Adsorption and reaction of CO and hydrogen on iron-based Fischer-Tropsch synthesis catalysts

Article abstract of DOI:10.1016/j.molcata.2010.05.020

H₂, CO chemisorptions and the carbon hydrogenation on promoted iron Fischer-Tropsch synthesis (FTS) catalysts (Fe/SiO₂ and FeK/SiO₂) were investigated using temperature programmed surface reaction with X-ray photoelectron spectroscopy and laser Raman spectroscopy. It is found that potassium, used as promoter, does not lead to a distinct variation in the carbon species but changes the surface H/C ratio of carburized catalysts. Besides bulk iron carbide, several carbon species with different cluster sizes exist in the carburized catalysts, which have different reactivities towards hydrogen. The surface atomic carbon, the oligomerized carbon species and the bulk iron carbide are more reactive to hydrogen, whereas the large-size amorphous carbons are relatively inert to hydrogen. There is a good correlation between the chemisorption of H₂ or CO and the corresponding feed gas conversion activity in FTS reaction. Meanwhile, the methane selectivity is correlated with the hydrogenation capability of catalysts, indicating that the surface H concentration has an important effect on the selectivity of hydrocarbons.

Full text of DOI:10.1016/j.molcata.2010.05.020

Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Adsorption and reaction of CO and hydrogen on iron-based Fischer–Tropsch
synthesis catalysts
Chenghua Zhang∗, Guoyan Zhao, Kangkai Liu, Yong Yang, Hongwei Xiang, Yongwang Li
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27#, Taiyuan 030001, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
H2, CO chemisorptions and the carbon hydrogenation on promoted iron Fischer–Tropsch synthesis (FTS)
catalysts (Fe/SiO2 and FeK/SiO2) were investigated using temperature programmed surface reaction with
X-ray photoelectron spectroscopy and laser Raman spectroscopy. It is found that potassium, used as
promoter, does not lead to a distinct variation in the carbon species but changes the surface H/C ratio of
carburized catalysts. Besides bulk iron carbide, several carbon species with different cluster sizes exist in
the carburized catalysts, which have different reactivities towards hydrogen. The surface atomic carbon,
the oligomerized carbon species and the bulk iron carbide are more reactive to hydrogen, whereas the
large-size amorphous carbons are relatively inert to hydrogen. There is a good correlation between the
chemisorption of H2 or CO and the corresponding feed gas conversion activity in FTS reaction. Meanwhile,
the methane selectivity is correlated with the hydrogenation capability of catalysts, indicating that the
surface H concentration has an important effect on the selectivity of hydrocarbons.
Received 11 March 2010
Received in revised form 30 April 2010
Accepted 27 May 2010
Available online 4 June 2010
Keywords:
Fischer–Tropsch synthesis
Iron catalyst
H2 adsorption
CO adsorption
Carbon hydrogenation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
there is evidence [12,13] that the bulk carbide does not appreciably
participate in the hydrocarbon synthesis. As for inactive carbons,
The depleting resource and increasing price of crude oil have
revived the research interest for petroleum substitutes [1,2]. Based
on Fischer–Tropsch synthesis (FTS) technology, carbon-containing
resources (natural gas, coal and biomass) could be converted to
clean liquid fuels and high-value chemicals. This process is con-
sidered as an alterative route to relax the pressure of depleting
petroleum and would act as the foundation of the energy and chem-
ical industry in post-oil era.
The FTS reaction is a typical CO hydrogenation on heteroge-
neous catalysts, which starts from the dissociation of CO and H2
on transient metal surface and follows with the hydrogenation and
polymerization of surface carbon species. Under reaction condi-
tions, the working catalysts, especially for iron, are great reservoirs
of carbons: bulk carbide, surface inactive carbon and surface active
carbon. Using temperature-programmed techniques or transient
methods, at least four kinds of carbon species have been indenti-
fied in iron catalysts [3–6]: (a) adsorbed, atomic carbon species; (b)
amorphous, lightly polymerized carbon species; (c) iron carbides;
and (d) graphitic surface carbon species. These carbon species play
different roles in FTS reaction. For example, it is widely accepted
that iron carbides are the active phases [7–11], which provide the
active sites for CO activation and hydrogenation. At the same time,
some are inert while some are notorious poisons, such as graphite
or coke. Different from the former two kinds of carbon species, sur-
face active carbons are the key intermediates in CO hydrogenation,
which have important influences on the activity, selectivity and
stability of catalysts. Dwyer and Haredenergh [14] suggested that
the competitive conversion of the reactive surface carbon to the
surface polymeric carbon or graphitic carbon over the iron carbide
surface determines the catalyst lifetime. Cao et al. [15] calculated
the competitive reactions of surface carbon hydrogenation and cou-
pling on Fe5C2(0 0 1) surface from first principles. They found that
the C–C coupling of the surface carbon is more favorable thermo-
dynamically while the surface hydrogen will hinder C–C coupling
and prevent the deactivation of Fe5C2(0 0 1) surface.
In FTS reaction, methane is one of main products but not the
objective products. Stockwell et al. [13] found that various hydro-
carbons are likely to be originated from the same CH precursor.
