Adsorbed redox particle

We consider, as an example, adsorbed redox particles comprising protons and hydrogen atoms as shown in Eqn. 5-53 ,... 

Fig. S-S8. Electron levels of dehydrated redox particles, H ld + bh /h = H,d, adsorbed on an interface of metal electrodes D = state density (electron level density) 6 = adsorption coverage shVi - most probable vacant electron level of adsorbed protons (oxidants) eH(d = most probable occupied electron level of adsorbed hydrogen atoms (reductants) RO.d = adsorbed redox particles.

In adsorption equUibrimn, the Fermi level c m) of electrons in the metal electrode equals the Fermi level ep(HyH ) oi redox electrons in the adsorbed redox particles the state density of the occupied electron level equals the state density of the vacant electron level at the Fermi level ( >b = Da). Assuming the Langmuir adsorption isotherm at low adsorption coverages and the Gaussian distribution for the state density, we obtain Eqn. 5-55 for the Fermi level ... 

Fig. 6-S8. Probability density for the energy level of interfadal redox electrons in adsorbed redox particles of proton-hydrogen and hydroxyl-hydroxide on the electrode interface of semiconductor ADS = adsorption > ost probable...

The reaction of electron transfer at electrodes in aqueous electrolytes proceeds either with hydrated redox particles at the plane of closest approach of hydrated ions to the electrode interface (OHP, the outer Helmholtz plane) or with dehydrated and adsorbed redox particles at the plane of contact adsorption on the electrode interface (IHP, the inner Helmholtz plane) as shown in Fig. 7-2. 

Fig. 7-2. Electron transfer of hydrated redox particles and of dehydrated adsorbed redox particles across an electrode interface (a) electron transfer of hydrated redox particles, (b) electron transfer of dehydrated and adsorbed redox particles on electrodes. (RED., OX,q) = hydrated redox particles (RED.d, OX.d) = dehydrated and adsorbed redox particles on electrode OHP = outer Helmholtz plane, IHP = inner Helmholtz plane.

As the adsorption affinity of redox particles on the electrode interface increases, the hydrated redox particles is adsorbed in the dehydrated state (chemical adsorption, contact adsorption) rather than in the hydrated state (ph3 ical adsorption) as shown in Fig. 7-2 (b). Typical reactions of redox electron transfer of dehydrated and adsorbed redox particles on electrodes are the hydrogen and the oxygen electrode reactions in Eqns. 7-6 and 7-7 ... 

The electron transfer of hydrated redox particles at the outer Helmholtz plane is occasionally called the outer-sphere electron transfer, while the electron transfer of dehydrated and adsorbed redox particles on electrodes is called the inner-sphere electron transfer. 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

The kinetic treatment for the electron transfer of ligand-coordinated redox particles described in Sec. 8.4.1 may, in principal, apply also to the electron transfer of adsorbed redox particles (inner-sphere electron transfer). The contact adsorption of redox particles on metal electrodes requires the dehydration of hydrated redox particles and hence inevitably shifts the standard Fermi level of redox electrons from in the hydrated state to in the adsorbed state. This shift of the Fermi level of redox electrons due to the contact adsorption of redox particles is expressed in Eqn. 8-83 similarly to Eqn. 8-79 for the complexation of redox particles (ligand coordination) ... 

Further, the contact adsorption may reduce the reorganization energy, ) , of redox particles in electron transfer the distribution of electron levels of adsorbed redox particles may be narrower than that of simply hydrated redox particles as shown in Fig. 8-36. Furthermore, the contact adsorption of redox particles produces (1) an increase in the effective cross section for electron capture due to the overlapping of the frontier oihital of adsorbed particles with the band orbital of... 

On account of these effects, the contact adsorption of redox particles frequently accelerates the redox electron transfer, as compared with the direct electron transfer between the hydrated redox particle and the electrode. In other words, the reaction current due to redox electron transfer will be greater with adsorbed redox particles than with simply hydrated redox particles if the contact adsorption shifts the energy level of redox electrons in the favorable direction. 

Fig. 9-19. Electron state density of adsorbed proton/lpndrogen redox particles on metal electrodes (a) the relative concentration of adsorbed reductant hydrogen atoms (Had) will be hitler if the Fermi level ef(h, of electrode is higher than the standard Fermi level of adsorbed redox particles, (b) the relative concentration of adsorbed oxidant...

Fig. 9-20. Electron state density D of adsorbed redox particles ) on semiconductor...

Fermi level of standard redox electrons in complexed redox particles Fermi level of standard redox electrons in adsorbed redox particles Fermi level of n-type or p-type semiconductor electrodes quasi-Fermi level of electrons in semiconductor electrodes quasi-Fermi level of holes in semiconductor electrodes energy of a particle i... 

