Size and promoter effects on stability of carbon-nanofiber-supported iron-based Fischer-Tropsch catalysts

Article abstract of DOI:10.1021/acscatal.6b00321

The Fischer-Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural gas, coal, and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based catalysts at elevated temperatures favor the selective production of C₂-C₄ olefins, which are important building blocks for the chemical industry. Bulk iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of supported iron nanoparticles (initial sizes of 3-9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar, H₂/CO = 1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, initial catalytic activities were high, but substantial deactivation was observed over a period of 100 h. In situ M?ssbauer spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100% carbidization, whereas for unpromoted catalysts, this was around 25%. In situ carbon deposition studies were carried out using a tapered element oscillating microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts, whereas for promoted catalysts, carbon deposition occurred mainly over the first 4 h and thus did not play a pivotal role in deactivation over 100 h. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20-50 nm, thereby supporting the proposal that the loss of active Fe surface area was the main cause of deactivation.

Full text of DOI:10.1021/acscatal.6b00321

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Article
Size and Promoter Effects on Stability of Carbon Nanofiber
Supported Iron-Based Fischer-Tropsch Catalysts
Jingxiu Xie, Hirsa M. Torres Galvis, Ard Koeken, Alexey Kirilin,
Achim Iulian Dugulan, Matthijs Ruitenbeek, and Krijn P. de Jong
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00321 • Publication Date (Web): 13 May 2016
Downloaded from http://pubs.acs.org on May 17, 2016
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ACS Catalysis
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Size and Promoter Effects on Stability of Carbon Nanofiber Support-
ed Iron-Based Fischer-Tropsch Catalysts
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†
†,
‡
‡
§
∥
Jingxiu Xie , Hirsa M. Torres Galvis , Ard C. J. Koeken , Alexey Kirilin , A. Iulian Dugulan ,
‡
†,*
Matthijs Ruitenbeek and Krijn P. de Jong
†
Inorganic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universteitsweg
99, 3584 CG, Utrecht, The Netherlands
‡
Dow Benelux B.V., P.O. Box 48, 4530 AA, Terneuzen, The Netherlands
§
Fundamental Aspects of Materials and Energy Group, Delft University of Technology, Mekelweg 15, 2629 JB, Delft,
The Netherlands
ABSTRACT: The Fischer-Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural
gas, coal and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based catalysts at elevated tempera-
tures favor the selective production of C – C olefins, which are important building blocks for the chemical industry. Bulk
2
4
iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or
carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth
may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of
supported iron nanoparticles (initial sizes 3 – 9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar,
H2/CO = 1/1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, ini-
tial catalytic activities were high but substantial deactivation was observed over a period of 100 hours. In-situ Mössbauer
spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100 % carbidization while for un-
promoted catalysts this was around 25% only. In-situ carbon deposition studies were carried out using a Tapered Element
Oscillating Microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts while for promoted
catalysts carbon deposition occurred mainly over the first 4 hours and thus did not play a pivotal role in deactivation over
1
00 hours. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20 - 50 nm, thereby sup-
porting the proposal that the loss of active Fe surface area was the main cause of deactivation.
KEYWORDS: Fischer-Tropsch, FTO, iron, lower olefins, synthesis gas, stability, sintering
based catalysts, dispersing Fe nanoparticles on supports
INTRODUCTION
was attempted. A concern to the use of oxidic supports,
such as silica and high surface area alumina, is the inhibi-
tion of critical phase transformation to active Fe carbide
The Fischer-Tropsch synthesis (FTS) is a catalytic sur-
face polymerization reaction which converts synthesis gas
1
1
(
CO and H2) into valuable hydrocarbons, such as lower
species due to strong support-metal interactions. Thus,
weakly interacting supports such as nanostructured car-
bon materials and low surface area alumina were pre-
1
olefins. Synthesis gas can be produced from a wide array
of carbon sources including natural gas, coal, and biomass
and product selectivity towards chemicals and fuels can
be adjusted by catalyst design, which makes this a flexible
1
2–15
ferred for supported Fe FT catalysts.
