Effect of preparation of Fe-Zr-K catalyst on the product distribution of CO2 hydrogenation

Article abstract of DOI:10.1039/c5ra12504a

The precursors of Fe-Zr-K catalysts were prepared by microwave assisted homogeneous precipitation followed by incipient wetness impregnation. The obtained mesoporous particles were small and uniformly shaped. After reduction, the catalyst showed high activity for the selective production of light olefins from CO₂ hydrogenation, superior to the catalyst obtained by co-precipitation. Characterization indicated that with Zr addition, the reducibility, surface basicity and surface atomic composition of the samples varied with the Zr content. The CO₂ adsorption ability of the samples was increased and the samples became hard to reduce as a result of the interaction between iron species and zirconia. In the case of using 5Fe-1Zr-K under selected conditions of 1000 h^-1, 320 °C and 2 MPa, the CO₂ conversion and the selectivity of C₂-C₄ olefins reached 54.36% and 53.63%. The olefin/paraffin ratio in the C₂-C₄ hydrocarbon product fraction reached 6.44. The selectivity of CO and C₅⁺ hydrocarbons were 3% and 19.78%, respectively. The catalytic activity remained stable up to 92 h in time-on-stream reaction.

Full text of DOI:10.1039/c5ra12504a

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Effect of preparation of Fe–Zr–K catalyst on the
product distribution of CO2 hydrogenation
Cite this: RSC Adv., 2015, 5, 80196
*
*
Xiaojuan Su, Jianli Zhang, Subing Fan, Qingxiang Ma and Tian-Sheng Zhao
The precursors of Fe–Zr–K catalysts were prepared by microwave assisted homogeneous precipitation
followed by incipient wetness impregnation. The obtained mesoporous particles were small and
uniformly shaped. After reduction, the catalyst showed high activity for the selective production of light
olefins from CO2 hydrogenation, superior to the catalyst obtained by co-precipitation. Characterization
indicated that with Zr addition, the reducibility, surface basicity and surface atomic composition of the
samples varied with the Zr content. The CO2 adsorption ability of the samples was increased and the
samples became hard to reduce as a result of the interaction between iron species and zirconia. In the
case of using 5Fe–1Zr–K under selected conditions of 1000 hꢀ1, 320 ꢁC and 2 MPa, the CO2 conversion
and the selectivity of C2–C4 olefins reached 54.36% and 53.63%. The olefin/paraffin ratio in the C2–C4
Received 28th June 2015
Accepted 7th September 2015
DOI: 10.1039/c5ra12504a
+
hydrocarbon product fraction reached 6.44. The selectivity of CO and C5 hydrocarbons were 3% and
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19.78%, respectively. The catalytic activity remained stable up to 92 h in time-on-stream reaction.
selectivity of light olens.15,19,20 Several structural supports and
chemical promoters have been employed to improve the cata-
lytic performance and to tailor the product distribution.11–18
1. Introduction
The increase in CO2 emissions with industrialization, and the
resulting greenhouse effect have given rise to ever-increasing
concerns about the disposal of CO2. The chemical trans-
formation of CO2 into other chemicals as an avenue for carbon
recycling has attracted durative interest.1–6
Compared with the H2/CO2–methanol-to-olens process,
direct synthesis of light olens from CO2 hydrogenation avoids
the methanol synthesis and is more cost-effective. Research
suggests that CO2 hydrogenation to light olens proceeds in two
steps: reduction of CO2 to CO via the reverse water–gas shi
(RWGS) reaction, followed by CO hydrogenation via the Fisher–
Tropsch (F–T) reaction7,8 as follows:
Manganese stabilizes the Fe–K catalyst due to a higher resis-
tance to bulk oxidation, but does not change the product
distribution.11 Alumina supported Fe–K shows high conversion
of CO2 and low selectivity of CO and methane at reaction
+
temperature of 400 ꢁC with the C5 selectivity above 33%.12
g-Al2O3 supported Fe–Mn–K lowers the formation of methane
and increases the product ratio of olen/paraffin.17 Zirconia
supported K–Fe also displays higher selectivity of light olens,
whereas the CO selectivity was 15%.23 Up to the present time,
the selectivity of CO or methane or both in CO2 hydrogenation
still remains higher. Thus, further reduction of these two side
products is desired.
