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Catalytically Enhanced Hydrogen Storage Properties of Mg(NH2)2 + 2LiH Material by Graphite-Supported Ru Nanoparticles Lai-Peng Ma, Hong-Bin Dai, Yan Liang, Xiang-Dong Kang, Zhan-Zhao Fang, Pei-Jun Wang, Ping Wang,* and Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China ReceiVed: July 28, 2008; ReVised Manuscript ReceiVed: September 17, 2008
Identification of effective catalyst has been a subject of great interest and challenge in developing metal-N-H systems as potential hydrogen storage media. Motivated by the mechanistic understanding of the essential amide/imide conversion, we experimentally examined the possibility of N-H bond activation by using metal catalyst. We prepared the graphite-supported Ru nanoparticles (Ru/C catalyst) and evaluated their effect on the hydrogen storage properties of Mg(NH2)2 + 2LiH material. Our studies show that the Ru/C catalyst is catalytically active toward both dehydrogenation and rehydrogenation reactions of Mg(NH2)2 + 2LiH. Moreover, the catalytically enhanced hydrogen sorption kinetics persists well over 10 de/rehydrogenation cycles. Careful examination of the isothermal dehydrogenation behaviors suggests that the enhanced dehydrogenation kinetics may result from the Ru-catalyzed interface reaction between amide and imide solid phases. This is consistent with the Fourier transform infrared results, which show clearly the promoting effect of Ru catalyst on the N-H bond reconstruction. Finally, the catalytic mechanism of Ru catalyst on the reversible dehydrogenation reactions of Mg(NH2)2 + 2LiH material is discussed. 1. Introduction al.,1
light-metal Since the revolutionary discovery by Chen et amides and the “hybrid” materials containing both NH2- and BH4- (or AlH4-) anionic units have attracted extensive interest as potential hydrogen storage media.2-8 Among the numerous newly identified nitrogen-containing systems (metal-N-H systems), Mg(NH2)2 + 2LiH received the most extensive studies due to its combined advantages of favorable thermodynamics, good reversibility, and moderate hydrogen capacity.9-13
Mg(NH2)2 + 2LiH / Li2Mg(NH)2 + 2H2
(1)
The Mg(NH2)2 + 2LiH system undergoes reversible dehydrogenation/ rehydrogenation reactions following eq 1. Compared to the LiNH2 + LiH system, Mg(NH2)2 + 2LiH possesses a much lower dehydrogenation reaction enthalpy, just around 44 kJ mol-1 H2.1,9,13 Thermodynamically, it predicts a plateau pressure of 0.1 MPa at 90 °C. Experimentally, however, this system requires an operation temperature of above 200 °C for proceeding the dehydrogenation reaction at an appreciable rate. This clearly indicates that the dehydrogenation reaction of Mg(NH2)2 + 2LiH is kinetically limited.14-18 Therefore, identification of effective catalysts is essentially significant for improving the reversible dehydrogenation properties of the Mg(NH2)2 + 2LiH material. In contrast to the successful thermodynamic modification, the kinetic enhancement of metal-N-H systems lacks progress. According to the literature reports, no catalyst has been identified to be highly effective for the reversible dehydrogenation reactions of metal-N-H systems. Clearly, a better understanding of the dehydrogenation/rehydrogenation reaction chemistry may favor the efforts in the identification of effective catalysts. Two competitive models have been proposed to describe the * To whom correspondence should be addressed. E-mail: pingwang@ imr.ac.cn. Fax: +86 24 2389 1320.
dehydrogenation processes of the metal-N-H systems. Chen et al. proposed that the dehydrogenation reaction proceeded via coupling reactions between the amide and hydride solid phases, which contained the oppositely charged H+ and H- species.9,13,14 Alternatively, Ichikawa et al. suggested an ammonia (NH3)mediated mechanism: the amide phase first decomposed to yield ammonia (NH3), which was then “captured” by the hydride phase to release H2.5,19 These two mechanism models differ in the specific reaction pathway. However, fundamental understanding of the dehydrogenation reaction indicates that the amide/imide conversion should essentially involve the N-H bond activation as an elementary and crucial step, regardless of the specific reaction pathway.20,21 On the rehydrogenation aspect, the dissociative hydrogen chemisorption should play a crucial role in determining the rehydrogenation performance of the material. Motivated by these considerations, we focus on catalyst screening to catalytically enhance the reversible dehydrogenation of Mg(NH2)2 + 2LiH material. Notably, the knowledge accumulated in ammonia synthesis/decomposition industry provides us with critical reference in terms of catalyst screening. It has been well established that Ru-based catalysts possess pronounced catalytic activity for the activation of N-H bond and for the dissociative chemisorption of hydrogen molecule.22-26 In the present study, we prepared the graphite-supported Ru nanoparticles (referred as Ru/C catalyst hereinafter) and examined its effect on the hydrogen storage properties of Mg(NH2)2 + 2LiH material. As expected, the Ru/C catalyst exhibits pronounced effect on promoting the reversible dehydrogenation reaction of Mg(NH2)2 + 2LiH. Moreover, the kinetic enhancement arising upon adding Ru/C catalyst is well preserved during de/rehydrogenation cycling. We herein report these experimental findings.
