Mechanistic surface electrochemistry

Inspired by these Surface Science studies at the gas-solid interface, the field of electrochemical Surface Science ( Surface Electrochemistry ) has developed similar conceptual and experimental approaches to characterize electrochemical surface processes on the molecular level. Single-crystal electrode surfaces inside liquid electrolytes provide electrochemical interfaces of well-controlled structure and composition [2-9]. In addition, novel in situ surface characterization techniques, such as optical spectroscopies, X-ray scattering, and local probe imaging techniques, have become available and helped to understand electrochemical interfaces at the atomic or molecular level [10-18]. Today, Surface electrochemistry represents an important field of research that has recognized the study of chemical bonding at electrochemical interfaces as the basis for an understanding of structure-reactivity relationships and mechanistic reaction pathways. 

The organic and organometallic complexes of transition metals are especially important in catalysis and photovoltaics, on the basis of their redox and electron-mediating properties. Whilst most complex compounds can be studied in (organic) solution-phase experiments, their solid-state electrochemistry (often in an aqueous electrolyte solution environment) is in general also easily accessible by attaching microcrystalline samples to the surface of electrodes. Quite often, the voltammetric characteristics of a complex in the solid state will differ remarkably from its characteristics monitored in solution. Consequently, chemical, physical or mechanistic data are each accessible via the voltammetry of immobilized microparticles. 

Realization that suitably modified or functionalized electrode surfaces could interact in a specific and non-degradative manner with proteins, to allow stable and reversible (i.e. well-behaved ) direct electrochemistry that is uncomplicated by artifacts, came about in the late 1970s. The chemical and mechanistic diversity of these so-called functionalities is evident from the three first demonstrations of such behaviour. 

Limitations to the acceptance of organic electrochemistry, particularly as a synthetic technique, may have been connected with the fact that electrode reactions are normally two-dimensional, i.e., they are restricted to a surface and therefore require mass transport (see elsewhere in this chapter) and also because many reactions yield a complex mixture of products when the electrolyses are carried out using a constant current. However, as early as 1898, Haber had pointed out the importance of control of the electrode potential for the overall process, in his work where nitrosobenzene, phenylhy-droxylamine and aniline were isolated selectively from the reduction of nitrobenzene. However, design of suitable controlled-potential equipment proved to be a practical barrier, even in laboratory studies, until 1942, when the potentiostat—an instrument capable of automatically controlling the electrode potential—was introduced.Without question, this instrument has facilitated electro-organic syntheses, mechanistic studies, and specific electrooxidation and electroreduction processes. More modern and electronically... 

If the electrochemistry is understood, one needs the application of surface analytical methods to learn about the chemical properties and chemical structure of passive layers. Then one has to take care that electrochemical specimen preparation has to occur with optimum control in order to get reliable results. This permits to draw clear mechanistic conclusions on the properties of the layers like their growth, reduction, changes of their composition, reactivity, degradation, and stability including realistic environmental conditions. Application of XPS, ISS, and RBS to a wide variety of pure metals and binary alloys has been described in Section 5.6. These techniques provide valuable results especially when applied together with a systematic change of the experimental parameters like the potential and time of passivation, the composition of the electrolyte, and alloy and conditions for layer degradation. 

---