Hydrogen peroxide decomposition compensating reactions

As another example of catalytic decomposition of hydrogen peroxide by compensating reactions, Bray and his school have studied the system iodine iodate-hydrogen peroxide. In solutions of moderate acidity ( 0.1 N) iodate ion decomposes hydrogen peroxide and is itself unaffected at the end. Bray and Caulkins (30) observed that during the... 

Compensation behavior occurs in the decomposition of hydrogen peroxide on Ag-Au alloys (25) and, unlike most other alloy systems, there is a systematic change in the Arrhenius parameters with proportions of metals present. This behavior is ascribed to the progressive transformation, with alloy composition, of the reaction mechanism from that characteristic of one metal to that which occurs on the other. In contrast, decomposition of hydrogen peroxide on Pd-Au alloys (27) does not correlate with ratios of metals present in the catalyst, and kinetic parameters are sensitive to surface pretreatment. 

Compensation behavior found for the decomposition of hydrogen peroxide on preparations of chromium (III) oxide, which had previously been annealed to various temperatures, was attributed to variations in the energy states of the active centers (here e 0.165). Compensation behavior has also been observed (284) in the decomposition of hydrogen peroxide on cobalt-iron spinels the kinetic characteristics of reactions on these catalysts were ascribed to the electronic structures of the solids concerned. 

Although it has been well established by the work described above that the catalytic decomposition of hydrogen peroxide by the halides arises as a result of the compensating reactions (1) and (2), it is obvious from the kinetics of these reactions that the detailed mechanisms are more complicated than the stoichiometric equations would indicate. The early workers in this field recognized that the kinetics of reaction (1) as given by (a) could be explained by the sequence... 

The ready reversibility of the ferrocyanide-ferricyanide redox system makes it a potential catalyst for the decomposition of hydrogen peroxide by the mechanism of compensating oxidation-reduction reactions. Moreover, the well-known facts that in acid solution ferrocyanide is oxidized to ferricyanide, whereas in alkaline solution the reverse reduction occurs, seem a good indication that at suitable pH s both reactions might occur to give catalytic decomposition. But from the investigations to date it would appear doubtful whether any such catalysis occurs to a measurable extent, and that what seems to be ready reactions of ferro- and ferricyanides are in fact those of partial hydrolysis products of these ions in which water molecules replace the cyanide ions in the coordination shell. 

These observations are readily explained in terms of reactions of the aquo salts and it would appear that the usually accepted rapid oxidation of ferrocyanide by hydrogen peroxide is misleading. The fact that this is a very slow reaction means that although the ferricyanide reduction is fairly rapid there is no possibility of appreciable catalytic decomposition of the peroxide by the compensating reactions mechanism. 

It is not clear whether catalysis can occur purely by formation and decomposition of these perchromate intermediates as suggested by Kobosev or whether at all acidities there is reduction of CrVI to Crni and subsequent regeneration of CrVI along the lines of the compensating reactions mechanism. It appears from Spitalsky s work described above that in the more acid solutions Cr+++ is in dynamic equilibrium with CrVI. Whether this is also the case in less acid solutions where no Cr+++ is produced finally cannot be concluded from the present evidence, but such a possibility is clearly present since Cr+++ is readily oxidized to CrCV by hydrogen peroxide in neutral solution (111). 

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