On the role of the stability of functional groups in multi-walled carbon nanotubes applied as support in iron-based high-temperature Fischer-Tropsch synthesis

Article abstract of DOI:10.1016/j.cattod.2015.09.023

The role of the stability of surface functional groups in oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) applied as support for iron catalysts in high-temperature Fischer-Tropsch synthesis was studied in a fixed-bed U-tube reactor at 340°C and 25 bar with a H₂:CO ratio of 1. Iron oxide nanoparticles supported on untreated oxygen-functionalized CNTs (OCNTs) and nitrogen-functionalized CNTs (NCNTs) as well as thermally treated OCNTs were synthesized by the dry impregnation method using ammonium ferric citrate as iron precursor. The properties of all catalysts were examined using X-ray diffraction, temperature-programmed reduction in H₂, X-ray photoelectron spectroscopy and temperature-programmed oxidation in O₂. The activity loss for iron nanoparticles supported on untreated OCNTs was found to originate from severe sintering and carbon encapsulation of the iron carbide nanoparticles under reaction conditions. Conversely, the sintering of the iron carbide nanoparticles on thermally treated OCNTs and untreated NCNTs during reaction was far less pronounced. The presence of more stable surface functional groups in both thermally treated OCNTs and untreated NCNTs is assumed to be responsible for the less severe sintering of the iron carbide nanoparticles during reaction. As a result, no activity loss for iron nanoparticles supported on thermally treated OCNTs and untreated NCNTs was observed, which even became gradually more active under reaction conditions.

Full text of DOI:10.1016/j.cattod.2015.09.023

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On the role of the stability of functional groups in multi-walled
carbon nanotubes applied as support in iron-based high-temperature
Fischer–Tropsch synthesis
∗
Ly May Chew, Wei Xia, Hendrik Düdder, Philipp Weide, Holger Ruland, Martin Muhler
Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44801 Bochum, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2015
Received in revised form 8 September 2015
Accepted 11 September 2015
Available online xxx
The role of the stability of surface functional groups in oxygen- and nitrogen-functionalized multi-walled
carbon nanotubes (CNTs) applied as support for iron catalysts in high-temperature Fischer–Tropsch syn-
thesis was studied in a fixed-bed U-tube reactor at 340 C and 25 bar with a H2:CO ratio of 1. Iron oxide
◦
nanoparticles supported on untreated oxygen-functionalized CNTs (OCNTs) and nitrogen-functionalized
CNTs (NCNTs) as well as thermally treated OCNTs were synthesized by the dry impregnation method
using ammonium ferric citrate as iron precursor. The properties of all catalysts were examined using
X-ray diffraction, temperature-programmed reduction in H2, X-ray photoelectron spectroscopy and
temperature-programmed oxidation in O2. The activity loss for iron nanoparticles supported on untreated
OCNTs was found to originate from severe sintering and carbon encapsulation of the iron carbide nanopar-
ticles under reaction conditions. Conversely, the sintering of the iron carbide nanoparticles on thermally
treated OCNTs and untreated NCNTs during reaction was far less pronounced. The presence of more sta-
ble surface functional groups in both thermally treated OCNTs and untreated NCNTs is assumed to be
responsible for the less severe sintering of the iron carbide nanoparticles during reaction. As a result,
no activity loss for iron nanoparticles supported on thermally treated OCNTs and untreated NCNTs was
observed, which even became gradually more active under reaction conditions.
Keywords:
High-temperature Fischer–Tropsch
synthesis
Iron-oxide based catalysts
Multi-walled carbon nanotubes
Surface functional groups
©
2015 Published by Elsevier B.V.
1
. Introduction
shift (WGS) activity, which results in less unwanted CO2 formed
in FTS compared with Fe-based catalysts. On the other hand, high
reaction temperatures (>230 C) are essential for iron-based cata-
◦
Fischer–Tropsch synthesis (FTS) has attracted increasing atten-
tion due to its potential in producing high quality fuels and
chemicals from coal, natural gas or biomass [1]. Moreover, the
diminishing crude oil resources and the volatility of the crude oil
price have raised the interest in FTS [2]. At present, FTS is a well
lysts for the production of short-chain hydrocarbons. The high WGS
activity of Fe-based catalysts renders them suitable for FTS using
CO-rich syngas [5].
In recent years, numerous studies have been performed focus-
ing on catalytic activity and selectivity mostly using iron or cobalt
supported on silica, alumina or titania [6–8]. In addition, carbon
was also used as support and showed promising performance
in FTS. An early study performed by Jones et al. [9] reported
that Fe/carbon catalysts achieved higher olefin selectivity than
unpromoted Fe/silica and Fe/alumina catalysts. Recently, carbon
nanotubes (CNTs) have been identified as a promising support for
catalysts used in FTS, because CNTs have a relatively large and freely
accessible surface area and are able to improve the dispersion of
the catalytically active nanoparticles [10]. The surface modifica-
tion of CNTs is essential to attain high dispersion of metal particles,
because CNTs are hydrophobic and inert in nature [11]. Oxygen and
nitrogen functionalization are the most widely used methods for
the modification of CNTs. Oxygen-containing functional groups can
established industrial process, in which synthesis gas (CO + H ) is
2
converted into mainly linear hydrocarbons with a broad chain-
length distribution [3,4]. The most common metals used for FTS
catalysts are iron, cobalt and ruthenium. Iron- and cobalt-based
catalysts are used commercially at present, although ruthenium
is the most active catalyst for FTS [2]. Ruthenium is excluded from
industrial application due to the limited resources and its high cost.
