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 437

recent studies, the definition of the dual-pathway mechanism was not always
consistent, and in some cases, the reaction pathway leading to incomplete oxidation
products was described as second reaction pathway in the dual-pathway mechanism
[Cao et al., 2005]. For clarity, we suggest that the formation of incomplete oxidation
products be included in the reaction scheme, in addition to the direct and the indirect
pathway, which results in the expanded reaction scheme presented in Fig. 13.8b.
A more detailed reaction scheme proposed recently by Housmans et al. [2006]
agrees in most aspects with that simple scheme. The dashed arrows indicate the poss-
ible formation of the incomplete oxidation products formaldehyde and formic acid
from the reaction intermediate in the direct pathway and their possible readsorption
(two-headed arrow), and the (irreversible) formation of CO 2 from these two reaction
products. The dotted arrow accounts for CO ad formation upon re-adsorption and
dehydrogenation of the incomplete oxidation products.
Furthermore, it is important to distinguish between adsorbed side products on the
one hand, which may be formed reversibly or irreversibly, but are not involved in
the actual reaction pathway (“spectator species”), and real reaction intermediates on
the other hand, where the latter are part of the reaction pathway to the final product.
In a reaction network including different parallel pathways to the final product, such
reaction intermediates may occur in each of these pathways, for example, in the domi-
nant “majority” reaction pathway and in minority pathways. As one example, it was
shown by quantitative ATR-IRS measurements that for formic acid oxidation on a
Pt film electrode at potentials around 0.6 V, the indirect pathway is a minority pathway
[Chen et al., 2006b, c; Samjesk´ et al., 2005, 2006], contributing less than 1% to the
total faradaic current [Chen et al., 2006b, c]. In that case, CO ad can be considered
as reaction intermediate in a minority pathway. Often reaction intermediates are
short-lived species, which, because of their very low steady-state coverage, are not
detectable spectroscopically. It is important to note that both spectator species and
reaction intermediates in a minority pathway can significantly affect the dominant
reaction pathway as well, for example by blocking the reactive surface. In the above
case, the reaction intermediate in the minority pathway, CO ad , affects the main reaction
path by blocking the surface with a reaction-inhibiting CO adlayer.
In the following, we will discuss four points that are relevant for the mechanistic
understanding of the C 1 oxidation reaction and where the present data can contribute.
Since formic acid oxidation leads to a single reaction product only and the mechanism
for formic acid oxidation has been discussed in detail in recent publications [Miki
et al., 2002, 2004; Samjesk´ and Osawa, 2005; Chen et al., 2006a, b, c; Samjesk´
et al., 2005, 2006], we will concentrate on methanol and formaldehyde oxidation.
For these reactions, one may formulate the following questions:
1. Does methanol (or formaldehyde) oxidation along the direct pathway lead
“directly” to CO 2 or are incomplete oxidation products formed first, which
are then oxidized further to finally result in CO 2 formation (dashed arrows in
Fig. 13.8b)?
2. A similar question may also be asked for the indirect pathway: Is CO ad directly
formed by methanol decomposition, or does it result from a follow-up reaction,