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Catalytically enhanced dehydrogenation of Li–Mg–N–H hydrogen storage material by transition metal nitrides
Journal of Alloys and Compounds 468 (2009) L21–L24
Letter
Catalytically enhanced dehydrogenation of Li–Mg–N–H hydrogen storage material by transition metal nitrides Lai-Peng Ma, Ping Wang ∗ , Hong-Bin Dai, Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China Received 26 November 2007; received in revised form 22 December 2007; accepted 7 January 2008 Available online 4 March 2008
Abstract Motivated by the understanding of dehydrogenation/rehydrogenation reaction mechanisms, we examine the effect of two commercial transition metal nitrides, TaN and TiN, on the dehydrogenation performance of Li–Mg–N–H system. Both nitrides are catalytically active for accelerating the dehydrogenation reaction of this system. Such catalytic enhancement well persists without hydrogen capacity loss. A catalytic mechanism is proposed with reference to the catalytic model for hydrotreating reactions by transition metal nitrides. © 2008 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Nitride materials
1. Introduction In 2002, Chen et al. reported that Li3 N could reversibly store over 10 wt.% hydrogen [1]. This revolutionary discovery immediately stimulated worldwide interest in light metal amide/imide systems (i.e., Metal–N–H systems) as potential hydrogen storage media. As a result of extensive research efforts over past years, many new systems have been identified via composition modification. Among these, the most encouraging one is the Li–Mg–N–H system, which shows favorable thermodynamics [2–9]. Eq. (1) describes the reversible dehydrogenation/rehydrogenation reactions. 2LiNH2 + MgH2 → Li2 Mg(NH)2 + 2H2 Mg(NH2 )2 + 2LiH
(1)
As reported, simply replacing LiH with 1/2 MgH2 results in a favorable modification of enthalpy change for the dehydrogenation reaction: from around 60 kJ/mol H2 for LiNH2 /LiH system to around 40 kJ/mol H2 for 2LiNH2 /MgH2 system [1,3,9,10]. Thermodynamic calculation suggests a plateau pressure around 0.3 MPa at 100 ◦ C. Experimentally, however, dehydrogenation with an appreciable rate requires a temperature above 220 ◦ C at 0.1 MPa [3,8,10,11]. Although some encouraging improvements ∗
Corresponding author. Tel.: +86 24 2397 1622; fax: +86 24 2389 1320. E-mail address: [email protected] (P. Wang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.031
have been achieved in its kinetic modification [10–15], no catalyst is identified to be effective for activating the Li–Mg–N–H system. Clearly, a better understanding of the dehydrogenation/rehydrogenation mechanism of Metal–N–H systems will assist experimental effort to explore effective catalysts. In this regard, several mechanisms have been proposed to describe the dehydrogenation and/or rehydrogenation reaction pathways of the Li–N–H “model” system. Typically, the “solid phase reaction” mechanism suggests that the dehydrogenation reaction proceeds via direct solid phase reaction, which is driven by the strong interaction between the oppositely charged H+ and H− from respective LiNH2 and LiH [16]. Kinetically, this process is suggested to be controlled by the diffusion of reactant species across the solid interfaces [17]. In contrast, the “ammoniamediated” mechanism claims that a necessary “NH3 ” generation from LiNH2 and subsequent “NH3 ” capture by LiH is required to evolve hydrogen [18], which is further extended by a Li+ /H+ ion migration model based on the non-stoichiometric bulk reactions [19]. However, understanding of these models suggests that the reversible dehydrogenation/rehydrogenation reactions of Li–N–H “model” system essentially involve the activation of N–H and H–H bonds, regardless of the specific reaction pathway. This may act as a guide in screening catalyst effective for Metal–N–H systems. It is well known that transition metal nitrides are capable of catalyzing hydrotreating processes such as ammonia synthesis/decomposition,
