Valence underpotential deposition

Fig. 32. Valence band spectra (UPS) of a polycrystalline Pt electrode emersed at a different potential where underpotential deposition of Cu (0.3V, 0.1 V) and bulk deposition (—0.2 V) of Cu occurs. Clean Pt and Cu surfaces are shown for comparison.

Chapter 3, by Rolando Guidelli, deals with another aspect of major fundamental interest, the process of electrosorption at electrodes, a topic central to electrochemical surface science Electrosorption Valency and Partial Charge Transfer. Thermodynamic examination of electrochemical adsorption of anions and atomic species, e.g. as in underpotential deposition of H and metal adatoms at noble metals, enables details of the state of polarity of electrosorbed species at metal interfaces to be deduced. The bases and results of studies in this field are treated in depth in this chapter and important relations to surface -potential changes at metals, studied in the gas-phase under high-vacuum conditions, will be recognized. Results obtained in this field of research have significant relevance to behavior of species involved in electrocatalysis, e.g. in fuel-cells, as treated in chapter 4, and in electrodeposition of metals. 

Mechanisms of electrochemical and photoelectrochemical deposition of metal selenide clusters (Me = Pb, Cd, Zn, Bi, In) onto the surface as well as into the selenium films have been studied. These clusters are formed as a result of underpotential and overpotential deposition of the metals onto Se. Photoinduced underpotential deposition of Bi onto Se was used to cover selenium colloidal particles with BiiSes clusters. The PbSe and Bi2Sc3 clusters modify the Se surface and form electronic surface states in the Se bandgap, thus promoting electron exchange processes between the valence band and redox species in solution and the rise of the subbandgap photocurrent. 

From theoretical considerations of bond formation in electrosorbates it was concluded that the underpotentially deposited metal ad-atoms can approximately be considered to be covalently bonded and nearly completely discharged if the difference in the Pauling s electronegativities of the substrate and adsorbed metal is I Ax I < 0.5 [6]. That means these systems have electrosorption valency i/z 1. Systems in which the electrosorption valency is equal to the ionic charge are those resulting from the UPD of heavy metal ions on noble and transition metal electrodes. 

In some electrode reactions it may be appropriate to consider that partial charge transfer occurs. In the case of the underpotential deposition of metals, the electrode process leads to the formation of a monolayer or sub-monolayer of metal atoms which interact with the substrate. In the one extreme, the metal-metal bond may have considerable ionic character, so that the reaction approaches ion adsorption, whereas at the other limit an essentially electroneutral surface atom is formed. These complex electrode processes have been discussed extensively in recent years, and the concept of the electrosorption valency [30] has emerged as a way of describing the nature of the under-potential deposit. 

Cd deposited underpotentially on Au(lll) reaches a limiting coverage of 0.66 monolayer. Electrosorption valency corresponding to this coverage is equal to 0.5, which indicates that Cd adatoms are not fully discharged at the surface [427]. 

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