CARBON
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The beneficial effect of CO2 in the low temperature synthesis
of high quality carbon nanofibers and thin multiwalled
carbon nanotubes from CH4 over Ni catalysts
Steven Corthals a, Jasper Van Noyen a,1, Jan Geboers a, Tom Vosch c,2, Duoduo Liang b,
Xiaoxing Ke b, Johan Hofkens c, Gustaaf Van Tendeloo b, Pierre Jacobs a, Bert Sels a,*
a
b
c
K.U. Leuven, Centre for Surface Chemistry and Catalysis, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium
Universiteit Antwerpen, Electron Microscopy for Materials Science (EMAT), Groenenborgerlaan 171, 2020 Antwerpen, Belgium
K.U. Leuven, Molecular and Nanomaterials, Celestijnenlaan 200f – Bus 2404, 3001 Heverlee, Belgium
A R T I C L E I N F O
A B S T R A C T
Article history:
A low temperature chemical vapor deposition method is described for converting CH4 into
Received 13 May 2011
high-quality carbon nanofibers (CNFs) using a Ni catalyst supported on either spinel or
Accepted 22 August 2011
perovskite oxides in the presence of CO2. The addition of CO2 has a significant influence
Available online 30 August 2011
on CNF purity and stability, while the CNF diameter distribution is significantly narrowed.
Ultimately, the addition of CO2 changes the CNF structure from fishbone fibers to thin multiwalled carbon nanotubes. A new ‘‘in situ’’ cooling principle taking into account dry
reforming chemistry and thermodynamics is introduced to account for the structural
effects of CO2.
2011 Elsevier Ltd. All rights reserved.
1.
Introduction
In the last few decades, carbon nanofibers (CNFs) have attracted great attention because of their interesting chemical,
mechanical and electrical properties [1]. Their high electrical
conductivity, high surface area and high mechanical strength,
chemical inertness and purity make CNFs promising materials for hydrogen storage [2], selective adsorption/absorption
[3], composite materials [4], field emission devices [5], sensors
and probes [6] and catalysis [7–10].
For most applications highly pure CNFs are required. Up to
now arc discharge [11], laser ablation [12] and chemical vapor
deposition (CVD) [13] are the most investigated methods to
synthesize CNFs. Catalytic CVD consists of the decomposition
of a carbon-containing source over a supported catalyst.
Whereas arc discharge and laser ablation yield high-quality
CNFs, the methods are facing scale-up difficulties. While
CVD is considered a promising route for large-scale, low-cost
synthesis of carbon nanofibers [1,14–16], it is not obvious
obtaining high carbon yields of CNFs with high purity
[14–18]. Compared to laser ablation and arc discharge, CNFs
obtained by CVD show higher carbon yields, but unavoidably
also a higher defect density due to the low synthesis temperatures [18]. Therefore, impurities should be removed from the
fibers during a post-synthesis step. As purification procedures
may affect the structure of nanofibers [19], much effort is still
put into the development of a direct low temperature synthesis route of high-quality CNFs.
By co-feeding oxidizing agents, which remove in situ the
impurities without damaging the CNFs, this goal may be
achieved. H2O and O2 are known to have a beneficial effect
on nanofiber purity as oxidizing agents due to the continuous
* Corresponding author: Fax: +32 16 32 1998.
E-mail address: [email protected] (B. Sels).
1
Present address: Flemish Institute for Technological Research (VITO), Boeretang 200, 2400 Mol, Belgium.
2
Present address: Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen,
Denmark.
0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2011.08.047
CARBON
5 0 (2 0 1 2) 3 7 2–38 4
removal of amorphous carbon at high temperature [20,21]. In
addition, it is reported that both oxidizing agents facilitate
nanofiber growth. CO2 shows great potential as oxidizing
agent for improving the quality of CNF synthesis [22–24] and
enables to catalytically decompose amorphous carbon by
the reverse Boudouard reaction (1). The occurrence of noncatalyzed gradual removal of C-layers from multiwalled carbon nanotubes (MWCNTs) has been demonstrated with CO2
at temperatures around 850 C [24].
CO2ðgÞ þ CðsÞ $ 2COðgÞ
DH298 ¼ 172 kJ=mol
CH4ðgÞ þ CO2ðgÞ $ 2COðgÞ þ 2H2ðgÞ
CO2ðgÞ þ H2ðgÞ $ H2 OðgÞ þ COðgÞ
CH4ðgÞ $ 2H2ðgÞ þ CðsÞ
DH298 ¼ 247 kJ=mol
DH298 ¼ 41 kJ=mol
DH298 ¼ 75 kJ=mol
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Numerous high purity (>99.9%) carbon sources (such as CO,
CH4, C2H6, C2H4, and C2H2) have been investigated in CVD research [1,25–33]. Compared to cheap sources like natural gas,
liquefied petroleum gas, coal and biogas, the aforementioned
pure gases are expensive [34].
Instead of co-feeding CO2 together with the primary carbon source for producing high-quality CNFs, in the present
work the conversion of natural gas and biogas is envisioned,
both containing CH4 and CO2, as cheap, abundantly available
and renewable feedstocks, into highly added-value carbon
structures with high carbon yields. Natural gas fields often
contain next to CH4 significant amount of CO2 [35]. Biogas,
abundantly available from various waste sources [36], is typically produced from biomass fermentation, its exact composition depending on the biomass source. It consists of CO2,
CH4, H2, N2 and trace impurities of NH3, H2S and halides.
H2S, an important impurity in both sources is, is expected
to poison Ni catalysts.
Ni catalyst supported on spinel-type MgAl2O4 or perovskite-type XTiO3 (X = Mg, Ca, Sr, Ba) oxides are demonstrated
here as promising candidates for the valorization of CH4/CO2
mixtures as feeds for high-quality CNFs, having H2 and CO as
valuable byproducts. Ni has been shown as one of the most
effective catalysts for CH4 decomposition [37], and is therefore the preferred element chosen in the present
contribution.
