Application of gallium nitride nanostructures and nitrogen doped carbon spheres as supports for the hydrogenation of cinnamaldehyde

Article abstract of DOI:10.1166/jnn.2013.7578

This paper reports on the synthesis and use of nanostructures of gallium nitride (GaN NSs) and nitrogen doped carbon spheres (NCSs) as support materials for the hydrogenation of cinnamaldehyde. This study provides the first investigation of GaN as a catalyst support in hydrogenation reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage furnace (750 °C) while NCSs were made by CVD in a single stage furnace (950 °C) respectively. TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3% loadings) were uniformly dispersed on acid functionalized GaN NSs and NCS. The Pd nanoparticles had average diameters that were influenced by the type of support material used. The GaN NSs and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and 60 °C under atmospheric pressure. A comparative study of the activity of the nanostructured materials revealed that the order of catalyst activity was 3% Pd/GaN >3% Pd/NCSs >1% Pd/NCSs >1% Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1% Pd/GaN at reasonable conversion rates. Copyright

Full text of DOI:10.1166/jnn.2013.7578

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 4990–4995, 2013
Application of Gallium Nitride Nanostructures and
Nitrogen Doped Carbon Spheres as Supports
for the Hydrogenation of Cinnamaldehyde
Thobeka Kente1ꢀ2, Sibongile M. A. Dube1,
Neil J. Coville1ꢀ2, and Sabelo D. Mhlanga1ꢀ2ꢀ∗
1Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand,
Private Bag 3, Wits, 2050, Johannesburg, South Africa
2DST/NRF Centre of Excellence in Strong Materials, University of the Witwatersrand,
Private Bag 3, Wits, 2050, Johannesburg, South Africa
This paper reports on the synthesis and use of nanostructures of gallium nitride (GaN NSs) and
nitrogen doped carbon spheres (NCSs) as support materials for the hydrogenation of cinnamalde-
hyde. This study provides the first investigation of GaN as a catalyst support in hydrogenation
reactions. The GaN NSs were synthesized via chemical vapour deposition (CVD) in a double stage
ꢀ
ꢀ
furnace (750 C) while NCSs were made by CVD in a single stage furnace (950 C) respectively.
TEM analysis revealed that the GaN NSs were rod-like with average diameters of 200 nm, while the
NCSs were solid with smoother surfaces, and with diameters of 450 nm. Pd nanoparticles (1 and 3%
loadings) were uniformly dispersed on acid functionalized GaN NSs and NCS. The Pd nanoparticles
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had average diameters that were influenced by the type of support material used. The GaN NSs
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and NCSs were tested for the selective hydrogenation of cinnamaldehyde in isopropanol at 40 and
Copyright: American Scientific Publishers
60 ꢀC under atmospheric pressure. A comparative study of the activity of the nanostructured materi-
als revealed that the order of catalyst activity was 3% Pd/GaN > 3% Pd/NCSs > 1% Pd/NCSs > 1%
Pd/GaN. However, 100% selectivity to hydrocinnamaldehyde (HCALD) was obtained with 1%
Pd/GaN at reasonable conversion rates.
Keywords: Gallium Nitride Nanostructures, N-Doped Carbon Spheres, Palladium,
Hydrogenation, Cinnamaldehyde, Catalyst Supports.
