Ti-Based Catalysts on Magnesium Hydride

Ti-Based Catalysts on Magnesium Hydride: History

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Magnesium-based hydrides are considered as promising candidates for solid-state hydrogen storage and thermal energy storage, due to their high hydrogen capacity, reversibility, and elemental abundance of Mg. To improve the sluggish kinetics of MgH2, catalytic doping using Ti-based catalysts is regarded as an effective approach to enhance Mg-based materials.

magnesium hydride
titanium-based hydride
catalysis
hydrogen storage properties

1. Introduction

Depletion of fossil fuels and changes in the global climate urge people to seek green, sustainable energy resources and high-efficiency energy systems. Hydrogen is one of the secondary energy solutions with high gravimetric energy density, high efficiency, and zero carbon emission ^[1]. However, the hydrogen economy relies on safe and mature technology to store hydrogen, which remains a great challenge ^[2]. Solid-state hydrogen storage using metal hydrides is considered to be a safe and efficient method in comparison to other storage technologies, such as compressed hydrogen gas or liquid hydrogen.

Among various solid-state hydrogen storage materials, magnesium hydride (MgH₂) is one of the metal hydrides that has been considered to be promising, due to its high storage capacity, abundant resources, and relative safety. MgH₂ was first prepared in 1912 ^[3], and was proposed that can be used as energy storage media since the 1960s ^[4]. MgH₂ is known for its high hydrogen storage content, up to 7.76 wt%. More importantly, Mg has a single and flat pressure plateau under desorption/absorption, and is an abundant resource in the crust, which makes it one of the most promising hydrogen storage materials comparing to others. Thus, Mg-based hydride is expected to play important roles in future hydrogen storage techniques. In past decades, research efforts have made significant progress on improving Mg-based hydrides in terms of thermodynamics, kinetics, and reversibility. The utilization of MgH₂ for “energy storage” relates to two aspects, namely, hydrogen storage (HS) ^[5] and thermal energy storage (TES) ^[6]. Despite the difference in material-level for HS and TES, both applications require Mg-based hydride with fast hydrogen absorption and desorption rates. This leads to a large demand for studying catalysis in the Mg-H₂ system.

Due to the extensive research activities on Mg-based hydrides, a series of review papers have been published ^[7]^[8]^[9]^[10]^[11]^[12]^[13]^[14]. A comprehensive review by Yartys et al. ^[15] provides a historical overview as well as future perspectives. Recent reviews have covered various directions for Mg-based hydrogen storage, such as downsizing (nanostructuring) ^[7]^[10], catalysis and kinetics ^[7]^[16]^[17], and destabilization ^[18]^[19].

2. Fundamentals of the Mg-H₂ System

2.1. Crystal Structure

MgH₂ is a stoichiometric compound with a H/Mg atomic ratio of 1.99 ± 0.01 ^[20]. The Mg-H bond is an ionic type that is similar to alkali and alkaline earth metal hydrides ^[21]. MgH₂ with different types of structures can be synthesized by the reaction of magnesium with hydrogen under different conditions. β-MgH₂, which is stable at ambient pressure (1 bar) and room temperature, has a tetragonal TiO₂-rutile-type structure with space group P4₂/mnm ^[22]. β-MgH₂ can be formed under moderate conditions during reversible hydrogen cycling. Nevertheless, MgH₂ has at least four high-pressure forms, and the corresponding crystal structure parameters are tabulated in Table 1. At high applied pressures exceeding 0.387GPa or milled under high energy, β-MgH₂ transforms into the orthorhombic γ-MgH₂ form with α-PbO₂-type structure ^[23]. Additionally, a subsequent phase transition from γ-MgH₂ to a modified-CaF₂-type structure was observed experimentally using in situ synchrotron diffraction when hydrogen pressure is above 3.84 GPa ^[24]. According to Varin et al., high energy ball-milling of MgH₂ produced γ-MgH₂ coexisted with nanocrystalline β-MgH₂. They suggested that the presence of the γ-MgH₂ phase contributed to reducing the hydrogen desorption temperature of MgH₂ ^[25].

Table 1. Optimized structural parameters, bulk modulus (B₀), and pressure derivative of bulk modulus (B^’₀) for MgH₂ in ambient and high-pressure phases. (Reprinted with permission from ref. ^[22]. Copyright 2006 American Physical Society).

