Underpotential deposition as two-dimensional phase formation

The thermodynamic description of UPD is based on a deposition of metal ions with total or partial discharge of the ions as was formulated in Eq. 4.48. In the broad range of publications on UPD the interpretation of the effect concentrated primarily on an adsorption 

An alternative view on the UPD process provides the concept of substrate supported low-dimensional phase formation from Staikov, Budevski and LorenzThis concept explains unexpected stabilities of underpotential deposits. In this concept the UPD layer is considered to be a two-dimensional phase stabilized by the bond between substrate and UPD metal. A linear phase is the adsorption of atoms on steps and a phase of zero dimension is a group of atoms around a kink site position or around a surface dislocation. This concept is used to explain the positively shifted potentials in the cyclic voltammogram of UPD. Higher stabUily of the low-dimensional phase is described by introducing activities of atoms in the substrate-supported phase of lower dimension OMe,od Me,id d The deposition potential is given by the equation 

The index i either is 2, 1, or 0. The activity of metal atoms in the lower dimensional phase is smaller than the activity in a higher dimensional phase and in the bulk phase, where Me 3d partial discharge connected with each step is described by Az- 

Based on this concept, thermodynamic functions of the UPD phase can be determined. 

The formation reaction of the UPD modification of metal B on a substrate metal A is obtained by the following reaction scheme 

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