Upcycling of Cr-Containing Sulfate Waste into Efficient FeCrO₃/Fe₂O₃ Catalysts for CO₂ Hydrogenation Reaction

Abstract

Upcycling Cr-containing sulfate waste into catalysts for CO₂ hydrogenation reaction benefits both pollution mitigation and economic sustainability. In this study, FeCrO₃/Fe₂O₃ catalysts were successfully prepared by a simple hydrothermal method using Cr-containing sodium sulfate (Cr-SS) as a Cr source for efficient conversion and stable treatment of Cr. The removal rate of Cr in Cr-SS can reach 99.9% at the optimized hydrothermal conditions. When the synthesized catalysts were activated and used for the CO₂ hydrogenation reaction, a 50% increase in CO₂ conversion was achieved compared with the catalyst prepared by impregnation with a comparable amount of Cr. According to the extraction and risk assessment code (RAC) of the Reference Office of the European Community Bureau (BCR), the synthesized FeCrO₃/Fe₂O₃ is risk-free. This work not only realizes the detoxification of the Cr-SS but transfers Cr into stable FeCrO₃ for application in a catalytic field, which provides a strategy for the harmless disposal and resource utilization of Cr-containing hazardous waste.

1. Introduction

Cr-containing sodium sulfate (Cr-SS) is a by-product in the production process of the chrome salt industry. Cr (VI) in Cr-SS has the characteristics of high mobility, high toxicity, and carcinogenesis [1,2]. If it is discharged into the environment without proper treatment, it will pose a serious threat to the ecological environment and human health. In fact, Cr has been designated as hazardous waste in the China National Hazardous Waste List (2016), making the safe treatment and resource utilization of Cr-SS a pressing issue within chromium-related industries.

Currently, treatment methods for Cr-containing waste are categorized into harmless [3] and extraction methods [4]. Commonly employed techniques include wet reduction [5], high-temperature roasting [6], bioremediation [7,8,9], immobilization/stabilization [10,11], and physico-chemical extraction methods [4,12,13,14]. Among these methods, wet reduction and physico-chemical extraction are commonly used disposal measures. Wet reduction involves the use of acids or bases to transfer Cr (VI) from the solid phase to the liquid phase and then chemically reduce it to Cr (III), which is less toxic. Although this method is simple, it usually produces acid–base waste liquid and consumes a large amount of reducing agent [15]. The physico-chemical extraction method achieves sufficient extraction of Cr by controlling the phase transition of solid particles [16]. Although this method realizes the effective extraction of chromium, it has not been fully explored in the utilization of resources. Considering that Cr is a strategic metal resource, the annual waste of Cr resources due to inefficient practices is enormous [17,18]. Therefore, it is urgent to develop an efficient, stable, and economical method to recover chromium from Cr-containing sodium sulfate.

We propose the idea of detoxifying Cr from Cr-containing sodium sulfate (Cr-SS) and selectively converting it into functional materials to achieve a balance between environmental and economic considerations. Since Cr exists mainly in the form of Fe-Cr crystals in the natural environment [19], the conversion of Cr into stable Fe-Cr crystals can significantly reduce its ecotoxicity due to oxidation. On the other hand, iron–chromium oxide (FeCrO₃) has a similar molecular formula to ilmenite, which display excellent chemical stability, thermal stability, magnetic properties, and specific catalytic capabilities [20,21,22]. Therefore, if the Cr in Cr-SS can be selectively converted into FeCrO₃ by direct reaction with iron, the simultaneous detoxification and stabilization of Cr can be effectively achieved, which provides a new strategy for the resource utilization of Cr-containing wastes. Some studies have mentioned the hydrothermal conversion of Cr(VI) to stable ferrite. Lan [23] synthesized FeCr₂O₄ by hydrothermal treatment of chromium-containing wastewater, while Xie [24] applied hydrothermal treatment to chromium sludge and synthesized FeCr₂O₄ for advanced persulfate oxidation. However, there has been no research on the conversion of chromium from Cr-containing waste to FeCrO₃ for high-value utilization.