The selection of the spinel support (MgAl2O4) for the CH4/
CO2 CVD process was based on its high resistance to sintering, high mechanical strength and low acidity. It has been
studied extensively as catalyst support in environmental
catalysis, petroleum processing and fine chemicals production [48–51]. Though carbon deposition and growth mechanisms have been studied on MgAl2O4 supported Ni catalysts,
it was only exceptionally done with the aim to produce structured nanocarbon [48,52]. In addition to its high thermal stability, MgAl2O4 shows pronounced Lewis basicity affording
activation of CO2 [53,54]. Next to their catalytic properties
for partial oxidation [38], steam reforming [39] and dry
reforming (DR) [40] of CH4 and due to their excellent electrical
properties as field emitters [41], perovskite oxides recently
have been introduced as precursor materials in CNF synthesis
using pure methane [42–47]. Carbon yields up to 600 wt.%
have been reported. The purity of the tubes, not often documented in detail, seems to be low based on inspection of
the relative intensity of the G and D bands in Raman spectra,
373
showing an intensity ratio larger than unity (ID/IG > 1). Synthesis temperatures above 900 C are required to obtain fibers
with appropriate structural quality, viz. for which ID/IG is substantially smaller than one.
In the present study perovskites are used as support material, not as precursor. It is expected that their unique high
oxygen mobility and stabilization of unusual cation oxidation
states in the structure [55] can be very helpful in assisting the
removal of amorphous carbon and defects during CVD CNF
growth. Among the screened Ni catalysts supported on spinel
or perovskite oxides, NiSrTiO3 was the catalyst of choice
based on carbon yield and CNF structural purity. Finally, the
effect on nanofiber properties of the amount of Ni loaded
on the catalyst support, the reaction temperature and the
concentration of CO2 is determined. Most surprisingly, upon
diluting CH4 with CO2, high yields of thin MWCNTs are
formed at a synthesis temperature as low as 600 C. It is
shown that by this CVD procedure yield, stability, purity and
diameter (distribution) of the synthesized CNFs can be tuned.
A working hypothesis explaining the different influences on
the structural properties of the CNFs is advanced with the
introduction of the ‘‘in situ cooling effect’’ taking into account
dry reforming.
2.
Experimental
2.1.
Catalyst preparation
Ni catalysts supported on perovskite-type or spinel-type oxides were prepared by a slurry type impregnation procedure
using a high-throughput synthesis platform [56]. Twenty milliliters of an aqueous solution of Ni(NO3)2Æ6H2O was added to
0.5 g of the support. The slurry was stirred for 1 h at room
temperature and dried overnight at 85 C (unless stated otherwise). The solid residue was calcined at 800 C for 5 h using a
temperature rise of 3 C/min.
Perovskite-type oxides supports were synthesized according to the sol–gel method proposed by Pechini [57]. To an
aqueous mixture of metal (Mg, Ca, Sr or Ba) nitrate and titanium isopropoxide with appropriate stoichiometry, an excess
amount of citric acid was added (citric acid:metals = 2.5). To
the solution heated to 90 C, ethylene glycol was added, using
a citric acid/ethylene glycol molar ratio of 2/3. After gel formation, drying was done at 100 C for 48 h. The solids were
heated in air, at 200 C and 500 C each time for 2 h, and finally calcined at 800 C using a heating rate of 2.5 C/min.
The MgAl2O4 support was prepared as reported earlier [50].
The pH of a 0.4 M aqueous mixture of Mg(NO3)2Æ6H2O (Chem
Lab) and Al(NO3)3Æ9H2O (Riedel-de-Häen) was adjusted to 9.5
with an aqueous ammonia solution. The solution was stirred
for 1 h and stored overnight. The precipitate was washed with
water and dried for 15 h at 120 C. The resultant powder was
then calcined at 800 C for 5 h. XRD shows the typical reflection pattern of a spinel phase, while nitrogen physisorption
is indicative of a characteristic pore distribution for a mesoporous support, with a specific surface of 120 m2/g and pores
greater than 10 nm as shown in the supporting info (Figs. S1–
S3). Catalysts will be denoted by the following code:
Ni(wt.%)support, viz. Ni(30)MgTiO3.
374
2.2.
CARBON
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
CNF synthesis
Carbon nanofibers were synthesized at atmospheric pressure
in a conventional quartz tube oven with an internal diameter
of 30 mm, equipped with an internal thermocouple. Hundred
and fifty milligrams of the catalyst was placed in a ceramic
boat in the center of the tube close to the thermocouple.
Reduction was done with a mixture of He and H2 gas in a 5/
1 volumetric ratio, using a flow of 120 ml/min. After reduction
at reaction temperature for 1 h, the reactor system was
purged with He. Finally, carbon nanofibers were formed in a
mixture of CH4 and CO2 using a total gas flow rate of
100 ml/min for 3 h. The yield, Y, of synthesized fibers was calculated according to equation (a).
Y¼
ðWfin Win Þ
100 ½%
Win
ðaÞ
With Win and Wfin the sample weight before and after reaction, respectively. When the specific surface area (SA) of the
support material (m2/g) is taken into account the yield, YSA,
of the produced amount of fibers is calculated according to
equation (b).
YSA ¼
Y
½mmolC=m2 mol feed
SA
ðbÞ
with Y*, the molar CNF yield, in mmol C/(mol feed g). Yg, in
gC/(gcat h), can be used as a measure of the production rate
of the catalyst, while YC (in %) expresses the percentage mole
of solid C formed per mole of C fed. The latter definition assumes formation of CNF only from CH4.
2.3.
Characterization
Crystal structures of fresh and spent catalysts were determined by X-ray powder diffraction (XRD). XRD measurements
were performed at room temperature with a STOE transmission powder diffractometer system Stadi P Combi equipped
with a Cu Ka radiation of 154 pm and a Ka1 Germanium
Johann-type monochromator.
N2 physisorption measurements were conducted with a
Micromeritics Tristar Surface and Porosity Analyser, allowing
determination of the specific surface area of the spinel and
perovskite materials. For the XTiO3 supports SA values of 19,
10, 7 and 16 m2/g were obtained for Mg-, Ca- Sr- and Ba-titanate, respectively. The SA value of the spinel support
amounted to 120 m2/g.
Thermogravimetric analyses (TGA) of spent catalysts with
a TGA Q500 from TA Instruments were carried out using a gas
flow of 100 ml/min of O2 and N2 in a volumetric ratio of 9.
Tmax, defined as the temperature at which the first derivative
of the TGA weight loss curve is maximal (d00 W/dT = 0), is a
measure for the stability of the produced CNFs. The CNFs
were not purified from metal and support prior to the oxidation test. TGA analyses are illustrated in supporting info in
Figs. S4–S7.
