Environmental Engineering Reference
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starts to increase steeply at 0.9 V, after reduction of the Pt oxides. It passes through a wide
peak with several ill-resolved maxima, and starts to decay again at E , 0.58 V, reaching
zero current at the negative potential limit of 0.06 V. In the low potential region, the elec-
trocatalytic activity of Pt is suppressed owing to surface blocking by reaction-inhibiting
side products/reaction intermediates, which in in situ IR spectroscopy studies were ident-
ified as CO ad species [Sun et al., 1988; Sun and Yang, 1999; Miki et al., 2002; Samjesk´
and Osawa, 2005; Chen et al., 2006a, b, c; Samjesk´ et al., 2005; Park et al., 2002] (see also
Section 13.3.1). The latter species are responsible for the complex current response on the
electrode potential and the pronounced hysteresis, with a low initial coverage of poisoning
adsorbate species immediately after PtO reduction in the negative-going potential scan,
but a high coverage at the negative potential limit. Further mechanistic implications
will be discussed in more detail in Section 13.4.
In Fig. 13.3a (lower panel), we show the m/z ¼ 44 mass signal for continuous
formic acid oxidation in 0.1 M HCOOH solution (solid line) and, in addition, the
10-fold magnified m/z ¼ 44 ion current for the oxidation of formic acid adsorption
residues developed after adsorption at 0.11 V (gray line, data taken from
Fig. 13.1b). Obviously, the steep increase in the formic acid oxidation rate at about
0.7 V in the positive-going scan (Fig. 13.3a) coincides with the oxidation of adsorbed
CO, and is therefore attributed to the appearance of CO ad -free, reactive Pt sites. Similar
conclusions were drawn by Willsau and Heitbaum from DEMS experiments, where
they investigated the oxidation of pre-adsorbed labeled formic acid adsorbate in non-
labeled formic acid solution [Wilhelm and Heitbaum, 1986]. On the other hand, there
is hardly any intensity in the m/z ¼ 44 signal for oxidation of formic acid adsorbate
(CO ad generated by “chemical” dehydration/adsorption of formic acid) at potentials
negative of 0.55 V, while CO 2 formation during continuous HCOOH oxidation
starts already at potentials as negative as 0.2 V in the positive-going scan. In agreement
with the interpretation in [Willsau and Heitbaum, 1986], this clearly demonstrates the
presence of an additional reaction pathway that does not proceed via formation and
oxidation of CO ad (the “dual-pathway mechanism”; for further discussion, see
Section 13.4 and [Samjesk´ and Osawa, 2005; Chen et al., 2006a, b; Samjesk´
et al., 2005, 2006]).
The dashed line in Fig. 13.3a (top panel) shows the faradaic current resulting from
this reaction, calculated by assuming that CO 2 is the only reaction product during
formic acid oxidation, independent of the reaction mechanism, and corrected by the
time constant of the DEMS setup (2 s). The calculated faradaic current obtained this
way is free from contributions arising from PtO formation/reduction, which appear
in the measured faradaic current at potentials positive of 0.8 V, or from pseudocapa-
citive double-layer charging effects in the potential region 0.2 - 0.8 V. These contri-
butions are determined by subtracting the calculated faradaic current from the
measured current (dotted line in the top panel of Fig. 13.3a). This difference signal
closely resembles the base CV of the same Pt/Vulcan electrode (Fig. 13.1a), except
for the H upd- related features due to CO ad blocking.
13.3.2.3 Formaldehyde Oxidation The general characteristics of the faradaic
current dependence on the potential (solid line in the top panel of Fig. 13.3b) are
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