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Article

Developing Multifunctional Fe-Based Catalysts for the Direct Hydrogenation of CO2 in Power Plant Flue Gas to Light Olefins

by
Likui Feng
1,2,
Shuai Guo
2,
Zhiyong Yu
1,2,
Yijie Cheng
1,2,
Julan Ming
2,
Xiaoning Song
2,
Qiuyang Cao
2,
Xiaofeng Zhu
1,
Guanghui Wang
1,
Di Xu
3,* and
Mingyue Ding
3,*
1
E. Energy Technology Co., Ltd., Hangzhou 310014, China
2
State Grid Zhejiang Electric Power Co., Ltd., Research Institute, Hangzhou 310014, China
3
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(3), 204; https://doi.org/10.3390/catal14030204
Submission received: 31 January 2024 / Revised: 22 February 2024 / Accepted: 1 March 2024 / Published: 20 March 2024
(This article belongs to the Special Issue Catalysis for Selective Hydrogenation of CO and CO2, 2nd Edition)
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Abstract

:
The hydrogenation of carbon dioxide (CO2) to produce light olefins is one of the most promising ways to utilize CO2 in power plant flue gas. However, the low concentration of CO2 (~10%) and the existence of water steam in the flue gas pose great challenges for the catalyst design. To address these problems, we introduced a Mg promoter and hydrophobic component into the Fe-based catalyst to improve the CO2 adsorption capacity and weaken the negative effects of water. The yield of light olefins on an optimized multifunctional Fe-based catalyst increased by 37% in low-concentration CO2 hydrogenation with water steam. A variety of characterizations proved that the Mg promoter played critical roles in regulating the adsorption capacity of CO2, increasing the surface electron density of Fe species, and promoting the formation of iron carbide active sites. The hydrophobic component mainly contributed to constraining the oxidation of iron carbides via water steam. It benefited from the rational design of the catalyst, showing how our multifunctional Fe-based catalyst has great potential for practical application in CO2 utilization.

Graphical Abstract

1. Introduction

Since the late 20th century, various environmental problems caused by a large amount of greenhouse gases, including CO2 emissions, have widely concerned the world. In this regard, carbon neutrality has been accepted by most countries. To achieve this goal, on the one hand, the use of fossil fuels should be reduced to decrease CO2 emissions at the source; on the other hand, effluent CO2 could be utilized by converting CO2 into high-value-added chemicals [1,2,3]. Light olefins are important chemical raw materials that are widely used in the production of various polymeric materials and fine chemicals [4,5,6]. Additionally, ethylene is an important symbol to evaluate the development level of the petrochemical industry. Using CO2 as raw material to produce light olefins is a promising way to achieve CO2 emissions reduction and CO2 utilization.
Catalytic hydrogenation is one of the feasible ways for CO2 conversion to light olefins. The reaction route is as follows: CO2 is first reduced to CO via a reverse water gas shift (RWGS) reaction, and then CO is converted to light olefins via the Fischer–Tropsch synthesis (FTS) reaction [7]. Fe-based catalysts have been extensively studied for CO2 hydrogenation to olefins due to the dynamic matching of RWGS and FTS reactions [8,9,10]. For example, Sun et al. developed alkali metals and modified Fe-based catalysts for CO2 hydrogenation to light olefins, which exhibited a CO2 conversion of 40.5% and a light olefin selectivity of 46.6% under reaction conditions of 3 MPa and 320 °C [11]. They proposed that alkali metal additives promoted CO dissociation, thus favoring the formation of an iron carbide active phase. Additionally, alkali metal promoters were proven to inhibit the hydrogenation of olefin intermediates, thus improving olefin selectivity. Ma et al. studied the synergistic effect of Mn and Na on Fe-based catalysts for CO2 hydrogenation [12]. The Mn promoter was supposed to disperse and stabilize Fe species, while the Na promoter mainly acted as an electron donator. The optimized Na and Mn co-modified Fe-based catalyst exhibited the highest olefin/paraffin ratio of 8.1. For iron-based catalysts, iron oxide and iron carbide are generally considered to be the active phases of RWGS and FTS reactions, respectively [13,14]. The presence of water steam in feed gas could lead to the oxidation of iron carbide to iron oxide, resulting in a significant decrease in catalytic activity [15,16,17]. Introducing water-conducting or hydrophobic components has been proven to be an effective strategy to restrain the negative effect of water on catalysts [17,18,19]. Therefore, the rational design of the catalyst according to the reaction environment is of great importance in CO2 hydrogenation to light olefins.
Coal-fired power generation occupies a leading position in power generation. In the meantime, coal-fired power plants emit large amounts of CO2 each year [20,21,22]. Therefore, it is of great significance to make efficient use of CO2 in flue gas. The concentration of CO2 in the flue gas of power plants is usually about 10%, and the rest is mostly made up of N2 [23,24,25]. Traditional CO2 catalytic hydrogenation uses high-purity CO2 as a raw material, which faces the problem of high capture costs [26,27,28]. Based on that, the direct use of plant flue gas containing low-concentration CO2 as raw material to prepare light olefins has potential economic benefits. Additionally, the effects of water steam in flue gas on the catalyst and reaction should not be ignored. Developing highly active Fe-based catalysts for the conversion of CO2 in flue gas has great significance but results in a big challenge. Herein, we designed and prepared a multifunctional Fe-based catalyst with Mg as the promoter and polydivinylbenzene (PDVB) as the hydrophobic component for converting CO2 in flue gas into light olefins. The optimized multifunctional catalyst achieved a CO2 conversion of 25.2% and a light olefin yield of 5.47% for H2O-containing low-concentration CO2 hydrogenation. The effects of Mg doping and hydrophobic PDVB introduction on catalytic performance and the internal regulatory mechanism were studied. This could accumulate theoretical knowledge for the development of catalysts in H2O-containing low-concentration CO2 hydrogenation to light olefins with industrial application prospects.