2.2. Thermodynamics of the Mg-H₂ System

The first experimental evaluation of the thermodynamics of the Mg-H₂ system was reported by Stampfer et al., showing the enthalpy of formation of MgH₂ to be −74.5 kJ/mol·H₂, and the entropy of formation is 136 J/K·mol·H₂ ^[20]. The thermodynamics parameters of the Mg-H₂ system have been reported, see Table 2. The pressure-composition-isotherm (PCI) method is commonly used to determine the enthalpy (ΔH) and entropy (ΔS) of the Mg-H₂ system. By measuring a series of equilibrium pressures at various temperatures, ΔH and ΔS can be derived by Van’t Hoff relation.

Table 2. Thermodynamic parameters and energy storage properties of MgH₂ ^[26].

Thermodynamic Parameters	Values
Formation enthalpy, kJ/(mol·H₂)	−74.5
Formation entropy, J/(mol·H₂·K)	−135
Hydrogen Storage Capacity (Theoretical)
Gravimetric capacity, wt%	7.6
Volumetric capacity, g/(L·H₂)	110
Thermal Energy Storage Capacity (Theoretical)
Gravimetric capacity, kJ/kg	2204
Volumetric capacity, kJ/dm³	1763

For on-board solid-state hydrogen storage, a thermodynamic window in the range of approximately 25–45 kJ/mol·H₂ is recognized for suitable metal hydride material ^[27]. Therefore, efforts have been directed to destabilize the MgH₂, or in other words, reducing the ΔH of MgH₂. It is expected that reducing ΔH can lower the working temperature for Mg-based hydride, which is crucial for on-board applications. Three typical approaches were proposed to destabilize MgH₂, namely, alloying, downsizing, and stress effect.

The alloying method refers to alloying other elements with Mg to form a new alloy or hydride compound with lower stability of its hydride. So far, alloying systems have been reported including Mg₂NiH₄ ^[28], Mg₂FeH₆, Mg₂CoH_5, Mg₂Cu ^[29], Mg(Al) ^[30], Mg₅₁Zn₂₀ ^[31], Mg₂Si ^[32], Mg(In) ^[33], Mg(Sn) ^[21], Mg(AgIn) ^[34], MgReNi ^[35], Mg₂M-xM_xH_y (M = Fe, Co, Ni), and so on. The principle is using a less-stable hydriding element A to form an Mg-A alloy. The energy diagram of the alloying method is illustrated in Figure 1. Since Mg-Ti is an immiscible system, Mg and Ti do not form an alloy. However, metastable Mg-Ti-H compounds have been reported. Kohta et al. ^[36]^[37]^[38]^[39] successfully synthesized Mg_xTi_100−x (35 ≤ x ≤ 80) alloys with hexagonal close-packed (HCP), face-centered cubic (FCC), and body-centered cubic (BCC) structures by ball milling. Vermeulen et al. ^[40] reported that Mg-Ti-H system has a very low plateau pressure (≈10⁻⁶ bar at room temperature). Additionally, it will have a higher plateau pressure and a reversible hydrogen storage capacity of more than 6 wt%, when forming ternary compositions with Al or Si.

Nano-sizing of Mg-based materials is not only a strategy to enhance kinetics, but also considered as an approach to destabilize MgH₂. It has attracted a great deal of effort in the past decades, despite its effectiveness and feasibility remaining controversial. The influence of nano-sizing on pressure-temperature dependence as well as ΔH is given in Figure 2. Theoretically, nanosizing to hydrides introduces excessive free energy to bulk or coarse particles. The excessive free energy may originate from lattice distortion ^[42]. Sadhasivam et al. ^[8] summarized the dimensional effects of nanostructured Mg/MgH₂ materials. They reported that Mg/MgH₂ with a particle size <5 nm has improved hydrogen storage properties. However, a great challenge remains in synthesizing such fine particles as well as maintaining the nano-size after thermal cycling for Mg-based materials. According to ^[8], the 1-dimensional Mg nanowire shows a promising hydrogen storage property. However, the nanowire structure would collapse into nanoparticles after a few cycles. Additionally, it is reported that reducing magnesium hydride structure to nanosize induces the stress/strain effect, which has been reviewed by Zhang et al. ^[43] It was pointed out that the stress/strain applied on MgH₂ leads to lattice deformation and volume change, which endows the extra strain energy for MgH₂. The research of Berube et al. ^[44] supported this claim. They reported that a 15% reduction of the formation enthalpy of nanostructured MgH₂ can be achieved by the introduction of surfaces, grain boundaries, as well as the presence of γ-MgH₂. Recent reviews ^[8]^[45]^[46], have provided thoughtful introduction and discussion into the thermodynamic aspects.

