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The Dehydrogenation Reactions and Kinetics of 2LiBH4-Al Composite Yao Zhang,*,† Qifeng Tian,‡,§ Jian Zhang,† Shu-Sheng Liu,† and Li-Xian Sun† Materials and Thermochemistry Group, Lab of Catalysis and New Materials for Aerospace, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China, Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, China, and Hubei Key Laboratory of NoVel Reactor and Green Chemical Technology, Wuhan Institute of Technology, Wuhan 430073, China ReceiVed: April 29, 2009; ReVised Manuscript ReceiVed: September 8, 2009
The dehydrogenation reactions and kinetics of the 2LiBH4-Al composite were investigated by means of thermogravimetry allied with mass spectroscopy, differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis, and isothermal dehydrogenation measurements. According to the analysis of the experimental results, the dehydrogenation of 2LiBH4-Al can be roughly divided into three stages that range from 553 to 648 K, 648 to 773 K, and 773 to 823 K. In the initial stage, it was observed that the decomposition of LiBH4 occurs simultaneously with the formation of AlB2. It was also found that the 3D diffusion mechanism in the form of the Jander equation dominates the kinetics of this stage. The second stage is the major dehydrogenation process. The well-fitted curves of the isothermal dehydrogenation by the Prout-Tompkins equation prove that this stage is mainly dominated by an autocatalysis reaction. The suggested reaction 2LiBH4 + AlB2 f 2LiH + Al + 4B + 3H2 takes place simultaneously with the reaction 2LiBH4 + Al f 2LiH + AlB2 + 3H2. AlB2 and Al serve not only as products but also as reagents for the decomposition of LiBH4. Within this stage, the activation energy is remarkably reduced to 94 ( 5 kJ/mol from that of bulk LiBH4. At the third stage, when the temperature is above 798 K, LiAl can be formed due to the decomposition of LiH. This stage was still characterized by the autocatalysis reaction. 1. Introduction Hydrogen is an environmentally clean energy carrier, which could attain nearly zero emission of pollutants if it is reversibly produced and converted to electricity, for instance in fuel cells. Nevertheless, its safe storage and transportation are major problems, which hinder more widespread applications. Many solid-state materials were developed for hydrogen storage over the last few decades.1-3 However, only those lightweight compounds, such as alanates,4,5 borohydrides,6 and amides,7,8 might be able to meet the gravimetric target for onboard hydrogen storage devices designated by the U.S. Department of Energy.9 These complex hydrides, containing low-atomicweight alkali or alkaline-earth metal cations and alane [AlH4]-, amide [NH2]-, or borohydride [BH4]- anions, have a high gravimetric and volumetric hydrogen content. They are, at present, the most promising hydrogen storage materials for mobile applications based on hydrogen fuel cell techniques. LiBH4 is competitive among these materials owing to its large theoretical hydrogen storage capacity (18.5 wt %) and efficient capacity (13.8 wt %).10 However, people have to overcome the tough issues of both thermodynamics and kinetics before practical application is possible.11 Some additives are introduced to catalyze or destabilize LiBH4, for example, SiO2,11,12 transition-metal oxides,13,14 LiNH2,15-17 and MgH2.18-20 Recently, Aldoped LiBH4 has attracted considerable attention for its
enhanced performance of reversible hydrogen storage.21-24 The total reaction was suggested as follows:
2LiBH4 + Al f 2LiH + AlB2 + 3H2
(1)
On the basis of the above reaction, it is believed that the enhancement of the dehydrogenation is mainly due to the destabilization by Al.22 However, it has been also suspected that Al was hardly involved in the decomposition of LiBH4,24 assuming that the self-decomposition of LiBH4 initially takes place, followed by the yielded B combining with Al to form AlB2. Besides this, the kinetic behaviors during the dehydrogenation are still unclear. The present work tried to give a more definite answer to the reaction mechanisms and kinetics during the dehydrogenation process. Thermogravimetry allied with mass spectroscopy (TG-MS) was used to describe the thermal dehydrogenation profiles. X-ray diffraction (XRD) characterizations, Fourier transform infrared spectrometry (FTIR) analyses, and isothermal dehydrogenation measurements were carried out on the as-dehydrogenated samples to identify the product phases and deduce the reaction mechanisms. Isothermal dehydrogenation measurements at different temperatures (T ) 598, 623, 648, 673, 698, 723, 748, 773, and 798 K) were conducted to reveal the kinetic models corresponding to different stages of the dehydrogenation. 2. Experimental Section
* To whom correspondence should be addressed. Tel: 86-411-84379215. Fax: 86-411-84379213. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology. § Hubei Key Laboratory of Novel Reactor and Green Chemical Technology, Wuhan Institute of Technology.
