Complex Ammine Titanium(III) Borohydrides as Advanced Solid

Article pubs.acs.org/cm

Complex Ammine Titanium(III) Borohydrides as Advanced Solid Hydrogen-Storage Materials with Favorable Dehydrogenation Properties Feng Yuan,† Qinfen Gu,‡ Xiaowei Chen,† Yingbin Tan,† Yanhui Guo,† and Xuebin Yu*,† †

Department of Materials Science, Fudan University, Shanghai, China, 200433 Australian Synchrotron, 800 Blackburn Road, Clayton 3168, Australia

‡

S Supporting Information *

ABSTRACT: Ammine metal borohydrides (AMBs), with high hydrogen contents and favorable dehydrogenation properties, are receiving intensive research efforts for their potential as hydrogen storage materials. In this work, we report the successful synthesis of three ammine titanium borohydrides (denoted as ATBs), Ti(BH 4 ) 3 ·5NH 3 , Li 2 Ti(BH4)5·5NH3, and Ti(BH4)3·3NH3 via metathesis reaction of metal chloride ammoniates (TiCl3·5NH3 and TiCl3·3NH3) and lithium borohydride. These ATBs present favorable stability, owing to the coordination with NH3 groups, compared to the unstable Ti(BH4)3 at room temperature. Dehydrogenation results revealed that Ti(BH4)3·5NH3, which theoretically contains 15.1 wt % hydrogen, is able to release ∼13.4 wt % H2 plus a small amount of ammonia. This occurred via a single-stage decomposition process with a dehydrogenation peak at 130 °C upon heating to 200 °C. For Li2Ti(BH4)5·5NH3, a three-step decomposition process with a total of 15.8 wt % pure hydrogen evolution peaked at 105, 120, and 215 °C was observed until 300 °C. In the case of Ti(BH4)3·3NH3, a release of 14 wt % pure hydrogen via a two-step decomposition process with peaks at 109 and 152 °C can be achieved in the temperature range of 60−300 °C. Isothermal TPD results showed that over 9 wt % pure hydrogen was liberated from Ti(BH4)3·3NH3 and Li2Ti(BH4)5·5NH3 within 400 min at 100 °C. Preliminary research on the reversibility of this process showed that dehydrogenated ATBs could be partly recharged by reacting with N2H4 in liquid ammonia. These aforementioned preeminent dehydrogenation performances make ATBs very promising candidates as solid hydrogen storage materials. Finally, analysis of the decomposition mechanism demonstrated that the hydrogen emission from ATBs is based on the combination reaction of B−H and N−H groups as in other reported AMBs. KEYWORDS: hydrogen storage, ammine metal borohydrides, ammine titanium borohydrides, metathesis reaction

1. INTRODUCTION The occurrence of hydrogen and oxygen fueled proton exchange membrane fuel cells (PEMFC) makes hydrogen one of the most promising alternative energy sources.1 However, to implement the hydrogen-based propulsion systems, a viable, highly efficient, safe, and inexpensive hydrogen storage method is still highly sought after.2 The main challenge for hydrogen storage materials to be practical is to release hydrogen at moderate temperature with suitably high contents.3 Among all the developed gas−solid hydrogen storage systems, the complex metal borohydride system has been considered to be one of the most promising candidates for hydrogen carriers due to their high gravimetric and volumetric hydrogen density.4 In particular, alkali and alkali-earth borohydrides, e.g., LiBH4, Mg(BH4)2, and Ca(BH4)2 (with gravimetric capacities of 18.3, 14.9, and 11.5 wt %, respectively), have received the most intensive investigation. Unfortunately, their high hydrogen desorption temperature obstructs their practical utilization for PEMFC under © 2012 American Chemical Society

reasonable physics conditions. Therefore, recently, more and more attention has been given to transition metal borohydrides because of their lower formation enthalpy than that of alkali and alkali-earth borohydrides.5,6 Nevertheless, with the exception of Y and Sc, almost all of these kinds of metal borohydrides are highly volatile and unstable (e.g., Zr, Ti, Mn, Fe, V, and Nb, etc.) at ambient conditions, nonexistent, or at least rarely reported (Ta and Cr, etc.).7 This clearly precludes their convenient preparation, intensive studies into physical and chemical characteristics, and furthermore their application as solid hydrogen storage materials. It is likely that by saturating and immobilizing the metal coordination sphere, soft electro-donating ligands may play an important kinetic role in stabilizing the borohydrides. Recently, we successfully stabilized the Al(BH4)3, which is liquid and Received: May 5, 2012 Revised: July 24, 2012 Published: August 2, 2012 3370

