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Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 Catalyst
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 [email protected] (X.-D.W.) [email protected] (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 photoirradiation. 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
Please cite this article in press as: Gao et al., Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 Catalyst, Chem (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 Xiao-Dong Wen,2,* and Ding Ma1,8,*
SUMMARY
The Bigger Picture
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.
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 FischerTropsch 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.
INTRODUCTION As the key building block chemicals, lower olefins (C2–4=) act as a pillar for the synthesis 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 demand 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 FischerTropsch 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., 230–450 C, 2–5 MPa) demand a more green and sustainable pathway to derive the aforementioned FTO process. Compared with the reactions driven by thermal energy, photo-driven reactions toward CO or CO2 utilization are sustainable routes by which to harvest the most abundant 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 syngas 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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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 utilization of light and excellent photothermal performance of Fe5C2 catalysts; the existence of surface O atoms on the Fe5C2 nanoparticles can promote olefin desorption 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 industrially 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 bromide under mild temperatures (up to 350 C).20,23 The element distributions of Fe5C2 catalyst were detected by scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy and are shown in Figure S1 and Table S1. As references, passivated Fe5C2 (passivated in 0.5% O2/He at room temperature for 12 hr), air-treated Fe5C2 (oxidized in air at 500 C for 2 hr), and hydrogen-treated Fe5C2 (reduced in 10.0% H2/Ar at 800 C 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 (passivated) 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 airand hydrogen-treated Fe5C2 samples, they were transformed into Fe2O3 and metallic Fe nanoparticles, respectively. 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 catalyst (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 relatively low in the FTO catalysts reported,4–6,8,10 ensuring a high carbon resource utilization 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 under 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,
2
Chem 4, 1–12, December 13, 2018
1Beijing
National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, and BIC-ESAT, Peking University, 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
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 8Lead
authors contributed equally
Contact
*Correspondence: [email protected] (X.-D.W.), [email protected] (D.M.) https://doi.org/10.1016/j.chempr.2018.09.017
Please cite this article in press as: Gao et al., Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 Catalyst, Chem (2018), https://doi.org/10.1016/j.chempr.2018.09.017
Figure 1. Catalyst Characterization and Reaction Performance (A) XRD patterns of Fe 5 C 2 and reference catalysts. (B) Monitoring of the catalyst bed temperature of Fe 5 C 2 catalyst under photo-irradiation. (C) UV-vis spectra of Fe5 C 2 and reference catalysts. (D) Recycling of the Fe 5 C 2 catalyst for five cycles. Reaction conditions: no external heating, catalyst mass 180 mg, CO/H 2 = 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-irradiation (Figure S7), the temperature reached approximately 200 C within 60 s under irradiation and rocketed up to 490 C 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 excitation, 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 addition, we tried to test the photoluminescence spectra, as given in 3D excitation emission 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 Fe 5 C 2 catalyst at different treated conditions. (B) O1s spectra of fresh and used Fe 5 C 2 catalyst. (C) In situ Fe K-edge XANES of Fe 5 C 2 catalyst under different reaction time (under photoirradiation). (D) In situ EXAFS of Fe 5 C 2 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 nanoparticles 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, suggesting that Fe5C2 is relatively stable during the reaction process, although a slight surface oxidation cannot be ruled out. Generally, in the thermal FTS reaction process, 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 reaction.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 production of olefins.
