Sodium Promoted FeZn@SiO₂-C Catalysts for Sustainable Production of Low Olefins by CO₂ Hydrogenation

Abstract

A prepared FeZnNa@SiO₂-C catalyst with graphitized carbon (C)-modified mesoporous SiO₂ supports metal nanoparticles with the sol–gel method. The effect of adding metal Na and Zn promoters as a dispersion on the CO₂ hydrogenation to low olefins was systematically studied. The results showed that Zn–Na, as a combination, could promote the absorption of CO₂ and improved the conversion rate of CO₂. Na as an alkaline substance can improve the absorption of more acidic CO₂, which could increase the conversion rate of CO₂ to 59.03%. Meanwhile, the addition of secondary metal Zn to Fe-based catalysts to form a surface alloy could alter the adsorption of CO₂ and the activation of C-O bonds, inhibit the subsequent hydrogenation of olefins to paraffins, and facilitate the reduction of Fe₂O₃ and the formation of active Fe₅C₂ species. The formation of active Fe₅C₂ species was found in TEM and XRD, and the selectivity of the target product was 41.07%. The deep hydrogenation of olefins was inhibited, and the space–time yield (STY) of low olefins was raised again by inhibiting their deep hydrogenations, up to 0.0436. However, the corresponding STY did not increase infinitely with the increase of Na doping, and higher catalytic performance for CO₂ hydrogenation could be exhibited when the Na doping reached 6.4%. Compared with Fe@SiO₂-C catalyst, Na- and Zn-promoted Fe-based catalysts, prepared by the modified sol-gel method, can be used directly for highly efficient CO₂ hydrogenation to low olefins and thus has a more promising application prospect in the future.

1. Introduction

Increasing carbon dioxide emissions (CO₂) have a significant impact on climate change. CO₂ capture and utilization has received much attention as a viable pathway towards a low carbon economy [1,2,3,4]. CO₂ hydrogenation is a promising route to convert CO₂ into value-added C₂₊ hydrocarbons, such as olefins, aromatics, and gasoline, and has attracted industry and academic research worldwide [4,5,6,7,8,9,10]. Among them, the process of CO₂ hydrogenation to olefins should be divided into two steps. Firstly, CO intermediates are formed by the Reverse Water Gas Shift (RWGS) reaction and then hydrogenated to hydrocarbons by the Fischer–Tropsch Synthesis (FTS) reaction [11].

Compared to other catalysts [12,13,14], iron-based catalysts remain a promising system for the hydrogenation of CO₂ to low olefins due to their high reverse water–gas shift activity, flexible operating conditions and low cost [15]. However, the competitive carburization of a single iron phase in the catalyst and the evolvement into active phase Fe₅C₂ are not efficient. Therefore, the addition of other promoters could improve the structural evolution of the iron phase to increase the selectivity of olefin and long-chain hydrocarbons, and the study of the effect of promoters on the dynamic transformation of the iron phase is important for the development of efficient catalysts.

To date, alkaline species (AMs, M = Na [16,17,18,19] and K [20,21,22]) and transition metals (TMs, M = Ni, Co, Cr and Mn) [23,24,25] have been commonly introduced into Fe-based catalysts to compete carburizing and the evolution into active phase Fe₅C₂ for improving the selectivity of olefins. In particular, the addition of Na promotes the ability of Fe₅C₂ formation as the interaction between Na and Fe species inhibited the hydrogenation of olefins by increasing the electron density of the active sites, leading to a higher CO₂ conversion rate and improved olefin selectivity [26,27]. Yao [18] and Zhang [11] reconfirmed that the low olefins/paraffins (O/P) and CO₂ conversion rate were significantly increased by the addition of the adjuvant Na doping. More recently, co-modification with TMs and AMs is gaining high popularity as an emerging strategy to accelerate CO₂ dissociative adsorption and improve iron carbide phase formation for favor olefin synthesis [28]. However, the effects of secondary metal and Na promoters on catalyst structures and performances of CO₂ hydrogenation remain ambiguous. For instance, the Zn promoter is usually considered as the structure promoter, but it has also been proposed as an electronic promoter or the active phase in the form of ZnO for RWGS. Furthermore, the studies on the synergistic effects of Zn and Na promoters were seldom in the previous literature.

Combined with our previous work [29,30], GC-modified mesoporous silica-encapsulated metal nanoparticles can effectively improve reducibility and electron conductivity properties similar to those of expensive noble metals (Pt [31] and Ru [32]). In this study, FeZnNa@SiO₂-C catalysts were prepared by the sol–gel method via regulating the amount of Na loading. The role of these Zn and Na promoters was rigorously analyzed by decoupling the rates of the RWGS and FTS steps and considering the proximity to the reaction equilibrium. In addition, a combination of quasi in situ structural characterization measurements, including X-ray photoelectron spectroscopy and Transmission Electron Microscope (TEM), etc., were used to establish a clear structure–property relationship for CO₂ hydrogenation on Fe-based catalysts. During this process, synergetic promoting effects by Zn and Na are elucidated, which were responsible for the concurrently enhanced CO₂ conversion rates and olefin selectivity. These results would contribute to the rational regulation of catalytic performance by multiple additives, especially for complex reaction systems that are generally encountered in heterogeneous catalysis.

