Enhanced HydridingâDehydriding Performance of CeAl2-Doped

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Enhanced Hydriding-Dehydriding Performance of CeAl2-Doped NaAlH4 and the Evolvement of Ce-Containing Species in the Cycling Xiulin Fan, Xuezhang Xiao, Lixin Chen,* Shouquan Li, Hongwei Ge, and Qidong Wang Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China

bS Supporting Information ABSTRACT: One of the major questions in a catalytically enhanced NaAlH4 system used for hydrogen storage that remains is where catalysts like Ti/Ce reside and present as what form improving the kinetics and reversible hydrogen storage performance. In the present study, by directly introducing Ce-Al species with a structure of CeAl2 into NaAlH4, a dramatic enhancement in the hydrogen release and uptake kinetics of NaAlH4 was achieved. CeAl2-doped NaAlH4 can be reloaded 4.9 wt % hydrogen at moderate conditions in 20 min, which is among the highest values ever reported for NaAlH4. Besides, the material exhibits an exceptional performance under low pressures. For example, a capacity of more than 4.0 wt % hydrogen can be achieved at a hydrogen pressure as low as 4.0 MPa. The apparent activation energy of NaAlH4 doped with 2 mol % CeAl2 is estimated to be 72.3-90.4 kJ/mol and 93.6-98.9 kJ/mol for the first and the second dehydrogenation step respectively by using Kissinger’s approach, much lower than those of pristine NaAlH4. After prolonged cycling, the Ce-Al species transforms to a more stable species of CeAl4. On the basis of these findings and the previous investigations, the active species and mechanism of catalysis in doped NaAlH4 were discussed.

1. INTRODUCTION Onboard hydrogen storage has been recognized as one of several scientific challenges in promoting hydrogen fuel cell-powered vehicles. As potential solid-state hydrogen storage materials, sodium alanate has offered a good perspective due to its high reversible hydrogen storage capacity and optimal thermodynamic stability for reversible hydrogen storage at medium temperatures,1-4 since Bogdanovic and Schwickardi demonstrated that transition-metal dopants can considerably lower kinetic barriers for both hydrogenation and dehydrogenation of NaAlH4.5 By doping a few mol % of selected transition metal halides, NaAlH4 can reversibly release and uptake hydrogen as described in the following two-step reactions, giving the material a theoretical hydrogen storage capacity of 5.6 wt %. NaAlH4 T 1=3Na3 AlH6 þ 2=3Al þ H2 ð3:7wt%Þ

ð1Þ

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

ð2Þ

Although the transition-metal halides can substantially improve the hydrogen release and uptake kinetics of NaAlH4, their addition reduces significantly the hydrogen capacity of the system due to the formation of Na halides as reaction byproducts between NaAlH4/NaH and halides during the milling process or cycling, which is a major drawback of these hydrogen storage materials.6,7 Therefore, there have been earnest efforts to find new effective catalysts that enhance the absorption/desorption kinetics while maintaining the hydrogen capacity at as high a level as possible.6 During the past decade, several transition-metal-Al species were detected in the doped NaAlH4 during the doping process or the subsequent cycling, such as Fe-Al alloy,8 Al-Ti alloy,9-19 r 2011 American Chemical Society

Al-Zr alloy,20 Al-Hf alloy,21,22 Al-V alloy,22 Al-La alloy.23 These alloys exhibit various forms of small disordered clusters, amorphous clusters, or crystalline transition-metal-Al at different experimental conditions. Extensive researches have indicated some relationship between these in situ formed transitionmetal-Al species and the catalysis of hydrogen uptake and release.10-14,21,23 However, to our knowledge, no experiments have definitely confirmed this relationship. Recently, we proposed that the in situ formed Ce-Al phase with a structure of CeAl4 might act as the active species in the CeCl3-doped NaAlH4, which exhibit similar kinetics to that of CeCl3 in the NaAlH4 system.24 However, whether other forms of Ce-Al species are also catalytically active for hydrogen release/uptake is still an open topic and needs to be investigated. Detailed explorations of these species are of great importance for the doped NaAlH4. First, in respect of the catalytic mechanism, by effectively doping these species into NaAlH4, the catalyzing capability can be systematically studied, which may provide in-depth understanding of the reaction mechanism and the development of a series of practical kinetics schemes. Second, on the utilitarian side, if these species are verified as the active species, which exhibit the catalytic effects much like the halides, the hydrogen capacity will be further increased for NaAlH4 system, which will make these systems much more suitable for practical applications. In this article, CeAl2-doped NaAlH4 was prepared using onestep direct synthesis method.25,26 To elucidate the role of Ce-Al species, the article will focus on the catalysis of CeAl2 on the Received: September 18, 2010 Revised: December 18, 2010 Published: January 19, 2011 2537

