Enhanced Catalytic Conversion of Ortho-Hydrogen to Para-Hydrogen by using Iron- Cobalt Bimetallic Catalysts

Enhanced Catalytic Conversion of Ortho-Hydrogen to Para-Hydrogen by using Iron- Cobalt Bimetallic Catalysts | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Enhanced Catalytic Conversion of Ortho-Hydrogen to Para-Hydrogen by using Iron- Cobalt Bimetallic Catalysts Liujing Yang, Xinbao Li, Kai Sun, Xiaoling Zheng, Ying Chen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4625849/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 09 Sep, 2024 Read the published version in Scientific Reports → Version 1 posted 12 You are reading this latest preprint version Abstract The catalytic conversion of ortho-hydrogen (o-H 2 ) to para-hydrogen (p-H 2 ) serves as a crucial step in the storage of liquid hydrogen over extended periods. A variety of iron-cobalt catalysts were synthesized using a precipitation technique, incorporating diverse levels of Co doping into Fe-based catalysts. The effects of Co doping on the crystal structure, porosity, and magnetism of FCO were examined through XRD, N 2 adsorption-desorption, FTIR, XPS, and VSM analyses. The efficacy of ortho-para hydrogen conversion within FCO at 77 K was analyzed using chromatography. Findings revealed that Co doping enhances the material’s lag coefficient, leading to an increase in active sites and larger magnetic moments. Notably, FCO-5 [n(Fe)/n(Fe + Co) = 0.5] exhibited the most efficient ortho-para hydrogen conversion performance. Specifically, at GHSV = 5400 h − 1 , FCO-5 achieved a reaction rate constant of 291.7 mol·L − 1 ·s − 1 , a conversion rate of 99.24%, and a post-conversion p-H 2 content of 49.7%. Physical sciences/Chemistry Physical sciences/Energy science and technology Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Hydrogen, with diverse sources and environmentally-friendly combustion characteristics [ 1 ] , is considered the most promising clean energy within the global decarbonization framework [ 2 , 3 ] 。Considering the potential of hydrogen energy, numerous countries and organizations have introduced strategic plans for its advancement. Notably, the European Union’s “EU Hydrogen Strategy” [ 4 ] and the U.S. Department of Energy’s (DOE) “Hydrogen Shot” initiative introduced in 2021 [ 5 ] . The storage and transportation of hydrogen, pivotal elements in the hydrogen energy sector, present challenges to its advancement. Liquid hydrogen storage technology, with its high energy density, is particularly well-suited for long-distance transportation [ 6 – 8 ] . Furthermore, low-temperature liquid hydrogen storage provides enhanced safety measures compared to high-pressure storage [ 9 ] . Nevertheless, the conversion of ortho-hydrogen (o-H 2 ) to para-hydrogen (p-H 2 ) during hydrogen liquefaction results in the release of heat due to their differing physical properties [ 10 , 11 ] . Thus, for industrial purposes, the para-hydrogen (p-H 2 ) content in liquid hydrogen storage should exceed 95% [ 12 ] . Hydrogen gas is categorized into ortho-hydrogen and para-hydrogen based on nuclear spin orientation [ 13 ] . Under equilibrium conditions, normal hydrogen (n-H 2 ) at room temperature and above consists of 75% o- H 2 and 25% p-H 2 [ 14 ] . With decreasing temperature, o-H 2 undergoes a gradual conversion to the lower spin state of p-H 2 , with the process occurring slowly [ 15 ] . The production and storage of liquid hydrogen require low-temperature conditions [ 11 , 16 ] . The gradual conversion of o-H 2 to p-H 2 leads to losses in liquid hydrogen storage [ 17 ] . Consequently, the utilization of catalysts is crucial for the efficient conversion of o-H 2 to p-H 2 , ensuring effective long-term liquid hydrogen storage. With the advancement of catalytic o-p hydrogen conversion in industrial settings, there is a growing number of proposed mechanisms for this process. Notably, paramagnetic centers are instrumental in facilitating rapid ortho-para conversion(o-p conversion). A prominent theory, suggested by Wigner in 1993, emphasizes the role of paramagnetic catalysis in molecular hydrogen o-p conversion [ 18 ] . According to this theory, the o-p conversion rate correlates with paramagnetic materials and proton magnetic moments. Further, in 1973, Petzinger and Scalapino noted that augmenting the catalytic metal’s magnetic moment and decreasing the distance between reactants and H 2 surfaces dramatically boosts the o-p conversion rate [ 19 ] . Thus, these represent the two fundamental approaches in developing o-p conversion catalysts. Transition metals, favored for o-p conversion catalysts, possess unpaired electrons leading to heightened magnetic moments [ 20 ] . Iron, known for its abundant availability and cost-effectiveness, is extensively employed in the industrial synthesis of o-p conversion catalysts [ 21 ] . Svadlenak et al. [ 22 ] conducted a study on various iron-zinc compounds, encompassing γ-Fe 2 O 3 , for o-p conversion at 78 K. The findings revealed that γ-Fe 2 O 3 demonstrated the highest catalytic activity, with α- Fe 2 O 3 , α- Fe 2 O 3 -ZnO, and ZnFe 2 O 4 showing progressively lower catalytic activity. The catalysts exhibited a range of magnetic properties including ferromagnetic, antiferromagnetic, weak antiferromagnetic, and paramagnetic with subtle ferromagnetism. The study conclusively demonstrated that ferromagnetic arrays are superior to antiferromagnetic arrays in promoting hydrogen nuclear spin conversion. Das et al. [ 23 ] synthesized LaFeO 3 /Al 2 O 3 catalysts via the citrate sol-gel technique and observed that the catalyst attained the highest spin conversion rate post-activation through calcination at 773 K. At 17 K, a catalyst with La:Fe ratio of 2:8 (20La 0.2 Fe 0.8 O 3 /Al 2 O 3 ) achieved a remarkable 99.8% conversion of o-H 2 to p-H 2 within a 120 min duration. Karlsson [ 24 ] examined the commercially available porous particle catalyst, IONEX®, predominantly comprising iron oxide (Fe 2 O 3 ). The conversion process involves spin conversion on magnetically aligned surfaces, with the catalyst’s magnetism emanating from iron and possessing a magnetic moment of 5.92 Bohr magnetons. The results showed that achieving a 99.8% conversion of normal hydrogen into para-hydrogen at 16 K took merely 80 min. In addition, we discovered that Xu et al. [ 25 ] employed ammonia hydroxide (NH 4 OH) as a precipitant to prepare a range of FeCo bimetallic catalysts through the precipitation method. The catalyst with a Co/Fe ratio of 3:7 achieved 38.5% conversion of hydrogen at the outlet under a H 2 flow rate of 500 ml·min − 1 . However, the series of catalysts prepared by Xu et al. exhibited distinct patterns in the X-ray diffraction (XRD) characterization results compared to the catalysts prepared in this study using sodium hydroxide (NaOH) as the precipitant. Furthermore, Xu et al. proposed that the addition of Co increased the proportion of Fe 3+ rather than Fe 2+ , resulting in higher magnetization intensity and improved catalytic performance. Conversely, our study posits that the unique Fe 2+ and Fe 3+ site structures within the characterization results of Fe 3 O 4 are pivotal to the excellent catalytic effects of FeCo bimetallic catalysts, rather than solely the superior magnetic properties of Fe 3+ compared to Fe 2+ . Polyukhov et al. [ 26 ] showcased the viability of Metal-Organic Frameworks (MOFs) as catalysts for o-p hydrogen conversion, introducing efficient catalysts such as M-MOF-74 (M = Mn, Co, Cu, Ni, Zn). Among these catalysts, Ni-MOF-74 displayed an o-p conversion rate constant (k) of 26000 min − 1 g- 1 at 77 K, suitable for use at temperatures lower than 15 K. Despite the excellent performance of MOF catalysts, their complex and costly preparation process necessitates the proposal of a straightforward, cost-effective production method for these catalysts. This study synthesized a series of FCO catalysts doped with Co elements using a precipitation method. The performance of the FCO catalyst in