Copper nanoparticles decorated inside or outside carbon nanotubes used for methyl acetate hydrogenation

Article abstract of DOI:10.1166/jnn.2013.5952

Copper nanoparticles loaded inside or outside carbon nanotubes (CNTs) were prepared by a simple wet chemistry method. The structures and physicochemical properties of the obtained materials were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Methyl acetate (MA) hydrogenation forming methanol and ethanol was chosen as the application to explore the catalytic performances of copper inside and outside CNTs. The reaction results indicated that the catalytic activity of Cu-inside-CNTs catalyst was significantly higher than that of Cu-outside- CNTs because of the space confined effects. Furthermore, the influences of Cu content on CNTs to the catalytic performance were also investigated. Copyright

Full text of DOI:10.1166/jnn.2013.5952

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Journal of
Nanoscience and Nanotechnology
Vol. 13, 1274–1277, 2013
Copper Nanoparticles Decorated Inside or
Outside Carbon Nanotubes Used for
Methyl Acetate Hydrogenation
1
2
1
1
1
3
Ding Wang , Xiaoyu Sun , Chuang Xing , Guohui Yang , Kai Tao , Tokimasa Kawabata ,
3
4
1ꢀ∗
Kenji Matsuda , Yisheng Tan , and Noritatsu Tsubaki
1
Department of Applied Chemistry, Graduate School of Engineering, University of Toyama, Gofuku 3190, 930-8555, Japan
2
China University of Petroleum, Dongying, Shandong 257061, China
3
Department of Material Engineering, School of Engineering, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
4
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China
Copper nanoparticles loaded inside or outside carbon nanotubes (CNTs) were prepared by a simple
wet chemistry method. The structures and physicochemical properties of the obtained materials
were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Methyl
acetate (MA) hydrogenation forming methanol and ethanol was chosen as the application to explore
the catalytic performances of copper inside and outside CNTs. The reaction results indicated that
the catalytic activity of Cu-inside-CNTs catalyst was significantly higher than that of Cu-outside-
CNTs because of the space confined effects. Furthermore, the influences of Cu content on CNTs
to the catalytic performance were also investigated.
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Keywords: Carbon Nanotubes, Copper, Methyl Acetate, Hydrogenation, Confinement Effect.
Copyright: American Scientific Publishers
1
. INTRODUCTION
CNTs is crucial to the study of space confined effect of
CNTs to catalyst performance.
Carbon nanotubes with unique properties such as large sur-
face area, unique hollow tubular structure, large adsorption
capacity, and high chemical, thermal and mechanical sta-
bility have received considerable interest as catalyst sup-
Here, we report a simple wet chemistry method which
can finely regulate the loading of Cu nanoparticles inside
or outside the CNTs. The Cu nanoparticles decorated
CNTs were studied by TEM and XRD, indentifying the
distribution of Cu on CNTs and its reductive ability.
The methyl acetate (MA) hydrogenation, as one of cat-
alytic applications of Cu decorated CNTs, was adopted to
explore the confinement effect of CNTs.
1
port in heterogeneous catalysis. The copper and its oxides
(CuO, Cu O) are one of the most important catalysts since
2
2
their hydrogenation property. To date, copper-based cat-
alysts also had been widely used for methanol synthesis,
water gas shift reaction. Using CNTs as catalyst support
3
–4
with Cu or CuO nanoparticles as catalytic sites, the Cu
x
decorated CNTs catalysts had been prepared by so many
physical and chemical ways: the physics methods such as
arc-discharge, chemical vapor deposition (CVD), super-
2. EXPERIMENTAL DETAILS
5
6
Multi-wall CNTs (Chengdu China) are used as support
materials for the preparation of Cu loaded CNTs cata-
lyst. Prior to impregnation, the CNTs were treated with
67% HNO3 at 393 K for 14 h, followed by washing
repeatedly with deionized water until pH = 7, drying at
7
critical CO fluid, and the chemical methods including
2
8
9
electrochemical co-deposition, ethylene glycol reduction,
1
0
solution infusion. All of these methods, however, are
very complicated processes, expensive costs, and espe-
cially difficult to control the fixed sites of metal nanopar-
ticles inside or outside CNTs channels. As we know, the
controlled loading of Cu nanoparticles inside or outside
3
1
33 K over 12 h and then vacuum drying at 353 K for
2 h. The obtained CNTs with opened ends were used
as the support for copper loading. For impregnation pro-
cess on this CNTs, 0.5 g of CNTs was impregnated with
1
mL Cu(NO ꢀ solution at the assistance of ultrasound.
3 2
∗
Author to whom correspondence should be addressed.
After 30 min ultrasound impregnation, another 0.5 mL
1
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J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 2
1533-4880/2013/13/1274/004
doi:10.1166/jnn.2013.5952

