Catalysis Communications 11 (2009) 7–10
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Shape effect of Pt nanocrystals on electrocatalytic hydrogenation
Cheonghee Kim, Hyunjoo Lee *
Department of Chemical and Biomolecular Engineering, The Specialized Graduate School of Hydrogen and Fuel Cell, Yonsei University, Seoul 120-749, South Korea
a r t i c l e
i n f o
Article history:
Received 23 June 2009
Received in revised form 29 July 2009
Accepted 30 July 2009
Available online 8 August 2009
Keywords:
Platinum
Nanoparticles
2-Cyclohexenone
Electrocatalytic hydrogenation
Shape
Selectivity
a b s t r a c t
Shape-controlled platinum nanocrystals have various surface atomic structures depending on their
shapes. We used cubic, cuboctahedra, and dendritic Pt nanoparticles to investigate the effect of shape
on selectivity in electrocatalytic reactions. As a model reaction, we used electrocatalytic hydrogenation
of 2-cyclohexenone. The surface-capping agents of the nanoparticles were removed by electrochemical
activation, and subsequently each shape of nanoparticle exhibited characteristic H adsorption/desorption
peaks consistent with the corresponding single-crystalline surface. 2-Cyclohexenone was electrocatalytically hydrogenated to cyclohexanone and cyclohexanol, and the selectivity showed a dependence on the
shape of the nanocrystals, resulting from the distinct surface atomic structures.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Surface crystalline structure profoundly influences electrocatalytic properties. Electrochemical structure-sensitive behavior has
been widely studied for various metallic catalysts [1–3]. In particular, studies on single-crystalline surfaces of Pt, the most common
electrocatalyst, have been conducted on the various surface structures of Pt(1 0 0), Pt(1 1 1), Pt(1 1 0), etc. [4–6]. Different surface
structures give distinct peak positions and heights for H adsorption/desorption when checked by cyclic voltammetry [7].
Recent developments in nanotechnology have enabled the size
and shape of metal nanoparticles to be controlled. For face-centered cubic metals (e.g. Pt, Pd, and Rh), cubic nanoparticles have
only (1 0 0) surfaces whereas octahedral or tetrahedral nanoparticles are completely bound by (1 1 1) surfaces, and cuboctahedral
nanoparticles have both (1 0 0) and (1 1 1) surfaces. Consequently,
the surface structure of nanoparticle catalysts can be readily controlled by varying their shapes.
Sun and co-workers observed that Pt nanocubes exhibited oxygen reduction activities approximately two times greater than
those of commercial Pt catalysts [8]. Sun and co-workers showed
that an increase in the fraction of Pt(1 0 0) sites on nanoparticles
leads to a significant enhancement in activity for ammonium oxidation [9]. Wang and co-workers synthesized tetrahexahedral Pt
nanocrystals by electrochemical treatment of Pt nanospheres.
These nanocrystals showed enhanced (4 times) catalytic activity
* Corresponding author.
E-mail address: [email protected] (H. Lee).
1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.07.032
for electro-oxidation of small organic fuels [10]. However, the purity and monodispersity of the nanoparticles can still be improved.
Furthermore, most of the work to date has focused on activity.
In the present study, we investigate the effect of shape on selectivity for electrocatalytic reactions using high quality shape-controlled Pt nanocrystals. The shape effect on selectivity of
electrocatalytic reactions has been rarely reported. As a model
reaction, we used electrocatalytic hydrogenation of 2-cyclohexenone to study competitive hydrogenation resulting in different
selectivity. 2-Cyclohexenone had been previously used to evaluate
the selective electrocatalytic hydrogenation when different kinds
of metals (Ni, Cu, Co) were used as electrocatalysts [11] or when
Pt nanoparticles were deposited with various conducting polymers
[12]. The difference in selectivity was investigated for cubic, cuboctahedral, and dendritic Pt nanoparticles.
