LiAlH₄ supported on TiO₂/hierarchically porous carbon nanocomposites with enhanced hydrogen storage properties

Abstract

We report the synthesis of LiAlH₄ supported on TiO₂/hierarchically porous carbon (LAH–TiO₂/HPC) nanocomposites using a one-step solvent method and their enhanced catalytic dehydrogenation performance. The as-prepared TiO₂/HPC nanocomposites show that TiO₂ nanoparticles (∼10 nm) are homogeneously distributed on the surface of hierarchically porous carbon (HPC). The results show that TiO₂/HPC nanocomposites exhibit better catalytic performance for the dehydrogenation of LiAlH₄ and rehydrogenation of the dehydrided sample than that of TiO₂ nanoparticles and HPC. The dehydrogenation temperature of 37LAH–25TiO₂/38HPC with 37 wt% LiAlH₄, 25 wt% TiO₂ and 38 wt% HPC is the lowest. Hydrogen started to be released at 64 °C, which is about 100 °C lower than that of pure LiAlH₄. In addition, 4.3 wt% of hydrogen could be released from 37LAH–25TiO₂/38HPC within 40 min at 130 °C, indicating fast kinetics with an activation energy of 47.1 ± 3.5 kJ mol⁻¹. Furthermore, it can re-adsorb H₂ at 300 °C under a hydrogen pressure of 4 MPa. The nanoconfinement of LiAlH₄ into hierarchically porous carbon with high surface areas and the high distribution of TiO₂ nanoparticles with a Ti(4+)/Ti(3+)/Ti(2+) defect site play a synergistic role in improving the hydrogen storage properties of LiAlH₄.

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Introduction

Hydrogen is regarded as a promising energy carrier due to its high energy density and environmental friendliness.^1–6 However, the storage of hydrogen remains a bottleneck in the widespread utilization of hydrogen energy. It is still urgent to develop a safe and efficient technique to store hydrogen.^7–10 Among all hydrogen storage materials, solid-state complex hydrides have attracted more attention because of their high gravimetric/volumetric hydrogen content, adequate kinetics, as well as reversible dehydrogenation/rehydrogenation properties.^11–13 Particularly, light metal complex hydride LiAlH₄ is considered to be a promising hydrogen storage material owing to the high theoretical hydrogen content (10.5 wt%).^14–16 The process of hydrogen generation from LiAlH₄ is presented below:¹⁷

LiAlH₄ → 1/3Li₃AlH₆ + 2/3Al + H₂↑ 187–218 °C (1)

1/3Li₃AlH₆ → LiH + 1/3Al + 1/2H₂↑ 228–282 °C (2)

LiH + Al → LiAl + 1/2H₂↑ 380–430 °C (3)

Studies mainly focus on the former two steps because the decomposition temperature of the third step is too high.¹⁸ However, the thermodynamics and kinetics of reactions (1) and (2) are not appropriate for practical applications. Recently, metal catalysts have been widely applied to the former two steps.^5,19 Among various metal catalysts, Ti-based catalysts are safe, of low cost and highly efficient.^20–23 For example, Amama and coworkers²⁴ investigated the effects of Ti, TiCl₃ and TiO₂ additives on the dehydrogenation properties of LiAlH₄. The onset temperature (T_onset) decreased from 185 °C for pure LiAlH₄ to 108 °C for 0.5 mol% TiO₂ doped LiAlH₄. Additionally, Chen et al. have reported the reversibility of LiAlH₄ with the catalyst of TiCl₃–1/3AlCl₃ under a low hydrogen pressure of 4 MPa.⁷ In order to improve the hydrogen storage properties, the interaction between catalysts and LiAlH₄ is a key factor.^25,26 However, in previous studies, bulk LiAlH₄ and Ti-based additives were mainly mixed by high-energy ball milling, which always caused the aggregation of catalysts and thus resulted in the loss of active sites.^27,28 It is of great significance to disperse the catalysts on supporting materials with high surface areas; carbon matrices would be good candidates.^29–31 Tan et al. reported the decoration of multiwall carbon nanotubes with TiCl₃ particles (TiCl₃-MWCNTs) as catalysts for the decomposition of LiAlH₄. They found that the composite of LiAlH₄-20 wt% TiCl₃-MWCNTs began to desorb hydrogen at 75 °C, which is 96 °C lower than that of pure LiAlH₄.³² Moreover, nanoconfining the hydrides to a carbon scaffold could reduce the particle size of hydrides to the nanoscale^33–35 and prevent the de/rehydrogenated products from agglomerating in de/rehydrogenation cycles.^36–38 In our previous work, we prepared hierarchically porous carbon (HPC) with large surface areas and multiscale pores, which is in favour of dispersing metal nanoparticles.³⁹ Thus, TiO₂/HPC nanocomposites as synergistic catalysts for the dehydrogenation of LiAlH₄ are interesting.