dx.doi.org/10.1021/cm301387d | Chem. Mater. 2012, 24, 3370−3379

Chemistry of Materials

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TiCl3·5NH3 was first prepared by exposing TiCl3 to NH3 atmosphere according to eq 3. Approximately 0.5 g (typical weight) of TiCl3 was transferred to the reaction vessel under an argon atmosphere. After the argon was evacuated, the purified ammonia was purged and kept for 3 h until the TiCl3 sample was saturated. After excessive ammonia was evacuated, the molar ratio of NH3 to TiCl3 for the saturated salt was determined gravimetrically to be 5.0, confirming the formation of TiCl3·5NH3. Heat treatment of the mixture of TiCl3/TiCl3·5NH3 (in a molar ratio of 2/3) at 200 °C in a closed vessel for at least 24 h was chosen for the synthesis of TiCl3·3NH3 (eq 4) according to the TPD results of TiCl3·5NH3 (Figure S1 in the Supporting Information). In addition, the formation of TiCl3·5NH3 and TiCl3·3NH3 was further confirmed by elemental analysis, which gave the ratio of Ti/N = 1:5 for TiCl3·5NH3 and Ti/N = 1:3 for TiCl3·3NH3, respectively. The high-resolution XRD profiles of TiCl3·nNH3 (n = 3 and 5), which are consistent with a previous report,15 are shown in Figure S2 in the Supporting Information. These ammine titanium borohydrides were prepared by ball milling the mixtures of TiCl3·3NH3/TiCl3·5NH3−LiBH4 (in molar ratios of 1:3, 1:4, and 1:5) using Planetary QM-3SP2 with a ball-to-sample ratio of 30:1, agitation frequency of 350 rpm. To avoid the temperature of the powder in the vessel increasing, the milling process was carried out by alternating 6 min of milling and 6 min of rest. Regeneration of the dehydrogenated Li2Ti(BH4)5·5NH3 by direct reaction with hydrazine and liquid ammonia was conducted in a sealed pressure vessel at 40 °C for 72 h. Hydrazine was obtained by the reaction between NH2NH2·H2SO4 and liquid ammonia. 2.2. Instrument and Analysis. Temperature-programmeddesorption (TPD) measurements were carried out using a semiautomatic Sievert’s apparatus, connected with a reactor filled with a sample (typical weight ∼0.2 g) under argon atmosphere (∼1 bar) with a heating rate of 5 °C/min. The H2 and NH3 contents within the emission gas were determined using gravimetric and volumetric results. First, the mass percent (Wp) and mole per gram (Mp) of gas released from the sample were calculated from the weights of the samples and volumetric results, then the mole proportion of H2 (C(H2)) and NH3 (C(NH3)) can be calculated from the follow two equations,10b

volatile at ambient temperature, by coordinating NH3 to form its hexammine, Al(BH4)3·6NH3.8 Similar to other BN-based chemical hydrides, for example, ammonia borane (BH3NH3, AB), in which the presence of the N−Hδ+···Hδ‑−B dihydrogen bond results in the dehydrogenation of these hydrides with favorable thermodynamics and kinetics,9 the dehydrogenation of Al(BH4)3·6NH3 is based on the combination mechanism between NH3 and BH4 groups.10 The favorable dehydrogenation performances displayed by the Al(BH4)3·6NH3 provide a prospective strategy to explore more efficient hydrogen storage candidates via an adduct of ammonia to various metal borohydrides. The traditional method of forming the ammine metal borohydrides is to react metal borohydrides with stoichiometric ammonia, such as LiBH 4 ·NH 3 , 11 Al(BH4)3·6NH3,8 Mg(BH4)2·2NH3,12 and so forth. This method requires obtaining the metal borohydride starting substance, which clearly limits the formation of most of the ammine transition metal borohydrides due to the difficulties in obtaining these unstable borohydrides as stated above. To overcome this problem, a novel method consisting of a metathesis reaction between metal chloride ammoniates and lithium or sodium borohydride by mechanical milling was developed by our group. This method has been confirmed to be a general strategy for synthesis of various ammine metal borohydrides.13 Our previous studies have confirmed that the Pauling electronegativity χp of the central metal cation is correlated with the thermal decomposition performance for ammine metal borohydrides.13a Therefore, improvements to the design and preparation of hydrogen storage materials will be achieved by synthesis, phase and structure analysis as well as comprehensive investigations of the dehydrogenation properties of new ammine metal borohydrides. TiCl3·n NH3 + 3LiBH4 → Ti(BH4)3 ·n NH3 + 3LiCl (n = 3 and 5)

(1)

Ti(BH4)3 ·n NH3 + mLiBH4 → Li mTi(BH4)3 + m ·n NH3 (n = 3 and 5; m = 1 and 2)

In this paper, we report the first synthesis of Ti(BH4)3·3NH3 and Ti(BH4)3·5NH3 according to the metathesis reaction of eq 1. Moreover, to optimize the dehydrogenation properties of pentammine, excessive LiBH4 was added according to eq 2,10a,14 and Li2Ti(BH4)5·5NH3 was obtained successfully. Dehydrogenation results showed that these synthesized ammine Ti-based borohydrides exhibit excellent dehydrogenation performances and potential reversibility, thereby making them very promising candidates for hydrogen storage applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Anhydrous starting materials (LiBH4, 95%; TiCl3, 99%) were purchased from Sigma-Aldrich and used directly without further purification, while NH3 (Alfa Aesar) was purified by soda lime before use. All solid samples were handled in a glovebox equipped with a recirculation and regeneration system, which maintained the oxygen and water concentrations below 1 ppm. (3)

2TiCl3 + 3TiCl3· 5NH3 → 5TiCl3· 3NH3

(4)

(5)

((C(H 2) × 2.02) + (C(NH3) × 17.03)) × M p = Wp

(6)

