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relatively higher C22C bond breaking activity. This increased ability to activate the
ethanol C22C bond is also reflected in cathodic scans, during which the sole reduction
product was methane.
The lower total activity for Rh electrodes may be partly due to increased CO
poisoning and slower CO electro-oxidation kinetics compared with Pt electrodes, as
demonstrated by the number of voltammetric cycles required to oxidize a saturated
CO adlayer from Rh electrodes (see Section 6.2.2) [Housmans et al., 2004]. In
addition, it is argued that the barrier to dehydrogenation is higher on Rh than on Pt,
leading to a lower overall reaction rate [de Souza et al., 2002]. These effects may
also explain the lower product selectivity towards acetaldehyde and acetic acid,
which require the dehydrogenation of weakly adsorbed species.
6.5.4 Acetaldehyde Oxidation on Platinum
Since acetaldehyde has repeatedly been observed as one of the main products of
ethanol electro-oxidation, it has become clear that investigations on the mechanism
of the oxidation of acetaldehyde may help to understand the processes involved in
ethanol oxidation. In contrast to the large number of papers on ethanol oxidation, rela-
tively little has been published on acetaldehyde oxidation. By following the electro-
oxidation of acetaldehyde with in situ FTIR spectroscopy at varying concentrations,
Farias and co-workers concluded that acetaldehyde is oxidized in two parallel path-
ways [Farias et al., 2007]. The pathway producing CO
2
requires acetaldehyde to be
adsorbed dissociatively on the ( polycrystalline) platinum electrode. From the depen-
dence of the amount of CO
2
formed on the acetaldehyde concentrations, it was
suggested that CO
2
is formed through a Langmuir - Hinshelwood mechanism. At
low acetaldehyde concentrations (,0.05 M), CO
2
production is the dominant
pathway. At higher concentrations, the main reaction product is acetic acid, generated
through an Eley - Rideal-like mechanism involving (weakly adsorbed) acetaldehyde
reacting directly with OH
ads
to form acetic acid. Similar conclusions were drawn for
Pt(111) and Pt(110) electrodes [Rodriguez et al., 2000].
Recently, a systematic study of the influence of electrode structure on acetaldehyde
electro-oxidation was performed in Leiden [Lai and Koper, 2009]. By employing a
series of Pt[n(111)
(111)] ; Pt[(n 2 1)(111)
(110)] single-crystal electrodes
(Pt(111), Pt(15, 15, 14), Pt(554), and Pt(553), with n ¼ 200 - 500, 30, 10, and 5,
respectively) in acidic media, the density of step sites could be controlled in a precise
manner. The results of this investigation are shown in Fig. 6.22. Surprisingly, in
contrast to ethanol electro-oxidation, oxidation activity was found to decrease with
increasing step density, although this decrease is relatively small. This decrease in
activity is reflected both in decreasing maximum current densities and increasing
peak potentials with decreasing terrace width. The dependences of both peak par-
ameters on the step density (defined as u
step
¼ 1/(n 2
3
) [Clavilier et al., 1990])
were found to be linear, suggesting a cumulative effect of steps on the oxidation beha-
vior of acetaldehyde. One possible explanation for this behavior could be the faster
decomposition of acetaldehyde on step sites, compared with ethanol, owing to a
relatively easy-to-access C22C bond. Combined with the preferential poisoning of
step sites, this restricts oxidation to the terrace sites, explaining the observed behavior.
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