Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat An efficient mechanochemical synthesis of alpha-aluminum hydride: Synergistic effect of TiF3 on the crystallization rate and selective formation of alpha-aluminum hydride polymorph Congwen Duana,b,⁎, Yizheng Caoc, Lianxi Hud, Dong Fua,b,⁎, Jinlong Mae, Jeffrey Youngbloodf aHebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, 071003, PR China bMOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, PR China c SSCI, Albany Molecular Research Inc., West Lafayette, IN 47906, USA d School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China e School of Information Science and Engineering, Hebei University of Science and Technology, Shijiazhuang, 050018, China f School of Materials Engineering, Purdue University, West Lafayette, IN 47907, USA G R A P H I C A L A B S T R A C T A R T I C L E I N F O Keywords: Synergistic effect Mechanochemical reaction ɑ-AlH3 nano-composite Dehydriding property A B S T R A C T α-AlH3 is one of the most promising hydrogen storage materials due to its high gravimetric hydrogen capacity and low dehydriding temperature. In present work, a convenient and cost-efficient solid-state mechanochemical reaction is proposed to obtain α-AlH3 nano-composite. With the addition of TiF3, α-AlH3 nano-composite was formed in a short period by milling of LiH and AlCl3. Based on XRD and NMR results, the average grain size of the α-AlH3 in the nano-composite was 45 nm. The reaction pathway as well as the synergistic effect of TiF3 on the solid state reaction between LiH and AlCl3 were confirmed. In the α-AlH3/LiCl nano-composite, TiF3 reduced the temperature of dehydriding reaction and improved dehydrogenation rate of α-AlH3. Within the temperature range between 80 and 160 °C, dehydrogenation of the as-milled α-AlH3 nano-composite showed fast kinetics. At 160 °C, a maximum hydrogen desorption of 9.92 wt% was obtained within 750 s, very close to the theoretical hydrogen capacity of α-AlH3. https://doi.org/10.1016/j.jhazmat.2019.03.064 Received 29 November 2018; Received in revised form 11 February 2019; Accepted 14 March 2019 ⁎ Corresponding authors at: Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, 071003, PR China. E-mail addresses: duancw@ncepu.edu.cn (C. Duan), fudong@ncepu.edu.cn (D. Fu). Journal of Hazardous Materials 373 (2019) 141–151 Available online 15 March 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved. T 1. Introduction Hydrogen, as a clean and renewable energy source, has received wide attention [1–3]. However, the major obstacle for a widespread hydrogen utilization is the challenges in storage [4]. The hydrogen storage technology includes gas storage, liquid storage and solid-state hydrogen storage method which is different from other storage technologies such as CO2 adsorption and storage technology [1,5,6]. Solidstate hydrogen storage methods offer advantages, such as the lower safety risk, over high-pressure and cryogenic approaches [7]. Additionally, solid materials have excellent volumetric and gravitational storage capacities. Therefore, a lot of efforts have been undertaken in research and development of solid-state hydrogen storage materials as well as storage systems [8–16]. Research of safe, recyclable and cheap hydrogen storage materials, such as the light metal hydrides and their complexes, is the focus of the most recent studies; however, several issues still remain unsolved [4,10–12]. Aluminum trihydride (AlH3), also known as alane, has a large hydrogen capacity (about 10wt%) as well as a high volumetric density equal to 148 kg/m3 of hydrogen [17–21], both of which exceed United States Department of Energy (US DOE) requirements, i.e., 7.5 wt.% and 70 g/l [7,20,22,23]. AlH3 completely decomposes with a desirable rate at relatively low temperatures (100–200 °C), which is essential for the compatibility requirements with proton-exchange membrane fuel cells [24]. AlH3 releases hydrogen leaving only metallic Al, a residue that is convenient to recycle. Thus, similar to RhAu, carbon nanotubes and transition metal composites [25–32], AlH3 is considered as an excellent reversible hydrogen storage source with potential applications in rocket and fuel cell industries [33–37]. Currently, seven different polymorphs for AlH3 are known: α, α′, β, γ, δ, ε and ζ [38–46]. All but α-polymorph are metastable at ambient conditions. α-AlH3 is the most studied polymorph because of the high stability. It is stable at room temperature but thermodynamically unstable at 150 °C under 10 kbar [40]. Although AlH3 has many promising attributes, it is still not fully utilized as a hydrogen storage material, mainly because of the lack of an efficient and convenient industrial production method. Until 1976, the most popular methods to prepare AlH3 polymorphs were the wet chemical synthesis proposed by Brower et al. It involved LiAlH4 and AlCl3 as the starting materials and LiBH4 as a desolvation aid, and ether as the solvent. Alaneether complexes form according to Eq. (1) [47]: 3LiAlH4+AlCl3+ nEt2O→4AlH3*nEt2O+3LiCl (1) Bulychev et al. [48–50] later modified this method and synthesized AlH3 by reacting lithium aluminum hydride (LiAlH4) with a strong acid like sulfuric acid (H2SO4) in ether, with crystallization from diethyl ether-benzene solution (see Eq. (2)): 2LiAlH4+H2SO4→ 2AlH3+ Li2SO4+ 2H2 (2) In general, AlH3 can be synthesized from alkali and/or alkaline earth metal tetrahydroaluminates, or hydrides based on wet chemical reactions [45–47] through the formation of alane–ether complexes. However, despite being less expensive and/or very reactive, these methods of α-AlH3 synthesis involve large amounts of environmentally harmful organic solvents, and are highly sensitive to temperature and time. It is impractical to handle volatile organic liquids and the methods are not suitable for large-scale AlH3 production. Thus, a solvent-free synthesis yielding non-solvated and adduct-free AlH3 is highly desirable. Saitoh reported synthesis of adduct-free AlH3 by hydrogenation of Al (see Eq. (3)) [51]. However, this method can only be accomplished under H2 pressure with at least 10 GPa at 25 °C because of the stabilizing surface aluminum oxide layer or low diffusivity of hydrogen [34]. Saitoh also demonstrated a H2 pressure as high as 6 GPa at 300–800 °C could be used to perform efficient Al hydrogenation [52]. Thermodynamically, hydrogenation of Al to obtain AlH3 is not a preferred method. Thus, such high pressures during AlH3 synthesis make this method less attractive for large-scale AlH3 synthesis. 2Al+3H2→2AlH3 (3) Mechanochemical method is a well-known technique, which is green and sustainable and offers several economic benefits [53]. It was demonstrated by several groups that this process can be solvent-free and consumes less energy comparing to solvent-based reactions [54,55]. Thus, mechanochemical method is gaining attention as an alternative to conventional solution-based processes and was proposed to synthesize metal hydrides by solid-state reactions with ball milling [53–58]. For example, Fernandez studied milling processes of LiAlH4 with different metal halides (VCl3, VBr3 and AlCl3) and observed AlH3 formation by a mechanochemical reaction between LiAlH4 and AlCl3 [59]. To date, mechanochemical milling has been successfully used for the solid-state synthesis of AlH3 [40,60–63]. Mechanochemical method for AlD3 synthesis was employed by Brinks [40]. AlD3 mixture of various polymorphs was synthesized by mechanically-assisted reaction between LiAlD4 and AlCl3 at 77 K and room temperature (see Eq. (4)) [36]. 3LiAlD4(s)+ AlCl3(s)→ 4AlD3+3LiCl (4) Lately, Paskevicius obtained a mixture of several polymorphs of AlH3 (α-, α'-, and γ) by a mechanochemical reaction of LiAlH4 and AlCl3 [61]. The final crystalline size of AlH3 was 15–17 nm. Sartori reported that the yield of (α+α')-AlH3 increased from 6.1–49.9mol% when NaAlH4+AlCl3 and LiAlD4+AlBr3 were used as reagents (see Eqs. (5) and (6)) [62,63]: 3LiAlD4+AlBr3→ 4AlD3+3LiBr (5) 3NaAlH4+AlCl3→ 4AlH3+3NaCl (6) Prior studies mainly focused on mechanochemical method for AlH3 composite synthesis rather than the selective synthesis of certain polymorphs during mechanochemically activated reaction process. Ti and its halides are well-known as effective catalysts for the synthesis of various hydrides. However, most of previous research focused on the effect of Ti-based additives on hydrogen sorption or desorption properties on/from the hydrogen storage materials. For example, Ma demonstrated that presence TiF3 significantly enhanced dehydrogenation of MgH2 [64]. Ismail confirmed catalytic role of Ti-containing compounds for performance of NaAlH4-TiF3 system and promotes dehydrogenation of NaAlH4 [65]. Balema [66,67] recently demonstrated synthesis of Li3AlH6 using solid state reaction at ambient temperatures, which proceeded faster with addition of TiCl4. Moreover, Sartori [63] added FeF3 to the cryomilled LiAlD4/AlCl3 mixture to accelerate nucleation of the as-milled product and alter its α/α'-AlH3 ratio. This could be attributed to the isostructure between FeF3 and α'-AlH3 which may lead to larger amount of α'-AlH3 in the as-milled product [63]. Because TiF3 and α-AlH3 have similar structure [68], it is possible that fine TiF3 crystals may act as nucleation seeds for αAlH3 formation. In this work, we studied formation of α-AlH3 by the mechanochemical process. We used solid-state reaction to obtain AlH3 by milling using cheaper AlCl3 and metal hydrides as starting materials. Without solvent addition, the separation is omitted and the cost of production can be reduced effectively. We introduced TiF3 into LiH/ AlCl3 reaction system to facilitate nucleation of α-AlH3. This preliminary finding demonstrates that the usage of TiF3 during mechanochemical process may be beneficial to the formation of nano-sized αAlH3. This simple and efficient synthesis of α-AlH3 for the first time offers a complete suppression of the parasitic formation of metallic Al. Moreover, the high selectivity of form makes it convenient for further extraction. Meanwhile we systematically studied effect of TiF3 on the ball-milling assisted reaction rates as well as the underlying mechanisms of the TiF3-assisted nucleation of α-AlH3. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 142 2. Experimental The Gibbs free energy of the reaction between LiH and AlCl3 with a range of 273–380 K is also shown in Fig. S1. The calculation of Gibbs free energy has been described in detail elsewhere [69]. The Gibbs free energy for the reaction (Eq. (7)) at 298 K is about −269 kJ/mol, and therefore the reaction can proceed at room temperature. 