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Hydrogen storage properties of [M(Py){Ni(CN)4}] (M=Fe , Co, Ni)
International Journal of Hydrogen Energy 32 (2007) 3411 – 3415 www.elsevier.com/locate/ijhydene
Hydrogen storage properties of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) Yan Li a , Yang Liu a , Yuntao Wang a , Yonghua Leng a , Lei Xie a , Xingguo Li a,b,∗ a Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b College of Engineering, Peking University, Beijing 100871, China Received 14 January 2007; received in revised form 2 March 2007; accepted 2 March 2007 Available online 17 May 2007
Abstract [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) frameworks were synthesized by solution method and their hydrogen storage properties were measured using a PCT measuring system. The series of frameworks were found to adsorb twice hydrogen predicted by monolayer adsorption model. Compared with the hydrogen adsorption property of FeFe(CN)6 , this strange phenomenon is elucidated at a molecular level. The pore diameter of Py series is just enough to contain two hydrogen molecules but only one nitrogen molecule is permitted to get in, which leads to the apparent double-layer adsorption. It is concluded that pores with free diameter smaller than 0.42 nm are not favorable for hydrogen storage because no more than one hydrogen molecule can get into them. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; Framework; Physisorption
1. Introduction Great enthusiasm to hydrogen energy has been stimulated in recent years by the increasingly rapid consumption of fossil fuels due to both economic and environmental reasons. However, the storage of hydrogen is still an unsolved problem which limits its wide application [1,2]. Many materials have been investigated for hydrogen storage, for example, nitrides, borohydrides, metal hydrides, carbon materials [3–7]. In particular, Yaghi et al. reported the hydrogen storage properties of a series of metal organic frameworks consisting of tetrahedral [Zn4 O]6+ units linked by linear aryldicarboxylates [8–10]. In comparison with other nanoporous materials such as zeolites, these materials have lower framework density and higher specific surface areas and attracted a lot of research interests [11]. Thereafter, many porous materials with high specific surface areas were reported to adsorb hydrogen, and became an important branch of hydrogen storage materials [3]. Recently, dehydrated Prussian blue ∗ Corresponding author. Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Tel./fax: +86 010 62765930. E-mail address: [email protected] (X. Li).
analogues were reported to have higher adsorption enthalpies [12–14]. This was partly attributed to the cyanides in the frameworks. Therefore, in this work, we built another framework with cyanides and studied its hydrogen storage property. Generally, physisorption is counted for the hydrogen storage in these porous materials [14]. Storage in this way has many advantages, such as the fast storage speed and excellent circulation properties, but the mass storage density is relatively smaller. Therefore, adsorbing more hydrogen is one of the most important aims in hydrogen storage via physisorption. In this work, the hydrogen adsorptions of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) (Py = pyrazine) were investigated (for short, they are named FePy, CoPy, NiPy, respectively). The 3D coordinating compound [Fe(Py){Ni(CN)4 }] was first reported by Kitazawa et al. [15]. Thereafter, its analogues were widely studied due to their superior spin crossover properties [16–18]. To our knowledge, this series of materials have not been investigated for their gas adsorption properties. In this research, it was found that the series of frameworks adsorb much more hydrogen in terms of mass storage per 1000 m2 /g specific surface area, which is twice hydrogen predicted by monolayer adsorption. This strange phenomenon is elucidated at a molecular level and some useful conclusions are drawn for further design of framework for hydrogen storage.
