Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 Catalyst

Article abstract of DOI:10.1016/j.chempr.2018.09.017

Photo-driven CO hydrogenation reaction toward the formation of olefins was realized over Fe₅C₂ catalyst under photo-irradiation. Comprehensive catalyst characterization studies show that under mild reaction conditions, the Fe₅C₂ catalyst undergoes a surface reconstruction, forming an oxygen-decorated Fe₅C₂ surface that is prone to the formation of olefins in photo-driven Fischer-Tropsch synthesis process. Theoretical calculations demonstrate that a small number of O atoms adsorbed on the Fe₅C₂ surface can promote the desorption of olefin, which plays an important role in tuning the production of this value-added product. The current work extends the knowledge of Fe-based Fischer-Tropsch synthesis catalyst and process under external field. With the depletion of crude oil and especially the growing demand for lower olefins, the FTO process is a promising alternative for the petroleum route. In this work, a photo-driven FTO process was realized over Fe₅C₂ catalyst under photo-irradiation. The unique slight oxygen-modulated Fe₅C₂ structure formed in situ under the photo-irradiation condition, resulting in high selectivity to olefins, making it an excellent catalyst in photo-driven FTO reaction. With the depletion of crude oil and especially the growing demand for lower olefins, the direct conversion of syngas into lower olefins via the Fischer-Tropsch to olefins (FTO) process is a promising alternative for the petroleum route. A photo-driven FTO process can dramatically change the product selectivity over Fe₅C₂ catalyst, leading to an olefin/paraffin ratio of 10.9 with CO conversion >49%. The selectivity toward CO₂ was as low as 18.9%, ensuring a high carbon resource utilization efficiency, and the catalyst showed good stability. Under the photo-irradiation condition, the surface of the Fe₅C₂ catalyst was spontaneously decorated by O atoms formed in situ, resulting in a high selectivity to olefins, which makes it an excellent catalyst for photo-driven FTO reaction.

Full text of DOI:10.1016/j.chempr.2018.09.017

Article
Photo-Driven Syngas Conversion to Lower
Olefins over Oxygen-Decorated Fe5C2
Catalyst
Wa Gao, Rui Gao, Yufei Zhao, ...,
Run Long, Xiao-Dong Wen, Ding
Ma
wxd@sxicc.ac.cn (X.-D.W.)
dma@pku.edu.cn (D.M.)
HIGHLIGHTS
A photo-driven lower olefin
production via direct FTS process
is obtained
Higher olefin selectivity is
dependent on O-decorated
Fe5C2 under photo-irradiation
Effect of surface O atoms on
Fe5C2 for the high selectivity of
olefins is discussed
With the depletion of crude oil and especially the growing demand for lower
olefins, the FTO process is a promising alternative for the petroleum route. In this
work, a photo-driven FTO process was realized over Fe5C2 catalyst under photo-
irradiation. The unique slight oxygen-modulated Fe5C2 structure formed in situ
under the photo-irradiation condition, resulting in high selectivity to olefins,
making it an excellent catalyst in photo-driven FTO reaction.
Gao et al., Chem 4, 1–12
December 13, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.09.017

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(2018), https://doi.org/10.1016/j.chempr.2018.09.017
Article
Photo-Driven Syngas
Conversion to Lower Olefins
over Oxygen-Decorated Fe5C2 Catalyst
Wa Gao,1,5,7 Rui Gao,2,4,7 Yufei Zhao,3,7 Mi Peng,1 Chuqiao Song,1 Mengzhu Li,1 Siwei Li,1 Jinjia Liu,2
Weizhen Li,1 Yuchen Deng,1 Mengtao Zhang,1 Jinglin Xie,1 Gang Hu,1 Zhaosheng Zhang,6 Run Long,6
1,8,
Xiao-Dong Wen,2, and Ding Ma
*
*
SUMMARY
The Bigger Picture
Photo-driven CO hydrogenation
reaction toward the formation of
olefins was realized over Fe5C2
catalyst under photo-irradiation.
