Effect of Milling Parameters on the Dehydrogenation Properties of the

Oct 12, 2009 - Small particle size which can be achieved by milling and small amounts of transition-metal compounds as catalysts result in increased ...
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J. Phys. Chem. C 2009, 113, 19344–19350

Effect of Milling Parameters on the Dehydrogenation Properties of the Mg-Ti-H System Young Joon Choi,† Jun Lu,† Hong Yong Sohn,*,† Zhigang Zak Fang,† and Ewa Ro¨nnebro‡ Department of Metallurgical Engineering, UniVersity of Utah, 135 South 1460 East Room 412, Salt Lake City, Utah 84112, and Pacific Northwest National Laboratory, 902 Battelle BouleVard, P.O. Box 999 MSIN K2-03, Richland, Washington 99352 ReceiVed: July 28, 2009; ReVised Manuscript ReceiVed: September 01, 2009

Magnesium-based alloys are promising candidates as potential hydrogen storage materials due to their inherent high hydrogen contents. Small particle size which can be achieved by milling and small amounts of transitionmetal compounds as catalysts result in increased hydrogen release/uptake kinetics. In this work, we examined the effects of various milling parameters and TiH2 content on the dehydrogenation properties of the Mg-Ti-H system. The samples were prepared with different amounts of TiH2 using various milling methods and conditions. The activation energy and the enthalpy change of dehydrogenation of the milled samples were determined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The results indicated that, among a variety of MgH2/TiH2 ratios and milling conditions, samples with 9.1 mol % TiH2 milled in a dual-planetary high energy mill for 4 h under 15 MPa hydrogen pressure were found to be the optimal materials, displaying a substantially reduced activation energy and enthalpy change for MgH2 dehydrogenation. 1. Introduction In the past several decades, research and development on the use of hydrogen as a fuel for various applications have gathered momentum in response to the demand for cleaner fuels and substitutes to fossil fuels. The use of hydrogen for automobiles, which is one of the most important applications of hydrogen fuel, requires an on-board hydrogen storage system that can be on-board or off-board regenerated. However, one of the key obstacles to this application is that current available storage technologies do not meet the capacity and efficiency requirements for achieving commercial viability. Hydrogen storage systems developed so far are liquid hydrogen, compressed gas cylinders, and solid-state storage materials. Compared to the physical approaches such as liquefaction and compression, hydrogen storage in the solid state has merits in terms of high volumetric and gravimetric contents and, most importantly, safety.1 The critical properties of a solid hydrogen storage material include the storage capacity, dehydrogenation reaction temperatures, hydrogen pressure generated by the dehydrogenation reactions, and the reversibility of dehydrogenation and hydrogenation reactions. The U.S. Department of Energy established requirements for on-board storage for vehicles, recently updated in April 2009: the storage capacity must be high (>4.5 wt % in 2010 and >5.5 wt % in 2015), the kinetics of hydrogen release/uptake reactions and the dehydrogenation temperatures (350 °C)

(1)

To overcome the poor kinetics attributed to the fact that MgH2 has strong ionically bound hydrogen, many researchers have focused on the modification of thermodynamics and kinetics by destabilizing, that is, by mixing with a second element in order to lower the dehydrogenation temperature. It has been shown that several previously unknown MgxTHy (T ) Ti, V, Cr, Mn, or Nb) compounds can be prepared by mixing MgH2 with a transition metal or binary transition metal hydride, and heated in hydrogen at a GPa level.28-31 The dehydrogenation temperatures of these materials were lower than that of MgH2. Several recent papers have reported that ternary alloys or hydrides based on Mg which possess hydrogen storage properties can be obtained in the form of thin films.21,32-35 In addition, by using first-principles modeling methods, Song et al.36 found that the addition of Ti into the MgH2 crystal structure weakened the bonding between Mg and H, resulting in a lower heat of formation. This is an encouraging result that has prompted many research groups to attempt to synthesize the same kinds of phases in powder form, which were recently demonstrated by Kalisvaart et al.,37 Rousselot et al.,38 and Choi et al.39 For example, work by Choi et al.39 has shown that a reversible hydrogen storage capacity of approximately 6.0 wt

