Selective reduction of carbon dioxide into amorphous carbon over activated natural magnetite

To some extent, CO₂ emission reduction can efficiently alleviate the global temperature rise and enable the use of nonrenewable fuels [1]. The approaches for reducing CO₂ emission mainly include reduction and capture. Moreover, CO₂ can be reduced to various chemical high-added-value fuels via electrocatalysis, photocatalysis, and thermal catalysis [2]. For the electrocatalytic or photocatalytic method, the reaction can only be carried out under a certain overpotential because CO₂ molecule is extremely stable. The products are generally a mixture of gas and liquid containing 1–4 carbons due to the product selectivity, leading to difficulty in separation and transport [3−4]. While CO₂ capture requires a medium (certain beneficial geographical conditions), it cannot be widely adopted, and government support is needed [2]. Therefore, studying CO₂ reduction by thermal catalysis with cheap raw materials is necessary.

Since the last decades, natural magnetite has attracted much attention because of its extensive use. It can be used for drug delivery [5], degradation of organic pollutants [6], decomposition of H₂O₂ [7], and decomposition of CO₂ [8−12]. In the early 1990s, Tamaura and Tahata [13] reported that CO₂ could be completely decomposed to carbon over the oxygen-deficient magnetite Fe₃O_4−δ (0 < δ < 1) at a low temperature near 300°C. This opened up a new study area of CO₂ decomposition as an expected route to address the “green-house effect” resulting from CO₂ emission. In a subsequent work, cation-deficient magnetite (Fe_3−δO₄, 0.2 > δ > 0) synthesized by the air oxidation of Fe(OH)₂ suspensions followed by H₂ reduction, oxygen-deficient magnetite, and wustite (Fe_1−xO) prepared by high-temperature thermal decomposition of iron oxalate (FeC₂O₄) were used for the reaction of CO₂ decomposition [14−15]. The study result showed that CO₂ could be decomposed to C by nearly 100% over these activated iron oxides at certain conditions. Álvarez-Torrellas et al. [16] enhanced the catalytic activity of natural magnetite by thermal treatments. In 2007, CO₂ was decomposed by mechanically milled wustite or ultrafine magnetite powders, and the decomposition products were confirmed to be graphite, amorphous carbon, and cementite (Fe₃C) [17−18], in which granularity and phase composition were regulated by tailoring the time and atmosphere of the high-power milling process. Compared with simple iron oxides such as magnetite or wustite, multi-metallic ferrites formed by isomorphism replacements are expected to produce more crystal defects and therefore provide more activated sites on their surfaces. Enhancing the ability and recycling stability of these iron oxides for CO₂ decomposition is advantageous. Therefore, multi-metallic ferrites with oxygen deficiency have attracted increasing attention. They are represented by the general formula M_xFe_3−xO_4−δ and can be fabricated via H₂ reduction for M_xFe_3−xO₄ compounds, where M is a transition metal, such as Cu, Co, Ni, Mn, Cr, and Zn, and δ is the reduction degree. For instance, in previous studies, a series of ferrites MFe₂O₄ (M = Mn, Zn, Co, Ni, Cr) was prepared and used in the decomposition of CO₂ to C. The result demonstrated that Ni_0.49Cu_0.24Zn_0.24Fe₂O₄ possessed a higher activity and stability compared with binary compounds (Ni, Cu, Zn)Fe₂O₄ [8−12]. In addition, Kim et al. [19−20] prepared ultrafine (Ni,Zn)Fe₂O₄ powders using co-precipitation and hydrothermal methods and found that the activity of the powder prepared by the hydrothermal method was higher than that prepared by co-precipitation, which agrees with the results reported by other researchers [21–26]. The aforementioned results show that the activity and recycling stability can be significantly improved by mono- or multi-element doping of the ferrite. Among the dopants, Zn²⁺, Ni²⁺, and Cr³⁺ are preferred candidates. Certainly, these oxygen-deficient ferrites can be synthesized using different chemical routes. Only considering our country the substantive emissions of CO₂ are more than 2 × 10¹⁰ t before in 2000 [27–28], conventional preparation methods cannot meet the tremendous demands of the catalysts used for CO₂ decomposition, and they also bring new pollutants originated from their preparation processes.

