Oxidation-reduction phenomena

The necessity to have more than one component in a catalyst arises from many needs those linked to the polyfunctionality often required for the different steps in a reaction, the need to enhance the rate of some reaction steps, inhibition of unwanted side reactions, provision of adequate thermal stability, to take advantage of observed synergetic effects. From a fundamental point of view, the presence of several metal elements in a common structure permits the adjustment of the local electronic properties, imposes well defined coordinations, limits the extent of oxidation-reduction phenomena, and may stabilize the whole catalyst by retarding sintering. Mixed oxide catalysts are used as such, or as precursors of active catalysts, for a whole range of important industrial processes, a representative selection of which is given in Table 1. 

Ethanal is the most important of these compounds (Table 2.6). The many ways it can be produced and its high reactivity (the CHO radical has extensive chemical affinities), as well as its rapid combination with sulfur dioxide at low temperatures and its organoleptic properties, make ethanal a very important component of wine. The presence of ethanal, produced by the oxidation of ethanol, is closely linked to oxidation-reduction phenomena. It is involved in the alcoholic fermentation mechanism. Furthermore, ethanal plays a role in the color changes occurring in red wines during aging by facilitating the copolymerization of phenols (anthocyanins and catechins) (Section 6.3.10). 

Considering oxidation-reduction phenomena, when A1 reacts with O2 to produce AljOj,... 

Explain what the two reaetions below show about oxidation-reduction phenomena in water. How are they tied in with pE in water How is pE conceptually defined ... 

Displacement is ordinarily associated with either acid-base or oxidation-reduction phenomena. For example ... 

Dead-stop-end-point method was first introduced by Foulk and Bawden in 1926. Evidently, this particular technique is a modification of the classical amperometric titration. This technique is specifically applicable to only such systems where the phenomenon of oxidation-reduction exists both before as well as after the equivalence point has been duly accomplished. 

Moreover, despite the many advances in electrochemical measurement and modeling, our understanding of SOFC cathode mechanisms remains largely circumstantial today. Our understanding often relies on having limited explanations for an observed phenomenon (e.g., chemical capacitance as evidence for bulk transport) rather than direct independent measures of the mechanism (e.g., spectroscopic evidence of oxidation/reduction of the electrode material). At various points in this review we saw that high-vacuum techniques commonly employed in electrocatalysis can be used in some limited cases for SOFC materials and conditions (PEEM, for example). New in-situ analytical techniques are needed, particularly which can be applied at ambient pressures, that can probe what is happening in an electrode as a function of temperature, P02, polarization, local position, and time. 

As a chemical phenomenon, weathering can be viewed as the result of the tendency of the rock-water-mineral system to attain equilibrium. This occurs through the usual chemical mechanisms of dissolution and precipitation, acid-base reactions, complexation, hydrolysis, and oxidation-reduction. 

Redox reactions are not limited to single replacement reactions. They really describe a wide variety of reactions, but each shares the common theme of involving an oxidation and a reduction. An oxidation occurs when a substance loses electrons and becomes more positively charged. Earlier in the book we discussed a similar phenomenon in the formation of ionic compounds. Substances don t just lose electrons for no reason. They lose electrons because another substance takes them. When a substance acquires additional electrons and becomes more negatively charged, it is called a reduction. An oxidation cannot take place without a reduction, so these processes must occur simultaneously. These reactions describe the simultaneous oxidation and reduction of materials, which has earned them the name oxidation-reduction reactions. 

The charge-transfer phenomenon also appears in many other adducts. If the charge-transfer transition actually transfers the electron permanently, the result is an oxidation-reduction reaction—the donor is oxidized and the acceptor is reduced. The sequence of reactions of [Fe(H20)6] (the acid) and aquated halide ions (the bases)... 

Certain biotransformation processes are reversible, and formation of an inactive metabolite that can be converted back to the active drug delays the removal of the drug from the body and probably prolongs the duration of exposure of the target tissues to the drug. The common processes that can contribute to this phenomenon are oxidation/reduction of secondary alcohols/ketones, sulfides/sulfoxides, and tertiary amines/N-oxides, all of which are reversible processes. 

Electrochromism is a phenomenon displayed by some materials reversibly changing colors. Various materials can be used to construct electrochromic devices, such as transition metal oxides, liquid crystals, photonic crystals, and polymers (Booth and Casey, 2009 Nicoletta et al., 2005 Arsenault et al., 2007 Gamier et al., 1983). Here, we will focus on the electrochromic materials based on polymers. There are several mechanisms to explain the color changes of polymer electrochromic materials like electro-induced oxidation-reduction and electrothermal chromatic transition and so on. 

Many studies have been reported during the last almost 20 years regarding the effect of the electrochemical promotion of catalysis and its origin and application to several types of reactions of environmental and industrial interest. The effectiveness of NEMCA for catalytic oxidations, reductions, hydrogenerations, decompositions, and isomerizations using numerous types of soUd electrolytes and catalysts underlines the importance of this phenomenon in both catalysis and electrochemistry. Application of these... 

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