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Highly selective conversion of CO2 to light olefins via Fischer-Tropsch synthesis over stable layered K–Fe–Ti catalysts
Applied Catalysis A, General 573 (2019) 32–40
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Highly selective conversion of CO2 to light olefins via Fischer-Tropsch synthesis over stable layered K–Fe–Ti catalysts ⁎
Xu Wang, Dakai Wu, Jianli Zhang , Xinhua Gao, Qingxiang Ma, Subing Fan, Tian-Sheng Zhao
T ⁎
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Ningxia University), Yinchuan, 750021, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 hydrogenation Layered K–Fe–Ti catalyst Light olefins Olefin adsorption
K–Fe–Ti layered metal oxides (LMO) were prepared using solid-state reaction followed by in situ reduction and applied in CO2 hydrogenation to light olefins via Fischer-Tropsch synthesis process. The results showed that the molar ratio of K/Fe/Ti had dramatic effects on the textural and structural properties of the LMO and the catalytic performance. Layered structure K–Fe–Ti exhibited high olefin selectivity and stability for CO2 hydrogenation. The light olefin selectivity reached approximately 60% with an olefin/paraffin value of 7.3 over 0.8K–2.4Fe–1.3Ti, and the exfoliated LMO through acid treatment was found to weaken the interaction between Fe and Ti. This enabled the reduction and activation of iron oxides easier to form iron carbide species and promoted a shift from the reverse water gas shift reaction regime to hydrocarbon synthesis regime, contributing to higher hydrocarbons while lower CO.
1. Introduction Carbon dioxide (CO2) is generally known as a greenhouse gas and earth abundant in the atmosphere so as to bring out climate change [1–7]. All of the problems brought out by excessive emission of CO2 in atmosphere compelled researchers to develop efficient way to CO2 capture and storage (CCS), and CO2 utilization in order to relieve environmental pressures [8–15]. Utilization of CO2 highly interests researchers to find scientific approaches to turn CO2 into serviceable chemical products [8,12,13,15–20]. However, CO2 is a very stable molecule due to its double bonds between carbon and oxygen atom which makes it chemically inactive. Therefore, activation of CO2 seems to be a vital progress in CO2 utilization. CO2 hydrogenation to light olefins through Fischer-Tropsch synthesis (CO2-FTS) is a progress that CO2 transforms into CO via the reverse water gas shift (RWGS) reaction and followed by CO hydrogenation into hydrocarbons by FTS process [8,12–16,18,20]. This process not only converted CO2 to CO but also made CO2 valuable and profitable, and has already become a focus. In the CO2-FTS progress, olefin selectivity is limited by Anderson-SchulzFlory (ASF) distribution, which is influenced by the side reaction like the secondary hydrogenation of primary olefins and structure properties, for instance, morphology, surface basicity, pore structure, and surface electronic density of the catalysts [20–27]. It is reported that granule structure influenced adsorption and diffusion of reactant over catalysts [25,26]. In FTS, catalysts with porous
⁎
structure usually cause a relatively long transmission distance for feed and product, leading to longer residence time which contributes to initiating the secondary reaction and decreasing the olefin selectivity unsatisfactorily [28]. Compared with the bulk structure materials, layered structure materials are preferred to make specific metal atoms homogeneously dispersed at an atomic level due to their crystal structure [29], which also have a relative shorter transmission distance and duct benefiting to the adsorption and diffusion of reactant and product. All the features of layered structure can be of advantage to the suppression of the secondary reaction during CO2 hydrogenation. Layered metal oxide (LMO) materials have been frequently applied in photocatalytic reaction [29–32]. One type of candidates, the layered K–Fe–Ti compound has been studied extensively as ion exchangers, metal ion adsorbents due to their excellent properties [33–35]. K–Fe–Ti layered oxide has been deemed to the formation of K2.3Fe2.3Ti5.7O16 phase which belongs to orthorhombic Cmcm symmetry. It shows the lattice parameters of a = 0.3796 nm, b = 1.5735 nm and c = 0.2964 nm [36]. In this crystal structure, equilibrium charge achieves by inserting potassium ion into layers constituted by the (Fe,Ti)O6 octahedron anion. Furthermore, K+ can be substituted for other suitable cations like Na+, Rb+. Crystal structure extends and forms platelet crystal which is responsible for the layered structure [29,37], and makes the iron species exposed and highly dispersed resulting in some new properties. Such features inspire us to apply it in catalyzing CO2 hydrogenation to olefins reaction. At the same time, its
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (T.-S. Zhao).
https://doi.org/10.1016/j.apcata.2019.01.005 Received 24 September 2018; Received in revised form 27 December 2018; Accepted 9 January 2019 Available online 09 January 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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from 50 °C to 800 °C at a heating rate of 10 °C min–1 in a flow of He and TPD data was recorded synchronously. C2H4-TPD was carried out by using a Micromeritics AutoChem II 2920 instrument. Detail process was as follows: Before adsorption, 50 mg of the catalyst sample was reduced with syngas maintained 40 min under conditions of H2/CO = 2, 400 °C in a flow of 30 mL min–1. Afterwards, the temperature was cooled down to 50 °C under a flow of He. Then ethylene was switched and introduced into reactor in a flow of 15 mL min–1 for adsorption. Finally, the temperature was increased from 50 °C to 600 °C at a heating rate of 10 °C min–1 in a flow of He and TPD data were recorded. The X-ray photoelectron spectra (XPS) were performed on a thermo scientific ESCALAB 250 spectrometer using an Al Kα X-ray source with the pressure of the chamber of 2 × 10–8 Pa. Meanwhile, the binding energies (BEs) were calibrated using the C1s peak of contamination carbon at 284.8 eV as a reference.
excellent structural phase stability has attracted much attention during the CO2 hydrogenation reaction. In this work, K–Fe–Ti LMO was prepared by employing solid phase reaction method. Stable phase structure and high selectivity for CO2 hydrogenation to olefins were obtained over K–Fe–Ti LMO catalysts. Furthermore, in order to further promote the activation of iron and then tune the product distribution, the layered structure was exfoliated by an acid treatment. The formation of slice structure and phase structure defect deeply contributed to CO2 converting to hydrocarbon, maintaining high olefin selectivity but with low CO selectivity at the same time. With the combination of several structure and surface property characterizations, the structure-performance relationships of K–Fe–Ti LMO catalysts were discussed. 2. Experimental 2.1. Catalyst preparation
2.3. Catalytic activity tests As reported [32,34], different molar ratios of K2CO3, Fe (NO3)3·9H2O and TiO2 were well mixed, dried at 100 °C for 24 h and then calcined at 500 °C for 3 h. The obtained power was signed as K–Fe–Ti–C. Afterwards, the samples were ground and calcined again with gradient elevation of temperature of 15 °C – 500 °C (5 °C/min), 500 °C – 800 °C (10 °C/min), 800 °C – 1000 °C (5 °C/min), and 1000 °C – 1100 °C (2 °C/min), respectively. Finally, the obtained powder above was treated by 1 M nitric acid solution according to the research reported [35]. The product was washed for 6 times with deionized water and ethanol alternately and collected by centrifugation, then dried in air at 100 °C for 12 h and signed as K–Fe–Ti–E. All the obtained samples were made into 20–40 meshes for use.