In this study, Cr (VI) detoxification and the preparation of FeCrO₃/Fe₂O₃ catalytic material were realized by a hydrothermal process using Cr-SS as raw material. The obtained material is activated and used for a carbon dioxide hydrogenation reaction. The research objectives include the following: (1) investigating the effects of different hydrothermal treatment conditions on the fixation and stabilization of Cr. (2) The prepared FeCrO₃/Fe₂O₃, Cr/Fe₂O₃, and Fe₂O₃ were analyzed utilizing analytical characterization methods. (3) Elucidating the mechanism of Cr(VI) conversion to FeCrO₃. (4) Conducting CO₂ hydrogenation experiments on FeCrO₃/Fe₂O₃, Cr/Fe₂O₃, and Fe₂O₃ to investigate differences in catalytic performance. (5) Determining the speciation of Cr in different Fe-Cr materials using the European Community Bureau of Reference (BCR) method and assessing their environmental risks using the risk assessment code (RAC) method. This study provides a convenient method for the efficient detoxification of Cr-SS and fixation of Cr elements, and also offers a feasible strategy for the effective disposal and resource utilization of Cr-containing solid waste.

2. Experimental

2.1. Materials

The Cr-SS was obtained from Chongqing Minfeng Chemical Co., Ltd. (Chongqing, China). It is a by-product of the process used to prepare chromate and dichromate from chrome ore using the alkali method. The collected samples were dried at 80 °C, crushed, and sieved to ensure uniform compositions. After ICP determination, the total Cr content in Cr-SS is 1.03 mg/g. The chemical reagents used in this study included anhydrous sodium sulfite (Na₂SO₃, AR, 99%), ferric sulfate (Fe₂(SO₄)₃, AR, 99%), sodium sulfate (Na₂SO₄, AR, 99%), and urea (CO(NH₂)₂, AR, 99%) purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Deionized water was used in all the experiments. All chemicals were used as received and without further purification. Unless otherwise noted, all samples tested herein were screened through a 200 mesh (0.074 mm) sieve.

2.2. Experimental

2.2.1. Preparation of FeCrO₃/FeOOH and FeCrO₃/Fe₂O₃

Firstly, 1.0 g of Cr-SS was dissolved in 20 mL of deionized water, then Cr(VI) was utterly reduced to Cr(III) by adding Na₂SO₃. Simultaneously, Fe₂(SO₄)₃ and CO(NH₂)₂ were introduced to form a homogeneous solution, transferred into a 100 mL Teflon-lined stainless autoclave, and maintained at 180 °C for 12 h. After air-cooling to room temperature, the precipitate was filtered and washed with deionized water and anhydrous alcohol several times and then dried in an oven at 80 °C for 10 h. The dried product is denoted as FeCrO₃/FeOOH. Finally, the FeCrO₃/Fe₂O₃ was obtained by calcinating the FeCrO₃/FeOOH powder in air at 500 °C for 2 h with a 10 °C/min heating rate (the process of calcination at 500 °C for 2 h was regarded as high-temperature activation).

The solution was collected after hydrothermal reaction, and then dried to obtain the sodium salt after removing Cr (Cr-SS-AR). After ICP testing, the Cr content in Cr-SS-AR is 3.18 × 10⁻⁴ mg/g. The Cr removal rate of the method can be calculated to be 99.9% based on Formula (1).

Cr removal rate = (c(Cr-SS) − c(Cr-SS-AR))/c(Cr-SS) × 100%

(1)

2.2.2. Preparation of Fe₂O₃ and Cr/Fe₂O₃

To compare the catalytic performance of FeCrO₃/Fe₂O₃ material in CO₂ hydrogenation synthesis, a control sample was prepared using Na₂SO₄ instead of Cr-SS. The following steps were followed to obtain Fe₂O₃ and Cr/Fe₂O₃ materials.

Cr-SS was replaced with Na₂SO₄ under the same experimental conditions as FeCrO₃/Fe₂O₃. The final product obtained is Fe₂O₃.

An isovolumetric impregnation method was employed, using the previously obtained Fe₂O₃ as the impregnation cocatalyst. The impregnated cocatalyst was dried in an oven at 120 °C for 2 h, and 1.0 g was taken for the determination of water absorption. Based on the water absorption of Fe₂O₃, Na₂CrO₄ was used as the Cr source, and deionized water was used as the solvent to formulate the impregnation solution with the corresponding concentration, so as to ensure that the final Cr loading of the catalyst was 0.3%. A total of 1.0 g of dried sample was put into an eggplant-shaped bottle, adding the calculated amount of impregnation solution, impregnated at room temperature for 1 h, stirring every 10 min to ensure full impregnation, then vacuum drying at the end of impregnation for 12 h to obtain a red powder, and the obtained red powder was put into a muffle furnace, and the temperature was raised to 500 °C at a rate of 10 °C/min and kept for 2 h, to obtain the final catalyst product, named as Cr/Fe₂O₃.