Earlier research showed that sodium and sulfur promot-
ers, as well as the particle size of iron nanoparticles sup-
ported on the carbon nanofibers (CNF) affects activity
2–5
option to the industry.
Iron-based FT catalysts are favored due to their low
cost, high abundance, low methane formation, and useful
water-gas shift (WGS) activity. Bulk Fe catalysts (often
modified with promoters and synthesized via co-
precipitation or sintering) were extensively studied and
displayed promising results, but these suffered from poor
mechanical stability. Carbon formation which occurs via
the Boudouard reaction (2 CO → C + CO2), leads to
blocking of active sites and disintegration of catalyst par-
ticles. To improve on the mechanical properties of Fe-
1
6
and lower olefins selectivity. This breakthrough in in-
creasing the lower olefins selectivity made the direct pro-
duction of lower olefins from synthesis gas (Fischer-
Tropsch to Olefins, FTO) a more attractive option. How-
ever, the stability of the improved Fe FT catalysts re-
mained a challenge.
6–9
1
0
Deactivation of Fe FT catalysts could be due to Fe parti-
cle growth and/or carbon deposition, and/or transfor-
mation of active Fe carbide species into inactive Fe car-
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1
7–20
bide/oxide/metallic species. . Sintering, either via Ost-
wald ripening or particle migration and coalescence, re-
sults in loss of active Fe carbide surface area, and thereby
loss of catalytic activity. Phase transformation and carbon
deposition are speculated to be the main causes of deacti-
Preparation of unpromoted supported catalysts. Four
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unpromoted catalysts with different iron loadings (2, 5, 10,
20 wt.% Fe) were prepared using incipient wetness im-
1
6
pregnation as described previously. 7.014 g ammonium
iron citrate (Fluka, purum p.a., 14.5−16 wt % Fe) was dis-
solved in 25 mL demineralized water to form a stock solu-
tion. Depending on the iron loading, different volumes of
this stock solution were impregnated onto CNF to achieve
the desired loading. Except for the 2 wt.% Fe loaded cata-
lyst, every catalyst required successive impregnation
steps. The samples were dried under static air at 120 °C
between impregnation steps and after the final impregna-
tion step for 1 h and 2h respectively. Heat treatment was
performed at 500 °C for 2 h (5 °C/min; 100 mL/min for 2 g
catalyst) under the nitrogen flow. After cooling to room
temperature, the catalyst was passivated by controlled
surface oxidation. The oxygen concentration was in-
creased stepwise (2% v/v increase every 30 min) until
reaching 20% v/v. The number in the sample code indi-
cates the surface area-average particle size of iron nano-
particles measured by TEM.
2
1–28
vation for bulk Fe catalysts,
while the surface chemis-
try of support material dictates the deactivation mecha-
nism for supported Fe catalysts. Fe nanoparticles sup-
ported on oxidic supports were stable due to strong sup-
port-metal interactions, hence sintering and phase trans-
formations were not expected and C deposition was likely
to be the dominant cause for activity loss.
ly, Fe nanoparticles supported on weakly interacting sup-
ports were prone to Fe particle growth.
of deactivation is affected by the support material, there
are different strategies to improve catalytic stability by
means of proper support selection. Carbon supports are
widely used and to improve on the catalytic stability,
catalyst design focused on encapsulating Fe nanoparticles
0
1
2
3
4
5
6
7
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9
0
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3
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0
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7
8
9
0
2
9,30,15
Converse-
3
1–33
Since the mode
3
4,35
to prevent Fe particle growth.