This indicates that the surface carbon hydrogenation to methane is
competitive with the C–C coupling to the valuable high-molecular-
weight hydrocarbons. Therefore, it is of great practical importance
to investigate the hydrogenation of surface carbon species. In addi-
tion, potassium or alkali metals are the most effective promoters to
suppress the formation of methane. At the moment, one question
arises in one’s mind. Which is the decisive factor for the methane
formation, surface carbon species or alkali promoters?
In this study, the surface carbon species on carburized Fe/SiO2
and FeK/SiO2 catalysts were investigated using temperature-
∗
Corresponding author. Tel.: +86 351 7117176; fax: +86 351 7560668.
E-mail address: zhangchh@sxicc.ac.cn (C. Zhang).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.020

36
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
programmed desorption and hydrogenation (TPD and TPH),
Mössbauer effect spectroscopy, X-ray photoelectron spectroscopy
(XPS) and laser Raman spectroscopy (LRS).
2. Experiments
Fig. 1. Treatment and characterization protocols of Fe/SiO2 and FeK/SiO2 catalysts
during carburization and hydrogenation.
2.1. Catalyst preparation
Two catalysts (Fe/SiO2 and FeK/SiO2) were used in this study.
The Fe/SiO2 precursor was prepared by continuous co-precipitation
using Fe(NO3)·9H2O (99.9%, Tianjin Chemical Co., PR China) and
acidic silica sol (30 wt% SiO2, Tianjin Chemical Co., PR China) as raw
materials. The precipitation was carried out in a 4 L precipitated
batch maintaining temperatures at 70 ◦C and pH value at 9.0 0.1.
The silica gel was added into ferric nitrate solution before precip-
itation with Fe/Si atomic ratio of 100/15. After precipitation, the
precipitate was filtered. The filtered cake was consequently dried
at 120 ◦C for 24 h and calcined at 500 ◦C for 5 h. The FeK/SiO2 precur-
sor was prepared through mechanically mixing certain amount of
Fe/SiO2 and KHCO3. This kind of potassium addition method has
been proved effective [16]. These catalysts have Fe/SiO2 atomic
ratio of 100/15 or Fe/K atomic ratio of 100/3.
treatments and the characterization protocols are presented in
Fig. 1.
The Mössbauer spectra of carburized catalysts were recorded at
room temperature with a MR 351 constant-acceleration Mössbauer
spectrometer (FAST, Germany), using a 25 mCi57Co (Pd) source. The
spectrometer was operated in a symmetric constant acceleration
mode. The spectra were collected over 512 channels in the mirror
image format.
The XPS spectra were collected on a XPS analyzer (VG, Milti-
Lab 2000), which has a monochromatized Mg K␣ X-ray source
(300 W). All binding energies of XPS spectra are calibrated with the
peak position of adventitious carbon, which normally is adjusted
to 284.6 eV. The relative surface concentrations of the metals were
determined using the whole peak area of Fe 2p and C 1s and their
corresponding sensitivity factors of 2.957 and 0.296, respectively.
The Raman spectra were recorded on a Raman microscope instru-
ment (Renishaw, UV–vis Raman 1000) equipped with an Argon ion
laser (ꢀ = 514 nm). The carbon content in the catalyst was analyzed
using an automatic elemental analyzer (Elementar, Vario EL Cube,
Germany).
2.2. Catalyst characterizations
A transient reaction system with on-line mass spectroscopy was
used for TPD and TPH measurements. Gaseous feeds were con-
trolled and metered using mass flow controllers. Gases were passed
through a series of purification traps containing de-oxygen agent
and zeolite to remove oxygen and water. The outlet of the microre-
actor was connected to a quadruple mass spectrometer (OmniStar
200, Balzers, Switzerland) through a capillary inlet system.
2.3. Catalyst performance testing
H
2 temperature-programmed desorption (H2-TPD) was used to
The FTS experiments were conducted in a 12 mm i.d. stainless
steel fixed-bed down-flow reactor with an effective bed length
of approximately 15 cm (15 cm3 bed volume). Approximately 3 g
of catalyst was loaded in the isothermal region of the reactor.
The remaining volume of reactor was filled with quartz granules
with particle size of 10–20 and 20–40 mesh. All the catalysts were
reduced in syngas with H2/CO ratio of 2.0 (v/v) at 280 ◦C, 0.5 MPa,
and 1000 h−1 for 16 h. The reaction conditions were controlled at
250 ◦C, 1.5 MPa, 4000 h−1 and H2/CO = 2.0 (v/v).
measure the hydrogen adsorption and desorption on H2-reduced
and CO-carburized catalysts. For H2-TPD on H2-reduced catalysts,
the catalyst sample (100 mg) was firstly reduced in pure H2 at
400 ◦C for 16 h and cooled to 50 ◦C. Then, the reduced sample was
purged with Ar until the baseline of H2 signal leveled off. Finally,
the sample was heated to 800 ◦C at ramp of 10 ◦C/min. For H2-TPD
on carburized catalysts, the catalyst sample was firstly carburized
in 5%CO/95%He at 300 ◦C for 5 h. Then, the feed gas was switched
to He and the sample was flushed to 50 ◦C. At this temperature,
the carburized sample was flushed with H2 for 30 min and conse-
quently purged with He until the baseline of H2 signal leveled off.