3 LiH+AlCl3→AlH3+ 3LiCl (7) The starting materials LiH (97%), AlCl3 (99%) and TiF3 (98%) were purchased from Sigma Aldrich for synthesis of α-AlH3 composite. The materials, LiH, AlCl3 and TiF3 were used as-received for this research. The materials were analyzed shortly prior to the use. Purity of materials can be seen from Fig. S2 in ESI. Because of potential sensitivity to moisture, reagents were handled and stored in sealed vials in Ar-filled glove box. Oxygen and water levels in a glove box were> 5 ppm. In an Ar-filled glove box, the reagents, LiH/AlCl3 as well as TiF3 were weighed and mixed in the various molar ratios firstly. The mixtures were subsequently sealed in a high-pressure ball milling canister for use. Ambient temperature milling was performed using planetary QMSP4 mixer containing custom-built high-pressure 500 cm3 stainless steel (316) canister [69–72]. Each canister was sealed on both ends using an o-ring. Milling balls were made from stainless steel 316. The containers were evacuated and filled with H2 or Ar to a designated pressure ranging between 5 and 40MPa. Different ranges of milling parameters, such as ball to powder mass ratio, milling speed and gas pressure, were explored to obtain different reaction products. Thus, ball milling was performed for variable durations with different speeds between 100 and 400 rpm. Ball to powder weight ratios were 40:1, 80:1, 120:1 and 200:1. To ensure that average temperatures in the milling vials were close to room temperature, the milling procedure involved alternating milling directions every 20min. Between every 20min the milling was discontinued for 10min. After completion of the mechanochemical reaction, the as-milled product was obtained and collected from vials for further analysis. For the isothermal desorption measurements, the as-milled samples were loaded into a stainless holder and sealed in an Argon-atmosphere glove box. The hydrogen desorption experiments were carried out in a vacuum apparatus, from which the temperature range from 25 °C to the specified temperature at a vacuum up to 1×10−2 Pa. To investigate the dehydriding property of α-AlH3 composite, the as-milled products were heated in a vacuum chamber from room temperature to different temperatures (80, 120, 140 and 160 °C). The time required for the full dehydriding reaction was fixed within 6500 s, respectively. Once the amounts of hydrogen decomposed from the composite were measured, the vial vacuum change in chamber can be calculated, the corresponding the contents of AlH3 in its composite could be obtained according to the ideal gas equation as well as stoichiometric weight of AlH3 calculated by the chemical reaction. The samples for X-ray diffraction (XRD), scanning electron microscopy (SEM) and solid-state NMR were also prepared in the glove box. The powder X-ray diffraction patterns were obtained by a Philips X'PERT diffractometer equipped with graphite monochromator with a rotating anode tube and operated with Cu Ka radiation source. The voltage and tube current were 45 kV and 40mA. The samples were scanned from 10 to 90° at 0.5°/min. The average grain size of the αAlH3 in the composite was obtained by XRD analysis and the following Scherrer formula. =D k cos (8) Where D is the average grain size, k a constant of 0.89, λ and β the X-ray wavelength and the half-width, θ the diffraction angle. The as-milled product was placed on carbon tape, coated with a thin layer of gold (Au) by sputtering (EMITECH K450X) and then observed on a scanning electron microscope (SEM, Quanta 200 FEG). The solidstate NMR spectra were collected with a Bruker Avance NMR spectrometer, equipped with a wide bore 14.1 T magnet and a magic angle spinning probe head for rotors of 2.5 mm diameter. To protect the samples from moisture, all samples were packed into a 2.5mm ZrO2 rotor and sealed with a tight fitting kel-F cap. NMR shifts were reported in parts per million (ppm). Moreover, this shift was referenced externally with potassium alum to −0.033 ppm, and recycle delay for 27Al was set to 3 s. TEM was conducted on a Tecnai G2 F30 instrument operating at 200 kV. Differential thermal analysis measurements were performed using a TG-DSC METTLER TOLEDO instruments, the samples were handled and transferred to the instrument in T-zero pans and were heated from 40 to 240 °C. Fig. 1. XRD patterns of LiH and AlCl3 with a molar ratio of 3:1 milled in various hydrogen pressures for 1 h. BPR and rotation speed were 120:1 and 300 rpm. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 143 3. Results and discussion 3.1. Mechanochemical synthesis Solid-state mechanochemical reaction of Li hydride and AlCl3 is regarded as a green pathway for producing adduct-free AlH3 [7,20,61]. Brinks reported that ball milling mixture of LiAlH4 and AlCl3 generates mainly Al and LiCl, with small amounts of α-AlH3 and α′-AlH3 [40,41]. Gupta et al. [20] suggested that gas pressure affects decomposition of the metastable hydrides into the metal and hydrogen under the mechanochemical conditions. To prevent AlH3 from decomposing during the ball milling and to find an efficient and cost-effective mechanochemical process for α-AlH3 synthesis, we conducted LiH/AlCl3 reaction under various hydrogen pressures. Fig. 1 shows XRD patterns of the LiH and AlCl3 mixture (with a 3:1M ratio) milled for 1 h at various hydrogen pressures. Ball-to-powder weight ratio (BPR) and rotation speed were 120:1 and 300 rpm, respectively. LiH/AlCl3 mixture milled under 10MPa of hydrogen pressure demonstrated only XRD peaks belonging to the metallic Al, LiCl and unreacted LiH. Thus, the following reaction occurred during the ball milling Eq. (9): 3LiH+AlCl3→ 3LiCl+Al+ 1.5H2 (9) Under higher hydrogen pressures, AlH3 XRD peaks were not observed as well. These results are