CO2 hydrogenation reaction was performed in a fixed-bed reactor with a stainless bed (8 mm i.d., 400 mm in length) loading 1 mL of catalyst sample. Prior to the reaction, the catalyst was pre-reduced in situ at 400 °C for 6 h in a flowing stream of 30% H2 in N2 (GHSV = 1000 h–1 and P = 0.1 MPa). After the temperature was cooled down to 320 °C, the feed gas (H2/CO2 = 3, molar ratio) was introduced into reactor with a pressure of 2 MPa and GHSV of 1000 h–1. The sampling interval was about 12 h. The effluent gas was analyzed online with a gas chromatograph (GC-9160-I). A TDX-01 column was packed on a thermal conductivity detector (TCD) for C1 products analysis, and a Al2O3 capillary column was packed on a flame ionization detector (FID) for C1–C5 hydrocarbons products. The liquid products were analyzed on an off-line gas chromatograph (GC-9160-II) packed on a TCD detector (2 m Porapak Q column) for the aqueous phase products analysis (oxygenates), and a FID detector (30 m SE-30 capillary column) for the oil phase products analysis. CO2 conversion presented was based on carbon in mass balance. Light olefin selectivity (C%) was calculated by olefin fraction in all hydrocarbon products and olefin to paraffin ratio (O/P) in the C2–C4 fraction. The calculation formula of product selectivity and CO2 conversion was given as follows:
2.2. Catalyst characterization The specific surface area of catalyst was analyzed by the BrunauerEmmett-Teller (BET) equation according to multi-point method. A type of JW-BK132 F N2 adsorption instrument was used to the measurement under the condition at −196 °C. 2.0 g sample was degassed primarily under a flow of He in a vacuum at 300 °C for 2 h before adsorption. The microscopic morphology of sample was observed by the scanning electron microscope (SEM) on a JEOL-JSM-7500 F instrument. Low voltage of 3.0 kV was to contribute to more sensitive signal to achieve clear images of the samples. Transmission electron microscope (TEM) image was obtained by the Hitachi HT7700 instrument at voltage of 100 kV. The samples were ultrasonically dispersed in an ethanol solution and supported on a Cu grid for the measurement. High resolution transmission electron microscopy (HRTEM) was conducted on the Tecnai G2 F30 instrument. The X-ray diffraction (XRD) of the samples were detected on a Rigaku D/MAX-2200PC diffractometer via Cu Kα radiation (λ = 1.5406 Å). In addition, all of the samples were under the condition of 45 kV, 200 mA and the scanning speed was ranged 8° min–1 from 3° (2θ) to 85°. H2 temperature-programmed reduction (H2-TPR) measurements were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. Firstly, 50 mg sample was pretreated by a flow of He (30 mL min–1) at 350 °C for 40 min and then cooled down to the room temperature. The gas of 5% H2 and 95% N2 (25 mL min–1) was switched to the reduction. The temperature was ramped from 25 to 800 °C at the heating rate of 10 °C min–1. CO2 temperature-programmed desorption (CO2-TPD) was performed by using a Micromeritics AutoChem II 2920 instrument. Firstly, a sample of 50 mg was pretreated with a flow of 10% H2 and 90% Ar (30 mL min–1) at 400 °C for 1 h. Secondly, the temperature was cooled down to 50 °C under a flow of He. The analysis was measured by altering of CO2 to the adsorption. Finally, the temperature was increased
XCO2 = (F in × y inCO2 – Fout × yout
CO2
) / (F in × y inCO2) × 100%
(1)
In which the F in and Fout represented the molar flow of inlet and outlet gas respectively. y inCO2 and yout CO2 were signs for the CO2 fraction in feed and tail gas. XCO2 represented the conversion of CO2. The selectivity of different hydrocarbon was represented by Si and symbol i represented each hydrocarbon. The equation was as follows: Si = Fout·y
i
out
/ (Fin·yCO2in·XCO2) × 100%
(2)
CO selectivity was calculated based on the Eq. (3) : Sco = Fout·yCOout / (Fin·yCO2in·XCO2) × 100%
(3)
3. Results and discussion 3.1. Phase structure of the catalyst samples The crystal structure of the fresh and spent catalysts was indicated in Figs. 1 and 2, respectively. It was observed that molar ratios of K/Fe/ Ti had dramatic effects on the phase composition and crystallinity for the fresh samples in Fig. 1. Furthermore, layered structure was revealed to be dominant in the whole detected phase. K2.3Fe2.3Ti5.7O16, with diffraction peaks at 2θ = 11.36°, 29.18°, 34.3°, 38.16° and 46.26°, was considered to be the structural unit and responsible for the layered structure [32,34,36]. The layered plane spacing was about 0.79 nm 33
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Fig. 3. N2 adsorption-desorption isotherms of fresh samples.
Fig. 1. XRD patterns of fresh samples.
Fig. 4. Pore diameter distribution of fresh samples. Fig. 2. XRD patterns of spent samples.
Table 1 Textural property of fresh and spent samples.
calculated by the Bragg equation [34]. Fe2TiO5 and K2Fe4O4 phases appeared in the 0.8K–2.4Fe–1.3Ti. Fe2O3 and KFeO2 were also found. After being treated by nitric acid solution, the (020) plane (2θ = 11.36°) shifted to higher position (2θ = 11.97°), illustrating that the crystal plane space was narrowed [36]. This alteration perhaps emerged when the H+ was substituted for K+ between layers undergoing the acid treatment [36]. Excellent layer structural stability was kept during CO2-FTS reaction for the spent samples (signed as AR–K–Fe–Ti) (Fig. 2). Fe3O4 phase presented after reaction, meanwhile, weak peak attributed to iron carbides was observed as well.
Catalyst
BET surface area SBET(m2 g–1)
Pore volume Vtotal(cm3 g–1)
Pore diameter Dmeso(nm)
0.8K–0.8Fe–1.3Ti 0.8K–1.6Fe–1.3Ti 0.8K–2.4Fe–1.3Ti 0.8K–4.0Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–C 0.8K–2.4Fe–1.3Ti–E
2.2 2.5 1.7 2.5 2.9 5.0
0.015 0.017 0.015 0.021 0.028 0.020
28.19 27.12 35.66 33.48 38.04 16.27
pore diameter. 3.2. Textural properties of fresh samples
3.3. Morphology of the samples
Fig. 3 indicates the N2 adsorption-desorption isotherms of fresh samples, which illustrated that all the samples were shaped with similar pore structure assigned to the H3 type hysteresis loop [38]. The pore diameter distribution shown in Fig. 4 revealed that the most probable pore size was 3 nm and pore mainly ranged from 2 to 10 nm. As shown in Table 1, low BET surface area around 2.0 m2 g–1 was obtained for all the samples excepted for 0.8K–2.4Fe–1.3Ti–E. There existed an increase in BET surface area and a contrary trend for pore diameter after being exfoliated. Accumulation of large particles probably induced the large
The SEM images and surface element component of three catalyst samples were shown in Fig. 5. 0.8K–2.4Fe–1.3Ti sample (Fig. 5a) formed by solid phase reaction method and stacked by crystal plateletshad a larger particle size in approximately 7 μm. However, the 0.8K–2.4Fe–1.3Ti–C (Fig. 5b) owned the different morphology and failed to the formation of layer structure because of the low calcined temperature less than 800 °C and insufficient energy for the rearrangement of ion species [32]. Although uncompleted exfoliation appeared, partial uniform flakes of the samples with size of about 34
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Fig. 5. SEM images and EDS analysis of the catalyst samples. (a): fresh 0.8K–2.4Fe–1.3Ti, (b): fresh 0.8K–2.4Fe–1.3Ti–C, (c): fresh 0.8K–2.4Fe–1.3Ti–E.
the reduction of Fe3O4 to α-Fe. Surface structural defects and lower potassium content created by nitric acid treatment led to more facile reduction of iron species for 0.8K–2.4Fe–1.3Ti–E in contrast with that of 0.8K–2.4Fe–1.3Ti. The first reduction peak occurred at approximately 400 °C.
130 nm in length could be observed in Fig. 5c after being exfoliated in nitric acid solution. α, β and γ detective areas provided the mean EDS surface element content of samples. It was clear that surface Fe content was lower over 0.8K–2.4Fe–1.3Ti–C than the other two samples. Surface potassium was enriched for 0.8K–2.4Fe–1.3Ti, nevertheless, a sharp drop to 6.09% could be found after acid treatment over 0.8K–2.4Fe–1.3Ti–E. As shown in Fig. 6, HRTEM images of fresh (Fig. 6a and b) and spent (Fig. 6c and d) 0.8K–2.4Fe–1.3Ti catalysts presented well-defined lattice planes with planar d of 0.79 nm, assigned to the (020) plane of K2.3Fe2.3Ti5.7O16. It showed that the layered structure exhibited good stability. Fe3O4 (311) was also observed in the spent catalyst.
