Electrooxidation proposed mechanism

Scheme 12 Proposed mechanism for Ni-catalyzed electrooxidation of H2 to protons. ...

The kinetic parameters are slightly dilferent for iron N4-macrocyclic complexes, compared to cobalt complexes. In previous investigation of the electrooxidation of hydrazine catalyzed by FeN4 macrocyclics, the proposed mechanism involved adduct formation between Fe and the hydrazine molecule, prior to the rate determining step [46]. It is evident that the formation of a bond between the metal active site and the hydrazine molecule is a crucial step in electrocatalysis phenomena [47-50]. The electrooxidation of hydrazine on iron N4 macrocyclic complexes results in a Tafel plot with slope of around 0.040 V/decade, instead of 0.060 V/decade. The order in hydrazine is still one, but the order with respect to OH is two, so a reaction mechanism was proposed as follows [44, 45] ... 

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

The mechanism of electrooxidation of methanol is now nearly well understood. From the considerable effort made during the past 20 years, it is now possible to propose electrocatalysts with acceptable activities for DMFCs, even though further improvement is still necessary. Despite considerable research efforts, R-Ru alloys are the only acceptable catalysts for the electrooxidation of methanol at low anode potentials. Two questions still remain unanswered ... 

The Ritter reaction [6] proceeds by the electrooxidation of alkyl iodides (56) in an MeCN-(Pt) system to form Ai-alkyl acetamides (58) (Scheme 21). Attack of carbenium ion intermediate - from dissociation of the initially formed alkyl cation radical - to acetonitrile would give the iminium cation (57). However, a different mechanism is proposed, whereby the alkyl iodide reacts with the electrogenerated iodo cation [I]" " [73]. 

Figure 6.23. Schematic reaction pathways for the electrooxidations of small oxygenated Cj molecules. The scheme comprises the dual path mechanism proposed for formic acid oxidation (steps 1,2,3,4) as well as the single path mechanism with CO as intermediate (steps 1,5,4).

It has been known that the electrolysis in an MeCN- NaClO system generates an acid The hydrogen has to originate from the solvent. A mechanism for hydrogen abstraction from acetonitrile by the electrooxidatively generated radical 104- to produce perchloric acid has been proposed, but no evidence for the succinonitrile formation appeared (Eq. (5)). The detection of the 104- radical by the aid of HSR was tried But it was found to be difficult to differentiate between the perchlorate radical and the radical from chlorine dioxide The electrolysis in a CH Clj—... 

From these results, a mechanism of methanol electrooxidation at PtRu can be proposed. The first step may consist in the dissociative adsorption of methanol at platinum and formation of an adsorbed CHO species according to the schema presented in Fig. 12. This mechanism of methanol adsorption and dehydrogenation is generally admitted." Then, for the co-reduced catalysts (alloy), the number of involved electrons from methanol stripping as determined by DBMS is higher than 2, then adsorbed CHO and CO species seem to be involved in the mechanism. Moreover, the number of electrons for the oxidation of bulk methanol is greater... 

Electrooxidations can also make use of mediators In this case they proceed at lower potentials and need milder conditions . Enamines derived from aldehydes and bearing a )5-hydrogen atom were oxidized, using KI as a mediator, into the corresponding -keto amines. In the mechanism proposed, an unknown active species, T is assumed to be formed by oxidation of 1 and to add to the double bond. The resulting iodohydrin cyclized into an a-amino epoxide and subsequently is ring opened into a jS-ketoamine, by a 1,2-hydrogen shift (Scheme 81). 

Interpretation of the spectra based on a comparison with vibrational spectra of the alcohol in pure form and dissolved in the electrolyte solution resulted in a proposed adsorbate structure with the r-bond system of the unsaturated alcohol interacting strongly with the electrode-side on (vibrational mode of the C=C bond around 1598 cm ) and with the C-OH bond (vibrational mode of the C-OH bond around 1030 cm ) at a tilted or perpendicular orientation with respect to the electrode surface towards the electrolyte solution. This information is helpful for understanding the mechanism of the electrooxidation of this alcohol [89]. 

A mathematical model for DEFC was proposed by Pramanik and Basu describing different overpotentials [191]. The assumptions of their model are (i) the anode compartment considered as a well-mixed reactor, (ii) 1 bar pressure maintained both at the anode and cathode compartments, (iii) the transport processes are modelled in one dimension. The model accounts for Butler-Volmer-based descriptions of the ethanol electrooxidation mechanisms, diffusive reactants transport and ohmic losses at the electrode, current collector and electrode-current collector interfaces. The experiment data on current-voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is studied by the authors (Fig. 8.19). 

Another method based on Fl-ECL inhibition method for determination of neurotransmitters, noradrenaline, and DA has been developed for Ru(bpy)3 /TPA and for Ru(phen)3 /TPA systems. The proposed inhibition mechanism shows the quenching effect of o-benzoquinone derivative generated from the direct electrooxidation reactions of noradrenaline and DA. This method has superiority of sensitivity over several detection methods, such as spectrometry, electro-chemical, CE, and HPLC methods, and has potential for sensitive and trace analyses of... 

