Effect of Ca Promoter on the Structure and Catalytic Behavior of FeK/Al2O3 Catalyst in Fischer-Tropsch Synthesis

Article abstract of DOI:10.1002/cctc.201900501

Fischer-Tropsch synthesis (FTS) to olefins is an alternative route for lower olefins production. The role of alkaline-earth metals as promoters in FTS was not so clear. Herein, we prepared a series of FeKCa/Al₂O₃ catalysts by excessive co-impregnation method to elucidate the promotion effect of Ca on catalytic performance. Characterization technologies of N₂ physisorption, XRD, Raman spectrum, TEM and CO chemisorption were used to study the textural properties, phase composition and dispersion of Fe species. The results showed that the addition of Ca promotes dispersion of iron species thus leads to much smaller crystallite size. Meanwhile, no electron donation effect was observed upon Ca introduction, as evidenced by XPS and H₂-TPR. A maximum of FTY is achieved at 1.0 wt.% Ca, followed by a decline when adding excessive Ca due to less exposed active sites. Notably, the product distribution is almost unchanged. The nearly constant TOF values with and without Ca addition indicates Ca works only as a structural promoter rather than chemical promoter.

Full text of DOI:10.1002/cctc.201900501

Accepted Article
Title: Effect of Ca promoter on the structure and catalytic behavior of
FeK/Al2O3 catalyst in Fischer-Tropsch synthesis
Authors: Hongyu Wang, Shouying Huang, Qiao Zhao, Jian Wang, Yifei
Wang, Yue Wang, and Xinbin Ma
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Effect of Ca promoter on the structure and catalytic behavior of
FeK/Al O catalyst in Fischer-Tropsch synthesis
2
3
[
a]
Hongyu Wang, Shouying Huang, Jian Wang, Qiao Zhao, Yifei Wang, Yue Wang and Xinbin Ma*
Abstract: Fischer-Tropsch synthesis (FTS) to olefins is an
alternative route for lower olefins production. The role of alkaline-
earth metals as promoters in FTS was not so clear. Herein, we
function as electronic promoters because of the electron
donation to iron that facilitates CO dissociation and suppresses
C‒C
coupling.[13-16] Dual
promoters
usually function
prepared a series of FeKCa/Al
impregnation method to elucidate the promotion effect of Ca on
catalytic performance. Characterization technologies of
2
O
3
catalysts by excessive co-
synergistically in literatures. Some of work employ a combination
of cationic and anionic species, such as alkali metals and S
promoter,[17-19] and some focus on bimetallic promoters such as
MnK, CuK, NaZn and so on.[20-21, 13] Some promoters, such as
Mn, were reported with a combination effect on both particle size
and local chemical environment, which contribute to catalytic
performance in FTO.[22, 23] Cu has a positive effect on the
N
2
physisorption, XRD, Raman spectrum, TEM and CO chemisorption
were used to study the textural properties, phase composition and
dispersion of Fe species. The results showed that the addition of Ca
promotes dispersion of iron species thus leads to much smaller
crystallite size. Meanwhile, no electron donation effect was observed
2
reduction of iron oxides because of enhanced H dissociation
upon Ca introduction, as evidenced by XPS and
H
2
-TPR.
A
ability,[21] and Zn could serve as a structural promoter for
reducing the size of iron species.[13] However, the study of
maximum of FTY is achieved at 1.0 wt.% Ca, followed by a decline
when adding excessive Ca due to less exposed active sites. Notably, alkaline earth metals in Fe-based FTS is relatively limited. The
the product distribution is almost unchanged. The nearly constant
TOF values with and without Ca addition indicates Ca works only as
a structural promoter rather than chemical promoter.
exact role of alkaline earth metals as chemical promoter or
structural promoter in iron species remains controversial. On the
one hand, alkaline earth metals may act as similar as alkali
metals, supplying electrons to Fe species or enhancing surface
basicity of the catalyst for promoting CO activation.[24, 25] On the
other hand, it has been reported that alkaline earth metals are
added to the iron-based catalysts as a structural promoter and
Introduction
have no chemical promotional effect, which normally increases
Fe dispersion.[26, 27] To date, there is not an exact explanation
Lower olefins (ethylene, propylene, and butylene) as essential
chemical blocks are widely applied to packing materials, rubber,
anti-freezing agents, coatings, etc.[1-3] Currently, lower olefins
are mainly produced through petroleum industry.[4, 5] Developing
alternative routes for olefin production from various resources is
desirable to ease depletion of oil. Fischer-Tropsch-to-olefins
about the effects of alkaline earth metals on the active sites,
which may subsequently affect catalytic activity and hydrocarbon
selectivity.