The dehydrogenation properties of the as-prepared samples were examined by simultaneous thermal gravimetric analysis (TGA) combined with a mass spectrometer (MS, QMS 403), for which samples of ∼6 mg were heated at a heating rate of 5 °C/min from room temperature to 300 °C under argon flow. Differential scanning calorimetry (DSC) measurements were performed by TAQ 2000 DSC under argon with a gas flow of 40 mL Ar min−1 at a heating rate of 5 °C/min. For phase identification and structure determination, the sample was loaded into a predried 0.7 mm boron-silicate glass capillary while inside an argon-filled glovebox. Synchrotron powder X-ray diffraction data were collected with a Mythen-II detector at a wavelength of 0.9997 Å at the powder diffraction beamline, Australian Synchrotron. The capillary was sealed with vacuum grease for X-ray diffraction measurements. In order to identify the reaction products and estimate the crystallinity of the ammine titanium borohydrides, time-resolved in situ measurements were conducted on milled-TiCl3·3NH3 + 3LiBH4 using a cyberstar hot-air blower to heat the capillary from 30 to 160 °C at a constant heating rate of 7 °C/min with a wavelength of 0.9528 Å. For milled-TiCl3·5NH3 + 3LiBH4 and TiCl3·5NH3 + 5LiBH4 composites, the wavelength used was 1.001 Å. Data were collected with an exposure time of 150 s at every 10 °C step. Solid-state nuclear magnetic resonance (NMR) spectra were measured using a Bruker Avance 300 MHz spectrometer, using a Doty CP-MAS probe with no probe background. The powder samples collected after the decomposition reactions were spun at 5 kHz using 4 mm ZrO2 rotors loaded in purified argon atmosphere glove boxes. A

(2)

TiCl3 + 5NH3 → TiCl3· 5NH3

C(H 2) + C(NH3) = 1

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TiCl3·5NH3 ratio to 5:1, Li2Ti(BH4)5·5NH3 became the only ATB present after ball-milling. Bragg peaks in the room temperature diffraction pattern from Li2Ti(BH4)5·5NH3 can be indexed to an orthorhombic unit cell using DICVOL06.16 Analysis of systematic extinction shows the structure has a space group of P222 or P2221. Lebail refinement was performed using TOPAS v4.2 (Figure S3 in the Supporting Information),17 and the refined lattice parameters are a = 18.283 Ǻ , b = 10.216 Ǻ , c = 7.954 Ǻ , V = 1485.64 Ǻ 3. The indexed unit cell volume for Li2Ti(BH4)5·5NH3 is 1485.64 Å3 and has a similar compound’s density. We tried a structure solution in space group P222 and P2221, with Z = 4. However, we did not solve the structure successfully using the direct space method in which the BH4 and NH3 units were input as rigid bodies with common bond lengths and bond angles and Ti as free atoms. There are some misfits of peak intensities. In addition, by using a similar PONKCS technique,18 we refined the room temperature data of the TiCl3·5NH3/5LiBH4 sample. The refinement results give 63 wt % Li2Ti(BH4)5·5NH3 and 37 wt % LiCl, which is equal to a mole ratio of 0.33:1. This value agrees well with the nominated reaction formula 1:3. Further elemental analysis also gave a ratio of Li/Ti/B/N = 2:1:5:5 (excluding the LiCl). These results undoubtedly confirm the formation of Li2Ti(BH4)5·5NH3. In order to balance the number of BH4− and NH3 groups for ammine titanium borohydrides to achieve superior dehydrogenation properties, Ti(BH4)3·3NH3 was also synthesized via ball-milling a mixture of TiCl3·3NH3 and LiBH4 in a molar ratio of 1:3. Figure 2a

0.55 ms single-pulse excitation was employed, with a repetition time of 1.5 s. Fourier transform-infrared (FT-IR) (Magna-IR 550 II, Nicolet) analyses were conducted to confirm the chemical bonds in all solid products. Products were pressed with KBr and then loaded into a sealed chamber with a CaF2 window for the measurement. Anhydrous KBr was used as a pellet material. A complete chemical analysis was performed for TiCl3·nNH3 (n = 3 and 5), the as-prepared ATBs, and the dehydrogenated ATBs to confirm their compositions. The Li, Ti and B contents were determined by inductively coupled plasma atomic emission spectroscopy analysis (P-4010, with a 40.68 MHz rf generator, Hitachi). Powders with a certain weight were prior dissolved into H2SO4 + H2O2/H2O solvent (10%), and then diluted to around 10−100 μg/mL concentration. The nitrogen content was measured with a standard inert gas fusion method (TC-436AR, Leco-Co).

3. RESULTS AND DISCUSSION 3.1. Structure and Phase Characterization of Ammine Titanium Borohydrides. The mechanochemical solid/solid reaction was successfully conducted to produce the desired ammine titanium borohydrides (ATBs) according to eqs 1 and 2. The products of these reactions were verified by highresolution XRD. As shown in Figure 1, the high-resolution

Figure 1. High-resolution XRD profiles for the ball milled TiCl3·5NH3/LiBH4 with molar ratios of 1:3 (a), 1:4 (b), and 1:5 (c). λ = 0.9997 Å.