Finally, the sample was heated to 800 ◦C at ramp of 10 ◦C/min.
CO-TPD was used to measure the CO adsorption and desorp-
tion behavior on carburized catalysts. The catalyst sample (50 mg)
was firstly carburized in 5%CO/95%He at 300 ◦C for 5 h and cooled
to 50 ◦C. Then, the carburized sample was purged with He until
the baseline of CO signal leveled off. Finally, the temperature was
increased to 800 ◦C at ramp of 10 ◦C/min.
TPH was used to investigate the hydrogenation behavior of cat-
alysts. Catalysts (50 mg) were loaded into the quartz tube reactor.
The catalyst sample was firstly carburized in 5%CO/95%He at 300 ◦C
for 5 h and cooled to 50 ◦C. Then, the carburized sample was flushed
with pure H2 until the signal leveled off. Finally, the temperature
was increased to 800 ◦C at the heating rate of 10 ◦C/min.
In order to quantify the surface carbon species in catalysts, the
catalyst samples were characterized with Raman spectrum and X-
ray photoelectron spectroscopy (XPS) at different treatment stages
in the TPH process. The objective of the work was to study the
surface/bulk carbon species in carburized catalysts and their trans-
formations in the TPH process. Therefore, three typical stages (after
carburization, during the TPH and after the TPH) were chosen
for the catalyst characterization. The carburization–hydrogenation
3. Results and discussion
3.1. H2 and CO chemisorptions
The catalysts were carburized in 5%CO/95%He at 300 ◦C for
300 min. Then the iron phase compositions of carburized catalysts
were measured using Mössbauer spectroscopy, as listed in Table 1.
It can be seen that catalysts after carburization contain iron carbide
(Fe5C2) and superparamagnetic Fe3+ and Fe2+ oxides. Specifically,
the Fe/SiO2 catalyst contains 21.4% of iron carbide and 78.6% of iron
oxides while the FeK/SiO2 catalyst contains 56.5% of iron carbide
and the remainder iron oxides.
In Fig. 2, the H2-TPD curves of H2-reduced catalysts are shown.
It can be seen that the TPD curve of Fe/SiO2 catalyst show an
intense peak at about 100 ◦C and a group of overlapping peaks at
above 300 ◦C. In the presence of potassium, all the peaks decrease
markedly in intensity. As reported in literature [17], the H2 thermal
desorption peaks on Fe single crystal surfaces occurred at around
100 ◦C. In the present study, H2 desorption peaks at about 100 ◦C are
corresponding to the H species adsorbed on the metallic iron sur-
face. The peaks at higher temperature may be due to the cleavage
of OH species on the difficultly reduced oxide surface in catalysts.

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
37
weak H adsorption site at lower temperature and a strong H adsorp-
tion site at higher temperature, on carburized iron catalyst surface.
Meanwhile, potassium suppresses the H2 adsorption on carbur-
ized iron catalysts. Although iron carbides are considered as the
FTS active phases, there is nearly no experimental report on the H2
adsorption on iron carbide surfaces. The reason for this situation
seems to be the difficulty to prepare a clean and single iron car-
bide surface or phase. However, there are a few of papers dealing
with the chemisorption of hydrogen on iron surfaces [17,18–23].
On Fe(1 1 0), (1 1 0) and (1 1 1) single crystal surfaces, the hydrogen
adsorption is dissociative and the initial adsorption energies are
between 80 and 110 kJ/mol [17,21]. The corresponding H2 ther-
mal desorption peaks occurred at below 250 ◦C with heating rates
in range of 5–20 ◦C/s [17,18,20,21]. Additionally, the H2 desorp-
tion peaks on the carbon contaminated Fe(1 0 0) surface occurred
at even lower temperatures [21,24]. In the present study, the H2
desorption peaks on carburized catalysts occurred at above 250 ◦C,
which are higher than those on iron crystal surfaces. These H2 des-
orption peaks are not likely to result from the H species on Fe sites
in carburized catalysts. Brucker and Rhodin [25] had suggested that
the adsorption of hydrogen on a “carbonated” Fe(1 0 0) surface at
−175 ◦C led to the formation of CH species. The theoretical calcula-
tion has indicated that, for H2 adsorption on Fe5C2(0 0 1), (1 1 0) and
(1 0 0) surfaces, the most stable surface species is CH. Meanwhile,
the activation energies of CHads decomposition on transition met-
als are generally higher than 100 kJ/mol [15,26–28]. In this study,
the MES results indicated that catalysts were not completely car-
burized and there were a large amount of iron oxides remained in
catalysts. Therefore, H2 desorption peaks on carburized catalysts
appear to be a combination of the decomposition of surface CH
species and the cleavage of surface OH species. From the H2-TPD
results, a direct evidence is that potassium apparently suppresses
the H2 adsorption on either H2-reduced or carburized catalyst sur-
faces. It is in good agreement with reports that potassium is a very
strong poison for H2 dissociation [29,30].
Fig. 2. H2-TPD profiles on Fe/SiO2 and FeK/SiO2 catalysts reduced in pure H2 at
400 ◦C for 16 h.
