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 445

Based on the number of electrons required for their oxidation, the stable adsorbates
remaining on the surface after adsorption in the H upd potential region and subsequent
electrolyte exchange were identified as CO ad . The saturation coverages reached
after 10 minutes' adsorption from 0.1 M C 1 solution decrease in the order
formaldehyde . formic acid . methanol, assuming coverages between 87% and
10% of the coverage of a saturated CO adlayer obtained by exposure to a CO-
containing electrolyte. The C 1 adsorbates are easier to oxidize than a CO adlayer of
similar coverage prepared by CO adsorption. This is reflected by a downshift of the
oxidation peak, both of the onset and of the maximum, in potentiodynamic stripping
experiments and in a faster oxidation with no induction period in potentiostatic
stripping experiments. The differences in oxidation behavior are tentatively assigned
to a different distribution of the CO ad molecules after C 1 and after CO adsorption,
respectively.
Potentiodynamic oxidation of methanol and formaldehyde reveals considerable
formation of incomplete oxidation species (formaldehyde and formic acid, respect-
ively) under the present reaction conditions, with potential-averaged CO 2 current
efficiencies of about 50% for methanol oxidation and 20% for formaldehyde
oxidation. In the positive-going scan, the onset of the oxidation reaction differs signifi-
cantly for the three reactants, with onset potentials of 0.1 V (formic acid), 0.4 V
(methanol), and 0.55 V (formaldehyde), respectively. The higher onset potential for
formaldehyde oxidation compared with methanol oxidation is attributed to the
much higher CO ad coverage, which inhibits the reaction start. For formic acid
oxidation, the reaction is possible already in the presence of a CO adlayer and at
potentials far below the onset of OH ad formation, which was the first evidence for a
direct pathway for formic acid oxidation. In the negative-going scan, the similar
onset potential for the main oxidation peak for all three reactants is correlated to the
reduction of the oxidized/OH ad -covered Pt surface, underlining also the much
higher activity of the bare metallic Pt surface compared with the oxidized surface.
In both scan directions, the maximum formaldehyde oxidation rate is at least twice
as high as those for methanol and formic acid oxidation.
The current efficiency for formic acid formation during methanol oxidation is
below 10%, leaving formaldehyde as the main incomplete oxidation product under
the present reaction conditions. The dominant formation of incomplete oxidation
products at the onset of the reaction in the positive-going scan points to a lower site
requirement for incomplete oxidation compared with complete oxidation to CO 2 .
For both methanol and formaldehyde oxidation, the CO 2 current efficiency exhibits
a characteristic double-peak structure in the positive-going scan, with maxima at
about 0.62/0.7 V and 0.8/0.9 V for methanol and formaldehyde oxidation, respect-
ively. The first maximum, which coincides with the CO ad stripping peak, is attributed
to the oxidation of CO ad that was formed in the low potential region of the preceding
negative-going scan and the present positive-going scan. The second maximum
reflects an inherently higher activity for CO 2 formation on the Pt surface in the absence
of reaction-hindering adlayers. It results mainly from CO 2 formation by direct
oxidation of the respective reactant. At increasing potential, both the overall reaction
rate and the CO 2 efficiency decrease owing to the increasing coverage of adsorbed