10.1021/jp806680n CCC: $40.75 2008 American Chemical Society Published on Web 10/28/2008
Catalytically Enhanced Hydrogen Storage 2. Experimental Section 2.1. Preparation of Ru/C Catalyst. The Ru/C catalyst was prepared by using a modified impregnation method, which involved the self-assembled monolayer (SAM) technique for surface modification.27-29 SAM treatment: Graphite powder (99.9% purity) was first treated with a 98 wt % H2SO4/30 wt % H2O2 solution (3:1, v/v) at 85 °C for 30 min to produce the -OH terminated surface. The hydroxylated graphite powder was then exposed to an ethanol solution containing 8% (v/v) 3-aminopropyltrimethoxysilane (APTES, NH2(CH2)3Si(OC2H5)3) and heated to reflux for 4 h. After overnight aging, a self-assembled monolayer was formed and covalently bonded to the graphite surface. Impregnation: The organosilane functionalized graphite powder was immersed in a 0.1 M RuCl3 solution (pH ) 1, adjusted by the addition of 10 wt % HCl solution) and stirred magnetically for 30 min at room temperature. The coordination reaction between Ru3+ and -NH2 terminal group then produced a Ru(III) f NH2(CH2)3Si(O)3 complex at the graphite surface. The subsequent reduction treatment was carried out by dropping the above graphite powder into a solution of 0.4 M NaBH4 and stirred until the bubbles generation ceased, which generally required about 20 min. The redox reaction between Ru3+ and BH4- yielded Ru nanoparticles on the graphite support. In each step, the treated powder was separated from the solution by vacuum filtration, followed by thorough rinsing with deionized water and ethanol. Finally, the prepared Ru/C catalyst was dried at 40 °C under dynamic vacuum for 48 h. According to the ICP-AES analysis, the Ru loading on the graphite support was ∼3 wt %. 2.2. Preparation of Mg(NH2)2 + 2LiH Samples. The starting material LiNH2 (95%, Sigma-Aldrich Corp.) was used as received. MgH2 was prepared by mechanically milling Mg powder (purity: >99.9%) under 1 MPa hydrogen, followed by hydrogenation at 400 °C under 8 MPa hydrogen. This process was repeated three times to achieve a hydrogenation ratio of ∼95%, as determined by volumetric measurements. The Mg(NH2)2 + 2LiH samples were prepared via a two-step process: first, a 2LiNH2/MgH2 mixture was mechanically milled for 5 h, followed by treatment at 200 °C under 5 MPa hydrogen to allow for a conversion to the Mg(NH2)2+2LiH mixture; second, the Mg(NH2)2 + 2LiH mixtures with and without the Ru/C catalyst were then milled for another 5 h prior to the property examination. The mechanical milling was performed using a Fritsch P7 planetary mill at 400 rpm under Ar atmosphere. The ball-to-powder weight ratio was about 200:1. To minimize the temperature increment of the samples, the milling process was paused for 5 min every 10 min of milling. All the sample handling was performed in an Ar-filled glovebox (purity: 99.999%) equipped with a circulative purification system, in which the typical H2O/O2 levels were below 0.1 ppm. 2.3. Property Measurements and Structural Characterization. The hydrogen storage properties of the samples, with a typical amount of around 200 mg, were measured using volumetric method. Dehydrogenation/rehydrogenation kinetics were measured at desired temperatures with an initial hydrogen pressure of 0.1 and 5.0 MPa, respectively. The hydrogen supply (initial purity: 99.999%) was further purified by using a hydrogen storage alloy system to minimize the H2O/O2 contamination. Other than specified, the H-capacity was calculated using the weight of the samples containing additives to allow for an evaluation of the practical hydrogen storage property. Property examination indicated that the as-milled Mg(NH2)2 + 2LiH samples exhibited a dehydrogenation rate higher that of
