Excellent Catalysis of Various TiO2 Dopants with Na0.46TiO2 In-situ

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C: Energy Conversion and Storage; Energy and Charge Transport

Excellent Catalysis of Various TiO2 Dopants with Na0.46TiO2 In-situ Formed on the Enhanced Dehydrogenation Properties of NaMgH3 Zhencan Hu, Haiying Qin, Xuezhang Xiao, Man Chen, Meijia Liu, Ruicheng Jiang, and Lixin Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06688 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Excellent Catalysis of Various TiO2 Dopants with Na0.46TiO2 Insitu Formed on the Enhanced Dehydrogenation Properties of NaMgH3 Zhencan Hu a, Haiying Qin b, Xuezhang Xiao a,d*, Man Chen a, Meijia Liu a, Ruicheng Jiang a, a

Lixin Chen a,c

State Key Laboratory of Silicon Materials; School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China

b

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, P.R. China. c Key

Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310013, China

d

Guangdong Provincial Key Laboratory of Advance Energy Storage Materials, South China University of Technology, Guangzhou, 510640, China

Abstract NaMgH3 has been considered to be a potential candidate for the solid-state hydrogen storage due to its considerable hydrogen gravimetric (6.0 wt.%) and volumetric (88.0 g L-1) densities. Meanwhile, NaMgH3 possesses an outstanding theoretical thermal storage density of 2881 kJ/kg, which makes it one of the most promising thermal energy storage materials. However, the sluggish dehydrogenation kinetics of NaMgH3 embarrasses the further practical application. Doping nano-size Tibased catalyst is treated to be one of the most effective methods to settle the poor dehydriding kinetics. In this work, different kinds of TiO2 catalysts, the 5 wt.% TiO2 microparticle (MP) (100 nm), TiO2 nanoparticle (NP) (5-10 nm) and TiO2 nanotube (NT) (5-10 nm), were doped into NaMgH3 in the process of ball milling and heattreatment, which in-situ formed Na0.46TiO2 significantly promoting the full hydrogen *Corresponding

author. Tel./fax: +8657187951152. E-mail address: [email protected] (X.Z. Xiao).

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desorption kinetics of NaMgH3. Among all samples, TiO2 NT doped sample shows the best performance, of which the onset decomposition temperature is reduced to 300 °C, and the first- and second-step decomposition peak temperatures are decreased to 346.3 °C and 355.8 °C. The TiO2 NT doped sample desorbs approximately 3.4 wt.% H2 at 350 °C within 10 min, while the pure NaMgH3 sample releases only 0.2 wt.% H2 in 10 min. The significant improvement in both two decomposition reactions kinetics of NaMgH3 can be attributed to tubular morphology of TiO2 NT and the in-situ formation of multi-valence Ti species (Na0.46TiO2). These two reasons can change the kinetic models of NaMgH3 from A2 to R2, and further dramatically decrease the activation energies of first- and second-step decomposition reactions of NaMgH3 to 91.7 and 142.1 kJ/mol, respectively. In particular, the in-situ formed Na0.46TiO2 can benefit the e‒ transfers among Na+, Mg2+ and H‒, tremendously enhancing dehydrogenation properties.

1. Introduction As we all know, the environment issue has been the hottest topic in the last few years. How to decrease the greenhouse gas emission and oil consumption are currently the important challenges. Therefore, the clean and sustainable energy, which is supposed to alternate the traditional fuels, should be discovered for the future. Hydrogen, the cleanest energy, has a great potential to be an excellent energy storage medium and efficient fuel1-3. However, large-scale practical use of hydrogen energy remains to be unsettled. Hydrogen storage in the solid-state is regarded as the best choice for future mobile applications, because of efficiency and safety. In order to satisfy the requirement of high hydrogen capacities and fast de-/hydrogenation kinetics under moderate conditions, various solid-state hydrogen storage materials4-7, such as light-weight metal hydrides, complex hydrides and chemical hydrides, have been widely discussed and researched to enhance their reversible or irreversible hydrogen


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storage properties8. Among them, the AMgH3 (where A = alkali element), which are typical Mg-based ternary perovskite-type hydrides, have attracted many researchers’ attentions9-13. NaMgH3, with a structure of being similar to GdFeO3-type perovskite (space group Pnma), is a prototype of Mg-based ternary hydrides and has considerable hydrogen gravimetric (6.0 wt.%) and volumetric (88.0 g L-1) densities with a reversible de-/hydrogenation process14-16. Sheppard et al.17 precisely determined the desorption enthalpy and entropy for the first hydrogen desorption reaction (NaMgH3 → NaH + Mg + H2), which were 86.6 ± 1.0 kJ/(mol H2) and 132.2 ± 1.3 kJ/(mol H2 K) respectively, demonstrating that the thermodynamic properties of NaMgH3 were more stable than those of MgH2. Besides its thermodynamic advantages, NaMgH3 also has advantages of the flat plateau, suitable re/dehydrogenation plateau pressure, no hysteresis and easily accessible raw materials, which make it available for being an excellent solar heat storage medium18-20 The reversible de-/hydrogenation reactions of NaMgH3 are the two-step reactions, as shown in Eq.1 and Eq.2: NaMgH3 → NaH + Mg + H2

(1)

NaH + Mg → Na + Mg + 1/2 H2

(2)