Cobalt-based catalysts are mainly used in FTS to produce long-
◦
chain hydrocarbons at low reaction temperatures (<230 C). One
of the advantages of using Co-based catalysts is its low water-gas
∗ Corresponding author. Tel.: +49 234 32 28754; fax: +49 234 32 14115.
E-mail address: muhler@techem.rub.de (M. Muhler).
http://dx.doi.org/10.1016/j.cattod.2015.09.023
0
920-5861/© 2015 Published by Elsevier B.V.
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be created by treating CNTs with oxidizing agents such as nitric acid
11,12]. Nitrogen doping of CNTs can be achieved by post-treatment
role in the deactivation of FT catalysts [23–26]. Torres Galvis
et al. [24] studied the carbon formation on iron-based catalysts
[
of oxygen-containing CNTs in flowing ammonia [13]. The generated
oxygen- and nitrogen-containing functional groups are assumed to
act as efficient anchoring sites for metal particles [11,14,15]. These
anchoring sites immobilize the metal particles preventing them
from excessive sintering upon heating [16].
Different functional groups such as carbonyl, carboxylic acid,
pyrone, phenol and ether species are present on the CNT surface
after nitric acid treatment. It has been reported that the nature
of surface functional groups on CNTs can be altered by thermal
treatments [11]. Kundu et al. [11] investigated the thermal sta-
bility of surface functional groups on oxygen-functionalized CNTs
using a thermal tapered element oscillating microbalance under
◦
plug-flow conditions at 350 C, 20 bar and a H :CO ratio of 1. They
2
observed that the supported iron catalyst promoted with sodium
had higher initial coking rates than the unpromoted supported
iron catalyst. Therefore, the low catalytic activity of the sodium-
promoted iron catalyst was ascribed to the higher extent of carbon
deposition.
In this work, we studied the role of the stability of surface
functional groups in CNTs applied as support for iron cata-
lysts in high-temperature FTS. The oxygen-containing functional
groups were first generated by gas-phase oxidation and then
partly removed by thermal treatment of the obtained OCNTs in
flowing helium. The nitrogen-containing functional groups were
introduced via NH3 post-treatment. The catalysts used in the FTS
reaction were prepared by the dry impregnation method using
ammonium ferric citrate as iron precursor. The catalytic perfor-
mance of the catalysts was studied in a fixed-bed isothermal U-tube
reactor. The properties of the iron catalysts before and after reaction
were characterized by X-ray diffraction (XRD), temperature-
programmed reduction (TPR), X-ray photoelectron spectroscopy
(XPS) and temperature-programmed oxidation (TPO).
(
OCNTs) after thermal treatment in ultra-high vacuum at temper-
◦
atures between 300 and 720 C for 30 min. The authors observed
that the surface atomic concentration of oxygen decreased with
increasing treatment temperatures. The carboxylic groups start to
decompose at about 300 C, whereas the decomposition of phenol
groups occurs between 300 and 600 C and ether decomposes at
◦
◦
even higher temperature. Hence, it is possible to selectively remove
some of the surface oxygen groups by heating OCNTs in inert
atmosphere. In comparison, nitrogen groups are significantly more
stable upon heating. Thermal treatment of nitrogen-functionalized
◦
CNTs (NCNTs) at 600 C did not cause a significant loss of nitro-
gen except for a slight decrease of pyridinic nitrogen and a minor
increase in quaternary nitrogen [13].
2. Experimental
Schulte et al. [17] studied high-temperature FTS using OCNT-
and NCNT-supported iron nanoparticles at 340 C and 25 bar with
2.1. Catalyst preparation
◦
a H :CO ratio of 1. They observed that the NCNT-supported iron
Multi-walled CNTs with inner diameters of 20–50 nm and outer
diameters of 70–200 nm were obtained from Applied Sciences Inc.
(Ohio). As-received CNTs were subjected to nitric acid vapor treat-
2
nanoparticles with 40 wt% of iron loading were capable of achiev-
ing a high and constant degree of CO conversion under the
high-temperature condition for a period of 80 h time on stream.
Furthermore, the iron carbide nanoparticles found on NCNTs
after reaction were smaller than on OCNTs suggesting stronger
iron-nitrogen interactions. Interestingly, the CNT-supported iron
nanoparticles without promoters obtained excellent olefin selec-
tivities and moderate methanation tendency as compared to the
conventional bulk iron FT catalysts. In addition, they found that
iron supported on non-functionalized CNTs was nearly inactive,
indicating that efficient CNT functionalization is mandatory.