converting ortho-para hydrogen at 77 K was evaluated by adjusting the Fe/(Fe + Co) molar ratio to identify the optimum FCO ratio. The influence of Co doping on Fe-based catalysts was analyzed through XRD, XPS, and VSM techniques. The impact of catalyst micro-porosity and surface area on catalytic performance was investigated using N 2 adsorption-desorption tests. The successful combination of Fe and Co was confirmed using FTIR technology. Experimental methods Material synthesis All reagents used are of analytical grade. Ferric nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) and Cobalt(II) nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O) are the precursors for iron and cobalt respectively. Sodium hydroxide (NaOH) is used as the precipitating agent. A series of Fe-Co catalysts were synthesized using precipitation method. (Fe(NO 3 ) 3 ·9H 2 O) was dissolved in deionized water and subjected to ultrasonic treatment for 10 min (Solution A). Similarly, (Co(NO 3 ) 2 ·6H 2 O) was dissolved in deionized water and subjected to ultrasonic treatment for 10 min (Solution B). Solution A was slowly poured into Solution B, resulting in Solution C. Afterwards, NaOH was dissolved in deionized water (Solution D) and dripped into Solution C using a peristaltic pump, while stirring. After complete dripping of Solution D, the mixture was further stirred for 30 minutes, followed by aging at room temperature for at least 6 hours. The precipitate was then filtered using deionized water. The obtained residues was dried overnight in a oven and crushed to a particle size of 40–60 mesh. The molar ratios of n(Fe) / n(Fe + Co) were 0.9, 0.7, 0.5, 0.3 and 0.1, and they were named as FCO-9, FCO-7, FCO-5, FCO-3, and FCO-1, respectively. Pure phase samples were prepared according to the above process without Co(NO 3 ) 2 ·6H 2 O and without Fe(NO 3 ) 3 ·9H 2 O, and named FO and Cob, respectively. Materials characterization Structural analysis of Fe-Co samples was conducted using X-ray diffraction (XRD, Smartlab 9 kW) with Cu Kα radiation. Nitrogen adsorption-desorption experiments at 77 K were performed using a Micromeritics ASAP 2460 analyzer. The surface area, total pore volume, and average pore size of the samples were calculated using the Brunauer-Emmett-Teller (BET) method, while the Barrett-Joyner-Halenda (BJH) method was employed for pore size distribution analysis. Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20) was used for qualitative and quantitative analysis of functional groups and chemical bonds in the samples. X-ray photoelectron spectroscopy (XPS) analyses were performed using the Thermo K-alpha spectrometer, while magnetic measurements were carried out at a temperature of 77 K using the PPMS-9T dc magnetometer from American Quantum Design. Catalytic performance at ultra-low temperatures The sample was tested using a fully automated o-p conversion and reduction device. Firstly, the sample (mass = 1 g) was placed in a furnace for activation. The activation conditions were vacuum, with a temperature of 130 ℃ for 6 hours. After activation, the reaction vessel containing the sample was switched from the furnace to a liquid nitrogen (77 K) low-temperature bath. Normal hydrogen was deoxygenated and dehydrated before being introduced into the reaction vessel for the reaction. The normal hydrogen flow rate was adjusted using a mass flow meter, and the content of para-hydrogen after catalysis was measured using a gas chromatograph (GC-9790Ⅱ, Fuli Instruments) equipped with a 5A molecular sieve packed column and a thermal conductivity detector (TCD). In this work, measurements are taken every 4 minutes for 6 consecutive times when the hydrogen flow rate is stable and the data is smoothed, and the average value is calculated. Results and discussions 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 Fe 2 O 3 (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 CoFe 2 O 4 (JCPDS No.03-0864) were detected in FCO-9, FCO-7 and FCO-5 (marked by asterisks). Furthermore, the characteristic diffraction peaks of Fe 2 O 3 vanished in the FCO-7 sample. As the Co content increased, characteristic diffraction peaks of Co 3 O 4 (JCPDS No.43-1003), Fe 3 O 4 (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 Co 3 O 4 and Fe 3 O 4 , 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 Co 3 O 4 decreased sequentially in FCO-5, FCO-3, and FCO-1. Table 1 Initial composition and grain size of CoOOH and Co 3 O 4 for FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob. Catalyst Fe/(Fe + Co) (%) CoOOH (nm) Co 3 O 4 (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. 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, N 2 adsorption and desorption tests were conducted. Figure 2 A shows the N 2 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. 2 B, 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 N 2 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/P 0 > 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. 2 B) was determined using the Barrett-Joyner-Halenda (BJH) method from the N 2 adsorption branch of the isotherms. The pores of FO and Cob are mainly composed of microspores ( 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 (m 2 ·g − 1 ) 256.76 267.96 242.40 137.36 129.97 89.98 180.54 Pore Volume (cm 3 ·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 Co 3 O 4 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 Co 3 O 4 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 4 a displays the Fe 2p spectra of FO and FCO-5. The main signals of Fe 2p 3/2 and Fe 2p 1/2 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 Fe 3 O 4 in FCO-5 [ 32 ] . The Fe 2p 3/2 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 Fe 3+ and tetrahedral Fe 3+ , respectively, while the peak at 709.6 eV is assigned to Fe 2+ . Furthermore, the peaks at 717.7 eV and 732.4 eV are attributed to the vibrational satellite of Fe 3+ . Notably, FCO-5 exhibits a heightened satellite peak of Fe 2+ at 715.6 eV compared to FO, indicating the presence of Fe 2+ in FCO-5 and supporting the existence of Fe 3 O 4 , in line with XRD findings. The inclusion of Co enhances the formation of Fe 3 O 4 . Fe 2 O 3 is characterized by the corundum structure, with octahedral voids occupied by Fe 3+ ions [ 33 ] . In contrast, Fe 3 O 4 adopts the inverse spinel structure, with Fe 2+ in octahedral sites and two Fe 3+ 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-H 2 to p-H 2 . Figure 4 b 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 2p 3/2 and Co 2p 1/2 , respectively, with satellite peaks at 786.9 eV and 802.7 eV, suggesting the existence of Co 3 O 4 [ 36 ] . The fitted peaks at 779.0 eV and 794.0 eV are attributed to Co 3+ , while those at 780.9 eV and 795.7 eV are assigned to Co 2+ . In the spectrum of Cob, peaks are identified at 780.1 eV and 796.1 eV for Co 2p 3/2 and Co 2p 1/2 , respectively, with an orbital separation of 16 eV, confirming the presence of Co 2+[ 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-H 2 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-H 2 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-H 2 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-H 2 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-H 2 content levels above 49.6%, approaching the equilibrium concentration of p-H 2 (~ 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-H 2 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 CoFe 2 O 4 , Co 3 O 4 , and Fe 3 O 4 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 CoFe 2 O 4 , Co 3 O 4 , and Fe 3 O 4 components, has an ion arrangement that is more favorable for the generation of large magnetic moments compared to Fe 2 O 3 and Co(OH) 2 , thereby promoting the rapid o-p conversion. Conclusion In summary, the doped FCO catalyst series with Co elements were successfully prepared using a co-precipitation method. The study investigated the effect of the amount of Co element doping on the hydrogen conversion. The results showed that Co doping altered the pore