nCO2 + nH2 4 nCO + nH2O (RWGS)
nCO + 2nH2 / (CH2)n + nH2O (F–T)
Based on the two-step reaction mechanism of CO2 hydro-
genation to light olens, the formation of CO or methane may
be reduced by tuning the rates of the two reactions. The F–T rate
and the WGS rate were shown to be altered with water addition
during the F–T synthesis reaction.24 A bifunctional catalyst of
ceria modied Fe/Mn/K for olen production from CO2 hydro-
genation was reported with combining the RWGS and F–T chain
growth activity.18 Zirconia alone provides adsorption sites for
the surface reaction intermediates, and the catalytic behavior of
metal/zirconia for CO2 hydrogenation is dependent on metal
type and inuenced by the interfacial contact area between the
metal and zirconia.25 Methane formation is suppressed by H2O
in the WGS reaction over Rh/Fe2O3/ZrO2.26 On the other hand,
iron-based catalysts with higher surface areas from micro-
emulsion technology exhibit higher activity for CO2 hydroge-
nation,14 and mesoporous xFeMnO by the sol–gel process
Various iron-based catalysts have been studied for this direct
process9–21 because they are usually adopted in the industrial
shi reaction as well as the F–T reaction.22 The catalysts'
formulation and preparation procedures obviously affect their
performance. Unpromoted iron-based catalysts exhibit high
methane selectivity;9,10,19 however, the introduction of a potas-
sium promoter results in the enhancement of the carburization
of the iron species, formation of iron carbide active phases9 and
State Key Laboratory Cultivation Base of Natural Gas Conversion, College of
Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021,
People's Republic of China. E-mail: zhaots@nxu.edu.cn; zhangjl@nxu.edu.cn
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enhances CO2 hydrogenation to C2–C5 hydrocarbons.21 With a pretreated in a quartz tube at 350 ꢁC in He (25 mL minꢀ1) for 1 h
faster reaction rate than the conventional heating, microwave and cooled to room temperature. Then, the sample was reduced
assisted-homogeneous precipitation has shown potential in by a mixed gas composed of 5% H2 and 95% N2 as the
ꢁ
obtaining catalytic materials with desirable properties such as temperature increased from room temperature to 800 C at a
ne particles and good chemical homogeneity.27,28
heating rate of 10 ꢁC minꢀ1. The H2 consumption was
In this study, to further understand the role of Zr promoters synchronously detected on a gas chromatograph with a thermal
in iron catalysts for CO2 hydrogenation to light olens, Fe–Zr–K conductivity detector.
catalysts were prepared using microwave assisted-
CO2 temperature-programmed desorption (CO2-TPD) was
homogeneous precipitation followed by incipient wetness performed on the same TP5000. A 200 mg sample was pretreated
impregnation methods. The effects of the added amounts of Zr at 400 ꢁC for 3 h in a ow of 30% H2 and 70% N2 (30 mL minꢀ1),
on the structural properties of the precursors of Fe–Zr–K cata- and purged by He (25 mL minꢀ1) for 1 h. CO2 was then pulse-
ꢁ
lysts and on the activity for CO2 hydrogenation were investi- adsorbed on the sample at 50 C until saturation, followed by
gated by means of physico-chemical characterization. The He purge for 1 h. Finally, the sample was heated in He
inuencing mechanism of zirconia on iron catalysts in product (25 mL minꢀ1) from room temperature to 800 C at a heating
ꢁ
rate of 10 C minꢀ1. The CO2 desorption amount was synchro-
ꢁ
selectivity and activity stability was correlated.
nously detected.
X-ray photoelectron spectra (XPS) were obtained on a
Thermo SCIENTIFIC ESCALAB 250 equipped with an Al Ka X-ray
source (h ¼ 1486.8 eV) at 15 kV and 10 mA. The background
vacuum of the analysis chamber was 2 ꢂ 10ꢀ9 mbar. The
binding energies (BE) were calibrated using the C1s peak at
284.8 eV from adventitious carbon.