Herein, we synthesized LiAlH₄-supported on TiO₂/HPC (LAH–TiO₂/HPC) composites using a one-step solvent method at room temperature and further investigated their enhanced hydrogen storage properties. For the LAH–TiO₂/HPC composites, LiAlH₄ was homogeneously supported on the surface of TiO₂/HPC. TiO₂/HPC nanocomposites show better catalytic performance on de/rehydrogenation of LiAlH₄ than individual TiO₂ nanoparticles and HPC. The composite with 37 wt% LiAlH₄, 25 wt% TiO₂ and 38 wt% HPC (denoted as 37LAH–25TiO₂/38HPC) exhibited superior performance. It started to release H₂ at 64 °C and generate 4.3 wt% H₂ within 40 min at 130 °C. Moreover, it can partially re-adsorb hydrogen at 300 °C under a hydrogen pressure of 4 MPa. HPC with high surface areas could downsize the hydride and restrain the dehydrogenated products from aggregation, while the highly distributed TiO₂ nanoparticles with close contact of LiAlH₄ provide a lot of active sites. The synergistic effect of the TiO₂/HPC nanocomposites guarantees the enhanced hydrogen storage properties of LiAlH₄.

Experiment

Synthesis of TiO₂/HPC nanocomposites

HPC was prepared via a silica source/carbon source self-assembly approach and the associated details are shown in the ESI.† To prepare the TiO₂/HPC nanocomposites, 50 mg HPC was dispersed in 15 mL absolute ethanol under ultrasonic treatment for 6 min. Then, 142 μL tetra-n-butyl titanate (TBT, 98%, Tianjin Guangfu Fine Chemical Research Institute) and 15 μL HNO₃ (65%, Tianjin 5th Chemical Plant) were added under ultrasonic treatment for another 4 min in an ice–water bath. The mixture was then stirred for 0.5 h, followed by addition of 45 μL deionized water (the molar ratio of TBT : HNO₃ : H₂O = 1 : 0.8 : 6)⁴⁰ and kept under stirring for 6.5 h at room temperature. The solvent was evaporated under powerful stirring at 50 °C. The obtained powder was calcined at 500 °C for 3 h in an argon atmosphere. The content of TiO₂ in TiO₂/HPC nanocomposites is 40 wt%. For comparison, TiO₂ nanoparticles and TiO₂/HPC nanocomposites with TiO₂ loading of 20, 50 and 60 wt% were synthesized by the same method.

Preparation of the LAH–TiO₂/HPC composite

The composite of LiAlH₄ loaded on TiO₂/HPC was prepared using a one-step solvent method. Firstly, 100 mg TiO₂/HPC composites was dispersed in 10 mL THF. A certain amount of LiAlH₄ (97%, Alfa) was dissolved in 30 mL distilled THF under vigorous stirring for 1 h. After that, the LiAlH₄–THF solution was added dropwise into a TiO₂/HPC–THF mixture. The mixture was then stirred for about 7 h at room temperature. Finally, LAH–TiO₂/HPC with various LiAlH₄ contents (29, 37, 45, and 55 wt%) was obtained after evaporation under vacuum overnight. All the above operations were carried out in an argon filled glove box (Mikrouna Universal 2440/750) with H₂O and O₂ content below 1 ppm.