XRD patterns of the ball-milled TiCl3·5NH3−n LiBH4 (n = 3, 4, and 5) show diffraction peaks assigned to LiCl and unidentified phases, suggesting that an interaction between the two starting materials, i.e., TiCl3·5NH3 and LiBH4, has occurred during the ball-milling process. Moreover, weak peaks assigned to residues of TiCl3·5NH3 are observed in Figure 1a,b but not in Figure 1c. For the ball-milled LiBH4/TiCl3·5NH3 composite with a mole ratio of 3:1, Ti(BH4)3·5NH3 was obtained in a homogeneous mixture with LiCl. This formation of Ti(BH4)3·5NH3 was confirmed by the elemental analysis, which gave a ratio of Ti/B/N = 1:3:5. On increasing the ratio of LiBH4/TiCl3·5NH3 to 4:1, peaks for Ti(BH4)3·5NH3 decreased and peaks of a new phase assigned to Li2Ti(BH4)5·5NH3 appeared. Meanwhile, a small quantity of unreacted LiBH4 remained, which may be due to insufficient solid reaction during the ball-milling. Further increasing the LiBH4/

Figure 2. High-resolution XRD profiles for the ball milled TiCl3·3NH3/LiBH4 with molar ratios of 1:3 (a), 1:4 (b), and 1:5 (c). λ = 0.9997 Å.

shows diffraction peaks assigned to Ti(BH4)3·3NH3 and LiCl, along with a small amount of residual TiCl3 originating from the starting substance of TiCl3·3NH3 (Figure S2 in the Supporting Information), indicating the successful completion of the reaction according to eq 1. However, no Ti−Li based dual-cation borohydride triammoniates were produced, even when the amount of LiBH4 was increased to 4 or 5 M based on the fact that there are only diffraction peaks from Ti(BH4)3·3NH3, LiCl, excessive LiBH4, and residual TiCl3 in Figure 2b,c. Unfortunately, endeavors to solve the structure of Ti(BH4)3·3NH3 were also failed due to poor crystallinity and 3372



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H2, indicating an entire transformation of Hδ+ in the amidebased species and Hδ‑ in the BH4− units in Li2Ti(BH4)5·5NH3 to H2 molecules. This entire transformation of NH3 to H219 in Li2Ti(BH4)5·5NH3 clearly overcomes the common drawbacks in AMBs that usually additionally evolve a fair amount of ammonia upon decomposition.8,10d,13a The observation in the DSC curve of a weak endothermic peak at 58 °C, which precedes the decomposition temperature, can be ascribed to a recrystallization of the Li2Ti(BH4)5·5NH3. This is further confirmed by high-resolution in situ XRD results (see in Figure 8a). The following two exothermal peaks at 91 and 127 °C agree well with the former two decomposition ranges derived from the combination of H(N) and H(B) atoms, while the weak endothermic phenomenon around 220 °C can be assigned to the decomposition effect of the surplus 1 mol of BH4 in Li2Ti(BH4)5·5NH3.14b,20 Likewise, given that the milled-TiCl3·5NH3/4LiBH4 is a mixture of LiCl (acts as inert bystander), LiBH4, Ti(BH4)3·5NH3, and Li2Ti(BH4)5·5NH3, its thermal behavior resembles that observed of a combination of Ti(BH4)3·5NH3 and Li2Ti(BH4)5·5NH3. The desorbed gases are largely made up of H2, peaks at 111, 130, and 215 °C, and a small amount of NH3 evolution in the second stage, with a total weight loss of 18.0 wt %. DSC profile of the milledTiCl3·5NH3/4LiBH4 also shows the behaviors of both Ti(BH4)3·5NH3 and Li2Ti(BH4)5·5NH3. The quantitative gas desorption for the penta-ammine titanium borohydrides system was determined using TPD measurement (Figure 4). These measurements show that the

peak overlap with impurities introduced in the preparation process of TiCl3·3NH3. 3.2. Dehydrogenation Properties of Ammine Titanium Borohydrides. The dehydrogenation properties of penta-ammine titanium borohydrides system were investigated by MS (Figure 3a) and TG-DSC (Figure 3b). As shown in

Figure 3. MS (a) and TG-DSC (b) spectra of milled-TiCl3·5NH3/ LiBH4 composite in molar ratios of 1:3 (i), 1:4 (ii), and 1:5 (iii) with a heating rate of 5 °C/min. These gravimetric contents are calculated with LiCl being viewed as an inert bystander.

Figure 3a for Ti(BH4)3·5NH3, a single hydrogen desorption step occurs, accompanied with a slight emission of ammonia, which peaked at 130 °C. No B2H6 and/or B3N3H6 byproducts were detected in this decomposition process. The emission of ammonia can be ascribed to the surplus NH3 groups related to BH4 groups in Ti(BH4)3·5NH3. A total weight loss of 22.0 wt %, consisting of H2 and NH3, is seen in the TG curve (Figure 3b). The exothermal peak at 131 °C, seen in the DSC profile, corresponds to the hydrogen evolution peak, which suggests that the hydrogen release is derived from an exothermic reaction of NH3 and BH4 species in Ti(BH4)3·5NH3, as confirmed in other reported AMBs systems.13a After increasing the ratio of LiBH4/TiCl3·5NH3 to 5:1, the formed Li2Ti(BH4)5·5NH3 exhibits a complete release of pure hydrogen via a three-step decomposition process, with peaks at 105, 120, and 215 °C. The temperature range studied was 75−300 °C. Therefore, the dehydrogenation capacity throughout the decomposition process was determined to be 15.8 wt % according to the TG result, which corresponds to 17.5 equiv of