10.1021/jp907218t CCC: $40.75  2009 American Chemical Society Published on Web 10/12/2009

Dehydrogenation Properties of the Mg-Ti-H System

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% is feasible with an onset temperature from TGA experiments (at 5 °C/min heating rate) of 126 °C by synthesizing a Mg-Ti-H system using a high energy high pressure (HEHP) milling method. Its dehydrogenation reaction is as follows:

7MgH2 + TiH2 ) 7Mg + TiH2 + 7H2 (theoretical H2 capacity: 5.98 wt %) (2) The above result indicates that the kinetics of MgH2 dehydrogenation is affected by the molar ratio of the reactants and material preparation techniques, which has promoted further research on the Mg-Ti-H system. In this work, the effects of milling and TiH2 content on the dehydrogenation properties of the Mg-Ti-H system were investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA). As indicated by the results presented below, among various MgH2/TiH2 ratios and milling conditions, the activation energy and enthalpy change of MgH2 dehydrogenation were significantly reduced when 9.1 mol % of TiH2 was added and the mixture was milled for 4 h in a dual-planetary high energy mill under 15 MPa hydrogen pressure. 2. Experimental Apparatus and Procedure The initial materials, magnesium hydride (MgH2, 98%) and titanium hydride (TiH2, 99%), were purchased from SigmaAldrich (Milwaukee, WI) and Alfa-Aesar (Ward Hill, MA), respectively, and used as received without any further purification. All of the material handling was carried out in a glovebox filled with purified argon (99.999%) in the presence of an oxygen scavenger and a drying agent to prevent raw materials and samples from undergoing oxidation and/or hydroxide formation. In order to study the effects of the milling on the dehydrogenation properties, mixtures of MgH2 and TiH2 in various molar ratios were prepared using low energy and high energy ballmilling methods under an argon or hydrogen atmosphere. For “low energy milling” and “high energy milling”, a jar-roll mill and Spex Vibratory mill/dual-planetary high energy mill, respectively, were used. Approximately 3.0 g mixtures were loaded into the milling jar with a ball to powder ratio of 35:1 by weight in the glovebox, and the milling time was varied from 30 min to 16 h. The milling jar, which had an inner volume of 160 mL, was sealed by a Vito-type O-ring, which kept the inside atmosphere inert during milling. Thirty stainless steel balls with a 1/8 in. diameter were used. The hydrogen release properties of the mixture were examined by the use of thermogravimetric analysis (TGA, Shimadzu TGA50) and differential thermal analysis (DTA, Shimadzu DTA50) in which 10 mg samples were heated under flowing argon up to 350-450 °C at a heating rate of 5-10 °C/min, followed by being held at this temperature for a predetermined length of time. This equipment was specially designed and built to be used inside the argon-filled glovebox equipped with a gas purification system, which permitted simultaneous TGA and DTA without exposing the sample to air. The identification of phases in the reactants and products before and after ball milling was carried out using an X-ray diffractometer (XRD, Siemens D5000) with Ni-filtered Cu KR radiation (λ ) 1.5406 Å). Each sample for XRD analysis was mounted on a glass slide and covered with a Kapton tape as a protective film in the glovebox. The X-ray intensity was measured over diffraction 2θ from 10 to 100° with a scanning rate of 0.02°/s. The crystallite size and effective internal strain

Figure 1. TGA profiles of milled 10MgH2/TiH2 under various heating rates. (The inset shows how the onset temperature is obtained. The straight line on the right is drawn through the inflection point with the same slope of the curve at that point.)