dx.doi.org/10.1021/jp1089382 | J. Phys. Chem. C 2011, 115, 2537–2543

The Journal of Physical Chemistry C

Figure 1. XRD patterns of the as-cast CeAl2 (a) and ball-milled CeAl2 (b).

Figure 2. SEM micrographs of the ball-milled CeAl2.

hydrogen release and uptake of NaAlH4 and the evolvement of Ce-Al species during prolonged cycling. By comparing the hydrogen release/uptake kinetics with that of CeCl3-doped NaAlH4 which possesses the state-of-art kinetics in the doped NaAlH4,26,27 we will show how the CeAl2 can improve the hydrogen release/uptake kinetics of NaAlH4. X-ray diffraction is applied to gain more insight into the transformations of Ce-Al species in the cycling. Moreover, in order to acquire detailed information about the kinetics of the reactions, nonisothermal Kissinger method28 is used to evaluate the activation energy. At the end of the article, on the basis of these findings and the previous investigations, the active species and the catalysis are discussed.

2. EXPERIMENTAL SECTION Materials handling and sample preparation were performed in an argon-filled glovebox, where the oxygen and water concentrations were kept below 1 ppm. Commercially available NaH (Aldrich, >95%, 99.9%, 99.5%) and CeH2 (Alfa Aesar, >99.5%) were used as received. The catalyst of the CeAl2 and CeAl4 were prepared by induction melting stoichiometric mixtures of pure Ce (99.9%) and Al (99.9%) metals in argon atmosphere. The as-prepared ingots were smashed and then mechanically milled for 20 h under 0.1 MPa of pure argon atmosphere by the Planetary mill (QM-3SP4J, Nanjing) at 300 rpm to prepare powder precursors. The powder mixtures of NaH, Al, and CeAl2 with a molar ratio of 1:1:0.02 were milled in the Planetary mill (QM-3SP4J, Nanjing) at 350 rpm under a hydrogen pressure of 3 MPa. Around 2 g of mixture was prepared each time. The vial is made of stainless steel and the volume is 120 mL. The ball is made of

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stainless steel with a diameter of 1 cm. The ball to powder weight ratio was about 35:1. The G-forces generated in the process of milling was about 8.57 g. For comparison, the powder mixtures of NaH, Al, and with 2 mol % other dopant (CeCl3, CeAl4, CeH2) and without the dopant were prepared in the same way. Dehydrogenation and rehydrogenation cycling of the prepared samples were carried out on a carefully calibrated Sievert’s type apparatus. The sample holder had a thermocouple located in the center of the sample to monitor temperature in the reaction zone. The dehydrogenation measurements proceeded against a constant pressure of 0.1 MPa. The rehydrogenation measurements were performed at 80-120 °C with different initial hydrogen pressures. After hydrogenation, the pressure decreased by about 0.40-1.00 MPa due to hydrogen uptake by the sample. It should be noted that the weight percent of hydrogen that is plotted versus time in the figures is calculated on the basis of the total weight of the samples including the weight of dopants. X-ray diffraction (XRD) experiments of the samples were performed on the ARL X0 TRA diffractometer (Thermo Electron Corp.) with Cu-KR radiation. To prevent the reaction with oxygen/moisture during XRD examination, the sample was covered with a special plastic tape layer, which has a negligible effect on diffraction patterns. The data were collected with a step width of 0.02° at a rate of 2.5°/min. The particle morphology and size distribution were analyzed with a scanning electron microscope (SEM, Hitachi S-4800). The DSC measurements were performed on a Netzsch STA 449F3 instrument, using a fully automated program for the evaluation of the DSC data. A sample of about 5 mg was tested using 0.1 MPa of argon as the purge gas with a rate of 40 mL/min. The preweighed sample was loaded into the aluminum pan and sealed with the aluminum lid inside the glovebox to prevent moisture and oxygen from getting into the sample during transfer.