Natural magnetite possesses a counter spinel structure, in which half of Fe³⁺ and all Fe²⁺ are located at the octahedral lattices, with the other half of Fe³⁺ at the tetrahedral positions. They are formed by the isomorphism substitutions of Al³⁺, Ti⁴⁺, Cr³⁺, and V³⁺ for Fe³⁺ and also the substitutions of Mg²⁺, Mn²⁺, Zn²⁺, Ni²⁺, Co²⁺, and Cu²⁺ for Fe²⁺. Natural magnetite is greatly expected to serve as a catalyst for CO₂ decomposition considering its special structure that is similar to the ferrites synthesized artificially via a series of complicated chemical processes. However, CO₂ decomposition over natural magnetite has not been sufficiently researched so far. Hence, in this study, natural magnetite was mechanically milled and reduced in H₂ atmosphere for CO₂ decomposition. The study aims at presenting a facile and “green” approach to fabricate multi-metallic ferrites for the reduction of CO₂ into carbon with high selectivity.

Natural magnetite concentrates served as the raw material, with the main chemical compositions (wt%) of Fe 51.56, TiO₂ 12.73, V₂O₅ 0.564, Co 0.02, Ni 0.013, S 0.53, P 0.004, SiO₂ 4.69, Al₂O₃ 4.69, CaO 1.57, and MgO 3.91. Prior to mechanical milling, the concentrates were dried at 120°C for 12 h. The mechanical milling was performed under air with a ball-to-sample mass ratio of 15:1 and a rotation speed of 580 r/min for 6 h. Afterward, the sample was reduced in H₂ with a flux of 30 mL/min at 450°C for 90 min. Subsequently, the sample was cooled to 300°C using N₂. Then CO₂ reduction reaction on this sample was carried out at 300°C for 90 min (in situ), with a CO₂ flux of 30 mL/min. The mass of the sample treated by H₂ or CO₂ was 100 mg. As the control experiment, natural magnetite without mechanical milling was subjected to H₂ reduction and CO₂ reduction under the same conditions.

The morphologies of the samples were observed using field emission scanning electron microscopy (FE-SEM; Inspect F, FEI Corporation, Japan). The chemical composition was analyzed by energy-dispersive X-ray spectroscopy (Inca 350, Oxford Corporation, UK) coupled with scanning electron microscopy. The crystal structure was characterized using a Philips X’Pert PRO diffractometer (Netherlands) with Cu K_α radiation, employing a step size of 0.03°. The accelerating voltage was set at 40 kV with 40 mA flux. The software JADE 5.0 was used to index diffraction peaks. The Raman spectra were recorded with Lab Raman HR-800 (France), excited using 514.532 nm radiation argon ion laser with a power of 3.9 mW. The infrared radiation (IR, America) spectra were taken on a NEXUS470 spectrophotometer, using KBr disc and working in the absorption mode. The experimental conditions in the temperature-programmed reduction of hydrogen (H₂-TPR) process were as follows: 100 mg sample was embedded in a quartz tube with ϕ6 mm; the reducing atmosphere was pure H₂, the flow rate was 50 mL/min, and the heating rate was 10°C/min. The temperature-programmed surface reaction of carbon dioxide (CO₂-TPSR) experiments were conducted using the same apparatus as the TPR. The sample (100 mg) was heated from the ambient temperature to 850°C with a ramp-up of 10°C/min and CO₂ flow rate of 25 mL/min after the natural magnetite (hereafter denoted as NM) and the mechanically milled natural magnetite (hereafter denoted as MNM) were reduced in H₂ at 450°C for 90 min with a flow rate of 30 mL/min.

The H₂-TPR spectra of the natural magnetite without and with mechanical milling are shown in Fig. 1. For this magnetite without mechanical milling, there are two reduction peaks, which are located at 466°C and 813°C, respectively. The peak at 466°C is very weak and corresponds to the reduction of iron oxides from a higher valence to a lower one, while the peak at 813°C is very strong and is ascribed to the formation of metallic iron. Correspondingly, for the mechanically milled natural magnetite, the peaks shift to lower temperatures. One obvious peak at 450°C is observed, with the other peak at 751°C. The initial reduction peak attributed to oxygen loss is at 373°C for the MNM but at 591°C for the NM. There is a markedly negative shift of 218°C, as compared with the NM. Accordingly, to enhance the oxygen-deficient degree as much as possible, while keeping its spinel structure from destruction, the temperature of H₂ reduction should be selected as 450°C.

Figure 1.  H₂-TPR profiles of (a) NM and (b) MNM.

Fig. 2 shows the CO₂-TPSR spectra of the NM and the MNM. The samples were reduced in H₂ atmosphere at 450°C for 90 min and then cooled to the ambient temperature before CO₂-TPSR measurements. The intensities of CO₂ and CO are presented as black and red lines, respectively. It is clear that from 150 to 750°C, the CO₂ intensity for the MNM was significantly lower than that for the NM, the gap between the two samples was the largest at about 300°C and then slightly decreased with increasing temperature, suggesting that the MNM possesses a higher activity of CO₂ reduction. At 400°C, the CO signals for both specimens were observed. When the temperature was further increased to 450°C, the signal became more obvious. As a rule, CO₂ decomposition into carbon on the oxygen-deficient magnetite (Fe₃O_4−δ, 0 < δ < 1) goes through an intermediate process that produces CO [22, 29–31]. The reaction course can be depicted as follows:

Figure 2.  CO₂-TPSR profiles of the samples reduced at 450°C: (a, a′) NM; (b, b′) MNM. The black lines are the intensity of CO₂, and the red lines correspond to the intensity of CO.