2. Experimental
2.1 Sample preparation
The Fe–Zr sample was prepared by the microwave-assisted
homogeneous precipitation (MAHP) method with urea as the
precipitator. The molar ratios of Fe/Zr included 7 : 1, 5 : 1, 3 : 1
and 1 : 1. The molar ratio of (Fe + Zr)/urea was 1 : 6. In the
typical procedure, desired amounts of Fe(NO3)3$9H2O,
ZrO(NO3)2$2H2O and urea were mixed in 200 mL of deionized
water under stirring until complete dissolution. The solution
was then transferred into a Teon-lined sample holder and
reacted under microwave conditions (2450 MHz, 500 W,
1.6 MPa, 170–180 ꢁC) for 2 h. The product was separated via
centrifugation, washed, and dried at 120 ꢁC overnight and
calcined at 500 ꢁC in air for 3 h. The sample without Zr addition
was prepared using the same procedure. For comparison, Fe–Zr
samples were also prepared by co-precipitation with NH3$H2O
as the precipitator and marked with suffix [C].
2.3 Catalytic activity evaluation
Activity tests were carried out in a ow type xed-bed system
with a stainless steel reactor (350 mm length ꢂ 8 mm i.d. ꢂ
12 mm o.d). Typical conditions were as follows: precursor
packing amount ¼ 2 mL (20–40 mesh), H2/CO2 (molar ratio) ¼
3/1, GHSV ¼ 1000 hꢀ1, T ¼ 320 ꢁC and P ¼ 2.0 MPa. The sample
ꢁ
was rst reduced at 400 C for 8 h by 30% H2 in N2 (GHSV ¼
1000 hꢀ1 and P ¼ 0.1 MPa). The feed gas was then switched on;
the reaction started and proceeded for 96 h of time-on-stream
(TOS). The sampling interval was 12 h. The data in Table 3
were obtained at TOS ¼ 68 h. Effluent gas was analyzed on an
on-line gas chromatograph (GC-9160) with a f3 mm ꢂ 2 m
TDX-01 packed column for C1 product analysis on TCD, and a
50 m ꢂ 0.53 mm ꢂ 40 mm Al2O3 capillary column for C1–C5
hydrocarbon product analysis on FID, respectively. Collected
liquid products were analyzed on an off-line GC-9160 with a f3
mm ꢂ 2 m Porapak Q packed column for aqueous phase
product analysis on TCD and a 50 m ꢂ 0.32 mm ꢂ 1.0 mm HJ-1
capillary column for oil phase product analysis on FID. The data
aer 48 h time-on-stream reaction were used for the activity
discussion. The conversion of CO2 and the selectivity of prod-
ucts were calculated based on carbon balance.
The Fe–Zr–K sample was prepared by incipient wetness
impregnation in a xed Fe/K molar ratio of 10 : 1. The aqueous
solution with the desired amount of K2CO3 was impregnated on
the Fe–Zr sample, and then dried at 120 ꢁC overnight. The
obtained samples were named as 7Fe–1Zr–K, 5Fe–1Zr–K, 3Fe–
1Zr–K, 1Fe–1Zr–K, Fe–K and 5Fe–1Zr–K[C].
2.2 Sample characterization
X-ray powder diffraction (XRD) patterns were recorded on a
Rigaku D/MAX 2200PCX diffractometer with Ni-ltered Cu Ka
radiation at 40 kV and 30 mA at scanning speed of 8ꢁ minꢀ1
.
Scanning electron microscopy (SEM) images were obtained
on a JEOL-JSM-7500F microscope at 1 kV.
CO2
CO2
CO2
CO2 conv. ¼ (Fin ꢂ yin ꢀ Fout ꢂ yout )/(Fin ꢂ yin ) ꢂ 100%
CH4(CO) sel. ¼ (Fout ꢂ youti)/(Fin ꢂ yin
ꢂ Conv.CO ) ꢂ 100%
CO2
N2 physical adsorption–desorption isotherms were
measured at ꢀ196 ꢁC on a JW-BK132F instrument aer each
2
sample was pretreated under vacuum at 300 ꢁC for 2 h. The BET where Fin was the mole ow rate of the feed gas, mol hꢀ1; Fout
surface area was calculated from the adsorption branch. The was the mole ow rate of the effluent, mol hꢀ1; and youti was the
mesopore and micropore parameter calculation was based on mole fraction of CH4(CO) in the effluent. Calculation for
the BJH and HK models, respectively.
hydrocarbon product selectivity included gaseous phase and
H2 temperature-programmed reduction (H2-TPR) measure- liquid phase products.
ments were carried out on a chemisorption analyzer (TP5000).