Figure 2. (a)Temperature dependence of the dissociation pressure of MgH₂ and associated evolution of such a dissociation pressure for various approaches investigated, and (b) evolution of ΔH and ΔS as a function of the particle size of Mg. (Reproduced with permission from ref. ^[47]. Copyright 2018 Elsevier).

2.3. Kinetics

Kinetics for hydrogen storage materials is generally defined as the dynamic rate where hydrogenation and dehydrogenation take place in time. Kinetics measurements provide critical information on the rates of hydrogen uptake or release from Mg-based materials. It is necessary to be rather explicit when investigating hydrogenation and dehydrogenation kinetics. For pure Mg and MgH₂ in the conventional form of coarse powders, they demonstrate very sluggish kinetics for hydrogen absorption and release, usually requiring over 400 °C for the reverse reactions. The slow hydrogenation rate of Mg, as well as dehydrogenation rate of MgH₂, can be attributed to several intrinsic factors: dissociation of the hydrogen molecule, penetration of hydrogen through the surface, diffusion of hydrogen in the matrix, in addition to possible contamination in the sample environment.

For hydrogenation of Mg, dissociation of the hydrogen molecule on the Mg surface is often considered as a rate-limiting step. Table 3 summarizes the energies for hydrogen molecule dissociation on Mg and modified Mg surfaces. The reported values of hydrogen dissociation energy on the Mg surface are in the range of 0.4–1.15 eV (38.59–110.96 kJ/mol), which is higher than most transition metals, such as Ti, V, Ni, and Fe ^[48]. This means that a large energy barrier needs to be overcome for dissociation of H₂ on pure Mg (0001) surfaces ^[49]. Another intrinsic issue is the slow hydrogen diffusion rate in MgH₂. Figure 3 shows the geometry model of the reaction for an Mg/MgH₂ particle. Based on the model, the hydride layer formed on the particle surface becomes the major barrier during hydrogenation, since the hydrogen atom diffusion rate in the hydride phase is much slower than that in the metallic phase. According to Spatz et al., the hydrogen diffusion coefficient (D_H) of MgH₂ is quite low (1.1 × 10⁻²⁰ m²/s at 305 K) ^[50]. Figure 4 shows that the D_H of MgH₂ is at magnitudes lower than the D_H of the Mg metal phase. It is also evident in this figure from the diffusion coefficient plots that most transition metals and their hydrides have D_H several magnitudes higher than the D_H of MgH₂.

Metal	Dissociation Energy (eV)
Pure Mg	0.87, 0.40, 0.50, 1.15, 1.05, 0.95, 1.00
Ti-doped Mg	Null, negligible
Ni-doped Mg	0.06
V-doped Mg	Null
Cu-doped Mg	0.56
Pd-doped Mg	0.39
Fe-doped Mg	0.03
Ag-doped Mg	1.18

Catalytic doping and nanosizing of Mg-based systems have been considered as important methods to improve their kinetics. In general, the catalyst is defined as an agent which reduces the activation barrier without participating in the chemical reaction, as illustrated in Figure 5. A common consensus is that transition metals (TM) and their compounds are effective catalysts. These catalysts can be doped into Mg/MgH₂ material by different synthetic approaches. Most TM catalysts are effective in both hydrogenation and dehydrogenation reactions. The roles of different Ti-based catalysts and the underlying mechanism will be reviewed in the following section.

Downsizing MgH₂ to nano-scale is also shown to be effective to improve the kinetics. It is believed that nano-sizing can enhance kinetics by the creation of a large amount of fresh surface, shortening hydrogen diffusion, and promoting nucleation of the hydride/metal phase ^[12]. It is noteworthy that a combination of nanosizing and catalytic doping is usually realized during synthesis. For example, using a high-energy ball milling technique, co-milling MgH₂ with transition metal powder could produce a nanocomposite with nano-size microstructure and homogeneously doped catalyst particles.

3. Catalytic Effects

3.1. Transition Metals Catalysts

Among various additives for improving Mg-based materials, TM catalysts have been intensively investigated. Interestingly, most of the transition metals and their compounds are found to be effective as both hydrogenation and dehydrogenation catalysts. In general, 1–5 at.% addition of TM catalyst leads to dramatic improvement while the hydrogen storage capacity is not sacrificed significantly. Research efforts have been directed to investigate the effectiveness of various TM-based catalysts. Table 4 compiles the reported results from Ti-based additive-enhanced MgH₂ systems as well as corresponding synthetic approaches and kinetic behaviors.

