Hydrogenation and dehydrogenation reactions catalyzed by CNTs supported palladium catalysts

Article abstract of DOI:10.1016/j.cattod.2011.11.001

Pd supported on carbon nanotubes (CNTs), Al₂O₃ and SiO₂ were prepared by wet-impregnation method. The synthesized catalysts were characterized by BET, XPS, SEM and TEM. Hydrogenation of benzaldehyde and dehydrogenation of ethylbenzene over Pd/CNTs, Pd/r-Al ₂O₃ and Pd/SiO₂ were carried out in fixed bed phase reaction under an atmosphere pressure to test the activity and selectivity of the catalysts. Influence of temperature on activity and selectivity was investigated in both hydrogenation of benzaldehyde and dehydrogenation of ethylbenzene. The selectivity and conversion of obtained products were dependent on the nature of the support and reaction temperature. The Pd/CNTs showed a higher activity (100%) and selectivity to toluene (94.4%) in the hydrogenation. High selectivity (100%) towards the desired the product of styrene was achieved with conversion of ethylbenzene over 45.7% in the dehydrogenation.

Full text of DOI:10.1016/j.cattod.2011.11.001

Catalysis Today 186 (2012) 109–114
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Hydrogenation and dehydrogenation reactions catalyzed by CNTs supported
palladium catalysts
Xiao-Feng Guo, Da-Young Jang, Hyung-Gyu Jang, Geon-Joong Kim∗
Department of Chemical Engineering, Inha University, Incheon 402-751, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 June 2011
Received in revised form 28 October 2011
Accepted 3 November 2011
Available online 29 December 2011
Pd supported on carbon nanotubes (CNTs), Al2O3 and SiO2 were prepared by wet-impregnation method.
The synthesized catalysts were characterized by BET, XPS, SEM and TEM. Hydrogenation of benzaldehyde
and dehydrogenation of ethylbenzene over Pd/CNTs, Pd/r-Al2O3 and Pd/SiO2 were carried out in fixed bed
phase reaction under an atmosphere pressure to test the activity and selectivity of the catalysts. Influence
of temperature on activity and selectivity was investigated in both hydrogenation of benzaldehyde and
dehydrogenation of ethylbenzene. The selectivity and conversion of obtained products were dependent
on the nature of the support and reaction temperature. The Pd/CNTs showed a higher activity (100%)
and selectivity to toluene (94.4%) in the hydrogenation. High selectivity (100%) towards the desired the
product of styrene was achieved with conversion of ethylbenzene over 45.7% in the dehydrogenation.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Carbon nanotubes
Pd catalyst
Hydrogenation
Dehydrogenaton
1. Introduction
superheated steam (steam-EBDH) using potassium-promoted iron
oxide catalysts at 550–650 ◦C [11]. However, the steam-EBDH is
Without any doubt, there is an increasing tendency to apply
catalytic methods for the synthesis and production of multifunc-
tional, complex fine, and specialty chemicals such as industry,
agrochemicals and pharmaceuticals. On the one hand, the cata-
lysts used are homogeneous metal complexes with organic ligands
(emphasis on industrial applications [1]; up-to-date monograph
[2]) and on the other hand heterogeneous catalysts, either in the
metallic state or as oxides [3]. From an industrial point of view,
catalysts not soluble in the same phase as the organic reactant
have the inherent advantage of easy separation and very often
also of better handling properties. In the field of fine chemicals,
palladium is arguably the most versatile and the most widely
applied catalytic metal in the hydrogenation/dehydrogenation
reactions [4]. Hydrogenation of benzaldehyde is of industrial
importance as the product benzyl alcohol is used as intermediate
in the synthesis of pharmaceuticals, flavor, fragrances, chemi-
cal intermediates and photographic chemicals [4,5]. It is also
used as raw material in the manufacture of various esters (such
as benzyl formate, acetate, propionate, and butyrate) which are
extensively used in the flavor and fragrance industries [7]. Cat-
alytic dehydrogenation of ethylbenzene (EBDH) is a representative
process to produce styrene, an important monomer for syn-
thetic polymers [6–8]. Styrene is now produced commercially
by vapor-phase EBDH dehydrogenation with a large excess of
a thermodynamically limited and energy-consuming endother-
mic process. The use of a large excess of superheated steam
(steam/EB = 7–12 mol/mol) has a drawback of losing latent heat of
condensation during subsequent separation particularly in a mass
process [10,12].