Ikeda et al.14 studied the pressure-composition-isotherms curve of NaMgH3 at 400 °C, and found that it clearly exhibited two plateaus at around 1.5 and 0.4 bar H2 corresponding to the decompositions of NaMgH3 and NaH, respectively. The hydrogen desorption process of NaMgH3 only starts above 371 °C at 1 bar H2, which is between the decomposition temperatures of the NaH (427 °C) and MgH2 (287 °C). NaMgH3 is expected to have a good dehydrogenation kinetic property due to the high H mobility which is related to its perovskite-type structure21-22. However, recent researches revealed that both decomposition reactions of NaMgH3 were shifted to higher temperatures than their theoretical values, resulted from the sluggish kinetics of this reaction23. Hence, the poor dehydrogenation kinetic properties of NaMgH3 could be the dominating obstacle for the further practical application, which are needed further studies. Previous studies on NaMgH3 have shown that incorporating other elements or



doping catalysts could significantly improve its hydrogen desorption kinetic properties24-28. For instance, Li’s group24 in-situ embedded of Mg2NiH4 and YH3 nanoparticles into NaMgH3, which enhanced the kinetics of hydrogen absorption and desorption cycling. Introducing Li or K as a dopant into NaMgH3 would partially replace Na by Li or K and form the distorted perovskite-type structure, improving the dehydriding kinetic properties25-26. Among them, Ti-based catalysts exhibited the best dehydriding kinetic properties. Wang et al.27 doped K2TiF6 into NaMgH3 by highenergy ball milling, which released approximately 3.8 wt.% H2 at 365 °C within 20 min. The apparent activation energy for NaMgH3 could be reduced to 104 kJ/mol, by doping TiF3 as a catalyst28. Among all Ti-based catalysts, the TiO2 has advantages that its reduction makes variations on the chemical environment during the de/hydrogenation process, which is known as an effective catalyst to improve the hydrogen storage performances of various hydride systems29-33. For example, Shahi et al.29 investigated the effects of various particle sizes of TiO2 (200, 25 and 7 nm) on de/rehydriding properties of Mg(NH2)2/LiH system, and the best catalyzing effect was found to be 25 nm TiO2. TiO2 nanorods were founded to have remarkable performances for hydrogen absorption/desorption kinetics on MgH2 with around 5.5 wt.% H2 absorbed and desorbed in 10 min at 300 °C30. Cui et al.31 significantly enhanced the hydrogen desorption properties of MgH2 by a nano-coating of multi-valence Ti-based dopants, including Ti, TiH, TiCl3 and TiO2. This system can release H2 at 175 °C and desorb approximately 5 wt.% H2 within 15 min at 250 °C. Hence, in this work, we aim to investigate the catalyzing effects of different particle sizes and shapes of TiO2 on the dehydrogenation properties of NaMgH3. Moreover, 5 wt.% commercial TiO2 microparticle (MP) (100 nm), 5 wt.% commercial TiO2 nanoparticle (NP) (5-10 nm) and 5 wt.% as-prepared TiO2 nanotube (NT) (5-10 nm) were doped during the in-situ synthesis of NaMgH3 for the first time, where various kinds of TiO2 dopants would be reduced to multi-valence Ti-based dopants. The morphology and phase components of different kinds of TiO2 catalysts were analyzed by TEM, HRTEM and XRD tests. The compositions of hydrogenated and dehydrogenated samples, as well as the dehydriding kinetics was investigated. Finally,


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the XPS test was carried to determine the changes of valence states of Ti in the de/hydrogenation samples.

2. Experimental details The TiO2 nanotube (NT) (5-10 nm) was prepared by an alkaline hydrothermal method. The TiO2 P25 (Anatase, 5-10 nm, 99.8 %, Aladdin) was first immersed in 90 ml of NaOH (11.25 mol L-1) and then transferred to a 150 mL Teflon-lined autoclave. The autoclave processed heat-treatment at 130 °C for 20 h. The as-formed TiO2 precipitate was washed with deionized water and HCl (0.1 mol L-1) via centrifugation and ultrasonic treatment for several times until the final pH of this solution reached a value of 6.5. The precipitate was dried at 80 °C for 24 h. In order to eliminate sodium totally, a second-washing with a 1.0 M HCl solution was necessary. Finally, the precipitate processed heat-treatment at 400 °C under the oxygen atmosphere (heating rate: 2 °C/min) to obtain the anatase TiO2 NT. Additionally, commercial TiO2 microparticle (MP) (Anatase, 100 nm, 99.8 %, Aladdin) and TiO2 nanoparticle (NP) (Anatase, 5-10 nm, 99.8 %, Aladdin) have also been used as the comparisons to explore the effects of size and morphology on the dehydrogenation properties of NaMgH3. The NaMgH3 samples with or without dopants were synthesized by milling stoichiometric mixtures of NaH (>95%, Aldrich), Mg (99%, Sinopharm) and catalysts. NaH and Mg were mixed by the molar ratio of 1:1, with 5 wt.% TiO2 MP, TiO2 NP, TiO2 NT added, respectively. The mixtures were milled under a base pressure of 10 bar H2 at 400 rpm for 20 h, which was operated on a planetary-mill (QM-3SP4, Nanjing), and denoted as NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT. The ball-to-powder of mixtures was 40:1. After ball milling, all samples processed heat-treatment at 400 °C under 40 bar H2. In order to characterize the phase evolution and structure of NaMgH3 doped/undoped samples and catalysts, X-ray diffraction (XRD) analysis was conducted on an X’Pert Pro X-ray diffractometer (PANalytical, the Netherlands) with Cu Kɑ radiation at 40 kV and 40 mA. During the transferring and phases characterizing, a