Table 4. Hydrogen storage properties of Mg with various types of Ti-based catalysts.

Materials	Synthetic Methods	Hydrogen Storage Properties				Reference
Materials	Synthetic Methods	Desorption Kinetics	Ea_des (kJ/mol)	Absorption Kinetics	Ea_abs (kJ/mol)	Reference
Titanium/Titanium Hydrides
Mg-2%Ti	Inert gas condensation	Des: 4.50%/320 °C/0.2 bar/25 min		Abs: 4.80%/320 °C/8 bar/21 min		^[53]
MgH₂ + 2 at% Ti	Ball milling (argon)	Des: 6.32 wt%/623 K/35 kPa/0.5 h		Abs: 6.32 wt%/623 K/2000 kPa/4 min		^[54]
MgH₂ + 2 at% Ti	Cold rolling (5 times, air)	Des: 6.00 wt%/623 K/35 kPa/0.5 h		Abs: 5.70 wt%/623 K/2000 kPa/4 min		^[54]
MgH₂-4 mol% Ti	Ball milling	Des: 1.10%/573 K/2 MPa/5 min		Abs: 6.40%/573 K/2 MPa/5 min		^[55]
MgH₂-5 at% Ti	Ball milling	Des Temperature: 235.6 °C	70.11			^[56]
MgH₂-5 at% Ti	Ball milling	Des: 5.50%/523 K/0.015 MPa/20 min	71.1	Abs: 4.20%/373 K/1.0 MPa/15 min		^[57]
MgH₂-5 at% Ti	Ball milling	Des: 5.20%/573 K/0.03 MPa/15 min		Abs: 6.70%/ 573 K/0.8 MPa/15 min		^[58]
Mg-5% Ti	Chemical vapor synthesis		104			^[59]
Mg-14 at% Ti	Gas phase condensation		35		52	^[60]
Mg-22 at% Ti	Gas phase condensation		31		47	^[60]
MgH₂-15% Ti	Ball milling	Des: 0.12%/573 K/1 bar/60 min		Abs: 3.48%/573 K/12 bar/60 min		^[61]
Mg_0.9Ti_0.1	Ball milling		76	Abs: 6.62% (after milling)		^[62]
Mg_0.75Ti_0.25	Ball milling		88	Abs: 6.18% (after milling)
Mg_0.5Ti_0.5	Ball milling		91	Abs: 5.21% (after milling)
MgH₂-20% Ti	Ball milling		72 ± 3			^[63]
MgH₂-coated Ti	Ball milling	Des: 5.00%/250 °C/15 min (TPD) Des Temperature: 175 °C				^[64]
Mg_83.5Ti_16.5	Inert gas condensation	Des: 2.50%/300 °C/0.15 bar/2 min		Abs: 2.20%/300 °C/9 bar/1 min		^[65]
15Mg-Ti	Chemical method				72.2	^[66]
MgH₂-4 mol% TiH₂	Ball milling	Des: 0.70%/573 K/2 MPa/5 min		Abs: 6.10%/573 K/2 MPa/5 min		^[55]
MgH₂-5 at% TiH₂	Ball milling	Des: 5.80%/270 °C/0.12 bar/10 min Des Temperature: 235.5 °C	67.24	Abs: 2.70%/25 °C/1 bar/250 min		^[56]
10MgH₂-TiH₂	Ball milling		73			^[67]
7MgH₂-TiH₂	Ball milling		71			^[68]
4MgH₂-TiH₂	Ball milling		68			^[68]
MgH₂-10 mol% TiH₂	Ball milling			Abs: 5.70%/240 °C/2 MPa/200 s	16.4	^[69]
MgH₂-10% TiH₂	Ball milling				24.2	^[70]
MgH₂-10% TiH₂	Ball milling				17.9	^[71]
Mg-9.2% TiH_1.971-3.7% TiH_1.5	Ball milling	Des: 4.10%/573 K/100 Pa/20 min	46.2	Abs: 4.30%/298 K/4 MPa/10 min	12.5	^[72]
Mg_0.65Ti_0.35D_1.2	Ball milling		17			^[73]
Titanium Oxides
MgH₂-10% TiO₂	Ball milling	Des: 6.00%/300 °C/vacuum/20 min		Abs: 6.00%/300 °C/0.84 MPa/5 min		^[74]
Mg-20% TiO₂	Reactive ball milling	Des: 4.40%/350 °C/1 bar/8.5 min		Abs: 3.80%/350 °C/20 bar/2 min		^[75]
MgH₂-6% TiO₂	Ball milling		145.8 ± 14.2			^[76]
MgH₂ + 10% TiO₂	Ball milling	Des Temperature: 200 °C	75.50			^[77]
Titanium Halides
MgH₂-10% TiF₄	Ball milling	Des Temperature: 216.7 °C	71	(Des: 6.6%)		^[78]