Figure 4. TPD results for milled-TiCl3·5NH3/LiBH4 in molar ratios of 1:3 (a), 1:4 (b), and 1:5 (c) with a heating rate of 5 °C/min to 350 °C. Ti(BH4)3·5NH3 after preserving for 1 week under argon at room temperature was also measured for comparison (d). These volumetric contents are calculated with LiCl being viewed as an inert bystander.

dehydrogenation temperature decreased with an increase in the LiBH4 ratio and that the onset of hydrogen release temperature occurred at approximately 75 °C, similar to that measured by TG-MS. After heating the samples to 350 °C, TiCl3·5NH3/ 4LiBH4 and TiCl3·5NH3/5LiBH4 display the same gas release content of 0.079 mol/g, while TiCl3·5NH3/3LiBH4 shows a release content of 0.069 mol/g. By combining the volumetric desorbed gas results and the gravimetric results of residual solids, the emission of H2 and NH3 contents from the three samples were calculated to be 13.4 wt % H2 and 7.3 wt % NH3 for TiCl3·5NH3/3LiBH4, 15.0 wt % H2 and 2.0 wt % NH3 for TiCl3·5NH3/4LiBH4, and 15.8 wt % pure H2 for TiCl3·5NH3/ 3373



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Figure 5. (a) Isothermal TPD results for Li2Ti(BH4)5·5NH3 and (b) a summary of hydrogen release capacity and purity of part a. These volumetric and gravimetric capacities are calculated relative to the Li2Ti(BH4)5·5NH3 weights only.

5LiBH4 (LiCl was viewed as an inert bystander for all three samples), respectively. This indicates that the evolution of ammonia content can be depressed by increasing the LiBH4 ratio in these composites. Furthermore, the stability of ATBs was evaluated by TPD measurement, and as an example the result for the Ti(BH4)3·5NH3 is shown in Figure 4. It shows that the dehydrogenation performance remained unchanged after the sample was preserved at room temperature under an argon atmosphere for a week (Figure 4d). This is in contrast to the instability of Ti(BH4)3,21 suggesting that the coordinative NH3 groups indeed stabilized Ti(BH4)3 to form its pentaammines. Isothermal hydrogen desorption profiles were employed to characterize the dehydrogenation and kinetic properties of Li2Ti(BH4)5·5NH3 as shown in Figure 5. As the chosen heating temperatures are above 100 °C, the temperature of the highest reaction rate recognized by TPD, no induction period was observed for each isothermal reaction. At 100 and 120 °C, about 9.5 wt % and 12.0 wt % pure hydrogen were released in 400 min, respectively. This kinetic performance can meet the practical requirements for an on-board hydrogen source material. Surprisingly, on further increasing the heating temperature to 140 °C, 12.1 wt % hydrogen and 3.1 mol % ammonia were released in about 300 min. The emission of ammonia may be due to the abruptly given high heating temperature that could supply a high energy input for the breaking of M−NH3 coordinate bonds together with the combination of NH···HB dihydrogen bonds. These results also shed new light on achieving the pure hydrogen evolution by choosing an appropriate dehydrogenation temperature in the ATBs and other AMBs system. The comparative dehydrogenation properties of Ti(BH4)3·3NH3 were measured by the combination of MS (a) and TG-DSC (b). As shown in Figure 6, Ti(BH4)3·3NH3 starts to release hydrogen at as low a temperature as 60 °C, which is 15 °C lower than that of Ti(BH4)3·5NH3, suggesting that the ammine titanium borohydride, which has a lower ammonia coordination number, possesses a lower dehydrogenation temperature, in accordance with our previous report.10c Since no byproducts, i.e., ammonia, diborane, and borazine, were detected throughout the desorption process based on the MS pattern, the weight loss of 14 wt % recorded by TG can be

Figure 6. MS (a) and TG-DSC (b) spectra of milled- TiCl3·3NH3/ 3LiBH4 with a heating rate of 5 °C/min. These gravimetric contents are normalized to the amounts of pure Ti(BH4)3·3NH3.

totally attributed to hydrogen emission, corresponding to ∼10 equiv of H2. These two explicit dehydrogenation stages, which peaked at 109 and 152 °C in the MS profile, are also confirmed as two corresponding exothermal peaks located at 106 and 146 °C in the DSC profile, indicating that the desorbed H2 in these stages may have originated from the exothermal combination of NH3 and BH4 groups.13a The dehydrogenation properties of Ti(BH4)3·3NH3 were further examined by volumetric measurement. Figure 7a shows the TPD curve of Ti(BH4)3·3NH3 that shows the two-step decomposition feature. The combination of a volumetric increase of gas products and a gravimetric decrease of residual solids reveals that a pure hydrogen release with a capacity of ∼14.0 wt % (10 equiv of hydrogen) was achieved by 350 °C. To further investigate the kinetics, content, and purity of hydrogen emission, isothermal TPD measurements from 100 to 130 °C were conducted as shown in Figure 7b. No induction periods for any reaction carried out at different temperatures was observed. For the reaction carried out at 100 and 110 °C, about 9.2 wt % and 10.4 wt % pure hydrogen were released in 3374



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Figure 7. Nonisothermal (a) and isothermal (b) TPD results for Ti(BH4)3·3NH3. These volumetric capacities are calculated relative to the Ti(BH4)3·3NH3 weights only.