Comprehensive catalyst
With the depletion of crude oil and especially the growing demand for lower
olefins, the direct conversion of syngas into lower olefins via the Fischer-Tropsch
to olefins (FTO) process is a promising alternative for the petroleum route. A
photo-driven FTO process can dramatically change the product selectivity
over Fe5C2 catalyst, leading to an olefin/paraffin ratio of 10.9 with CO conver-
sion >49%. The selectivity toward CO2 was as low as 18.9%, ensuring a high car-
bon resource utilization efficiency, and the catalyst showed good stability. Un-
der the photo-irradiation condition, the surface of the Fe5C2 catalyst was
spontaneously decorated by O atoms formed in situ, resulting in a high
selectivity to olefins, which makes it an excellent catalyst for photo-driven
FTO reaction.
characterization studies show that
under mild reaction conditions,
the Fe5C2 catalyst undergoes a
surface reconstruction, forming an
oxygen-decorated Fe5C2 surface
that is prone to the formation of
olefins in photo-driven Fischer-
Tropsch synthesis process.
INTRODUCTION
Theoretical calculations
As the key building block chemicals, lower olefins (C2–4=) act as a pillar for the syn-
thesis of polymers and pharmaceutical products.1–3 Generally, lower olefins can be
obtained commercially from naphtha cracking and, very recently, from the methanol
to olefins process.1–3 With the depletion of crude oil and especially the growing de-
mand for lower olefins, the direct conversion of syngas (a mixture of H2 and CO) into
lower olefins via the Fischer-Tropsch synthesis (FTS) process (also known as Fischer-
Tropsch to olefins [FTO] process) is a promising alternative for the petroleum
route.1–6 It has been reported that K- and S-promoted iron catalysts or Na- and
Zn-promoted Fe5C2 catalysts are highly selective for olefin production.5,7–11 More
recently, Mn-modified cobalt carbide catalysts have been reported to be able to
convert syngas to lower olefins with the olefin/paraffin ratio up to a record high level
of 30–50.6 However, the massive thermal energy required by the reaction condition
(e.g., 230–450ꢀC, 2–5 MPa) demand a more green and sustainable pathway to derive
the aforementioned FTO process.
demonstrate that a small number
of O atoms adsorbed on the Fe5C2
surface can promote the
desorption of olefin, which plays
an important role in tuning the
production of this value-added
product. The current work extends
the knowledge of Fe-based
Fischer-Tropsch synthesis catalyst
and process under external field.
Compared with the reactions driven by thermal energy, photo-driven reactions to-
ward CO or CO2 utilization are sustainable routes by which to harvest the most abun-
dant energy form on earth and to produce desired fuels and chemicals.12–14
Recently, we reported that modified Ni catalysts can catalyze the conversion of syn-
gas under visible or UV-visible (UV-vis) light irradiation, and produce methane or C2+
hydrocarbons depending on the different catalyst structures used.14 Significantly,
reactions can also be driven by photothermal process or plasmonic effect, which is
another important method of photo-energy utilization.15–18 Indeed, group VIII
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1

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metals show outstanding photothermal performances and a unique capability to
convert CO2 by an effective utilization of the whole range of solar energy.19
Is it possible to use catalysts composed of the earth-abundant elements, such as
iron, for the photo-driven or photo-assisted conversion of syngas to value-added
chemicals, for example, lower olefins? Our previous experience indicates that
Fe5C2 is the active phase of iron-based catalyst for FTS, which is responsible for
the high activity of hydrocarbon production in FTS reaction.5,20–22 Here we report
that by using Fe5C2 nanoparticles as catalysts, it is possible to obtain lower olefins
from a photo-driven syngas conversion process, with the olefin to paraffin (o/p) ratio
up to 10.9. The key for the unique catalytic performance was the highly efficient uti-
lization of light and excellent photothermal performance of Fe5C2 catalysts; the ex-
istence of surface O atoms on the Fe5C2 nanoparticles can promote olefin desorp-
tion and restrict the over-hydrogenation of the primary product of olefins. This
work heralds the introduction of high-performance, energy-efficient, and green
FTO processes with solar as the energy source and cost-effective iron carbide as
the catalyst, which shows the potential of solar energy in the production of industri-
ally important chemicals.
