Ethanol oxidation mechanism

Since several steps in the ethanol oxidation mechanism require the presence of an OHads species, the use of an alkaline medium has also attracted some attention, owing to the ubiquitous hydroxide ions leading to significantly higher oxidation... 

One recent example is notable for its iimovative application and has already been mentioned here. A new stack concept from the University of Applied Science in Offenburg (Germany), included a new catalyst from the Italian company ACTA and an AEM membrane from the Japanese membrane producer Tokuyama. This consortium developed an AEM-based ethanol-powered AFC and successfully inserted it into an electric vehicle. This stack was not able to work for an extended time because the system was built without an ethanol loop and without a KOH re-concentrator, but it is an impressive demonstration of AEM technology and the direct use of ethanol. DLR together with the University of Diisseldorf are working with laboratory-scale fuel cells to understand the ethanol oxidation mechanism in detail. 

Regarding the ethanol oxidation mechanism, adsorbed CO and CHx fragments have been identified as the main poisoning reaction intermediates [77, 84—90]. 

Ethanol is oxidized by alcohol dehydrogenase (in the presence of nicotinamide adenine dinucleotide [NAD]) or the microsomal ethanol oxidizing system (MEOS) (in the presence of reduced nicotinamide adenine dinucleotide phosphate [NADPH]). Acetaldehyde, the first product in ethanol oxidation, is metabolized to acetic acid by aldehyde dehydrogenase in the presence of NAD. Acetic acid is broken down through the citric acid cycle to carbon dioxide (CO2) and water (H2O). Impairment of the metabolism of acetaldehyde to acetic acid is the major mechanism of action of disulfiram for the treatment of alcoholism. 

The microsomal ethanol oxidizing system is another mechanism of ethanol metabolism. CYP2E1 may be an important enzyme in the metabolism of ethanol in heavy drinkers, who may have a 10-fold increase in activity. Two aUehc variants in the gene cl and c2) are associated with differing enzymatic activity. Approximately 40% of Japanese have the more active c2 allele, which is rare in individuals of European heritage (Sun et al. 2002). It is not believed to be a risk or protective factor in the development of alcohohsm, although current studies are examining its relationship to a variety of ethanol-related diseases. 

At this stage, it should be pointed out that modihcation of a Pt-Sn catalyst by Ru atoms increases cell performance (and hence catalytic activity with regard to ethanol electro-oxidation), but has no effect on the OCV or on product distribution [Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be produced when the anode potential is increased and noncatalytically active tin oxides are formed. 

Figure 8.2 Reaction mechanism for ethanol oxidation on an Mo dimer/Si02 catalyst as an example of the reaction mode (a) in Figure 8.1...

The mechanism of ethanol oxidation is less well established, but it apparently involves two mechanistic pathways of approximately equal importance that lead to acetaldehyde and ethene as major intermediate species. Although in flow-reactor studies [45] acetaldehyde appears earlier in the reaction than does ethene, both species are assumed to form directly from ethanol. Studies of acetaldehyde oxidation [52] do not indicate any direct mechanism for the formation of ethene from acetaldehyde. 

Reactions alcohols, 29 36-49 adsorption, 29 36-37 clean surfaces, 29 37-38 ethanol oxidation, 29 44—48 methanol oxidation, 29 38-44 oxidation on copper and silver, 29 38-48 oxidation reaction, silver, 29 48-49 base-catalyzed, of hydrocarbons, 12 117 free radical mechanism in, of hydrogen peroxide, 4 343... 

It must be finally shown that Song et reported that the structural water in Ti02 nanotubes co-catalyzes ethanol oxidation and improves the tolerance to poisoning, but the mechanism is not fully clear. 

Reactions of anthocyanins and flavanols take place much faster in the presence of acetaldehyde that is present in wine as a result of yeast metabolism and can also be produced through ethanol oxidation, especially in the presence of phenolic compounds, or introduced by addition of spirit in Port wine technology. The third mechanism proposed involves nucleophilic addition of the flavanol onto protonated acetaldehyde, followed by protonation and dehydration of the resulting adduct and nucleophilic addition of a second flavonoid onto the carbocation thus formed. The resulting products are anthocyanin flavanol adducts in which the flavonoid units are linked in C6 or C8 position through a methyl-methine bond, often incorrectly called ethyl-link in the literature. 

