Thermodynamics, Kinetics, and Modeling Investigation on the

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Thermodynamics, Kinetics, and Modeling Investigation on the Dehydrogenation of CeAl4-Doped NaAlH4 Hydrogen Storage Material Xiulin Fan, Xuezhang Xiao, Lixin Chen,* Leyuan Han, Shouquan Li, Hongwei Ge, and Qidong Wang Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China ABSTRACT: The CeAl4-doped NaAlH4 has been synthesized by mechanical milling NaH/Al mixture with 4 mol % CeAl4 as catalyst under hydrogen pressure of 3 MPa. The hydrogen desorption thermodynamics and kinetics of as-synthesized NaAlH4 were systematically investigated. The enthalpies for the first and second dehydrogenation steps of CeAl4-doped NaAlH4 system are estimated to be 40.56 ( 1.62 and 51.48 ( 1.92 kJ/mol H2, respectively. By regulating the desorption temperatures, the two dehydrogenation steps were studied separately under a constant hydrogen backpressure of 0.1 MPa. The apparent activation energy, Ea, for the first and second step is estimated to be 87.9 and 103.6 kJ/mol, respectively, by using Arrhenius equation. Isothermal dehydrogenation measurements show that no induction period is observed in the first step or the second step under the measuring conditions. Both of the decomposition steps conform to the JohnsonMehlAvrami (JMA) formalism with Avrami exponent n ≈ 1, indicating that the nucleation of decomposition process is of the site saturation type. Detailed modeling study presents that the firststep dehydrogenation kinetics is most likely controlled by the reaction at a moving boundary, whereas the second-step decomposition follows the first-order reaction mechanism. Change in the dehydrogenation temperature does not alter the nature of decomposition mechanism.

1. INTRODUCTION Complex hydrides have attracted much attention because of their potential for use as solid-state hydrogen storage materials, since Bogdanovic et al.1 discovered that NaAlH4 could reversibly store hydrogen at mild temperature in the presence of transition metal species, such as Ti or Zr. The NaAlH4 doped with selected additives can reversibly release and uptake hydrogen in the following two-step reactions, giving the material a theoretical hydrogen capacity of 5.6 wt % NaAlH4 T 1=3Na3 AlH6 þ 2=3Al þ H2 ð3:7 wt %Þ

ð1Þ

1=3Na3 AlH6 T NaH þ 1=3Al þ 1=2H2 ð1:9 wt %Þ

ð2Þ

Despite numerous studies on alanates, the solid-state decomposition in these systems is not totally understood, and the mechanism by which the dopants enhance the dehydrogenation of NaAlH4 remains an enigma.24 An understanding of the solidstate decomposition and the mechanistic role of the dopants would aid the development of alanates that are suitable for practical applications. From all previous experimental findings, three substantial features have been generally ascertained in the doped NaAlH4 system. One is that the high-valence transition-metal TMn+ of the dopants will be reduced and react with Al forming TM-Al species. During the past decade, several transition metal-Al species have been detected in the doped NaAlH4 during the doping process or the subsequent cycling, such as TiAl,517 FeAl,17,18 ZrAl,19 HfAl,20,21 VAl,21 LaAl,22,23 CeAl,2426 and ScAl27 species. Moreover, theoretical calculations revealed that some r 2011 American Chemical Society

transition metals, such as Ti, would react with the parent hydride and generate TiAl near-surface alloy on the aluminum surface,28 which can modify the properties of Al surface and promote the reactions. The observations were further supported by recent high-resolution TEM experimental studies,17 demonstrating that the transition metal-Al species was located at the surface of grains. The second feature is that during the process of dehydrogenation and rehydrogenation, long-range transport of Al will proceed. X-ray diffraction measurements of the decomposed materials exhibit sharp Bragg peaks for Al phase, indicating that the crystallite size of Al precipitate reaches a few hundred nanometers.5,29,30 Electron microscopy studies confirmed that Al crystallites in 100300 nm size range are present after several cycles of hydrogenation and dehydrogenation.3133 The last feature is the identification of highly mobile species of AlHx vacancies, which arise in the dehydrogenation and hydrogenation process and play a critical role in the large scale Al atom transfer.2,3438 These aforementioned observations broaden our knowledge on the doped NaAlH4 system, yet the decomposition kinetic model and the related rate-limiting step in the process are still controversial. Kiyobayashi et al.39 reported that at high temperature the decomposition of NaAlH4 followed a first-order type model, but at low temperatures, the model failed to predict the desorption behavior. Kircher and Fichtner40 analyzed the absorption and desorption data of Ti-doped NaAlH4 and concluded that the Received: September 6, 2011 Revised: October 7, 2011 Published: October 10, 2011 22680