3.5. Surface properties of the catalyst samples 3.5.1. Surface basicity of the catalysts The surface basicity of pre-reduced samples was confirmed by CO2TPD as shown in Fig. 8a. A peak at temperature about 100 °C and a peak at about 620 °C presented in all of the profiles. According to the previous studies [42–47], the small peaks at lower temperature corresponded to desorption of weakly adsorbed CO2, and peaks at higher temperature might be ascribed to the decomposition of a small amount of metal carbonates formed during CO2 adsorption [43]. A large amount of potassium promoter in layered structure catalysts strongly increased the contribution of the second high-temperature peak in the TPD profile [43,48]. Wang and co-workers [42] hold that strongly chemisorbed CO2 or carbonate species at about 600–700 °C might be difficult to be hydrogenated under reaction conditions and probably not contribute to CO2 catalytic conversion. However, Herskowitz et al. [43] regarded that reduced Fe-ions interact with CO2 molecules according to the redox mechanism formed relatively stable surface carbonate species, which decomposed at temperatures higher than 350 °C and then caused RWGS reaction. Combined with catalytic performance results, we considered that the high temperature desorption of CO2 had little relationship with catalytic activity, but was related to K content on the surface (see Table 2). As it could be seen for 0.8K–2.4Fe–1.3Ti–C, a
3.4. Reduction behavior of the samples The reduction behaviors of different K–Fe–Ti samples demonstrating reducibility of iron oxide species as H2-TPR profiles are shown in Fig. 7. Distinct representations of the samples in the reduction behavior profiles were divided by multi-peaks Gaussian fitting. All the catalysts were more difficult to be reduced compared with that of the single iron oxides like Fe2O3 or Fe3O4 samples [39,40], suggesting that LMO structure and high K content inhibited the reduction of iron species [41]. The first reduction temperature at around 500–550 °C of the layered structure catalyst shifted towards lower temperature with the increase of Fe content, due to the appearance of hematite. The overlapped set of peaks for 0.8K–2.4Fe–1.3Ti–C could be found at the range of 550 °C – 800 °C. The peaks at 550 °C and 590 °C could be ascribed to the reduction of small and big microscopic iron particles for Fe2O3 to Fe3O4, respectively. The peak at more than 600 °C was corresponding to 35
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Fig. 6. HRTEM images of fresh (a, b) and spent (c, d) 0.8K–2.4Fe–1.3Ti catalysts.
catalyst, much larger peak area was obtained for 0.8K–2.4Fe–1.3Ti–C at the same temperature around 100 °C, suggesting that desorption of ethylene area was relatively triple as much as 0.8K–2.4Fe–1.3Ti (see Table 3). Even though surface basicity was being of advantage for olefin desorption, higher surface desorption of C2H4 was taken place in 0.8K–2.4Fe–1.3Ti–C from C2H4-TPD results. More C2H4 adsorption stimulated the secondary hydrogenation of primary olefins reaction occurrence and was more likely to the formation of alkane during CO2FTS for 0.8K–2.4Fe–1.3Ti–C (see Fig. 9). On the contrary, higher C2H4 selectivity and O/P ratio presented over 0.8K–2.4Fe–1.3Ti with less ethylene (C2H4) adsorption. Therefore, we conclude that the layered structure contributes to the formation of higher light olefins besides the potassium promotion.
large number of CO2 adsorption at high temperature decreased the catalytic activity due to excessive K promotion [42]. According to XRD results in Fig. 1, K2Fe10O16 was the only detective phase except for layered structure peaks and may be responsible for the medium adsorption in 350–450 °C. After being exfoliated, further enhancement in CO2 conversion achieved due to higher Fe and lower K contents exposed on surface of 0.8K–2.4Fe–1.3Ti–E (Fig. 5 and Table 2). Weak peak in 0.8K–2.4Fe–1.3Ti–E was attributed to the low content of K after acid treatment. Meanwhile, the state of Fe species altered and was beneficial to the reduction as shown in Fig. 7.
3.5.2. Structural effects on C2H4 adsorption Relative shorter transmission distance, duct and lower surface area in layered structure is beneficial to the adsorption and diffusion of reactant and product, which gives less opportunity to the side reaction like the secondary reaction of primary olefins. Thus, to investigate the effect of layered structure on the adsorption of olefins, C2H4-TPD experiment was carried out to indicate the surface adsorption and desorption of C2H4 over the two samples with different structure in the same component. C2H4-TPD profiles which were fit in two peaks of α and β and is shown in Fig. 8b. Syngas reduction before C2H4 adsorption was meant to imitatively make the catalyst reach a similar state for CO2FTS. It was reported that the weak desorption peak of C2H4 appeared at approximately 100 °C [49,50]. Compared with 0.8K–2.4Fe–1.3Ti
3.5.3. Chain growth possibility The calculated chain growth possibility is shown in Fig. 10. It is observed that the product distributions of hydrocarbons deviated from the Anderson–Schulz–Flory (ASF) equation. The occurrence of secondary reactions gives the most reasonable explanation for these deviations of the ASF distribution [51]. From the slope of the plots, the values of probability of chain growth, α, were increased from 0.609 to 0.653 for the 0.8K–2.4Fe–1.3Ti and 0.8K–2.4Fe–1.3Ti–C, respectively. That is, olefin re-adsorption was favored to participate into the chain propagation to form higher hydrocarbons over 0.8K–2.4Fe–1.3Ti–C. 36
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Fig. 7. H2-TPR profiles of the catalyst samples.
However, over layered 0.8K–2.4Fe–1.3Ti, C2=–C4= selectivity was enhanced to approximately 59.3%. The calculated results were in agreement with the experimental data of olefin adsorption.
surface K on 0.8K–2.4Fe–1.3Ti–E significantly decreased to 1.6% due to the partial exfoliates of the structure and surface acid-base neutralization by nitric acid treatment. Carbon deposition appeared in all the spent catalysts, excepted for 0.8K–2.4Fe–1.3Ti–E. The loss of potassium content in the 0.8K–2.4Fe–1.3Ti–E probably prohibited the formation of carbon deposition [57,58].