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.03.006
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2. Experimental Synthesis: The series of frameworks were prepared by a solution method. The compound deposited when a 0.01 mol/l stoichiometric M nitrate (M = Fe, Co, Ni) and pyrazine solution (H2 O: CH3 OH = 1: 1) was added dropwise to potassium tetracyanonickelate (II) solution (H2 O: CH3 OH = 1: 1) under constant stirring. The color of the precipitate varied with the added ions, FePy orange, CoPy gray, NiPy light purple. The obtained precipitate was then filtered and washed by 100 ml water for three times and by 100 ml C2 H5 OH for one time after aging in the mother liquid for 3 days. Then, the precipitate was dried in the air for 24 h at room temperature before other tests. Some other bivalent transition metal ions, such as Mn2+ , Cu2+ , Zn2+ , Cd2+ , were also used in the synthesis. The compounds of these ions are hard to form due to their weak coordinating properties. Structure determination: Powder X-ray diffraction (XRD) was used to determine the structure of the samples. The tests were taken on Rigaku D/Max2500 VB2 + PC from 10◦ to 60◦ at a scanning rate of 6◦ / min. The XRD of the samples were tested again after gas adsorption test to monitor whether the compounds were destroyed during the dehydration process at high temperature. Thermogravimetric analysis (TGA) was used to determine the stability of the samples on thermal analysis SDT2960, which is useful in the determination of the proper temperature for dehydration. Gas adsorption: After dehydration at 493 K for 4 h, nitrogen adsorptions of the samples were tested to get the information about pore structure of the samples on a COULTER SA 3100 apparatus at 77 K. After the same dehydration process, hydrogen storage properties of the samples were taken with pressure-content-temperature (PCT) measuring system based on volumetric method. Before each hydrogen storage test, hydrogen was charged into the equipment to the pressure of 7 MPa and the pressure changed less than 0.003 MPa during 12 h, which demonstrated that there is no hydrogen leaking in the equipment. In the experiment, the powder sample was put into the reaction container, and the container was immersed in liquid nitrogen in a Dewar vessel to get the adsorption at 77 K. 3. Results and discussion In the series of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni), coordinating planes are formed by Ni(CN)4 2− ions tetragonally coordinated with M2+ [18]. At each side of the plane, there are coordinating vacancies of M2+ ions. In this research, the vacancies are occupied by the nitrogen atoms of pyrazines. The coordinating polymers form 3D pores with tetragonal crystal structure. The coordinating ability of M2+ ions to the ligands increases in the following order: Fe2+ < Co2+ < Ni2+ . This difference determines the crystallization and stability of the frameworks. The conclusion is confirmed by the XRD patterns of the as-prepared series, which are shown in Fig. 1. The patterns are in good accordance with the ones that are reported by other researchers, which demonstrates the success of synthesis [17]. The peaks of each curve move right in the order: FePy < CoPy < NiPy. This fact is in accordance with the
Fig. 1. XRD patterns of the as-prepared samples FePy, CoPy and NiPy. The marked by arrows: some unknown impurities.
Fig. 2. TGA curves of sample FePy, CoPy and NiPy.
prediction because the unit cell parameters decrease with the radii of M2+ ions, and the diffraction peaks move right according to Bragg equation. The crystallization of NiPy is the best one of the three frameworks. The adjacent peaks in the curves, such as the diffractions at about 24 and 37, are recognizable. The crystallization of FePy is the worst. The peaks of some unknown impurity are found in the curve, which are marked with arrows in Fig. 1. The TGA tests in nitrogen flow give a further proof to the prediction about stability of the series, and the curves are shown in Fig. 2. The samples have good stability at high temperature. The framework of FePy does not collapse up to 523 K. Due to the increased coordinating abilities, the decomposing temperatures of CoPy and NiPy are even higher, up to 600 and 620 K, respectively. The series were prepared in aqueous solution and the pores of the framework are filled with solution molecules, water molecules. Therefore, the series must be dehydrated to free
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Fig. 4. BET curves of samples FePy, CoPy and NiPy.
Fig. 3. XRD patterns of sample FePy, CoPy and NiPy after dehydration. The marked by arrows: the unknown impurities.
