Environmental Engineering Reference
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delay (about 2 s) between faradaic current and mass spectrometric current detection.
For methanol oxidation (Fig. 13.3c), the partial current for CO
2
formation (complete
methanol oxidation) constitutes only about half of the measured faradaic current; for
formaldehyde oxidation (Fig. 13.3b), it is even less than 30%. As expected, for formic
acid oxidation (Fig. 13.3a), CO
2
formation accounts for almost all of the faradaic cur-
rent, except for the small contributions from double-layer charging and from PtO for-
mation/reduction, which also shows the precision of the partial current determination.
Similarly, the m/z ¼ 60 ion current signal was converted into the partial current for
methanol oxidation to formic acid in a four-electron reaction (dash - dotted line in
Fig. 13.3c; for calibration, see Section 13.2). The resulting partial current of methanol
oxidation to formic acid does not exceed about 10% of the methanol oxidation current.
Obviously, the sum of both partial currents of methanol oxidation to CO
2
and formic
acid also does not reach the measured faradaic current. Their difference is plotted in
Fig. 13.3c as a dotted line, after the PtO formation/reduction currents and pseudoca-
pacitive contributions, as evident in the base CV of a Pt/Vulcan electrode (dotted line
in Fig. 13.1a), were subtracted as well. Apparently, a significant fraction of the faradaic
current is used for the formation of another methanol oxidation product, other than
CO
2
and formic acid. Since formaldehyde formation has been shown in methanol oxi-
dation at ambient temperatures as well, parallel to CO
2
and formic acid formation [Ota
et al., 1984; Iwasita and Vielstich, 1986; Korzeniewski and Childers, 1998; Childers
et al., 1999], we attribute this current difference to the partial current of methanol
oxidation to formaldehyde. (Note that direct detection of formaldehyde by DEMS
is not possible under these conditions, owing to its low volatility and interference
with methanol-related mass peaks, as discussed previously [Jusys et al., 2003]).
Assuming that formaldehyde is the only other methanol oxidation product in addition
to CO
2
and formic acid, we can quantitatively determine the partial currents of all three
major products during methanol oxidation, which are otherwise not accessible.
Similarly, subtraction of the partial current for formaldehyde oxidation to CO
2
from
the measured faradaic current for formaldehyde oxidation yields an additional current,
which corresponds to the partial oxidation of formaldehyde to formic acid. The charac-
teristics of the different C
1
oxidation reactions are presented in more detail in the
following sections.
13.3.2.2 Formic Acid Oxidation
Despite the relatively simple oxidation reaction
(two-electron oxidation of formic acid leads to CO
2
as the only reaction product), the
potential sweep results in a rather complicated faradaic and mass spectrometric current
response (Fig. 13.3a). It closely resembles that reported previously for polycrystalline
Pt [Feliu et al., 2003; Capon and Parsons, 1973b, c; Willsau and Heitbaum, 1986;
Wolter et al., 1985; Okamoto et al., 2004; Lu et al., 1999; Chen et al., 2006a, b, c;
Samjesk´ et al., 2006] and Pt nanoparticle electrodes [Park et al., 2002], with the
faradaic current starting to increase slowly at 0.15 V and then more strongly at E .
0.35 V and again at E . 0.6 V. After passing through two maxima at about 0.72 and
0.80 V, it then decays again. The current decay at more positive potential is attributed
to increasing OH
ad
accumulation and PtO formation, where the latter is not active for
formic acid oxidation in this potential regime. In the negative-going scan, the current
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