Environmental Engineering Reference
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(111) planes with significantly less CO poisoning compared to that of other planes
such as (100) and (110) respectively [
115
]. The geometrical arrangement of four
sites on a square unit lattice of the (100) plane and on a rectangular unit lattice of
the (110) plane are favourable for CO intermediate formation, whereas that on a
hexagonal unit lattice of the (111) plane is not so favourable [
116
]. Thus, Pt/
C-hexagons show higher activity towards formic acid oxidation compared to Pt
multipods, Pt discs and commercial platinized carbon. In contrast, for ethanol
oxidation, it is clear that Pt multipods show better electrocatalytic activity com-
pared to that on Pt discs, Pt hexagons and commercial platinized carbon. The
present observation could also be explained in terms of the proportion of crys-
tallographic planes exposed on the surface. It is found that compared to the (111)
and (110) planes (100) shows higher activity, while (111) shows least activity
towards the C-C bond cleavage involved during ethanol oxidation [
117
] In the
present case, comparison of the density ratio of (100)/(110) planes (calculated
from XRD results) reveals a higher value for multipods than for discs, hexagons
and commercial Pt/C. As a result, Pt multipods show enhanced activity for ethanol
oxidation compared to other shapes.
In addition to these various anisotropic shapes of platinum, we have also
compared the electrocatalytic capability of high aspect ratio nanostructures such as
Pt Y-junction and Pt Nanowires [
114
]. Accordingly, Fig.
10
a, c shows a com-
parison of transient current density response of Pt-Y/C, Pt-NW/C and Pt/C
towards formic acid and ethanol oxidation reactions at a particular potential, where
for both cases the oxidation current density on Pt-Y/C is significantly higher
compared to those on both Pt-NW/C and Pt/C. Furthermore, these Pt-Y junction
nanostructures show a significantly higher R, which varies up to a maximum of
270 % with respect to Pt/C and up to 200 % with respect to Pt-NW/C. Similarly
for ethanol oxidation, the factor R increases up to a maximum of 180 % for Pt-Y
with respect to Pt/C and 130 % for Pt-Y/C with respect to Pt-NW/C. This clearly
suggests the importance of junction structures in controlling the kinetics of these
oxidation reactions (shape-dependent reactivity). In addition to kinetic feasibility,
these reactions are also thermodynamically more feasible on Y-junction nano-
structures compared to that on nanowires and commercial Pt/C, as shown by the
dotted lines in Fig.
10
b, d. It is clear that at a given current density, the corre-
sponding potential on Pt-Y/C is shifted negatively by ca. 90 mV with respect to
that of Pt/C, whereas the shift is ca. 40 mV as compared with Pt-NW/C. Similarly,
for ethanol oxidation, Pt-Y/C is shifted negatively by 70 mV as compared to that
of Pt/C, while there is a 20 mV shift with respect to Pt-NW/C, thus indicating the
order of thermodynamic stability.
Hence, Pt-Y nanostructures exhibit much enhanced catalytic activity per unit
surface area for the oxidation of formic acid and ethanol. This could perhaps be due
to the higher density of active sites on the surface of Y-junction Pt (large surface
area is expected for these high aspect ratio nanostructures), and in addition it is
presumed that the branched regions also enhance the activity due to a large field
gradient. This is obvious on comparison of the performance of both Y-junctions and
linear structures (nanowires) of Pt as electrocatalysts for the same reaction.
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