Raman measurements were performed by putting the
samples on top of a cleaned glass cover slip on an Olympus
IX71 microscope. As excitation source a 633 nm wavelength
laser (35 mW) was used. A narrow bandpass filter centered
at 633 nm was used to spectrally clean the laser source. The
laser light was reflected in the microscope on a 633 nm dichroic mirror towards a 100· 1.3 NA Olympus objective
(immersion oil objective UplanFL N) that focused the laser
on the sample and collected the Raman signal. A 655 nm long
pass filter was used to block the 633 nm laser light in the
detection path. The Raman spectrum was recorded by using
a Chromex Inc. Model 250IS polychromator (600 lines per
mm blazed at 500 nm) and a liquid N2 cooled CCD camera
from Princeton Instruments Inc. (Model LN/CCD-512-TKB/1/
VISAR). Raman spectra showed two characteristic bands.
The band at 1360 cm1 is caused by defects and disorder in
the graphene-like structure of the fiber and is denoted as D
band. The G band at 1590 cm1 is associated with graphite
sheets of the CNFs. The intensity ratio of the bands, ID/IG,
gives a measure of the degree of crystallinity of the graphite
layers. Low ID/IG ratios correspond to high-quality CNFs with
only few structural defects.
SEM investigations were performed with a Phillips XL30
FEG microscope. Before analysis samples were coated with a
thin layer of gold. Based on SEM pictures, diameter size distributions of CNFs were determined. For each sample 50 CNF
diameters were measured.
Transmission electron spectroscopy (TEM) was conducted
with a Philips Tecnai 10 and a Jeol 4000EX microscope.
3.
Results and discussion
3.1.
CNF growth on MgAl2O4 supported Ni catalysts from
CH4 in presence and absence of CO2
The impact in CVD of biogas composition, presented as a mixture of pure CO2 and CH4, and compared to pure methane, on
CNF stability, purity and yield, is shown in Table 1 using a Ni
catalyst supported on MgAl2O4. Firstly, the presence of high
amounts of CO2 enhances the oxidative stability of the CNFs,
as evidenced by weight analysis experiments in O2-rich conditions. Tmax, the temperature of maximum weight loss in
TGA, shifts from 601 C to 550 C for CVD with equimolar
CH4/CO2 mixtures or pure CH4, respectively. When CO2 is
substituted for He, Tmax remains around 550 C, indicating
that dilution is not at the origin of this temperature shift.
Thus adding CO2 directly into the system provides CNFs with
a higher oxidative stability.
The oxidative stability is in line with a decreasing number
of defects in the CNFs, as measured with Raman spectroscopy. Typically, the intensity ratio ID/IG of two Raman signals
at 1360 (D band) and 1590 cm1 (G band) measures the degree
of crystallinity of the graphite layers, since decreasing ID/IG ratios correspond to lower fractions of sp3-like carbon and
hence less structural defects [58,59]. More specifically, the D
band at 1360 cm1 is caused by defects and disorder in the fiber structure, while the G band at 1590 cm1 is associated
with the graphene sheets of the CNFs [60]. The intensity ratio
ID/IG clearly decreases from 1.0 to 0.7 with the addition of CO2
(Table 1), thus pointing to a lower number of structural
defects.
CO2 co-fed to a CH4 CVD process also impacts the structural arrangement of the graphitic planes and the CNF diameters and their distribution. TEM pictures (Fig. 1) show a clear
CARBON
375
5 0 (2 0 1 2) 3 7 2–38 4
Table 1 – Effect of CH4/CO2 ratio on CNF yield, stability, purity and diameter for Ni(25)MgAl2O4.a
CH4/CO2
1
2
1
0.5
1b
a
b
c
d
Y (wt.%)c
552
272
229
84
224
YSA (mmol C/m2 mol feed)
Yg (gC/gcat h)
6.9
3.4
2.9
1.1
2.8
1.84
0.91
0.76
0.28
0.75
Tmax (C)d
550
552
601
609
558
ID/IG
1.01
0.86
0.75
0.73
0.86
Diameter (nm)
88 ± 22
74 ± 14
69 ± 14
55 ± 10
44 ± 12
Total flow of 100 ml/min at 600 C for 3 h; drying temperature of 70 C.
CH4/He.
Standard deviation of 20 wt.%.
Standard deviation of 5 C.
Fig. 1 – TEM picture of CNF synthesized by CH4 (a) or biogas (CH4/CO2 = 1) CVD (b) with Ni(25)MgAl2O4 at 600 C.
decline of a when the biogas mixture with CH4/CO2 ratio of 1,
is used compared to CH4. a is defined as the angle between the
graphitic planes and the fiber axis. The change in angle suggests a change in CNF growth mechanism upon adding CO2,
producing at the mild synthesis temperature of 600 C CNFs
with almost MWCNTs characteristics with a value for a of
0, instead of the fishbone-like carbon structure.
Addition of CO2 also results both in a decrease of CNF
diameters and a narrowing of the diameter distribution. The
average CNF diameter (Table 1) based on CNF diameter estimations from the corresponding SEM pictures, decreases
from 88 ± 22 nm for a CH4/CO2 ratio of 1 to 69 ± 14 and
55 ± 10 nm for a CH4/CO2 ratio of 1 and 0.5, respectively. These
results not only show that the average diameter of the CNFs
decreases upon addition of CO2 compared to a pure CH4 feed,
but that the diameter distribution narrows as well.
3.2.
CNF growth on NiXTiO3 catalysts in presence or
absence of CO2
In order to increase the yield of CNFs, Ni(30)XTiO3 perovskitetype oxide catalysts with X = Mg, Ca, Sr, or Ba were investigated. Carbon nanofibers were synthesized in a series of
CVD experiments at 600 C with CH4 or an equimolar mixture
of CH4 and CO2, using either Ni(30)XTiO3 perovskite-type
oxide catalysts with X = Mg, Ca, Sr, or Ba, or a Ni(30)MgAl2O4
reference spinel catalyst with SA of 120 m2/g.
Fig. 2 shows XRD patterns of as-synthesized Ni(30)XTiO3.