home-made argon-filled container was used to protect samples from being poisoned by oxygen and moisture. The dehydrogenation properties of NaMgH3 doped/undoped samples were measured by a home-made Sieverts-type apparatus with around 250 mg samples tested each time. The temperature-programmed dehydrogenation (TPD) measurements were carried on vacuum and heated from 25 °C to 450 °C (heating rate: 5 °C/min). The dehydrogenation kinetic tests were performed at a given temperature (such as 325 °C, 350 °C, 365 °C and 400 °C) at a base pressure of 7×10-2 bar H2. Moreover, the further dehydrogenation properties and activation energies of hydrogen desorption reactions were determined by differential scanning calorimeter (DSC). The DSC analyses were heated at a set X rate (X= 1, 2, 4 and 5 °C/min) from 30 °C to 500 °C under flowing Ar gas (>99.999% purity, 20 mL/min). The thermodynamic stability of NaMgH3 doped/undoped samples was determined by the pressure-contenttemperature measurements. The microstructure of as-prepared catalysts was further analyzed via transmission electron microscopy (TEM, Tecnai G2 F20 working at 200 kV). To determine the ionic state of elements before and after the dehydrogenation process, the X-ray Photoluminescence spectroscopy (XPS) was used with Mg Kɑ radiation at 1253.6 eV under a base pressure of 1×108 Torr. To prevent the samples coming into contact with oxygen and moisture, the tests were carried out carefully with protective actions.

3. Results and discussion After the successful synthesis of TiO2 NT, the XRD measurements were carried out to confirm the structure and phase composites of different kinds of TiO2 catalysts. Figure 1 shows the XRD patterns of TiO2 MP, TiO2 NP and TiO2 NT (as-formed), of which the crystal phase is further confirmed, all belonging to TiO2 (Anatase, JCPDS: 21-1272). At the same time, the diffraction peaks of TiO2 MP are both sharp and intense, indicating their highly crystalline nature. However, the diffraction peaks of TiO2 NP and TiO2 NT are relatively broader and weaker, to some extent, which can be ascribed


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to the nano-size of particles and tubes. The XRD analyses can be further proved by the following TEM and HRTEM tests. Moreover, there are no other impurity peaks of all catalysts, suggesting the high purity of three catalysts.

Figure 1. XRD patterns of (a) TiO2 MP, (b) TiO2 NP, (c) TiO2 NT.

The TiO2 MP, TiO2 NP and TiO2 NT samples underwent the TEM and HRTEM characterizations for further exploring the morphology and structure of different kinds of TiO2 catalysts. Figure 2 a and b show the typical TEM images of TiO2 MP, which can be easily measured that the mean of the particle size of TiO2 MP is about 100 nm. In Figure 2 c and d, TiO2 NP exhibits a much smaller particle size than TiO2 MP, with the mean of the particle size being around 5-10 nm. However, the smaller particle size of TiO2 NP tends to severe aggregation, which may be adverse to the catalytic action. In Figure 2 e and f, TEM images demonstrate that the TiO2 NT are successfully synthesized, where a batch of nanotubes are developed from nanosheets, forming a core-like morphology, and the pore diameter of the nanotubes is 5-10 nm. To characterize the fine structure of TiO2 MP, TiO2 NP and TiO2 NT samples, the HRTEM images were recorded and displayed in Figure 2 g and h. The TiO2 MP and TiO2 NP both exhibit a spherical morphology, with well-defined (103), (112) and (103) lattice fringes of TiO2 (Anatase), respectively. The HRTEM image of Figure 2 i reveals that TiO2 NT, displaying a tubular morphology, is well formed from TiO2 (P25, Anatase)



with no phase transformation which is proved by the appearance of obvious (101) lattice fringe of TiO2 (Anatase). In addition, the TEM and HRTEM results are in high consistency with the XRD analyses, demonstrating that both TiO2 NP and TiO2 NT have a smaller size than TiO2 MP and three TiO2 catalysts all belong to TiO2 (Anatase).

Figure 2. TEM images of (a and b) TiO2 MP, (c and d) TiO2 NP, (e and f) TiO2 NT and HRTEM images of (g) TiO2 MP, (h) TiO2 NP, (i) TiO2 NT.

The NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT samples were checked by XRD tests to confirm the phase compositions. As shown in Figure 3, the XRD patterns of all samples show obvious diffraction peaks of NaMgH3 (JCPDS: 52-0873), suggesting the successful synthesis of NaMgH3 from NaH and Mg. Additionally, the diffraction peaks corresponding to TiO2 (Anatase, JCPDS: 21-1272) are not detected in the XRD patterns of NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT samples. However, the new diffraction peaks of Na0.46TiO2 can be detected in the XRD patterns of NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3TiO2 NT composites after ball milling, indicating that the reduction reaction between NaMgH3 and TiO2 occurs during ball-milling and heat-treatment. This result suggests that the valence states of TiO2 have been changed during the in-situ synthesis of


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NaMgH3-TiO2 composites, with some Ti4+ reduced to Ti3+, corresponding to the reaction between NaH and TiO2, as shown in Eq.3: 2 NaH + 4.348 TiO2 → 4.348 Na0.46TiO2 + H2

(3)

The weak peaks corresponding to MgO (JCPDS: 71-1176) are probably due to the impurity of Mg.

Figure 3. XRD patterns of (a) NaMgH3, (b) NaMgH3-TiO2 MP (100 nm), (c) NaMgH3-TiO2 NP (5-10 nm), (d) NaMgH3-TiO2 NT (5-10 nm).

