Ethanol at a Platinum Electrode

The study of the adsorption and electrooxidation of ethanol on catalytic surfaces, such as platinum, is of considerable interest, both because of its possible use in fuel cells and because it is the first member of the series of aliphatic alcohols with a C—C bond. 

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Taking into acconnt previous IR reflectance spectroscopy studies and the results obtained here, a general mechanism can be proposed for the electrocatalytic oxidation of ethanol at a platinum electrode (Scheme 2). 

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... 

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. 

Perez JM, Beden B, Hahn F, Aldaz A, Lamy C. 1989. In situ infrared reflectance spectroscopic study of the early stages of ethanol adsorption at a platinum electrode in acid medium. J Electroanal Chem 262 251-261. 

Perez, J., Beden, B., Hahn, R, et al. (1989). In situ Infrared Reflectance Spectroscopic Study of the Early Stages of Ethanol Adsorption at a Platinum Electrode in Acid Medium, J. Electroanal. Chem., 262, pp. 251-261. 

K.D. SneU, A.G. Keenan, Chloride inhibition of ethanol electrooxidation at a platinum electrode in aqueous add solution, Electrochim. Acta 26 (1981) 1339-1344. 

In the indirect amperometric method [560], saturated uranyl zinc acetate solution is added to the sample containing 0.1-10 mg sodium. The solution is heated for 30 minutes at 100 °C to complete precipitation. The solution is filtered and the precipitate washed several times with 2 ml of the reagent and then five times with 99% ethanol saturated with sodium uranyl zinc acetate. The precipitate is dissolved and diluted to a known volume. To an aliquot containing up to 1.7 mg zinc, 1M tartaric acid (2-3 ml) and 3 M ammonium acetate (8-10 ml) are added and the pH adjusted to 7.5-8.0 with 2 M aqueous ammonia. The solution is diluted to 25 ml and an equal volume of ethanol added. It is titrated amperometrically with 0.01 M K4Fe(CN)6 using a platinum electrode. Uranium does not interfere with the determination of sodium. 

After heterogeneous electron transfer at a solid electrode, the resulting reduced/oxidized species may then transfer an electron homogeneously to or from another molecule in solution to regenerate the starting electroactive species. Such a mechanism is described as EC, where C represents a catalytic step. A typical example of such a reaction involves the oxidation of phenylamine, N,N-diethylphenylamine, histidine and histamine by ferricyanide electrogenerated at a platinum working electrode in basic aqueous/ethanolic mixtures see (60) and (61) (Rashid and Kalvoda, 1970). 

Figure 38. Voltammograms of a platinum electrode recorded in a 0.1 M perchloric acid solution at 0.05 V s" and 10°C with various concentrations of ethanol (1) 5 x 10 M (2)...

Figure 39. Concentration profiles of the different products involved in the prolonged electrolysis of 3.6 mM ethanol at 0.8 V vs RHE on a platinum electrode in 0.1 M HCIO4 and at 10°C ( ) ethanol consumption, ( ) acetaldehyde (AAL) and (A)acetic acid production (AA).

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. 

An additional problem arises from ethanol crossover through the proton exchange membrane. It results that the platinum cathode experiences a mixed potential, since both the oxygen reduction and ethanol oxidation take place at the same electrode. The cathode potential is therefore lower, leading to a decrease in the cell voltage and a further decrease in the voltage efficiency. 

The electrociiemical oxidation of ethanol has been extensively studied at platinum electrodes [22-34]. The first step is the dissociative adsorption of ethanol, either via an 0-adsorption or a C-adsorption process [25, 26], to form acetaldehyde (AAL) according to the following reaction equations. Indeed, it was shown by Hitmi ef al. [34] that AAL was formed at potentials lower than 0.6 V vs RHE. Thus ... 

Several added metals were investigated to improve the kinetics of ethanol oxidation at platinum-based electrodes, including ruthenium [27, 28], lead [29] and tin [22, 30]. Of these, tin appeared to be very promising. Figure 1.11 shows the polarization curves of ethanol electro-oxidation recorded at a slow sweep rate (5 mV s ) on different platinum-based electrodes. Pt-Sn(0.9 0.1)/C displays the... 

EMIRS studies of ethanol on platinum electrodes have demonstrated the presence of linearly bonded carbon monoxide on the surface [106]. An important problem in the use of EMIRS to study alcohol adsorption is the choice of a potential window where the modulation is appropriate without producing faradaic reactions involving soluble products. Ethanol is reduced to ethane and methane at potentials below 0.2 V [98, 107] and it is oxidized to acetaldehyde at c 0.35 V. Accordingly, a potential modulation would be possible only within these two limits. Outside these potential region, soluble products and their own adsorbed species complicate the interpretation of the spectra. The problem is more serious when the adsorbate band frequencies are almost independent of potential. In this case, the potential window (0.2-0.35 V) is too narrow to obtain an appropriate band shift and spectral features can be lost in the difference spectrum. 

Thienyl)ethanol as a starting material will give monomers with an ether linkage in the substituent at the 3-position. Such monomers, once polymerized, have exhibited the ability to complex cations such as Li in a loose crown ether type structure [70]. This in turn leads to enhanced conductivity of the polymer when such cations are part of the supporting electrolyte. An added benefit of electropolymerization of polythiophene originates from the fact that sulfur has a tendency to physisorb to metals such as gold and platinum, which are electrode materials. Hence they may enhance the adsorption of polymer to the electrode and thus improve the physical stability of the system, as well as the extent of polymer/electrode interaction. The synthesis of these type of monomers (e.g., 60) is shown in Scheme 10-28. 

Rotating disk electrode polarography and cyclic voltammetry at platinum electrodes of [Tc2Cl.s] solutions in hydrochloric acid/ethanol mixtures suggested a quasi-reversible oxidation at Ey2 = 0.140 V vs SCE involving one electron ... 

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