Chronologically there were almost no reports on this topic prior to the early 1980s, and the first of these concerned studies on xanthate electrooxidation to assist froth flotation in mineral-ore recovery. The mechanisms of flotation are not completely imderstood, and for xanthates the involvement of dixanthogen and other species has been proposed. In an early paper, combined use of ultrasound and electrooxidation was found to increase the amount of dixanthogen and give a uniform emulsion with 80% of droplets below 5 pm diameter, improving the recoveries of galena, sphalerite, and chalcopyrite. ... 

A mechanism was proposed [143] for electrooxidation of adenine at the graphite electrode that involves two sequential 2e — 2H oxidations to give first 2-oxy- and 2,8-dioxyadenine (Fig. 11 A). Then, a further two electron oxidation at the C(4)=C(5) double bond occurs to give a dicarbonium ion. Thus the mechanism for oxidation of adenine involves three primary 2e"-2H oxidations. The final product (the diimine) may undergo hydrolysis and fragmentation which results in further products. 

At present, the Pt-Ru bimetallic system is recognized as the most promising CO-tolerant anode catalyst for the DMFCs. A large body of hterature exist demonstrating improvement of the CO oxidation on the Pt-Ru alloy and Ru-modified Pt catalysts. The superior CO tolerance of the Pt-Ru bimetallic catalysts compared with the monometallic Pt catalyst is frequently explained with concepts of bifunctional mechanism [17] and ligand effect [22, 23]. The former mechanism proposed by Watanabe and Motoo is widely accepted. They claimed that the Ru has higher reactivity with water than Pt and that formation of Ru-OH at a lower potential promotes the electrooxidation of the chemisorbed CO on the Pt (formulas (4) and (5)). 

It has been widely reported that electrooxidation of the thiocyanate ion at pH 4 yields hydrogen cyanide and (or) cyanide and sulfate ions as main products [134]. However, there is a huge gap in terms of the mechanism that takes place between the binding of thiocyanate to the catalyst and the products experimentally found. In order to get insight about the elemental reactions between these processes, it is proposed based on experimental evidence that the oxidation of thiocyanate particularly catalyzed by MPc and MPc like systems, leads to the production of thiocyanate radicals that dimerize to form the pseudohalogen molecule thio-cyanogen (SCN)2 [132, 135-139]. 

The role of organometallie eomplexes as co-catalysts with Pt for methanol electrooxidation was mentioned earlier. Can they also act alone and replace Pt completely For the anodic oxidation of methanol, ethanol, and formic acid, studies looking at CO oxidation by various porphyrin complexes with Ir, Rh, and Co in aqueous electrolytes (both acid and alkaline) are relevant [90, 203, 204]. The mechanism proposed by Shi and Anson for the activity Co-octaethylporphyrin considers the oxidation of Co(II) to Co(III) in conjunction with coordination of CO to the Co(III) centres, followed by nueleophilic H2O attack leading to catalytic oxidation forming CO2 (see Equations 4.15-4.18) [90]. 

Cobalt is used to promote CO oxidation in reformers [284, 285], suggesting PtCo alloys may be useful catalysts for H2 oxidation in the presence of CO. PtCo alloys have been proposed as improved methanol oxidation catalysts [286] because cobalt may assist with CO removal (CO is an intermediate in meflianol electrooxidation) through a mechanism analogous to the PtRu bifunctional mechanism. PtCo alloys have also been studied as improved ORR catalysts [200, 287, 288]. In addition to their improved ORR kinetics, these alloys have been shown to be more tolerant to methanol crossover in direct methanol fuel cells (DMFCs), again possibly through improved CO removal kinetics [289]. However, Stevens et al. [235] observed no impact on CO-stripping with the addition of eobalt to Pt, and explained this as due to surface cobalt dissolving away. 

Based on kinetic data of the investigated aniline oxidation reaction as obtained with a variety of methods, including spectro-electrochemical ones, numerous researchers [252-270] have proposed an autocatalytic mechanism of oxidation and growth. In this scheme, polymer growth occurs without further electrooxidation of aniline monomers. The polymer film in its oxidized form contains oxidized aniline imits, most likely also at the ends of polymer chains. These units react basically like monomeric radical cations with further monomer molecules from solution (radical cation-parent molecule coupling). Subsequently, the chain has to be reoxidized, that is, one electron has to be transferred per monomer unit. An alternative proposal is that pemigraniline sites act as oxidants for monomer units [271]. The characterization of this pro-... 

Our work on the electrooxidation of alcohols with homogeneous heterobimetallic catalysts was initially motivated by the advances in DMFC anodes 8,21-23). The strategy initially employed for this project was to utilize the bifunctional oxidation mechanism observed in DMFC anodes to design discrete bimetallic complexes as catalysts for the electrooxidation of renewable fuels. These complexes can be viewed as utilizing all of the metal as opposed to bulk metal anodes, where the reaction occurs only at active surface sites. Although bimetallic complexes are not accurate models of the proposed surface binding site on bulk metal anodes, our investigations address the question of whether it is possible to reproduce the essential functions of the proposed electrooxidation mechanism in discrete heterobinuclear complexes 24-26). 

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