J. Phys. Chem. C 2010, 114, 6869–6873
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Reversible Vacancy Formation and Recovery during Dehydrogenation-Hydrogenation Cycling of Ti-Doped NaAlH4 Kouji Sakaki,* Yumiko Nakamura, and Etsuo Akiba National Institute of AdVanced Industrial Science and Technology, AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan
Meredith T. Kuba and Craig M. Jensen Department of Chemistry, UniVersity of Hawaii, 2545 McCarthy Mall, Honolulu, Hawaii 96822 ReceiVed: January 27, 2010; ReVised Manuscript ReceiVed: February 22, 2010
The mechanism of the dehydrogenation-rehydrogenation of Ti-doped NaAlH4 was investigated through X-ray diffraction (XRD) studies and Doppler broadening measurements of positron annihilation. Phase transformations during dehydrogenation and hydrogenation half-cycles were studied by the Rietveld analysis of XRD profiles. Changes in lattice defects were monitored by determination of the S parameter of the Doppler broadening spectra, which shows the ratio of positron annihilation to valence electrons. No significant changes in lattice parameters and the S parameter were observed directly following Ti doping. However, the S parameter increased upon dehydrogenation and decreased following rehydrogenation. These results indicate that vacancies are not introduced into NaAlH4 upon Ti doping, but rather, they arise during dehydrogenation and disappear during rehydrogenation. I. Introduction NaAlH4 contains 7.5 mass % of hydrogen and releases hydrogen gas upon heating as follows:
1 2 NaAlH4 f Na3AlH6 + Al + H2 3 3
(1)
1 1 1 Na AlH6 f NaH + Al + H2 3 3 3 2
(2)
1 NaH f Na + H2 2
(3)
However, the kinetics of these reactions are slow and irreversible. In 1997, Bogdanovic et al. reported that Ti doping of NaAlH4 renders the reactions reversible and enhances the kinetics.1 The first two reactions release greater than 5 mass % of hydrogen at moderately low temperatures,2,3 and thus, Tidoped NaAlH4 has been considered as a candidate hydrogen carrier for utilization in fuel cell powered vehicles. However, information about the location of the active Ti atoms and the fundamental basis of why Ti doping improves the hydrogenation properties of NaAlH4 is still lacking. Three different mechanistic possibilities have been advanced to account for the improved hydrogenation kinetics that arises upon Ti doping. Sun et al. have suggested that Ti doping into the lattice forms vacancies.4 Alternatively, Brinks et al. have hypothesized that Ti works as a catalyst on the surface of NaAlH4.5 Lastly, the activation energy and the pre-exponential factor of the relaxation process obtained by the anelastic spectroscopy suggest that the formation of AlH6-x defects may be important to the acceleration of the * To whom correspondence should be addressed. E-mail: kouji.sakaki@ aist.go.jp.
dehydrogenation process.6 This latter possibility focuses on the kinetics of hydrogen atoms. In situ X-ray diffraction (XRD) measurements during the dehydrogenation half-reaction revealed relatively sharp Bragg peaks for the Al phase with an fcc structure.7 This finding suggests that the crystallite size of the Al precipitate must be a few hundred nanometers. Electron microscopy studies have also shown that Al crystallites in the 100-300 nm size range are present after several cycles of dehydrogenation and rehydrogenation.8,9 Thus, the constituent elements diffuse long distances in the lattice in order to form precipitates. Kiyobayashi et al. have previously suggested that the rate-limiting step in the dehydrogenation is the long-range transport of heavy atoms.10 In other words, the dehydrogenation reactions are not expected to be limited by hydrogen diffusion but rather by Al and/or Na diffusion. Generally, atomic diffusion is dependent on vacancy concentration and mobility because atomic diffusion proceeds by the exchange of atoms and their neighboring vacancies. We propose that the dehydrogenation reactions are enhanced by vacancies that are created during dehydrogenation of Ti-doped NaAlH4. The positron annihilation technique is one of the best methods to observe vacancy formation and recovery. This technique utilizes the characteristic that positrons are drawn toward and trapped in open volume defects, such as dislocations and vacancies. We have previously utilizied the positron annihilation technique to detect vacancy formation during hydrogenation of LaNi5-based alloys and Pd11-13 as well as vacancy recovery during dehydrogenation in hydrogenated LaNi4.93Sn0.27.14 To clarify the dehydrogenation-rehydrogenation mechanism of Ti-doped NaAlH4, we have studied the change in microstructure, including lattice defects, in NaAlH4 by Ti doping and during cycling, by XRD and Doppler broadening measurements of the positron annihilation technique.