2. Results and Discussion

2.1. CO2 Hydrogenation Performance under Simulant Power Plant Flue Gas Conditions

The prepared KFMMx were generally tested for CO2 hydrogenation at 320 °C, 3 MPa, and 4000 mL/(gcat·h) with a CO2 content of 10%. As shown in Figure 1a,b, the CO2 conversion produced a volcanic curve with a Mg doping amount and peaked at 25.2% when the molar ratio of Mg/Fe was 0.15. Therefore, with the addition of Mg, CO selectivity decreased to a certain extent, indicating the accelerated conversion of CO2 to hydrocarbons via the CO intermediate [7]. Light olefins selectivity and the olefins/paraffin (o/p) ratio both increase after doping Mg, suggesting the promotion effect of Mg on olefin synthesis. Among the KFMMx catalysts, KFMM0.15 shows the best performance with a CO2 conversion of 25.2%, a light olefins selectivity of 24.7%, and an o/p ratio of 4.33. The light olefins yield of KFMM0.15 reached 6.22%, which is 38.8% higher than that of the undoped Mg catalyst (4.48%). All these results prove that the introduction of appropriate Mg is conducive to the conversion of low-concentration CO2 to light olefins. The effects of the reaction temperature and weight hour space volume (WHSV) on the light olefin synthesis of KFMM0.15 were further studied. As shown in Figure 1c,d, high temperature favored the conversion of CO2, but the light olefin selectivity and o/p ratio both decreased. As there is a reduction in WHSV, the yield of light olefins clear improves to 7.68% due to the accelerated conversion of CO2 to 33.1%, with light olefin selectivity stabilizing at around 23%.
Considering that the flue gas contains water steam, we further introduced water steam in reactant gas to study the effect of water steam on light olefin synthesis. The catalytic performances were tested at 340 °C, 3 MPa, and 4000 mL/(gcat·h) with a CO2 content of 10% and water steam content of 4%. As shown in Figure 2a,b, the addition of water had a significantly negative effect on catalytic performance. Specifically, the CO2 conversion decreased from 33.1% to 20.5%, and the CO selectivity increased from 22.9% to 42.1%. Although the light olefin selectivity slightly increased to 33.7%, the yield of light olefins was significantly reduced from 7.68% to 3.99%. In order to weaken the negative effect of water steam, we introduced hydrophobic PDVB to construct composite catalysts. As for composite catalysts with the powder mixing of KFMMx and PDVB (KFMMx/PDVB-P), the conversion of CO2 increases to 25.2%, and the selectivity of CO decreases to 31.9% compared to KFMMx alone, indicating the promoted conversion of CO2 via CO. Moreover, the yield of light olefins increased to 5.47%, which is 36.9% higher than that of KFMMx. We speculate that the addition of hydrophobic PDVB could avoid sufficient contact between the water steam and the catalyst surface, thus weakening the oxidation effect of H2O on the catalyst. However, when KFMMx is mixed with PDVB by mortar mixing (KFMMx/PDVB-M), the catalytic performance is severely weakened. Specifically, the CO2 conversion and CO selectivity on KFMMx/PDVB-M are 15.2% and 78.8%, respectively. The selectivity and yield of light olefins are as low as 19.8% and 0.64%. In addition, the catalytic performance of granule mixing for the composite catalyst (KFMMx/PDVB-G) is between the above two catalysts. This reminds us that the distance between KFMMx and PDVB is critical to light olefin synthesis, and KFMMx/PDVB-P shows the best catalytic performance in H2O-containing low-concentration CO2 hydrogenation. More than that, we further studied the effect of H2O dosage. As shown in Figure 2c,d, the yield of CO2 conversion and light olefins both decreased with the increase in H2O dosage, while the H2O dosage had little effect on light olefin selectivity. The optimized KFMMx/PDVB-P catalyst exhibited good stability even in a water steam atmosphere (Figure 2e). In summary, optimized KFMMx/PDVB-P exhibited great potential in CO2 hydrogenation to light olefins under simulant flue gas conditions compared to conventional Fe-based catalysts.