Chain growth probability (a) was calculated according to the
In a typical procedure, a sample (20–40 mesh) of 70 mg was Anderson–Schulz–Flory model for the product distribution:
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ln(Wn/n) ¼ n ln a + ln[(1 ꢀ a)2/a], where n is the number of not a signicant change compared with 24.0 nm of 5Fe–1Zr[C]
carbon atoms in a product; Wn is the weight fraction of the by co-precipitation.
corresponding product.
3.2 Textural properties of the samples
To check the effect of the MAHP method on preparation, the
3. Results and discussion
SEM images of fresh samples prepared by the MAHP method
3.1 Phases of the samples
before K impregnation were taken, as shown in Fig. 2. Samples
The XRD patterns of the samples are shown in Fig. 1. The
characteristic diffraction peaks at 2q of 24.2ꢁ, 33.3ꢁ, 35.7ꢁ, 40.9ꢁ,
49.6ꢁ, 54.1ꢁ, 62.6ꢁ and 64.1ꢁ were ascribed to the a-Fe2O3
phase29 and those at 2q of 30.7ꢁ, 51.2ꢁ and 60.8ꢁ were attributed
to the tetragonal ZrO2 phase.30
For samples (a)–(e) before reduction and reaction, as the Zr
content increased, the peak strength of a-Fe2O3 was gradually
diminished, whereas that of ZrO2 increased. K impregnation
did not obviously affect the phases, as shown in samples (b) and
(c). The diffraction peaks of a-Fe2O3 by the MAHP method were
stronger than that obtained by co-precipitation, as shown in
samples (b) and (d). For samples (f)–(h) aer reaction, the Fe3O4
phase was observed via the diffraction peaks at 2q of 18.34ꢁ,
30.18ꢁ, 36.56ꢁ, 43.24ꢁ, 53.54ꢁ, 57.10ꢁ and 62.74ꢁ, whereas the
a-Fe2O3 phase disappeared, suggesting that the iron species are
transformed to Fe3O4.23,31 In contrast, the ZrO2 phase did not
change distinctly before and aer the reaction.
(a)–(c) were made up of small and uniformly shaped particles
with a microscopic morphology; particle size was in the range of
50–60 nm. In contrast, sample (d) prepared by co-precipitation
showed large and irregular particles. During MAHP, the OHꢀ
ions required for precipitation were gradually released by the
hydrolysis of urea, and the pH value remained consistent.
Moreover, when the hydrothermal temperature reached 170–
180 ꢁC, the nucleation might occur faster than the crystal
particle growth. These two conditions cause the formation of
small and uniform particles.32
The N2 adsorption–desorption isotherms of fresh samples
before K impregnation are shown in Fig. 3. Samples (a)–(e)
prepared by the MAHP method showed the type IV isotherm
pattern with H3-type hysteresis loop, indicating the presence of
mesopores in the samples.21
The average crystal particle sizes of the a-Fe2O3 phase in the
samples are shown in Table 1. As the Zr content increased, the
particle size of the a-Fe2O3 crystal phase, corresponding to the
(104) crystal face, increased rst and then decreased. With the
MAHP preparation method, the crystal particle size of the
a-Fe2O3 phase in the 5Fe–1Zr sample was 28.1 nm, which was
Fig. 2 SEM images of the samples. (a) Fe2O3 (b) 7Fe–1Zr (c) 5Fe–1Zr
(d) 5Fe–1Zr[C].
Fig. 1 XRD patterns of the samples. (a)–(e) Before reduction and
reaction; (f)–(h) after reaction.
Table 1 Crystal particles of the a-Fe2O3 phase
Sample
Size/nm
Fe2O3
24.9
32.7
28.1
24.0
11.1
24.0
7Fe–1Zr
5Fe–1Zr
3Fe–1Zr
1Fe–1Zr
5Fe–1Zr[C]
Fig. 3 N2 adsorption–desorption isotherms of the samples.