consistent with the ones obtained by Hlova [7], who also found that replacing AlH3 with metallic Al resulted in LiH and AlCl3 during ball-milling under 30MPa hydrogen pressure. Paskevicius elaborated that high-energy milling at room temperature might be needed for AlH3 dehydrogenation [61]. Although local temperatures are often quite high during high-energy ball milling, such temperature spikes are generally very brief. Moreover, each vial has enough time to cool down during paused milling cycles. Considering kinetic stabilization of AlH3 at room temperature, raising temperature by a few degrees should not decompose AlH3. The final product obtained after 1 h milling under 40MPa of hydrogen pressure was light grey in color and consisted of αand α′-AlH3 as well as LiCl, LiH and LiAlCl4 with no metallic Al. Grain sizes of α and α′ AlH3 phases were up to 83 and 87 nm, respectively. Thus, we conclude that α and α′-AlH3 phases transformed into their corresponding nano-crystalline forms. To explore whether the milling parameters, especially milling intensity, were related to the formation of metallic Al, we decreased rotation speed from 300 to 100 rpm. BPR was also reduced from 120:1 to 40:1. The hydrogen pressure was set at 10MPa. Fig. 2 shows the XRD patterns of products synthesized by milling LiH/LiCl mixture with various ratios for 1 h. At 300 rpm and 80:1 ratio, intensities of LiH and AlCl3 XRD peaks are bigger, suggesting a lower reaction rate. However, metallic Al was formed in these as-milled samples. As the ratio was further reduced to 40:1, diffraction peaks of LiH and AlCl3 were observed. Complete reaction of LiH and AlCl3 with metallic Al as a product was not achieved during the final milling stage. Fig. S3 shows XRD results of the as-milled LiH/LiCl mixture milled at various rotation speeds with the same milling time. The results were the same as shown in Fig. 2: only Al, LiCl, unreacted LiH and AlCl3 diffraction peaks were observed. Although the rotation speed and the BPR have large impacts on the kinetics of the reaction, these parameters cannot suppress formation of metallic Al during mechanochemical reaction between LiH and AlCl3 under a lower hydrogen pressure at room temperature. Thus, pressure is more influential on the mechanochemical synthesis of AlH3 than milling intensity. Therefore, control of the reaction by changing the pressure during the reaction may be feasible to obtain pure phase of AlH3. Despite the fact that hydrogen pressure has such a dramatic influence on the mechanochemical reaction outcome, pressure applied in this work was too high to be used in large-scale AlH3 production. Mechanically activated solid-gas reactions were studied by several groups [73–76], and many of them confirmed that no gas-solid exchanges occur during mechanochemical reaction of LiAlH4/AlCl3 [61], NaAlH4/AlCl3 and LiH/AlCl3 [7,62,63]. Thus, mechanochemical reaction of the LiH/AlCl3 mixture can also be performed in other inert atmospheres, such as Ar. Therefore, we performed mechanochemical reaction between LiH and AlCl3 (the molar ratio is 3:1) at 300 rpm with BPR of 120:1 for 1 h under various Ar pressures. XRD patterns of the products are very similar to those prepared by milling the LiH/AlCl3 mixture under 10–30MPa hydrogen pressure (Fig. 3). XRD peaks attributable to metallic Al were observed in samples milled in Ar at 5MPa. When Ar pressure was 10MPa, in contrast to the mechanochemical reaction under H2, unreacted LiH, α, α′-AlH3, LiCl and LiAlCl4 instead of metallic Al were observed. Thus, pressure is a critical factor that efficiently suppresses formation of Al during the ball milling of LiH/AlCl3 mixture. The value of the critical pressure decreases as the gas molecular weight increases, demonstrating that the critical pressure is related to the physical properties of the ambient atmosphere. During the mechanochemical reaction in the LiH-AlCl3 system at 10–15MPa, the reaction process did not accelerate based on the XRD results. Complete reaction of LiH and AlCl3 was not achieved by milling the Fig. 2. XRD patterns of LiH and AlCl3 with a molar ratio of 3:1 milled at 300 rpm with various BPR for 1 h. The hydrogen pressure was 10MPa. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 144 mixture for 1 h, where unreacted LiH was observed in the final milled product. Therefore, this serves as evidence that the increment in critical pressure is not important to obtain a lower reaction rate, which is more likely controlled by the milling intensity. It was demonstrated by several groups that the rotation speed and BPR are important parameters in the mechanochemical process. They effectively enhances the chemical reactivity between the reagents, increase the kinetics of the reaction and deduce the time of reaction [7,20,69]. To accelerate mechanochemical reaction of LiH/AlCl3 and obtain AlH3 composite in a short milling time, the LiH/AlCl3 mixture (the molar ratio is 3:1) was milled at 400 rpm with a BPR 200:1 in Ar at 10MPa with different milling times. When milling time was 0.5 h, LiH and AlCl3 XRD peaks disappeared (see Fig. 4), indicating acceleration of the mechanochemical reaction rate as the milling intensity increased. Additionally, peaks attributable to LiAlCl4 and αand α′AlH3 phase were observed in the sample milled for 1 h. Thus, a complete reaction with the formation of α-AlH3 was not achieved. Phases observed after 3 h milling were the same as those obtained by milling LiH/AlCl3 mixture for 1 h but