Facile Synthesis and Superior Catalytic Activity of Nano-TiN@NâC for

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Facile synthesis and superior catalytic activity of nano-TiN@N-C for hydrogen storage in NaAlH4 Xin Zhang, Zhuanghe Ren, Yunhao Lu, Jianhua Yao, Mingxia Gao, Yongfeng Liu, and Hongge Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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ACS Applied Materials & Interfaces

Facile Synthesis and Superior Catalytic Activity of Nano-TiN@N-C for Hydrogen Storage in NaAlH4 Xin Zhang,†‡ Zhuanghe Ren,† Yunhao Lu,*† Jianhua Yao,‡ Mingxia Gao,† Yongfeng Liu,*†§ and Hongge Pan† †

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and

Applications for Batteries of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡

Institute of Laser Advanced Manufacturing, Collaborative Innovation Center of High-end

Laser Manufacturing Equipment, Zhejiang University of Technology, Hangzhou 310014, China §

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) Nankai

University, Tianjin 300071, China KEYWORDS: hydrogen storage, complex hydrides, alanates, catalyst doping, cycling stability

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ABSTRACT: Herein, we synthesize successfully ultrafine TiN nanoparticles (< 3 nm in size) embedded in N-doped carbon nanorods (nano-TiN@N-C) by a facile one-step calcination process. The prepared nano-TiN@N-C exhibits superior catalytic activity for hydrogen storage in NaAlH4. Adding 7 wt% nano-TiN@N-C induces more than 100 °C reduction in the onset dehydrogenation temperature of NaAlH4. Approximately 4.9 wt% H2 is rapidly released from the 7 wt% nano-TiN@N-C-containing NaAlH4 at 140 °C within 60 min, and the dehydrogenation product is completely hydrogenated at 100 °C within 15 min under 100 bar of hydrogen, exhibiting significantly improved desorption/absorption kinetics. No capacity loss is observed for nano-TiN@N-C-containing sample within 25 de-/hydrogenation cycles because nano-TiN functions as an active catalyst instead of a precursor. A severe structural distortion with extended bond lengths and reduced bond strength for Al-H bonding when the [AlH4]- group adsorbs on the TiN cluster is demonstrated for the first time by DFT calculations, which well explains the reduced de-/hydrogenation temperatures of the nanoTiN@N-C-containing NaAlH4. These findings provide new insights into designing and synthesizing high-performance catalysts for hydrogen storage in complex hydrides.


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1. INTRODUCTION Seeking a compact, efficient and safe manner to store hydrogen is a key challenge for using it as an energy carrier in a more sustainable future society.1-5 One feasible option is solid-state storage in hydrides.6-12 Numerous hydrides have been studied and evaluated for their hydrogen storage properties, and the scopes of study have expanded from conventional metal hydrides and sorbent materials to complex hydrides, including alanates, borohydrides, amides/imides, ammonia borane and amidoboranes.11-22 In particular, sodium alanate, NaAlH4, was the first complex hydride to be considered for mobile hydrogen storage applications because of suitable thermodynamics and relatively high hydrogen density.17 Theoretically, 7.5 wt% H2 or 94 g H2/L is stored in NaAlH4 by the following three-step reaction, but only the first two steps (5.6 wt% H2) are practically available due to the high decomposition temperature of NaH (> 400 °C).23 1 2 (1) NaAlH 4 → Na 3 AlH 6 + Al + H 2 3 3 3 (2) Na 3 AlH 6 → 3NaH + Al + H 2 2 1 (3) NaH → Na + H 2 2 For pure NaAlH4, the release and uptake of hydrogen have to surmount a high kinetic barrier with reported reaction activation energy of approximately 120 kJ/mol, which makes the materials reversible only under extreme conditions (270 °C and 175 bar of H2). Hope that NaAlH4 could be utilized as a practical hydrogen storage medium was generated in 1997 by Bogdanović and co-worker, who successfully achieved reversible hydrogen storage with it under much milder conditions by doping in a few mole percent of β-TiCl3 or Ti(OBu)4.17 After that, tremendous effort has been devoted to developing catalyst-doped NaAlH4, and a large number of catalysts, ranging from transition metal compounds, rare earth metal compounds to carbon-based materials, have been screened and studied.12,17,18,21-39



To date, Ti-based species are the most frequently studied catalytic additives for improving the kinetics of hydrogen release and uptake of NaAlH4, and much attention has been paid to halides and oxides.21-35 The original work by Bogdanović and Schwickardi demonstrated that doping 2 mol% β-TiCl3 reduced the dehydrogenation temperature of NaAlH4 by more than 80 °C, and the rehydrogenation was achieved at 170 °C and 152 bar of H2 pressure.17 Sandrock et al. obtained the release of ~ 3 wt% H2 from 2-4 mol% TiCl3-doped NaAlH4 prepared by dry ball milling within ~ 1 h at 125 °C and 1.3 bar pressure.23 Majzoub et al. compared different Ti halide catalyst-precursors, including TiCl2, TiCl3, TiF3 and TiBr4, and found that they all effectively improved sorption kinetics behaving in essentially the same manner.29 Wang et al. observed that TiF3 was even superior to TiCl3 as a dopant precursor in catalytically enhanced NaAlH4.30 Moreover, the effectiveness of TiO2 at catalysing hydrogen storage in NaAlH4 has also been evaluated.31-33 Nano-TiO2-catalysed NaAlH4 exhibited good kinetics comparable to TiCl3-containing sample, greatly superior to that with micro-sized TiO2.29 Similar phenomena were also observed by Rangsunvigit et al.32 and Xiong et al.33 More encouragingly, ultrafine nanocrystalline TiO2@C-catalysed NaAlH4 was completely hydrided (4.5 wt% H2) under 100 bar H2 even at temperatures as low as 50 °C.34 Although great success has been achieved in catalysing hydrogen release from NaAlH4, the remaining problem, when using these high-valence Ti compounds as dopant precursors, is the limited practical available capacity of 4.5 wt%, which is far from the theoretical value of 5.6 wt% due to the additional ‘dead weight’ and the chemical reaction between dopants and NaAlH4. In general, the dopants add weight to the sample, and they are not used for the storage of H2. Moreover, non-zero valent Ti dopants are readily reduced by reacting with NaAlH4 during ball milling and heating processes. This consumes the active hydrogen storage elements of Na and/or Al and further reduces the available hydrogen capacity. As a consequence, designing and preparing much more effective and thermodynamically stable active catalysts instead of catalyst precursors still remains of crucial importance for the development of a practical 4 ACS Paragon Plus Environment