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Table 2 Pore parameters of the samples
Samples
Surface area (m2 gꢀ1
)
Mesopore volume (cm3 gꢀ1
)
Micropore volume (cm3 gꢀ1
)
Mesopore diameter (nm)
Fe2O3
22.1
39.7
52.6
76.7
83.5
95.3
0.092
0.156
0.190
0.191
0.168
0.153
0.008
0.014
0.020
0.029
0.032
0.037
2.54
2.31
2.25
11.58
11.94
7.32
7Fe–1Zr
5Fe–1Zr
3Fe–1Zr
1Fe–1Zr
5Fe–1Zr[C]
The pore parameters of fresh samples before reduction and demonstrate that there exists an interaction between the iron
reaction are summarized in Table 2. As the Zr content species and zirconia, and it was the strongest in 5Fe–1Zr–K. The
increased, the surface area and the micropore volume reducibility of sample 5Fe–1Zr–K[C] by co-precipitation was
increased. The mesopore volumes of all samples were much different from that by the MAHP method. Similar to Fe–K (b),
larger than the micropore volumes, and 5Fe–1Zr and 3Fe–1Zr two reduction peaks appeared at low temperatures for 5Fe–1Zr–
showed the maximal mesopore volume. Moreover, 5Fe–1Zr had K[C] (e). The second peak was at 500–650 ꢁC, lower than that of
the minimal mesopore diameter, whereas 1Fe–1Zr had the the corresponding 5Fe–1Zr–K, implied that the reduction of
maximal mesopore diameter. In contrast, 5Fe–1Zr[C] prepared Fe3O4 to a-Fe is relatively easy.
by co-precipitation had the largest surface area, micropore
volume, mesopore volume and mesopore diameter.
3.4 Surface adsorption behavior of the samples
Fig. 5 shows the CO2-TPD proles of fresh samples aer the H2
pretreatment but before the reaction. Sample Fe–K (a) displayed
3.3 Reduction behavior of the samples
two CO2 desorption peaks at 120–170 ꢁC and 610–800 ꢁC. Aer
From the H2-TPR proles of the samples in Fig. 4, it can be seen
that all the fresh samples produced two reduction peaks.
the addition of Zr, the peak slightly shied to high temperatures
and the CO2 desorption amount increased, as shown for sample
ꢁ
For Fe2O3 (a), the sharp peak at 280–420 C and the broad
(b). In addition, two new desorption peaks appeared at 200–
one at 450–800 ꢁC were assigned to the initial reduction of
320 ꢁC and 510–620 ꢁC. The data suggest that the adsorption of
a-Fe2O3 to Fe3O4, and then the reduction of Fe3O4 to a-Fe.31 For
CO2, which is related to surface basicity,12 is intensied due to
Fe–K (b), two peaks appeared at low temperatures and were
ascribed to the reduction of a-Fe2O3 to Fe3O4, and the partial
reduction of Fe3O4 to FeO.13 For K-containing samples (b)–(g),
the ZrO2 formation by Zr addition. With further addition of Zr
in sample (d), the high temperature desorption peak dramati-
cally decreased. By comparison, the high temperature desorp-
the rst peaks shied to high temperatures, suggesting that K
tion peak strength of 5Fe–1Zr–K[C] (c) was weaker since the
modication causes the reduction to be more difficult. The use
of a K promoter has also been proved to suppress the adsorption
of H2 to a certain extent.12,23 Moreover, the second peaks of 7Fe–
desorption peak temperature was lower.
3.5 Surface composition of the samples
1Zr–K (c) and 5Fe–1Zr–K (d) shied to high temperatures,
indicating the inhibition effect on the reduction of Fe2O3 and
the formation of the iron phase through the FeO phase. With
increasing the Zr content in samples (f) and (g), the second peak
shied to lower temperatures, which is coincident with the
reported results that Zr addition improves the dispersion of iron
phases and makes iron species easily reduced.33 These results
The XPS spectra of fresh samples before reduction and reaction
are shown in Fig. 6. For all samples in Fig. 6-A, the binding
energies (BE) at 710.9 eV and 724.5 eV were ascribed to Fe(2p3/2
)
and Fe(2p1/2), respectively. The satellite peak at 718.9 eV char-
acterized the existence of Fe2O3.21 In Fig. 6-B, the BE at 182.1 eV
was attributed to Zr(3d5/2) of ZrO2. K addition in Fig. 6-A(b)
Fig. 4 H2-TPR profiles of the samples.