structure of FCO, reducing the catalyst’s specific surface area, increasing the lag coefficient, and thus increasing the contact time between the catalyst and n-H 2 . FCO-5 possesses a large specific surface area and the highest lag coefficient, making it the optimal catalyst for hydrogen conversion. Additionally, the incorporation of Co into FCO brings forth an increased number of active sites and larger magnetic moments, which facilitate the o-p conversion process. At a GHSV of 5400 h − 1 , the reaction rate constant for FCO-5 can reach 291.7 mol·L − 1 ·s − 1 , and the p-H 2 content can reach 49.7%. The preparation process of the FCO catalyst series is straightforward, the raw materials are inexpensive, and it has potential for industrial production and application. Declarations Declaration of competing interest No potential conflict of interest was reported by the author(s). Author Contribution Conceptualization:L.Y.,X.L.,X.Z.,methodology:K.S.,Y.C.,X.L.,formal analysis:L.Y.,X.Z.,synthesis:L.Y.,K.S.,X.Z.,writing—original draft preparation:X.Z.,L.Y.,writing—review and editing:L.Y.,X.L. All authors have read and agreed to the published version of the manuscript. Acknowledgement The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51976094). Data Availability Our analyzed datasets are available from the corresponding author on reasonable request References Singh, A., Shivapuji, A. M.&Dasappa, S., VPSA process characterization for ISO quality green hydrogen generation using two practical multi-component biomass gasification feeds, Sep. Purif. Technol. 315, 123667 (2023). Li, Z., Zhang, W., Zhang, R.&Sun, H., Development of renewable energy multi-energy complementary hydrogen energy system (A Case Study in China): A review, Energy Explor. & Exploit. 38, 2099–2127 (2020). Alkhaledi, A. N., Sampath, S.&Pilidis, P., A hydrogen fuelled LH2 tanker ship design, Ships and Offshore Struct. 17, 1555–1564 (2022). 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J., Para- to Ortho-Hydrogen Conversion on Magnetic Surfaces, Phys. Rev. B 8, 266–279 (1973). Bar, A. K., Pichon, C.&Sutter, J.-P., Magnetic anisotropy in two- to eight-coordinated transition–metal complexes: Recent developments in molecular magnetism, Coord. Chem. Rev. 308, 346–380 (2016). Liu, J., Peng, C.&Shi, X., Preparation, characterization, and applications of Fe-based catalysts in advanced oxidation processes for organics removal: A review, Environ. Pollut. 293, 118565 (2022). Svadlenak, R. E.&Scott, A. B., The Conversion of Ortho- to Para hydrogen on Iron Oxide-Zinc Oxide Catalysts1, J. Am. Chem. Soc. 79, 5385–5388 (1957). Das, T., Kweon, S.-C., Choi, J.-G., Kim, S. Y.&Oh, I.-H., Spin conversion of hydrogen over LaFeO3/Al2O3 catalysts at low temperature: Synthesis, characterization and activity, Int. J. Hydrog. Energy 40, 383–391 (2015). Karlsson, E. 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J. 461, 141989; 10.1016/j.cej.2023.141989 (2023). Yu, H., et al., Dual-Pore Carbon Shells for Efficient Removal of Humic Acid from Water, Chem. A Eur. J. 23, 16249–16256 (2017). Athar, T., Hakeem, A., Topnani, N.&Hashmi, A., Wet synthesis of monodisperse cobalt oxide nanoparticles, Int. Sch. Res. Netw. 2012, 691032; 10.5402/2012/691032 (2012). Somnath, Ahmad, M.&Siddiqui, K. A., Synthesis of Mixed Ligand 3D Cobalt MOF: Smart Responsiveness towards Photocatalytic Dye Degradation in Environmental Contaminants, J. Mol. Struct. 1265, 133399; 10.1016/j.molstruc.2022.133399 (2022). Tan, P., Active phase, catalytic activity, and induction period of Fe/zeolite material in nonoxidative aromatization of methane, J. Catal. 338, 21–29 (2016). Kinebuchi, I.&Kyono, A., Study on magnetite oxidation using synchrotron X–ray diffraction and X–ray absorption spectroscopy: Vacancy ordering transition in maghemite (γ–Fe2O3), J. of Mineral. and Petrol. Sci. 116, 211–219 (2021). Lei, W., et al., Synthesis and magnetic properties of octahedral Fe3O4 via a one-pot hydrothermal route, Phys. Lett. A 381, 314–318 (2017). Sahadevan, J., et al., Magnetic property of Fe2O3 and Fe3O4 nanoparticle prepared by solvothermal process, Mater. Today: Proc. 58, 895–897 (2022). Duan, Q., Chen, H. J. I. C. S. M. S.&Engineering, Synthesis and electrochemical properties of Co3O4 nanoparticles by hydrothermal method at different temperatures, IOP Conf. Ser.: Mater. Sci. Eng. 207, 012020; 10.1088/1757-899X/207/1/012020 (2018). Wang, L., et al., Construction of bundle-like cobalt/nickel hydroxide nanorods from metal organic framework for high-performance supercapacitors, J. of Mater. Sci.: Mater. in Electron. 33, 10540–10550 (2022). Wang, J., Xie, T., Deng, Q., Wang, Y.&Liu, S., Three-dimensional interconnected Co(OH)2 nanosheets on Ti mesh as a highly sensitive electrochemical sensor for hydrazine detection, New J. Chem. 43, 3218–3225 (2019). Feyerherm, R., Loose, A., Rabu, P.&Drillon, M., Neutron diffraction studies of canted antiferromagnetic ordering in CoII hydroxide terephtalate, Solid State Sci. 5, 321–326 (2003). Zhao, Z., et al., A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials, Nat. Commun. 14, 958; 10.1038/s41467-023-36619-5 (2023). Chitu, L., et al., Structure and magnetic properties of CoFe2O4 and Fe3O4 nanoparticles, Mater. Sci. and Eng.: C 27, 1415–1417 (2007). S. Nantogma, B. Joalland, K. Wilkens, E. Y. Chekmenev, Clinical-Scale Production of Nearly Pure (> 98.5%) Parahydrogen and Quantification by Benchtop NMR Spectroscopy, Anal. Chem. 93 3594–3601 (2021). Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4625849","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":327671274,"identity":"65cc1aab-d738-4255-9bba-c08d2c1c5a14","order_by":0,"name":"Liujing Yang","email":"","orcid":"","institution":"China Jiliang University","correspondingAuthor":false,"prefix":"","firstName":"Liujing","middleName":"","lastName":"Yang","suffix":""},{"id":327671275,"identity":"361d7ca3-465c-433d-aab5-0bdcd1ea44b6","order_by":1,"name":"Xinbao Li","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA20lEQVRIie3RMQrCMBSA4SeBdGntWod6hkpAF/EsCQV3N8HBgqCbs4KHKAji+CRDlxzAsZOTmwfQ16LgFDIK5l8ayvuShgL4fD9YlwFgpwCIARTSi2Ztj39IrwDpSOA9ltG8IwnCgZ6dJ31RYa1DGKclsltt/zAu9d7kYmikJDIVJfJRZicMdbRm6nSFhmhVYsgTO+kURJbquGvJ04W0p9DmSUvQhdBdIlOJhO5yOWS52Gs+tJI4NuIRnRf9eGNUfZ9P0m21ulnJV6Gkv0NP5jhPBeg+6/P5fH/VC3NfR7bL3IsrAAAAAElFTkSuQmCC","orcid":"","institution":"China Jiliang University","correspondingAuthor":true,"prefix":"","firstName":"Xinbao","middleName":"","lastName":"Li","suffix":""},{"id":327671277,"identity":"96f55e08-cadc-4fef-8582-1fe038e7e777","order_by":2,"name":"Kai Sun","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Kai","middleName":"","lastName":"Sun","suffix":""},{"id":327671279,"identity":"ce7e5fb0-245d-43ad-a299-68b8e01bf63a","order_by":3,"name":"Xiaoling Zheng","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Xiaoling","middleName":"","lastName":"Zheng","suffix":""},{"id":327671281,"identity":"a465fde7-d866-4ece-8fe9-4ea49f33afd5","order_by":4,"name":"Ying Chen","email":"","orcid":"","institution":"Ningbo University","correspondingAuthor":false,"prefix":"","firstName":"Ying","middleName":"","lastName":"Chen","suffix":""}],"badges":[],"createdAt":"2024-06-23 15:41:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4625849/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4625849/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-71790-9","type":"published","date":"2024-09-09T15:58:29+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":60518287,"identity":"c68403f1-536c-48fd-9f09-19114796412b","added_by":"auto","created_at":"2024-07-17 15:56:25","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":428350,"visible":true,"origin":"","legend":"\u003cp\u003eXRD pattern of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.\u003c/p\u003e","description":"","filename":"floatimage1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/11971552b31d2232939dfabc.jpg"},{"id":60517027,"identity":"3e5b44fa-4e69-45e1-8618-19ce05d5913d","added_by":"auto","created_at":"2024-07-17 15:40:24","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":454416,"visible":true,"origin":"","legend":"\u003cp\u003e(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.