Fischer-Tropsch synthesis: Effect of CO conversion on CH4 and oxygenate selectivities over precipitated Fe-K catalysts

Article abstract of DOI:10.1016/j.apcata.2018.04.042

The explanation for CH₄ selectivity for iron based Fischer-Tropsch catalysts in the low conversion region (i.e., <50%) remains elusive. In this contribution, the CO conversion effect was carefully examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1-5) over a wide range of CO conversion (i.e., 4-85%). Moreover, the effect of CO conversion on oxygenate selectivity of the Fe-K catalysts was carefully studied as well. The change in CH₄ selectivity with CO conversion was found to resemble asymmetric “V” shaped curves, with the minimum values occurring at approximately 50% CO conversion. Adding greater than x = 2 K significantly alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing the surface H coverage while improving CO adsorption. The unique CH₄ selectivity trend suggests a complicated CH₄ formation process that results from different aspects of the catalyst (i.e., chain growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range of 0.7-2.8% and varied with the CO conversion and K loading. The addition of K up to x = 3 was found to promote oxygenate formation and chain growth. The overall oxygenate distribution up to C₁₇ follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation (e.g., CO insertion versus CO dissociation) were proposed to explain the experimental results.

Full text of DOI:10.1016/j.apcata.2018.04.042

Accepted Manuscript
Title: Fischer-Tropsch Synthesis: Effect of CO Conversion on
CH4 and Oxygenate Selectivities over Precipitated Fe-K
Catalysts
Authors: Wenping Ma, Wilson D. Shafer, Gary Jacobs, Jia
Yang, Dennis E. Sparks, Hussein H. Hamdeh, Burtron H.
Davis
PII:
S0926-860X(18)30214-X
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.04.042
APCATA 16648
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
29-11-2017
27-4-2018
28-4-2018
Please cite this article as: Ma W, Shafer WD, Jacobs G, Yang J, Sparks DE, Hamdeh HH,
Davis BH, Fischer-Tropsch Synthesis: Effect of CO Conversion on CH4 and Oxygenate
Selectivities over Precipitated Fe-K Catalysts, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.04.042
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

Fischer-Tropsch Synthesis: Effect of CO Conversion on CH4 and Oxygenate Selectivities
over Precipitated Fe-K Catalysts
Wenping Ma,1 Wilson D. Shafer,1,2 Gary Jacobs,1,3 Jia Yang,1,4 Dennis E. Sparks,1 Hussein H.
Hamdeh,5 and Burtron H. Davis1,*
1University of Kentucky, Center for Applied Energy Research, Lexington, KY
40511, USA.
2Health and Science Department, Asbury University, Wilmore, KY 40390
3Chemical Engineering Program – Department of Biomedical Engineering & Department of
Mechanical Engineering, University of San Antonio, San Antonio, TX 78249
4
Department of Chemical Engineering, Norwegian University of Science and Technology,
Trondheim, Norway
5 Department of Physics, Wichita State University, Wichita, KS 67260
* Corresponding author: tel: +1 (859) 257-0251; email: burtron.davis@uky.edu
Highlights
CH4 selectivity displayed an asymmetric “V” trend with CO conversion
Adding K (x = 2+) suppressed the CO conversion effect on CH4 formation
Oxygenate selectivity was < 3% and was enhanced for K loadings up to x= 3
K promoted CO adsorption and inhibited H adsorption




Abstract
The explanation for CH4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion
region (i.e., < 50%) remains elusive. In this contribution, the CO conversion effect was carefully
examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1 – 5) over a wide
range of CO conversion (i.e., 4 - 85%). Moreover, the effect of CO conversion on oxygenate
selectivity of the Fe-K catalysts was carefully studied as well. The change in CH4 selectivity with
CO conversion was found to resemble asymmetric “V” shaped curves, with the minimum values
occurring at approximately 50% CO conversion. Adding greater than x = 2 K significantly
1

alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing
the surface H coverage while improving CO adsorption. The unique CH4 selectivity trend suggests
a complicated CH4 formation process that results from different aspects of the catalyst (i.e., chain
growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range
of 0.7 to 2.8% and varied with the CO conversion and K loading. The addition of K up to x = 3
was found to promote oxygenate formation and chain growth. The overall oxygenate distribution
up to C17 follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant
oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation (e.g.,
CO insertion versus CO dissociation) were proposed to explain the experimental results.
Keywords: Fischer-Tropsch synthesis; iron catalysts; CO conversion effect; CH4 selectivity
oxygenate selectivity; K promoter
Introduction
Despite the fact that the Fischer-Tropsch synthesis (FTS) has been studied extensively
studied the past nine decades [1-17], the influence of process conditions, such as the CO
conversion, on hydrocarbon (HC) selectivity for iron based catalysts is still not well established.
Significant advances in XTL (X = coal, natural gas (NG), biomass, and CO2) technology have
been made and many XTL plants have been commissioned worldwide in South Africa, Nigeria,
Malaysia, Qatar, and China [1-9]. Considering this rapid industrial growth, the issue of variation
of methane selectivity with conversion becomes increasingly important in order to optimize
heavier HC productivity and limit Greenhouse Gas emissions.
The CO conversion effect on CH4 formation for cobalt catalysts has been studied in a
systematic manner and consistent conclusions have been drawn [10-13]; the CH4 selectivity
2