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Table 1. Catalytic Performance of Various Catalysts in the Photo-Driven FTO Reaction Entry
1
Catalysts
blank
Catalyst Bed Temperature (oC)a
b
CO Conversion (%)
–
0
CO2 Selectivity (%)
–
Olefin/Paraffin Ratio
Product Selectivity (%, CO2-free) CH4
C2–4=
C2–40
C5+
C2–4
–
–
–
–
–
2
Fe/Al2O3
356
0.3
–
–
–
–
–
–
3
Fe5C2c
490
0
–
–
–
–
–
–
4
Fe5C2
490
49.5
18.9
33.1
55.5
5.1
6.3
10.9
5
Fe5C2(passivated)d
488
13.6
16.9
33.7
51.4
7.8
7.1
6.6
6
Fe5C2 (air treated)
e
404
<0.1
–
–
–
–
–
–
7
Fe5C2 (H2 treated)f
421
0.4
–
–
–
–
–
–
8
Fe5C2g
491
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. a Monitoring of the catalyst bed temperature of various catalysts under photo-irradiation. b Blank experiment; irradiation in the absence of any catalysts under above conditions. c Reaction conditions: H2/Ar (10/90), irradiation time 0.5 hr, 300-W Xe lamp. d The fresh Fe5C2 catalyst was passivated with O2/He (0.5/99.5) for 12 hr before reaction. e The fresh Fe5C2 catalyst was calcined in air at 500 C for 2 hr before reaction. f The fresh Fe5C2 catalyst was reduced in H2/Ar (10/90) at 800 C for 5 hr before reaction. g Reaction 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 energy 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 structure 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 surface 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 (O2 ) at 529.8 eV, peaks at 532.1 and 533.5 eV corresponding to OH and COOH were observed.36–38 After the reaction, the sample was transferred to an XPS chamber without exposure to air. Significantly, 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 increase 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 dramatic 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 CH 2 .
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 ( 0.59 eV) was much lower than that on Fe5C2(111) ( 1.07 eV), indicating that the adsorbed O atoms can promote C2H4 desorption and avoid the further over-hydrogenation to C2H6. Subsequently, we computed the C2H4 hydrogenation into C2H6, and the effective barriers 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 atmosphere 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 ( 0.52, 0.57 versus 0.60) suggest the electron transfer from the Fe atom to O and C2H4. From a frontier 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.
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Figure 4. Energy Profiles for C2H4 Adsorption and Hydrogenation Forming C2H6 under ThermalDriven (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 CH 2 .
Therefore, we believe that a small number of O atoms adsorbed on the Fe5C2 surface 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 using 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 ( 0.41 versus 0.72 eV and 1.30 versus 1.71 eV). This suggests that the surface O atoms can modify the local electronic structure and optical band gap of the surface.41,42 Under photoirradiation, the domain products are preferred over olefins on oxygen-decorated Fe5C2(111), whereas for pure Fe5C2(111), the formation of alkane is more favorable. 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 surface. 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 formation of olefins in the photo-driven FTO process. In a control experiment, a thermal FTS experiment of Fe5C2 catalyst was carried out at 300 C–500 C without photo-irradiation (and all other reaction conditions the same). The result demonstrated 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 500 C), the CO conversion reached 80.5%; however, the dominant products were CH4 with high selectivity of 95.1% in hydrocarbons 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 temperature (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 selectivity. 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 ammonium bromide (0.113 g) was stirred sufficiently and degassed under a flow of Ar. The mixture was heated to 120 C and Fe(CO)5 (0.5 mL, 3.6 mmol) was injected under an Ar blanket. The mixture was heated to 180 C at 10 C/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 350 C at 10 C/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 60 C for 12 hr and then calcinated in air at 500 C 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
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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 300 C 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 catalysts 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 injected. The light source was a 300-W Xe lamp (l = 200–1,100 nm, light intensity 2.9 W cm 2) 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-wavebased 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-BurkeErnzerhof 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 characterize 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 molecular 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 [Eslab + EX], where EX/slab is the total energy of the slab with adsorbates in its equilibrium geometry, 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 EIS and DEr = EFS 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) G(Fe5C2) 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) m(H2). On the basis of the equation m(P, T) = m0(T) + EZPE + kTln(P/P0), we can find the m(H2O) and m(H2) from the
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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 = 97.60 , showing an excellent agreement with experiments, in which a, b, and c equal 11.562, 4.573, and 5.060 A˚, respectively, and b equals 97.74 .49 The calculated 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 geometry optimization and transition-state searching. In particular, the Fe5C2(111) surface 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 figures, 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, HundredTalent Program of Chinese Academy of Sciences, and Shanxi Hundred-Talent Program. The computational resources for the project were supplied by the Tianhe-2 in Lvliang, Shanxi. XAFS experiments were performed at the 1W1B beamline of Beijing 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
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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 [email protected] (X.-D.W.) [email protected] (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 photoirradiation. 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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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 Xiao-Dong Wen,2,* and Ding Ma1,8,*
SUMMARY
The Bigger Picture
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.