3.6. XPS measurement of the catalysts As shown in Fig. 11, the existence of Fe3+ in fresh catalysts was evidenced by a shoulder peak observed on the main peak at 710.5 eV and 724.6 eV [52–55], which was also consistent with the reduction behaviors in Fig. 7. In addition, a distinct satellite peak at a binding energy 8 eV higher than the main Fe2p3/2 peak could be observed at about 719.0 eV assigned to Fe3+ species [52]. The higher BE (711.0 eV) for 0.8K–2.4Fe–1.3Ti–C and 0.8K–2.4Fe–1.3Ti–E provided evidence of relatively weak interaction between iron and titanium species, which could also be seen from Fig. 12. Remarkable decrease of K for 0.8K–2.4Fe–1.3Ti–E by acid treatment probably contributed to higher BEs (Table 2). As shown in Fig. 12, the BEs of Ti 2p3/2 was about 458.0 eV, indicating that Ti4+ was in the form of TiO2 for 0.8K–2.4Fe–1.3Ti–C [54,56]. BEs shifted to lower temperature in the layered structure catalyst in both Fe and Ti, due to its strong electron donor effect of K, while surface K was enriched due to formation of the layered structure. At the same time, the interaction between Fe and Ti was enhanced. According to XRD result of 0.8K–2.4Fe–1.3Ti–E (Fig. 1), layered structure was the mainly detective phase associated with the BEs of Fe2p3/2 shifted from 711.0 eV to 710.5 eV, contrary to this, the BEs of Ti shifted to higher position in LMO. Surface compositions of the fresh and spent catalysts in Table 2 by XPS measurement exhibited that surface molar content of Fe and molar ratio of Fe/K in layered structure catalysts had positive correlation with increase of Fe addition. After being treated by acid, the content of
3.7. Catalytic activity The catalytic performances of the catalysts during CO2-FTS were evaluated at 320 °C, 2.0 MPa and 1000 h–1. All catalysts showed high light olefin selectivity (Fig. 13a). Selectivity towards C2–C4 olefins was obviously enhanced by the formation of LMO structure. A maximum C2–C4 olefin selectivity of 59.3% with O/P of 7.3 was obtained over 0.8K–2.4Fe–1.3Ti LMO sample. Apparently, increase in Fe content brought out the CO2 conversion ranging from 12.5% to 36.3%, which had positive correlations with increase of Fe content. Enrichment of surface potassium probably contributed to RWGS, restraining the reduction of Fe3O4, thus causing high CO selectivity (Fig. 13b). Another reason was that it was difficult to reduce and carburize iron oxides for LMO catalysts (Figs. 7 and 2). Iron species were mainly stabilized in Fe3O4, the active phase of RWGS [43,59]. Therefore, the FTS activity was low but with high RWGS activity. After being exfoliated, CO2 conversion was significantly increased to 41.4%, simultaneously, CO selectivity remarkably decreased to 36.3%. The reason was that the exfoliated LMO by acid pretreatment reduced the interaction of Fe and Ti (Figs. 11 and 12), causing the reduction and activation easier to form more iron carbide species, which led to a shift from the RWGS reaction regime to hydrocarbon synthesis regime, contributing to higher CO2 conversion to hydrocarbons and lowering 37
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Fig. 9. Structure effects on light olefin distribution over catalysts.
Fig. 8. Profiles of CO2-TPD (a) and C2H4-TPD (b) of catalyst samples. Fig. 10. ASF distribution over 0.8K–2.4Fe–1.3Ti and 0.8K–2.4Fe–1.3Ti–C. Table 2 Surface compositions of the fresh and spent samples. Sample
0.8K–0.8Fe–1.3Ti 0.8K–2.4Fe–1.3Ti 0.8K–4.0Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–E 0.8K–2.4Fe–1.3Ti–C AR–0.8K–0.8Fe–1.3Ti AR–0.8K–2.4Fe–1.3Ti AR–0.8K–4.0Fe–1.3Ti AR–0.8K–2.4Fe–1.3Ti–E AR–0.8K–2.4Fe–1.3Ti–C
Surface atom content (mol %) Fe
Ti
K
O
C
Fe/Ti
Fe/K
1.59 4.16 5.10 8.73 8.87 0.49 0.98 1.06 5.44 1.85
4.24 4.66 4.64 7.75 5.50 1.49 1.72 1.75 7.80 2.33
19.04 16.24 12.01 1.61 10.25 16.44 12.03 10.29 11.03 16.85
37.20 39.69 38.98 47.12 47.89 26.44 24.64 25.94 45.37 32.97
37.94 35.24 39.27 34.79 23.39 55.14 60.62 60.96 30.35 45.99
0.37 0.89 1.10 1.13 1.61 0.03 0.57 0.61 0.70 0.79
0.08 0.26 0.42 5.42 0.87 0.03 0.08 0.10 0.49 0.11
Table 3 Peak areas of C2H4 desorption over different structure catalysts. Catalysts
α
β
0.8K–2.4Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–C
0.132 0.397
0.272 0.405
Fig. 11. Fe (2p) XPS spectra of the catalyst samples.
Although C2–C4 hydrocarbons was limited by ASF distribution, highly selective conversion of CO2 to light olefins about 60% was obtained over stable layered structure K–Fe–Ti catalysts, higher than the reported Fe–based catalysts in previous reports [13,14,42,47,48], where the selectivity of C2–C4 olefins were less than 55%. The excellent
the CO selectivity as shown in Fig. 13b. The stability experiment suggested that 0.8K–2.4Fe–1.3Ti LMO catalyst not only exhibited high selectivity of olefins but also showed high stability with 202 h time on stream during the reaction as shown in Fig. 14. 38
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Fig. 14. Stability of 0.8K–2.4Fe–1.3Ti with time on stream. Reaction conditions: H2/CO2 = 3/1, 320 °C, 2.0 MPa and 1000 h–1. Fig. 12. Ti (2p) XPS spectra of the catalyst samples.
4. Conclusions K–Fe–Ti LMO catalysts by solid phase reaction exhibited high catalytic activity, olefin selectivity and good stability toward CO2-FTS. The LMO structure showed low BET surface area and caused iron species hard to be reduced and carburized, which causes high CO selectivity via RWGS reaction. C2H4 adsorption was inhibited on LMO structure, which contributed to higher olefin selectivity by inhibiting the secondary hydrogenation of primary olefins. The formation of slice structure by acid treatment preferred to CO2 converting to hydrocarbons and decreased the CO selectivity. Acknowledgments Financial supports from the National Natural Science Foundation of China (No.21666030), the National First-rate Discipline Construction Project of Ningxia (Chemical Engineering & Technology, NXYLXK2017A04), the East-West Cooperation Project of Ningxia Key R & D Plan (2017BY063) and the Key Research and Development Program in Ningxia (2018BEE03010) are greatly acknowledged. References [1] S. Manabe, R.T. Wetherald, J. Atmos. Sci. 37 (1980) 99–118. [2] J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, D. Russell, Science 213 (1981) 957–966. [3] W. Cramer, A. Bondeau, F.I. Woodward, I.C. Prentice, R.A. Betts, V. Brovkin, P.M. Cox, V. Fisher, J.A. Foley, A.D. Friend, C. Kucharik, M.R. Lomas, N. Ramankutty, S. Sitch, B. Smith, A. White, C. Young–Molling, Global Change Biol. 7 (2001) 357–373. [4] K. Caldeira, A.K. Jain, M.I. Hoffert, Science 299 (2003) 2052–2054. [5] Q. Roberta, P. Sierra, Energ. Policy 35 (2007) 5938–5952. [6] S.J. Davis, H.D. Matthews, Science 329 (2010) 1330–1333. [7] G.P. Peters, C.L. Quéré, R.M. Andrew, J.G. Canadell, P. Friedlingstein, T. Ilyina, R.B. Jackson, F. Joos, J.I. Korsbakken, G.A. McKinley, S. Sitch, P. Tans, Nat. Clim. Change 7 (2017) 848–852. [8] C. Zhang, K.W. Jun, K.S. Ha, Y.J. Lee, S.C. Kang, Environ. Sci. Technol. 48 (2014) 8251–8257. [9] Y. Belmabkhout, V. Guillerm, M. Eddaoudi, Chem. Eng. J. 296 (2016) 386–397. [10] B. Adeniran, R. Mokaya, Nano Energy 16 (2015) 173–185. [11] J.L. Míguez, J. Porteiro, R. Pérez-Orozco, D. Patiño, S. Rodríguez, Appl. Energ. 211 (2018) 1282–1296. [12] Y.H. Choi, Y.J. Jang, H. Park, W.Y. Kim, Y.H. Lee, S.H. Choi, J.S. Lee, Appl. Catal. B Environ. 202 (2017) 605–610. [13] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Util. 5 (2014) 66–81. [14] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B Environ. 200 (2017) 530–542. [15] W. Li, H. Wang, X. Jiang, J. Zhu, Z. Liu, X. Guo, C. Song, RSC Adv. 8 (2018) 7651–7669. [16] J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Sun, Nat. Commun. 8 (2017) 15174.
Fig. 13. Catalytic activity (a) and products distribution (b) of different catalyst samples.