1. INTRODUCTION
provides a range of easily identifiable (and important)
chemicals to evaluate catalyst behaviour.3
The selective hydrogenation of ꢀ,ꢁ-unsaturated aldehydes
is of interest due to the wide industrial applications of their
corresponding hydrogenation products in the fine chem-
icals and pharmaceuticals industries.1 The hydrogenation
of ꢀ,ꢁ-unsaturated aldehydes to generate ꢀ,ꢁ-unsaturated
alcohols and saturated alcohols depends on the method
used to prepare the catalysts, the amount of catalyst
used, the type of support, the catalyst precursors, sol-
vents, additives (or promoters) and the reaction conditions
employed e.g., temperature.2 A standard reaction used to
evaluate catalysts and supports is the hydrogenation of
cinnamaldehyde (CALD) (Fig. 1). CALD hydrogenation
can yield hydrocinnamaldehyde (HCALD), cinnamyl alco-
hol (CALC) and 3-phenyl-1-propan-1-ol (3P1P) and thus
HCALD is of interest because it has been found to be
an important intermediate for the preparation of chemi-
cals used in the treatment of the HIV. It is also commonly
used as an additive in the food industry as a flavouring
agent.3 CALC is also an important chemical but the pro-
duction of CALC via selective hydrogenation is a difficult
task because the formation of the saturated aldehyde is
thermodynamically favoured over that of the unsaturated
alcohol.4 Considerable efforts have been made to search
for a catalytic system that is able to actively and selectively
carry out the preferential hydrogenation of the C O bond
in the presence of a conjugate C C bond.5 Selectivity
towards the unsaturated alcohol can be improved signif-
icantly using designed heterogeneous catalysts.4 Previous
studies have shown that it is much easier to hydrogenate
the unsubstituted isolated C C bond than to hydrogenate
∗Author to whom correspondence should be addressed.
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doi:10.1166/jnn.2013.7578

Kente et al.
Application of GaN NSs and Nitrogen Doped Carbon Spheres as Supports for the Hydrogenation of Cinnamaldehyde
doping of carbons leads to better catalyst stability/activity
than when undoped carbons are used. We have thus cho-
sen to use N doped carbon spheres (NCSs) as our model
system. These are easy to prepare and are easy to study by
electron microscopy techniques. In this work, we have thus
compared Pd/GaN NSs and Pd/NCSs (1 wt% and 3 wt%
Pd loadings) for the preferential selective hydrogenation of
CALD to the corresponding saturated aldehyde (HCALD)
at different temperatures under atmospheric pressure.
2. EXPERIMENTAL DETAILS
2.1. Synthesis and Functionalization of
GaN Nanostructures
Fig. 1. Reaction scheme proposed for the selective hydrogenation of
cinnamaldehyde.12
GaN nanorods were synthesized in a quartz tube placed in
a two-stage furnace. The first zoneꢀ(zone 1) of the double
stage furnace was heated to 1100 C to preheat the NH3
and cause its dissociation into NH2, NH, and N reactive
speciꢀes. The second reactor zone (zone 2), was heated to
750 C. Ga2O3 powder was placed in a quartz crucible
placed in the center of the quartz tube in zone 2. N2was
allowed to flow through the quartz tube (to remove air) at
a flow rate of 12 ml/min while the two zones of the reac-
tor reached their respective reaction temperatures. When
the desired temperatures were reached NH3, at different
flow rates, was introduced. The NH3 and decomposed NH3
the isolated C O aldehydic or ketonic group.6 The hydro-
genation of CALD in polar solvents is known to favour
the transformation of the C O bond, while hydrogenation
in non-polar solvents favours transformation of the C
C
bond.7 Further hydrogenation of CALC and HCALD pro-
duces 3P1P (Fig. 1). While also a useful chemical, the
products HCALD and CALC are the target compounds in
this reaction.
Many catalysts (e.g., Pd on carbon nanotubes) have been
reported for the hydrogenation of CALD but selectivity
to the final required products remains a challenge.3ꢂ8–11
Indeed, carbon supported Pd catalysts are known to be
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species reacted with Ga2O3 in zone 2 to convert the reac-
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tant to GaN NSs. The NH3 flow rate was varied from
one of the most effective catalysts for the selective hydro-
genation of the C C bond.3 Thus, carbon supported Pd
catalysts provide a measure of the catalyst selectivity of
other Pd supported catalysts.
Copyright: American Scientific Publishers
12–210 ml/min while N2 was kept at the same flow rate.
