Decomposition reactions electrochemical

In order to find out in any particular case whether a semiconductor is liable to anodic or cathodic decomposition (both in darkness and under illumination), it is convenient to use the energy diagram (Fig. 15), which plots the energies of band edges and electrochemical potential levels for decomposition reactions. Various situations are possible here, as is schematically shown in Fig. 15. A semiconductor is stable with respect to anodic decomposition if the electrochemical potential level for the corresponding reaction... 

Figure 9-10. Mechanism of an electrochemically driven internal decomposition reaction in AgBr h +AgBr = Br+ Ag. ...

The electrochemical redox potential of several possible decomposition reactions at pH = 0 (relative to the potential of the saturated calomel electrode), which have been estimated from thermodynamic parameters (6,17-21), are shown schematically in Figure A. The band levels are shown for open-circuit conditions. The standard electrode potentials were calculated from the free energies of formation, which are summarized below in Table III. 

This complex has been widely used in sensing applications since both radical ions of the complex are relatively stable to decomposition reactions. Many systems using this chromophore exist in which ECL is produced at a single electrode via coreactant oxidation or reduction schemes as discussed in the first segment of this section [Eqs. (5) through (9)]. For example, the reduction product of the peroxydisulfate dianion, S2Og, can function as an oxidant in the ECL reaction by annihilation with the electrochemically generated Ru1+ to yield the MLCT excited state of the Ru(II) complex by the mechanism [24] ... 

The electrochemical water decomposition reaction can be reversed according to... 

Decomposition potential (voltage) — The onset voltage for electrochemical decomposition of the electrolytic solution or the electrodes. The decomposition can take place due to either oxidation or reduction, or both. The decomposition potentials define the electrochemical window of the system. Its value depends on the salt, solvent, electrode material, temperature, and the existence of materials that can catalyze decomposition reactions, such as Lewis acids. Exact decomposition voltages are hard to reproduce as the onset current of the process is very sensitive to the experimental conditions (e.g., scan rate, temperature, type of electrode, etc.). Decomposi-... 

Fig. 1. Schematic representation of the connection between electrode potential E and current density I in the decomposition reaction of the medium. Hydrogen (oxygen) evolution starts at Ei if the substrate requires for the electrochemical step a potential (numerically) higher than E2, transfer of electrons to the substrate becomes appreciable only if / > I2.

Platinum is not the inert material that it is often considered to be. There are a great many oxidation-reduction and decomposition reactions in solution that are catalyzed by metallic platinum. Examples are the CeIV—Br reaction and the decomposition of N2Ht to N2 and NH3. It is possible to predict whether catalysis can occur from a knowledge of the electrochemical properties of the reacting couples. 

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

The electrochemical potentials of redox couples in solution are calculated in a known manner from the thermodynamic characteristics of the substances involved. For certain reactions, which proceed with the participation of the semiconductor electrode material, in particular anodic or cathodic decomposition reactions, the values of Fredox are as reported in References 15-18. 

In order to find out whether a semiconductor is liable to electrolytic or corrosion decomposition in any particular system, let us consider the energy-band diagram which plots the electrochemical potential levels for the decomposition reactions (39a) and (39b). 

Figure 14 schematically shows that various situations are possible here. The semiconductor is stable against cathodic decomposition if the electrochemical potential level of the corresponding reaction lies in the conduction band and against anodic decomposition if it lies in the valence band. In both cases, if the whole potential change occurs in the semiconductor (band edge pinning at the surface), this level is inaccessible to the Fermi level of the semiconductor.t For example, in the case of Fig. 14a, the semiconductor is absolutely stable because the levels of both decomposition reactions lie outside the forbidden band. However, more frequent are the cases where the semiconductor is stable against only one type of decomposition cathodic (Fig. 14b) or anodic (Fig. 14c). Finally, if both levels Fdec, and Fjec.p He in the forbidden band (Fig. 14d) the semiconductor can, in principle, suffer decomposition both under anodic and cathodic polarization.

Let us consider, as an example, the photocorrosion behavior of GaAs and GaP in aqueous solutions. The levels of the electrochemical potential of decomposition reactions for these materials are shown in Fig. 16 together with the levels and... 

The H2S decomposition reaction has also been studied utilizing electrochemical membrane reactors. Alqahtany et al. [2.319], for example, studied the reaction in a Pt/YSZ/Pt... 

Here, it is assumed that the solvent decomposition reactions (release of dihydrogen and dioxygen) are immeasurably slow. The battery voltage in equilibrium is obtained by writing the electrochemical equilibrium of all the interfaces ... 

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