Commercial LiBH4 (95%, Alfa Aesar) and Al (99%, Tianjin Delan) powders were directly used without further purification. The 2LiBH4-Al mixture was ground in a QM-1SP planetary ball mill for 20 h at a rate of 500 rpm. In each stainless milling pot (100 mL), the ball-to-powder weight ratio was 50:1 and
10.1021/jp903967y CCC: $40.75 2009 American Chemical Society Published on Web 09/29/2009
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Figure 1. SEM morphology and EDX element maps of the as-milled 2LiBH4-Al composite. (a) SEM morphology. (b) The enlarged image of (a). (c) The element map of boron for the image of (b). (d) The element map of Al for the image of (b).
the protection atmosphere was 0.1 MPa of Ar. All handlings of the sample were conducted in a glovebox (MBraun unilab) filled with high-purity Ar (99.9999%) and low-density H2O and O2 (both 0.99), as exhibited in Table 2. In Figure 7, one can observe the linear relationships for the measurements at 598 and 623 K. Many models, as summarized in Table 3, have also been tried on the measurements achieved at 598 K. However, the values of the correlation coefficient are much lower than the one obtained by using the Jander equation. It means that only a 3D diffusion mechanism can represent the dominant reactions in this stage. The poor fit of the sample at 648 K by the Jander equation is likely due to the transition between the first and the second stages. We have tried to fit the curve with several models listed in Table 3, but none can fit better than the Jander equation (R2 ) 0.871). At 648 K, the curves cannot be fitted with only the Jander model. At
Figure 7. Kinetic model and the activation energy for the 2LiBH4-Al composite in the first stage of dehydrogenation (598, 623, and 648 K).
TABLE 3: Kinetic Models Examined in the Isothermal Dehydrogenation Curves of 2LiBH4-Al Composite at 598 K symbol
reaction model
D1 D2 D3
one-dimensional diffusion two-dimensional diffusion Jander equation for three-dimensional diffusion Ginstling-Braunshtein equation for threedimensional diffusion Mampel unimolecular law two-dimensional phase boundary three-dimensional phase boundary Prout-Tompkins equation Avarami-Erofeev Avarami-Erofeev
D4 F1 R2 R3 A1 A2 A3
integral f(R) form
R2
R2 0.512 R + (1-R)ln(1-R) 0.626 1 [1-(1-R) /3]2 0.993 2
(1-2R/3)-(1-R) /3
0.638
-ln(1-R) 1 1-(1-R) /2
0.611 0.754
1
1-(1-R) /3
0.767
ln[a/(1-a)] 1 [-ln(1-R)] /2 1 [-ln(1-R)] /3
0.710 0.715 0.764
this temperature, it seems that the kinetics is driven by several mechanisms that could not be identified herein. In the second stage ranging from 598 to 773 K, however, the Jander equation in a 3-D diffusion model cannot describe the kinetics mechanism anymore. To select a proper model for the kinetics mechanism of the second stage, the comparison of the shapes of the plots of alpha (fraction of the product) against reduced time has been done. By plotting the experimental values of (t/t0.5)exp versus the theoretical ones (t/t0.5)theo, as suggested by Sharp et al.33 and Jone et al.,34 the reaction mechanism can be identified. Figure 8 shows the graph for the P-T equation. The (t/t0.5)theo values plotted against the (t/t0.5)exp ones exhibit a linear relationship with a slope of 1. This finding confirms that the bulk dehydrogenation reaction of the 2LiBH4-Al sample corresponds to the autocatalytic Prout-Tompkins model. The isothermal kinetics in Figure 9 can be satisfactorily fitted by the Prout-Tompkins equation35
