Environmental Engineering Reference
In-Depth Information
electrode is completely dominated by the kinetics of the oxidation process.
Experiments carried out with a Pt(111) hanging meniscus rotating disk electrode
showed that the voltammogram is independent of the rotation rate (formic acid
concentration 0.2 M), which indicated that the reaction is controlled by the electron
transfer process [Maci´ et al., 2003].
On the other hand, the Pt(100) electrode showed almost no currents in the positive-
going scan, a clear indication that the surface is completely blocked by the poisoning
intermediate, which is accumulated on the surface at low potentials. Once the poison is
oxidized, above 0.7 V (vs. RHE), currents in the negative-going scan are almost one
order of magnitude higher than those recorded for Pt(111) [Clavilier et al., 1981].
This indicates that both paths of the reaction mechanism are much faster for the
Pt(100) electrode.
For the Pt(110) electrode, there are some contradictory results regarding its
catalytic performance compared with Pt(100); some studies indicate that the activity
is higher for Pt(110), whereas others suggest the opposite [Chang et al., 1990;
Clavilier et al., 1981; Lamy et al., 1983]. The differences are probably associated
with different surface states of the Pt(110) electrode. The actual surface structure of
the Pt(110) electrode is strongly dependent on the electrode pretreatment. Since
formic acid oxidation is a surface-sensitive reaction, different electrocatalytic behavior
can be obtained for the same electrode after different treatments.
The studies with Pt single crystals showed some correlation between the measured
current, which, obviously, is a measure of the total CO
2
produced, and the poisoning
rate; i.e., the surfaces with higher catalytic activity showed higher poisoning rates.
It may happen that the poisoning intermediate is the only reactive intermediate, and
the reactivity through the active intermediate path would be negligible. However, the
significant currents measured for the electrode at low potentials would indicate that
the reactivity through the active intermediate is significant. In order to gain insight
into the mechanism, we will discuss the two paths separately.
The first step towards disentangling the mechanism is identification of the poison-
ing intermediate. Several candidates were proposed—CO, COH, and CHO—but the
only one identified by electrochemically modulated infrared spectroscopy (EMIRS)
and FTIR spectroscopy was CO [Kunimatsu, 1986; Chang et al., 1990; Sun et al.,
1988; Beden et al., 1983]. In fact, CO can be considered as the typical poisoning inter-
mediate, since its oxidation to CO
2
on platinum takes place at significant rates only
above 0.6 V (see Section 6.2). Furthermore, CO could be detected by IR spectroscopy
at potentials as low as 0.2 V, and the total elimination occurs only above 0.7 V. The
confirmation that CO was the poisoning intermediate and not an active intermediate
was given by differential electrochemical mass spectroscopy (DEMS) [Willsau and
Heitbaum, 1986; Wolter et al., 1985]. In these experiments (Fig. 6.14), H
13
COOH
was initially put in contact with a platinum electrode until the maximum amount of
CO was formed. Then, the solution was replaced with H
12
COOH and the potential
swept positively. The formed CO
2
was analyzed with a mass spectrometer. In this
case,
12
CO
2
was detected prior to the formation of
13
CO
2
, which clearly indicates
that there is a path going through an active intermediate different from CO, yielding
CO
2
. In fact, recent FTIR studies indicate that the contribution of the CO path to
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