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hydrodenitrogenation (HDN) and hydrogenation (HYD) reactions [20–23]. Their high catalytic activity towards N–H and H–H bonds suggests that transition metal nitrides may be the potential catalyst for metal amide/imide conversion. Here, we select two commercial transition metal nitrides, TaN and TiN, and investigate their catalytic effect on the dehydrogenation performance of Li–Mg–N–H system. As expected, both nitrides are catalytically active for accelerating the dehydrogenation reaction. 2. Experimental The starting material LiNH2 (95%, Sigma–Aldrich Corp.) was used as received. MgH2 was prepared by milling Mg powder (>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 >90%, as determined by volumetric measurements. Two additive-free Li–Mg–N–H samples were prepared. In sample 1, a 2LiNH2 /MgH2 mixture was mechanically milled for 5 h (Fritsch P7 planetary mill, 300 rpm, Ar atmosphere). In sample 2, a 2LiNH2 /MgH2 mixture was first mechanically milled for 5 h and then cycled one time, left in the hydrogenated state (nominally Mg(NH2 )2 + 2LiH, as indicated by Eq. (1)). During milling, a period of 5 min rest was preformed every 10 min to limit the temperature increment. The additives of TiN (99.8%, Alfa-Aesar Corp.), TaN (99.5%, Alfa-Aesar Corp.) and TiCl3 (99.999%, Sigma–Aldrich Corp.) were all pre-milled for 10 h before using. The Li–Mg–N–H samples with additives were prepared from the Mg(NH2 )2 + 2LiH mixture via a two-step process: first, a 2LiNH2 /MgH2 mixture was mechanically milled for 4 h under the conditions described above, followed by one de-/hydrogenation cycle into the Mg(NH2 )2 + 2LiH mixture; then, 10 wt.% pre-milled additive was added into this mixture and milled for 1 h. Thus, the total milling time remains the same for the samples with and without additives. All the sample handling was performed in an Ar (99.999%)-filled glove box equipped with a circulative purification system, in which the typical H2 O/O2 levels are below 0.1 ppm. Hydrogen storage properties of the samples, with a typical amount of around 200 mg, were examined using a custom-made Sievert-type apparatus. Dehydrogenation performance was measured at 200 ◦ C with an initial hydrogen backpressure of 0.1 MPa, followed by rehydrogenation at the same temperature under 5 MPa. The hydrogen supply (initial purity: 99.999%) was further purified by using a hydrogen storage alloy system to minimize H2 O/O2 contamination. Other than specified, we calculated the H-capacity using the weight of the samples containing additives to allow an evaluation of the practical hydrogen storage capacity. The samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu K␣ radiation). In the preparation of the samples for XRD examination, a small amount of grease was used to cover the sample surface to minimize H2 O/O2 contamination during measurement.
3. Results and discussion We first examine the additive-free Li–Mg–N–H sample to provide a reference for subsequent catalyst screening. Fig. 1 presents the dehydrogenation curves of samples 1 and 2. Note that the initial incubation-like sections in both curves do not reflect the intrinsic dehydrogenation feature, but an artifact resulting from the measuring method, in which the measurements started from the moment of pushing the reactor into the furnace set at a desired temperature. In this case, it takes 30 min for the sample inside reactor to reach 200 ◦ C. Moreover, no significant hydrogen release can be observed during this period. So the starting point for the determination of dehydrogenation time is selected to be 30 min. Clearly, sample 1 (primarily 2LiNH2 /MgH2 ) and sample 2 (Mg(NH2 )2 + 2LiH) exhibit distinct dehydrogenation kinetics. The average dehydrogenation
Fig. 1. Dehydrogenation curves of the as-milled 2LiNH2 /MgH2 mixture (sample 1) and the cycled 2LiNH2 /MgH2 mixture (sample 2) at 200 ◦ C and 0.1 MPa.