\u003c/p\u003e","description":"","filename":"floatimage2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/1ea0fce4c9139665a1fb2722.jpg"},{"id":60517028,"identity":"80712830-920e-4d08-a9f9-47befc91f5ec","added_by":"auto","created_at":"2024-07-17 15:40:25","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":635954,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.\u003c/p\u003e","description":"","filename":"floatimage3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/d11b7380204b632da83812ba.jpg"},{"id":60517631,"identity":"d862288b-a50f-410b-967d-2af009cbeee2","added_by":"auto","created_at":"2024-07-17 15:48:25","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":595286,"visible":true,"origin":"","legend":"\u003cp\u003eXPS spectra of the (a) Fe 2p: FO and FCO-5 and (b) Co 2p: FCO-5 and Cob (oct, octahedral; tet, tetrahedral; sat, satellite).\u003c/p\u003e","description":"","filename":"floatimage4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/5eb42ce90b2202e5feb985bc.jpg"},{"id":60517033,"identity":"530d15eb-62ad-416f-88ff-c40fccf75ddb","added_by":"auto","created_at":"2024-07-17 15:40:25","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":551747,"visible":true,"origin":"","legend":"\u003cp\u003eHysteresis loops of FO, FCO-5 and Cob at 77 K.\u003c/p\u003e","description":"","filename":"floatimage5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/7c87ebd41efec13b68ddd16e.jpg"},{"id":60517633,"identity":"6d5125ef-48cb-4a93-8417-cf9e882eb82c","added_by":"auto","created_at":"2024-07-17 15:48:25","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":612546,"visible":true,"origin":"","legend":"\u003cp\u003e(A) O-P conversion and (B) content of p-H\u003csub\u003e2\u003c/sub\u003e in the outlet after catalytic reaction of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.\u003c/p\u003e","description":"","filename":"floatimage6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/fc5801d4de067add290df01e.jpg"},{"id":64619352,"identity":"782b5cea-685e-4f91-86dc-0fe4cd624696","added_by":"auto","created_at":"2024-09-16 16:14:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3885641,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/665d3ea5-88ad-45dd-9037-e98f4d683265.pdf"},{"id":60517030,"identity":"e50473f2-1077-4461-9256-d08b3125e1f5","added_by":"auto","created_at":"2024-07-17 15:40:25","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":29873,"visible":true,"origin":"","legend":"","description":"","filename":"supportinginformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4625849/v1/61e06fe5ab573d19c331e9ba.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhanced Catalytic Conversion of Ortho-Hydrogen to Para-Hydrogen by using Iron- Cobalt Bimetallic Catalysts","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHydrogen, with diverse sources and environmentally-friendly combustion characteristics\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e, is considered the most promising clean energy within the global decarbonization framework\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e。Considering the potential of hydrogen energy, numerous countries and organizations have introduced strategic plans for its advancement. Notably, the European Union\u0026rsquo;s \u0026ldquo;EU Hydrogen Strategy\u0026rdquo;\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e and the U.S. Department of Energy\u0026rsquo;s (DOE) \u0026ldquo;Hydrogen Shot\u0026rdquo; initiative introduced in 2021\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. The storage and transportation of hydrogen, pivotal elements in the hydrogen energy sector, present challenges to its advancement. Liquid hydrogen storage technology, with its high energy density, is particularly well-suited for long-distance transportation\u003csup\u003e[\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Furthermore, low-temperature liquid hydrogen storage provides enhanced safety measures compared to high-pressure storage\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. Nevertheless, the conversion of ortho-hydrogen (o-H\u003csub\u003e2\u003c/sub\u003e) to para-hydrogen (p-H\u003csub\u003e2\u003c/sub\u003e) during hydrogen liquefaction results in the release of heat due to their differing physical properties\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. Thus, for industrial purposes, the para-hydrogen (p-H\u003csub\u003e2\u003c/sub\u003e) content in liquid hydrogen storage should exceed 95%\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eHydrogen gas is categorized into ortho-hydrogen and para-hydrogen based on nuclear spin orientation\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Under equilibrium conditions, normal hydrogen (n-H\u003csub\u003e2\u003c/sub\u003e) at room temperature and above consists of 75% o- H\u003csub\u003e2\u003c/sub\u003e and 25% p-H\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. With decreasing temperature, o-H\u003csub\u003e2\u003c/sub\u003e undergoes a gradual conversion to the lower spin state of p-H\u003csub\u003e2\u003c/sub\u003e, with the process occurring slowly\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. The production and storage of liquid hydrogen require low-temperature conditions\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. The gradual conversion of o-H\u003csub\u003e2\u003c/sub\u003e to p-H\u003csub\u003e2\u003c/sub\u003e leads to losses in liquid hydrogen storage\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. Consequently, the utilization of catalysts is crucial for the efficient conversion of o-H\u003csub\u003e2\u003c/sub\u003e to p-H\u003csub\u003e2\u003c/sub\u003e, ensuring effective long-term liquid hydrogen storage.\u003c/p\u003e \u003cp\u003eWith the advancement of catalytic o-p hydrogen conversion in industrial settings, there is a growing number of proposed mechanisms for this process. Notably, paramagnetic centers are instrumental in facilitating rapid ortho-para conversion(o-p conversion). A prominent theory, suggested by Wigner in 1993, emphasizes the role of paramagnetic catalysis in molecular hydrogen o-p conversion\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. According to this theory, the o-p conversion rate correlates with paramagnetic materials and proton magnetic moments. Further, in 1973, Petzinger and Scalapino noted that augmenting the catalytic metal\u0026rsquo;s magnetic moment and decreasing the distance between reactants and H\u003csub\u003e2\u003c/sub\u003e surfaces dramatically boosts the o-p conversion rate\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e. Thus, these represent the two fundamental approaches in developing o-p conversion catalysts. Transition metals, favored for o-p conversion catalysts, possess unpaired electrons leading to heightened magnetic moments\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Iron, known for its abundant availability and cost-effectiveness, is extensively employed in the industrial synthesis of o-p conversion catalysts\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSvadlenak et al.\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e conducted a study on various iron-zinc compounds, encompassing γ-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, for o-p conversion at 78 K. The findings revealed that γ-Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e demonstrated the highest catalytic activity, with α- Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, α- Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-ZnO, and ZnFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e showing progressively lower catalytic activity. The catalysts exhibited a range of magnetic properties including ferromagnetic, antiferromagnetic, weak antiferromagnetic, and paramagnetic with subtle ferromagnetism. The study conclusively demonstrated that ferromagnetic arrays are superior to antiferromagnetic arrays in promoting hydrogen nuclear spin conversion. Das et al.