RESULTS
Fe5C2 catalysts were synthesized by a facile chimie douce route, which involves the
reaction of iron carbonyl, Fe(CO)5, with octadecylamine with the presence of bro-
mide under mild temperatures (up to 350ꢀC).20,23 The element distributions of
Fe5C2 catalyst were detected by scanning electron microscopy (SEM) with energy-
dispersive X-ray spectroscopy and are shown in Figure S1 and Table S1. As refer-
ences, passivated Fe5C2 (passivated in 0.5% O2/He at room temperature for
12 hr), air-treated Fe5C2 (oxidized in air at 500ꢀC for 2 hr), and hydrogen-treated
Fe5C2 (reduced in 10.0% H2/Ar at 800ꢀC for 5 hr) samples were also prepared to
tune the surface structure of Fe5C2 catalyst. It was clear from the X-ray diffraction
(XRD) profile that the reflections of Fe5C2 phase (JCPDS no. 36-1248) were well
matched with those of pure-phase iron carbides (Figure 1A).20 For Fe5C2 (passiv-
1Beijing National Laboratory for Molecular
Sciences, College of Chemistry and Molecular
Engineering, and BIC-ESAT, Peking University,
ated) catalyst, the bulk phase was still Fe5C2 from the XRD pattern (Figure 1A), while
the surface was mostly transformed into iron oxide (Figure 2A). In the case of the air-
and hydrogen-treated Fe5C2 samples, they were transformed into Fe2O3 and
metallic Fe nanoparticles, respectively.
Beijing 100871, China
2State Key Laboratory of Coal Conversion,
Institute of Coal Chemistry, Chinese Academy of
Sciences, 27 South Taoyuan Road, Taiyuan,
Shanxi 030001, China
3State Key Laboratory of Chemical Resource
Engineering, Beijing University of Chemical
Technology, Beijing 100029, China
Fe5C2 and the reference catalysts were used in CO hydrogenation reaction under
photo-irradiation (atmospheric pressure, molar ratio CO/H2 = 1:2), and the results
are shown in Tables 1 and S2. Under photo-irradiation, the reference Fe/Al2O3 cata-
lyst (Figure S2) was almost inactive for CO activation with CO conversion of less than
1% in 0.5 hr (Table 1, entry 2). Surprisingly, the Fe5C2 catalyst displayed an
outstanding catalytic performance with the lower olefins as the dominant products
(Figure S3). The conversion of CO was as high as 49.5%, and the o/p ratio in the
C2–4 hydrocarbons reached 10.9, indicating that most of the C2–4 hydrocarbons
were olefins (Table 1, entry 4). The selectivity toward CO2 was 18.9%, which is rela-
tively low in the FTO catalysts reported,4–6,8,10 ensuring a high carbon resource uti-
lization efficiency. As shown in Figure S4, the CO conversion presented an increase
while prolonging the reaction time, and the syngas was completely converted within
2.5 hr. The performance of Fe5C2 under flow conditions was also tested. It displayed
good selectivity toward olefin but higher methane selectivity in the flow system un-
der photo-irradiation. The photo-driven reaction of Fe5C2 catalyst in pure hydrogen
(Table 1, entry 3), as well as isotope-tracing experiments (Figure S5) using 13CO,
4College of Chemistry and Chemical
Engineering, Inner Mongolia University, Hohhot
010021, China
5College of Biological Science and Engineering,
Key Laboratory of Urban Agriculture (North
China) Ministry of Agriculture, Beijing University
of Agriculture, Beijing 102206, China
6College of Chemistry, Key Laboratory of
Theoretical & Computational Photochemistry of
Ministry of Education, Beijing Normal University,
Beijing 100875, China
7These authors contributed equally
8Lead Contact
*Correspondence: wxd@sxicc.ac.cn (X.-D.W.),
dma@pku.edu.cn (D.M.)
https://doi.org/10.1016/j.chempr.2018.09.017
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Figure 1. Catalyst Characterization and Reaction Performance
(A) XRD patterns of Fe5C2 and reference catalysts.
(B) Monitoring of the catalyst bed temperature of Fe5C2 catalyst under photo-irradiation.
(C) UV-vis spectra of Fe5C2 and reference catalysts.
(D) Recycling of the Fe5C2 catalyst for five cycles. Reaction conditions: no external heating, catalyst
mass 180 mg, CO/H2 = 1/2, irradiation time 0.5 hr, 300-W Xe lamp; after each reaction, new
reactants were added and a new reaction began.
conclusively demonstrated that the carbon source of olefins and hydrocarbons in the
CO hydrogenation reaction was the dissociation of CO.
