Mixed-potential mechanism

The mixed potential mechanism was described above, using CO as an example. However, the mechanism can be applied to any pair of oxidation and reduction reactions. Thus, mixed potential sensors have been reported for other reducing gases, such as hydrocarbons. Figure 13.20 shows that gold and platinum electrodes can be used to measure the amount of propylene (CsHe) [228, 229, 231, 233, 236-238]. Mixed potential hydrocarbon sensors have also been reported using proton-conducting electrolytes [239-242]. 

The elecirochemical mechanism, also called the mixed-potential mechanism [477], assumes charge transfer within a particle from the cathodic patch, at which oxygen is reduced, to the anodic one, at which the sulfide itself and/or xanthate anion are oxidized. This mechanism describes a broad spectrum of interfacial phenomena involving, as an intermediate step, a redox reaction in which the anodic and cathodic processes are spatially separated. Some examples of this include electrocatalytic chemisorption of xanthate and synthesis of dixanthogen and precipitation of xanthate-metal complexes (nncleation of a microphase of metal xanthate). In the latter reaction, the anodic sulfide dissolution is initiated with the ionization of surface metal atoms, and the metal ions thus produced on the surface are transferred into aqueous solution to form hydrated metal ions or metal-ion complexes associated with anions [478]. The ionization of surface metal atoms is an electrochemical oxidation, whereas the hydration or complex-ation of metal ions is a chemical process (an acid-based reaction). 

Figure 1. Schematic representation of the mixed-potential mechanism for the interaction of thiol collectors with sulfide minerals in which the anodic process is chemisorption (a), a single-step reaction to form a metal collector compound (b), the latter reaction occurring in two stages, comprising oxidation of the mineral (c), ion exchange with the collector (d), and formation of the dithiolate (e). (FrOTi Woods.

The mixed-potential mechanism has two important implications. First, the potential across the mineral/solution interface will be an important parameter in determining flotation recovery. Second, the reaction imparting floatability, the anodic process involving the collector, is amenable to investigation using electrochemical techniques. 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

The isotope independent potential energy surface was evaluated using a mixed quantum mechanics/molecular mechanics (QM/MM) method. The system (N atoms) was partitioned into Nqm quantum mechanical atoms and Nmm classical mechanical atoms. Nqm consisted of the 15 atom substrate (phospho-D-glycerate)... 

Ever since the mixed potential model has been proposed, the interaction mechanism between thio-collector and sulphide minerals has been usually explained on the basis of this model. The principle of the mixed potential model can be schematically shown in Fig. 4.1. Here, E respectively... 

It follows that the corrosion potential on a heterogeneous metal corroding by local-cell action is virtually equal to the mixed potential at an electrode on which electronation and deelectronation reactions are occurring on spatially separated sinks and sources and is identical to a mixed potential when the metal is corroding homogeneously by a Wagner-Traud mechanism. The concept of the corrosion current /corr and the corrosion potential 40corr will now be treated quantitatively. 

The rest of the chapter has been devoted to special topics and in materials science there are many possibilities. Those selected include the mechanism of the flotation of minerals in which the addition of a certain organic to the solution causes a specific mineral to become hydrophobic so that it is exposed to air bubbles, the bubbles stick to it and buoy the mineral up to the surface, leaving unwanted minerals on the bottom of the tank. It turns out that the mechanism of this phenomenon involves a mixed-potential concept in which the anodic oxidation of the organic collector, often a xanthate, allows it to form a hydrophobic film upon a semiconducting sulfide or oxide, but only if there is a partner reaction of oxygen reduction. This continues until there is almost full coverage with the dixanthate, and the surface is thereby made water-repelling. 

Many of these approaches have been used in mixed potential models to predict the behavior of copper nuclear waste containers in a compacted clay environment (22), and to predict the corrosion rate of nuclear fuel inside these containers once they have failed and water allowed to contact the nuclear fuel (U02) wasteform (6). The container is lined with a carbon shell liner to give it mechanical integrity. Consequently, when the container floods with water on failure, two corrosion processes are possible, corrosion of the U02 wasteform (conservatively assumed to be unprotected by the Zircalloy cladding within which it is encapsulated) and corrosion of the carbon steel liner. The reaction scheme underlying... 

Considerable progress has been made during the past decade toward a better insight into the basic concepts and mechanism involved in metallic dissolution and corrosion. More emphasis has been placed on the "fundamental particles (metallic ions, electrons, and electron acceptors) and on the use of current-voltage characteristics. The wide recognition of dissolution and corrosion as electrode processes, and the idea of a polyelectrode exhibiting a mixed potential, have augmented the use of electrochemical techniques in the study and interpretation of corrosion phenomena. There is even some evidence that the phenomenon of passivity may soon be clarified. 

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