2. Results and Discussion

2.1. Characterization of the Fe-Based Catalysts

2.1.1. Textural Property, Structure and Morphology

As shown in Figure 1a,b, the calcined SiO₂-C, Zn@SiO₂-C, Na@SiO₂-C, Fe@SiO₂-C and FeNa@SiO₂-C catalysts exhibit the characteristic diffraction patterns of the Fe₂O₃ phase (JCPDS No. 39-1346) and SiO₂ phase (JCPDS No. 39-1425), while the Zn@SiO₂-C and FeZnNa_x@SiO₂-C catalysts provide diffraction peaks mainly assigned to ZnFe₂O₄ spinel phases (JCPDS No. 22-1012), in addition to the weak signals for the ZnO (JCPDS No. 36-1451) phase. The specific results are shown in Figure 1a,b and

Table 1. Physicochemical characteristics of the Fe@SiO₂-C, FeNa@SiO₂-C and FeZnNa_x@SiO₂-C catalysts.

Catalyst	Fe Loading (wt%) a	Na Loading (wt%) a	Total CO Chemisorbed (μmol/g) b	% Fe Dispersion b	Fe2O3 Size (nm) c	SSA (m2/g) d	PV (cm3/g) d	APS (nm) d
Fe@SiO2-C	25.4	0	472	5.2	23	75.5	0.087	3.19
FeNa@SiO2-C	24.2	1.5	570	6.6	33	78.4	0.111	4.18
FeZn@SiO2-C	25.3	0	551	6.1	15	48.5	0.144	4.19
FeZnNa1.5@SiO2-C	23.7	1.5	576	6.8	21	9.2	0.035	4.46
FeZnNa3.1@SiO2-C	23.5	3.1	596	7.1	23	16.4	0.035	2.95
FeZnNa6.4@SiO2-C	22.1	6.4	576	7.3	16	20.5	0.047	2.95
FeZnNa7.6@SiO2-C	23.4	7.6	568	6.8	18	14.6	0.035	2.95

^a Measured by XRF; ^b Based on total CO chemisorbed (corrected), $n_{CO}/n_{Fe}=0.5$, % dispersion = $2\times n_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{C}\mathrm{O} \mathrm{c}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}}/n_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{F}\mathrm{e} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}}$; ^c Measured with the Scherrer equation by XRD; ^d Measured by N₂ physical adsorption.

. The particle size of Fe₂O₃ in FeZn@SiO₂-C catalysts, evaluated by the Scherrer equation ($D=K\times \gamma /(B\times \cos \theta )$formula [33], means that K, B, θ, γ are the Scherrer constant, the half peak width or integral width of the measured sample diffraction peak, and the Prague angle, which was ~15 nm, respectively, much smaller than those of the Fe@SiO₂-C (~23 nm), FeNa@SiO₂-C (~33 nm) and FeZnNax@SiO₂-C (~20 nm). These results indicate that the Zn promoter significantly decreases the size of Fe species over Fe catalysts [11]. However, the addition of Na changed the particle size of the phase in the FeZnNa_x@SiO₂-C catalysts. The particle size of Fe₂O₃ generally showed a trend of first increasing and then decreasing with the increase in Na doping. Xiong [34] reported that the introduction of alkali metals promoted the agglomeration of iron particles, thereby increasing the size of iron oxide particles. The presence of alkali metals appears to promote the agglomeration of the iron precursor, thus enlarging the crystallite size of the iron oxide. In conjunction with Figure 1b, no Na₂O (JCPDS No. 03-1074) phase was found in FeNa_x@SiO₂-C and FeZnNa_x@SiO₂-C samples, which is mainly due to the low Na doping [11]. Moreover, it is also possible that there are no XRD patterns associated with the Na₂O/Na phase, which suggests a high dispersion of Na₂O/Na in these catalysts [18]. Figure 1b shows that the diffraction peaks of ZnO (36.25° and 34.42°) are more obvious when the Na doping in FeNa_x@SiO₂-C and FeZnNa_x@SiO₂-C samples reached 6.4%. It can be found that the specific surface area (SSA) of FeZnNa_x@SiO₂-C catalysts increased from 9.261 m²/g to 20.572 m²/g as the particle size of Fe₂O₃ decreases. Combined with the larger specific surface area of the remaining catalysts and the final actual CO₂ conversion rate, it can be found that although the remaining catalysts have a larger specific surface area, the actual CO₂ conversion rate is lower than that of any of FeZnNa_x@SiO₂-C catalysts, which shows that Zn and Na as promoters can overcome the disadvantage of its own smaller specific surface area and obtain a more practical advantage of higher CO₂ conversion rate. If we only consider alkaline Na as a promoter of the absorption of acidic CO₂, it can be found that there is still a clear difference between their CO₂ conversion rate by comparing FeZnNa_x@SiO₂-C catalysts. Therefore, the increase in CO₂ conversion rate was still attributed to the bifunctional effect of Zn and Na.