In Fig. 4, CO–TPD curves of CO-carburized catalysts are pre-
sented. As shown in Fig. 4, the CO desorption peaks of catalysts
all locate in temperature range of 300–600 ◦C. For Fe/SiO2 cata-
lyst, the CO desorption curve demonstrates a broad peak at 375 ◦C
and a more intense peak at 468 ◦C. With the addition of potassium,
the CO desorption peak intensity increases obviously and the peak
position shifts to higher temperatures. Specifically, the FeK/SiO2
catalyst shows a multi-peak overlapped curve with the maximum
peak position at 548 ◦C and somewhat less defined shoulders at
lower temperature. There are lots of papers studying on the CO
adsorption on iron surfaces [19,21,31,32] but little work on iron car-
bide surfaces [33]. As reported in these studies, the CO adsorption
on a clean Fe(1 0 0) surface resulted in four desorption peaks, which
were designated as three molecular states at −23, 67, and 157 ◦C
and a dissociative state at 527 ◦C [21,31,32]. The CO adsorption
on a H2-reduced iron catalyst [19] produced a similar CO desorp-
Fig. 3. H2-TPD profiles on Fe/SiO2 and FeK/SiO2 catalysts carburized in 5%CO/95%He
at 300 ◦C for 5 h.
In Fig. 3, H2-TPD curves of CO-carburized catalysts are pre-
sented. As shown in Fig. 3, the H2 adsorption on carburized catalysts
produces H2 desorption peaks at range of 250–500 ◦C. For Fe/SiO2
catalyst, the H2-TPD curve demonstrates a sharp peak at 455 ◦C and
a somewhat less defined shoulder at lower temperature. In con-
trast, FeK/SiO2 catalyst shows a peak at 435 ◦C and a broad shoulder
at higher temperature. Moreover, the peak intensity of FeK/SiO2
decreases sharply relative to that of Fe/SiO2 catalyst. From the H2-
TPD curves, it is clear that there are two kinds of H adsorption sites, a
Table 1
MES parameters and iron phase composition of Fe/SiO2 and FeK/SiO2 catalysts carburized in 5%CO/95%He at 300 ◦C for 5 h.
Catalysts
Fe/SiO2
Phase
Mössbauer parameters
IS, mm/s
QS, mm/s
Hhf, kOe
Area, %
␹-Fe5C2
0.40
0.22
0.19
0.30
1.02
−0.26
0.22
185
218
105
4.2
11.0
6.2
69.3
9.3
−0.16
Fe3+ (spm)
Fe2+ (spm)
0.94
1.66
FeK/SiO2
␹-Fe5C2
0.29
0.25
0.12
0.35
0.87
−0.15
0.10
185
219
108
21.5
11.3
23.7
42.7
0.9
−0.04
Fe3+ (spm)
Fe2+ (spm)
0.92
1.54

38
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
Fig. 4. CO–TPD profiles on Fe/SiO2 and FeK/SiO2 catalysts carburized in 5%CO/95%He
Fig. 5. TPH profiles of Fe/SiO2 and FeK/SiO2 catalysts carburized in 5%CO/95%He at
at 300 ◦C for 5 h.
300 ◦C for 5 h.
[5]. Thus, the CH4 desorbed peak in range of 340–360 ◦C can be
ascribed to the hydrogenation desorption of atomic carbon on sur-
face, which is signed as ␣ carbon (C␣). The reaction temperature
of carbon species with H2 at 390–480 ◦C is similar with that of
the oligomerized carbon species, which is signed as ␤ carbon (C␤).
There is one main carbon species reacted with H2 at 490–550 ◦C,
which is very close to the temperature of hydrogenation reaction of
bulk carbides. Thus, this kind of carbon species is signed as ␥ carbon.
The last carbon species on the catalysts surface is hydrogenated at
above 600 ◦C close to the temperature of hydrogenation reaction
of graphite. Thus, this kind of carbon species could be attributed to
the graphitic carbon.
As shown in Table 2, the Fe/SiO2 catalyst has relatively higher
content of ␣ carbon than the FeK/SiO2 catalyst. Meanwhile, in
the presence of potassium, the peak positions of different carbon
species all shift to higher temperatures. It should be mentioned
that there is a small amount of graphitic carbon on the Fe/SiO2
catalyst rather than on the FeK/SiO2 catalyst. One possible reason
is that the Fe/SiO2 catalyst produces graphite-like carbon species
during the carburization while the FeK/SiO2 does not. Another
probable reason is that graphitic carbon forms on both catalysts,
whereas the hydrogenation capability of the Fe/SiO2 catalyst is
strong enough to hydrogenate the graphitic carbon to methane.