The number of anchoring sites on CNT surfaces is related to
the thermal stability of the supported metal particles. It has been
reported that the metal particle size is one of the most important
aspects that influences the catalytic activity and selectivity in FTS
◦
ment at 200 C for 24 h to generate oxygen-containing functional
groups [27]. In order to introduce nitrogen-containing functional
groups, the OCNTs were loaded into a quartz boat and placed in
the centre of a horizontal quartz reactor. The sample was treated at
◦
400 C for 6 h in 10% NH3 in He with a flow rate of 25 sccm to obtain
NCNTs [13]. To remove a fraction of the surface oxygen groups, the
◦
◦
−1
obtained OCNTs were heated to 600 C at 10 C min in helium
using a flow rate of 100 sccm and treated for 1 h before cooling
in the same atmosphere. The obtained OCNTs were denoted as
OCNT 600. Additionally, untreated OCNTs and NCNTs were used
as reference supports for comparison with the thermally treated
OCNTs. The supports were loaded with iron by the dry impreg-
nation method using sodium- and sulphur-free ammonium ferric
citrate (Fluka, 14.5–16% Fe basis, purum p.a. >95.0%) as iron precur-
sor [28]. Briefly, the supports were mixed with the aqueous solution
of ammonium ferric citrate aiming at a theoretical Fe loading of
[
8,18,19]. Park et al. [8] studied the effect of particle size on the
catalytic activity and selectivity in FTS over iron oxide supported
on ␦-Al O . The authors observed that the turnover frequency and
2
3
◦
C5+ selectivity increased with increasing iron oxide particle size
20 wt%. The mixture was dried at 50 C overnight and subsequently,
◦
in the range of 2–6 nm, whereas the CH4 and C –C4 selectivities
the solid product was collected and calcined at 300 C in synthetic
2
decreased correspondingly. Similar results were also obtained by
Cheng et al. [18], who studied the effect of pore size in mesoporous
silicas on the structure and catalytic performance of iron catalysts in
high-temperature FTS. They showed that the overall catalyst reac-
tion rate, C –C olefin selectivity and C5+ selectivities increased
air (20.5% O2 in N2) with a flow rate of 100 sccm for 90 min. The
three obtained samples were labelled as Fe/OCNT, Fe/OCNT 600
and Fe/NCNT, respectively.
2.2. Characterization
2
4
with increasing iron carbide particle size in the range of 4–22 nm.
The authors attributed the higher catalytic activity of iron catalysts
containing larger iron particles to the higher degree of carbidiza-
tion. Torres Galvis et al. [19] reported that the turnover frequency
decreased, while C –C olefin and C5+ selectivities increased with
Atomic absorption spectroscopy (AAS) was used to determine
the actual iron loading on the support. Powder X-ray diffraction
patterns were recorded with a Panalytical MPD diffractometer
◦
using Cu K␣ radiation in the 2ꢀ range of 10–70 . Temperature-
2
4
increasing iron carbide particle size up to 17 nm.
programmed reduction was performed by heating approximately
40 mg of the sample to 800 C with a heating rate of 10 C min
◦
◦
−1
Catalyst stability is also an important issue in the design of FTS
catalysts. The loss of catalytic activity for catalysts under FTS reac-
tion conditions can be attributed to iron phase changes, deposition
of inactive carbonaceous compounds, sintering of the active phase,
and poisoning [20–22]. A number of studies claimed that deposition
of inactive carbon species on the catalyst surface plays a significant
in a gas mixture of 4.5% H2 in Ar with a flow rate of 84.1 sccm at
atmospheric pressure. X-ray photoelectron spectroscopy measure-
ments were conducted in an ultra-high vacuum set-up utilizing
a high-resolution Gammadata-Scienta SES 2002 analyzer and a
monochromatic Al K␣ X-ray source (1486.6 eV, anode operating
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at 14 kV and 30 mA). The base pressure in the measurement
−
10
chamber was maintained at about 7 x 10
mbar. The measure-
ments were performed in the fixed transmission mode with a
pass energy of 200 eV. Charging effects during the measurements
were compensated by applying a flood gun. The measured spec-
tra were calibrated based on the graphite C 1s peak at 284.5 eV.
The CasaXPS software was used to analyze the XPS data. The
XP spectra were normalized to the corresponding C 1s peaks.
Temperature-programmed oxidation was conducted in a fixed-bed
reactor equipped with quantitative online gas analytics with 10 mg
of the sample diluted with 490 mg of pure silicon carbide [29]. The
◦
◦
−1
sample was heated to 800 C with a heating rate of 5 C min in
a gas mixture of 4.5% O2 in Ar with a total flow rate of 20 sccm.
The final temperature was held until no more O2 was consumed.
Transmission electron microscopy (TEM) analysis was performed
with a FEI Tecnai F20 microscope operated at 200 kV. The samples
for the TEM investigation were prepared by ultrasonically dispers-
ing the powder material in ethanol and subsequently dropping the
solution on a carbon-coated Au grid.
Fig. 1. XRD patterns of the calcined Fe/OCNT, Fe/OCNT 600 and Fe/NCNT precursors
before reaction.
2.3. Catalytic tests
Typically, 40 mg of catalyst diluted with 160 mg of silicon car-
bide were used in each FTS experiment. Prior to the catalytic tests,
which are present on the nitrogen-doped carbon surface. In con-
trast to the OCNT-supported iron oxide nanoparticles, the hematite
reflections are not visible for the calcined Fe/NCNT precursor, which
may be attributed either to a very small crystallite size or to a low
content of hematite in the calcined Fe/NCNT precursor.