Figure 2 shows the TEM images and high-resolution TEM images of FeZnNa_x@SiO₂-C; it can be determined that the FeZnNa_x@SiO₂-C series catalysts have a mesoporous structure with a large number of particles distributed. The nanoparticles are not easy to produce carbon deposits with critical particle size. The small particle size is attributed to the fact that P123 is rich in hydroxyl groups (-OH), which are able to bind tightly with TEOS molecules to form micelles. The C overlay produced by polymer carbonation inhibits the sintering of metal nanoparticles and metal, and support interaction is enhanced during high-temperature treatment. The well-dispersed Fe₂O₃ nanoparticles, with an average diameter of 15–21 nm on Fe₂O₃, can be clearly observed, as shown in Figure 2. The lattice spacing of the Fe₂O₃ particle on SiO₂ is 0.26 nm, which can be attributed to the (400) planes, suggesting that the catalyst exists in the form of Fe₂O₃ and retains good crystallinity, consistent with the XRD results. Figure 2f,h shows that there are two different oxides, Fe₂O₃ and ZnO. These metal particles have a very small particle size and are dispersed on mesoporous SiO₂, including Fe₂O₃ and ZnO or Na₂O/Na. In the reduction step, these metal oxides are reduced to Fe₂O₃/ZnO at a temperature of 673 K. In the catalytic process, more active Fe₅C₂ particles are generated to improve the selectivity of the target product.

Apparently, in Figure 3, the FeZnNa_x@SiO₂-C nanoparticles give a type-IV isotherm with a long hysteresis loop at relative pressure (P/P₀) of 0.4–1.0, which is characteristic of mesoporous materials according to the classification of International Union of Pure and Applied Chemistry (IUPAC). As shown in Table 1, the specific surface area generally shows a trend of first increasing and then decreasing with the increase in Na doping; FeZnNa₆.₄@SiO₂-C had the highest specific surface area compared with other catalysts at 20.572 m²/g. Liang [35] found that when the Na doping was 0.1%, 0.5% and 5%, respectively, the specific surface area also increased first and then decreased, which were 31, 45, 28 m²/g. Although the specific surface area of the other three catalysts was higher than that of FeZnNa₆.₄@SiO₂-C catalysts, the actual CO₂ conversion rate was lower than that of FeZnNa₆.₄@SiO₂-C catalysts, which indicated that ZnNa can significantly improve the CO₂ conversion rate. Herein, the change in conversion rate is also consistent with Liang’s study [35] that alkaline additives can promote the adsorption and conversion of CO₂.

The results are consistent with the related reports that an Na promoter will increase the CO₂ adsorption and adsorption strength. Therefore, the adsorption of CO₂ on the surface of the FeZnNa_x@SiO₂-C catalyst is stronger than other catalysts, and it is easier to activate CO₂, which is conducive to the subsequent conversion. The pore volume of FeZnNa₆.₄@SiO₂-C also reached the maximum value of 0.047 cm³/g. The average pore size reached a maximum at 1.5%, and the remaining three groups of catalysts did not differ much, and still achieved the maximum value of 2.956 nm at 6.4%, and the pore volume decreased from 0.047 cm³/g at 6.4%Na to 0.035 cm³/g at 7.6% Na. However, the change in pore volume was very small, which showed that the excessive increase in Na doping did not have much effect on the volume of the pore. However, the pores of the catalyst do got clogged by excess Na. When the Na doping were 1.5%, 3.1 and 6.4%, which was not excessive, the pores were not clogged. Therefore, the decrease in surface area is suggested to be due to crystallite size growth rather than pore clogging [34].