Actually, the MES results have proved a relatively lower carburiza-
tion degree of the Fe/SiO2 catalyst. At the same time, Table 2 also
lists the carbon contents and the total peak areas of CH4 evolution
on both carburized Fe/SiO2 and FeK/SiO2 catalysts. As shown in the
table, the carbon content in the carburized Fe/SiO2 catalyst is one
fourth of that in the carburized FeK/SiO2 catalyst while the total
amount of carbon hydrogenation to CH4 during TPH process on the
Fe/SiO2 catalyst is fourfold higher than that of the FeK/SiO2 cata-
lyst. According to MES and TPH results, the FeK/SiO2 catalyst has
higher carbide content after the carburization but produces lower
CH4 amount in TPH, indicating that a large amount of carbon species
in the FeK/SiO2 catalyst could not be hydrogenated to CH4 dur-
ing the TPH process. It also indicates that potassium suppresses
the carbon hydrogenation, which is consistent with the H2-TPD
results.
tion shape as that on the Fe(1 0 0) surface. In our previous study
[33], the CO–TPD curves on carburized iron catalysts were differ-
ent from those on iron single crystal facets. In the present study, the
CO adsorption on the carburized iron catalyst produced a totally
different shape from that on the metallic iron surface. There are
groups of overlapping peaks with maximum peak position at tem-
perature range of 370–550 ◦C. The desorption temperatures of CO
on carburized catalysts are far higher than those of the molecular
CO on the Fe(1 0 0) surface and close to that of the dissociative CO.
Benzigerand Madix [21] estimated theactivationenergyforthe dis-
sociative CO desorption as 220 20 kJ/mol (about 2.28 eV) on the
Fe(1 0 0) surface. The first principle calculation of the CO adsorp-
tion on Fe5C2 surfaces [34] found that the adsorption energies of
CO at multiple-iron-atom sites are generally below −2.10 eV. If des-
orbed in thermal flashing, the required temperature for the CO on
Fe5C2 surfaces should be at about 500 ◦C, which is very close to the
CO desorption temperatures in this study. Therefore, the CO–TPD
peaks on carburized catalysts in the present study are likely to
result from the strongly bound CO on Fe5C2 surfaces. A certain fact
is that potassium largely improves the CO adsorption on carburized
catalysts, which is consistent with the effect of potassium on the
CO adsorption on iron surfaces [21,32,35].
3.2. TPH of surface carbon
Temperature-programmed surface reaction with hydrogen was
used to study the hydrogenation behavior of surface carbon species
on carburized catalysts. In this process, the main product of the car-
bon hydrogenation is CH4. The curves of CH4 evolution are shown
in Fig. 5. It can be seen from the figure that CH4 evolution peaks
of the Fe/SiO2 catalyst lie in the temperature range of 300–540 ◦C.
These peaks are sharp and present a quite broad bottom, indicating
that several overlapping peaks coexist. The CH4 desorption curve of
the FeK/SiO2 catalyst presents a group of overlapping broad peaks
in the temperature range of 300–600 ◦C. It is evident that the CH4
peaks of FeK/SiO2 catalyst shift to higher temperature compared
with those of the Fe/SiO2 catalysts. To quantitatively analyze the
overlapping peaks, these TPH spectra were fitted with Gaussian
curves to yield up several peaks and the corresponding peak param-
eters (temperatures and percent compositions) are listed in Table 2.
From the table, it can be seen that four kinds of carbon species
formed on catalyst surface according to the peak temperatures.
It was reported that the reaction temperatures of adsorbed CO,
atomic C and surface carbide with H2 are in range of 270–390 ◦C
3.3. Surface carbon species
Although TPH provides the information of most carbon species
on the catalyst surface, a large amount of carbon species are not
shown in TPH spectra. So, XPS and Raman methods were used to

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
39
Table 2
The carbon content of carburized catalysts, peak parameters of individual carbon species and CH4 total areas in TPH process for Fe/SiO2 and FeK/SiO2 catalysts carburized in
5%CO/95%He at 300 ◦C for 5 h.
Catalysts
Ca, wt.%
Peak parametersb,c
CH4 total aread
C␣
C␤
C␥
C␦
Fe/SiO2
FeK/SiO2
1.19 0.07
4.34 0.02
5.2/335
1.8/352
16.3/356
19.9/396
29.7/382
28.3/444
27.9/466
30.7/502
20.2/491
19.3/534
0.7/621
418.8
104.5
a
The carbon content was measured by element analysis.
The TPH profiles were fitted by multiple Gaussian shapes.
b
c
The percentage composition (%)/the peak position (◦C) of individual carbon species.
The total area (a.u./g-Fe) is obtained by integrating the whole CH4 evolution curves.
d
identify the detailed structure of these carbon species on carburized
catalysts.
calated hydrogen. In the present study, the low binding energy
components with EB at 283.5–284.0 eV and below 283.0 eV could be
attributed to carbidic carbon–hydrogen and Fe–CHx, respectively,
considering these species being abundant after hydrogenation. The
corresponding fitting parameters of C 1s XPS spectra of catalysts
with different treatment protocols are given in Table 3. As shown
in the table, the most abundant surface species is graphitic car-
bon on catalysts with treatment A. With treatment B, there is little
effect on surface carbonates and carbonyls. However, the relative
intensity of graphitic carbon band decreases and the band posi-
tion shifts to lower binding energy. With treatment C, the relative
intensities of carbonates, carbonyls and graphitic carbon decrease
apparently and the most abundant species shifts to hydrogen-rich
species (carbidic carbon or Fe–CHx). Comparison of two catalysts,
there is no essential difference in surface carbon species except
that the Fe/SiO2 catalyst has slightly higher content of Fe–CHx
species than the FeK/SiO2 catalyst. Through integrating Fe 2p and
C 1s spectra, the surface C/Fe mole ratios of catalysts are calcu-
lated and listed in Table 3. It can be seen that the C/Fe mole ratios
of both catalysts increased with sequential treatments. It is rea-
sonable for the increased C/Fe ratio after carburization because
the carburization of catalysts is a process of carbon deposition
on metal surface and subsequent surface carbon dissolving into
bulk phase. With the treatments of the hydrogenation, the sur-
face C/Fe ratios also increased greatly. The hydrogenation of the
carburized catalysts is a reaction of surface carbon with hydro-
gen to methane and a concomitant diffusion of bulk carbon out
to surface. The increased surface C/Fe ratios after hydrogenation
indicate that a considerable carbon species are inert to hydrogen.