intensity of AlH3 peaks increases with milling time. Upon milling for 5 h, diffraction peaks of the intermediate LiAlCl4 were still observed. However, its peak between 10° and 20° broadened significantly, indicating formation of an amorphous intermediate compound. Although the BPR and rotation speed accelerate reaction kinetics, it is difficult for LiH/AlCl3 mixture to form AlH3 nano-composite within 5 h of the mechanochemical reaction. Based on calculation, the average grain size of α-AlH3 can get 124 nm. Thus, it was ineffective to achieve a complete reaction to generate AlH3 by increasing reaction intensity. Moreover, formation of different polymorphs of AlH3 will ultimately lead to the complexity of the hydrogen extraction. This mechanochemical method is considered environmentally friendly because no solvents were used during the whole milling process. This method, involving a addition of AlCl3 into LiH, is a simple way to synthesize α-AlH3 nano-composite. This method may not be fully applicable to the LiH-AlCl3 system, which was confirmed by our in-depth investigation on formation of α-AlH3 during short milling periods. Fig. 3. XRD patterns of LiH and AlCl3 with a molar ratio of 3:1 milled at various Ar pressures for 1 h with a BPR of 120. The rotation speed is 300 rpm. Fig. 4. XRD patterns of the LiH and AlCl3 with a molar ratio of 3:1 milled at 400 rpm with a BPR 200:1 in Ar at 10MPa for different milling times. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 145 3.2. Effect of TiF3 on mechanochemical reaction Sartori demonstrated that FeF3 not only accelerated nucleation of the as-milled metal hydride [63], but also affected crystalline structure during cryo-milling processes. TiF3 can act as a seed crystal for formation of α-AlH3. To investigate the influence of TiF3 effect on the reaction rate of the mechanochemical reaction of the LiH/AlCl3 mixture and crystallization of the AlH3 more clearly, we studied XRD patterns of products of mechanochemical reactions of the LiH/AlCl3 mixtures with the presence of the TiF3 powder at a 3:1:0.1M ratio milled at 400 rpm with a BPR 200:1 in Ar at 10MPa for 0.5, 1, 3 and 5 h (see Fig. 5). After milling for 0.5 h, LiH and AlCl3 XRD peaks disappeared, however, complete conversion of the LiH/AlCl3 mixture into AlH3 and LiCl phases did not occur. Similar diffraction peaks were observed when TiF3 was not added. Thus, TiF3 could not accelerate diffusion rate between LiH and AlCl3 after 0.5 h of ball-milling. XRD pattern of the LiH/AlCl3 mixture after milling for 1 h contained LiAlCl4 and α-AlH3 peaks (Fig. 5), indicating that crystallization of amorphous phases (formed during the first 30min of ball-milling) occurred within the second 30min. Thus, TiF3 can induce quick crystallization of the product from the amorphous phases resulting in α-AlH3 phase. After ball-milling for 3 h, diffraction peaks of the intermediate phases, such as LiAlCl4, disappeared, leaving only α-AlH3 and LiCl phases in the final product. As the milling time was increased to 5 h, no substantial changes in the XRD patterns were observed comparing to the products obtained after 3 h milling. However, peak intensity at  30° decreased suggesting formation of nano-sized α-AlH3. The average grain size of α-AlH3 formed after 5 h milling with the presence of TiF3 was up to 45 nm. Thus, TiF3 addition into the LiH/AlCl3 mixture can induce nano-sized α-AlH3 formation with much smaller crystallite size than without TiF3. Crystallization rate and selective formation of only α-AlH3 polymorph occurs because of the synergetic mechanism. 3.3. Analysis of the mechanochemical process To better understand the mechanochemical process, LiH/AlCl3/TiF3 mixtures (molar ratio 3:1:0.1) milled at 400 rpm with a BPR of 200:1 for various times were analyzed by SEM (Fig. 6). Milling for 0.5 h did not reduce particle sizes (Fig. 6a). Moreover, a large amount of agglomerates formed with irregular shapes, which can be attributed to the brittleness of the rods. Average particle size of the mixture milled for 0.5 h was  40 μm. After 1 h milling, significant amount of large agglomerates were still present (Fig. 6b and c), with finer individual particles adhered to the agglomerates. After 3 h milling, only fine particles were observed with no agglomerates. With longer milling time, the powders became granular in shape with uniform size distribution. The average particle size of the sample milled for 5 h is smaller than 1 μm, which is in agreement with the XRD analysis. We also analyzed milled samples by 27Al NMR (Fig. 7). Hlova et al. observed an NMR peak centered at   −2 ppm for the mixtures milled for 8 h [7]. They attributed this peak to AlCl3. However, we observed no AlCl3, likely because AlCl3 was consumed after 0.5 h of milling (Fig. 7a). Instead, we observed a dominant spectral band at  95 ppm. It was demonstrated by Hlova that the peak at  95 ppm is attributed to the four-coordinated Al [7]. The four-coordinated Al signal may be assigned to a superposition of the central transition powder patterns from LiAlCl4 or LiAlH4. With the above XRD analysis, this band corresponds to the four-coordinated Al in LiAlCl4, which is the intermediate product. The same results were observed using DPMAS spectra obtained by Hlova et al. [7]. These results also agree with our XRD analysis. With 1 h milling, NMR spectrum also confirmed the presence of LiAlCl4 but the peak intensity was smaller. NMR spectrum of sample milled for 1 h exhibited broad peaks