3. RESULTS AND DISCUSSION 3.1. Characterization of CeAl2 Additive. Figure 1 shows the XRD patterns of as-cast and ball-milled CeAl2 precursors. For the as-cast CeAl2, all the detected peaks can be assigned to the diffraction of CeAl2 phase according to JCPDS 65-5379. The sharp and intense peaks indicate good crystallinity of CeAl2 alloy. However, after ball-milling for 20 h, these peaks changed greatly, exhibiting much lower intensity and a wide diffraction peak. Only two of the most intense peaks at 2θ = 31.4° and 37.0° are detectable, indicating the grain pulverization and even amorphousization happened during ball-milling. The particle morphology and size distribution of ball-milled CeAl2 are studied by SEM, as shown in Figure 2. The SEM micrographs show that the sample has a wide range of size: some smaller particles have 100-200 nm diameters, while some particles as large as 2 μm are also present. In higher magnification of SEM (part b of Figure 2), we could see that most of these larger particles are formed by agglomeration of the smaller ones. 3.2. Direct Synthesis of CeAl2-Doped NaAlH4 and its Hydriding-Dehydriding Performance. Direct synthesis of metal-doped NaAlH4 from NaH/Al and additives by ball-milling has been utilized as a novel method to prepare the hydrogen storage materials.26 This novel method not only further improves the kinetics of the rehydrogenation process of the material but also provides a perspective to probe and testify the nature of active metal species. 2538

dx.doi.org/10.1021/jp1089382 |J. Phys. Chem. C 2011, 115, 2537–2543


Figure 3. Selection of XRD patterns obtained during the synthesis of CeAl2-doped NaAlH4.

Figure 4. Typical comparison on the hydrogenation and dehydrogenation profiles between the synthesized CeCl3-doped NaAlH4 (a) and CeAl2-doped NaAlH4 (b).

During the synthesis of the CeAl2-doped NaAlH4, XRD data were collected with an interval of every 10 h to probe the species transition in the process. The selected patterns are shown in Figure 3. As milling time increases, the diffraction peaks of the starting materials become gradually broadened and weaker due to the reduction of their crystalline; meanwhile, the intermediate phase of Na3AlH6 shows up and becomes dominate after a milling time of about 40 h. After 100 h milling time, the diffraction peaks for Na3AlH6 become almost undetectable, signifying the direct synthesis of NaAlH4 neared completion. In a comparative investigation, we failed in our attempt to synthesize NaAlH4 just from the mixture of NaH/Al without any dopants under identical milling conditions. After 100 h milling time, only part of Na3AlH6 came into being (Figure S1 of the Supporting Information). Therefore, it can be concluded that the doped CeAl2 plays a critical role in the formation of NaAlH4. Figure 4 gives a typical comparison on the hydrogen uptake and release performance between the synthesized CeAl2-doped NaAlH4 and CeCl3-doped NaAlH4, respectively. To exclude the cycling effect on the hydrogenation and dehydrogenation, the hydrogen uptake data shown are all collected in the second cycle and the hydrogen release data are collected in the following third cycle. For hydrogen uptake, the materials were first heated to

