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Room Temperature Hydrogen Absorption of Titanium with Surface Modification by Organic Solvents Keita Shinzato, So Hamamoto, Hiroki Miyaoka, and Takayuki Ichikawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02014 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 30, 2019

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The Journal of Physical Chemistry

Room Temperature Hydrogen Absorption of Titanium with Surface Modification by Organic Solvents， Keita Shinzato†, So Hamamoto†, Hiroki Miyaoka‡, Takayuki Ichikawa*†‡ †Graduate

School of Engineering, Hiroshima University, Hiroshima, 739-8527, Japan

‡Institute for Advanced Materials Research, Hiroshima University, Hiroshima, 739-8530, Japan

ABSTRACT

As a thermochemical heat storage system by hydrogenating reaction of titanium (Ti), surface modification methods are investigated in order to improve the hydrogen absorption kinetics of Ti. It is clarified that Ti with fresh surface has high reactivity with not only oxygen and water but also hydrogen. However, the reactivity with hydrogen is lost after 1 day even under highly purified Ar atmosphere. Typical catalysts for hydrogen dissociation observed in Mg systems are not effective for Ti. On the other hand, when Ti powder was ball-milled with acetone, the surface is effectively modified and the characteristic surface shows high reactivity and selectivity for hydrogen even at room temperature. In the X-ray photoelectron spectroscopy of the Ti powder, metallic state of Ti stabilized in this characteristic surface can be observed although a pristine surface showed only Ti4+ state as TiO2. This surface is composed of TiC precursor, transforming to crystalline TiC phase after annealing at 650 C.

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1. INTRODUCTION Metal hydrides have been investigated as hydrogen storage materials, especially for onboad use so far. After marketing fuel cell vehicles with high pressure H2 tank (70 MPa) in 2014, various kinds of application with respect to hydrogen storage materials has been proposed such as chemical compressor 1, 2, hydrogen storage systems 3, 4, and high-temperature thermochemical heat storage

5, 6.

These research fields are dramatically developing. Magnesium hydride (MgH2) has

been studied as promising hydrogen storage materials because of its high hydrogen capacity and low cost 7, 8. MgH2 can reversibly absorb and desorb 7.6 wt. % of hydrogen via a following reaction (1),

Mg + H2  MgH2.

(1)

The reaction heat of the hydrogen absorption/desorption reaction is 76  2 kJ 9. The Mg-H system has been studied as thermochemical heat storage medium as well because of its relatively large reaction heat 10. Here, the relationship between equilibrium pressure (Peq) and the enthalpy change (Habs) and entropy change (Sabs) of hydrogen absorption can be described as following equation (2),

ln(Peq/P0) = Habs/RT  (Sabs/R)

(2)

P0 is the standard pressure (~ 0.1 MPa), R is gas constant, and T is reaction temperature. According to the above equation (2), the thermodynamic properties of metal-H systems are directly related to the reaction heat and dissociation pressure at designated temperature, namely stable metal hydrides


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can store high temperature heat and decrease the partial pressure of H2 at room temperature. The MgH2 system reported by the group in Max-Planck Institute offers a heat storage material for concentrating solar thermal energy 10. However, practical use of the Mg-H system is limited by following drawbacks 11, slow reaction kinetics, requirements of Fe- or Ni-doping, sintering at high temperature region, and high operating pressure around 10 MPa for utilizing around 500 C. Titanium hydride (TiH2) is also recognized as one of promising materials for storage media of high temperature heat. The enthalpy change (Habs) of the reaction between Ti and H2 is 142 kJ/mol

12,

which is much larger than that of MgH2, and thus TiH2 is expected as heat storage

materials at higher temperature region than 600 C under 0.1 MPa of H2. The high H value leads to low dissociation pressure at room temperature, suggesting that H2 with even low partial pressure can be absorbed and removed from reaction fields. In fact, Ti can decrease the hydrogen concentration below the explosion limit at 25 C, thermodynamically. Therefore, it is possible that Ti is also useful for hydrogen capture material in the case of accidents due to hydrogen leakage. So far, the hydrogen desorption properties of TiH2 have been studied for aluminum foaming agent and production of high-performance Ti alloys by thermal decomposition process 13-16. However, few studies have been reported for hydrogen absorption of Ti, and these reports focus on thermodynamics properties at more than 300 C

15-17.