around at  33 ppm and  6 ppm (see Fig. 7b), indicating formation of the 6-coordinated Al. The amorphous intermediate products might have more than one Al species because Li-Al-H and Al-H both have priorities in the formation during the mechanochemical reaction. However, the coordinated Al species in Li-Al-H is different from that in Al-H. Grubisic demonstrated that for the other aluminum hydride clusters such as AlxHx+2 (3≤×≤8), the polyhedral structure of Al-H cannot be formed with six polymorphic modifications [77]. Thus, it can be deduced that polymers Al-Hx (x= 1, 2, 3 the coordinated Al species is six polymorphic modification) in an amorphous state was formed during the milling process. After milling for 3 h, LiAlCl4 NMR peak disappeared (Fig. 7c), thus, it is very likely that LiAlCl4 as an intermediate product were entirely consumed in the reaction. Additionally, peak corresponding to the 6-coordinated Al changed significantly, which was due to the phase conversion of the intermediate products during extended milling. Thus, NMR can be used as an indicator of the mechanochemical reaction termination. Humphries and Hwang associated the resonance bands at  6.0 ppm with the α-AlH3 phase [78,79]. Thus, after milling for only 60min, only αAlH3 was observed in the product. Therefore, TiF3 induced crystallization of the amorphous phase and accelerated crystallization of the αAlH3 phase. Fig. 5. XRD patterns of the LiH, AlCl3 and TiF3 with a molar ratio of 3:1:0.1 milled at 400 rpm with a BPR 200:1 in Ar at 10MPa for different milling times. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 146 3.4. Reaction and synergetic mechanism Even though we thoroughly analyzed solid-state mechanochemical reaction by XRD and NMR, the actual details of the reaction mechanism and synergistic effect of TiF3 were still somewhat unclear. Typically, AlH3/LiCl composite can form directly during the milling process and mechanism of the AlH3 formation during this process is similar to the one reported by Paskevicius [61,80]. XRD results (Fig. 1) indicated that metallic Al instead of AlH3 formed during the milling process. To investigate the mechanism for the Al formation during ball milling of LiH/AlCl3, this mechanochemical reaction was controlled by a stepwise reduction of Ar pressure. The as-milled LiAlCl4/LiCl composites comes from 3:1:0.1 LiH:AlCl3:TiF3 mixture at 400 rpm under a 10MPa Ar atmosphere, with a BPR of 200:1 for 0.5 h milling conditions were used as the original compounds to be milled at a lower gas pressure. During the second stage milling Ar pressure was reduced to 5MPa but the other milling parameters remained unchanged. Fig. 8 shows LiAlCl4/LiCl composites milled at 5MPa for 0.5 and 1 h with the same BPR and the rotation speeds. LiCl remained present, while some of the LiAlCl4 was consumed after 30min, based on the lower intensities of peaks at  29° and  48°. New peaks, corresponding to the metallic Al, appeared at 38 and 40°. Additional 30min of milling resulted in disappearance of the LiAlCl4 peaks. At the same time, peaks corresponding to metallic Al and LiCl by-products showed high intensities. Thus, this mechanically activated reaction involves formation of LiAlCl4 and amorphous intermediate product, and their subsequent decomposition. Thus, comparing to the decomposition of AlH3 during the milling, the above experimental evidence indicated that metallic Al formed at lower pressures appeared as a result of the transformation of the LiAlCl4 intermediate products. It appears that TiF3 has a synergistic effect on the mechanochemical reaction of LiH and AlCl3. According to the NMR analysis (Fig. 7b), when TiF3 was added into the reaction system, almost all of the 6-coordinated Al in AlCl3 was converted into the 4-coordinated Al in LiAlCl4 and amorphous Al-Hx (x= 1, 2, 3). With furthermore milling, LiAlCl4 and amorphous Al-Hx were first converted to amorphous but then to the 6-coordinated Al in α-AlH3. The intermediate LiAlCl4 and amorphous Al-Hx were identified by both XRD and NMR, while, its role in the Fig. 6. Morphology of the LiH, AlCl3 and TiF3 with a molar ratio of 3:1:0.1 milled at 400 rpm with a BPR 200:1 in Ar at 10MPa for different milling times: (a) 0.5 h, (b) 1 h, (c) 3 h, (d) 5 h. Fig. 7. Solid-state 27Al NMR spectra of the LiH, AlCl3 and TiF3 with a molar ratio of 3:1:0.1 milled at 400 rpm with a BPR 200:1 in Ar at 10MPa for the 0.5, 1, 3 h. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 147 mechanochemical process still remains unclear. Ashby et al. [81,82] demonstrated that the transformation between AlIV and AlVI species may occur as a result of Cl replacement by H in LiAlCl4, which leads to LiAlCl4-xHx (x= 1, 2, 3) formation. Thus, it is assumed that a LiAlCl4xHx amorphous intermediate formed during the furthermore milling of LiAlCl4 and Al-Hx. It is very likely that LiAlCl4-xHx amorphous intermediate species are unstable during the ball milling. Compared with more stable α-AlH3, this intermediate product cannot maintain its crystalline state during the extended milling times. Therefore, this LiAlCl4-xHx intermediate eventually transforms into α-AlH3. TiF3 was introduced into the reaction system to shorten transformation time of the amorphous intermediate phases into α-AlH3 phase. The fine TiF3 might act as seed crystals and accelerate substitution of the Cl by H in LiAlCl4-xHx. Moreover, because of the isostructuralism of TiF3 and αAlH3, TiF3 prompts α-AlH3 formation during a short milling time. Thus, an intermediate amorphous LiAlCl4-xHx phase formed during the milling process. By introducing TiF3, this intermediate phase can be crystallized easily. All Al atoms in the final α-AlH3 phase are six-coordinated with hydrogen atoms. The mechanism of this mechanochemical reaction can be described according to Eq. (10). 