Preparations of platinum nanoparticles and their catalytic performances

Article abstract of DOI:10.1166/jnn.2015.10394

This work investigates the effect of reducing agents and stabilizing agent on the preparation of platinum nanoparticles. We used H₂PtCl₆as a precursor and hydrogen and sodium borohydride as reducing agents to prepare colloidal platinum nanoparticles. Polyvinylpyrrolidones (PVPs) is used as a stabilizing agent. Hydrogen and sodium borohydride are used as reducing agents. The prepared platinum nanoparticles are characterized by transmission electron microscopy (TEM) and X-ray diffractometer (XRD). The concentrations of the precursor and the stabilizing agent influence the size of platinum nanoparticles, while the reducing agents influence the morphologies and structures of platinum nanoparticles. Supported platinum catalysts (CPt-NaBH₄, CPt-H₂) are prepared from colloidal platinum nanoparticles and γ-Al₂O₃. For comparison, another supported platinum catalyst (IPt) is prepared by the conventional impregnation method with an aqueous H₂PtCl₆ solution and γ-Al₂O₃. The catalytic activities of CPts are superior to that of IPt on the basis of benzene conversion.

Full text of DOI:10.1166/jnn.2015.10394

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 5461–5465, 2015
Copyright © 2015 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Preparations of Platinum Nanoparticles and
Their Catalytic Performances
1
2
3
4
Sang Chul Jung , Seung Won Nahm , Ho Young Jung , Young-Kwon Park ,
5
2ꢀ∗
Seong Gyu Seo , and Sang Chai Kim
1
Department of Environmental Engineering, Sunchon National University, Sunchon 540-742, Korea
Department of Environmental Education, Mokpo National University 61 , Muan 534-729, Korea
2
3
Department of Environment and Energy Engineering, Chonnam National University, Gwangju 500-757, Korea
4
School of Environmental Engineering, University of Seoul, Seoul 130-743, Korea
5
Department of Environmental System Engineering, Chonnam National University, Yeosu 550-749, Korea
This work investigates the effect of reducing agents and stabilizing agent on the preparation of
platinum nanoparticles. We used H PtCl as a precursor and hydrogen and sodium borohydride as
2
6
reducing agents to prepare colloidal platinum nanoparticles. Polyvinylpyrrolidones (PVPs) is used
as a stabilizing agent. Hydrogen and sodium borohydride are used as reducing agents. The pre-
pared platinum nanoparticles are characterized by transmission electron microscopy (TEM) and
X-ray diffractometer (XRD). The concentrations of the precursor and the stabilizing agent influence
the size of platinum nanoparticles, while the reducing agents influence the morphologies and struc-
Delivered by Publishing Technology to: University of Waterloo
tures of platinum nano IpPa :r t5i c0l e. 1s .2 S2 .u 2p 1p 6o .r 3t e Od np :l a Tt i un ue m, 2 c0 a Ot a lcy ts 2t s0 (1C5 P 0t - 9N : a2 B6 H: 4 7, CPt-H ) are prepared
4
2
from colloidal platinum nan oC p oa pr tyi cr il eg sh ta :n Ad mꢀ -e Ar il c Oa n. SF oc ri ec no tmi f ipc a Pr i su obn l ,i sa hn eo rt sh er supported platinum cat-
2
3
alyst (IPt) is prepared by the conventional impregnation method with an aqueous H PtCl solution
2
6
and ꢀ-Al O . The catalytic activities of CPts are superior to that of IPt on the basis of benzene
2
3
conversion.
Keywords: Platinum, Nanoparticle, Hydrogen, Sodium Borohydride, Catalytic Oxidation.
1
. INTRODUCTION
petrochemical, pharmaceutical, electronic and automotive
Nanoscale metal particles (nanoparticles) are promising
advanced materials that have electronic, magnetic, optic,
and thermal properties as well as catalytic properties.
industries because of its outstanding catalytic and elec-
trical properties and superior resistant characteristics to
1
0ꢀ11
1
–3
corrosion.