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Contents lists available at ScienceDirect
Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Highly selective conversion of CO2 to light olefins via Fischer-Tropsch synthesis over stable layered K–Fe–Ti catalysts ⁎
Xu Wang, Dakai Wu, Jianli Zhang , Xinhua Gao, Qingxiang Ma, Subing Fan, Tian-Sheng Zhao
T ⁎
State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Ningxia University), Yinchuan, 750021, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 hydrogenation Layered K–Fe–Ti catalyst Light olefins Olefin adsorption
K–Fe–Ti layered metal oxides (LMO) were prepared using solid-state reaction followed by in situ reduction and applied in CO2 hydrogenation to light olefins via Fischer-Tropsch synthesis process. The results showed that the molar ratio of K/Fe/Ti had dramatic effects on the textural and structural properties of the LMO and the catalytic performance. Layered structure K–Fe–Ti exhibited high olefin selectivity and stability for CO2 hydrogenation. The light olefin selectivity reached approximately 60% with an olefin/paraffin value of 7.3 over 0.8K–2.4Fe–1.3Ti, and the exfoliated LMO through acid treatment was found to weaken the interaction between Fe and Ti. This enabled the reduction and activation of iron oxides easier to form iron carbide species and promoted a shift from the reverse water gas shift reaction regime to hydrocarbon synthesis regime, contributing to higher hydrocarbons while lower CO.
1. Introduction Carbon dioxide (CO2) is generally known as a greenhouse gas and earth abundant in the atmosphere so as to bring out climate change [1–7]. All of the problems brought out by excessive emission of CO2 in atmosphere compelled researchers to develop efficient way to CO2 capture and storage (CCS), and CO2 utilization in order to relieve environmental pressures [8–15]. Utilization of CO2 highly interests researchers to find scientific approaches to turn CO2 into serviceable chemical products [8,12,13,15–20]. However, CO2 is a very stable molecule due to its double bonds between carbon and oxygen atom which makes it chemically inactive. Therefore, activation of CO2 seems to be a vital progress in CO2 utilization. CO2 hydrogenation to light olefins through Fischer-Tropsch synthesis (CO2-FTS) is a progress that CO2 transforms into CO via the reverse water gas shift (RWGS) reaction and followed by CO hydrogenation into hydrocarbons by FTS process [8,12–16,18,20]. This process not only converted CO2 to CO but also made CO2 valuable and profitable, and has already become a focus. In the CO2-FTS progress, olefin selectivity is limited by Anderson-SchulzFlory (ASF) distribution, which is influenced by the side reaction like the secondary hydrogenation of primary olefins and structure properties, for instance, morphology, surface basicity, pore structure, and surface electronic density of the catalysts [20–27]. It is reported that granule structure influenced adsorption and diffusion of reactant over catalysts [25,26]. In FTS, catalysts with porous
⁎
structure usually cause a relatively long transmission distance for feed and product, leading to longer residence time which contributes to initiating the secondary reaction and decreasing the olefin selectivity unsatisfactorily [28]. Compared with the bulk structure materials, layered structure materials are preferred to make specific metal atoms homogeneously dispersed at an atomic level due to their crystal structure [29], which also have a relative shorter transmission distance and duct benefiting to the adsorption and diffusion of reactant and product. All the features of layered structure can be of advantage to the suppression of the secondary reaction during CO2 hydrogenation. Layered metal oxide (LMO) materials have been frequently applied in photocatalytic reaction [29–32]. One type of candidates, the layered K–Fe–Ti compound has been studied extensively as ion exchangers, metal ion adsorbents due to their excellent properties [33–35]. K–Fe–Ti layered oxide has been deemed to the formation of K2.3Fe2.3Ti5.7O16 phase which belongs to orthorhombic Cmcm symmetry. It shows the lattice parameters of a = 0.3796 nm, b = 1.5735 nm and c = 0.2964 nm [36]. In this crystal structure, equilibrium charge achieves by inserting potassium ion into layers constituted by the (Fe,Ti)O6 octahedron anion. Furthermore, K+ can be substituted for other suitable cations like Na+, Rb+. Crystal structure extends and forms platelet crystal which is responsible for the layered structure [29,37], and makes the iron species exposed and highly dispersed resulting in some new properties. Such features inspire us to apply it in catalyzing CO2 hydrogenation to olefins reaction. At the same time, its
Corresponding authors. E-mail addresses: [email protected] (J. Zhang), [email protected] (T.-S. Zhao).
https://doi.org/10.1016/j.apcata.2019.01.005 Received 24 September 2018; Received in revised form 27 December 2018; Accepted 9 January 2019 Available online 09 January 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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from 50 °C to 800 °C at a heating rate of 10 °C min–1 in a flow of He and TPD data was recorded synchronously. C2H4-TPD was carried out by using a Micromeritics AutoChem II 2920 instrument. Detail process was as follows: Before adsorption, 50 mg of the catalyst sample was reduced with syngas maintained 40 min under conditions of H2/CO = 2, 400 °C in a flow of 30 mL min–1. Afterwards, the temperature was cooled down to 50 °C under a flow of He. Then ethylene was switched and introduced into reactor in a flow of 15 mL min–1 for adsorption. Finally, the temperature was increased from 50 °C to 600 °C at a heating rate of 10 °C min–1 in a flow of He and TPD data were recorded. The X-ray photoelectron spectra (XPS) were performed on a thermo scientific ESCALAB 250 spectrometer using an Al Kα X-ray source with the pressure of the chamber of 2 × 10–8 Pa. Meanwhile, the binding energies (BEs) were calibrated using the C1s peak of contamination carbon at 284.8 eV as a reference.
excellent structural phase stability has attracted much attention during the CO2 hydrogenation reaction. In this work, K–Fe–Ti LMO was prepared by employing solid phase reaction method. Stable phase structure and high selectivity for CO2 hydrogenation to olefins were obtained over K–Fe–Ti LMO catalysts. Furthermore, in order to further promote the activation of iron and then tune the product distribution, the layered structure was exfoliated by an acid treatment. The formation of slice structure and phase structure defect deeply contributed to CO2 converting to hydrocarbon, maintaining high olefin selectivity but with low CO selectivity at the same time. With the combination of several structure and surface property characterizations, the structure-performance relationships of K–Fe–Ti LMO catalysts were discussed. 2. Experimental 2.1. Catalyst preparation
2.3. Catalytic activity tests As reported [32,34], different molar ratios of K2CO3, Fe (NO3)3·9H2O and TiO2 were well mixed, dried at 100 °C for 24 h and then calcined at 500 °C for 3 h. The obtained power was signed as K–Fe–Ti–C. Afterwards, the samples were ground and calcined again with gradient elevation of temperature of 15 °C – 500 °C (5 °C/min), 500 °C – 800 °C (10 °C/min), 800 °C – 1000 °C (5 °C/min), and 1000 °C – 1100 °C (2 °C/min), respectively. Finally, the obtained powder above was treated by 1 M nitric acid solution according to the research reported [35]. The product was washed for 6 times with deionized water and ethanol alternately and collected by centrifugation, then dried in air at 100 °C for 12 h and signed as K–Fe–Ti–E. All the obtained samples were made into 20–40 meshes for use.
