Metal-organic frameworks (MOFs) provide highly selective catalytic activity due to their porous crystalline structure. There is particular interest in metal nanoparticle-MOF composites (M NP@MOF) that could take advantage of synergistic effects for enhanced catalytic properties. We present an investigation into the local structure and electronic properties of Mg NP@Mg-MOF-74, which is composed of Mg nanoparticles and Mg-MOF-74. A theoretical study on the adsorption of multiple Mg2–Mg10 clusters at one pore in a 1 × 1 × 2 Mg-MOF-74 supercell is conducted, clearly showing that the small clusters tend to aggregate together when stabilized by bonds between Mg and O in the MOF. Considering the size and shape of the pore in the MOF, HCP-Mg nanoparticles with 60 Mg atoms are embedded in one pore of 1 × 1 × 2 Mg-MOF-74 to form nanowires. Results show that the mixture Mg NP@Mg-MOF-74 exhibits a better hydrogen adsorption performance than the isolated Mg nanoparticle, with a considerable estimated theoretical hydrogen storage capacity of 3.98 wt. %. The corresponding electronic structure analysis reveals that the accumulation of charges on H in the hybrid system is clearly enhanced with respect to the isolated Mg nanoparticles.

Hydrogen storage is critical in fuel cell vehicles for the automotive industry.1–7 Hydrogen storage by metal hydrides has been the focus of recent intensive research.8,9 Among them, MgH2 has a theoretical storage capacity of 7.6 wt. % of hydrogen, demonstrating high reversible hydrogen absorption and desorption capabilities. In addition, magnesium is also attractive for thermal energy storage due to its high enthalpy of hydride formation (ΔH = −75 kJ/mol). Combined with other features, such as low cost and abundant availability, magnesium has been widely studied for hydrogen storage.10,11 To date, however, magnesium hydride has been providing limited application for hydrogen storage. This is mainly due to the slow kinetics in its hydrogenation and dehydrogenation reaction and the relatively high temperature (about 300 °C) of hydrogen desorption.11–13 Many efforts, such as improving the kinetic properties by ball milling,14 alloying with other transition metals and their oxides,15–17 using catalysts,18,19 and forming thin film hydrides,20,21 have focused on reducing the desorption temperature and accelerating the reaction. The size confinement approach provides a feasible treatment for magnesium-based hydrogen storage materials. Actually, by converting magnesium-based hydrogen storage materials into nanopores or nanotubes, the interaction between Mg and the pores or tubes profits from the adsorption of hydrogen, in addition to size confinement.

Meanwhile, porous metal-organic frameworks (MOFs) have been considered as an effective material for hydrogen storage22–26 because of their high specific surface area and their ability to physically adsorb large amounts of hydrogen (>7 wt. %) at low temperatures (77 K) with high pressures.27,28 However, the hydrogen storage capacity of MOFs decreases drastically to less than 1 wt. % at room temperature when the interaction energy between hydrogen and MOFs is very low (4–8 kJ/mol). To improve the efficiency of hydrogen storage in MOF at room temperature, approaches such as adjusting ligands,29 generating open metal sites,30 and embedding metal nanoparticles31–35 have been employed.

To obtain a highly efficient material for hydrogen storage, hybrid materials that store hydrogen by both physical adsorption and chemisorption are produced. It is interesting to combine physisorption and chemisorption in MOF systems to enhance hydrogen storage properties.31–35 The combination of Mg nanoparticles and MOFs for hydrogen storage has already been reported. Mg NCs@MOF prepared by Lim et al.35 is a hybrid hydrogen storage material that stores H2 by both physical adsorption and chemisorption, exhibiting a synergistic effect to increase the isosteric heat of H2 physisorption and reduce the temperatures for chemisorption/desorption of H2. Besides, Wang et al. prepared Mg/MOF nanocomposites as composites of Mg and MOFs (ZIF-8, ZIF-67, and Mg-MOF-74) using a deposition-reduction method.19 The addition of MOFs can enhance the hydrogen storage properties of Mg. For example, Mg/Mg-MOF-74 can absorb 2.0 wt. % of hydrogen within 3966 s, which is 1209 s quicker than pure Mg’s absorbance under 175 °C. The combination of Mg and Mg-MOF-74 in experiments does show a better hydrogen storage performance.

