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These trends agree well with recent theoretical studies, which found that several
neighboring (three) Pt atoms are required for methanol dehydrogenation [Desai
et al., 2002; Greeley and Mavrikakis, 2002, 2004; Okamoto et al., 2003; Cao et al.,
2005], while these requirements are less stringent for formaldehyde dehydrogenation
[Desai et al., 2002; Greeley and Mavrikakis, 2002].
13.3.1.2 Potential-Step Electro-Oxidation of Adsorbed Species
Additional information on the adsorbate layer resulting from adsorption of the
different C 1 reactants is obtained from potential step oxidation transients, recorded
after adsorption of the respective reactants at a potential in the H upd regime (10 minutes
at 0.16 V), subsequent electrolyte exchange, and finally a potential step to 0.6 V.
Figure 13.2 shows faradaic (Fig. 13.2a) and m/z ¼ 44 mass spectrometric
(Fig. 13.2b) current transients measured during oxidation of the adlayer at 0.6 V.
For comparison, we also include similar transients recorded after CO ad adsorption
from a CO-containing supporting electrolyte and a control experiment performed
with a clean Pt/C catalyst in pure supporting electrolyte. The insets show the corre-
sponding current transients on an expanded time scale, during the first 2 - 3 minutes.
(For better separation, the current transients in the insets are progressively shifted by
30 s against each other.)
The control experiment in pure supporting electrolyte (dotted lines in Fig. 13.2)
shows a sharp faradaic current spike, which is mainly due to pseudocapacitive contri-
butions (adsorption of (bi)sulfate and rearrangement of the double layer) plus oxidation
of adsorbed H upd (dotted lines in Fig. 13.2a), but no measurable increase in the CO 2
partial pressure (m/z ¼ 44 current) above the background level (dotted lines in
Fig. 13.2b). Therefore, a measurable adsorption of trace impurities from the base elec-
trolyte can be ruled out on the time scale of our experiments. Moreover, this experiment
also demonstrates the advantage of mass spectrometric transient measurements
compared with faradaic current measurements, since the initial reaction signal is not
obscured by pseudocapacitive effects and the related faradaic current spike.
The initial current spike appears as a dominant feature also in the faradaic current
transients obtained upon oxidation of pre-adsorbed CO and of the adsorbates resulting
from adsorption of C 1 molecules. For oxidation of a saturated CO adlayer (CO adsorp-
tion from a CO-saturated electrolyte), the faradaic current decays within about 30 s to
0.5 mA and remains approximately constant for about 3 minutes (solid lines in
Fig. 13.2a). Later on, the faradaic current increases slowly, passes through a maximum
value of about 1.2 mA about 10 minutes after the potential step, and then decreases
again to zero. The characteristic shape of the CO oxidation transient recorded
during oxidation of a saturated CO adlayer (CO adsorption from a CO-saturated
electrolyte), with a pronounced induction period after the initial current spike and a
distinct oxidation peak, closely resembles previous observations on supported Pt/C
catalysts [Lanova et al., 2006; Friedrich et al., 2000; Gustavsson et al., 2004;
Maillard et al., 2004a, 2005; Arenz et al., 2005] and solid Pt electrodes
[Santos et al., 1991; Petukhov et al., 1998; Koper et al., 1998; Bergelin et al., 1999;
Korzeniewski and Kardash, 2001; Lebedeva et al., 2002; Heinen et al., 2007]. For
oxidation of formaldehyde adsorbate, the current transient still shows a distinct
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