Oxidation electrocatalytic

An advantage in electrocatalysis is realized with redox catalysts anchored to the surface of the solid face. The most important examples of these anchored systems are those where an oxide layer covers the surface. In this respeet the niekel oxide/hydro-xide electrode should be mentioned [6]. A Ni(OH)2 layer ean be easily formed at any inert electrode surface and its oxidation can be achieved by shifting the potential of the electrode to an appropriate value (Eq. 9-20)  

The nickel(in)-oxide-hydroxide thus prepared can be regarded as an active oxidation agent. Holding the potential of the electrode at this value and adding a reacting organic substrate to the solution phase the oxidation of the organic species takes place. The first steps of this oxidation can be formulated as follows (Eqs. 9-21 and 9-22)  

With regard to Eq. (9-20) a steady state coverage with NiO(OH) will be attained, i.e. continuous oxidation reaction with a continuous current flow will be observed under potentiostatic conditions. All this means that the nickel oxide catalyst is turned into a nickel oxide electrocatalyst, that can be used in electrosynthesis. The most important synthetic reactions emplo)dng such electrodes are as follows  

A technologycally important version of the first reaction is the oxidation of diace-tone-L-sorbose to diaceto-2-keto-L-gulonic acid. 

The oxidation of alcohols to aldehydes or ketones assmnes new importance with long term attention to biomass based economics. Acetaldehyde, the product of ethanol oxidation, is of special interest since it can be utilized in the synthesis of other basic chemikals such as acetic acid, butanol, etc. [6]. The following Equations 9-23 and 9-24 can be formulated  

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Beden, B. Electrocatalytic Oxidation of Oxygenated Aliphatic Organic Compounds at Noble Metal Electrodes 22... 

E. Lamy-Pitara, S.E. Mouahid, and J. Barbier, Effect of anions on catalytic and electrocatalytic hydrogenations and on the electrocatalytic oxidation and evolution of hydrogen on platinum, Electrochim. Acta 45, 4299-4308 (2000). 

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

The electrocatalytic oxidation of methanol has been widely investigated for exploitation in the so-called direct methanol fuel cell (DMFC). The most likely type of DMFC to be commercialized in the near future seems to be the polymer electrolyte membrane DMFC using proton exchange membrane, a special form of low-temperature fuel cell based on PEM technology. In this cell, methanol (a liquid fuel available at low cost, easily handled, stored, and transported) is dissolved in an acid electrolyte and burned directly by air to carbon dioxide. The prominence of the DMFCs with respect to safety, simple device fabrication, and low cost has rendered them promising candidates for applications ranging from portable power sources to secondary cells for prospective electric vehicles. Notwithstanding, DMFCs were... 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

The electrocatalytic oxidation of methanol has been thoroughly investigated during the past three decades (see reviews in Refs. 21-27), particularly in regard to the possible development of DMFCs. The oxidation of methanol, the electrocatalytic reaction, consists of several steps, which also include adsorbed species. The determination of the mechanism of this reaction needs two kinds of information (1) the electrode kinetics of the formation of partially oxidized and completely oxidized products (main and side products) and (2) the nature and the distribution of intermediates adsorbed at the electrode surface. 

Figure 10. Electrocatalytic oxidation of 1 A/ CH3OH in 0.5 A/ HCIO4 on a platium electrode dispersed in polanil-ine (R loading 5 mg cm", sweep rate 5 mV s" ).

Figure 12. Electrocatalytic oxidation of CO (from a CO-saturated 0.1 M HCIO4 solution) on different Pt-based electrodes (sweep rate 5 mV/s, 25 "C) ( ) smooth Pt, (0) 0.1 mg cm Pt dispersed in a polyaniline film,(A) 0.1 mg cm" R-Sn dispersed in a polyaniline film.

Beden, C. Lamy, and J.-M. Leger, Electrocatalytic Oxidation of Oxygenated Aliphatic Organic Compounds at Noble Metal Electrodes, in Modem Aspects of Electrochemistry, Vol. 22, Ed. by J. O M. Bockris, B. E. Conway, and R. E. White, Plenum Press, New York, 1992, pp. 97-264. 

Perhaps the most important paradigm in research on the mechanism of the electrocatalytic oxidation of small organic molecules is the dual pathway mechanism introduced in Capon and Parsons [1973a, b], and reviewed in Parsons and VanderNoot [1988]. In terms of methanol oxidation, the dual pathway may be summarized in a simplified way by Fig. 6.1. The idea is that the complete oxidation of methanol to carbon dioxide may follow two different pathways ... 

Beden B, Lamy C, Leger JM. 1992. Electrocatalytic oxidation of oxygenated aliphatic organic compounds at noble metal electrodes. In Bockris JO M, Conway BE, White RE, eds. Modem Aspects of Electrochemistry. Volume 22. New York Plenum Press, p 97-264. 