TABLE 2: Kinetic Models and Parameters in Different Dehydrogenation Stages of the 2LiBH4-Al Composite temperature (K) 598 623 648 673 698 723 748 773 823
model
integral f(R) form
R2
rate constant k
D3
3D diffusion (Jander equation)
[1-(1-R) 3]2 + c
A1
Prout-Tompkins equation
ln[a/(1-a)] + c
A1
Prout-Tompkins equation
ln[a/(1-a)] + c
0.993 0.996 0.871 0.994 0.996 0.995 0.993 0.995 0.996
2.8 × 10-7 1.1 × 10-6 4.1 × 10-6 1.29 × 10-3 2.63 × 10-3 4.39 × 10-3 7.69 × 10-3 11.6 × 10-3 11.1 × 10-3
symbol
1/
activation energy (kJ/mol)
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k(t - t0) ) ln(R) - ln(1 - R) + c
Zhang et al.
(3)
The equation is in integration form, where k is the temperature-dependent rate constant. To avoid modeling “negative reaction time”, the Prout-Tompkins equation incorporated a term t0 into their equation, which relates to the location of the inflection point of the experimental curve and can be regarded as an induction period for the sigmoid curve. R in the equation denotes the fraction of the product measurable at time t. The integration constant c serves as a vertical offset for the plots of the P-T equation. A deviation of the fit from the experimental curves can be observed in Figure 9, especially for t < 2000 s. These deviations are attributed to the melting of LiBH4 during the dehydrogenation. Although the fusion occurs at around 550 K, it needs some times to be completely finished (e.g., t > 2000 s for the curves b, c, d, and e). Such a process can somewhat disturb the dehydrogenation kinetic curves. One may notice that the reaction fractions tend to final values of about 0.8, whereas the theoretical value is 1.0. The measurements of the isothermal dehydrogenation are performed in a closed system. Even though the initial state is nearly vacuum (100 Pa), the pressure in the chamber increases during the dehydrogenation and the final pressure can reach up to 0.1 MPa. This value can be close to the equilibrium pressure of the
Figure 10. XRD patterns for the 2LiBH4-Al composite obtained at different temperatures (673, 698, 723, 748, 773, 798, and 823 K).
remaining compounds to decompose and prevent their complete dehydrogenation. Furthermore, the dehydrogenation of the LiBH4-Al is very sensitive to the hydrogen pressure.24 That should be the reason why the rate of reaction is increasing close to a reaction completion but not 1.0. The modified Prout-Tompkins model has been previously applied in solid-fluid reactions and catalysis, such as the carbothermic reduction of nickel oxide.36 In this case, additives such as alkali metal carbonates melt and cover the metallic surface.37 If including the catalyst, calcium lignosulphonate, the system is actually in solid-liquid-gas phases because a solidgas reaction was also observed: CO2(g) + C(s) f 2CO(g). In this case, the kinetics was well-described by the modified P-T equation. That case is similar to the reaction studied herein. LiBH4 melts, first, covers the surface of Al, and subsequently desorbs, leading to a solid-liquid-gas (Al-LiBH4-H2) reaction. These cases demonstrate that the use of the P-T equation is not restrained to solid-state reactions. Several other solid-liquid or solid-gas systems were also described as autocatalysis reactions by modified P-T equations.37 The derivative form of eq 4 can be expressed as follows.
dR/dt ) kR(1 - R) Figure 8. Plot of (t/t0.5)theo vs (t/t0.5)exp calculated using the equations listed in Table 3 for the second stage of the isothermal dehydrogenation of 2LiBH4-Al.