10.1021/jp100810u 2010 American Chemical Society Published on Web 03/12/2010
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II. Experimental Section A. Preparation of Samples. We prepared four varieties of NaAlH4: as-received, mechanically doped, wet-doped, and ballmilled NaAlH4 without TiCl3. NaAlH4 was obtained from Albemarle Corp. and TiCl3 was obtained from Aldrich Chemical Inc. Two mole percent of TiCl3 was doped into NaAlH4 by two processesswet doping and mechanically doping by ball-milling. The wet-doping was carried out in diethyl ether, according to the previous study reported by Bogdanovic et al.1 For the mechanically doped samples, NaAlH4 and 2 mol % TiCl3 were loaded into a tungsten carbide bowl (250 mL) with 10 mm diameter tungsten carbide balls. Ball-milling was performed by a Fritsch planetary mill for 30 min at a speed of 350 rpm. To investigate the effect of ball-milling on the microstructure, ballmilling of NaAlH4 without TiCl3 was also performed at the same conditions. The dehydrogenated and rehydrogenated samples were prepared as follows. Mechanically doped and wet-doped NaAlH4 were dehydrogenated by isothermal heating at 393 and 423 K under vacuum until the phase transformation from NaAlH4 to Na3AlH6 and Al was complete. The dehydrogenated mechanically doped sample was rehydrogenated at 10 MPa of hydrogen at 373 K. Furthermore, subsequent dehydrogenation and rehydrogenation were carried out up to five cycles at 393 K under vacuum for 24 or 16 h and 373 K at 10 MPa of hydrogen for 40 or 48 h, respectively. After each process, Doppler broadening spectra and XRD profiles were obtained. B. X-ray Diffraction Measurement. The samples were handled under an Ar atmosphere or vacuum, without any exposure to air, during XRD measurement. XRD data were collected at room temperature using a rotating Cu anode. The measurement conditions were 50 kV and 200 mA from 2θ ) 15° to 100° with a 0.02° step. The samples were contained in a sample holder sealed in an Ar atmosphere. The XRD data were analyzed by the Rietveld refinement program, RIETAN-2000,14-17 and lattice parameters and phase fractions of each phase were determined. A pseudo-Voigt function was used for expressing peak profiles. Mass fractions of each phase Wp were calculated from the scale factors obtained in the Rietveld refinement18
Wp )
spZpMpVp
∑ sjZjMjVj
(4)
j
where sp is the scale factor of the pth phase, Zp is the number of the chemical formula in the unit cell, Mp is the mass of the chemical formula, and Vp is the unit cell volume. From the obtained Wp, the fraction of each phase, fp, was recalculated to remove the contribution of the NaCl phase, which forms during Ti doping but has no contribution to the following dehydrogenation/rehydrogenation:
fp )
Wp 1 - WNaCl
(5)
In this paper, diffraction from only metal atoms was taken into account. C. Doppler Broadening Measurement.19 Positrons in materials annihilate with electrons, and then, two γ-rays are immediately emitted in opposite directions. The energy distribution of the emitted γ-rays is recorded as the Doppler broadening
Figure 1. Positron annihilation Doppler broadening spectra obtained in mechanically doped NaAlH4: (a) before dehydrogenation and (b) 1920 min heating.