After Confirming the structure and morphology of different kinds of TiO2 catalysts and different kinds of NaMgH3-TiO2 samples, the dehydrogenation properties of NaMgH3 and different kinds of NaMgH3-TiO2 samples were studied by DSC, TPD and isothermal dehydrogenation measurements. As shown in Figure 4 a, the DSC curves of the pure NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT at a heating rate of 5 °C/min, exhibit two endothermic peaks below 400 °C. The first and the second endothermic peaks are corresponding to the dehydrogenation of NaMgH3 and NaH, respectively, confirming the two-step decomposition of NaMgH3, as shown in Eq.1 and 2. In the case of pure NaMgH3 sample, a strong endothermic peak at 385.3 °C can be detected before the weak endothermic peak at 395.4 °C. The NaMgH3-TiO2 MP composite exhibits the desorption peak temperatures of 369.4 °C and 377 °C,



reducing by 15.9 °C and 18.4 °C respectively, compared with those of pure NaMgH3 sample. NaMgH3-TiO2 NP composite shows better dehydriding performance than that of NaMgH3-TiO2 MP composite. The desorption peak temperatures of NaMgH3-TiO2 NP are decreased to 350 °C and 360.9 °C, respectively. In particular, the NaMgH3-TiO2 NT composite exhibits the lowest desorption peak temperatures of 346.3 °C and 355.8 °C, which are 39 °C and 39.6 °C lower than those of pure NaMgH3 sample, respectively. The observed trend of endothermic peaks suggests that peak temperatures of the first and the second dehydrogenation reactions of NaMgH3 are significantly reduced, implying the effective catalysis of TiO2 MP, TiO2 NP, TiO2 NT. In order to further explore the hydrogen desorption properties of NaMgH3 with and without dopants, the TPD experiment was carried out and displayed in Figure 4 b. The TPD curves show that the dopants remarkably decrease the onset desorption temperature of NaMgH3. The pure NaMgH3 did not release any hydrogen below 350 °C, while NaMgH3-TiO2 MP starts to release hydrogen at 325 °C. The onset decomposition temperature is further lowered to 300 °C with TiO2 NP or TiO2 NT doped in NaMgH3. Moreover, as the temperature rises to 350 °C, around 1.5 wt.%, 0.9 wt.% and 0.5 wt.% H2 are released by TiO2 MP, TiO2 NP and TiO2 NT doped composites, respectively. As expected, TiO2 NT shows the best catalytic activity, tremendously reducing the onset and peak temperatures. Nevertheless, the dehydrogenation capacity suffers approximately 4.3 % loss, from 5.7 wt.% H2 to 5.45 wt.% H2, due to the dead weight of TiO2 NT and existence of MgO. Even that, the thermal storage density of NaMgH3TiO2 NT composite can be as high as 2616.91 kJ/kg, corresponding to the actual capacity of 5.45 wt.% H2. Further pressure-content-temperature measurements and Van’t Hoff fitting reveal that the enthalpies of both the two-step reactions of NaMgH3TiO2 NT are barely changed compared with pure NaMgH3 (Figure S1 and S2). It’s confirmed that TiO2 NT won’t break the thermodynamic stability of NaMgH3. These results suggest the NaMgH3-TiO2 NT composite has a potential candidate for Thermal Energy Storage application18-20. The effects of TiO2 MP, TiO2 NP and TiO2 NT on the dehydrogenation kinetic properties of NaMgH3 are evaluated by isothermal dehydrogenation measurements at 350 °C (Figure 4 c). It’s obvious that the desorbed


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hydrogen capacities within 10 min are found to be 0.2, 1.2, 1.6 and 3.4 wt.% for pure NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT, respectively. Besides, pure NaMgH3 desorbs 3.4 wt.% H2 in 100 min, of which dehydriding kinetics is 10 times slower than that of the NaMgH3-TiO2 NT. The results of isothermal dehydrogenation measurements support that the dehydriding kinetics of NaMgH3 is remarkably improved through the in-situ doping with TiO2 NT. The analyses of isothermal dehydrogenation measures at 350 °C highly agree with the TPD and DSC results, demonstrating that TiO2 can improve the hydrogen desorption kinetics of NaMgH3 and TiO2 NT is confirmed to be the most effective one. The different catalytic effects of TiO2 MP, TiO2 NP and TiO2 NT may be resulted from various sizes and morphologies, which will be deeply explored and discussed in the next section.

Figure 4. (a) DSC，(b) TPD and (c) Isothermal dehydrogenation at 350 °C curves of NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT.

The isothermal hydrogen desorption kinetics of NaMgH3 with and without dopants at different temperatures was investigated. As shown in Figure 5, all samples exhibit good hydrogen desorption kinetics at the temperature of 400 °C, at which the desorbed hydrogen capacities within 10 min are found to be 5.3, 5.0, 5.1 and 5.0 wt.% for pure NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT composites, respectively. Further tests confirm that the doped samples display faster dehydrogenation kinetics at even lower temperature (325 °C, 350 °C and 365 °C), as compared with pure NaMgH3. In particular, TiO2 NT shows the best catalytic effect among the three doped composites at all temperatures. When the temperature goes



down to 365 °C, the dehydrogenation kinetics of NaMgH3 stays excellent with only the first-step desorption reaction happening. There is around 3.5 wt.% H2 released in 10 min. However, the dehydrogenation kinetics of pure NaMgH3 gets worsened with 3.5 wt.% H2 desorbed in 80 min, while the temperature goes down. At 350 °C, the NaMgH3-TiO2 NT sample releases approximately 3.4 wt.% H2 in 10 min, while the pure NaMgH3 sample releases only 0.2 wt.% H2 in 10 min. The NaMgH3-TiO2 NT sample releases about 3.1 wt.% H2 in 50 min at a temperature as low as 325 °C, while NaMgH3-TiO2 NP, NaMgH3-TiO2 MP, NaMgH3 samples just release 2.5, 1.3 and 0.2 wt.% H2 in 50 min, respectively. Based on the DSC, TPD and isothermal dehydrogenation kinetic measurements, it can be reasonably concluded that the TiO2 NT exhibits promising effects as compared with other additives.