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 FischerTropsch 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.
INTRODUCTION As the key building block chemicals, lower olefins (C2–4=) act as a pillar for the synthesis 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 demand 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 FischerTropsch 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., 230–450 C, 2–5 MPa) demand a more green and sustainable pathway to derive the aforementioned FTO process. Compared with the reactions driven by thermal energy, photo-driven reactions toward CO or CO2 utilization are sustainable routes by which to harvest the most abundant 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 syngas 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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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 utilization of light and excellent photothermal performance of Fe5C2 catalysts; the existence of surface O atoms on the Fe5C2 nanoparticles can promote olefin desorption 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 industrially 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 bromide under mild temperatures (up to 350 C).20,23 The element distributions of Fe5C2 catalyst were detected by scanning electron microscopy (SEM) with energydispersive X-ray spectroscopy and are shown in Figure S1 and Table S1. As references, passivated Fe5C2 (passivated in 0.5% O2/He at room temperature for 12 hr), air-treated Fe5C2 (oxidized in air at 500 C for 2 hr), and hydrogen-treated Fe5C2 (reduced in 10.0% H2/Ar at 800 C 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 (passivated) 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 airand hydrogen-treated Fe5C2 samples, they were transformed into Fe2O3 and metallic Fe nanoparticles, respectively. 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 catalyst (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 relatively low in the FTO catalysts reported,4–6,8,10 ensuring a high carbon resource utilization 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 under 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,
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1Beijing
National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, and BIC-ESAT, Peking University, 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
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 8Lead
authors contributed equally
Contact
*Correspondence: [email protected] (X.-D.W.), [email protected] (D.M.) https://doi.org/10.1016/j.chempr.2018.09.017
Please cite this article in press as: Gao et al., Photo-Driven Syngas Conversion to Lower Olefins over Oxygen-Decorated Fe5C2 Catalyst, Chem (2018), https://doi.org/10.1016/j.chempr.2018.09.017
Figure 1. Catalyst Characterization and Reaction Performance (A) XRD patterns of Fe 5 C 2 and reference catalysts. (B) Monitoring of the catalyst bed temperature of Fe 5 C 2 catalyst under photo-irradiation. (C) UV-vis spectra of Fe5 C 2 and reference catalysts. (D) Recycling of the Fe 5 C 2 catalyst for five cycles. Reaction conditions: no external heating, catalyst mass 180 mg, CO/H 2 = 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-irradiation (Figure S7), the temperature reached approximately 200 C within 60 s under irradiation and rocketed up to 490 C 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 excitation, 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 addition, we tried to test the photoluminescence spectra, as given in 3D excitation emission 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 Fe 5 C 2 catalyst at different treated conditions. (B) O1s spectra of fresh and used Fe 5 C 2 catalyst. (C) In situ Fe K-edge XANES of Fe 5 C 2 catalyst under different reaction time (under photoirradiation). (D) In situ EXAFS of Fe 5 C 2 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 nanoparticles 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, suggesting that Fe5C2 is relatively stable during the reaction process, although a slight surface oxidation cannot be ruled out. Generally, in the thermal FTS reaction process, 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 reaction.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 production of olefins.
