Simultaneous reduction and oxidation

Fig. 9. Principle of single catalytic bed for simultaneous reduction and oxidation with oxygen sensor and feedback control on air-to-fuel ratio.

Photocatalysis, i.e., using semiconductor particles under band gap irradiation as little micro reactors for the simultaneous reduction and oxidation of different redox systems, has been intensively studied during the last 25 years since the pioneering work of Carey et al [1]. The main focus of these studies seems to be the investigation of the principal applicability of photocatalytic systems for the efficient treatment of water and air streams polluted with toxic substances. Several review articles on this topic have recently been published [2]. In some cases, pilot-scale or even commercially available reactors have already been constructed, especially when titanium dioxide is used as the photocatalyst [3]. 

If the polarity of the current is not allowed to change too rapidly, it is possible, since oxidation and reduction occur successively at each pole, to accomplish electrolyses with alternating currents. Experiments with this end in view have been made b Drechsel.1 Dehydration is a case of simultaneous reduction and oxidation. The supposition that in living organisms carbamide is produced from ammonium carbamate by the splitting off of water prompted Drechsel to make experiments in this direction. When an aqueous solution of ammonium carbamate is electrolyzed with a current from a battery of 4-6 Grove cells, and platinum electrodes used, carbamide is obtained independently of the electrode material when alternating currents are employed. The reactions are supposed to be either... 

Tennakone et al. reported simultaneous reductive and oxidative photo-catalytic N2 fixation in aerated water in the presence of Nafion films loaded with hydrous iron(III) oxide [84]. (We note that Nafion film is a per-fluorinated ion-exchange membrane that, in the hydrogen-ion form, is a superacid. Some Nafion films are reinforced with Teflon.) Nafion film [85,86] was refluxed in concentrated HC1, washed repeatedly, and boiled in distilled water. The film was immersed in aqueous 0.1 MFeCl3 for 30 min, rinsed with water, and immersed in 0.8 M NaOH for 45 min. The treated film was found to contain particles of hydrous iron(HI) oxide with a diameter of about 48 A. The density distribution of Fe3+ in the treated film was approximately 2/imolcm"2. 

M is a transition metal complex A and D are an electron acceptor and an electron donor, respectively. In order to store for a reasonable period of time, one must either avoid the reaction between A" and (energy wasting process) or rapidly convert those two species into A and D with simultaneous reduction and oxidation of an appropriate substrate (H2O, for instance). 

Although absence of any redox reactions is ideal for electrode lifetime, these reactions are very hard to avoid. As soon as the electrode materials come in contact with an ionic solution, lots of different reactions may occur, even if no current flows into the electrode. Consider a piece of iron immersed in saline solution. After enough time, rust appears on the iron surface. Corrosion occurs following simultaneous reduction and oxidation reactions on the surface. While the metal is oxidized, oxygen (O2) or H+ ions are reduced at the same time to complete the redox reaction. 

Transition metal oxides attract great interests mainly due to their redox nature, which is thought to be related with their flexible stmcture modiflcation under reductive and oxidative conditions. Such stmcture modiflcation takes place by forming so called crystallographic shear (CS) stmctures to accommodate anion vacancies in speciflc crystallographic planes by simultaneous shear displacement and crystal stmctural collapse [30-32]. High-resolution transmission electron microscopy (HRTEM) is a... 

Redox processes affect contaminant solubility and may result from fluctuating saturation and drying processes in the subsurface due to natural or anthropogenic factors. Reduction and oxidation processes also may occur simultaneously in a partially... 

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

Oxidation and reduction are two sides of the same coin, in that oxidation is about the loss of electrons and reduction is about the gain of electrons. Within any chemical reaction, if oxidation is occurring, reduction is also taking place. This simultaneous occurrence of oxidation and reduction is called redox, a combination of the words reduction and oxidation. The redox reactions of our everyday life include respiration, rusting, all combustion, and most metabolic reactions in living organisms. 

Treatment of pentaacetyl-D-galactopyranose with liquid hydrogen bromide at room temperature yielded 6-deoxy-6-bromo-2,3,4-triacetyl-a-D-galactopyranosyl bromide (LXXVIII). The action of silver carbonate and methanol produced methyl 6-deoxy-6-bromo- 8-D-galacto-pyranoside 2,3,4-triacetate (LXXIX). Simultaneous reduction and deacetylation in alcoholic sodium hydroxide with Raney nickel gave methyl 0-D-fucopyranoside, which with silver oxide and methyl iodide afforded methyl trimethyl-/ -D-fucopyranoside (LXXX), from which... 

Since photogenerated MV" " and Ru(bipy) can be used for water reduction and oxidation respectively, it is tempting to examine a system where the two catalytic processes can take place simultaneously following photoinduced electron transfer. As was pointed out above, the RUO2 and Pt cateOysts have to be active enou to intercept the back reaction. Also, their intervention has to be specific in that MV reacts selectively with the Pt particles while RuCbipy) " " interacts with RUD2. Cross reactions have to be avoided since they lead to short-circuitry of the back reaction. 

It is also possible to consider a simultaneous Hg(II) reduction and oxidation of the organic moiety, taking place on the initial organomercurial. However, it is not possible to distinguish between simultaneous or consecutive steps. 

In some cases, a chemical species may simultaneously undergo reduction and oxidation. For example, when chlorine gas is bubbled through a basic solution, the resulting products contain chlorine in the —1 and +1 oxidation states (i.e., Cl-... 

Solid solutions of transition metal carbides and carboni-trides, which also play an important role in the hardmetal industry, can be advantageously manufactured by the simultaneous reduction of oxide or oxide-carbide mixtures in the presence of carbon (equation 3). 

While an ovapotential may be applied electrically, we are interested in the overpotential that is reached via chemical equilibrium with a second reaction. As mentioned previously, the oxidation of a metal requires a corresponding reduction reaction. As shown in Figure 4.34, both copper oxidation, and the corresponding reduction reaction may be plotted on the same scale to determine the chemical equilibrium between the two reactions. The intersection of the two curves in Figure 4.34 gives the mixed potential and the corrosion current. The intersection point depends upon several factors including (the reversible potential of the cathodic reaction), cu2+/cu> Tafel slopes and of each reaction, and whether the reactions are controlled by Tafel kinetics or concentration polarization. In addition, other reduction and oxidation reactions may occur simultaneously which will influence the mixed potential. 

The Simultaneous Occurrence of Oxidation and Reduction. Oxidation or reduction of a substance could be carried out without simultaneous reduction or oxidation of another substance if one had at hand a very large electrical condenser from which to remove electrons or in which to store them. Ordinarily such an electron reser oir... 

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