The optimized flow rate used in this study was 210 ml/min.
After 2 h, the furnace was cooled to room temperature
while N2 was passed through the reactor. The crucible was
then removed from the reactor and weighed to establish
the amount of GaN product formed. A piranha solution
(i.e., mixture of 0.52 M HNO3 + 0.12 M H2SO4ꢃ was
used to create functional groups on the GaN NS support
prior to catalyst synthesis stage.13
We have recently commenced a study to investigate the
use of classical semiconductors as supports in heteroge-
neous catalysis. It is believed that the semi-conducting
power of a support will affect the metal-support interac-
tion and hence influence the activity and selectivity of a
metal catalyst. The new synthetic strategies that allow for
the synthesis of nano shaped/sized semi-conductors thus
opens up the use of these materials as catalyst supports.
These studies will thus complement the typical studies of
semiconductors as sensors etc.
In the first of these studies we have explored the
synthesis and use of nano GaN as catalyst support for
hydrogenation reactions. GaN is a stable material at high
temperatures and it is not easily oxidized to gallium oxide
or reduced to Ga metal by oxidizing or reducing atmo-
spheres. The ability to make nanoscale GaN, as seen with
other nanomaterials, gives high surface area GaN with tun-
able electrical and optical properties, making it a good can-
didate for various applications including catalysis; hence
our choice to exploit its ability to act as a support material
for Pd catalysts.
2.2. Synthesis and Functionalization of NCSs
The synthesis method for producing NCSs was adopted
from Deshmukh et al. with minor modifications.14 NCSs
were synthesized using a non-catalytic CVD method from
acetylene (C2H2ꢃ as a carbon source and acetonitrile
(CH3CN) as the N (and C) source. N2 was first flowed
through the quartz tube (to remove air) at 100 ml/min
while the furnace was heated from room temperature to
ꢀ
ꢀ
950 C at a heating rate of 10 C/min. Once the desired
temperature was attained the N2 flow was switched off and
ꢀ
C2H2 was bubbled through the CH3CN solution (80 C)
at a flow rate of 100 ml/min for 90 minutes. After the
required carbonization time, the C2H2 was switched off
and N2 was flowed through the system at 100 ml/min until
the furnace had cooled down to room temperature. NCSs
were then collected from the walls of the quartz tube and
As mentioned earlier, Pd/C catalysts provide a good
model for comparing with other catalyst/support combina-
tions. In earlier studies we have shown that nitrogen (N)
J. Nanosci. Nanotechnol. 13, 4990–4995, 2013
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Application of GaN NSs and Nitrogen Doped Carbon Spheres as Supports for the Hydrogenation of Cinnamaldehyde
Kente et al.
weighed. Functionalization of the NCSs was carried out
3. RESULTS AND DISCUSSION
by treatment with 55% nitric acid (HNO3ꢃ for 24 h at
ꢀ
3.1. XRD and TEM Analysis
40 C. The functionalized NCSs were then filtered and
washed with deionized water until the pH of the filtrate
was neutral. The NCSs were then dried in an oven for 12 h
at 80 C.
Powder XRD was used to study the chemical composition
and purity of the GaN NSs and NCSs. Figure 2 shows
that the as-synthesized materials were the required mate-
rials. The peaks observed in Figure 2(a) were indexed
and found to correspond to crystalline hexagonal lattice
planes of the wurzite structure of GaN. In Figure 2(b),
the two diffraction peaks at 25.5ꢀ and 43.3ꢀ correspond to
the (002) and (101) hexagonal lattice planes of graphitic
carbon respectively.15
Since the materials were relatively free of impurities no
purification process was required. However, it was neces-
sary to modify the surface of the materials through acid
treatments to make them dispersible in solution and to
allow uniform dispersion of Pd catalysts on the supports.