In this work, the aim is to comprehend the effect of iron
particle size and the presence or absence of promoters on
catalyst stability. Various causes of deactivation, such as
Fe phase transformations, carbon deposition, and Fe par-
ticle growth will be assessed. To eliminate the contribu-
tion from oxidic supports, carbon nanofibers were used to
support the iron nanoparticles. Iron nanoparticles sup-
ported on CNF with/without Na and S promoters were
prepared via incipient wetness impregnation. By varying
the loading of iron between 2 to 20 %wt, iron oxide nano-
particles between 3 – 9 nm were obtained. Catalytic tests
were performed to determine the catalytic activity, selec-
tivity and stability. The as-synthesized and spent catalysts
were characterized ex-situ by transmission electron mi-
croscopy (TEM). In-situ Mössbauer spectroscopy was
used to characterize the iron phases under FTO condi-
Preparation of promoted supported catalysts. Four pro-
moted catalysts with different iron loadings (2, 5, 10, 20
wt.% Fe) were prepared using incipient wetness impreg-
nation as described above. The desired promoter loading
is shown in Table 1. 6.954 g ammonium iron citrate
(Fluka, purum p.a., 14.5−16 wt % Fe), 0.199 g sodium cit-
rate tribasic dihydrate (Sigma Aldrich, > 99.0 %), and
0.056 g iron(II) sulfate heptahydrate (Merck) were dis-
solved in 25 mL demineralized water to form a stock solu-
tion. Subsequent steps were performed as described
above. In addition to the number in the sample code
which indicates the surface area-average particle size of
Fe O nanoparticles measured by TEM, the letter P was
2
3
included to identify promoted catalysts.
Characterization. TEM was used to determine the iron
particle size distribution and the spatial distribution of
iron nanoparticles on the support. At least 300 iron nano-
particles per catalyst were measured to obtain an average
particle size. The images were attained with a Philips
Tecnai-20 FEG (200 kV) microscope equipped with an
EDX and HAADF detector. Temperature programmed
reduction (TPR) measurements were carried out with a
Micromeritics AutoChem II equipped with a TCD detec-
tor. Relevant reduction conditions were used, i.e. 350°C (5
tions.
A
tapered element oscillating microbalance
(
TEOM) was used to monitor the rate of carbon deposited
3
6
during the FTO process.
EXPERIMENTAL METHODS
Catalyst Preparation and Characterization.
Growth of CNF support. A 5 wt.% Ni/SiO catalyst (sieve
2
fraction of 425 – 825 μm) was synthesized via homoge-
3
7
nous deposition precipitation as reported previously. 5 g
of the catalyst was reduced under the flow of 190 mL/min
H and 625 mL/min N at a pressure of 2.8bar and 700 °C
°
C/min), 5 % H in Ar, 2 hours. The composition of the Fe
2
phases before reaction, after reduction, and after FTO
2
2
57
reaction was determined in-situ with transmission Fe
for 2 h (5 °C/min). Temperature was lowered to 550 °C
3.5 °C/min) after reduction, and carbon nanofibers with
57
Mössbauer spectroscopy. Transmission Fe Mössbauer
(
spectra were collected at 300 and 4.2 K with a sinusoidal
fishbone structure were grown by flowing diluted syngas
57
velocity spectrometer using a Co(Rh) source. Velocity
with the composition 102 mL/min H , 266 mL/min CO,
2
calibration was carried out using an α-Fe foil. The source
and the absorbing samples were kept at the same temper-
ature during the measurements. The Mössbauer spectra
and 450 mL/min N for 24 h. To remove SiO , the CNF
2
2
grown was refluxed thrice in 400 mL of 1 M KOH followed
by washing with demineralized water to pH 7. To remove
Ni and to introduce oxygen-containing groups on the
surface, the purified CNF was refluxed in 400 mL of 65 %
HNO3 for 1.5 h followed by washing with demineralized
water to pH 7.
38
were fitted using the Mosswinn 4.0 program. The exper-
iments were performed in a state-of-the-art high-pressure
Mössbauer in-situ cell – recently developed at Reactor
39
Institute Delft. The high-pressure beryllium windows
used in this cell contain 0.08% Fe impurity and its spec-
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ACS Catalysis
tored for 4 hours at 340 °C, 20 bar, CO/H = 1, and GHSV
tral contribution was fitted and removed from the final
2
-
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spectra. The Mössbauer transmission cell has a tubular
reaction chamber with an internal diameter of 15 mm, and
the catalyst bed lengths were 1.5 – 3 mm (catalyst loading
of 100-300 mg). Although the reactant gases pass through
the catalyst bed, the Mössbauer cell is not a plug-flow
= 54 000 h . Finally, the feed was switched back to pure
N gas feed after 4 hours of synthesis gas exposure. The
2
CNF support was also tested in a blank experiment and
no mass difference was observed during reduction and
Fischer-Tropsch reaction. To prevent accumulation of
hydrocarbon products in the catalyst bed, a high GHSV
was required. As the quartz element may break due to an
increase in the catalyst volume from coking, time of
stream (TOS) was limited to 4 hours.