◦
the catalyst was first reduced in pure H at 380 C and 25 bar for
2
◦
−1
5
h using a 2 C min heating rate. Subsequently, the reactor was
cooled to the reaction temperature of 340 C in flowing Ar. Then, the
FTS reaction was performed at 340 C and 25 bar using a mixture of
5% CO, 45% H and 10% Ar at a total flow rate of 50 L gcat
◦
◦
TPR experiments were conducted to investigate the reducibility
of all three supported iron oxide samples. The TPR profiles of all
−
1
−1
4
h . The
2
reaction was carried out for 80 h. Online gas analysis was performed
with a Shimadzu gas chromatograph equipped with a thermal con-
ductivity detector and two flame ionization detectors using Ar as
internal standard. The experimental set-up was described in detail
elsewhere [30].
◦
samples after calcination in air at 300 C are shown in Fig. 2, which
suggest a stepwise reduction of iron oxide nanoparticles according
to Eq. (1) [31,32]:
Fe O → Fe O → FeO → Fe
(1)
2
3
3
4
◦
In the first temperature region (300–400 C) the reduction of
Fe O to Fe O occurs, and in the second temperature region
(
3
. Results
2
3
3
4
◦
400–600 C) the reduction of Fe O to metallic Fe. The TPR profiles
3 4
3.1. Characterization of the catalysts before reaction
of OCNT-supported iron oxide nanoparticles demonstrate that the
first region of the Fe/OCNT profile is slightly larger than the first
region of Fe/OCNT 600. The amount of surface oxygen species in
Fe/OCNT is higher than in Fe/OCNT 600 as some of the surface oxy-
The actual iron loadings of the calcined Fe/OCNT, Fe/OCNT 600
and Fe/NCNT precursors determined by AAS were 25.1 wt%,
9.0 wt% and 16.1 wt%, respectively. The XRD patterns of OCNT-
and NCNT-supported iron oxide nanoparticles before reaction are
1
◦
gen was removed by the thermal treatment of OCNTs at 600 C. A
◦
◦
second peak at about 500 C with a clear shoulder at around 600 C
◦
◦
shown in Fig. 1. The reflections at about 26.2 and 44.8 were
observed in all calcined precursors, which can be assigned to the
(
0 0 2) and (1 1 1) reflections of hexagonal graphite, respectively. A
mixture of hematite (Fe O ) and magnetite (Fe O ) was detected
2
3
3
4
◦
in all calcined precursors as indicated by the reflections at 35.4 ,
6.9 , and 62.5 in the XRD patterns. The XRD patterns for the
◦ ◦
5
two OCNT-supported iron oxide nanoparticles samples are rather
similar indicating that the iron oxide nanoparticles in both sam-
ples have a fairly similar crystallite size. However, the reflections
of the calcined Fe/NCNT precursor are slightly broader than that
of the Fe/OCNT and Fe/OCNT 600 precursors, which implies that
the crystallite size of the iron oxide nanoparticles on NCNTs is
slightly smaller than those on OCNTs and OCNT 600. This obser-
vation is in agreement with a study by Sánchez et al. [16], who
observed less agglomeration of iron nanoparticles on NCNTs than
on OCNTs. The higher stability against sintering can be attributed to
the stronger iron-nitrogen interaction, the higher thermal stability
of nitrogen groups and also the surface defects, which were created
by the incorporation of nitrogen into the graphene sheet. The aggre-
gation of neighbouring nanoparticles can be prevented by strong
metal–support interactions. The authors further claimed that the
mobility of the nanoparticles may be hindered by the edge planes,
Fig. 2. TPR profiles of the calcined Fe/OCNT, Fe/OCNT 600 and Fe/NCNT precursors
before reaction.
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Fig. 3. (a) XP O 1s spectra, (b) XP N 1s spectra and (c) XP Fe 2p spectra of calcined Fe/OCNT, Fe/OCNT 600 and Fe/NCNT before reaction.
is observed for Fe/OCNT. Conversely, there is no clear shoulder vis-
ible in the second peak observed for Fe/NCNT at about 490 C. This
The XP Fe 2p spectra of all three calcined catalysts are shown in
Fig. 3(c). The Fe 2p3/2 peaks and the shake-up satellites are observed
at 711 eV and 719 eV, respectively, for Fe/OCNT, Fe/OCNT 600 and
Fe/NCNT catalysts. These features illustrate that all the calcined cat-
◦
observation is in agreement with our previous study [33], in which
the shoulder was attributed to the reduction of Fe O to metal-
3
4
lic iron via FeO for Fe/OCNT. Additionally, the whole TPR profile of
Fe/NCNT is slightly shifted towards lower temperatures compared
with the Fe/OCNT catalyst indicating that iron oxide nanoparticles
supported on NCNTs are reduced faster than those supported on
OCNTs. Our previous in situ XANES reduction results confirmed that
iron oxide nanoparticles on NCNTs are easier to reduce than those
on OCNTs [33]. Moreover, the TPR profiles show that the first region
of the calcined Fe/NCNT precursor is slightly lower than the first
region of the two OCNT samples. This observation is consistent with
the XRD results (Fig. 1) demonstrating that the calcined Fe/NCNT
precursor possesses less hematite than the two OCNT samples. The
TPR profiles of Fe/OCNT 600 and Fe/NCNT catalysts are relatively
alysts contain Fe O3 [16,35]. The intensities of the Fe 2p spectra
2
of Fe/OCNT 600 and Fe/NCNT are lower compared with Fe/OCNT
reflecting the different iron loadings found by AAS, which are in
the order Fe/OCNT > Fe/OCNT 600 > Fe/NCNT.