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170 °C under vacuum to ensure all the hydrogen was released and then entailed under an initial hydrogen pressure of 12 MPa with temperature of 120 °C. It was found that these two materials exhibited quite similar hydrogen absorption kinetics. The hydrogenation time is about 20 min, with 90% hydrogen uptake in less than 10 min, which is among the best reported in the literature for the metal-doped NaAlH4. For hydrogen release, it also shows very pronounced kinetics; in the first 6 min, the materials can release about 70% of the hydrogen capacity, which is mainly evolved by the first step. It should be noted that the dehydrogenation measurements were carried out against a constant hydrogen pressure of 0.1 MPa. If this pressure is reduced, the release kinetics can be further improved, especially for dehydrogenation of the second step. As for the capacity, because the dopant of CeCl3 will consume the effective hydrogen storage component of NaH and Al and give rise to the byproduct of NaCl,24 CeAl2-doped NaAlH4 exhibits a much higher hydrogen capacity in comparison with that of CeCl3-doped NaAlH4. It is known that, for the CeCl3-doped NaAlH4, the catalyzing agent can be produced in situ during the ball-milling or the subsequent cycling, thus a much higher dispersion of catalyst can be achieved, which results in a dramatic catalytic enhancement on hydrogen release and uptake for NaAlH4.27 We believe that the pronounced kinetics for the CeAl2-doped NaAlH4 is related to the following two reasons. For one thing, it is shown that the Ce-Al species is an intrinsic catalyst for the hydrogenation of NaAlH4 in comparison with the undoped materials during the direct synthesis. For another, directly introducing the Ce-Al species into NaAlH4 system using one-step method will further facilitate the catalysis of Ce-Al species. In the direct hydrogenation process, the Ce-Al species will undergo a series of varied structural and chemical environments and may come into a favorable interaction with the host hydrides. For technical applications, the storage materials should be able to rehydrogenate at relatively low hydrogen pressures. Compared with TiCl3-doped NaAlH4, Bogdanovic et al.27 showed that CeCl3-doped NaAlH4 has a superior advantage, which can reload hydrogen at rather low hydrogen pressures. For CeAl2, high catalytic efficiencies were also found at low pressures, the results are shown in Figure 5. At a constant hydrogenation temperature of 120 °C, as the hydrogen pressure decreases, the rate of hydrogenation is reduced due to the proximity to the thermodynamic equilibrium. However, when the pressure decreases to 4 MPa and even less, the CeAl2-doped NaAlH4 exhibits a more improved kinetics compared with CeCl3-doped NaAlH4.27 This reason may be due to a more favorable interaction between the catalyzing agent and parent hydrides formed in the process of one-step synthesis. When the pressure is further reduced to 2.5 MPa, a capacity of 1.68 wt % is reached in 30 min, indicating that under this pressure only the first hydrogenation step can happen. Under a constant hydrogen pressure of 4 MPa, decreasing the hydrogenation temperature, a maximum of hydrogenation rate can be achieved at about 100 °C. At this temperature, a capacity of 4.0 wt % hydrogen can be uploaded in 230 min. Raising the hydrogenation temperature can improve the hydrogen uptake rate of the first hydrogenation step according to the Arrheniums law. Whereas, for the second hydrogenation step, the hydrogenation rate will be governed by two aspects. On the one hand, the thermodynamic driving force of hydrogenation will increase as the temperature decreases. On the other hand, the low temperature has adverse effects for the kinetics. Therefore, a compromise between the two aspects can be 2539



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Figure 6. Dehydrogenation curves of CeAl2-doped NaAlH4 with cycling. The circles with the corresponding numbers indicate the stages at which samples were taken for XRD analysis. All samples were previously hydrogenated at 120 °C under an initial hydrogen pressure of 12 MPa for 2 h. Solid line, hydrogen release; dashed line, sample temperature.

Figure 5. Hydrogenation curves of desorbed NaAlH4 sample doped with 2 mol % CeAl2 (a) at a constant temperature of 120 °C under different hydrogen pressures, (b) under a constant initial hydrogen pressure of 4 MPa at different temperatures.

reached with an optimum hydrogenation rate at a certain temperature, which is shown around 100 °C. To the best of our knowledge, such a pronounced property improvement has never been achieved in metal-doped NaAlH4 under such low pressure. Further study indicates that the catalytic enhancement arising upon adding Ce-Al species persists well in the following hydrogenation/dehydrogenation cycles. As shown in Figure 6, the recycled samples exhibited well maintained kinetics. After 40 cycles, only a slight capacity loss was observed compared to the second cycle. To understand the loss in hydrogen capacity and probe the phase transformations during cycling, the XRD analysis was performed. Figure 7 presents the XRD patterns of the CeAl2-doped NaAlH4 sample at varied states, which have been denoted in Figure 6. These results confirm the two-step dehydrogenation reactions of CeAl2-doped NaAlH4. Two distinct discrepancies can be observed in the XRD patterns between the second cycle and the 40th cycle. In respect of reversibility, the peaks of Na3AlH6 phase in B1 indicate the incomplete rehydrogenation of the second hydrogenation step after prolonged cycling, whereas, for A1, no peaks of Na3AlH6 can be found. This may qualitatively account for the capacity decrease in comparison with that of cycle 2. More interestingly, after prolonged cycling, a very broad peak appears at 2θ = 34°, which can be assigned to the overlap of (112) and (103) diffraction of the CeAl4 phase according to ICDD-JCPDS 65-2678. In the XRD patterns of cycle 2, no diffractions of CeAl4 were detected. Thus, it can be concluded that the CeAl4 phase is not generated

