Oxygen critical levels

The rate of H2 escape from the upper atmosphere and thus oxygenation from decomposition of H2O vapour is directly dependent on the density of H2 at the critical level of escape. It may be that the Great Oxidation Event from 2.4 Ga to 2.0 Ga reflects elevated H2 in the hydrosphere / atmosphere during much of this period. 

We believe that the primary reason of deactivation is the formation of irreversibly adsorbed species and oxygen poisoning occurs when the blocking of active sites reaches a critical level. By-products can be formed during the oxidation reaction. A frequently observed side-reaction is the aldol-dimerization of the carbonyl compound and a further oxidation of the product. The process is catalyzed by bases, including the basic functional groups of a carbon support (25). 

Once the aldehyde concentration builds up to a critical level (and even in the oxidation of secondary alcohols, small quantities of aldehydes are produced by side-reactions), then autocatalysis is observed. It is suggested that the aldehydes bring about chain-branching by reacting directly with oxygen... 

The acceleration of the reaction after the induction period appears to be the result of the build-up of acetaldehyde to a critical level. The amount of acetaldehyde present when the reaction begins to accelerate is about the same as the quantity of butyraldehydes formed at the corresponding stage of reaction of n- and iso-butanol. Furthermore, the addition of acetaldehyde to a sec-butanol—oxygen mixture results in a considerable decrease in the induction period, whereas the addition of even quite large amounts of methyl ethyl ketone has only a slight effect. This is not unexpected, since, below about 400 °C the combustion of methyl ethyl ketone is preceded by a lengthy induction period (see Sect. 2.2). 

In the Cambrian period life began to develop very quickly. For this reason the oxygen concentration increased rather rapidly. Thus, in the late Silurian (420 millions years ago) the oxygen level was as high as 0.1 PAL (Fig. 3) which is termed the second critical level. With the increase of the oxygen concentration the quantity of ozone in the atmosphere increased, together with an increase in the altitude of maximum ozone production. This latter, in the late Silurian period reached 20 km level, which made the spread of life onto dry land possible. At the same time the thermal structure of the atmosphere was drastically changed, which resulted in the appearance of the stratosphere. It was shown previously that our atmosphere has an... 

The right side of Fig. 1 shows the next switch, as 0 (g) replaces C0(g) over the CO-covered surface. As already observed in other studies (refs.5,6,2) oxygen finds very few adjacent sites for dissociative adsorption until the gaseous CO concentration falls to a critical level, so that a small decrease in CO coverage can occur. At this point there is a sudden production of CO and consumption of oxygen. 

In fungi, xylose is reduced to xylitol by NADH- or NADPH-dependent xylose reductase (XR) and thereafter is oxidized to xylulose by NAD -dependent xylitol dehydrogenase (XDH). The xylulose is phosphorylated, channeled into the pentose phosphate pathway [3]. XR of most fungi, including most yeasts, prefers NADPH to NADH. Because of the cofactor preference of XR (NADPH) and XDH (NAD, redox imbalance occurs under anaerobic condition [4]. Therefore, the oxygen-limited rather than anaerobic condition is ideal for bioconversion of xylose to ethanol, so that the accumulated reduced cofactor can be oxidized to reach redox balance. A critical level of oxygen should exist for the highest ethanol yield and productivity. 

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