The surface atomic concentrations of C, O, N and Fe in all three
◦
catalysts after calcination at 300 C (Table 1) were derived by inte-
grating the peak areas of the C 1s, O 1s, N 1s and Fe 2p XP region
◦
spectra. The treatment of the OCNTs at 600 C in helium removed a
significant fraction of the oxygen-containing surface groups. It can
be seen from Table 1 that the atomic concentration of oxygen in
Fe/OCNT 600 decreased by a factor of almost 2 from 22.1% to 13.6%.
◦
It has been reported that the post-treatment of OCNTs at 400 C for
◦
similar. The peak appearing at high temperature above 600 C cor-
responds to the gasification of CNTs [12,34].
6 h in flowing ammonia removed a significant amount of oxygen-
containing surface groups, too, especially those with low thermal
stability and simultaneously introduced large amounts of nitrogen-
containing surface groups on the CNT surface [13]. It can be seen
that the atomic concentration of nitrogen in Fe/NCNT (8.0%) is sig-
nificantly higher than in Fe/OCNT (2.9%) and, on the other hand, the
oxygen atomic concentration in the former (16.0%) is lower than in
the latter (22.1%).
The surface composition of the three calcined catalysts before
reaction was characterized by XPS. The spectra were normalized to
the intensity of the C 1s peak at 284.5 eV. Similar to our previous
study [33], nitrogen species were also detected in both Fe/OCNT
and Fe/OCNT 600 catalysts, which originate from the decomposi-
tion of the ammonium ferric citrate precursor during calcination at
◦
3
00 C releasing NH that forms surface nitrogen species. Fig. 3(a)
TPO experiments were performed with all the calcined cata-
lysts before reaction. The unloaded OCNTs and NCNTs were also
studied by TPO experiments as reference for comparison with
all the catalysts. It can be seen from Fig. 4 that the evolution of
CO and CO2 was observed for both unloaded OCNTs and NCNTs
samples, but for all three iron oxide catalysts only the formation
of CO2 was detected due to iron oxide-catalyzed CO oxidation.
Fig. 4 shows that only one peak was found for all the samples.
For the unloaded OCNTs and NCNTs samples, the peak at about
3
shows the XP O 1s spectra of all the calcined catalysts. The peaks at
30 eV, 531.5 eV and 533.3 eV in the O 1s spectra can be assigned
5
to the contributions of oxygen anions in iron oxides, oxygen dou-
bly bound to carbon (C O) and oxygen singly bound to carbon
(
C O), respectively. It can be seen from Fig. 3(a) that the inten-
sities of all three peaks for Fe/OCNT 600 and Fe/NCNT are lower
than for Fe/OCNT. The decrease of the peak intensity at 530 eV is
mainly due to the decrease in iron dispersion, whereas the decrease
of the C O and C O peaks originates from the decomposition of
surface oxygen groups such as carboxyl groups, carboxylic anhy-
drides and ester groups during the thermal treatment of the OCNTs
◦
670 C can be assigned to the non-catalytic oxidation of CNTs. In
comparison to the unloaded OCNTs and NCNTs samples, the TPO
◦
at 600 C in helium flow and the post-treatment of the OCNTs in
◦
Table 1
flowing NH at 400 C. Fig. 3(b) shows the XP N 1s spectra of the cal-
3
XPS-derived C, O, N and Fe surface atomic concentrations of calcined Fe/OCNT,
Fe/OCNT 600 and Fe/NCNT before reaction.
cined Fe/OCNT, Fe/OCNT 600 and Fe/NCNT catalysts. It can be seen
that all the catalysts contain pyridinic-type N and pyrrolic-type N
with binding energies of 398.5 eV and 399.8 eV, respectively. The
intensity of the N 1s spectrum of Fe/NCNT is considerably higher
compared with Fe/OCNT and Fe/OCNT 600 demonstrating that the
Catalyst
C (%)
O (%)
N (%)
Fe (%)
Fe/OCNT
Fe/OCNT 600
Fe/NCNT
65.1
77.5
70.4
22.1
13.6
16.0
2.9
2.5
8.0
9.9
6.4
5.6
◦
post-treatment of the OCNTs in flowing NH3 at 400 C for 6 h gen-
erated a significant amount of nitrogen-containing surface groups.
Atomic sensitivity factors: carbon 0.25, oxygen 0.66, nitrogen 0.42, iron 3.00.