In this work, we investigated the effects of
polyvinypyrrolidone (PVP) as a stabilizing agent and
hydrogen and sodium borohyride as a reducing agent
on the preparation of colloidal nano-size platinum met-
als. We also compared the catalytic activities of the
catalyst prepared from colloidal nano-size platinum
metals and prepared by the conventional impregnation
method for benzene oxidation. They were characterized
by Transmission Electron Microscopy (TEM), and X-ray
diffractometer (XRD).
Accordingly, the properties of narrowly distributed metal
nanoclusters and their preparations have been inten-
sively studied. Moreover, the morphology of a metal
4
nanocrystal is critical to modern materials properties
because its physical and chemical properties can be
easily and widely tuned by controlling the size and
5
shape, and fractal nanostructures can be formed by met-
als. Because of their higher structural complexity com-
pared to nanospheres, nanowires, and nanodiscs, these
branched nanostructures are expected to have a wide
6
range of technological applications. Especially, their elec-
tronic and catalytic properties, and their semiconductor
heterogeneity have potential uses in fuel cells and solar
cells.7 Platinum has been widely applied in chemical,
2
. EXPERIMENTAL DETAILS
2
.1. Preparation of Platinum Nanoparticle and
–9
Catalysts
Colloidal nano-size platinum metals were prepared from
H PtCl (Aldrich Chemical Company, 99.99%) as a
∗
Author to whom correspondence should be addressed.
2
6
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 7
1533-4880/2015/15/5461/005
doi:10.1166/jnn.2015.10394
5461

Preparations of Platinum Nanoparticles and Their Catalytic Performances
Jung et al.
precursor and polyvinylpyrrolidone (PVP: molecular
weight = 40,000, Sigma-Aldrich Chemical Company) as
a stabilizing agent. Hydrogen and sodium borohydride
temperature of the saturator and the additional air stream.
0.1 g of catalyst sample was loaded in the middle of the
reactor with quartz support, and the total flow rate of the
3
−1
(
NaBH , Aldrich Chemical Company) were used as reduc-
gas mixture through the reactor was 100 cm min , which
4
−
1
ing agents, respectively. An aliquots of PVP and H PtCl
gave a gas hourly space velocity (GHSV) of 50,000 h .
2
6
ꢀ
were soluble with each 10 mL deionized water, respec-
tively. Two solutions were mixed with 10 mL deionized
water in a 50 mL glass beaker. The resulting solution was
held at 150 rpm with magnetic stirrer and simultaneously
flushed with a 50 mL/min flow rate of nitrogen for 1 h,
and then reduced by a 50 mL/min flow rate of hydrogen
for 2 h. In the case of using sodium borohydride as a
reducing agent, computer programmed syringe pump was
All the lines were heated sufficiently at 120 C to pre-
vent the adsorption and condensation of the reactant and
the product in the tubes. The concentration of inlet and
exit gas stream was determined using a gas chromato-
graph GC-14A model (Shimadzu) equipped with thermal
conductivity. The GC/MS (Shimadzu, QP5050) was also
employed for quantitative and qualitative analysis of the
products and the by-products. The conversion was calcu-
lated on the basis of carbon consumption.