We monitored the temperature of the catalyst bed (Figures 1B and S6) at various
conditions. For the CO hydrogenation reaction under full-spectrum photo-irradia-
tion (Figure S7), the temperature reached approximately 200ꢀC within 60 s under
irradiation and rocketed up to 490ꢀC within 28 min. Under photo-irradiation, the
remarkable increase in the local temperature of Fe5C2 resulted from excitation of
the surface plasmon band and the consequential fast thermal relaxation of the exci-
tation, which can provide energy to overcome the reaction barrier and initiate the
reaction process.17,18,24–26 Indeed, fresh Fe5C2 had a strong absorbance at
300–2,500 nm, covering UV light, visible light, and infrared radiation (Figure 1C),
which implies a high utilization efficiency of solar energy over Fe5C2 catalyst. In addi-
tion, we tried to test the photoluminescence spectra, as given in 3D excitation emis-
sion photoluminescence spectra (Figure S8). Even under excitation wavelength from
250 to 500 nm, we could not obtain any emission signal (from 250 to 800 nm); this
non-photoluminescence property of Fe5C2 indicates that the Fe5C2 catalysts give
the whole photo-induced heat conversion without obvious photo-induced charge
(electro/hole) occurring.
The catalyst under photo-irradiation condition is very stable. Figure 1D shows that
the CO conversion was almost intact in the five reactions that were run. The product
distributions from the first to the fifth run were almost unchanged as well, indicating
that the nature of the catalyst does not change during the recycling test.
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Figure 2. XPS Profiles of Different Catalysts and Operando X-Ray Adsorption Experiments
(A) Ex situ XPS spectra of Fe5C2 catalyst at different treated conditions.
(B) O1s spectra of fresh and used Fe5C2 catalyst.
(C) In situ Fe K-edge XANES of Fe5C2 catalyst under different reaction time (under photo-
irradiation).
(D) In situ EXAFS of Fe5C2 catalyst under different reaction time (under photo-irradiation).
High-resolution transmission electron microscopy (Figure S9), SEM (Figure S10), and
XRD (Figure S11) results show that the structure and particle size of the Fe5C2 nano-
particles remained unchanged after the five reaction cycles. To gain further insight
about the structure of Fe5C2 catalyst during reaction condition, we performed in
situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption
fine structure (EXAFS) experiments in the photo-driven CO hydrogenation reaction.
As shown in Figure 2C, the XANES spectra suggest that the valence state of Fe5C2
catalyst remained stable under a continuous photo-irradiation of 5 hr. The inset
shows that the working catalyst had an electronic structure similar to that of fresh
catalyst during the photo-driven CO hydrogenation reaction process. In the EXAFS
˚
part (Figure 2D), the Fe-C and Fe-Fe scatterings (Fe5C2) at 1.58 and 2.18 A were
clearly resolved in fresh Fe5C227 and were only slightly changed in the process, sug-
gesting that Fe5C2 is relatively stable during the reaction process, although a slight
surface oxidation cannot be ruled out. Generally, in the thermal FTS reaction pro-
cess, Fe5C2 can be easily oxidized by the active products, such as CO2 and water,
leading to the formation of a mixture of iron carbides and oxides in the reac-
tion.20–22,28,29 In the current case, the result indicates that the bulk phase of Fe5C2
stayed almost unchanged during the photo-driven FTO process. This is different
from the thermal reaction, showing the superiority of the photo-driven reaction
processes.
More importantly, in order to obtain olefins, the modification of Fe5C2 catalyst by Na
or K is normally mandatory in the thermal FTS process.2,11 Therefore, it is of interest
to know why the unprompted iron carbide in the current case is prone to the produc-
tion of olefins.
4
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Table 1. Catalytic Performance of Various Catalysts in the Photo-Driven FTO Reaction
Entry Catalysts
Catalyst Bed
CO Conversion (%) CO2 Selectivity (%) Product Selectivity
(%, CO2-free)
Olefin/Paraffin Ratio
Temperature (oC)a
0
CH4
C2–4
=
C2–4
–
C5+ C2–4
1
2
3
4
5
6
7
8
blankb
–
0
–
–
–
–
–
–
–
–
–
Fe/Al2O3
356
490
490
488
0.3
–
–
–
–
c
Fe5C2
0
–
–
–
–
Fe5C2
49.5
13.6
<0.1
0.4
18.9
16.9
–
33.1 55.5
33.7 51.4
5.1
7.8
–
6.3
7.1
–
10.9
6.6
–
Fe5C2(passivated)d
Fe5C2 (air treated)e 404
–
–
–
–
Fe5C2 (H2 treated)f
421
491
–
–
–
–
g
Fe5C2
52.2
28.9
30.9 54.0
8.8
6.3
6.1
Reaction conditions: no external heating, catalyst mass 180 mg, CO/H2 = 1/2, irradiation time 0.5 hr, 300-W Xe lamp.
