Hydrogen redox reactions

Figure 10-3 juxtaposes the Fermi levels of the following redox reactions in aqueous solutions and the quasi-Fermi levels of interfacial electrons and holes in an n-type semiconductor electrode erhjo/Hj) of the hydrogen redox reaction F(0a/H20) of the oxj en redox reaction ersc) of the n- q)e semiconductor and... 

Fig. 10-16. Polarization curves for anodic oxygen and cathodic hydrogen redox reactions on an n-type semiconductor electrode of titanium oxide in the dark and in a photoex-cited state i = anodic current in the dark (zero) = anodic current...

In order for the photoelectrolytic decomposition of liquid water to proceed, the Fermi levels of the redox reactions in Eqns. 10-53a and 10- 3b need to be located within the band gap of the n-type semiconductor anode. In Fig. 10-26(a), we have assumed that the Fermi level ep(ac) of the n-type semiconductor anode at the flat band potential is higher than the Fermi level ep(h-/h2) of hydrogen redox reaction we have also assumed that the Fermi level e,(M) of the metallic cathode is lower than ekh /Hj)- Further, we have assiuned that the edge level of the conduction band is higher than the Fermi level of hydrogen redox... 

Fig. 10-26. Energy diagrams of a cell for photoelectrolytic decomposition of water consisting of a metal cathode (M) and an n-type semiconductor anode (n-SC) of which the Fermi level is higher than the Fermi level of hydrogen redox reaction ( R8o>ep(H /H2)) (a) cell circuit is open in the dark, (b) cell circuit is closed in the daric, (c) cell circuit is closed in a photoezdted state (cell reaction proceeds.). potential hairier of a space charge layer.

Since the highest possible Fermi level of the photoexcited n-type anode corresponds to the flat band potential of the semiconductor anode, the Fermi level of the metallic cathode short-circuited with the photoexcited n-lype anode can also be raised up to the level equivalent to the flat band potential of the semiconductor anode. In order for the cathodic electron transfer of hydrogen redox reaction to proceed at the metallic cathode, the Fermi level 1 of the cathode needs to be higher than the Fermi level of hydrogen redox reaction. Consequently, in... 

With n-type anodes that do not meet this requirement, an external voltage, V , must thus be applied to the photoelectrols c cell until the Fermi level erm) of the metallic cathode exceeds the Fermi level ef(h /h2) of hydrogen redox reaction (ep(M)>EF(H+/H2) for the photoelectrolytic decomposition of water to proceed as shown in Pig. 10-27. 

In order for this mixed electrode reaction shown in Fig. 11-2 to proceed, the Fermi level efod of the iron electrode must be higgler than the Fermi level erh-zhj) of the hydrogen redox reaction and also must be lower than the Fermi level for the transfer reaction of iron ions. In other words, the potential E of the iron electrode must be lower than the equilibrium potential of the... 

The most cited reference electrode is the platinum-hydrogen electrode, and electrode DC potentials are often given relative to such an electrode. It is an important electrode for absolute calibration, even if it is impractical in many applications. The platinum electrode metal is submerged in a protonic electrolyte solution, and the surface is saturated with continuously supplied hydrogen gas. The reaction at the platinum surface is a hydrogen redox reaction H2 2H (aq) + 2e, of course with no direct chemical participation of the noble metal. Remember that the standard electrode potential is under the condition pH = 0 and hydrogen ion activity 1 mol/L at the reference electrode. Thus the values found in tables must be recalculated for other concentrations. Because of the reaction it is a hydrogen electrode, but it is also a platinum electrode because platinum is the electron source or sink, and perhaps a catalyst for the reaction. 

In the case of cathode starvation, it is likely that, instead of oxygen reduction, hydrogen evolution from water present at the cathode will take place. Since the hydrogen redox reactions are taking place with high exchange current density, the... 

Oxidoreduciases. Enzymes catalysing redox reactions. The substrate which is oxidized is regarded as the hydrogen donor. This group includes the trivially named enzymes, dehydrogenases, oxidases, reductases, peroxidases, hydrogenases and hydroxylases. 

Heat treatment of related glasses melted under reducing conditions can yield a unique microfoamed material, or "gas-ceramic" (29). These materials consist of a matrix of BPO glass-ceramic filled with uniformly dispersed 1—10 p.m hydrogen-filled bubbles. The hydrogen evolves on ceranarning, most likely due to a redox reaction involving phosphite and hydroxyl ions. These materials can have densities as low as 0.5 g/cm and dielectric constants as low as 2. 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

Redox reactions occur in the reduction of ores (metal oxides) into pure metals and the corrosion (oxidation) of pure metals in the presence of oxygen and water. Rusting iron, 4Fe + 30, + 611,0 —> 4Fe(OH), is a good example of metal oxidation. Strong oxidizing agents can be used as antiseptics (hydrogen peroxide, Fd,0,) or bleaches (sodium hypochlorite, NaOCl). 

Metallic elements taking part in redox reactions, such as zinc in the reaction above, commonly act as reducing agents they are oxidized to cations such as Zn2+. Other reducing agents include hydrogen gas, which can be oxidized to H+ ions ... 

Like ammonia, hydrogen sulfide (oxid. no. S = —2) can act only as a reducing agent when it takes part in redox reactions. Most often the H2S is oxidized to elementary sulfur, as in the reaction... 

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

Flavin Adenine Dinucleotide (FAD) (C27 H33 N9 O15P2) is a coenzyme that acts as a hydrogen acceptor in dehydrogenation reactions in an oxidized or reduced form. FAD is one of the primary cofactors in biological redox reactions. 

Common components of many redox systems are a peroxide and a transition metal ion or complex. The redox reactions of peroxides are covered in the sections on those compounds. Discussion on specific redox systems can be found in sections on diacyl peroxides (3,3.2.1.5), hydroperoxides (3,3.2.5) persulfate (3.3.2.6.1) and hydrogen peroxide (3.3.2.6,2). 

Hydroxy radicals are produced by redox reactions involving hydrogen peroxide (see 3.3.2.6.2). They can also be generated in organic solution by thermal decomposition of a-hydroperoxydiazenes (see 3.3.3.1). 

One of the most used systems involves use of horseradish peroxidase, a 3-diketone (mosl commonly 2,4-pentandione), and hydrogen peroxide." " " Since these enzymes contain iron(II), initiation may involve decomposition of hydrogen peroxide by a redox reaction with formation of hydroxy radicals. However, the proposed initiation mechanism- involves a catalytic cycle with enzyme activation by hydrogen peroxide and oxidation of the [3-diketone to give a species which initiates polymerization. Some influence of the enzyme on tacticity and molecular... 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

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