2.2. Structural Characterization

The N2 absorption–desorption isotherms of series KFMMx catalysts are shown in Figure 3a. All these catalysts exhibit type IV isotherms with hysteresis belonging to type H2b, indicating the complex porous structure. In addition, the BET-specific surface area of KFMMx catalysts improves with the introduction of Mg (Table 1). Specifically, the BET-specific surface area of KFMM0.15 is 82 m2/g, which is 1.7 times that of KFMM0 (48 m2/g). The pore size distributions of KFMMx catalysts clearly confirm the newly emerged mesoporous with a pore size of around 13 nm (Figure 3b). The increased BET-specific surface area and newly emerged mesoporous structure after introducing Mg are due to the pore-forming effect of carbonate decomposition during calcination, which is conducive to the adsorption of reactant molecules and has positive effects on catalytic performance.
The SEM images of KFMM0 and KFMM0.15 catalysts are shown in Figure 4. Figure 4a shows that KFMM0 is dominated by uniform nanoparticles with a particle size of about 100 nm. By contrast, the dispersion of KFMM0.15 is very uneven (Figure 4b). The SEM images of spent KFMM0.15 catalysts are shown in Figure 4c. It can be found that the morphology of KFMM0.15 is almost unchanged after the CO2 hydrogenation reaction.
The bulk phase structures of the catalysts were studied using XRD. As shown in Figure 5a, the diffraction peaks at 30.0°, 35.4°, 43.0°, 56.9°, and 62.5° are attributed to (220), (311), (400), (511), and (440) planes of Fe3O4, which is the active phase of the RWGS reaction [13,14,29]. No diffraction peaks attributed to MnO and MgO appeared on fresh KFMMx catalysts, indicating good dispersion. As for the catalysts after the CO2 catalytic hydrogenation reaction, the phase structures clearly changed. As shown in Figure 5b, the appearance of diffraction peaks at 40~46° indicates the formation of Fe5C2, which is the active phase of the FTS reaction [30]. In addition, the diffraction peaks at 24.6°, 31.7°, and 51.9° belong to MnCO3, suggesting the phase change in MnO to MnCO3 during CO2 hydrogenation. As for KFMM0.15/PDVB composite catalysts, the broad peak at around 20° was assigned to the amorphous structure of PDVB (Figure 5c). The XRD patterns of catalysts after the H2O-containing CO2 hydrogenation reaction are shown in Figure 5d. It can be found that the peak intensity of Fe5C2 species decreased significantly after introducing H2O, indicating the oxidation effect of H2O on Fe5C2. This is strongly related to the weakened catalytic activity. As for KFMM0.15/PDVB-P, the peak intensity of Fe5C2 species was greater than the sole KFMM0.15. This proves that the introduction of PDVB can reduce the oxidation effect of H2O on Fe5C2, maybe by avoiding sufficient contact between the water steam and KFMM0.15 surface. Moreover, the Fe5C2 species is almost absent, and Fe3O4 is the only Fe phase on KFMM0.15/PDVB-M. This suggests that the close distance between KFMM0.15 and PDVB could restrain the transformation of Fe3O4 to Fe5C2, thus exhibiting high CO selectivity. In summary, Fe3O4 and Fe5C2 undergo catalytic CO2 hydrogenation to light olefins by coupling RWGS and FTS reactions. The existence of H2O in feed gas goes against the formation of Fe5C2, resulting in lower activity. The introduction of hydrophobic PDVB was found to weaken the oxidation effect of H2O and make the formation of Fe5C2 available. Note that a moderate distance between KFMM0.15 and PDVB is crucial to ensure the quick removal of H2O and avoid the negative effect of PDVB. Based on this, the KFMM0.15/PDVB-P exhibits the best catalytic performance in H2O-containing low-concentration CO2 hydrogenation.