decreases monotonously with increasing CO conversion up to 80%. This has been explained by a
negative kinetic effect of water on CH4 formation during FTS [18]. Unlike cobalt based catalysts,
the effect of CO conversion on iron based FTS catalysts has not yet been thoroughly studied; this
is in part because it is a more complex reaction system in which the water gas shift (WGS) reaction
is significantly involved. In several previous promoter effect studies [14-16] using continuously
stirred tank reactors (CSTR), the findings were seemingly inconsistent and therefore require
further investigation. Raje et al. [14] reported that CH4 selectivity increased monotonically from
10 to 15% with increasing CO conversion from 10 to 90% at 270 oC, 1.3 MPa and H2/CO = 0.67
for a 100 Fe/4.4 Si/0.5K catalyst; on the other hand, a virtually negligible change in CH4 selectivity
(i.e., ~5%) below 50% CO conversion was reported in subsequent K (0.36 to 2.2) and Cu (0.1-2.0)
promoter effect studies [15,16] as well as in a recent review [17]. Therefore, there is no affirmative
conclusion on the effect of CO conversion on CH4 formation of iron-based catalyst; furthermore,
how promoters impact the CO conversion effect remains unclear.
Oxygenates are produced simultaneously with hydrocarbons on iron based FTS catalysts.
The oxygenate selectivity of an iron catalyst is generally small (i.e., less than x = 3); however, this
is not negligible, especially considering the production capacity of a commercial plant. Thus,
determining the oxygenate selectivity and distribution is of great importance from an industrial
viewpoint. Ma et al. [19] reported that about 3% of converted carbon went to water soluble
oxygenates on x = 1 to 2 K promoted Fe/AC catalysts and K promoter was found to inhibit
o
oxygenate formation at 260-300 C and 2 MPa. Yang et al. [20] reported quite high oxygenate
selectivities (i.e., 7-31%) on x = 0.2 to 3 K promoted Fe-Mn catalysts under the conditions: 270-
300 oC, 2.50 MPa, and H2/CO = 2.0. The effects of CO conversion on oxygenate selectivity and
distribution over iron based catalysts have not been systemically studied.
3

Consequently, the current study was undertaken to carefully investigate the effect of CO
conversion on CH4 selectivity and selectivity for oxygenates and hydrocarbons for K promoted Fe
catalysts using a 1L CSTR. The process conditions 260-270 oC, H2/CO = 0.67 and 1.3 MPa were
employed. The upper limit of CO conversion was 85% while the lower limit of CO conversion
was 4%, a conversion that is low enough to give new insight into the CO conversion impact on
selectivity. The results for all catalysts were reproduced in the same run as well as in separate runs
by repeating the same conditions. Different mechanisms for oxygenate and hydrocarbon formation
were proposed to interpret the selectivity changes of the catalysts.
Experimental
Catalyst preparation
Four potassium promoted iron catalysts, 100 Fe/5.1Si/2Cu/ x K, where x = 1, 2, 3 and 5,
were used to investigate the CO conversion and K loading effects. To prepare the catalysts, a base
Fe catalyst (100Fe/5.1Si) was first prepared using a precipitation method. Details of catalyst
preparation can be found elsewhere [21-22]. The 100Fe/5.1Si/2Cu/xK catalyst was prepared by
sequential impregnation of appropriate amounts of Cu (Cu(NO3)22.5 H2O, 99.9% purity) and K
(KNO3, 99.9% purity) containing solutions. Between each step, the catalyst was dried under
vacuum in a rotary evaporator at 80°C and the temperature was then slowly increased to 95 °C.
After the impregnation/drying step, the catalyst was calcined under an air flow at 350 °C for 4 h
and sieved to a size range of 45-90 m. ICP results of the four Fe-K catalysts are provided in Table
1. The experimental compositions of all catalysts are consistent with the nominal compositions.
The BET surface areas of the Fe-K catalysts were ~110 m2/g. Note that the atomic ratio of Cu in
all iron catalysts (i.e., 100Fe/5.1Si/2Cu/xK) was maintained at 2 to ensure that the added Cu
promoter did not affect a determination of the K loading effect in this study; it has been reported
4

that changes in Cu loading alter CH4 selectivity to a certain extent by presenting either the Cu0 or
the Cu+1 state under syngas activation, respectively [23]. The 100 Fe/5.1Si/2Cu/1K, /2 K, /3K and
/5K catalysts are abbreviated as 1K, 2K, 3K and 5K catalysts, respectively.
Catalyst Testing
The FTS reaction for the iron catalysts (45-90 m) was carried out using a 1-L CSTR.
Durasyn-164 C30 oil (~300 g) was used as the startup solvent. The iron catalysts were charged to
o
the reactor for in-situ reduction by CO at 270 C and 1 atm for 24 h. After activation, the iron
catalysts were tested at 260-270 oC, 1.3 MPa, H2/CO = 0.67 over a wide range of CO conversion
(i.e., 485%). Steady state was achieved after 120 h. To ensure accuracy of the data, the process
conditions were repeated within the same run or by using a different run. The results were
reproduced and the mass balance period for each set of conditions lasted approximately 816 h.
The total and elemental closures were generally 100 ± 2%. The hydrogen and carbon monoxide
conversions (Xi), CH4 and oxygenate selectivities, partial pressures of hydrogen, carbon monoxide
and water (푃퐻 , 푃퐶푂, 푃퐻 푂) used in this paper are defined by the following formulas:
2
2
Xi, % = 100* (Ni, in - Ni,out)/Ni, in
(1)
(2)
(3)
(4)
(5)
(6)
CH4 selectivity, % = 100 * 푁퐶퐻 /(NCO, in - NCO,out -푁퐶푂
)
4
2
Oxygenate selectivity, % = 100 * Noxy/(Noxy + NHC)
푃퐻 = 푃퐻 0(1 − 푋퐻 )
2
2
2
0
(
)
푃퐶푂 = 푃퐶푂 1 − 푋퐶푂
0
0
푃퐻 푂 = 푃퐻 푁퐻 푂푁퐻
2
2
2
2
where i refers to CO or H2, Ni, in and Ni out are the molar flow rates of CO or H2 at the inlet and
outlet, respectively; 푁퐶퐻 and 푁퐶푂 are the molar formation rates of CH4 and CO2, respectively;
4
2
Noxy and NHC are the molar formation rates of oxygenate and hydrocarbon, respectively; 푃퐻 0 and
2
5