used to inject 1 mL of NaBH solution at an injection rate
4
of 10 ꢀL/min. It was observed that its color was changed
from yellow to dark brown. The product was centrifuged
at 17,000 rpm for 20 min. The supernatant was sepa-
rated and the precipitate was collected. Acetone (90 mL)
was then added to the precipitate, and the precipitate was
redispersed and then separated again by centrifugation at
3
. RESULTS AND DISCUSSION
3
.1. Sodium Borohydride as a Reducing Agent
3.1.1. Effect of the Platinum Precursor Concentration
Figure 1 shows the TEM images of the colloids prepared
by varying the amount of H PtCl , while 0.3 g of PVP
1
7,000 rpm for 20 min. The black products were further
2
6
washed three times by using acetone. The nanocrystals
were then redispersed in 10 mL of ethanol for characteri-
zation. CPt-NaBH and CPt-H catalysts are prepared from
(MW = 40,000) was used as a stabilizing agent. As shown
in Figure 1(a)–(c), when 0.05, 0.1 and 0.15 g of H PtCl
2
6
were used as precursors, the particles in all colloids are
well-dispersed. Metal size distribution diagrams indicate
that the average particle sizes for 0.05 g, 0.1 and 0.15
of H PtCl are 2.9, 3.3 and 3.9 nm, respectively. In addi-
4
2
colloidal platinum nanoparticles using NaBH and H as
4
2
a reductant under the same conditions, respectively, and
-Al O is used as a support. For comparison, another cat-
ꢁ
2
3
2
6
Delivered by Publishing Technology to: University of Waterloo
alyst (IPt) is prepared by the conventional impregnation
tion, three samples show dominantly polyhedron-like
IP: 50.122.216.3 On: Tue, 20 Oct 2015 09:26:47
method with an aqueous H2PtCl6 solution and ꢁ-Al O .
2
3
Copyright: American Scientific Publishers
Platinum content of CPt-NaBH , CPt-H or IPt catalyst is
4
2
(
(
(
a) (%)
00
1
wt%.
1
8
6
4
2
0
0
0
0
0
2
.2. Characterization Technique
Colloidal nano-size platinum metals were characterized by
UV-vis spectroscopy (Shimadzu, 2401PC). The images of
transmission electron microscope (TEM) were obtained
using a JEOL JEM-2000FAXII at an acceleration volt-
age of 200 kV. The crystal structures of samples used
in this work were examined by X-ray diffraction (XRD)
using a Phillips PW3123 diffractometer equipped with a
graphite monochrometer and Cu Kꢂ radiation of wave-
length 0.154 nm.
0
-2
2-3
3-4
4-5
(
nm)
b) (%)
00
1
8
0
60
40
2
0
0
2
.3. Catalytic Oxidation
0-1.5
1.5-3.0 3.0-4.5 4.5-6.0
(
nm)
The purity of benzene (Aldrich Chemical Co.) is 99.5%,
which was used as received from the manufacturer with-
out further treatment. Complete oxidation of toluene was
carried out using a conventional fixed bed flow reactor
c) (%)
00
1
8
6
0
0
1
2
as reported in our previous work. The catalytic reactor
quartz tube with the shape of a I) consisted of a verti-
(
40
20
0
cal tubular of 1.2 cm diameter and 35 cm in length in
an electrical heating system controlled by a proportional
integral derivative (PID) controller. For accurate and stable
controlling of the gas flow rates, the mass flow controllers
0
-2
2-5
5-8
8-17
(nm)
(
UNIT Instrument, UFC-8100) were used. The concentra-
Figure 1. TEM images. (a) PVP: 0.3 g, H PtCl : 0.05 g, (b) PVP:
2
6
tion of benzene was 1,000 ppm, which is controlled by the
0.3 g, H PtCl : 0.1 g, (c) PVP: 0.3 g, H PtCl : 0.15 g.
2
6
2
6
5462
J. Nanosci. Nanotechnol. 15, 5461–5465, 2015

Jung et al.
Preparations of Platinum Nanoparticles and Their Catalytic Performances
platinum nanoparticles irrespective of the precursor con-
centration. Subsequently, an increase in the concentra-
tion of H PtCl leads to increased platinum particle
(a)
(%)
100
8
6
4
2
0
0
0
0
0
2
6
size.
3
.1.2. Effect of the PVP Concentration
0
-30
30-50 50-70 70-90
nm)
Figure 2 shows the TEM images of the colloids prepared
(
by varying the amount of PVP (MW = 40,000) while
0
.03 g of H PtCl is used. When 0.1 g, 0.3 and 0.5 g
(b) (%)
00
2
6
1
of PVP are used, the average particle sizes of platinum
are 3.4, 2.7 and 2.1 nm, respectively. This means that an
increase in the concentration of the PVP brings about a
decrease in the average particle size of platinum.