aMonitoring of the catalyst bed temperature of various catalysts under photo-irradiation.
bBlank experiment; irradiation in the absence of any catalysts under above conditions.
cReaction conditions: H2/Ar (10/90), irradiation time 0.5 hr, 300-W Xe lamp.
dThe fresh Fe5C2 catalyst was passivated with O2/He (0.5/99.5) for 12 hr before reaction.
eThe fresh Fe5C2 catalyst was calcined in air at 500ꢀC for 2 hr before reaction.
fThe fresh Fe5C2 catalyst was reduced in H2/Ar (10/90) at 800ꢀC for 5 hr before reaction.
gReaction conditions: 180 mg, CO/H2 = 1/3, irradiation time 0.5 hr, 300-W Xe lamp.
It has been demonstrated that plasmon-excited nanoparticles can be an efficient
source of hot electrons and energy, and they have emerged as a new breed of
photocatalysts enabling chemical reactions.25,26,30–32 Sometimes this form of en-
ergy can even change the reactivity or selectivity in a catalytic reaction.33 Besides
this route for altered selectivity, another possibility is that the unique surface struc-
ture in photo-driven CO hydrogenation reaction confers the catalyst with the
ability to stop the reaction in the olefin step (instead of further hydrogenation to
alkanes). We studied the surface structures of the fresh and used catalysts by ex
situ X-ray photoelectron spectroscopy (XPS). Figure 2A clearly depicts that the sur-
face of fresh Fe5C2 catalyst was dominated by the peak at ꢁ707 eV in the Fe2p
spectrum assigned to iron in iron carbide. Meanwhile, a tiny shoulder peak of
FeO at ꢁ709.3 eV was resolved.34,35 On the O1s spectrum (Figure 2B),
besides a small peak of surface oxide (O2À) at ꢁ529.8 eV, peaks at ꢁ532.1 and
ꢁ533.5 eV corresponding to OH and COOH were observed.36–38 After the reac-
tion, the sample was transferred to an XPS chamber without exposure to air. Signif-
icantly, we did not observe the formation of a large amount of iron oxide as in the
thermal catalytic FTS process.20–22 Instead, the surface of catalyst was still Fe5C2
but decorated with a small amount of surface oxygen, as evidenced by the in-
crease of peaks at ꢁ709.3 and 529.8 eV. Given these results, we hypothesized
that Fe5C2 decorated with surface oxygen will promote the formation of the olefins
under photo-irradiation.
To understand the effect of surface O atoms on Fe5C2 for the corresponding dra-
matic change to the selectivity of FTO products, we further carried out density
functional theory (DFT) calculations on Fe5C2 catalysts (Figures 3, 4, S12, and
S17). On the basis of the experimental characterization results, we constructed
two possible surface models (Figures S12 and S13) of Fe5C2(111) and
Fe5C2(111)-4Oads, the former of which represents the clean iron carbide catalyst
and the latter of which represents the surface decorated with O atoms. By
comparing the C2H6 formation mechanisms from CH2 coupling and hydrogenation
on three models (2CH2 + 2H / C2H4 + 2H / C2H5 + H / C2H6 in Figure 3), we
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Figure 3. Energy Profiles for Two CH2 Coupling and Hydrogenation Forming C2H6 on Fe5C2(111) and Fe5C2(111)-4Oads, as well as the Corresponding
Intermediate Structures
Fe, C, O and H atoms are blue, brown, red, and white, respectively; the C atom is colored black to distinguish it from CH2.
found that the CH2-CH2 coupling was more favorable with the lowest barriers
(0.10 eV) on the Fe5C2(111)-4Oads than with the Fe5C2(111) (0.35 eV). Furthermore,
the adsorption energy of C2H4 on the Fe5C2(111)-4Oads (À0.59 eV) was much lower
than that on Fe5C2(111) (À1.07 eV), indicating that the adsorbed O atoms can pro-
mote C2H4 desorption and avoid the further over-hydrogenation to C2H6. Subse-
quently, we computed the C2H4 hydrogenation into C2H6, and the effective bar-
riers were 0.79 and 0.71 eV on the Fe5C2(111) and Fe5C2(111)-4Oads surfaces,
respectively. For comparison, we found that C2H4 will be easily desorbed to atmo-
sphere on Fe5C2(111)-4Oads because of its lower adsorption energy, whereas C2H4
tends to further hydrogenate to form C2H6 on Fe5C2(111). Furthermore, the
computed charge density differences (Figure S16) and the Bader charge of the
Fe atom in Fe5C2, Fe5C2-4Oads and Fe5C2-4Oads-C2H4 (À0.52, À0.57 versus
À0.60) suggest the electron transfer from the Fe atom to O and C2H4. From a fron-
tier orbital point of view (Figure S17), we infer that the doping O atoms lead to the
electron densities of the z2 band over Fe sites and reduce them greatly in order to
weaken the bonding between the z2 band of Fe and the ps state of C2H4.