2. Experimental
The various shapes of cubic, cuboctahedra, and dendritic nanoparticles were synthesized as reported previously [13] by using
tetradecyltrimethylammonium bromide (TTAB, Sigma–Aldrich) as
surface-capping agents. The nanoparticle shape and size distributions were examined with transmission electron microscopy (JEOL
2100) operated at 200 kV. The cubic nanoparticle solution
(1.5 lmol of Pt based on the concentration of Pt precursor) was
washed, concentrated, then dropped on a gold working electrode
(5 mm in diameter) and dried overnight at room temperature.
The same procedure was followed for the cuboctahedral and the
dendritic nanoparticle solution. The use of binding agents was
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C. Kim, H. Lee / Catalysis Communications 11 (2009) 7–10
avoided in order to prevent any interference with the surface reaction [14]. The surface characteristics of each nanoparticle shape
were evaluated by cyclic voltammetry using VersaSTAT3 potentiostat in 100 ml of 0.5 M H2SO4 (DUKSAN, 95%) solution at a scan rate
of 50 mV/s. A conventional three-electrode cell was used with a
standard calomel electrode (SCE) as the reference electrode (all
potentials in this paper are quoted versus the RHE electrode) and
a platinum wire inside a glass frit as the counter electrode. TTAB
on the nanoparticle surface was removed by applying an activation
procedure in which the potential is cycled up to 1.5 V three times
in 0.5 M H2SO4 at a scan rate of 50 mV/s [15]. Electrocatalytic
hydrogenation was performed in 20 ml of 2 mM 2-cyclohexenone
(Sigma–Aldrich, 95+%) with the electrode polarized at 0.6 V (versus RHE) under vigorous stirring (1000 rpm). A 0.5 ml of aliquot
was collected every hour, neutralized with saturated NaHCO3 (Sigma–Aldrich), and extracted with CH2Cl2 (Sigma–Aldrich, P99.5%).
The product distribution in CH2Cl2 solution was analyzed by GC–
MS (Agilent Technology, HP6890/5973MS) with capillary column
(J&W 122-5532 DB-5 ms). The standard solution in CH2Cl2 was
prepared by mixing the same mole number of cyclohexenone (Sigma–Aldrich, 95+%), cyclohexanone (Sigma–Aldrich, 99.8%), and
cyclohexanol (Sigma–Aldrich, 99%) in aqueous solution, and then
extracting with CH2Cl2. Since cyclohexenol was not detected after
electrocatalytic hydrogenation, cyclohexenol was not included in
the standard solution. The peak area ratio of cyclohexenone, cyclohexanone, cyclohexanol was corrected after electrocatalytic reaction with the peak area ratio of the standard solution, and then
conversion and selectivity were calculated. Any other peak was
not observed in GC–MS data.
3. Results and discussion
Catalytically active cubic, cuboctahedral, and dendritic Pt nanoparticles were prepared as shown in Fig. 1. TTAB, which has a weak
interaction with the metal surface, was used as a surface-capping
agent to protect the catalytically active sites. Each type of nanoparticle was washed, concentrated, and deposited on a gold electrode.
The distinct electrochemical characteristics of each shape were
investigated, along with the effect of shape on the electrocatalytic
hydrogenation of 2-cyclohexenone.
3.1. Removal of surface-capping agents
The different surface crystalline structures of Pt catalysts are
known to exhibit distinctive signatures for H adsorption/desorption
in sulfuric acid [7]. Therefore, nanoparticles with different shapes
are expected to give H adsorption/desorption profiles that correspond to those of their single-crystalline analogues. For example,
Pt nanocubes should have the same signature as a Pt(1 0 0) surface,
and Pt cuboctahedra should show a mixed signature of Pt(1 0 0) and
Pt(1 1 1) surfaces. The H adsorption/desorption cyclic voltammograms (CVs) for the as-prepared nanoparticles showed no characteristic peaks, and only contained a broad peak, probably
reflecting the organic layers (TTAB) capping the surface of the Pt
nanoparticles. The removal of organic layers without affecting the
crystalline surface structure has previously been performed by
applying CV cycles up to 1.5 V, although too many cycles may degrade the surface structure [15,16]. Applying a high voltage imparts
a more positive charge to the Pt surface, which then repulses the
positively charged TTAB surfactant more strongly than when
neutral surface-capping agents are used. By repeating the CV cycle
up to 1.5 V only three times, the organic capping molecules were
successfully removed.