Figure 5. Isothermal dehydrogenation curves for (a) NaMgH3, (b) NaMgH3-TiO2 MP, (c) NaMgH3-TiO2 NP, (d) NaMgH3-TiO2 NT at different temperatures.

As demonstrated above, TiO2 NT obviously enhances the hydrogen desorption


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properties of NaMgH3, however, the catalytic mechanism of TiO2 NT doped composite is still unsettled. To investigate the improved kinetic mechanism of the NaMgH3-TiO2 NT composite, further analyses based on the isothermal dehydrogenation measurements were carried out to determine the reaction models. The reaction models are represented as the form of F(a) = kt, where a is the fraction of material reacted in time, t. Furthermore, the function F(a) is mainly affected by the mechanism controlling reaction and by sizes and morphologies of the reacting particles. On the basis of Sharp and Jone’s method34-35, the relationship between (t/t0.5)theo and (t/t0.5)exp of nine kinetic models (Table 1) for all samples were displayed in Figure 6. Therefore, the most suitable hydrogen desorption kinetic model for pure NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT samples are determined to be A2, A2, A2 and R2 model, respectively. It’s implied that the hydrogen desorption kinetic models are changed by the nano-size and tubular morphology of TiO2 NT. According to Jone’s research, the dehydrogenation kinetic mechanism of pure NaMgH3, NaMgH3-TiO2 MP and NaMgH3-TiO2 NP samples are the phase-boundary controlled reaction with the nucleation step occurring virtually instantaneously34. However, the hydrogen desorption mechanism of NaMgH3-TiO2 NT sample is controlled via the movement of two-dimensional phase boundary34, which would be the phase boundaries between NaMgH3 phase and Na0.46TiO2 phase.



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Figure 6. (t/t0.5)theo vs (t/t0.5)exp at 365 ℃ for (a) NaMgH3, (b) NaMgH3-TiO2 MP, (c) NaMgH3TiO2 NP and (d) NaMgH3-TiO2 NT. Table 1. Kinetic models examined in the isothermal desorption curves34 Symbol

Model

g(ɑ)

Sharp’expression

D1

one-dimensional diffusion

ɑ2

0.2500(t/t0.5)

D2

two-dimensional diffusion

ɑ+(1-ɑ)ln(1-ɑ)

0.1534(t/t0.5)

[1-(1-ɑ)1/3]2

0.0.0426(t/t0.5)

(1-2ɑ/3)-(1-ɑ)2/3

0.0367(t/t0.5)

three-dimensional diffusion D3 (Jander equation) three-dimensional diffusion D4 (Ginstling-Braunshtein equation) F1

First-order reaction

-ln(1-ɑ)

-0.6931(t/t0.5)

R2

two-dimensional phase boundary

1-(1-ɑ)1/2

0.2929(t/t0.5)

R3

three-dimensional phase boundary

1-(1-ɑ)1/3

0.2063(t/t0.5)

A2

Avarami-Erofe’ev

[-ln(1-ɑ)]1/2

0.8326(t/t0.5)

A3

Avarami-Erofe’ev

[-ln(1-ɑ)]1/3

0.8850(t/t0.5)


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In order to obtain the better insight of the enhanced hydrogen desorption kinetics of TiO2 NT doped NaMgH3, the activation energies (Ea) corresponding to the first- and second-step decomposition reactions of NaMgH3 were quantitatively measured by Kissinger method36, as Eq.4 shown. ln(β/Tp2) = -Ea/RTp + ln(AR/Ea)

(4)

where the β is the heating rate, Tp is the absolute temperature at the endothermic peaks, A is the pre-exponential factor and R is the gas constant. The DSC curves of all samples at different heating rates (1, 2, 4 and 5 °C/min) are displayed in Figure 7 a-d, with argon flowing continuously as a protective measure. The dependence of ln(β/Tp2) against 1000/RT was exhibited in Figure 7 e and f. All Kissinger plots are well fitted, with linear coefficient R2 larger than 0.95. The activation energies can be calculated from the slope (Ea/R) of the fitted line, which are displayed in Table 2. Based on the data in Table 2, Ea is remarkably lowered by doping with various kinds of TiO2 additives. The activation energies for hydrogen desorption of NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT samples are estimated to be 172.1, 156.1, 151.9 and 91.7 kJ/mol for the first-step dehydrogenation, respectively. The activation energies of the second-step dehydrogenation can be calculated to be 163.5, 160.0, 155.7 and 142.1 kJ/mol for NaMgH3, NaMgH3-TiO2 MP, NaMgH3-TiO2 NP and NaMgH3-TiO2 NT samples, respectively. It’s obvious that the NaMgH3 doped TiO2 NT sample exhibits the lowest activation energies for both two decomposition reactions, which are 80.4 and 21.4 kJ/mol lower than those of NaMgH3, respectively. These results quantitatively evidence the remarkable effect of TiO2 NT additive. Furthermore, the superior enhancement of dehydrogenation kinetics is probably due to the TiO2 NT reducing the energy barrier.



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Figure 7. DSC curves for (a) NaMgH3, (b) NaMgH3-TiO2 MP, (c) NaMgH3-TiO2 NP, (d) NaMgH3-TiO2 NT at different rates; And Kissinger plots of the (e) first- and (f) second-step dehydrogenation of all samples.