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NaAlH4 system with both high available hydrogen capacity and appropriate operating temperature. In this regard, several attempts have been focused on TiN, TiC and TiB2 with higher thermodynamic stability compared with Al3Ti.21-24 The results showed good catalytic stability for these catalysts, without appreciable capacity loss upon cycling. For example, Bogdanović reported that with TiN as a dopant, a reversible storage capacity of 4.9 wt% H2 persisted after 16 cycles when operating at 170 °C and 132 bar of H2 pressure.35 Li et al. evidenced that TiN did not react with NaAlH4 during the preparation and dehydrogenation process, therefore no additional dead-weight by-products were generated.36 Similar results were also obtained in the TiC or TiB2-containing samples.37,38 More importantly, Kim et al. observed that TiN nanopowders were much more effective at catalysing hydrogen release from NaAlH4 than micron-sized samples as there was an additional 30-60 °C reduction in the dehydrogenation temperature for the nano-TiN-doped sample.39 Thus, the utilization of TiN as active catalyst achieved a better catalytic effect on the available hydrogen capacity and stable cyclability of NaAlH4 than the well-studied dopant TiCl3. Unfortunately, the working temperatures of the currently reported TiN-containing NaAlH4 are still high, and the kinetics of release and uptake are too sluggish for practical mobile applications. Therefore, it still is of particular scientific interest and practical importance to further enhance the catalytic activity of TiN. In principle, the activity of a catalyst is strongly particle-size dependent as the smaller the particle size, the higher the catalytic effect is. Nanoparticles generally possess a significant advantage due to remarkably increased specific surface area; however, the high surface energy usually induces severe aggregation. Hence, supporting materials were often used for the preparation of ultrafine and well-dispersed catalyst nanoparticles that not only function as barriers to prevent aggregation but also improve the chemical and thermal stability.25 Carbon materials have attracted much attention as excellent catalyst supports because of their light 5 ACS Paragon Plus Environment


weight and high chemical and thermal stability.40-42 However, the design and development of ultrafine and well-defined catalyst nanoparticles homogeneously dispersed on carbon materials still remains a great challenge. In this work, we successfully synthesized ultrafine TiN nanoparticles embedded in Ndoped carbon nanorods (denoted as nano-TiN@N-C) by a simple calcination and impurity removal process and evaluated their activity in catalysing hydrogen storage in NaAlH4. The prepared TiN particles were determined to be < 3 nm in size and showed remarkable catalytic activity for the hydrogen storage reaction of NaAlH4. Adding 7 wt% nano-TiN@N-C into NaAlH4 induces more than 100 °C reduction in the onset dehydrogenation temperature from 175 °C to 72 °C, and all available hydrogen (4.9 wt%) was released within 60 min at 140 °C. The dehydrogenated sample was completely hydrogenated even at temperatures as low as 60 °C. The reversible hydrogen capacity stabilized at 4.9 wt% after 25 cycles, corresponding to 100% of capacity retention, which is greatly superior to that of other Ti-based compoundadded samples. The existing state and role of TiN@N-C were systematically analysed and discussed in a series of structure and composition characterizations and theoretical calculations. The favourable performance and mechanistic understanding presented in this work provide new insights into designing and synthesizing novel high-performance catalysts for hydrogen storage in complex hydrides.

2. EXPERIMENTAL SECTION 2.1. Material preparation: All reagents and solvents were commercially available and used as received without further purification. Nano-TiN@N-C was synthesised in our own laboratory by using titanocene dichloride (Cp2TiCl2, 97%, Aladdin), lithium nitride (Li3N, 99.4%, Alfa Aesar) and dicyandiamide (C2H4N4, 98%, Aladdin) as raw materials, which were first uniformly mixed at a given molar ratio of 1:1:1 by ball milling at 300 rpm for 3 h on a 6 ACS Paragon Plus Environment

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planetary ball mill (QM-3SP4, Nanjing) and then calcined in a tube furnace under Ar atmosphere. In a typical procedure, approximately 0.5 g sample was sealed into a stainlesssteel tubular reactor, then heated to 550 °C at a ramping rate of 5 °C min-1 and kept for 2 h. After cooling to room temperature, the black solid-state powder was collected and washed twice with ethanol to remove the by-product LiCl. The final product was obtained by drying the filter residue under vacuum at 150 °C for 12 h. The fabricated nano-TiN@N-C was introduced into NaAlH4 (hydrogen storage grade, Sigma Aldrich) to evaluate its catalytic effectiveness. Six samples with compositions of NaAlH4-x wt% TiN@N-C (x = 0, 1, 3, 5, 7, 9) were prepared by ball milling the corresponding chemicals at 500 rpm for 24 h. To ensure even mixing and prevent dehydrogenation evolution initiated by energetic colliding balls, ball milling was operated first under 1 bar of Ar for 12 h and then under 50 bar of hydrogen for another 12 h. Here, approximately 1 g of mixture was loaded into the milling jar inside an MBRAUN glovebox (Germany) filled with pure argon (H2O and O2: < 1 ppm). The ball-to-sample weight ratio was approximately 120:1. To minimize the temperature rise, the milling process was set to rotate for 0.3 h in one direction, pause for 0.1 h, and then revolve in the reverse direction for another 0.3 h. For comparison, the samples with the addition of TiF3 (98%, Alfa Aesar), TiCl3 (contains 25 wt%AlCl3, Alfa Aesar) and nano-Ti (60 nm, 99.8%, Aladdin) were also prepared by a same milling process.