The choice of an appropriate support upon which to dis-
perse a metal can have a significant effect on both the activity
and selectivity in certain reactions, such as the hydrogenation
of molecules containing carbonyl bonds or dehydrogenation of
paraffin [7].
Since their discovery at the beginning of the last decade [13],
carbon nanotubes seemed to be promising candidate for use
as catalyst support for hererogeneous catalyst reaction [14]. It
was expected to be efficient due to their high external surfaces
which can allow a significant decrease in critical mass trans-
fer limitation, also leading to an increase in the rate and the
selectivity of the reactions [15] CNTs possess unique proper-
ties which make them competitive with conventional catalyst
support. Hybrid of metal particles and CNTs were used as
catalysts in the hydrogenation [16]. Recently raw CNTs or mod-
ified CNTs were directly used to catalyze the dehydrogenation
[17].
In this paper, CNTs was used to disperse and stabilize the Pd
particles. Pd/conventional supports (r-Al2O3 and SiO2) were cho-
sen to compare with the Pd/CNTs. Activity and selectivity of the
catalysts were investigated in the hydrogenation of benzaldehyde
and dehydrogenation of ethylbenzene in fixed bed reaction under
an atmosphere pressure.
∗
Corresponding author. Tel.: +82 32 860 7472; fax: +82 32 872 4046.
E-mail address: kimgj@inha.ac.kr (G.-J. Kim).
0920-5861/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2011.11.001

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X.-F. Guo et al. / Catalysis Today 186 (2012) 109–114
Fig. 1. Morphologies of pristine CNTs (a) SEM images and (b) TEM images.
2. Experimental details
3. Results and discussion
2.1. Catalyst preparation
3.1. Catalysts characterization
CNTs (Ctube 300, CNT Co., Ltd, Korea) were refluxed in concen-
trated nitric acid (100 ml gCNT−1) for 2 h, washed with deionized
water to pH 6–7, and dried in air at 393 K overnight (oCNTs).
oCNTs, r-Al2O3 (JRC-ALO4)and SiO2 (silica gel 60,Sigma Aldrich)
supported catalysts were prepared by incipient wetness impreg-
nation using aqueous solutions of Pd(NO3)2·XH2O (Sigma Aldrich,
>99%), to achieve loadings of 2 wt% Pd to their supports. The sam-
ples were impregnated samples were dried in air at 373 K overnight
(Pd/CNTs, Pd/r-Al2O3 and Pd/SiO2). The catalysts were reduced at
100 ◦C for 30 min under the H2 atmosphere before they were used
to test the activity and selectivity.
Morphology and microstructures of pristine CNTs were char-
acterized by SEM and TEM. As shown in Fig. 1, the SEM image
revealed that external diameters of CNTs were varied from 8 to
50 nm (Fig. 1a). TEM images revealed that inner diameter of CNTs
were changed from 2 to 5 nm (Fig. 1b).
To obtain quantitative evidence of pore structure for each sup-
ports, N2 adsorption/desorption isotherm was measured as shown
in Fig. 2. The pristine CNTs (Fig. 2a) show type II adsorption isotherm
according to IUPAC classification by revealing a gradual uptake
of N2 at the high relative pressure (P/P0 = 0.9) which is related
to the multilayer adsorption on macropores. The appearance of
macropores is attributed to agglomerate between CNTs. N2 adsorp-
tion/desorption isotherm of the r-Al2O3 and Silica (Fig. 2b and c)
exhibit hysteresis loop (P/P0 = 0.7–0.9). The pores are attributed to
the interparticle voids due to an agglomerate of nanometer size
particles. Specific area of CNTs, r-Al2O3 and SiO2 is 172, 178 and
500 m2/g. The specific area is much larger than r-Al2O3 and SiO2
regarding many micropores exited in both supports.
2.2. Apparatus and procedure
The reaction was carried out using a flow-type reactor (quartz
tube, 1.0 cm ID, 1.5 cm OD, 50 cm length) and was operated at atmo-
spheric pressure. A catalyst (50 mg) was placed in the center of
the reactor using a quartz glass wool plug. Hydrogenation of ben-
zaldehyde was reduced at the range of 100–300 ◦C under H2 flow.