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Figure 1. Pressurecomposition isotherms for the dehydrogenation of CeAl4-doped NaAlH4.

transformation kinetics of NaAlH4 is governed by nucleation and growth of the new phases. Using transition-state theory, Dather and Mainardi3 suggested that the rate-determining step for the decomposition of NaAlH4 is the hydrogen evolution from associated AlH3 species. By modeling the hydriding and dehydriding kinetics of the first two decomposition steps in NaAlH4, Yang et al.4 believed that the absorption kinetics is most likely controlled by reaction at a moving boundary, and the desorption kinetics is more complex and cannot be displayed by a single model. Recently, Lozano et al.41 developed an empirical kinetic model for the doped NaAlH4 system. The model shows that the first desorption step follows linear kinetics like a surface controlled reaction, whereas the second step follows the JMA model with the order n = 1. Li et al.42 studied the kinetics of the space-confined nano NaAlH4 and showed that the dehydrogenation process is controlled by multiple mechanisms rather than a single mechanism. The puzzle of the kinetics for the decomposition of NaAlH4 hindered the understanding of the decomposition mechanism and the development of more reliable complex alanates for hydrogen storage. In 2006, Bogdanovic et al.43 demonstrated that the CeCl3-doped NaAlH4 exhibits a superior kinetics compared with Ti-doped NaAlH4 and can be reloaded hydrogen at lower hydrogen pressures. Our recent investigation25 showed that CeAl species with a form of CeAl4 will come into being and may act as the active species in CeCl3-doped NaAlH4 system. Directly doping CeAl species will not generate byproducts in the system, and thus CeAl4-doped NaAlH4 gives a higher capacity in comparison with halide-doped material. In this work, we investigated the thermodynamics and kinetics on the decomposition of CeAl4-doped NaAlH4, with much attention paid to the decomposition kinetic models and rate-limiting step in the process, hoping to give a relatively clear description of the dehydrogenation process of CeAl4-doped NaAlH4 system.

2. EXPERIMENTAL SECTION Materials handling and sample preparation were performed in an argon-filled glovebox, where the oxygen and water concentration were kept below 1 ppm. Commercially available NaH (Aldrich, >95%, 99.9%, 100 nm, which is in the same size range as the in situ formed species in the chloride-doped NaAlH4 after several hydrogenation and dehydrogenation cycles.23 The homogeneous distribution of the catalyst and the morphology of the material after dehydrogenation strongly suggest that the decomposition process of the doped material complies with isotropic characteristics. Therefore, the mechanism of nucleation and growth along one-dimension can be basically excluded. To examine the mechanisms underlying the dehydrogenation process, we conducted kinetic modeling studies on CeAl4-doped NaAlH4. It is generally accepted in literature that thermally activated solid-state reactions can be described by a reaction rate dα/dt, which depends on the temperature, T, and the reaction fraction, α, in the following way dα ¼ kðTÞf ðαÞ dt

ð6Þ

Figure 9. Plots of kinetic modeling equation g(α) for reacted fraction versus time (the first step, 116 °C): (a) first-order mechanism, (b) twodimension phaseboundary mechanism, and (c) three-dimension phaseboundary mechanism.

integrating the above equation gives the integral rate law gðαÞ ¼ kðTÞt

ð7Þ

In the equation, α is reaction fraction ranging from 0 to 1, corresponding to the beginning and the completion of the reaction, t is the time, T is the absolute temperature, k(T) is a temperature-dependent reaction rate constant, and f(α) and g(α) are the reaction modeling equations related to the specific mechanism. 22684

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Figure 11. Time dependence of kinetic modeling equations g(α) for the decomposition of CeAl4-doped NaAlH4 at different temperatures: (a) the first-step dehydrogenation, two-dimension phaseboundary mechanism and (b) the second-tep dehydrogenation, the first-order mechanism.