the pores for gas adsorption tests. According to TGA results, the dehydration of Py series was carried out at 493 K, which is 30 K below the decomposition temperature. In this condition, the frameworks will not be damaged and the solution molecules will be taken out. The colors of Py series change obviously after dehydration, FePy from orange to dark red, CoPy from gray to dark green and NiPy from light purple to light pink, which indicates the impact of adsorbates. It is noticeable that the colors of Py series changed back after the compounds were exposed to the air, and if steam is accessible, this process finishes immediately. The reversibility of the color change indicates that the series keeps the structure during the dehydration at high temperature. After the dehydration and hydrogen storage test, the XRD patterns of the series are tested again to monitor the structure change. The XRD patterns are shown in Fig. 3. The patterns are almost identical to that of the as-prepared ones. However, the peaks of the impurity in FePy move left, which are marked by arrows in Fig. 3. Despite this little change, the series keeps the structure, which confirms the stability of Py series. To get more information about the surface states of Py series, nitrogen adsorptions were tested at liquid nitrogen temperature and the curves are shown in Fig. 4. According to BET equation, the specific surface areas are calculated and the data are listed in Table 1. The specific surface areas of the series increase in the order of FePy < CoPy < NiPy. This result conflicts with the prediction because NiPy have smaller unit cells and larger molecular weight. However, as mentioned above, the series have increasingly better crystallization, which is the main factor that accounts for the specific surface area changes. The large reduce of FePy is attributed both to the imperfectness of the crystalline and the little amount of unknown impurity in the sample. The hydrogen storage properties of the series are tested at liquid nitrogen temperature and the curves are shown in
Fig. 5. The max storages of Py series at 7.7 MPa are 1.46, 2.19, 2.34 wt% respectively, which are listed in Table 1. All the samples have high storage ability at low pressure, which is typical for the gas adsorption process in microporous materials. The storage abilities of the series increase with hydrogen pressure and are not saturated even up to 7.7 MPa, which indicates the frameworks will adsorb more hydrogen at higher pressure. As mentioned above, physisorption is counted for the hydrogen storage in porous materials. According to BET theory, gas molecules are adsorbed layer by layer on the adsorbent. The adsorption energy of the first layer is determined by the interaction between gas and adsorbent, while that of the second and after layers is similar to the latent heat of vaporization of the adsorbate. Therefore, only a single monolayer is formed in the situation where the adsorbate cannot be liquefied, for example, the supercritical adsorption. The hydrogen storage properties of Py series are tested at 77 K. This is much higher than hydrogen’s critical temperature, 33 K, which means only monolayer adsorption happens in this research. Therefore, based on the monolayer hypothesis, the hydrogen adsorption amount on unit specific surface area is estimated to be 2.27 wt% per 1000 m2 /g [2]. However, this is a saturated amount and cannot be accomplished in the situation far above critical temperature. In terms of hydrogen storage amount on unit specific surface area, the capacity of Py series is very high and the storages at 7.7 MPa are 4.41% for FePy, 4.41% for CoPy, 4.52% for NiPy per 1000 m2 /g, respectively, which are almost twice as much as the monolayer adsorption. Double-layer adsorption seems to occur in Py series but this can hardly happen. To get further information about this abnormal phenomenon, the hydrogen storage properties of a Prussian blue analogue, FeFe(CN)6 , is measured and the curve is shown in Fig. 5, and the corresponding information is listed in Table 1. The storage capacity of FeFe(CN)6 is 2.25 per 1000 m2 /g, only half of NiPy, but it is almost the same as monolayer prediction. In the gas adsorption of the Py series and FeFe(CN)6 , the entire surface exposed to gas is the electron cloud of the K-bands (including cyanides and pyrazine) and there is only a few of the C–H bands in Py series. The interaction of K-bands with hydrogen molecules contributes most to the adsorption which determines the intensity of adsorption.
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Table 1 Properties of the samples Sample (m2 /g)
Specific surface area Hydrogen storage at 7.7 MPa (wt%) Hydrogen storage per 1000 m2 /g (wt%) Nitrogen molecules per unit cell (P /P0 = 0.2) Hydrogen molecules per unit cell (P = 7.7 MPa) Ratio of hydrogen and nitrogen molecules
FePy
CoPy
NiPy
FeFe(CN)6
331 1.46 4.41 1.28 2.16 1.69
497 2.19 4.41 1.90 3.28 1.73
518 2.34 4.52 1.95 3.5 1.79
405 0.91 2.25 5.54 4.84 0.87
FeFe(CN)6 can only absorb about half of the hydrogen that Py series absorb on per 1000 m2 /g. It is concluded that pores with free diameter smaller than 0.42 nm are not favorable for hydrogen storage because no more than one hydrogen molecule can get into them. On the other hand, pore with too large diameter is not favorable for hydrogen storagebecause the enhancement of adsorbing ability in the pore will be weak, and the adsorption of hydrogen will not be very strong [10]. It is noticeable that the hydrogen/nitrogen ratio adsorbed in FeFe(CN)6 is not 1 but 0.87. Although the ratio of Py series is twice of FeFe(CN)6 ’s, it is about 1.7 not 2. The weak interaction between hydrogen and adsorbent is accounted for this phenomenon. Fig. 5. Hydrogen adsorption curves of samples FePy, CoPy, NiPy and FeFe(CN)6 at liquid nitrogen temperature.
