The Electro-Oxidation of Ethanol

The reaction mechanisms of the anodic oxidation of other alcohols than methanol is more difficult to elucidate since the number of electrons exchanged greatly increases and the activation of the C-C bond breaking is relatively difficult at moderate temperatmes (50-90°C). The complete oxidation of ethanol involves 12 electrons per molecule, i.e., 

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth- 

These pioneering results on smooth platinum obtained by EMIRS were confirmed by the new IR Reflectance Spectroscopy developed by Pons et al. (SNIFTIRS) and Weaver et al. (SPAIRS), able to observe intermediate species produced on carbon (e.g. Vulcan XC-72) supported Pt-based nanoparticles. 

These infrared reflectance teclmiques allowed us to observe the adsorption and oxidation of ethanol and showed that other in 

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At the anode, the electro-oxidation of ethanol takes place as follows, leading, in the case of complete oxidation, to CO2 ... 

Table 1.2 Chemical yields in acetaldehyde, acetic acid and CO2 for the electro-oxidation of ethanol at Pt/C, Pt-Sn (90 10)/C and Pt-Sn-Ru (86 10 4)/C catalysts under DEFC operating conditions at 80 °C for 4 h.

A quantitative study of the electro-oxidation of ethanol on carbon-supported Pt, PtRu and PtsSn catalysts was made by Behm and co-woikers " nsing combined voltarmnetric and on-line Differential Electrochemical Mass Spectrometry measnrements (DEMS). 

Figure 35 shows a typical set of results obtained during the electro-oxidation of ethanol on PtRu/C catalyst with the voltam-mogram (Fig.35a) and three mass spectrometric cyclic voltammo-grams (MSCVs) (Figs. 35b, 35c and 35d). 

From the mass spectrometric data and Eqs. (27) and (28), it is possible to calculate the relative current due to the formation of caibon dioxide, acetaldehyde and finally acetic acid from the ethanol oxidation reaction. These partial currents are shown in Fig. 36 for Pt/C, PtRu/C and PtsSn/C catalysts. " This figure clearly presents the efficiency of the three different catalysts towards the formation of reaction products resulting from the electro-oxidation of ethanol. This quantitative analysis allows us to evaluate the total number of exchanged electrons during the oxidation reaction and the global current efficiency (Aq) and product yield (Wq) of the reaction calculated from the total charge involved for each partial current (Table 4). 

The analytical results concerning the electro-oxidation of ethanol in a Direct Ethanol Fuel Cell (DEFC) were obtained by online HPLC analysis coupled with the fuel cell test bench (Scheme 3). 

The electro-oxidation of ethanol at platinum-based catalysts dispersed in an electron-conducting polymer was relatively sparsely investigated. It seems that... 

PtSn/C electrocatalysts with R Sn atomic ratios of 50 50 and 90 10 were prepared by alcohol-reduction process, using ethylene glycol as solvent and reducing agent, and by borohydride reduction. The electrocatalysts were characterized by EDX, XRD and cyclic voltammetry. The electro-oxidation of ethanol was studied by cyclic voltammetry using the thin porous coating technique. The electrocatalysts performance depends greatly on preparation procedures and R Sn atomic ratios. 

Sen Gupta S, Datta J (2005) An investigation into the electro-oxidation of ethanol and 2-propanol for application in direct alcohol fuel cells (DAECs). J Chem Sci 117(4) 337-344... 

Bozzini B, Gaudenzi GPD, Busson B, Humbert C, Six C, Gayral A, Tadjeddine A (2010) In situ spectroelectrochemical measurements during the electro-oxidation of ethanol on WC-supported Pt-black, based on sum-frequency generation spectroscopy. J Power Sources 195(13) 4119-4123... 

Kim JW, Park SM (2003) In situ XANES studies of electrodeposited nickel oxide films with metal additives for the electro-oxidation of ethanol. J Electrochem Soc 150(11) E560-E566... 

Zhu LD, Zhao TS, Xu JB, Liang ZX (2009) Preparation and characterization of carbon supported submonolayer palladium decorated gold nanoparticles for the electro-oxidation of ethanol in alkaline media. J Power Sources 187 80-84... 

In this mechanism the adsorbed acetyl plays a key role and its further oxidation is favored by the addition to platinum of metal atoms more easily oxidizable at low potentials, such as Ru, Sn, Mo, etc. Other electrocatalysts were considered for the electro-oxidation of ethanol, such as rhodium, iridium [18] or gold [19], leading to similar results in acid medium. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and carbon monoxide, which is further oxidized to carbon dioxide when the rhodium surface begins to oxidize, at 0.5-0.7 V/RHB [18]. On gold in acid medium the oxidation reaction leads mainly to the formation of acetaldehyde [19]. 

Hitmi H, Belgsir EM, Leger J-M, Lamy C, Lezna RO (1994) A kinetic analysis of the electro-oxidation of ethanol at a platinum electrode in acid medium. Electrochim Acta 39 407 15... 

Spinace EV, Linardi M, Oliveira Neto A (2005) Co-catalytic effect of nickel in the electro-oxidation of ethanol on binary Pt-Sn electrocatalysts. Electrochem Commun 7 365-369... 

Oliveira Neto A, Giz MJ, Perez J, Ticianelli EA, Gonzalez ER. The electro-oxidation of ethanol on Pt-Ru and Pt-Mo particles supported on high-surface-area carbon. J Electrochem Soc 2002 149 A272-9. 

On a pure Pf/C catalyst, Rousseau et showed that the electro-oxidation of ethanol at the anode of a DEFC working at 80°C mainly led to the formation of acetaldehyde, acetic acid and carbon dioxide, with chemical yields of 47.5%, 32.5% and 20.0%, respectively. By comparing the mass yield and the faradic yields, they concluded that no other products were formed in a significant amount. This result confirms that Pt is able to break the C-C bond to some extent In situ infrared measurements on ethanol adsorption and electro-oxidation at platinum electrodes have clearly shown that the adsorbed CO species are formed from 0.3 V vs RHE at the platinum surface moreover Iwasita and Pastor found some traces of CH4 at potentials lower than 0.4 V vs RHE. Previous studies showed that the initial steps of ethanol adsorption and oxidation on Pt can follow two different modes ... 

Hitmi, H., Belgsir, E., Leger, J.-M., et al. (1994). A Kinetic Analysis of the Electro-oxidation of Ethanol at a Platinum Electrode in Acid Medium, Electrochim. Acta, 39, pp. 407 15. 

Several studies on the electro-oxidation of ethanol have been mainly devoted to the identification of the adsorbed intermediates onto the electrode and to the elucidation of the reaction mechanism by means of various techniques, such as differential electrochemical mass spectrometry (DBMS) [7-12] and in situ Fourier transform infrared spectroscopy (FTIR) [13-15]. On the basis of the foregoing work, the global mechanism of ethanol oxidation in acidic solution may be summarized according to the following scheme of parallel reactions [2] ... 

The electro-oxidation of ethanol is studied recently by few investigators in alkaline medium. Tripkovic et al, (2001) used Pt (111) for the electro-oxidation of ethanol, methanol, propanol and butanol in alkaline solution. Gupta et al. (2004) used CuNiPt and CuNiPtRu alloys for the electro-oxidation of ethanol in alkaline medium. The cell performances using these anodes are mediocre. 

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