8
6
4
0
0
0
20
0
0
-20
20-40 40-60
(nm)
60-80
3
.2. Hydrogen as a Reducing Agent
3
.2.1. Effect of the Platinum Precursor Concentration
(c)
Figure 3 shows the TEM images of the colloids pre-
pared by varying the amount of H PtCl at 0.5 g of PVP
2
6
(
MW = 40,000), when hydrogen is used as a reducing
agent. When 0.05 g and 0.1 g of H PtCl are used, the
2
6
average particle sizes of platinum are 65.6 and 47.4 nm,
respectively. This means that an increase in the concen-
tration of the H PtCl results in a decrease in the average
2
6
particle size of platinum. However, as shown in Figure 3,
it should be noticed that the morphology and structure of
the platinum nanostructures prepared by using hydrogen
Figure 3. TEM images. (a) PVP: 0.5 g H PtCl : 0.05 g, (b) PVP: 0.5 g
2
6
H PtCl : 0.1 g, (c) high-magnification of (b).
2
6
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as a reducing agent are quite different from those of the
IP: 50.122.216.3 On: Tue, 20 Oct 2015 09:26:47
platinum nanostructures prepared by using sodium borohy-
Copyright: American Scientific Publishers
dride. The high-magnification TEM image in Figure 3(c)
clearly indicates that the platinum nanomaterials reduced
(a)
(b)
(c)
(
(
(
%)
1
00
13
by hydrogen are spherical dendritic structures. Shen et al.
8
6
4
2
0
0
0
0
0
1
4
and Borodko et al. have reported that this may be due
to the platinum-polymer chains interactions that induce
the nanoparticle aggregation. Their mechanistic studies
suggested that the platinum nanoparticles are assembled
together by attaching to the polymer chains, and by aggre-
gating them to form dentric platinum nanoparticles. Our
experimental results also indicate that hydrogen leads to
aggregating the platinum nanoparticlces for the formation
of dendritic structures, and sodium borohydride did not
lead to aggregation of the platinum nanoparticlces.
0
-2
2-4
4-6
6-8
(
nm)
%)
00
1
8
6
4
2
0
0
0
0
0
3.2.2. Effect of the PVP Concentration
0-1
1-2
2-3
3-4
4-6
(nm)
Figure 4 shows the TEM images of the colloids prepared
by varying the amount of PVP (MW = 40,000) when
0.05 g of H PtCl is used as a precursor. When 0.1 g and
%)
00
2
6
1
0
.3 g of PVP are used, the average sizes of dentric plat-
8
6
4
2
0
0
0
0
0
inum nanoparticles are 72.2 and 56.3 nm, respectively. An
increase in the concentration of PVP leads to decreasing
the size of the platinum polycrystalline dendrites.
Figure 5 shows the XRD patterns of platinum nanopar-
ticles (a) and nanodendrites (b) prepared by using sodium
borohydride and hydrogen as reducing agents. As shown
in Figures 5(a) and (b), the XRD profiles obtained for plat-
inum nanoparticles and nanodendrites show three peaks
0-1
1-2
2-3
3-5
(nm)
Figure 2. TEM images. (a) H PtCl : 0.03 g PVP: 0.1 g, (b) H PtCl :
2
6
2
6
0
.03 g PVP: 0.3 g, (c) H PtCl : 0.03 g PVP: 0.5 g.
2 6
J. Nanosci. Nanotechnol. 15, 5461–5465, 2015
5463

Preparations of Platinum Nanoparticles and Their Catalytic Performances
a)(%)
Jung et al.
(
100
100
8
6
4
2
0
0
0
0
0
8
0
60
40
0-40
40-60
60-80 80-100
nm)
(
(
b)(%)
20
100
8
6
4
2
0
0
0
0
0
0
1
00 120 140 160 180 200 220 240 260 280
Temperature (ºC)
10-20
20-40
40-60
60-80
Figure 6. Benzene oxidation over the catalysts reaction condi-
tion: Benzene concentration = 1,000 ppm, catalyst weight = 0.1 g,
GHSV = 50,000 hr− , • CPt-NaBH , ꢀ CPt-H , ꢁ IPt.
