Underpotential Deposition of Metals on Foreign Substrates

The development of the ultrasensitive potential sweep technique, capable of detecting submonolayer amounts of substance on electrode surfaces and its application to metal deposition studies, resulted in detailed investigations of the phenomenon of deposition of metals on foreign substrates at potentials more positive than the thermodynamic reversible potential for the given conditions.This phenomenon has been termed underpotential deposition (UPD). 

This should be distinguished from findings about the incorporation of alkali metals into the lattices of some other metals (e.g., lead, aluminum, zinc) occurring at potentials more than 1 V more positive than the normally expected deposition potentials of alkali metals, which represents a form of surface alloying (cf. amalgams). 

The UPD is usually limited to a monolayer of metal (although there are certain indications that, in some cases, it can extend to 2 -- 3 monolayers) and as soon as larger amounts are deposited, bulk metal phase is formed with a resulting shift of the potential to normal values. Sometimes a partial second layer is distinguished, e.g., on Pt. 

For lead with n = 12, the bond energy between atoms is obtained as 13.4 kJ mol while that between lead and gold atoms (with m = 4) has been estimated to be 33.4 kJ moF  

This argument assumes that adatoms on the surface do not carry any charge and hence are neither complexed nor solvated. In the case of lead, this assumption seems to be fulfilled, since the so-called electrosorption valency equals 2, i.e., the ideal normal valency of the discharging plumbous ion. However, in most cases, this assumption is not quite fulfilled. The electrosorption valency differs usually from the full valency of the metal ion and, hence, some attraction from the solution side partly compensates the attractive force exerted by the substrate atoms. Thus, in such cases, the latter should actually be somewhat larger than could be calculated from Eq. (13). 

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