2.1.2. XPS Results

The valence states of components in catalysts were investigated by XPS, the FeZn@SiO₂-C catalyst delivered a Fe 2p3/2 peak at the binding energy of 711.23 eV and a satellite peak around 719.07 eV, which was slightly higher than the corresponding peak of the standard Fe₂O₃ sample, which confirmed the existence of the Fe species in a form of Fe³⁺ species. The Fe 2p3/2 binding energy of the FeZnNa1.5@SiO₂-C catalyst in Figure 4a was reduced by about ~0.18 eV compared to that of the FeZn@SiO₂-C catalyst, which indicates that the introduction of Na promoter reduced the Fe 2p3/2 binding energy of the FeZnNa₁.₅@SiO₂-C catalyst. The decrease in binding energy suggests the charge transfer from Na ions to the vacant d orbits of Fe species [11]. Among them, the existence of C-O bonds indicates that there may be close interactions between C-Fe₂O₃-SiO₂ in the form of C-O-Fe/Fe-C-O or C-O-Si (Figure 4b). The O/Si ratio is higher than that of 2.0 for SiO₂, indicating that it may be due to the enhanced interaction between C and Fe nanoparticles and SiO₂. The satellite peak is approximately 718.81 eV, it is consistent with standard Fe₂O₃ samples, which confirms the existence of the Fe species in a form of Fe³⁺ species [16]. The catalyst has part of ZnFe₂O₄, so there is Fe²⁺ inside (Figure 1), and in Figure 4a, the binding energy peak (B.E.P.) at 711.05, 714.82, 718.81 and 724.49 eV are associated with Fe²⁺ and Fe³⁺ species.

2.1.3. Reduction Behavior

H₂-TPR experiments (Figure 5) showed that with Na doping in the FeZnNa₁.₅@SiO₂-C, the peaks ascribed to H₂ consumption rose up to 550 °C, related to the reduction of the Fe₂O₃ to Fe₃O₄ and/or to FeO, and Fe₂O₃ to FeO and/or to Fe⁰ shifted to a lower temperature, which suggests that the Na promoter concentration favored the initial reduction of the iron species [20]. On the contrary, the reduction peaks displaced towards high temperature for the FeZnNa₁.₅@SiO₂-C, FeZnNa₃.₁@SiO₂-C, FeZnNa₆.₄@SiO₂-C and FeZnNa₇.₆@SiO₂-C catalysts as the Na concentration increased. This displacement could be attributed to the closer contact between the iron oxides and potassium, as discussed in the previous XPS section. As for the peaks located at the high-temperature region (>550 °C), which were related to the reduction of Fe³⁺ species to Fe⁰. The noticed shift to high temperature and the increase in their intensities in FeZnNa_x@SiO₂-C catalysts could be attributed to a stronger interaction between the iron oxides and the SiO₂-C support, as suggested by the XPS results. Combined with TEM image (Figure 2), it can be found that the variation trend of Fe₂O₃ particle sizes in FeZnNa_x@SiO₂-C is almost the same as that of the H₂ reduction temperature.

2.1.4. Chemisorption Behavior

The CO₂ adsorption properties of FeZnNa_x@SiO₂-C with different Na doping were further investigated by CO₂ temperature-programmed desorption (CO₂-TPD). As shown in Figure 6, three distinct peaks were observed for all the samples. The lower temperature peak around 130 °C (α peak) was ascribed to the desorption of CO₂ weakly adsorbed in the bulk phase. The peaks in the temperature range of 300–400 °C (β peak) and 500–700 °C (γ peak) were due to the decomposition of SiO₂ or the desorption of CO₂, interacting strongly with the surface basic sites [36], which shift gradually to higher temperatures with increasing Na content. The increased Na loading also enhances the amount of CO₂ uptake at strong basic sites (

Table 2. CO₂ uptake feature of the FeZnNa_x@SiO₂-C catalysts with different Na doping.

Catalysts	The Amount of CO2 Desorbed (μmol/g)
	α Peak	β Peak	γ Peak	Total
FeZnNa1.5@SiO2-C	58.5	60.4	396.4	515.3
FeZnNa3.1@SiO2-C	36.6	91.3	446.0	573.9
FeZnNa6.4@SiO2-C	51.4	128.6	576.7	756.6
FeZnNa7.6@SiO2-C	295.4	43.1	621.7	960.2

) with the increased Na doping as well. The amounts of desorbed CO₂ for β peak listed in Table 2 follow the sequence: FeZnNa₆.₄@SiO₂-C >  FeZnNa₃.₁@SiO₂-C >  FeZnNa₁.₅@SiO₂-C >  FeZnNa₇.₆@SiO₂-C, which was consistent with the variation pattern of CO₂ conversion in

Table 3. Comparison of CO₂ hydrogenation Catalytic Performances: STY, selectivity of CH₄, C₂-C₄ olefins (${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$), C₂-C₄ paraffins (${\mathrm{C}}_2^0-{\mathrm{C}}_4^0$) and C₅₊.

