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 418

The different CO ad coverages obtained after CO and C 1 molecule adsorption are
sufficient to qualitatively account for the observed change in the overall shape of
the respective stripping transients. For a more quantitative comparison, we included
aCO ad stripping transient recorded upon oxidation of a medium coverage CO adlayer
obtained by adsorption from CO containing solution (gray lines in Fig. 13.2, CO sub ).
Although the CO ad submonolayer coverage is between those obtained after formic acid
and after formaldehyde adsorption, respectively, the general shape of the transient is
much closer to that obtained upon saturated CO adlayer oxidation than to those for C 1
adsorbate oxidation. In agreement with the potentiodynamic stripping experiments, C 1
adsorbate stripping is significantly faster at the same potential as that of a similar
coverage CO adlayer produced by CO adsorption.
In total, the differences between C 1 adsorbate stripping and CO ad stripping, in both
potentiodynamic measurements and potentiostatic transients, can be qualitatively
explained by the lower CO ad coverage after C 1 adsorption. A quantitative comparison
with a lower coverage CO adlayer reveals, however, that these coverage effects are not
sufficient and that contributions from other effects, most likely related to the structure
of the CO adlayer, are important for the different oxidation behavior of the respective
adsorbates as well.

13.3.2 Bulk Oxidation of Formic Acid, Formaldehyde, and Methanol:
Potentiodynamic Measurements

In this section, we present results of potentiodynamic DEMS measurements on the
continuous (bulk) oxidation of formic acid, formaldehyde and methanol on a Pt/
Vulcan catalyst, and compare these results with the adsorbate stripping data in
Section 13.3.1. We quantitatively evaluate the partial oxidation currents, product
yields, and current efficiencies for the respective products (CO 2 and the incomplete
oxidation products). In the presentation, the order of the reactants follows the increas-
ing complexity of the oxidation reaction, with formic acid oxidation discussed first
(one reaction product, CO 2 ), followed by formaldehyde oxidation (two reaction pro-
ducts) and methanol oxidation (three reaction products).

13.3.2.1 General Characteristics DEMS data recorded during the potentio-
dynamic electro-oxidation of the three C 1 molecules on a Pt/Vulcan electrode are
shown in Fig. 13.3a (formic acid oxidation), 13.3b (formaldehyde oxidation), and
13.3c (methanol oxidation). They include the faradaic current (solid lines in the top
panels), the CO 2 þ -related m/z ¼ 44 ion current [solid lines in the lower panels in
(a) and (b) and the middle panel in (c)], and, for methanol oxidation [bottom panel
in (c)], also the m/z ¼ 60 signal of methyl formate. The latter is commonly used to
detect formic acid in methanol-containing solutions [Iwasita and Vielstich, 1986;
Iwasita, 2002, 2003]. The faradaic current signals closely resemble those reported
earlier for methanol oxidation [Iwasita and Vielstich, 1986; Jusys and Behm, 2001;
Jusys et al., 2003; Iwasita, 2002, 2003], formaldehyde oxidation [Loucka and
Weber, 1968; Beltowska-Brzezinska and Heitbaum, 1985; Nakabayashi, 1998;
Nakabayashi et al., 1998; Batista and Iwasita, 2006; Samjesk´ et al., 2007;