The frontier electron level of adsorbed particles splits itself into an occupied level (donor level) in a reduced state (reductant, RED) and a vacant level (acceptor level) in an oxidized state (oxidant, OX), because the reduced and oxidized particles differ from each other both in their respective adsorption energies on the interface of metal electrodes and in their respective interaction energies with molecules of adsorbed water. The most probable electron levels, gred and eqx, of the adsorbed reductant and oxidant particles are separated from each other by a magnitude equivalent to the reorganization energy 2 >. ki in the same way as occurs with hydrated redox particles described in Sec. 2.10. 

The same approach may also apply to the adsorption of redox particles other than the adsorption of proton-hydrogen atom on metal electrodes. To understand electrosorption phenomena, various concepts have been proposed such as the charge transfer coefficient and the adsorption valence [Vetter-Schultze, 1972]. The concept of the redox electron level in adsorbed particles introduced in this textbook is usefiil in dealing with the adsorption of partially ionized particles at electrodes. 

In addition to simple reactions of electron transfer (outer-sphere electron transfer) between an electrode and hydrated redox particles, there are more complicated reactions of electron transfer in which complexation or adsorption of redox particles is involved. In such transfer reactions of redox electrons, the redox particles are coordinated with ligands in aqueous solution or contact-adsorbed on the electrode interface before the transfer of their redox electrons occurs after the transfer of electrons, the particles are de-coordinated from ligands or desorbed from the electrode interface. 

For the transfer of redox electrons (inner-sphere electron transfer) in which redox particles are adsorbed on a thin superficial film that covers a metal electrode, the transfer current of redox electrons is not always decreased but rather increased by the presence of the thin film. Such an increase in the reaction ciurent will occur, if the film acts as a reaction catalyst providing the adsorbed state of redox particles favorable for the redox electron transfer. For example, the anodic oxidation of carbon monoxide is catalyzed by the presence of an anodic oxide film on... 

Particle reactive metals such as lead and trace metals with nutrient-like behavior (e.g.. Cd, Kremling and Pohl, 1989) are mostly removed from surface waters by adsorption or incorporation into particles. They accumulate at horizontal interfaces such as the pycnocline or the redox boundary because of slow sinking rates. On their way through the water column, trace metals adsorbed to particles can be influenced by the change of important variables such as salinity and pH and several other processes including agglomeration and modification of redox-sensitive elements. 

In contrast to a mixture of redox couples that rapidly reach thermodynamic equilibrium because of fast reaction kinetics, e.g., a mixture of Fe2+/Fe3+ and Ce3+/ Ce4+, due to the slow kinetics of the electroless reaction, the two (sometimes more) couples in a standard electroless solution are not in equilibrium. Nonequilibrium systems of the latter kind were known in the past as polyelectrode systems [18, 19]. Electroless solutions are by their nature thermodyamically prone to reaction between the metal ions and reductant, which is facilitated by a heterogeneous catalyst. In properly formulated electroless solutions, metal ions are complexed, a buffer maintains solution pH, and solution stabilizers, which are normally catalytic poisons, are often employed. The latter adsorb on extraneous catalytically active sites, whether particles in solution, or sites on mechanical components of the deposition system/ container, to inhibit deposition reactions. With proper maintenance, electroless solutions may operate for periods of months at elevated temperatures, and exhibit minimal extraneous metal deposition. 

The local structure around the lanthanide ions with differing redox potentials Eu(III)/(II) (-0.35 V vs. NHE), Yb(III)/(II) (-1.05 V), and Sm(III)/(II) (-1.55 V) in Ti02 particles was investigated by EXAFS. The photocatalytic reaction and EXAFS studies were also carried out for a calcined Yb(III) ion adsorbed-Ti02 catalyst [157], The photocatalytic activity of these lanthanides toward methyl blue photodecomposition was very similar, suggesting that adsorbed lanthanide ions on TiOz particles scarcely assist the high-photocatalytic activity of TiOz catalyst. However, the way in which the catalysts were activated was of high importance. The photocatalytic activity of the calcined Yb/TiOz... 

These levels of interfacial redox electrons are connected with the hydrogen and oxygen electrode reactions. As noted in Sec. 5.1.2, the electron level of adsorbate particles is broadened by contact adsorption and undergoes the Franck-Condon level splitting due to a difference in adsorption energy between the oxidized particle and the reduced particle on the interface of semiconductor electrodes as shown in Fig. 5-59. 

Fig. 8-36. Shift of redox electron levels due to contact adsorption of reductant and oxidant particles (1) adsorption affinity is greater with oxidants than with reductants, (2) adsorption affinity is greater with reductants than with oxidants. ADS = adsorbed particle = standard Fermi level of adsorbed particles X = reorganization energy of redox partides.

The relative solubilities reported are very crude estimates based on equilibrium solubility products. These estimates do not take into account variations in solubility as a function of pH, ionic strength, activities of various solution species (e.g., HCO "), redox state, particle size, surface defect types and concentrations, the concentration of various types of adsorbates, including natural organic matter, on mineral surface, or the presence of different types of bacteria or microbial biofilms on mineral surfaces. 

---