The reaction pathway, reactivity of the active sites, and the nature of adsorbed intermediates constitute the catalytic reaction mechanism. Our study has been focused on the investigation of the nature of adsorbed intermediates under reaction conditions. We report the results of in situ infrared study of CO and ethanol oxidation on Au/Ti02 catalysts. This study revealed the high activity of Au/Ti02 is related to the presence of reduced Au and oxidized Au sites which may promote the formation of carbonate/carboxylate intermediates during CO oxidation. 

The induction of liver enzymes has been demonstrated in many species, including humans, and probably represents a homeostatic, defense mechanism. Induction usually requires multiple exposures to the inducing agent over a period of several days, the time required for the synthesis of new protein. Enzymes induced include the cytochrome P450 monooxygenase system glucuronyltransferase the microsomal ethanol oxidative system and the steroid-metabolizing system. 

Fig. 34. Reaction mechanism of ethanol oxidation at illuminated TiOj in the presence of O2 [159]...

Henglein et al. have also found dimerization-products at ZnS-colloids, using 2-propanol as an electron donor and CO2 as an electron acceptor [190]. The dimerization produced was pinacol, and its formation has been interpreted by the same mechanism as given for the ethanol oxidation. Inoue et al. performed the same experiment, but did not find pinacol as a dimerization-product [191]. According to Muller et al., these differences may be due to slight differences in the preparation of the ZnS-colloids, which seems to be extremely critical [187]. 

Finally, the combined voltammetric and on-line differential electrochemical mass spectrometry measnrements allow a quantitative approach of the ethanol oxidation reaction, giving the partial current efficiency for each product, the total number of exchanged electrons and the global product yields of the reaction. But, it is first necessary to elucidate the reaction mechanism in order to propose a coherent analysis of the DBMS results. In the example exposed previously, it is necessary to state on the reaction products in order to evaluate the data relative to acetic acid production which cannot be directly detected by DBMS measurements. However, experiments carried out at high ethanol concentration (0.5 mol L" ) confirmed the presence of the ethyl acetate ester characterized by the presence of fragments at m/z = 61, 73 and 88 at ratios typical of the ethyl acetate mass spectrum. " This ethyl acetate ester is formed by the following chemical reaction between the electrochemically formed acetic acid and ethanol (Bq. 29) and confirms the formation of acetic acid. 

These fuel cell results, completed by the different spectroscopic and chromatographic results, allowed us to propose a detailed reaction mechanism of ethanol oxidation, involving parallel and consecutive oxidation reactions, on Pt-based electrodes, where the key role of the adsorption steps was underlined. 

In Methanogenium thermophilum, which can use both H2 and ethanol as electron donor for CO2 reduction, the energetics and the mechanism of ethanol oxidation was... 

Figure 2 Ethanol oxidation. Temperature programmed experiments on alumina supported catalysts. MM mechanical mixture of Pt/AfiOr and Rh/Al203 Cl Pt-Rh/Al20j catalyst prepared by coimpregnation, a = fresh catalysts b = aged catalysts white column first oxidation cycle black column second oxidation cycle.

Although the major metabolic pathway for ethanol is via alcohol dehydrogenase (see below) there is also a microsomal ethanol oxidizing system (MEOS) which metabolizes ethanol to ethanal. The mechanism may involve hydroxylation at the carbon atom, although this is uncertain. Although this enzyme system is of minor importance in naive subjects, exposure to ethanol can induce the enzyme system such that it becomes the major enzyme system metabolizing ethanol. 

Indirect evidence for a relatively slow isomerization step that is rate-limiting under some conditions has also been obtained 35). The dissociation velocity constant. A -, for the compound E-NADH is increased threefold in the presence of sodium chloride, but the maximum rate of ethanol oxidation is only slightly increased thus, dissociation of NADH can no longer be the sole rate-determining step. Since the fast hydride transfer step was not affected by sodium chloride, and reasonable evidence that aldehyde dissociation is also relatively fast was obtained. Shore et al. 35) concluded that a new step in the mechanism had been revealed. This could be isomerization of either the ternary product complex or the binary complex E-NADH. Evidence of a similar slow step in the oxidation of ethanol and propanol with APAD as coenzyme 71) was referred to in Section II,E,1. 

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