of a sample were obtained from the XRD peak broadening by applying Stokes and Wilson’s formula.40 A scanning electron microscope (SEM, TOPCON SM-300) was employed to determine the particle size and to observe the morphology of the samples after milling. The samples were protected from exposure to air during the transfer to the SEM sample chamber by a conductive tape applied in the glovebox. The microstructure was analyzed further to confirm the crystallite size of the sample by a Tecnai 30 transmission electron microscope (TEM, FEI, Hillsboro, OR) coupled with energy dispersive spectrometry (EDS) at an accelerating voltage of 300 kV. For the TEM observation, the sample was suspended in heptane using an ultrasonic bath for several hours, dropped onto a TEM grid, and dried before the measurement. 3. Results and Discussion 3.1. Effect of TiH2 Content on the Activation Energy and Onset Temperature of MgH2 Dehydrogenation. As noted in section 1, the kinetics of MgH2 dehydrogenation has been found to improve significantly by TiH2.39 For example, the onset dehydrogenation temperature of the 7MgH2/TiH2 mixture (126 °C) determined by TGA is much lower than that of MgH2 alone (381 °C). The activation energy of dehydrogenation (71 kJ/mol) was also much smaller than that of as-received MgH2 (153 kJ/ mol) or milled MgH2 (96 kJ/mol). These results showing the beneficial effect of TiH2 prompted further work to investigate the effects of TiH2 content, milling time, and methods. To investigate the effect of the TiH2 content on the dehydrogenation properties of MgH2, mixtures of MgH2 with TiH2 added in the amount of 0-20 mol % were milled using high energy high pressure (HEHP) milling. A canister under 15 MPa hydrogen pressure, which contained the Mg-Ti-H mixtures or just MgH2, was employed for the HEHP milling. The milled samples were subsequently analyzed using TGA. The activation energy of dehydrogenation of the Mg-Ti-H mixtures was determined by applying the Ozawa-Flynn-Wall method41-43 to the results of the nonisothermal runs. TGA profiles of the milled 10MgH2/TiH2 mixtures under various heating rates are shown in Figure 1. The method is based on the following rate equation:

( )

-Ea dR ) Af(R) exp dt RT

(3)

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Figure 2. Onset dehydrogenation temperature (at 5 °C/min heating rate) vs TiH2 content.

Figure 3. Activation energy (Ea) for the dehydrogenation of milled Mg-Ti-H mixtures with various TiH2 contents.

where R is the fractional conversion, t the reaction time, A the pre-exponential factor, f(R) a kinetic function that is related to the reaction mechanism, and R the gas constant. Integration of eq 3 under a constant heating rate (T ) To + βt; β ) the heating rate; To ) the starting temperature) results in the following equation:

log β ) -

(

0.457Ea R - 2.315 - log RT AEa

dR ∫0a f(R)

)

(4)

On the basis of eq 4, the activation energy, Ea, can be calculated from the slope of a plot of log β vs 1/T at a given value of R. As seen in Figure 1, the TGA curves of the milled 10MgH2/ TiH2 move to a higher temperature with increasing heating rate from 1 to 20 °C/min, as expected. The activation energy is then evaluated using the heating rates (β ) 1, 2, 5, 10, and 20 °C/ min) and a fractional conversion (R ) 0.4) from the TGA profiles by plotting log β versus 1/T. Figure 2 shows the change of onset dehydrogenation temperature, which represents the temperature at which a material begins to release hydrogen at a significant rate and is defined as shown in the inset of Figure 1, as a function of TiH2 content based on the TGA curves obtained with 5 °C/min heating rate for the milled Mg-Ti-H mixtures. The TGA experiment was run under an argon atmosphere at a heating rate of 5 °C/min up to 350 °C, after which the temperature was held at this value for 1 h. It is seen that the onset temperature (at 5 °C/min heating rate) of the mixture drops significantly from 267 to 163 °C when only 2 mol % TiH2 was added, and decreases gradually as the TiH2 content is further increased. Moreover, the onset temperature of the milled 10MgH2/TiH2 (corresponding to 9.1 mol % TiH2) is approximately 280 and 166 °C lower than those of as-received MgH2 and milled MgH2, respectively. Figure 3 presents the activation energy (Ea) for the dehydrogenation of the milled Mg-Ti-H mixtures with various TiH2 contents using the above method. The most noticeable result is that the activation energy is reduced significantly when only 2 mol % TiH2 is added, and decreases gradually as the TiH2 content is increased, which is similar in character to the plot of onset temperature (at 5 °C/min heating rate) vs TiH2 content shown in Figure 2. In addition, the activation energy was further reduced from 81 to 68 kJ/mol when the molar ratio Mg:Ti was decreased from 50:1 to 4:1 (corresponding to 2 and 20 mol % TiH2, respectively), although this lowers the hydrogen storage capacity to below 5 wt %. Thus, it can be concluded that the