3
reactor due to a large dead volume (~7 cm ) before the
catalyst bed. A total flow rate of 100 ml/min was used
during treatments, corresponding to a gas hourly space
-
1
velocity (GHSV) of about 12000 - 24000 h . The reaction
conditions were as described in the catalytic tests below.
0
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0
RESULTS AND DISCUSSION
Catalyst Performance. Catalytic experiments were
performed using high throughput fixed-bed reactors as
described elsewhere. The catalysts were first reduced in-
An overview of the fresh catalysts and their properties is
presented in Table 1. More information on elemental
loadings and the particle size distributions can be found
in Supporting Information, SI 1.
40
situ at 340 °C (5 °C/min), 3 bar, He/H = 2, GHSV = 7 200
2
-1
h
for 2 hours. Synthesis gas mixture (H /CO/He =
2
4
5/45/10) was introduced at 280 °C and 3 bar, and tem-
perature and pressure were subsequently increased over
.5 h to 340 °C and 20 bar. Two different gas hourly space
Table 1. Properties of as synthesized promoted and
unpromoted CNF-supported Fe catalysts.
0
-1
velocities (GHSV) were employed, specifically 7 200 h
and 54 000 h for mimicking industrially relevant condi-
a
Average particle
Wt. % loading
-1
size (nm)
tions and C deposition test conditions respectively. A
blank experiment using CNF support showed zero activity
under relevant FTO conditions. Catalytic activity, ex-
pressed as iron time yield (FTY), was expressed as moles
of CO converted per gram of Fe per second. CO conver-
sion (%) was calculated as:
b
Fe
2
Na
S
Fe O3
2
3Fe
4Fe
0.03
0.03
0.02
0.03
0.10
0.24
0.41
0.68
< 0.005
< 0.006
< 0.004
0.007
0.02
3
4
7
9
3
5
7Fe
9
9
Fe
15
2

CCO,R CHe,blk




3FeP
4FeP

XCO = 1−
⋅
⋅100%

5
0.04
4
7
9
CHe,R CCO,blk

(
Eq. 1)
7FeP
9
0.07
Where CCO, R and CHe,R correspond to concentration of
CO and He at the reactor outlet, respectively. COHe,blk and
CCO, blk correspond to concentrations of CO and He at the
outlet of the blank reactor. The product selectivity to
hydrocarbons up to C was determined with online gas
chromatography (GC) and was calculated on a carbon
atom basis. Selectivity towards CO was also measured.
This analysis method is consistent with previous litera-
ture.
9FeP
16
0.11
a
Wt. % loading determined using ICP-MS.
Surface area - average determined by TEM.
b
9
Tables 2 and 3 summarize the activity and product se-
lectivity of these supported Fe catalysts at low conver-
sions during the initial period (TOS = 12 h) and steady
state (TOS = 100 h), respectively. Activity and product
selectivity of these supported Fe catalysts at high conver-
sions are included in SI 2. Data in Table 2 show that the
addition of promoters resulted in increased activity and
product selectivity for CO , C -C olefins and C hydro-
2
16
C deposition. Carbon deposition was measured using a
TEOM (TEOM series 1500 Pulse Mass Analyzer, Rup-
precht & Patashnick Co.,Inc.). The procedure started
with flushing the tapered element with N at room tem-
perature and pressure. The pressure and temperature
were increased to 2 bar and 340 °C (10 °C/min) respective-
ly. The resulting decrease in gas density was reflected by a
decrease in the mass signal. Upon stabilization of the
mass signal which took ~4 hours, the gas feed was
switched to a mixture of N and H , resulting in a sharp
3
6
2
2
2
4
5+
16
carbons, and these promoted catalysts exhibited stable
product selectivities after 12 hours on-stream (Table 3).
Product selectivities were influenced by various factors,
for example, CO conversion and Fe particle size at differ-
ent times on-stream.
