Reduction peroxide

The reaction in equation 55 occurs when there is insufficient hydrogen peroxide reductant in the absorber solution. Monitoring of the hydrogen peroxide concentration is important because is unstable in strong alkaline solutions, decomposing to water and oxygen. A small excess of sodium hydroxide is... 

Adzic RR, Markovic NM. 1982. Structural effects in electrocatalysis Oxygen and hydrogen peroxide reduction on single crystal gold electrodes and the effects of lead ad-atoms. J Electroanal Chem 138 443-447. 

Markovic NM, Tidswell IM, Ross PN. 1994. Oxygen and hydrogen peroxide reduction on the gold(lOO) surface in alkaline electrolyte the roles of surface structure and hydroxide adsorption. Langmuir 10 1-4. 

The use of hydrogen peroxide as an oxidant is not compatible with the operation of a biocatalytic fuel cell in vivo, because of low levels of peroxide available, and the toxicity associated with this reactive oxygen species. In addition peroxide reduction cannot be used in a membraneless system as it could well be oxidized at the anode. Nevertheless, some elegant approaches to biocatalytic fuel cell electrode configuration have been demonstrated using peroxidases as the biocatalyst and will be briefly reviewed here. 

The kinetics of hydrogen peroxide reduction catalyzed by Prussian blue has been investigated [12, 113]. In neutral media the reaction scheme of H202 reduction has been found to be the following ... 

The electrochemical rate constants for hydrogen peroxide reduction have been found to be dependent on the amount of Prussian blue deposited, confirming that H202 penetrates the films, and the inner layers of the polycrystal take part in the catalysis. For 4-6 nmol cm 2 of Prussian blue the electrochemical rate constant exceeds 0.01cm s-1 [12], which corresponds to the bi-molecular rate constant of kcat = 3 X 103 L mol 1s 1 [114], The rate constant of hydrogen peroxide reduction by ferrocyanide catalyzed by enzyme peroxidase was 2 X 104 L mol 1 s 1 [116]. Thus, the activity of the natural enzyme peroxidase is of a similar order of magnitude as the catalytic activity of our Prussian blue-based electrocatalyst. Due to the high catalytic activity and selectivity, which are comparable with biocatalysis, we were able to denote the specially deposited Prussian blue as an artificial peroxidase [114, 117]. 

Prussian blue-based nano-electrode arrays were formed by deposition of the electrocatalyst through lyotropic liquid crystalline [144] or sol templates onto inert electrode supports. Alternatively, nucleation and growth of Prussian blue at early stages results in nano-structured film [145], Whereas Prussian blue is known to be a superior electrocatalyst in hydrogen peroxide reduction, carbon materials used as an electrode support demonstrate only a minor activity. Since the electrochemical reaction on the blank electrode is negligible, the nano-structured electrocatalyst can be considered as a nano-electrode array. 

Except for Prussian blue activity in hydrogen peroxide, reduction has been shown for a number of transition metal hexacyanoferrates. The latter were cobalt [151], nickel [152], chromium [150], titanium [153], copper [154], manganese [33], and vanadium [28] hexacyanoferrates. However, as was shown in review [117], catalytic activity of the mentioned inorganic materials in H202 reduction is either very low, or is provided by impurities of Prussian blue in the material. Nevertheless, a number of biosensors based on different transition metal hexacyanoferrates have been developed. 

A.A. Karyakin, E.E. Karyakina, and L. Gorton, The electrocatalytic activity of Prussian blue in hydrogen peroxide reduction studied using a wall-jet electrode with continuous flow. J. Electroanal. Chem. 456, 97-104 (1998). 

The reversible O2 potential has not been observed on palladium, due to the existence of a local cell, but O2 evolves from water oxidation on a Pd02 covered surface and the reduction mechanism is supposed to be the same as on Pt, the rate determining step being the first electron addition [31]. In basic media, the reduction on Pd proceeds mainly through the 4e mechanism, which has been attributed to a high catalytic activity for the peroxide reduction, or disproportionation [83, 84]. 

Electrocatalytic Reduction of Dioxygen and Hydrogen Peroxide These two processes must be emphasized because reduction of dioxygen, and eventually hydrogen peroxide, features the usually claimed pathway for reoxidation of reduced POMs after the participation of the latter in oxidation processes. As a consequence, electrocatalysis of dioxygen and hydrogen peroxide reduction is a valuable catalytic test with most new POMs [154, 156,161]. 

Pb Lead, and particularly underpotentially deposited Pb, exhibits electrocatalytic properties in numerous electrode processes. The model reaction can be oxygen reduction with slow step of peroxide reduction ]374-376] or reduction of nitrobenzene and other nitrocompounds [377, 378]. In the case of... 

Alkoxyl radicals can be generated by a variety of methods including peroxide reduction, nitrite ester photolysis, hypohalite thermolysis, and fragmentation of epoxyalkyl radicals (for additional examples of alkoxyl radical generation, see Section 4.2.S.2). Hypohalites are excellent halogen atom donors to carbon-centered radicals, and a recent example of this type of cyclization from the work of Kraus is illustrated in Scheme 43.182 Oxidation of the hemiketal (57) presumably forms an intermediate hypoiodite, which spontaneously cyclizes to (58) by an atom transfer mechanism. Unfortunately, the direct application of the Barton method for the generation of alkoxyl radicals fails because the intermediate pyridine-thione carbonates are sensitive to hydrolytic reactions. However, in a very important recent development, Beckwith and Hay have shown that alkoxyl radicals are formed from N-alkoxypyridinethiones.183 Al-... 

R. Prabhakar, T. Vreven, M. J. Frisch, K. Morokuma and D. G. Musaev, Is the protein surrounding the active site critical for hydrogen peroxide reduction by selenoprotein glutathione peroxidase An ONIOM study, J. Phys. Chem. B, 110 (2006) 13608-13613. 

Photo-oxidation of heterocycles leads to unstable endoperoxides whose existence has been generally proven by low temperature NMR. In addition to the classical transformations of peroxides (reduction, hydrolysis, deoxygenation, generally performed at low temperature), the bicyclic peroxides from heterocycles can afford characteristic rearranged products... 

---