Figure 4. Onset dehydrogenation temperature (at 5 °C/min heating rate) of milled 10MgH2/TiH2 vs milling time.

kinetics of dehydrogenation of Mg-Ti-H mixtures is improved as more TiH2 is added. The 10MgH2/TiH2 was selected for further investigation because it showed the lowest onset temperature (at 5 °C/min heating rate) at around 101 °C and a reasonable reaction rate with over 6 wt % theoretical H2 capacity. 3.2. Effect of Milling Time on the Dehydrogenation Properties of 10MgH2/TiH2. As described in our previous work,39 the most significant reduction in particle size occurred during the first 12 h of milling and this milling time induced sufficient contact between starting chemicals; i.e., further milling did not cause significant differences in the results. However, it was still uncertain how this ball-milling parameter would affect the dehydrogenation properties of the 10MgH2/TiH2 mixture in terms of kinetic improvement. Therefore, the effects of milling time on the onset temperature and the average grain size were investigated in this work. The 10MgH2/TiH2 mixtures were prepared by changing the milling time from 30 min to 16 h. TGA experiments were run under the conditions described in section 3.1. As shown in Figure 4, the onset temperature is about 225 °C for 30 min milling time, while milling for 4 h lowers the onset temperature (at 5 °C/min heating rate) to about 101 °C. A longer milling time over 4 h, however, results in the increase of the onset temperature from 101 to 236 °C. Thus, a short milling time in the range from 2 to 4 h results in a better dehydrogenation property in terms of the onset temperatures (at 5 °C/min heating rate) being below 150 °C. The substantial effect of milling time to lower the onset temperature is likely due to the reduced particle size, which in turn decreases the path lengths for hydrogen diffusion. X-ray diffraction analysis was therefore

Dehydrogenation Properties of the Mg-Ti-H System

Figure 5. XRD patterns of 10MgH2/TiH2 milled for (a) 10MgH2/TiH2 before milling, (b) 30 min, (c) 1, (d) 2, (e) 4, (f) 8, and (g) 16 h. (The broad peak on the far left is from a Kapton tape used to cover the powders.)

Figure 6. Average grain size of milled 10MgH2/TiH2 vs milling time.

