Yaran
Zhao
,
Mo
Han
,
Haixia
Wang
,
Chengcheng
Chen
and
Jun
Chen
*
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: chenabc@nankai.edu.cn; Fax: +86-22-23506808; Tel: +86-22-23506808
First published on 30th September 2016
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.
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 (Tonset) 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 nanoscale33–35 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.
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.
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. |
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 |
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).
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
lnk = lnA − Ea/RT | (4) |
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.
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.
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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