Mg-MOF-74 has a Brunauer–Emmett–Teller surface area of 1206 m2g−1, which is the highest in comparison with other types of MOF-74.36 Its crystal structure pore size is 10.8 Å.37 Mg-MOF-74 consists of an unsaturated metal oxide cluster [Mg2O2(CO2)2] joined to a 2,5-dioxido-1,4-benzenedicarboxylate (DOBDC) linker to form a periodic framework. The hydrogen uptake of Mg-MOF-74 at 77 K and 1 bar is slightly enhanced to 1.8 wt. % compared with Zn-MOF-74 under these conditions.38 Here, we find small Mg clusters in one pore of Mg-MOF-74, which tend to cluster together instead of uniform distribution. Considering the size and volume of the pore in the 1 × 1 × 2 Mg-MOF-74 supercell, nanoparticles with a size of 60 Mg/cell are found to be stable when filled within the pore. Consequently, a flower-like Mg nanowire embedded in Mg-MOF-74 is employed to simulate hydrogen absorption, which uses Mg nanowire mainly due to a significant charge redistribution.

The first-principles calculations are conducted with the generalized gradient approximation (GGA) of PBE39 as implemented in the VASP code.40,41 The electron–ion interaction was described by projector-augmented wave potential.42 The DFT-D3 method by Grimme with a zero-damping function has been employed in the calculation of hydrogen adsorption. The unit cell of Mg-MOF-74 is trigonal with the space group R3̄, which has a six-fold rotational symmetry (see Fig. 1). There are 162 atoms, including 18 Mg, 54 O, 72 C, and 18 H in the unit cell of Mg-MOF-74. Its lattice parameters are a = b = 26.221 Å, and c = 6.958 Å, following the study by Queen et al.43 The plane wave cutoff-energy was set to 750 eV. The energy convergence criterion was set to 10−6 eV for all of the relaxations. A 1 × 1 × 3 k-grid was used to optimize the unit cell. Gamma-point calculations are sufficient for all adsorption studies in the 1 × 1 × 2 supercell. VESTA44 and VASPKIT45 were employed to perform post-processing on the results obtained by VASP, and the iterative Hirshfeld (Hirshfeld-I) method46 was additionally used for charge analysis.

FIG. 1.

Top view (a) and side view (b) of the crystal structure of Mg-MOF-74. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

FIG. 1.

Top view (a) and side view (b) of the crystal structure of Mg-MOF-74. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

Close modal
In order to find the preferred adsorption site for Mg, we have considered 16 adsorption sites in the cell of a single Mg atom-inserted Mg-MOF-74. It turns out that Mg prefers to adsorb at the adsorption sites a, b, and c, as shown in Fig. 2. It is clear that Mg prefers to adsorb at the site close to the O atom and forms a Mg–O bond. The bond length of the Mg–O bond ranges from 2.10 to 2.25 Å. Here, the three preferred sites are employed to adsorb the Mgn (n = 2–10) cluster. Hydrogen storage in magnesium clusters has been widely studied.47–49 The Mgn cluster is initially constructed according to the structures reported by Shen et al.49 The calculated average binding energies (Eab) of Mgn clusters agree well with those obtained in the earlier work (see Fig. 3). The average binding energy for the magnesium clusters is defined as
Eb=EMgnnEMgn=EMgnnEMg,
(1)
with EMgn and EMg being the energy of the Mgn cluster and the Mg atom alone in the same supercell as the empty Mg-MOF-74 box, respectively. n refers to the number of Mg atoms in the Mgn cluster.
FIG. 2.

Mg preferred adsorption sites: a, b, and c. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

FIG. 2.

