One-pot synthesis of supported sub-micron LaNi₅ for hydrogen storage in a carbazole-type liquid organic hydrogen carrier

Abstract

Metal hydrides are promising catalysts in hydrogen-involving reactions. However, downsizing and loading metal hydrides is difficult due to their sensitivity towards oxygen and water. Here, a simple one-pot molten salt synthetical method is proposed to synthesize porous La(OH)₃-supported LaNi₅. The as-synthesized catalyst shows an evident improvement in catalyzing reversible hydrogen storage in liquid organic hydrogen carrier N-ethylcarbazole.

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Metal hydrides (MHs) have long been utilized as efficient hydrogen storage materials due to their unique hydrogen absorption–desorption features.^1–4 The multiple H-bonding sites and surface H-species of MHs render them promising hydrogenation and dehydrogenation catalysts, especially in reactions such as olefin hydrogenation, CO₂ hydrogenation, NH₃ synthesis, and H₂ storage in liquid organic hydrogen carriers (LOHCs).^5,6 LOHCs are newly-emerging liquid-state hydrogen storage materials that utilize the hydrogenation and dehydrogenation of aromatic and N-heterocycle rings to store H₂, which is considered a possible solution to large-scale, long-distance H₂ transportation.^7,8 The hydrogenation and dehydrogenation of LOHCs generally occur under 200 °C with no oxygen-containing species involved, making them an appropriate model system to investigate MH-type catalysts. Moreover, studying MH-type catalysts for LOHC is beneficial for developing MH/LOHC hybrid H₂ storage systems adaptable to diverse application scenarios in the future.

Based on this concept, Chen et al. investigated LaNi₅, MlNi₅, and Mg–Ni alloy as catalysts for the hydrogenation of benzene and toluene.^9–11 An et al. utilized NH₄F-treated LaNi₅ as a hydrogenation catalyst for N-ethylcarbazole (NEC).¹² Recently, our group has developed a series of YH₃-based materials for reversible H₂ storage in NEC.^13–16 Some of the MH-based catalysts show interesting dual functional catalytical activity in both the hydrogenation of NEC and the dehydrogenation of its hydrogen-rich counterpart 12H-NEC due to the fluctuating H-content in the solid solution region of MH catalysts.^15,16 Nevertheless, current MH-type catalysts suffer from two intrinsic drawbacks: (1) MH alone usually exhibits low or negligible catalytic activity and requires coupling with transition metals (TM). (2) Most MHs are sensitive to O₂ and H₂O, which hinders their activity and stability enhancement owing to limited downsizing and loading methods. The coupling methods of MH and TM are often confined to mechanical approaches like simple mixing and ball milling, making it difficult to precisely construct the interface.

We have partially solved these problems by bottom-up synthesis of sub-μm TM-containing LaNi₅-based catalysts.^17–19 However, considering the high reducing temperature (600–800 °C) and strong-reductive environment (CaH₂ as reducing reagent) during the synthetical process, further activity enhancements via dispersing sub-μm LaNi₅ on supports with a large surface area is demanding. Conventional loading methods like incipient wetness, sol–gel, and ion exchange²⁰ are not suitable for synthesizing supported-LaNi₅, as common oxide support Al₂O₃, TiO₂ will be simultaneously reduced, forming other unintended byproducts like Ni–Al and Ni–Ti alloys during the synthetical process. Although there are several reports on the synthesis of supported MgH₂ nanoparticles that utilize the decomposition of organometallic precursors,^21–23 it is hard to apply it to LaNi₅ alloy due to the highly negative reduction potential of La³⁺/La²⁺ (−2.38 eV) and the lack of corresponding organic compounds with the exact composition.

Here in the current paper, we propose a novel one-pot method to synthesize sub-μm LaNi₅ supported on porous metal hydroxide La(OH)₃. Under the catalysis of the as-synthesized LaNi₅/La(OH)₃, NEC exhibits a hydrogenation rate of 0.028 mol h⁻¹ g⁻¹ (180 °C, 7 MPa H₂) and dehydrogenation rate of 0.019 mol h⁻¹ g⁻¹ (200 °C, 0.1 MPa H₂), with a final H₂ uptake and release amount of 5.75 wt%. Loading of sub-μm LaNi₅ on La(OH)₃ significantly increases its catalytic activity and selectivity towards fully hydrogenated and dehydrogenated products as a result of the synergetic effect between LaNi₅ and La(OH)₃, together with the etching of supports by molten salt to create porous La(OH)₃ with a relatively large surface area.