For the practical development of Ti as

thermal storage and hydrogen capture material, control of the hydrogen absorption kinetics around room temperature is the most important issue. Several methods of surface modification have been reported in order to control the hydrogen absorption/desorption kinetics of materials. The main issues of MgH2 as hydrogen storage material are the improvement of absorption/desorption kinetics and the high reactivity with moisture and oxygen in the air

18.

Over the decades, many scientists have studied surface


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modification by solid additives such as transition metals 7, 19, metal oxides 20, and metal halides 21, to improve hydrogen absorption/desorption kinetics of MgH2. Among them, Mg with 1 mol% of Nb oxide synthesized by ball-milling method has the best performance, and the hydrogen absorption and desorption proceed even below 0 C and around 200 C, respectively

9, 20.

Wet-

milling is also one of surface modification methods, and the interesting effects are reported for titanium iron (TiFe), which is attractive hydrogen storage alloy because of its low cost and abundance 22. Application of TiFe as hydrogen storage material is limited by difficulty of initial activation due to stable oxide layer on the surface. By the surface modification using wet-milling with acetone, the properties of initial hydrogen absorption are drastically improved, and the activity of TiFe surface is preserved even after one month in the air. It is noteworthy that acetone shows quite drastic effect although it is thought to have worse effect for the kinetics due to the oxygen atom in the molecule. The above results suggest that the wet-milling in acetone is useful for making active surface for hydrogen with high stability for oxygen and moisture in the air, however the modification mechanism is not clarified yet 22. In this work, effects of various additives and organic solvents are investigated as the surface modification method of Ti to improve the kinetics.

2. EXPERIMENTAL SECTION TiH2 (98%, 325 mesh) were purchased from Sigma Aldrich and used as a starting material without further purification. Ti was prepared by dehydrogenation of TiH2 at 580 C under dynamic vacuum condition. After the dehydrogenation, the reactor was transferred into the glove box (Miwa MFG, MDB-2BL, O2  5ppm, H2O  2ppm) filled with purified Ar, and the hydrogenation temperature was immediately examined after taking out the sample from the reactor (within 5


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minutes) to minimize the influence of contamination even in the glove box. In addition, to investigate degradation of properties with time for hydrogenation of Ti, the dehydrogenated sample was kept in the glove box for 1 and 7 days. Solid additives were dispersed on the Ti surface in order to improve the hydrogenation properties of Ti. Palladium (Pd ; 99.9 ≥ %), nickel (Ni ; 99 ≥ %), vanadium oxide (V2O5 ; 99.6 ≥ %), and niobium oxide (Nb2O5 ; 99.99%) were purchased from Sigma Aldrich and used as solid additives, where these additives were chosen as catalysts on the basis of previous works about Mg

7, 20.

These mixtures of 300 mg TiH2 and 1mol% of each

additive were put into a Cr steel vessel (30 cm3 inner volume) together with 20 steel balls (7 mm in diameter). Then, these mixtures were milled for 20 h at 370 rpm by using planetary ball-milling apparatus (Fritsch P7) under 1 MPa H2 (7N) at room temperature, where this technique was referred to MgH2 system

23.

To suppress an increase in inside temperature during milling, the

milling process was stopped for 30 min every 1 h. After the ball-milling, the TiH2 samples with 1 mol% additive were dehydrogenated at 550 C for 4 h under the dynamic vacuum condition to prepare Ti with additives. As a different approach from surface modification with solid additives, wet-milling was performed by using various kinds of organic solvents. Acetone (CH3COCH3), ethanol (CH3CH2OH), tetrahydrofuran (THF ; C4H8O), hexane (C6H14), and cyclohexane (C6H12), which include water below 0.001% as impurity, were purchased from Wako Pure Chemical Industries, Ltd. 300 mg of Ti (99.7%, 100 mesh, Sigma Aldrich) was ball-milled with 20 wt. % of various kinds of solvents and 20 pieces balls made of zirconia (8 mm in diameter) for 3 h (10 min milling / 1 min pause) at 200 rpm by using the planetary ball-milling apparatus. To remove the residual solvents, these samples were heat-treated at 300 C for 2 h under the dynamic vacuum before evaluation of the hydrogenation temperature. The hydrogenation temperature was evaluated by a thermogravimetry with differential