0
PCO are inlet partial pressures of H2 and CO; and 푁퐻 푂 and 푁퐻 0 are the molar formation rate of
2
2
water and the inlet molar flow rate of H2, respectively.
Analysis of FT products.
Quantification of the oil products was done using a 6890 Agilent (Santa Clara, CA) GC
with flame ionization detector (FID). No sample preparation was needed before injection of the
oil sample on the GC column. One microliter of sample was injected on an Agilent JW Scientific
DB-5 GC Column (Agilent, Santa Clara) at 35 oC and 8 psi with a 1.5 ml/min split flow.
Aqueous samples were injected onto the SRI8610 GC with thermal conductivity detector
(TCD) and packed (1/8” x 6m Porapak) column at 20 psig, and 100 °C, and also to an 8610C SRI
GC using the same column and method. The water component was identified by matching
retention times of known standards. Calibration of the SRI8610 GC was performed by injecting a
standard solution containing 91.6% water, 2.0% C1-OH, 3.7% C2-OH, 1.9% C3-OH and 0.8% C4-
OH to quantify the water product composition.
Gas from the CSTR was analyzed online with a Hewlett-Packard Quad Series Micro GC
(MicroGC 3000 from Agilent) specifically used as a refinery gas analyzer (RGA). This GC has
four columns that run in parallel (MolSieve 10 m 5A, Plot U (PPU) 8 m, Alumina 10 m, OV-1 10
m). Two pumps in the GC allow the same volume of sample to be injected into the four columns
(being held at a constant temperature and pressure), quickly separating specific compounds in each
column.
Specific oxygenates for all the runs were only identified (i.e., not quantified) using an
Agilent 5975N MSD directly connected to a 6890 GC using the same GC conditions and column
as the 6890 Agilent GC-FID. By doing this, the peaks observed in the chromatographs could be
matched between the FID and MSD, where the retention times and patterns of specific compounds
6

are the same. Thus, this allows for specific product verification (by the MSD) and quantification
(by FID) for as many of the FT products as possible. Note that calibration of each component
detected by the oil GC FID was not possible. Unit response factors for each hydrocarbon product
(C4-C35) were applied; alcohol components were suppressed in the FID signal, and thus the
response factors of 0.46-0.86 taken from reference [24] were used to calculate oxygenate
composition (C4-C17) in the oil phase.
Catalyst characterization
Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to analyze
Fe-K catalyst composition. Meanwhile, powder X-ray diffraction (XRD) was carried out for the
reduced Fe-K catalysts at room temperature using a Rigaku Diffractometer (DMAX-B) operating
with Cu K radiation (1.54 Å). This was used to determine if size of iron carbide changed greatly
with K loading. Before the XRD experiment, the Fe-K samples were reduced at the same
conditions (270 oC for 24 h by CO) as used for the FTS reaction tests followed, with the exception
that they were passivated using 1%O2 in nitrogen.
Mössbauer spectroscopy measurements were conducted in transmission mode, with a 57Fe
source mounted in a standard constant acceleration velocity transducer, in order to find out whether
iron phases change significantly with CO conversion. The used 3K Fe catalysts, in slurry form,
were collected after separate runs at a high (i.e., 80%) or low (i.e., 30%) conversion for 192 h. The
samples were placed in a copper holder and mounted near the finger of a vibration-free closed-
cycle refrigerator which provided temperatures as low as 20 K. Structural analysis of the samples
was performed by least-squares fitting of the Mössbauer spectra to a summation of hyperfine
sextets. The least squares fitting procedure employed user-defined functions within the Peak Fit
program. The parameters for each sextet in the fit consisted of the position, width and height of
7

the first peak, the hyperfine magnetic field, and the quadrupole electric field. These parameters
were allowed to vary freely to obtain the best fit of the experimental data. Errors in the determined
percentages of Fe are about ±3 % for well-resolved spectra; in those that contain several iron oxide
and carbide phases, the uncertainty increases with the complexity of the spectrum (i.e., depending
on the degree of overlap and the weakness of the signal).
Results and Discussion
Effects of K loading and CO conversion on CH4 selectivity.
CH4 selectivity for the K promoted iron based catalysts is generally low (i.e., 2-6%) at
typical FTS conditions due to the significant role of K promoter in promoting CO dissociation and
o
chain growth. Table 2 gives CH4 selectivities at 250 to 270 C over the four Fe-K catalysts.
Apparently, both the 1K and 3K Fe catalysts displayed low CH4 selectivity within a narrow range
over the temperature range 250-270 oC. For example, between 75 and 80% CO conversion, CH4
selectivity varied between 5.1 and 5.6% for the 1K catalyst and between 2.3 and 3.3% for the 3K
catalyst. Therefore, the CH4 selectivity of the Fe-K catalysts is not very sensitive to temperature.
o
At 260 C, CH4 selectivity decreased from 5.3 to 2.1% with increasing K from x = 1 to 5. This
strongly suggests a role played by K in enhancing CO dissociation and promoting chain growth
for iron catalysts.
The CO conversion effect on CH4 selectivity in the low CO conversion range of 10-15%
was examined in previous studies [14-16]. In this contribution, the four Fe-K catalysts (i.e., 1K,
2K, 3K and 5K) were tested and CO conversion levels as low as 4% were used to obtain a more
pronounced CO conversion effect on selectivity and the impact of K loading. All data were
reproduced in the same run or in separate runs (reproduced results not shown), demonstrating
8

reliability of the data. Note that the K promoter also affects the activity of the iron catalyst, possibly
by altering the degree of iron reduction/carburization, or the dispersion. However, this is outside
the scope of the current study. Additional information regarding the effect of K on Fe catalyst
activity can be found elsewhere [19-21,26].
The changes in CH4 selectivity with CO conversion over 1K, and 2K, 3K and 5K catalysts
were measured at 270 oC and 260 oC, respectively, and the results are displayed in Figure 1. For
the 1K catalyst, a well-defined parabolic shaped curve was obtained, which indicates a prominent
CO conversion effect on CH4 selectivity for the iron catalyst. In particular, the CO conversion
effect was quite distinct in the lower CO conversion range of 4.5 to 50%. CH4 selectivity was as
high as 9.0% at a CO conversion of 4.5% but at a conversion of 50% it decreased to 4.6%.
Interestingly, CH4 selectivity tends to increase after this point, reaching 6% at 85% CO conversion.
A similar parabolic shaped CH4 selectivity versus CO conversion curve was also observed over
the 2K and 3K catalysts with the minimum occurring at about the same conversion, 50% (Figure
1). However, compared to Figure 1 (top), a notable difference is that CH4 selectivity of the 2K
and 3K catalysts was much lower (i.e., 3-4%, and 2.6 -3.5% for the 2K and 3K catalysts,
respectively) compared to the 1K catalyst, and the degree of change was smaller as the conversion
increased from 4 to 85%, respectively. When K was further increased to x = 5, CH4 selectivity was
decreased further to ~1.9% and the degree of change became even smaller, which indicates that
the conversion effect can be virtually neglected in the case of the 5K catalyst. Therefore, CO
conversion has a distinct effect on CH4 selectivity for the K promoted iron catalysts, and it was
dramatically suppressed by increasing to a 5K loading, where the CO conversion effect was almost
eliminated. Note that Cu was present in all four Fe-K catalysts; however the Cu promoter is not a
dominant reason for the CO conversion effect with diffeent Fe-K catalysts. Rather, the K loading
9