2.2. Property evaluation: The temperature-dependent dehydrogenation behaviour was qualitatively evaluated using a home-built temperature-programmed desorption (TPD) system attached to an online gas chromatograph (GC). A continuous flow of pure Ar with a flow rate of 20 ml min-1 was used as carrier gas. For each test, approximately 40 mg of the as-prepared sample was gradually heated from room temperature to 400 °C at 2 °C min-1. Further quantitative dehydrogenation/hydrogenation properties were measured and compared with a 7 ACS Paragon Plus Environment


Sieverts-type apparatus under isothermal and non-isothermal conditions by using approximately 70 mg of sample. The non-isothermal data were acquired by gradually heating the sample from room temperature to a preset temperature at an average rate of 2 °C min-1 under a primary vacuum (~10-3 Torr) for dehydrogenation and 1 °C min-1 with 100 bar of hydrogen pressure for hydrogenation. The isothermal measurements were conducted by rapidly heating the sample to a desired temperature and then maintaining during the entire test. The temperature and pressure of samples were monitored and recorded simultaneously, and the amount of hydrogen release/uptake was calculated using the equation of state.

2.3. Structural and morphological characterization: An X'Pert Pro X-ray diffractometer (Rigaku, Japan) with Cu Ka radiation (40 kV, 40 mA) was employed to identify the samples' phase structure. XRD data were collected in a 2θ range of 10-90° with 0.05° step increments at room temperature. The samples were sealed in a custom-designed container with a window covered by Scotch tape for transmission of X-rays to prevent air and moisture contamination. Elemental analysis was performed on a Vario MICRO cube elementary analyser (Elementer, Germany) to quantify the contents of Ti, C and N in the prepared TiN@N-C. A SEM (Hitachi, S-4800) and a TEM (FEI, Tecnai G2 F20 S-TWIN) were used to observe samples’ morphology and nanostructure. In SEM experiments, the sample was transferred quickly to the SEM facility under the protection of an Ar atmosphere. For TEM observation, the sample was protected with a double tilt vacuum transfer holder (Gatan 648, USA). The distribution of elemental C, Ti and N was identified with an energy-dispersive EDS attached to the Tecnai G2 F20 S-TWIN TEM facility. XPS analyses were carried out on a Kratos AXIS Ultra DLD spectrometer. The powder sample was first compacted into a pellet at room temperature and then mounted on the sample holder in an argon atmosphere, then it was transferred from the glovebox to the XPS facility with a transfer vessel for preventing air contamination. The XPS data were recorded using a monochromatic Al Kα X-ray source with a base pressure of 8 ACS Paragon Plus Environment

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6.8×10-9 Torr. The binding energy spectra were fitted by XPSPEAK software. Raman spectra were recorded using a confocal Raman microscope (Via-Renishawplc, UK) at a laser excitation wavelength of 532 nm. The N2 adsorption/desorption isotherms were collected at 77 K, maintained by a liquid nitrogen bath, with the relative pressures ranging from 0.0424 to 0.987 (P/P0) using a NOVA-1000e automated surface area analyzer (Quantachrome, USA).

2.4. Theoretical calculation: First-principles calculations based on density functional theory (DFT) were performed using the projected augmented plane-wave (PAW) as implemented in Vienna ab initio simulation package (VASP).43,44 Generalized gradient approximation was employed for the exchange correlation function with the Perdew, Burke, and Ernzerhof (PBE) form.45 The plane wave cutoff energy was set above 400 eV for all calculations. The atoms were relaxed until the residual forces were less than 0.05 eV Å-1. The k-points were generated using a Gamma centred grid of 1×1×1. The crystal orbital overlap population (COOP) was analysed by Local Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER) code.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of Nano-TiN@N-C. Figure 1a schematically illustrates the strategy for preparation of ultrafine TiN nanoparticles embedded in N-doped carbon nanorods (denoted as nano-TiN@N-C). The raw materials, Cp2TiCl2, Li3N and C2H4N4 at a molar ratio of 1:1:1, were first uniformly mixed by ball milling at 300 rpm for 3 h. Subsequently, the prepared mixture was loaded into a stainless-steel reactor to calcine at 550 °C for 4 h under Ar atmosphere. Finally, the calcined solid-state residue was washed with ethanol to remove the by-product LiCl and dried in dynamic vacuum at 150 °C for 12 h to produce the resultant well-dispersed nanocrystalline TiN@N-C.