Dehydrogenation of ethylbenzene was performed at the range of
400–600 ◦C under air stream for 1–8 h. Benzaldehyde or ethylben-
zene was fed (ca. 3 ml/h) to the reactor by H2 or air (30 ml/min)
through the syringe pump preheated at 100 ◦C. The condensed
material was cooled externally in an ice water bath. Reaction prod-
ucts were quantified by gas chromatography equipped with FID
detector, using a 0.32 mm 30 m glass column packed with HP-5.
To determine the Pd dispersity and particle size, TEM images
were taken in Fig. 3. The TEM images of Pd/CNTs (Fig. 3a and d)
evidenced the high and homogeneous dispersion of spheroidal pal-
ladium metal particles, with a narrow particle size distribution
800
(a)
700
(b)
600
(c)
500
400
300
200
100
0
2.3. Characterization
The morphology and microstructures of samples were char-
acterized by field emission transmission electron microscopy
(FE-TEM, S-4200), and field emission scanning electron microscopy
(FE-SEM, JEM-2100F). The nitrogen adsorption/desorption analysis
was performed at −196 ◦C by using a surface area and poros-
ity equipment (Micromeritics, ASAP 2010). The specific surface
areas were calculated according to BET theory. X-ray photoelec-
tron spectroscopy (XPS, K-Alpha) was performed in high vacuum.
The content of Pd in the Pd/CNTs was determined by inductively
coupled plasma (ICP, Elan 6100).
0.0
0.2
0.4
0.6
0.8
1.0
P/P0
Fig. 2. Nitrogen adsorption/deporption isotherms of catalyst supports (a) pristine
CNTs; (b) r-Al2O3; and (c) SiO2.

X.-F. Guo et al. / Catalysis Today 186 (2012) 109–114
111
Fig. 3. TEM images of 2 wt% Pd loading on different supports (as-synthesized): CNTs (a) and (d); SiO2 (b) and (e); r-Al2O3 (c) and (f).
Fig. 4. TEM images of 2 wt% Pd/CNTs (used without regeneration) after catalytic reaction at different temperature: (a) 100 ◦C; (b) 200 ◦C; and (c) 300 ◦C.
centered at around 2–3 nm diameter. The homogenous disper-
sion was attributed to a relatively strong metal-support interaction
between the metal salt precursor and the graphite edges of CNTs,
increasing the resistance to the growth of the Pd particles[18]. The
smaller the Pd particle size could be due to the high specific surface
area of CNTs. The particle size was evaluated from centered on at
least 100 Pd particles in TEM images. Comparing with the Pd/CNTs,
the TEM photographs of Pd/SiO2 (Fig. 3b and e) and Pd/r-Al2O3
(Fig. 3c and f)even gave quite uniform dispersion but a wide dis-
tribution of Pd particle size ranged from 5.0–6.5 nm to 6.5–8.5 nm,
respectively.
To investigate the influence on Pd particles size by the high reac-
tion temperature, the samples of Pd/CNTs which had been used to
test activity for benzaldehyde hydrogenation were recovered and
used for TEM analysis. TEM images at 100–300 ◦C were shown in
Fig. 4. The particle size was evaluated from centered on at least 100
Pd particles in TEM images. It was apparently observed that parti-
cles size increased from 2–3 to 3–4 and 4–4.5 nm with increasing
of the reaction temperature from 100 to 300 ◦C. Two or three pri-
mary particles aggregated into another new bigger particle due to
the temperature increasing effect. The change in particles size will
have influence on activity and selectivity of catalysts partly.
The XPS characterization data of Pd supported on different sup-
ports were presented in Fig. 5 and Table 1. The oxidation state
of Pd and interaction between Pd particle and supports are veri-
fied through the Pd 3d XPS measurements. In the three catalysts
(Pd/CNTs, SiO2 and r-Al2O3), the doublet peaks of binding energy
(BE) for Pd 3d5/2 is equal to 335.8, 335.3, 335. 4 eV and for Pd
3d3/2 is 341.2, 340.6 and 340.8 eV. These BE values are agreement
with those reported for Pd0 (Pd 3d5/2 BE = 335.3 eV and Pd 3d3/2
BE = 340.6 eV) [19]. Chemical shift of BE is attributed to the local
chemical and physical environment difference. The higher chemi-
cal shift of BE is attributed to the interaction between Pd and CNTs
[18].