In terms of modifier concentration, some researchers
reported that catalytic activity achieves the maximum at K or Ca
content of 1.0 wt.% and further increasing promoter content will
(FTO) process is a direct conversion of coal- and biomass-
based syngas into lower olefins, which has been studied for
decades.[6] Fe-based catalyst is attractive and competitive for
FTO, owing to its low cost, appropriate hydrogenation activity,
high performance of water-gas-shift reaction and considerable
selectivity to lower olefins.[7, 8]
block the active sites due to enhanced carbon deposition on
catalyst surface.[28-29, 24]
Herein, we thus prepared a series of FeKCa/Al
containing fixed 1.0 wt.% K loading and variable Ca content
0~2.0 wt.%) by co-impregnation method to investigate the
promoter effect of Ca upon K existence. XRD, TEM, Raman
spectra, XPS spectra and H -TPR were employed to shed light
on nature of Fe species with Ca decoration, in comparison with
FeK/Al . The activity, lower olefins selectivity and hydrocarbon
2 3
O catalysts
(
Promoters are always necessary for Fe-based FTO
catalysts to achieve desired catalytic properties. The activity,
selectivity and stability are strongly affected by promoters in
terms of electronic and structural modification.[9, 10] Metal oxides
2
2
O
3
(
2 2
i.e. ZrO , TiO ) are usually considered as structural promoters
product distribution were discussed and correlated with the
characterization results to confirm the role of Ca in FTS.
to disperse and stabilize Fe species,[11, 12] while alkali metals (e.g.
K, Na) and several non-metal elements (e.g. N, S) always
Results and Discussion
[
a]
H. Wang, Dr. S. Huang, J. Wang, Q. Zhao, Y. Wang, Y. Wang, Prof.
Dr. X. Ma
Key Laboratory for Green Chemical Technology of Ministry of
Education Collaborative Innovation Center of Chemical Science and
Engineering, School of Chemical Engineering and Technology
Tianjin University
Tianjin 300072 (China)
E-mail: xbma@tju.edu.cn
Characterization of catalysts
X-ray diffraction patterns of the fresh catalysts with different Ca
loadings are shown in Figure 1a, and the Fe/Al
2 3
O and
FeK/Al were taken as reference. Only rhombohedral
2
O
3
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2
Figure 1. (a) XRD patterns, (b) Raman spectra, (c) XPS spectra and (d) H -TPR curves of all fresh catalysts
hematite (α-Fe
2
O
3
) with a corundum-type structure (JCPDS No.
promoter to Fe
Raman spectra were observed in this study, excluding electronic
effect of promoters on Fe species.
2 3
O . In contrast, few changes in peak location of
3
3-0664) was observed for all the samples without additional
iron-related reflections.[30] It is also evidenced by lattice fringes
2 3
O
with d-spacing of 0.270 nm in HR-TEM (Figure S1),
The peaks located at ~710.9 eV and ~724.5 eV in XPS
spectra (Figure 1c) can be ascribed to the Fe 2p3/2 and Fe 2p1/2
levels of Fe3+ respectively, coupling with the satellite peaks at
2 3
corresponding to the (104) plane of α-Fe O . There are no
reflections attributable to K or Ca related phase because of the
low loading (shown Table 1). Note that the intensities of the
2 3
around 718.9 eV, which confirms the existence of Fe O
.[10] No
diffraction peaks of α-Fe
additive, and the trend becomes more significant after Ca
addition, implying the modified dispersion of α-Fe . Table 1
shows the crystallite sizes of α-Fe according to the Scherrer
Equation on the basis of the (116) lattice plane. With the addition
of K and Ca, the crystalline size of α-Fe particles decreased
from 14.8 nm (Fe/Al ) to 10.6 nm (FeK/Al ) and 7.8 nm
FeKCa1.0/Al ), respectively. Further increasing Ca loading
hardly influenced the crystal size of α-Fe , as evidenced by
almost identical linewidth of α-Fe diffraction peak.