Next to the characteristic patterns of the crystalline phases
of the different perovskite structures [61–64], small reflections
assigned to NiO (o) are clearly visible at 2h values of 43.2,
37.3 and 62.7 [65]. Although the same amount of Ni is loaded
on each Ni(30)XTiO3 catalyst, a difference in peak areas can be
observed in the diffraction patterns of the catalysts. XRD peak
areas of equal amounts of catalyst weight decrease with the
molecular weight of X. With increasing molecular weight of
X, increased scattering of the X-rays occurs, resulting in a decreased intensity of the detector signal.
In the diffraction patterns of the catalysts used in carbon
nanofiber synthesis (Fig. 3), the typical perovskite phases
are preserved, pointing to their stability under the applied
CNF growth conditions. Peaks diagnostic for Ni (x) metal
phase, which serves as the catalyst for CNF growth, are visible
at 2h values of 44.6, 51.9 and 76 [65]. In addition, some NiO
is still present in the patterns, pointing to a partial reoxidation or incomplete reduction. As unchanged reflection patterns of the used catalysts were found after 6 months,
reoxidation of Ni in air after CNF synthesis seems less likely.
Also no difference is observed between the patterns of the
catalysts used for fiber growth with pure CH4 or a CH4/CO2
mixture. Reoxidation during CNF growth can be excluded,
as it would imply clearly different XRD patterns of the catalysts used with pure CH4 or CH4/CO2 because of the presence
in the latter case of a mild oxidant, viz. CO2, during fiber
376
CARBON
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
Fig. 2 – XRD patterns of Ni(30)MgTiO3 (a), Ni(30)CaTiO3 (b), Ni(30)SrTiO3 (c) and Ni(30)BaTiO3 (d) calcined at 600 C (d: MgTiO3
[61]; j: CaTiO3 [62]; : SrTiO3 [63]; m: BaTiO3 [64]; o: NiO [65]).
Fig. 3 – XRD patterns of used Ni(30)MgTiO3 (a), Ni(30)CaTiO3 (b), Ni(30)SrTiO3 (c) and Ni(30)BaTiO3 (d) (CH4/CO2 = 1, 600 C, 3 h);
d: MgTiO3; j: CaTiO3; : SrTiO3; m: BaTiO3; o: NiO; x: Ni; g: graphite.
growth. Probably not all NiO is reduced during the reduction
step. An additional peak, typical for crystalline graphite, appears at a 2h value of 26.3 in all diffraction patterns of used
catalysts.
The effect of composition of feed and perovskite support
on yield (Y, YSA, YC and Yg), stability (Tmax) and purity (ID/IG)
of the carbon nanofibers is shown in Table 2. Compared to reported catalysts in literature, the perovskite-type oxide supports show promising results with regard to CNF yield and
stability. The catalytic CH4 CVD procedure in this work yields
4.27 and 12.50 gC/gcat over Ni(30)SrTiO3 and Ni(30)CaTiO3,
respectively. These values, higher than the ones for CVD
growth with the Ni on MgAl2O4 catalyst, are comparable with
the highest yields for Ni-based catalysts recently reported by
Gallego et al. [47]. Reshetenko et al. reported very high carbon
yields of the order of 500 gC/gcat on NiCu(90)Al2O3 for CVD
with pure CH4 at a temperature of 625 C [66]. The very high
ID/IG ratios (>1) indicate that such CNFs are very rich in defects [29]. Toebes et al. investigated the methane decomposi-
tion over Ni(20)SiO2 catalysts at 550 C resulting in a carbon
yield, YSA, of 0.765 mmol C/(m2 mol feed) [67]. Takenaka et
al. reported values of 10.2 mmol C/(m2 mol feed) for the CH4
decomposition over Ni(40)SiO2 [68].
As shown in Table 2, TGA in O2 atmosphere and Raman
spectroscopy indicate that pure methane conversion over
the perovskite-like supported Ni catalysts also leads to an
improvement of the oxidative stability and the structural purity of CNFs, respectively. Indeed, Tmax is invariably higher for
CNFs produced by Ni on perovskite supports, SrTiO3 being
most preferred. In addition, the lower ID/IG ratios in the Raman spectra for carbon fibers obtained with perovskite-type
oxide also suggest a higher structural purity. Compared to
pure CH4, an equimolar CH4/CO2 mixture as carbon source
yields a somewhat lower amount of nanofibers, in line with
the CO2 effect for Ni on MgAl2O4. CNF yield reduction (YSA)
at constant CH4 contact time is also obvious for Ni perovskite
catalysts (Table 2). The lower yields are in agreement with
thermodynamics, as will be explained in Section 3.3.
CARBON
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Table 2 – Effect of support and feed composition on yield, stability and purity of the CNFs synthesized on Ni(30)MgAl2O4 and
Ni(30)XTiO3 (X = Mg, Ca, Sr, Ba).a
a
b
Y (wt.%)
YSA (mmol C/m2 mol feed)
YC (%)
Yg (gC/gcat h)
Tmax
b
Support
Feed
(C)
ID/IG
MgAl2O4
CH4
CH4/CO2
760
185
10.2
2.5
13
6
2.53
0.62
546
588
0.83
MgTiO3
CH4
CH4/CO2
824
434
70.0
36.9
14
15
2.75
1.45
564
579
0.66
0.67
CaTiO3
CH4
CH4/CO2
1246
293
201.1
47.3
21
10
4.15
0.98
572
596
0.62
0.65
SrTiO3
CH4
CH4/CO2
424
150
97.7
34.6
7
5
1.41
0.50
557
605
0.64
0.59
BaTiO3
CH4
CH4/CO2
161
135
16.2
13.6
3
5
0.54
0.45
556
584
0.79
0.56
600 C, total flow rate of 100 ml/min, reaction time of 3 h, atmospheric pressure, catalyst drying temperature of 85 C.
Standard deviation of 5 C.
Also in presence of CO2, higher values for YSA are obtained
with Ni on perovskite catalysts compared to the reference
MgAl2O4 support. The highest yield is obtained with NiCaTiO3,
corresponding to 12.5 gC/g within 3 h. The use of CH4/CO2
mixtures has a clear beneficial effect on the oxidative stability
of CNFs. As shown in Table 2, values for Tmax increase when
CO2 was added during CNF growth. For CNFs synthesized on
NiSrTiO3, a shift of Tmax from 557 to 605 C is seen for CH4
and CH4/CO2 CVD, respectively. Possibly, the significant increase of Tmax for (unpurified) CNFs synthesized on NiSrTiO3,
is related to the high oxygen mobility in the support [55]. As
explained below, oxygen mobility facilitates the beneficial impact of CO2 on the nanofiber (structural) properties.