J. Phys. Chem. C, Vol. 112, No. 46, 2008 18281 the following cycles. However, the dehydrogenation kinetics of the sample persisted well from the second cycle. To allow for a reliable evaluation of the dehydrogenation property, we selected the dehydrogenation curves starting from the second cycle for comparison. The samples in the hydrogenated state were also examined using synchronous thermal analyses (thermogravimetry/differential scanning calorimetry/mass spectroscopy, TG/DSC/MS, Netzsch STA 449C/QMS 403C). Approximately 2 mg of powder was loaded into the sample pan and heated at 5 °C /min under a flow of Ar gas (purity: 99.9999%). Special caution was taken to minimize the H2O/O2 contamination during sample transferring from the glovebox into the chamber. The Ru/C catalysts were characterized by using transition electron microscopy (TEM, JEOL 2010) equipped with an energy dispersive X-ray (EDX, Oxford) analysis unit, X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu KR radiation) and X-ray photoelectron spectroscopy (XPS, VG ESCALAB 250, Al KR X-ray source). In the XPS measurements, all of the binding energy values were referred to the C 1s peak (284.6 eV). Curve fitting was performed using nonliner least-squares fitting technique on Gaussion-Lorentzian sum functions provided by the XPSPEAK software. The composition of the catalyst was analyzed using inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Intrepid). The Mg(NH2)2 + 2LiH samples with Ru/C catalyst were analyzed using XRD and Fourier transform infrared spectrometer (FTIR, Bruker TENSOR 27, MCT detector) equipped with an in situ cell. All of the sample preparation was performed in the Ar-filled glovebox. FTIR spectra were collected in diffuse reflectance infrared Fourier transform (DRIFT) mode at a resolution of 4 cm-1. Peak area integration was performed using the OPUS spectroscopy software supplied by the manufacturer. 3. Results and Discussion 3.1. Catalyst Characterization. Figure 1 presents a typical TEM image of the Ru/C catalyst prepared by the SAM-modified impregnation method. According to the EDX results, the observed small particles with an average size of ∼5 nm are Ru nanoparticles. This is consistent with the XRD results, which show no diffraction peak of Ru (the results not shown here). The small amounts of silicon and oxygen were incorporated into the sample during the SAM treatment. The formation of uniformly dispersed Ru nanoparticles, particularly with a narrow size distribution, on the graphite surface should be attributed to the use of SAM technique. Additionally, the presence of coupling agent SAM between Ru and graphite may help stabilize the Ru nanoparticles.27-29 Such favorable morphology/structure characteristics lay an important foundation for exerting the catalytic function of the Ru catalyst. The Ru/C catalyst is further examined by XPS to characterize the bonding state of Ru. As shown in Figure 2, the predominant Ru 3d5/2 signal at 280.2 eV indicates that most Ru remains metallic even in the presence of SAM monolayer.30 The weak peak at 281.2 eV is assigned to a RuOx component, indicating a slight oxidation of Ru nanoparticles. 3.2. Reversible Dehydrogenation of the Ru-Catalyzed Mg(NH2)2 + 2LiH Material. Property examination shows that the Ru/C catalyst is effective for enhancing the hydrogen sorption kinetics of the Mg(NH2)2 + 2LiH material. Figure 3 compares the dehydrogenation kinetics curves of the neat sample and those with varying amounts (2-10 wt %) of catalysts. With increasing the addition amount of the Ru/C catalyst, the dehydrogenation performance of Mg(NH2)2 + 2LiH becomes
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Figure 3. Effect of the Ru/C catalyst on the dehydrogenation performance of the Mg(NH2)2 + 2LiH sample at (a) 180 and (b) 160 °C in the 2nd cycle.
Figure 1. TEM image of the as-prepared Ru/C catalyst, together with the representative EDX result.