promising candidates for hydrogen storage once the byproduct LiCl is successfully removed and reversibility is achieved. Unfortunately, attempts to separate ATBs from LiCl by dissolution or flotation, even assisted with ultrasound or centrifuge, in various solvents failed, most likely due to the presence of microcrystals.5a,13 In addition, regeneration of the dehydrogenated ATBs, particularly for the Li2Ti(BH4)5·5NH3 or Ti(BH4)3·3NH3, may be possible due to the fact that there was no emission of ammonia and borane during the dehydrogenation process, and there must remain equivalent amounts of B and N in the residual amorphous solid. In considering the fact that the dehydrogenated ATB systems may possess similar amorphous B−N containing products to the spent fuel of AB that has been regenerated successfully by reacting with hydrazine (N2H4) in liquid ammonia recently,22 we have tried to regenerate the dehydrogenated ATBs by using the same route. The regeneration results of Li2Ti(BH4)5·5NH3 is given here as an example. As shown in Figure S4a in the Supporting Information, three distinct B−H bending absorption peaks at 1183, 1140, and 1085 cm−1 are presented, indicating the broken B−N bonds and rebuilding the B−H groups. However, the residual B−N stretching absorption is also existent, suggesting incomplete reduction of the B−N bonds. Furthermore, as shown in Figure S4b in the Supporting Information, the regenerated Li2Ti(BH4)5·5NH3 presents a three-step dehydrogenation process, with peaks at 110, 159, and 227 °C, in the temperature range of 85−350 °C. This is accompanied by distinct ammonia emission which is derived from the decomposition of surplus N2H4 and NH3.10h Though this regeneration route is partially feasible due to the appearance of the regenerated BH groups and the dehydrogenation performance, which is similar to that of the fresh-obtained Li2Ti(BH4)5·5NH3, the actual achievement of a full regeneration was unsuccessful. This may be due to the fact that the dehydrogenated ATBs are different from polyborazylene (PB), which can be converted to AB by this regeneration route. Given that it is easy to add protons by adding ammonia to metal, the real challenge comes from the breaking of the B−N bond and the restoration of the BH groups. Therefore, further efforts to combine a digestant with a reductant, e.g., alcohol with a strong reductant hydride, are expected to address this challenge. In this combination, the digestant can cut the B−N bond and the

400 min, respectively. Upon elevating the temperature to 130 °C, the sample could reach the equilibrium of 11.2 wt % hydrogen content after holding for 400 min. The released gas from Ti(BH4)3·3NH3 measured by isothermal TPD at different temperatures was confirmed to be pure hydrogen by the volumetric and gravimetric results. Table 1 lists a summary of dehydrogenation properties of all the studied ATBs, and a comparison with other reported BNTable 1. Summary of H2 and NH3 Evolution from Ammine Titanium Borohydridesabystander calculated gases contentsd

samples Ti(BH4)3·3NH3 Ti(BH4)3·5NH3 Ti(BH4)3·5NH3 + LiBH4 Li2Ti(BH4)5·5NH3

TG resultsb (wt %)

TPD resultsc (mol/g)

H2 capacity (wt %)

NH3 capacity (wt %)

H2 purity (mol %)

14 22.0 18.0

0.07 0.069 0.079

14 13.4 15.0

0 7.3 2

100 96 98.5

15.8

0.079

15.8

0

100

a

LiCl in these systems was viewed as inert. bThe samples were heated under Ar flow from RT to 300 °C with a heating rate of 5 °C/min. c The samples were heated under 1 bar argon atmosphere from RT to 350 °C with a heating rate of 5 °C/min by TPD in a closed system. d These contents are calculated based on the combination of a TPD volumetric increase and gravimetric decrease of the reacted residual solid.

based hydrogen storage candidates. Clearly, compared with AB and other reported BN-based hydrides, the main advantages offered by the ATBs are their favorable dehydrogenation properties. For example, releasing of over 14 wt % pure hydrogen can be achieved from Li2Ti(BH4)5·5NH3 and Ti(BH4)3·3NH3, while only 11.6 wt % hydrogen is released from AB at the same conditions with a large amount of undesirable byproducts, i.e., ammonia, boranes, and borazine, which are harmful for the operation of the fuel cell. Furthermore, similar to AB, the studied ATBs, i.e. Ti(BH4)3·5NH3 and Li2Ti(BH4)5·5NH3, are neither flammable nor explosive at room temperature when exposed to air. Therefore, the advanced dehydrogenation performances for these ATBs at moderate temperatures makes them very 3375