The presence of CO2 during CNF synthesis not only improves the oxidative stability, but also the structural purity
of synthesized CNFs. Raman spectroscopy shows a decrease
of the ID/IG intensity ratios from 0.64 to 0.59 and 0.79 to 0.56,
for NiSrTiO3 and NiBaTiO3 catalysts, respectively, when
substituting CH4/CO2 for CH4. This increase in fiber purity
seems to be related to the adsorption enthalpy of CO2 on
the support, as the order in purity follows the basicity of the
perovskites. The latter according to XPS measurements (Table
3) increases in the following sequence: MgTiO3, CaTiO3 < SrTiO3, BaTiO3 [69]. A tentative explanation of the support effect
will be discussed below.
A detailed TEM investigation of the nanofibers provides an
explanation for the improved oxidative stability and structural purity of the CNF samples synthesized on Ni on perovskite catalysts. The graphite sheets of the fishbone CNF,
prepared by pure CH4 CVD, make a sharp angle, a, of 25 with
the axis along the fiber (Fig. 4a). Almost exclusive formation
Table 3 – Binding energies (BE) of the O1s signal for different
XTiO3 perovskite materials using XPS (after [69]).
BE O1s (eV)
MgTiO3
CaTiO3
SrTiO3
BaTiO3
530.1
530.5
529.6
529.6
of MWCNT, essentially with a = 0, was obtained over the Nicontaining perovskites, using equimolar mixtures of CO2
and CH4. It is shown in Fig. 4b, that MWCNTs, synthesized
on Ni(30)MgTiO3 are very thin with only a few graphene walls.
The presence of such MWCNTs explains the higher purity and
oxidative stability of the carbon material. Their formation
also accords with the slightly lower density of the tube-like
material. In the same conditions, formation of such MWCNTs
was not observed for the spinel-like materials.
Because CNFs synthesized on NiSrTiO3 by CH4/CO2 CVD
demonstrate satisfactory yield, high purity and the highest
oxidative stability, NiSrTiO3 is selected as model catalyst to
further explore CNF synthesis. The effect of the amount of
Ni on the SrTiO3 support on CNF yield prepared by CH4/CO2
CVD is demonstrated in Fig. 5.
With increasing Ni loading a higher yield of carbon fibers is
obtained, with a maximum value for Ni(30)SrTiO3 (Fig. 5). This
maximum is obviously associated with the specific surface
area of the support used. This phenomenon was not further
investigated.
The Ni amount was slightly affecting the purity of the fibers, as evident from the ID/IG ratios that are slightly increasing from 0.66 for Ni(10)SrTiO3 to 0.70 for Ni(50)SrTiO3, leaving
the high oxidative stability of the formed fibers unchanged.
The influence of reaction temperature and CO2 concentration on CNF properties for Ni/SrTiO3 is shown in Table 4. CH4/
CO2 CVD experiments performed with Ni(30)SrTiO3 at temperatures ranging from 500 to 700 C, show an optimal yield at
600 C. A decrease of CNF yield with increasing temperature
between 600 and 800 C contradicts with most other reports
on CNF growth in absence of CO2 over Ni catalysts [47].
Stability and purity of the fibers increase with rising reaction temperatures (Table 4). Increasing Tmax values, associated with enhanced oxidative stabilities, are in line with the
high purity of the CNFs as evidenced by the decreasing ID/IG
ratios observed using Raman spectroscopy.
Thus, at a reaction temperature of 700 C with Ni-containing SrTiO3 catalysts, not only moderate CNF yields of 2 gC/g
catalyst within 3 h period were obtained from CH4 in presence
of CO2, but also compared to literature high purity MWCNTs
as evidenced by the very low ID/IG ratio of 0.29 in the Raman
378
CARBON
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
Fig. 4 – TEM pictures of a fishbone CNF synthesized on Ni(30)SrTiO3 by CH4 CVD (a) and a MWCNT synthesized on
Ni(30)MgTiO3 by CH4/CO2 CVD (b) (T = 600 C, total flow rate = 100 ml/min, reaction time = 3 h, P = 1 atm).
Yg (gC/gcat.h)
2.00
1.50
1.00
0.50
0.00
10
20
30
40
50
Ni (wt%)
Fig. 5 – Effect of the amount of Ni on SrTiO3 on CNF yield
(600 C; CH4/CO2 = 1; total flow rate = 100 ml/min; reaction
time = 3 h; P = 1 atm; drying temperature = 100 C).
spectra. With arc discharge typical ID/IG ratios of 0.45 are obtained, while with catalytic methods these values range from
0.8 to 3.5 [47,70].
The purity of the MWCNTs is also illustrated by the TEM
pictures in Fig. 6.
The influence of CO2 present in the feed is studied on
Ni(30)SrTiO3. Just as for fibers grown on NiMgAl2O4 catalysts
(Table 1), lower amounts of CO2 in the feed result in higher
yields and decreasing nanofiber stability. For a molar CH4/
CO2 feed ratio of 2, the yield, Y, amounts to 1116% compared
to 573% for an equimolar feed ratio (Table 5). The stability of
the fibers increases when the feed is changed from pure
CH4 to an equimolar CH4/CO2 mixture, as indicated by the
shift of Tmax values from 580 to 597 C. It is important to note
that, as in case of the NiMgAl2O4 catalysts, the beneficial effect on fiber oxidative stability is not caused by a CH4 dilution
effect, but by a CO2 effect.
Next to the positive effect on stability and purity, the presence of CO2 in the CVD process has an advantage on the
diameter (distribution) of the nanofibers, as illustrated in
Fig. 7. Distribution plots based on SEM pictures, are shown
for CNFs synthesized on Ni(30)SrTiO3 by CH4 or CH4/CO2
CVD. The distribution peaks at smaller diameters and becomes narrower with added CO2, allowing tuning of the CNF
diameter distribution by adjusting the CH4/CO2 ratio in the
synthesis feed mixture.
3.3.
