Anions specific adsorption

The process of monolayer deposition of metal ions in underpotential deposition is strongly affected by anion-specific adsorption, and the two processes at the electrode interface must be elucidated if one is to understand underpotential deposition phenomena in a unified way. 

As found in preceding sections, the underpotential deposition process is strongly influenced by anion adsorption, or controlled sometimes by the presence of the adsorbed anions. Anion-specific adsorption/desorption seems to take place as an electron transfer process and may be accompanied by the underpotential deposition formation/removal process. ... 

Induced adsorption — Enhancement of the - adsorption of a component induced by another adsorbed species. These phenomena are treated in terms of induced anion and cation adsorption. Interrelation of anionic -> specific adsorption and -> underpotential deposition of metal ions is a typical example for the induced adsorption of anions [i]. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Very interesting studies have been made of the pattern for UPD on single crystal metal substrates [49]. A commensurate pattern is usually observed for the formation of a partial monolayer. The theory of UPD and formation of the initial monolayer is an area of active research. Of course, the phenomenon of UPD is restricted to formation of a monolayer. Once this has formed, the deposition process reverts to one of metal ion M"+ on metal M. By comparing the UPD process with anion-specific adsorption, the role of partial charge transfer in these processes is clarified. 

In electrocatalysis, the structure sensitivity of an adsorption process at the platinum-electrolyte interface is evidenced by the effect of surface orientation on the distribution of the configuration states in current vs. potential profiles [1-4]. The electrolyte composition at constant pH affects these adsorption states because of the anion-specific adsorption [2],... 

In the case of Pt(l 11), voltammograms for flame-cleaned electrodes were obtained under UHV conditions [53] providing evidence that such voltammograms correspond to well-ordered (lxl) structures. Since the same voltammetric features were obtained with various surface preparation procedures, there is a large stability of the (111) (lxl) structure [60], Conversely, UHV studies have shown the existence of various surface structures for Pt(100) [42] or Pt(110) [61]. Such structural changes may influence the electrochemical response of these orientations. Similar to Pt(lll), voltammograms obtained with Pt(100) electrodes have been found to depart on the electrolyte composition that is, a decrease of the anion-specific adsorption shifts the hydrogen adsorption states to more positive potentials. 

In these simplest cases the evaluation of the specific adsorption is based on the assumption that one type of ion in the electrolyte is not involved in the specific adsorption. Mostly anionic specific adsorption is calculated assuming that the cation (often an alkali ion) is not specifically adsorbed. This means that the cation is only in the diffuse layer. Determining F( and r( from Eq. (37) (by measuring the interfacial tension), it is possible to calculate, from r(. the extent of ri, excess concentration of the anion in the diffuse layer, by the relationships of GC theory. 

Interrelation of Anion Specific Adsorption and UPD of Metal Ions The... 

Anion specific adsorption induced by adatoms. Figure 6 demonstrates the increase in the sulfate adsorption (determined by radiotracer technique) following the formation of Cd, Cu, Hg adlayer on a polycrystalline Au support [147]. 

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