Environmental Engineering Reference
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poison, while the latter does not. Formation of CO ads in the course of formic acid
electro-oxidation results in strong self-inhibition effects. Takasu and co-workers,
and later Park and co-workers, demonstrated that SA for the electro-oxidation of
formic acid on carbon-supported Pt nanoparticles increases as the particle size
decreases,
with
no
significant
difference
in
electro-oxidation
kinetics
between
HClO 4 and H 2 SO 4 [Yahikozawa et al., 1991; Park et al., 2002b].
Strong PSEs in the FOR have been attributed to the structural sensitivity of the
reaction, previously confirmed for Pt(hkl) single crystals by Clavilier, Adzic, and
co-workers [Clavilier et al., 1981b; Adzic et al., 1983]. The reaction kinetics has
been studied with cyclic voltammetry and chronoamperometry. The peak current den-
sities for Pt(hkl) in CVs follow the order (110) . (111) . (100) in the positive-going
sweep and (110) . (100) . (111) in the negative-going sweep [Adzic et al., 1983].
Pt(111) has the lowest activity but the highest tolerance to self-poisoning. Sun and
Yang obtained similar results, with Pt(110) . Pt(111) . Pt(100), in a chronoampero-
metric study [Sun and Yang, 1999]. Not only the reaction rate, but also the apparent
activation energy demonstrates remarkable structure sensitivity [Sun and Yang, 1999].
Park and co-workers attributed the PSE in the FOR to the “ensemble effect,” where
reactant dehydrogenation to form poisoning CO ads species is impeded by the sharply
decreasing availability of contiguous Pt terrace sites for particles with d , 4 nm [Park
et al., 2002b]. This structural model is consistent with infrared measurements, which
for small particles below about 4 nm show a decreased rate of surface poisoning with
CO ads formed upon particle contact with formic acid [Park et al., 2002b]. Since the
oxidation of formic acid to CO 2 does not necessarily require its dehydrogenation,
the decreased contribution of this pathway results in the enhancement of the overall
reaction rate.
It is interesting that for carbon-supported Pd nanoparticles, PSEs are manifested
in a different manner [Zhang et al., 1995; Zhou et al., 2006]. SA gradually increases
as the particle size decreases to about 4 nm, but for smaller particles it falls
steeply. Zhang et al. and Zhou et al. attributed the observed changes to the variation
of the electronic structure of Pd clusters, in particular the DOS at the Fermi level,
as accessed by XPS and UPS. Unfortunately, they did not attempt to separate
initial and final state effects in the photoelectron spectra, and therefore one may
challenge their conclusions. Dissimilarities in the behavior of Pd and Pt nanoparticles
may be tentatively ascribed to differences in the mechanism of the FOR for
these metals. Indeed, it is known from single-crystal studies that for Pd electrodes
the contribution of the dehydrogenation pathway is marginal, resulting in their
insignificant poisoning.
15.5.3.3 Methanol Oxidation (MOR) The MOR on noble metal electrodes
has been studied for decades (see, e.g., the reviews [Petrii et al., 1965; L´ger, 2001;
Wasmus and Kuver, 1999; Iwasita, 2003; Waszczuk et al., 2002; Gasteiger et al.,
1993] and references therein):
CH 3 OH þ H 2 O ! CO 2 þ 6H þ þ 6e
(15 : 21)
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