Catalyst	CO2 Con (%)	Selectivity a (%)				O/P b	TOF c (10−3 s−1)	STY (g/gcat·h)
		CH4	${\mathbf{C}}_2^{=}-{\mathbf{C}}_4^{=}$	${\mathbf{C}}_2^{=}-{\mathbf{C}}_4^{=}$	${\mathbf{C}}_5^{+}$
Fe@SiO2-C	52.17	83.06	5.93	5.59	5.45	1.06	0.56	0.0057
FeZn@SiO2-C	53.78	64.32	1.54	30.87	3.29	0.05	0.12	0.0015
FeNa@SiO2-C	52.26	66.56	6.48	20.23	8.33	0.32	0.53	0.0090
FeZnNa1.5@SiO2-C	58.41	22.89	29.15	14.50	33.42	2.01	2.53	0.0315
FeZnNa3.1@SiO2-C	59.03	33.00	29.40	13.67	23.83	2.15	2.49	0.0321
FeZnNa6.4@SiO2-C	58.32	18.30	40.40	9.60	31.69	4.21	3.50	0.0436
FeZnNa7.6@SiO2-C	58.4	18.63	41.07	8.68	31.72	4.73	3.61	0.0400

Reaction condition: T = 300 °C, P = 2.5 MPa, H₂/CO₂ = 3, GHSV = 1500 mL/g·h_, TOS = 100 h; Reduced in 5% H₂/Ar for 16 h at 400 °C before CO₂ hydrogenation. ^a Hydrocarbon selectivity was normalized with the exception of CO. ^b O/P is the molar ratio of olefin (${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$) to paraffin (${\mathrm{C}}_2^0-{\mathrm{C}}_4^0$). ^c TOF: turnover frequencies for CO₂ conversion, i.e., the number of CO₂ molecules converted per surface catalytic site per second at present CO₂ hydrogenation.

. Considering that FeZnNa₁.₅@SiO₂-C has a weaker CO₂ adsorption in the early stage, the FeZnNa₆.₄@SiO₂-C catalyst has better CO₂ adsorption and support structure than other catalysts (Figure 2e), which may be related to the SSA enhancement and particle size reduction of the catalyst. The advantage of specific surface area also proved the adsorption of CO₂ was better, and the two characterizations could support each other.

2.2. Catalytic Performances for CO₂ Hydrogenation

Table 3 shows the catalytic performance of this series of catalysts for CO₂ hydrogenation reaction. The ultimate goal of this series of catalysts is to improve the selectivity of olefin, so this section needs to focus on STY (space–time yield) of ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$. The catalysts discussed were divided into seven types, each of which was an average of multiple samples. Among them, the maximum value of STY was 0.0436 when the Na doping was 6.4%, and the observation of FeZnNa₇.₆@SiO₂-C revealed that although the corresponding Na doping exceeded that of FeZnNa₆.₄@SiO₂-C, the STY was relatively low, which shows that this series of catalysts are the best-quality catalyst when Na doping is less than 7.6%. The results of four catalysts with different Na doping (1.5–7.6%) were analyzed, and it was found that a further increase in Na doping did not improve the conversion rate of CO₂ any further, but rather suppressed the methane selectivity, improved the selectivity of C₂₊, and significantly increased the value of low-carbon olefin STY. Compared with FeZnNa₁.₅@SiO₂-C, FeZnNa₃.₁@SiO₂-C and FeZnNa₇.₆@SiO₂-C, FeZnNa₆.₄@SiO₂-C exhibited higher CO₂ conversion, which probably had appropriate particle sizes and surface areas, providing more active sites and thereby increasing the CO₂ hydrogenation catalytic activity. Moreover, it is noteworthy in Table 3 that turnover frequencies of CO₂ conversion (TOF) for the FeZnNa_x@SiO₂-C catalyst (3.61 × 10⁻³ s⁻¹) is lower than that of FeSb (3.6 × 10⁻⁵ mol/g·s), and FeBi (3.2 × 10⁻⁵ mol/g·s) [37]. Although a high TOF value of 3.6 × 10⁻⁵ mol/g·s was reported for FeSb, it should be noted that this value was obtained by the addition of the promoter Sb. All of these indicate that the catalysts (addition of promoters such as Na, Sb and Bi) can obviously promote TOF and STY, but Na was the lowest cost in all the above promoters.

2.3. Effect of Different Na Doping on Final Catalytic Performance

Figure 7 shows the effects of different Na-doping levels on the selectivity, CO₂ conversion rate and STY of CO, CH₄, ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$, ${\mathrm{C}}_2^0-{\mathrm{C}}_4^0$ and C₅₊ in the same series of catalysts. The CO₂ conversion has increased from about 54% to about 59% with the addition of Na, but the subsequent increase in Na doping has not increased significantly. With the increase in Na doping in the catalyst, the selectivity of the target product ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$ raised from 1.54% to 41.07%. STY achieved the maximum value when Na doping was 6.4%, which showed that although Na increased the activity of the catalyst, it inhibited the activity of the catalyst when the Na doping was too high. From above catalytic characterization, it can be found that FeZnNa₆.₄@SiO₂-C had the most excellent performance with excellent mesoporous structure, highly dispersed metal nanoparticles, and the active component Fe₅C₂ (labeled as “●”, Figure 8a) particles were also more obvious, and the metal and support had a strong interaction, which shows the advantages of accelerating the electron transport, improving the hydrothermal stability and catalytic activity, and the results of the characterization are consistent with the convincing experimental results. Furthermore, the synergistic effect of Na and Zn promoters not only increased the CO₂ adsorption amount, thereby promoting the complete transformation of Fe species into Fe₅C₂, but also reduced the Fe₅C₂ size and exposed more active sites, which was conducive to the FTO to consume CO and accelerated the conversion of CO₂. Therefore, the FeZnNa_x@SiO₂-C catalyst gave a higher CO₂ hydrogenation rate and ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$ formation rate.