They deposit on catalyst surfaces even under severe hydrogenation
In Fig. 6, the K 2p and C 1s XPS spectra of catalysts are shown.
Each XPS spectrum was fitted by means of an iterative nonlin-
ear least-squares procedure with Gaussian bands. As shown in the
figure, all C 1s spectra show a group of overlapping peaks with
binding energies (EB) from 280 to 289 eV. The energy difference
of C 1s core level is associated with its change from an oxidized
state (C␦+) in the O–C–metal bond to a reduced state (C␦−) in the
C-metal bond [32]. Using Gaussian fitting, the C 1s spectrum could
be divided into several peaks, which are attributed to oxide species,
elemental carbon, and reduced species, respectively. As identified
in literature for carbonaceous species on iron surface, the bands
with EB in range of 286.0–289.0 eV are attributed to carbonates
[36], in range of 285.0–286.0 eV to carbonyl species [36–38], and
at 284.6 eV to graphitic carbon. The low binding energy compo-
nents (<284.0 eV) are attributed to the carbonaceous species in
a reduced state whereas the more detailed chemical structure of
these species is difficult to be obtained from the XPS measure-
ment. Through controlling carbon deposition parameters (CO/H2
ratio, temperature and time), Krebs et al. [39,40] obtained, on iron
crystals, three significantly different surface carbon phases (named
as phases I, II and III) depending on the amount of the bonded
hydrogen. With the aid of AES and XPS, the phase I was desig-
nated to atomic carbon, CH, CH2, CH3 as well as polymerized CnHm
groups with EB (C 1s) = 283.9 eV, which contain a large amount
of hydrogen. The phase II contains less bonded hydrogen than
phase I and was characterized as a carbidic carbon layer with EB
(C 1s) = 284.2 eV. The phase III was attributed to graphitic carbon
with EB (C 1s) = 284.7–285.0 eV depending on the amount of inter-
Table 3
The surface C/Fe ratioa and C 1s XPS band parametersb of surface carbon species for Fe/SiO2 and FeK/SiO2 catalysts with different treatments during carburization and
hydrogenation.
Catalysts
Treatment A
Treatment B
Treatment C
Fe/SiO2
C/Fe ratio
Fe–CHx
0.72
1.95
2.26
282.7/1.3/14
281.3/1.8/23
283.8/1.3/21
284.6/1.2/19
285.4/1.2/9
288.0/1.35/6
286.6/1.4/9
Carbidic carbon
Graphitic carbon
Carbonyl
284.0/1.3/15
284.6/1.2/37
285.4/1.2/20
288.4/1.35/17
286.6/1.4/11
283.9/1.3/45
284.6/1.2/26
285.5/1.2/20
288.3/1.35/9
Carbonates
FeK/SiO2
C/Fe ratio
Fe–CHx
0.84
0.99
282.5/1.3/6
353.22
282.8/1.3/12
281.6/1.3/20
283.7/1.3/25
284.6/1.2/15
285.7/1.2/8
288.3/1.35/7
286.9/1.4/13
Carbidic carbon
Graphitic carbon
Carbonyl
283.7/1.3/23
284.6/1.2/41
285.4/1.2/19
288.8/1.35/7
287.7/1.4/11
283.9/1.3/18
284.6/1.2/33
285.4/1.2/13
288.3/1.35/17
286.9/1.4/12
Carbonates
a
The surface C/Fe ratio was determined from the C 1s and Fe 2p XPS spectra.
The binding energy (eV)/bandwidth (eV)/percentage of each band area relative to the total C 1s spectrum area.
b

40
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
Fig. 6. C 1s and K 2p XPS spectra of Fe/SiO2 and FeK/SiO2 catalysts with different treatments during carburization and hydrogenation.
conditions. Comparison of two catalysts, especially for catalysts
with treatment C, it is found that the surface C/Fe ratio of the
FeK/SiO2 catalyst is far higher than that of the Fe/SiO2 catalyst.
These results indicate that potassium has no essential effect on
the surface carbon species but suppresses their reactivities towards
hydrogen.
in the intensity of hydrocarbons on the Fe/SiO2 catalyst while there
is little change on the FeK/SiO2 catalyst.