3LiH+AlCl3→ LiAlCl4+AlHx+ 2LiCl→ LiAlCl4-xHx+ 2LiCl→ 3LiCl+ α-AlH3 (10) 3.5. Dehydrogenation properties of the α-AlH3 nano-composite Considering the influence of Ti-based additives on the desorption properties of hydrogen storage materials, the α-AlH3/LiCl nano-composite with TiF3 might have different dehydriding behavior than pure AlH3. Thus, we studied dehydriding properties of TiF3-doped α-AlH3/ LiCl nano-composite under isothermal conditions in the temperature range 80–160 °C. Amount of H2 released from the composite was obtained by dividing actual H2 content by the stoichiometric AlH3 weight. As shown in Fig. 9, isothermal dehydriding curves exhibit sigmoidal features with distinct introduction, acceleration and decay periods. This behavior is similar to decomposition of other comparable materials. Compared with α-AlH3/LiCl nano-composite obtained without TiF3 (inserting in the Fig. 9), additive TiF3 accelerated decomposition kinetics of α-AlH3/LiCl nano-composite. Additionally, desorption kinetics improved as the temperature was increased to 160 °C. Thus, temperature can have a large impact on the dehydrogenation reaction kinetics of α-AlH3/LiCl composite. The reaction of AlH3 dehydrogenation can be expressed in Eq. (11): 2AlH3→ 2Al+ 3H2↑ (11) As the reaction temperature increased from 80 to 120 °C, the time of full decomposition decreased rapidly, that a complete decomposition was obtained within 3000 s, suggesting significantly higher decomposition rate at higher temperature (see the Fig. 9). In comparison, only 8.0 wt% of H2 was released at 20–160 °C during α-AlH3 decomposition reported by Chen [83]. After we annealed our as-milled α-AlH3 at 140 °C, the shape of the dehydriding curve became step-wise for 1160 s with the same hydrogen amount as at 120 °C. Graetz reported that full decomposition time for α-AlH3 at 138 °C could be achieved within 1800s [84]. After we annealed our composite at 160 °C, decomposition curve in Fig. 9 was also step-wise for 750 s with higher hydrogen content equal to 9.92 wt%. Shahi found that among all Ti based additives, TiF3 is found to be the most effective catalyst for dehydrogenation kinetics of nano MgH2 [85]. However, full decomposition for the MgH2/TiF3 at 300 °C could be achieved within 1800s with a maximal hydrogen content of 6.3 wt%. Therefore, TiF3 not only influenced decomposition temperature of α-AlH3 but also improved dehydriding kinetics of α-AlH3. Although numerous papers showing a dehydriding rate enhancement with the addition of transition metal additives in AlH3 [86,87], the excess transition metal additives have little influence on the dehydriding property of AlH3. The α-AlH3/LiCl nano-composite with different amounts of TiF3 (molar ration of LiH/ AlCl3/TiF3 are 3:1:0.1, 3:1:0.2, 3:1:0.3) were heated at 160 °C for different times. As shown in Fig. S4, desorption kinetics were improved as the molar ratio of TiF3 increased. However, increment of TiF3 results in a relatively small increase in the rate. After we annealed our as-milled α-AlH3/LiCl/TiF3 composite at the same temperature, the full dehydriding time of the curve(III) was reduced from 750 s to 570 s. DSC analysis was performed to further understand the dehydriding process [88,89]. Fig. 10a shows DSC thermograms of as-milled α-AlH3 composite with TiF3 at several heating rates(composite synthesized from LiH/AlCl3/TiF3 with a molar ration of 3:1:0.1). The DSC thermograms contain only one endothermic peak within the range of 40–240 °C. The endothermic peak between 80 and 190 °C corresponds to the dehydriding reaction of the α phase [90]. This result indicates that no new phases formed, and α-AlH3 does not react with TiF3 during its dehydrogenation. Based on the above non-isothermal analysis, the dehydriding temperature of α-AlH3 is remarkably reduced with the TiF3 Fig. 8. XRD patterns of LiAlCl4/LiCl composites milled at 400 rpm and 5MPa Ar pressure with a BPR of 200:1 for different times: a) 0.5 h, b) 1 h. The asmilled LiAlCl4/LiCl composites comes from 3:1:0.1 LiH:AlCl3:TiF3 mixture at 10MPa Ar pressure with the same BPR and the rotational speeds for 0.5 h milling conditions. Fig. 9. The dehydriding curves of final α-AlH3/LiCl/TiF3 nano-composite at different temperatures. Insert is the dehydriding curves of α-AlH3/LiCl composite without TiF3. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 148 in the composite. To determinate the apparent activation energy (Ea) of this process, we studied desorption kinetics of the α-AlH3/LiCl nanocomposite using the Kissinger's method. Moreover, the relationship between the activation energy (Ea, KJ/mol), the heating rate (c, K/ min), the gas constant (R, 8.314 J/mol K) and the peak temperature of dehydriding (TP, K) in the DSC curve is described by following Kissinger's equation: = +ln(c/T ) -(E /RT ) Ap2 a p (12) Ea for the dehydrogenation reaction calculated from DSC data are shown in Fig. 10b. The Ea for the hydrogen desorption of α-AlH3 in the composite was 52.1 KJ/mol. This value is significantly lower than the value 104 KJ/mol of the α-AlH3 without TiF3 reported by Gabis [91]. It was reported by Nakagawa that with NbF5 in α-AlH3, the α-AlH3/NbF5 present a quite high hydrogen desorption capacity and fast kinetics. It is worth noting that the activation energy of dehydriding of α-AlH3 was reduced from 104 to 96 kJ/mol(ΔE is 8 kJ/mol) [92]. Based on above analysis, this decrease in the kinetic barrier contributed to the improvement in the hydrogen desorption kinetics of α-AlH3/LiCl/TiF3 nano-composite. 