Ethanol oxidation concentration effect

Camara GA, Iwasita T. 2005. Parallel pathways of ethanol oxidation The effect of ethanol concentration. J Electroanal Chem 578 315-321. 

If the re-oxidation of NADH to cofactor NAD is the rate-limiting step of ethanol oxidation in vivo (82), the different capabilities of alcohol dehydrogenase may not be fully reflected in the elimination rate of ethanol. There are also the possibilities that the effect of the variants depends on ethanol concentration, or that beta2 causes an initial spurt of ethanol oxidation in Orientals which is not seen in Caucasians who have the betai allele and in Blacks who have the beta3 allele. 

Pt-Ru catalysts also received much attention in ethanol oxidation [48, 67, 78, 79]. Iwasita et al. [48] investigated the EOR activity of ft-Ru electrodeposits as a function of their atomic composition and demonstrated that the catalytic activity of ft—Ru is strongly dependent on the Ru content. The optimum ft Ru compositimi was suggested to be 3 2. At low Ru concentration, there are insufficient Ru sites to effectively assist the oxidaticai of adsorbed residues, while ethanol adsorption can be inhibited when Ru concentration is too high (i.e., >ca. 40 %) due to the diminution of Pt sites. 

Figure 4.40. Ethanol oxidation currents (at 0.65 V vs. SSCE) on poly-NiSalen films deposited on vitreous carbon. The effect of ethanol concentration and NiSalen surface coverage Fni (mol cm ) a) 0.9x10 h) 1.8x10 c) 4.75x10 d) 6.2x10 . 0.1 M NaOH, 293 K [206]. (Trevin S, Bedioui F, Villegas MGG, Charreton-Bied C. Electropolymerized nickel macrocycle complex-based films design and electrocataljhic application. J Mater Chem 1997 7 923-8. Reproduced by permission of The Royal Society of Chemistry.)...

Additionally, Ni and CuNi supports were also explored for ethanol oxidation in alkaline media using RRu and PtMo eatalysts [201, 298]. EDX analysis showed that Ni was mostly present in metallie state, with some contribution from an oxide layer. The ethanol oxidation current density increased linearly on a logarithmic scale with the NaOH concentration (10 M to 2 M) for both PtRu and PtMo supported on CuNi (70 30 wt%) [201]. Unfortunately, no direct comparison was performed with carbon-supported catalysts. Thus, the contribution of the support to the observed electrocatalytic effect could not be assessed. PtMo had a higher initial activity however, after about 200 minutes its activity dropped below fliat of PtRu. Anodes with PtRu atomic ratios between 1.1 1 and 2.1 1 supported on Ni gave the lowest Tafel slopes for ethanol oxidation [298]. 

Apparently, Acetobacter is protected from the toxic effects of high concentration of acetic add by its own ethanol-oxidizing system, through maintenance of pH homeostasis. As the acetic add concentration increases, the pH outside approaches 2—4, whereas the pH inside the cells must be maintained at pH 6.5-7. Thus, for survival, a ApH of 4—2.5 must be maintained very much as for other acidophiles (White 1995). 

Rao et al. studied ethanol oxidation reaction in a real fuel cell using 40% Pt/C as cathode and Pt/C (20% and 40%), PtRu/C, and PtaSn/C as anodes [51]. Their DBMS sensor consisted on a cylindrical detection volume through which anode outlet flow passes. This volume was separated from the vacuum system of the mass spectrometer by a microporous Teflon membrane (pore size 0.02 (im and thickness of 110 (im) supported by a Teflon disk. For Pt/C and 0.1 M ethanol the carbon dioxide selectivity increased with the reaction temperature. The selectivity was highest at 0.5-0.6V and doubled from 60°C (40%) to 90°C (ca. 85%). At higher potentials the CO2 selectivity decreased and increased the acetaldehyde production. CH3CHO formation also increased at lower temperatures (at 90 °C and low, ethanol concentration was almost absent). At high ethanol concentrations the selectivity to carbon dioxide decreased but this effect was less significant than temperature effect at least for ethanol concentrations lower than 1M. 

The extent of coupling is also influenced by the solvent. In the hydrogenation of aniline over ruthenium oxide, coupling decreased with solvent in the order methanol > ethanol > isopropanol > t-butanol. The rate was also lower in the lower alcohols, probably owing to the inhibiting effect of greater concentrations of ammonia (44). Carboxylic acid solvents increase the amount of coupling (42). 

Induced oxidation of alcohols by hydrogen peroxide was studied by Kolthoff and Medalia . According to their measurements the value of F-, increases with the increase in the concentration of ethanol, while it decreases with increase in the acid concentration (see Table 16). In acetic acid medium the value of F[ is considerably lower. Chloride ions effectively suppress the induced oxidation of alcohols. The main product of the oxidation of ethanol is acetaldehyde which can be further oxidized to acetic acid. The data on the induced oxidation of alcohol (H2A) can be interpreted by reactions (53), (98), (99) and (57). 

Cu-CuO% nanoparticles (with a content of about 10 wt.%) on titania are effective for the production of hydrogen under sacrificial conditions [176-178], A fairly low concentration of Cu (2.5 wt.%) was sufficient to allow promising H2 production from ethanol-water and glycerol-water mixtures in the case of CuO% nanoparticles encapsulated into porous titania [179]. A key limitation of this system is photocorrosion under oxidizing conditions (oxygen and carboxylic adds as by-products of partial oxidation of the sacrificial agent). However, in the presence of UV irradiation, Cu photodeposition can occur, preventing loss of Cu [179]. 

Since anaerobic azo dye reduction is an oxidation-reduction reaction, a liable electron donor is essential to achieve effective color removal rates. It is known that most of the bond reductions occurred during active bacterial growth [48], Therefore, anaerobic azo dye reduction is extremely depended on the type of primary electron donor. It was reported that ethanol, glucose, H2/CO2, and formate are effective electron donors contrarily, acetate and other volatile fatty acids are normally known as poor electron donors [42, 49, 50]. So far, because of the substrate itself or the microorganisms involved, with some primary substrates better color removal rates have been obtained, but with others no effective decolorization have been observed [31]. Electron donor concentration is also important to achieve... 

A very high activity of mitochondrial aldehyde dehydrogenases (together with its low ensures very efficient oxidation in the liver so that the concentration of acetaldehyde in blood remains very low. Nonetheless, it is possible that some of the pathological effects of ethanol are due to acetaldehyde (ethanal). In contrast, a large proportion of the acetate escapes from the liver and is converted to acetyl-CoA by acetyl-CoA synthetase in other tissues ... 

Some catabolic reactions depend upon ADP, but under most conditions its concentration is very low because it is nearly all phosphorylated to ATP. Reactions utilizing ADP may then become the rate-limiting pacemakers in reaction sequences. Depletion of a reactant sometimes has the effect of changing the whole pattern of metabolism. Thus, if oxygen is unavailable to a yeast, the reduced coenzyme NADH accumulates and reduces pyruvate to ethanol plus C02 (Fig. 10-3). The result is a shift from oxidative metabolism to fermentation. 

---