played the major role. To confirm this conclusion, Table 3 lists the FTS data of 100Fe/5.1Si/1.3K
with and without Cu. Cu had very little effect on various hydrocarbon selectivities for the
100Fe/5.1Si/1.3K catalyst at 75% and 42-48% CO conversion levels. This is consistant with Cu
promoter effect studies of Fe-K catalysts [25, 26].
The asymmetric V-shaped CH4 selectivity versus CO conversion curves for the Fe-K
catalysts are presumed to be governered by the process conditions and the nature of the Fe-K
catalyst. In the low CO conversion region (i.e., < 50%), the H2/CO ratio shown in Figure 2, which
was adjusted by the WGS reaction, decreased slightly from 0.61 to 0.55 for the 1K and 2K catalysts,
and this contributes in part to the decrease in CH4 selectivity. For the 3K and 5K catalysts, the
H2/CO ratio in the low conversion region remained nearly constant (i.e., 0.650.67) due to the
promoting effect of K on the WGS reaction, which in turn adjusts to a greater extent the 푃퐻 . This
2
implies that the decrease in, or relatively unchanged, CH4 selectivity with CO conversion on the
two catalysts was less affected by the change in process conditions, but rather that the K promoter
provided a greater role in changing CO or H2 adsorption and chain growth, which is consistent
with many studies regarding the role of K on the adsorption of H2 and CO on iron catalysts (e.g.,
K increases CO chemisorption and increases Fe-C bond strength [26-31], and K decreases H
chemisorption and weakens Fe-H bond strength on iron based catalysts [26-28,31]). This
conclusion is further evidenced by the significant decrease in the CO conversion effect with
increasing K loading. For example, the percentage change in the CH4 selectivity (100 x (CH4
selectivity at t0 - CH4 selectivity at t1)/CH4 selectivity at t0) in the low conversion region decreased
greatly from 49 to 3% as the K loading increased from 1 to 5% (Table 4). The results are again
consistent with previous studies [27-31] on the role of K in strengthening CO adsorption and
weakening H2 adsorption, respectively, with K promoting chain growth [19,26] on iron catalysts,
10

resulting in lower CH4 selectivity and lower extents of change on the higher K loading catalysts.
At the high CO conversion level (i.e., > 50%), the WGS activity of the Fe-K catalysts was
enhanced significantly, which led to a precipitous increase in the H2/CO ratio (0.6 to 0.88 for 1 K,
0.67 to 1.09 for 2-5 K in Figure 2) with increasing CO conversion; this reversed the CH4 selectivity
trend such that it increased with further increases in CO conversion. Table 4 shows that CH4
selectivity had similar percentage increases (i.e., 3033%), which was accompanied by different
extents of increase in the H2/CO ratio for the 1-3 K catalysts (i.e., greater change in H2/CO ratio
on 23 K than 1 K), while it was only 20% for the 5 K catalyst. This result again demonstrates
that higher K loading weakens the CO conversion effect at the high CO conversion range by
promoting CO adsorption and suppressing H2 adsorption.
From the kinetic point of view, CH4 selectivity is a function of the coverages of surface
species CH3, C or CH if assuming CH4 is formed through the hydrogenation of surface CH3 groups
(CH3-s + H-s -> CH4 +2s, where s is an active site):
[
]
[ ]
푠퐶퐻 = 푟퐶퐻 /푟퐹푇 = 푘푀 퐶퐻3 /푘퐹푇 푥
(7)
4
4
Where kM, kFT are rate constants for the CH4 formation and overall FT reaction, x can be a species
such as C, CH, CH2, CO or CHO, etc. in terms of surface carbide (also known as alkyl or CH2
insertion) [27,28], CO insertion [29,30], enol [31,32], alkenyl [33], or H-assisted CO dissociation
(HCO-s + H2 (or H-s) –> CH2-s) [34-37] mechanisms. Since the surface [CH3] is associated with
the surface [H] and chain growth probability 1, [CH3] = 1 [H], eq (7) can be rewritten as
follows:
(
)([ ] [ ])
푠퐶퐻 = 푘푀/푘퐹푇 퐻 훼1/푘퐹푇 푥
(8)
4
where 1 = (kg [CH2]/(kg[CH2] + kM[H]) if it is assumed that the CH2 intermediate is a chain growth
monomer (Cn-1H2n-1-s + CH2-s -> CnH2n+1 + 2s). Since the surface concentration of the CH2 species
11

is associated with the chain growth rate and chain termination rate constants (i.e. catalyst intrinsic
property) and partial pressures of various reactants and products (process conditions), the
formation of CH4 is complicated and governed by intrinsic factors related to the catalyst as well
as process conditions. Thus, the previously discussed asymmetric V-shaped CH4 selectivity versus
conversion patterns over different Fe-K catalysts shown in Figure 1 and Table 2 are likely
consistent with this kinetic view, and suggest a rather complicated CH4 formation process that is
determined by both the nature of the catalyst and process conditions. It appears that the catalyst
properties played a greater role relative to process conditions in controlling CH4 formation at low
conversion. On the other hand, process conditions had a greater influence on CH4 formation at the
high conversions for the low K containing catalyst. The high K loading significantly diminished
CH4 formation and the effect of conditions on its formation. This could suggest that a) surface H
coverage was higher on the low K loading catalyst, b) surface adsorbed CO on the high K loading
Fe catalyst was dominant and a lean H environment might exist, and c) CH4 was formed mainly
through the hydrogenation of a monomer such as CH2 with adsorbed H but not with molecular
gas-phase H2 since 푃퐻 on the 1K catalyst was even lower than the other high K loading catalysts
2
(푃퐻 results not shown) due to lower WGS activity.
2
The water produced by FTS could slightly change the CH4 selectivity of the iron catalyst
as well. Isotopic and water cofeeding studies [38-40] reported that indigenous water positively or
negatively affected the FTS rate and CH4 formation for cobalt catalyst, respectively, which was
attributed to the water increasing surface active carbon species. Taking into account the negative
role of water on CH4 selectivity, a better explanation of the CH4 selectivity trend is expected.
Figure 3 shows the variation of 푃퐻 푂with XCO for the Fe-K catalysts. For the 1 and 2 K catalysts,
2
volcano shaped 푃퐻 푂 versus XCO curves were observed, peaking in the range of 4050% CO
2
12