performed to estimate the average crystallite size using Stokes and Wilson’s formula40 in combination with XPowder software,44 and to analyze the reaction products as discussed below. Figure 5 shows the XRD profiles of the raw materials as well as the products obtained with milling times of 30 min to 16 h. Crystalline phases were identified by comparing the experimental data with the JCPDS files from the International Center for Diffraction Data. The XRD patterns (b-g in Figure 5) show that the milled powder consists of MgH2 and TiH2 like the raw material. There is no indication of impurities that may come from the erosion of the milling tools and the formation of any new phases. Additionally, as the milling time increases, some of the peaks of the MgH2 and TiH2 phases disappear compared with the raw material before milling shown in Figure 5a. The crystal structures of MgH2 and TiH2 phases were gradually changed into amorphous structures upon milling. The milled 10MgH2/TiH2 mixtures present lower diffraction peaks with increased widths at most 2θ values, although the peak for the (2,1,1) face at 55° changed less than other peaks, compared with 10MgH2/TiH2 before milling. The broadening of diffraction peaks indicates the refinement of the crystallite size and the presence of a lattice microstrain in the milled 10MgH2/TiH2 mixture. On the basis of the peak broadening, the grain sizes of the milled 10MgH2/TiH2 mixtures were estimated using Stokes and Wilson’s formula,40 and plotted in Figure 6. The average grain size was calculated by averaging the grain sizes from the three main peaks.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19347 It can be seen in Figure 6 that the crystallite size of the 10MgH2/TiH2 mixture is reduced to 8.2 nm after 4 h of milling, which lowers the onset temperature of dehydrogenation, as shown in Figure 4. Further milling up to 16 h did not cause a significant difference in the average grain size. However, the onset dehydrogenation temperature increased with milling time between 4 and 8 h, indicating a slower dehydrogenation rate, possibly due to the fact that the particles become much more tightly aggregated without a further decrease in the grain size as milling continued. Thus, the substantial effect of milling time to lower the onset temperature during the first 4 h can be attributed to the reduced particle size and increased contact between MgH2 and TiH2. SEM was used to examine the micrographs of (a) the 10MgH2/TiH2 mixture before milling, (b) the 10MgH2/TiH2 mixture after 4 h of milling, and (c) the 10MgH2/TiH2 mixture after 16 h of milling, as shown in Figure 7. While the coarse particle size of the 10MgH2/TiH2 mixture before milling was in the range 20-200 µm, the primary particle sizes of 10MgH2/ TiH2 mixtures milled by the HEHP milling in Figure 7b and c have been reduced to 0.3-6 µm. In addition, Figure 7b and c show ultrafine particle dimensions of below 1 µm. The crystallite size of the 10MgH2/TiH2 mixture obtained by the HEHP milling for 4 h was also studied by TEM. Figure 8a shows the high resolution transmission electron microscope (HRTEM) image of the milled 10MgH2/TiH2 mixture, which indicates that the mixture consists of agglomerated particles of about 10-20 nm in size. The energy dispersive spectroscopy (EDS) analysis in Figure 8b shows that there is no indication of Fe, which may come from the erosion of the milling tools, or other impurities. 3.3. Effect of Milling Methods on the Dehydrogenation Properties of 10MgH2/TiH2. As shown above, the formation of nanocrystalline structure produced by ball milling results in dramatic improvement of hydrogen release and uptake kinetics, as also shown by others.18,45,46 However, high energy ball milling may cause unexpected reactions and changes in the original phases. For example, magnesium oxide formed during milling causes degradation of hydrogen capacity.10 In order to better understand how different milling methods affect particle size, strain, and lattice defects, the following four different milling methods were tested in this work: (1) high energy high pressure milling (HEHPM), (2) high energy milling (HEM), (3) low energy high pressure milling (LEHPM), and (4) low energy milling (LEM). It is noted that “high pressure” refers to “15 MPa H2” in the canister. The samples milled by these different milling methods, which are summarized in Table 1, were analyzed using TGA and DTA. The onset temperature (at 5 °C/min heating rate) and activation energy of samples milled for 4 h in the four different mills are presented in Figure 9. The onset temperature (at 5 °C/min heating rate) of the HEHPM mixture is approximately 43 and 114 °C lower than those of the HEM and LEHPM mixtures, respectively. In addition, the results show that the activation energy (Ea) for the dehydrogenation of the HEHPM mixture is approximately 73 kJ/mol, which is much lower than the results determined for the LEM mixture (131 kJ/mol) and LEHPM mixture (115 kJ/mol). This comparison indicates that HEHPM results in better dehydrogenation kinetics compared to the other milling techniques. DTA was performed in combination with TGA to measure the enthalpy changes of the products from the four different milling methods, as shown in Figure 10. Enthalpy changes as a function of time were measured while the samples were heated from room temperature to 450 °C under an argon atmosphere

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Figure 7. SEM micrographs of (a) 10MgH2/TiH2 before milling, (b) 10MgH2/TiH2 after 4 h of milling, and (c) 10MgH2/TiH2 after 16 h of milling.