rate of sample 2 is about twice that of sample 1. For sample 2, the time required for 90% dehydrogenation stabilizes at 80 min, which increases slightly in the following cycles (results not shown here). Thus, in the subsequent examinations, the dehydrogenation curve of sample 2 in the second cycle is selected as that of the additive-free Li–Mg–N–H sample (i.e., Mg(NH2 )2 + 2LiH sample) for subsequent comparison. This activation-like behavior agrees well with previous result [7]. Probably, it results from the composition/structure rearrangement that occurs in the first irreversible dehydrogenation reaction in Eq. (1). As illustrated in Fig. 2, the dehydrogenation kinetics is assessed using volumetric measurement for the Li–Mg–N–H samples with TaN or TiN additive in the second cycle, which provides a detailed description of the dehydrogenation feature. For comparison, the dehydrogenation behavior of Li–Mg–N–H sample with TiCl3 additive is also included. Small additions of TiCl3 are reported to be effective for improving the desorption kinetics of Li–N–H system [18,24]. It is, therefore, a natural choice as an additive to promote the dehydrogenation of Li–Mg–N–H material [25]. However, our volumetric measurements in Fig. 2 clearly demonstrate that TiCl3 has little positive effect in this system. Although it enhances the dehydrogenation rate in the first cycle, the cyclic measurements indicate that its effect substantially diminish upon cycling. In fact, TiCl3 even results in a nearly 50% capacity loss from the second cycle, as shown in Fig. 2(a). Preliminary investigation suggests that this unusual capacity loss effect of TiCl3 should be attributed to a combination of the in situ formed chloride by-product, the consumption of reactant LiH and the subsequent deviation of stoichiometry between Mg(NH2 )2 and LiH. In contrast, TaN and TiN have favorable effect for the dehydrogenation of Li–Mg–N–H system. As seen in Fig. 2(b and c), the time required for 90% dehydrogenation is reduced from 80 min to around 60 min for the samples with TaN or TiN additive. More importantly, such enhancement is highly stable, without marked degradation between the second and third cycles. It is noteworthy that this kinetic enhancement is obtained without capacity loss compared to the additive-free sample. In
L.-P. Ma et al. / Journal of Alloys and Compounds 468 (2009) L21–L24
L23
Fig. 3. XRD patterns for the dehydrogenated Li–Mg–N–H samples (a) without additive, (b) with 10 wt.% TiN, (c) with 10 wt.% TaN. (Dehydrogenation was performed at 200 ◦ C and 0.1 MPa.)
Fig. 2. Dehydrogenation curves of Li–Mg–N–H samples with 10 wt.% additives: (a) TiCl3 , (b) TaN and (c) TiN at 200 ◦ C and 0.1 MPa in the second and third cycles.
fact, because the two metal nitrides are not expected to absorb hydrogen, this enhancement indicates that the addition of TaN or TiN activates around 10% more Li–Mg–N–H component for reversible hydrogen storage. Interestingly, the samples with 10 wt.% TaN or TiN additive exhibit nearly identical hydrogen storage performance. Nevertheless, TaN appears to be more
effective than TiN in view of its lower molar content. Without definite evidence on favorable thermodynamic modification, we attribute the accelerated desorption rate to merely catalytic enhancement arising upon adding TaN or TiN. Fig. 3 presents the XRD patterns of the Li–Mg–N–H samples with and without nitride additives after dehydrogenation measurement. From these results, neither newly formed phase nor appreciable change in the diffraction pattern of Li2 Mg(NH)2 can be identified in the samples with nitride additives, suggesting the high phase stability of TaN and TiN. This agrees well with the stable H-capacity in both samples during cycling. Furthermore, SEM examination indicates that the samples with or without additives exhibit similar particle size distribution. Because the phase and microstructure characterizations provide no valuable hint, we propose a possible interpretation for the present system with reference to the catalytic model for hydrotreating reactions by transition metal nitrides. For the dehydrogenation reaction of Li–Mg–N–H system, dissociation of N–H bond is required in the generation of the reactant species (i.e., H+ or NH3 species, depending on specific reaction pathway [16,19]). The high activation energy in the decomposition reaction of Mg(NH2 )2 demonstrates a strong N–H bond in the amide [9], suggesting that enhanced dissociation of N–H bond may facilitate the dehydrogenation reaction between Mg(NH2 )2 and LiH. In this regard, transition metal nitrides may be a promising catalyst. The transition metal nitrides typically possess N-deficient patch of metal atoms on the surface and edges, which are attributed to be active sites in hydrotreating reactions [20–23] and can be further enriched by high-energy ball milling (pre-milling treatment applied here). Presumably, the defective Metal–N pairs act as active sites in favor of dissociating the N–H bond of the amide in this system, which then promotes the generation of reactant species. 4. Conclusions Transition metal nitrides TaN and TiN are promising catalysts for the Li–Mg–N–H system. Both additives are catalytically
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active for accelerating the dehydrogenation of Li–Mg–N–H sample. Such kinetic enhancement persists well without hydrogen capacity loss. These results are consistent with our speculation about the catalytic activity of metal nitrides towards metal amide/imide conversion, thus highlighting the guide of mechanism understanding in screening effective catalysts for Metal–N–H systems. Acknowledgements We are grateful to Mr. Xue-Wen Wang for kindly providing TaN and TiN, to Mr. Zhan-Zhao Fang for his experimental assistance. The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant no. 50671107) are gratefully acknowledged. References [1] [2] [3] [4]
P. Chen, Z. Xiong, J. Luo, J. Lin, K. Tan, Nature 420 (2002) 302–304. Z. Xiong, G. Wu, J. Hu, P. Chen, Adv. Mater. 16 (2004) 1522–1525. W. Luo, J. Alloys Compd. 381 (2004) 284–287. Y. Nakamori, G. Kitahara, K. Miwa, S. Towata, S. Orimo, Appl. Phys. A 80 (2005) 1–3. [5] H. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe, H. Fujii, J. Phys. Chem. B 108 (2004) 8763–8765. [6] J. Rijssenbeek, Y. Gao, J. Hanson, Q. Huang, C. Jones, B. Toby, J. Alloys Compd. 454 (2008) 233–244. [7] Y. Chen, C.Z. Wu, P. Wang, H.M. Cheng, Int. J. Hydrogen Energy 31 (2006) 1236–1240.