Wang et al.
Copper Nanoparticles Decorated Inside or Outside Carbon Nanotubes Used for Methyl Acetate Hydrogenation
of deionized water was orderly dropped in. Then a dry
sample could be obtained after further 30 min ultrasound
treatment. Subsequently, the obtained sample was dried
at 373 K in air for 10 h, and calcined at 623 K under
Ar atmosphere for 3 h. The final sample was defined as
Cu-inside-CNTs.
The Cu-outside-CNTs was obtained by the similar
impregnation method to that of Cu-inside-CNTs, but hav-
ing a specific pre-filling procedure on CNTs. Here, the
pre-filling way used on CNTs by filling xylene in its chan-
1
1
nels are similar to the precursor report. Briefly, before the
Cu(NO ꢀ solution impregnation process, the acid treated
3
2
CNTs with opened ends were first pre-impregnated with
ml xylene at the assistance of ultrasound, inhibiting the
entrance of Cu(NO ꢀ solution into its inner wall. After
1
3
2
that, the CNTs were impregnated with Cu(NO ꢀ solution,
3
2
following by drying at 353 K in air for 2 h and calcining
at 623 K in Ar atmosphere for 3 h. The final samples were
defined as Cu-outside-CNTs. Both of these two types of
Cu loaded CNTs, Cu-outside-CNTs and Cu-inside-CNTs,
have the same Cu-content of 10 wt%. Moreover, another
two samples with higher Cu loading amount, 20 wt% and
4
0 wt%, as reference, were also prepared by the same
method to that of 10 wt% Cu-inside-CNTs.
The microstructures of carbon nanotubes samples
were characterized with transmission electron microscopy
Fig. 1. TEM photos of (a) Cu-outside-CNTs after reduction, (b) Cu-
inside-CNTs after reduction and (c) XRD patterns of Cu-outside-CNTs
(
TOPCON EM-002B working at 120 kV). The crystal
Delivered by Publishing Technologyat fot e:r Mr e ad ui nc ti Co nI Da n ids C8 u0 - i0n 0s i 4d e8- C0 N5 T (s J Pa f Pt e r) reduction. (Reduction with
structure of the materials was confirmed by X-ray diffrac-
tion (XRD) with a Rigaku D/max-2550 V diffractometer
IP: 128.135.12.127 On: Fri ,1 01 0 0% OH c
ta t2 507 13 4K 0f o2 r: 13 01 h: 5) .6
2
Copyright: American Scientific Publishers
employing Cu Kꢁ radiation (ꢂ = 1ꢃ54056 Å).
illustration of the preparation process of Cu-outside-CNTs
was showed in Figure 2. The xylene, here, was used to
lock the CNTs channels temporarily before the decora-
tion of the exterior CNTs surface with Cu clusters, which
can effectively prevent the access of Cu(NO ꢀ solution
All the catalytic reactions were conducted with a fixed-
bed stainless steel reactor (9.5 mm OD). 0.5 g catalyst
was loaded into the reactor. Before the reaction, the cata-
lysts were reduced in situ by a flow of pure hydrogen at
5
73 K for 10 h. After being cooled to the reaction tem-
3 2
1
1
infiltrating into CNTs channels. Furthermore, the higher
boiling point of xylene than that of water can also allows
the preferential evaporation of water on the outside of
nanotubes, leading to the unique copper loading only on
the external surfaces of CNTs. Different from Cu-outside-
CNTs, almost all of the Cu nanoparticles of Cu-inside-
CNTs were filled inside the channel of CNTs shown in
Figure 1(b). The formation of Cu nanoparticles inside
CNTs should be attributed to the following reasons: First,
the bigger size of CNTs channels was beneficial for solu-
tion entering to the inner cavities of CNTs. Second, most
of the ends of CNTs have been opened after the concen-
trated nitric acid treatment. Relatively low surface tension
of water and the capillary force donated by CNTs chan-
nels ensured that an equal volume liquid can be sucked
by CNTs channels entirely. Third, the additional water
added in the second step for Cu-inside-CNTs preparation
shown in Figure 2 helped to wash the residual copper
salt, shifting them into CNTs channels. For these two type
catalysts, Cu-outside-CNTs and Cu-inside-CNTs, their Cu
nanoparticles size are in the range from 10 to 18 nm
perature, the flow rate of pure hydrogen was changed to
−1
8
0 ml · min and the reaction pressure were maintained
at 3 MPa. Then, the MA was injected by a pump with
−1
the flow rate of 0.9 g·h . The liquid MA was evaporated
to MA gas by heater band, and carried into the reactor
by the hydrogen flow. The CO, CH , CO and Ar were
4
2
analyzed online by the gas chromatograph equipped with
TCD detector. The liquid products after catalytic reaction
were collected with an ice water trap using 1-butanol as
solvent, and analyzed by another gas chromatograph with
flame-ionization detector (FID), in which 1-propanol was
employed as the internal standard.
3. RESULTS AND DISCUSSION
Figures 1(a) and (b) exhibit TEM images of Cu-outside-
CNTs and Cu-inside-CNTs catalysts after reduction at
5
73 K with 100% H2 for 10 h, respectively. For Cu-
outside-CNTs catalyst, Cu nanoparticles with the size from
0 to 15 nm had been deposited evenly on the outer sur-
1
face of CNTs, as shown in Figures 1(a). The schematic
J. Nanosci. Nanotechnol. 13, 1274–1277, 2013
1275