As shown in Figure 1b, only the characteristic peaks at 225,
46, 294 and 410 cm-1 of the α-Fe
were observed in Raman
spectra, which indicates that the majority of Fe species existed
as α-Fe
phase in all samples.[31] This agrees with the XRD
results. Note that the four characteristic bands become much
weaker with K addition, compared to the spectrum of Fe/Al
2
O
3
become weaker and broader with K
signals related to other Fe species were observed, in line with
the XRD and Raman analysis. Note that when K was added as a
promoter, the peak of Fe 2p1/2 shifted from 724.5 eV toward a
2
O
3
2
O
3
2 3
lower binding energy (BE) of 724.0 eV, compared to Fe/Al O
without promoter. The decreased BE could be ascribed to the
electron transfer from K to the vacant d orbits of iron, thus
2
O
3
2
O
3
2
O
3
enhancing the dissociative adsorption of CO and lowering the H
2
(
2
O
3
adsorption capability.[33, 34] However, it is noteworthy that the
addition of Ca did not further cause any change in the BE of Fe
2p1/2, manifesting that Ca promoter has a negligible effect on the
electronic properties of iron species.
2
O
3
2
O
3
2
2
O
3
The reduction behaviors of fresh catalysts were measured
2
by H -TPR. All the profiles showed three overlapped reduction
2
O
3
peaks (Figure 1d), implying representative three-step process of
(270~430 oC), then Fe
Fe O : Fe O to Fe O O to FeO
2 3 2 3 3 4 3 4
O
3
(430~530 oC), and finally to metallic Fe (530~720 oC).[35]
A
2
o
sample. The change is more pronounced after adding Ca,
indicating that doping Ca promoter effectively inhibits
complete reduction achieved under 720 C is indicative of weak
interaction between Fe and support, with reference to ref..[36, 3]
agglomeration of Fe
2
O
3
phase during impregnation and
For FeK/Al
2
O
3
catalyst, K promoter hindered the reduction
to Fe , supported by the first reduction peak
calcination. Liu et al.[32] reported that strong interaction between
process of Fe
O
2
3
3 4
O
Fe species and additives contributes to a shift of Raman peaks
at about 22 oC higher temperature. In literatures, the hindered
reduction of iron oxide is due to the electrostatic interaction,[37,38]
to
a lower wavenumber, owing to electron transfer from
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Table 1. Physico-chemical properties of the unpromoted and promoted catalysts.
Element content
(
wt.%)[a]
CO uptake
Dispersion of
Fe (%)[d]
2
[b]
d (nm)[e]
d (nm)[f]
d (nm)[g]
Catalysts
S
BET (m /g)
(
μmol/g)[c]
Fe
10.4
8.4
8.0
7.3
9.6
K
Ca
-
Fe/Al
2
O
3
-
129
131
138
130
121
35.5
42.7
47.7
51.6
51.1
3.8
5.7
6.7
7.9
5.9
19.6
13.2
11.2
9.5
16.4
10.4
8.7
14.8
10.6
8.1
FeK/Al
2
O
3
1.5
1.5
1.6
1.7
-
FeKCa0.5/Al
FeKCa1.0/Al
FeKCa2.0/Al
O
2 3
O
2 3
O
2 3
0.4
1.1
2.1
7.3
7.8
12.6
9.0
8.5
[
a] Determined by ICP-OES. [b] Measured by BET method. [c] Measured by CO chemisorption. [d, e] Calculated by Eq.1 and Eq.2, respectively. [f] Mean diameter
of iron oxide particles, as determined by HR-TEM. [g] Particle size of Fe
2 3
O
, as determined by XRD with reference to the diffraction peak (116) at 2θ = 54.09O
direct bonding between the K and the adatom,[39, 40] or K induced
molecular polarization.[41-43] However, Ca promoter has
2 3
because of transformation of Fe O to Fe carbides as well as
a
coke deposition under reaction conditions. The partciles size
statistics was obtained by measuring both core layer and shell
structure. It is obvious that the particle sizes for all samples
increases, indicating a certain content of sintering is unavoidable.