The contact angle test is performed to confirm the hydrophobic properties of the catalyst. As shown in Figure 6, the addition of PDVB can effectively increase the hydrophobicity from 66.13° to 125.21° for KFMM0.15/PDVB-P and to 123.39° for KFMM0.15/PDVB-M, which helps to speed up the discharge of H2O from the catalyst surface. Moreover, the catalyst remains hydrophobic even after the catalytic reaction, indicating the good stability of PDVB during the CO2 hydrogenation reaction. The retention of characteristic peaks of PDVB after the catalytic test further confirms its good stability (Figure 6g) [31]. Note that the retention of hydrophobicity of KFMM0.15/PDVB-P and KFMM0.15/PDVB m implies that the great difference in catalytic performance has nothing to do with hydrophobicity but is related to the iron phase.
XPS is used to analyze the electronic structure of catalysts. Figure 7a shows the Fe2p spectra of fresh KFMM0 and KFMM0.15 samples, in which 710.2~710.4 eV and 711.8~712.0 eV are attributed to Fe2+ and Fe3+, respectively [30,32]. After Mg is added, all peaks shift at 0.2 eV to a lower binding energy, indicating the electron transfer from Mg to Fe. The increased electron density on the surface of iron species can facilitate the activation of CO2 and CO molecules on the one hand, thus accelerating the conversion of CO2 and CO [33]. On the other hand, it can inhibit the hydrogenation reaction and facilitate the desorption of olefins, thus favoring the formation of olefin products [34]. Figure 7b shows the Fe2p spectra of spent samples in which 707.8~708.3 eV is attributed to FeCx [33,34]. Compared with the KFMM0 catalyst, the relative content of FeCx in KFMM0.15 is higher, indicating that the addition of Mg is conducive to the formation of the FeCx active phase. This can be reasoned by the facilitation of the CO dissociation on the electron-rich Fe surface [34,35]. Figure 7c shows the C1s spectra of spent samples. The peaks at 283.5~283.7 eV are attributed to the FeCx species, which is consistent with the Fe2p spectra [36]. In addition, the peaks at 289.0~289.1 eV and 288.1 eV belong to carbonate and formate, both of which are important intermediates in the process of CO2 hydrogenation [9,37,38]. Note that the surface of KFMM0.15 contains more carbonate species, indicating that the addition of Mg can enrich CO2 on the catalyst surface. This enrichment effect of Mg is conducive to the adsorption and activation of CO2 at a low concentration. As shown in Figure 7d, the introduction of water steam in feed gas leads to a decrease in the iron carbide fraction from 9.46% to 8.05%, while the introduction of PDVB increases the iron carbide fraction to 8.93%. These results indicate that water steam is not conducive to the formation of the iron carbide active phase, which is consistent with XRD results. And the introduction of PDVB could weaken the negative effect of water steam on active phase formation by avoiding prolonged contact between the water and the catalyst.
The H2-TPR results of KFMM0 and KFMM0.15 catalysts are shown in Figure 8a. Three reduction peaks were observed in the temperature range of 300~800 °C, which reflected the gradual reduction process of Fe3O4 to metallic Fe. Among them, the reduction peak at 300~400 °C was attributed to a reduction in the Fe3O4 surface, while the reduction peaks at 500~550°C and after 600°C were attributed to a reduction in bulk Fe3O4. With the addition of the Mg promoter, the reduction in the Fe3O4 surface shifted to a high temperature by 40 °C, while the reduction in bulk Fe3O4 shifted to a low temperature by 31 °C. Moreover, the fraction reduction peak at 300~400 °C in KFMM0.15 was greater than KFMM0, indicating the promotion effect of Mg on catalyst reduction. The CO2-TPD results of the KFMM0 and KFMM0.15 catalysts are shown in Figure 8b. There are three CO2 desorption peaks corresponding to weakly adsorbed CO2 (α) below 200°C, moderately strongly adsorbed CO2 (β) at 200~500 °C, and strongly adsorbed CO2 (γ) above 550 °C [39,40]. It was found that the addition of Mg slightly weakened CO2’s adsorption strength, leading to the desorption temperature (325 °C) being closer to the reaction temperature (320 °C). We can conclude that the addition of the Mg promoter could optimize the CO2 adsorption capacity of the catalyst and is more conducive to the activation of CO2.