Fig. 2 shows the CVs for H adsorption/desorption after activation. According to single-crystalline surface studies performed by
Fig. 1. TEM images of platinum nanoparticles with (a) cubic (average size:
8.6 ± 1.1 nm face to face, 85% cubes, 10% tetrahedra, and 5% irregular shapes), (b)
cuboctahedral (average size: 11.9 ± 1.3 nm vertex to vertex, 92% cuboctahedra, 2%
tetrahedra, and 7% irregular shapes), and (c) dendritic nanoparticles (average size:
15.8 ± 2.3 nm longest axis, 100% dendritic shapes).
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C. Kim, H. Lee / Catalysis Communications 11 (2009) 7–10
Current (mA/cm2 )
0.08
(a)
0.04
0.00
-0.04
-0.08
0.0
0.2
0.4
0.6
0.8
Potential (E vs RHE)
0.08
3.2. Electrocatalytic hydrogenation of 2-cyclohexenone
(b)
0.04
Current (mA/cm2 )
represent Pt(1 0 0) surfaces. But another peak is also observed
around 0.45 V, which is a characteristic of small (111) ordered surface domains. These characteristic peaks indicate that cuboctahedral particles have both Pt(1 0 0) and Pt(1 1 1) surfaces. Similar
peak allocation was previously shown in Ref. [17] for Pt nanoparticles. In contrast to the cubes and cuboctahedra, the dendritic nanoparticles do not have well-defined peaks in their CV. The dendritic
nature imparts a high radius of curvature to their surfaces. Consequently, the surfaces of the dendritic nanoparticles may consist of
high index surfaces with many steps and narrow terraces. The high
index surfaces are generally considered as a combination of (1 0 0),
(1 1 1), and (1 1 0) surfaces. The CV shown in Fig. 2c has large peaks
at 0.12 V and 0.25 V; these are thought to be mainly due to (1 1 0)
surfaces on the high index surfaces of the dendritic nanoparticles.
0.00
-0.04
After the TTAB layers had been removed, an electrode coated
with Pt nanoparticles was immersed in a solution of 2-cyclohexenone for electrocatalytic hydrogenation. 2-Cyclohexenone was
hydrogenated to cyclohexanone and cyclohexanol when the working electrode was polarized at 0.6 V. Production of 2-cyclohexenol was not observed. The conversions of 2-cyclohexenone for
each shape were shown in Fig. 3a. The conversion was almost
the same. Selectivity, measured by the ratio of cyclohexanone to
cyclohexanol, was evaluated for the different nanoparticle shapes
as shown in Fig. 3b. At a reaction time of 1 h, selectivity was 3.1
80
-0.08
0.2
0.4
0.6
Potential (E vs RHE)
0.08
(c)
0.04
(a)
60
50
40
30
Cube
Cuboctahedra
Dendritic
20
0.00
10
1
-0.04
2
3
4
5
Reaction Time (hr)
6
5
-0.08
0.0
0.2
0.4
0.6
0.8
Potential (E vs RHE)
Fig. 2. Cyclic voltammograms in 0.5 M H2SO4 after activation for (a) cubic and (b)
cuboctahedral, and (c) dendritic platinum nanoparticles with a scan rate of 50 mV/s.
The electrochemical activation was performed by repetitively scanning three times
from 0.05 V to 1.5 V at 50 mV/s.