Mg preferred adsorption sites: a, b, and c. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

Close modal
FIG. 3.

Average binding energies of Mg in the Mgn cluster and Ref. 49. Optimized geometries of Mg2–Mg10 clusters. One Mg atom is connected with its neighbor atoms with orange sticks.

FIG. 3.

Average binding energies of Mg in the Mgn cluster and Ref. 49. Optimized geometries of Mg2–Mg10 clusters. One Mg atom is connected with its neighbor atoms with orange sticks.

Close modal

The bond lengths of Mg2–Mg8 clusters are in good agreement with Ref. 50, as optimized by the B3PW91 functional. The bond length of the Mg2 dimer is 3.50 Å, that of the equilateral triangle Mg3 is 3.26 Å, that of the relatively more stable and compact tetrahedron Mg4 is 3.07 Å, that of the slightly elongated triangular bipyramid Mg5 is 2.99–3.30 Å, that of Mg6 in which three pyramids are connected by their faces is 2.92–3.62 Å, that of the pentagonal bipyramid Mg7 is 2.89–3.32 Å, and that of the capped pentagonal bipyramid Mg8 is 3.00–3.35 Å. Lyalin et al.50 also investigated the size evolution of the Mg cluster geometry where the hexagonal ring structure determines the cluster growth starting from Mg15; this ring is one of the basic elements of the HCP lattice for the bulk magnesium.

As discussed above, there are three adsorption sites for the Mgn cluster to adsorb on Mg-MOF-74. The average adsorption energy of each Mg atom in Mg-MOF-74 is defined as
EadsMg=EmMgn+MOFEMOFm×n,
(2)
where EmMgn+MOF and EMOF are the total energies of the MOF systems before and after adsorbing the Mgn cluster with a quantity of m (m = 2–6), respectively.

It is clear from Table Ⅰ that the overall trend of the magnitude of average adsorption energy increases as n increases. This may be related to the amount of bonding between some Mg and O in the cluster. So far, we have only considered the adsorption of a single Mgn cluster on one side of the MOF pore. Consequently, the adsorption of 2–6 Mgn clusters on the MOF pore is considered. As shown in Fig. 4, one Mgn cluster is considered for adsorption on one side, and there are three types of adsorption configurations for 2, 3, and 4 clusters, respectively. Only one configuration is available for the adsorption of Mg5 or Mg6 clusters. Mg2 and Mg4 clusters are exemplified to investigate the stability of various configurations.

TABLE I.

Average adsorption energies (in eV/Mg) of one Mg2–Mg10 cluster adsorbing on Mg-MOF-74.

Cluster sizeEadsMgCluster sizeEadsMg
−0.429 −0.606 
−0.352 −0.662 
−0.492 −0.728 
−0.613 10 −0.695 
−0.601 ⋯ ⋯ 
Cluster sizeEadsMgCluster sizeEadsMg
−0.429 −0.606 
−0.352 −0.662 
−0.492 −0.728 
−0.613 10 −0.695 
−0.601 ⋯ ⋯ 
FIG. 4.

Configurations where 2–6 clusters are adsorbed separately in one pore. The number in the hexagon represents the number of adsorbed clusters, and the black stick represents the position of the adsorbed clusters.

FIG. 4.

Configurations where 2–6 clusters are adsorbed separately in one pore. The number in the hexagon represents the number of adsorbed clusters, and the black stick represents the position of the adsorbed clusters.

Close modal

The relaxed configurations of 2–6 Mg2 or Mg4 clusters adsorbed on a Mg-MOF-74 pore are shown in Fig. 5. Relaxed structures show that the bonding between the O and Mg atoms forms the Mg2 or Mg4 clusters. Furthermore, when two adsorbents are placed on adjacent sides of MOF pores, the Mg atoms in the two clusters attract each other and tend to aggregate together. It can be inferred that the Mg clusters adsorbed in the Mg-MOF-74 pore should grow as much as possible to fill the pores. It is known that Mgn forms hexagonal rings when n > 15.50 This plays an important role in the evolution of the magnesium cluster structure into the bulk lattice. Here, the Mg nanoparticle embedded in Mg-MOF-74 is also supposed to be hexagonal when n is large.