On the other hand, the graphite or amorphous carbon layer
deposited on external surface of Fe nanoparticles may also
contribute to the increased particle size. It is noted that changes
in particle size still follow the trend as the fresh catalysts.
negligible effect on iron reduction behaviors. The slight
difference with increasing Ca contents may be due to the similar
particle size. Taking XRD, Raman and XPS results together, we
proposed Ca serves as a structural promoter, which favors
dispersion of iron species without significant influence on
electronic properties. In addition, typical IV isotherms of N
2
adsorption-desorption of all fresh catalysts indicate mesoporous
characteristic (Figure S2a). The similar specific surface area
2
3
(
~130 m /g) and pore volume (~0.28 cm /g) as well as pore
distribution (Figure S2b) for all samples (Table 1 and Table S1)
suggest limited influence on textural properties upon promoter
addition.
Figure
2
shows the TEM images of unpromoted,
K
promoted and
K
plus Ca promoted samples. Figure 2a,
corresponding to unpromoted catalyst, shows obviously
aggregation of Fe species in comparison with promoted samples.
Instead, K and dual promotes result in uniformly distributed Fe
nanoparticles. The corresponding particle size distribution from
TEM images was obtained by measuring more than 300
nanoparticles, which was also shown in Figure 2. The calculated
average particle sizes were listed in Table 1. Adding promoter K
and dual promoters stabilizes Fe species dispersion, evidenced
by a decline of average particle size from 16.4 nm to 10.4 nm
and 7.3 nm. CO chemisorption was also carried out to determine
the dispersion of Fe nanoparticles. The CO uptake, estimated
dispersion of Fe and particle diameter were summarized in
2 3 2 3 2 3
Figure 2. TEM images of (a) Fe/Al O , (b) FeK/Al O and (c) FeKCa1.0/Al O
Taking TEM and chemisorption results with XRD patterns
together, we found that the change trends of nanoparticle size
obtained by various characterizations agree well. For alkali
metals such as K, numerous work have been devoted to reveal
their positive effect on F-T synthesis, some of which proposed
they function as not only electronic but also structural
promoter.[21, 29] However, which is the key factor that governs
catalytic properties on Ca promoted catalysts? We will discuss in
Table 1. Compared with Fe/Al
with dual promoters show smaller particle size. Notably, the
FeKCa1.0/Al catalyst shows the maximum CO uptake (~51
2 3 2 3
O and FeKAl O , the samples
2
O
3
μmol/g) thus obtains the highest dispersion of Fe (~8%) among
these catalysts, manifesting that introduction of Ca promotes the
2 3
dispersion of Fe species on Al O support.
After reaction, it is found from TEM images (Figure S3) that
Fe nanopaticles of all samples emerge core-shell-like structure
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Table 2. Effects of promoters on FTO catalytic performance.
2
Select. % (free CO )
FTY
CO
CO
2
Catalyst
O/P
(
μ mol/gFe·S)
Conv.%
Select. %
o
=
CH
4
C
2
-C
4
C
2
-C
4
C
5+
Oxy.
1.6
2.1
1.9
2.4
2.4
Fe/Al
2
O
3
124
225
246
288
203
33.0
63.0
66.9
75.1
67.4
29.2
46.1
47.3
49.7
48.3
24.7
16.8
16.4
16.0
18.6
15.3
13.0
12.5
12.9
12.8
17.2
29.2
30.0
30.5
30.4
41.2
38.9
39.2
38.2
35.8
1.1
2.2
2.4
2.4
2.4
FeK/Al
2 3
O
FeKCa0.5/Al
FeKCa1.0/Al
FeKCa2.0/Al
O
2 3
O
2 3
O
2 3
Activation conditions: H
stream (TOS) = 30 h.