2
2
decrease in the mass signal. Each catalyst was reduced at
-
1
2
bar, 340 °C, N /H = 2, and GHSV = 54 000 h for 2
2 2
hours, and this reduction step was apparent from the
gradual decrease in the mass signal. After reduction, the
gas feed was switched from N and H mixture to pure N
2
to determine the mass loss during reduction. Stabilization
of the mass signal was again needed (~0.5 h) before syn-
thesis gas feed was introduced. C deposition was moni-
2
2
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Table 2. Catalytic performance of CNF-supported Fe catalysts under FTO conditions (340 °C, 20 bar, H /CO/He =
2
-1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
45/45/10, GHSV = 54 000 h , TOS = 12 h).
-3
Product Selectivity (% C , CO free)
CO conv.
FTY (10
at
2
CO sel. (%)
2
(
%)
molCO/g .s)
Fe
CH4
52
50
47
46
20
11
C – C olefins C – C paraffins
C5+
0
2
4
2
4
3
Fe
4Fe
Fe
Fe
3FeP
FeP
FeP
FeP
4
3
0.8
0.3
0.2
0.1
12
18
26
22
24
22
14
14
6
26
28
36
54
61
1
7
4
20
23
35
38
40
42
3
9
4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
7
1.7
12
22
25
27
4
7
0.7
0.5
0.3
7
10
16
8
60
60
6
9
8
5
Table 3. Catalytic performance of CNF-supported Fe catalysts under FTO conditions (340 °C, 20 bar, H /CO/He =
2
-
1
45/45/10, GHSV = 54 000 h , TOS = 100 h).
-
3
Product Selectivity (% C , CO free)
CO conv.
(%)
FTY (10
at
2
CO sel. (%)
2
molCO/g .s)
Fe
CH4
49
40
38
35
14
C – C olefins C – C paraffins
C5+
5
2
4
2
4
3
Fe
Fe
7Fe
Fe
FeP
4FeP
FeP
FeP
6
8
1.3
0.7
0.5
0.4
0.8
0.3
0.3
0.2
15
21
25
21
20
14
15
7
4
31
36
36
72
66
62
62
9
13
18
2
30
33
33
37
41
40
12
14
7
9
3
4
11
7
16
22
24
7
7
10
6
9
8
9
5
Figure 1. Iron time yield (FTY) of (a) unpromoted catalysts and (b) promoted catalysts (340 °C, 20 bar, H /CO/He =
2
-
1
45/45/10, and GHSV = 54 000 h ).
Figure 1 shows the catalytic activity as a function of
time, thereby providing insights in the stability of these
catalysts at high temperature and pressure. The un-
promoted catalysts had the lowest catalytic activity dur-
ing the initial period. On the other hand, the promoted
catalysts had the highest catalytic activity during the
initial period, but catalytic activity decreased over time
(Figure 1b). It is noted that the catalytic activity after 100
hours on-stream of promoted and unpromoted catalysts
was similar, except for 3Fe(P). Formation of different Fe
species, carbon accumulation, and Fe particle growth are
possible causes of deactivation, and these were further
investigated.
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plete reduction was observed and similar content of Fe
1
2
3
4
5
6
7
8
9
carbide phase was measured (Table S8 and Figure S11).
However, after 20 h of FTO conditions, 4FeP was fully
carburized and 4Fe had only 23 mol. % Fe carbide species
(
Figures 2b and 2d; and SI 3). The remaining Fe atoms
2+
were present as Fe species having Mössbauer spectra
similar to those of Wüstite and its nonstoichiometric
equivalent(Fe O). The promoters induced formation of
4
1
1
-x
more Fe carbide species in the initial period, which gave
higher catalytic activity. Alkali promoters such as Na are
4
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known to increase Fe carburization rate, but the addi-
4
3
tion of S showed the opposite effect. It is noted that
FeP had four times more Fe carbide species but its cata-
4
lytic activity was only twice higher than 4Fe.
