Electrocatalytic oxidation reduced

Figure 3.13 — Set-up for the electrocatalytic oxidation of NADH catalysed by the reduced form of diaphorase (Dp). Dp (red) and Dp (ox) reduced and oxidized form of diaphorase. (Reproduced from [88] with permission of the Royal Society of Chemistry).

However, a pH of 8 or 9 is more favorable to shift the overall equilibrium toward the oxidized product. Taking this remark into account, Keita and Nadjo showed recently that efficient POMs exist that meet these conditions [185]. A preliminary work is being published. The series of V-substituted derivatives selected in this work contain several POMs stable in their oxidized and reduced forms in alkahne media, thus guaranteeing the stability of the species generated during the catalytic process. The preliminary study is devoted to the electrocatalytic oxidation of NADPH, a substrate closely related to NADH, with the same importance in vivo and in vitro. 

Chemically modified electrodes (CMEs) for electrocatalytic oxidation of the reduced form of the nicotinamide adenine dinucleotide cofactor (NADH) are discussed. The work of the authors in the field is reviewed. CMEs based on adsorbed polyaromatic redox mediators (phenoxazines and phenothiazines) and the deposition of aqueous insoluble redox polymers are described. 

E. Katz, T. Lotzbeyer, D. Schlereth, W. Schuhmann, and H.-L. Schmidt, Electrocatalytic oxidation of reduced nicotinamide coenzymes at gold and platinum electrode surfaces modified with a monolayer of pyrroloquinoline quinone. Effect of Ca2+ cations, J. Electroanal. Chem. 373, 189-200 (1994). 

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

Gorton, L., Torstensson, A., Jaegfeldt, H., Johansson, G., Electrocatalytic Oxidation of Reduced Nicotinamide Coenzymes by Graphite Electrodes Modified with an Adsorbed Phenoxazinium Salt, Meldola Blue , / Electroanal. Chem. 161 (1984) 103-120. 

In order to improve the electrocatalytic properties of methanol electrodes, and to reduce the poisoning phenomenon usually observed with bulk platinum, different platinum based alloys were considered such as Pt-Ru, and Pt-Sn, etc. [153]. Therefore such alloys were also dispersed into electron conducting polymers. Hable et al. [53] were apparently the first authors to disperse Pt-Sn catalyst particles in a polyaniline matrix, in order to activate the oxidation of methanol. They evaluated the Pt/Sn ratio by X-ray Photoelectron Spectroscopy and found that small amounts of Sn (e.g. Pt/Sn ratios of 10/1) were sufficient to enhance the electrocatalytic oxidation of methanol. Pt was found to be in the Pt(0) state whereas Sn was in an oxidized form. Similar observations concerning the enhanced electrocatalytic activity of Pt-Sn particles incorporated in PAni films were made by Laborde et al. [154]. Such Pt-Sn alloys are also very active for the electrocatalytic oxidation of ethanol [68,154]. 

Chi Q. and Dong S., Electrocatalytic oxidation of reduced nicotinamide coenzymes at methylene green-modified electrodes and fabrication of amperometric alcohol biosensors, Anal. Chim. Acta., 285, 125-133, 1994. 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

Electrocatalytic oxidation of reduced nicotinamide-adenine glycolhydrolase by polymers from 51a (M = Co, Ni, Zn) [161]. 

Alternatively, electrochemical detection by using an amperometric biosensor has been proposed using modified electrodes for the electrocatalytic oxidation of the reduced cofactors (NADH, NADPH). The oxidation current reflects the rate of glucose conversion. Additionally, covalent coupling of the coenzyme is a precondition of more advanced reagentless measuring devices. Further developments use an electron mediator such as ferrocyanide and PQQ/ PQQHi (pyrroloquinoline quinone) as the cofactor pair. 

Unfortunately, the catalytic direct oxidation of CySH at the above solid electrode surface is kineticaUy slow and needs an overpotential. Hassan et al. (2007) developed a new, fast, simple, and highly selective potentiometric biosensor to determine the CySH in Tricho-sporon jirovecii yeast cells. Deng et al. constructed the boron-doped CNT-modified electrode for the electrocatalytic determination of CySH using chronoamperometric method. Ardakani et al. constructed a carbon paste electrode modified with quinizarine for the measurement of cysteine in the presence of tryptophan and measured the CySH in blood samples and CySH tablets. The common biological interfering substrates perhaps interfere and reduce the sensitivity of the CySH detection. Therefore, it is important to look for the electrocatalytic oxidation that might decrease the overpotential and increase the sensitivity of CySH detection. Earher, we studied the thiol oxidase-peroxidase activities of SOD. Based on that, we have developed a novel method for the measurement of CySH. 

Yang et al. [1018] prepared a nanoparticulate-Pt catalyst dispersed on a P(Py) film by using PtCl4 as a dopant during P(Py) electropolymerization, then reducing the dopant to metallic Pt. This catalyst was used in the electrocatalytic oxidation of MeOH. In later work, Hepel [1019] duplicated this effort. In both these works, however, it was not evident that the CP served any purpose other than an inactive matrix or host for the active Pt catalyst. 

Methane can be oxidatively coupled to ethylene with very high yield using the novel gas recycle electrocatalytic or catalytic reactor separator. The ethylene yield is up to 85% for batch operation and up to 50% for continuous flow operation. These promising results, which stem from the novel reactor design and from the adsorptive properties of the molecular sieve material, can be rationalized in terms of a simple macroscopic kinetic model. Such simplified models may be useful for scale up purposes. For practical applications it would be desirable to reduce the recycle ratio p to lower values (e.g. 5-8). This requires a single-pass C2 yield of the order of 15-20%. The Sr-doped La203... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

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