Figure 8b shows the TEM images of the spent sample; the metal particles had a slight enlargement, but did not congregate uncontrollably, which showed that graphitized carbon played a certain role, and when combined with the 100 h working condition diagram of Figure 9, it can be concluded that the catalyst has good structural stability.

XRD showed that iron-based catalysts containing Fe₂O₃ and ZnO components were successfully prepared. The approximate metal particle size (Fe₂O₃, ZnO) was calculated by the corresponding formula. The presence of Na and the proportion of components of each element were confirmed by XPS testing. From BET and TEM, it can be concluded that the FeZnNa_x@SiO₂-C catalysts were indeed mesoporous in structure. The pore size, pore volume, specific surface area and precise metal particle size were also obtained; the high-resolution TEM diagram displayed the carbon layer wrapped on the surface of the catalyst and the highly dispersed metal nanoparticles. From the H₂-TPR test, the increase in Na doping significantly changed the reduction temperature of Fe₂O₃ species. The lowest reduction temperatures were observed for FeZnNa₆.₄@SiO₂-C: 580 °C (Fe₂O₃→FeO), 741 °C (FeO→αFe). The initially prepared Fe₂O₃ phase was reduced to Fe₃O₄ at about 400 °C [11]. CO₂-TPD showed that more basic sites were provided for adsorption of more acidic CO₂ in the early stage due to the introduction of Na, which once again proved that the introduction of Na enhanced the CO₂ adsorption. In the later stage, support was decomposed with the increase in temperature. Compared with other catalysts, FeZnNa₆.₄@SiO₂-C has a more stable support structure and CO₂ adsorption. Combining the above analyses, it is found that the reduction temperature, specific surface area, pore volume, average pore size and CO₂ adsorption are closely related to the particle size of Fe₂O₃. The reason for the change in particle size is due to the addition of alkaline elements (Na), Fe particle promotion, and the increase in metal particle size [34]. When Na reached a certain amount, Na acted as a dispersant and inhibited the aggregation of Fe particles, thereby reducing the particle size of Fe₂O₃. But as the Na doping increased again, the amount that acted as a dispersant was broken, and then the pore was blocked. Fe particles with a slightly larger particle size do not have extra positions to enter, and therefore the agglomeration phenomenon is produced again, and the particle size of Fe particles increases again. Therefore, the phenomenon of particle size change in Fe₂O₃ was first increasing, then reducing, and finally increasing. The reduction temperature, specific surface area, pore volume and average pore size are all affected by the Fe₂O₃ particle size and change accordingly. The specific internal connection is referred to above.

3. Experimental

3.1. Catalyst Preparation

FeZnNa₁.₅@SiO₂-C was prepared by dissolving Fe(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O, NaNO₃ and P123 in a mixed solvent formed by ethanol and deionized water and vigorously stirring for two hours to form a homogeneous solution. The mass of P123, Fe(NO₃)₃·9H₂O, Zn(NO₃)₂·6H₂O and NaNO₃ were 2.5 g, 6.87 g, 4.36 g, 0.21 g, respectively. The volume ratio of ethanol deionized water in 40 mL mixed solvent was 1:1, and 4.46 mL of tetraethyl orthosilicate was added dropwise into the solution hydrolyzed in the previous step, and a homogeneous solution was formed by stirring for 1 h. After the end of the reaction, the solution was put into a reaction dish and placed in a fume hood for ventilation for 12–24 h, with the purpose of removing the surface ethanol and deionized water. The sample with the surface solvent removed were placed in an oven for heating and holding at 80 °C for 12 h. The sample was taken out and placed in a tubular furnace for calcination at 600 °C in air for 4 h at a rate of 2 °C/min to obtain the final catalyst material.

Two groups of catalysts were prepared according to the different Zn doping, namely Fe@SiO₂-C and FeZn@SiO₂-C; Fe accounted for 25% of the entire catalyst mass. Four groups of catalysts were prepared with Na doping as variables, with loading of 1.5%, 3.1%, 6.4% and 7.6%, respectively. Both Fe and Zn accounted each for 25% of the mass of the entire catalyst, named FeZnNa₁.₅@SiO₂-C FeZnNa₃.₁@SiO₂-C, FeZnNa₆.₄@SiO₂-C and FeZnNa₇.₆@SiO₂-C. Among them, the FeNa catalyst was additionally produced, named FeNa@SiO₂-C; Fe accounted for 25% of the mass of the entire catalyst, and Na accounted for 6.4% of the mass of the entire catalyst.