2
, 350 °C, 10 h; reaction conditions: 340 oC, 1.0 MPa, H
2
/CO = 1, WHSV = 9000 mL/(g·h). O/P = olefins/paraffins for C
2 4
~C . Time on
detail by combining catalytic evaluation with characterizations in
the following section.
literature.[46] This demonstrates K modified the instinct properties
of active sites by electronic effect. Remarkably, the Fe-based
catalysts with dual promoters possess almost identical TOFs to
2 3
that on FeK/Al O , which is indicative of similar surface nature
Catalytic performance
with or without Ca modification. In other words, the enhanced
activity is attributed to more exposed surface sites, which makes
Ca acting as structural promoter more convincing. As reported
de Jong’ s work,[46] the structure sensitivity can be neglected
when the nanoparticle size is beyond 7 nm. Therefore, in this
study, we think that electronic environment of active sites
caused by size effect could be excluded.
The catalytic performance for unpromoted and promoted
catalysts were displayed in Table 2 and Figure 3. Apparently,
2 3 2 3
FeK/Al O outperforms unpromoted Fe/Al O with a higher FTY
value of 225 μmol/gFe·s and FTY(C2~4=) of 72 μmol/gFe·s. The
result is expected K always acts as an electronic donator to Fe
Figure 3b illustrates the effect of K and Ca promoter on
product distribution under same reaction conditions at TOS = 30
species, which facilitates the CO activation and iron carbides
formation, thus enhancing the FTS activity.[14, 15] An improved
2 3 4
h. Fe/Al O shows the highest selectivity of CH and C5+,
selectivity of CO
2
suggests that the Boudouard reaction is
compared with others catalysts. Once K was introduced, the
significantly accelerated upon K addition, which is in line with
literatures.[44, 34] Note that the activity of the catalysts continues
hydrocarbons distribution shifts to lower olefins with less CH4
formation. And the O/P ratio is significantly improved as well.
This is reasonable because alkali metal is generally accepted as
an electronic additive for promoting C‒C coupling reaction and
weakening hydrogenation ability in FTS. However, the value of
2
increasing with almost constant CO selectivity as Ca content
increases, passing through a maximum FTY at a Ca mass
fraction of 1.0%. On basis of characterization analysis, favorable
Fe dispersion of Ca-doped catalysts with more exposed active
sites may account for improved activity. However, as the content
of Ca equals to 2.0 wt.%, the catalytic performance decreases. It
can be inferred that excessive Ca will cover partial active sites of
catalyst, which gives rise to negative influence on CO
conversion. This is in line with lower Fe dispersion measured by
CO chemisorption (Table 1). To further elucidate the role of Ca
promoter, we calculated the “apparent” or “nominal” TOF at 30 h
for all catalysts by using the particle sizes from chemisorption.
C5+ selectivity slightly decreases after adding K in this work,
which might be due to the change of particle size. F-T synthesis
on Fe-based catalysts has been proved as a structure-sensitive
reaction. Generally, the C‒C coupling probability declines with
decreasing the particle size.[47, 48] In some literatures, a similar
influence of Ca promoter on selectivity of hydrocarbons was
reported. They attributed the favored carbon-chain growing and
higher O/P ratio to a stronger surface basicity.[24, 25] Therefore,
Ca functions as an electronic promoter in their catalytic system.
However, in this study, adding Ca promoter gives rise to
negligible changes in hydrocarbon product distribution in the
presence of K, possibly due to the stronger electron donation
effect of K than Ca. For confirming this issue, we supplement
0
The density of Fe was used by assuming the particles consist of
Fe0 instead of complex phases. The results were shown in
Figure 3a. We used “apparent TOF” with reference to de Jong’s
work,[45, 46] as accurate quantifying active sites is not available
due to complex phase transformation for Fe-based catalysts. A
significant increase of the TOF value upon K-promoted catalyst
FeK2.0/Al
the effect of K and Ca. As presented in Figure S4, CO
conversion on FeK2.0/Al is almost the same as that on
2 3 2 3
O and FeCa/Al O catalysts as references to clarify
2 3
(FeK/Al O ) was observed, similar to the results reported in
2
O
3
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FeK/Al
2
O
3
. It means that an optimal content of alkaline additives
Figure 4 shows catalytic stabilities of different catalysts in
long time run. There is no apparent deactivation on
shows positive effect of electron donation. Further increasing the
alkaline additives, the activity improvement caused by electronic
FeKCa1.0/Al
decline in CO conversion on FeK/Al
possibly due to better dispersion of Fe species caused by Ca.
