Ethanol electrodes, electrooxidation

Fig. 1.1 Schematic representatirais of the parallel pathways for methanol (a) and ethanol (b) electrooxidation on Pt electrode [5, 6]...

Gold is generally considered a poor electro-catalyst for oxidation of small alcohols, particularly in acid media. In alkaline media, however, the reactivity increases, which is related to that fact that no poisoning CO-hke species can be formed or adsorbed on the surface [Nishimura et al., 1989 Tremihosi-Filho et al., 1998]. Similar to Pt electrodes, the oxidation of ethanol starts at potentials corresponding to the onset of surface oxidation, emphasizing the key role of surface oxides and hydroxides in the oxidation process. The only product observed upon the electrooxidation of ethanol on Au in an alkaline electrolyte is acetate, the deprotonated form of acetic acid. The lack of carbon dioxide as a reaction product again suggests that adsorbed CO-like species are an essential intermediate in CO2 formation. 

In-situ FTIR studies on the electrooxidation of ethanol on polycrystalline Pt [97-99] as well as on single-crystal Pt electrodes [lOO/lOl] have shown the formation of acetaldehyde and acetic acid in addition to cmbon dioxide as soluble products. Figure 29 shows the typical features for thesq/products, which were assigned according to Ihble 1. 

Electrooxidative desulfenylation of Michael-type thiol adducts of activated olefins may be performed in ethanol with TBABr as supporting electrolyte in an undivided cell at platinum electrodes. The activated olefin is formed in excellent yield most of the sulfur is found as sulfinate [133]. 

Simoes FC, dos Anjos DM, Vigier F, Leger JM, Hahn F, Coutanceau C, Gonzalez ER, Tremiliosi-Filho G, de Andrade AR, Olivi P, Kokoh KB (2007) Electroactivity of tin modified platinum electrodes for ethanol electrooxidation. J Power Sources 167 1-10... 

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). 

Shao MH, Adzic RR (2005) Electrooxidation of ethanol on a Pt electrode in acid solutions in situ ATR-SEIRAS study. Electrochim Acta 50 2415-2422... 

Methanol and Ethanol Electrooxidation on Bulk Platinum Electrode... 

To improve the electrocatalytic activity of platinum and palladium, the ethanol oxidation on different metal adatom-modified, alloyed, and oxide-promoted Pt- and Pd-based electrocatalysts has been investigated in alkaline media. Firstly, El-Shafei et al. [76] studied the electrocatalytic effect of some metal adatoms (Pb, Tl, Cd) on ethanol oxidation at a Pt electrode in alkaline medium. All three metal adatoms, particularly Pb and Tl, improved the EOR activity of ft. More recently, Pt-Ni nanoparticles, deposited on carbon nanofiber (CNE) network by an electrochemical deposition method at various cycle numbers such as 40, 60, and 80, have been tested as catalysts for ethanol oxidadmi in alkaline medium [77]. The Pt-Ni alloying nature and Ni to ft atomic ratio increased with increasing of cycle number. The performance of PtNi80/CNF for the ethanol electrooxidation was better than that of the pure Pt40/CNF, PtNi40/CNF, and PtNi60/CNF. 

The ethanol electrooxidation mechanism on platinum electrodes in acidic solution has been studied by various techniques and a number of adsorbed intermediates have been identified [16-25]. Carbon dioxide (CO2), acetaldehyde (CH3CHO), and acetic acid (CH3COOH) are the main products of the reaction. The global... 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

A variant of the enhanced reaction zone concept is to utilize as catalyst support various porous three-dimensional electrodes with thickness between 200 to 2,000 pm. Thus, the electric contact resistance between the individual layers is eliminated. The three-dimensional matrix (such as various graphite felts, reticulated vitreous carbon, metal mesh, felt, and foam) supporting uniformly dispersed electrocatalysts (nanoparticles or thin mesoporous coating) could assure an extended reaction zone for fuel (methanol, ethanol, and formie aeid) electrooxidation, providing an ionic conductor network is established to link the catalytically active sites and the proton exchange membrane. The patent by Wilkinson et al. also suggests such electrode configurations (e.g., carbon foam, expended metal and reticulated metal) but experimental results were not provided [303]. 

Figure 9.16. Schematic mechanism of ethanol electrooxidation at a Pt/C electrode.

ELECTROOXIDATION OF ETHANOL ON POLYCRYSTALLINE PT, PT (HKL) ELECTRODES AND PT/C ELECTRODES. IDENTIFICATION AND OXIDATION OF ETHANOL ADSORBATE(S)... 

Early studies about ethanol electrooxidation detected only acetic acid and acetaldehyde as the reaction products 110,12,131 and concluded that their relative production depended upon the working potential. Rightmire et al. 112] analyzed by gas chromatography the liquid products resulting from the electrolysis of ethanol with platinized Ft electrodes in 0.5 M H2SO4 and identified acetaldehyde as the primary product with small amounts of acetic acid at potentials of 0.75 V. They went further and reported that polarographic scans in acetic acid solutions showed no reaction leading to the conclusion that acetic acid is a final oxidation... 

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