2.3. Characterization

The total metal concentrations were analyzed using an inductively coupled plasma emission spectrometer (ICP-OES, AvioTM200, PerkinElmer, Waltham, MA, USA). The Cr(VI) concentrations were quantified at 540 nm using a UV–vis spectrophotometer (UV2600, Shimadzu, Kyoto, Japan) based on the diphenylcarbazide colorimetric method (GB7467 [25] and GB/T15555.4-1995 [26]). The mineral phase compositions of the samples were determined using an X-ray diffractometer (XRD, D8Advance, Bruker, Saarbrücken, Germany) in continuous scanning mode with Cu-Kα radiation (λ = 1.5418 Å, 40 kV, 40 mA). The samples’ microstructure, morphology, and elemental analysis were examined using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS, Sigma300, Zeiss, Oberkochen, UK). The microstructure of the samples was observed using a transmission electron microscope (TEM, FEI Talos F200X, Waltham, MA, USA) in STEM mode at 200 kV. The valence changes of heavy metals on the sample surface were analyzed using X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).

2.4. CO₂ Hydrogenation Test

Catalytic performance evaluation of the synthesized catalyst for CO₂/H₂ methanation synthesis was conducted in a fixed-bed reactor. A 0.3 g sample of fresh catalyst and a 2.0 g sample of quartz sand were evenly blended and loaded into the reaction tube. The reaction was conducted under constant H₂ flow rate conditions (100 mL/min). The temperature was increased at a rate of 2 °C/min until reaching 400 °C for a reduction period of 4 h. Subsequently, the temperature was lowered to 240 °C for evaluation under atmospheric pressure. The feed gas composition was V(H₂): V(CO₂) = 3:1 with a 100 mL/min flow rate. The Agilent (Santa Clara, CA, USA) GC-7820A gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID) was used to analyze the online feed gas and gas products. The TCD, equipped with a Porapak Q column and an MS 5A packed column, was used to analyze components such as CO₂, CO, CH₄, etc. The FID, equipped with an Rt-Q-Bond column, was used to analyze hydrocarbon products ranging from C₂ to C₇. CO₂ conversion ($X_{C{O}_2}$, %) and product selectivity (S, %) were calculated [27] as shown in Equations (2)–(5):

$$X_{C{O}_2}=\frac{A_{C{O}_{2}in}-A_{C{O}_{2}out}}{A_{C{O}_{2}in}}\times 100\%$$

(2)

$$S_{CO}=\frac{A_{CO out} }{A_{CO in}-A_{CO out }}\times 100\%$$

(3)

$$S_{C{H}_4}=\frac{A_{C{H}_{4}out} }{A_{C{H}_4 in}-A_{C{H}_{4}out}}\times 100\%$$

(4)

$$S_{C_2-C_7}=100\% -S_{C{H}_4}-S_{CO}$$

(5)

In the equations, $X_{C{O}_2}$ represents the conversion rate of CO₂. S_CO, $S_{C{H}_4}$, and $S_{C_2-C_7}$ represent the selectivity of each component. $A_{C{O}_2}$, A_CO, and $A_{C{H}_4}$ represent the molar concentration of each component, and the subscripts in and out represent the feed gas and the outlet gas, respectively.

2.5. Stability Test

Different forms of Cr in FeCrO₃/Fe₂O₃ and Cr/Fe₂O₃ were extracted by a modified BCR four-step extraction method [28]. A sample weighing 0.8 g was mixed with 32 mL of a 0.1 mol/L acetic acid solution. The mixture was shaken for 16 h and then centrifuged to obtain extract 1 (exchangeable fraction F1) and residue 1. In residue 1, 32 mL of a 0.5 mol/L hydroxylamine hydrochloride solution was added, followed by shaking for 16 h. The mixture was centrifuged to obtain extract 2 (reducible fraction F2) and residue 2. For residue 2, 8 mL of an 8.8 mol/L hydrogen peroxide solution was added, and the mixture was heated until the volume was reduced to 1 mL. Then, 25 mL of a 1 mol/L ammonium acetate solution was added, followed by shaking for 16 h. The mixture was centrifuged to obtain extract 3 (oxidizable fraction F3) and residue 3. The dried sample from residue 3 was placed in a digestion bottle and mixed with 8 mL of aquaregia. The mixture was allowed to digest at room temperature for 5 h and then diluted to obtain extract 4 (residual fraction F4).