conversion (0.07-0.075 MPa). The rapid increase in 푃퐻 푂 with CO conversion likely resulted in an
2
additional contribution on reducing the CH4 selectivity if the negative impact of water on CH4
formation is considered. For the 3K and 5K catalysts, 푃퐻 푂 was significantly lower and increased
2
almost linearly with increasing CO conversion over the entire range of conversion (Figure 3). Thus,
the increased 푃퐻 푂 partial pressure might slightly decrease CH4 selectivity in the low CO
2
conversion range for the 3K and 5K catalysts; above 50% CO conversion, the WGS reaction likely
played a dominant role in adjusting CH4 selectivity for the 3K catalyst. Nevertherless, the
conditions did not greatly impact CH4 selectivity with the 5K catalyst due to the dominant role of
K on CO dissociation and chain growth.
The selectivity trends for the CH4 and oxygenates as discussed above did not result from
the iron phase changes occurring at different conversion levels. To confirm this, the 3K iron
catalyst was selected to run at high (80%) and low (30%) CO conversion level for 192 h,
respectively, with the other conditions being identical. The two used catalysts were characterized
by Mössbauer spectroscopy, and the results are shown in Figure 4. Three phases of iron, i.e. χ-
Fe5C2, -carbide, and magnetite (Fe3O4), were detected in both samples and are represented by
three blue sextets, two green sextets and one red sextet, respectively. Interestingly, the intensities
of the three iron phases in the two spectra were similar; the composition was 49-56% magnetite,
5-6% -Fe2.2C and 39-45% χ-Fe5C2; namely, the iron catalyst phases did not change significantly
over a wide range of CO conversion. Since these iron phases were formed during FTS conditions
as evident in the current and previous Mössbauer spectroscopy data, the observed CH4 selectivity
trends were likely caused by the CO conversion effect, but not the minor iron phase transformation
during the reaction.
13

The XRD spectra of four reduced Fe-K catalysts are shown in Figure 5. All four reduced
Fe-K catalysts show similar XRD patterns in the 2 range of 25o - 85o. The two most intense
reflections observed at 2 of 35.5o and 43.5o represent the characteristic peak of Fe3O4 (PDF# 00-
o
003-0863), while the small peak observed at 40 C could be identified as Fe2C5 (PDF# 00-051-
0997). The diffraction peak for the iron carbides expected to occur at 2 = 42.6 o 45o are not
observed in this study, which could be due to the effect of O passivation or air oxidation of the
samples after the passivation. The intensity of the corresponding magnetite at 43.5 o in all cases
are similar indicating similarly sized iron oxide species. To illustrate if K changed the size of iron
carbide domains which are usually assumed to be FTS active sites, the crystal size of iron carbides
at the characteristic peak at 40o were calculated using the Scherrer equation. The iron carbides
were estimated to be 20.5, 16.3, 15.0 and 17.1 nm for the reduced 1K, 2K, 3K and 5 K catalysts,
respectively, which indicates that the iron carbide domain size did not change greatly with K
loading. The XRD of the used iron catalysts for this study were not successfully obtained, since
not enough sample was obtained after extraction. However, comparing the above XRD results with
that of the used 3K catalyst reported previously [41], it is found the domain size of iron carbide
for these reduced iron catalysts (15-21 nm) and the used 3K catalyst at 40o (19.8 nm) are similar.
Thus, the XRD results of the Fe-K catalysts is in line with the above discussed Mossbauer results,
and suggests that the CO conversion effect played a dominant role in changing catalyst selectivity;
moreover, the effect was affected by the K loading rather than the domain size.
Effect of CO conversion on oxygenate selectivity of Fe catalysts
Oxygenates are products of the FTS reaction with precipitated-iron based catalysts and
can be upgraded to useful chemicals [19,42,43]. However, information about the effect of CO
14

conversion on oxygenate selectivity is rather scarce. In this paper, we explored the changes in
oxygenate selectivity and distribution with conversion and K loading in the 485% range. The
oxygenates were found to be present in aqueous and oil phases of the iron catalysts (see Figure
6a-b). The water phase was generally comprised of up to C5 oxygenates while detectable amounts
up to C17 were found in the oil phase. The amounts in both phases were quantified and summed
for a comparison. Figure 7 depicts the change in oxygenate selectivity with CO conversion for
all four Fe-K catalysts.
o
With the 3K catalyst at 260 C, the oxygenate selectivity was low (i.e., ~0.5%) at 4% CO
conversion; it increased rapidly with increasing CO conversion and reached 2.3% at 20% of CO
conversion. After that point, it continued to increase to 2.7% at a slower rate up to ~50% CO
conversion; the oxygenate selectivity then reversed its trend and decreased with further increases
in CO conversion, dropping to 1.95% at 80% of CO conversion. On the 1 K and 2 K catalysts,
the oxygenate selectivity displayed similar patterns, but the peak values of 2.9% and 2.2%
occurred at 270 and 260 oC, respectively for 1 K; on the other hand, a peak value of 2.5% for 2
o
K at 260 C was observed. Compared to the 3 K catalyst, the 1 K and 2 K catalyst displayed
o
lower oxygenate selectivity at the same temperature (260 C), with more oxygenate products
being present in the aqueous phase. This is consistent with the finding of Dry et al. (i.e., for
precipitated FeSiK) [42] that oxygenate formation was enhanced by increasing potassium content.
Dictor and Bell [43] (using supported Fe2O3/K), Bukur et al. [26] (using precipitated FeCuK)and
Ma et al. [19] (using supported FeCuK/AC) suggested that K promoter suppresses oxygenate and
methanol formation. The different conclusions about the K effect on oxygenates in these studies
might originate from using a different basis, for example, in the latter case, with the conclusion
being made only on the basis of the water phase analysis, while the oxygenates in the oil phase
15