Moreover, the lower the reduction degree δ, the greater the amounts of CO [32−33]. As shown in Fig. 2, for the two samples, the CO signal was much lower than the CO₂ signal from 150 to 500°C. Furthermore, from 150 to 687°C, the CO intensity of the MNM was always weaker than that of the NM. By contrary, when using (Ni_xCu_1−x)Fe₂O₄ or M ferrites (M = Ni and Cu) as the catalyst for CO₂ decomposition, the marked CO intensity, which is close to CO₂, is detected within a wider temperature region, and the lower the temperature the stronger the CO signal [30, 34]. Accordingly, it can be inferred that for the mechanically milled and freshly reduced natural magnetite, their higher selectivities of CO₂ reduction should be ascribed to their higher oxygen-deficient degrees and isomorphism substitutions of Fe, Ti, V, Co, Ni, and Al.

The X-ray diffraction (XRD) patterns of the NM and the MNM at various stages are illustrated in Figs. 3 and 4. For the NM, diffraction peaks from Fe₃O₄, FeTiO₃, Mg_1.55Fe_1.6O₄, and MgFe₂O₄ are observed, of which Fe₃O₄ and FeTiO₃ are the main crystalline phases, and only one compound of AB₂O₄ type (i.e., MgFe₂O₄) exists besides Fe₃O₄. After H₂ reduction, the diffraction peak of Mg_1.55Fe_1.6O₄ (JCPDS-80-0073) disappears while (Co_0.2Fe_0.8)Co_1.2Fe_1.2O₄ (JCPDS-77-0426) phase appears. This increases the number of spinel phases, which is beneficial to CO₂ decomposition. After CO₂ decomposition, all diffraction peaks, which appeared in the former stages, become very weak, with disappearances of MgFe₂O₄ and (Co_0.2Fe_0.8)Co_0.8Fe_1.2O₄ of spinel structure. Surprisingly, a new phase [Fe,Ni] exhibits the strongest signal as shown in curve (c) in Fig. 3. This suggests that for the NM, the spinel structure is destroyed greatly after CO₂ reduction reaction. As for the MNM, the phase evolvements at corresponding stages are distinctly different. Apart from Fe₃O₄, MgFe₂O₄ and (Co_0.2Fe_0.8)Co_0.8Fe_1.2O₄ with AB₂O₄ spinel structures occur just after H₂ reduction. Note that the metallic phases of [Fe,Ni] and Fe were detected, while for the NM, the metal phases were observed only after CO₂ decomposition. Also, the stability of the crystalline structure, especially for the spinel phases Fe₃O₄, (Co_0.2Fe_0.8)Co_0.8Fe_1.2O₄, and MgFe₂O₄, was evidently enhanced, while the diffraction peak of Fe₃O₄ phase for the NM has become very weak and markedly widened, with the latter two phases disappearing after CO₂ decomposition. Based on the above results, for the MNM after H₂ reduction, its isomorphism substitutions of multi-metals effectively promote adsorption and decomposition of CO₂ due to the oxygen flooding effect formed on the surface [29]. Furthermore, the appearance of metallic phases of [Fe,Ni] and Fe indicates that there is a higher oxygen-deficient degree, which conduces to heighten the reaction selectivity of CO₂ decomposition to C. Then, the formed C could reduce the oxides of Fe or FeNi to metal phase Fe or [Fe,Ni]. These results are in good agreement with the results of the H₂-TPR and the CO₂-TPSR.

Figure 3.  XRD patterns of the NM at various stages: (a) NM; (b) after H₂ reduction at 450°C; (c) after CO₂ decomposition at 300°C.

Figure 4.  XRD patterns of the MNM at various stages: (a) MNM; (b) after H₂ reduction at 450°C; (c) after CO₂ decomposition at 300°C.

The FE-SEM images of the samples reduced in H₂ atmosphere at 450°C for 90 min and subsequently oxidized in CO₂ atmosphere at 300°C for 90 min (in situ) are shown in Fig. 5. As shown in Fig. 5(a), the size distribution of the NM was very uneven and ranged from 10 to 180 μm with irregular shapes, whereas the MNM (Fig. 5(b)) took a spherical shape with an even distribution, whose particle diameter is about 0.5 μm. Although there are some larger agglomerates, their particle sizes are lower than 2.0 μm. Table 1 presents the energy-dispersive X-ray spectroscopy (EDS) results of the samples after CO₂ reduction. The increase in carbon content and decrease in oxygen content indicate that the MNM possessed a higher oxygen-deficient degree and the CO₂ reduction reaction occurs. The carbon content obtained over the MNM was 2.87wt% higher than that of NM.