3.2. Nanocomposite Characterization

TEM images were obtained on a JEM-2010 electron microscope (JEOL (BEIJING) CO., Ltd., Beijing, China) with an acceleration voltage of 200 kV. XRD patterns were recorded on a Rigaku D/Max-2rB-II XRD meter (Cu Kα radiation, λ = 1.5406 Å, Rigaku, Tokyo, Japan), with 2θ = 10° to 90° scanned at 8°·min⁻¹. The texture properties of the nanocomposites were determined by N₂ adsorption/desorption isotherms on a Micromeritics ASAP 2010 apparatus (Micromeritics instrument (Shanghai) Ltd., Shanghai, China). The pore size distribution, pore volume and specific surface area were determined by the multipoint Brunauer–Emmett–Teller (BET) method. The concentration of surface metal atoms was determined using CO chemisorption in a Micromeritics ASAP 2010 (Micromeritics instrument (Shanghai) Ltd., Shanghai, China) automated system. A 0.1 g sample of a freshly calcined catalyst was first evacuated to 10⁻⁶ mm Hg at 100 °C for 30 min and then reduced under flowing H₂ by ramping at 2 °C/min to 280 °C and holding there for 12 h. The catalyst was then evacuated at 280 °C for 60 min to desorb H₂. CO chemisorption was carried out at 35 °C. X-ray photoelectron spectroscopy (XPS, Shimadzu (Shanghai) Experimental Equipment Co., Ltd., Shanghai, China) characterization was carried out in an UHV chamber equipped with a monochromatized Al Kα X-ray source and a Sphere 2 analyzer. All the binding energies were calibrated by the adventitious C1s line at 284.8 eV. Hydrogen temperature-programmed reduction (H₂-TPR, BSD Instrument Technology (Beijing) Co., Ltd., Beijing, China) experiments were conducted on homemade apparatus equipped with a thermal conductivity detector (CO₂-TCD, BSD Instrument Technology (Beijing) Co., Ltd., Beijing, China). The reactor was loaded with 50 mg of sample at a time and heated to 1073 K at 10 K/min with a gas of 10% H₂/Ar.

3.3. Catalytic Reaction

It was used as a catalyst for the hydrogenation reaction of carbon dioxide in order to evaluate its catalytic performance. Firstly, the catalyst was placed in a mesh screen with the help of a grinding rod to obtain an experimental catalyst of about 0.5 g (40–50 mesh). Prior to CO₂ hydrogenation, 300 mg of catalyst was diluted with 600 mg of quartz powder (60–80 mesh) and reduced on site in flowing 5 vol % H₂/Ar (gas hourly space velocity (GHSV) = 2500 mL/g·h) at 400 °C for 18 h with a heating rate of 2 °C·min⁻¹. Catalytic testing was performed under industrially relevant operation conditions of 300 °C, 2.5 MPa, and a H₂/CO₂/N₂ ratio of 23/69/8 by volume (GHSV = 1500 mL/g·h) in a tubular fixed-bed reactor with an inner diameter of 10 mm. N₂ was used as the internal standard during gas-phase product analysis. All tubing and connections were wrapped with heating tape and heated at 150 °C to avoid condensation. The effluent of the reactor was analyzed on-stream by using a gas chromatograph (GC, Agilent 7890B, Agilent Technologies, Inc., Santa Clara, California, United States) equipped with a thermal conductivity detector (TCD, GC-2014AT, SHIMADZU, a TDX-01 column) and a flame ionization detector (FID, Agilent, a capillary column of PONA type). H₂, N₂, CO, CH₄ and CO₂ were detected by the TCD, hydrocarbons (C_nH_m), and the FID. To reduce the uncertainty in the product analysis, we collected the liquid products during every 12 h for one sample. The calculated carbon balance for all catalytic performance tests were between 95% and 105%. All listed data were obtained at 100 h on-stream when achieving the steady state.