EDS results indicated that the final product was mainly composed of Ti, N, and C elements (Figure 1b). XRD examination (Figure 1c) identified unambiguously the TiN phase with considerable intensity although its reflections were broad, representing poor crystallization and/or small particle/grain size. Moreover, an additional bump centred at 2θ = 26° was also detected, which can be assigned to layered graphite carbon, as reported previously.46 Further Raman analysis confirmed the presence of graphite-like carbon because its characteristic D and G bands emerged at ~ 1350 and 1590 cm-1 (Figure 1d), respectively. It is known that the G band at 1590 cm-1 originates from the in-plane vibration of sp2 C atoms in a 2D hexagonal lattice, such as layered graphite, and the D band at 1350 cm-1 represents the non-perfect crystalline structure of graphite caused by defects.46 Moreover, the results obtained by elemental analysis determined a molar ratio of 0.74:1.36:3.77 for Ti:N:C. Here, it is noteworthy that the molar ratio of Ti and N is greatly smaller than the 1:1 of the TiN compound, representing the existence of excessive N, which is possibly doped into the carbon structure. Such conjecture was further evidenced by XPS measurements as C-N bonding was identified from the peaks of C 1s (285.6 eV) and N 1s (401 and 399 eV) (Figure 1e and g).47 In the meantime, the Ti-N bonding was confirmed by the XPS peaks of Ti 2p (455.7 and 461.1 eV) and N 1s (396.5 eV). Moreover, the very weak peak of C-O bonding shown in the C 1s spectrum was mainly ascribed to adsorbed CO2 impurity. Thus, we believe that the final product in the present study was composed of TiN and N-doped carbon. Figure 2 displays SEM and TEM images and corresponding EDS element mapping of the prepared final product. The SEM image shown in Figure 2a displays clear nanorod-like morphology for the as-synthesized nano-TiN@N-C. TEM observation indicated that the diameter of the representative nanorods was approximately 50 nm, and some small black particles were well distributed in the grey matrix (Figure 2b and c). These particles were identified to be nano TiN single crystals by interplanar spacing of 0.21 nm, as seen in 10 ACS Paragon Plus Environment

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HRTEM image (Figure 2d), which corresponds to an interplanar distance of (200) between the planes of TiN. The particle size of the TiN nanoparticles was measured to be < 3 nm, with good dispersion. Further EDS mapping results revealed that the matrix was mainly composed of C and N elements because they nearly covered the whole nanorod (Figure 2e), exhibiting very similar distribution. However, for Ti, it only partially covered the area of the nanorods. These results further confirmed that nano-TiN single-crystal particles embedded in N-doped carbon nanorods (denoted as nano-TiN@N-C) were successfully synthesized by calcining uniformly mixed Cp2TiCl2, Li3N and C2H4N4 (1:1:1 in molar ratio) to 550 °C.

3.2. Catalytic effect of TiN@N-C on dehydrogenation of NaAlH4. The prepared nanoTiN@N-C was introduced into NaAlH4 to determine its catalytic effectiveness. The designed samples were composed of NaAlH4-x wt% TiN@N-C (x = 0, 1, 3, 5, 7 and 9), which was prepared by ball milling at 500 rpm for 12 h first under 1 bar of Ar, and then for another 12 h under 50 bar of hydrogen. The milled samples were collected for structural characterization and property evaluation. Figure 3a shows the XRD patterns of the resultant samples. All samples exhibited very similar patterns because the NaAlH4 phase still dominated the XRD profiles although the intensity of the characteristic reflections was gradually weakened with increasing addition of nano-TiN@N-C due to a dilution effect. No additional diffraction peaks were detected while the addition amount of nano-TiN@N-C was below 9 wt%, representing the persistence of NaAlH4 and TiN@N-C. Unfortunately, no Ti-, N- or C-containing phases were identified by XRD alone, possibly due to their low content and/or amorphous form. Moreover, it should be mentioned that for the sample with x > 9 wt%, the strongest reflection of Al with quite weak intensity was observed at 38.5°, which can be ascribed to the decomposition of a minor amount of NaAlH4 caused by the catalysis of nano-TiN@N-C. To elucidate the distribution of nano-TiN@N-C in the prepared TiN@N-C-containing NaAlH4 samples, the NaAlH4-7 wt% TiN@N-C sample was taken as a representative 11 ACS Paragon Plus Environment


example for SEM and TEM observation and EDS analysis. For TEM observation, the sample was dispersed onto a carbon-coated 200-mesh copper grid inside a glove box, and then placed in a Gatan 628 double tilt vacuum transfer holder to protect the sample from moisture and oxygen contamination. The results are shown in Figure S1 (Supporting Information). After ball milling, the resultant product exhibits irregular solid particles measuring 200 nm - 2 μm in size as observed in an SEM image (Figure S1(a), Supporting Information). TEM observation (Figure S1(b) and (c), Supporting Information) reveals that there are many small dark particles (~ 5 nm) in the larger grey matrix; these small dark particles were identified to be composed of Ti, C and N by EDS analysis (Figure S1(d), Supporting Information) and are consequently believed to be TiN@N-C species. Thus, we speculate that during energetic ball milling, the TiN-containing N-doped carbon nanorods were broken and fractured into much smaller particles and distributed diffusely into the NaAlH4 matrix. Figure 4 illustrates the qualitative and quantitative dehydrogenation behaviour of the prepared TiN@N-C-containing NaAlH4 measured by TPD and volumetric techniques. Three dehydrogenation peaks were observed at 175 - 367 °C in the TPD curve (Figure 4a), corresponding to the decomposition of NaAlH4, Na3AlH6 and NaH as reported previously.20 After adding 1 wt% nano-TiN@N-C, the dehydrogenation peaks corresponding to NaAlH4 and Na3AlH6 were significantly reduced from 254 and 267 °C to 146 and 162 °C, respectively; however, only 10 °C reduction was obtained for the decomposition of NaH. This fact indicated that the prepared nano-TiN@N-C offers significant catalytic activity only for the decomposition of [AlH4] and [AlH6] groups and has little effect on NaH. Adding more nano-TiN@N-C up to 7 wt% induced a further reduction in the decomposition temperatures of the [AlH4] and [AlH6] groups. The peak decomposition temperatures of the [AlH4] and [AlH6] groups were determined to be 114 and 148 °C, respectively, for the NaAlH4-7 wt% TiN@N-C sample, representing additional reduction of 32 and 14 °C compared to the 1 wt% 12 ACS Paragon Plus Environment