Mechanistic proposal for CNF growth under CO2 rich
conditions
As demonstrated for Ni catalysts supported on MgAl2O4 and
XTiO3, CO2 has an effect on the structural properties of the
produced CNFs. In presence of CO2, CNFs with higher purity,
oxidative stability and a smaller fiber diameter size (distribution) are synthesized, while the structure changes from fishbone CNFs into MWCNTs in case of XTiO3.
In the generally accepted top-growth mechanism (Fig. 8)
for CNF synthesis by CH4 CVD, one side of the Ni particle
serves as CH4 adsorption site while its rear end functions as
decomposition site. The decomposition of CH4 results in the
gradual formation of surface carbon atoms with concomitant
desorption of hydrogen molecules [1]. The carbon atoms dissolve into the nickel particle and diffuse either through the
Table 4 – Effect of reaction temperature on CNF properties synthesized on Ni(30)SrTiO3.a
a
b
c
Temperature (C)
Y (%)b
500
600
700
390
573
190
Yg (gC/gcat h)
1.3
1.91
0.63
Tmax (C)c
538
597
623
CH4/CO2 = 1, total flow rate of 100 ml/min, reaction time of 3 h, atmospheric pressure, catalyst drying temperature of 100 C.
Standard deviation of 20%.
Standard deviation of 5 C.
ID/IG
0.82
0.69
0.29
CARBON
379
5 0 (2 0 1 2) 3 7 2–38 4
Fig. 6 – TEM pictures of MWCNTs synthesized on Ni(30)SrTiO3 by CH4/CO2 CVD (T = 700 C, total flow rate = 100 ml/min,
reaction time = 3 h, P = 1 atm).
Table 5 – Effect of CO2 concentration on CNF properties synthesized on Ni(30)SrTiO3 at 600 C.a
a
b
c
Feed
Y (%)b
CH4
CH4/CO2 = 2
CH4/CO2 = 1
CH4/He = 1
1044
1116
573
552
Tmax (C)c
Yg (gC/gcat h)
3.84
3.72
1.91
1.84
580
538
597
576
Total flow rate of 100 ml/min, reaction time of 3 h, atmospheric pressure, drying temperature of 100 C.
Standard deviation of 20%.
Standard deviation of 5 C.
bulk of the metal or along the metal surface. It is generally accepted that carbon diffusion is the rate limiting step during
CNF growth, since the activation energy for carbon diffusion
is in reasonable agreement with the energy needed for CNF
formation, as shown by experimental and theoretical approaches [52,71–73]. The carbon atoms precipitate as graphite
at the backside of the nickel particle, leading to top grown
CNFs. The occurrence of this top-growth mechanism presently is evidenced by TEM analysis (Figs. 1 and 4). By co-feeding CO2 this top-growth mechanism remains valid. Probably,
the cleaning and cooling ability of CO2 affects CNF yield, stability, purity, fiber type and the shift from fishbone fiber towards MWCNT.
(a)18
Both the effect of the temperature and presence of CO2 on
the yield of CNFs, as well as their oxidative stability and purity
can be explained by dry reforming (DR) thermodynamics [74–
76], taking into account reactions (1)–(4), viz. the reverse Boudouard reaction (1), the DR reaction (2), the reverse water gas
shift reaction (3) and the CH4 decomposition reaction (4). To
support the occurrence of these reactions, a CH4/CO2 CVD
experiment (comparable to that mentioned in Section 2.2)
was done in a TGA system equipped with online mass spectroscopy (MS), allowing analysis of the outlet gases during
the carbon nanofibers growth process. As shown in Fig. 9, during the initial phase of the reaction, CO, H2O and H2 are
formed simultaneously upon CNF growth, the latter being
(b) 20
16
14
15
Counts
Counts
12
10
8
10
6
5
4
2
0
0
20
40
60
Diameter (nm)
80
100
20
40
60
Diameter (nm)
80
100
Fig. 7 – Effect of CO2 on the diameter(distribution) of CNFs synthesized by CH4 CVD (a) or CH4/CO2 CVD (b) on Ni(30)SrTiO3
(T = 600 C, total flow rate = 100 ml/min, reaction time = 3 h, P = 1 atm, drying temperature = 100 C).
380
CARBON
CH4
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
CO2
ΔG (kJ)
c
Ni
b
CO2
CO2
a
Fig. 10 – Calculated DG values against temperature. j, DR
reaction; , reverse Boudouard reaction; m, methane
decomposition reaction; d, reverse water gas shift reaction.
Support
Fig. 8 – Schematic representation of the CNF top-growth
mechanism on supported Ni catalysts with CH4/CO2 CVD, by
CO2 cleaning via support (a), CNF (b) or Ni particle (c).
Intensity
t (a.u.)
monitored by the increase of weight during the TGA analysis.
Similar results are obtained for NiMgAl2O4 catalysts (not
shown).
These results support the thermodynamic calculations
presented in Fig. 10. DG values of the four reactions are
slightly positive, approaching equilibrium around 600 C.
The graph predicts the lower CNF yields obtained at lower
temperatures, because the endothermic CH4 decomposition
(4) is not favored in such conditions. The graph also explains
why the expected increase in CNF yield is not observed at
higher temperatures in CO2 rich conditions. At 700 C not only
the CH4 decomposition is favored, but also the reverse Boudouard reaction (1), i.e. the removal of carbon deposits under
influence of CO2, gains importance, the balance (kinetics) between both reactions determining the final CNF yield
eventually.
The importance of the reverse Boudouard reaction (1) increases with higher CO2 concentrations, resulting in a lower
yield of CNFs, in agreement with the data in Tables 1 and 5.
Compared to pure CH4 CVD, CO2 induces a competitive reaction with the carbon species adsorbed on Ni, reducing the
0
500
1000
Time (a.u.)
Temperature (°C)
1500
Fig. 9 – MS analysis of gas exhaust (H2 - - -, CO: —, H2O: — •)
during CNF growth on Ni(30)SrTiO3 using an equimolar CH4/
CO2 mixture at 600 C.
amount of carbon atoms available for carbon nanofiber
growth. The availability of high amounts of CO2, provides
more oxidative power, shifting the system towards conditions
where carbon deposition is thermodynamically avoided. The
equilibrium between carbon deposition (4) and carbon gasification (1) reactions provides H2/CO, diminishing the total
amount of carbon deposited. As less structured amorphous
carbon deposits are obviously oxidized more easily, more pure
CNFs are synthesized, yielding high structural purity and
oxidative stability, in agreement with the Raman and TGA
measurements presented. A temperature increase provides
an effect on the purity and stability, similar to that of elevated
CO2 concentration. At higher temperatures the catalytic system is in conditions where the reverse Boudouard reaction
(1) gains in importance, resulting into more stable and pure fibers as shown in Table 4.