MgH₂-10% TiF₄	Ball milling (2 h, argon)	Des Temperature: 154 °C	70			^[79]
MgH₂ + 10% TiF₄	Ball milling	Des Temperature: 150 °C	70			^[77]
MgH₂-4 mol% TiF₃	Ball milling	Des: 4.50%/573 K/2 MPa/5 min		Abs: 5.10%/573 K/2 MPa/5 min		^[55]
MgH₂-4 mol% TiCl₃	Ball milling	Des: 3.70%/573 K/2 MPa/5 min		Abs: 5.30%/573 K/2 MPa/5 min		^[55]
MgH₂-7% TiCl₃	Ball milling	Des temperature: 274 °C	85			^[80]
Titanium Alloys
MgH₂-5a% TiAl	Ball milling	Des: 4.90%/270 °C/0.12 bar/10 min Des Temperature: 219.6 °C	65.08	Abs: 2.50%/25 °C/1 bar/250 min		^[56]
MgH₂-5 a% Ti₃Al	Ball milling	Des Temperature: 232.3 °C	70.61			^[56]
Mg₈₅Al_7.5Ti_7.5	DC-magnetron co-sputtering	Des: 5.30%/200 °C/vacuum/20 min		Abs: 5.60%/200 °C/3 bar/0.5 min		^[81]
Mg_0.63Ti_0.27Si_0.10D_1.1	Ball milling		27			^[73]
MgH₂-5 at%TiNi	Ball milling	Des Temperature: 242.4 °C	73.09			^[56]
15Mg-Ti-0.75Ni	Chemical method				63.7	^[66]
Mg_0.63Ti_0.27Ni_0.10D_1.3	Ball milling		21			^[73]
MgH₂-5at%TiNb	Ball milling	Des: 5.90%/27 °C/0.12 bar/10 min Des Temperature: 231.3 °C	71.72	Abs: 2.80%/25 °C/1 bar/250 min		^[56]
MgH₂-5at% Cr-5a% Ti	Film	Des: 6.00%/200 °C/5 mbar/25 min		Abs: 6.20%/200 °C/3 bar/10 min		^[82]
MgH₂-7 at% Cr-13 at% Ti	Film	Des: 5.00%/200 °C/5 mbar/25 min		Abs: 5.60%/200 °C/3 bar/10 min		^[82]
MgH₂-5 at% TiFe	Ball milling	Des: 5.20%/270 °C/0.12 bar/10 min Des Temperature: 237.7 °C	72.63	Abs: 3.00%/25 °C/1 bar/250 min		^[56]
MgH₂-5% FeTi	Ball milling			Abs: 2.30%/150 °C/2 MPa/5 min	21	^[83]
MgH₂-5 at% TiMn₂	Ball milling	Des: 4.80%/270 °C/0.12 bar/10 min Des Temperature: 219.7 °C	74.22	Abs: 3.20%/25 °C/1 bar/250 min		^[56]
MgH₂-10% TiMn₂	Ball milling				22.6	^[70]
MgH₂-5% VTi	Ball milling			Abs: 3.30%/150 °C/2 MPa/5 min	10.4	^[83]
Mg_87.5Ti_9.6V_2.9	Hydrogen plasma metal reaction	Des: 4.00%/300 °C/1 mbar/5 min	73.8	Abs: 4.80%/200 °C/40 bar/5 min	29.2	^[84]
MgH₂-5 at% TiVMn	Ball milling	Des: 5.70%/270 °C/0.12 bar/10 min Des Temperature: 216.7 °C	85.20	Abs: 3.00%/25 °C/1 bar/250 min		^[56]
Multiple Catalysts
Mg-10% Ti-10% Pd	Ball milling		114 ± 4			^[85]
Mg-TiH_1.971-TiH_1.5-ZrH_1.66	Arc melting		36.6		21.2	^[86]
Mg_0.9Ti_0.1 + 5% C	Ball milling		88	Abs: 6.43% (after milling)		^[62]
MgH₂-6% NiTiO₃	Ball milling		74 ± 4			^[87]
MgH₂-6% CoTiO₃	Ball milling		100 ± 2			^[87]
MgH₂-10 mol% TiH₂-6 mol% TiO₂	Ball milling		118			^[88]
MgH₂-5% VTi-CNTs	Ball milling			Abs: 5.10%/150 °C/2 MPa/5 min	10.2	^[83]
MgH₂-5% FeTi-CNTs	Ball milling			Abs: 0.60%/150 °C/2 MPa/5 min	65.5	^[83]
MgH₂-10% Ni-TiO₂	Ball milling	Des: 6.50%/265 °C/0.02 bar/7 min	43.7 ± 1.5	Abs: 5.00%/100 °C/60 bar/7 min		^[76]
MgH₂-4% Ni-6% TiO₂	Ball milling		91.6 ± 8.5			^[76]
MgH₂-10% Co-TiO₂	Ball milling	Des: 6.20%/250 °C/0.02 bar/15 min	77	Abs: 4.24%/100 °C/60 bar/10 min		^[89]

