XRD
X-ray diffraction (XRD) was utilized for the compositional and crystal structure characterization of the samples, with results presented in Fig. 1. The analysis of Fig. 1 reveals that Co doping altered the crystalline phases of the samples and enhanced their crystallinity. All diffraction peaks of Cob were found to align with those of Co(OH)2 (JCPDS No.89-8616), demonstrating good crystallinity. The prominent diffraction peaks observed at 2θ = 35.63° and 62.93° in the FO samples were attributed to Fe2O3 (JCPDS No.39-1346), identifying it as the primary crystalline phase[27]. Introduction of Co led to the emergence of new phases in the FCO series of samples. Unique diffraction peaks corresponding to CoFe2O4 (JCPDS No.03-0864) were detected in FCO-9, FCO-7 and FCO-5 (marked by asterisks). Furthermore, the characteristic diffraction peaks of Fe2O3 vanished in the FCO-7 sample. As the Co content increased, characteristic diffraction peaks of Co3O4 (JCPDS No.43-1003), Fe3O4 (JCPDS No.26-1136), and CoO(OH) (JCPDS No.07-0169) were observed in FCO-5, FCO-3, and FCO-1 samples, respectively. The XRD results indicated a trend towards transformation into Co(OH)2 with increasing Co doping. In FCO-1, the CoO(OH) phase displaced the coexistence of Co3O4 and Fe3O4, resulting in the formation of new crystal facets. Additionally, a novel peak (311) emerged at 2θ = 35.45° in FCO-9. Incrementing Co content led to the appearance of new crystal facets (003), (220), (222), (400), (015), (511), and (113) at 2θ = 20.24°, 31.27°, 38.55°, 44.81°, 50.58°, 59.35°, and 69.17° in FCO-5. Conversely, in FCO-1, new crystal facets (012), (104), and (110) appeared at 2θ = 38.89°, 45.86°, and 65.34°, respectively. Notably, as Co content increased, the intensity of diffraction peaks rose, and the peak shape sharpened, indicating an enhancement in crystallinity due to Co doping[25, 28].
The grain sizes of Co species in the samples were calculated using the Scherrer formula, and the results are shown in Table 1. It was observed that FCO-9 and FCO-7 did not exhibit distinct characteristic diffraction peaks of Co species. Meanwhile, the grain sizes of CoOOH and Co3O4 decreased sequentially in FCO-5, FCO-3, and FCO-1.
Table 1
Initial composition and grain size of CoOOH and Co3O4 for FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.
Catalyst | Fe/(Fe + Co) (%) | CoOOH (nm) | Co3O4 (nm) |
FO | 100 | / | / |
FCO-9 | 90 | N.A. | N.A. |
FCO-7 | 70 | N.A. | N.A. |
FCO-5 | 50 | 20.1 | 23.9 |
FCO-3 | 30 | 21.3 | 16.8 |
FCO-1 | 10 | 14.8 | 13.0 |
Cob | 0 | / | / |
Note: N.A. stands for “not available,” indicating that data cannot be provided due to very small grain size. |
N2 physical adsorption
In order to investigate the effect of Co doping and different Fe/Co ratios on the surface area and pore structure of the samples, N2 adsorption and desorption tests were conducted. Figure 2A shows the N2 adsorption-desorption isotherms of samples. All samples exhibit type IV isotherms, indicating a typical mesoporous structure. With the increase of Co content, the hysteresis loop transitions from an H2 type to an H4 type, indicating a transformation from “ink-bottle” pores to slit-like pores. This suggests that the variation in Co doping levels brings about different pore structures in the samples. Combining with Fig. 2B, the pore size distribution of FCO-9 is mainly concentrated between 1.7–4.9 nm, with smaller pore sizes leading to the formation