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Table 2
5
CO conversion, average specific reaction rate, chain growth probabilities, product
selectivities and olefin fractions of Fe/OCNT, Fe/OCNT 600 and Fe/NCNT obtained at
steady state averaging between 65 and 80 h time on stream. Reaction conditions:
◦
−1 −1
h .
3
40 C, 25 bar, H2/CO = 1, total flow of 50 L g
Parameter
Catalyst
Fe/OCNT
Fe/OCNT 600
Fe/NCNT
XCO (%)
18.4
15.4
0.44
25.2
31.6
23.5
15.4
40.0
53.5
44.6
0.47
22.0
39.4
19.3
18.6
50.0
56.5
46.9
0.50
20.3
39.3
18.4
20.4
53.0
−
1
−1
Reaction rate (mmolCO gFe min )
˛
SCH4 (%)
SCO2 (%)
SC2–C6 paraffins (%)
SC2–C6 olefins (%)
C2 –C6 /C2–C6 (%)
the lowest among all three catalysts. All catalysts achieved rather
similar olefin fractions of 40–60% in the hydrocarbon range of
C –C . It is worth noting that all catalysts achieved moderate olefin
2
6
selectivity without adding additional promoters such as potassium
and manganese, which are well known to improve the dissocia-
tive adsorption of CO and impede the dissociative adsorption of H2
Fig. 4. TPO profiles of the unloaded OCNTs and NCNTs, and the calcined catalysts
before reaction. Effluent mole fractions of CO2 (black) and CO (grey).
[
38,39]. The methane selectivities and chain growth probabilities of
all catalysts were fairly similar. Nevertheless, a closer comparison
of all three catalysts showed that the Fe/NCNT catalyst achieved the
highest chain growth probability and the lowest methane selectiv-
ity.
peaks observed for all three calcined catalysts are shifted towards
lower temperature. The burn-off temperature was lowered by
◦
◦
1
30 C from 670 to 540 C in the presence of 20 wt% of iron oxide
due to the catalytic oxidation of CNTs [29,36,37]. Therefore, the
TPO profiles of all the catalysts are rather identical.
3.3. Characterization of the catalysts after reaction
3.2. Catalytic tests
XRD measurements were conducted with the catalysts after 80 h
time on stream under high-temperature FTS reaction conditions.
Fig. 6 presents the XRD patterns of all three catalysts after reac-
The catalytic activity of all three iron oxide catalysts was stud-
ied in the high-temperature FTS. Fig. 5 shows the degrees of CO
conversion as a function of time on stream for all the catalysts. For
the Fe/OCNT catalyst, the CO conversion decreased from 38% to 18%
during a period of 80 h time on stream. Surprisingly, the CO con-
version on Fe/OCNT 600 catalyst increased with time on stream
from 33% to 54%. Furthermore, high degrees of CO conversion and
increasing catalytic activity were observed for the Fe/NCNT catalyst
compared with the Fe/OCNT catalyst, and the CO conversion over
Fe/NCNT was slightly higher than that over Fe/OCNT 600.
tion. It can be seen that Hägg carbide (Fe5C ) and magnetite were
2
detected in all used catalysts. Iron carbide has been reported as
the active phase for FTS, whereas magnetite is responsible for the
WGS reaction [40,41]. It has to be noted that the dominating iron
◦
phase in all three catalysts is the Hägg carbide. The peaks at 43 ,
◦ ◦
5
3 and 57 , which are identified as the magnetite phase, are rela-
tively weak. The carbide reflections of Fe/OCNT 600 and Fe/NCNT
are slightly broader than that of Fe/OCNT indicating that the crys-
tallite size of the Hägg carbide on Fe/OCNT is larger than that on
Fe/OCNT 600 and Fe/NCNT. The average crystallite sizes of Hägg
carbide in the used catalysts were estimated based on the Scherrer
The degrees of CO conversion, average specific reaction rates,
chain growth probabilities and selectivities for all three catalysts
are summarized in Table 2. The catalytic activity of Fe/OCNT was
Fig. 5. CO conversion as a function of time on stream for the Fe/OCNT, Fe/OCNT 600
◦
and Fe/NCNT catalysts. Reaction conditions: 340 C, 25 bar, H2/CO = 1, total flow of
Fig. 6. XRD patterns of the Fe/OCNT, Fe/OCNT 600 and Fe/NCNT catalysts after 80 h
time on stream under FTS conditions. SiC was used as diluent.
−
1
−1
h .
5
0 L g
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the TPO profile of grown CNTs over Ni, which were assigned to the
catalyzed oxidation of CNTs and to the removal of encapsulating
graphite, respectively. For Fe/OCNT, the intensity of the peak at
◦
◦
5
5
10 C is considerably lower while the intensity of the peak at
80 C is significantly higher compared with the other two catalysts
as shown in Fig. 7 demonstrating that the encapsulating carbon
formation on Fe/OCNT is much higher than that on Fe/OCNT 600
and Fe/NCNT. It is likely that the encapsulating carbon was block-
ing the active iron sites and consequently hindered the catalyzed
oxidation of relatively more reactive CNTs leading to the lower
◦
intensity of the CNTs oxidation peak at 510 C for Fe/OCNT. After
the removal of encapsulating carbon shells, the active iron sites
were exposed to oxygen. Subsequently, the catalyzed oxidation of
CNTs occurred resulting in an apparent increase of the oxidation
◦
temperature of CNTs from 510 to 580 C. This finding clearly
reflects that the Fe/OCNT catalyst was less active in FTS.