Table 1
XPS data of Pd supported on different supports.
Catalyst
Binding energy
Pd 3d5/2
Pd 3d3/2
CNTs
SiO2
r-Al2O3
335.8
335.3
335.4
341.2
340.6
340.8

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X.-F. Guo et al. / Catalysis Today 186 (2012) 109–114
100
90
80
70
60
50
40
30
20
10
0
3d5/2
3d3/2
(a)
(a)
(b)
(c)
(b)
(c)
330
332
334
336
338
340
342
344
346
1
2
3
4
5
6
7
8
Time (h)
Binding energy (eV)
Fig. 7. Selectivity (benzyl alcohol) of different catalysts for catalytic benzaldehyde
hydrogenation: Pd/CNTs (a); Pd/SiO2 (b); and Pd/r-Al2O3 (c).
Fig. 5. X-ray photoelectron spectroscopy of 2 wt% Pd loading on different supports
(reduced by H2 for 1 h at 100 ◦C): CNTs (a); SiO2 (b); and r-Al2O3 (c).
attributed to the lack of micropores as well as the high surface
area of the CNTs comparing with SiO2 and r-Al2O3. Lack of micro-
pores affects the mass transfer of reactants and products on the
surface of the support as well as affecting the manner in which
desorption occurs. In another words, the high external surface area
of Pd/CNTs inhibits the mass transfer limitation of the reactant to
the active sites and lead to the obtention of an active catalyst. As
it is well-known, both SiO2 and r-Al2O3 exists many micropores.
One the one hand micropores can trap active Pd particles; on the
other hand they limit the mass transfer of reactants and products to
affect the activity of catalyst. In addition, such an order seems to be
related to the crystallite size of the palladium phase. The order of Pd
particles size is as following: Pd/CNT < Pd/SiO2 < Pd/r-Al2O3 (Fig. 3).
The particle size is inversely in proportion to activity of catalysts.
The smaller particle size usually would be given a better activity.
The results of influence on the catalytic activity by the temperature
have been shown in Table 2. The TOF data indicated that Pd/SiO2
(2519 h−1) and Pd/r-Al2O3 (939 h−1) showed the best activity at
200 ◦C reaction temperature. The deactivation appeared at 300 ◦C in
both Pd/SiO2 (163 h−1) and Pd/r-Al2O3 (166 h−1). The bigger num-
ber of TOF means higher active for the catalyst. It is attributed to
influence of temperature and Pd particle size. When temperature
is not very high (200 ◦C), the activity was mainly promoted by the
temperature. As the temperature was increased into 300 ◦C, Pd par-
ticle size grew up by sintering to deactivate the catalysts. However,
the Pd/CNTs exhibited very a little change for activity of the cataly-
sis with temperature ranged from 100 to 300 ◦C. It can be attributed
to the strong interaction between Pd particle and CNTs to inhibit
the aggregate of Pd even in high temperature.
3.2. Hydrogenation of benzaldehyde
3.2.1. Activity of catalyst
The supports were inactive in hydrogenation of benzaldehyde
in the whole range of reaction temperature. All activities were
obtained in the presence of palladium indicating that the latter was
the only component of the active site. The conversions obtained
by catalytic hydrogenation of benzaldehyde at 100 ◦C by different
catalysts are presented in Fig. 6 and Table 2. It can be noted in
the figure that Pd/r-Al2O3 was the least active (15.5% conversion)
whereas Pd/CNTs was the most active with a high stabilized con-
version (100%) versus time on H2 stream. The Pd/CNTs catalyst was
by far the most active and the order of activity was as follows:
Pd/CNTs (100%) > Pd/SiO2 (70.3%) > Pd/Al2O3 (15.5%)
It is well agreement with turn-over frequency (TOF) of catalysts
(Pd/CNTs (3129 h−1) > Pd/SiO2 (2200 h−1) > Pd/Al2O3 (491 h−1)).
