Environmental Engineering Reference
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and co-workers for Pt/Vulcan nanoparticles of different sizes provided by E-Tek
[Kabbabi et al., 1994], and later by other authors for Pt particles supported on various
carbon materials [Frelink et al., 1995; Takasu et al., 2000; Gloaguen et al., 1997;
Gojkovic and Vidakovic, 2001; Maillard et al., 2002; Cherstiouk et al., 2003a].
Frelink and co-workers prepared Pt/C electrocatalysts with particle sizes ranging
between 1.2 and 10 nm by different elaboration techniques (ion exchange, impreg-
nation, impregnation followed by sintering, and aging of colloids) [Frelink et al.,
1995]. The plot of specific activity versus particle size was bell-shaped, with a
maximum between 4.5 and 10 nm [Frelink et al., 1995].
Most of these authors ascribed the decrease in SA to the increased coverage of oxy-
genated species on the surface of small Pt/C particles and concomitant blockage of the
methanol dissociative adsorption. Takasu and co-workers [Yahikozawa et al., 1991]
related the observed decrease in SA for the MOR to the decrease in the number of
Pt(100) planes, which are the most active among the three low index planes for this
reaction [Housmans et al., 2006]. Park and co-workers investigated the MOR using
cyclic voltammetry and FTIR spectroscopy, and attributed the decrease in SA to an
“ensemble effect,” whereby methanol dehydrogenation to form CO
ads
is impeded
by the sharply decreasing availability of contiguous Pt terrace sites for d , 4nm
[Park et al., 2002b]. Indeed, using FTIR spectroscopy, they observed slower accumu-
lation of CO
ads
on the surface of small Pt particles. This explanation is based on con-
sidering CO
ads
as the reactive intermediate, while it acts also as a poison. Hence,
Cherstiouk and co-workers suggested that the lower activity of small Pt particles in
the
MOR
may
be
related
to
their
lower
activity
in
CO
ads
electro-oxidation
[Cherstiouk et al., 2003a].
Recently, Bergamaski and co-workers investigated the MOR using differential
electrochemical mass spectrometry (DEMS) over commercial Pt/Vulcan XC-72 cat-
alysts with Pt weight percentage ranging from 10% to 80% [Bergamaski et al., 2006].
It was found that the partitioning among the end products depends strongly on the
metal percentage in the catalysts and thus on mean particle size and particle size dis-
tribution. CO
2
efficiency was minimal for 10% and 80% catalysts, but increased
greatly for the intermediate metal percentages of 30% and 60%. In order to account
for this remarkable observation, Bergamaski et al. proposed a model of diffusional
coupling between large and small particles on the nanometer scale, where formal-
dehyde desorbs from large particles and is readsorbed and further oxidized to CO
2
on smaller particles. We find this idea very interesting. It should be noted, however,
that the total Pt surface area in porous electrodes employed by Bergamaski et al.
varied strongly with metal percentage. Comparison of the data on CO
2
efficiency
with total Pt area shows a one-to-one correlation between them (see Fig. 15.10 and
Table 15.1 in Bergamaski et al. [2006]), suggesting that the effect observed by
Bergamaski et al. may be explained within the readsorption - reoxidation model
proposed earlier by Jusys and co-workers when studying the Pt/C loading effect
[Jusys et al., 2003]. Note that Jusys et al. did not change the type of electrocatalyst
but rather changed its amount in the thin layer, and observed that with increasing Pt
loading, the current efficiency for partial electro-oxidation to formaldehyde decayed
significantly (from 40% to almost 0%), while that for complete electro-oxidation to
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