The Raman spectra could be fitted by asymmetric
Breit–Wigner–Fano curves for the G peak and Lorentzian curves for
D and other peaks [41]. For Raman spectra of amorphous carbon,
the intensity ratio of D- and G-peaks (ID/IG) is proportional to the
number and clustering of rings when the dimensions are under
20 Å. Ferrari and Robertson [41] proposed a relation between ID/IG
and the in-plane correlation length or cluster diameter (La):
In Fig. 7, the Raman spectra of catalysts with different treat-
ments are shown. It can be seen that Raman spectra of carburized
catalysts show two broad bands at 1350 and 1600 cm−1 and a shoul-
der at 1450 cm−1. The Raman spectra of the FeK/SiO2 catalyst do
not change with the variation of treatment protocols. However, the
Raman spectrum of the Fe/SiO2 catalyst totally changes after treat-
ment C. The bands at 1350 and 1600 cm−1 become weaker while
the band at 1450 cm−1 becomes stronger apparently accompanied
several new bands at 1050, 1160 and 1300 cm−1. As reported in lit-
erature [41,42], the band at around 1600 cm−1 could be attributed
to the in-plane bond-stretching motion of pairs of C sp2 atoms, usu-
ally occurring in graphite and named as G band. This mode does not
require the presence of sixfold ring, and so it occurs at all sp2 sites,
not only those in rings. The band at 1350 cm−1 could be assigned
to the breathing mode of A1g symmetry (D band) of graphitic car-
bon. This mode is forbidden in perfect graphite and only becomes
active in the presence of disorder. Its intensity is directly depen-
dent on the presence of sixfold aromatic ring. The width of the D
band is correlated to a distribution of sp2 bonded clusters with dif-
ferent ring sizes [41,43]. The band at 1450 cm−1 could be assigned
to the vibration of the hydrogens of the –CH2– group in hydro-
carbons [44]. The band at 1300 cm−1 could be attributed to the
vibration of a methyl group against the rest of the molecule and
the bands at 1000–1200 cm−1 could be ascribed to the vibration of
carbon atoms in straight chain hydrocarbons with more than seven
carbon atoms [44]. From Raman analysis, the carbon species on car-
burized catalysts are mostly amorphous carbon and a small amount
of hydrocarbons. After hydrogenation, there is a marked increase
ID
= C(ꢀ) La2
(1)
IG
where ID and IG are heights or integrated intensities of the D-peak
and G-peak, respectively. The parameter C is wavelength depen-
dant and in this work C(514 nm) is about 0.0055. Using Eq. (1), the
cluster diameter of surface amorphous carbon on catalysts was cal-
culated and shown in Table 4. It can be seen from the table that
the average cluster size of amorphous carbon is close to 11 Å for
both carburized catalysts, and the size increases after hydrogena-
tion treatments (B and C). This is possibly in that the smaller carbon
cluster is easily removed with hydrogenation, whereas the larger
carbon cluster remained on catalyst surfaces. Comparison of both
catalysts, it can be also found that, after treatment C, the carbon
cluster of the Fe/SiO2 catalyst is larger than that of the FeK/SiO2 cat-
alyst. These results indicate that the Fe/SiO2 catalyst has stronger
hydrogenation capability while potassium inhibits the hydrogena-
tion of surface carbon species.
3.4. FTS performance
The FTS activity and selectivity of catalysts are listed in Table 5. It
can be seen that the Fe/SiO2 catalyst shows CO conversion of 34.3%
and H2 conversion of 30.9%. On FeK/SiO2 catalyst, the CO conversion
is markedly increased to 58.3% while the H2 conversion is decreased

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
41
Fig. 7. Raman spectra of Fe/SiO2 and FeK/SiO2 catalysts with different treatments during carburization and hydrogenation.
Table 4
In-plane correlation length (La) of surface amorphous carbon by fitting Raman spectra of Fe/SiO2 and FeK/SiO2 catalysts with different treatments during carburization and
hydrogenation.
Catalysts
Treatment A
Treatment B
Treatment C
G position, cm−1
ID/IG
La, Å
G position, cm−1
ID/IG
La, Å
G position, cm−1
ID/IG
La, Å
Fe/SiO2
FeK/SiO2
1597.4
1598.8
0.7
0.7
11.5
11.3
1597.7
1599.4
0.7
0.9
11.5
12.8
1608.0
1607.0
2.4
1.0
20.9
13.2
to 25.4%. For the product selectivity, Fe/SiO2 catalyst produces rel-
atively higher CH4 and lower C5 hydrocarbons. It is evident that
role in determining the hydrocarbon selectivity of the iron FTS cat-
alyst. The role of potassium is likely to change the surface H/C ratio
on the catalyst surface.
+
potassium suppresses the hydrogenation activity, lowers the CH4
+
selectivity, and enhances the C5 selectivity.
Based on above results and discussion, a scheme for
As indicated by characterization results, potassium apparently
suppresses the H2 adsorption, largely enhances the CO adsorption,
and hence decreases the hydrogenation capability of catalysts. It
was found that H2 or CO conversion in FTS reaction correlates
well with the corresponding gas chemisorption. Meanwhile, the
CO conversion to long-chain hydrocarbons is improved and the
CO hydrogenation to methane is suppressed by the promotion of
potassium in FTS reactions. However, there is no obvious evidence
to indicate a distinct carbon species on the potassium promoted
catalyst. It is evident that the surface H/C ratio plays an important
the formation and transformation of carbon species on
iron FTS catalysts could also be outlined as shown in
Fig. 8.
