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Effect of hydrogen on the synthesis of carbon nanofibers by CO disproportionation on ultrafine Fe3O4.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
Published online 14 May 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.295
Special Theme Research Article
Effect of hydrogen on the synthesis of carbon nanofibers
by CO disproportionation on ultrafine Fe3O4
Wenxin Lu,1 Zhijun Sui,1 Jinghong Zhou,1 Ping Li,1 Xinggui Zhou1 * and De Chen2
1
2
State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, 200237 Shanghai, China
Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
Received 13 August 2008; Revised 16 February 2009; Accepted 17 February 2009
ABSTRACT: Carbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3 O4 catalyst at
a hydrogen concentration of 0–29.17%, and the time-depending rates of CNFs growth are continuously monitored and
the morphologies of the as-synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation
energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs,
but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon
diffusion in the catalyst particle is rate limiting. As a result of H2 -induced catalyst reconstruction and carbon deposition,
the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2
concentration is increased.  2009 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: carbon nanofibers; hydrogen; CO disproportionation; catalysts; morphology
INTRODUCTION
Carbon nanofibers (CNFs), which can be synthesized
from catalytic decomposition of certain hydrocarbons
or carbon monoxide over small metal particles such as
iron, cobalt, nickel, or their alloys,[1] have been known
for about several decades as a novel material with some
unique properties (e.g. large surface area and strong
mechanical intensity) and potential applications (e.g.
polymer additives, gas storage materials, and catalyst
supports).[2 – 5] However, large-scale synthesis of CNFs
at low costs and with well-controlled morphologies is
still a problem to overcome.
During synthesis of CNFs by CO disproportionation, H2 can either accelerate or suppress growth of
CNFs.[6 – 11] Three mechanisms have been proposed
for the accelerating effect, that is, decomposition of
inactive metal carbides by hydrogen to form catalytic
metals,[6,7] removal of encapsulating carbon that causes
catalyst deactivation by gasification,[8] and acceleration of CO adsorption by lowering the chemisorption
energy.[9] However, the suppressing effect has been
reported because of the competitive adsorption between
hydrogen and CO and hydrogasification of newly grown
carbon to form methane.[8,10] Hydrogen will also change
*Correspondence to: Xinggui Zhou, State Key Laboratory of Chemical Engineering, East China University of Science and Technology,
130 Meilong Road, 200237 Shanghai, China.
E-mail: xgzhou@ecust.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
the structure of the catalyst and the morphology of
CNFs that grow on it. Ci[11] reported that H2 flow
rate could affect the particle size of catalyst. Wahed
et al .[12] found that the H2 concentration could apparently affect the morphology of multi-walled carbon nanotubes (MWCNTs).
In this paper, the effects of the H2 concentration on
the growth rate of CNFs, the rate of catalyst deactivation, and the morphology of CNFs are investigated.
The results would be useful for efficient production and
precise control of the morphology of CNFs.
EXPERIMENTAL
Catalyst and CNFs characterization
Fe3 O4 (purity >99.8%, particle size 20–30 nm) catalyst was purchased from Alfa Aesar Corp. and used
without further treatment. X-ray diffraction (XRD) analysis was performed on Rigaku D/Max 2550 VB/PC
diffractometer equipped with a Cu Kα radiation source
(λ = 0.154056 nm), which was operated at a voltage of
40 kV and a current of 100 mA. To determine the purity
of the Fe3 O4 catalyst, temperature thermogravimetry
(TG) was carried out on SDT Q600 (TA Corporation,
USA). The sample was heated up to 900 ◦ C and reduced
in a H2 (20 vol.%)–Ar mixture. Temperature programmed reduction (TPR) experiments were conducted
Asia-Pacific Journal of Chemical Engineering
EFFECT OF HYDROGEN ON THE SYNTHESIS OF CARBON NANOFIBERS
on Micromeritics Autochem II 2920 (USA) using a H2
(10 vol.%)–Ar mixture (30 ml/min) at a heating rate of
10 ◦ C/min from 35 to 950 ◦ C. N2 adsorption was carried
out using Micromeritics ASAP 2010 (USA). Scanning
Electronic Microscope (SEM) pictures were taken on a
JOEL JSM3360LV electron microscope and Transmission Electron Microscopy (TEM) pictures taken on a
JEOL JEM 2010 electron microscope.
of H2 on CO disproportionation. In each experiment,
0.15 g of Fe3 O4 catalysts was used. The temperature
of the sample was first raised from room temperature
to 600 ◦ C at 10 ◦ C/min in 50 ml/min Ar atmosphere,
and then 10% H2 –90% Ar mixing gas (50 ml/min)
was introduced to the sample tube to reduce Fe3 O4 for
30 min. Then the gas flow was changed to Ar, and the
temperature was raised to 620 ◦ C to sweep the adsorptive H2 . After sweeping by Ar (50 ml/min) for 20 min,
the temperature was decreased to 600 ◦ C, and the gas
flow was changed to 100% CO or 80% CO–20% H2 to
adsorb CO or the mixing gas of CO–H2 . After 5 min,
the gas flow was changed to Ar and the temperature
was decreased to 40 ◦ C. Finally, the temperature was
raised to 1000 ◦ C in Ar flow (50 ml/min) and the offgas was monitored continuously by an online quadruple
mass spectrometer (Questor, ABB Corp, USA).