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nano-TiN@N-C-containing sample. However, no appreciable change was observed upon further increasing nano-TiN@N-C to 9 wt%. It is therefore believed that NaAlH4-7 wt% TiN@N-C should be the optimized composition in the present study. Because the decomposition temperature of NaH was still higher than 300 °C, which is meaningless for practical mobile applications, the follow-up quantitative dehydrogenation measurements were concentrated on the first two steps. Figure 4b presents the volumetric release curves of the prepared NaAlH4-x wt% TiN@NC samples. As expected, the volumetric release curve of NaAlH4 shifted towards lower temperatures with the addition of nano-TiN@N-C. The 7 wt% TiN@N-C-containing sample exhibited the optimal dehydrogenation properties because no additional reduction was detected in the operating temperature with further addition of nano-TiN@N-C to 9 wt%; however, a gradual reduction in the dehydrogenation capacity continued due to the dead weight of TiN@N-C. Approximately 4.9 wt% of hydrogen was released from the 7 wt% TiN@N-C-containing sample at 70 – 170 °C. Such dehydrogenation performance is remarkably superior to those of well-studied TiCl3, TiF3 and nano-Ti-doped NaAlH4 (Figure S2a, Supporting Information). Further isothermal dehydrogenation measurements showed that the 7 wt% TiN@N-Ccontaining sample released all available hydrogen (4.9 wt%) within 60 min at 140 °C (Figure 4c). However, no more than 0.2 wt% of hydrogen was liberated from the pristine NaAlH4 under identical conditions. Even at 100 °C, the 7 wt% TiN@N-C-containing sample released approximately 3.4 wt% of hydrogen within 60 min, exhibiting remarkably improved dehydrogenation kinetics. This is also greatly superior to the previously reported TiN-doped sample because its dehydrogenation amount was only 1 wt% under identical conditions.22 Calculations based on Kissinger’s method48 determined the activation energy values to be 79.7 ± 1.6 and 81.3 ± 0.9 kJ/mol for the first and second dehydrogenation reactions of the 13 ACS Paragon Plus Environment


NaAlH4-7 wt% TiN@N-C sample, respectively (Figure 4d and e). These values are 41.1 and 32.1% lower than those of the pristine NaAlH4 as reported previously.49 The decreased activation energy is responsible for the significantly reduced temperature and improved reaction kinetics for hydrogen desorption from the nano-TiN@N-C-containing samples.

3.3 Hydrogen storage reversibility of nano-TiN@N-C-containing NaAlH4. The effect of nano-TiN@N-C on the hydrogenation properties of NaAlH4 was also studied. As shown in Figure 5a, the dehydrogenated NaAlH4-7 wt% TiN@N-C sample started absorbing hydrogen at only 33 °C under 100 bar of H2, and full hydrogenation was achieved at 95 °C, exhibiting good reversibility. This was confirmed by XRD measurement as the hydrogenation sample was completely converted to NaAlH4 without additional diffraction peaks (Figure 5b). As a comparison, the dehydrogenated pristine NaAlH4 was also hydrogenated under identical conditions. The results showed the starting temperature for hydrogen uptake by the dehydrogenated pristine NaAlH4 to be approximately 100 °C, and only 1.1 wt% of hydrogen was recharged even heating to 150 °C, representing poor hydrogen storage reversibility (Figure 5a). The fact that NaH and Al still dominated the XRD profile of the hydrogenated product provided additional evidence for the incomplete hydrogenation of the pristine dehydrogenation sample (Figure 5b). Further isothermal hydrogenation was performed under 100 bar of H2. As shown in Figure 5c, the dehydrogenated NaAlH4-7 wt% TiN@N-C sample was completely hydrogenated even at temperatures as low as 60 °C; approximately 4.9 wt% of hydrogen was absorbed within 50 min. When operating at 100 °C, it only took 15 min to complete hydrogenation. Structural analysis confirmed that NaAlH4 dominated the XRD profile of the hydrogenated samples (Figure S3a, Supporting Information). The hydrogenated sample released 4.9 wt% of H2 in the follow-up cycle, exhibiting good cycling stability (Figure S3b). Moreover, it should be noted that reducing the hydrogenation pressure induced a slow 14 ACS Paragon Plus Environment

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reaction rate, which is possibly related to the equilibrium pressure. When hydrogenation proceeded at 25 bar H2 and 100 °C, the dehydrogenated sample rapidly absorbed 2 wt% of H2 within 20 min, and then the hydrogenation rate was distinctly slower (Figure 5d), exhibiting a two-stage reaction process. Approximately 4.0 wt% of H2 was absorbed in 160 min at this temperature and pressure, greatly superior to those of well-studied TiCl3, TiF3 and nano-Tidoped NaAlH4 (Figure S2b, Supporting Information).24-28,33 Further re-dehydrogenation measurement also confirms the improved low-pressure hydrogenation performance (Figure S3c and d, Supporting Information).

3.4. Dehydrogenation process and existing state of TiN. The dehydrogenation process of nano-TiN@N-C-containing NaAlH4 was understood by characterising the dehydrogenation samples stopped at different stages with XRD and XPS. The results are illustrated in Figure 3b-d. It can be observed from Figure 3b that with increasing the sample temperature, the characteristic reflections of NaAlH4 gradually disappeared and those of Na3AlH6 and Al emerged and intensified at 105-135 °C. Interestingly, further increasing the sample temperature to 175 °C, the newly generated Na3AlH6 was invisible, and NaH was observed along with the further intensification of Al. We therefore believe that upon heating, NaAlH4 first decomposed to produce Na3AlH6 and Al, and then further converted to NaH and Al in association with hydrogen release. In other words, the dehydrogenation process of NaAlH4, which can still be described by reactions (1)-(2), did not change due to the presence of nanoTiN@N-C. A similar phenomenon was observed even increasing the addition of nanoTiN@N-C to 15 wt% which shows an identical dehydrogenation behavior to NaAH4-7 wt% TiN@N-C (Figure S4 and S5, Supporting Information). Unfortunately, no Ti-containing species was identified by XRD. To shed light on the state of Ti, high-resolution XPS spectra of 15 wt% TiN@N-C-containing sample before and after dehydrogenation were further acquired and analysed (Figure 3c and d). It is clear that the dehydrogenated sample exhibited 15 ACS Paragon Plus Environment