TEM measurements were performed with Fe/OCNT after 80 h
time on stream under FTS reaction conditions to provide further
evidence for the formation of encapsulating carbon shells. The TEM
micrographs of Fe/OCNT after reaction (Fig. 8) clearly demonstrate
the presence of carbon-encapsulated iron carbide nanoparticles in
agreement with the TPO profile (Fig. 7).
Fig. 7. TPO profiles of all the catalysts after 80 h time on stream under FTS conditions.
equation resulting in 21, 17, and 14 nm for Fe/OCNT, Fe/OCNT 600,
and Fe/NCNT, respectively.
The TPO profiles of the catalysts after 80 h time on stream under
FTS reaction conditions are shown in Fig. 7. One clearly visible peak
◦
at about 580 C was observed for the Fe/OCNT catalyst. In contrast
4. Discussion
◦
to Fe/OCNT, one peak at approximately 510 C and two shoulders at
◦
◦
about 370 C and 580 C were observed for both Fe/OCNT 600 and
Fe/NCNT. It has been reported that different kinds of carbonaceous
compounds are formed under typical FTS operating conditions
The XRD and the TPR results of the catalysts after calcination
revealed that the content of hematite was slightly lower in the
Fe/NCNT catalyst than in the Fe/OCNT catalyst. Our previous in situ
XANES results disclosed that the content of hematite is indeed
slightly lower in the NCNT-supported iron oxide nanoparticles
(74.8% Fe O3 and 25.2% Fe O ) than in the OCNT-supported iron
[
5,20,24]. Amorphous carbon is known to possess lower oxidation
resistance as compared to crystalline carbon. Hence, it is reason-
◦
able to assume that the shoulder at about 370 C is the oxidation
2
3
4
of amorphous carbon deposits. An estimated quantitative analysis
oxide nanoparticles (88.5% Fe O3 and 11.5% Fe O ) [33]. Several
2 3 4
of the TPO profile of Fe/OCNT 600 shows that the amount of CO
produced at 370 C is about 33 ␮mol, whereas a theoretical estima-
tion of the amount of CO2 formed by the oxidation of Hägg carbide
studies reported that the interaction between iron and nitrogen is
stronger than the interaction between iron and oxygen [13,16,42].
It is therefore likely that the strong iron-nitrogen interaction
retards the oxidation of iron during calcination and consequently
the NCNT-supported iron oxide nanoparticles had not been fully
oxidized to hematite.
In contrast to the Fe/OCNT catalyst, the CO conversion achieved
by both Fe/OCNT 600 and Fe/NCNT catalysts increased over
80 h time on stream. The increasing CO conversion over the
Fe/OCNT 600 and Fe/NCNT catalysts is probably due to the ongo-
ing carbiding process over 80 h time on stream. This observation is
consistent with a recent study by Cheng et al. [18], who observed a
2
◦
in 10 mg of catalyst with 20 wt% of iron is about 15 ␮mol. It can
◦
therefore be assumed that the shoulder at 370 C is the oxidation
of amorphous carbon deposits originating from the formation of
short-chain olefins rather than the oxidation of iron carbide.
◦
Furthermore, the peak at 510 C can be assigned to the catalyzed
◦
oxidation of CNTs, whereas the peak at 580 C can be attributed
to both the oxidation of encapsulating carbon and CNTs. This
observation is in agreement with a recent study by Düdder et al.
◦ ◦
29], who found a peak at 580 C with a shoulder at 650–680 C in
[
Fig. 8. TEM micrographs of the Fe/OCNT catalyst after 80 h time on stream under FTS conditions.
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gradual increase in CO conversion for the hydrogen-activated iron
catalyst over 60 h time on stream, which was attributed to gradual
carbidization of the iron catalyst during FTS. Schulz [43] proposed
that iron-based catalysts undergo considerable self-organization
during FTS involving the disappearance of metallic iron and the
formation of iron carbide, which leads to an enhancement in FT
activity. Deactivation of Fe/OCNT 600 and Fe/NCNT catalysts was
not observed, which can be attributed to the presence of thermally
more stable surface functional groups in both catalysts. The thermal
that iron nanoparticles supported on thermally treated OCNTs
(OCNT 600) achieve a rather similar catalytic activity and sta-
bility as iron nanoparticles supported on NCNTs. It is therefore
reasonable to assume that the high thermal stability of surface func-
tional groups is the major reason for the outstanding performance
of both Fe/OCNT 600 and Fe/NCNT catalysts in high-temperature
FTS. Accordingly, the high thermal stability of nitrogen functional
groups and of oxygen functional groups such as carbonyl and ether
groups has a significant impact on the catalytic activity and stability
of iron catalysts in high-temperature FTS.