Formula (6) can be used to calculate the proportion of metal instability in the total content of the material, so as to assess the environmental risk level after the material is discarded [29].

RAC = F1 fraction/Total amount of heavy metals

(6)

3. Results and Discussion

3.1. Detoxification and Targeted Transfer of Cr in Cr-SS

We determined the effect of conditions on Cr removal from Cr-SS by controlling the hydrothermal temperature, hydrothermal time, and the amounts of CO(NH₂)₂ and Fe₂(SO₄)₃; the detailed information can be found in the ).

As shown in Figure S1, when the mass ratio of Cr-SS, Fe₂(SO₄)₃, and CO(NH₂)₂ is 1 g:1 g:0.6 g, and the hydrothermal conditions are 180 °C and 12 h, the best Cr removal effect can be obtained. The XRD pattern of Cr-SS (Figure S2a) shows that the main phase is Na₂SO₄, and no crystalline compound of Cr was found. However, as shown in Figure S2b, Cr-SS is a yellow granular substance. This indicates that in Cr-SS, the content of Cr is relatively low, or Cr exists as amorphous Na₂CrO₄. After processing, the XRD pattern of Cr-SS-AR (Figure S2c) shows that the main phases of Cr-SS are Na₂SO₄ and (NH₄)₂SO₄. The optical image of Cr-SS-AR (Figure S2d) shows the appearance of white particle crystals, indicating that Cr in Cr-SS has been removed. The Cr content in Cr-SS decreased from 41.2 mg/L to 1.27 × 10⁻² mg/L before and after hydrothermal treatment, and no Fe content was detected (Table S1). In conclusion, the removal rate of Cr is 99.9% when the method is used for detoxification of Cr-SS, which is well-matched to the similar literature (see Table S2 for details on what the literature compares) [23,24,30,31,32].

The cocatalysts were prepared using Cr-SS and Na₂SO₄ under the above optimum dechromiumization conditions and the XRD patterns and SEM images are shown in Figure 1. As shown in Figure 1, the characteristic peak of the material prepared by Na₂SO₄ corresponded to FeOOH, and the material was named FeOOH, and a new characteristic peak corresponding to FeCrO₃ appeared in the material prepared by Cr-SS, and the material was named FeCrO₃/FeOOH. By comparing the XRD spectra of the two materials, it was found that the peak corresponding to FeOOH in FeCrO₃/FeOOH was shifted to a lower angle. According to the Bragg equation (2dsinθ = nλ), doping with heteroatoms of larger atomic radii increases the lattice constant, which leads to a shift in the diffraction peaks of the phases [33], which can indicate that Cr has been successfully doped into FeOOH to form FeCrO₃. Surface morphology analyses were carried out on both FeOOH and FeCrO₃/FeOOH, as shown in Figure 1b, short rod-shaped crystals are observed on the surface of FeOOH, whereas the surface of FeCrO₃/FeOOH (Figure 1c) shows a regular hexahedral structure, and the size of the crystal size is also significantly increased. This suggests that Cr doping changes the surface morphology of the materials.

FeOOH and FeCrO₃/FeOOH were activated at high temperature, and the two materials were transformed into Fe₂O₃ and FeCrO₃/Fe₂O₃, respectively. Figure 1d shows the XRD patterns of Fe₂O₃ and FeCrO₃/Fe₂O₃. The inability to identify the characteristic peaks of FeCrO₃ alone may be due to the fact that the characteristic peaks of Fe₂O₃ are too strong, or due to the proximity of the characteristic peaks of FeCrO₃ and Fe₂O₃. However, in FeCrO₃/Fe₂O₃, the characteristic peaks corresponding to the (110) and (104) crystal faces of Fe₂O₃ are shifted to a lower angle, suggesting that the material was successfully doped with chromium. Compared with FeCrO₃/Fe₂O₃, the diffraction peaks of pure Fe₂O₃ are wider (as shown in Figure 1e,f), which may be attributed to the larger grain size of FeCrO₃/Fe₂O₃, resulting in the reduction in internal stresses inside the crystal, which leads to the narrowing of the diffraction peaks. In addition, the original morphological features of both Fe₂O₃ and FeCrO₃ remain intact after the high-temperature activation treatment, indicating no signs of sintering or melting. This result indicates that the materials have good thermal stability and are suitable for thermal catalyst synthesis.