Furthermore, TOF of the catalysts for dehydrogenation of ben-
zaldehyde at 100 ◦C was calculated by activity divided by surface
Pd atoms (Table 3). The conversion of benzaldehdye was fixed
at 50% by increasing or decreasing the corresponding catalyst
amount. The number of surface Pd atoms was calculated by the
Jin et al. method [20]. The order of the TOF was as follows:
Pd/CNTs (3.1 s−1) > Pd/SiO2 (2.0 s−1) > Pd/Al2O3 (0.8 s−1). It could be
100
80
3.2.2. Selectivity
The catalytic reaction products of benzaldehyde hydrogenation
(Table 2 and Figs. 7 and 8) were benzyl alcohol, toluene, and methyl-
cyclohexane. They resulted from two types of reaction:
60
(a)
(b)
40
(c)
(i) C O double bond reduction to benzyl alcohol and toluene.
(ii) Reduction of the aromatic ring to methylcyclohexane.
20
0
The distribution of the production for benzaldehyde hydrogena-
tion depends on activity of catalysts and reaction temperature.
1
2
3
4
5
6
7
8
Fig.
7 showed the selectivity to benzyl alcohol versus the
Time (h)
time by different catalysts at 100 ◦C reaction temperature. The
order of selectivity to benzyl alcohol is Pd/CNT (4.7%) < Pd/SiO2
(88.4%) < Pd/r-Al2O3 (92.9%). The analysis of the results reveals that
Fig. 6. Conversion of benzaldehyde using different catalysts: Pd/CNTs (a); Pd/SiO2
(b); and Pd/r-Al2O3 (c).

X.-F. Guo et al. / Catalysis Today 186 (2012) 109–114
113
Table 2
Catalytic results for benzaldehyde hydrogenation over Pd supported on various supports.
c
Catalyst
Pd loading (wt%)a
T (◦C)
Selectivity (%)b
BA
Conversion (%)
TOF (h−1
)
TOL
MC
0.9
17.3
87.1
100
200
300
4.7
38.6
0
94.4
44.1
12.9
100
97.8
99.5
3129
3060
3113
CNT
SiO2
2.04
1.95
1.97
100
200
300
88.4
90.5
37.6
11.6
9.5
3.4
0
0
59.0
70.3
80.5
5.2
2200
2519
163
100
200
300
92.9
96.6
19.1
7.1
3.4
4.5
0
0
76.4
15.5
30.0
5.3
491
939
166
r-Al2O3
a
Determined by inductively coupled plasma atomic emission spectrometry.
BA, benzyl alcohol; TOL, toluene; MC, methylcyclohexane.
Mol benzaldehyde converted per mol Pd per hour.
b
c
the highest selectivity to benzyl alcohol (92.9%) appears on the
least active catalyst Pd/r-Al2O3 (15.7%). As a general trend, ben-
zyl alcohol appears as a low reaction temperature. The highest
selectivity to benzyl alcohol for Pd/SiO2 (90.5%) and Pd/r-Al2O3
(96.6%) appeared at 200 ◦C reaction temperature. The methylcy-
clohexane became main product when reaction temperature was
increased to 300 ◦C. However, the Pd/CNTs exhibited a different
product distribution with Pd/SiO2 and Pd/r-Al2O3. A highest selec-
tivity to toluene (94.4%) was obtained at 100 ◦C for the Pd/CNTs
(Fig. 8). It was attributed to the properties of CNTs surface and high
activity of Pd/CNTs. Now, it is well established that toluene is the
product of the consecutive reaction of benzyl alcohol hydrogenol-
ysis [21]. The mass transfer of reactant and product can be carried
on the surface of Pd/CNTs (high external surface area) very quickly.
Consecutively, produced benzyl alcohol favorable adsorbed on the
surface of Pd/CNTs, and then further was converted into toluene.
The Pd/CNTs catalyst was by far the most active and the order of
selectivity to toluene was as follows:
due to increasing reaction temperature. The methyl cyclohexane
appeared as a main product at high temperature (300 ◦C) as well as
Pd/SiO2 and Pd/r-Al2O3.
Mechanism of benzaldehyde hydrogenation was provided in
Fig. 9. Firstly, at low temperature the vapored benzaldehyde was
adsorbed on the surface, and then active H2 reacted with the ben-
zaldehyde. The benzyl alcohol was produced and desorbed on
surface of catalyst. The benzyl alcohol adsorbed on surface of cat-
alyst can be further converted into toluene with H2. At higher
temperature, Toluene adsorbed on the surface of catalyst was
reduced by H2 into methylcyclohexane. The produced toluene
desorbed on surface of catalyst. The benzyl alcohol is reaction
intermediate of toluene and toluene are reaction intermediate of
methyl cyclohexane. The reaction of benzaldehyde hydrogena-
tion is a pathway of temperature dependency. This mechanism is
in good agreement with the experimental data on benzaldehyde
hydrogenation over Pd/CNTs.