Material characterization

The phase of all the samples was analyzed by powder X-ray diffraction (XRD) using a Mini Flex 600 X-ray generator with Cu-Kα radiation (λ = 1.5406 Å) between 10° and 80° at a scan rate of 2° min⁻¹. The sample of LAH–TiO₂/HPC was covered with a parafilm to prevent it from reacting with oxygen and water in the air during measurement. The morphology and structure were observed by field-emission scanning electron microscopy (SEM, JEOL JSM7500F) and transmission electron microscopy (TEM, Philips Tecnai-F20). Fourier transform infrared (FTIR) spectroscopy was performed to analyze the chemical bonding using a Tensor II (Bruker) at a resolution of 4 cm⁻¹. N₂ adsorption–desorption isotherms were measured with a BELSORP-Mini instrument at 77 K. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method. The pore size distributions were derived from the desorption branches of the isotherms using the Barrett–Joyner–Halenda (BJH) model. The oxidation state of the titanium component was detected with X-ray photoelectron spectroscopic (XPS) measurement in a Versa Probe PHI 5000 system (Al Kα radiation of 1486.6 eV). The binding energy of the C 1s core level is at 484.8 eV.

Hydrogen storage property testing

The decomposition performance was tested on a temperature-programed desorption-mass spectroscopy (TPD-MS, Quantachrome) instrument. Typically, about 20 mg powder was loaded into the sample chamber, which is heated from room temperature to 300 °C with a heating rate of 5 °C min⁻¹ under an Ar flow. The isothermal dehydrogenation kinetics and rehydrogenation properties were analysed on automated Sieverts’ apparatus (PCTPro-2000). For the dehydrogenation process, about 100 mg powder was firstly loaded on the reactor in the glove box and then transferred to the apparatus. After evacuating the system to vacuum, the sample was rapidly heated and maintained at a given temperature. For the rehydrogenation process, the fully dehydrogenated sample was heated at 300 °C under the hydrogen pressure of 4 MPa.

Results and discussion

The morphology of HPC is shown in Fig. 1a, which is composed of honeycomb-like hollow hemispheres with an average diameter of 600 nm. Fig. 1b displays the SEM image of TiO₂/HPC nanocomposites, in which TiO₂ nanoparticles are homogeneously dispersed on the surface of HPC. The loading content of TiO₂ is 40 wt%. The TEM image (Fig. 1c) demonstrates TiO₂ nanoparticles with a narrow size distribution of ∼10 nm (Fig. S1†). The HRTEM image (inset of Fig. 1c) shows that the d-spacing of 0.3520 nm is attributed to the (111) lattice plane of TiO₂. Fig. 1d shows XRD patterns of HPC, TiO₂ nanoparticles and TiO₂/HPC nanocomposites with 40 wt% TiO₂. The TiO₂/HPC nanocomposites exhibit distinct diffraction peaks of TiO₂ indexing to an anatase phase (JCPDS card no. 21-1272). Furthermore, the Raman spectra of the TiO₂/HPC nanocomposite in Fig. 1e displays four characteristic Raman scattering peaks at 147 cm⁻¹ (E_g), 399 cm⁻¹ (B¹_g), 516 cm⁻¹ (B¹_g) and 639 cm⁻¹ (E_g) assigned to anatase.⁴¹ XPS analysis (Fig. 1f) shows that the peaks at 463.9 eV and 458.2 eV correspond to Ti⁴⁺.⁴² When the loading content of TiO₂ in the TiO₂/HPC nanocomposites was increased to 50–60 wt%, the TiO₂ nanoparticles suffered from serious aggregation (Fig. S2†). Since the highly dispersed catalysts are favourable for the production of hydrogen, the TiO₂/HPC nanocomposites with 40 wt% TiO₂ (the weight ratio of TiO₂ : HPC is 2 : 3) are selected to be further studied as catalysts for improving the hydrogen storage properties of LiAlH₄.

Fig. 1 (a) SEM image of HPC, (b) SEM and (c) TEM images of the TiO₂/HPC nanocomposite with 40 wt% TiO₂ (inset of c is the HRTEM image), (d) XRD patterns and (e) Raman spectra of HPC and TiO₂ nanoparticles and the TiO₂/HPC nanocomposite with 40 wt% TiO₂, (f) XPS spectra of the TiO₂/HPC nanocomposite with 40 wt% TiO₂.