spectrum. The obtained Doppler broadening spectra in the mechanically doped NaAlH4 are shown in Figure 1 as an example. The shape of the Doppler broadening spectrum depends on the momentum of annihilating electrons with positrons. Valence electrons contribute to the center part of the Doppler broadening spectrum because they have lower momentum than core electrons. When lattice defects are introduced, positrons are trapped around them and annihilate with more valence electrons than core electrons. In this case, the Doppler broadening spectrum becomes higher and narrower. The contribution of valence electrons to Doppler broadening spectrum is evaluated by the S parameter defined by the equation below. When lattice defects are introduced in a lattice, the fraction of annihilating valence electrons with positrons increases and the S parameter increases. The S parameter is defined as the area of the central lowmomentum part of the Doppler broadening spectrum divided by the area below the whole curve after background subtraction, which was performed as subtraction of a straight line
S)
central part of spectrum , whole spectrum central part of spectrum )
∫EE-E+E NDdE 0
0
s
s
(6)
where ND is intensity of the Doppler broadening spectra in each energy channel, E0 is the energy of the γ-ray for positron annihilation, and Es is the energy limit to estimate the S parameter. The interval limits are chosen symmetrically around E0, as E0 ( Es. The chosen interval limits are shown in Figure 1 and was 511 ( 0.65 keV for all the spectra in this study. The samples were handled under an Ar atmosphere or vacuum, without any exposure to air, during Doppler broadening measurement. 22NaCl (30 µCi), which was sealed with Kapton film, was used as the positron source and was put into the sample holder. Doppler broadening measurements were carried out under vacuum at room temperature using a Ge detector cooled with liquid nitrogen. The Doppler broadening spectra consisted of more than 106 positron annihilation events, and several spectra were obtained for all samples in order to ensure the reproducibility of the data. III. Results and Discussion A. Effect of Ti Doping into NaAlH4. The as-received and ball-milled NaAlH4 without TiCl3 materials were observed to be a single phase, whereas Ti-doped NaAlH4 was seen to contain small amounts of impurity phases, such as Al, NaCl, and/or Na3AlH6. The phase fractions of NaAlH4, evaluated by Rietveld
Dehydrogenation-Hydrogenation Cycling of Ti-Doped NaAlH4
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TABLE 1: Lattice Constants and S Parameters for Various As-Prepared NaAlH4 as-received mechanically doped wet-doped ball-milled
a (Å)
c (Å)
S parameter
5.0210(5) 5.0194(4) 5.0196(3) 5.0231(3)
11.342(1) 11.3430(8) 11.3417(8) 11.3510(7)
0.5065(44) 0.5067(19) 0.5054(22) 0.5082(21)
refinement of XRD profiles, were determined to be 98.4 and 96.1 mol % in wet-doped and mechanically doped NaAlH4, respectively. Table 1 shows lattice constants and S parameters evaluated from XRD data and Doppler broadening spectra. The lattice constants of the NaAlH4 in the four samples were a ) 5.0207 Å and c ) 11.3444 Å on average. The deviations from the average are smaller than 0.007 Å. Ti doping did not influence the lattice constants of NaAlH4, which agrees well with previous reports by synchrotron radiation powder XRD (a ) 5.0232 Å and c ) 11.3483 Å).5 No significant change was observed in the S parameter after ball-milling or Ti doping. This indicates that positron trapping sites, such as vacancies and dislocations, are not introduced into NaAlH4 by Ti doping. As mentioned above, Sun et al. observed a slight reduction in the lattice constants of NaAlH4 upon doping and suggested this resulted from vacancy formation.4 However, our present studies by XRD and Doppler broadening measurements do not support this finding and hypothesis. B. Dehydrogenation of Ti-Doped NaAlH4. To observe the microstructure change in Ti-doped NaAlH4 during dehydrogenation, the mechanically doped and wet-doped NaAlH4 were subjected to prolonged heating at 393 and 423 K under vacuum. Ti-doped NaAlH4 was transformed to Na3AlH6 and Al with hydrogen desorption, as shown in Figure 2. Figure 3 shows the changes in the phase fractions against the heating time. In mechanically doped NaAlH4, the phase fraction of NaAlH4 was decreased to 4.7 mol % upon heating for 960 min, while the phase fractions of Al and Na3AlH6 were increased. The lattice constants during dehydrogenation were determined to be a ) 5.0222 ( 0.0025 Å and c ) 11.3509 ( 0.0036 Å for NaAlH4; a ) 5.4096 ( 0.0025 Å, b ) 5.5333 ( 0.0028 Å, and c ) 7.7543 ( 0.0031 Å for Na3AlH6; and a ) 4.0475 ( 0.0019 Å for Al. The lattice constants for these phases did not change so much during dehydrogenation. The crystallite size of Al estimated by the Rietveld analysis as 100-300 nm did not change. This agrees with the reported values that were determined by electron microscopy.8,9 As shown in Figure 3b, the dehydrogenation of wet-doped NaAlH4 proceeded in a similar way to that of mechanically doped sample.