LaNi₅/La(OH)₃ is synthesized via a one-pot high-temperature molten salt reduction method with excessive La in the precursor. In a typical experiment, 12.0 mmol La(NO₃)₃·6H₂O and 5.00 mmol Ni(NO₃)₂·6H₂O were dissolved in 15.0 mL deionized water (20 wt% LaNi₅ loading amount), then Ni²⁺ and La³⁺ ions were co-precipitated by 3.33 mol L⁻¹ NaOH aqueous solution, the precipitates were dried at 60 °C overnight and calcined at 600 °C for 3 hours to convert into a black La–Ni–O precursor powder. 1.20 g of precursor was mixed with 2.00 g KCl and 0.40 g CaH₂, then heated at 700 °C for 2 h under Ar protection (flow rate 70 sccm). The calcined mixture was washed with 0.100 mol L⁻¹ acetic acid under Ar protection, and the final products were obtained after centrifugation, washing, and vacuum drying.

The ratio of precursor, molten salt, and reducing agent is vital in synthesizing LaNi₅/La(OH)₃. As shown in Fig. 1a, when the input of CaH₂ is only 0.05 g, no reducing products are observed, as the CaH₂ input increases to 0.25 g, Ni is generated, further increasing the CaH₂ input to 0.40 g, La-precursor is reduced and alloyed with existing Ni to form LaNi₅ alloy with some of the Ni phase preserved. A byproduct that cannot be ignored is LaOCl, which has a matlockite-type tetragonal structure of alternating (LaO)_nⁿ⁺ cation and Cl⁻ anion layers aligning along the crystallographic c direction.^24,25 In the comparative experiment, catalysts with a high LaOCl content demonstrate poor catalytic activity, especially in 12H-NEC dehydrogenation (Fig. S1, ESI†). To minimize the generation of LaOCl, the input of KCl is adjusted, according to XRD in Fig. 1b, too much or too little KCl can lead to significant formation of LaOCl, and the optimized KCl input is 2.00 g. The formation of LaOCl is inevitable as in the Cl⁻-rich environment, the following reaction reaches the equilibrium:

La₂O₃ + 2Cl⁻ → 2LaOCl + O²⁻ (1)

Fig. 1 XRD of the reduction products of 1.20 g La–Ni–O precursor (La : Ni = 12 : 5) in (a) 2.00 g KCl with varying amounts of CaH₂, and (b) 0.50 g CaH₂ with varying amounts of KCl.

In reaction (1), the generated LaOCl dissolved in molten salt KCl. KCl serves both as a reactant and solvent. Decreasing solvent KCl leads to the precipitation of LaOCl, and increasing reactant KCl leads to an increase of Cl⁻ concentration, both shifting the equilibrium in reaction (1) towards LaOCl formation. Under the optimized feeding ratio of 1.2 : 2.0 : 0.4 (precursor : KCl : CaH₂), the product shows the lowest amount of about 3% LaOCl by semi-quantitative XRD analysis.

As demonstrated in Fig. 2a, the obtained LaNi₅ alloy exhibits particle sizes at the magnitude of tens nanometers with a short column morphology, which is slightly smaller than unsupported sub-μm LaNi₅¹⁷ due to the dispersive effect of excessive La(OH)₃. HRTEM shown in Fig. 2c and d confirms the core–shell structure of sub-μm LaNi₅. The resolved interplanar spacings of 0.318 nm and 0.264 nm outside sub-μm LaNi₅ are designated to the (011) and (012) planes of La(OH)₃ and LaOCl, respectively (Fig. 2e). Fig. 2b depicts the deconvoluted XPS results of the as-synthesized catalysts, and the spectrum is complicated due to the overlap of the La 3d_3/2 and Ni 2p_3/2 orbitals. The La 3d_5/2 peaks at 834.3 eV and 837.8 eV represent La(III) and its satellite peaks, and the binding energy difference between the two peaks is 3.5 eV, indicating that surface La mainly exists as La(OH)₃.^26,27 The intensity of Ni 2p_1/2 is too feeble to resolve the exact forms of Ni on the surface.