XRD and SEM information for the Fe₂O₃ and Cr/Fe₂O₃ materials used for the comparison of catalytic properties can be found in the SI (Figures S4 and S5). Based on Figure S4, it is evident that the characteristic peaks of Fe₂O₃ completely align with the standard card, signifying that the prepared Fe₂O₃ material is a pure phase. The Cr/Fe₂O₃ obtained by the impregnation method did not show the characteristic peaks of the Cr phase, which may be caused by the low content of Cr and its uniform dispersion on the surface of the material. Nonetheless, the characteristic peaks corresponding to the Fe₂O₃ (104) and (110) crystal planes shift towards lower angles, indicating the successful loading of Cr onto Fe₂O₃. It can be seen from Figure S5a–c that the Fe₂O₃ material has good crystallinity and uniform element distribution. Figure S5d shows that the Cr/Fe₂O₃ surface does not appear as a regular hexahedron like the FeCrO₃/Fe₂O₃ (Figure 1f) material, which indicates that Cr only uniformly covers the short rod-like crystals on the surface of the material.

TEM analysis reveals the presence of hexahedral-shaped Fe-Cr crystals on the surface of the Fe matrix, as depicted in Figure 2a. The HPTEM images (Figure 2b) were Fourier transformed to determine two sets of lattice spacings of 0.25 nm and 0.27 nm, matching the (110) and (104) planes of FeCrO₃. From Figure 2c,e, it can be seen that the crystallinity of FeCrO₃/Fe₂O₃ is good. In addition, the EDX results (Figure 2g) indicate that the O, Fe, and Cr elements are uniformly distributed within the selected area of the nanocrystal.

The XPS spectrum of FeCrO₃/Fe₂O₃ is shown in Figure 3a. Three peaks at 531 eV, 577 eV, and 711 eV, corresponding to the binding energies of O 1s, Cr 2p, and Fe 2p, respectively, indicate the presence of O, Cr, and Fe elements. The O 1s XPS spectra shown in Figure 3b indicate that the peak at 530 eV may be attributed to lattice oxygen, while the peaks at 531.7 eV, 532.7 eV, and 533.7 eV are attributed to O-H, oxygen vacancies, and chemisorbed oxygen [34,35]. As shown in Figure 3c, the Cr 2p XPS peaks observed at 586 eV and 577 eV correspond to the Cr 2p_1/2 and Cr 2p_3/2 peaks [36]. The peaks at 576.5 eV and 578.1 eV are attributed to Cr³⁺, which suggests that the valence state exists predominantly in the form of Cr³⁺ and that a small amount of Cr⁶⁺ may be the key to the better catalytic performance of the material in later stages. As shown in Figure 3d, the Fe 2p XPS peaks observed at 725.1 eV and 711.6 eV correspond to the Fe 2p_1/2 and Fe 2p_3/2 peaks [37,38,39], respectively. In conclusion, during the crystallization process, Fe and Cr are mainly present as Fe³⁺ and Cr³⁺, which is consistent with the elemental valence state of FeCrO₃, and further proves that the combination of Cr and Fe forms stable FeCrO₃ crystals instead of FeCr₂O₄ crystals [40]. The XPS spectra of the Fe₂O₃ and Cr/Fe₂O₃ catalysts are detailed in SI (Figures S6 and S7). In Figure S6b, it can be determined that Fe₂O₃ contains lattice oxygen, O-H, oxygen vacancies, and chemisorbed oxygen, and from Figure S6c, it can be determined that Fe₂O₃ contains only Fe³⁺. As shown in Figure S7, Cr/Fe₂O₃ contains lattice oxygen, O-H, and oxygen vacancies, and no Cr⁶⁺ peak is detected in this material, which may be an important reason for inhibiting its catalytic performance.