Copper Nanoparticles Decorated Inside or Outside Carbon Nanotubes Used for Methyl Acetate Hydrogenation
Wang et al.
Cu(NO3)2
Washing
Heat treatment
Impregnation
Argon atmosphere
CNTs
Cu-inside-CNTs
Cu(NO3)2
Dry
Xylene
Filling
Heat treatment
Argon atmosphere
Impregnation
CNTs
Fig. 2. Schematic illustration of the formation process of Cu-inside-CNTs and Cu-outside-CNTs.
Cu-outside-CNTs
according to the TEM images in Figures 1(a) and (b). The
average Cu crystallite sizes of these two catalysts after
reduction had also been estimated according to their XRD
patterns shown in Figure 1(c). The values of Cu-outside-
CNTs and Cu-inside-CNTs after reduction were 14.3 nm
and 15.1 nm respectively, which are in good agreement
with the values obtained from their TEM images.
auto-reduced than outside of CNTs channels. The similar
results for other different metal oxides had also been found
1
2
by other researchers. Furthermore, the XRD patterns also
prove that the valence state change of Cu from Cu +
Cu O in 10% Cu-inside-CNTs to CuO + Cu O in 40%
2
2
Cu-inside-CNTs, which means the reducibility of catalysts
decrease with the increase of Cu loading amount. And
the average crystallite size of 20% Cu-inside-CNTs and
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Figure 3 presents the XRD profiles of 10 wt%
Cu-outside-CNTs, 10 wt% Cu-inside-CNTs, 20 wt%
IP: 128.135.12.127 On: Fri, 10 Oct 2014 02:31:56
40% Cu-inside-CNTs were about 16.4 nm and 22.3 nm,
respectively.
Copyright: American Scientific Publishers
Cu-inside-CNTs and 40 wt% Cu-inside-CNTs catalysts
only after calcination in Ar atmosphere without further
reduction by hydrogen. During the oxygen-free calcina-
tion of Cu loaded CNTs, auto-reduction of copper oxide
using CNTs as reductant may be happen. Comparing the
XRD pattern of 10 wt% Cu-inside-CNTs with that 10 wt%
Cu-outside-CNTs, more metallic Cu can be detected on
the Cu-inside-CNTs than Cu-outside-CNTs, indicating that
the Cu loaded inside CNTs channels is better to be
The catalytic performances of catalysts in MA hydro-
genation reaction were compared in Table I. Clearly, the
catalysts with Cu nanoparticles inside CNTs channels
have better activity than that the Cu nanoparticles outside
CNTs channels even under the same reaction conditions.
Although the same Cu loading amount, MA conversion
on 10 wt% Cu-inside-CNTs catalyst is 13.9%, about 3
times bigger than that of 10 wt% Cu-outside-CNTs. This
result should be attributed to the space confined effect
of CNTs, which make Cu clusters inside CNTs channels
Table I. Methyl acetate hydrogenation reaction performances over var-
ious catalysts.
Coversion (%)
Molecular selectivity (mol%)
Catalysts
MA
MeOH EtOH EA CO CH4 CO2
1
1
2
4
0% Cu-
outside-CNTs
0% Cu-
inside-CNTs
0% Cu-
inside-CNTs
0% Cu-
4ꢃ8
49.8
55.3
54.7
52.4
43.7
4ꢃ1 2.4
0
0
0
0
0
0
13ꢃ9
35ꢃ0
10ꢃ6
31.9 12ꢃ8
0
32.5 11ꢃ9 0.8 0.1
37.4 8ꢃ3 1.9
0
inside-CNTs
Notes: Reaction conditions: 493 K; 3 MPa; 2 h; catalysts weight = 0.5 g; F (H ꢀ =
2
Fig. 3. XRD profiles of 10% Cu-outside-CNTs, 10% Cu-inside-CNTs,
80 ml/min; F (MA) = 0.9 g/h; MA = methyl acetate; EA = ethyl acetate; MeOH =
methanol; EtOH = ethanol.
20% Cu-inside- CNTs and 40% Cu-inside-CNTs.
1
276
J. Nanosci. Nanotechnol. 13, 1274–1277, 2013