Figure 8. (a) HRTEM image of 10MgH2/TiH2 after 4 h of HEHP milling and (b) its EDS analysis.

TABLE 1: Summary of Four Different Milling Methods HEHPM

HEM

LEHPM

LEM

high energy low energy high energy low energy milling under milling under milling under milling under atmospheric Ar 15 MPa H2 atmospheric Ar 15 MPa H2

at a ramping rate of 10 °C/min. Two endothermic peaks, caused by the dehydrogenation of MgH2 followed by that of TiH2, were observed in both samples of HEHPM and HEM. On the other hand, only one endothermic peak appeared in the case of LEHPM and LEM samples, indicating that the dehydrogenation of TiH2 has not happened up to 450 °C. This is attributed to the fact that the LEHPM and LEM did not enhance the kinetics of TiH2 dehydrogenation by lattice defects and the reduction of crystallite size. At the same time, it can be seen that the area of endothermic effect in the HEHPM is smaller than that in the LEHPM. This is further confirmed by the fact that integration of each endothermic peak in both cases gave values for the heat of reaction of 69.2 ( 0.5 and 72.8 ( 0.7 kJ/mol of H2, respectively. It is noted that the sample weight was quite similar and the heat of reaction was obtained from the integration of the peak based on only the MgH2 dehydrogenation in these two cases. Thus, it can be concluded that the energy needed for dehydrogenation of MgH2 from a 10MgH2/TiH2 mixture is substantially smaller when HEHPM is applied in comparison with the other milling methods. This suggests that more energy is absorbed by the sample during HEHP milling.

Figure 9. Onset dehydrogenation temperature (at 5 °C/min heating rate) vs activation energy: (a) HEHPM 10MgH2/TiH2; (b) HEM 10MgH2/TiH2; (c) LEHPM 10MgH2/TiH2; (d) LEM 10MgH2/TiH2.

3.4. Dehydrogenation Enthalpy of MgH2 with and without TiH2. It was reported that the hydrogen release of MgH2 can occur at a much lower temperature by the addition of TiH2.39 Similar effects of other elements have been observed.17,18 In order to better understand the effect of TiH2, four different samples of HEHPM 10MgH2/TiH2, LEHPM 10MgH2/TiH2, HEHPM MgH2, and as-received MgH2 were prepared and their dehydrogenation properties were investigated by DTA. The

Dehydrogenation Properties of the Mg-Ti-H System

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19349 TABLE 2: Enthalpies of Dehydrogenation (kJ/mol of H2)

Figure 10. DTA results for (a) HEHPM 10MgH2/TiH2, (b) HEM 10MgH2/TiH2, (c) LEHPM 10MgH2/TiH2, and (d) LEM 10MgH2/TiH2 with a heating rate of 10 °C/min up to 450 °C and holding for 1 h. Curve e shows the temperature profile corresponding to curves a, b, c, and d. (Two endothermic peaks, caused by the dehydrogenation of MgH2 followed by that of TiH2, were observed in curves a and b.)

Figure 11. DTA profiles for (a) HEHPM 10MgH2/TiH2, (b) HEM MgH2, and (c) as-received MgH2 with a heating rate of 10 °C/min up to 450 °C and holding for 1 h. Curve d shows the temperature profile corresponding to curves a, b, and c. (Two endothermic peaks, caused by the dehydrogenation of MgH2 followed by that of TiH2, were observed in curve a.)