Fig. 5 CO2-TPD profiles of the samples.
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Fig. 8 C(1 s) binding energy in 5Fe–1Zr–K.
Fig. 6 XPS spectra of the samples.
From the atomic concentration of the XPS spectra, the
atomic Zr/Fe ratios of the samples were calculated. As shown in
Fig. 9, the calculated Zr/Fe ratios are higher than the prepara-
tion ratios, revealing that Zr is enriched on the sample surface
and the enrichment is stronger on 5Fe–1Zr[C] than on 5Fe–1Zr.
caused a slight decrease in the BE of Fe(2p1/2) compared with
Fe2O3 (a). It was proposed that K lowers the metal work function
by donating electrons to the vacant d orbits of the iron,
enhancing the dissociative adsorption of CO, whereas lowering
the H2 adsorption ability.34 Furthermore, the BE of Fe(2p3/2) and
Zr(3d5/2) were lowered for the Fe–Zr samples in Fig. 6-A(c)–(e)
and in Fig. 6-B(a)–(c), indicating the increased interaction
between the iron species and zirconia.
3.6 Catalytic activity for CO2 hydrogenation
The catalytic activities of the samples for CO2 hydrogenation
under selected conditions are shown in Table 3. It can be seen
that all samples prepared by the MAHP method showed higher
activity with CO2 conversion above 43%. The main products
were hydrocarbons, except for CO and small amounts of
oxygenates. The 5Fe–1Zr–K sample displayed the maximum CO2
conversion of 54.36%. In addition to presenting uniform
particles with better dispersion, 5Fe–1Zr–K had the strongest
surface basicity as the CO2-TPD prole shows in Fig. 5, which
should be favorable for the CO2 adsorptive activation and
conversion.
It can be seen from Fig. 7 that aer the reaction, the BE of
Fe(2p3/2) in 5Fe–1Zr–K shied slightly towards low energy,
compared with those before reduction and reaction, suggesting
the existence of Fe3O4 in accordance with the XRD results. The
binding energy of the carbon species in the 5Fe–1Zr–K sample is
shown in Fig. 8. Before and aer reaction, there was carbon
(C1s, BE: 284.8 eV) on the sample surface, whereas carbidic
carbon (BE: 283.1–283.4 eV)21 was not observed. This suggests
that the carburization of Fe probably occurs during the reaction,
and aer reaction the Fe species transform to iron oxides.
Fig. 7 Fe(2p) binding energy in 5Fe–1Zr–K.
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Fig. 9 Atomic Fe/Zr molar ratios on the sample surface.
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Table 3 Catalytic activity of the samples for CO2 hydrogenationa
Sel./%
Hydrocarbon distributiond/C%
HCb
Oxy.c
CH4
C]2 –C]4
C02–C04
C5
O/P
+
Catalysts
CO2 Conv./%
CO
Fe–K
48.33
49.78
54.36
51.55
43.43
34.02
7.45
7.10
3.00
6.16
9.88
4.47
89.17
91.84
96.58
90.39
90.11
94.61
3.38
1.06
0.42
3.45
0.01
0.92
16.12
19.62
19.78
21.77
24.74
19.74
50.53
52.70
53.63
50.61
46.61
48.82
6.69
8.45
8.33
9.94
14.20
7.92
26.67
19.21
18.26
17.68
14.46
23.51
7.55
6.24
6.44
5.09
3.28
6.17
7Fe–1Zr–K
5Fe–1Zr–K
3Fe–1Zr–K
1Fe–1Zr–K
5Fe–1Zr–K[C]
a
Reaction conditions: H2/CO2 ¼ 3/1, GHSV ¼ 1000 hꢀ1, 2 MPa, 320 ꢁC. b HC: hydrocarbons. c Oxy.: oxygenates. d C]2 –C4]: C2–C4 olens; C02–C40: C2–
C4 paraffins; C5+: C5+ hydrocarbons.