2.3. Reaction Mechanism Study

We further performed in situ CO2 hydrogenation DRIFTS tests to study the reaction mechanism on KFMM0 and KFMM0.15 catalysts. As shown in Figure 9a,b, as for the KFMM0 catalyst, the IR bands at 2954/2932/2855/2723/1618/1374 cm−1, assigned to the formate intermediate (*HCOO), appeared rapidly with the introduction of CO2/H2 [41,42]. In addition, weak signals attributed to gaseous CO (2220~2050 cm−1) and linearly adsorbed CO (2075 cm−1) also appeared [43,44,45,46]. With the extension of the reaction time, the peak intensity of the above adsorbates was almost unchanged, and no CH4 was detected. This suggests that the subsequent conversions of intermediates to hydrocarbons are difficult in the KFMM0 catalyst. As for the KFMM0.15 catalyst (Figure 9c,d), a carbonate (*CO32−, 1500/1268 cm−1) species was observed, indicating CO2’s stronger adsorption ability, which is consistent with the XPS and CO2-TPD results [47,48]. More than that, the formation rate of the CO species was much faster than KFMM0. This reminds us that improved CO2 adsorption favors the formation of CO species. In addition, the gaseous CH4 product (3015 cm−1) was detected as the reaction continued [39,49]. We speculate that the CO species was the intermediate of CH4 formation, while the formate acted as a spectator in our catalyst [50,51]. We conclude that the introduction of Mg promotes CO2 adsorption and the RWGS reaction, thus accelerating the subsequent conversion of the CO intermediate to light olefins.

3. Experimental Section

3.1. Catalyst Preparation

A series of KFMMx (KFMM is an abbreviation of KFeMnMg) catalysts were synthesized by a coprecipitation method, where x (varies from 0 to 0.20) represents the atomic ratio of Mg in metal elements. In total, 16.9 mmol FeCl3·6H2O, 9.2 mmol FeCl2·4H2O, 7.8 mmol MnCl2·4H2O, 14.5 mmol MgCl2·6H2O, and hydrochloric acid were dissolved in 20 mL of deionized water, which was called solution A. 0.4 mol KOH was dissolved in 250 mL of deionized water to form solution B. Solution A was added into solution B with stirring at 50 °C. After 3 h of stirring, the product was washed with a large amount of deionized water and dried at 100 °C overnight to obtain FMMx. Then, K2CO3 was dissolved in an appropriate amount of deionized water (according to the saturated water adsorption of FMMx). FMMx was added to the above solution and stirred for 24 h. The suspension was dried at 100 °C overnight and calcined in N2 at 350 °C for 3 h to obtain KFMMx. The weight percentage of K was fixed at 3%.
PDVB was synthesized according to the reported method [31]. In total, 0.5 g of azobisisobutyronitrile (AIBN) was added to 10 g of divinylbenzene (DVB) and stirred at room temperature for 1 h. Then, the solution was transferred to a reactor and heat-treated at 100 °C for 24 h. The obtained solids were washed with methanol and then dried at 100 °C for 8 h to prepare PDVB. The composite catalysts were typically prepared by the physical mixing of KFMMx and PDVB with a mass ratio of 1:1, including powder mixing (donated as KFMMx/PDVB-P), granule mixing (donated as KFMMx/PDVB-G) and mortar mixing (donated as KFMMx/PDVB-M).

