Beyond Oxygen Evolution

The high overpotential for O2 evolution could be avoided if the reaction were replaced with a different anodic reaction. This replacement could in turn reduce AE, the minimum cell potential difference, which depends on the nature of the electrode reactions. Such a strategy has already been applied with success in the chlor-alkali industry, where the CI2-H2 couple (A = 1.35 V) has been replaced with CI2-O2 (A ri0.90 V) (O2 is reduced at the so-called air cathode). 

A few attempts to apply the same strategy have been made recently. In one case, anodic O2 evolution was replaced with oxidation of carbonaceous materials 

In another case, the electrolysis of an aqueous solution of NH3 was proposed [75-77]. The net reaction would be 

Further examples of recent attempts to reduce the consumption of electrical energy are the electrolysis of aqueous solutions of methanol (but CO2 is still produced at the anode) [78, 79] and water electrolysis using ionic liquids as electrolytes [80]. In the latter case, the authors claimed the possibility of obtaining high hydrogen production efficiencies using an inexpensive material such as low-carbon steel. 

The financial support of MIUR (Rome), PRIN Projects, is gratefully acknowledged. References 

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Molten salts are favored electrolytes in cases where aqueous solutions cannot be used because the decomposition voltage of water is lower than that of the salt in question. Although high overvoltages for hydrogen and oxygen evolution allow us to extend the use of aqueous electrolytes somewhat beyond the limits set by thermodynamics, there are many substances which cannot be electrowon or plated from aqueous solutions. 

Ti—O is also an excellent candidate as an initiator of H2O2 decomposition (Scheme 18.20). The subsequent decomposition of H02 species into molecular oxygen could follow, at least in part, known pathways [1], The details of oxygen evolution, however, is an issue that goes beyond the scope of this chapter. 

In comparison with the ionic solid corrosion discussed earlier, it is also worth noting that nickel ion transfer from the film into the solution is assumed to control the transpassive nickel dissolution whose rate increases with increasing interfacial potential. We may also see that the rate-determining process changes from the metal ion transfer to the oxide ion transfer near the oxygen evolution potential, beyond which the dissolution rate of the transpassive oxide film decreases with increasing interfacial potential, AH. 

Photosynthetic oxygen evolution measured as mass 32 in a particle preparation of Oscillatoria chalybea. An oxygen background of 50 mV represents a nearly complete anaerobic condition. Reactivation of the assay occurs by only increasing the oxygen tension to 53 mV. Normal oxygen content of air (21%) corresponds to a background value far beyond the upper limit of our device which lies at 12500 mV under these conditions. 

On illumination of Chlorella, a green alga, at wavelengths above 680 nm at which only chlorophyll a may absorb light, Emerson and coworkers in 1943 observed a progressive decline in photosynthesis as measured by oxygen evolution. This phenomenon was termed the red drop effect. By the simultaneous use of additional wavelengths below 680 nm, photosynthesis was enhanced by over 30% beyond the sum of the rates obtained by separate monochromatic illumination. This observation,... 

What are some of the unsolved problems in membrane bioenergetics in the 1980 s and beyond In photosynthesis, there are three, namely (i) the primary steps of photon conversion, (ii) electron transport and oxygen evolution, and (iii) ATP synthesis. In respiration, the main unsolved... 

SNII events alone explain the observed solar abundance distribution between oxygen and chromium. This can be taken as a major theoretical achievement. Complementary sources of hydrogen, helium, lithium, beryllium, boron, carbon and nitrogen are required, and these have been identified. They are the Big Bang, cosmic rays and intermediate-mass stars. Around iron and a little beyond, we must invoke a contribution from type la supernovas (Pig. 8.5). These must be included to reproduce the evolution of iron abundances, a fact which suggests... 

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