Figure 3. Phase fractions during dehydrogenation of Ti-doped NaAlH4: (a) mechanically doped NaAlH4 at 393 K and (b) wet-doped NaAlH4 at 423 K.
Figure 4. S parameters during dehydrogenation of Ti-doped NaAlH4: (a) mechanically doped NaAlH4 at 393 K and (b) wet-doped NaAlH4 at 423 K.
Figure 2. XRD profiles during dehydrogenation of mechanically doped NaAlH4 at 393 K.
Figure 4 shows the changes in the S parameter against the dehydrogenation time. During dehydrogenation from NaAlH4 to Na3AlH6 and Al, the S parameter increased from 0.5067 to 0.5590 in mechanically milled NaAlH4. This indicates that the
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positron annihilation site changed during dehydrogenation. There are two possible explanations for this phenomenon: (1) the change of the positron annihilation site from NaAlH4 to Na3AlH6 and/or Al without any formation of lattice defects during dehydrogenation, and (2) positron trapping at the lattice defects, such as vacancies, are formed in the dehydrogenated products. To verify the first hypothesis, which is the positron annihilation site changes to Na3AlH6 and/or Al without any formation of lattice defects, the Na3AlH6 was synthesized by ball-milling of NaAlH4 and NaH and then fully annealed Al powder at 723 K for 5 h was mixed into this synthesized Na3AlH6. In this verification, the effect of the positron annihilation site at the interstitial site was evaluated. It means that the effect of phase distribution and the formation of the lattice defects were not considered. Doppler broadening spectra were obtained for the synthesized Na3AlH6 and the mixture with Al powder. The synthesized Na3AlH6 had a lower S parameter, S ) 0.5198, than the material obtained from the dehydrogenation reaction after 1920 min (S ) 0.5590). The mixing of Al into Na3AlH6 decreased the S parameter from 0.5198 to 0.5047. Therefore, the increase in the S parameter during dehydrogenation cannot be explained by a change of positron annihilation sites from NaAlH4 to Na3AlH6 and/or Al without formation of lattice defects. Therefore, the increase in the S parameter must originate from the formation of the lattice defects during dehydrogenation. The S parameter increases with decreasing phase fraction of NaAlH4 and is still a higher value even when the phase fraction of NaAlH4 is below 5 mol % after 960 min of heating. In particular, as described below, the S parameter is still a higher value (S ) 0.5545) even when the phase fraction of NaAlH4 is around 1 mol % after 480 min of heating in the case of wetdoped NaAlH4. Therefore, NaAlH4 must not be related to the increase in the S parameter during dehydrogenation. NaH must not also have a relationship to the increase in the S parameter during dehydrogenation because the S parameter increases before the precipitation of NaH was observed in XRD profiles. Therefore, it is thought that the products arising from the dehydrogenation of NaAlH4 have much higher S parameters than pure NaAlH4. The formation of defects based on hydrogen vacancies, such as AlH6-x in Na3AlH6, was reported by Palumbo et al.6 Hydrogen vacancies are too small to be detected by the positron annihilation technique we used in this study. Therefore, the observed increase in the S parameter must be caused by the formation of metal atom vacancies in Na3AlH6 and/or Al. As shown in Figure 4b, the S parameter in wet-doped NaAlH4 increased from 0.5045 to 0.5545 during 480 min of reaction time, similar to the mechanically doped NaAlH4. No significant difference of the microstructure change during dehydrogenation was observed in mechanically doped and wet-doped NaAlH4 by positron and XRD techniques. These results show that vacancies arise in the products of dehydrogenation of doped NaAlH4 and the Ti doping method has no effect on the microstructure change that occurs during dehydrogenation. C. Rehydrogenation of Ti-Doped NaAlH4. The changes in the phase fractions and S parameters during rehydrogenation of the product mixture at 10 MPa of hydrogen and 373 K are shown in Figure 5. The final XRD profile clearly shows that the products are largely converted back to NaAlH4, although small amounts of unreacted Na3AlH6 and Al are present. The phase fraction for NaAlH4 increased, while the other phases decreased during rehydrogenation. The phase fractions for Al and NaH markedly decreased and that for NaAlH4 increased during the first 600 min of rehydrogenation. The S parameter decreased to 0.5161 over the first 600 min of rehydrogenation
Sakaki et al.