FIG. 5.

Optimized structures of 2–6 Mg2 or Mg4 clusters adsorbed on a Mg-MOF-74 pore. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

FIG. 5.

Optimized structures of 2–6 Mg2 or Mg4 clusters adsorbed on a Mg-MOF-74 pore. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

Close modal

The diameter of the pore of Mg-MOF-74 is around 10.8 Å.37 We estimate that the number of Mg atoms that can be filled into a pore is about 54.8 per supercell according to the size of Mg in its elemental crystal. Considering the shape of the pore, it is found that 60 Mg atoms are appropriate to fill in a pore with the HCP structure maintained. The Mg nanoparticle obtained above was placed into a pore in the MOF for structural optimization, and the atomic layer of the Mg nanoparticle in the obtained structure is no longer flat. As shown in Fig. 6, the middle part of the Mg atomic layer of the Mg nanoparticle in Mg NP@Mg-MOF-74 bulges up from the side view, while the Mg nanoparticle looks like a blooming flower from the top view. It is obvious that the Mg nanoparticle in Mg-MOF-74 is slightly enlarged with respect to the isolated one as the Mg–Mg bond lengths have become longer. Meantime, several Mg–O bonds were formed between the Mg nanoparticle and the Mg-MOF-74, with bond lengths of about 2.07–2.13 Å. Besides, the average adsorption energy of a Mg atom that adsorbs on Mg-MOF-74 is −1.174 eV per Mg. Given that one pore of 1 × 2 × 2 Mg-MOF-74 can accommodate 60 Mg atoms, the chemical formula for all pores filled with Mg atoms is C144H36O108Mg216. Assuming that the embedded Mg nanoparticles react fully with hydrogen and undergo a phase transition to MgH2, the theoretical hydrogen storage capacity of the Mg NP@Mg-MOF-74 mixture can reach 3.98 wt. %, which is considerable.

FIG. 6.

Optimized structure of the isolated Mg nanoparticle and Mg NP@Mg-MOF-74 (top) and the deformation charge density of the isolated Mg nanoparticle and Mg NP@Mg-MOF-74 (below).

FIG. 6.

Optimized structure of the isolated Mg nanoparticle and Mg NP@Mg-MOF-74 (top) and the deformation charge density of the isolated Mg nanoparticle and Mg NP@Mg-MOF-74 (below).

Close modal

In order to further understand the impact of Mg-MOF-74 on the electronic structure of the Mg nanoparticle, the deformation charge density of the isolated Mg nanoparticle and Mg NP@Mg-MOF-74 is compared, as shown in Fig. 6. Charge distribution shows charges concentrated in site O and site T2 of the Mg nanowire, while those in Mg NP@Mg-MOF-74 are distinctly different from the redistribution. The adsorbing H atom is the one that gains electrons, so the site where the charges gather should be the preferred adsorption site for H. Next, the H adsorption properties of these two systems are discussed.

Three H adsorption sites were considered for each layer in the isolated Mg nanoparticle of Mg NP@Mg-MOF-74, corresponding to the T1, T2, and O sites in each layer. In detail, the adsorption sites of H in the Mg nanoparticle include sites T1, T2, and O between each layer, resulting in a total of 15 adsorption sites with five atomic layers. H adsorption energy is defined as
EadsH=Esubstrate/HEsubstrate12EH2,
(3)
in which Esubstrate/H and Esubstrate correspond to the total energy of the Mg nanoparticle or Mg NP@Mg-MOF-74 with and without H adsorption, respectively. EH2 is the energy of the isolated H2 molecule in a large box.