3.6. Microstructural evolution during the dehydriding process Morphology of the α-AlH3/LiCl/TiF3 composite during the dehydriding process was studied with SEM images. Fig. 11 shows the SEM images of product heated at 160 °C for various times. As observed in the Fig. 11a, the particles after desorption for 10min at 160 °C with a range of 400–500 nm is generally smaller than those of the as-milled α-AlH3/ LiCl/TiF3 sample, indicating particle refinement of the product during dehydrogenation of AlH3. When the heating time was increased to 20min, it is observed form Fig. 11b that the difference in surface morphology is significant due to the aggregation of the composite after further desorption. The morphological characteristics of the product in Fig. 11c, indicate that when heating time was extended to 30min, the inhomogeneity in particle distributions is significant and most individual particles had an average size between 5–7 μm. This may contribute to the further aggregation of the composite. Based on above SEM observation, it can be seen that the particles in the composite tend to aggregate with the longer heating time. Graetz found that the aggregation of particles will result in the reduction of the gateways for hydrogen diffusion [36]. This can explain the phenomenon that the dehydriding rates of the α-AlH3/LiCl/TiF3 composite declined with dehydriding time in the later stage of the isothermal dehydriding process. Thus, particle aggregation may give rise to the decline of the dehydriding kinetics of the α-AlH3/LiCl/TiF3 nano-composite. After the desorption process of the α-AlH3/LiCl/TiF3 composite, the phase transformation, structural and morphological changes were not clearly understood. Fig. 12 shows the TEM of the α-AlH3/LiCl/TiF3 nano-composite heated at 160 °C for 10min. A large amount of much finer crystallines are observed from Fig. 12a. The nucleation and quite uniform in size distribution of the crystallites indicates that the dehydrogenation of the α-AlH3 has occurred simultaneously after heating. This phenomenon has a good agreement with the above dehydriding outcomes. The average size of nano-composite after heating process is approximately to be 50 nm. A selected area diffraction pattern (SADP) of the same material is also shown in Fig. 12b. Lattice fringes confirmed that finer crystalline particles matched Al, LiCl and TiF3 phases (see Fig. 12b). Lattice spacings equal to 2.34 and 2.02 Å correspond to (111) and (200) planes of metallic Al. Moreover, the typical lattice spacing which is determined to be 1.55, 1.29 and 3.88, 1.94 Å are attributed to the (311) and (400) planes of the LiCl and (012) and (024) planes of the metal TiF3, respectively. These observations indicated that the α-AlH3 phase decomposed without reacting with TiF3, which is in agreement with the DSC results. Fig. 10. (a) DSC curves and (b) The Kissinger plot of the α-AlH3/LiCl/TiF3 nano-composite doped with TiF3. Composite synthesized from LiH/AlCl3/TiF3with a molar ration of 3:1:0.1. Fig. 11. SEM images of as-milled α-AlH3/LiCl/TiF3 nano-composite heated at 160 °C for various times:a) 10min, b) 20min, c) 30min. C. Duan, et al. Journal of Hazardous Materials 373 (2019) 141–151 149 4. Conclusions We demonstrated that milling could be used as a simple and efficient method to prepare nanocrystalline α-AlH3. The type of gas and the pressure are important for the mechanochemical synthesis of α-AlH3 nano-composite. Using proper gas and pressures, we were able to entirely suppress formation of metallic Al. When TiF3 was introduced into the reaction system, LiH and AlCl3 underwent several intermediate stages before the final product nano-structured α-AlH3/LiCl was formed. The average grain size of α-AlH3 nanoparticles embedded in LiCl reached  45 nm after milling in Ar for 5 h. TiF3 has significantly affected dehydriding properties of AlH3, compared to the α-AlH3/LiCl nano-composite without TiF3. The as-milled α-AlH3/LiCl-TiF3 composite has a hydrogen desorption of 9.92wt% at 160 °C within 750 s, which is very close to the theoretical hydrogen capacity of AlH3. Apparent activation energy of α-AlH3 hydrogen desorption in the composite was 52.1 KJ/mol with the addition of TiF3. Acknowledgements The present work was supported financially by the Natural Science Foundation of Hebei Province (Grant E2018502054), the National Major Science and Technology Program for Water Pollution Control and Treatment (Grant 2017ZX07101-001-001) and the National Nature Science Foundation of China (Grant 21506043).This work was also supported by the Fundamental Research Funds for the Central Universities (Grant 2017MS141), Opening Project of the Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan (KF201702), and Shihezi University 2017 High Level Intelligence Supporting Project (RCSX201731). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2019.03.064. References [1] G. Landucci, T. Alessandro, C. Valerio, Inherent safety key performance indicators for hydrogen storage systems, J. Hazard. Mater. 159 (2008) 554–566. [2] J. Zheng, X. Xiao, L. 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