3.2. Formation Mechanism of FeCrO₃

The chemical formula for iron titanium-type crystals is FeMO₃ (M refers to metal elements), representing the substitution of a Fe atom in Fe₂O₃ with a metal atom M that possesses properties similar to Fe [41]. The proportion of metal cations in the crystals is influenced by factors such as the metal atoms’ radius, valence state, and gap size [42]. Consequently, substitution phenomena can occur among metal atoms with comparable properties under specific physical and chemical conditions. Common ilmenite-type crystals include FeTiO₃, FeCrO₃, FeGeO₃, etc. [41,43,44]. Due to their similar atomic numbers (Fe-26 and Cr-24), Fe and Cr have analogous ionic radii and properties. Therefore, stable FeCrO₃ crystals can form when appropriate molar ratios and hydrothermal conditions are present. Using a proper amount of ferric sulfate as a cocatalyst powder not only accelerates the formation of the ferrite matrix but also enhances the presence of free Cr³⁺, promotes its binding with FeOOH sites, and further accelerates the formation of FeCrO₃.

In Figure 4, the detoxification mechanism of Cr(VI) from Cr-SS and its conversion into FeCrO₃ is illustrated. The Cr-SS solution contains a high concentration of free Cr⁶⁺. Sodium sulfite (Na₂SO₃) provides SO₃^2- during the reaction process, reducing free Cr⁶⁺ in chromic acid to Cr³⁺ (see reaction Formula (7)). During the hydrothermal stage, CO(NH₂)₂ is gradually hydrolyzed to provide an alkaline environment for the reaction (see reaction Formula (8)). Fe₂(SO₄)₃, utilized as a cocatalyst powder, forms Fe(OH)₃ under alkaline conditions (see reaction Formula (9)). As the hydrothermal reaction progresses, Fe(OH)₃ begins to transform into FeOOH (see reaction Formula (10)). As shown in Figure S8, an iron matrix composed of Fe(OH)₃ is initially formed, and FeOOH grows on its surface as the hydrothermal reaction continues. In an alkaline environment, free Cr³⁺ first forms Cr(OH)₃ (see reaction Formula (11)). When Fe ions are abundant, under alkaline hydrothermal conditions, FeOOH and Cr(OH)₃ are converted to the target product FeCrO₃ at 180 °C (see reaction Formula (12)).

The chemical reaction formulas are described as follows:

Cr₂O₇²⁻ + 3SO₃²⁻ + 8H⁺ → 3SO₄²⁻ + 2Cr³⁺ + 4H₂O

(7)

CO(NH₂)₂ + 3H₂O → CO₂ + 2NH₄⁺ + 2OH⁻

(8)

Fe³⁺ + 3NH₄⁺ + 3OH⁻ → Fe(OH)₃ + 3NH₄⁺

(9)

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}_3 \stackrel{180 ℃ \mathrm{hydrothermal}}{\leftrightarrow } \mathrm{FeOOH}+{\mathrm{H}}_2\mathrm{O}$$

(10)

Cr³⁺ + 3NH₄⁺ + 3OH⁻ → Cr(OH)₃ + 3NH₄⁺

(11)

$$\mathrm{FeOOH}+\mathrm{Cr}{\left(\mathrm{OH}\right)}_3\stackrel{180 ℃ \mathrm{hydrothermal}}{\to }{\mathrm{FeCrO}}_3+2{\mathrm{H}}_2\mathrm{O}$$

(12)

3.3. CO₂ Hydrogenation Reaction Performance

As shown in Figure 5a, FeCrO₃/Fe₂O₃ exhibits higher CO₂ conversion compared to Fe₂O₃ and Cr/Fe₂O₃. With the increase in reaction time, the CO₂ conversion of FeCrO₃/Fe₂O₃ did not change significantly, indicating its good stability. The CO₂ conversion ability of Cr/Fe₂O₃ decreased in the first 2 h, then increased slowly, and reached the maximum at 8 h. This may be since Cr loaded by the impregnation method only deposits on the surface of the material, which inhibits the conversion of Fe₂O₃ to Fe-carbide and, thus, reduces the CO₂ conversion. However, with the increase in reaction time, some chromium will be detached from the Fe₂O₃ surface, which may expose more iron sites, thus, improving the CO₂ conversion.