◦
treatment of OCNTs at 600 C in helium flow and the post-treatment
◦
of OCNTs at 400 C in flowing ammonia removed those surface func-
tional groups with low thermal stability such as carboxylic groups
and anhydrides. A significant fraction of the oxygen-containing sur-
face functional groups was removed by thermal treatment of the
functionalized CNTs in helium flow as verified by the XPS results.
The atomic oxygen concentration in the surface functional groups
of the Fe/OCNT 600 catalyst amounting to 13.6% O was considerably
lower than that of the Fe/OCNT catalyst of 22.1% O before reac-
tion. Consequently, the surface functional groups present in both
Fe/OCNT 600 and Fe/NCNT catalysts have higher thermal stabil-
ity and are able to resist the high reaction temperature. It is likely
5
. Conclusions
Iron oxide nanoparticles supported on OCNTs, OCNT 600 and
NCNTs were synthesized by the dry impregnation method using
ammonium ferric citrate as iron precursor. The prepared catalysts
were applied in high-temperature FTS. The XPS results disclosed
that the oxygen-containing surface functional groups were par-
tially removed by heating the functionalized CNTs in helium flow.
The encapsulating carbon formation on Fe/OCNT was much more
pronounced than that on Fe/OCNT 600 and Fe/NCNT as revealed by
the TPO results after reaction, and the XRD results of the catalysts
after reaction disclosed that sintering of the iron carbide nanopar-
ticles in Fe/OCNT was more severe than in the other two catalysts.
TEM micrographs obtained after reaction further confirmed the
formation of encapsulating carbon shells on Fe/OCNT. Whereas a
decrease of CO conversion was observed for Fe/OCNT over 80 h
time on stream, the Fe/OCNT 600 and Fe/NCNT catalysts became
more active under reaction conditions. It is likely that the pres-
ence of more stable surface functional groups in both Fe/OCNT 600
and Fe/NCNT catalysts was the reason for the less severe sintering
of the iron carbide nanoparticles during reaction. Severe sintering
of the iron carbide nanoparticles and their encapsulation by car-
bon during reaction played a significant role in the deactivation of
the Fe/OCNT catalyst. Hence, CNTs possessing surface functional
groups of high thermal stability are beneficial when used as sup-
port for iron catalyst in high-temperature FTS as they are able to
prevent a significant loss of anchoring sites on CNT surface dur-
ing reaction and consequently stabilize iron carbide nanoparticles
under FTS conditions.
◦
that the applied severe reaction conditions (340 C and 25 bar under
H2 flow) lead to a significant loss of less stable surface functional
groups. Kundu et al. [11] reported that the oxygen-containing sur-
face functional groups can be removed in the presence of H2 at
elevated temperature. As a result, sintering of the iron nanopar-
ticles in the Fe/OCNT catalyst is believed to be more severe than
in the Fe/OCNT 600 and Fe/NCNT catalysts due to the removal of
anchoring sites on the CNT surface during reaction. This hypothesis
is verified by the XRD results of the catalysts after reaction (Fig. 6)
indicating that the crystallite size of Hägg carbide on OCNTs is
larger than those on OCNT 600 and NCNTs. Additionally, the strong
iron-nitrogen interaction also contributes to the less severe sin-
tering of the iron carbide nanoparticles on NCNTs during reaction.
Schulte et al. [17] also studied high-temperature FTS over OCNT-
and NCNT-supported iron oxide nanoparticles and observed that
the iron nanoparticles found on NCNTs after reaction were smaller
than on OCNTs.
The similar TPO profiles of all the catalysts before reaction
demonstrate that the deposited FeOx nanoparticles were cat-
alytically active in carbon combustion. The TPO profiles of the
Fe/OCNT 600 and Fe/NCNT catalysts after reaction are nearly iden-
tical. However, the TPO profile of the Fe/OCNT catalyst after reaction
is clearly different from the others. It has to be noted that the
TPO results are in accordance with the catalytic activity obtained
with all the catalysts. Moreover, the TPO results disclosed that the
encapsulating carbon formation on Fe/OCNT was higher than that
on Fe/OCNT 600 and Fe/NCNT, suggesting that large iron particles
are more prone to carbon deposition as it has been observed for
nickel catalysts [44–46]. A decreasing CO conversion over 80 h time
on stream was observed for Fe/OCNT, whereas Fe/OCNT 600 and
Fe/NCNT became more active under reaction conditions. It is most
likely that the sintering of the iron carbide nanoparticles on OCNTs
occurred at the beginning of the reaction leading to the strong
initial activity loss observed for Fe/OCNT, and the further gradual
decrease in activity is due to the encapsulation of the iron carbide
nanoparticles. It can thus be suggested that the severe sintering
and carbon encapsulation of the iron carbide nanoparticles under
reaction conditions are the main reasons for the activity loss of
Fe/OCNT.
Acknowledgements
The work was performed under the project “Sustainable Chemi-
cal Synthesis (SusChemSys)”, which is co-financed by the European
Regional Development Fund (ERDF) and the state of North Rhine-
Westphalia, Germany, under the Operational Programme “Regional
Competitiveness and Employment” 2007–2013.
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walled carbon nanotubes applied as support in iron-based high-temperature Fischer–Tropsch synthesis, Catal. Today (2015),
http://dx.doi.org/10.1016/j.cattod.2015.09.023