6
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Figure 4. Energy Profiles for C2H4 Adsorption and Hydrogenation Forming C2H6 under Thermal-
Driven (Ground-State) and Photo-Driven (Excited-State) Conditions on Fe5C2(111)-4Oads and
Fe5C2(111) Surfaces, as well as the Corresponding Intermediate Structures
Fe, C, O, and H atoms are blue, brown, red, and white, respectively; the C atom is colored black to
distinguish it from CH2.
Therefore, we believe that a small number of O atoms adsorbed on the Fe5C2 sur-
face can dramatically improve the o/p ratio, which is well in agreement with our
experimental results. Furthermore, to further explore the mechanism responsible
for different product distribution under thermal and photo-driven reactions, by us-
ing either a DFT or constrained DFT approach,39,40 we optimized ground-state
(thermal-reaction) and excited-state (photo-reaction) structures of C2H4, C2H5,
and C2H6 on pristine and O-decorated Fe5C2(111) surfaces. In Figure 4, the
excited-state energies of the pristine Fe5C2(111) with adsorbed C2H4, C2H5, and
C2H6 are close to their corresponding ground-state energies on the pristine
Fe5C2(111), whereas the excited-state energies of C2H5 and C2H6 are obviously
suppressed on the O-decorated Fe5C2(111) surface (À0.41 versus À0.72 eV and
À1.30 versus À1.71 eV). This suggests that the surface O atoms can modify the
local electronic structure and optical band gap of the surface.41,42 Under photo-
irradiation, the domain products are preferred over olefins on oxygen-decorated
Fe5C2(111), whereas for pure Fe5C2(111), the formation of alkane is more favor-
able. We believe that (1) the presence of Fe5C2 in the oxygen-decorated catalyst
is responsible for the light-heat transformation and (2) the existence of a small
amount of FeOx species on Fe5C2 as a heterostructure also absorbed light, was
stimulated, and interacted with the reactants, resulting in a different reaction
path and different product distribution.
When Fe5C2 is intended to be oxidized before the reaction—for example, the
surface was covered with a larger amount of oxygen through the passivation
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process (passivated Fe5C2 catalyst)—the obvious shift of Fe binding energy (from
ꢁ707.02 eV to ꢁ707.2 eV) can be clearly observed because of the significantly
increasing amount of surface oxygen. However, the catalytic performance
decreased drastically (Table 1, entry 5, CO conversion = 13.6% and o/p ratio = 6.6;
Table S3), indicating the negative role in FTO of extensive oxidation of Fe5C2 sur-
face. Significantly, when the catalyst was totally oxidized to iron oxide (Fe5C2
treated with air) or totally hydrogenated to iron (Fe5C2 treated with H2) (Figures
1A and 2A), it lost the ability to form olefins (Table 1, entries 6 and 7), showing
the importance of the proper amount of surface oxygen modification on the for-
mation of olefins in the photo-driven FTO process. In a control experiment, a ther-
mal FTS experiment of Fe5C2 catalyst was carried out at 300ꢀC–500ꢀC without
photo-irradiation (and all other reaction conditions the same). The result demon-
strated that the Fe5C2 catalyst has relatively low CO conversion of less than 5%
at lower temperature. When the temperatures gradually reached the temperature
of the photothermal process (around 500ꢀC), the CO conversion reached 80.5%;
however, the dominant products were CH4 with high selectivity of 95.1% in hydro-
carbons and CO2 (36.0%, in all products) with a lower o/p ratio (0.1) (Table S4). Ex
situ XPS experiments of the Fe5C2 catalyst after traditional thermal reaction
showed that the Fe5C2 surface was oxidized to oxides seriously at this high tem-
perature (Figure S18), which explains its poor catalytic performance. These results
demonstrate that under a proper photo-driven CO hydrogenation condition, the
surface of the Fe5C2 catalyst was spontaneously decorated by surface oxygen
formed in situ, resulting in a high selectivity to olefins, which makes it an excellent
catalyst in photo-driven FTO reactions. Compared with other recent FTO work
(Table S5), the photo-driven FTO based on the Fe5C2 catalyst is a sustainable
route for the harvest of the most abundant energy on Earth and the production
of valuable fuels and chemicals. Moreover, compared with thermal FTO reaction
on the Fe5C2 catalyst, the presence of light has totally changed the reaction selec-
tivity. This is the major benefit of the photo-driven reaction, and we anticipate
more research and results in this direction.