Fig. 2 (a) SEM, (b) deconvoluted XPS results of Ni 2p and La 3d orbital, and (c)–(e) HRTEM of the as-synthesized LaNi₅/La(OH)₃ (the core–shell-structure is highlighted).

The comparison sample Ni/La₂O₃ (Ni loading 20 wt%) was synthesized by the same co-precipitation-reduction method. The Ni/La₂O₃ catalyst was reduced directly from La–Ni–O precursor with H₂/Ar (20 sccm: 50 sccm) in a tube furnace. XRD and SEM of the La–Ni–O precursor (Fig. 3b and c) confirms that it consists of La(OH)₃ and La₃NiO₈ with an aggregated ball-like morphology. Reduction in H₂/Ar doesn’t change the overall morphology, which consists of Ni particles with a diameter of several nanometers attached on the surface of the La(OH)₃ support (Fig. 3b and d). On the other hand, reduction by KCl and CaH₂ breaks down the aggregated La(OH)₃ into smaller particles (Fig. 2a). The etching effect of molten-mixture KCl–CaH₂ is more prominent by comparing the surface area of the as-synthesized catalysts, as it increases from 11.4 m² g⁻¹ for Ni/La₂O₃ to 93.9 m² g⁻¹ for LaNi₅/La(OH)₃ (Fig. S2, ESI†). La and Ni disperse evenly on the supports in both catalysts as shown in EDS mapping (Fig. S3 and S4, ESI†). The schematic structures of the precursor and the two catalysts are illustrated in Fig. 3a.

Fig. 3 (a) Schematic structure and (b) XRD of the La–Ni–O precursor and Ni/La₂O₃. SEM of (c) the La–Ni–O precursor and (d) Ni/La₂O₃.

The catalytical performances of Ni/La₂O₃, sub-μm LaNi_5.5, and LaNi₅/La(OH)₃ for NEC hydrogenation and 12H-NEC dehydrogenation are summarized in Fig. 4a and b. The reaction rate is calculated based on the converted amount of substances (1 mol) in unit time (1 h) on the unit mass of the catalyst (1 g). As we use consumed and produced H₂ to draw hydrogenation and dehydrogenation kinetics, the accurate consumption of reactants isn’t recorded; here, the multi-step reaction is simplified to a single-step one. The conversion of the reactant is calculated based on H₂ consumption/production, taking the NEC hydrogenation reaction as an example:

m_NEC represents the mass of the reactant, M_NEC represents the relative molecular mass of NEC, x₁ represents the actual H₂ uptake amount (wt%) in the first hour of reaction, x₂ represents theoretical capacity 5.8 wt%, t₁ is 1 h, and m_catalyst is the mass of the catalyst.

Fig. 4 (a) Hydrogen uptake and (b) release kinetics of NEC/12H-NEC catalyzed by Ni/La₂O₃, unsupported sub-μm LaNi_5.5, and LaNi₅/La(OH)₃. Hydrogenation and dehydrogenation reactions are carried out at 180 °C, 7 MPa H₂, and 200 °C, 0.1 MPa H₂, respectively, and the catalyst loading is 10 wt%. (c) and (d) Hydrogenation and dehydrogenation performances of this work compared with dual-functional catalysts for reversible H₂ storage of NEC reported in the literature.

In NEC hydrogenation, LaNi₅/La(OH)₃ displays the highest catalytical activity of 0.028 mol h⁻¹ g⁻¹, compared to 0.0096 mol h⁻¹ g⁻¹ of the unsupported sub-μm LaNi_5.5 and 0.015 mol h⁻¹ g⁻¹ of Ni/La₂O₃. In 12H-NEC dehydrogenation, LaNi₅/La(OH)₃ shows the highest catalytical activity of 0.019 mol h⁻¹ g⁻¹, which equals that of unsupported sub-μm LaNi_5.5 and is higher than that of Ni/La₂O₃. LaNi₅/La(OH)₃ also demonstrates the highest selectivity towards the fully hydrogenated/dehydrogenated product; the 12H-NEC selectivity in NEC hydrogenation is 96.5%, and the NEC selectivity in 12H-NEC dehydrogenation is 100%, yielding 5.75 wt% reversible H₂ storage capacity in total. The NEC hydrogenation and 12H-NEC dehydrogenation rate in this work and those of other dual-functional catalysts reported in the literature^{15–19,28–30} are compared in Fig. 4c and d.