Carbon deposition over time was monitored in-situ us-
ing the TEOM. No carbon deposition was measured dur-
ing 4 hours for the unpromoted catalysts with different
particle sizes (Figure S10) and it is proposed to be due to
the lack of active χ-Fe C phase. This proposal was sup-
5
2
ported by in-situ Mössbauer spectroscopy data (Figure 2)
and catalyst performance data. The promoted catalysts
showed carbon accumulation over time (Figure S11). Dur-
ing the early stages (first 30 minutes approximately),
there were 2 opposite occurrences for mass changes
which resulted in a negligible net mass change. Further
reduction of Fe oxide species to Fe carbide species caused
a decrease in mass, whilst carbon laydown due to the Fe
carbide species already present after reduction caused an
increase in mass. Thus, the net effect was a lower mass
change rate than when the catalyst reached a state where
no further reduction took place. Subsequently, the coking
rate decreased over 4 hours. It is believed that the meas-
ured mass change was caused directly by carbon build-up
and not hydrocarbons because of the reaction conditions
36
used. Carbon laydown rate decreased with increasing
st
particle size initially (1 hour), but increased with increas-
th
Figure 2. in-situ Mossbauer spectra of (a) 4Fe after reduc-
tion, (b) 4Fe after 20 h TOS, (c) 4FeP after reduction, and
ing initial particle size at steady state (4 hour). This
indicates that the effect of particle size on carbon deposi-
tion rate was different when the surfaces were relatively
clean and after reaction had proceeded for a longer peri-
od. Possible reasons for this decrease in coking rate in-
clude blocking of active sites (surface covered with amor-
phous or graphitic carbon), growth of Fe carbide particles,
and phase transformations that do not convert CO to
carbon. Thus, it appeared that the carbon deposition was
most severe in the initial period and was not the main
reason for deactivation over a longer period (10-100 h
TOS).
(
d) 4FeP after 20 h TOS. Reduction: 350 °C, 2 bar, Ar/H =
2
2
, 2 h. FTO: 340 °C, 20 bar, H /CO = 1, TOS = 20 h. (3 Fe
2
sites for Hägg carbide: magenta, red and purple; 1 Fe site
for ε’ carbide: dark cyan; 3 – 4 Fe sites for Fe O: violet,
1
-x
olive, navy, blue)
In-situ Mössbauer spectroscopy determined quantita-
tively the various Fe species present during reduction and
FTO conditions. Upon reduction, 25 mol. % Fe carbide
species were measured for both 4FeP and 4Fe (Figure 2a
and 2c; and SI 3). It is thus suggested that the addition of
promoters did not have a significant influence on the
reduction step. This observation was confirmed by the
similar TPR profiles obtained for 4PFe and 4Fe (Figure
S10). To further investigate the reduction behaviour of a
promoted catalyst (7PFe), Mössbauer spectra were meas-
ured after 2 hours and 24 hours reduction (Ar/H2 = 2, 350
The spent catalysts after carbon deposition studies (TOS
=
4 h) and after catalytic tests (TOS = 100 h) were charac-
terized with TEM, and images are shown in Figure S2 and
Figure 3 respectively. It was mentioned earlier that the
addition of promoters did not lead to a significant change
in Fe particle size distribution of the fresh catalysts. How-
ever, the Fe particle size distributions of spent promoted
°
C, 2 bar). Despite the longer reduction duration, incom-
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Figure 3. TEM images of spent (a to d) unpromoted catalysts and (e to h) promoted catalysts after carbon deposition studies
-
1
(
340 °C, 20 bar, H /CO = 1, GHSV = 54 000 h , TOS = 100 h). TEM-EDX maps of spent 7FeP catalyst (i to m).
2
and unpromoted catalysts were strikingly different. The
promoted catalysts displayed a higher degree of sintering
compared to the unpromoted catalysts. In the extreme
case (9FeP) depicted in Figure 3h, Fe particles of approx-
imately 100 nm were observed. In addition, the promoted
Fe nanoparticles seemed to have a core-shell structure
(
Figure 3e-h). TEM-EDX maps (Figure 3i-m) showed that
the core was rich in Fe and the shell was mainly Fe oxide.
The formation of Fe oxide was most likely due to expo-
sure to air after reaction.