Figure 9. Fitted isothermal curves by the Prout-Tompkins equation for the 2LiBH4-Al composite in the second stage of dehydrogenation (T ) 673, 698, 723, 748, and 773 K).
(4)
Equation 4 shows clearly the dependence of the rate on both the amount of reactant left and the amount of product formed, known as autocatalysis.37 For the purpose of understanding the autocatalytic process, it is necessary to study the reactions occurring in this stage. The phases observed by XRD analysis in Figure 10 can give some clues of the reaction pathway. Among these phases summarized in Table 1, LiH and B hardly facilitate the decomposition of LiBH4 because both are the decomposed products of LiBH4. Only Al and AlB2 are possibly involved in the decomposition of LiBH4. This suggests that two decomposition reactions might take place simultaneously, as expressed in eqs 5 and 6.
2LiBH4 + AlB2 f 2LiH + Al + 4B + 3H2
(5)
2LiBH4 + Al f 2LiH + AlB2 + 3H2
(6)
It can be seen in eqs 5 and 6 that both AlB2 and Al serve not only as products but also reagents in the dehydrogenation
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Figure 11. Fitted isothermal curve described by the Prout-Tompkins equation for the 2LiBH4-Al composite in the third stage of dehydrogenation (T ) 823 K).
process. Such behavior coincides well with the mechanism of the autocatalytic reaction, which usually emphasizes the involvement of the product as a reactive phase. In this case, both Al and AlB2 can be regarded as the reactive phases mutually participating in the LiBH4-decomposed reactions. By means of the Arrhenius equation, the dependency of ln(k) upon 1/T (as found in the inset of Figure 9) was plotted and the activation energy was calculated from the slope of the fitting line. The activation energy Ea (94 ( 5 kJ/mol) is drastically reduced from the previously reported value of LiBH4 (156 ( 20 kJ/mol),11 which exhibits a greatly enhanced kinetics in this stage. In the last stage of dehydrogenation (temperature above 798 K), the LiAl phase can be characterized in the XRD profile of the 2LiBH4-Al sample. Such a phenomenon probably results from the dissociation of LiH in the stage. It was deduced that LiAl comes from the reaction between LiH and Al, as shown in eq 7.
LiH + 2Al f 2LiAl + H2
(7)
The presence of Al seems to be beneficial to the destabilization of LiH, as mentioned by Vajo et al.38 in a LiH-Si system because the onset temperature is remarkably lower than that of the self-decomposition of LiH (around 873 K).26 The reflections of AlB2 weakened drastically in the XRD patterns with the increase of the dehydrogenation temperature from 798 to 823 K. Meanwhile, the reflections of LiBH4 disappeared in the XRD patterns as well. Such phenomena suggest that both LiBH4 and AlB2 can be thoroughly consumed in this stage. FTIR analysis still detected the B-B vibration in the as-dehydrogenated composites, indicating the formation of elemental B. On the basis of the above analyses, it is believed that the reactions relevant to LiBH4 in this stage are still in the pathways of eqs 5 and 6. In Figure 11, the isothermal dehydrogenation curve at 823 K can be fitted well using the P-T equation, as shown in eq 3 with an R2 ) 0.996[0]. This demonstrates that the same autocatalytic mechanism as in the second stage still dominates the final dehydrogenation stage. 4. Conclusions The present work investigated the reactions and kinetic behaviors in the 2LiBH4-Al composite by means of TG-MS,
