Heterogeneous redox reactions

Thus in all corrosion reactions one (or more) of the reaction products will be an oxidised form of the metal, aquo cations (e.g. Fe (aq.), Fe (aq.)), aquo anions (e.g. HFeO aq.), Fe04"(aq.)), or solid compounds (e.g. Fe(OH)2, Fej04, Fe3 04-H2 0, Fe203-H20), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/non-metal interface in which the metal is oxidised and the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products, and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will clearly be important in determining the form, extent and rate of the reaction. 

In heterogeneous redox reactions similar reaction sequences are observed usually an encounter (outer-sphere or inner-sphere) surface complex is formed to facilitate the subsequent electron transfer. 

Equation (2.1) defines current as the rate of charge movement. An electroanalyst could have re-expressed equation (2.1) with, in words, the magnitude of an electrochemical current represents the number of electrons consumed or collected per second . Each electron consumed or collected represents a part of a heterogeneous redox reaction at an electrode (equations (2.3) or (2.4)), so the magnitude of the current also tells us about the amounts of material consumed or formed at the electrode surface per unit time. 

Peterson, M.L. Brown Jr., G.E. Parks, G.A. (1996) Direct XAFS evidence for heterogeneous redox reaction at the aqueous chro-... 

Comparison of the Rates of Homogeneous and Heterogeneous Redox Reactions... 

In direct electrochemical reactions, the substrate undergoes a heterogeneous redox reaction at the electrode surface within the Helmholtz layer. The reac-... 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Scheme 2.3 A schematic view of an heterogeneous redox reaction at a liquid/ liquid interface...

In the case of microparticulate deposits, there is a possibility for different electrochemical pathways. First, we consider the catalyst immobilized and the substrate dissolved. The catalyst is electrochemically reduced via the process described by Equation (3.11) and, as previously noted, the direct reduction of S (Equation (3.1) does not run electrochemically but only via heterogeneous redox reaction with Cab " ... 

The resulting quinone diffuses across the mesoporous system and reacts with the NADH catalyst. As a result, the direct reduction of the immobilized NADH substrate does not proceed electrochemically but only via heterogeneous redox reaction with the oxidized form of the hydroquinone catalyst in solution (see Figure 3.5). 

Figure 24. Mini-soil column used for in-situ pXRF and p.XANES studies of heterogeneous redox reactions (courtesy of T. Tokunaga and J. Wan, LBNL).

To increase the stability of the phenothiazine dye CMEs, investigations of the electrochemical reactions of redox proteins at polymer CMEs have been undertaken. One example of such a study is the heterogeneous redox reaction of Cyt c at an electrochemically polymerized polypyrrole-methylene blue (PPy-MB) film CME. Figure 17 shows the cyclic voltammetric response that occurs during the preparation of this CME by potential... 

In direct electrochemical reactions the substrate undergoes a heterogeneous redox reaction at the electrode surface within the Helmholtz layer. The thus formed reactive intermediate, i.e. a radical ion, undergoes the chemical follow-up reaction to the product in die reaction layer. There are steep concentration gradients near the electrode. Second-order reactions of the intermediates can thus be obtained at high current densities. 

The reduction on aqueous transition metal species at the surfaces of Fe(II)-containing oxides is defined in terms of heterogeneous redox reactions that occur concurrently with oxidation and weathering of the mineral surfaces. Electrochemical measurements made on mineral electrodes document that such reactions are coupled half cells in which reductive dissolution can be decoupled and substituted for reduction of aqueous species. This is confirmed experimentally in aqueous/mineral suspensions in which Cr(VI), V(V) Fe(III) and Cu(II) are reduced to lower valance states. 

The reactivity of the surface with respect to heterogeneous redox reactions is often significantly altered by the presence of surface states. The highest occupied states at the... 

The energy-level distribution factor is now recognized as a fundamental factor in the quantum-mechanical representation of electron-transfer rates both in heterogeneous redox reactions(2 ) at electrode surfaces and in homogeneous ones in bulk solution, as well as at semi-conductors(18). 

The theory of linear-sweep voltammetry (LSV) applied to heterogeneous redox reactions accompanied by the nondissociative adsorption of the reactant or the product is developed. The basic criterial relationships of LSV in this case are invariant with respect to the type of adsorption isotherm and the number of adsorption sites occupied by one species. The degree of irreversibility of the discharge-ionization step can be evaluated from the effect of the potential scan rate on the peak potential. The nature of the adsorbate can be deduced from the effect of the reactant concentrations on the peak current. 

So one the purpose of this work is to develop LSV theory as applied to heterogeneous redox reactions accompanied by the preferential adsorption of the reactant (or product) for an arbitrary form of the dependence of the standard Gibbs energy of adsorption on the coverage of the electrode surface. 

Examination of Eqs. (20), (21), (29), (30), (37), (38), (45), and (46) indicates that the basic criterial relationships of linear chronovoltammetry, which are represented in the form of Eqs. (3) and (4), are also valid for the general form of the isotherm that is, they are invariant with respect to the choice of an adsorption isotherm. Physically, this means that the crucial circumstance dictating that relationships (3) and (4) should be used is the very fact of the adsorption of the reactant or of the product of the heterogeneous redox reaction, no matter what the particular type of the adsorption isotherm. 

The autocatalytic deposition is basically a heterogeneous redox reaction in which metal cations are reduced by a suitable reducing agent in a controlled way. Schematically, see also Figure 11.2, the deposition reaction of a metal cation AF+ is made possible by the oxidation of a reducing agent Red present in solution, which supplies ze electrons and produces the Ox species. Control over the reaction is granted by the catalytic nature of the reaction itself, which is catalyzed by the metal or... 

---