Figure 7. XRD patterns of CeAl2-doped NaAlH4 at different dehydrogenation stages.

in a single cycling, much probably formed gradually during cycling at the Al-rich conditions. However, this transformation shows no relevant differences in the dehydrogenation kinetics. The reaction can be assumed to be CeAl2 þ 2Al f CeAl4

ð3Þ

The observations indicate that, in the dehydrogenation process, one of the roles the Ce-Al species may play is acting as the nucleation centers and leading to an easy growth of Al crystallites, about the catalysis we will discuss in the following section. To acquire detailed information about the kinetics of the reactions, the apparent activation energy (Ea) related to the firstand the second-step dehydrogenations were calculated using the 2540



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Figure 8. Kissinger plots for the first and second dehydrogenation steps of CeAl2-doped NaAlH4. The powder histories are given on the graph.

Table 1. Apparent Activation Energies (Ea) for the Dehydrogenation of NaAlH4 (First Step) and Na3AlH6 (Second Step) Activation energy Ea (kJ/mol) NaAlH4

Na3AlH6

2 mol % CeAl2-as prepared

72.3 ( 3.8

98.9 ( 7.2

2 mol % CeAl2- 4th cycle

88.9 ( 2.4

93.6 ( 2.3

2 mol % CeAl2- 40th cycle 2 mol % CeCl3- 4th cycle

90.4 ( 2.4 85.6 ( 2.9

94.3 ( 2.7 97.3 ( 2.6

2 mol % CeAl4- 4th cycle

91.0 ( 2.3

93.0 ( 3.2

TiCl329,30

78.5-85.6

88.3-98.2

Ti(OBun)431

100 ( 7

99 ( 13

ScCl332

85.56-89.67

-

TiCl332

89.56-96.87

-

Undoped (pristine)29

118.1

120.7

Dopant

nonisothermal Kissinger method28 according to the following equation. d ln ðβ=Tm 2 Þ Ea ¼ d ð1=Tm Þ R

ð4Þ

Here, β, Tm, and R are the heating rate, the absolute temperature for the maximum desorption rate, and the gas constant. In this work, Tm was obtained using DSC measurement with the selected heating rates of 2, 3, 5, 8, 10 K/min. The detailed DSC curves are shown in Figures S2-S4 of the Supporting Information. Figure 8 plots the dependence of ln(β/Tm2) on 1/Tm. The intrinsic linearity of all of the curves indicates that the dehydrogenation kinetics of doped NaAlH4 is well represented by the nonisothermal Kissinger equation and they follow a first order decomposition reaction. The derived values of Ea are given in Table 1. For comparison, Ea of pristine NaAlH4 and NaAlH4 doped with CeCl3, CeAl4, and other additives are also recapitulated in Table 1. The DSC results and the nonisothermal Kissinger plots of NaAlH4 doped with CeCl3 or CeAl4 are shown in Figures S5-S7 of the Supporting Information. The comparison in the activation energy for dehydrogenation reveals several phenomena as follows: (1) All of the presented additives significantly enhance the dehydrogenation kinetics of NaAlH4 and Na3AlH6 (reducing Ea). (2) The first several cycles

Figure 9. Hydrogenation curves of 2 mol % CeH2-doped NaH/Al under 12 MPa at 120 °C.

