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 431

efficiency for formic acid oxidation to CO 2 has been reported also for potentiostatic
formic acid oxidation on a polycrystalline Pt electrode based on transmission IR spec-
troscopy measurements in a micro flow cell, where the intensities were calibrated
against solutions with known CO 2 concentrations [Huang et al., 2002].
The saturation values for both faradaic and mass spectrometric currents obtained in
the potentiostatic experiment (Fig. 13.5a, b) are about three times higher than the
potentiodynamic formic acid oxidation currents at the same potential in the posi-
tive-going scan, but lower than those in the negative-going scan (Fig. 13.3a). These
differences can be explained by the higher CO ad coverage in the positive-going
scan and the lower CO ad coverage in the negative-going scan compared with the
steady-state situation. Therefore, the increase in the formic acid oxidation rate after
the potential step is caused by the increasing removal of reaction-inhibiting CO ad
species, which allows “direct” formic acid oxidation (see the discussion in Section
13.4) on an increasing area of the Pt surface. Because of the steady supply of CO ad ,
due to continuous adsorption and dehydrogenation of formic acid, the faradaic current
and the CO 2 formation rate decrease with time, and finally reach ( potential- and con-
centration-dependent) steady-state values once the rates for CO ad formation and CO ad
oxidation are equivalent. The lower steady-state formic acid oxidation rate compared
with the peak rate in the negative-going scan in the potentiodynamic measurements
can be explained by the lack of time to build up a CO adlayer of comparable coverage
in the latter case. In the same way, the higher steady-state formic acid oxidation rate
compared with that at 0.6 V in the positive-going scan is attributed to the higher
CO ad coverage in the potentiodynamic measurement, resulting from the low CO ad oxi-
dation rate at potentials E , 0.6 V and the CO ad coverage of about 50% CO ad of the
saturation value in the positive-going scan before the onset of CO ad oxidation [Chen
et al., 2006a, b, c]. Finally, the much smaller faradaic current and CO 2 formation rate
upon formic acid adsorbate oxidation compared with the steady-state bulk oxidation
current/rate indicates that CO ad oxidation makes only a minor contribution to
formic acid oxidation under these conditions and that most of the formic acid oxidation
current results from direct oxidation of formic acid. This result agrees well with recent
conclusions based on quantitative data from in situ IR spectroscopy measurements
[Chen et al., 2006a, b, c], where, depending on the reaction conditions, the indirect
pathway was found to contribute only in the range of 1% or less (for details, see
Samjesk´ et al. [2005, 2006]).
13.3.3.2 Formaldehyde Oxidation The faradaic current for formaldehyde
oxidation (solid line in Fig. 13.6a) is largely suppressed during the first minute after
stepping to 0.6 V, and then increases with time, resulting in an S-shaped transient.
It reaches its maximum current about 4 - 5 minutes after the potential step, followed
by a slow and nearly linear decay of the faradaic current with time (solid line in
Fig. 13.6a). In comparison with formic acid oxidation (solid line in Fig. 13.5a),
three major differences can be noted:
(i) Formaldehyde oxidation is more suppressed during the initial stages of the
oxidation reaction than formic acid oxidation.