Fig. 2a shows the SEM image of the 37LAH–25TiO₂/38HPC composites with 37 wt% LiAlH₄, indicating that LiAlH₄ is uniformly coated on the surface of TiO₂/HPC. Fig. 2b displays that the wall of HPC becomes thicker, suggesting that LiAlH₄ is successfully supported on the surface of TiO₂/HPC. This is proved by Fig. 2c, in which the elements Al, Ti and O are evenly distributed on the carbon matrix. Furthermore, N₂ adsorption/desorption analysis (Fig. S3†) shows that the BET specific surface area and the total pore volume of 37LAH–25TiO₂/38HPC are much smaller than that of HPC, which demonstrates that LiAlH₄ was successfully impregnated into the hierarchical pores of HPC.^43,44 The morphologies of pure LiAlH₄ and the LAH–TiO₂/HPC composites with other loading weights (29, 45 and 55 wt%) of LiAlH₄ are displayed in Fig. S4.† It is obvious that the particle size of LiAlH₄ in LAH–TiO₂/HPC is decreased in comparison with that of pure LiAlH₄. A high degree of contact between nanosized LiAlH₄ and TiO₂ nanoparticles would be beneficial for dehydrogenation.

Fig. 2 (a) SEM image, (b) TEM image and (c) TEM elemental mapping images of 37LAH–25TiO₂/38HPC. (d) XRD patterns and (e) FTIR spectra of pure-LAH and LAH–TiO₂/HPC nanocomposites with different loading weights of LiAlH₄.

XRD was employed to investigate the phase structure of pure LiAlH₄ and LAH–TiO₂/HPC nanocomposites with LiAlH₄ loading of 29, 37, 45 and 55 wt%. In Fig. 2d, for pure-LAH, all diffraction peaks correspond to LiAlH₄ (JCPDS card no. 12-473) except for the diffraction peaks of parafilm at 21.4°, 23.8° and 73°. With the decrease of the LiAlH₄ loading weight, the peaks of LiAlH₄ become weaker. When the loading weight of LiAlH₄ decreases to 29 wt% in 29LAH–28TiO₂/43HPC, the peaks of Al/LiH start to appear. This indicates that the decomposition of LiAlH₄ occurred during the process of impregnation with abundant TiO₂/HPC. Meanwhile, diffraction peaks of TiO₂ are observed at 25.1°. To further confirm the decomposition process of LiAlH₄, LAH–TiO₂/HPC composites are further investigated by FTIR spectroscopy. In Fig. 2e, the infrared vibrations of Al–H stretching modes (1757 cm⁻¹ and 1621 cm⁻¹) and Li–Al–H bending modes (879 cm⁻¹ and 625 cm⁻¹) attribute to LiAlH₄.^15,18 In addition, a weak stretching mode at about 1410 cm⁻¹ appears and becomes stronger with the increasing amount of TiO₂/HPC, which can be assigned to Li₃AlH₆. This result demonstrates that a certain amount of LiAlH₄ was decomposed into Li₃AlH₆ during the impregnation process, indicating the high reactivity of the TiO₂/HPC nanocomposites. Furthermore, the stability of the LAH–TiO₂/HPC nanocomposites is studied by FTIR spectroscopy after aging for 5 months in a glovebox, as shown in Fig. S5.† The representative peaks belonging to LiAlH₄ indicate that the LAH–TiO₂/HPC nanocomposites are relatively stable at room temperature.

The effect of the TiO₂/HPC nanocomposites on the dehydrogenation behaviour of LiAlH₄ was investigated by TPD at a constant heating rate. As shown in Fig. 3, two main peaks before 300 °C represent the first two steps of hydrogen desorption of LiAlH₄. For pure-LAH, it begins to release hydrogen at 163 °C and reaches the largest hydrogen-desorption rate at 210 °C for step one. With the increasing content of the TiO₂/HPC catalyst, a remarkable drop in the dehydrogenation temperature can be found. In particular, the 37LAH–25TiO₂/38HPC nanocomposite with 37 wt% LiAlH₄, 25 wt% TiO₂ and 38 wt% HPC displays the optimal performance. It starts releasing hydrogen at 64 °C, which is about 100 °C lower than that of pure LiAlH₄. The dehydrogenation properties of 37LAH–25TiO₂/38HPC are superior to many LiAlH₄-based systems (Table 1). It is noted that the desorption peak of 29LAH–28TiO₂/43HPC is weak and broad, indicating slow kinetics of hydrogen release.⁴⁴ To illustrate the good thermodynamic performance of 37LAH–25TiO₂/38HPC, LiAlH₄ mixed with TiO₂ (LAH–TiO₂, the weight ratio of LiAlH₄ : TiO₂ = 37 : 25) or HPC (LAH–HPC, the weight ratio of LiAlH₄ : HPC = 37 : 28) were also prepared (Fig. S6†). The desorption temperatures are summarized in the line chart in Fig. S7.† The LAH–TiO₂/HPC nanocomposites display much better thermodynamic properties than those of LAH–HPC and LAH–TiO₂. Therefore, TiO₂/HPC has a more efficient catalytic effect than that of HPC and TiO₂.

