Oxidation 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]. 

Fig. 3. Polarization curves for anodic oxidation of hypophosphite on different metals (bath composition 0.2 mol dm NaH2P02, 0.2 mol dm sodium citrate, 0.5 mol dm H3BO3, pH 9, 70°C). Dashed lines current density attributed to anodic dissolution of silver or copper (I. Ohno et al., 1985 [5]).

In the following text, we will present the data on catalytic hypophosphite oxidation on nickel using deuterium tracer and online electrochemical mass spectrometry studies of partial reactions and their mutual interaction as a function of electrode potential, the modeling of the catalyst surface state upon the oxidation of hypophosphite, isotopic gas composition during electroless Ni-P alloy... 

The differences between hydrogen production rate from hypophosphite and water are reflected in the time dependence of deuterium content in the gas evolving during the catalytic oxidation of hypophosphite by heavy water on nickel catalyst (Figure 19.2A(c)), as calculated from the corresponding mass spectra using... 

Minor variations in hypophosphite oxidation rate on the electrode potential (Figures 19.2A(b) and 19.3A(a)) could be interpreted as the evidence of a non-electrochemical rate-determining step in the anodic oxidation of hypophosphite, for example, catalytic dehydrogenation [19], possibly, via hypophosphite radical intermediate formation, as was evidenced from the electron spin-trap resonance spectroscopy measurements [73], although this was not confirmed by in situ IR spectroscopy, apparently, due to a short lifetime of the radical intermediate [74]. 

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]. 

To decrease the pH of the bath, hypophosphite was tried as reducer [37]. Because the oxidation of hypophosphite only occurs on nickel particles, nickel had to be added to the bath resulting in coatings with a small... 

The reactions which take place are usually represented by (12.43) and (12.44), which is the equivalent of an electric current. In (12.43), two electrons, supplied by the oxidation of hypophosphite to phosphite, are used to reduce Ni " to Ni°. 

The oxidation of hypophosphite by [FeCCNlg] " in basic media obeysthe rate law... 

Complex formation is considered essential for the redox process. The role of the chloride ions as inhibitors is that they act as competitors in blocking the co-ordination sites on the metal ion. The situation is clearly more complicated than this, however. In a study from the same laboratory, the reverse order of reactivity of Tl chloro-complexes has been observed in the oxidation of hypophosphite. The explanation for this behaviour involves a unique reaction of hypophosphite in that... 

The copper(n)-catalysed oxidation of hypophosphite by SaO in perchloric acid media has been described, the rate law having the form... 

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