This step impacted on the Pd-support interaction and hence
the dispersion and activity of the Pd. We have shown in
our previous studies on carbon nanotubes that the degree
of functionalization of the support materials affects the
activity of the catalyst.15
ꢀ
2.3. Preparation of Pd/GaN and Pd/NCS Catalysts
The catalysts were prepared using the liquid phase reduc-
tion method in an ethylene glycol solution. The supports
(500 mg) were placed into a 500 ml round bottomed flask
and ethylene glycol (150 ml) was added to the flask. The
flask contents were sonicated for 15 min followed by mag-
netic stirring for 30 min. The metal precursor solution
(0.05 M palladium acetate) was then added drop-wise to
the mixture of carbon support and ethylene glycol. The
concentrations of the precursor solutions in the ethylene
glycol solutions were calculated in order to prepare cata-
lysts with metal loadings of 1 and 3 wt%. The resultant
solution was magnetically stirred vigorously for a further
ꢀ
3 h at room temperature and then refluxed at 195 C in
an oil bath for 3 h. The solution was then cooled to room
temperature. The resultant solution was filtered and thor-
oughly washed with 20 ml of acetone followed by excess
distilled water. The supported metal catalysts were dried
in air for 8 h at 80 ꢀC and then ground in a mortar to form
The TEM images of the GaN and NCS supports revealed
the rod like structure of the GaN (comprised of smaller par-
ticles) and the spherical shape of NCSs (not shown here).
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a powder.
Copyright: American Scientific Publishers
2.4. Characterization of the Supports and Catalysts
The structural morphology of the support materials
and catalysts was ascertained by transmission electron
microscopy (FEI Tecnai G2 Spirit electron microscope at
120 kV). Powder X-ray diffraction (Bruker D2 Phaser)
was used to study the chemical composition and crys-
tallinity of the materials. A Brunauer–Emmett–Teller
(BET) TRISTAR 3000 analyzer was used to measure the
surface area and porosity of the catalysts.
2.5. Hydrogenation of Cinnamaldehyde
Hydrogenation of CALD was carried out in a three necked
round bottomed flask. The catalyst (0.01 g) and a 0.01 M
CALD (dissolved in solvent) were added into the reaction
flask. The air inside the system was expelled by passing
N2 through the flask for 30 min while the contents were
heated to the required temperature. After this, hydrogen
gas (50 ml/min) was bubbled through the system while the
flask contents were stirred. The course of the reaction was
monitored by withdrawing liquid samples from the flask
every hour for 6 h. The products were analyzed on a gas
chromatogram equipped with an FID detector. The column
used was a ZB-1 capillary column (30 m × 0ꢄ53 mm ×
1ꢄ5 ꢅm). Reaction products were identified by comparison
with authentic GC standards of CALD, HCALD, 3P1P,
and CALC.
Fig. 2. PXRD patterns of (a) as-synthesized GaN NSs (synthesis
temp = 750 ꢀC) and (b) as-synthesized NCSs.
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Kente et al.
Application of GaN NSs and Nitrogen Doped Carbon Spheres as Supports for the Hydrogenation of Cinnamaldehyde
Fig. 3. TEM images of (a) 1% Pd/GaN, (b) 3%Pd/GaN, (c) 1%Pd/NCSs
and (d) 3%Pd/NCSs. The dark spots are Pd nanoparticles.
The TEM images of the four Pd supported samples studied
(1% Pd/GaN; 3% Pd/GaN; 1% Pd/NCSs and 3% Pd/NCSs)
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are shown in Figure 3. The Pd loaded on the NCSs can
Copyright: American Scientific Publishers
readily be seen while the Pd loaded on the GaN is more
difficult to observe due to
(i) the small Pd particle size
(ii) the distribution of Pd on the rough surface and
(iii) the colour contrast of Pd and GaN.