The conversion of ${\mathrm{C}\mathrm{O}}_2$ (${\mathrm{C}\mathrm{O}}_2 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}.$) and the selectivity of hydrocarbons (${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}} \mathrm{s}\mathrm{e}\mathrm{l}.$), CO ($\mathrm{C}\mathrm{O} \mathrm{s}\mathrm{e}\mathrm{l}.$) and ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$(${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=} \mathrm{s}\mathrm{e}\mathrm{l}.$) were calculated via the following equations:

$$\begin{gathered} {\mathrm{C}\mathrm{O}}_2 \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}.(\mathrm{\%})=\frac{{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}-{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}}\times 100 \\ \mathrm{C}\mathrm{O} \mathrm{s}\mathrm{e}\mathrm{l}.(\mathrm{\%})=\frac{{\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}-{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{o}\mathrm{u}\mathrm{t}}}\times 100 \\ {\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}} \mathrm{s}\mathrm{e}\mathrm{l}.(\mathrm{\%})=\frac{{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\times \mathrm{n}}{{\sum }_1^{\mathrm{n}}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\times \mathrm{n}}\times 100 \\ {\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=} \mathrm{s}\mathrm{e}\mathrm{l}.(\mathrm{\%})=\frac{{\sum }_2^4{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\times \mathrm{n}}{{\sum }_1^{\mathrm{n}}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\times \mathrm{n}}\times 100 \\ \mathrm{C}\mathrm{a}\mathrm{r}\mathrm{b}\mathrm{o}\mathrm{n} \mathrm{b}\mathrm{a}\mathrm{l}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}(\mathrm{\%})=\frac{{\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}}+{\sum }_1^{\mathrm{n}}{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}\times \mathrm{n}+{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}}\times 100 \end{gathered}$$

where ${{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}$ and ${{\mathrm{C}\mathrm{O}}_2}_{\mathrm{o}\mathrm{u}\mathrm{t}}$ expressed the mole of ${\mathrm{C}\mathrm{O}}_2$, the inlet and the outlet of the reactor, respectively. ${\mathrm{C}\mathrm{O}}_{\mathrm{o}\mathrm{u}\mathrm{t}}$ was the mole of CO in the outlet. ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{\mathrm{m}}$ was the mole of the hydrocarbon product with carbon number n.

$$\begin{gathered} {\mathrm{S}\mathrm{T}\mathrm{Y}}_{{\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}}={\sum }_2^4{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}}\times \mathrm{n}\times {\mathrm{M}}_{{\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}} \\ \mathrm{T}\mathrm{O}\mathrm{F}=\frac{{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{i}\mathrm{n}}-{{\mathrm{C}\mathrm{O}}_2}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\mathsf{\omega }}_{\mathrm{F}\mathrm{e}}\times {\mathrm{D}}_{\mathrm{F}\mathrm{e}}÷{\mathrm{M}}_{\mathrm{F}\mathrm{e}}} \end{gathered}$$

where ${\mathrm{S}\mathrm{T}\mathrm{Y}}_{{\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}}$ is the time–space yield of ${\mathrm{C}}_2^{=}-{\mathrm{C}}_4^{=}$ based on unit mass catalyst ($\mathrm{g}\cdot {\mathrm{g}}_{\mathrm{c}\mathrm{a}\mathrm{t}}^{-1}\cdot {\mathrm{h}}^{-1})$. TOF is the conversion frequency of catalyst (${\mathrm{h}}^{-1}$). ${\mathsf{\omega }}_{\mathrm{F}\mathrm{e}}$ is the mass fraction of Fe (%). ${\mathrm{M}}_{\mathrm{F}\mathrm{e}}$ is the relative atomic mass of Fe (56 g·mol⁻¹), and D_Fe is the dispersion of Fe (%), which was based on total CO₂ chemisorbed, CO₂/Fe_s = 0.5, ${\mathrm{n}}_{\mathrm{C}\mathrm{O}}/{\mathrm{n}}_{\mathrm{F}\mathrm{e}}=0.5$, D_Fe (%) = $2\times {\mathrm{n}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{C}\mathrm{O} \mathrm{c}\mathrm{h}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}}/{\mathrm{n}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r} \mathrm{o}\mathrm{f} \mathrm{F}\mathrm{e} \mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}}$.

4. Conclusions

The iron-based catalysts containing sodium and zinc additives have a stable mesoporous structure with graphitized carbon on the surface of the active component and are highly selective for the production of high-value olefins by CO₂ hydrogenation. In this paper, the CO₂ conversion of the FeZnNa catalyst was stable at about 59%, and the CH₄ selectivity was significantly inhibited compared with other catalysts, and its STY value was the highest 0.0436. The results showed that the presence of Na and Zn additives favored the carburizing of Fe species to generate smaller active Fe₅C₂ particles, improved the adsorption of carbon dioxide, and inhibited the deep hydrogenation of olefins, which is the key to the conversion of carbon dioxide into olefins. Catalysts with a sinter resistance and mesoporous resistance to collapse have more stable and long-lasting catalytic performance than other catalysts. The catalyst in this paper has sinter resistance and mesoporous collapse resistance, so it has a more stable structure than other catalysts. However, during the above reaction, the generated water inhibits the RWGS step, which reduces the conversion rate of CO₂ and hinders the formation of olefins in the FTO process, affecting the final catalytic performance. Therefore, the development of catalysts with hydrophobic surfaces to ensure the stability and durability of the catalytic performance is a difficult point for further development.