Fig. 3 TPD hydrogen desorption curves of (a) pure-LAH, (b) 55LAH–18TiO₂/27HPC, (c) 45LAH–22TiO₂/33HPC, (d) 37LAH–25TiO₂/38HPC, (e) 29LAH–28TiO₂/43HPC.

Table 1 Comparison of LAH–TiO₂/HPC composites and other LiAlH₄-based materials

Sample	Thermodynamics (°C)		Method	Kinetics	Ref.
	R1 Tonset	R1 Tpeak
LiAlH4-20 wt% TiCl3-MWCNTs	75	—	TGA, 5 °C min^−1	3.6 wt% at 110 °C	32
LAH-confined-Ni-MCS	66	154	TPD, 3 °C min^−1	3.82 wt% at 150 °C	45
TiCl3·1/3AlCl3-doped LiAlH4	100	—	TG, 2 °C min^−1	<4 wt% in 60 min at 125 °C	7
2% TiN–LiAlH4	90	137.2	TPD, 2 °C min^−1	<3 wt% in 1 h at 130 °C	46
Fe2O3-doped LiAlH4	80	—	PCT, 5 °C min^−1	4.7 wt% in 70 min at 90 °C	47
5 wt% CAs/TiO2–LiAlH4	95	—	TPD, 2 °C min^−1	—	16
37LAH–25TiO2/38HPC	64	115	TPD, 5 °C min^−1	4.3 wt% in 40 min at 130 °C	This work

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As 37LAH–25TiO₂/38HPC shows the best thermodynamics, we further study its kinetic properties at different holding temperatures. As shown in Fig. 4a, the quantity of released hydrogen (calculated from pure LiAlH₄ without containing TiO₂ or HPC) as well as the rate of dehydrogenation rises as the holding temperature increases. At a temperature of 100 °C, it can release 2.3 wt% H₂ within 100 min. When the temperature increases to 130 °C, 4.3 wt% H₂ can be released within 40 min, exhibiting a fast and efficient hydrogen release process. The capacity of released hydrogen is competitive to previous reports (Table 1).

Fig. 4 (a) Hydrogen desorption kinetic curves of 37LAH–25TiO₂/38HPC at different temperatures, (b) XRD patterns of 37LAH–25TiO₂/38HPC dehydrogenated at 100 °C, 110 °C, 120 °C and 130 °C. (c) The Arrhenius plots of pure-LAH, LAH–HPC, LAH–TiO₂ and 37LAH–25TiO₂/38HPC.

XRD was implied to detect the dehydrogenated products at different holding temperatures in Fig. 4b. When dehydrogenated at 100 °C, distinct diffraction peaks of Li₃AlH₆ and Al/LiH are detected, while no peaks of LiAlH₄ have appeared, confirming the reaction of reaction (1). Moreover, with the increase of the holding temperature, the intensity of Li₃AlH₆ peaks gradually gets weaker and Al/LiH becomes the main product. This demonstrates that the decomposition degree of LiAlH₄ was related to various temperatures.

