 350 mV, the UPD is characterized by a quasi-reversible adsorption/desorption of Cd whereas at low underpotentials. Δ E < 50 mV, an increase of the anodic stripping charge with the polarization time is found due to the formation of an AgCd alloy at the substrate surface. The time dependence of this process can be described by a parabolic rate law, the rate constant of which is a function of Δ E and temperature T. Relatively low activation energies of about 70 kJmol −1 were determined from measurements at 293 ⩽ T ⩽ 338 K. The results are discussed in terms of a semi-infinite-linear diffusion model. The alloy formation process is assumed to be initiated by a place exchange between Ag substrate atoms and vacancy sites within a mobile Cd adsorbate layer thus forming the initial stage of a highly distorted AgCd alloy. The further growth will take place by the movement of Ag atoms through the vacancy-rich surface alloy and the simultaneous deposition of Cd at the interface AgCd/Cd 2+.
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