The Pd nanoparticles were found to be well dispersed
on the surface of the nanostructures with an average size
range of between 4–15 nm. The Pd particle sizes were
smaller on the GaN than on the NCSs (Fig. 4). For exam-
ple 3% Pd/GaN gave Pd particles with an average diam-
eter of 10 nm, while the diameters of the Pd on the 3%
Pd/NCSs were 15 nm (Fig. 4). The 1% Pd/NCS catalysts
had Pd average particle sizes of 4.7 nm; the Pd on the 1%
Pd/GaN was difficult to see and thus a meaningful particle
size analysis could not be achieved. As the loading of Pd
was increased, agglomeration occurred and larger particles
formed, and the average particle size increased.16
Fig. 4. Particle size distribution graphs of (a) 3% Pd/GaN, (b) 1%
Pd/NCSs and (c) 3% Pd/NCSs.
that most of Pd nanoparticles that were deposited on the
surface did not block any of the mesopores.17 The higher
surface area of GaN would suggest a better dispersion of
Pd particles on GaN than NCSs (as seen for the 3% loaded
sample).
3.2. BET Surface Area Analysis
Table I shows the surface areas and pore volumes of the
supports and catalysts. Comparison of the surface areas
of the supports revealed that the GaN NS had a higher
surface area (∼20 m2/g) than the NCSs (∼1 m2/g). The
surface area increased when the Pd was added to the sup-
port materials. The highest surface area (i.e., 63.5 m2/g)
was observed with the 3% Pd/GaN catalyst. This suggests
3.3. Comparison of Catalyst Activity:
Cinnamaldehyde Hydrogenation
Figure 5 and Table II shows the CALD hydrogenation
results using a 1% wt loading of Pd on the GaN and NCS
supports carried out at reaction temperatures of 40 and
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Application of GaN NSs and Nitrogen Doped Carbon Spheres as Supports for the Hydrogenation of Cinnamaldehyde
Kente et al.
Table I. BET surface areas and pore volumes of the supports and
Table II. A summary of the % CALD conversions and selectivities
catalysts.
of the Pd/GaN and Pd/NCSs catalysts using different Pd loadings and
reaction temperatures.
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Product selectivity (%)a
GaN
NCSs
1% Pd/GaN
1% Pd/NCSs
3% Pd/GaN
3% Pd/NCSs
19ꢄ6
1ꢄ5
23ꢄ9
3ꢄ8
63ꢄ5
4ꢄ2
0.01
0.003
0.1
0.07
0.02
0.02
Temperature
(ꢀC)
CALD
conversion (%) HCALD
Catalyst
3P1P
1% Pd/GaN
40
60
8
23
100
100
0ꢄ00
0ꢄ00
1% Pd/NCSs
40
60
27
61
86
94
14
5ꢄ6
3% Pd/GaN
3% Pd/NCSs
60
60
97
60
86
90
13ꢄ9
9ꢄ5
ꢀ
60 C. The 1% Pd/GaN catalyst gave lower conversions at
both temperatures when compared to the 1% Pd/NCSs cat-
alyst. This was unexpected as the Pd particles are smaller
on the GaN and hence the dispersion and turnover number
would be expected to be larger on the GaN. This suggests
a strong Pd-GaN interaction (relative to the Pd-NCS inter-
action) that modifies the catalyst behaviour. This was also
suggested by the selectivity data—the Pd/GaN does not
over hydrogenate the HCALD to 3PIP.
Note: aNo CALC observed.
hydrogenated the HCALD-Pd intereaction must be weak
and the product will dissociate from the Pd. At the higher
temperatures 3PIP is formed. Thus the size of the Pd par-
ticle and the Pd-support interaction detemine the course of
the reaction. In future studies the stability of the catalyst
will be evaluated under the reaction conditions.
