Solid-solution interface, redox reactions

Mam heterogeneous processes such as dissolution of minerals, formation of he solid phase (precipitation, nucleation, crystal growth, and biomineraliza-r.on. redox processes at the solid-water interface (including light-induced reactions), and reductive and oxidative dissolutions are rate-controlled at the surface (and not by transport) (10). Because surfaces can adsorb oxidants and reductants and modify redox intensity, the solid-solution interface can catalyze rumv redox reactions. Surfaces can accelerate many organic reactions such as ester hvdrolysis (11). 

In illuminated dispersions of semiconductors, light induces an electron-hole charge separation at the surface of the particles and within their bulk. Redox reactions at the solid/solution interface can then occur upon capture of the electrons and holes by an oxidant and a reductant, respectively. Since a reduction and an oxidation reaction proceed simultaneously on the same surface (at the same rate, at steady-state conditions), the dispersed system can be compared to an ensemble of semiconductor microelectrodes at open circuit (8). 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

Before a heterogeneous electron-transfer reaction can take place, be it oxidation or reduction, we must appreciate that the redox reaction occurs at the interface that separates the electrode and the solution containing the electroanalyte. Some electrochemists call this interface a phase boundary since either side of the interface is a different phase (i.e. solid, liquid or gas). An electrochemist would usually indicate such a phase boundary with a vertical line, . Accordingly, the interface could have been written as solution electrode . 

The kinetics found for the reactions at the solution/solid interface show some marked similarities with those at gas/solid [9, 49], gas/liquid, and liquid/liquid interfaces [268]. Whenever one of the phases is a liquid rather than a gas, mass transport is apt to become rate-controlling because of the smaller diffusion coefficients of species in liquids. Many of the catalysed redox reactions in Sect. 4 were indeed partly or wholly diffusion-controlled. These systems could be converted to surface-controlled ones simply by reducing the size of the catalysing material by using colloidal catalysts, for... 

Kavan [28] and Kijima et al. [29] have used the electrochemical method to synthesize carbyne. This technique may be realized by classical electrochemistry whereby the charge transfer reaction occurs at interface of a metal electrode and liquid electrolyte solution. Electrons in reaction were supplied either through redox active molecules or through an electrode, which contacts an ionically conducting solid or liquid phase and the precursor. In general, the structure and properties of electrochemical carbon may differ considerably from those of usual pyrolytic carbons. The advantage of this technique is the synthesis of carbyne at low (room) temperature. It was shown that the best product was prepared by cathodic defluorination of poly(tetrafluoroethylene) and some other perhalo-//-alkanes. The carbyne... 

It is worth noting that, as far as they are less than several nanometers thick, the passive films are subject to the quantum mechanical tunneling of electrons. Electron transfer at passive metal electrodes, hence, easily occurs no matter whether the passive film is an insulator or a semiconductor. By contrast, no ionic tunneling is expected to occur across the passive film even if it is extremely thin. The thin passive film is thus a barrier to the ionic transfer but not to the electronic transfer. Redox reactions involving only electron transfer are therefore allowed to occur at passive film-covered metal electrodes just like at metal electrodes with no surface film. It is also noticed, as mentioned earlier, that the interface between the passive film and the solution is equivalent to the interface between the solid metal oxide and the solution, and hence that the interfacial potential is independent of the electrode potential of the passive metal as long as the interface is in the state of band edge pinning. 

Pore-water concentration profiles of redox-sensitive ions (nitrate, Mn, Fe, sulphate and sulphide) and nutrients (ammonium and phosphate) demonstrate the effects of degradation of OM. In freshwater sediments, the redox zones generally occur on a millimetre to centimetre scale due to the high input of reactive OM and the relatively low availability of external oxidators, especially nitrate and sulphate, compared to marine systems. A typical feature for organic-rich freshwater sediments deposited in aerobic surface waters, is the presence of anaerobic conditions close to the sediment-water interface (SWI). This is indicated by the absence of dissolved oxygen and the presence of reduced solutes (e.g. Mn, Fe and sulphides) in the pore water. Secondary redox reactions, like oxidation of reduced pore-water and solid-phase constituents, and other postdepositional processes, like precipitation-dissolution... 

In IP, there exist two paths by which current may pass the interface between the solid particle and the electrolyte the faradaic and nonfaradaic paths. Current passage in the faradaic path is the result of electrochemical reactions (redox reactions) and the diffusion of charge toward or off the Helmholtz double layer and aqueous solution interface, that is, Warburg impedance. In the nonfaradaic case, charged particles do not cross the interface. Instead, the current is carried by... 

Unlike the situation at a solid/electrolyte interface where a three-electrode system is used, four- and two-electrode systems have been widely employed for large and small liquid/ liquid interfaces. Most of the four-electrode potentiostats are homemade and only a few instruments with such functions have been commercialized (98). This is probably one of the reasons why this field has not been very popular since most electrochemical laboratories are equipped with a three-electrode potentiostat. In 1998, Anson et al. reported that charge transfer reactions at a liquid/liquid interface could also be studied by a three-electrode system with a thin-layer cell (99,1(X)). Later, Scholz et al. reported a three-phase junction setup (101, 102). Shao et al. supported a small droplet of aqueous solution (pL) containing a certain concentration ratio of redox couples on a Pt surface and demonstrated that charge transfer could be studied by a three-electrode setup (103). Girault et al. extended this to a supported small droplet of aqueous (organic) phase on the surface of... 

For an aqueous droplet supported on a solid (Pt or C) electrode surface, the potential difference at the solid/liquid interface is fixed because the concentration ratio of the redox couples in the droplet is constant and it can be used as a pseudo-reference electrode. It forms a W/0 interface when the assembly is immersed in an organic solution. The W/O interface formed between the aqneons droplet and the organic phase can be studied with a three-electrode system. The FT process at the solid/liquid interface and the charge transfer processes at the liquid/liquid interface are coupled as reactions in series. The disadvantage of this setup is the cations associated with the redox couples usually limit the potential window and very few redox couples can be chosen (103). The droplet supported on Ag/AgCl or Ag/AgX (X is a big anion) can function similarly and only needs Cl (or X ) to be present in the aqueous (organic) droplet to fix the potential difference at the solid/liquid interface. In this way one can obtain a wider potential window and study the effect of the phase volume ratio on charge transfer reactions at a L/L interface. 

The accumulation is a dynamic process that may turn into a steady state in stirred solutions. Besides, the activity of accumulated substance is not in a time-independent equilibrium with the activity of analyte in the bulk of the solution. All accumulation methods employ fast reactions, either reversible or irreversible. The fast and reversible processes include adsorption and surface complexation, the majority of ion transfers across liquid/liquid interfaces and some electrode reactions of metal ions on mercury. In the case of a reversible reaction, equilibrium between the activity of accumulated substance and the concentration of analyte at the electrode surface is established. It causes the development of a concentration profile near the electrode and the diffusion of analyte towards its surface. As the activity of the accumulated substance increases, the concentration of the analyte at the electrode surface is augmented and the diffusion flux is diminished. Hence, the equilibrium between the accumulated substance and the bulk concentration of the analyte can be established only after an infinitely long accumulation time (see Eqs. II.7.12-II.7.14 and II.7.30). The reduction of metal ions on mercury electrodes in stirred solutions is in the steady state at high overvoltages. Redox reactions of many metal ions, especially at solid... 

An elementary electrochemical reaction is a redox reaction occurring at the interface between an electron-conducting solid, called the electrode, and a solution of ions, called the bath or electrolyte solution. 

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