Environmental Engineering Reference
In-Depth Information
TABLE 13.1 Number of Electrons per Resulting CO 2 Molecule Required for
Oxidizing the Stable Adsorbed Decomposition Product from Adsorption of C 1
Molecules to CO 2 , and C 1 Adsorbate Coverage Relative to that of a Saturated CO
Adlayer after Adsorption of C 1 Reactants a
CO ad from Molecules b
C 1 n e +0.3
u CO,rel +0.02
u CO,rel +0.05
HCHO
1.9
0.87
0.70
HCOOH
2.4
0.36
0.47
CH 3 OH
2.1
0.10
0.22
a As determined from potentiodynamic (data from Fig. 13.1, adsorption potential 0.11 V) and potentiostatic
(data from Fig. 13.2c, adsorption potential 0.16 V) stripping experiments.
b c ¼ 0.1 M, E ad ¼ 0.11 V, 10 minutes.
compared with a saturated CO adlayer (Fig. 13.1a). Correspondingly, the oxidation of
adsorbed hydrogen in the potential range 0.06 - 0.3 V during the positive-going scan is
less suppressed than for saturated CO adlayer stripping. The numbers of electrons
formed per CO 2 molecule are summarized in Table 13.1. Based on the number of
about 2 electrons per CO 2 product molecule, the stable, adsorbed residues developed
upon dehydrogenative adsorption of the three C 1 molecules are predominantly CO ad
species. The somewhat higher value (about 2.4 e 2 molecule 21 )determinedfor
formic acid adsorbate oxidation, which was obtained in a number of experiments,
might point to contributions from a non-CO ad adsorbate requiring 3 electrons per CO 2
molecule formation (e.g., COH), as had been proposed previously [Willsau and
Heitbaum, 1986]. Considering the error margin in these experiments of about +0.3
electron per CO 2 molecule, however, the deviation is still close to the precision of
these measurements, and thus is not considered as proof for any other type of adsorbate
than CO ad .
The conclusion that CO ad represents the only stable adsorbate after interaction with
these three C 1 molecules is supported also by the findings in previous in situ IR
studies, where adsorbed CO was detected as the only stable adsorbate on various Pt
surfaces, including single-crystal surfaces, after adsorption of methanol, formic
acid, or formaldehyde over a wide potential range ( 0.6 V) [Xia et al., 1997;
Waszczuk et al., 2001; Rice et al., 2000; Iwasita and Nart, 1997; Beden et al.,
1981; Nichols and Bewick, 1988; Hamnett et al., 1990; Park et al., 2002]. Similar
results were also obtained very recently from highly sensitive in situ IR measurements
on a Pt film electrode in an ATR-IRS configuration [Chen Y-X, et al., to be published].
Despite their high surface sensitivity, these measurements did not show any indication
of other stable adsorbates after removal of the reactant solution. It should be noted that
in early DEMS studies on porous Pt electrodes, values of about 3 electrons per CO 2
product molecule were found for both methanol and formic acid adsorbate oxidation
[Willsau and Heitbaum, 1986; Wolter et al., 1985; Willsau et al., 1985]. Most plausi-
bly, the higher values can be explained by an incomplete removal of the respective
reactants from the porous layer after electrolyte exchange [Jusys and Behm, 2001],
and/or variations in the collection efficiency [Wolter and Heitbaum, 1984], which
can occur for very high CO 2 formation rates on these porous electrodes.
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