Early work by Liang et al. ^[57] evaluated the catalytic effects of 3d-TM elements (Ti, V, Mn, Fe, and Ni) on the reaction kinetics of ball-milled catalyzed MgH₂ (see Figure 6). The MgH₂-Ti composite showed superior hydrogen desorption/absorption kinetics, exhibiting the best desorption kinetics at 573 K, followed in order by V, Fe, Ni, and Mn. The activation energies (E_a) of MgH₂-Ti, MgH₂-V, MgH₂-Mn, MgH₂-Fe, and MgH₂-Ni are calculated to be 71.1 kJ/mol, 62.3 kJ/mol, 104.6 kJ/mol, 67.6 kJ/mol, and 88.1 kJ/mol, respectively, which are significantly reduced compared to that of the ball-milled pure MgH₂ (120 kJ/mol). It was indicated that the TM catalysts could drastically improve the kinetic properties of MgH₂, among which Ti-catalyzed MgH₂ shows superior performance. Rizo-Acosta et al. ^[58] compared hydrogenation properties of MgH₂ with the addition of early transition metals (Sc, Y, Ti, Zr, V, and Nb). As shown in Figure 7a,b, their results indicated that full reactions finished within less than 120 min in all cases and the hydrogen absorption rate increased along the sequence Y < V < Ti < Nb < Sc < Zr. However, an apparent degradation was observed when the cycling number increases. Interestingly, this evolution is less pronounced in the Ti-doped system, as shown in Figure 7c, which was attributed to the lattice mismatch between Mg and TiH₂ hydride that limits Mg grain growth. Among all cases, MgH₂-TiH₂ nanocomposite presented the best cycling properties with a reversible capacity of 4.8 wt% after 20 cycles and the reaction time arbitrarily limited to 15 min.

Figure 6. Hydrogen desorption curves ((a), desorption pressure of 0.015 MPa, 573 K) and absorption curves ((b), absorption pressure is 1.0 MPa, 302 K) of Mg–Tm composites. (Reproduced with permission from ref. ^[57]. Copyright 1999 Elsevier).

Figure 7. (a) Hydrogen uptake curves of 95Mg-5ETM powder mixtures during reactive ball milling synthesis; (b) the corresponding absorption rates (derivative curves of a); and (c) hydrogen sorption curves at 573 K of MgH₂-ETMH_x NCs for different sorption sweeps. (Reproduced from ref. ^[58]. Copyright 2019 RSC).

Zhou et al. ^[90] prepared 49 additive-doped MgH₂ samples by ultra-high-energy-high-pressure ball milling, in order to conduct a comprehensive survey on a wide range of additives and corresponding dehydrogenation temperatures of the catalyzed MgH₂. The plot of the Thermogravimetric Analysis (TGA) dehydrogenation temperatures is shown in Figure 8, indicating that the additives containing the IV-B and V-B group elements are the most effective catalysts while the VII-B (Mn), VIII-B (Fe, Co, and Ni) groups show moderate catalytic effects. Besides, Ti and its compounds are more effective compared to those catalysts based on heavier elements (Zr, ZrH₂, ZrO₂, and Ta) in the same periodic group.

Cui et al. ^[91] synthesized micro-sized Mg particles coated with nano-sized TM catalyst, showing that the nano-coating of TM on the Mg/MgH₂ surface is more effective than co-ball-milling of Mg with TMs. The authors also suggested that the catalytic improvement on dehydrogenation kinetics can be ranked as Mg-Ti, Mg-Nb, Mg-Ni, Mg-V, Mg-Co, and Mg-Mo, and the hydrogenation kinetics is in a sequence of Mg-Ni, Mg-Nb, Mg-Ti, Mg-V, Mg-Co, and Mg-Mo.