CNFs growth
The CNFs were grown in a quartz tube reactor (8 mm
internal diameter). In each experiment, 5 mg of Fe3 O4
catalyst was loaded on a quartz-sintered plate fixed in
the middle of the reactor. The catalyst was reduced
in situ in a H2 (20 mol%)–Ar mixture (with a total
flow rate 50 ml/min) at 600 ◦ C for 30 min. Then, a
gas mixture of CO, H2 , and balance Ar was introduced
to the reactor at a total flow rate of 120 ml/min. CO,
CO2 , H2 , and CH4 concentrations of the effluent were
monitored continuously by an online quadruple mass
spectrometer (Questor, ABB Corp, USA) at a sampling
rate of 0.1 s. The growth of CNFs was stopped, when
the concentration of CO2 in the off-gas became less
than 0.5%. Because no methane was found in the offgas, the production rate of CNFs was calculated by
the CO2 production rate according to the stoichiometry
of CO disproportionation. After each experiment, the
produced CNFs were discharged and weighted. The
results indicated that the weight of CNFs was consistent
with the result calculated from the integration of CO2
concentration curve.
RESULTS AND DISCUSSION
Catalyst characterization
Figure 1 shows the XRD pattern of the catalyst, which
indicates that the main composition was Fe3 O4 . The
existence of Fe2 O3 in the catalyst was shown by TPR
result (not shown here), and the proportion of Fe2 O3 in
the catalyst was calculated as 1.73% according to TG
analysis in which the catalyst was thoroughly reduced
and the amount of the residue (pure Fe) was found
to be 72%. The BET surface area and pore volume
of the Fe3 O4 catalyst were 40.9 m2 /g and 13.2 cm3 /g,
respectively.
TPD-MS results
Experiments were also carried out on AUTOCHEM
(Micromeritic Corporation, USA) to study the effect
Figure 2 shows the TPD-MS results of the Fe3 O4
catalyst that was treated in 100% CO or 80% CO–20%
Fe3O4(440)
Fe3O4(531)
Fe3O4(442)
Fe3O4(620)
Fe3O4(533)
Fe3O4(622)
Fe3O4(444)
Fe3O4(511)
Fe3O4(422)
100
Fe3O4(331)
200
Fe3O4(222)
300
Fe3O4(220)
400
Fe3O4(111)
Intensity(Counts)
500
Fe3O4(400)
Fe3O4(311)
TPD-MS analysis
0
10
20
30
40
50
60
70
80
2-Theta( ° )
Figure 1. XRD pattern of the Fe3 O4 catalyst.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
DOI: 10.1002/apj
591
W. LU ET AL.
Asia-Pacific Journal of Chemical Engineering
0.03
1.6
A1
100% CO
80% CO-20% H2
0.02
A4
B1
0.01
A2 B2
A3
B3
B4
r(mmol/s • g cat)
CO2 Concentration/ %
592
0% H2
4.17% H2
8.33% H2
12.50% H2
16.67% H2
20.83% H2
25.00% H2
29.17% H2
32.50% H2
1.2
0.8
0.4
0.00
200
400
600
Temperature/ °C
800
1000
0.0
0
40
80
120
Time/min
Figure 2. TPD results of the Fe3 O4 catalyst treated with
100% CO or 80% CO–20% H2 mixing at 600 ◦ C. This figure
is available in colour online at www.apjChemEng.com.
H2 at 600 ◦ C. The position and area of each desorption
peak are listed in Table 1. The CO2 concentration peak
A1 is 41 ◦ C earlier than B1 and A2 is 6 ◦ C earlier
than B2. In addition, the area of A1 is much larger
than B1. These results indicate that the presence of H2
can lower the CO dissociation energy and promote the
disproportionation of CO on Fe surface. There may be
another two reasons, which can lead to these results,
one is that in the presence of H2 , the surface is rich
in the deposited carbon (big area in Fig. 3) reducing
the interaction with adsorbed CO2 molecules and the
other one is the adsorption sites (probably defective) on
the surface are saturated by hydrogen or hydrocarbon
species and thus exclude the bonding with the produced
CO2 .