CO2 hydrogenation reaction was performed in a fixed-bed reactor with a stainless bed (8 mm i.d., 400 mm in length) loading 1 mL of catalyst sample. Prior to the reaction, the catalyst was pre-reduced in situ at 400 °C for 6 h in a flowing stream of 30% H2 in N2 (GHSV = 1000 h–1 and P = 0.1 MPa). After the temperature was cooled down to 320 °C, the feed gas (H2/CO2 = 3, molar ratio) was introduced into reactor with a pressure of 2 MPa and GHSV of 1000 h–1. The sampling interval was about 12 h. The effluent gas was analyzed online with a gas chromatograph (GC-9160-I). A TDX-01 column was packed on a thermal conductivity detector (TCD) for C1 products analysis, and a Al2O3 capillary column was packed on a flame ionization detector (FID) for C1–C5 hydrocarbons products. The liquid products were analyzed on an off-line gas chromatograph (GC-9160-II) packed on a TCD detector (2 m Porapak Q column) for the aqueous phase products analysis (oxygenates), and a FID detector (30 m SE-30 capillary column) for the oil phase products analysis. CO2 conversion presented was based on carbon in mass balance. Light olefin selectivity (C%) was calculated by olefin fraction in all hydrocarbon products and olefin to paraffin ratio (O/P) in the C2–C4 fraction. The calculation formula of product selectivity and CO2 conversion was given as follows:
2.2. Catalyst characterization The specific surface area of catalyst was analyzed by the BrunauerEmmett-Teller (BET) equation according to multi-point method. A type of JW-BK132 F N2 adsorption instrument was used to the measurement under the condition at −196 °C. 2.0 g sample was degassed primarily under a flow of He in a vacuum at 300 °C for 2 h before adsorption. The microscopic morphology of sample was observed by the scanning electron microscope (SEM) on a JEOL-JSM-7500 F instrument. Low voltage of 3.0 kV was to contribute to more sensitive signal to achieve clear images of the samples. Transmission electron microscope (TEM) image was obtained by the Hitachi HT7700 instrument at voltage of 100 kV. The samples were ultrasonically dispersed in an ethanol solution and supported on a Cu grid for the measurement. High resolution transmission electron microscopy (HRTEM) was conducted on the Tecnai G2 F30 instrument. The X-ray diffraction (XRD) of the samples were detected on a Rigaku D/MAX-2200PC diffractometer via Cu Kα radiation (λ = 1.5406 Å). In addition, all of the samples were under the condition of 45 kV, 200 mA and the scanning speed was ranged 8° min–1 from 3° (2θ) to 85°. H2 temperature-programmed reduction (H2-TPR) measurements were performed on a Micromeritics AutoChem II 2920 chemisorption analyzer. Firstly, 50 mg sample was pretreated by a flow of He (30 mL min–1) at 350 °C for 40 min and then cooled down to the room temperature. The gas of 5% H2 and 95% N2 (25 mL min–1) was switched to the reduction. The temperature was ramped from 25 to 800 °C at the heating rate of 10 °C min–1. CO2 temperature-programmed desorption (CO2-TPD) was performed by using a Micromeritics AutoChem II 2920 instrument. Firstly, a sample of 50 mg was pretreated with a flow of 10% H2 and 90% Ar (30 mL min–1) at 400 °C for 1 h. Secondly, the temperature was cooled down to 50 °C under a flow of He. The analysis was measured by altering of CO2 to the adsorption. Finally, the temperature was increased
XCO2 = (F in × y inCO2 – Fout × yout
CO2
) / (F in × y inCO2) × 100%
(1)
In which the F in and Fout represented the molar flow of inlet and outlet gas respectively. y inCO2 and yout CO2 were signs for the CO2 fraction in feed and tail gas. XCO2 represented the conversion of CO2. The selectivity of different hydrocarbon was represented by Si and symbol i represented each hydrocarbon. The equation was as follows: Si = Fout·y
i
out
/ (Fin·yCO2in·XCO2) × 100%
(2)
CO selectivity was calculated based on the Eq. (3) : Sco = Fout·yCOout / (Fin·yCO2in·XCO2) × 100%
(3)
3. Results and discussion 3.1. Phase structure of the catalyst samples The crystal structure of the fresh and spent catalysts was indicated in Figs. 1 and 2, respectively. It was observed that molar ratios of K/Fe/ Ti had dramatic effects on the phase composition and crystallinity for the fresh samples in Fig. 1. Furthermore, layered structure was revealed to be dominant in the whole detected phase. K2.3Fe2.3Ti5.7O16, with diffraction peaks at 2θ = 11.36°, 29.18°, 34.3°, 38.16° and 46.26°, was considered to be the structural unit and responsible for the layered structure [32,34,36]. The layered plane spacing was about 0.79 nm 33
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Fig. 3. N2 adsorption-desorption isotherms of fresh samples.
Fig. 1. XRD patterns of fresh samples.
Fig. 4. Pore diameter distribution of fresh samples. Fig. 2. XRD patterns of spent samples.
Table 1 Textural property of fresh and spent samples.
calculated by the Bragg equation [34]. Fe2TiO5 and K2Fe4O4 phases appeared in the 0.8K–2.4Fe–1.3Ti. Fe2O3 and KFeO2 were also found. After being treated by nitric acid solution, the (020) plane (2θ = 11.36°) shifted to higher position (2θ = 11.97°), illustrating that the crystal plane space was narrowed [36]. This alteration perhaps emerged when the H+ was substituted for K+ between layers undergoing the acid treatment [36]. Excellent layer structural stability was kept during CO2-FTS reaction for the spent samples (signed as AR–K–Fe–Ti) (Fig. 2). Fe3O4 phase presented after reaction, meanwhile, weak peak attributed to iron carbides was observed as well.
Catalyst
BET surface area SBET(m2 g–1)
Pore volume Vtotal(cm3 g–1)
Pore diameter Dmeso(nm)
0.8K–0.8Fe–1.3Ti 0.8K–1.6Fe–1.3Ti 0.8K–2.4Fe–1.3Ti 0.8K–4.0Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–C 0.8K–2.4Fe–1.3Ti–E
2.2 2.5 1.7 2.5 2.9 5.0
0.015 0.017 0.015 0.021 0.028 0.020
28.19 27.12 35.66 33.48 38.04 16.27
pore diameter. 3.2. Textural properties of fresh samples
3.3. Morphology of the samples
Fig. 3 indicates the N2 adsorption-desorption isotherms of fresh samples, which illustrated that all the samples were shaped with similar pore structure assigned to the H3 type hysteresis loop [38]. The pore diameter distribution shown in Fig. 4 revealed that the most probable pore size was 3 nm and pore mainly ranged from 2 to 10 nm. As shown in Table 1, low BET surface area around 2.0 m2 g–1 was obtained for all the samples excepted for 0.8K–2.4Fe–1.3Ti–E. There existed an increase in BET surface area and a contrary trend for pore diameter after being exfoliated. Accumulation of large particles probably induced the large
The SEM images and surface element component of three catalyst samples were shown in Fig. 5. 0.8K–2.4Fe–1.3Ti sample (Fig. 5a) formed by solid phase reaction method and stacked by crystal plateletshad a larger particle size in approximately 7 μm. However, the 0.8K–2.4Fe–1.3Ti–C (Fig. 5b) owned the different morphology and failed to the formation of layer structure because of the low calcined temperature less than 800 °C and insufficient energy for the rearrangement of ion species [32]. Although uncompleted exfoliation appeared, partial uniform flakes of the samples with size of about 34
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Fig. 5. SEM images and EDS analysis of the catalyst samples. (a): fresh 0.8K–2.4Fe–1.3Ti, (b): fresh 0.8K–2.4Fe–1.3Ti–C, (c): fresh 0.8K–2.4Fe–1.3Ti–E.
the reduction of Fe3O4 to α-Fe. Surface structural defects and lower potassium content created by nitric acid treatment led to more facile reduction of iron species for 0.8K–2.4Fe–1.3Ti–E in contrast with that of 0.8K–2.4Fe–1.3Ti. The first reduction peak occurred at approximately 400 °C.
130 nm in length could be observed in Fig. 5c after being exfoliated in nitric acid solution. α, β and γ detective areas provided the mean EDS surface element content of samples. It was clear that surface Fe content was lower over 0.8K–2.4Fe–1.3Ti–C than the other two samples. Surface potassium was enriched for 0.8K–2.4Fe–1.3Ti, nevertheless, a sharp drop to 6.09% could be found after acid treatment over 0.8K–2.4Fe–1.3Ti–E. As shown in Fig. 6, HRTEM images of fresh (Fig. 6a and b) and spent (Fig. 6c and d) 0.8K–2.4Fe–1.3Ti catalysts presented well-defined lattice planes with planar d of 0.79 nm, assigned to the (020) plane of K2.3Fe2.3Ti5.7O16. It showed that the layered structure exhibited good stability. Fe3O4 (311) was also observed in the spent catalyst.