DSC, XRD, FTIR analyses, and isothermal dehydrogenation measurements. According to the results, the dehydrogenation of 2LiBH4-Al can be divided into three stages: the temperature ranges from 553 to 648 K, 648 to 773 K, and 773 to 823 K. The first stage is a minor dehydrogenation stage, in which B and AlB2 are formed concurrently. Such observation differs from previous understanding that LiBH4 may decompose itself and, during a slower second step, form AlB2. The work suggested that the decomposition and the reaction of LiBH4 with Al occur simultaneously. The 3D diffusion mechanism in the form of the Jander equation dominates the kinetics in this stage. The second stage is the major dehydrogenation process. The satisfactory fits of the isothermal dehydrogenation curves by the Prout-Tompkins equation reflect that the stage is dominated by an autocatalytic reaction. The simultaneously occurring reactions are suggested as 2LiBH4 + AlB2 f 2LiH + Al + 4B + 3H2 and 2LiBH4 + Al f 2LiH + AlB2 + 3H2. The enhancement in the dehydrogenation kinetics is demonstrated by a remarkably reduced activation energy of 94 ( 5 kJ/mol. At the third stage, when the temperature is above 798 K, the reaction between LiH and Al would lead to the formation of LiAl. Both LiBH4 and AlB2 can be fully consumed. The autocatalysis reaction still dominates the dehydrogenation process. Acknowledgment. The authors would like to acknowledge the financial support received from the National Natural Science Foundations of China (Grant Nos. 50901070 and 20833009), the National High Technology Research and Development Program of China (Grant No. 2007AA05Z120), and the National Basic Research Program of China (Grant No. 2010CB631303). The authors are also grateful to Mr. Markus Karahka from Dalhousie University in Canada for his kind correction on the writing. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353–358. (2) Schu¨th, F.; Bogdanovic´, B.; Felderhoff, M. Chem. Commun. 2004, 2249–2258. (3) Bogdanovic´, B.; Ritter, A.; Spliethoff, B. Angew. Chem., Int. Ed. 1990, 29, 223–234. (4) Bogdanovic´, B.; Schwikardi, M. J. Alloys Compd. 1997, 253-254, 1–9. (5) Chen, J.; Kuriyama, N.; Xu, Q.; Takeshita, H. T.; Sakai, T. J. Phys. Chem. B 2001, 105, 11214–11220. (6) Muller, A.; Havre, L.; Mathey, F.; Petit, V. I.; Bensoam, J. U.S. Patent 4,193,978, 1980. (7) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002, 420, 302–304. (8) Xiong, Z. T.; Wu, G. T.; Hu, J. J.; Chen, P. AdV. Mater. 2004, 22, 1522–1525. (9) http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/ storage.pdf. (10) Zu¨ttel, A.; Wenger, P.; Rentsch, S.; Sudan, P.; Mauron, Ph.; Emmenegger, Ch. J. Power Sources 2003, 118, 1–7. (11) Zu¨ttel, A.; Rentsch, S.; Fischer, P.; Wenger, P.; Sudan, P.; Mauron, Ph.; Emmenegger, Ch. J. Alloys Compd. 2003, 356-357, 515–520. (12) Zhang, Y.; Zhang, W. S.; Fan, M. Q.; Liu, S. S.; Chu, H. L.; Zhang, Y. H.; Gao, X. Y.; Sun, L. X. J. Phys. Chem. C 2008, 112, 4005–4010. (13) Au, M.; Jurgensen, A. J. Phys. Chem. B 2006, 110, 7062–7067. (14) Au, M.; Jurgensen, A.; Zeigler, K. J. Phys. Chem. B. 2006, 110, 26482–26487. (15) Aoki, M.; Miwa, K.; Noritake, T.; Kitahara, G.; Nakamori, Y.; Orimo, S.; Towata, S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1409– 1412. (16) Meisner, G. P.; Scullin, M. L.; Balogh, M. P.; Pinkerton, F. E.; Meyer, M. L. J. Phys. Chem. B 2006, 110, 4186–4192. (17) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P. J. Phys. Chem. B 2006, 10, 7967–7974. (18) Vajo, J. J.; Skeith, S. L. J. Phys. Chem. B 2005, 109, 3719–3722.
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