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(BH4)5·5NH3 disappear, while another two peaks with 2θ at 5.8 and 6.8° appear at 60 °C. These changes may be due to the formation of an unknown high-temperature phase of Li2Ti(BH4)5·5NH3. Following this, the peak intensity of this unidentified phase decreased with the increasing temperature until 110 °C, at which point the peaks belonging to this phase vanished entirely. This observation is in accordance with the first dehydrogenation stage (75−115 °C). At the same temperature, three new peaks with 2θ at 15.7, 16.9, and 17.9° appeared and existed throughout the heating process, indicating that the high-temperature phase of Li2Ti(BH4)5·5NH3 has decomposed into another phase accompanied with gas release. In the case of milled-TiCl3·5NH3-3LiBH4 composites (Figure 8b), diffraction peaks assigned to Ti(BH4)3·5NH3 and some residual TiCl3·5NH3 are presented at ambient temperature, the former being stable up to its decomposition temperature of 120 °C, while the TiCl3·5NH3 disappears before the decomposition temperature, indicating the formation of Ti(BH4)3·5NH3 via an exchange reaction throughout the heating process. With the decomposition of Ti(BH4)3·5NH3, a small, new peak with 2θ around 10° appeared accompanied with the disappearance of Ti(BH4)3·5NH3. This is identical with the hydrogen evolution, indicating the generation of boron containing decomposition products as a new phase. Figure 9 shows the evolution of high-

reductant can be used to hydrogenate B back to BH4 groups.23 Further investigations on the regeneration of this system are still ongoing. 3.3. Decomposition Mechanism of Ammine Titanium Borohydrides. To clarify the reaction mechanism in detail and to obtain information about the decomposition products during the dehydrogenation process, in situ high-resolution XRD was conducted on the studied ATBs. The selected part of the in situ high-resolution XRD profile for Li2Ti(BH4)5·5NH3, shown in Figure 8a, reveals that the orthorhombic structure of Li2Ti(BH4)5·5NH3 starts to distort with the increase in temperature from 30 to 60 °C. This is represented by a decrease in the intensity and an increase in the fwhm of the diffraction peaks for this phase. On the other hand, some Bragg reflections, e.g., peaks with 2θ at 13.1, 14.4, 18.0, and 18.8°, of Li2Ti-

Figure 9. In situ high-resolution XRD pattern for milled-TiCl3·3NH3− 3LiBH4 composite heated from 30 to 160 °C with a constant heating rate of 7 °C/min in a closed capillary tube under Ar atmosphere. The diffraction peaks of LiCl are marked with arrows. λ = 0.9528 Å. Ti(BH4)3·3NH3 (◊) and an unknown decomposition product (γ).

resolution XRD profiles of TiCl3·3NH3−3LiBH4 as a function of temperature from 30 to 160 °C. Diffraction peaks assigned to Ti(BH4)3·3NH3 and LiCl are presented at ambient temperature. As the heating temperature of the milled composite is increased to 80 °C, the overall profiles become narrower and the peak intensity increases, indicating that the crystallinity of the milled sample increases during annealing. Afterward, when heated to 90 °C, a very crystalline product with sharp peaks appears, and peaks of this unidentified product weaken upon further heating and vanish at 140 °C, accompanied with the beginning of hydrogen emission, as derived from the dehydrogenation reaction of BH and NH groups, in good accordance with the observation of TG-MS. This unknown

Figure 8. Selected part (5−19 deg) of in situ high-resolution XRD profiles for (a) milled-TiCl 3 ·5NH 3 -5LiBH 4 and (b) milledTiCl3·5NH3−3LiBH4 composites heated from 30 to 160 °C with a constant heating rate of 7 °C/min in a closed capillary tube under Ar atmosphere. λ = 1.001 Å. Li2Ti(BH4)5·5NH3 (▽), Ti(BH4)3·5NH3 (#), TiCl3·5NH3 (∗), and unknown intermediate phase (○). 3376



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dehydrogenation reactions were completed (Figures S6 and S7 in the Supporting Information). The formation of the B−N polymer upon dehydrogenation was further confirmed by 11B NMR measurement as shown in Figure 11 (Li2Ti(BH4)5·5NH3

compound can be indexed to be an orthorhombic unit cell with lattice parameters of a = 18.401 Ǻ , b = 8.414 Ǻ , c = 4.336 Ǻ , and V = 671.36 Ǻ 3 (Figure S5 in the Supportinfg Information). Unfortunately, endeavors to solve the structure of this compound failed due to the lack of precise chemical constitution and complexity of the in situ XRD patterns (Ti(BH4)3·3NH3 starts to decompose at 60 °C, see Figure 6). This unknown product begins to decompose at 110 °C, and peaks belonging to it vanished at 140 °C accompanied with the generation of amorphous boron containing compounds. In addition, the peaks of LiCl in these three ATBs (peaks assigned to LiCl in milled-TiCl3·5NH3/3LiBH4 and milled-TiCl3·5NH3/ 5LiBH4 composites are not given here) remained unchanged throughout the heating process, indicating that LiCl indeed acted as an inert bystander. The dehydrogenated ammine titanium borohydrides were further examined by FT-IR and 11B NMR since the thermal decomposition yielded solid substances with an amorphous state that prevented sufficient information being collected using XRD. As shown in Figure 10, for the FT-IR spectra of

Figure 11. 11B MAS solid-state NMR data for the as-prepared Li 2 Ti(BH 4 ) 5 ·5NH 3 (i) and the postdehydrogenated Li 2 Ti(BH4)5·5NH3 at 120 °C (ii) and 300 °C (iii).