2
O
3
during 120 h under FTS conditions, but a slight
modification can be neglected. On the other hand, FeCa/Al
shows higher CO conversion and similar selectivity in
comparison with Fe/Al , and the enhancement of activity
2
O
3
2 3
O
can be observed. This is
2
O
3
becomes more pronounced with time on stream (Figure S5).
The negligible change of electronic properties (XPS, Figure S6a)
o
and reducibity at 350 C (H
2
-TPR, Figure S6b) after adding Ca
suggests that whether or not doped with K, Ca serves as
structural promoter to obtain superior activity.
For confirming phase composition of spent catalysts, the
XRD patterns of three used catalysts (Fe/Al
FeK1.0Ca/Al ) were shown in Figure S7a. All samples contain
Fe phase (JCPDS 51-0997), which is considered to be active
species for F-T reaction.[6] Note that the Fe
phase (JCPDS
9-0629) almost completely disappears with the addtion of K,
2 3 2 3
O , FeK/Al O and
2
O
3
5
C
2
3
O
4
1
suggesting that
carburization of Fe
K
as electron promoter facilitates the
3
O and formation of iron carbides, thus leads
4
to a higher FTS activity. Moreover, the weakened peaks of
Fe phase confirms the fact that particle size turns smaller
after Ca adding. Visible lattice fringes with d-spacing of 0.205
nm corresponding to the (510) planes of Fe (Figure S7b) also
indicated that Fe phase is the dominated Fe species.
5
C
2
2 3 2 3
Figure 4. Stability of FeK/Al O and FeKCa1.0/Al O on 120 h TOS
5
C
2
5
C
2
Conclusions
The role of Ca as a promoter was investigated in the presence
of K on Fe/Al in FTS. An obvious increase in FTY with almost
2
O
3
unchanged products distribution and better stability was
observed with Ca addition. The maximum FTY value of 288
μmol/gFe·s was obtained at Ca content of 1.0 wt.%. Further
increasing Ca loading resulted in a decline in activity. XPS,
2
Raman and H -TPR results confirmed the addition of Ca results
in a negligible change on electronic properties of catalyst
surface. On the other hand, XRD, TEM and CO chemisorption
reveal that dispersion of Fe species are improved, which is
responsible for the enhanced activity. With reference to
2 3
FeK/Al O catalysts, the almost identical product selectivity and
TOF demonstrated that Ca functions as a structural promoter
rather than electron promoter.
Experimental Section
FeKCa/Al
via calcining pseudoboehmite at 900 oC) with the corresponding nitrates
of Fe, K and Ca. Typically, 0.72 g Fe(NO ·9H O, 0.052 g KNO , 0.060 g
Ca(NO ·4H O were dissolved in 20 mL distilled water. Their theoretical
loadings were 10 wt.%, 1 wt.% and 1 wt.%, respectively. Then 1.0 g γ-
2 3 2 3
O was prepared by co-impregnation of γ-Al O (as obtained
3
)
2
2
3
3
)
2
2
Al
2
O
3
was added to the solution and stirred at room temperature for 10 h.
o
The water of mixture was evaporated at 60 C under vacuum using rotary
evaporator. The resulting solid product was dried at 100 oC for 6 h.
Finally, the calcination process was performed in air from room
o
o
o
temperature to 550 C with a ramping rate of 2 C/min and kept at 550 C
Figure 3. Catalytic performance of all catalysts. (a) Iron time yield (FTY) (blue
histogram), FTY(C2-4=) (orange histogram) and Turnover frequency (TOF)
2 3 2 3 2 3
for 6 h, denoted as FeKCa1.0/Al O . Similarly, Fe/Al O , FeK/Al O ,
(blue ball) (b) Products distribution at TOS = 30 h
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FeKCa0.5/Al
2
O
3
and FeKCa2.0/Al
2
O
3
were prepared according to the
chromatograph (Agilent GC-7890B) with a flame ionization detector (FID)
and two TCD. Agilent DB-FFAP, HP-AL/S and DB-1 columns were used
to separate oxygenates, hydrocarbons and other gaseous components,
respectively. All products were separated by a hot trap to prevent
possible condensation of the products before entering the gas
chromatograph, ensuring analysis safety and reliability. The carbon
balance for all reactions is above 98%. The catalytic activity was
expressed by FTY (moles of CO converted to hydrocarbons per gram of
Fe per second). TOF was estimated by the total amount of active site on
the basis of CO chemisorption results. The iron dispersion D% was
determined by Equation (Eq.) (1):
process above.