milling time was 4 h, the canister was pressurized under 15 MPa H2, and DTA was run under the same conditions as in section 3.3. Figure 11 presents the plot of enthalpy changes vs time to examine the effect of TiH2 on the dehydrogenation properties. Two endothermic peaks were observed in the HEHPM 10MgH2/ TiH2 sample, whereas only one peak appeared in the case of HEM MgH2 and as-received MgH2, suggesting that one reaction occurred in the HEM and as-received samples due to the MgH2 dehydrogenation before 450 °C. It is seen that the dehydrogenation of HEHPM 10MgH2/TiH2 took place at lower temperatures than that of HEHPM MgH2 by observing the change of peak temperature. In addition, the dehydrogenation temperature of HEHPM 10MgH2/TiH2 is much lower than that of as-received MgH2. The peak is also broader. This is attributed to the fact that lattice defects and the reduction of crystallite size are produced during ball milling, which enhance the kinetics of dehydrogenation of MgH2.45 More importantly, the area of endothermic effect in the HEHPM 10MgH2/TiH2 is significantly smaller than that in the HEHPM MgH2, indicating less energy is needed for the dehydrogenation reaction when TiH2 is added. The overall enthalpy of dehydrogenation obtained by integrating each endothermic peak for the four different samples is

HEHPM 10MgH2/TiH2

HEM MgH2

LEHPM 10MgH2/TiH2

as-received MgH2

69.2 ( 0.5

72.7 ( 1.0

72.8 ( 0.7

75.0 ( 1.0

compared in Table 2. The comparison suggests that the heat of reaction of the HEHPM 10MgH2/TiH2 (69.2 ( 0.5 kJ/mol of H2) is approximately 3.5 ( 0.5 and 5.8 ( 0.5 kJ/mol of H2 lower than those of HEHPM MgH2 and as-received MgH2, respectively. This result implies that the thermodynamics of MgH2 dehydrogenation is affected by the addition of TiH2 and the application of the HEHPM. Therefore, it can be concluded that the hydrogen release can occur at a much lower temperature with less energy consumption not only by the addition of TiH2 but also by the particle size reduction and crystal structure modification using high energy high pressure milling. 3.5. Possible Mechanism for the Effect of TiH2 on the Kinetics and Thermodynamics of MgH2 Dehydrogenation. The fact that the hydrogen release of MgH2 occurs at a much lower temperature by the addition of TiH2 is in contrast to the fact that other Ti species, such as Ti and TiCl3, do not exhibit similar effects.39 Song et al.36 demonstrated that the incorporation of Ti into the MgH2 crystal structure weakened the bonding between Mg and H, resulting in a lower heat of formation. A possible explanation of the role of TiH2 in improving the kinetics of hydrogen release is that TiH2 is incorporated into the crystal structure of MgH2 during HEHPM, thus weakening the Mg-H bond. The incorporation of TiH2 is likely to be easier than that of the Ti element because TiH2 brings hydrogen with Ti into the MgH2 crystal structure. This interpretation is supported in part by the weakening crystallinity of the milled samples with increased milling time, shown in Figure 5. Additional supporting evidence is presented by the reduced enthalpy of dehydrogenation with the incorporation of TiH2, as discussed in the previous section. 4. Conclusions In this work, the effects of milling parameters and TiH2 content on the dehydrogenation properties of the Mg-Ti-H system were investigated. The samples were made with different amounts of TiH2 using various milling methods and conditions. Dehydrogenation characteristics were significantly improved, releasing a large amount of hydrogen (∼6.0 wt %) starting at 101 °C, when 9.1 mol % TiH2 was added and the samples were milled for 4 h by dual-planetary high energy milling under 15 MPa hydrogen pressure. The TGA and DTA data indicate that the 10MgH2/TiH2 mixture had an activation energy value of 73 kJ/mol and a heat of reaction of 69.2 ( 0.5 kJ/mol of H2, which is lower by about 80 kJ/mol and 5.8 ( 0.5 kJ/mol of H2 than those of as-received MgH2, respectively. It has been determined that hydrogen release occurs at a much lower temperature with significantly less energy consumption by the addition of TiH2 and by the particle size reduction and crystal structure modification using a high energy high pressure milling method. Acknowledgment. This research was supported by U.S. Department of Energy (DOE) under Contract No. DE-FC3605GO15069. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353. (2) U.S. Department of Energy, Multi-Year Research, Development and Demonstration Plan: Planned program activities for 2004-2015, http:// www1.eere.energy.gov/hydrogenandfuelcells/mypp/.

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