3.5. Surface properties of the catalyst samples 3.5.1. Surface basicity of the catalysts The surface basicity of pre-reduced samples was confirmed by CO2TPD as shown in Fig. 8a. A peak at temperature about 100 °C and a peak at about 620 °C presented in all of the profiles. According to the previous studies [42–47], the small peaks at lower temperature corresponded to desorption of weakly adsorbed CO2, and peaks at higher temperature might be ascribed to the decomposition of a small amount of metal carbonates formed during CO2 adsorption [43]. A large amount of potassium promoter in layered structure catalysts strongly increased the contribution of the second high-temperature peak in the TPD profile [43,48]. Wang and co-workers [42] hold that strongly chemisorbed CO2 or carbonate species at about 600–700 °C might be difficult to be hydrogenated under reaction conditions and probably not contribute to CO2 catalytic conversion. However, Herskowitz et al. [43] regarded that reduced Fe-ions interact with CO2 molecules according to the redox mechanism formed relatively stable surface carbonate species, which decomposed at temperatures higher than 350 °C and then caused RWGS reaction. Combined with catalytic performance results, we considered that the high temperature desorption of CO2 had little relationship with catalytic activity, but was related to K content on the surface (see Table 2). As it could be seen for 0.8K–2.4Fe–1.3Ti–C, a
3.4. Reduction behavior of the samples The reduction behaviors of different K–Fe–Ti samples demonstrating reducibility of iron oxide species as H2-TPR profiles are shown in Fig. 7. Distinct representations of the samples in the reduction behavior profiles were divided by multi-peaks Gaussian fitting. All the catalysts were more difficult to be reduced compared with that of the single iron oxides like Fe2O3 or Fe3O4 samples [39,40], suggesting that LMO structure and high K content inhibited the reduction of iron species [41]. The first reduction temperature at around 500–550 °C of the layered structure catalyst shifted towards lower temperature with the increase of Fe content, due to the appearance of hematite. The overlapped set of peaks for 0.8K–2.4Fe–1.3Ti–C could be found at the range of 550 °C – 800 °C. The peaks at 550 °C and 590 °C could be ascribed to the reduction of small and big microscopic iron particles for Fe2O3 to Fe3O4, respectively. The peak at more than 600 °C was corresponding to 35
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Fig. 6. HRTEM images of fresh (a, b) and spent (c, d) 0.8K–2.4Fe–1.3Ti catalysts.
catalyst, much larger peak area was obtained for 0.8K–2.4Fe–1.3Ti–C at the same temperature around 100 °C, suggesting that desorption of ethylene area was relatively triple as much as 0.8K–2.4Fe–1.3Ti (see Table 3). Even though surface basicity was being of advantage for olefin desorption, higher surface desorption of C2H4 was taken place in 0.8K–2.4Fe–1.3Ti–C from C2H4-TPD results. More C2H4 adsorption stimulated the secondary hydrogenation of primary olefins reaction occurrence and was more likely to the formation of alkane during CO2FTS for 0.8K–2.4Fe–1.3Ti–C (see Fig. 9). On the contrary, higher C2H4 selectivity and O/P ratio presented over 0.8K–2.4Fe–1.3Ti with less ethylene (C2H4) adsorption. Therefore, we conclude that the layered structure contributes to the formation of higher light olefins besides the potassium promotion.
large number of CO2 adsorption at high temperature decreased the catalytic activity due to excessive K promotion [42]. According to XRD results in Fig. 1, K2Fe10O16 was the only detective phase except for layered structure peaks and may be responsible for the medium adsorption in 350–450 °C. After being exfoliated, further enhancement in CO2 conversion achieved due to higher Fe and lower K contents exposed on surface of 0.8K–2.4Fe–1.3Ti–E (Fig. 5 and Table 2). Weak peak in 0.8K–2.4Fe–1.3Ti–E was attributed to the low content of K after acid treatment. Meanwhile, the state of Fe species altered and was beneficial to the reduction as shown in Fig. 7.
3.5.2. Structural effects on C2H4 adsorption Relative shorter transmission distance, duct and lower surface area in layered structure is beneficial to the adsorption and diffusion of reactant and product, which gives less opportunity to the side reaction like the secondary reaction of primary olefins. Thus, to investigate the effect of layered structure on the adsorption of olefins, C2H4-TPD experiment was carried out to indicate the surface adsorption and desorption of C2H4 over the two samples with different structure in the same component. C2H4-TPD profiles which were fit in two peaks of α and β and is shown in Fig. 8b. Syngas reduction before C2H4 adsorption was meant to imitatively make the catalyst reach a similar state for CO2FTS. It was reported that the weak desorption peak of C2H4 appeared at approximately 100 °C [49,50]. Compared with 0.8K–2.4Fe–1.3Ti
3.5.3. Chain growth possibility The calculated chain growth possibility is shown in Fig. 10. It is observed that the product distributions of hydrocarbons deviated from the Anderson–Schulz–Flory (ASF) equation. The occurrence of secondary reactions gives the most reasonable explanation for these deviations of the ASF distribution [51]. From the slope of the plots, the values of probability of chain growth, α, were increased from 0.609 to 0.653 for the 0.8K–2.4Fe–1.3Ti and 0.8K–2.4Fe–1.3Ti–C, respectively. That is, olefin re-adsorption was favored to participate into the chain propagation to form higher hydrocarbons over 0.8K–2.4Fe–1.3Ti–C. 36
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Fig. 7. H2-TPR profiles of the catalyst samples.
However, over layered 0.8K–2.4Fe–1.3Ti, C2=–C4= selectivity was enhanced to approximately 59.3%. The calculated results were in agreement with the experimental data of olefin adsorption.
surface K on 0.8K–2.4Fe–1.3Ti–E significantly decreased to 1.6% due to the partial exfoliates of the structure and surface acid-base neutralization by nitric acid treatment. Carbon deposition appeared in all the spent catalysts, excepted for 0.8K–2.4Fe–1.3Ti–E. The loss of potassium content in the 0.8K–2.4Fe–1.3Ti–E probably prohibited the formation of carbon deposition [57,58].
3.6. XPS measurement of the catalysts As shown in Fig. 11, the existence of Fe3+ in fresh catalysts was evidenced by a shoulder peak observed on the main peak at 710.5 eV and 724.6 eV [52–55], which was also consistent with the reduction behaviors in Fig. 7. In addition, a distinct satellite peak at a binding energy 8 eV higher than the main Fe2p3/2 peak could be observed at about 719.0 eV assigned to Fe3+ species [52]. The higher BE (711.0 eV) for 0.8K–2.4Fe–1.3Ti–C and 0.8K–2.4Fe–1.3Ti–E provided evidence of relatively weak interaction between iron and titanium species, which could also be seen from Fig. 12. Remarkable decrease of K for 0.8K–2.4Fe–1.3Ti–E by acid treatment probably contributed to higher BEs (Table 2). As shown in Fig. 12, the BEs of Ti 2p3/2 was about 458.0 eV, indicating that Ti4+ was in the form of TiO2 for 0.8K–2.4Fe–1.3Ti–C [54,56]. BEs shifted to lower temperature in the layered structure catalyst in both Fe and Ti, due to its strong electron donor effect of K, while surface K was enriched due to formation of the layered structure. At the same time, the interaction between Fe and Ti was enhanced. According to XRD result of 0.8K–2.4Fe–1.3Ti–E (Fig. 1), layered structure was the mainly detective phase associated with the BEs of Fe2p3/2 shifted from 711.0 eV to 710.5 eV, contrary to this, the BEs of Ti shifted to higher position in LMO. Surface compositions of the fresh and spent catalysts in Table 2 by XPS measurement exhibited that surface molar content of Fe and molar ratio of Fe/K in layered structure catalysts had positive correlation with increase of Fe addition. After being treated by acid, the content of
3.7. Catalytic activity The catalytic performances of the catalysts during CO2-FTS were evaluated at 320 °C, 2.0 MPa and 1000 h–1. All catalysts showed high light olefin selectivity (Fig. 13a). Selectivity towards C2–C4 olefins was obviously enhanced by the formation of LMO structure. A maximum C2–C4 olefin selectivity of 59.3% with O/P of 7.3 was obtained over 0.8K–2.4Fe–1.3Ti LMO sample. Apparently, increase in Fe content brought out the CO2 conversion ranging from 12.5% to 36.3%, which had positive correlations with increase of Fe content. Enrichment of surface potassium probably contributed to RWGS, restraining the reduction of Fe3O4, thus causing high CO selectivity (Fig. 13b). Another reason was that it was difficult to reduce and carburize iron oxides for LMO catalysts (Figs. 7 and 2). Iron species were mainly stabilized in Fe3O4, the active phase of RWGS [43,59]. Therefore, the FTS activity was low but with high RWGS activity. After being exfoliated, CO2 conversion was significantly increased to 41.4%, simultaneously, CO selectivity remarkably decreased to 36.3%. The reason was that the exfoliated LMO by acid pretreatment reduced the interaction of Fe and Ti (Figs. 11 and 12), causing the reduction and activation easier to form more iron carbide species, which led to a shift from the RWGS reaction regime to hydrocarbon synthesis regime, contributing to higher CO2 conversion to hydrocarbons and lowering 37
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Fig. 9. Structure effects on light olefin distribution over catalysts.