Figure 5.  FE-SEM images of the samples treated in H₂ at 450°C and CO₂ atmosphere: (a) NM; (b) MNM.

To investigate the nature of carbon-containing phases (graphite, amorphous carbon, or cementite), the powder samples that were reduced in H₂ atmosphere at 450°C for 90 min and subsequently oxidized in CO₂ atmosphere at 300°C for 90 min (in situ) were dissolved using adequate aqueous hydrochloric acid. The XRD analysis was conducted on the undissolved substance [17−18]. No phase containing carbon was discovered, as shown in Fig. 6. Detecting the deposited carbon by XRD is difficult; thus, the Raman spectra and IR spectra were employed to further investigate the phase form of the deposited carbon [9–12]. Fig. 7 shows the Raman spectrum profiles of the NM and the MNM after treatment in H₂ and CO₂ atmosphere. Two characteristic peaks were observed for both specimens, which is well consistent with the fact that amorphous carbon shows two broad peaks, between 1340 and 1600 cm⁻¹. The mode at about 1600 cm⁻¹, often referred to as the G mode, is assigned to the “in-plane” displacement of the carbons strongly coupled in the hexagonal sheets, while the mode at around 1340 cm⁻¹ corresponds to the D mode induced disorder carbon [9–12, 35–36]. Compared with the MNM, the bandwidths of both D and G modes for the NM are greater, and the intensity rates of D band to G band (I_D/I_G) are higher, indicating that deposited carbons over the NM possessed a higher defect density or a smaller crystal granule size [35−36]. As shown in Fig. 8, the species containing carbon was further confirmed by bands at about 985, 1070, and 1625 cm⁻¹ in the IR profiles, which are attributed to the in-plane bending vibration of C−H, the asymmetric stretching mode of C–O–C, and the flexing oscillation of conjugated C=C, respectively [37−38]. The formation of C–H and C−O−C suggests that the deposited carbon and the magnetite substrate interacted. Accordingly, it is believed that deposited carbon is only in the amorphous form, neither graphite nor cementite form. This is significantly different from the products when unitary, binary, or ternary ferrite serves as catalyst for CO₂ reduction reaction [8–14, 17–21, 24, 30–31]. This gives the important meaning for the ferrite recycle because the formation of graphite or cementite will lead to deactivation of the ferrite for CO₂ decomposition [11−12,23,39]. For the MNM, its high selectivity of CO₂ reduction reaction can be principally ascribed to its isomorphism substitutions of Fe, Ti, V, Co, Ni, and Al, and the high activity of MNM, ascribed to higher oxygen-deficient degree, smaller granule size, and a more stable spinel structure [9–12, 19–22, 24–25].

Figure 6.  XRD patterns of the undissolved substance after HCl wash for the samples, then treatment in H₂ at 450°C and CO₂ atmosphere at 300°C: (a) NM; (b) MNM.

Figure 7.  Raman spectra for the samples after treatment in H₂ at 450°C and CO₂ atmosphere at 300°C: (a) NM; (b) MNM.

Figure 8.  IR spectra for the samples after treatment in H₂ at 450°C and CO₂ atmosphere at 300°C: (a) NM; (b) MNM.

Natural magnetite formed by isomorphism substitutions of Fe, Ti, V, Co, Ni, and Al was used as raw materials and was activated by mechanical milling and then reduced in H₂ atmosphere. Afterward, H₂-TPR and CO₂-TPSR were carried out to investigate the oxygen loss and CO₂ decomposition processes. The evolvement of the crystalline structure was investigated from mechanical milling to H₂ reduction and then to CO₂ decomposition processes. Both the NM and the MNM exhibited high selectivity of decomposing CO₂ to amorphous C. The amount of carbon deposited over the mechanically milled natural magnetite freshly reduced in H₂ gas was 2.87wt% higher than that deposited over the natural magnetite. Moreover, the high selectivity and enhanced activity of the MNM should be due to the isomorphism substitutions of Fe, Ti, V, Co, Ni, and Al; smaller granule size; higher oxygen-deficient degree; and a more stable spinel structure. Herein, a facile and “green” path is easily available for the catalyst to selectively decompose CO₂ to amorphous C.

This work was supported by the National Key Research and Development Program of China (No. 2016YFB0600904). The authors gratefully acknowledge the support of the Analytical and Test Center of Sichuan University.