\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e synthesized LaFeO\u003csub\u003e3\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e catalysts via the citrate sol-gel technique and observed that the catalyst attained the highest spin conversion rate post-activation through calcination at 773 K. At 17 K, a catalyst with La:Fe ratio of 2:8 (20La\u003csub\u003e0.2\u003c/sub\u003eFe\u003csub\u003e0.8\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e) achieved a remarkable 99.8% conversion of o-H\u003csub\u003e2\u003c/sub\u003e to p-H\u003csub\u003e2\u003c/sub\u003e within a 120 min duration. Karlsson\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e examined the commercially available porous particle catalyst, IONEX\u0026reg;, predominantly comprising iron oxide (Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e). The conversion process involves spin conversion on magnetically aligned surfaces, with the catalyst\u0026rsquo;s magnetism emanating from iron and possessing a magnetic moment of 5.92 Bohr magnetons. The results showed that achieving a 99.8% conversion of normal hydrogen into para-hydrogen at 16 K took merely 80 min.\u003c/p\u003e \u003cp\u003eIn addition, we discovered that Xu et al.\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e employed ammonia hydroxide (NH\u003csub\u003e4\u003c/sub\u003eOH) as a precipitant to prepare a range of FeCo bimetallic catalysts through the precipitation method. The catalyst with a Co/Fe ratio of 3:7 achieved 38.5% conversion of hydrogen at the outlet under a H\u003csub\u003e2\u003c/sub\u003e flow rate of 500 ml\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. However, the series of catalysts prepared by Xu et al. exhibited distinct patterns in the X-ray diffraction (XRD) characterization results compared to the catalysts prepared in this study using sodium hydroxide (NaOH) as the precipitant. Furthermore, Xu et al. proposed that the addition of Co increased the proportion of Fe\u003csup\u003e3+\u003c/sup\u003e rather than Fe\u003csup\u003e2+\u003c/sup\u003e, resulting in higher magnetization intensity and improved catalytic performance. Conversely, our study posits that the unique Fe\u003csup\u003e2+\u003c/sup\u003e and Fe\u003csup\u003e3+\u003c/sup\u003e site structures within the characterization results of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e are pivotal to the excellent catalytic effects of FeCo bimetallic catalysts, rather than solely the superior magnetic properties of Fe\u003csup\u003e3+\u003c/sup\u003e compared to Fe\u003csup\u003e2+\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003ePolyukhov et al.\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e showcased the viability of Metal-Organic Frameworks (MOFs) as catalysts for o-p hydrogen conversion, introducing efficient catalysts such as M-MOF-74 (M\u0026thinsp;=\u0026thinsp;Mn, Co, Cu, Ni, Zn). Among these catalysts, Ni-MOF-74 displayed an o-p conversion rate constant (k) of 26000 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003eg-\u003csup\u003e1\u003c/sup\u003e at 77 K, suitable for use at temperatures lower than 15 K. Despite the excellent performance of MOF catalysts, their complex and costly preparation process necessitates the proposal of a straightforward, cost-effective production method for these catalysts.\u003c/p\u003e \u003cp\u003eThis study synthesized a series of FCO catalysts doped with Co elements using a precipitation method. The performance of the FCO catalyst in converting ortho-para hydrogen at 77 K was evaluated by adjusting the Fe/(Fe\u0026thinsp;+\u0026thinsp;Co) molar ratio to identify the optimum FCO ratio. The influence of Co doping on Fe-based catalysts was analyzed through XRD, XPS, and VSM techniques. The impact of catalyst micro-porosity and surface area on catalytic performance was investigated using N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption tests. The successful combination of Fe and Co was confirmed using FTIR technology.\u003c/p\u003e"},{"header":"Experimental methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eMaterial synthesis\u003c/h2\u003e \u003cp\u003eAll reagents used are of analytical grade. Ferric nitrate nonahydrate (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO) and Cobalt(II) nitrate hexahydrate (Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) are the precursors for iron and cobalt respectively. Sodium hydroxide (NaOH) is used as the precipitating agent.\u003c/p\u003e \u003cp\u003eA series of Fe-Co catalysts were synthesized using precipitation method. (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO) was dissolved in deionized water and subjected to ultrasonic treatment for 10 min (Solution A). Similarly, (Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO) was dissolved in deionized water and subjected to ultrasonic treatment for 10 min (Solution B). Solution A was slowly poured into Solution B, resulting in Solution C. Afterwards, NaOH was dissolved in deionized water (Solution D) and dripped into Solution C using a peristaltic pump, while stirring. After complete dripping of Solution D, the mixture was further stirred for 30 minutes, followed by aging at room temperature for at least 6 hours. The precipitate was then filtered using deionized water. The obtained residues was dried overnight in a oven and crushed to a particle size of 40\u0026ndash;60 mesh. The molar ratios of n(Fe) / n(Fe\u0026thinsp;+\u0026thinsp;Co) were 0.9, 0.7, 0.5, 0.3 and 0.1, and they were named as FCO-9, FCO-7, FCO-5, FCO-3, and FCO-1, respectively. Pure phase samples were prepared according to the above process without Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and without Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO, and named FO and Cob, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eMaterials characterization\u003c/h2\u003e \u003cp\u003eStructural analysis of Fe-Co samples was conducted using X-ray diffraction (XRD, Smartlab 9 kW) with Cu Kα radiation. Nitrogen adsorption-desorption experiments at 77 K were performed using a Micromeritics ASAP 2460 analyzer. The surface area, total pore volume, and average pore size of the samples were calculated using the Brunauer-Emmett-Teller (BET) method, while the Barrett-Joyner-Halenda (BJH) method was employed for pore size distribution analysis. Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS20) was used for qualitative and quantitative analysis of functional groups and chemical bonds in the samples. X-ray photoelectron spectroscopy (XPS) analyses were performed using the Thermo K-alpha spectrometer, while magnetic measurements were carried out at a temperature of 77 K using the PPMS-9T dc magnetometer from American Quantum Design.\u003c/p\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003eCatalytic performance at ultra-low temperatures\u003c/h2\u003e \u003cp\u003eThe sample was tested using a fully automated o-p conversion and reduction device. Firstly, the sample (mass\u0026thinsp;=\u0026thinsp;1 g) was placed in a furnace for activation. The activation conditions were vacuum, with a temperature of 130 ℃ for 6 hours. After activation, the reaction vessel containing the sample was switched from the furnace to a liquid nitrogen (77 K) low-temperature bath. Normal hydrogen was deoxygenated and dehydrated before being introduced into the reaction vessel for the reaction. The normal hydrogen flow rate was adjusted using a mass flow meter, and the content of para-hydrogen after catalysis was measured using a gas chromatograph (GC-9790Ⅱ, Fuli Instruments) equipped with a 5A molecular sieve packed column and a thermal conductivity detector (TCD).\u003c/p\u003e \u003cp\u003eIn this work, measurements are taken every 4 minutes for 6 consecutive times when the hydrogen flow rate is stable and the data is smoothed, and the average value is calculated.