EXPERIMENTAL PROCEDURES
Synthesis of Fe5C2 Nanoparticles
In a four-neck flask, a mixture of octadecylamine (14.5 g) and cetyltrimethyl ammo-
nium bromide (0.113 g) was stirred sufficiently and degassed under a flow of Ar.
The mixture was heated to 120ꢀC and Fe(CO)5 (0.5 mL, 3.6 mmol) was injected
under an Ar blanket. The mixture was heated to 180ꢀC at 10ꢀC/min and kept at
this temperature for 10 min. A color change from orange to black was observed
during the process, implying the decomposition of Fe(CO)5 and the nucleation
of Fe nanocrystals. Subsequently, the mixture was further heated to 350ꢀC at
10ꢀC/min and kept there for 10 min before it was cooled to room temperature.
The product was washed with ethanol and hexane, and collected for further
characterization.
Synthesis of Reference Catalysts
Fe/Al2O3 was prepared by incipient impregnation method using ferric nitrate as the
precursor. After impregnation, the sample was dried at 60ꢀC for 12 hr and then cal-
cinated in air at 500ꢀC for 5 hr.
Catalytic Tests
Photo-driven CO hydrogenation reactions were carried out in a stainless-steel
reactor (volume, ꢁ70 mL) with a quartz window on the top of the reactor. The
photo-driven reaction was performed in a gas (vapor)-solid heterogeneous reaction
8
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(2018), https://doi.org/10.1016/j.chempr.2018.09.017
mode. Typically, 180 mg of solid catalyst was spread as a very thin layer (<1 mm)
onto round-shaped quartz glassware with an area of 14 cm2. Before the catalytic
test, the catalyst was activated in situ with C2H4/H2/Ar (10:10:80) at 300ꢀC for 2 hr.
After cooling to room temperature, the two globe valves were closed to keep the
gas in the reactor before the sample was taken out in a glovebox. The activated cat-
alysts were transferred to stainless-steel reactor in a glovebox. After evacuation of
the reaction system, syngas (CO/H2/Ar = 25:50:25, atmospheric pressure) were in-
jected. The light source was a 300-W Xe lamp (l = 200–1,100 nm, light intensity
2.9 W cmÀ2) to drive the photo-driven CO conversion. The tip of thermometer
was maintained in intimate contact with the sample. The photo-driven reaction
was typically performed for 0.5 hr. We monitored the temperature of the catalysis
bed by using a thermocouple in intimate contact with the catalyst bed. For the
cycling test of the Fe5C2 catalyst, the reactor was evacuated after each test, and
then syngas (CO/H2/Ar = 25:50:25) was injected into the reactor. No regeneration
treatment was done between the catalytic cycles.
Computational Details
Methods
All spin-polarization calculations were performed according to the plane-wave-
based periodic DFT method as implemented in the Vienna Ab Initio Simulation
Package.43,44 The electron-ion interaction was described with the projector
augmented wave method.45,46 The electron exchange and correlation energies
were treated within the generalized gradient approximation in the Perdew-Burke-
Ernzerhof functional (GGA-PBE).47 The energy cutoff of the plane-wave basis
was set up to 400 eV. Electron smearing width of s = 0.2 eV was used via the
Methfessel-Paxton technique to speed up the convergence of the metallic systems.
All transition states were estimated by the climbing image nudged elastic band
method (CI-NEB),48 and we analyzed the stretching frequencies in order to charac-
terize whether a stationary point is a minimum state without imaginary frequency or
a transition state with only one imaginary frequency. We obtained all excited-state
calculations with the constrained DFT39,40 by promoting five spin-down electrons
from the highest occupied molecular orbitals to the spin-up lowest unoccupied mo-
lecular orbitals at each k point. Such a situation is reasonable because a heavy Fe
atom has strong spin-orbit coupling and should experience a spin-flip process upon
photoexcitation.