of “ink-bottle” pores at relative pressures between 0.4–0.6[29]. Therefore, the isotherm exhibits a typical H2 type hysteresis loop. The saturated adsorption plateau at the high-pressure stage indicates a relatively uniform pore size distribution for FCO-9. As the Co content increases, FCO-5 starts to exhibit an H3 type hysteresis loop, which is typically associated with slit-like pores formed by the stacking of sheet-like particles. The results calculated according to the lag coefficient (equation S1) are shown in Table 2. The results indicate that as the Co content increases, the N2 adsorption decreases continuously. The lag coefficient first increases and then decreases, reaching the highest value at FCO-5. This indicates that FCO-5 has a smaller degree of pore openness, allowing for better gas interaction. In the P/P0 > 0.8 range, the adsorption rate for FCO-1 continuously increases without significant adsorption limitation, due to capillary condensation occurring in the pores. This implies the presence of larger and diverse pore types within FCO-1. The pore distribution (Fig. 2B) was determined using the Barrett-Joyner-Halenda (BJH) method from the N2 adsorption branch of the isotherms. The pores of FO and Cob are mainly composed of microspores (< 2 nm) and macrospores (> 50 nm), while the FCO series are mainly composed of mesoporous (2–50 nm). Among them, FCO-9, FCO-7, and FCO-3 contain a small amount of microspores, while FCO-1 contains a small amount of macrospores. The analysis above shows that the pores in FCO-5 are mainly mesoporous, and the pore structure is predominantly slit-shaped. Slit-shaped pores have smaller sizes, which obstruct the flow of gas in the pores, allowing the catalyst to fully react with hydrogen. Among the samples with slit-shaped pores (FCO-5, FCO-3, and FCO-1), FCO-5 has the highest BET specific surface area. These advantages not only provide abundant catalytic active sites, but also provide sufficient time for subsequent hydrogen reaction with FCO-5. Therefore, even though the BET specific surface area of FCO-5 is not the largest and the average pore size is not the smallest among the samples, it has the best performance in hydrogen conversion.
Table 2
Surface area, pore volume, pore size and lag coefficient of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.
Catalyst | FO | FCO-9 | FCO-7 | FCO-5 | FCO-3 | FCO-1 | Cob |
BET Surface Area (m2·g− 1) | 256.76 | 267.96 | 242.40 | 137.36 | 129.97 | 89.98 | 180.54 |
Pore Volume (cm3·g− 1) | 0.22 | 0.19 | 0.22 | 0.27 | 0.19 | 0.26 | 0.30 |
Pore Size (nm) | 3.49 | 2.79 | 3.67 | 7.95 | 5.87 | 11.7 | 6.56 |
Lag Coefficient | 0.005 | 0.006 | 0.025 | 0.075 | 0.070 | 0.064 | 0.015 |
FTIR Spectral Analysis
The FTIR spectrum of the sample in the range of 500–4000 cm− 1 is shown in Fig. 3. The stretching mode at 3422 cm− 1 and a weak asymmetric peak at 1620 cm− 1 are characteristic of the O-H stretching vibration, due to the absorption of water molecules during the sample preparation process[30]. The peak at 1380 cm− 1 corresponds to the Fe-O stretching vibration. The presence of strong Co-O stretching and bending modes at 663 cm− 1 and 570 cm− 1 indicates the formation of the Co3O4 phase, with a high degree of phase purity in the cubic structure[31]. This suggests that FCO-5, FCO-3, and FCO-1 have good stability and the Co3O4 produced by them has uniform particle size, consistent with the previous BET analysis results.