It has been recognized that early transition metals (ETM) belong to the group of most effective catalysts. Despite some discrepancies in reported data, Ti-based catalysts, involving not only elemental Ti but also Ti hydrides, oxides, halides, and intermetallic compounds have shown great benefits in improving the hydrogen storage properties of MgH₂. In-depth investigations of Ti-based catalysts are also beneficial for understanding the catalysis mechanism for the Mg-H₂ system.

3.2. Catalytic Effects of Ti-Based Compounds

A large number of Ti-based catalysts have been explored for enhancing the hydrogen storage properties of MgH₂. Early attempts using elemental Ti powder to ball-mill with MgH₂ received encouraging results ^[57]. Soon, researchers found that TiH₂ powder additive is very effective as well. Lu et al. ^[92] reported exceptional room temperature hydrogenation properties of MgH₂-0.1TiH₂ material prepared by ultra-high-energy-high-pressure (UHEHP) ball milling. Liu et al. ^[72] studied the effects of two different Ti hydrides (TiH_1.971 and TiH_1.5) on the hydrogenation kinetics of Mg. It pointed out an important fact that elemental Ti can easily react with hydrogen to form various Ti hydrides under certain temperatures and hydrogen pressures. During the reverse hydrogen reaction, the following equations can be summarized:

According to the Mg-Ti phase diagram, neither Ti nor Ti hydrides are immiscible with Mg or MgH₂ phases. Furthermore, no ternary Mg-Ti hydride exists in the phase diagram. However, under a metastable condition, it is possible for Ti to dissolve into Mg and form a solid solution. Ponthieu et al. ^[93] reported Ti solubility in β-MgD₂ up to 7 at.%, and Mg solubility in TiD₂ up to 8%, which suggested shortened D-diffusion path due to the introduction of TiD₂. An Nuclear Magnetic Resonance (NMR) study of MgD₂/TiD₂ composite found lattice coherent fluorite (fcc) structured TiD₂ and MgD₂, which is expected to be a fast H-diffusion pathway to accelerate the kinetics ^[94].

Another focus is discovering a novel metastable Mg-Ti-H hydride with a new structure. Kyoi et al. ^[95] synthesized Mg₇-Ti-H FCC hydride using a high-pressure anvil cell. Asano and Akiba reported the ball-milling synthesis of a series of Hexagonal Closest Packed (HCP), Face-centered Cubic (FCC), and Body-centered Cubic (BCC) Mg_xTi_100−x alloys, and Mg-Ti-H FCC hydride phases with chemical formulae of Mg₄₀Ti₆₀H₁₁₃ and Mg₂₉Ti₇₁H₅₇. These ternary hydrides had lower stabilities in comparison to MgH₂ and thus show lower desorption temperatures.

TiO₂ was considered an effective catalyst. Wang et al. ^[75] prepared ball-milled Mg-TiO₂ and showed good hydrogenation and dehydrogenation kinetics. For the past two decades, however, the investigation of oxide catalysts paid more attention to Nb₂O₅, since it seems to be more efficient among transition metal oxides ^[96]. Actually, doping of TiO₂ would present a similar effect comparing to the Nb₂O₅ catalyst. As suggested by Pukazhselvan et al. ^[97], TiO₂ can be partially reduced to a lower 3⁺/2⁺ state (TiO and Ti₂O₃). The presence of Mg_xTi_yO_{x + y} oxide was also suspected, but no direct support was seen by X-ray Diffraction (XRD) results. More recently, Zhang et al. ^[98] showed good catalytic activity of carbon-supported nanocrystalline TiO₂ (TiO₂@C). It was reported that the dehydrogenation temperature of MgH₂-10 wt%TiO₂@C can be lowered to 205 °C and hydrogen uptake took place at room temperature. Berezovets et al. ^[99] reported that the Mg-5 mol% Ti₄Fe₂O_x was able to absorb hydrogen even at room temperature after hydrogen desorption at 300–350 °C and its cycling stability could be substantially improved by introduction of 3 wt% graphite into the composite.