The adsorption energy was calculated according to Eads = EX/slab À [Eslab + EX],
where EX/slab is the total energy of the slab with adsorbates in its equilibrium geom-
etry, Eslab is the total energy of the bare slab, and EX is the total energy of the free
adsorbates in the gas phase. Therefore, the more negative the Eads, the stronger
the adsorption. The barrier (Ea) and reaction energy (DEr) were calculated according
to Ea = ETS À EIS and DEr = EFS À EIS, where EIS, EFS, and ETS are the total energy of
the corresponding initial state (IS), final state (FS), and transition state (TS),
respectively.
To choose a reasonable O coverage on the Fe5C2 surface, we used ab initio atomistic
thermodynamics simulation. The surface oxidation process obeys the reaction of
Fe5C2 + nO / nO/Fe5C2. The related reaction Gibbs free energy can be expressed
as DG = G(nO/Fe5C2) À G(Fe5C2) À nm(O), where G(nO/Fe5C2) and G(Fe5C2)
stand for the Gibbs free energies of the Fe5C2 slab with and without adsorbed nO
atoms, whereas m(O) stands for the chemical potential of O atom, which can be
calculated according to the formula m(O) = m(H2O) À m(H2). On the basis of the equa-
tion m(P, T) = m0(T) + EZPE + kTln(P/P0), we can find the m(H2O) and m(H2) from the
Chem 4, 1–12, December 13, 2018
9

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(2018), https://doi.org/10.1016/j.chempr.2018.09.017
thermodynamic data table. Shown in Figure S11, under our experimental conditions,
the p(231) Fe5C2(111) tends to firmly adsorb 2O atoms. Therefore, it is expected
that the p(232) Fe5C2(111) is capable of anchoring 4O atoms on its surface. The
detailed description is presented in the computational models.
Models
The optimized Fe5C2 unit cell is a monoclinic crystal and has C2/c crystallographic
˚
˚
˚
symmetry, whose lattice constants are a = 11.545 A, b = 4.496 A, c = 4.982 A, and
b = 97.60ꢀ, showing an excellent agreement with experiments, in which a, b, and c
49
ꢀ
˚
equal 11.562, 4.573, and 5.060 A, respectively, and b equals 97.74 . The calcu-
lated average magnetic moment on all Fe atoms inside the unit cell is 1.73 mB, which
is very close to the experimentally measured values (1.72–1.75 mB).49 To rationalize
the experimental observations of this work, we used the same unit cell p(2 3 2) Fe5C2
and Fe5C2(111)-4Oads (Figure S13) supercell and 3 3 3 3 1 k-point sampling for ge-
ometry optimization and transition-state searching. In particular, the Fe5C2(111) sur-
face contains 96 Fe and 48 C atoms, in which the bottom 40 Fe and 16 C were fixed,
whereas both the Fe5C2(111)-4Oads and Fe5C2(111)-4Oembed have 96 Fe, 48 C, and 4
O atoms. Similarly, the bottom 40 Fe and 16 C were fixed to represent their bulk
configurations.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 18 fig-
ures, and 5 tables and can be found with this article online at https://doi.org/10.
1016/j.chempr.2018.09.017.
ACKNOWLEDGMENTS
This work was financially supported by the National Key R&D Program of China
(2017YFB0602200) and Natural Science Foundation of China (21725301,
21821004, 91645115, 21473003, 91545121, 21573022, and 21473229). We also
acknowledge the National Thousand Young Talents Program of China, Hundred-
Talent Program of Chinese Academy of Sciences, and Shanxi Hundred-Talent Pro-
gram. The computational resources for the project were supplied by the Tianhe-2
in Lvliang, Shanxi. XAFS experiments were performed at the 1W1B beamline of Bei-
jing Synchrotron Radiation Facility.
AUTHOR CONTRIBUTIONS
D.M. designed this work; W.G. designed the experiments, catalyst preparation,
and catalytic reactions; W.G., Y.Z., and R.G. wrote the paper; R.G., X.-D.W.,
J.L., Z.Z., and R.L. did the DFT calculations; Y.Z., M.P., C.S., S.L., M.L., and
G.H. did part of the catalyst preparation and catalytic evaluation, as well as other
characterization measurements; W.L., Y.D., M.Z., and J.X. did the in situ X-ray
absorption and XPS measurements; all authors participated in the analysis of
the experimental data and discussions of the results, as well as preparation of
the paper.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: May 16, 2018
Revised: June 19, 2018
Accepted: September 22, 2018
Published: October 18, 2018
10
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(2018), https://doi.org/10.1016/j.chempr.2018.09.017
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