Environmental Engineering Reference
In-Depth Information
TABLE 15.1 Comparison of Surface Areas A
i
and Charges Required for
Electrochemical Desorption of H
UPD
(Q
H
), CO
ads
(Q
CO
), and Surface Oxidation (Q
ox
)
a
¯
N
/
nm
b
¯
S
/
nm
b
A
H
/
m
2
g
21
A
CO
/
m
2
g
21
A
TEM
/
m
2
g
21
2Q
H
/
Q
CO
Q
H
/
Q
ox
Q
CO
/
Q
ox
5.5
c
4.6
c
Pt foil
—
—
1.2
0.83
2.0
Pt(ed)
—
—
—
—
0.96
0.91
1.9
3.2
4.3
80
87
88
0.92
0.87
1.9
3.1
3.8
64
84
90
0.76
0.74
1.9
2.4
2.8
98
127
117
0.77
0.76
2.0
1.8
2.3
77
144
156
0.53
0.54
2.0
a
Data have been recalculated from voltammograms in Maillard et al. [2004a, 2005]—Reproduced with per-
mission from The Royal Society of Chemistry and the PCCP Owner Societies. The scan rate was 0.1 V s
21
and the positive limit was 1.23 V. See the text for details.
b
The
¯
N
¯
S
by d
N
¼
average
particle
size
and
surface
average
particle
size
are
given
P
i
N
i
d
i
P
i
N
i
, d
S
¼
P
i
N
i
d
i
P
i
N
i
d
i
:
c
Total surface area in cm
2
.
of two) of the surface areas, while CO stripping data are in very good agreement
with electron microscopy. We rule out contamination as a possible reason for
H
UPD
suppression on small Pt particles on the grounds of the excellent agreement
between surface areas measured by CO adsorption and calculated from TEM data.
Note that CVs were obtained after potential cycling and were stable throughout the
measurements.
Thus, we conclude that the traditional method of surface evaluation based on H
UPD
coulometry is inappropriate for measurement of the surface areas of particles smaller
than 3 nm. This conclusion may have a major influence on the comparison of the
activities of Pt nanoparticles. Instead, we suggest calculating specific surface areas
of Pt particles in the size range 1 - 4 nm from the CO stripping charge. It should be
stressed that our conclusion is based on measurements on model Pt nanoparticles
supported on a clean nonporous GC substrate. These samples comprise particles
that have relatively narrow size distributions, with very low fractions of agglomerated
particles at moderate Pt loadings, and are fully accessible to electrolyte. The last of
these may not always be the case for porous electrodes. For example, Bergamaski
and co-workers reported A
CO
/A
TEM
¼ 0.35 and 0.52 for 10 and 30 wt% Pt/Vulcan
XC-72 catalysts from E-Tek, respectively [Bergamaski et al., 2006]. They attributed
these discrepancies to factors such as carbon particle wetting and pore blockage that
depend on the electrode preparation. Meanwhile, for a 20 wt% Pt/Vulcan XC-72
containing Pt particles with average size 3.7 nm and purchased from the same
manufacturer, Jusys and co-workers observed excellent agreement between the sur-
face areas calculated from H
UPD
, CO stripping, and TEM [Jusys and Behm, 2001].
For saturation CO coverage, obtained by adsorption in the H
UPD
region, they found
2Q
H
/Q
CO
¼ 1.05, which is in very good agreement with the data of Table 15.1 for
“large” particles. These data underline the importance of control of the electrode
preparation procedure.
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