[8] J. Yang, A. Sudik, C. Wolverton, J. Alloys Compd. 430 (2007) 334– 338. [9] P. Chen, Z. Xiong, L. Yang, G. Wu, W. Luo, J. Phys. Chem. B 110 (2006) 14221–14225. [10] A. Sudik, J. Yang, D. Halliday, C. Wolverton, J. Phys. Chem. C 111 (2007) 6568–6573. [11] S. Luo, T.B. Flanagan, W. Luo, J. Alloys Compd. 440 (2007) L13– L17. [12] Y. Liu, J. Hu, Z. Xiong, G. Wu, J. Mater. Res. 22 (2007) 1339–1345. [13] Y. Chen, P. Wang, C. Liu, H.M. Cheng, Int. J. Hydrogen Energy 32 (2007) 1262–1268. [14] G.J. Lewis, J.W.A. Sachtler, J.J. Low, D.A. Lesch, S.A. Faheem, P.M. Dosek, L.M. Knight, L. Halloran, C.M. Jensen, J. Yang, A. Sudik, D.J. Siegel, C. Wolverton, V. Ozolins, S. Zhang, J. Alloys Compd. 446–447 (2007) 355–359. [15] Z. Xiong, G. Wu, J.J. Hu, P. Chen, W. Luo, J. Wang, J. Alloys Compd. 417 (2006) 190–194. [16] P. Chen, Z. Xiong, J. Luo, J. Lin, K. Tan, J. Phys. Chem. B 107 (2003) 10967–10970. [17] K.F. Aguey-Zinsou, J. Yao, Z.X. Guo, J. Phys. Chem. B 111 (2007) 12531–12536. [18] T. Ichikawa, N. Hanada, S. Isobe, H. Leng, J. Fujii, J. Phys. Chem. B 108 (2004) 7887–7892. [19] W. David, M. Jones, D. Gregory, C. Jewell, S. Johnson, A. Walton, P. Edwards, J. Am. Chem. Soc. 129 (2007) 1594–1601. [20] G.W. Hadix, J.A. Reimer, A.T. Bell, J. Catal. 108 (1987) 50–57. [21] C.W. Colling, J.G. Choi, L.T. Thompson, J. Catal. 160 (1996) 35–42. [22] M. Nagai, Y. Goto, O. Uchino, S. Omi, Catal. Today 43 (1998) 249– 259. [23] E. Furimsky, Appl. Catal. A 240 (2003) 1–28. [24] T. Ichikawa, N. Hanada, S. Isobe, H.Y. Leng, H. Fujii, J. Alloys Compd. 404–406 (2005) 338–435. [25] W. Lohstroh, M. Fichtner, J. Alloys Compd. 446–447 (2007) 332–335.
