Ad-Atoms and Underpotential Deposition

In the previous chapter, the equilibrium between the solid and the liquid phase was described. The interface between the two bulk phases is the mediator and any processes between the two neighboring phases wiU be influenced by the properties of this interface. In this chapter the properties of this interface and methods of investigation will be described. 

---

The importance of the oxidation of Ci molecules in electrochemical energy conversion has prompted intense research efforts to find new more active electrocatalysts or to improve the activity of existing catalysts, for instance, platinum. Efforts to enhance the catalytic properties of metal electrodes toward C electrooxidation include both bulk alloys and pure metal surfaces modified by ad-atoms (modifiers), usually deposited in the underpotential region. 

Underpotential deposition Underpotential deposition (upd) occurs when monolayers (or submonolayers) of a metal ad-atom are deposited on a foreign metal substrate at potentials positive of the reversible Nernst potential for bulk deposition [16]. Monolayers will only form when a low work function metal is deposited onto the surface of a higher work function substrate. In this case, the metal ad-atom-substrate bond is greater than the ad-atom-ad-atom bond formed in bulk metal deposition. Upd phenomena have been the subject of extensive work using SPMs and of particular interest is the role of coadsorbed anions on this process, as a function of electrode potential. 

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. 

Modified electrodes with ad-atoms (usually deposited in the underpotential region) exhibit enhanced electrocatalytic activity for several categories of electrochemical reactions. The most extensively studied reactions are those related to the development of low-temperature fuel cell technology, namely, the reduction of oxygen and the oxidation of organic fuels. The ad-atoms may influence the rate and the mechanism of electrochemical reactions through [26-29]... 

Ag, Au, and Pt electrodes has been studied during the last two decades [55-61]. It was found that ad-atom layers of Pb, Tl, and Bi deposited at underpotentials have a strong influence on the performance of the above substrates for the nitro and nitroso group electroreduction, both in terms of the improvement of the catalytic activity and the catalytic selectivity of these electrodes. 

Reduction of heterocyclic nitro compounds on Au/M(upd) Overlayers of Pb and T1 ad-atoms, deposited on Au in the underpotential region, exert pronounced catalytic effects on the reduction of some iV-heterocyclic nitro compounds (56, 60, 61],... 

Generally, submonolayers of ad-atoms of heavy metals (such as Pb, Tl, Bi, Cd, etc.) deposited in the underpotential region increase the catalytic activity of Pt toward the electrooxidation of alcohols and sugars both in aqueous, acid, and alkaline solutions. The catalytic effect is more... 

Underpotentially deposited ad-atom layers of Pb, Tl, and Bi improve markedly the reversibility of the redox systems hydroquinone/p-benzoquinone [119, 120], pyrocatechol/o-benzoquinone [120], adrenaline/adrenalinequinone [121], hexa-hydroxybenzene/tetrahydroxy-1,4-benzoq-uinone [122], p-benzoquinone dioxime/ p-dinitrosobenzene [123], as well as the... 

---