Table 2. Activation energies (Ea) for the dehydrogenation of the in-situ hydrogenated NaMgH3 samples doped/undoped with 5 wt.% catalysts Apparent activation energy Ea (kJ/mol) Dopant NaMgH3(first-step)

R2

NaMgH3(second-step)

R2

Undoped

172.1

0.96997

163.5

0.99617

TiO2 MP

156.1

0.98417

160.0

0.95627

TiO2 NP

151.9

0.9889

155.7

0.96501

TiO2 NT

91.7

0.9532

142.1

0.96271

Combing with the above analyses, it should be noted that the catalytic effect of size and morphology of TiO2 additives has been demonstrated clearly that TiO2 NT has changed the hydrogen desorption kinetic model of NaMgH3 and remarkably reduced the energy barrier. In addition, to further investigate the mechanism for the catalysis of


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TiO2 NT during the dehydrogenation of NaMgH3, the XRD and XPS studies were performed (Figure 8) at NaMgH3-TiO2 NT before and after dehydrogenation samples. The XRD patterns of TiO2 NT doped dehydrogenation sample (Figure 8 a) show the peaks of Ti (JCPDS:88-2321), Na (JCPDS:22-0948), MgO (JCPDS:71-1176), Mg (JCPDS:35-0821) and Na0.46TiO2 (JCPDS:37-0354), where the peaks of NaMgH3 are not detected, in comparison with the XRD pattern of TiO2 NT doped hydrogenation sample. It is evidenced that in the process of ball-milling and heating treatment, the introduction of TiO2 NT will in-situ form the Na0.46TiO2 which significantly promotes the full hydrogen desorption of NaMgH3. As shown in Figure 8 b, the as-prepared TiO2 NT doped hydrogenation sample presents Ti 2p signals of Ti, Ti3+ and Ti4+. The relative intensities of Ti and Ti3+ 2p are obviously higher than that of Ti4+ 2p, which indicates that during the in-situ synthesis of NaMgH3, the TiO2 NT was reduced and the Na0.46TiO2 phase was formed. Figure 8 b shows that the Ti 2p signals of Ti, Ti3+ and Ti4+ exist in the TiO2 NT doped dehydrogenation samples. Moreover, it is found that the relative intensity of Ti 2p signals to Ti3+ and Ti4+ 2p signals remarkably differs between NaMgH3-TiO2 NT before and after dehydrogenation samples. The XPS result indicates that the content of high valence Ti 2p signals of Ti4+ is decreased after dehydrogenation. Therefore, from the changes of valence states between de-/hydrogenation samples, it could be deduced that the electronic structure of Ti-based catalysts has been changed during the dehydrogenation process.



Figure 8. (a) XRD patterns of (i) NaMgH3-TiO2 NT, (ii) NaMgH3-TiO2 NT dehydrogenation at 400 °C and (b) XPS spectra of Ti 2p in (iii) NaMgH3-TiO2 NT, (iv) NaMgH3-TiO2 NT dehydrogenation at 400 °C.

Further discussions were carried to demonstrate the mechanism for the catalyzing effect of TiO2 NT during the dehydrogenation of NaMgH3, the schematic diagram is drawn in Figure 9. Firstly, as we all know, the electronegativity of Ti (1.54) is stronger than Na (0.93) and Mg (1.31), but weaker than H (2.2), which can create an available condition for weakening the Na-H and Mg-H bonds29. Furthermore, due to a splitting of the 3d state of Ti ions, the Ti ions are able to gain negative electrons (e‒) more easily than Na and Mg ions, and lose e‒ more easily than H ions. According to the results of the kinetic model simulation in Figure 6, the hydrogen desorption mechanism of NaMgH3-TiO2 NT lies in the movement of two-dimensional phase boundary which can provide numerous sites among NaMgH3, high-valence (Ti3+ and Ti4+) and low-valence (Ti0) state Ti compounds in the TiO2 NT doped NaMgH3 sample. Based on the XRD and XPS results above (Figure 8 a and b), the high-valence and low-valence state Ti compounds are determined to be Na0.46TiO2 and Ti, respectively. The active sites between Ti and Na0.46TiO2 can promote the transformation of the valence state of Ti, which enhances the transfers of e‒ among Na+, Mg2+ and H‒. The transfers process of e‒ among Na+, Mg2+ and H‒ during the dehydrogenation of NaMgH3-TiO2 NT should be described as following step31: (1) The H‒ on active sites between NaMgH3 and Na0.46TiO2 gives e‒ to Ti3+/4+ which gets e‒ and is reduced to Ti0. (2) The Na-H and Mg-


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H bonds are weakened and dissociative H are formed easily so that the hydrogen desorption process is initiating. (3) The dissociative H is recombined into the H2 molecule. (4) Na and Mg nucleates and grows.

Figure 9. The schematic diagram of the catalytic mechanism in the dehydrogenation of NaMgH3TiO2 NT samples.

Conclusion In this study, NaMgH3 doped with the 5 wt.% TiO2 MP (100 nm), TiO2 NP (5-10 nm) and TiO2 NT (5-10 nm) were synthesized in the process ball milling and heattreatment. The following tests of dehydrogenation properties prove that TiO2 NT has the most promising catalytic effect. The onset temperature (300 °C) and hydrogen desorption peak temperature (346.3 °C and 355.8 °C) are significantly reduced. In addition, isothermal dehydrogenation measurement displays that NaMgH3-TiO2 NT composite stays excellent dehydrogenation kinetics even at a relative low temperature. It can desorb approximately 3.4 wt.% H2 at 350 °C in 10 min, while the pure NaMgH3 composite just desorbs 0.2 wt.% H2. The mechanism for the catalytic effect of TiO2 NT is deeply investigated via Sharp and Jone’s kinetic models, DSC, XRD and XPS



analysis. The superior dehydrogenation properties of NaMgH3-TiO2 NT composite can be ascribed to three reasons shown as follow: Firstly, TiO2 NT changes the hydrogen desorption kinetic model from A2 to R2 which is controlled by the movement of the two-dimensional phase boundary. This change should provide more edgy sites and new boundaries to increasing more channels for hydrogen diffusion. Secondly, TiO2 NT with nano-size and tubular morphology notably decreases the activation energies which are 91.7 and 142.1 kJ/mol for the first- and second-step dehydrogenation reactions, respectively. The reduction of energy barrier significantly improves dehydriding kinetic properties of NaMgH3. Finally, the Na0.46TiO2, formed in the in-situ synthesis process, can play as an intermediate during the transfers of e‒ among Na+, Mg2+ and H‒, remarkably enhancing dehydrogenation properties.