an identical Ti 2p XPS spectrum to the milled sample as two peaks corresponding to the binding energy of TiN were observed at 455.7 and 461.5 eV.40 This indicates that TiN still persisted during the ball milling and dehydrogenation process and worked only as a catalyst to reduce the reaction barrier for hydrogen storage in NaAlH4. To further understand the catalyst role played by TiN, DFT calculations and Crystal Orbital Overlap Population (COOP) analyses were conducted with a focus on interaction between the negatively charged [AlH4]- ionic group and the TiN nanoparticle. Here, the TiN nanoparticle is modelled as 14 Ti atoms and 13 N atoms (Figure 6a) with lateral size ~ 1 nm, within the same size scale of TiN nanoparticles observed in the TEM image (Figure 2). The [AlH4]- ionic group with different adsorption structures on the TiN cluster was constructed and geometrically optimized, and the most stable one is shown in Figure 6a. The results show that the [AlH4]- group retains a typical tetrahedral geometry but suffers from a severe structural distortion after adsorption on the TiN cluster. The bond lengths of Al-H exhibit roughly an increasing tendency, especially for the Al-H1 and Al-H4 bonds because they are extended more than 15% compared with those of a pristine [AlH4]- group. It is generally accepted that bond length is highly related to bond strength, and its elongation decreases the interaction between corresponding atoms. Specifically, the bond lengths of Al-H1 and Al-H4 are over 1.9 Å, much larger than the sum of covalence radius of isolated Al (1.18 Å) and H (0.32 Å) atoms. It is therefore much easier for an H atom to detach from an [AlH4]- group when it adsorbs on the surface of a TiN cluster. The effect of TiN on the Al-H bonding character was further elucidated by COOP analysis. Figure 6b presents the COOP curves of Al-H bonds in the [AlH4]- group before and after adsorption on a TiN cluster. It is observed that two peaks at around -1 and -4.3 eV contribute the bonding of Al-H in pristine [AlH4]- before adsorption. However, there is a dramatic weakening in bonding character once the ionic group is adsorbed on the TiN cluster. 16 ACS Paragon Plus Environment

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For the Al-H1 and Al-H4 bonds, the bonding peaks become quite weak associated with the presence of a little anti-bonding character that is slightly below the Fermi energy. The overlap population (OP) gives a quantitative description of bond strength (Table 1). The bond strengths of Al-H1 and Al-H4 of [AlH4]- adsorbed on a TiN cluster were reduced by more than 60% compared with those of pristine [AlH4]-. Moreover, there is a large reduction for AlH2 and Al-H3 although their bond lengths remained nearly constant. Thus, adsorbing on a TiN cluster induces a remarkable reduction in bond strength for Al-H bonding, which facilitates the liberation of H atoms and gives rise to the formation of H2. This is in excellent agreement with experimental observations that TiN nanoparticles work as an active catalyst to dramatically reduce the initial dehydrogenation temperature and improve the desorption kinetics (Figure 4).

3.5. Cyclability of hydrogen storage in NaAlH4-7 wt% TiN@N-C. The cycling stability of hydrogen storage in TiN@N-C-catalysed NaAlH4 was also measured by operating dehydrogenation at 140 °C under vacuum and hydrogenation at 120 °C under 100 bar of H2. Figure 7a illustrates the cycling dehydrogenation and hydrogenation curves of an NaAlH4-7 wt% TiN@N-C sample within 25 cycles. It was observed that the available hydrogen capacity still stayed at approximately 4.9 wt% after 25 cycles, which corresponds to capacity retention of 100%. Interestingly, comparison with the non-isothermal dehydrogenation behaviour presented a slightly lower operating temperature after 25 cycles as an additional 15 °C reduction in the onset dehydrogenation temperature was achieved (Figure 7b). This should closely correlate with the morphology change of the cycled sample because the TiN still remained nearly constant upon cycling as observed in the XPS spectrum (Figure 7c). An SEM image displayed a loosely porous morphology made of a large number of nanoparticles (< 200 nm) for the 25-cycle sample (Figure 7d), which is distinctly different from the as-milled sample (Figure S1, Supporting Information). This result was confirmed by TEM observation 17 ACS Paragon Plus Environment


as the particle size was measured to be approximately 50-100 nm (Figure 8a and b). Additionally, a 4-fold increase in the specific surface area was achieved with respect to the asmilled sample as determined by the N2 sorption measurement (Figure S6, Supporting Information). Thus, the porous morphology and reduced particle size enlarged the specific surface area and facilitated the diffusion of H, consequently further improving the reaction kinetics of the nano-TiN@N-C-containing sample as shown in Figure 7b. Moreover, it was also observed in HRTEM images (Figure 8d) that numerous fine nanoparticles measuring < 3 nm in size were dispersedly distributed on the larger particles of the matrix, which were identified to be TiN (fine nanoparticles) and NaAlH4 (matrix) by EDS analyses (Figure 8c and d). The dispersive distribution of TiN offered high catalytic activity for hydrogen storage in NaAlH4 upon cycling, which is reasonably responsible for the good long-term cycling stability of the TiN@N-C-containing sample (Figure 7a).