3.2. Catalyst Characterization

The N2 adsorption–desorption isotherms were determined by the JW-BK100B-specific surface analyzer. The morphology of the catalyst was characterized by the MIRA 3 LMH mode field emission scanning electron microscope (SEM) and FEI Talos F200X transmission electron microscope (TEM). Before characterization, the sample was dispersed in ethanol and then deposited on a carbon film. The X-ray diffraction (XRD) patterns of KFMMx were carried out on the Rigaku Ultima IV X-ray diffractometer with the Cu target (40 mV, 40 mA). The contact angle (CA) tests were taken on a POWEREACH JC2000C contact angle measuring instrument. Before the test, the catalyst was pressed, and then a 50 μL injector was used to drop demineralized water to test the contact angle between the water and the catalyst. Fourier-transform infrared spectroscopy (FT-IR) was used to analyze the phase composition and structure of PDVB and was taken by the Nicolet™ iS50 instrument with a DTGS detector. X-ray photoelectron spectroscopy (XPS) was carried out by using an ESCALAB250Xi X-ray photoelectron spectrometer equipped with an Al Kα (1486.6 eV) quartz monochrometer source.
H2 temperature-programmed reduction (H2-TPR), H2 temperature-programmed desorption (H2-TPD), and CO2 temperature-programmed desorption (CO2-TPD) experiments were tested on a DAS-7000 instrument with a TCD detector. As for the H2-TPR test, the catalysts were first cleaned by He at 350 °C for 1 h. After that, the samples were cooled to 50 °C and injected with a 10% H2/Ar mixture (50 mL/min) for 0.5 h. After baseline stabilization, the samples were heated up to 800 °C at 10 °C/min. As for H2-TPD and CO2-TPD tests, catalysts were firstly reduced by 10% H2/He at 350 °C for 1 h and purged by He for 1 h. After cooling down to 50 °C, the catalysts were treated with 10% H2/He or 10% CO2/He for 1 h; then, He was introduced for 1h to remove the weak physically adsorbed H2 or CO2 on the surface. The desorption was performed by heating from 50 °C to 600 °C. In situ diffuse reflection infrared Fourier transform spectroscopy (in situ DRIFTS) was tested using the Nicolet™ iS50 instrument with an MCT detector. The catalysts were first reduced in pure H2 at 350 °C for 1 h and purged in Ar for 1 h. The background spectra were collected by 32 scans at a resolution of 4 cm−1 after cooling down to 320 °C in Ar flow. The DRIFTS tests were carried out at 320 °C in a CO2/H2 (1:3 by volume) flow at 1 MPa.

3.3. Catalyst Activity Test

The CO2 hydrogenation reaction over the KFMMx and KFMMx/PDVB composite catalysts was carried out in a fixed-bed reactor. Before the reaction, 0.5 g of the catalyst was treated with pure H2 at 350 °C for 8 h and reactant gas (CO2:H2:N2 = 10:39:51) was then fed in at a temperature of 320~340 °C and a weighted hourly space velocity (WHSV) of 2000~4000 mL/(gcat·h) as the system was pressurized to 3 MPa. The water was added by a peristaltic pump and heated to 150 °C to form steam. All products were heated to 150 °C and analyzed by online gas chromatography (GC2060), where N2, CO, CH4, and CO2 were detected by a thermal conducted detector (TCD), and hydrocarbons were detected by two flame ionization detectors (FIDs). CO2 conversion and product selectivity were calculated according to the full product analysis. The carbon balance of all catalysts was between 96 and 97%.