Figure 2. XPS spectrum of the as-prepared Ru/C catalyst.
increasingly favorable. For example, as seen in Figure 3a, the neat sample requires 165 min to desorb 3 wt % hydrogen at 180 °C. For the samples containing 2 or 10 wt % Ru/C catalyst, however, release of the same amount of hydrogen is completed
within 130 and 90 min, respectively. The catalytic effect of Ru/C catalyst becomes more pronounced upon reducing the dehydrogenation temperature. As shown in Figure 3b, the neat sample desorbs only 1.3 wt % hydrogen within 300 min at 160 °C, whereas the sample containing 10 wt % Ru/C releases 2.5 wt % hydrogen over the same time interval, showing a nearly 2-fold increase in the average dehydrogenation rate. It is noteworthy that the catalytically enhanced kinetics arising upon adding Ru/C catalyst is achieved without penalty of the practical capacity of the materials. Particularly, the samples with catalyst show higher hydrogen capacity than the neat sample at 160 °C. Our comparative study demonstrates no detectable de/rehydrogenation for the sole Ru/C catalyst under the applied conditions. Therefore, this finding clearly indicates that the added catalyst activates more Mg(NH2)2 + 2LiH component to participate in the reversible dehydrogenation reaction. The addition of Ru/C catalyst also results in considerable enhancement on the rehydrogenation kinetics. This is evident from the increased hydrogen recharging amount with increasing the catalyst amount, as shown in Figure 4. Here, it should be noted that the use of high-pressure hydrogen troubles the accurate determination of rehydrogenation kinetics, as small temperature variation may cause considerable pressure fluctuation. We therefore determined the restored hydrogen amount by measuring the subsequent half-cycle of dehydrogenation. In this regard, plotting the restored hydrogen amount versus the discrete hydrogenation time (Figure 4) offers an indirect but
Catalytically Enhanced Hydrogen Storage
Figure 4. Effect of the Ru/C catalyst on the rehydrogenation performance of the Mg(NH2)2 + 2LiH sample in the 2nd cycle. The rehydrogenation was performed at 180 °C under an initial hydrogen pressure of 5 MPa.
Figure 5. Comparison of the dehydrogenation performance of the Mg(NH2)2 + 2LiH sample containing 10 wt % Ru/C catalyst in the 2nd and 10th cycles at (a) 180 and (b) 160 °C.
reliable way for the comparative rehydrogenation kinetics study. Currently, without definite evidence on the favorable thermodynamic modification, we temporarily ascribe the improved reversible dehydrogenation properties of Mg(NH2)2 + 2LiH sample to the catalytically enhanced kinetics arising upon adding Ru/C catalyst. Further study indicates that the hydrogen storage properties of the Mg(NH2)2 + 2LiH sample with Ru/C catalyst are highly stable during de/rehydrogenation cycling. As shown in Figure 5, the dehydrogenation curves of the sample containing 10 wt % Ru/C catalyst at the second and the 10th cycle are nearly identical at both 180 and 160 °C. The well persisted catalytic activity of the Ru/C catalyst may be partially ascribed to the stabilized nanostructure upon using SAM technique. The catalytic effect of Ru/C catalyst on the dehydrogenation reaction of the Mg(NH2)2 + 2LiH sample is further verified by the synchronous thermal analyses. As shown in Figure 6, increasing the Ru/C catalyst amount leads to a successively downward shift of the peak temperatures of dehydrogenation reaction. When the catalyst amount is increased to 10 wt %, the dehydrogenation peak temperature is reduced to 213 °C, about 14 °C lower than that of the neat sample. Notably, the DSC profiles of all the catalyzed samples are characterized by an asymmetrical feature with the appearance of a shoulder peak
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Figure 6. DSC/TG profiles of the rehydrogenated samples with and without Ru/C catalyst (ramping rate of 5 °C /min under Ar flow).