Letter
Catalytically enhanced dehydrogenation of Li–Mg–N–H hydrogen storage material by transition metal nitrides Lai-Peng Ma, Ping Wang ∗ , Hong-Bin Dai, Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China Received 26 November 2007; received in revised form 22 December 2007; accepted 7 January 2008 Available online 4 March 2008
Abstract Motivated by the understanding of dehydrogenation/rehydrogenation reaction mechanisms, we examine the effect of two commercial transition metal nitrides, TaN and TiN, on the dehydrogenation performance of Li–Mg–N–H system. Both nitrides are catalytically active for accelerating the dehydrogenation reaction of this system. Such catalytic enhancement well persists without hydrogen capacity loss. A catalytic mechanism is proposed with reference to the catalytic model for hydrotreating reactions by transition metal nitrides. © 2008 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Nitride materials
1. Introduction In 2002, Chen et al. reported that Li3 N could reversibly store over 10 wt.% hydrogen [1]. This revolutionary discovery immediately stimulated worldwide interest in light metal amide/imide systems (i.e., Metal–N–H systems) as potential hydrogen storage media. As a result of extensive research efforts over past years, many new systems have been identified via composition modification. Among these, the most encouraging one is the Li–Mg–N–H system, which shows favorable thermodynamics [2–9]. Eq. (1) describes the reversible dehydrogenation/rehydrogenation reactions. 2LiNH2 + MgH2 → Li2 Mg(NH)2 + 2H2 Mg(NH2 )2 + 2LiH
(1)
As reported, simply replacing LiH with 1/2 MgH2 results in a favorable modification of enthalpy change for the dehydrogenation reaction: from around 60 kJ/mol H2 for LiNH2 /LiH system to around 40 kJ/mol H2 for 2LiNH2 /MgH2 system [1,3,9,10]. Thermodynamic calculation suggests a plateau pressure around 0.3 MPa at 100 ◦ C. Experimentally, however, dehydrogenation with an appreciable rate requires a temperature above 220 ◦ C at 0.1 MPa [3,8,10,11]. Although some encouraging improvements ∗
Corresponding author. Tel.: +86 24 2397 1622; fax: +86 24 2389 1320. E-mail address: [email protected] (P. Wang).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.01.031
have been achieved in its kinetic modification [10–15], no catalyst is identified to be effective for activating the Li–Mg–N–H system. Clearly, a better understanding of the dehydrogenation/rehydrogenation mechanism of Metal–N–H systems will assist experimental effort to explore effective catalysts. In this regard, several mechanisms have been proposed to describe the dehydrogenation and/or rehydrogenation reaction pathways of the Li–N–H “model” system. Typically, the “solid phase reaction” mechanism suggests that the dehydrogenation reaction proceeds via direct solid phase reaction, which is driven by the strong interaction between the oppositely charged H+ and H− from respective LiNH2 and LiH [16]. Kinetically, this process is suggested to be controlled by the diffusion of reactant species across the solid interfaces [17]. In contrast, the “ammoniamediated” mechanism claims that a necessary “NH3 ” generation from LiNH2 and subsequent “NH3 ” capture by LiH is required to evolve hydrogen [18], which is further extended by a Li+ /H+ ion migration model based on the non-stoichiometric bulk reactions [19]. However, understanding of these models suggests that the reversible dehydrogenation/rehydrogenation reactions of Li–N–H “model” system essentially involve the activation of N–H and H–H bonds, regardless of the specific reaction pathway. This may act as a guide in screening catalyst effective for Metal–N–H systems. It is well known that transition metal nitrides are capable of catalyzing hydrotreating processes such as ammonia synthesis/decomposition,
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hydrodenitrogenation (HDN) and hydrogenation (HYD) reactions [20–23]. Their high catalytic activity towards N–H and H–H bonds suggests that transition metal nitrides may be the potential catalyst for metal amide/imide conversion. Here, we select two commercial transition metal nitrides, TaN and TiN, and investigate their catalytic effect on the dehydrogenation performance of Li–Mg–N–H system. As expected, both nitrides are catalytically active for accelerating the dehydrogenation reaction. 2. Experimental The starting material LiNH2 (95%, Sigma–Aldrich Corp.) was used as received. MgH2 was prepared by milling Mg powder (>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 >90%, as determined by volumetric measurements. Two additive-free Li–Mg–N–H samples were prepared. In sample 1, a 2LiNH2 /MgH2 mixture was mechanically milled for 5 h (Fritsch P7 planetary mill, 300 rpm, Ar atmosphere). In sample 2, a 2LiNH2 /MgH2 mixture was first mechanically milled for 5 h and then cycled one time, left in the hydrogenated state (nominally Mg(NH2 )2 + 2LiH, as indicated by Eq. (1)). During milling, a period of 5 min rest was preformed every 10 min to limit the temperature increment. The additives of TiN (99.8%, Alfa-Aesar Corp.), TaN (99.5%, Alfa-Aesar Corp.) and TiCl3 (99.999%, Sigma–Aldrich Corp.) were all pre-milled for 10 h before using. The Li–Mg–N–H samples with additives were prepared from the Mg(NH2 )2 + 2LiH mixture via a two-step process: first, a 2LiNH2 /MgH2 mixture was mechanically milled for 4 h under the conditions described above, followed by one de-/hydrogenation cycle into the Mg(NH2 )2 + 2LiH mixture; then, 10 wt.% pre-milled additive was added into this mixture and milled for 1 h. Thus, the total milling time remains the same for the samples with and without additives. All the sample handling was performed in an Ar (99.999%)-filled glove box equipped with a circulative purification system, in which the typical H2 O/O2 levels are below 0.1 ppm. Hydrogen storage properties of the samples, with a typical amount of around 200 mg, were examined using a custom-made Sievert-type apparatus. Dehydrogenation performance was measured at 200 ◦ C with an initial hydrogen backpressure of 0.1 MPa, followed by rehydrogenation at the same temperature under 5 MPa. The hydrogen supply (initial purity: 99.999%) was further purified by using a hydrogen storage alloy system to minimize H2 O/O2 contamination. Other than specified, we calculated the H-capacity using the weight of the samples containing additives to allow an evaluation of the practical hydrogen storage capacity. The samples were characterized using X-ray diffraction (XRD, Rigaku D/MAX-2500, Cu K␣ radiation). In the preparation of the samples for XRD examination, a small amount of grease was used to cover the sample surface to minimize H2 O/O2 contamination during measurement.