can slightly increase the Ea of NaAlH4, but have little effect on the Ea of Na3AlH6. For the as-synthesized NaAlH4, the reduction of activation energy may be attributed to the particle size reduction and the catalysis of the Ce-Al species. Balde et al.33 showed that the Ea can be reduced by decreasing the particle sizes of NaAlH4. After several cycles, the size effects caused by long-time ballmilling will diminish, resulting in the slight increase of Ea. (3) Doping CeAl2, CeAl4, or CeCl3 brings about almost the same Ea for the first (86-91 kJ/mol) and second (93-97 kJ/mol) dehydrogenation. (4) Further comparison shows that doping the additives of Ce/Ti/Sc reduces the activation energy into the same range. For the first dehydrogenation step, it is in the range of 78.5-100 kJ/mol. For the second step, this range is 88.399 kJ/mol. Consider that the first several cycles exert some inferior effects on the Ea of the first step, if we evaluate the Ea in the same cycle, this range may be even narrow. In other words, it seems that the Ea is insensitive to the presented additives. About the third and fourth interesting phenomena, we will further discuss in the following section. 3.3. Active Species and the Catalysis Discussion. Identifying the nature of the active transition-metal species and understanding the rate-limiting steps in the decomposition of materials evolving hydrogen are two challenging tasks when studying complex metal hydrides as viable onboard hydrogen storage materials. Because the Ti compounds are the first group discovered as the effective dopants, much attention has been paid on the Tidoped NaAlH4. Up to now, several Ti-containing species are proposed as the active species, including Ti-Al clusters,10-14 Ti hydrides,34,35 and Ti cations in NaAlH4 lattice.36,37 However, none has been firmly confirmed. Recently, rare earth halides are also found to be very efficient in catalyzing hydrogenation/ dehydrogenation.23,26,27 By exploring and comparing the transformation of dopant species in different metal-doped NaAlH4, in-depth understanding may be achieved on the clarification of active species. Previously, Wang et al.34,35 proposed TiH2 as the active species in the metallic Ti-doped NaAlH4, which shows a catalytic effect on hydrogenation and dehydrogenation. To study this possible active species, the CeH2-doped NaAlH4 was investigated. We tried to synthesize the CeH2-doped NaAlH4 using one-step synthesis method, but failed within a milling time of 100 h, indicating that CeH2 exerts an inferior catalytic effect on 2541


The Journal of Physical Chemistry C hydrogenation of NaAlH4. The hydrogenation curves of ballmilled CeH2-doped NaH/Al are presented in Figure 9. In 6 h, a hydrogen capacity of 4.0 wt % can be reloaded, much inferior to the CeAl2-doped hydrides. For the halides-doped NaAlH4, it only takes about 20 min to complete the hydrogenation, whereas for the transition-metal hydrides (TiH2- or CeH2-) doped NaAlH4, no transition-metal hydride was detected.34 So, we can conclude that transition-metal hydride has some catalytic effects on the hydrogen release and uptake of NaAlH4, but the effects are rather limited. They may act like some group of activators in hydrogen spillover. These hydrides can split up the hydrogen molecules and may have some catalytic effects on the hydrogen atom diffusion into the desorbed NaAlH4. However, for the halide-doped NaAlH4, a number of theoretical calculations and supposed mechanisms suggest that diffusion of heavier hydrogen-containing species, such as AlHx or NaH, will proceed in H2 release and uptake38-42 and the classical catalytic effect of promoting the dissociation and the recombination of hydrogen molecules has been ruled out.40,43 On the basis of the phase analysis and the kinetic experiments, especially the much pronounced catalytic efficiencies obtained by directly introducing Ce-Al species of CeAl2 or CeAl424 into NaAlH4, we firmly believe that the Ce-Al species are the active catalyzing species during the process of hydrogenation and dehydrogenation in CeCl3-doped NaAlH4. These species may stay on the hydride surfaces or grain boundaries and serve as both the catalytic species in splitting hydrogen from AlH4/AlH3 groups and the initiators for Al nucleation sites during the dehydrogenation process in NaAlH4. This performance can be partially supported by the formation of CeAl4 after prolonged cycling in the CeAl2- and CeCl3-doped24 NaAlH4. The generated Al atoms in the dehydrogenation first nucleate around the Ce-Al clusters and grow into Al nanocrystallite. This process may even alter the thermodynamics of the system by dilution of alloy.44 During the rehydrogenation process, the Al atoms will recombine the NaH/Na3AlH6 to form NaAlH4. However, because of the affinity between the Ce and Al atoms, not all of the Al atoms reverse to NaAlH4, which causes the formation of CeAl4. The similarity of Ea indicates that all of the presented Ce-/Ti-/ Sc-doped materials may be governed by the same underlying mechanisms. Two scenarios may be able to explain this phenomenon. One possible explanation is that, after doping the presented additives, the rate-limiting step of dehydrogenation changes and has nothing to do with the catalysts. This scenario can be supported by the proposed mass-transport rate-limiting step,38-42 the calculated activation energy of which is 85 ( 2 kJ/mol,42 in excellent agreement with the measured activation energies in Ti/Ce/Scdoped NaAlH4. The other possibility is that the Ce, Ti, and Sc species act as the catalyst and reduce the major dehydrogenation barrier of NaAlH4 to similar values. This barrier may be the nucleation of product phases or the scission of the AlH4/AlH3 bond. Recently, Dathar et al.45 suggested that the hydrogen evolution from associated AlH3 species was the rate-determining step. The calculated the activation energy is about 82 kJ/mol, which is also in line with the presented Ea. To confirm and identify what reason causes this interesting result, more information is needed.