3.3. Dehydrogenation of ethylbenzene
Pd/CNTs (94.4%) > Pd/SiO2 (11.6%) > Pd/Al2O3 (7.1%)
Both SiO2 and r-Al2O3 are not active for dehydrogenation of
ethylbenzene. The CNTs is active for the reaction with (5% conver-
sion of ethylbenzene and 65% selectivity to styrene). The catalytic
activity and selectivity results are presented in Table 4. In general,
the conversion of ethylbenzene increased with increasing reaction
temperature and all catalysts exhibited a similar trend. Conver-
sions of ethylbenzene and distributions of products by the different
catalysts were shown in Table 4. In case of Pd/CNTs, the conver-
sion of ethylbenzene increased from 15.2% to 45.7% with increasing
temperature from 400 ◦C to 600 ◦C. The activity is attributed to rel-
atively high external surface area of Pd/CNTs. The high external
surface area of the catalyst inhibits the mass transfer limitation of
the reactant to the active sites and leads to the obtention of an
active sites. The selectivity to styrene remained almost constant
(100%) against increasing temperature. It was due to strong inter-
action between Pd particle and CNTs. The strong interaction can
result in formation of a small, uniform and homogenous dispersed
Pd on the surface of CNTs. Conversion and distribution of products
by Pd/SiO2 and Pd/r-Al2O3 was shown in Table 4. The Pd/SiO2 and
It indicated that the higher active catalyst gave the higher
selectivity to toluene. In addition, the temperature still played
an important role on the selectivity of Pd/CNTs (Table 2). Benzyl
alcohol showed higher selectivity (38.6%) at modest temperature
(200 ◦C) for Pd/CNTs. On the one hand it is attributed to reducing
adsorptive capacity of benzyl alcohol on the surface of Pd/CNTs;
on the other hand the activity of the Pd/CNTs was decreasing
100
90
80
(a)
70
(b)
60
(c)
50
40
30
20
10
0
Table 3
Parameter of catalyst activity for benzaldehyde hydrogenation over Pd supported
on various supports at 100 ◦C.
a
Pd catalyst
Conversion (%)
Number of surface atoms
TOF (s−1
)
1
2
3
4
5
6
7
8
CNTs
SiO2
r-Al2O3
50
50
50
1.1 × 1018
2.0 × 1018
4.2 × 1018
3.1
2.0
0.8
Time (h)
Fig. 8. Selectivity (toluene) of different catalysts for catalytic benzaldehyde hydro-
genation: Pd/CNTs (a); Pd/SiO2 (b); and Pd/r-Al2O3 (c).
a
TOF was calculated by activity divided by per surface atoms.

114
X.-F. Guo et al. / Catalysis Today 186 (2012) 109–114
PhCHOg
C6H11CH3g
PhCHOads
H2
C6H11CH3ads
K1
H2
K3
PhCH2OHg
PhCH3g
PhCH3ads
PhCH2OHads
H2
K2
Fig. 9. Mechanism of benzaldehyde hydrogenation catalyzed by Pd catalysts.
Table 4
reaction is pathway of temperature dependency. The Pd/CNTs also
showed not very high activity (45.7%) but high selectivity (100%) in
the dehydrogenation of ethylbenzene.
Catalytic results for ethylbenzene dehydrogenation over Pd supported on various
supports.
Catalyst Pd loading (wt%) T ( ◦C) Selectivity (%)
Conversion (%) TOF (h−1
)
a
Acknowledgments
Styrene Others
400
500
600
100
100
99.5
0
0
0.5
15.2
23.1
45.7
409
602
1191
This research was supported by Korea Ministry of Environment
as “converging technology project”, and supported by Inha Univer-
sity Research Grant.
CNT
SiO2
2.04
1.95
1.97
400
500
600
98.2
95.7
88.2
1.8
4.3
11.8
10.5
20.3
35.6
274
529
928
References
400
500
600
97.5
90.4
80.5
2.5
9.6
19.5
7.9
17.4
20.7
206
453
539
r-
Al2O3
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