Adsorption of redox particle

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. 

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. 

In Chapter 7 general kinetics of electrode reactions is presented with kinetic parameters such as stoichiometric number, reaction order, and activation energy. In most cases the affinity of reactions is distributed in multiple steps rather than in a single particular rate step. Chapter 8 discusses the kinetics of electron transfer reactions across the electrode interfaces. Electron transfer proceeds through a quantum mechanical tunneling from an occupied electron level to a vacant electron level. Complexation and adsorption of redox particles influence the rate of electron transfer by shifting the electron level of redox particles. Chapter 9 discusses the kinetics of ion transfer reactions which are based upon activation processes of Boltzmann particles. 

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 ... 

However, there remains a possibility that the appearance of two pairs of voltammetric peaks may arise from particle adsorption onto the electrode surface. As in solid films, the ferrocene moieties might exhibit different energetic states and accessibility to counterions because of spatial effects. This hypothesis is discounted by results from two additional experiments. First, the cathodic and anodic current density of the redox peaks was found to be linearly proportional to the square root of potential scan rates, suggesting that the charge-transfer processes were under diffusion control. Second, after the electrochanical measurements in the Ru=CH-Fc particle solution, the Au electrode was taken out and rinsed with a copious amount of DMF and then immersed into a same electrolyte solution without the nanoparticles. Only featureless voltammetric responses were observed, as shown in Figure 3.13 (long dashed curve). In short, both measurements signify minimal surface adsorption of the particles. 

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. 

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 ... 

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. 

The contact adsorption of particles other than hydroxyl groups also generates the energy levels of interfacial redox electrons in the same reason as occurs with the hydroxyl groups. 

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. 

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. 

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.

Various studies and some patents have been published on the use of membrane catalysts for the direct synthesis of H202 [73-81]. The redox treatment of the membrane influences the properties both in the synthesis and decomposition of H202. Formation of a hydrophobic layer improves the selectivity, because it limits the consecutive decomposition of hydrogen peroxide, limiting the chemisorption of H2 and re-adsorption of H202 [73]. Either polymeric or ceramic-type membranes could be used, but the latter are preferable to allow more robust operations. The mono- or bi-metallic Pd-based active component could be deposited either in the form of dispersed particles (e.g., by precipitation-deposition) or of a thin film (e.g., by... 

One may assume that slow conformational transformations in the surfactant macromolecules may affect considerably the adsorption-desorption equilibria at the surface of the semiconductor particles under consideration and thus affect the course of redox processes generated by these particles under the action of light. We present below an attempt in a semiquantitative description of the observed processes. 

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