Figure 5. S parameters and phase fractions during rehydrogenation at 373 K at 10 MPa in mechanically doped NaAlH4: (a) S parameters and (b) phase fractions.
and then slightly decreased to 0.5125 at the end of the rehydrogenation reaction. This result shows that, even though at least the dehydrogenated products contain vacancies, there are no vacancies in the lattice of the NaAlH4 formed by rehydrogenation. The difference in S parameters of as-received NaAlH4 and the rehydrogenated NaAlH4 is attributed to the vacancies that are present in the small amount of unreacted dehydrogenation products. It is not possible that vacancies in the dehydrogenated products are eliminated from the crystal lattice by a thermal activation process because the rehydrogenation temperature, 373 K, is lower than the dehydrogenation temperature, 393 K. It is likely that the vacancy-containing phase itself is removed with vacancies by rehydrogenation. As mentioned previously, NaH and NaAlH4 phases have no contribution to the changes in the S parameter during dehydrogenation. However, the increase of the phase fraction of NaAlH4 was roughly consistent with the decrease of the S parameter. It suggests that at least the decomposed products had vacancies. The decrease in the phase fractions of Al during rehydrogenation over time matches the observed decrease in the S parameter. This result indicates that the vacancies are possibly present in the Al phase. On the other hand, the vacancy migration temperature in pure Al has been determined to be 275 K,20 indicating that vacancies in Al cannot settle down in a lattice in our experimental conditions. Although our studies do not directly identify Al as the species containing vacancies, we can conclude that at least vacancies are introduced into the dehydrogenated products of the first step of the dehydrogenation process, that is, Al and/or Na3AlH6. The coincidence Doppler broadening (CDB) method, which measures the two annihilation γ-rays using two detectors, can identify the elements around a vacancy. We plan to identify the vacancy-containing phase in the dehydrogenated products using the CDB method. D. Dehydrogenation and Rehydrogenation Cycling of TiDoped NaAlH4. Figure 6 shows the changes in the S parameters and XRD profiles during cyclic dehydrogenation-hydrogenation.
Dehydrogenation-Hydrogenation Cycling of Ti-Doped NaAlH4
J. Phys. Chem. C, Vol. 114, No. 14, 2010 6873 NaAlH4 does not create vacancies into a lattice prior to the dehydrogenation reaction, vacancies are introduced in the products arising from the dehydrogenation of Ti-doped NaAlH4 during dehydrogenation. During the rehydrogenation process, the products recombine to form vacancy-free NaAlH4. Furthermore, this reversible formation and recovery of vacancies occurs repeatedly during dehydrogenation-rehydrogenation cycling. Acknowledgment. A part of this research was financially supported by the Ministry of Economy Trade and Industry (METI) of Japan. A portion of this work was also supported by the New Energy and Industrial Technology Development Organization (NEDO) under “Advanced Fundamental Research Project on Hydrogen Storage Materials” and the Office of Hydrogen Fuel Cells and Infrastructure Technology of the U.S. Department of Energy. References and Notes
Figure 6. S parameters and XRD profiles during dehydrogenation and rehydrogenation cycles: (a) S parameters and (b) XRD profiles.
XRD profiles show that the dehydrogenated products are Na3AlH6, Al, and NaH and the species present following rehydrogenation are NaAlH4, Na3AlH6, and Al. The lattice constants of all phases did not change during the cycling. The S parameter reversibly increased to 0.56 during dehydrogenation and returned to 0.51 after rehydrogenation through five cycles. Thus, although vacancies are repeatedly introduced in the dehydrogenated products, they do not persist in the products of the rehydrogenation reaction. Our findings indicate that the repeated formation and recovery of vacancies during dehydrogenation and rehydrogenation must play an important role in the kinetic enhancement of the reversible reaction of NaAlH4. Apparently, the diffusion of constitutional elements during dehydrogenation and rehydrogenation proceeds by the exchange of positions between atoms and neighboring vacancies. IV. Conclusions We have investigated the phase transformations and the behavior of lattice defects during dehydrogenation and hydrogenation of Ti-doped NaAlH4 by using X-ray diffraction (XRD) and Doppler broadening measurements. Although Ti doping in
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JP100810U
