Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles - Fuel Cell Catalysis: A Surface Science Approach - page 538

Figure 15.10 Schematic representation of the “dual pathway” mechanism of the MOR.

An in-depth review of the mechanism of the MOR is beyond the scope of this chapter
and will be discussed only inasmuch as it is necessary for understanding PSEs. For a
more detailed discussion, the reader is referred to Chapters 6, 11, and 13 and to the
original papers and reviews cited in this section.
A simplified scheme of the “dual pathway” electrochemical methanol oxidation on
Pt resulting from recent advances in the understanding of the reaction mechanism [Cao
et al., 2005; Housmans et al., 2006] is shown in Fig. 15.10. The term “dual pathway”
encompasses two reaction routes: one (“indirect”) occurring via the intermediate
formation of CO ads , and the other (“direct”) proceeding through partial oxidation
products such as formaldehyde.
Adsorption via dehydrogenation was proposed many years ago by Petrii and
co-workers [Petrii et al., 1965]. This is a surface-sensitive reaction, as underlined,
for example, in Papoutsis et al. [1987], and requires three adjacent sites [Beden
et al., 1981; L´ger, 2001], as confirmed recently in an elegant study [Cuesta, 2006].
Methanol dehydrogenation appears as an anodic current in the positive-going scan
of the CV starting from the H UPD region. The simultaneously registered mass spectro-
metric current shows zero at m/z ¼ 44, suggesting that CO 2 is not formed in this
potential interval [Krausa and Vielstich, 1994]. Stepwise dehydrogenation via C22H
bond breaking results in a hydroxymethyl (CH 2 OH) intermediate, which is then
further dehydrogenated to CO ads , the latter acting as a reaction intermediate but also
as a poison. Formation of CO ads has been confirmed in numerous electrochemical
and spectroscopic investigations (see Biegler and Koch [1967]; Beden et al. [1981]
and the reviews [Petrii et al., 1965; L´ger, 2001; Wasmus and Kuver, 1999;
Iwasita, 2003; Waszczuk et al., 2002; Gasteiger et al., 1993]). As outlined above,
CO ads electro-oxidation proceeds in a surface reaction with OH ads and thus does not
occur below the threshold potential of water splitting. CO ads electro-oxidation in the
potential interval relevant to DMFCs (,0.35 V vs. RHE) is very slow, and thus
CO ads accumulates on the surface, inhibiting further reaction. Thus, some authors
have identified water dissociation on Pt electrodes as the rate-determining step for
the MOR [Redondo et al., 1989; L´ger, 2001]. It should be noted, however, that the
problem is not necessarily the slow OH ads formation, but rather its low coverage in
the potential interval relevant to the MOR.
Recent experimental and theoretical studies [Cao et al., 2005; Housmans et al.,
2006] suggest that, depending on the surface structure of the electrode and on the