Underpotential alloy formation in the system Ag( hkl)/Cd 2+

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 AgCd 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 AgCd 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 AgCd/Cd 2+.