Mixed-oxide theory

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. 

In the mixed potential theory (MPT) model, both partial reactions occur randomly on the surface, both with respect to time and space. However, given the catalytic nature of the reductant oxidation reaction, it may be contended that such a reaction would tend to favor active sites on the surface, especially at the onset of deposition, and especially on an insulator surface catalyzed with Pd nuclei. Since each reaction strives to reach its own equilibrium potential and impose this on the surface, a situation is achieved in which a compromise potential, known as the mixed potential (.Emp), is assumed by the surface. Spiro [27] has argued the mixed potential should more correctly be termed the mixture potential , since it is the potential adopted by the complete electroless solution which comprises a mixture of reducing agent and metal ions, along with other constituents. However, the term mixed potential is deeply entrenched in the literature relating to several systems, not just electroless deposition. 

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]. 

Marmatite has a narrower band gap than (Zn, Cu)S, thus it should be more easily oxidized than (Zn, Cu)S. But in fact, the sphalerite after Cu activation has the most excellent flotation response using xanthate. These phenomena can be explained by the mixed potential theory. 

According to the mixed potential theory, an anodic reaction can occur only if there is a cathodic reaction proceeding at finite rate at that potential (Rand and Woods, 1984). For the flotation systems, the cathodic reaction is usually given by the reduction of oxygen. The corresponding anodic reaction involves interaction of xanthate on the sulphide minerals in various ways, including the reaction of xanthate with the sulphide mineral (MS) to form metal xanthate and the oxidation of xanthate to dixanthogen (X2) at the mineral surface. 

Several arguments against the last theory may be raised. First, the proposed mixed oxide contains nickel atoms that are fully reduced. If so, what makes this oxide stable Secondly, no direct observation was provided for the reduction of this oxide into an alloy. Thirdly, the origin of the adsorbed hydrogen atoms included in Eq. (45) is not clear. Finally, it should be obvious that Eqs. (44) and (45) cannot be considered as elementary steps in the reaction sequence. Four Ni ions could not be possibly reduced simultaneously. Neither could the eight-electron reduction of Mo04 + 3Ni + occur in one step. Thus, there seem to be absolutely no basis... 

Online mass spectrometry data presented and discussed in the previous sections suggest that catalytic hypophosphite oxidation on nickel in D2O solutions proceeds via the coupling of anodic (19.11) and cathodic (19.12) half-reactions at the catalyst surface. The classical mixed-potential theory for simultaneously occurring electrochemical partial reactions [14] presupposes the catalyst surface to be equally accessible for both anodic (19.11) and cathodic (19.12) half-reactions. Equilibrium mixtures of H2, HD, and D2 should be formed in this case due to the statistical recombination of Hahalf-reactions (19.11) and (19.12) for example, the catalytic oxidation of hypophosphite on nickel in D20 solution under open-circuit conditions should result in the formation of gas containing equal amounts of hydrogen and deuterium (H/D=l) with the distribution H2 HD D2= 1 2 1 (the probability of HD molecule formation is twice as high as for either H2 or D2 formation [75]). Therefore, to get further mechanistic insight, the distribution of H2, HD, and D2 species in the evolved gas was compared to the equilibrium values at the respective deuterium content [54]. 

The theoretical interpretation of the high temperature superconductors is still under development. The copper oxide ceramic superconductors obtain their paired conducting electrons from copper in mixed oxidation states of I and II or II and III, depending on the particular system. The paired conducting electrons are called Cooper pairs, after Leon N. Cooper. Cooper s name also gives us the C of BCS the BCS theory is an interpretation of superconductivity for low temperature superconductors (having Tc s of less than 40 K). 

Till now no theory exists which permits an explanation or an insight into why some mixed oxides have greater basicity or greater basic strength of sites, than each component oxide. ... 

In part I above, c. Wagner s theory of mixed conduction was reviewed in terms of an equivalent circuit approach. The implications of mixed conduction theory for parabolic scaling of metals in high temperature atmospheres were also detailed. It was pointed out, however, that current interest in mixed conduction theory is no longer motivated by corrosion considerations because far too few systems of practical interest conform to the conditions required for pareibolic oxidation. 

Thimsen E, Biswas S, Lo CS, Biswas P (2009) Predicting the band structure of mixed transition metal oxides theory and experiment. J Phys Chem C 113 2014-2021... 

In the active region, the anodic electrochemical reaction is metal oxidation. Mixed potential theory governs the alloy corrosion in this r on. The corrosion potential and... 

According to mixed potential theory, any electrochemical reaction consists of partial reduction and oxidation reactions. In any redox reaction, such as the corrosion of a metal, there is no net accumulation of electric chaise and the rate of the oxidation must equal the rate of reduction. At the intersection of the cathodic and anodic kinetic lines (see Fig. 3.8), the rates of oxidation and reductions are equal. This point represents the corrosion potential, Eco .> and the corrosion current, At the... 

Solution pH, velocity, and oxidizer concentration change the properties of the anodic curve of the active-passive metal. For example, the equilibrium potential of the cathodic reaction shifts according to the Nemst equation in the noble direction by increasing the oxidizer concentration. Mixed potential theory, in this case, may predict the intersection of the cathodic and anodic Tafel fines and corrosion rate or extent of passivation of the metal. 

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