In the production distribution, the CH4 selectivity over Fe–K
was lower than that over Fe–Zr–K but the CO and C5+ selectivity
were high. As the Zr content increased, the CH4 selectivity was
+
gradually increased, whereas the C5 selectivity decreased,
indicating that the Zr addition in the catalyst samples
+
suppresses the formation of the C5 (heavy) products. As the
Fe/Zr ratio increased, the CO2 conversion and the selectivity of
light olens (C]2 –C4]) rst increased and then decreased. At the
Fe/Zr ratio of 5, the CO2 conversion and the C]2 –C4] selectivity
reached the maxima of 54.36% and 53.63%, respectively, while
Fig. 10 CO2 hydrogenation over 5Fe–1Zr–1K with time-on-stream.
the selectivity of CO was the lowest (3%). The O/P ratio in the
C2–C4 hydrocarbon products was 6.44. Considering the inter-
action between iron species and zirconia indicated by the
aforementioned XPS results (Fig. 6) as well as the H2-TPR results
(Fig. 4), it is likely that during the reduction of the samples
zirconia would also lower the metal work function by donating
electrons to iron and enhance the adsorption of CO while
reducing the H2 adsorption, resulting in an increase in the light
olen products.
In contrast, the 5Fe–1Zr–K[C] catalyst prepared by co-
precipitation showed the lowest CO2 conversion, although
better product distribution. As shown in Fig. 9, Zr tends to have
more enrichment on the catalyst surface prepared by co-
precipitation. This enrichment might reduce the active site
number on the catalyst surface, inuencing the Fe–Zr interac-
tion and the catalytic activity. Furthermore, the weaker surface
basicity of 5Fe–1Zr–K[C] than that of 5Fe–1Zr–K (see Fig. 5) is
unfavorable for the CO2 adsorptive activation.
indicates that there exists an interaction between iron species
and zirconia, which might play a stable role in the active phase
of the catalysts.26
Chain growth probability (a) for CO2 hydrogenation over the
catalyst samples is shown in Fig. 11. With Zr addition, a tends
to decrease. As the Zr content increases, a decreases from 0.64
over Fe–K to 0.53 over 1Fe–1Zr–K. This is well in accordance
with the hydrocarbon product distribution in Table 3, i.e., CH4
+
increases, while C5 hydrocarbons decrease. The a over 5Fe–
1Zr–K[C] is higher than that over 5Fe–1Zr–K, and there is more
+
formation of C5 . Moreover, the ln(Wn/n) values for C2 hydro-
carbons show negative deviation to the tted line. As the a
decreases, the deviation tends to diminish, implying that the
secondary reactions of C2 towards heavy products are likely
suppressed.
These results demonstrate that on the one hand, addition of
certain amounts of Zr to Fe–K by the MAHP method can
It has been reported that the steady state establishment of
F–T synthesis over iron catalysts can be divided into ve
episodes of distinct kinetic regimes, which last for about 60 h.
As the reaction time is extended, Fe species undergo trans-
formation from iron oxide to iron carbide (Fe5C2).35
Fig. 10 shows the catalytic activity of CO2 hydrogenation over
5Fe–1Zr–K and the production changes at set reaction condi-
tions with time-on-stream. It is clear that the catalytic activity
exhibited an induction period of about 60 h. At the initial
reaction stage, the CO2 conversion increased as the active phase
iron carbide was gradually formed. Aer 60 h, both the CO2
conversion and the C]2 –C4] selectivity remained above 55% and
53%, respectively. The CO selectivity was retained at 3% from
the incipient about 8%. The preceding characterization
Fig. 11 a value of CO2 hydrogenation over the catalyst samples.
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Acknowledgements
Financial supports from the Special Prophase Project on Basic
Research of Department of Science and Technology of China
(Grant No. 2012CB723106) and the Natural Science Foundation
of China (Grant No. 21366025) are greatly acknowledged.
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80202 | RSC Adv., 2015, 5, 80196–80202
This journal is © The Royal Society of Chemistry 2015