XPS
In order to further investigate the impact of Co doping on FCO catalyst, the surface elemental composition and chemical state of FO, FCO-5, and Cob samples were analyzed using X-ray photoelectron spectroscopy (XPS). Figure 4a displays the Fe 2p spectra of FO and FCO-5. The main signals of Fe 2p3/2 and Fe 2p1/2 for FO and FCO-5 are observed at 710.0 eV and 723.4 eV, with the Fe 2p peak in FCO-5 appearing broader and the satellite peak smaller compared to FO. This suggests that the introduction of Co promotes the formation of Fe3O4 in FCO-5[32]. The Fe 2p3/2 peak observed in both FO and FCO-5 at 710.0 eV can be fitted into three distinct peaks at 711 eV, 712.9 eV, and 709.6 eV. The peaks at 711 eV and 712.9 eV correspond to octahedral Fe3+ and tetrahedral Fe3+, respectively, while the peak at 709.6 eV is assigned to Fe2+. Furthermore, the peaks at 717.7 eV and 732.4 eV are attributed to the vibrational satellite of Fe3+. Notably, FCO-5 exhibits a heightened satellite peak of Fe2+ at 715.6 eV compared to FO, indicating the presence of Fe2+ in FCO-5 and supporting the existence of Fe3O4, in line with XRD findings. The inclusion of Co enhances the formation of Fe3O4. Fe2O3 is characterized by the corundum structure, with octahedral voids occupied by Fe3+ ions[33]. In contrast, Fe3O4 adopts the inverse spinel structure, with Fe2+ in octahedral sites and two Fe3+ ions in tetrahedral and octahedral sites, respectively[34], facilitating the arrangement of magnetic materials and resulting in increased magnetism[35], and is more conducive to the catalytic conversion of o-H2 to p-H2.
Figure 4b illustrates the Co 2p spectra of FCO-5 and Cob. The signals observed at 779.1 eV and 794.2 eV in FCO-5 correspond to Co 2p3/2 and Co 2p1/2, respectively, with satellite peaks at 786.9 eV and 802.7 eV, suggesting the existence of Co3O4[36]. The fitted peaks at 779.0 eV and 794.0 eV are attributed to Co3+, while those at 780.9 eV and 795.7 eV are assigned to Co2+. In the spectrum of Cob, peaks are identified at 780.1 eV and 796.1 eV for Co 2p3/2 and Co 2p1/2, respectively, with an orbital separation of 16 eV, confirming the presence of Co2+[37]. Furthermore, the satellite peaks at 785.6 eV and 802.1 eV support the presence of the Co(OH)2 phase[38], in agreement with XRD findings.
VSM
The magnetic properties of FO, FCO-5, and Cob were analyzed at 77 K by varying the magnetic field, disclosing distinct magnetic characteristics (Fig. 5). FO exhibits negligible coercivity and remanence values, displaying an S-shaped hysteresis loop indicative of superparamagnetic traits[17]. Despite reaching 90,000 Oe, the magnetization intensity of FO did not attain saturation, suggesting the presence of a spin-disordered region that remains active at low temperatures, resulting in a magnetization intensity of 14.12 emu·g-1. Conversely, the hysteresis loop of Cob displays a linear pattern, indicating antiferromagnetism attributed to its primary component, Co(OH)2, where the Co2 + ions possess a 3d7 electron configuration with a spin value of 3/2. At reduced temperatures, the Co2 + ions tend to align their spins in an antiparallel orientation due to electron interactions, resulting in the formation of an antiferromagnetic structure characterized by antiferromagnetism[39]. FCO-5, conversely, portrays a symmetric hysteresis loop with a coercivity of 538.08 Oe and a remanence value of 7.29 emu·g-1, placing it within the category of hard magnetic materials due to the inclusion of CoFe2O4[40]. The introduction of Co atoms within the Fe-O matrix of CoFe2O4 enhances the material’s anisotropy. CoFe2O4 adopts a spinel structure, with Co and Fe ions occupying tetrahedral and octahedral positions, respectively. This specific configuration amplifies the magnetic moments of Co2 + and Fe3 + ions, resulting in a substantial net magnetic moment[41].