4. CONCLUSIONS The hydrogen storage properties of CeAl2-doped NaAlH4 were systematically studied. It was found that, by directly

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introducing CeAl2 into NaAlH4, the hydrogen uptake and release kinetics can be significantly improved with a much high hydrogen capacity. A hydrogen capacity of 4.9 wt % can be achieve in around 20 min under moderate conditions. Detailed investigations show that CeAl2-doped NaAlH4 also exhibits pronounced kinetics under low pressures and highly stable kinetics during cycling. Kinetic investigations revealed that the apparent activation energy for the first and the second dehydrogenation step was decreased from ∼118 kJ/mol and ∼120 kJ/mol for the pristine sample to 72.3-89 kJ/mol and ∼95 kJ/mol for CeAl2-doped NaAlH4, providing quantitative evidence for the decrease in the kinetic barriers during dehydrogenation. Previously, we have showed that Ce-Al species can be generated in the CeCl3-doped NaAlH4 and doping CeAl4 also result in pronounced catalytic kinetics.24 On the basis of these systematically kinetic investigations and the probing of Ce-containing species, it can be concluded that, by doping CeAl2 or CeCl3, it just provides a source of Ce-Al species, which probably exhibit as amorphous particles or nanoclusters and serve as a catalyzing agent in the process of hydrogen release and uptake. After prolonged cycling, both of the dopants will react with Al and transform to CeAl4, which is more stable in Al-rich conditions. These findings give a clear evidence that the in situ formed transition-metal-Al species play a crucial role in catalyzing the doped NaAlH4, on which it has been intensively explored and debated for 10 years. The very similar dehydrogenation Ea indicates that desorption reactions of the materials are determined by a similar mechanism and even probably by the same rate-limiting steps. However, why the catalytic abilities of Ce-Al species are so impressive, the understanding of atomic level and information of the local structure around Ce-Al clusters are still needed. On these unresolved puzzles, theoretical calculations and the designed experiments are in progress. Because NaAlH4 is the archetypical complex alanate for hydrogen storage, it is entirely possible that these species are also good catalysts for other alanates.

’ ASSOCIATED CONTENT

bS

Supporting Information. Figure of XRD patterns obtained during ball-milling NaH/Al mixtures without any dopants, and figures of DSC analysis of CeAl2-doped NaAlH4 at different heating rates to be used in the Kissinger equation. Figures of DSC analysis of CeCl3- and CeAl4-doped NaAlH4 and the corresponding Kissinger plots. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*To whom correspondence should be addressed. Tel./fax: þ86 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 and 2010CB631300), from the National Natural Science Foundation of China (51001090, 50871099, and 50631020), from the program for New Century Excellent Talents in Universities (NCET-07-0741), and from the University Doctoral Foundation of the Ministry of Education (20090101110050). 2542



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