Environmental Engineering Reference
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onset of Pt oxidation, and therefore the structure of the CO
2
efficiency is rather simple,
with a more or less continuous increase to more cathodic potentials.
13.3.2.4 Methanol Oxidation
Methanol oxidation on a Pt/Vulcan catalyst was
studied and discussed extensively in our previous papers [Jusys and Behm, 2001;
Jusys et al., 2003]. Therefore, we will focus her on the most important features and
refer to those previous studies for more detailed discussions.
At potentials negative of 0.5 V, methanol oxidation is largely hindered by a reaction
inhibiting CO adlayer, which results from dehydrogenative methanol adsorption in the
preceding potential scan. Comparison with the 10-fold magnified methanol adsorbate
stripping peak, both for the faradaic current (dotted line in the top panel of Fig. 13.3c)
and for the CO
2
mass spectrometric signal (gray line in the middle panel of Fig. 13.3c),
shows that both methanol adsorbate and methanol bulk oxidation start at the same
potential in the positive-going scan. At potentials positive of 1.0 V, methanol
oxidation is suppressed by PtO formation. It is re-activated again by PtO reduction
in the negative-going scan. Overall, these effects result in the well-known bell-
shaped polarization curves. The corresponding m/z ¼ 44 and m/z ¼ 60 mass spectro-
metric currents (solid lines in the middle and bottom panels of Fig. 13.3c) generally
follow the faradaic current, but exhibit some differences in the ratio of the peak heights
and in the peak shape, which point to a potential-dependent variation of the product
distribution for the two scan directions (for details, see [Jusys and Behm, 2001;
Jusys et al., 2003]).
The calibrated m/z ¼ 44 and m/z ¼ 60 ion currents were converted into the respect-
ive partial reaction faradaic currents as described above, and are plotted in Fig. 13.3c as
dashed (m/z ¼ 44) and dash - dotted (m/z ¼ 60) lines, using electron numbers of 6
electrons per CO
2
molecule and 4 electrons per formic acid molecule formation. The
calculated partial current for complete methanol oxidation to CO
2
contributes only
about one-half of the measured faradaic current. The partial current of methanol oxidation
to formic acid is in the range of a few percent of the total methanol oxidation current. The
remaining difference, after subtracting the PtO formation/reduction currents and
pseudocapacitive contributions as described above, is plotted in Fig. 13.3c (top panel)
as a dotted line. As mentioned above (see the beginning of Section 13.3.2), we attribute
this current difference to the partial current of methanol oxidation to formaldehyde.
This way, we were able to extract the partial currents of all three major products during
methanol oxidation reaction, which are otherwise not accessible.
The current efficiencies for the different products during the positive-going scan in
potentiodynamic methanol oxidation are plotted in Fig. 13.4b. (To reduce the noise
level of the formic acid current efficiency, which results from the poor signal-to-
noise ratio of the m/z ¼ 60 mass signal, this was fitted by a Gaussian.) At potentials
of 0.4 - 0.5 V, where the Pt surface is largely blocked by adsorbed CO, the (very slow)
reaction is dominated by incomplete oxidation of methanol to mainly formaldehyde,
with a current efficiencies of about 80 - 100%. With increasing potential, the current
efficiency for CO
2
formation increases, reaching a double peak with maxima of
about 60% at 0.62 and 0.8 V. At the same time, the current efficiency for formaldehyde
formation decreases gradually. In the potential range E . 0.5 V, it mirrors that for
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