Figure 7 shows the adsorption energies of H adsorbed at the 15 sites of the Mg nanoparticle or Mg NP@Mg-MOF-74. All the H adsorption energies of Mg NP@Mg-MOF-74 are lower than those of the corresponding sites in the isolated Mg nanoparticle except sites T22–3 and T23–4. The lower adsorption energy suggests a better hydrogen adsorption performance. Besides, the site T1 in the Mg nanoparticle system seems to be an unwelcome site for H, which reverses when the Mg nanoparticle is embedded in Mg-MOF-74.

FIG. 7.

Adsorption energy of the H atom at various sites in the isolated Mg nanoparticle or Mg NP@Mg-MOF-74 systems. “T11–2” means site T1 between the first and second layer; this is the same with other sites. Mg and H atoms are shown as orange and pink balls, respectively.

FIG. 7.

Adsorption energy of the H atom at various sites in the isolated Mg nanoparticle or Mg NP@Mg-MOF-74 systems. “T11–2” means site T1 between the first and second layer; this is the same with other sites. Mg and H atoms are shown as orange and pink balls, respectively.

Close modal

The H adsorption at the O1–2 position is a good example for analyzing the electronic structure. The charge density difference of H adsorption at O1–2 shown in Fig. 8 reveals that the H atom at octahedral sites gains electrons from six nearby Mg atoms, which in turn obtained electrons from their surrounding Mg atoms. Hirshfeld-I charges show that the adsorbate H obtains 0.406 e from the isolated Mg nanoparticle, while H in the Mg NP@Mg-MOF-74 system gains 0.510 e. More charge transfer indicates stronger interactions. The additional 0.104 e indicates that the combination of Mg nanoparticle and Mg-MOF-74 enhances hydrogen absorption performance.

FIG. 8.

Charge density difference between H and the Mg nanoparticle or Mg NP@Mg-MOF-74 system when H adsorbs on site O1–2. The unit is 4 × 10−4 e/bohr.3 The blue part represents the areas where electrons are missing, while the yellow part represents the areas where electrons gather. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

FIG. 8.

Charge density difference between H and the Mg nanoparticle or Mg NP@Mg-MOF-74 system when H adsorbs on site O1–2. The unit is 4 × 10−4 e/bohr.3 The blue part represents the areas where electrons are missing, while the yellow part represents the areas where electrons gather. Mg, O, C, and H atoms are shown as orange, red, brown, and pink balls, respectively.

Close modal

The adsorption of a Mg atom at various sites of Mg-MOF-74 has been studied with DFT calculations for effective hydrogen storage materials. The preferred sites are then employed to adsorb Mg2–Mg10 clusters. The simulated stability of various configurations for 2–6 Mg2 or Mg4 clusters shows that Mg atoms will gather together as much as possible, in line with the experimental observation that Mg forms nanoparticles in the MOF. After considering the number of Mg atoms that can be filled in one 1 × 1 × 2 Mg-MOF-74 pore and the size of the Mg nanoparticle that maintains a certain Mg crystal structure, Mg nanoparticles with 60 atoms is embed in a 1 × 1 × 2 cell, and this was used to simulate subsequent hydrogen absorption. The mixture Mg NP@Mg-MOF-74 could theoretically store 3.98 wt. % hydrogen gas. Consequently, H adsorption energies reveal a stronger interaction between H and Mg NP@Mg-MOF-74 than H and the isolated Mg nanoparticle. The deformation charge density demonstrated that the charge of the Mg nanoparticle redistributes when it embeds in Mg-MOF-74. Clearly, H storage capacity is improved with more available interstitial sites in Mg NP@Mg-MOF-74.

This work was financially supported by the National Natural Science Foundation of China (Grant No. 12074126). The computing resources from the National Supercomputer Center in Guangzhou (NSCCGZ) are gratefully acknowledged.

The authors have no conflicts to disclose.

Xingyu Zhou (周星宇): Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Yu-Jun Zhao (赵宇军): Funding acquisition (lead); Project administration (lead); Resources (lead); Software (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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