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thermal analysis equipment (TG-DTA, Rigaku, TG8120) under H2 gas flow. At first, 0 point in the TG measurements was adjusted under Ar flow. And then, the samples were heated up to 40 C and kept for 10 min. In this region, the carrier gas was switched from Ar to H2 to investigate the hydrogen absorption properties around room temperature. Then, the samples were heated up to 400 C with 5 C/min heating rate under H2 flow. For the TG measurement, there is error of approximately 0.5 wt. %. In addition, wt. % of all samples were calculated by weight gain per total amount of sample. From this measurement, although the starting temperature of H2 absorption can be evaluated, some detailed kinetic analyses were not able to be performed due to its drastic exothermic heat. Here, the hydrogen absorption properties of the Ti samples milled with additives and solvents were evaluated after keeping more than 1 day in the glove box. The phase identification of the products on each process was carried out by powder X-ray diffraction (XRD, Rigaku RINT 2000, Cu-K:  = 1.57 Å). The samples were covered by polyimide sheet (Kapton, Du Pont-Toray Co. Ltd.) to prevent the influence of air during the measurements. To characterize the surface state of the Ti samples, X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250Xi, Al-K: 1486.6 eV) was performed. The powder sample was fixed by carbon tape (Nisshin EM Co. Ltd.) on a sample holder. The charging effects of the samples were minimized by a low-energy flood gun equipped with the XPS apparatus. The spectra was calibrated by using a peak of C 1s (285.0 eV). In order to minimize oxidation of the surface, the samples were transferred directly from the glove box to the XPS chamber without exposing the samples to the air by a home-made vessel.

3. RESULTS AND DISCUSSION The hydrogenation properties of Ti after the dehydrogenation of pristine TiH2 were


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immediately examined by TG under H2 flow as shown in Figure 1. About 4 wt. % of hydrogen was absorbed by all the samples during TG measurements, and this value is corresponding to the theoretical weight gain due to the hydrogen absorption by Ti. It was clarified by the XRD measurements that Ti is totally converted to TiH2 as shown in Figure 2. When the TG measurement was immediately performed after the hydrogen desorption from pristine TiH2, hydrogen was able to be absorbed even below 50 C. This result suggests that the Ti should have an active surface to hydrogen just after dehydrogenation, and then any catalysts and heat-activation are not necessary. However, the hydrogenation temperature was increased around 125 and 225 C after keeping in the glove box for 1 and 7 days, respectively. It is considered that surface oxidation of Ti proceeds by tiny amount of oxygen (less than 5 ppm) and water (less than 2 ppm) included even in the glove box. In other words, the surface of Ti just after dehydrogenation has quite clean surface and it is easily deactivated under the conditions with very low O2 and H2O concentration due to the high reactivity. As a result, hydrogenation temperature was increased for the Ti samples kept for long time even in the glove box. Thus, the special surface modification of Ti is necessary in order to enhance the reactivity with only H2 gas than the other gases. Considering the above results, the hydrogen absorption properties were evaluated after keeping more than 1 day in the glove box for the below Ti samples to clarify the surface modification effect.


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Temp.

Weight gain (wt. %)

5 4

400

300

3 200

2 1

immediately after 1 day after 7 days

0 0

20

40 60 Time (min)

Temperature (ºC)

100

80

Figure 1. TG results of dehydrogenated TiH2, performed under H2 flow with a heating.

dehydrogenated pristine TiH2 before TG

 Ti



TiH2

after TG

Ti milled with 1 mol% of Pd Intensity (arb. units)

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

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Ti milled with acetone

after heating to 650 C under Ar flow TiC



20

30

40

50 60 2  (degree)

70

80

Figure 2. The XRD profiles of dehydrogenated pristine TiH2, the Ti with 1mol% of Pd, and the Ti milled with acetone, before and after TG measurements. The XRD measurement of the Ti milled


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with acetone after heating to 650 C under Ar flow was also performed. The XRD results of other samples were shown in supporting information.

The TG results under H2 flow of Ti ball-milled with and without solid additives are shown in Figure 3. All the Ti samples show weight gain due to hydrogenation at more than 200 C, which is higher than that of the Ti kept in the glove box for 1 day as shown in Fig. 1. Because they were prepared by dehydrogenation of the TiH2 samples milled with additives, these samples should have an active surface for H2 similar to the above dehydrogenated pristine TiH2. However, the higher temperatures were required for the hydrogenation, indicating that the deactivation more easily proceeds than that of the dehydrogenated pristine TiH2 by surface oxidation due to the increasing of surface area during the ball-milling. Namely, remarkable additive effects were not found. From the above results, it is expected that the influence of surface oxidation is much larger than the effects of those additives. Light element based hydrogen storage materials such as Mg require the effective catalysts to improve the kinetics because of poor dissociation ability of metallic Mg surface 20. On the other hand, it was clarified that Ti essentially possesses active surface for the dissociation of H2 molecules considering the results shown in Fig. 1. When the oxide layer was formed on the surface of Ti, hydrogenation is inhibited because hydrogen cannot penetrate into the oxide layer. In fact, the oxidation of the Ti surface might be accelerated by the additives as mentioned above. In order to control the kinetics of Ti, the surface modification to prevent the oxidation is important rather than the activation for the reaction with H2. Therefore, the characteristic Ti surface with selective reactivity of H2 and protection against O2 and H2O are required.


