Environmental Engineering Reference
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onset of CO 2 evolution, indicating that C22C bond breaking and CO 2 production are
decoupled processes.
Most studies on ethanol oxidation on Pt single crystal electrodes have focused on
the structure sensitivity of the formation of the different products, mainly on the basal
planes of Pt. By studying the early stages of adsorption and oxidation of ethanol on Pt
with in situ FTIR spectroscopy, Xia and co-workers found that the onset potential for
ethanol dissociation, as followed by the CO ads infrared absorption intensities,
coincides with the hydrogen desorption potential [Xia et al., 1997]. In addition, the
presence of adsorbed CO strongly inhibited further oxidation on all surfaces. By
relating the spectroscopic measurements with the voltammetric data, it was concluded
that the number of surface sites covered with CO ads , and thus the capacity to cleave the
C22C bond, is higher on Pt(100) than on Pt(111) or Pt(110). It is also worth noting that
the amount of CO 2 produced keeps increasing after the surface becomes free of CO ads ,
indicating the presence of other strongly adsorbed species, likely CH x , which is not
observable by infrared spectrometry.
More recently, the dissociation of ethanol was studied by SERS [Lai et al., 2008].
By employing isotopically labelled ethanol, it was found that C22C bond breaking
already occurs at low potentials, resulting in chemisorbed CH and CO. Upon oxidation
the CH fragments are converted to CO at a potential below that of CO oxidation,
suggesting that, at least on platinum, the potential limiting step in the oxidation of
the adsorbed C species is the oxidation of CO.
Infrared spectroscopy has also been employed to follow the formation of acet-
aldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal
planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well
before the onset of CO oxidation, while the onset of acetic acid formation closely
follows CO 2 formation [Chang et al., 1990; Xia et al., 1997]. This is readily explained
by the fact that both CO oxidation and acetic acid formation require a common
adsorbed co-reactant, OH ads , whereas the formation of acetaldehyde from ethanol
merely involves a relatively simple proton - electron transfer.
So far, few studies have focused on the effect of (the density of ) defect sites, as
modeled by using Pt single-crystal electrodes with varying step density. A notable
exception is a study by Tarnowski and Korzeniewski, who followed the quantities
of acetate formed at different potentials in potential step experiments using ion chrom-
atography on Pt(111), Pt(755) ; Pt[6(111) (100)] and Pt(533) ; Pt[4(111)
(100)] [Tarnowski and Korzeniewski, 1997]. It was shown that, although the maxi-
mum currents increased with step density, the relative contribution of acetic acid for-
mation decreased. Since step sites are assumed to facilitate C22C bond breaking in
ethanol oxidation [Shin et al., 1996], this decreased activity was partly attributed to
increased surface poisoning, blocking sites for water adsorption and thereby inhibiting
acetic acid formation. Since the maximum currents do increase with step density, it is
likely that other processes, such as acetaldehyde formation, become more pronounced
on (partially blocked) stepped surfaces [Leung et al., 1989].
Since several steps in the ethanol oxidation mechanism require the presence of
an OH ads species, the use of an alkaline medium has also attracted some attention,
owing to the ubiquitous hydroxide ions leading to significantly higher oxidation
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