were not considered. However, it must be noted that much higher K loadings (e.g., x = 5) did not
result in additional increases in oxygenates. Furthermore, according to Figure 7, high temperature
slightly suppressed oxygenate formation in the high conversion range, i.e. >50%.
The variation in oxygenate selectivity with CO conversion and K loading might suggest
that different mechanisms operate for oxygenate versus hydrocarbon formation. It has been
postulated that CO insertion is a chain growth path to form CH3-CO or R-CO intermediates,
which can be hydrogenated by H or OH groups to form different aldehydes, alcohols, acids, esters
and oxygenates. Assuming this is the case, the fraction of CO insertion at low CO conversion
could be low due to low contact time, resulting in low oxygenate selectivity. Increasing CO
conversion appeared to promote CO insertion that favored oxygenate formation. However, a
further increase in CO conversion (i.e., above 20-50%) likely hindered CO insertion but promoted
CO adsorption and dissociation to form hydrocarbons. The result is consistent with the view that
competitive reactions operate between oxygenate and hydrocarbon formation. However, given
that iron carbides are active sites for hydrocarbon formation, the active sites for oxygenate
formation on Fe-K catalysts remain unclear. It is speculated that some iron carbides, or more
likely an interface between iron phases, could be responsible for oxygenate formation. On cobalt
catalysts, while hydrocarbon formation is proposed to occur on metallic surface sites, oxygenate
formation has been proposed to occur on cobalt carbides or a cobalt-promoter interface such as
Co-Ce, or Co-Cu [44-46]. In the case of Co-Ce, cobalt has been proposed to be metallic whereas
the ceria is a partially reduced oxide, where water and oxygenates are activated on ceria, with the
metal assisting in H-transfer reactions. In one view, CO reacts with bridging OH groups on
partially reduced ceria to form formates or a related species, which serves as an intermediate for
molecular CO transfer from the support across the interface to the metal; this assists in terminating
16

the growing hydrocarbon chains by molecular CO insertion. One might envision a similar
scenario with iron, whereby iron carbide serves to promote H-transfer, with Fe3O4 serving as the
partially reduced oxide. However, further research is needed to test this hypothesis.
In light of oxygenate and hydrocarbon formation mechanisms as discussed above, a CO
hydrogenation pathway for different products is illustrated in Figure 8.
Effect of CO conversion and K loading on oxygenate distribution
The representative oxygenate distribution obtained on the 1K and 3K catalysts is provided
in Figure 9. C1-C5 alcohols were the main oxygenates detected for both Fe-K catalysts, varying
from 0.5-8%, while 2-butanone, acid, acetone, and acetaldehyde propionaldehyde were lower
(e.g., less than 0.5%). Among the oxygenates, the distribution of C1-C4 alcohols was 1-1.5%; 1.5-
4.0%, 0.6-2.0% and 0.5-1.0% for the 1K catalyst and 0.5-1.0%, 1-8%, 1-2% for the 3K catalyst.
Ethanol was the dominant alcohol for both catalysts and increased with CO conversion, especially
for the 3K catalyst. It is speculated that the surface concentration of methyl (CH3) groups on the
catalyst surface was higher due to high K or high CO driving CO dissociation; this in turn leads
to greater ethanol production than other alcohols by the CO insertion mechanism. The
assumption is in line with a mechanistic study for oxygenate formation on iron catalysts using in-
situ DRIFTS [47], which proposes that acetyl (CH3CO) was the main chain growth monomer and
hydrogenation of the monomer was a crucial step for oxygenate formation.
The total amount of oxygenates in the aqueous phase for the Fe-K catalysts was about 3-
15% under typical FTS conditions. If 10-20% aqueous products are produced with FTS
hydrocarbons on Fe-K catalysts, the low percentage of water still represents a significant amount
of high value oxygenates being formed during FTS. For example, if an FTS plant has a capacity
of 1 MT synfuels (HCs) per year, the plant would produce 3000 - 20,000 tons of alcohols each
17

year in addition to synfuels. Thus, the current results highlight the significance of utilizing XTL
technology to convert coal, natural gas, or biomass to transportation fuels.
Combining C1-C5 oxygenates present in the water phase with the C4-C17 oxygenates
present in the oil phase of the iron FTS, a representative overall oxygenate distribution of the 3K
catalyst is presented in Figure 10. Similar to the hydrocarbon distribution, the overall alcohol
distribution of the 3 K catalysts follows the trend predicted by the ASF kinetic model quite well.
The chain growth probability had a slight decrease with increasing CO conversion above 20%.
The  values for the oxygenates and CO conversion for the 3 K catalyst were: 0.32 (4.3%), 0.79
(16.1%), 0.77 (34.5%), 0.75 (53.6%) and 0.72 (78%). This low  value is likely due to holdup of
oxygenates in the liquid phase. This is consistent with increased hydrocarbon chain growth with
increasing CO conversion. As previously mentioned, only C1-C8 oxygenates at various
conversions for the 1 K catalyst were detected (results not shown), indicating low oxygenate chain
growth on the 1 K catalyst relative to the 3K catalyst.
4. Conclusions
The effects of CO conversion on CH4 selectivity were studied over four K promoted iron
catalysts (i.e., 1K, 2K, 3K and 5K) using a 1 L CSTR. A wide range of CO conversion (i.e., 4-
85%) was examined at 260-270 oC, 1.3 MPa, and H2/CO = 0.67. The CH4 selectivity versus CO
conversion was found to display an asymmetric “V” shaped curve, with the lowest CH4 selectivity
occurring at ~50% CO in all cases. However, the CO conversion effect was suppressed remarkably
with increasing K loading. At lower conversion levels (i.e., less than 50%), CH4 selectivity
exhibited a greater change (from 49% to 3%), and it depended on K loading. At higher conversions,
greater than 50%, the changes were relatively smaller (from 20% to 30%). From a kinetic point
18

of view, the unique CH4 selectivity trend highlights the complexity of CH4 formation, which can
be explained by the nature of the Fe-K catalyst and process conditions (e.g., CO conversion level).
CH4 selectivity decreased greatly from 5.2 to 1.9% with increasing K loading from 1 to 5K. The
results suggest that (1) high K loading remarkably suppressed H2 adsorption such that a hydrogen-
lean environment was created, with H2 pressure being a main factor to adjust CH4 selectivity; (2)
high K loading greatly promoted CO adsorption and dissociation and promoted chain growth; and
(3) surface H, but not gas phase H2, likely participated in hydrogenation to CH4 product.
The effect of CO conversion on oxygenate selectivity for the Fe-K catalysts was studied as
well. The product spectrum of the Fe-K catalyst featured low oxygenate selectivity (i.e., less than
3%) that was comprised of mainly alcohols, with ethanol being the dominant alcohol. Up to C17
oxygenates were detected and followed an ASF distribution. The oxygenate selectivity versus CO
conversion exhibited a volcano-shaped curve, and a maximum occurred in the range of 20-50%
CO conversion. Up to 3K loading enhanced oxygenate formation and promoted oxygenate chain
growth. The CO insertion mechanism was found to satisfactorily explain the oxygenate trend with
CO conversion.
The Mossbauer and XRD results show that structural changes and particle size effects on
the CO conversion effect were negligible.
Acknowledgments
This work was made possible by the Commonwealth of Kentucky. The authors are like to greatly
thank G. Thomas for the ICP analyzing work and S. Hopps for the XRD experiment.
19

