Environmental Engineering Reference
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similar to those for formic acid oxidation, with a few distinct differences. First, formal-
dehyde oxidation is more suppressed at low potentials than formic acid oxidation.
Second, the faradaic current peak for formaldehyde oxidation is about four to five
times higher than the respective formic acid oxidation faradaic current maxima.
Third, at potentials positive of 1.0 V, the formaldehyde oxidation current increases
again, up to the upper potential limit of 1.16 V, and decreases reversibly in the nega-
tive-going scan to a current minimum at about 0.95 V. The re-start of formaldehyde
oxidation in the negative-going scan coincides with the reduction of the PtO cover
layer at potentials lower than 0.9 V; it leads to a broad peak centered at about 0.6 V.
The subsequent suppression of the formaldehyde oxidation reaction at lower potentials
is explained by the increasing coverage of poisoning CO
ad
species (see Section
13.3.2.2). The earlier decay of the reaction current in the negative-going scan
compared with formic acid oxidation can be explained by the higher tendency of
formaldehyde for CO
ad
formation, which was observed also in the formaldehyde
adsorption experiments (see Section 3.1.2 and Table 13.1) and in recent in situ IR
spectroscopy experiments [Miki et al., 2004; Chen et al., to be published; Park
et al., 2002]. The increasing formaldehyde oxidation current at potentials .1.0 V indi-
cates a significant activity of the oxidized Pt surface for formaldehyde oxidation at
these potentials, in contrast to the negligible activity for formic acid oxidation
under similar conditions (Fig. 13.3a).
In the mass spectrometric trace (solid line in the lower panel of Fig. 13.3b), CO
2
formation starts at 0.6 V, almost simultaneously with the onset of formaldehyde adsor-
bate oxidation (gray line in the lower panel of Fig. 13.3b, data from Fig. 13.1b). The
main CO
2
formation peak coincides with the main faradaic current peak. Its intensity,
however, exhibits a rather different behavior compared with that for formic acid
oxidation, with rather similar peak currents in the positive- and negative-going
scans. While the peak current is about similar to that for formic acid oxidation in
the positive-going scan, it is almost three times lower in the negative-going scan
(solid lines in the lower panels of Fig. 13.3a, b). Compared with the formaldehyde
adsorbate stripping peak (gray line in the lower panel of Fig. 13.3b, panel, data
from Fig. 13.1b), the main onset of formaldehyde bulk oxidation in the positive-
going scan is shifted to slightly more anodic potentials. Most likely, this results
from re-formation of CO
ad
species by decomposition of formaldehyde during the
potential scan. Finally, the m/z ¼ 44 ion current (solid line in the lower panel of
Fig. 13.3b) does not show the significant increase in the high potential region observed
in the faradaic current. Under these conditions, incomplete oxidation of formaldehyde
to formic acid prevails (two-electron reaction). The same is true down to about 0.85 V
in the negative-going scan (solid line in the lower panel of Fig. 13.3b).
The partial faradaic current for formaldehyde oxidation to CO
2
, calculated from the
m/z ¼ 44 ion current, is plotted as a dashed line in Fig. 13.3b (upper panel). Complete
oxidation of formaldehyde to CO
2
contributes only one-third ( positive-going scan) or
one-quarter (negative-going scan) of the corresponding faradaic current peaks (solid
line in the upper panel of Fig. 13.3b). The difference between the measured net current
and the calculated faradaic current, which is plotted as a dotted line in Fig. 13.3b
(upper panel), reflects the partial current for incomplete formaldehyde oxidation to
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