4. CONCLUSION In this work, ultrafine TiN nanoparticles embedded in N-doped carbon nanorods (denoated as nano-TiN@N-C) was successfully synthesized by calcining a mixture of Cp2TiCl2, Li3N and C2H4N4 (1:1:1 in molar ratio) at 550 °C. The particle size of TiN nanoparticles was measured to be < 3 nm. The prepared nano-TiN@N-C showed superior catalytic activity for the hydrogen storage reaction of NaAlH4 as there was more than 100 °C reduction in the onset dehydrogenation temperature from 175 °C to 72 °C for the NaAlH4-7 wt% TiN@N-C sample, and hydrogen uptake amounted to 4.9 wt% while heating to 170 °C in non-isothermal mode. Isothermal dehydrogenation experiment revealed that all available hydrogen (4.9 wt%) was released within 60 min at 140 °C with 100% of capacity retention within 25 cycles. The dehydrogenated sample was completely hydrogenated under 100 bar of hydrogen even at temperatures as low as 60 °C. During ball milling and dehydrogenation process, TiN still persisted and worked only as a catalyst to reduce the reaction energy barriers for hydrogen 18 ACS Paragon Plus Environment

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storage in NaAlH4. Further DFT calculations and COOP analyses presented more than 15% extension in the bond length and 60% reduction in the bond strength for the [AlH4]- group adsorbed on TiN cluster. This is reasonably responsible for the reduced dehydrogenation operating temperatures and improved hydrogen storage kinetics of nano-TiN@C-containing NaAlH4.

ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ Experimental data, TEM images and XRD patterns (PDF)

AUTHOR INFORMATION: Corresponding Author *

E-mail: [email protected] (Y.H.L.), [email protected] (Y.F.L.).

AUTHOR CONTRIBUTIONS X.Z. and Z.H.R. contributed equally to this work.

NOTES The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the National Natural Science Foundation of China (51671172, U1601212), the Zhejiang Provincial Natural Science Foundation of China (LR16E010002) and the National Youth Top-Notch Talent Support Program.

REFERENCES 19 ACS Paragon Plus Environment


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Page 25 of 30

(a)

LiCl

Li3N

Heating

Ball Milling

Intensity (a.u.)

Intensity (a.u.)

N kα

1

(e)

292

2

3 4 5 Energy (keV) C-C 284.8 eV

6

7 10 C 1s

C-O 286.7 eV

288 286 284 Binding Energy (eV)

20

(f)

282

280

30

40

∋

∋

∀ After calcination ∀ ∀ ∀ ∀

∋

C-N 285.6 eV

290

Washed with ethanol

∋ ∀

50 2θ (°)

Ti 2p

60

70

80

466

90

900

TiN 2p3/2 455.7 eV

462 458 454 Binding Energy (eV)

C-N 399.0 eV

450

G-band -1 1590 cm

1100 1300 1500 1700 -1 Raman shift (cm )

(g) N 1s

TiN 2p1/2 461.5 eV

Intensity (a.u.)

0

D-band -1 1350 cm

∀: LiCl ∋: TiN

∋

Ti kα

(d)

XRD ∀

Ti kα

TiN@N-C

Ethanol

∀

(c)

Intensity (a.u.)

(b)

EDS

Intensity (a.u.)

Raw Material C kα

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


406

1900

Ti-N 396.5 eV

C-N 401.0 eV

404

402 400 398 396 Binding Energy (eV)

394

Figure 1. Schematic illustration of the preparation of nano-TiN@N-C (a) and its EDS profile (b); XRD patterns (c); Raman spectrum (d); and Ti 2p (e), N 1s (f) and C 1s (g) XPS spectra .

Figure 2. SEM image (a), TEM images (b, c), HRTEM image (d) and EDS element mapping images of C, Ti, and N elements (e) of the nano-TiN@N-C.



(a)

∗: Al φ: NaAlH4

φ φ

φ

φ φ φ

∗

φφ

(b) +7 wt%TiN@N-C +9 wt% #

Intensity (a.u.)

φ φ φ

φφ

φ

φ

φ

φ

φ

φ

φ

φ

10

20

30

40

φ φ φ

φφ

φ φ φ

φφ

φ

φ φ

φφ

φ φ φ

φφ

50

60

φ

+5 wt% φ

+3 wt% φ

+1 wt%

Deh -155 °C

#&

∗

#

∗

Deh -135 °C

& #

Deh -115 °C

φ & φ φ

∗

Pristine NaAlH4

φ& φ

BM-24h

φ

10

90

468

466

464

462

460

458

456

∗

∗ &φ

φ &φφ

∗ &

∗ ∗

∗

∗ & φ φ &φ

φφ

∗ &

φ

φφ

20

30

∗

φ

φ

40

φ

φ

50

60

70

80

90

454

452

TiN 2p3/2 455.7 eV

Ti 2p

454

∗

∗

φ

φ

Intensity (a.u.)

Intensity (a.u.) 470

∗

∗ &

(d) Dehydrogenation

TiN 2p1/2 461.5 eV

∗

2θ (°)

TiN 2p3/2 455.7 eV

Ti 2p

#∗ #

&

2θ (°) (c) As-milled

#

# : NaH ∗: Al ∗ ∗

&

#

φ

80

#∗ #

φ

Deh -105 °C

φ

70

#

∗

φ φ

∗

#

+7 wt%

Intensity (a.u.)

φ

φ

φ: NaAlH4 &: Na3AlH6

∗

Deh -175 °C

φ

φ

TiN 2p1/2 461.5 eV

470

452

468

466

464

462

460

458

456

Binding Energy (eV)

Binding Energy (eV)

Figure 3. XRD patterns of nano-TiN@N-C-containing NaAlH4 samples (a) and dehydrogenated samples at various stages (b), Ti 2p XPS spectra of the NaAlH4-15 wt% TiN@N-C sample before (c) and after (d) dehydrogenation. 3 2

Intensity (a.u.)

