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.
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 (Fe3O4−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 (NixCu1−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, Mg1.55Fe1.6O4, 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 Mg1.55Fe1.6O4 (JCPDS-80-0073) disappears while (Co0.2Fe0.8)Co1.2Fe1.2O4 (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 (Co0.2Fe0.8)Co0.8Fe1.2O4 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 (Co0.2Fe0.8)Co0.8Fe1.2O4 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, (Co0.2Fe0.8)Co0.8Fe1.2O4, 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 . 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.
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.
Sample C O Mg Al Si Ti V Fe NM 3.73 30.22 2.59 2.54 0.79 7.55 0.34 52.69 MNM 6.60 25.47 1.97 2.45 2.01 7.21 0.58 54.73
Table 1. EDS results of the samples reduced at 450°C then oxidized at 300°C
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 (ID/IG) 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.
Selective reduction of carbon dioxide into amorphous carbon over activated natural magnetite
15 October 2019
Revised: 3 March 2020
Accepted: 4 March 2020
Available online: 10 March 2020
Abstract: Natural magnetite formed by the isomorphism substitutions of transition metals, including Fe, Ti, Co, etc., was activated by mechanical grinding followed by H2 reduction. The temperature-programmed reduction of hydrogen (H2-TPR) and temperature-programmed surface reaction of carbon dioxide (CO2-TPSR) were carried out to investigate the processes of oxygen loss and CO2 reduction. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS). The results showed that the stability of spinel phases and oxygen-deficient degree significantly increased after natural magnetite was mechanically milled and reduced in H2 atmosphere. Meanwhile, the activity and selectivity of CO2 reduction into carbon were enhanced. The deposited carbon on the activated natural magnetite was confirmed as amorphous. The amount of carbon after CO2 reduction at 300°C for 90 min over the activated natural magnetite was 2.87wt% higher than that over the natural magnetite.