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basal planes decreases as follows: Pd(100) . Pd(111) and Pd(110) [Hoshi et al.,
2006]. The activity of Pd nanoparticles has also been studied, and it has been
shown that there is a small dependence of their catalytic activity on size [Zhou
et al., 2006]. However, the most active electrode material for formic acid oxidation
is 1 - 2 layers of Pd deposited on a Pt(111) or PtRu(111) electrode [Llorca et al.,
1994b; Baldauf and Kolb, 1996; Kibler et al., 2005]. In this case, high currents are
obtained at very low potentials (as low as 0.2 V) without poisoning.
Although gold normally shows very little activity for the oxidation of small organic
molecules, it is able to oxidize formic acid to CO
2
in acidic media. Interestingly, as
Hamelin, Weaver, and co-workers demonstrated, there is no indirect pathway on Au
and no formation of a CO poisoning intermediate [Hamelin et al., 1992]. They
observed a relatively weak but significant crystal face dependence of the formic
acid oxidation current on Au: the Au(111) surface was the most active of the low
index planes, and Au(110) the least active. Combining SERS and DFT, Beltramo
and co-workers identified the low frequency mode between 280 and 320 cm
21
observed during formic acid oxidation on polycrystalline Au with adsorbed formate,
HCOO
2
[Beltramo et al., 2005].
6.4 METHANOL OXIDATION
6.4.1 Methanol Oxidation on Platinum
In the oxidation of methanol to CO
2
, six electrons are involved. This high number of
electrons implies that the mechanism is inevitably very complex, with several inter-
mediate species participating in the mechanism. In spite of its complexity, it has
been proposed that the oxidation mechanism follows the same general scheme as
the oxidation of formic acid, i.e., a dual path mechanism with active and poisoning
intermediates (see the reaction Scheme 6.16) [Parsons and VanderNoot, 1988].
For that reason, we will compare the behavior with that of formic acid to highlight
the similarities and differences.
The qualitative voltammetric behavior of methanol oxidation on Pt is very similar
to that of formic acid. The voltammetry for the oxidation of methanol on Pt single crys-
tals shows a clear hysteresis between the positive- and negative-going scans due to the
accumulation of the poisoning intermediate at low potentials and its oxidation above
0.7 V (vs. RHE) [Lamy et al., 1982]. Additionally, the reaction is also very sensitive to
the surface structure. The order in the activity of the different low index planes of Pt
follows the same order than that observed for formic acid. Thus, the Pt(111) electrode
has the lowest catalytic activity and the smallest hysteresis, indicating that both paths
of the reaction are slow, whereas the Pt(100) electrode displays a much higher catalytic
activity and a fast poisoning reaction. As before, the activity of the Pt(110) electrode
depends on the pretreatment of the surface (Fig. 6.17).
The first IR studies detected the formation and adsorption of CO, and therefore CO
was proposed as the poisoning intermediate [Beden et al., 1981; Nichols and Bewick,
1988; Corrigan and Weaver, 1988]. The formation of CO is structure-dependent and
takes place at open circuit, and the maximum amount accumulated on the electrode
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