(nm)
1
4
2
Figure 4. TEM images. (a) H PtCl : 0.05 g, PVP: 0.1 g, (b) H PtCl :
2
6
2
6
0.05 g, PVP: 0.3 g.
that are attributed to 111, 200 and 220 reflections. The
three diffraction peaks corresponds well to the diffraction
peaks expected for fcc platinum crystals.
3
.3. Catalytic Oxidation
The benzene conversion as a function of reaction temper-
ature is shown in Figure 6. DC eOl iv ew r ae sd t hb ey oP n ul yb l pi sr ho di nu gc t T, ea nc dh nology to: University of Waterloo
(a)
(b)
(c)
Figure 7. STEM images. (a) CPt-H , (b) CPt-NaBH , (c) IPt.
2
4
2
IP: 50.122.216.3 On: Tue, 20 Oct 2015 09:26:47
no CO was detected at any point during this experimental
run. As shown in Figure 6, T50 and T conversions (the
Copyright: American Scientific Publishers
Figure 7 shows the STEM images of CPt-NaBH , CPt-
4
90
value of the temperature when conversion approaches 50
and 90%) of benzene over CPt-NaBH (CPt-H ) catalyst-
H
2
and IPt catalysts. We found that platinum dendritic
structure exist in CPt-H , platinum nanoparticles in CPt-
NaBH
NaBH
4
2
2
ꢀ
ꢀ
ꢀ
ꢀ
is 158 C (190 C) and 186 C (246 C), respectively.
Also, in the case of IPt catalyst, T and T90 conversions
4
, and nanoparticles larger than particles of CPt
in IPt. Accordingly, the oxidation state and size of
5
0
4
ꢀ
ꢀ
platinum seems to be associated with the catalytic activity.
are 264 C and 269 C, respectively. This indicates that the
catalytic activity of CPt-NaBH is the highest, followed
4
by CPt-H , and then IPt. CPt catalysts are found to be
superior to IPt catalyst for benzene oxidation.
2
4. CONCLUSION
In our study of nano-size platinum metals, we found
that the amounts of a precursor and a stabilizing agent
influence the formation of nano-size platinum metals.
An increase in the concentration of H PtCl led to an
2
6
increase in the platinum particle size. The average particle
size of platinum decreased with the increasing concentra-
tion of the PVP added. In addition, it was observed that the
morphology and structure of nano-size platinum metals is
significantly dependent on the reducing agent used. Hydro-
gen led to aggregating platinum nanoparticlces for the for-
mation of dendritic structures, and sodium borohydride
did not lead to aggregation of the platinum nanoparticlces.
CPt catalysts prepared from colloidal Pt nanoparticles was
found to be superior to IPt catalyst prepared by using the
conventional impregnation method.
(
111)
(
220)
(
b)
(
200)
(
a)
20
30
40
50
60
70
80
2
Theta
Acknowledgments: This research was supported by
Basic Science Research Program through the National
Figure 5. XRD patterns. (a) Pt particles after reduction by hydrogen.
b) Pt particles after reduction by sodium borohydride.
(
5464
J. Nanosci. Nanotechnol. 15, 5461–5465, 2015

Jung et al.
Preparations of Platinum Nanoparticles and Their Catalytic Performances
Research Foundation of Korea (NRF) funded by the Min-
istry of Education (2013R1A1A2057577).
8. O. Vidoni, K. Philippot, C. Amiens, B. Chaudret, Q. Balmes, J. O.
Malm, J. O. Bovin, F. Senocq, and M. J. Casanove, Angew. Chem.
Int. Ed. 38, 3736 (1999).
9
. D. J. Milliron, S. M. Hughes, Y. Cui, L. Manna, J. B. Li,
L. W. Wang, and A. P. Alivisatos, Nature (London) 430, 190
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Received: 18 November 2013. Accepted: 23 May 2014.
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Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 15, 5461–5465, 2015
5465