Figure 10. Plots of kinetic modeling equation g(α) for reacted fraction versus time (the second step, 157 °C): (a) first-order mechanism, (b) two-dimension phaseboundary mechanism, and (c) three-dimension phaseboundary mechanism.

Kinetic data are fitted by the theoretical equation g(α) to identify the reaction mechanism. If the proposed mechanism was controlling the rates, then according to eq 7, a plot of g(α) versus t should be linear. The most common modeling equations consistent with the JMA Avrami exponent n ≈ 1 are listed in Table 3.53 The first-order reaction rate depends on the concentration of the reactant. The phase-boundary mechanisms assume

that nucleation occurs rapidly on the surface of the particles. The rate of decomposition is controlled by the resulting reaction interface progress toward the center of the particle. Depending on the particle shape and restriction of dimension, two- and three-dimension phase-boundary models can be derived. Figures 9 and 10 show the plots of modeling equations. For the first-step decomposition, the model of two-dimension phaseboundary mechanism versus time exhibits a good linearity at α < 0.84, implying that the isothermal decomposition process of the first step is controlled by the moving phase boundary mechanism under the experiment conditions. For the second step, the linearity of plotting ln(1  α) against the time t is achieved in the range α < 0.82, suggesting that the second-step decomposition conforms to the first-order reaction. The departure from the proposed mechanism when conversional fraction α is above 0.8 is due to the mutual impingement of regions transforming from separate nuclei, which must ultimately interfere with each other’s growth. Figure 11 presents the plots of proposed mechanism versus time for the two-step decomposition. It shows that all curves exhibit a good linearity with linear coefficient R2 > 0.99, implying that the reaction mechanisms do not change within the temperature range studied. 22685

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4. DISCUSSION Since the discovery of the dehydrogenation and rehydrogenation of doped NaAlH4 at moderate conditions, many researchers have attempted to unveil the mechanism of reaction. Among the dopants, the most common used are the Ti halides. Studies showed that these halides would react with the hydride and generate TiAl clusters.517 Our recent investigation showed that in CeCl3-doped NaAlH4 system, CeAl species come into being after cycling, which is similar to the Ti halides doped NaAlH4.25 Although it is believed that these TM-Al species play a significant role in catalyzing the reaction, the detailed mechanistic process and the rate-controlling step are not totally revealed. The above modeling study shows that the decomposition of NaAlH4 is controlled by the moving boundary model. This model assumes the reaction is first aroused at the surface of the particles. Previously, we have shown that the undoped NaH/Al composite cannot hydrogenate or dehydrogenate under the moderate conditions.44,55 It is believed that the decomposition of the NaAlH4 will first be triggered at the point of the catalyst on the surface of the hydride grains, the operation of which might be the splitting of the AlH bonds3 and simultaneously arouse the AlHx vacancies2 in the process. Then, the decomposition will proceed. It is believed that the generated Al atoms in the dehydrogenation process first nucleate around the CeAl clusters and grow into Al nanocrystallite. This process will alter the thermodynamics of the system by dilution of CeAl alloy and may even accelerate the reaction.44 As for the decomposition of NaAlH4, the motion of interface boundary requires long-range transport of atoms of Al species away from the growing regions so that we have to consider the two processes in the decomposition. One is the transformation of AlH bond (AlH4 f AlH63), which means that the hydride lattice has to be rearranged in the interface boundary. The other is the diffusion process, which leads to the Al segregation. Two extreme cases can be distinguished in principle. One is that the boundary moves very slowly. The rate of motion will then be largely independent of the diffusion rate, and the growth can be described as interface-controlled. The other extreme case is where the boundary is highly mobile when compared with the rate of diffusion so that it will move as rapidly as the required segregation can be accomplished. The reaction rate is then determined almost entirely by the diffusion conditions and can be recognized as diffusion controlled reaction. In general, the diffusion rate of the solid-state reaction is proportional to the magnitude of defects, which require considerable energies to be excited. Besides, on average, an atom will have to make many hundreds or thousands of atomic jumps in the parent phase and only one or two jumps in crossing the interphase boundary. Therefore, it is rather more likely that the solid-state reaction will be diffusion-controlled when the composition difference is appreciable. However, for the doped NaAlH4, our present study shows that the interphase boundary moving is the rate-controlling step. In other words, although long-range transport of Al will proceed in the decomposition process, it is not the limiting step in the decomposition process. Highly mobile species of AlHx aroused in the decomposition process plays a significant role in the long-range transport of Al and makes the Al segregation much easier,2,34,3638 resulting in the diffusion not the overriding restriction in the decomposition process. In this Article, we performed the modeling investigation on the dehydrogenation of NaAlH4, selecting the CeAl4 as the catalyst.

