Methanol, Formaldehyde, and Formic Acid Adsorption/Oxidation on a Carbon-Supported Pt Nanoparticle Fuel Cell Catalyst: A Comparative Quantitative DEMS Study - Fuel Cell Catalysis: A Surface Science Approach - page 427

CO 2 , at least qualitatively, with two minima coincident with the maxima in the CO 2
current efficiency. The pronounced decrease in current efficiency for CO 2 formation
at more positive potentials, anodic of 0.8 V, is accompanied by a comparable increase
in formaldehyde formation, while that for formic acid formation remains about
constant. Simultaneously, the current efficiency for formic acid formation increases
to about 10% at about 0.6 V, then decreases to about 5% and remains at this value
up to 0.9 V, finally decaying to zero at 1.1 V. It should be mentioned, however, that
the absolute currents are rather small for potentials cathodic of 0.5 V or anodic of
1.0 V, respectively. This is true, of course, also for the partial current for formaldehyde
formation, which results in larger uncertainties in the calculated current efficiencies in
these potential regions. In the negative-going scan (Fig. 13.4c), formaldehyde
formation decreases continuously and reaches zero at 0.5 V, while the current
efficiency for CO 2 formation increases steadily from about 10% at 1.16 V to about
50% at 0.7 V. It then remains at that level. The formic acid current efficiency behaves
similarly as in the positive-going scan, starting to increase at about 0.7 V and reaching
a value of 50% at 0.5 V.
The distinct double-peak structure in the CO 2 current efficiency (Fig. 13.4b) is in
clear contrast to the slightly asymmetric peak for the partial current for CO 2 formation
(dashed line in top panel of Fig 13.3c) and the rather symmetric faradaic current peak
(solid line in the top panel of Fig. 13.3c) in the positive-going scan. Similar to formal-
dehyde oxidation, the additional peak in the current efficiency for CO 2 formation at
more negative potentials is attributed to the oxidation of CO ad that was formed
upon adsorption of methanol at more negative potentials (for comparison, see the
methanol adsorbate stripping peak in Fig. 13.3c: the gray line in the middle panel)
and which is superimposed on the signal for bulk oxidation of methanol to CO 2 .As
a result of the difference in electron number for the oxidation of CO ad and of methanol
to CO 2 , with a two-electron reaction for CO ad oxidation and a six-electron reaction for
methanol oxidation, the current efficiency for CO 2 formation is overestimated in the
more cathodic current efficiency peak, since for conversion of the m/z ¼ 44 ion
current to faradaic current, a six-electron reaction was assumed. This is different for
the (almost simultaneous) CO ad formation and oxidation during the potential scan,
since in that case the four electrons released during CO ad formation would be included
in the current signal as well. (Note that such problems are not encountered in the evalu-
ation of the steady-state current efficiencies during potentiostatic bulk oxidation.)
13.3.3 C 1 Molecule Bulk Oxidation: Potential-Step Transients
In order to distinguish more clearly between effects induced by the varying potential
and kinetic contributions, the continuous oxidation of the three C 1 molecules was
followed at a constant potential after the potential step. The corresponding faradaic
and mass spectrometric (m/z ¼ 44) current transients recorded after 3 minutes'
adsorption at 0.16 V and a subsequent potential step to 0.6 V (see Section 13.2) are
reproduced in Figs. 13.5 - 13.7. In all cases, the faradaic current exhibits a small initial
spike, which is associated with double-layer charging when stepping the electrode
potential to 0.6 V.