Furthermore, the isothermal dehydrogenation measurements of pure-LAH, LAH–HPC and LAH–TiO₂ were also conducted (Fig. S8 and Table S2†). Compared to pure-LAH, the amount of released hydrogen and the rate of hydrogen release have obviously improved after adding TiO₂ or HPC. The composite LAH–HPC has a higher hydrogen releasing capacity, while the dehydrogenation rate of LAH–TiO₂ is slightly faster. This suggests that the TiO₂ nanoparticles in TiO₂/HPC enhance the desorption kinetics of 37LAH–25TiO₂/38HPC, while HPC in TiO₂/HPC could improve the hydrogen desorption capacity. Thus, the synergistic effect of TiO₂/HPC enables 37LAH–25TiO₂/38HPC to display a better desorption performance.²⁵

The activation energy has been calculated from various isotherm curves through the Arrhenius equation:⁷

ln k = ln A − E_a/RT (4)

where k is the rate constant, A is a temperature-independent coefficient, E_a is the activation energy, R is the gas constant, and T is the absolute temperature. From the slope of the linear of ln k − 1/T in Fig. 4c, the activation energy is obtained (Table S3,† the error bars for E_a calculation are presented in Fig. S9†). The activation energy of pure LiAlH₄ is calculated to be 113.2 ± 6.4 kJ mol⁻¹, which is consistent with the value of 116.2 kJ mol⁻¹ reported.¹³ With the additives of HPC or TiO₂, the activation energies decrease to 63.5 ± 0.4 or 57.8 ± 2.1 kJ mol⁻¹, respectively. While, the E_a for 37LAH–25TiO₂/38HPC is 47.1 ± 3.5 kJ mol⁻¹, which is about 66.1 kJ mol⁻¹ lower than that of pure LiAlH₄. This confirms that the high dispersion of TiO₂ nanoparticles on the surface of HPC as a mixed catalyst plays a critical role in enhancing the kinetic properties of 37LAH–25TiO₂/38HPC.

The composite of 37LAH–25TiO₂/38HPC with 37 wt% LiAlH₄ is further studied after full dehydrogenation. Fig. 5a displays the SEM image, in which HPC maintains its hierarchically porous structure. The EDS elemental mapping images (Fig. S10†) indicate that the Al, Ti, and O are uniformly distributed on the matrix of C, suggesting no aggregation of dehydrogenated products and TiO₂ nanoparticles. This confirms that the HPC is a stable nanoscaffold material for dispersing TiO₂ nanoparticles and preventing the dehydrogenated products from aggregation. In Fig. 5b, the dehydrogenated products of 37LAH–25TiO₂/38HPC are composed of Al and LiH without LiAlH₄, Li₃AlH₆ or other additional phases. It is noteworthy that there is no diffraction peak of TiO₂ or the Ti-containing phase, which may be in the form of an amorphous state.

Fig. 5 (a) SEM image of 37LAH–25TiO₂/38HPC after dehydrogenation, (b) XRD patterns of pure-LAH and 37LAH–25TiO₂/38HPC after dehydrogenation, (c) Ti 2p XPS spectra of 37LAH–25TiO₂/38HPC before dehydrogenation (I) and after dehydrogenation (II).

To understand the nature of the Ti-catalyst, 37LAH–25TiO₂/38HPC was further studied by XPS before and after dehydrogenation in Fig. 5c. Compared to TiO₂/HPC (Fig. 1f), it is apparent that the shape of the Ti 2p spectra has greatly changed. The broadening of XPS spectra suggests the presence of multiple oxidation states of Ti. Before dehydrogenation (I) (a.c. after impregnation), in addition to two main peaks at 463.5 eV and 457.9 eV for Ti⁴⁺, another pair of spin–orbit doublets at 461.4 eV and 456.6 eV could be corresponded to Ti³⁺. The lower binding energy values indicate the partial reduction of TiO₂ resulting from the reaction between TiO₂ and LiAlH₄ during the impregnation process. After full dehydrogenation (II), the spectra became more complicated. Peaks located at 460.0 and 455.1 eV, attributed to Ti²⁺, appeared,⁴¹ illustrating that a deep reduction of TiO₂ took place with the increase of the temperature. From Gaussian fitting of the peak area, the Ti(4+)/Ti(3+)/Ti(2+) atomic ratio is 0.62 : 0.22 : 0.16, demonstrating the multiple valence on the oxide surface with dominant Ti⁴⁺. Since a microstructured composite with Ti⁰/Ti²⁺/Ti³⁺ defect sites could enhance the dehydriding kinetics,^7,24 the reduction of Ti⁴⁺ and the in situ formation of Ti-containing lower oxidation valence species have played a significant role in improving the dehydrogenation properties of LiAlH₄.