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Table 1. Catalytic Performance of Various Catalysts in the Photo-Driven FTO Reaction Entry
1
Catalysts
blank
Catalyst Bed Temperature (oC)a
b
CO Conversion (%)
–
0
CO2 Selectivity (%)
–
Olefin/Paraffin Ratio
Product Selectivity (%, CO2-free) CH4
C2–4=
C2–40
C5+
C2–4
–
–
–
–
–
2
Fe/Al2O3
356
0.3
–
–
–
–
–
–
3
Fe5C2c
490
0
–
–
–
–
–
–
4
Fe5C2
490
49.5
18.9
33.1
55.5
5.1
6.3
10.9
5
Fe5C2(passivated)d
488
13.6
16.9
33.7
51.4
7.8
7.1
6.6
6
Fe5C2 (air treated)
e
404
<0.1
–
–
–
–
–
–
7
Fe5C2 (H2 treated)f
421
0.4
–
–
–
–
–
–
8
Fe5C2g
491
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. a Monitoring of the catalyst bed temperature of various catalysts under photo-irradiation. b Blank experiment; irradiation in the absence of any catalysts under above conditions. c Reaction conditions: H2/Ar (10/90), irradiation time 0.5 hr, 300-W Xe lamp. d The fresh Fe5C2 catalyst was passivated with O2/He (0.5/99.5) for 12 hr before reaction. e The fresh Fe5C2 catalyst was calcined in air at 500 C for 2 hr before reaction. f The fresh Fe5C2 catalyst was reduced in H2/Ar (10/90) at 800 C for 5 hr before reaction. g Reaction 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 energy 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 structure 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 surface 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 (O2 ) at 529.8 eV, peaks at 532.1 and 533.5 eV corresponding to OH and COOH were observed.36–38 After the reaction, the sample was transferred to an XPS chamber without exposure to air. Significantly, 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 increase 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 dramatic 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 CH 2 .
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 ( 0.59 eV) was much lower than that on Fe5C2(111) ( 1.07 eV), indicating that the adsorbed O atoms can promote C2H4 desorption and avoid the further over-hydrogenation to C2H6. Subsequently, we computed the C2H4 hydrogenation into C2H6, and the effective barriers 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 atmosphere 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 ( 0.52, 0.57 versus 0.60) suggest the electron transfer from the Fe atom to O and C2H4. From a frontier 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.
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Figure 4. Energy Profiles for C2H4 Adsorption and Hydrogenation Forming C2H6 under ThermalDriven (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 CH 2 .
Therefore, we believe that a small number of O atoms adsorbed on the Fe5C2 surface 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 using 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 ( 0.41 versus 0.72 eV and 1.30 versus 1.71 eV). This suggests that the surface O atoms can modify the local electronic structure and optical band gap of the surface.41,42 Under photoirradiation, the domain products are preferred over olefins on oxygen-decorated Fe5C2(111), whereas for pure Fe5C2(111), the formation of alkane is more favorable. 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 surface. 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 formation of olefins in the photo-driven FTO process. In a control experiment, a thermal FTS experiment of Fe5C2 catalyst was carried out at 300 C–500 C without photo-irradiation (and all other reaction conditions the same). The result demonstrated 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 500 C), the CO conversion reached 80.5%; however, the dominant products were CH4 with high selectivity of 95.1% in hydrocarbons 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 temperature (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 selectivity. 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 ammonium bromide (0.113 g) was stirred sufficiently and degassed under a flow of Ar. The mixture was heated to 120 C and Fe(CO)5 (0.5 mL, 3.6 mmol) was injected under an Ar blanket. The mixture was heated to 180 C at 10 C/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 350 C at 10 C/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 60 C for 12 hr and then calcinated in air at 500 C 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
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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 300 C 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 catalysts 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 injected. The light source was a 300-W Xe lamp (l = 200–1,100 nm, light intensity 2.9 W cm 2) 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-wavebased 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-BurkeErnzerhof 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 characterize 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 molecular 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 [Eslab + EX], where EX/slab is the total energy of the slab with adsorbates in its equilibrium geometry, 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 EIS and DEr = EFS 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) G(Fe5C2) 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) m(H2). On the basis of the equation m(P, T) = m0(T) + EZPE + kTln(P/P0), we can find the m(H2O) and m(H2) from the
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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 = 97.60 , showing an excellent agreement with experiments, in which a, b, and c equal 11.562, 4.573, and 5.060 A˚, respectively, and b equals 97.74 .49 The calculated 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 geometry optimization and transition-state searching. In particular, the Fe5C2(111) surface 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 figures, 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, HundredTalent Program of Chinese Academy of Sciences, and Shanxi Hundred-Talent Program. The computational resources for the project were supplied by the Tianhe-2 in Lvliang, Shanxi. XAFS experiments were performed at the 1W1B beamline of Beijing 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
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