The occurrence of a cleaning effect implies the following
events (Fig. 8): (i) removal of amorphous carbon at the Ni surface (routes a and c) simultaneously with CNF synthesis, or
(ii); removal of amorphous carbon from the fiber structure
(routes a–c), after fiber growth. The etching routes of the fibers
after CNF growth, either non-catalytically or catalytically by
the activation of CO2 at the Ni surface or support, can be ruled
out for the following reasons. Activation of CO2 is feasible
both on the support (a) and the Ni surface (c). If CO2 is activated on the support by Lewis basic sites, the reactive oxygen
atoms have to be transported via the support towards the fibers, since on Ni catalysts the fibers grow according to a
top-growth mechanism. Firstly, the direct non-catalytic etching of the fibers (route b), as reported by Tsang et al. at 850 C
[24], can be eliminated in milder synthesis conditions, because no weight loss is observed in TGA when at 600 C a metal-free amorphous carbon source is contacted with CO2
[77,78]. In addition, this potential cleaning route would lead
to a fiber surface rougher than shown in TEM pictures (Figs.
1, 4 and 6).
As a result of the well-known shuttling capacity of Ni for
both oxygen and hydrogen, a dynamic cleaning of the CNFs
after growth could occur [79]. In this scenario, Ni continuously
shuttles surface oxygen species to the fibers (route c), removing amorphous carbon species from the fiber surface as CO
molecules. In addition, through CO2 activation on the support
(route a), O species very mobile on carbon surfaces can also be
CARBON
5 0 (2 0 1 2) 3 7 2–38 4
delivered to the fiber surface. However, catalytic etching
would yield a rough carbon surface of the CNFs, which is
not observed.
Cleaning is assumed to occur simultaneously with fiber
synthesis on the nickel surface. CO2 molecules will be activated, while the active surface oxygen species will eliminate
(part of the) reactive carbon atoms as CO at the Ni surface.
Verification was done with TGA experiments in which as-synthesized CNFs on Ni-containing MgAl2O4 and Ni impregnated
activated carbon are contacted with CO2 under the same
growth conditions (data not shown). While no mass reduction
was observed for the pure activated carbon, a fast weight decrease is observed in both experiments, showing that amorphous and structured carbon can be oxidized with CO2 in
presence of Ni. Graphite-like materials can be oxidized easily
with a stronger oxidant, viz. O2, in metal catalyst-free conditions [77].
Although CO2 can be activated directly on Ni particles
(route c) and serve as a cleaning agent (Fig. 8), it can not be ruled out that CO2 molecules are activated on the basic sites of
the support material (route a). In this case oxygen atoms
should move either as carbonate or O species towards the
growing CNF at the interphase with the Ni particle. MgAl2O4
is a support known for its Lewis basic properties, enabling
CO2 activation [53], while perovskite materials exhibit
extraordinary high oxygen mobility, as determined by a CO2
pulse method [55]. Compared to other XTiO3 perovskite structures, SrTiO3 showed the highest amount of mobile oxygen
(285 lg/g). This high capacity of mobile oxygen on SrTiO3,
combined with the high oxygen atom mobility on carbon-like
surfaces, exhibiting typical surface diffusivities in the order of
106 m2/s [80], may deliver an extra amount of activated oxygen species to clean the Ni surface from unstable carbon,
leaving only well-structured carbon at the Ni surface. Removal of carbon at the Ni surface is thus most effective at
SrTiO3.
To rationalize further the CO2 effect on the CNF diameter
and purity, an ‘‘in situ cooling’’ principle is introduced, based
on the growth mechanism of carbon nanofibers [81] and the
occurrence of DR reaction chemistry. CNF growth has been
described as a 12-step process, for which temperature and
heat transfer are key parameters, determining rate, yield,
selectivity, quality and diameter [82,83]. Due to the occurrence of the endothermic reverse Boudouard reaction (1),
the DR reaction (2) and the reverse water gas shift reaction
(3) and supported by thermodynamic calculations and TGA/
MS, the presence of CO2 in the feed provides an extra cooling
at the Ni particle compared to pure CH4 CNF synthesis, where
only endothermic CH4 decomposition (4) takes place. The extra cooling influences nanofiber nucleation, leading to formation of nanofibers with smaller diameter. Due to the lower
effective temperature, Ni particles will expand less and form
smaller Ni particles, leading to CNFs with smaller diameters.
The relationship between metal particle size and fiber diameter is well established in literature [13].
Evidence for this ‘‘in situ cooling’’ principle can be found in
Table 1 when pure CH4 CVD, equimolar CH4/CO2 CVD and
CH4/He CVD are compared, taking into account the heat conductivity of CO2 (25 mW/m at 400 K) and He (191 mW/m at
400 K). Dilution of the pure CH4 feed, either with CO2 or He,
381
results in a lower CNF yield due to the decrease in the amount
of carbon available for CNF growth. This dilution effect has no
influence on the purity of the fibers since both CH4 and CH4/
He CVD produce fibers with comparable purity (vide supra),
indicating that only the presence of CO2 is responsible for
the increased fiber purity. When CO2 or He is present in the
feed, fibers with a smaller diameter are synthesized due to
the heat removal at the Ni surface, either by the endothermicity of the dry reforming reactions in case of CO2 or by the
higher heat conductivity of He.
Generally, it is accepted that carbon nanotubes grow on
elongated metal particles and fishbone fibers are synthesized
on pear-shaped particles [52]. In addition, during fiber growth
Ni particles show a dynamic behavior, since they continuously undergo an elongation/contraction process [52]. To clarify the influence of CO2 on the fiber structure change from
fishbone fiber to MWCNT, and thus also on CNF purity and
stability, two hypotheses can be advanced, involving the
influence of gas composition on CNF growth, or a decrease
in carbon arrival rate. From literature it is known that the
gas composition has a major impact on the final structure
of carbon deposits produced [84]. From TGA/MS experiments
in CH4/CO2 CVD conditions, it follows that not only CH4 and
CO2, but also CO is present in the reactor system. This is
not the case for CH4 CVD, where only CH4 and H2 are available. Therefore, the presence of CO in the gas phase could
also be invoked for explaining the structural change from
fishbone fiber towards MWCNT.