As can be seen in Figure 5b, FeCrO₃/Fe₂O₃ maintains a high CH₄ selectivity throughout the reaction period, further highlighting its excellent stability. The results also show that Cr/Fe₂O₃ lacks selectivity for CH₄, while Fe₂O₃ has more than 40% selectivity for CH₄. This suggests that a single Cr species loaded on the surface of Fe₂O₃ has low selectivity for CH₄ and even inhibits the original catalytic selectivity of the cocatalyst. As shown in Figure S8, the selectivity of FeCrO₃/Fe₂O₃ for the C₂–C₇ product is twice that of Fe₂O₃. At the same time, the product selectivity of FeCrO₃/Fe₂O₃ did not change significantly with the increase in reaction time, indicating that the material had good catalytic stability. The poor CO₂ conversion and product selectivity of Cr/Fe₂O₃ may be due to the simple formation of CrOx particles rather than the formation of catalytically active chromium species. FeCrO₃/Fe₂O₃ not only has a larger contact area but also prevents the cocatalyst from sintering by forming hexahedral crystals, which is conducive to the catalytic reaction. Meanwhile, we compared the performance of the catalysts prepared in this paper with that of the catalysts in the literature (see Table S3 for specific data) [27,45,46,47,48]. The results show that the low-temperature CO₂ hydrogenation catalyst prepared by Cr-SS had better CH₄ selectivity and certain CO₂ conversion ability compared with the catalyst prepared by pure reagents.

3.4. Stability Evaluation

In order to better understand the mobility, stability, and toxicity of Cr in FeCrO₃/Fe₂O₃ and Cr/Fe₂O₃ materials, an improved BCR method was used to determine the speciation of heavy metals in the samples, and toxicity risk assessment was conducted using the RAC method [49]. The content of different forms of Cr in FeCrO₃/Fe₂O₃ and Cr/Fe₂O₃ and the RAC value of the material are detailed in SI (Table S4).

As shown in Figure 6, the values of F1 and F2 in FeCrO₃/Fe₂O₃ are significantly lower than those in Cr/Fe₂O₃. The exchangeable metal form in the F1 part is usually considered the most easily transferable metal. Compared with FeCrO₃/Fe₂O₃, Cr/Fe₂O₃ contains significantly higher Cr content in the exchangeable state, indicating a higher potential environmental risk. At the same time, it can be seen from the figure that the proportion of easily transitive forms (F1 and F2) of Cr in FeCrO₃/Fe₂O₃ is relatively low, indicating that the directional conversion of Cr into FeCrO₃ through hydrothermal pathways can better fix Cr, increase the proportion of stable state (F3 and F4), and effectively reduce the environmental risk level of the material [50]. At the same time, as shown in Table S5, the Cr⁶⁺ content of FeCrO₃/Fe₂O₃ as an active site is very low, and the material exists in the form of iron–chromium crystals, indicating that it is not easy to be oxidized and the risk of poisoning is very low. Additionally, the RAC value of FeCrO₃/Fe₂O₃ is below 1%, indicating it is a non-risk substance. In contrast, the RAC value of Cr/Fe₂O₃ is between 1–10%, and when the material is discarded and enters the natural environment, improper treatment can easily cause secondary environmental pollution.

4. Conclusions

In conclusion, this study introduces the direct conversion of Cr into FeCrO₃/Fe₂O₃ through a straightforward hydrothermal reaction utilizing Cr-containing sodium sulfate (Cr-SS) as the raw material, followed by its application in low-temperature CO₂ hydrogenation reactions. The incorporation of Cr prompts the transition of short rod-like crystals into hexahedral crystals, significantly increasing the crystal particle size. Moreover, the hydrothermal synthesis of FeCrO₃ preserves the original O-H, oxygen vacancies, and adsorbed oxygen content of Fe₂O₃, ensuring the maintenance of its inherent catalytic performance. When employed in catalytic reactions, FeCrO₃ achieves a CO₂ conversion rate of 12.4% and a CH₄ selectivity of 45.9%, surpassing the performance of Fe₂O₃ and Cr/Fe₂O₃ used as controls in the study. Furthermore, the proposed mechanism and corresponding equations elucidate the directional conversion of Fe³⁺ and Cr³⁺ into FeCrO₃ during the hydrothermal process. Stability assessments conducted using the BCR and RAC methods confirm the risk-free nature of the FeCrO₃ materials. This study not only accomplishes the removal and stabilization of chromium in Cr-SS but also yields a catalyst suitable for low-temperature CO₂ hydrogenation reactions, presenting a valuable solution for the detoxification and recycling of chromium-containing wastes.