4 5 6

(b)

0

1

NaAlH4

2 3 4 5 6

+1 wt%TiN@N-C +3 wt% TiN@N-C +5 wt% TiN@N-C +7 wt% TiN@N-C +9 wt% TiN@N-C

NaAlH4 +1 wt%TiN@N-C +3 wt%TiN@N-C +5 wt%TiN@N-C +7 wt%TiN@N-C +9 wt%TiN@N-C

-1

H/M (wt%)

(a)

1

-2 -3 -4 -5 -6

50

100

150

200

250

300

350

50

400

100

150

200

250

300

350

Temperature (°C)

Temperature (°°C) (c)

0

(e)

(d)

1st-step y=-9.59x+13.98

-10.0

2

R =0.999 Ea=79.7 ±1.6 kJ/mol

-3 -4

-10.5

4 °C/min

2

Intensity (a.u.)

Pristine NaAlH4140 °C +7 wt% TiN@N-C 100 °C +7 wt% TiN@N-C 120 °C +7 wt% TiN@N-C 140 °C

-2

ln(β /Tm)

8 °C/min

-1

H/M (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

-11.0

2 °C/min 1 °C/min

-11.5 2nd-step y= -9.78x+12.22

-12.0

2

-5

R =0.999 Ea=81.3±0.9 kJ/mol

0

20

40

60

80

100

120

Time (min)

50 75 100 125 150 175 200 2.3

Temperature (°C)

2.4

2.5

2.6

2.7

-1

1000/Tm(k

)

Figure 4. TPD (a), volumetric release (b) and isothermal dehydrogenation curves (c) of NaAlH4-x wt%TiN@N-C samples; and TPD curves (d), Kissinger’s plots (e) of NaAlH4-7 wt% TiN@N-C sample.


Page 27 of 30

(a)

φ

(b)

φ: NaAlH4 &: Na3AlH6

4 3

NaAlH4 milled 24h NaAlH4 - 7wt% TiN@N-C

2

∗: Al

Intensity (a.u.)

Hydrogen absorbed (wt%)

5

1

φ φ

φ φ

60

75

90

105

120

135

150

10

20

30

φ

φ φφ

40

50

φφ

Pristine NaAlH4 ∗ # # ∗ ∗

#

60

70

80

90

2θ (°)

(c)

(d)


5

4 3 60 °C 80 °C 100 °C

2

+7 wt%TiN@N-C

φ

#

Temperature (°C) 5

φ

φ

#

∗

0 45

# : NaH

∗

&

30


1

4 3 Hydrogenation at 100 °C

2

25 bar 50 bar 100 bar

1 0

0 0

5

10

15

20

25

30

35

40

45

0

50

20

40

60

80

100

120

140

160

Time (min)

Time (min)

Figure 5. (a) Non-isothermal hydrogenation curves and (b) XRD patterns of hydrogenated products of NaAlH4 ball milled with and without TiN@N-C; isothermal hydrogenation curves of NaAlH4-7 wt% TiN@N-C at different temperatures (c) and under different hydrogen pressure (d).

(a) H4 H3 H2 H1

[AlH4]

(b) 0

Side view

a

c

b

Top view

e

d

eV eV)) Energy ((eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 -4 -6 -8

-10

-

+

-

+

+ COOP

-

+

-

+

Figure 6. (a) The optimized geometry of [AlH4]- before and after adsorbing on a TiN cluster with H ions in white, Al ions in yellow, N ions in grey and Ti ions in cyan; (b) COOP curves for a: Al-H bond in isolated [AlH4]-, b-e: Al-H1, Al-H2, Al-H3 and Al-H4 bonds in [AlH4]adsorbed on a TiN cluster, respectively.



1

Dehydrogenation Hydrogenation

(a)

(b)0

0 -1

H/M (wt%)

H/M (wt%)

-1

-2

-3

-2

-3

Deh-1st cycle Deh-10th cycle Deh-25th cycle

-4

-4

-5

-5

Cycle 1-25

0

200

400

600

800 1000 1200 1400 1600 1800 2000

40

60

80

Time (min)

(c)

100

120

140

160

180

200

Temperature (°C)

(d)

Ti 2p TiN 2p3/2 455.7 eV TiN 2p1/2 461.5 eV

Intensity (a.u.)

2 µm 470

468

466

464

462

460

458

456

454

500 nm

452

Binding Energy (eV)

Figure 7. Isothermal (a) and non-isothermal (b) de/hydrogenation cyclic curves of TiN@N-Cdoped NaAlH4; Ti 2p XPS spectrum (c) and (d) SEM image of TiN@N-C-doped NaAlH4 after 25 cycles.

(b)

(a)

A

50 nm

20 nm

B

(c)

(d) Al

EDS-A

EDS-B

Al Na C

Cu

Cu

Ti

Cu C

Ti

N

0

Na

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

2

4

Cu

Ti Ti 6

8

10

0

2

Energy (keV)

4

6

8

10

Energy (keV)

Figure 8. TEM images (a, b) and EDS patterns (c, d) of NaAlH4-7 wt%TiN@N-C sample after 25 cycles. 28 ACS Paragon Plus Environment

Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. The bond distance and OP of Al-H bonds in [AlH4]- before and after adsorbing on a TiN cluster. Before adsorption After adsorption Chemical Bond Al-H Al-H1 Al-H2 Al-H3 Al-H4 Bond distance (Å)

1.652

1.906

1.710

1.601

1.905

OP

0.360

0.133

0.194

0.254

0.135



ToC figure 0 Pristine NaAlH4-140 °C

-1

H/M (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 30

-2 -3

[AlH4]

50 nm Side view

Nano-TiN@CN

-4 +7 wt% TiN@N-C-140 °C

-5 0

20

40

60

80

100

Time (min)


120

Facile Synthesis and Superior Catalytic Activity of Nano-TiN@NâC for

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