3. Results and discussion We first examine the additive-free Li–Mg–N–H sample to provide a reference for subsequent catalyst screening. Fig. 1 presents the dehydrogenation curves of samples 1 and 2. Note that the initial incubation-like sections in both curves do not reflect the intrinsic dehydrogenation feature, but an artifact resulting from the measuring method, in which the measurements started from the moment of pushing the reactor into the furnace set at a desired temperature. In this case, it takes 30 min for the sample inside reactor to reach 200 ◦ C. Moreover, no significant hydrogen release can be observed during this period. So the starting point for the determination of dehydrogenation time is selected to be 30 min. Clearly, sample 1 (primarily 2LiNH2 /MgH2 ) and sample 2 (Mg(NH2 )2 + 2LiH) exhibit distinct dehydrogenation kinetics. The average dehydrogenation
Fig. 1. Dehydrogenation curves of the as-milled 2LiNH2 /MgH2 mixture (sample 1) and the cycled 2LiNH2 /MgH2 mixture (sample 2) at 200 ◦ C and 0.1 MPa.
rate of sample 2 is about twice that of sample 1. For sample 2, the time required for 90% dehydrogenation stabilizes at 80 min, which increases slightly in the following cycles (results not shown here). Thus, in the subsequent examinations, the dehydrogenation curve of sample 2 in the second cycle is selected as that of the additive-free Li–Mg–N–H sample (i.e., Mg(NH2 )2 + 2LiH sample) for subsequent comparison. This activation-like behavior agrees well with previous result [7]. Probably, it results from the composition/structure rearrangement that occurs in the first irreversible dehydrogenation reaction in Eq. (1). As illustrated in Fig. 2, the dehydrogenation kinetics is assessed using volumetric measurement for the Li–Mg–N–H samples with TaN or TiN additive in the second cycle, which provides a detailed description of the dehydrogenation feature. For comparison, the dehydrogenation behavior of Li–Mg–N–H sample with TiCl3 additive is also included. Small additions of TiCl3 are reported to be effective for improving the desorption kinetics of Li–N–H system [18,24]. It is, therefore, a natural choice as an additive to promote the dehydrogenation of Li–Mg–N–H material [25]. However, our volumetric measurements in Fig. 2 clearly demonstrate that TiCl3 has little positive effect in this system. Although it enhances the dehydrogenation rate in the first cycle, the cyclic measurements indicate that its effect substantially diminish upon cycling. In fact, TiCl3 even results in a nearly 50% capacity loss from the second cycle, as shown in Fig. 2(a). Preliminary investigation suggests that this unusual capacity loss effect of TiCl3 should be attributed to a combination of the in situ formed chloride by-product, the consumption of reactant LiH and the subsequent deviation of stoichiometry between Mg(NH2 )2 and LiH. In contrast, TaN and TiN have favorable effect for the dehydrogenation of Li–Mg–N–H system. As seen in Fig. 2(b and c), the time required for 90% dehydrogenation is reduced from 80 min to around 60 min for the samples with TaN or TiN additive. More importantly, such enhancement is highly stable, without marked degradation between the second and third cycles. It is noteworthy that this kinetic enhancement is obtained without capacity loss compared to the additive-free sample. In
L.-P. Ma et al. / Journal of Alloys and Compounds 468 (2009) L21–L24
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Fig. 3. XRD patterns for the dehydrogenated Li–Mg–N–H samples (a) without additive, (b) with 10 wt.% TiN, (c) with 10 wt.% TaN. (Dehydrogenation was performed at 200 ◦ C and 0.1 MPa.)