Catalyst Activity Testing
The catalyst samples were tested for activity at 77 K, and the results are shown in Fig. 6. FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 all exhibited good catalytic activity. The catalytic activity of Cob was significantly lower than that of the Fe-containing catalysts. As the Co doping content increased, the conversion rate and the content of outlet p-H2 of the FCO series catalysts initially increased, then decreased. The results indicated that FCO-5 had the best catalytic performance. The O-P conversion rate of Cob remained below 75%, and the content of outlet p-H2 after conversion remained below 40%. Under conditions of GHSV (gaseous hourly space velocity) at 1800 h− 1, the normal para-hydrogen conversion rates of FO and FCO-1 were similar, both around 90%. But the content of outlet p-H2 after conversion of FCO-1 (48%) was higher than FO (41%). FCO-9, FCO-7, FCO-5, and FCO-3 all had O-P conversion rates exceeding 97%, with outlet para-hydrogen content above 49%. With the increase of GHSV, the catalytic activity of FCO-9, FCO-1, and Cob gradually decreased. However, the catalytic activity of FO increased with the increase of GHSV. When GHSV = 5400 h− 1, the conversion rate of FO was close to FCO-9, reaching 95%, but the outlet p-H2 content of FCO-9 was higher than that of FO. Therefore, it can be concluded that the catalytic performance of Fe-based catalyst doped with Co is superior to the single-metal FO catalyst, and far higher than Cob catalyst. The increase of GHSV did not have a significant impact on the activity of FCO-7, FCO-5, and FCO-3, as they all maintained good catalytic activity. The catalytic performance of FCO-5 demonstrates exceptional stability, consistently achieving conversion rates exceeding 99% and maintaining p-H2 content levels above 49.6%, approaching the equilibrium concentration of p-H2 (~ 50%) at 77 K[42].
Table 3 shows the O-P conversion rate constants (K) of the sample catalysts. The highest O-P conversion rate constant was observed in FCO-5, GHSV = 5400 h− 1, reaching 291.7 mol·L− 1·s− 1. The O-P conversion rate constants of FO and Cob were lower than the FCO series at all GHSV, indicating that the doping of Co improved the performance of Fe-based catalysts. FCO-7, FCO-5, and FCO-3 all showed a trend of increasing reaction rate constants with increasing GHSV. However, the reaction rate constant of FCO-9 showed a trend of first increasing and then decreasing, with the maximum value occurring at GHSV = 3600 h− 1, reaching 280 mol·L− 1·s− 1. The O-P conversion rate constant of Cob was much lower than that of the Fe-containing catalyst.
Table 3
The reaction rate constant K (mol·L− 1·s− 1) of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob at 77 K.
GHSV (h− 1) | K(FO) | K (FCO-9) | K (FCO-7) | K (FCO-5) | K (FCO-3) | K (FCO-1) | K(Cob) |
1800 | 20.5 | 134.2 | 97.3 | 98.1 | 81.2 | 52.3 | 13.2 |
3600 | 31.6 | 280.7 | 235.4 | 211.3 | 235.8 | 115.7 | 29.7 |
5400 | 86.5 | 180.2 | 255.0 | 291.7 | 253.5 | 107.9 | 15.8 |
Based on the above experimental results, it is concluded that the catalytic activity of the bimetallic catalysts in the FCO series remains at a high level. Among them, the reaction rate constant of FCO-5 is 291.7 mol·L− 1·s− 1, which is the highest value among the Fe-Co series catalysts, so the optimal ratio for Fe-Co bimetallic catalysts is n(Fe)/n(Fe + Co) = 0.5.
The doping level of Co has a significant impact on the catalytic effect of FCO. On one hand, different doping levels of Co result in different pore structures of FCO. Specifically, the addition of Co reduces the BET surface area of FCO but increases the lag coefficient of FCO. A high lag coefficient means that n-H2 has sufficient contact time with FCO, which is more conducive to o-p conversion. On the other hand, the addition of Co introduces new phases such as CoFe2O4, Co3O4, and Fe3O4 into FCO. The mixture of these phases compared to single components makes the sample more disordered internally, bringing more active sites and larger magnetic moments. In particular, FCO-5, which contains CoFe2O4, Co3O4, and Fe3O4 components, has an ion arrangement that is more favorable for the generation of large magnetic moments compared to Fe2O3 and Co(OH)2, thereby promoting the rapid o-p conversion.