Reaction Mechanisms of Ethanol Oxidation

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

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. 

This reaction mechanism of ethanol oxidation on a Pt-based electrode can be summarized by the following scheme [4] (Scheme 1). 

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. 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

Several studies on the electro-oxidation of ethanol have been mainly devoted to the identification of the adsorbed intermediates onto the electrode and to the elucidation of the reaction mechanism by means of various techniques, such as differential electrochemical mass spectrometry (DBMS) [7-12] and in situ Fourier transform infrared spectroscopy (FTIR) [13-15]. On the basis of the foregoing work, the global mechanism of ethanol oxidation in acidic solution may be summarized according to the following scheme of parallel reactions [2] ... 

Neither the reactant-pair mechanism [19] nor the water discharge mechanism [25] are in agreement with the pH dependence for the oxidation of type I species. The pH dependence may be explained as suggested by Frumkin and Podlovchenko [4] in their discussion of the mechanism of ethanol oxidation. A chemical reaction between adsorbed carbonaceous species and OH d is supposed rate-determining on a heterogeneous surface of the Temkin type [34]. The discussion is the same as that for the oxidation mechanism of chemisorbed carbonaceous species in section 11 of chapter IX. 

The reaction mechanisms of the anodic oxidation of other alcohols than methanol is more difficult to elucidate since the number of electrons exchanged greatly increases and the activation of the C-C bond breaking is relatively difficult at moderate temperatmes (50-90°C). The complete oxidation of ethanol involves 12 electrons per molecule, i.e.,... 

Dehydrogenases, classified under E.C.1.1., are enzymes that catalyze reduction and oxidation of carbonyl groups and alcohols, respectively I5l The natural substrates of the enzymes are alcohols such as ethanol, lactate, glycerol, etc. and the corresponding carbonyl compounds, but unnatural ketones can also be reduced enantiose-lectively. To exhibit catalytic activities, the enzymes require a coenzyme most of the dehydrogenases use NADH or NADPH, and a few use flavin, pyrroloquinoline quinone, etc. The reaction mechanism of the dehydrogenase reduction is as follows ... 

The Wacker-type oxidation of the olefins is one of the oldest homogeneous transition metal-catalyzed reactions. The mechanism of the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system is shown in Figure 23. Interestingly, the selectivity of the oxidation of olefins with longer alkyl chains is dependent on their solubility in water. Furthermore, the production of chlorinated side-products and isomerized olefins has also occurred for olefins with low water solubility. In order to avoid the solubility issues, co-solvents such as DMSO, acetone, THF, dioxane, acetonitrile, DMF, and ethanol were used and DMF seemed to be the best. ... 

The mechanism of this oxidation for the enzyme liver alcohol dehydrogenase is shown for the reaction of 83, where ethanol is bound to the active site of the enzyme to give 84 via proton abstraction and then hydride transfer to generate acetaldehyde (see 85). NAD+ binds to the active site of the enzyme to induce a conformational change (see Chapter 8 for conformation) to close the active site. The oxidation of ethanol to acetaldehyde (ethanal) is accompanied by reduction of NAD+ to NADH, as shown in the illustration. 

The role of Ru in the mechanism of ethanol electro-oxidation is similar to that of Sn. The adsorption and decomposition of ethanol and its intermediate reaction products happen on Pt active sites, while the dissociative adsorption of water occurs over Sn or Ru sites, to form oxygen-containing surface species. Antolini et al. [24] have shown that the Ru addition of Ru to PtSn catalysts can enhance the catalytic activity of a certain composition. However, this enhancement is related to the Ru/Sn ratio that is present in the alloy, as well as to the synergetic effect of Ru and Sn oxides. 

De Lima, S. M., Da Silva, A. M., Da Costa, L. O. O., et al., 2009, "Study of catalyst deactivation and reaction mechanism of steam reforming, partial oxidation, and oxidative steam reforming of ethanol over Co/Ce02 catalyst". Journal of Catalysis, v. 268, n. 2, p.268-281. 

Wang HP, Liu ZP Comprehensive mechanism and structure-sensitivity of ethanol oxidation on platinum new transition-state searching method for resolving the complex reaction network,/Am Chem Soc 130(33) 10996-11004, 2008. 

Facial dushing after ingestion of alcohol occurs in up to one-third of patients taking chlorpropamide. The mechanism, like that of the disulfiram reaction, probably involves inhibition of the oxidation of acetaldehyde, a metaboUte of ethanol. The plasma concentration of chlorpropamide may be correlated with chlorpropamide—alcohol dushing. 

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