Two-step redox process

At n-type electrodes, the complete reaction already occurs in the dark because sufficient electrons are available in the conduction band. In the latter case the participation of the valence band has been proved by luminescence measurements. Since in the second reaction step electrons are transferred from the valence band to the OH radicals, hole are injected into the valence band of the n-type electrode which finally recombine with the electrons (majority carriers). In the case of n-GaP, this recombination is a light-emitting process, as has been found experimentally. The same result has been obtained with 820 [68] and for quinones [69]. Since the reduction of H2O2 consists of two consecutive steps, it is reasonable to describe its redox properties by two standard potentials, given by 

It should be emphasized that the description of such a redox system by two standard potentials is of general importance. The application of semiconductor electrodes, however, offers the possibility of proving the resulting two-step process. Since the reorganization energies are not known, values of the two formal potentials could only be esti- 

These results also lead to consequences concerning the electron transfer between these redox systems and metal electrodes. Since in a metal only the energy levels below the Fermi level are occupied, a rather small overlap between these levels and the occupied levels i.ox in the redox system is expected at potentials corresponding to ZiFredox-Hence, the exchange current should be relatively small (compared with that found that with GaP). The rather large overpotential found for the cathodic reduction of H2O2 at metal electrodes supports this assumption [1,14]. 

At low intensities holes are injected from the HO intermediate (Eq. 7.111b). At higher intensities the concentration of electrons in the conduction band becomes very large and the HO radical is reduced via a conduction band process (Eq. 7.11 Ic). 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

At n-type electrodes, the complete reaction already occurs in the dark because sufficient electrons are available in the conduction band. In the latter case, the participation of the valence band has been proved by luminescence measurements. 

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C04-0149. Surface deposits of elemental sulfur around hot springs and volcanoes are believed to come from a two-step redox process. Combustion of hydrogen sulfide (H2 S) produces sulfur dioxide and water. 

However, only the latter can be employed in conjunction with lithium, LiAl or LiCe negative electrodes A two-step redox process is discussed according to Section 6.3 and Eq. (70). The first two-electron step leads to the (green) emeraldine form, where the A -protons have disappeared and an alternating chain of benzoid and quinoid Ce rings have formed. The second two-electron process leads to an anion-doped... 

R. Memming and F. Mollers, Two-step redox processes with quinones at semiconductor electrodes, Ber. Bunsenges. 76(7), 609, 1972. 

Interestingly, the anodic dark current at n-Ge electrodes increases considerably upon addition of the oxidized species of a redox system, for instance Ce" ", to the electrolyte, as shown in Fig. 8.4 [7]. The cathodic current is due to the reduction of Ce. The latter process occurs also via the valence band (see Chapter 7), i.e. since electrons are transferred from the valence band to Ce", holes are injected into the Ge electrode. Under cathodic polarization these holes drift into the bulk of the semiconductor where they recombine with the electrons (majority carriers) and the latter finally carry the cathodic current. In the case of anodic polarization, however, the injected holes remain at the interface and are consumed for the anodic decomposition of germanium, as illustrated in the insert of Fig. 8.4. Accordingly, the cathodic and anodic current should be compensated to zero. Since, however, the anodic current is increased upon addition of the redox system there is obviously a current multiplication involved, similarly to the case of two-step redox processes (see Section 7.6). Thus, in step (e) (Fig. 8.1) electrons are injected into the conduction band. This experimental result is a very nice proof of the analytical result presented by Brattain and Garrett [3]. 

Two-step redox processes are understood as reactions in which two electrons are exchanged at the electrode. Typical examples involve H2O2, 208 , quinones, HCOOH, and various other organic substances. Taking H2O2 as an example, its reduction can be studied by using photoexcitation techniques as follows ... 

Further information about the energetics of two-step redox process is given in Ref. 9. 

L = AsEts, PPrg, PEtg, or SEta, X = Cl or Br) have been studied in hydroxylic solvents such as methanol, ethanol, and n-propanol. The rates of the reactions, which are outer-sphere in nature, are strongly solvent dependent, the rate law being consistent with a two-step redox process in which a plati-num(iii) species is formed as an intermediate ... 

Two experimental systems have been used to illustrate the theory for two-step surface electrode mechanism. O Dea et al. [90] studied the reduction of Dimethyl Yellow (4-(dimethylamino)azobenzene) adsorbed on a mercury electrode using the theory for two-step surface process in which the second redox step is totally irreversible. The thermodynamic and kinetic parameters have been derived from a pool of 11 experimental voltammograms with the aid of COOL algorithm for nonlinear least-squares analysis. In Britton-Robinson buffer at pH 6.0 and for a surface concentration of 1.73 X 10 molcm, the parameters of the two-step reduction of Dimethyl Yellow are iff = —0.397 0.001 V vs. SCE, Oc,i = 0.43 0.02, A sur,i =... 

Figure 8.11 shows the cyclic voltammogram of the upper phase of the biphasic mixture of A1C13 /1 -bu tyl -1 -methylpyrrolidinium bis(trifluoromethylsulfonyl)imide on a gold substrate at room temperature. At a potential of —0.7 V (vs. Al), the cathodic current rises with two small cathodic steps at —0.9 and —1.3 V which are correlated to two different redox processes before the bulk growth of Al sets... 

Fig. 2. Schematic plots outlining outer-shell free energy-reaction coordinate profiles for the redox couple O + e R on the basis of the hypothetical two-step charging process (Sect. 3.2) [40b]. The y axis is (a) the ionic free energy and (b) the electrochemical free energy (i.e. including free energy of reacting electron), such that the electrochemical driving force, AG° = F(E - E°), equals zero. The arrowed pathways OT S and OTS represent hypothetical charging processes by which the transition state, T, is formed from the reactant.

The conversion of L-lactate to pyruvate is a two-electron redox process. One could consider this occurring as two one-electron steps (a radical mechanism) or as one two-electron step. There are two options for a single two-electron step, and these are hydride transfer (H ) or proton (H+) abstraction followed by a two-electron transfer from a carbanion intermediate. These two alternatives for lactate are shown formally in Eqs. (1) and (2) for hydride transfer and the carbanion mechanism, respectively. 

Fig. 33 A Set-up of the redox-mediated tunneling experiment with a viologen-modified Au tip B schematic energy level diagram of a two-step ET process mediated by a redox-active molecule. The electron is transferred from the Fermi level of the substrate (left) ep,s to the LUMO of the molecule and after partial vibrational relaxation to the Fermi level of the tip ept (right). C Average ix vs. Eg curves recorded in constant bias spectroscopy mode, ixo = 0.1 nA, Ebias = 0.050 V. The sweep started in the stability region of V+ D average constant bias spectroscopy curve C after baseline correction. The blue line represents the fit using Eq. 8 with k = 0.42 eV, = 1.0, y = 1.0 [269]...

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