Water reduction to hydrogen

Reactions using the anthracene carboxylate as the primary sensitizer are also known as for instance in the water reduction to hydrogen AC- triplet is quenched by MV2+. The obtained viologen radical cation initiates hydrogen formation mediated by platinum/polyvinyl alcohol. The AC- anion is regenerated from the corresponding radical by electron transfer from EDTA and the H2 production quantum yield is close to unity [165]. 

An example of a possible system for photocatalytic water decomposition is shown in Fig. 6. The photocatalyst in Fig. 6 is a CdS nanoparticle, which is located, e.g., in the inner aqueous phase. A sacrificial electron, donor (D) is also located in the inner phase. In the presence of a suitable water oxidation catalyst, the role of the donor could be served by the molecules of water. The molecular carriers of electrons (C in the figure) are built into the lipid membrane by the principle of a cascade, providing a certain gradient of redox potentials. In the outer aqueous phase, an electron acceptor and a catalytic agent of water reduction to hydrogen are placed. Thus, at light quantum absorption by the semiconductor PhC, the charge separation derives an electron hole which passes to the catalyst of... 

A reduction to hydrogen peroxide is obtained with enzymes with one copper atom of type-2 (Table 1). A reduction to water requires enzymes with a dimeric site, with the three types of copper or with a four-metal center (2 heme, 2 Cu) (Table 1). 

As shown in Figure 6.49a, the cracks grow by slip dissolution due to diffusion of active water molecules, halide ions, etc., to the crack tip, followed by a rupture of the protective oxide film by strain concentration, fretting contact between the crack faces. This is followed by dissolution of the fresh exposed surface and growth of the oxide on the bare surface. For the alternative mechanism of hydrogen embrittlement in aqueous media, the critical steps involve diffusion of water molecules or hydrogen ions to the crack tip reduction to hydrogen atoms at the crack tip surface diffusion of adsorbed atoms to preferential surface locations absorption and diffusion to critical locations in the... 

The principal sources of sulfate in formation waters are dissolved marine sulfate, sulfate derived from the dissolution of evaporites, and sulfate formed by the oxidation of sulfides. Sulfate is destroyed by reduction to hydrogen sulfide. The value of 5 " S in gypsum is only — 1.6%o heavier than sulfate in solutions from which it precipitates. The isotopic composition of sulfur in gypsum in Phanerozoic deposits precipitated from seawater during evaporation thus tracks the secular changes in the isotopic composition of sulfur in seawater ( 10% to 30%o). 

The four-electron reduction of O2 to water has a very favorable potential of + 1.229 volts (see Table 7.1), making O2 a powerful oxidant, at least in principle. Two-electron reduction to hydrogen peroxide (H2O2) is much less favorable ( = +0.68... 

From the chemical viewpoint sulphates are quite stable. However, under anaerobic conditions they fairly easily undergo biochemical reduction to hydrogen sulphide and can almost disappear from water. Under certain conditions elementary sulphur can also separate as a transient oxidation state. These processes can be represented as follows ... 

The case may be exemplified by an oxide semiconductor electrode in an indifferent electrolyte solution. Band diagram of the cell with the aqueous electrolyte solution is shown in Fig. 7, Now, two electrochemical potential levels in the solution are the characteristics of the two reactions of water transformation its oxidation to oxygen, and reduction to hydrogen ... 

The reduction of dioxygen to water may occur via a number of intermediate states having various thermodynamic stabilities. These are compounds with hydrogen and their different ionized (deprotonated) forms. The standard redox potentials relevant to reduction to hydrogen peroxide and subsequently to water are listed in Table II [20,21]. 

Figure 4 Experimental derivative chronopotentiograms of the reduction of mercurous cysteine thiolate in 0.1 mol I phosphate buffer, pH 7.4 (A) before and (B) after deaeration. The presence of dissolved oxygen, which is simultaneously being reduced at the working electrode, enhances the response. The dissolved oxygen is reduced in two steps (peaks on either side of a). The peak on the left is the reduction to hydrogen peroxide and the peak on the right is the reduction of hydrogen peroxide to water. (Reproduced with permission from Honeychurch MJ and Ridd MJ (1995) The effect of a non-adsorbing electroactive species on the transition time in derivative adsorptive chronopotentiometric stripping analysis. Electroanalysis 1047-1054 Wiley-VCH.)...

In general, biosensors contain a biological (usually polymeric) entity in the recognition environment of the sensor [166, 167]. Enzymes are the most frequently used biomodifiers, in particular, such ones that catalyze oxidation or reduction reactions (oxidoreductases), that is, oxidases and dehydrogenases, when the former ones exploit oxygen as the electron acceptor with consequent reduction to hydrogen peroxide or water. 

With this set-up, light absorption induces electron injection from the photosensitizer to the conduction band of the semiconductor while these high energy electrons are transferred to the cathode for proton reduction to hydrogen, the holes in the photosensitizer are filled by electron transfer from the catalyst that evolves to its active form capable of water oxidization. 

The related electron transfer mechanism for the reduction to hydrogen peroxide is shown in Fig. 16.12. The authors concluded that there are no evidences for a direct reduction to water, but that the integration of transition metals might enhance the catalytic interaction to enable the four-electron transfer process [135],... 

The concentration dependence of the indirect reduction mechanism for TCE can be understood from the data in Figure 9. At low TCE concentrations, there is sufficient atomic hydrogen produced from water reduction to measurably contribute to TCE dechlorination. For example. Figure 9 shows that at concentrations less than 1 mM, the current for water reduction was greater than the direct current for TCE reduction, whereas, at higher TCE concentrations, the water reduction current was small compared to the direct reduction current for TCE. 

A further difficulty is that reduction of moleaalar oxygen coidd easily lead to the 2-electron reduction to hydrogen peroxide instead of the 4-electron reduction to water or OH . This is to be expected, since the reaction... 

Compare the effect of pH on the cathode potential for oxygen reduction to water or to hydrogen peroxide (5 mM) over a pH range of 4 to 11. 

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