References
[1]
A.P. Steynberg, M.E. Dry (eds.), Fischer-Tropsch Technology, Stud. Surf. Sci. Catal., 152,
2004, pp. 1–680.
[2]
R. Overtoom, N. Fabricius, W. Leenhouts, Shell GTL, from Bench scale to World scale, in
Advances in Gas Processing, Proceedings of the 1st Annual Gas Processing Symposium,
(Eds.: H.E. Alfadala, G.V.R. Reklaitis, M.M. El-Halwagi), 10–12 January 2009, Doha,
Qatar, 2009, pp. 378–386
[3]
[4]
[5]
H.C. Heng, S. Idrus, J. Nat. Gas Chem. 13 (2004) 63–70.
A.Y. Khodakov, Catal. Today 144 (2009) 251–257.
D. A. Wood, C. Nwaoha, B.F. Towler, J. Nat. Gas Sci. Eng. 9 (2012) 196–
208.
[6]
[7]
[8]
[9]
D.B. Bukur, B. Todic, N. Elbashir, Catal. Today 275 (2016) 66–75.
J. Xu, Y. Yang, Y.-W. Li, Fuel 152 (2015) 122-130.
Z.Y. Liu, S.D. Shi, Y.W. Li, Chem. Eng. Sci. 65 (2010) 12–17.
Significant
coal-to-liquid
project
in
production
in
Ningxia,
http://www.china.org.cn/china/2016-12/29/content_40003229.htm
[10] Ø. Borg, S. Eri, E.A. Blekkan, S. Storsæter, H. Wigum, E. Rytter, A. Holmen, J. Catal. 248
(2007) 89-100.
[11] W. Ma, G. Jacobs, Y. Ji, T. Bhatelia, D.B. Bukur, S. Khalid, B.H. Davis, Top. Catal. 54
(2011) 757-767.
[12] D.B. Bukur, Z. Pan, W. Ma, G. Jacobs, B.H. Davis, Catal. Lett. 142 (2012) 1382-1387.
[13] J. Yang, G. Jacobs, T. Jermwongratanachai, V.R.R. Pendyala, W. Ma, D. Chen, A. Holmen,
B.H. Davis, Catal. Lett. 144 (2014) 123-132.
20

[14] A.P. Raje, B.H. Davis, Catal. Today 36 (1997) 335-345.
[15] A.P. Raje, R.J. O’Brien, B.H. Davis, J. Catal. 180 (1998) 36.
[16] R.J. O’Brien, B.H. Davis, Catal. Lett. 94 (2004) 1-6.
[17] J. Yang, W.P. Ma, D. Chen, A. Holmen, B.H. Davis, Appl. Catal., A: Gen. 470 (2014)
250– 260.
[18] W. Ma, G. Jacobs, T.K. Das, C.M. Masuku, J. Kang, V.R.R. Pendyala, B.H. Davis, J.L.S.
Klettlinger, C.H. Yen, Ind. Eng. Chem. Res. 53 (2014) 2157-2166.
[19] W. Ma, E.L. Kugler, D.B. Dadyburjor, Energy Fuels 21 (2007) 1832-1842.
[20] Y. Yang, H.-W. Xiang, Y.-Y. Xu, L. Bai, Y.-W. Li, Appl. Catal. A: Gen. 266 (2004) 181-
194.
[21] B.H. Davis, Technology Development for Iron Fischer-Tropsch Catalysis, DOE final
report, 1998.
[22] B.H. Davis, G. Jacobs, W. Ma, D. Sparks, K. Azzam, J.C. Mohandas, W. Shafer, V.R.R.
Pendyala, Sensitivity of Fischer-Tropsch synthesis and water-gas shift catalysts to poisons
from high-temperature high-pressure entrained-flow (EF) oxygen blown gasifier
gasification of coal/biomass mixtures, DOE final report, 2011.
[23] V.R.R Pendyala, G. Jacobs, M.K. Gnanamani, Y.Hu, A. MacLennan, B.H. Davis, Appl.
Catal. 495 (2015) 45–53.
[24] J.T. Scanion, D.E. Willis, J. Chromat. Sci. 23 (1985) 333.
[25] W.P. Ma, E.L. Kugler, D.B. Dadyburjor, Energy Fuels 2011, 25, 1931–1938.
[26] D.B. Bukur, D. Mukesh, S.A. Patel, Ind. Eng. Chem. Res. 29 (1990) 194-204.
[27] F. Fischer, H. Tropsch, Brennstoff Chem. 7 (1926) 97-104.
[28] R.C. Brady, R. Pettit, J. Am. Chem. Soc. 103 (1981) 1287-1289.
21