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Results and discussions","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eXRD\u003c/h2\u003e \u003cp\u003eX-ray diffraction (XRD) was utilized for the compositional and crystal structure characterization of the samples, with results presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The analysis of Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e 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)\u003csub\u003e2\u003c/sub\u003e (JCPDS No.89-8616), demonstrating good crystallinity. The prominent diffraction peaks observed at 2θ\u0026thinsp;=\u0026thinsp;35.63\u0026deg; and 62.93\u0026deg; in the FO samples were attributed to Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (JCPDS No.39-1346), identifying it as the primary crystalline phase\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. Introduction of Co led to the emergence of new phases in the FCO series of samples. Unique diffraction peaks corresponding to CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (JCPDS No.03-0864) were detected in FCO-9, FCO-7 and FCO-5 (marked by asterisks). Furthermore, the characteristic diffraction peaks of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e vanished in the FCO-7 sample. As the Co content increased, characteristic diffraction peaks of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (JCPDS No.43-1003), Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (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)\u003csub\u003e2\u003c/sub\u003e with increasing Co doping. In FCO-1, the CoO(OH) phase displaced the coexistence of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, resulting in the formation of new crystal facets. Additionally, a novel peak (311) emerged at 2θ\u0026thinsp;=\u0026thinsp;35.45\u0026deg; in FCO-9. Incrementing Co content led to the appearance of new crystal facets (003), (220), (222), (400), (015), (511), and (113) at 2θ\u0026thinsp;=\u0026thinsp;20.24\u0026deg;, 31.27\u0026deg;, 38.55\u0026deg;, 44.81\u0026deg;, 50.58\u0026deg;, 59.35\u0026deg;, and 69.17\u0026deg; in FCO-5. Conversely, in FCO-1, new crystal facets (012), (104), and (110) appeared at 2θ\u0026thinsp;=\u0026thinsp;38.89\u0026deg;, 45.86\u0026deg;, and 65.34\u0026deg;, 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\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe grain sizes of Co species in the samples were calculated using the Scherrer formula, and the results are shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. 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 Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e decreased sequentially in FCO-5, FCO-3, and FCO-1.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eInitial composition and grain size of CoOOH and Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e for FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCatalyst\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFe/(Fe\u0026thinsp;+\u0026thinsp;Co) (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCoOOH (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCo\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFCO-9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFCO-7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFCO-5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e23.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFCO-3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e21.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFCO-1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e14.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e13.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCob\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e/\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"4\"\u003eNote: N.A. stands for \u0026ldquo;not available,\u0026rdquo; indicating that data cannot be provided due to very small grain size.\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eN\u003csub\u003e2\u003c/sub\u003e physical adsorption\u003c/h2\u003e \u003cp\u003eIn order to investigate the effect of Co doping and different Fe/Co ratios on the surface area and pore structure of the samples, N\u003csub\u003e2\u003c/sub\u003e adsorption and desorption tests were conducted. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA shows the N\u003csub\u003e2\u003c/sub\u003e 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 \u0026ldquo;ink-bottle\u0026rdquo; 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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB, the pore size distribution of FCO-9 is mainly concentrated between 1.7\u0026ndash;4.9 nm, with smaller pore sizes leading to the formation of \u0026ldquo;ink-bottle\u0026rdquo; pores at relative pressures between 0.4\u0026ndash;0.6\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. 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\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The results indicate that as the Co content increases, the N\u003csub\u003e2\u003c/sub\u003e 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/P\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026gt;\u0026thinsp;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.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB) was determined using the Barrett-Joyner-Halenda (BJH) method from the N\u003csub\u003e2\u003c/sub\u003e adsorption branch of the isotherms. The pores of FO and Cob are mainly composed of microspores (\u0026lt;\u0026thinsp;2 nm) and macrospores (\u0026gt;\u0026thinsp;50 nm), while the FCO series are mainly composed of mesoporous (2\u0026ndash;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.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSurface area, pore volume, pore size and lag coefficient of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCatalyst\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFO\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eFCO-9\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFCO-7\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eFCO-5\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFCO-3\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eFCO-1\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eCob\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBET Surface Area (m\u003csup\u003e2\u003c/sup\u003e\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e256.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e267.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e242.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e137.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e129.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e89.98\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e180.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePore Volume (cm\u003csup\u003e3\u003c/sup\u003e\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePore Size (nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.49\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.67\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e7.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e11.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e6.