XRDX-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 phase29. 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 doping25,30.Fig. 1XRD pattern of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.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.
N
2
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 and 4.9 nm, with smaller pore sizes leading to the formation of “ink-bottle” pores at relative pressures between 0.4 and 0.631. 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 S5) 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. FCO-5, FCO-3, and FCO-1 feature a slit-type pore structure. Compared to “ink bottle” pores (FCO-9 and FCO-7), these slits are smaller, hindering gas flow through the pores and facilitating thorough catalyst reaction with hydrogen. Among samples primarily featuring slit-type pores (FCO-5, FCO-3, and FCO-1), FCO-5 has the largest BET surface area, providing enough contact between n-H2 and FCO-5. Additionally, FCO-5 exhibits the highest lag coefficient, indicating an extended contact time with H2. Therefore, considering both pore structure and BET surface area, FCO-5 shows the most effective ortho-para hydrogen conversion performance.Fig. 2(a) Nitrogen adsorption–desorption isotherms and (b) pore size distribution graphs of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.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.FTIR spectral analysisThe FTIR spectrum of the sample in the range of 400–4000 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 process32. The vibration absorption peak at 1380 cm−1 corresponds to COO–33. Additionally, Cob exhibits stretching vibration bands of O–OH bonds at 3630 cm−1 and Co–O bonds at 499 cm−1, which are characteristic of Co(OH)234. With the increase in Co content in Fe, strong Co–O stretching and bending modes appear at 570 cm−1 and 663 cm−1, indicating the formation of Co3O4. The cubic structure of Co3O4 exhibits phase purity with a monodisperse nature35.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.Fig. 3FTIR spectra of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.XPSIn 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 Fe 2p3/2 and Fe 2p1/2 main signals of FO and FCO-5 can be observed at 710.0 eV and 723.4 eV, but the main peak of Fe 2p in FCO-5 is wider, and the satellite peak is smaller compared to FO. This indicates that the addition of Co promotes the formation of Fe3O4 in FCO-536, consistent with XRD results. The Fe 2p3/2 peak at 710.0 eV for FO and FCO-5 can be further decomposed into two components: Fe3+ (711 eV) and Fe2+ (709.6 eV). The Fe 2p1/2 peak at 723.4 eV for FO and FCO-5 can be resolved into two peaks: Fe3+ (725.0 eV) and Fe2+ (723.2 eV). In addition, the peaks at 717.5 eV and 732.4 eV are attributed to the vibration satellites of iron37. The addition of Co can regulate the chemical properties and distribution of grain boundary phases in FCO, thereby altering its magnetic properties38. This regulation is conducive to catalyzing the conversion of o-H2 to p- H2.Fig. 4XPS spectra of the (a) Fe 2p: FO and FCO-5 and (b) Co 2p: FCO-5 and Cob (sat, satellite).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 Co3O439. The fitted peaks at 779.0 eV and 794.0 eV are attributed to Co3+, while those at 780.2 eV and 795.7 eV are assigned to Co2+. In the spectrum of Cob, peaks are identified at 780.2 eV and 795.7 eV for Co 2p3/2 and Co 2p1/2, respectively, with an orbital separation of ~ 15.5 eV, confirming the presence of Co2+40. Furthermore, the satellite peaks at 785.4 eV and 802.1 eV support the presence of the Co(OH)2 phase41, in agreement with XRD findings.VSMThe 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 traits17. 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. The hysteresis loop of Cob shows a straight line, with magnetization (M) proportional to magnetic field strength (H), indicating the material exhibits paramagnetism42. The hysteresis loop of FCO-5 exhibits a symmetric magnetic hysteresis curve, with a coercivity (Hc) of 538.08 Oe and a remanent magnetization of 7.29 emu·g−1, indicating that FCO-5 is a hard magnetic material. This is attributed to the presence of materials like CoFe2O4 in FCO-543. Taking CoFe2O4 as an example, the Co and Fe ions in CoFe2O4 occupy tetrahedral and octahedral sites, respectively. This unique arrangement allows for the mutual reinforcement of the magnetic moments of Co2+ and Fe3+ ions, resulting in a significant net magnetic moment44.Fig. 5Hysteresis loops of FO, FCO-5 and Cob at 77 K.Catalyst activity testingThe 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. FCO-5 in particular exhibited stable catalytic activity, with a conversion rate of over 99% and an outlet p-H2 content levels above 49.6%, approaching the equilibrium concentration of p-H2 (~ 50%) at 77 K45.Fig. 6(a) O-P conversion and (b) content of p-H2 in the outlet after catalytic reaction of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.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.Based on the experimental results above, the catalytic activity of the Fe-Co bimetallic catalysts in the FCO series remains consistently high. Among them, FCO-5 exhibits the highest reaction rate constant of 291.7 mol·L−1·s−1 among Fe-Co catalysts. Moreover, FCO-5 demonstrates exceptional stability with conversion rates consistently above 99% and outlet p-H2 content consistently above 49.6%. Therefore, the optimal Fe/Co ratio for the Fe-Co bimetallic catalysts is determined to be 1:1. Xu et al.25 prepared the FeCo bimetallic catalyst CFO-3 using NH4OH as a precipitant. At 77 K and a hydrogen flow rate of 100 mL·min−1, the p-H2 content was 47.61%. A comparison revealed that the catalysts prepared by Xu et al. exhibited entirely different patterns in XRD characterization with varying Co/Fe ratios compared to those in this study, demonstrating different crystalline phases due to the use of different precipitants. Additionally, Xu et al. suggest that the addition of Co increases the proportion of Fe3+, enhancing the catalyst’s magnetic strength and thus improving its catalytic performance. In contrast, this study proposes that the complex crystalline phases introduced by Co, which contribute to internal disorder and magnetic moments, are crucial factors in the excellent catalytic performance of FeCo bimetallic catalysts, rather than simply the enhanced magnetic characteristics of Fe3+ over Fe2+. Additionally, Xu et al.46 studied the ortho-para hydrogen conversion effects of different metal dopants and prepared the FeMn bimetallic catalyst Mn-FO, which achieved a p-H2 content of 49.45% after catalytic conversion at 77 K with a hydrogen flow rate of 100 mL·min−1. The p-H2 content at the outlet of the FCO-5 catalyst prepared in this study surpasses that of the aforementioned catalysts.The doping level of Co significantly affects the catalytic performance of FCO. On one hand, varying amounts of Co doping result in different pore structures in FCO. Specifically, the addition of Co increases FCO’s lag coefficient and enhances surface porosity (Figure S1). The exposed pores and higher lag coefficient facilitate sufficient contact and interaction time between n-H2 and FCO, favoring ortho-para conversion. On the other hand, based on XRD (Figure S2) and FTIR (Figure S4) results of FCO-5 before and after the reaction, there is minimal difference, indicating no new phases, chemical bonds, or bond breaks occurred. This suggests a magnetic conversion process for o-H2 to p-H247. The addition of Co introduces new phases such as CoFe2O4, Co3O4, and Fe3O4 in FCO. Compared to single-component phases, this mixture increases internal disorder, leading to larger magnetic moments. Particularly in FCO-5, which contains components such as CoFe2O4, Co3O4, and Fe3O4, the arrangement of ions favors the generation of larger magnetic moments compared to Fe2O3 and Co(OH)2, thereby facilitating rapid o-p conversion.
Ortho to para hydrogen conversion over bimetallic iron and cobalt catalysts