However, based on the amount of CNFs produced and on
the assumption that all deposited carbon originates from
CH4, this hypothesis is unlikely. Indeed, during CH4 CVD
and CH4/CO2 CVD, comparable amounts of the initial CH4 feed
are converted into CNFs, viz. 17 and 19 vol.%, respectively.
Taking into account the maximum thermodynamic conversion at 600 C (XCH4 = 41%, XCO2 = 52%), the stoichiometry of
the DR reaction (2) and the reverse water gas shift reaction
(3), only 28 vol.% of CO will be present in the gas phase in case
of CH4/CO2 CVD. The real experimental value will be significantly lower because of the insufficient contact between the
reagents and catalyst in the experimental set-up, hindering
the catalyst to operate in thermodynamic equilibrium conditions. Therefore, the change of fiber structure into MWCNT
will not be caused by CO. This is also confirmed by Guo et
al. [85], stating that in CH4/CO2 conditions carbon deposition
originates from CH4 decomposition (4) rather than from CO
disproportionation (1).
Therefore, it is suggested that the change in fiber structure
upon introduction of CO2 into the feed preferable is explained
by the lower carbon arrival rate at the nucleation sites located
at the backside of the particle. After decomposition of CH4,
carbon atoms diffuse towards the sites at the Ni/support/
gas interface, since nucleation at these interface sites is thermodynamically favored due to the high interfacial energy [86].
Diffusion of carbon atoms occurs either through the Ni bulk
or at the Ni surface. As indicated in a first-principles study,
surface diffusion of carbon has a lower energy barrier compared to bulk diffusion, making it the dominant diffusion process [73]. In CH4 CVD experiments, decomposition of
methane is fast and ensures a high carbon arrival rate, as
confirmed by the high yields of CNFs (Tables 1, 2 and 5). The
382
CARBON
5 0 ( 2 0 1 2 ) 3 7 2 –3 8 4
high carbon production rate leads towards blockage of surface
diffusion pathways, promoting bulk diffusion pathways, giving rise to fishbone fibers [73]. Compared to CH4 CVD, in
CH4/CO2 CVD the rate of carbon arrival at the nucleation sites
is reduced, since lower CNF yields are obtained due to DR
thermodynamics (vide supra). The lower carbon availability
makes surface diffusion the dominant pathway, favoring carbon nanotube synthesis [73].
In conclusion, temperature and CO2 concentration have a
significant influence on carbon nanofiber properties. Elevated
reaction temperatures cause increased CNF stability and purity, due to the increasing importance of the reverse Boudouard reaction (1). Due to the balance between CH4
decomposition and reverse Boudouard reaction, a maximum
yield is obtained at 600 C. The influence of the CO2 concentration can be summarized as follows: (1) it diminishes CNF
yield; this decrease can be explained by DR thermodynamics,
as CO2 addition removes the catalytic system from carbon
deposition favoring conditions; (2) it avoids co-production of
amorphous carbon impurities, due to the occurrence of the
reverse Boudouard reaction, resulting in more stable and pure
carbon nanofibers; (3) it allows tuning the diameter (distribution) of the CNFs by the in situ cooling mechanism and producing thin MWCNTs at fairly low temperatures, viz. 600 C,
on Ni catalysts.
4.
of the Ni catalyst and nanofiber nucleation, resulting in smaller fiber diameters. Interestingly, in presence of CO2, also a
shift of the fiber structure from fishbone to nanotube-type
is sometimes observed. This can be explained by the decrease
in carbon arrival rate at the nucleation sites, making carbon
surface diffusion the dominant diffusion pathway.
Acknowledgements
S.C. and J.G. acknowledge a Ph.D. grant from the Institute for
the Promotion of Innovation through Science and Technology
in Flanders (IWT-Vlaanderen). J.V.N. acknowledges CARBonCHIP (EUFP6 STREP program) for financial support. S.C. and
J.V.N. thank Rudy De Vos for his advice during SEM measurements (Department of Metallurgy and Materials Engineering,
K.U. Leuven). This research is also sponsored by the following
research programmes: IAP-PAI by BELSPO (Belgian Federal
Government), GOA, CECAT and long term structural Methusalem funding (Flemish Regional Government).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.carbon.2011.08.047.
Conclusions
R E F E R E N C E S
It is demonstrated that Ni catalysts supported on spinel-type
or perovskite-type oxides are promising materials for the conversion of CH4/CO2 mixtures into high-quality carbon nanofibers. Using CH4/CO2 mixtures as carbon source allows
adjusting carbon yield, stability, purity and diameter(distribution) of CVD synthesized CNFs. Screening of the NiMgAl2O4
and NiXTiO3 catalysts (X = Mg, Ca, Sr, Ba) pointed out that
NiSrTiO3 is the most promising catalyst. CNFs synthesized
with this catalyst by CH4/CO2 CVD show the highest increase
in stability compared to pure CH4 CVD. The important impact
on nanofiber properties of the amount of Ni, the reaction temperature and the CO2 concentration is illustrated.
Maximum yield and stability are obtained with Ni(30)SrTiO3. Stability and purity increase when reaction temperatures are high, though the carbon yield decreases as a result
of dry reforming thermodynamics. CO2 has a beneficial effect
on fiber stability and purity, since CO2 continuously removes
the most defective carbon impurities in an early stage of the
fiber synthesis at the catalyst surface, due to the Ni-catalyzed
reverse Boudouard reaction, possibly assisted by the CO2 activation ability of the spinel structure or the high oxygen mobility in the perovskite structure.
The use of CO2 allows tuning diameter (distributions) of
the synthesized CNFs. Average carbon nanofiber diameters
decrease and diameter distributions narrow by adjusting the
CH4/CO2 ratio. A so-called ‘‘in situ cooling’’ principle is introduced, explaining the effects on the structural properties of
the fiber. Due to the endothermic reverse Boudouard reaction,
DR reaction and the reverse water gas shift reaction, extra
cooling is supplied at the catalyst particle when CO2 is present in the feed. This temperature decrease affects reshaping
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