The carbon species stem from the dissociation of chemisorbed
CO, which involves two weakly bound molecular CO states (␣1 and
␣2), a strongly bound ␲-CO state (␣3) and a dissociated CO state (␤)
on iron surface [31,32]. The adsorbed carbon atoms (Cads) are very
reactive and further transform at least in three pathways. Firstly,
these surface atomic Cads species can dissolve in the interstitial
vacancies of iron phase and form iron carbides (FexC) indepen-
Table 5
Activity and selectivity of Fe/SiO2 and FeK/SiO2 catalysts for FTS reaction in a fixed-bed reactor.
Catalystsa
Activity, %
CO2b, %
HC selectivityc, wt.%
C2–4=/C2–4
+
CO conversion
H2 conversion
CH4
C2–4
C5
Fe/SiO2
FeK/SiO2
34.3
58.3
30.9
25.4
7.1
33.1
11.1
2.8
25.2
17.8
63.7
79.4
0.58
1.00
a
Reaction condition: H2/CO = 2.0 (v/v), 250 ◦C, 1.50 MPa, 4000 h−1
CO2 is the molar selectivity of CO conversion to CO2.
Hydrocarbon selectivities are reported on the basis of total hydrocarbon in weight.
.
b
c

42
C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
Fig. 8. The proposed conversion of surface and bulk carbon species on iron-based FTS catalysts in carburization and hydrogenation.
dent of hydrogen [45]. Secondly, in the presence of hydrogen the
Cads species can easily convert to surface methylidyne (CH) since
CH is the most stable species on iron surfaces [46]. Thirdly, if in
absence of hydrogen, the surface carbon species could also poly-
merize to large size carbon clusters because these carbon species
are thermodynamically favorable [15]. The surface CH species can
be subsequently hydrogenated to surface CH2, CH3 and gaseous
CH4. This pathway is the main channel of methane production in
FTS reaction. The C–C coupling of C␣ species to C␤ species involves
not only the atomic carbon but the CH species, i.e. C + C, CH + CH
and C + CH reactions. This is plausible because the C and CH species
[27,40,46] as building blocks rather than CH2 or CH3 species [47]
was supported by experimental and theoretical studies on Fe, Co
and Ru surfaces. Here, one would find that CH is a key crotch for
the branching of product pathways to methane or to long-chain
hydrocarbons. The modification of the reactivity of CH towards
hydrogen may be a valid method to control the production of
methane (for example, the addition of potassium in this study).
The oligomer of carbon (C␤) is difficult to detect or distinguish
from amorphous carbon (a-C) and graphite in the present exper-
iment. The C␤ species may be the common reactive intermediate
for hydrocarbons and amorphous carbon depending on the sur-
face concentration of hydrogen. Under the FTS atmosphere, the
building blocks (C or CH) polymerize to C␤ and further hydro-
genates to hydrocarbons. In the lean hydrogen atmosphere, the
C␤ further condense to larger size amorphous carbon cluster and
finally to graphite. Under more severe condition (higher tempera-
tures) and with excess of hydrogen, the amorphous carbon could
be hydrogenated to amorphous carbon–hydrogen (a–C:H) film and
hydrocarbons while graphite is relatively inert. The bulk carbon
in iron carbide can diffuse out from the bulk phase, hydrogenate
to methane or directly form amorphous carbon. The C–C bond in
hydrocarbon and amorphous carbon could cleave to methane by
hydrogenolysis at different temperatures depending on their reac-
tivities to hydrogen [48].
4. Conclusions
Two Fe/SiO2 catalysts promoted with or without potassium
were used to study the surface chemisorption and the surface car-
bon hydrogenation behavior.
On H2-reduced catalysts, hydrogen mainly adsorbs on the sur-
face iron sites and the surface oxide sites. On CO-carburized
catalysts, hydrogen probably exists as the most stable CH or OH
species. The CO adsorption on carburized catalysts may be the
strongly bound CO on Fe5C2 surfaces.
According to the reactivity towards hydrogen, the carbon
species on carburized catalysts could be categorized into four
classes: (a) atomic carbon, (b) oligomerized carbon, (c) iron car-
bide, and (d) graphitic carbon. According to the binding energy of
C 1s XPS spectra, the surface carbon species include carbonates,
carbonyl, graphite, carbide, and Fe–CHx. In LRS results, the surface
carbon species are mainly amorphous carbons with different clus-
ter sizes and long-chain hydrocarbons. The small-size amorphous
carbons are more reactive to hydrogen while the large-size carbons
are inert to hydrogen.
Potassium apparently suppresses the H2 adsorption, largely
improves the CO adsorption and decreases the hydrogenation
activity of catalysts. The role of potassium is to decrease the surface
H/C ratio while it dose not lead to any distinct carbon species on
the catalyst surface. With the decrease of the surface H/C ratio, the
H2 conversion decreases, the CO conversion increases, and hydro-
carbon products shift to long-chain molecules.
Acknowledgements
We thank the National Outstanding Young Scientists Foundation
of China (20625620), and the National Natural Science Founda-
tion of China (20703054). This work is also supported by Synfuels
CHINA. Co., Ltd.

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 328 (2010) 35–43
43
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