X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max-2500
diffractometer with a CuKα radiation (40 kV, 200 mA). Diffraction patterns
o
o
were collected in the 2θ angle range of 10-90 with scan speed of 8 /min
and 1 s per step. Visible Raman spectra were collected at room
temperature on a Spex 1877 D triplemate spectrograph with spectral
resolution of 2 cm-1 (Renishaw, InVia reflex). A 532 nm-1 diode-pump
solid semiconductor laser was used as the excitation source and the
power output was about 80 mW. Before measurements, the samples
were well ground and mounted into a spinning holder to avoid thermal
damage during the scanning. X-ray photoelectron spectroscopy (XPS)
was conducted by using PHI-1600 ESCA XPS equipment with
monochromated MgKa X-ray radiation. The binding energy was
calibrated using the C1s photoelectron peak at 284.6 eV as the reference.
4
D% = (M×n×10 )/SF×W×wt.%
(1)
where M is the molecular weight of iron, 56 g/mol, n is mole of CO
adsorbed (μmol), SF is stoichiometric factor (0.5), W is the weight of
sample (g), and wt.% is the iron percentage in the sample.
2
Hydrogen temperature programmed reduction (H -TPR) was performed
by using a Micromeritics AutoChem 2910 equipped with a thermal
Average iron particle size (dFe (nm)) was calculated by Eq. (2) on the
o
conductivity detector (TCD). The samples (50 mg) were purged at 200 C
2
basis of D%, where iron site density was assumed to be 0.094 nm /Fe
for 1 h under Ar flow for eliminating water and impurities then cooled to
atom.[50]
6
8
0 oC. The TPR measurement was conducted by heating the sample to
00 oC with a ramp of 10 oC/min in 10 % H
/Ar. CO chemisorption was
2
dFe = 75/D%
(2)
performed using a Micromeritics AutoChem 2910 equipped with a TCD to
determine the number of active sites on the surface in the Fe-based
o
catalysts. The samples (100 mg) were pretreated in situ at 350 C for 5 h
under flowing pure H
2
, and then cooled to 10 oC under He atmosphere
Acknowledgements
for CO chemisorption. The areas of the adsorption peaks were integrated,
and the amounts of adsorbed CO were quantified by comparing this area
with the assumption of a CO/Fe stoichiometry of 1:2.[49] The morphology
of samples was measured by a JEM-2100F high-resolution transmission
electron microscope operated at an accelerating voltage of 200 kV. The
sample for TEM measurement was prepared by ultrasonic dispersion of
the catalyst in ethanol. Then the suspension was dropped onto a micro-
grid. PSD histograms were constructed by randomly measuring more
than 300 nanoparticles. Thermogravimetric analysis (TGA) was carried
out using thermal analysis system (STA449F3, NETZSCH Crop.). The
sample was heated from the room temperature to 800 oC with a heating
This work was supported by National Natural Science
Foundation of China (U1462204) and National Natural Science
Foundation of Tianjin (18JCQNJC05900)..
Keywords: Fischer-Tropsch • lower olefins • Ca • structural
promoter • dispersion
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Entry for the Table of Contents
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H. Wang, S. Huang, J. Wang, Q. Zhao,
Y. Wang, Y. Wang and X. Ma*
Page No. – Page No.
Effect of Ca promoter on the
structure and catalytic behavior of
FeK/Al
2 3
O catalyst in Fischer-Tropsch
The addition of Ca promotes dispersion of iron species thus leads to the higher
catalytic performance, and Ca works only as a structural promoter rather than
electronic promoter.
synthesis
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