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Temp.

Weight gain (wt. %)

5 without additives 1mol % Ni 1mol % Pd 1mol % V2O5 1mol % Nb2O5

4 3

400

300

200

2 1

Temperature (ºC)

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

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100

0 0

20

40 60 Time (min)

80

Figure 3. TG results of the Ti milled without additives and with 1mol % of additives, performed under H2 flow with a heating. As another surface modification, Ti was milled in various kinds of organic solvents (wetmilling). It is expected that solvents react with Ti and the characteristic surface is possibly generated on the Ti surface. Figure 4 shows the results of TG under H2 flow of the Ti milled with various kinds of organic solvents. Although the significant effects of the wet-milling for TiFe alloy have been reported 22, it is speculated that the solvents including oxygen atoms in the molecule accelerate the oxidation and the deactivation of the Ti surface during the ball milling considering the results about Ti metal discussed above. In fact, the hydrogenation temperature of the Ti milled with ethanol (C2H5OH) was higher than 300 C (Fig. 4). However, the Ti milled with acetone (CH3COCH3) and THF (C4H8O) can absorb hydrogen at lower temperature compared with that of the Ti milled with hexane (C6H14) and cyclohexane (C6H12). It is noteworthy that the hydrogen can be absorbed even around room temperature when Ti is milled with acetone, suggesting that the Ti surface can be modified by the reaction with acetone during the wet-milling process. As mentioned above, the hydrogenation properties of the samples were evaluated after keeping in the glove box


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for more than 1 day. Thus, the influence of O2 and H2O was suppressed compared with that of the other samples, in other words, O2 and H2O molecules would not pass through the modified Ti surface although H2 can selectively penetrate into the surface and react with Ti. These phenomena are similar to a membrane separation of H2. The H2 separation method by means of membrane can be classified into two types according to the separation processes. One is H2 separation using metal membrane such as palladium thin film. In this case, H2 is dissociated on the surface of membrane, the H atoms diffuse into the metals, and then H2 molecules are reformed at the opposite side 24. By using this method, high selectivity for H2 can be obtained via a solution-diffusion mechanism although the high temperature and or thin membrane are required to separate H2 with moderate flow rate 24. Another is the method using micro-pores in materials such as zeolites like molecular sieve. In this case, the difference between permeation rates of H2 and the other gases is used for the separation 25. In contrast with the above metal membrane, not only H2 but also small amount of other gases are essentially permeated. Although the hydrogenation temperature of the Ti milled with acetone was the lowest among the samples, it was clearly increased after keeping for 7 days (Figure 5). These results suggest that O2 and/or H2O slowly pass through the modified surface of Ti, and then the inactive oxide layers were formed. Therefore, the surface layer having similar properties to porous membrane would be generated by the milling with acetone in this experimental condition.


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4 Weight gain (wt. %)

Temp.

hexane cyclohexane ethanol acetone THF

5

400

300

3 200

2 1

Temperature (ºC)

100

0 0

20

40 60 Time (min)

80

Figure 4. TG results of the Ti milled with various kinds of solvents, performed under H2 flow with a heating. Temp.

Weight gain (wt. %)

5 4

400

300

3 200

2 immediately after 1 day after 7 days

1

Temperature (ºC)

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

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100

0 0

20

40 60 Time (min)

80

Figure 5. TG results of the Ti milled with acetone, performed immediately and after 1 day, after 7 days after taking out sample from the reactor under H2 flow with a heating.