4. Conclusions

According to the present composition of power plant flue gas, we prepared multifunctional KFMMx/PDVB catalysts for H2O-containing low-concentration CO2 hydrogenation to produce light olefins. The optimized KFMM0.15 catalyst achieved a light olefin yield of 6.22%, which was 38.8% higher than that without the Mg catalyst in low-concentration CO2 hydrogenation. Several characterization results confirmed that the Mg promoter played critical roles in regulating the adsorption capacity of CO2, increasing the surface electron density of Fe species, and promoting the formation of iron carbide active sites. The introduction of water steam in feed gas was found to weaken the catalytic performance severely. In response, the optimized KFMM0.15/PDVB-P multifunctional catalyst exhibited a light olefin yield of 5.47%, which was 36.9% higher than that of the sole KFMM0.15 catalyst in H2O-containing CO2 hydrogenation. The structural characterization results proved that water steam could oxidize the Fe5C2 active phase, thus severely reducing the catalytic performance, while the introduction of hydrophobic PDVB weakened the negative effect to some extent. Note that the distance between KFMM0.15 and PDVB greatly affected the catalytic performance of the multifunctional catalyst, which could be reasoned by the balance of the H2O removal rate and Fe5C2 formation. It benefited from the rational design of the catalyst, indicating how our multifunctional Fe-based catalyst shows great potential for practical application in CO2 utilization.

Author Contributions

L.F. and S.G. carried out the synthesis, catalytic test, and characterization of the catalysts. Z.Y., Y.C., J.M., X.S., Q.C., X.Z. and G.W. participated in the investigation of synthesis. L.F. wrote the manuscript text with the assistance of D.X. and M.D. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by [Study on catalyst and process for light olefins synthesis from carbon dioxide in flue gas of power plant] grant number [CY790000JS20220235] And The APC was funded by [Study on catalyst and process for light olefins synthesis from carbon dioxide in flue gas of power plant].

Data Availability Statement

All the data can be found in the manuscript.

Conflicts of Interest

The authors declare no competing financial interest.