LaNi₅/La(OH)₃ realizes superior NEC hydrogenation and equivalent 12H-NEC dehydrogenation catalytical performances compared to unsupported sub-μm LaNi_5.5, proving the advantages of dispersing the catalysts on La(OH)₃ supports. As alkaline oxides, La₂O₃ and La(OH)₃ are not ideal supports for NEC (de)hydrogenation,³¹ but the loading process reduces the alloy particle size and increases the surface area, which is especially beneficial for enhancing catalytical activity in NEC hydrogenation. In the literature, Ni-based catalysts with large surface areas demonstrate considerable NEC hydrogenation activity but seldom serve as efficient 12H-NEC dehydrogenation catalysts.³² The same phenomenon is observed in the current work, as under the catalysis of Ni/La₂O₃, only 4.14 wt% and H₂ are released after reacting for 10 hours. The outstanding catalytic dehydrogenation performance of LaNi₅/La(OH)₃ comes from LaNi₅ alloy. Under the dehydrogenation conditions (200 °C, 0.1 MPa H₂), the abundant H-bonding sites in the LaNi₅ lattice offer a low-energy-barrier H-transfer pathway to facilitate the dehydrogenation reaction.¹⁹ The equilibrium metal-to-hydride transition pressure calculated by the CALPHAD method of LaNi₅ at 180 °C and 200 °C is 10.3 MPa and 14.7 MPa, which means that LaNi₅ utilizes the hydrogen-lean solid solution region in both NEC hydrogenation and 12H-NEC dehydrogenation, explaining the more evident improvement of LaNi₅/La(OH)₃ in 12H-NEC dehydrogenation compared to La₂O₃-supported Ni-based catalysts. However, the cycling stability of LaNi₅/La(OH)₃ is unsatisfactory, and the LaNi₅ alloy phase completely decomposes into Ni after one H₂ storage cycle (Fig. S5, ESI†). Due to the intensified H₂-induced amorphization, weak metal-support interaction, and unstable metal-support interface, LaNi₅/La(OH)₃ is even more unstable than unsupported sub-μm LaNi_5.5.¹⁷ It is necessary to further investigate support surface modification and post-synthetical methods to increase its thermal stability in the future.

In conclusion, supported-MH catalyst LaNi₅/La(OH)₃ is synthesized via a one-pot high-temperature molten salt reduction method with excessive La in the precursor. The etching effect of the molten mixture and the heat generated during LaNi₅ alloy formation greatly increased the surface area of La(OH)₃. The existence of the La(OH)₃ support helps decrease the particle size of LaNi₅ alloy at the magnitude of tens of nanometers. The NEC hydrogenation rate is boosted from 0.0096 mol h⁻¹ g⁻¹ of unsupported sub-μm LaNi_5.5 to 0.028 mol h⁻¹ g⁻¹ of LaNi₅/La(OH)₃, and the 12H-NEC dehydrogenation rate remains at 0.019 mol h⁻¹ g⁻¹. La(OH)₃ with an elevated surface area is an appropriate support of Ni-based catalysts for reversible H₂ storage in NEC. The one-pot molten salt reduction method is a direct and effective way of synthesizing supported-MH catalysts, which is beneficial for future industrial applications.

This work was supported by NSFC (No. 22409094 and No. 22179002), the startup foundation for introducing talent of NUIST, MOST of China (No. 2021YFB4000603 and 2021YFB4000601), and the PetroChina-Peking University strategic joint fundamental research project.

Data availability

The data supporting this article have been included in the paper.

Conflicts of interest

The authors declare no conflicts of interest.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc05419a

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