The XPS measurements were performed to characterize chemical state of the Ti surface modified by acetone. The XPS spectra of Ti 2p are shown in Figure 6. The clear peaks were


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observed around 459.0 eV (and 464.8 eV) in the spectra of pristine Ti and the Ti milled without additives, which should be assigned to Ti4+ corresponding to TiO2 26. Namely, the surface of these Ti is covered by the TiO2 layer. The small peak corresponding to Ti0 was also observed in addition to Ti4+ in the spectrum of milled Ti, suggesting that the oxide layer was partially broken and metallic state was found as the new surface by the milling process. For the Ti milled with acetone, the peak of low oxidation state (454.5 eV) was significantly large compared with the pristine and milled Ti. Since the metallic surface should not exist on the top-layer of the Ti surface due to the high reactivity, the above results suggest the surface layer on Ti metal is thinner than that of other Ti samples. In the high binding energy region, the small peak was observed at 458.4 eV, which was shifted from Ti4+ of TiO2 to slightly lower energy side. Thus, the characteristic surface was formed by the wet-milling with acetone, which prevented the oxidation of Ti. It is expected that the functional surface would be composed of carbon (C) and hydrogen (H) atoms in addition to oxygen (O) atom with lower electronegativity than the others. In fact, existence of C in the surface was clarified because the TiC was observed by XRD after heating to 650 C as shown in Figure 2. It has been reported by Emami et al. that TiFe can be activated by forming active oxide TiFeOx or an oxycarbide TiFe(O, C)x on the surface during the milling with acetone. The modified surface of TiFe is possibly similar to that of Ti synthesized in this work. Although the detailed surface state is not characterized yet, it is demonstrated that the wet-milling is useful to control hydrogenation kinetics.


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pristine Ti

Ti 2p3/2 Ti 2p1/2

Ti oxide

metallic Ti

Intensity (arb. units)

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

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milled Ti

Ti milled with acetone

468 466 464 462 460 458 456 454 452 Binding Energy (eV)

Figure 6. XPS analysis of the binding energy of the Ti 2p electron of pristine Ti, the Ti milled without solvents and additives, and the Ti milled with acetone.

From the above results, it was clarified that the Ti surface was modified by the wet-milling with acetone. As a result, the hydrogen absorption proceeds around room temperature due to suppression of the surface oxidation. The excellent hydrogenation properties would originate in the surface layer composed of C, O, and/or H, and features of the modified surface are similar to the membrane of porous materials. Here, the reaction properties of Ti samples prepared by only fixed conditions for different solvents were investigated in this work. The properties of modified surface can be improved by optimizing the synthesis conditions such as weight of solvent and milling time, and then the moderate surface with high reactivity for H2 and a perfect protection for other gases is possibly realized. As future work further analyses for the surface state are necessary to characterize the modified Ti surface in detail. In addition, the cycling of hydrogen absorption-


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desorption performance is one of the important properties for practical use although we focus on only the initial hydrogenation of Ti in this work. The investigation of cycle performance are also necessary in next step.

4. CONCLUSIONS In this work, surface modification by additives and wet-milling with organic solvents were carried out to control the hydrogenation kinetics of Ti. The Ti sample with fresh surface prepared from TiH2 dehydrogenation successfully absorbed hydrogen even below 50 C without any catalysts and heat-activation. However, the Ti surface was easily deactivated even in the glove box due to its high reactivity. As a result, the hydrogen absorption was inhibited by the oxide layer. Various kinds of solid additives, which were typical metal and oxide catalysts for hydrogen dissociation, were dispersed on the Ti with clean surface and the hydrogenation properties were evaluated after keeping more than 1 day in the glove box. However, these samples absorbed hydrogen at more than 200 C. Thus, it was found that the influence of surface oxidation is much larger than effects of the additives. Wet-milling was performed as the other method for modifying surface. Although it is expected that the solvents including oxygen atoms in the molecule such as ethanol, acetone, and THF accelerate the oxidation and the deactivation of the Ti surface, the hydrogen can be absorbed around room temperature even after keeping for 1 day in the glove box when Ti is milled with acetone. The Ti surface can be modified by the reaction with acetone during the milling process and has a protection feature against O2 and H2O. As a result, H2 can selectively penetrate into the surface and react with Ti. The XPS results suggest that the characteristic surface differently from TiO2 was formed by the milling with acetone, and the functional surface would be composed of C, O, and H atoms. The further characterization is required to understand the


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surface state in details.

ASSOCIATED CONTENT Supporting Information. Figure S1 shows XRD results of the Ti milled with various kinds of additives before and after TG measurements. Figure S2 shows XRD results of the Ti milled with various kinds of organic solvents before and after TG measurements.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +81 82 424 5744 ORCID Takayuki Ichikawa: 0000-0003-3425-8758

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol)

Acknowledgement This work was supported by The Fujikura Foundation.


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Room-Temperature Hydrogen Absorption of ... - ACS Publications

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