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLag Coefficient\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.005\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.025\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.075\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.070\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.064\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.015\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eFTIR Spectral Analysis\u003c/h2\u003e \u003cp\u003eThe FTIR spectrum of the sample in the range of 500\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The stretching mode at 3422 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and a weak asymmetric peak at 1620 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are characteristic of the O-H stretching vibration, due to the absorption of water molecules during the sample preparation process\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. The peak at 1380 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the Fe-O stretching vibration. The presence of strong Co-O stretching and bending modes at 663 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 570 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates the formation of the Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e phase, with a high degree of phase purity in the cubic structure\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. This suggests that FCO-5, FCO-3, and FCO-1 have good stability and the Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e produced by them has uniform particle size, consistent with the previous BET analysis results.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eXPS\u003c/h2\u003e \u003cp\u003eIn 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\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea displays the Fe 2p spectra of FO and FCO-5. The main signals of Fe 2p\u003csub\u003e3/2\u003c/sub\u003e and Fe 2p\u003csub\u003e1/2\u003c/sub\u003e 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 Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e in FCO-5\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. The Fe 2p\u003csub\u003e3/2\u003c/sub\u003e 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 Fe\u003csup\u003e3+\u003c/sup\u003e and tetrahedral Fe\u003csup\u003e3+\u003c/sup\u003e, respectively, while the peak at 709.6 eV is assigned to Fe\u003csup\u003e2+\u003c/sup\u003e. Furthermore, the peaks at 717.7 eV and 732.4 eV are attributed to the vibrational satellite of Fe\u003csup\u003e3+\u003c/sup\u003e. Notably, FCO-5 exhibits a heightened satellite peak of Fe\u003csup\u003e2+\u003c/sup\u003e at 715.6 eV compared to FO, indicating the presence of Fe\u003csup\u003e2+\u003c/sup\u003e in FCO-5 and supporting the existence of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, in line with XRD findings. The inclusion of Co enhances the formation of Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e is characterized by the corundum structure, with octahedral voids occupied by Fe\u003csup\u003e3+\u003c/sup\u003e ions\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. In contrast, Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e adopts the inverse spinel structure, with Fe\u003csup\u003e2+\u003c/sup\u003e in octahedral sites and two Fe\u003csup\u003e3+\u003c/sup\u003e ions in tetrahedral and octahedral sites, respectively\u003csup\u003e[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]\u003c/sup\u003e, facilitating the arrangement of magnetic materials and resulting in increased magnetism\u003csup\u003e[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/sup\u003e, and is more conducive to the catalytic conversion of o-H\u003csub\u003e2\u003c/sub\u003e to p-H\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb 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 2p\u003csub\u003e3/2\u003c/sub\u003e and Co 2p\u003csub\u003e1/2\u003c/sub\u003e, respectively, with satellite peaks at 786.9 eV and 802.7 eV, suggesting the existence of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]\u003c/sup\u003e. The fitted peaks at 779.0 eV and 794.0 eV are attributed to Co\u003csup\u003e3+\u003c/sup\u003e, while those at 780.9 eV and 795.7 eV are assigned to Co\u003csup\u003e2+\u003c/sup\u003e. In the spectrum of Cob, peaks are identified at 780.1 eV and 796.1 eV for Co 2p\u003csub\u003e3/2\u003c/sub\u003e and Co 2p\u003csub\u003e1/2\u003c/sub\u003e, respectively, with an orbital separation of 16 eV, confirming the presence of Co\u003csup\u003e2+[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]\u003c/sup\u003e. Furthermore, the satellite peaks at 785.6 eV and 802.1 eV support the presence of the Co(OH)\u003csub\u003e2\u003c/sub\u003e phase\u003csup\u003e[\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]\u003c/sup\u003e, in agreement with XRD findings.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eVSM\u003c/h2\u003e \u003cp\u003eThe magnetic properties of FO, FCO-5, and Cob were analyzed at 77 K by varying the magnetic field, disclosing distinct magnetic characteristics (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). FO exhibits negligible coercivity and remanence values, displaying an S-shaped hysteresis loop indicative of superparamagnetic traits\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e. 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\u0026middot;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\u0026thinsp;+\u0026thinsp;ions possess a 3d7 electron configuration with a spin value of 3/2. At reduced temperatures, the Co2\u0026thinsp;+\u0026thinsp;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\u003csup\u003e[\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]\u003c/sup\u003e. FCO-5, conversely, portrays a symmetric hysteresis loop with a coercivity of 538.08 Oe and a remanence value of 7.29 emu\u0026middot;g-1, placing it within the category of hard magnetic materials due to the inclusion of CoFe2O4\u003csup\u003e[\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]\u003c/sup\u003e. The introduction of Co atoms within the Fe-O matrix of CoFe2O4 enhances the material\u0026rsquo;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\u0026thinsp;+\u0026thinsp;and Fe3\u0026thinsp;+\u0026thinsp;ions, resulting in a substantial net magnetic moment\u003csup\u003e[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eCatalyst Activity Testing\u003c/h2\u003e \u003cp\u003eThe catalyst samples were tested for activity at 77 K, and the results are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. 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-H\u003csub\u003e2\u003c/sub\u003e 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-H\u003csub\u003e2\u003c/sub\u003e after conversion remained below 40%. Under conditions of GHSV (gaseous hourly space velocity) at 1800 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the normal para-hydrogen conversion rates of FO and FCO-1 were similar, both around 90%. But the content of outlet p-H\u003csub\u003e2\u003c/sub\u003e 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\u0026thinsp;=\u0026thinsp;5400 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the conversion rate of FO was close to FCO-9, reaching 95%, but the outlet p-H\u003csub\u003e2\u003c/sub\u003e 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-H\u003csub\u003e2\u003c/sub\u003e content levels above 49.6%, approaching the equilibrium concentration of p-H\u003csub\u003e2\u003c/sub\u003e (~\u0026thinsp;50%) at 77 K\u003csup\u003e[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the O-P conversion rate constants (\u003cem\u003eK\u003c/em\u003e) of the sample catalysts. The highest O-P conversion rate constant was observed in FCO-5, GHSV\u0026thinsp;=\u0026thinsp;5400 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, reaching 291.7 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. 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\u0026thinsp;=\u0026thinsp;3600 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, reaching 280 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The O-P conversion rate constant of Cob was much lower than that of the Fe-containing catalyst.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eThe reaction rate constant K (mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) of FO, FCO-9, FCO-7, FCO-5, FCO-3, FCO-1 and Cob at 77 K.