Ti halides have been reported to offer a positive effect on the kinetics of MgH₂. TM fluorides usually present superior catalytic effects and satisfactory kinetics. Malka et al. ^[80] reported the catalytic effects of a group of TM fluorides (FeF₂, NiF₂, TiF₃, NbF₅, VF₄, ZrF₄, CrF₂, CuF₂, CeF₃, and YF₃) on the kinetics of MgH₂. The best catalysts for magnesium hydride decomposition were selected to be ZrF₄, TaF₅, NbF₅, VCl_3, and TiCl₃. In another investigation by Jin et al. ^[100], it was suggested that TiF₃ and NbF₅ showed better effects over other TM fluorides. It was found that the hydride, for example, TiH₂, formed after co-milling MgH₂ with the fluorides, with an in situ reaction described as follows:

Moreover, Wang et al. ^[101] conducted a comparison study on the elemental Ti, TiO₂, TiN, and TiF₃ catalyzed MgH₂ materials, showing that TiF₃ had the strongest catalytic effect among them.

Ti-based intermetallics as catalysts have been receiving active attention in recent years. Early researchers used TiFe ^[102], (Fe_0.8Mn_0.2)Ti ^[103], Ti₂Ni ^[104], and TiMn_1.5 ^[105] additives to improve hydrogen storage properties of MgH₂, showing that all these intermetallics were effective catalysts. Interestingly, some Ti-based intermetallics themselves, including TiFe and TiMn_1.5, are known as hydrogen storage alloys. Zhou et al. ^[56] conducted a systematic investigation focusing on a series of Ti-based intermetallic catalysts (i.e., TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn₂, and TiVMn). The results found that TiMn₂-doped Mg demonstrated extraordinary hydrogen absorption capability at room temperature and 1-bar hydrogen pressure while its apparent activation energy is 20.59 kJ/mol·H₂. The strong catalytic effect of TiMn₂ is also confirmed by another experimental work by El-Eskandarany et al. ^[106]^[107] and first principles calculation by Dai et al. ^[108].

4. Synthetic Approaches

The synthesis methods of Mg-based hydrides have a great impact on their hydrogen storage properties. With expanding research scope of hydrogen storage materials, there are emerging preparation methods in recent years. Many hydrogen storage alloys can be prepared by physical methods, including ball milling ^[109], induction melting ^[110], arc melting ^[111], et cetera. Complex hydrides are usually prepared by chemical methods, such as organic synthesis, hydrothermal method, and solvothermal method ^[112]. However, conventional high-temperature preparations such as sintering or melting have been largely restricted due to the low melting temperature and high vapor pressure of magnesium ^[15]. Widely-used methods for Mg-based hydride preparation include ball milling, thin film deposition, and chemical methods.

5. Mechanisms of Catalysis

Understanding the catalysis is critical to improving hydrogen absorption and desorption kinetics for Mg-based systems. Based on the understanding of the hydrogen reaction in the metal-hydrogen system ^[113], the hydrogenation of metal should go through the following five steps: (1) Physisorption of the H₂ molecule, (2) dissociation of the H₂ molecule, (3) surface penetration of H atoms, (4) diffusion of H atoms in the host lattice, and (5) hydride formation at metal/hydride interface, as shown in Figure 9. For the dehydrogenation reaction, a hydride particle could go through the following steps: (1) Hydride decomposition, (2) diffusion of hydrogen atom, (3) surface penetration, (4) recombination to hydrogen molecule, and (5) desorption to the gas phase. Either hydrogen absorption or desorption should be controlled by a rate-limiting step while other steps are likely in equilibrium.

Figure 9. Reaction partial steps for the absorption (left) and desorption (right) of hydrogen by a spherical metal/hydride powder particle. (Reproduced with permission from ref. ^[113]. Copyright 1996 Elsevier).

However, the rate-controlling mechanisms in hydrogenation and dehydrogenation may not necessarily be the same. The physisorption of a H₂ molecule on a metal surface needs a very low activation energy, so it is generally not considered a limiting step. The rest of the steps can be rate-limiting which is worthy of discussion. For dehydrogenation, steps 1, 2, and 3 (illustrated in Figure 9) can be considered as possible rate-limiting steps. Note that the hydrogen atoms should diffuse across the metal phase, in which the diffusion coefficient is much higher compared to that in the hydride phase. Moreover, the dehydrogenation has a H₂ recombination step instead of dissociation. The recombination of H atoms into a molecule does not have an energy barrier to overcome ^[114]. From these aspects, it seems reasonable that the kinetic barrier of dehydrogenation could be lower than that of hydrogenation. However, dehydrogenation is an endothermic reaction whereas hydrogenation is exothermic, which means the hydrogenation of Mg is favored in respect of thermodynamics. These fundamental differences may change the activation barrier and lead to different reaction behaviors.

This entry is adapted from the peer-reviewed paper 10.3390/inorganics9050036

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