at the low-temperature side. Synchronous TG results indicate that more hydrogen is released at corresponding temperature. This is an important feature, as it provides a valuable hint in understanding the enhanced dehydrogenation reaction of the Rucatalyzed Mg(NH2)2 + 2LiH material. In the following section, this will be discussed in combination with the volumetric measurement results. Ammonia contamination of the hydrogen source is one of major concerns in the development of metal-N-H hydrogen storage systems. In the present study, we carefully examined the ammonia release during the dehydrogenation process by using synchronous TG/DSC/MS. Our study indicates that, for both the neat Mg(NH2)2 + 2LiH sample and those containing Ru/C catalyst, no ammonia can be detected within the detection limit of our MS apparatus throughout the heating process up to 275 °C. The high purity of the hydrogen released from the samples is further confirmed by the consistence of the results from parallel volumetric and gravimetric measurements. Presumably, the suppressed ammonia release should be ascribed to the combined effects of particle refinement, thorough mixing and close contact between Mg(NH2)2 and LiH particles in the intensively milled samples. Figure 7 presents the XRD patterns of the Mg(NH2)2 + 2LiH sample containing 10 wt % Ru/C catalyst in both the dehydrogenated and rehydrogenated states. Preliminary phase analysis demonstrates the reversible phase transformation between Mg(NH2)2/LiH and Li2Mg(NH)2. The addition of Ru/C catalyst leads to no appreciable change in the diffraction pattern of the host materials. 3.3. Mechanistic Study of the Ru-Catalyzed Mg(NH2)2 + 2LiH Material. The Ru/C catalyst used in the present study is a mixture containing ∼97 wt % graphite support and 3 wt % Ru nanoparticles. A parallel study of the Mg(NH2)2 + 2LiH sample with graphite additive showed that graphite had no effect on the reversible dehydrogenation kinetics of this system.17 Therefore, the observed property improvements arising upon adding Ru/C catalyst should be attributed to the catalytic effect of Ru nanoparticles. However, according to the comparative studies on the samples containing Ru/C and Ru powder additives (not shown here), the graphite support plays an important role in stabilizing the Ru nanoparticles, which is essential for maximizing the catalytic function of the Ru catalyst. Careful examination of the isothermal dehydrogenation profiles provides a valuable hint in understanding the dehydro-
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Figure 7. XRD patterns of the Mg(NH2)2 + 2LiH sample containing 10 wt % Ru/C catalyst in the 2nd cycle: (a) dehydrogenated and (b) rehydrogenated. A small amount of MgO was detected, which should primarily originate from the air contamination during XRD measurements.
genation behavior, as well as the catalytic function of Ru catalyst. As shown in Figure 3, panels a and b, a two-step dehydrogenation becomes increasingly obvious with increasing the catalyst amount, as outlined by regions A and B, respectively. This is consistent with the DSC/TG results, in which hydrogen release is enhanced at the low-temperature side of the peak reaction. Interestingly, varying the catalyst amount significantly influences the dehydrogenation rate in region A but has limited effect on that in region B, as manifested by the overlapping or parallel dehydrogenation curves at 180 and 160 °C, respectively. Such dehydrogenation behaviors suggest that the dehydrogenation may involve two kinetic processes: interface reaction and mass transport.13,20 At the initial stage, the dehydrogenation reaction is dominated by the interface reaction between Mg(NH2)2 and imide product, which generally follows a linear law between the reaction fraction and time (as observed in region A). This speculation is supported by the results of H-D isotopic exchange experiments.13 As the imide product layer grows, the barrier of mass transport through the interfacial layer finally surpasses that of the interface reaction. As a result, the diffusion of reactant species becomes the rate-determining step. At this stage, the time dependence of reaction fraction gradually deviates from the linear relationship. This can be seen more clearly in region B over an extended time scale (not shown here). The observed dependence of dehydrogenation rate on the catalyst amount in regions A and B suggests that Ru catalyst effectively reduces the energy barrier associated with the interface reaction, but contributes little to the mass transport of reactant species. In our further efforts to understand the catalytic function of Ru catalyst on the interface reaction of Mg(NH2)2 + 2LiH material, we examined the evolution of N-H bonds during dehydrogenation by using FTIR. Figure 8a-c compares the FTIR spectra of the samples with and without Ru/C catalyst at selected points of the dehydrogenation process. Compared to the neat sample, the Ru-catalyzed sample exhibits a more rapid intensity decrease in the characteristic N-H stretches of Mg(NH2)2 at 3272 and 3326 cm-1.13,16,18 Meanwhile, the broad vibration band of imide product gains more rapid intensity increase in the catalyzed sample. Particularly, the vibration at ∼3200 cm-1 is significantly enhanced relative to the neat sample. According to the literature report, this new vibration should be ascribed to the N-H stretch of a Mg-rich imide intermediate formed during amide/imide conversion.31 Here, we
Figure 8. FTIR spectra of the Mg(NH2)2 + 2LiH sample containing 2 wt % Ru/C catalyst (solid line) and the neat sample (dash line) collected at selected points of the dehydrogenation process (in the 2nd cycle) at 160 °C: (a) 30, (b) 60, and (c) 90 min. (d) Intensity ratio of N-H vibration between imide product and starting amide. The signal intensity of the sample with Ru/C catalyst has been normalized to that of the neat sample.