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGHSV (h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eK(FO)\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eK\u003c/em\u003e (FCO-9)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eK\u003c/em\u003e (FCO-7)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eK\u003c/em\u003e (FCO-5)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eK\u003c/em\u003e (FCO-3)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003e\u003cem\u003eK\u003c/em\u003e (FCO-1)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u003cem\u003eK(Cob)\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1800\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e134.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e97.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e98.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e81.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e52.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e13.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3600\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e31.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e280.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e235.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e211.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e235.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e115.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e29.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5400\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e86.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e180.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e255.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e291.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e253.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e107.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e15.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eBased 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\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 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\u0026thinsp;+\u0026thinsp;Co)\u0026thinsp;=\u0026thinsp;0.5.\u003c/p\u003e \u003cp\u003eThe 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-H\u003csub\u003e2\u003c/sub\u003e 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 CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e 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 CoFe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, and Fe\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e components, has an ion arrangement that is more favorable for the generation of large magnetic moments compared to Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Co(OH)\u003csub\u003e2\u003c/sub\u003e, thereby promoting the rapid o-p conversion.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn summary, the doped FCO catalyst series with Co elements were successfully prepared using a co-precipitation method. The study investigated the effect of the amount of Co element doping on the hydrogen conversion. The results showed that Co doping altered the pore structure of FCO, reducing the catalyst\u0026rsquo;s specific surface area, increasing the lag coefficient, and thus increasing the contact time between the catalyst and n-H\u003csub\u003e2\u003c/sub\u003e. FCO-5 possesses a large specific surface area and the highest lag coefficient, making it the optimal catalyst for hydrogen conversion. Additionally, the incorporation of Co into FCO brings forth an increased number of active sites and larger magnetic moments, which facilitate the o-p conversion process. At a GHSV of 5400 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the reaction rate constant for FCO-5 can reach 291.7 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the p-H\u003csub\u003e2\u003c/sub\u003e content can reach 49.7%. The preparation process of the FCO catalyst series is straightforward, the raw materials are inexpensive, and it has potential for industrial production and application.\u003c/p\u003e "},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eDeclaration of competing interest\u003c/h2\u003e \u003cp\u003eNo potential conflict of interest was reported by the author(s).\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization:L.Y.,X.L.,X.Z.,methodology:K.S.,Y.C.,X.L.,formal analysis:L.Y.,X.Z.,synthesis:L.Y.,K.S.,X.Z.,writing\u0026mdash;original draft preparation:X.Z.,L.Y.,writing\u0026mdash;review and editing:L.Y.,X.L. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors acknowledge financial support from the National Natural Science Foundation of China (No. 51976094).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eOur analyzed datasets are available from the corresponding author on reasonable request\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSingh, A., Shivapuji, A. M.\u0026amp;Dasappa, S., VPSA process characterization for ISO quality green hydrogen generation using two practical multi-component biomass gasification feeds, Sep. Purif. 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A variety of iron-cobalt catalysts were synthesized using a precipitation technique, incorporating diverse levels of Co doping into Fe-based catalysts. The effects of Co doping on the crystal structure, porosity, and magnetism of FCO were examined through XRD, N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption, FTIR, XPS, and VSM analyses. The efficacy of ortho-para hydrogen conversion within FCO at 77 K was analyzed using chromatography. Findings revealed that Co doping enhances the material\u0026rsquo;s lag coefficient, leading to an increase in active sites and larger magnetic moments. Notably, FCO-5 [n(Fe)/n(Fe\u0026thinsp;+\u0026thinsp;Co)\u0026thinsp;=\u0026thinsp;0.5] exhibited the most efficient ortho-para hydrogen conversion performance. Specifically, at GHSV\u0026thinsp;=\u0026thinsp;5400 h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, FCO-5 achieved a reaction rate constant of 291.7 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, a conversion rate of 99.24%, and a post-conversion p-H\u003csub\u003e2\u003c/sub\u003e content of 49.7%.\u003c/p\u003e","manuscriptTitle":"Enhanced Catalytic Conversion of Ortho-Hydrogen to Para-Hydrogen by using Iron- Cobalt Bimetallic Catalysts","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-17 15:40:20","doi":"10.21203/rs.3.rs-4625849/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-07-23T12:28:41+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-22T04:22:32+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-06T16:17:13+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-05T18:44:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"175728943897922908144642732267958191669","date":"2024-07-01T13:33:29+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"97980771044140685747049252031395292838","date":"2024-07-01T11:58:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"145502969193034960558430284884454511520","date":"2024-06-30T23:49:12+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-30T23:45:22+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-06-30T22:31:13+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-06-27T19:28:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-06-26T06:00:39+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-06-23T15:40:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"87e5f252-27d4-4c8f-8fca-f691f1335279","owner":[],"postedDate":"July 17th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":34679957,"name":"Physical sciences/Chemistry"},{"id":34679958,"name":"Physical sciences/Energy science and technology"}],"tags":[],"updatedAt":"2024-09-16T16:06:25+00:00","versionOfRecord":{"articleIdentity":"rs-4625849","link":"https://doi.org/10.1038/s41598-024-71790-9","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-09-09 15:58:29","publishedOnDateReadable":"September 9th, 2024"},"versionCreatedAt":"2024-07-17 15:40:20","video":"","vorDoi":"10.1038/s41598-024-71790-9","vorDoiUrl":"https://doi.org/10.1038/s41598-024-71790-9","workflowStages":[]},"version":"v1","identity":"rs-4625849","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4625849","identity":"rs-4625849","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}