Direct reduction of oxygen

This means that aqueous 02 will not penetrate the hydrophobic compartment of a cell, and that 02 is not likely to be efficiently formed from the direct reduction of oxygen in most biological membranes where strong reducing agents are not in high steady-state concentrations. 

Based on all these points above, various authors proposed different schemes for the ORR pathway. Among the schemes proposed, a modified scheme proposed by Wroblowa et al. seems to be the interesting one to describe the complicated reaction pathway for the reduction of O2 at the metal surface (refer to Figure 4.15). In this scheme, at high potentials, k >k2, the portion of direct reduction of oxygen to water will be more than the two-electron reduction to peroxide at intermediate potentials, the decrease in kilkz ratio will indicate an increase in the... 

In spite of the success in the optimization of platinum catalysts, a major breakthrough in the field of fuel cells is yet to be achieved. Especially, the desire for a significant cost reduction by the replacement of platinum motivates international research activities investigating new catalyst concepts for cathodes. Thereby, the cathodes have to be sufficiently stable under fuel cell conditions the alternative non-noble metal catalysts (NNMC) need to have a high selectivity for direct reduction of oxygen to water. The US department of energy (DOE) defined 25% of the achievable current density of a commercial platinum catalyst as target value for 2015. For fuel cell application, the catalysts should be producible in such a nano-structured form that suitable gas diffusion structures can be built. 

Platinum, which belongs to the second group of materials, " is considered to have the best catalytic activity for the ORR in an acid as well as in an alkaline medium. This metal possesses surface properties which facilitate the adsorption of molecular oxygen, and thus it is able to catalyse the direct reduction of oxygen into water. Moreover, it has a greater stability in an 02-saturated acid medium, a very oxidant medium, than less noble materials and its surface properties are less altered, or with a lower kinetics. However, despite these properties, the ORR on the platinum surface... 

At the electrode the reduction of oxygen proceeds in two 2-electron steps. This process would imply hydrogen peroxide as the first reduction product, which is then reduce to water in the second step. In the presence of one specific porphyrin mediator, practically no hydrogen peroxide was found, which in this case could indicate that a direct reduction of oxygen to water has taken place . 

In another approach, a DO sensor was proposed by modifying a glassy carbon (GC) electrode with cobalt tetrasulfonate phthalocyanine (CoTSPc). The direct reduction of oxygen at a solid electrode is a slow process and also requires a high negative potential which can be lowered by electron transfer mediators that can shuttle the electrons between oxygen and electrode. Among these mediators, phthalocyanines acquired a lot of attention because of their catalytic ability and... 

Direct reaction of oxygen with most organic materials to produce radicals (eq. 13) is very slow at moderate temperatures. Hydrogen-donating antioxidants (AH), particularly those with low oxidation—reduction potentials, can react with oxygen (eq. 14), especially at elevated temperatures (6). 

Two techniques, electrochemical reduction (section IIl-C) and Clem-mensen reduction (section ITI-D), have previously been recommended for the direct reduction of isolated ketones to hydrocarbons. Since the applicability of these methods is limited to compounds which can withstand strongly acidic reaction conditions or to cases where isotope scrambling is not a problem, it is desirable to provide milder alternative procedures. Two of the methods discussed in this section, desulfurization of mercaptal derivatives with deuterated Raney nickel (section IV-A) and metal deuteride reduction of tosylhydrazone derivatives (section IV-B), permit the replacement of a carbonyl oxygen by deuterium under neutral or alkaline conditions. 

Since sulphones 204 are easily available compounds one would expect that they could be used as starting materials for the preparation of sulphoxides via the selective removal of one oxygen atom from the sulphonyl group (equation 112). Up to now, there is only one example reported of a direct reduction of a sulphone to a sulphoxide. The bicyclic dideuterio sulphone 205 after 24 h treatment with three-fold excess of diisobutyl aluminium hydride in boiling dichloromethane gave the corresponding sulphoxide 206 in 36% yield (equation 113). A two-step procedure for the selective reduction of sulphones to sulphoxides, which involves an initial reaction of sulphone 204 with aryldiazonium tetrafluoroborate 207 to form aryloxysulphoxonium salt 208 and its subsequent reduction (equation 114), was alluded to by Shimagaki and coworkers and... 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electro-oxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Catalytic reduction of oxygen directly to water, while not as yet possible with traditional catalyst technology at neutral pH, is achieved with some biocatalysts, particularly by enzymes with multi-copper active sites such as the laccases, ceruloplasmins, ascorbate oxidase and bilirubin oxidases. The first report on the use of a biocatalyst... 

The biomembrane-like films can provide a favorable microenvironment for proteins and enhance direct electron-transfer rate between proteins and electrodes, so many biopolymers such as methyl cellulose (MC) and dihexadecylphospate (DHP) have been used to immobilize Mb and make biosensors [230-232], The electrochemical catalytic reduction of oxygen by the Mb-MC/EPG was examined by CVs. Mb-MC... 

Controversial results of oxygen radical detection in mitochondria have been described in the literature. Owing to experimental difficulties many authors were obliged to work with sub-mitochondrial particles instead of the whole mitochondria. However, it is quite possible that oxygen radical production by submitochondrial particles may be artificially enhanced due to exposure to oxygen. On the other hand, some analytical methods of superoxide detection such as cytochrome c reduction cannot be used due to the direct reduction of cytochrome by mitochondrial components. 

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