Environmental Engineering Reference
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oxidation under similar conditions, although the faradaic current for formaldehyde
oxidation (solid line in Fig. 13.6a) is about twice as high as that for formic
acid oxidation (solid line in Fig. 13.5a). In total, this results in a steady-state current
efficiency for CO
2
formation of 14% for formaldehyde oxidation. This is significantly
less than the current efficiency for CO
2
formation of about 55% ( product yield of 37%)
reported from IR spectroscopy measurements on Pt(111) at a similar potential [Batista
and Iwasita, 2006]. While we cannot rule out differences in the electrochemical
properties of the two electrodes, we mainly attribute the much lower CO
2
efficiency
in our measurements to the different mass transport conditions in the two experiments.
In the IR spectroscopy measurements, which were performed in a thin-layer configu-
ration with very slow diffusion of reactants/products to and from the electrode surface,
the probability of further oxidation of formic acid via re-adsorption is much higher
than in our flow cell measurements, where the off-transport of reaction products is
much more rapid.
We speculate that the high selectivity for formic acid formation during formaldehyde
oxidation under steady-state conditions is related to a relatively high CO adlayer cover-
age under these conditions compared with the other C
1
reactants (see Fig. 13.1). This
statement is based on the higher tendency of formaldehyde towards oxidative CO
ad
formation compared with the other reactants. Assuming that the CO
ad
oxidation rate
does not depend on the nature of the initial C
1
species, this should result in a higher
steady-state CO
ad
coverage during formaldehyde oxidation than during formic acid or
methanol oxidation under the present conditions (for comparison, see also [Chen
et al., to be published]). Considering further that formaldehyde oxidation to CO
2
is
more site-demanding than oxidation to formic acid (see above), this directly explains
the high current efficiency for formic acid formation. The nevertheless rather high far-
adaic current, which is significantly higher than the steady-state reaction currents for
methanol or formic acid oxidation, could be explained by considering that in aqueous
solution formaldehyde is largely hydrated to methylene glycol [Batista and Iwasita,
2006], which does not require OH
ad
for formic acid or CO
2
formation.
13.3.3.3 Methanol Oxidation
The faradaic current transient (solid line in
Fig. 13.7a) exhibits the most pronounced initial current spike among the C
1
bulk
oxidation transients, which may be due to the higher H
upd
coverage (itself due to
the lower initial CO
ad
coverage). Subsequently, the faradaic current drops within
about 30 s to 0.12 mA and then stays about constant for the remaining time of the
experiment (10 minutes). In that respect, the MOR transient is distinctly different to
the initial faradaic current increase during formic acid oxidation (Fig. 13.5a) or the
delayed current increase observed during formaldehyde oxidation. Because of the sig-
nificant pseudocapacitive double-layer charging current contributions, it is hardly
possible to interpret the magnitude of the initial spike [Franaszczuk et al., 1992;
Housmans and Koper, 2003]. It is clear, however, from the similar feature in the
mass spectrometric m/z ΒΌ 44 ion current transient that this feature is not due solely
to pseudocapacitive charging effects, but contains reactive contributions as well
[Jusys et al., 2003], similar to the observation for formaldehyde oxidation (see
Section 13.3.3.2). (Note that because of the limited time resolution (1 s
21
) of the
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