Fig. 2. Dehydrogenation curves of Li–Mg–N–H samples with 10 wt.% additives: (a) TiCl3 , (b) TaN and (c) TiN at 200 ◦ C and 0.1 MPa in the second and third cycles.
fact, because the two metal nitrides are not expected to absorb hydrogen, this enhancement indicates that the addition of TaN or TiN activates around 10% more Li–Mg–N–H component for reversible hydrogen storage. Interestingly, the samples with 10 wt.% TaN or TiN additive exhibit nearly identical hydrogen storage performance. Nevertheless, TaN appears to be more
effective than TiN in view of its lower molar content. Without definite evidence on favorable thermodynamic modification, we attribute the accelerated desorption rate to merely catalytic enhancement arising upon adding TaN or TiN. Fig. 3 presents the XRD patterns of the Li–Mg–N–H samples with and without nitride additives after dehydrogenation measurement. From these results, neither newly formed phase nor appreciable change in the diffraction pattern of Li2 Mg(NH)2 can be identified in the samples with nitride additives, suggesting the high phase stability of TaN and TiN. This agrees well with the stable H-capacity in both samples during cycling. Furthermore, SEM examination indicates that the samples with or without additives exhibit similar particle size distribution. Because the phase and microstructure characterizations provide no valuable hint, we propose a possible interpretation for the present system with reference to the catalytic model for hydrotreating reactions by transition metal nitrides. For the dehydrogenation reaction of Li–Mg–N–H system, dissociation of N–H bond is required in the generation of the reactant species (i.e., H+ or NH3 species, depending on specific reaction pathway [16,19]). The high activation energy in the decomposition reaction of Mg(NH2 )2 demonstrates a strong N–H bond in the amide [9], suggesting that enhanced dissociation of N–H bond may facilitate the dehydrogenation reaction between Mg(NH2 )2 and LiH. In this regard, transition metal nitrides may be a promising catalyst. The transition metal nitrides typically possess N-deficient patch of metal atoms on the surface and edges, which are attributed to be active sites in hydrotreating reactions [20–23] and can be further enriched by high-energy ball milling (pre-milling treatment applied here). Presumably, the defective Metal–N pairs act as active sites in favor of dissociating the N–H bond of the amide in this system, which then promotes the generation of reactant species. 4. Conclusions Transition metal nitrides TaN and TiN are promising catalysts for the Li–Mg–N–H system. Both additives are catalytically
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active for accelerating the dehydrogenation of Li–Mg–N–H sample. Such kinetic enhancement persists well without hydrogen capacity loss. These results are consistent with our speculation about the catalytic activity of metal nitrides towards metal amide/imide conversion, thus highlighting the guide of mechanism understanding in screening effective catalysts for Metal–N–H systems. Acknowledgements We are grateful to Mr. Xue-Wen Wang for kindly providing TaN and TiN, to Mr. Zhan-Zhao Fang for his experimental assistance. The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant no. 50671107) are gratefully acknowledged. References [1] [2] [3] [4]
P. Chen, Z. Xiong, J. Luo, J. Lin, K. Tan, Nature 420 (2002) 302–304. Z. Xiong, G. Wu, J. Hu, P. Chen, Adv. Mater. 16 (2004) 1522–1525. W. Luo, J. Alloys Compd. 381 (2004) 284–287. Y. Nakamori, G. Kitahara, K. Miwa, S. Towata, S. Orimo, Appl. Phys. A 80 (2005) 1–3. [5] H. Leng, T. Ichikawa, S. Hino, N. Hanada, S. Isobe, H. Fujii, J. Phys. Chem. B 108 (2004) 8763–8765. [6] J. Rijssenbeek, Y. Gao, J. Hanson, Q. Huang, C. Jones, B. Toby, J. Alloys Compd. 454 (2008) 233–244. [7] Y. Chen, C.Z. Wu, P. Wang, H.M. Cheng, Int. J. Hydrogen Energy 31 (2006) 1236–1240.
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