In order to examine the reversibility for hydrogen storage, the dehydrogenated product of 37LAH–25TiO₂/38HPC (LiH–Al–TiO₂/HPC) was rehydrogenated at 300 °C for 24 h under a hydrogen pressure of 4 MPa. Fig. 6 shows the FTIR spectra of the dehydrogenated product LiH–Al–TiO₂/HPC and the rehydrogenated product. Fig. 6a shows no peak of [AlH₄]⁻ or [AlH₆]³⁻, suggesting the complete dehydrogenation of 37LAH–25TiO₂/38HPC. Fig. 6b displays the infrared vibrations of the Al–H bond in [AlH₆]³⁻ between 1600–1400 cm⁻¹ in the rehydrogenated products, indicating the partial reversibility of reaction (2). Additionally, the SEM image and TEM image (Fig. S11†) show no aggregation or growth of the TiO₂ nanoparticles after re-adsorbing H₂. In comparison, little H₂ could be re-adsorbed for the dehydrogenated products of LAH–HPC and LAH–TiO₂ since a higher pressure was necessary.⁴⁸ These results further confirm the important effect of the TiO₂/HPC nanocomposites in the reversibility of the 37LAH–25TiO₂/38HPC.

Fig. 6 FTIR spectra of (a) dehydrogenated product LiH–Al and (b) rehydrogenated product of 37LAH–25TiO₂/38HPC.

From the above investigation, the as-synthesized TiO₂/HPC nanocomposites exhibit excellent catalytic activity in the de/rehydrogenation of LiAlH₄. The interpretations from two aspects are summarized to comprehend the catalytic function.^49–51 On one hand, the high distribution of TiO₂ nanoparticles is correlated with abundant active sites. The unique structure of TiO₂/HPC provides both the inner and the outer surface contact with LiAlH₄. Thus, a great deal of active sites is favorable to the kinetic enhancement of LiAlH₄. Meanwhile, the hierarchically porous morphology and structure of the obtained carbon material offers a stable scaffold to highly disperse LiAlH₄ and provide a hydrogen transporting pathway. This not only decreases the particle size of LiAlH₄ but also prevents the dehydrogenated products from agglomerating. On the other hand, the reduction of Ti⁴⁺ during the impregnation and dehydriding process is accompanied by the formation of Ti(4+)/Ti(3+)/Ti(2+) defect sites. As a result, the decomposition pathway of LiAlH₄ in LAH–TiO₂/HPC was altered and the reaction energy barrier significantly decreased. For the rehydrogenation process, LiH–Al on TiO₂/HPC is much easier to re-adsorb H₂ at a relatively low hydrogen pressure.

Conclusions

LAH–TiO₂/HPC nanocomposites with LiAlH₄ supporting on TiO₂/HPC were prepared. The optimized nanocomposite of 37LAH–25TiO₂/38HPC with 37 wt% LiAlH₄ started to release H₂ at 64 °C, which is about 100 °C lower than that of pure LiAlH₄. Furthermore, it could release 4.3 wt% H₂ within 40 min at 130 °C and 6.2 wt% H₂ within 60 min at 160 °C. The activation energy of the dehydrogenation process is 47.1 ± 3.5 kJ mol⁻¹. In addition, we demonstrated the partial reversibility of this nanocomposite at 300 °C under a hydrogen pressure of 4 MPa. The enhanced hydrogen storage properties of LAH–TiO₂/HPC are attributed to the synergistic effect of TiO₂/HPC nanocomposites. HPC acted as a stable nanoscaffold to disperse LiAlH₄ on the surface of TiO₂/HPC. TiO₂ nanoparticles with high distribution provide numerous active sites and Ti-containing defect sites, decreasing the energy barrier of the de/rehydrogenation of LiAlH₄. This catalyst is useful to promote the de/rehydrogenation performance of complex hydrides.

Acknowledgements

This work was supported by National NSFC (51371100 and 51271094), and MOE (B12015 and IRT13R30).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis procedures of HPC and dehydrogenation performances of LAH–HPC and LAH–TiO₂, and additional XRD patterns, SEM images, FTIR, BET, EDS, and N₂ adsorption/desorption curves. See DOI: 10.1039/c6qi00200e

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