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The results shows the first-step decomposition of NaAlH4 is controlled by a moving boundary mechanism, which is in line with empirical model of TiCl4-doped NaAlH4 proposed by Lozano et al.41 The decomposition mechanism of moving boundary indicates that after doping the effective catalyst into NaAlH4, the kinetic barrier in the system will be lowered to a relatively constant value and independent of the catalyst. This mechanism may be able to explain the similarity of activation energy of NaAlH4 doped with different additives.44,5659 The distribution and the particle size of the catalyst may affect the decomposition rate of system via altering the pre-exponential rate factor A in Arrhenius equation.

5. CONCLUSIONS The hydrogen desorption thermodynamics and kinetics of NaAlH4 doped with 4 mol % CeAl4 have been systematically investigated. The enthalpy changes for the first and second dehydrogenation step were deduced from the slopes of van’t Hoff plots to be 40.56 ( 1.62 and 51.48 ( 1.92 kJ/mol H2, respectively. The second-step decomposition exhibits a higher stability, the dissociation pressure of which is ∼0.096 MPa at 120 °C. The isothermal kinetic study shows that no induction period is observed in the first step or the second step, even in relatively low temperatures. It is shown that both of the decomposition steps comply with the JohnsonMehlAvrami (JMA) formalism with Avrami exponent n ≈ 1. SEM micrographs show that the nanoparticles of CeAl4 catalyst are embedded on the surface of hydrides. All of the hydride grains exhibit equiaxed morphology, suggesting the decomposition follows isotropic characteristics. The detailed modeling study presents that the first-step dehydrogenation kinetics is most likely controlled by reaction at a moving boundary, whereas the second-step decomposition conforms to the first-order reaction mechanism. Change in the dehydrogenation temperature does not alter the nature of decomposition mechanism. ’ AUTHOR INFORMATION Corresponding Author

*Tel/Fax: +86 571 8795 1152. E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the financial support for this research from the National Basic Research Program of China (2007CB209701), from the National Natural Science Foundation of China (51171173, 50871099 and 51001090), and from the University Doctoral Foundation of the Ministry of Education (20090101110050). ’ REFERENCES (1) Bogdanovic, B.; Schwickardi, M. J. Alloys Compd. 1997, 253254, 1–9. (2) Sakaki, K.; Nakamura, Y.; Akiba, E.; Kuba, M. T.; Jensen, C. M. J. Phys. Chem. C 2010, 114, 6869–6873. (3) Dathar, G. K. P.; Mainardi, D. S. J. Phys. Chem. C 2010, 114, 8026–8031. (4) Yang, H.; Ojo, A.; Ogaro, P.; Goudy, A. J. J. Phys. Chem. C 2009, 113, 14512–14517. (5) Bogdanovic, B.; Felderhoff, M.; Germann, M.; H€artel, M.; Pommerin, A.; Sch€uth, F.; Weidenthaler, C.; Zibrowius, B. J. Alloys Compd. 2003, 350, 246–255. 22686

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