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 434

MS measurements, the actual height of the initial spike may be significantly greater
than indicated in Fig. 13.7b.) The steady-state MOR current of 0.12 mA at the end
of the measurement is significantly lower than that for formic acid oxidation and
much lower than that for formaldehyde oxidation under similar conditions.
Comparison with the methanol adsorbate stripping trace (gray solid line in
Fig. 13.7b) shows that also for methanol oxidation the continuous CO 2 formation
rate under steady-state conditions is much higher than the maximum oxidation rate
of preformed CO ad , which resembles our observations for formic acid (solid line in
Fig. 13.5b) and formaldehyde (solid line in Fig. 13.6b) oxidation to CO 2 . However,
in contrast to the initial increase in the m/z ¼ 44 ion current with time for formic
acid and formaldehyde oxidation, the CO 2 formation rate for methanol oxidation
drops within half a minute to a value that is at most half of that attained initially (in
the spike). Apparently, the higher CO 2 efficiency during this initial phase not only
results from pre-adsorbed CO ad oxidation (for comparison, see the methanol adsorbate
stripping curve with its much lower current during the initial spike, which is included
as gray line in Fig. 13.7b), but also reflects a higher selectivity for CO 2 formation
during initial methanol bulk oxidation.
The m/z ¼ 60 ion current (Fig. 13.7c), which results from methyl formate and
hence is indicative of formic acid formation, equally increases in a pronounced step
after raising the potential, and then decays slowly with time. (The higher noise in
these data is due to the very low concentration of methyl formate species.)
The current efficiencies for the different reaction products CO 2 , formaldehyde, and
formic acid obtained upon potential-step methanol oxidation are plotted in Fig. 13.7d.
The CO 2 current efficiency (solid line) is characterized by an initial spike of up to
about 70% directly after the potential step, followed by a rapid decay to about 54%,
where it remains for the rest of the measurement. The initial spike appearing in the
calculated current efficiency for CO 2 formation can be at least partly explained by a
similar artifact as discussed for formaldehyde oxidation before, caused by the fact
that oxidation of the pre-formed CO ad contributes only two electrons per CO 2 mol-
ecule rather than six electrons, as assumed in the calculation of the current efficiency.
The current efficiency for formic acid oxidation steps to a value of about 10% at the
initial period of the measurement, and then decreases gradually to about 5% at the
end of the measurement. Finally, the current efficiency for formaldehyde formation,
which was not measured directly, but calculated from the difference between total
faradaic current and partial reaction currents for CO 2 and formic acid formation,
shows an apparently slower increase during the initial phase and then remains about
constant (final value about 40%). The imitial increase is at least partly caused by
the same artifact as discussed above for CO 2 formation, only in the opposite sense.
13.4 DISCUSSION
13.4.1 Mechanistic Aspects
The data presented in the preceding sections provide clear proof that the oxidation of
both methanol and formaldehyde over carbon-supported Pt/C catalysts can lead to