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Catalysts 14 00204 g001
Figure 1. (a,b) Catalytic performance of KFMMX catalysts. Reaction conditions: CO2/H2/N2 = 10/30/60, 320 °C, 3 MPa, and 4000 mL/(gcat·h). (c,d) Effect of reaction temperatures and pressures on catalytic performance over KFMM0.15.
Figure 1. (a,b) Catalytic performance of KFMMX catalysts. Reaction conditions: CO2/H2/N2 = 10/30/60, 320 °C, 3 MPa, and 4000 mL/(gcat·h). (c,d) Effect of reaction temperatures and pressures on catalytic performance over KFMM0.15.
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Catalysts 14 00204 g002
Figure 2. (a,b) Catalytic performance of KFMM0.15/PDVB catalysts with different stacking manners. Reaction conditions: CO2/H2/N2/H2O = 9.6/28.8/57.6/4, 340 °C, 3 MPa, and 4000 mL/(gcat·h). (c,d) Effect of water steam dosage on catalytic performance over the KFMM0.15/PDVB-P catalyst. (e) Stability test of KFMM0.15/PDVB-P catalyst. Reaction conditions: CO2/H2/N2 = 10/30/60 or CO2/H2/N2/H2O = 9.6/28.8/57.6/4, 340 °C, 3 MPa, and 4000 mL/(gcat·h).
Figure 2. (a,b) Catalytic performance of KFMM0.15/PDVB catalysts with different stacking manners. Reaction conditions: CO2/H2/N2/H2O = 9.6/28.8/57.6/4, 340 °C, 3 MPa, and 4000 mL/(gcat·h). (c,d) Effect of water steam dosage on catalytic performance over the KFMM0.15/PDVB-P catalyst. (e) Stability test of KFMM0.15/PDVB-P catalyst. Reaction conditions: CO2/H2/N2 = 10/30/60 or CO2/H2/N2/H2O = 9.6/28.8/57.6/4, 340 °C, 3 MPa, and 4000 mL/(gcat·h).
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Figure 3. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of series KFMMx catalysts.
Figure 3. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of series KFMMx catalysts.
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Catalysts 14 00204 g004
Figure 4. SEM images of (a) fresh KFMM0, (b) fresh KFMM0.15, and (c) spent KFMM0.15 catalysts.
Figure 4. SEM images of (a) fresh KFMM0, (b) fresh KFMM0.15, and (c) spent KFMM0.15 catalysts.
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Catalysts 14 00204 g005
Figure 5. XRD patterns of (a) fresh and (b) spent KFMMx catalysts, (c) fresh. and (d) spent KFMM0.15/PDVB catalysts.
Figure 5. XRD patterns of (a) fresh and (b) spent KFMMx catalysts, (c) fresh. and (d) spent KFMM0.15/PDVB catalysts.
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Catalysts 14 00204 g006
Figure 6. The contact angle of (a) fresh and (d) spent KFMM0.15 catalysts, (b) fresh and (e) spent KFMM0.15/PDVB−P catalysts, and (c) fresh and (f) spent KFMM0.15/PDVB−M catalysts. (g) FT-IR spectra of fresh and spent PDVB.
Figure 6. The contact angle of (a) fresh and (d) spent KFMM0.15 catalysts, (b) fresh and (e) spent KFMM0.15/PDVB−P catalysts, and (c) fresh and (f) spent KFMM0.15/PDVB−M catalysts. (g) FT-IR spectra of fresh and spent PDVB.
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Figure 7. XPS spectra of Fe2p of (a) fresh and (b) spent KFMMx catalysts, (c) the C1s of spent KFMMx catalysts, and (d) Fe2p of spent KFMM0.15 and KFMM0.15/PDVB-P catalysts under CO2/H2/H2O atmosphere.
Figure 7. XPS spectra of Fe2p of (a) fresh and (b) spent KFMMx catalysts, (c) the C1s of spent KFMMx catalysts, and (d) Fe2p of spent KFMM0.15 and KFMM0.15/PDVB-P catalysts under CO2/H2/H2O atmosphere.
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Catalysts 14 00204 g008
Figure 8. (a) H2-TPR and (b) CO2-TPD results of KFMM0 and KFMM0.15 catalysts.
Figure 8. (a) H2-TPR and (b) CO2-TPD results of KFMM0 and KFMM0.15 catalysts.
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Figure 9. In situ CO2 hydrogenation DRIFTS results of (a,b) KFMM0 and (c,d) KFMM0.15 catalysts. * represented adsorbed species.
Figure 9. In situ CO2 hydrogenation DRIFTS results of (a,b) KFMM0 and (c,d) KFMM0.15 catalysts. * represented adsorbed species.
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Table 1. Textual properties of KFMMx catalysts.
Table 1. Textual properties of KFMMx catalysts.
CatalystsElement Contents (%)BET Specific Surface Area
(m2/g)		Total Pore Volume (cm3/g)
FeMnMgK
KFMM073.922.5-3.647.60.25
KFMM0.0571.321.73.53.670.60.30
KFMM0.1068.621.36.83.368.70.35
KFMM0.1566.120.79.73.581.80.40
KFMM0.2064.620.012.13.3145.70.46
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Feng, L.; Guo, S.; Yu, Z.; Cheng, Y.; Ming, J.; Song, X.; Cao, Q.; Zhu, X.; Wang, G.; Xu, D.; et al. Developing Multifunctional Fe-Based Catalysts for the Direct Hydrogenation of CO2 in Power Plant Flue Gas to Light Olefins. Catalysts 2024, 14, 204. https://doi.org/10.3390/catal14030204

AMA Style

Feng L, Guo S, Yu Z, Cheng Y, Ming J, Song X, Cao Q, Zhu X, Wang G, Xu D, et al. Developing Multifunctional Fe-Based Catalysts for the Direct Hydrogenation of CO2 in Power Plant Flue Gas to Light Olefins. Catalysts. 2024; 14(3):204. https://doi.org/10.3390/catal14030204

Chicago/Turabian Style

Feng, Likui, Shuai Guo, Zhiyong Yu, Yijie Cheng, Julan Ming, Xiaoning Song, Qiuyang Cao, Xiaofeng Zhu, Guanghui Wang, Di Xu, and et al. 2024. "Developing Multifunctional Fe-Based Catalysts for the Direct Hydrogenation of CO2 in Power Plant Flue Gas to Light Olefins" Catalysts 14, no. 3: 204. https://doi.org/10.3390/catal14030204

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