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 420

de Lima et al., 2007; Miki et al., 2004], and formic acid oxidation [Okamoto
et al., 2004; Samjesk´ and Osawa, 2005; Breiter, 1967; Loucka and Weber, 1968;
Spasojevic et al., 1980; Napporn et al., 1995; Okamoto et al., 2005; Park et al.,
2002b] on polycrystalline Pt and Pt nanoparticle electrodes. For all three reactants,
they exhibit a distinct hysteresis between positive-going and negative-going
scan, which had been associated with the different states of the catalyst prior to the
onset of the reaction in the two scan directions, with the Pt surface being CO ad -covered
at low potentials, before the positive-going scan, and OH ad covered/partly oxidized
but CO ad -free at high potentials, before the negative-going scan [Parsons and
VanderNoot, 1988; Capon and Parsons, 1973]. For methanol and formaldehyde oxi-
dation, the reaction is largely hindered by adsorbed CO at potentials negative of 0.5 V
and 0.6 V, respectively, while formic acid oxidation starts already at much lower
potentials, at around 0.2 V in the positive-going scan. The current decay at more posi-
tive potentials (.0.8 - 0.9 V) is attributed to an increasing hindrance of the C 1 electro-
oxidation reaction by PtO formation [Angerstein-Kozlowska et al., 1973]. In the
reverse, negative-going scan, the oxidation current starts simultaneously with PtO
reduction on an initially CO ad -free (CO ad can be efficiently oxidized at these potentials
[Jusys et al., 2001; Gilman, 1964; Stonehart and Kohlmayr, 1972; Gasteiger et al.,
1995; Schmidt et al., 1999b]) and increases steadily. It then passes through a maxi-
mum and decreases to zero at lower potentials, where OH electrosorption and therefore
also oxidation of the CO ad species are inhibited by the CO adlayer. For all three reac-
tants, the reaction extends to potentials negative of the onset of CO ad stripping in the
positive-going scan, which is attributed to the finite time required for the build-up of a
reaction-inhibiting CO adlayer in the negative-going scan. Interestingly, the ratio of
the peak heights in the positive- and negative-going scans changes significantly
from methanol to formic acid oxidation, becoming increasingly higher in the nega-
tive-going scan. The oxidation of formic acid even occurs on a CO ad -blocked surface
at potentials lower than 0.4 V, where OH ad generation on the Pt catalyst electrode is
negligible. This can be rationalized by the presence of two oxygen atoms in the car-
boxylic acid group, which allows CO 2 formation without requiring the formation of
and reaction with adsorbed oxygen species. The same is true also for formaldehyde
oxidation, considering that formaldehyde is largely hydrated to methylene glycol in
aqueous formaldehyde solutions, with two OH groups attached to the carbon atom
(see Batista and Iwasita [2006]).
The mass spectrometric currents follow largely, but not completely the faradaic
current signals. The contributions to the respective faradaic currents resulting from
complete oxidation to CO 2 , which are calculated using the calibration constant K
(see Section 13.2), are plotted as dashed lines in the top panels in Fig. 13.3. For the
calculations of the partial reaction currents, we assumed six electrons per CO 2 mol-
ecule formation and considered the shift in the potential scale caused by the time
Figure 13.3 (Continued ) (a) and (b) and in the middle panel in (c) show the m/z ¼ 44 ion
current response to the electrode potential, the gray lines illustrate the oxidation of pre-
formed CO, derived upon C 1 adsorption at 0.11 V, in reactant-free H 2 SO 4 solution. The
bottom panel in (c) shows the m/z ¼ 60 ion current response to the electrode potential.