use the relative intensity change of the N-H bonds to denote the amide/imide conversion during the dehydrogenation reaction. As summarized in Figure 8d, the addition of Ru catalyst significantly accelerates the N-H bond reconstruction in the amide f imide conversion. The catalytic mechanism established for Ru-catalyzed NH3 dissociation provides an important reference in our mechanistic understanding of the Ru-catalyzed Mg(NH2)2 + 2LiH system. According to the structure characterization results, the facilitated NH3 dissociation involves the stepwise formation of NHx (NH2, NH, and N) species on Ru surface.32,33 Typically, the loss of the first H from NH3 is assumed to be rate-limiting. 22,23 This step proceeds via the chemisorption of NH3 followed by charge transfer from the Ru 4d orbital to the antibonding orbital of NH3 molecule. This greatly weakens the N-H bond, and results in the formation of Ru-NH2/Ru-H intermediates.22,25,34,35 In view of the high affinity of Ru toward NHx species, similar consideration may also apply for the present system. It is speculated that the Ru catalyst may develop interaction with the amide phase. As a result of the charge transfer between Ru and NH2-, the N-H bond of amide will be weakened, which facilitates the amide/imide conversion. In this regard, the applied high-energy milling may further promote the interaction between solid phases. On the rehydrogenation aspect, the pronounced effect of Ru catalyst on promoting the dissociative hydrogen chemisorption has been well established.26 As H2 molecule is adsorbed on Ru
Catalytically Enhanced Hydrogen Storage surface, electron is donated from the bonding orbital of H2 to the empty 4d orbital of Ru, which in turn donates 4d electron to the antibonding orbital of H2 molecule. The electron backdonation greatly promotes the dissociation of H2 molecule into mobile H atoms on Ru surface. At the initial hydrogenation stage, the dissociative H atoms locally bond with the component elements. The subsequently dissociated H atoms must diffuse away to proceed the hydrogenation reaction at adjacent regions, which may involve the “spillover” mechanism.36,37 For the first time, our study experimentally demonstrates that Ru catalyst is catalytically active toward the reversible dehydrogenation reactions of Mg(NH2)2 + 2LiH material. This finding offers a potential approach for addressing the kinetic limitation of metal-N-H systems. Furthermore, it may shed light on the reaction mechanism of amide/imide conversion. Currently, our ongoing efforts focus on the structure/composition optimization of metal-based catalysts. It may involve the use of high-specific-surface-area support materials to increase the catalyst loading, and the incorporation of alkaline promoter (e.g., K and Cs) to improve the intrinsic activity of the metal catalysts.24 4. Conclusions Our study shows that Ru catalyst is catalytically active toward the reversible dehydrogenation reactions of Mg(NH2)2 + 2LiH material. The Ru/C catalyst was prepared by using a modified impregnation method, in which the use of SAM technique facilitates the uniform dispersal of Ru nanoparticles on the graphite support, as well as the high catalyst-support adhesion. Compared to the neat Mg(NH2)2 + 2LiH, the samples containing Ru/C catalyst exhibit considerable enhancement on the dehydrogenation/rehydrogenation kinetics, particularly at lowered temperatures. Moreover, the catalytically enhanced kinetics arising upon adding Ru/C catalyst is highly stable during de/ rehydrogenation cycling. Combined property/structure examinations suggest that the enhanced dehydrogenation property of Mg(NH2)2 + 2LiH material results from the Ru-catalyzed interface reaction between the amide/imide solid phases. Specifically, it may originate from the facilitated N-H bond reconstruction by the Ru catalyst. These findings demonstrate the potential of using heterogeneous metal catalyst to catalytically enhance the hydrogen storage properties of metal-N-H systems. Furthermore, the observed catalytic activity of Ru catalyst toward the reversible dehydrogenation reactions of Mg(NH2)2 + 2LiH provides a valuable hint for the mechanistic understanding of the amide/imide conversion reactions. Acknowledgment. L.M. thanks Dr. Li-Chang Yin for helpful discussions. The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences, the National Natural Science Foundation of China (Grant Nos. 50571099, 50671107, and 50771094) and National High-Tech Research and Development Program of China (863 Program, Grant No. 2006AA05Z104) are gratefully acknowledged. References and Notes (1) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. Nature 2002, 420, 302–304.
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