Love B, Lipkowski J. 1988. Effect of surface crystallography on electrocatalytic oxidation of carbon monoxide on Pt electrodes. ACS Symp Ser 378 484. 

Sun SG. 1998. Studying electrocatalytic oxidation of small organic molecules with in-situ infrared spectroscopy. In Lipkowski J, Ross PN, eds. Electrocatalalysis. New York Wiley-VCH. p 243-291. 

Sun SG, Clavilier J, Bewick A. 1988. The mechanism of electrocatalytic oxidation of formic acid on Pt(lOO) and Pt(lll) in sulphuric acid solution An EMIRS study. J Electroanal Chem 240 147-159. 

Kabbabi A, Faure R, Durand R, Beden B, Hahn F, Leger JM, Lamy C. 1998. In situ FTIRS study of the electrocatalytic oxidation of carbon monoxide and methanol at platinum-ruthenium bulk alloy electrodes. J Electroanal Chem 444 41-53. 

Leung L-WH, Weaver MJ. 1990. Influence of adsorbed carbon monoxide on the electrocatalytic oxidation of simple organic molecules at platinum and palladium electrodes in acidic solution A survey using real-time FITR spectroscopy. Langmuir 6 323-333. 

Mishina F, Karantonis A, Yu Q-K, Nakabayashi S. 2002. Optical second harmoitic generation during the electrocatalytic oxidation of formaldehyde on Pt(lll) Potentiostatic regime versus galvanostatic potential oscillations. J Phys Chem B 106 10199-10204. 

Samjeske G, Miki A, Ye S, Osawa M. 2006. Mechanistic study of electrocatalytic oxidation of formic acid at platinum in acidic solution by time-resolved surface-enhanced infrared absorption spectroscopy. J Phys Chem B 110 16559-16566. 

Attwood PA, McNicol BD, Short RT. 1980. Electrocatalytic oxidation of methanol in acid electrolyte—Preparation and characterization of noble-metal electrocatalysts supported on pretreated carbon-fiber papers. J Appl Electrochem 10 213-222. 

Clavilier J, Lamy C, Leger JM. 1981a. Electrocatalytic oxidation of methanol on single-crystal platinum-electrodes—Comparison with polycrystalline platinum. J Electroanal Chem 125 (1) 249-254. 

Takasu Y, Zhang XG, Minoura S, Murakami Y. 1997. Size effects of ultrafine palladium particles on the electrocatalytic oxidation of CO. Appl Surf Sci 121 596-600. 

Hayden BE, Fletcher D, Suchsland J-F. 2007c. Enhanced activity for electrocatalytic oxidation of carbon monoxide on titania supported gold nanoparticles. Angew Chem Int Ed 46 3530-3532. 

Lamy, C., Electrocatalytic oxidation of organic compounds on noble metals in aqueous solutions, Electrochim. Acta, 29, 1581 (1984). 

The electrocatalytic oxidation of methanol was discussed on page 364. The extensively studied oxidation of simple organic substances is markedly dependent on the type of crystal face of the electrode material, as indicated in Fig. 5.56 for the oxidation of formic acid at a platinum electrode. 

There are reports of polymerisation of pyrrole [161, 162] and aniline [163] onto polyacetylene, to give oxygen and water stability [161], although there is some evidence for the polyacetylene acting electrocatalytically, oxidizing the pyrrole with no concomitant polymerisation. 

ELECTROCATALYTIC OXIDATION OF WATER AND ORGANICS 9.10.8.1 Oxidation of Water... 

Among the metal complexes used in electrocatalytic oxidation of organic compounds, polypyridyl oxo-ruthenium complexes have attracted special attention,494"508 especially [RuIV(terpy)(bpy)0]2+.495 197,499,500,502,504 This high oxidation state is reached from the corresponding Run-aqua complex by sequential oxidation and proton loss (Equations (75) and (76)). 

By using various polypyridyl oxo complexes a relationship between redox potentials ( 1/2) of the complexes and the efficiency and the selectivity of the electrocatalytic oxidation of alcohols and diols has been established.506 Higher 1/2 gives higher reactivity. The best results, from the point of view of synthesis, were obtained with the complex /ra ,v-[Ru (terpy)(0)2(0I I2)]2 which is characterized by a high redox potential and a relatively high stability. 

E. Casero, F. Pariente, and E. Lorenzo, Electrocatalytic oxidation of nitric oxide at indium hexacyanof-errate film-modified electrodes. Anal. Bioanal. Chem. 375, 294—299 (2003). 

S.-M. Chen, Electrocatalytic oxidation of thiosulfate by metal hexacyanoferrate film modified electrodes. J. Electroanal. Chem. 417, 145-153 (1996). 

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