Electro-Oxidation Mechanisms

Figure 2. (a) Addition-Elimination (A-E) and (b) Hydride-Transfer (H-T) methanol electro-oxidation mechanisms by Methanol Dehydrogenase Enzyme proposed in the literature. 

N.M. Markovic, H.A. Gasteiger, P.N. Ross, X.D. Jiang, I. Villegas, M.J. Weaver, Electro-oxidation mechanisms of methanol and formic acid on Pt-Ru alloy surfaces. Electrochim. Acta 1995, 40(1), 91-98. 

ELECTRO-OXIDATION MECHANISM OF AMMONIA NITROGEN 2.1 Direct electro-oxidation of ammonia nitrogen... 

AU experiments to be described below are interpreted on the basis of the Langmuir-Hinshelwood (LH) mechanism for CO electro-oxidation suggested by GUman more than 40 years ago [Gihnan, 1964]. According to GUman s model, water needs to be activated on a free site on the surface, leading to surface-bonded OH ... 

Weaver and co-workers have carried out extensive smdies of CO electro-oxidation on Au single crystals [Chang et al., 1991 Edens et al., 1996]. Continuous oxidation of CO on Au starts at potentials where the formation of surface oxides or surface-bonded hydroxyl (OH) is not apparent from voltammetry. Weaver suggested the following mechanism ... 

Lai SCS, Kleyn SEF, Rosea V, Koper MTM. 2008. Mechanism of the dissociation and electro-oxidation of ethanol and acetaldehyde on platinum as studied by SERS. J Phys Chem C 112 19080-19087. 

At this stage, it should be pointed out that modihcation of a Pt-Sn catalyst by Ru atoms increases cell performance (and hence catalytic activity with regard to ethanol electro-oxidation), but has no effect on the OCV or on product distribution [Rousseau et al., 2006]. It seems, then, that the oxidation mechanism is the same on Pt-Sn and Pt-Sn-Ru, which supports the proposition that Ru allows OH species to be produced when the anode potential is increased and noncatalytically active tin oxides are formed. 

Vigier F, Coutanceau C, Hahn F, Belgsir EM, Lamy C. 2004a. On the mechanism of ethanol electro-oxidation on Pt and PtSn catalysts Electrochemical and in situ IR reflectance spectroscopy studies. J Electroanal Chem 563 81-89. 

It is now widely accepted that COads electro-oxidation proceeds via the F-H mechanism, which includes the reaction steps of CO adsorption, water sphtting (15.18), and COads + OHads recombination (15.19) on two adjacent sites to yield CO2 ... 

Anderson AB, Neshev NM. 2002. Mechanism for the electro-oxidation of carbon monoxide on platinum, including electrode potential dependence—Theoretical determination. J Electrochem Soc 149 E383-E388. 

Kumiawan, P., "A Study of In-Situ Brightening of Mechanical Pulp via the Electro-Oxidation of Sodium Carbonate , M.A.Sc. Thesis, Dept, of Chemical and Bio-Resource Engineering, The University of British Columbia, 1998. 

One example of the application of in situ electrochemical epr concerns the study of the Kolbe reaction. As was discussed in section 1.3, the Kolbe reaction involves some extremely complex processes and considerable effort has been expended in the search for the identities of the radical intermediates. Evidence for such intermediates remains sparse but one system that has provided such evidence is the electro-oxidation of triphenyl acetic acid (TPA) at a platinum electrode in acetonitrile (Waller and Compton, 1989). The case history of epr in the study of this system is a very good example of the application of the technique to provide details of a reaction mechanism. In... 

One example of the application of in situ FTIR to the study of the near-electrode region concerns the study of the electro-oxidation of ethylene glycol (EG) at a platinum electrode in base. This work clearly illustrates the relative ease with which the products of an electrochemical reaction can be detected and identified, and a mechanism deduced. 

The mechanism of the polymerization was discussed based on electrochemical measurements. Applications of the electro-oxidative polymerization were also described. 

The following questions on the electro-oxidative polymerization arose. First, why various phenol derivatives were smoothly polymerized which could not occur by the oxidation with the copper catalyst or lead dioxide. Secondly, why the activated phenol was reacted preferentially through C-0 coupling to form the poly(phenyleneoxide). The mechanism of the electro-oxidative polymerization is discussed below by using the example of 2,6-dimethylphenol. 

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

On the other hand, the molecular weight for the electro-oxidative polymerization remains constant throughout the reaction course. That is, the electro-oxidative polymerization proceeds seemingly through a chain reaction mechanism. (In practice the polymerization proceeds heterogeneously only in the diffusion layer of the electrode. The details are described below.)... 

The polymerization mechanism of phenols is described as follows. The phenol is adsorbed on the electrode surface and accumulated in the diffusion layer. The adsorbed phenol undergoes one-electron oxidation to the phenoxy radical on the electrode surface. The concentrated phenoxy radical is coupled with each other at p-position to form the dimer, and the dimer repeats the electro-oxidation and coupling. The phenoxy radical is assumed to he adsorbed or oriented upon the electrode surface thus resulted in the selective coupling reaction. 

Costa, M. C., Botelho, do Rego A. M., Abrants, L. M., 2002. Characterization of a natural and an electro-oxidized arsenopyrite a study on electro-chemical and X-ray photoelectron spectroscopy. Inter. J. Miner. Process, 65 83 - 108 Dai Jingping, Sun Wei, Cao Limei, Hu Yuehua, 2000. Influence of mechanical excitation on adsorption of sodium diethyl dithioformate on galena and sphalerite. Trans. Nonferrous Met. Soc., China, 10 101 - 105... 

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