Effect of H2 concentration on CNFs growth
The influences of H2 on the growth of CNFs are shown
in Fig. 3. After a short induction period, the growth
rate reaches its maximum. The maximum growth rate
increases when H2 concentration is increased from
0 to 25% but decreases when H2 concentration is
further increased. This can be explained by the lowered
CO dissociation energy and the competing adsorption
between the CO and the H2 . In the absence of H2 , the
growth rate of CNFs is very low, whereas addition of
Time-depending CNFs growth rate under
different H2 concentrations (vol.%) at 600 ◦ C, CO% =
66.67%. This figure is available in colour online at
www.apjChemEng.com.
Figure 3.
4.17% H2 in the feed makes the maximum CNF growth
rate increase from 0.08 to 0.81 mmol/(s· gcat).
It is interesting to note that the Fe3 O4 catalysts are
easier to be deactivated at higher H2 concentration.
This is different from the Cu–Ni alloys for benzene
decomposition that have a longer life at higher H2
concentration.[8] It was suggested that H2 could help
hydrogasification of the surface carbon species so as
to prolong the catalyst life. However, on the Fe catalysts, it is less possible as evidenced by the absence
of methane formation during CNFs growth. The difficulty of hydrogasification on Fe catalysts was probably
due to the higher binding energy of Fe–C, which is
119.8 Kcal/mol, higher than that of Cu/Ni–C between
72.0 and 102.6 Kcal/mol.
For catalytic growth of CNFs, it is generally accepted
that the formation of carbon filament involves three
steps: Carbon atoms deposition from the gas phase
on gas/metal interface of a catalyst particle, diffusion
through the crystal lattice of the catalyst, and precipitation at the metal/carbon interface side of the catalyst
particle. The diffusion of carbon atoms through the crystal lattice is generally considered as the rate-determining
step. Adding H2 will lower the CO chemisorption
energy and therefore increase the rate of CO dissociation. However, the rate of carbon species diffusion,
which is the rate-limiting step of CNF formation, is
Table 1. Peak position and peak area of each CO2 concentration curve.
Peak of CO2 concentration curve
(100% CO curve)
Peak of CO2 concentration curve
(80% CO–20% H2 )
Peak position (◦ C)
Peak area
A1
A2
A3
A4
B1
B2
B3
B4
435
12.33
606
3.00
800
6.94
935
8.78
476
4.83
612
5.08
800
4.88
935
8.48
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF HYDROGEN ON THE SYNTHESIS OF CARBON NANOFIBERS
A1 H2%=0%
A2 H2%=0%
B1 H2%=4.17%
B2 H2%=4.17%
C1 H2%=8.33%
C2 H2%=8.33%
D1 H2%=12.50%
D2 H2%=12.50%
Figure 4a. SEM and TEM pictures of CNFs synthesized at different
H2 concentrations (CO% = 66.67%, 873 K), C2 and G2 are TEM
pictures, the others are SEM pictures.
unchanged. As a result, the carbon species produced by
CO dissociation will accumulate on the catalyst surface
and cause catalyst deactivation. That is why the life of
the catalyst becomes shorter as the H2 concentration is
increased.
Increasing H2 concentration will also decrease the
induction period, as seen from Fig. 3. When the H2
concentration is increased from 4.17 to 25%, the
induction period is decreased from 30 to 10 min.
Because the induction process is correlated with the
fragment rate of catalyst particle,[13] this phenomenon
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
implies that the fragment rate of catalyst particle
is different at different H2 concentrations. It is also
reported that H2 will change the size of catalyst
particles,[11] and smaller the particle size, the faster
the catalyst deactivation.[14] Therefore, one can assume
that the size of the fragmented catalyst particles is
related to the fragment rate. Increasing H2 concentration
will increase the fragment rate of catalyst particle
and produce smaller fragmented particles, which have
a shorter induction period but a faster deactivation
rate.
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
DOI: 10.1002/apj
593
594
W. LU ET AL.
Asia-Pacific Journal of Chemical Engineering
E1
H2%=20.83%
E2
H2%=20.83%
F1
H2%=25.00%
F2
H2%=25.00%
G1
H2%=29.17%
G2
H2%=29.17%
Figure 4b. (Continued).