Wang et al.
Copper Nanoparticles Decorated Inside or Outside Carbon Nanotubes Used for Methyl Acetate Hydrogenation
more reducible than outside, resulting to the same Cu
content but different active pure Cu sites of these two
catalysts for MA hydrogenation. Ethyl acetate (EA) was
from the secondary reaction of the ethanol product, via its
trans-esterification with methyl acetate, consequently low-
ering the apparent ethanol selectivity. Different Cu loading
content inside CNTs channels can obviously effects the
conversion speed of methyl acetate, as shown in Table I.
Increasing the Cu content inside CNTs channels from
the copper clusters within the channels of CNTs. Reac-
tion results indicated that the catalytic activity of Cu-
inside-CNTs catalyst was significantly higher than that of
Cu-outside-CNTs. Furthermore, the effects of Cu content
inside CNTs channels on catalyst activity had also inves-
tigated, in which 20 wt% Cu-inside-CNTs showed the
highest catalytic activity.
References and Notes
1
0 wt% to 20 wt% effectively improve the MA conversion
from 13.9% to 35.0%, which should only be attributed to
the increase of active Cu content inside CNTs channels.
However, when the Cu content reached up to 40 wt%,
the catalyst activity of Cu-inside-CNTs does not increase
as we expected. Conversely, it decreases sharply. It seems
that the Cu clusters size of this 40 wt% Cu-inside-CNTs,
about 22.3 nm similar to the aperture of CNTs channels,
is too bigger that most of the CNTs channels are blocked,
inhibiting the access of reactants into CNTs channels and
then decreasing the efficiency of this catalysts.
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4
. CONCLUSIONS
1
1
1
In this work, the Cu nanoparticles loaded inside or out-
side CNTs were prepared by a simple wet chemistry
method. Methyl acetate hydrogenation was selected as
the application to investigate the confinement effect of
2. W. Chen, X. Pan, M. Willinger, D. Su, and X. Bao, J. Am. Chem.
Soc. 128, 3136 (2006).
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Copyright: American Scientific Publishers
Received: 12 September 2011. Accepted: 30 November 2011.
J. Nanosci. Nanotechnol. 13, 1274–1277, 2013
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