[29] H. Pichler, H. Schulz, Chem. Ing Tech. 42 (1970) 1162-1174.
[30] B. Todic, W.P. Ma, G. Jacobs, B.H. Davis, D.B. Bukur, Catal. Today 228 (2014) 32–39.
[31] R.B. Anderson, R.A. Friedel, H.H. Storch, J. Chem. Phys. 19 (1951) 313-319.
[32] B.H. Davis, Catal. Today 141 (2009) 25–33.
[33] P.M. Maitlis, R. Quyoum, H.C. Long, M.L. Turner, Appl. Catal., A: Gen. 186 (1999) 363-
374.
[34] G.P. van der Laan, A.A.C.M. Beenackers, Appl. Catal. A: Gen. 193 (2000) 39–53.
[35] F.G. Botes, B.B. Breman, Ind. Eng. Chem. Res. 45 (2006) 7415-7426.
[36] M. Ojeda, R. Nabar, A.U. Nilekar, A. Ishikawa, M. Mavrikakis, E. Iglesia, J. Catal. 272
(2010) 287–297.
[37] L.P Zhou, X. Hao, J.H Gao, Y. Yang, B.S. Wu, J. Xu, Y.Y. Xu, Y.W. Li, Energy Fuels 25
(2011) 52–59.
[38] C.J. Bertole, C.A. Mims, G. Kiss, J. Catal. 210 (2002) 8496.
[39] S. Loegdberg, M. Boutonnet, J.C. Walmsley, S. Jaeras, A. Holmen, E.A. Blekkan, Appl.
Catal. A: Gen. 393 (2011) 109.
[40] W. Ma, G. Jacobs, D.E. Sparks, R.L. Spicer, B.H. Davis, J.L.S. Klettlinger, C.H. Yen,
Catal. Today 228 (2014) 158-166.
[41] M.E. Dry in J.R. Anderson, M. Boudart (Eds.), Catalysis Science and Technology, Vol. 1,
Springer-Verlag, New York, 1981, pp 175-183.
[42] Ma, W.; Jacobs, G.; Thomas, G.A.; Shafer, W.D.; Sparks, D.E.; Hamdeh, H.H.; Davis,
B.H., ACS Catalysis 5 (2015) 3124-3136.
[43] R.A. Dictor, A.T. Bell, J. Catal. 97 (1986) 121-136.
[44] G.G. Volkova, T.M. Yurieva, L.M. Plyasova, M.I. Naumova, V.I. Zaikovskii, J. Mol. Catal.
22

158 (2000) 389-393.
[45] G. Prieto, S. Beijer, M.L. Smith, M. He, Y. Au, Z. Wang, D.A. Bruce, K.P.de Jong, J.J.
Spivey, P.E.de Jongh, Angew. Chem. Intl. Ed. 53 (2014) 6397-6401.
[46] M.K. Gnanamani, M.C. Ribeiro, W.P. Ma, W. D. Shafer, G. Jacobs, U.M. Graham, B.H.
Davis, Appl. Catal., A: Gen. 393 (2011) 17–23.
[47] W.Y. Mao, Q.W. Sun, W.Y. Ying, D.Y. Fang, Ranliao Huaxue Xuebao 41 (2013) 314-322.
23

Table 1. ICP analysis results for FeSiCuK catalysts
Catalyst
ICP analysis of Fe catalysts, atomic%
Fe
Si
Cu
1.8
2.3
1.9
2.1
K
100 Fe/5.1Si/2Cu/1.3K
100 Fe/5.1Si/2Cu/2 K
100 Fe/5.1Si/2Cu/3K
100 Fe/5.1Si/2Cu/5K
100
100
100
100
5.3
5.0
5.1
4.8
1.2
1.8
3.1
4.4
Table 2. CH4 selectivity of 1K and 3K iron catalysts(a)
K loading
x =
Time
h
XCO
%
CH4 selectivity, %
250 oC
260 oC
270 oC
1.0
2.0
3.0
5.0
100-220
100-150
90-150
75
82
80
78
5.1
-
5.3
3.9
3.0
2.1
5.6
-
2.3
3.3
120-140
-
-
(a) Other reaction conditions: 1.31 Mpa, H2/CO = 0.67, and 2.0-8.0 NL/g-cat/h.
24

Table 3. Selectivities of the iron catalysts with and without Cu(a)
Selectivity, %
XCO
(%)
CO2
selectivity, %
Catalyst, %
TOS (hrs)
CH4 C2-C4 C5+
100Fe5.1Si1.25K
72-97
76.45
48.35
74.65
42.75
6.06 21.07
5.45 23.82
5.78 21.80
5.77 24.04
72.88 45.44
70.72 45.51
477-501
121-335
435-464
100Fe5.1Si2Cu1.25K
72.43 47.70
70.19 44.59
(a) Reaction conditions: 270 oC, 1.31 Mpa, H2/CO = 0.67, and 2.0-19 NL/g-cat/h.
Table 4. Percentage change in CH4 selectivity in the low and high CO conversion ranges
CH4 sel. change%
CO conversion level
1K
2K
-40
30
3K
-25
33
5K
-3.2
20
Low XCO region (4-50%)
High XCO region (50-85%)
-48.8
30
25

5
4
3
2
1
0
2 K
3 K
5 K
0
20
40
60
80
100
CO conversion, %
Figure 1. Change in CH4 selectivity with CO conversion over 1 K catalyst (top) at 270 oC, 1.3 MPa,
H2/CO = 0.67, and 2K, 3K and 5K catalysts at 260 oC, 1.3 MPa, H2/CO = 0.67.
26

1.4
1.2
1
1 K
2 K
3 K
5 K
0.8
0.6
0.4
0
20
40
60
80
100
CO conversion, %
Figure 2. Change of 푃퐻 푂/푃퐶푂 ratio in the reactor with CO conversion over 1 K catalyst (270 oC,
2
1.3 MPa, H2/CO = 0.67) and 2K, 3K and 5 K catalysts (260 oC, 1.3 MPa, H2/CO = 0.67).
27

1K
2K
3K
5K
0.08
0.06
0.04
0.02
0.00
0
20
40
60
80
100
CO conversion, %
Figure 3. Change in 푃퐻 푂 in the reactor with CO conversion over 1 K catalyst (270 oC, 1.3 MPa,
2
H2/CO = 0.67) and 2K, 3K and 5 K catalyst (260 oC, 1.3 MPa, H2/CO = 0.67).
28

-10
-5
0
5
10
-10
-5
0
5
10
Velocity (mm/s)
Velocity (mm/s)
Figure 4. Mössbauer spectra of used 3 K catalyst at high (left) and low (right) conversions. Left:
(XCO = 80%): Fe3O4 (red line): 56%; Fe2C5 (blue line): 38%; Fe2.2C (green line): 6% and low
CO conversion (i.e., 30%). Right: (XCO = 30%): Fe3O4 (red line): 49%; Fe2C5 (blue line): 44%,
Fe2.2C (green line): 7%. Other process conditions: 260 oC, H2/CO = 0.67 and 1.3 MPa.
29