Effect of H2 concentration on the CNFs
morphology
The morphologies of the CNFs grown at different H2
concentrations were observed by SEM and TEM, which
are shown in Fig. 4. The CNFs (A1–A2) are twisted
and entangled if there is no H2 in the feed. When
H2 concentration is at 4.17 and 8.33% (B1–C2), the
CNFs are still twisted but less entangled when H2
concentration is increased. The CNFs become helical
(D1 and D2) as the H2 concentration is increased to
12.50%. When the H2 concentration is increased to
20.83 and 25%, both straight and tight helical (E1–F2)
CNFs appear. The fibers contain more tight helical ones
at H2 % = 20.83% and more straight ones at H2 % =
25%. As the H2 concentration is further increased
to 29.17%, most of the CNFs are straight (G1 and
G2). Generally, the morphology of CNFs changes from
twisty to helical, then to straight as H2 concentration
is increased. The different morphologies of the CNFs
are the combining effect of catalyst restructuring in H2
and competing adsorption between H2 and CO. The
former leads to different particle sizes, morphologies,
and surface structures, whereas the later changes the
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
rate of CO dissociation. All of these change the rate of
carbon decomposition and the mechanism and rate of
carbon fiber formation and thus producing CNFs with
different morphologies.
CONCLUSION
In this paper, the effects of H2 on the rate of CO
disproportionation over ultrafine Fe3 O4 catalyst and
the morphologies of the formed CNFs were investigated by continuously monitoring the time-depending
CNFs growth rate and inspecting the as-grown CNFs
with SEM and TEM. TPD analysis was performed
on the catalyst in an atmosphere with or without H2
to show the effect of H2 on the rate of CO dissociation. At higher H2 concentration, the CNFs grow
faster at the beginning but the rate drops more rapidly
because of the catalyst deactivation. H2 increases the
rate of carbon deposition by lowering CO dissociation
energy and thus increases the rate of catalyst reconstruction in the induction period and increases the growth
rate of CNFs at the beginning. However, if the H2
concentration is higher than 25%, the rate of carbon
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECT OF HYDROGEN ON THE SYNTHESIS OF CARBON NANOFIBERS
deposition will be lowered because of the competing
adsorption between H2 and CO. On the Fe catalyst,
hydrogasification of deposited carbon does not happen, as a result increasing the H2 concentration will
not increase the catalyst life. In contrast, because the
rate of carbon deposition is increased and the rate of
carbon diffusion through the Fe crystal lattice is less
influenced by the presence of hydrogen, more encapsulating carbons are formed, leading to faster catalyst deactivation. By increasing the H2 concentration,
the morphology of the as-synthesized CNFs changes
from twisty to helical then to straight and becomes
less entangled, which is the result of hydrogen-assisted
or hydrogen-induced catalyst reconstruction and CO
dissociation.
Acknowledgements
This work is supported by NSFC (No. 20490200), the
MOE of China (No. IRT0721), and the SAFEA of China
(No. B08021).
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
REFERENCES
[1] N.M. Rodriguez. J. Mater. Res., 1993; 8, 3233–3250.
[2] K.P. De Jong, J.W. Geus. Catal. Rev. Sci. Eng., 2000; 42,
481–510.
[3] J.H. Zhou, Z.J. Sui, P. Li, D. Chen, Y.C. Dai, W.K. Yuan.
Carbon, 2006; 44, 3255–3262.
[4] A. Koganemaru, Y.Z. Bin, H. Tohora, F. Okino, S. Komiyama,
J. Zhu, M. Matsuo. Asia-Pac. J. Chem. Eng, 2008; 3, 521–526.
[5] M.S. Saha, Y.G. Chen, R.Y. Li, X.L. Sun. Asia-Pac. J. Chem.
Eng, 2009; 4, 12–16.
[6] P.L. Walker Jr., J.F. Rakszawski, G.R. Imperial. J. Phys.
Chem., 1959; 63, 140–149.
[7] K.L. Yang, R.T. Yang. Carbon, 1986; 24, 687–693.
[8] Y. Nishiyama, Y. Tamai. J. Catal., 1976; 45, 1–5.
[9] M.T. Tavares, I. Alstrup, C.A. Bernardo, J.R. Rostrup-Nielsen.
J. Catal., 1996; 158, 402–410.
[10] M.S. Kim, N.M.R. Rodriguez, R.T.K. Baker. J. Catal, 1991;
131, 60–73.
[11] L.J. Ci, J.Q. Wei, B.Q. Wei, J. Liang, C.L. Xu, D.H. Wu.
Carbon, 2001; 39, 329–335.
[12] W. Wasel, K. Kuwana, P.T.A. Reilly, K. Saito. Carbon, 2007;
45, 833–838.
[13] M. Audier, M. Coulon, L. Bonnetain. Carbon, 1983; 21,
99–103.
[14] D. Chen, K.O. Christensen, E. Ochoa-Fernandez, Z.X. Yu,
B. Tøtdal, N. Latorre, A. Monzón, A. Holmen. J. Catal, 2004;
229, 82–96.
Asia-Pac. J. Chem. Eng. 2009; 4: 590–595
DOI: 10.1002/apj
595
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