Environmental Engineering Reference
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adsorbed H
2
CO (Reaction 9), which may desorb as formaldehyde. Formaldehyde is
then nearly completely hydrated to methylene glycol, which will be oxidized to
formic acid in solution, either directly (Reaction 12) or through adsorbed formate
(Reactions 11 and 13). Finally, this scheme also explains why formate was observed
on Pt surfaces during the electrochemical oxidation of methanol (Reactions 8 - 12),
formic acid (Reaction 13), and formaldehyde (Reactions 10 - 13), as well as why
CO
ads
was
found
during
the
oxidation
of
formaldehyde
and
of
formic
acid
(Reactions 10, 11, and 16) by Osawa and co-workers [Chen et al., 2003].
6.4.2 Methanol Oxidation on Other Transition Metals
Methanol oxidation on any other transition metal than platinum is rather sluggish, and
none of these metals displays significant catalytic oxidation activity. Generally, cur-
rent densities are very small in the region of interest. The only case where significant
activity is found is for the oxidation of methanol on Au nanoparticles or rough surfaces
in alkali solutions [Borkowska et al., 2004b; Hern
´
ndez et al., 2006]. For these elec-
trodes, significant currents can be recorded at potentials as low as 0.2 V (vs. RHE)
[Hern
´
ndez et al., 2006]. This situation contrasts with that observed for single-crystal
electrodes, where the onset of the oxidation is always above 0.8 V [Borkowska et al.,
2004a]. The activity of these electrodes has to be associated to the presence of a high
number of low coordinated atoms on the surface, which are probably the responsible
for this unusual catalytic activity. The final product of methanol oxidation on Au in
alkaline media has been suggested to be formate [Hernandez et al., 2006], formed
through a formaldehyde intermediate, similar to the pathway suggested in Fig. 6.21.
Interestingly, formate cannot be further oxidized to CO
2
in alkaline solution.
6.5 ETHANOL OXIDATION
6.5.1 Ethanol Oxidation on Platinum
Ethanol, being a renewable fuel, is often mentioned as one of the potential candidates
for low temperature fuel cell applications. Besides practical advantages in the employ-
ment of ethanol, such as the ease of transport of large quantities of ethanol and its
nontoxicity, the interest is justified by the high energy content of ethanol
(6.09 kWh/kg), corresponding to 12 electrons per molecule for total oxidation.
Furthermore, ethanol is the smallest alcohol containing a C22C bond, and can there-
fore serve as a model for the electro-oxidation of compounds containing C22C bonds.
Although the C22C bonds are, theoretically, the weakest bond in small alcohols
[Tsang, 1976], they are difficult to access (electro)catalytically, posing a major
challenge for the complete oxidation of ethanol.
It is well established that the main products of ethanol electro-oxidation on Pt in
acidic media are acetaldehyde and acetic acid, partial oxidation products that do not
require C22C bond breaking, with their relative yields depending on the experimental
conditions [Iwasita and Pastor, 1994]. Apart from the loss of efficiency associated with
the partial oxidation, acetic acid is also unwanted, as it constitutes a catalyst poison.
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