Acknowledgements The authors gratefully acknowledge the financial supports for this research from the National Basic Research Program of China (2018YFB1502104), the National Natural Science Foundation of China (51571179), and the Open Fund of the Guangdong Provincial Key Laboratory of Advance Energy Storage Materials.


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References (1) Jacobson, M. Z.; Colella, W. G.; Golden, D. M. Cleaning the Air and Improving Health with Hydrogen Fuel-Cell Vehicles. Science. 2005, 308, 1901-1905. (2) Felderhoff, M.; Weidenthaler, C.; Von Helmolt, R.; Eberle, U. Hydrogen Storage: The Remaining Scientific and Technological Challenges. Phys. Chem. Chem. Phys. 2007, 9, 2643-2653.

(3) Liu, T.; Ma, X.; Chen, C.; Xu, L.; Li, X. Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen Storage Properties of Mg-Based Nanocomposite. J. Phys. Chem. C. 2015, 119, 14029-14037. (4) Wang, H.; Lin, H. J.; Cai, W. T.; Ouyang, L. Z.; Zhu, M. Tuning Kinetics and Thermodynamics of Hydrogen Storage in Light Metal Element Based Systems – a Review of Recent Progress. J. Alloy Compd. 2016, 658, 280-300. (5) Ouyang, L. Z.; Yang, X. S.; Zhu, M.; Liu, J. W.; Dong, H. W.; Sun, D. L.; Zou, J.; Yao, X. D. Enhanced Hydrogen Storage Kinetics and Stability by Synergistic Effects of in Situ Formed CeH2.73 and Ni in CeH2.73-MgH2-Ni Nanocomposites. J. Phys. Chem. C. 2014, 118, 7808-7820. (6) Wang, Z.; Ren, Z.; Jian, N.; Gao, M.; Hu, J.; Du, F.; Pan, H.; Liu, Y. Vanadium Oxide Nanoparticles Supported on Cubic Carbon Nanoboxes as High Active Catalyst Precursors for Hydrogen Storage in MgH2. J. Mater. Chem. A. 2018, 6, 16177-16185. (7) Zhang, Q.; Huang, Y.; Xu, L.; Zang, L.; Guo, H.; Jiao, L.; Yuan, H.; Wang, Y. Highly Dispersed MgH2 Nanoparticle–Graphene Nanosheet Composites for Hydrogen Storage. ACS Appl. Nano Mater. 2019, 2, 3828-3835. (8) Rusman, N. A. A.; Dahari, M. A Review on the Current Progress of Metal Hydrides Material for Solid-State Hydrogen Storage Applications. Int. J. Hydrogen Energ. 2016, 41, 12108-12126. (9) Vajeeston, P.; Ravindran, P.; Kjekshus, A.; Fjellvag, H. First-Principles Investigations of the MMgH3 (M = Li, Na, K, Rb, Cs) Series. J. Alloy Compd. 2008, 450, 327–337. (10) Bouhadda, Y.; Kheloufi, N.; Bentabet, A.; Boudouma, Y.; Fenineche, N.; Benyalloul, K. Thermodynamic Functions from Lattice Dynamic of KMgH3 for Hydrogen Storage Applications. J. Alloy Compd. 2011, 509, 8994-8998. (11) Reshak, A. H.; Shalaginov, M. Y.; Saeed, Y.; Kityk, I. V.; Auluck, S. First-Principles Calculations of Structural, Elastic, Electronic, and Optical Properties of Perovskite-Type KMgH3 Crystals: Novel Hydrogen Storage Material. J. Phys. Chem. B. 2011, 115, 2836-2841. (12) Wu, H.; Zhou, W.; J. Udovic, T.; Rush, J.; Yildirim, T. Crystal Chemistry of Perovskite-Type Hydride NaMgH3: Implications for Hydrogen Storage. Chem. Mater. 2008, 20, 2335-2342. (13) Li, D.; Zhang, T.; Yang, S.; Tao, Z.; Chen, J. Ab Initio Investigation of Structures, Electronic and Thermodynamic Properties for Li–Mg–H Ternary System. J. Alloy Compd. 2011, 509, 8228-8234. (14) Ikeda, K.; Kogure, Y.; Nakamori, Y.; Orimo, S. Reversible Hydriding and Dehydriding Reactions of Perovskite-Type Hydride NaMgH3. Scripta Mater. 2005, 53, 319-322. (15) Reshak, A. H. NaMgH3 a Perovskite-Type Hydride as Advanced Hydrogen Storage Systems: Electronic Structure Features. Int. J. Hydrogen Energ. 2015, 40, 16383-16390. (16) Bouamrane, A.; Laval, J.; Soulie, J. P.; Bastide, J. P. Structural Characterization of NaMgH2F and NaMgH3. Mater. Res. Bull. 2000, 35, 545-549. (17) Sheppard, D.; Paskevicius, M.; Buckley, C. E. Thermodynamics of Hydrogen Desorption from NaMgH3 and Its Application as a Solar Heat Storage Medium. Chem. Mater. 2011, 23, 4298-4300. (18) Sheppard, D.; Humphries, T.; Buckley, C. E. Sodium-Based Hydrides for Thermal Energy Applications. Appl. Phy. A. 2016, 122, 406-419.