Solla-Gullon et al. [7], Pt(1 0 0) surface shows a peak at 0.37 V
which represents (1 0 0) terrace sites and a shoulder at 0.27 V associated to (1 0 0) edge. Pt(1 1 1) surface show no characteristic peak
except a small peak at 0.45 V which represents bidimensionally ordered (1 1 1) domain. Pt(1 1 0) has a very intense peak at 0.12 V.
For the shape-controlled nanoparticles, the Pt nanocubes show a
sharp peak at 0.23 V, being characteristics of Pt(1 0 0) edges and
corner Pt surface sites. Also apparent is a shoulder at around
0.32 V, characteristic of Pt(1 0 0) terrace sites. Cuboctahedral nanoparticles show a peak at 0.24 V and the shoulder at 0.32 V, which
Selectivity (-anone/-nol)
Current (mA/cm2 )
70
0.8
Conversion (%)
0.0
(b)
4
Cube
Cuboctahedra
Dendritic
3
2
1
0
1
2
3
4
5
6
Reaction Time (hr)
Fig. 3. (a) Conversion and (b) selectivity for electrocatalytic hydrogenation of 2cyclohexenone. Selectivity was calculated as mole number of cyclohexanone
produced/mole number of cyclohexanol produced (square: cube, circle: cuboctahedra, triangle: dendritic nanoparticles).
10
C. Kim, H. Lee / Catalysis Communications 11 (2009) 7–10
for cubes, 1.1 for cuboctahedra, and 0.9 for dendritic nanoparticles.
It is well known that a double bond is hydrogenated by being
chemisorbed on the surface of a catalyst. When competitive
adsorption of C@C and C@O bonds occur on a Pt surface for hydrogenation, C@C bonds are generally more easily hydrogenated than
are C@O bonds. Delbecq and Sautet calculated the binding energy
of a carbonyl group on Pt(1 0 0), Pt(1 1 1), and Pt(1 1 0) surfaces
using semi-empirical extended Hückel calculations [18]. The binding energy varies with the orientation of the carbonyl group on the
surface, but the binding energies generally increase in the order
Pt(1 0 0) < Pt(1 1 1) < Pt(1 1 0). Therefore, carbonyl groups will be
most stabilized on a Pt(1 1 0) surface and least stabilized on a
Pt(1 0 0) surface. Consequently, cubic nanoparticles, which have
Pt(1 0 0) surfaces, produce the least amount of alcohol, whereas
dendritic nanoparticles, which have many stepped sites that include Pt(1 1 0) surfaces, produce the greatest amount of alcohol.
After electrocatalytic hydrogenation, the electrode was transferred to a fresh 0.5 M H2SO4 solution, and the H adsorption/
desorption CV was re-measured. The characteristic peaks for each
surface showed no change, indicating that the surface crystalline
structures were preserved.
4. Conclusions
Pt nanoparticles of different shapes (cubes, cuboctahedra, and
dendritic nanoparticles) were used for electrocatalytic hydrogenation of 2-cyclohexenone. TTAB on the Pt surface was removed by
electrochemical activation. The cubes show characteristic H
adsorption/desorption peaks consistent with a Pt(1 0 0) singlecrystalline surface, while cuboctahedra show peaks from both
Pt(1 0 0) and Pt(1 1 1) surfaces. The dendritic nanoparticles show
large Pt(1 1 0) and Pt(1 0 0) peaks in the H adsorption/desorption
CV. Electrocatalytic hydrogenation of 2-cyclohexenone was performed on Pt nanoparticles following TTAB removal. The selectivity (cyclohexanone/cyclohexanol) varied (3.1 for cubes, 1.1 for
cuboctahedra, and 0.9 for dendritic nanoparticles) depending on
the shape. The surface crystalline structures were found to be preserved after electrocatalytic hydrogenation.
Acknowledgments
This work was supported by DAPA/ADD of Korea and Korea Research Foundation Grant funded by the Korean Government
(MOEHRD) (KRF-2008-313-D00196).
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