Surface Redox Processes

In the previous section, the electrosorption of mobile ionic species onto an electrode surface was discussed. It is also possible to imagine the oxidation and reduction (with charge transfer) of immobile surface-bonded species. 

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For amperometric techniques at a fixed potential (e.g., liquid chromatography detectors), the classical charging current is zero since dE/dt = 0. There is still a background current, however, due to surface redox processes or slow... 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

BDD electrodes are semiconductor electrodes with microcrystalline structure and relatively rough surfaces on the micrometric scale. Diamond-coated electrodes used for disinfection are chemically, mechanically, and thermally very resistant and show very low corrosion even under high electric charge. Diamond electrodes present no surface redox processes as known from other carbon electrodes (for example glassy carbon). 

Electrochemical Behavior of Nitrite Ions. - The electrochemical behavior of NO2 ions in neutral, " alkaline, and acid media at platinum single crystal surfaces proved to be structure sensitive. In a phosphate-buffered medium, a surface redox process has been found for Pt(l 11) while no reaction was observed for Pt(lOO) or Pt(l 10). In acid medium the formation of an irreversible adlayer on Pt(l 10) surface was found. ... 

In this respect the special role of irreversible adsorption of foreign metal adatoms should be mentioned as this type of adsorption provides the possibihty of producing stable modified electrodes for electrocatalytic purposes (for instance Sn, Bi adatom modified electrodes). In some cases, the irreversibly adsorbed adatom undergoes well-defined surface redox processes that could be structure sensitive (for instance. As, Sb, and Bi adatoms on platinum substrates or two-dimensional Bi -i- As alloys supported on Pt(lll). ... 

Bronsted and Lewis acids and bases are frequently encountered in the chemistry of solid surfaces, but in some instances these terms are used in a slightly different context from their original proposals. From the point of view of the Bronsted definition, a solid is acidic if it is able to donate (or at least transfer partially) a proton to a basic probe molecule to which the proton bonds (or with which it, at least, becomes partially associated). According to the Lewis definition, a solid is acidic if it is able to accept an electron pear from a basic molecule and also form a coor dinative bond with it. The transfer of an electron pair in the meaning of the Lewis definition should be clearly distinguished from surface redox processes in which electron transfer (ofone or two electrons) occurs without coordination. 

For catalyst activity toward the ORR, the first step is normally to put the catalyzed electrode into the O2- or air-saturated electrolyte solution to record the cyclic votammograms. For example. Figure 5.13 shows the cyclic voltammograms recorded on a GC disk electrode (0.28 cm ) coated with Co-N-S/C catalyst layer in N2-saturated and 02-saturated 0.1 M KOH solutions, respectively. It can be seen that in the presence of O2, a large reduction wave appears with an onset potential close to that of surface Wave I/I in the presence of O2, suggesting that the surface redox process expressed by Wave I/I is probably responsible for the ORR catalytic activity. However, there is no significant ORR... 

Busalmen JP, Esteve-Nunez A, Bema A, FeUu JM. ATR-SEIRAs characterization of surface redox processes in G. sulfurreducens. Bioelectrochemistry 2010 78 25-29. 

A splitting of the net voltammetric peak was also observed by increasing the SW amplitude at a given frequency. A value of (-0.260 0.011) V was determined for the formal potential of the adsorbed redox couple from the split net voltammetric peak. A full characterization of the surface redox process was obtained by applying the methods of the quasi-reversible maximum and the split SW peak . In 1 M HCIO4 aqueous solution, the formal rate constant and the anodic transfer coefficient were (3.5 0.5) x 10 s and (0.50 0.03), respectively. Besides, the munber of electrons exchanged by the surface redox couple was calculated as n = 1. 

SWV was used to perform a full thermodynamic and kinetics characterization of the BU surface redox couple. Therefore, from the combination of the quasi-reversible maximum and the splitting of the net square wave voltammetric peak methods, values of (0.386 + 0.003) V, (0.46 0.04), and 2.7 x 10 s were calculated for the formal potential, the anodic transfer coefficient, and the formal rate constant, respectively, of the BU overall surface redox process in pH 4.00 citrate buffer solutions. 

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