is presented as an example). For the as-synthesized Li2Ti(BH4)5·5NH3, an asymmetric resonance peak centered at −38.1 ppm, accompanied with a slight peak around −20.7 ppm, was observed. The slight downfield shift of the 11B resonance in Li2Ti(BH4)5·5NH3 (−38.1 ppm) compared with that in LiBH4 (−41.3 ppm)24 indicates the definite difference in the chemical environment of boron in these two compounds, which further supports the formation of the new compound Li2 Ti(BH4)5·5NH3. The weak resonance peak at −20.7 ppm, corresponding to another boron site (BH3),10e,25 was derived from partial decomposition of Li2Ti(BH4)5·5NH3, which starts to dehydrogenate at as low as 75 °C, during the NMR test process (from the additional energy available). This partial decomposition was also confirmed by the increase of airpressure in the inner sample tube during the NMR measurement. After heat treatment at 120 °C, another two kinds of B nuclei peaks around 15 and −1 ppm appeared, indicating the generation of tetravalent B−N based substances (BN3 or N2BH).25,26 On further heating to 300 °C, these two broad peaks at 18 and −1 ppm finally became dominant, suggesting the dehydrogenation mechanism in ATBs may be similar to that of other M−B−N−H systems,8,10a−d,11b,13,14b,25,27 i.e., hydrogen evolution originated from the combination of B−H and N−H species accompanied with the final formation of BNsubstances. In addition, the Ti/B/N ratios for the dehydrogenated products of Ti(BH4)3·3NH3 and Li2Ti(BH4)5·5NH3 were also determined to be 1:3:3 and 1:5:5, respectively, further confirming the release of pure hydrogen.

Figure 10. FT-IR spectra of Li2Ti(BH4)5·5NH3 and the the postdehydrogenated Li2Ti(BH4)5·5NH3 at 125, 250, and 300 °C.

Li2Ti(BH4)5·5NH3 at room temperature, typical features of BH4 and NH3 groups can be identified in the spectra, i.e., the stretching and bending of B−H bands in the regions between 2190 and 2480 cm−1 and between 1000 and 1280 cm−1, respectively, and the bending of N−H bands peak at 1404 cm−1. Furthermore, the B−H stretching vibration is split into four peaks at 2220, 2288, 2380, and 2446 cm−1; and the B−H bending absorption is split into four peaks at 1028, 1122, 1187, and 1255 cm−1. The intensities of peaks assigned to the B−H and N−H vibrations decreased equally with the increase of the thermal treatment temperature from RT to 125 and 250 °C, accompanied with the occurrence of B−N absorption peaks. This indicates that the hydrogen release originated from the combination of B−H and N−H species and is accompanied by the generation of B−N containing solid residues. Finally, this dehydrogenation reaction was complete at 300 °C symbolized by the disappearance of N−H and B−H (very weak) vibration peaks and the only existing B−N absorption peak, as verified by the previous TG and TPD results. Ti(BH4)3·3NH3 and Ti(BH4)3·5NH3 possess identical FT-IR features to that of Li2Ti(BH4)5·5NH3 with the only exception that there are some surplus absorptions due to N−H vibrations at 350 °C after the

4. CONCLUSIONS In this article, ammine titanium borohydrides (denoted as ATBs), formulated as Ti(BH4)3·5NH3, Li2Ti(BH4)5·5NH3, and Ti(BH4)3·3NH3 were successfully synthesized via a metathesis reaction of metal chloride ammoniates (TiCl3·5NH3 and TiCl3·3NH3) with lithium borohydride, from which Li2Ti(BH4)5·5NH3 crystallized in an orthorhombic structure with lattice parameters of a = 18.283 Ǻ , b = 10.216 Ǻ , c = 7.954 Ǻ , 3377



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and V = 1485.64 Ǻ 3. The prepared ATBs presented superior dehydrogenation properties. Particularly, the Li 2 Ti(BH4)5·5NH3 and Ti(BH4)3·3NH3 released ∼15.8 wt % and 14 wt % pure hydrogen below 300 °C and over 9 wt % pure hydrogen at a constant temperature of 100 °C, which meets the practical requirements for on-board hydrogen source materials. Preliminary regeneration experiments revealed that the dehydrogenated ATBs could be partly recharged by reacting with N2H4 in liquid ammonia. These favorable dehydrogenation properties and potential regeneration ability make ATBs very promising candidates as hydrogen storage materials. In situ high-resolution XRD, FT-IR, and 11B NMR measurements confirmed H2 evolution from ATBs derived from the combination of NH3 and BH4 species in ATBs.

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S Supporting Information *

TPD profile of TiCl3·5NH3, high-resolution XRD profiles of TiCl3·nNH3 (n = 3 and 5), Lebail fit XRD profile for Li2Ti(BH4)5·5NH3, summary and comparison of H2 evolution from various kinds of favorable dehydrogenation candidates, FT-IR (a) and TG-MS (b) spectra of regenerated Li2Ti(BH4)5·5NH3, Lebail fit XRD profile for intermediate product of Ti(BH4)3·3NH3 heated to 100 °C, and FT-IR spectra of Ti(BH4)3·3NH3 and Ti(BH4)3·5NH3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Phone and fax: +86-21-5566 4581. E-mail: yuxuebin@fudan. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology of China (Grant 2010CB631302), the National Natural Science Foundation of China (Grant 51071047), the PhD Programs Foundation of Ministry of Education of China (Grants 20110071110009 and 20110071120008), and the Science and Technology Commission of Shanghai Municipality (Grants 11JC1400700 and 11520701100). We wish to thank Helen Brand for her valuable comments.

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Complex Ammine Titanium(III) Borohydrides as Advanced Solid

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