Fig. 8. Profiles of CO2-TPD (a) and C2H4-TPD (b) of catalyst samples. Fig. 10. ASF distribution over 0.8K–2.4Fe–1.3Ti and 0.8K–2.4Fe–1.3Ti–C. Table 2 Surface compositions of the fresh and spent samples. Sample
0.8K–0.8Fe–1.3Ti 0.8K–2.4Fe–1.3Ti 0.8K–4.0Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–E 0.8K–2.4Fe–1.3Ti–C AR–0.8K–0.8Fe–1.3Ti AR–0.8K–2.4Fe–1.3Ti AR–0.8K–4.0Fe–1.3Ti AR–0.8K–2.4Fe–1.3Ti–E AR–0.8K–2.4Fe–1.3Ti–C
Surface atom content (mol %) Fe
Ti
K
O
C
Fe/Ti
Fe/K
1.59 4.16 5.10 8.73 8.87 0.49 0.98 1.06 5.44 1.85
4.24 4.66 4.64 7.75 5.50 1.49 1.72 1.75 7.80 2.33
19.04 16.24 12.01 1.61 10.25 16.44 12.03 10.29 11.03 16.85
37.20 39.69 38.98 47.12 47.89 26.44 24.64 25.94 45.37 32.97
37.94 35.24 39.27 34.79 23.39 55.14 60.62 60.96 30.35 45.99
0.37 0.89 1.10 1.13 1.61 0.03 0.57 0.61 0.70 0.79
0.08 0.26 0.42 5.42 0.87 0.03 0.08 0.10 0.49 0.11
Table 3 Peak areas of C2H4 desorption over different structure catalysts. Catalysts
α
β
0.8K–2.4Fe–1.3Ti 0.8K–2.4Fe–1.3Ti–C
0.132 0.397
0.272 0.405
Fig. 11. Fe (2p) XPS spectra of the catalyst samples.
Although C2–C4 hydrocarbons was limited by ASF distribution, highly selective conversion of CO2 to light olefins about 60% was obtained over stable layered structure K–Fe–Ti catalysts, higher than the reported Fe–based catalysts in previous reports [13,14,42,47,48], where the selectivity of C2–C4 olefins were less than 55%. The excellent
the CO selectivity as shown in Fig. 13b. The stability experiment suggested that 0.8K–2.4Fe–1.3Ti LMO catalyst not only exhibited high selectivity of olefins but also showed high stability with 202 h time on stream during the reaction as shown in Fig. 14. 38
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Fig. 14. Stability of 0.8K–2.4Fe–1.3Ti with time on stream. Reaction conditions: H2/CO2 = 3/1, 320 °C, 2.0 MPa and 1000 h–1. Fig. 12. Ti (2p) XPS spectra of the catalyst samples.
4. Conclusions K–Fe–Ti LMO catalysts by solid phase reaction exhibited high catalytic activity, olefin selectivity and good stability toward CO2-FTS. The LMO structure showed low BET surface area and caused iron species hard to be reduced and carburized, which causes high CO selectivity via RWGS reaction. C2H4 adsorption was inhibited on LMO structure, which contributed to higher olefin selectivity by inhibiting the secondary hydrogenation of primary olefins. The formation of slice structure by acid treatment preferred to CO2 converting to hydrocarbons and decreased the CO selectivity. Acknowledgments Financial supports from the National Natural Science Foundation of China (No.21666030), the National First-rate Discipline Construction Project of Ningxia (Chemical Engineering & Technology, NXYLXK2017A04), the East-West Cooperation Project of Ningxia Key R & D Plan (2017BY063) and the Key Research and Development Program in Ningxia (2018BEE03010) are greatly acknowledged. References [1] S. Manabe, R.T. Wetherald, J. Atmos. Sci. 37 (1980) 99–118. [2] J. Hansen, D. Johnson, A. Lacis, S. Lebedeff, P. Lee, D. Rind, D. Russell, Science 213 (1981) 957–966. [3] W. Cramer, A. Bondeau, F.I. Woodward, I.C. Prentice, R.A. Betts, V. Brovkin, P.M. Cox, V. Fisher, J.A. Foley, A.D. Friend, C. Kucharik, M.R. Lomas, N. Ramankutty, S. Sitch, B. Smith, A. White, C. Young–Molling, Global Change Biol. 7 (2001) 357–373. [4] K. Caldeira, A.K. Jain, M.I. Hoffert, Science 299 (2003) 2052–2054. [5] Q. Roberta, P. Sierra, Energ. Policy 35 (2007) 5938–5952. [6] S.J. Davis, H.D. Matthews, Science 329 (2010) 1330–1333. [7] G.P. Peters, C.L. Quéré, R.M. Andrew, J.G. Canadell, P. Friedlingstein, T. Ilyina, R.B. Jackson, F. Joos, J.I. Korsbakken, G.A. McKinley, S. Sitch, P. Tans, Nat. Clim. Change 7 (2017) 848–852. [8] C. Zhang, K.W. Jun, K.S. Ha, Y.J. Lee, S.C. Kang, Environ. Sci. Technol. 48 (2014) 8251–8257. [9] Y. Belmabkhout, V. Guillerm, M. Eddaoudi, Chem. Eng. J. 296 (2016) 386–397. [10] B. Adeniran, R. Mokaya, Nano Energy 16 (2015) 173–185. [11] J.L. Míguez, J. Porteiro, R. Pérez-Orozco, D. Patiño, S. Rodríguez, Appl. Energ. 211 (2018) 1282–1296. [12] Y.H. Choi, Y.J. Jang, H. Park, W.Y. Kim, Y.H. Lee, S.H. Choi, J.S. Lee, Appl. Catal. B Environ. 202 (2017) 605–610. [13] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Util. 5 (2014) 66–81. [14] C.G. Visconti, M. Martinelli, L. Falbo, A. Infantes-Molina, L. Lietti, P. Forzatti, G. Iaquaniello, E. Palo, B. Picutti, F. Brignoli, Appl. Catal. B Environ. 200 (2017) 530–542. [15] W. Li, H. Wang, X. Jiang, J. Zhu, Z. Liu, X. Guo, C. Song, RSC Adv. 8 (2018) 7651–7669. [16] J. Wei, Q. Ge, R. Yao, Z. Wen, C. Fang, L. Guo, H. Xu, J. Sun, Nat. Commun. 8 (2017) 15174.
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