(19) Sheppard, D. A.; Buckley, C. E. The Potential of Metal Hydrides Paired with Compressed Hydrogen as Thermal Energy Storage for Concentrating Solar Power Plants. Int. J. Hydrogen Energ. 2019, 44, 9143-9163. (20) Poupin, L.; Humphries, T. D.; Paskevicius, M.; Buckley, C. E. A Thermal Energy Storage Prototype Using Sodium Magnesium Hydride. Sustain. Energ. Fuels. 2019, 3, 985-995. (21) Hao, S. Q.; S. Sholl, D. Role of Schottky Defects in Hydrogen and Metal Diffusion in NaH, MgH2, and NaMgH3. J. Phys. Chem. Lett. 2010, 1, 19, 2968-2973. (22) Wang, H.; Zhang, J.; Liu, J.; Ouyang, L. Z.; Zhu, M. Catalysis and Hydrolysis Properties of Perovskite Hydride NaMgH3. J. Alloy Compd. 2013, 580, S197-S201. (23) Pottmaier, D.; Pinatel, E. R.; Vitillo, J.; Garroni, S.; Orlova, M.; Vaughan, G. B. M.; Fichtner, M.; Lohstroh, W.; Baricco, M. Structure and Thermodynamic Properties of the NaMgH3 Perovskite: A Comprehensive Study. Chem. Mater. 2011, 23, 2317-2326. (24) Li, Y.; Zhang, L.; Zhang, Q.; Fang, F.; Sun, D.; Li, K.; Wang, H.; Ouyang, L. Z.; Zhu, M. In Situ Embedding of Mg2NiH4 and YH3 Nanoparticles into Bimetallic Hydride NaMgH3 to Inhibit Phase Segregation for Enhanced Hydrogen Storage. J. Phys. Chem. C. 2014, 118, 23635-23644. (25) Wang, Z. M.; Li, J. J.; Tao, S.; Deng, J. Q.; Zhou, H.; Yao, Q. Structure, Thermal Analysis and Dehydriding Kinetic Properties of Na1−XLiXMgH3 Hydrides. J. Alloy Compd. 2016, 660, 402-406. (26) Tao, S.; Wang, Z. M.; Wan, Z. Z.; Deng, J. Q.; Zhou, H.; Yao, Q. Enhancing the Dehydriding Properties of Perovskite-Type NaMgH3 by Introducing Potassium as Dopant. Int. J. Hydrogen Energ. 2017, 42, 3716-3722. (27) Wang, Z.; Tao, S.; Deng, J.; Zhou, H.; Yao, Q. Significant Improvement in the Dehydriding Properties of Perovskite Hydrides, NaMgH3, by Doping with K2TiF6. Int. J. Hydrogen Energ. 2017, 42, 8554-8559. (28) Ismail, M.; Zhao, Y.; Yu, X.; Dou, S. Improved Hydrogen Storage Performance of MgH2-NaAlH4 Composite by Addition of TiF3. Int. J. Hydrogen Energ. 2012, 37, 8395-8401. (29) Shahi, R. R.; Mishra, R. K.; Shukla, V.; Bhatnagar, A.; Srivastava, O. N. Enhanced Hydrogenation Characteristics of Li-Mg-N-H System Catalyzed with TiO2 Nanoparticles; a Mechanistic Approach. Int. J. Hydrogen Energ. 2017, 42, 29350-29359. (30) Jardim, P. M.; da Conceicao, M. O. T.; Brum, M. C.; dos Santos, D. S. Hydrogen Sorption Kinetics of Ball-Milled MgH2-TiO2 Based 1d Nanomaterials with Different Morphologies. Int. J. Hydrogen Energ. 2015, 40, 17110-17117. (31) Cui, J.; Wang, H.; Liu, J. W.; Ouyang, L. Z.; Zhang, Q. A.; Sun, D. L.; Yao, X. D.; Zhu, M. Remarkable Enhancement in Dehydrogenation of MgH2 by a Nano-Coating of Multi-Valence Ti-Based Catalysts. J. Mater. Chem. A. 2013, 1, 5603-5611. (32) Vujasin, R.; Novakovic, J. G.; Novakovic, N.; Giusepponi, S.; Celino, M. Ab-Initio Study of Hydrogen Mobility in the Vicinity of MgH2-Mg Interface: The Role of Ti and TiO2. J. Alloy Compd. 2017, 696, 548-559. (33) Daryani, M.; Simchi, A.; Sadati, M.; Hosseini, H. M.; Targholizadeh, H.; Khakbiz, M. Effects of Ti-Based Catalysts on Hydrogen Desorption Kinetics of Nanostructured Magnesium Hydride. Int. J. Hydrogen Energ. 2014, 39, 21007-21014. (34) Sharp, J. H.; Brindley, G. W.; Achar, B. N. N. Numerical Data for Some Commonly Used Solid State Reaction Equations. J. Am. Ceram. Soc. 1966, 49, 379-382. (35) Jones, L. F.; Dollimore, D.; Nicklin, T. Comparison of Experimental Kinetic Decomposition Data with Master Data Using a Linear Plot Method. Thermochim. Acta. 1975, 13, 240-245.


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(36) Kissinger, H. E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, 17021706.



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Excellent Catalysis of Various TiO2 Dopants with Na0.46TiO2 In-situ

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