In an attempt to increase the catalyst activity 3% Pd
loaded catalysts were studied (Fig. 6, Table II). The 3%
loaded catalysts show superior activity as expected but
(a)
90
ꢀ
a reduced selectivity to the HCALD. At 60 C the 3%
80
Pd/GaN has a slightly higher activity than the 3% Pd/NCS
catalyst, but the reason for this is not obvious. Figure 6(b)
indicates that this is true throughout the full reaction
timeframe. The selectivity data suggest that there is lit-
tle difference between the two catalysts. It appears that
3% Pd/GaN at 60 ºC-
HCALD
70
60
3% Pd/GaN at 60 ºC-
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3P1P
50
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3% Pd/NCSs at 60 ºC-
Copyright: American Scientific Publishers
HCALD
the HCALD/3PIP ratio did not vary with time on stream
40
30
20
10
3% Pd/NCSs at 60 ºC-
3P1P
(or conversion), perhaps suggesting that the secondary
hydrogenation step only occurs when the CALD is bound
to the Pd.
The data thus suggest that the Pd was bound to the
CALD and provided the interaction is strong enough
conversion to HCALD occurs. Not unexpectedly, any
1
2
3
4
5
6
Time on stream (hours)
C
O–Pd interactions must be weak as no CALC was
formed in the reaction. Once the C C bond has been
70
60
50
40
30
20
10
0
(b)
3% Pd/GaN at 60 ºC
3% Pd/NCSs at 60 ºC
100
80
60
40
1% Pd/GaN at 40 ºC
1% Pd/NCSs at 40 ºC
20
0
1% Pd/GaN at 60 ºC
1% Pd/NCSs at 60 ºC
1
2
3
4
5
6
1
2
3
4
5
6
Time on stream (hours)
Time on stream (hours)
Fig. 6. Graphs showing (a) the product selectivity to HCALD and 3P1P
(b) CALD remaining as a function of time on stream at 60 ꢀC for
3%Pd/GaN and 3% Pd/NCSs catalysts.
Fig. 5. Graphs showing CALD remaining as a function of time on
stream at 40 and 60 ꢀC for 1% Pd/GaN and 1% Pd/NCSs catalysts.
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Kente et al.
Application of GaN NSs and Nitrogen Doped Carbon Spheres as Supports for the Hydrogenation of Cinnamaldehyde
4. CONCLUSIONS
the DST/NRF South Africa–India bilateral (grant number
76157).
This study has shown for the first time that GaN NSs
can be used as support materials for Pd nanoparticles
for the hydrogenation of CALD. The data revealed that
the GaN inhibits the hydrogenation relative to NCSs
and in doing so gives a catalyst with better selectiv-
ity to HCALD. Higher conversions of CALD and poorer
selectivity towards HCALD were obtained using the 3%
Pd/GaN and 3% Pd/NCSs catalysts. TEM images showed
that the Pd metal nanoparticles are well dispersed on the
surface of the support materials and that small Pd particles
give lower conversions but higher selectivities. The uni-
form dispersion of the Pd nanoparticles was achieved by
support functionalization using different acid treatments.
The GaN NSs can thus be used as supports for catalytic
reactions in solution (and presumably in the gas phase).
This study suggests that the use of GaN NSs may not only
be used in electronic applications but can also be exploited
in heterogeneously catalysed reactions such as this one.
The interaction between the Pd nanoparticles and the GaN
NSs is yet to be established.
References and Notes
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Acknowledgments: The authors would like to thank
the University of the Witwatersrand, the Council for Sci-
entific and Industrial Research (Pretoria), Carnegie Cor-
poration (University of the Witwatersrand), the DST/NRF
Centre of Excellence in Strong Materials (CoE-SM) and
DST/NRF Nano—Architecture Flagship Program in the
beneficiation of platinum group metals for financial sup-
port. The work was also conducted with support from
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17. M. Moyo, M. A. M. Motchelaho, H. Xiong, L. L. Jewell, and N. J.
Coville, Appl. Catal. A: General 413–414, 223 (2012).
Received: 31 October 2012. Accepted: 12 February 2013.
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