In Figure 4, the average iron particle sizes of fresh and
spent catalysts are compared. The particle size distribu-
tions of spent catalysts can be found in SI 1. It was thus
shown that the promoted Fe nanoparticles produced
more carbon and displayed more sintering while the un-
promoted catalysts showed limited sintering even after
Figure 4. Iron particle size of ◊ promoted catalysts and
□ unpromoted catalysts after carbon deposition studies,
TOS = 4 h. Iron particle size of ♦ promoted catalysts and ■
unpromoted catalysts after catalytic tests, TOS = 100 h
-1
(340 °C, 20 bar, H /CO = 1, GHSV = 54 000 h ).
2
1
00 hours TOS.
Figure 5 reveals the effects of the particle size on the
stability of unpromoted Fe FT catalysts. The unpromoted
catalysts showed increased catalytic activity and limited
sintering over time, and it is proposed tentatively that the
increased catalytic activity is related to increased Fe car-
bidization.
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explain the relatively low carbon deposition rates when
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compared to other Fe-based catalysts. The sintering of Fe
nanoparticles supported on O-functionalized CNTs was
concluded previously to be more severe due to weak sup-
port-metal interactions and a low concentration of sur-
3
3
face defects for anchoring of Fe nanoparticles. Although
a weakly interacting support CNF was used in this study,
very different extent of particle growth was observed for
the promoted and unpromoted Fe nanoparticles. This
proves that the severe sintering of promoted Fe nanopar-
ticles was not solely due to weak metal-support interac-
tion. Intrinsically, the FTO process would be a high tem-
perature FT route, thus sintering is expected to be more
prominent than at the less severe conditions of low-
temperature FT, which is typical for Coal-to-Liquids
10
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Figure 5. Catalytic activity as a function of average iron
particle size of spent unpromoted catalysts □ at the initial
period, TOS = 4 – 10 h, and ■ at steady state, TOS = 100 h
-1
(
CTL).
(
340 °C, 20 bar, H /CO = 1, GHSV = 54 000 h ). Lines were
2
added to guide the eye.
CONCLUSION
Figure 6 depicts the effects of the particle size on the
stability of promoted Fe FT catalysts. The decrease in
catalytic activity appeared to be in agreement with the
increase in Fe particle size over time. The loss of surface
area per gram Fe follows the relation: surface area/volume
α 1/Fe particle diameter; thus, FTY was fitted to be in-
versely proportional to particle size (trend line in Figure
The activity, product selectivity and stability of CNF-
supported Fe catalysts under industrially relevant Fischer
Tropsch to Olefins (FTO) conditions were investigated. It
was observed that the activity of unpromoted catalysts
increased over time, regardless of particle size. With addi-
tion of promoters, maximum activity was attained in the
initial period and deactivation was prominent.
6
). The first data point (smallest average particle size)
appeared to be an anomaly, and that is attributed to the
presence of small particles around 3 and 4 nm. These
small Fe particles were shown previously to be highly
In-situ Mössbauer spectroscopy revealed that both pro-
moted and unpromoted catalysts attained similar Fe car-
bidization levels after reduction. However, after 20 h of
synthesis gas treatment, the promoted catalyst was fully
carbided and the carbidization level of the unpromoted
catalyst did not increase beyond 25 %. This difference in
phase transformation upon exposure to synthesis gas
resulted in a higher initial activity of the promoted cata-
lyst. As the correlation of activity and Fe carbide species
was not linear, deactivation via carbon deposition and/or
Fe particle growth is proposed to occur simultaneously
with phase transformations.
16
active and produced mainly methane.
A Tapered Element Oscillating Microbalance (TEOM)
was utilized to measure the rate of carbon deposition
under industrially relevant FTO conditions. No carbon
laydown was detected for the unpromoted catalysts, and
this was rationalized by the lack of active Fe carbide
phases. In contrast, the presence of promoters facilitated
Fe activation which resulted in significant carbon deposi-
tion over the first hours of operation. While different
particle sizes resulted in different coking rates, coking
rates decreased over time. This suggests that carbon dep-
osition is not the leading cause of deactivation over long-
er periods.
