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 444

for a systematic modeling of the reaction process in a DMFC or direct formic acid fuel
cell. In contrast to standard modeling procedures, where the oxidation of the reactant is
described by an overall kinetic rate law (most simply some kind of a power law), data
of the present type would allow the reaction process to be described in a much more
realistic way, including also the formation and subsequent further oxidation of the
incompletely oxidized reaction side products. Measurements such as those described
here can provide rate constants for all different partial reactions; methanol oxidation to
formaldehyde, formic acid, and CO 2 ; further oxidation of formaldehyde to formic acid
and CO 2 ; and finally oxidation of formic acid to CO 2 .
For a simple description, one could use a segmented plug flow reactor model, where
the reaction-induced changes in reactant and product concentrations are calculated for
each segment and where the outlet concentrations of the nth segment are identical to
the input concentrations of the (n þ 1)th segment [Levenspiel, 1972]. Considering
the measured steady-state reaction currents of 100 mA, which are equivalent to a
reaction rate of about 0.5 nmol s 21 , and an electrolyte flow rate of 5 mLs 21 , the
reaction-induced decrease in reactant concentration in the present setup and under
present reaction conditions is negligible (about 1 part per thousand conversion).
Hence, the DEMS cell is operated under differential reaction conditions (constant
reactant concentration throughout the reactor). Therefore, the reaction rates and the
product distributions measured under steady-state conditions for the different C 1
molecules are characteristic for reaction of 0.1 M reactant solution under the present
reaction conditions (space velocity and temperature). For modeling the reaction
under technical conditions, with significant conversion in the cell (integral reaction
conditions), a number of measurements of a similar type would be required, covering
the full range of reactant concentrations present in the reaction cell.
The situation becomes more complicated by the fact that the incompletely oxidized
side products are not only reactive, but may also affect the activity of the catalyst for the
main reaction, for example by modifying the composition of the steady-state adlayer.
This may be particularly important in the later part of the reactor, where the concen-
tration of these side products will be higher than at the reactor inlet. As a result, the reac-
tions of the different reactive components in the electrolyte can no longer be considered
as independent. Therefore, in order to properly describe the accumulation and further
oxidation of the incompletely oxidized reaction products along the reaction cell,
measurements using representative mixtures of the three C 1 molecules are required.
Finally, for relevant fuel cell modeling, the kinetic model studies should be
performed under similar temperature and pressure conditions, i.e., at temperatures in
the range of 80- 120 8C and pressures up to 3 bar, and at comparable space velocities.
The first flow-cell and DEMS measurements under such temperature and pressure con-
ditions are currently underway in our laboratory.
13.5 SUMMARY
The adsorption and oxidation of the C 1 molecules methanol, formaldehyde, and
formic acid over a carbon-supported Pt/C fuel cell catalyst under continuous electro-
lyte flow have been investigated in a quantitative, comparative online DEMS study.