Figure 6. Catalytic activity as a function of average iron
particle size of spent promoted catalysts ◊ at the initial peri-
od, TOS = 4 – 10 h, and ♦ at TOS = 100 h (340 °C, 20 bar,
-1
H /CO = 1, GHSV = 54 000 h ). The trend line is fitted to y α
2
-1
x for all data points except the first point.
For the promoted catalysts, Fe particle growth was con-
cluded to be the main cause of deactivation. Phase trans-
formations, which occurred when water was produced
but not removed from the catalyst bed, were reported
Limited sintering was observed for the unpromoted cat-
alysts while severe sintering was seen for the promoted
catalysts. This indicated that the promoters led to for-
mation of mobile and active Fe phases which resulted in a
higher degree of particle growth.
25–27
previously to be a reason for deactivation;
however it
is not expected to be relevant here because of the use of
high space velocities and low conversion conditions. Car-
bon deposition rates decreased over the initial hours and
were not expected to play a significant role over a longer
period. The use of sulfur, in the absence of sodium, to
increase the resistance against carbon deposition was
For the unpromoted catalysts, phase transformation was
considered to be the leading cause for the increase in
catalytic activity over time. For the promoted catalysts,
catalytic activity was shown to be inversely proportional
29
demonstrated earlier, and this may be a possibility to
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to Fe particle diameter of spent catalysts which leads to
Page 8 of 16
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Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. Rev. 2007,
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107, 1692–1744.
the conclusion that here Fe particle growth is the main
reason of deactivation over time.
(
(
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In this work, the catalytic performance of CNF-
supported Fe catalysts under industrially relevant FTO
conditions were thoroughly and critically assessed. Alt-
hough the activity and C -C olefins selectivity of the
11)
2
4
12)
promoted Fe catalysts were highly encouraging, the sta-
bility needs improvement. Carbon deposition for Fe na-
noparticles was less significant compared to bulk Fe cata-
lysts, and is proposed not to play a pivotal role in the
deactivation. Sintering was, however, the major cause of
deactivation and it is hence believed that sintering is a
vital factor affecting stability of these highly active and
selective promoted Fe catalysts. Thus, the direction of
future research is on designing highly active and selective
Fe catalysts which are more resistant to sintering. It is of
interest for future studies to unravel the mechanism and
details of particle growth of promoted and unpromoted
Fe nanoparticles.
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ASSOCIATED CONTENT
Supporting Information. TEM characterization of fresh
and spent catalysts, catalytic performance, in-situ Mössbauer
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AUTHOR INFORMATION
(23)
Corresponding Author
(
(
24)
25)
* k.p.dejong@uu.nl
Present Addresses
∥
Albemarle Catalysts Company B.V., Nieuwendammerkade 1 –
, 1022 AB, Amsterdam
(26)
3
(
(
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28)
ACKNOWLEDGMENT
This research received funding from the Netherlands Organi-
sation for Scientific Research (NWO) in the framework of the
TASC Technology Area “Syngas, a Switch to Flexible New
Feedstock for the Chemical Industry (TA-Syngas)”. Dow and
Johnson Matthey are also acknowledged for the funding
received. KPdJ acknowledges the European Research Coun-
cil, EU FP7 ERC Advanced Grant no. 338846. H. Meeldijk
and W. Lamme are thanked for the TEM images. Peter
Bramwell is acknowledged for the TPR measurements. G.
Bonte (Dow Benelux) is thanked for catalyst performance
testing. M. Watson and L. van de Water (Johnson Matthey)
are acknowledged for fruitful discussion.
(29)
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2009, 355, 33–41.
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Am. Chem. Soc. 2010, 132, 935–937.
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X.; Hakeem, A. a.; Koeken, A. C. J.; Ruitenbeek, M.;
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2
54x175mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 13 of 16
ACS Catalysis
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138x73mm (300 x 300 DPI)
ACS Paragon Plus Environment

ACS Catalysis
Page 14 of 16
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1
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1
27x74mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 15 of 16
ACS Catalysis
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1
25x73mm (300 x 300 DPI)
ACS Paragon Plus Environment

ACS Catalysis
Page 16 of 16
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254x120mm (300 x 300 DPI)
ACS Paragon Plus Environment