Therefore, the storage capacity of Py series and FeFe(CN)6 should be similar. The experimental results are inconsistent with the theoretical prediction. To solve this problem, the gas molecules adsorbed into the unit cell of the samples are calculated and the data are listed in Table 1. Nitrogen adsorption values at P /P0 = 0.2 are used in the calculation of nitrogen adsorbed, because multilayer adsorption does not occur at this relative pressure and the adsorption can be considered occurring mainly in the pores. According to the results, about two hydrogen molecules get into the unit cell of Py series as one nitrogen molecule gets in. However, in FeFe(CN)6 , only one hydrogen molecule can get in at the same condition. The comparison of gas molecule sizes and pore diameters leads to further understanding of this problem. The van der Waals diameter of nitrogen and hydrogen molecules is about 0.29 and 0.24 nm, respectively. In FeFe(CN)6 , the pore diameter is about 0.23 nm, which is determined by four tetragonally coordinated cyanides. Because the pore diameter is small, the pore that permits one nitrogen molecule to get in allows only one hydrogen molecule to get in. However, in Py series the pore diameter is determined by two adjacent pyrazines, which is about 0.42 nm, nearly twice as big as that of FeFe(CN)6 . Here, still only one nitrogen molecule can get into the hole, while two hydrogen molecules can get into the same pore. The pore that contains one nitrogen molecule in FeFe(CN)6 can contain only one hydrogen molecule, while the pore that contains one nitrogen molecule in Py series can contain two hydrogen molecules. This phenomenon may be considered as another kind of molecule-sieve effect. That is the reason why
4. Conclusion [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) coordinating frameworks were synthesized by solution method. The series have strong hydrogen storage of 4.52% per 1000 m2 /g, which is nearly twice the amount predicted by monolayer adsorption, 2.27% per 1000 m2 /g. Compared with hydrogen adsorption property of FeFe(CN)6 (2.25% per 1000 m2 /g), it is found that this abnormal strong adsorption is caused by the proper pore size of the frameworks. The pore that contains one nitrogen molecule in FeFe(CN)6 can hold only one hydrogen molecule, while the pore that contains one nitrogen molecule in Py series can contain two hydrogen molecules. It is concluded that the free pore diameters of hydrogen storage materials should not be smaller than about 0.42 nm because no more than one hydrogen molecule can get into pores of that size. Acknowledgment The authors acknowledge the National Natural Science Foundation of China (Nos. 20221101, 10335040 and 20671004) and MOST of China (No. 2006AA02Z130). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2007.03.006. References [1] Schlapbach L, Züttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414(6861):353–8. [2] Züttel A. Materials for hydrogen storage. Mater Today 2003;6:24–33.
Y. Li et al. / International Journal of Hydrogen Energy 32 (2007) 3411 – 3415 [3] Bogdanovic B, Felderhoff M, Kaskel S, Pommerin A, Schlichte K, Schuth F. Improved hydrogen storage properties of Ti-doped sodium alanate using titanium nanoparticles doping agents. Adv Mater 2003;15:1012–5. [4] Chen P, Xiong ZT, Luo JZ, Lin JY, Tan KL. Interaction of hydrogen with metal nitrides and imides. Nature 2002;420:302–4. [5] An M, Jurgensen A. Modified lithium borohydrides for reversible hydrogen storage. J Phys Chem B 2006;110(13):7062–7. [6] Shao HY, Xu HR, Wang YW, Li XG. Preparation and hydrogen storage properties of Mg2 Ni intermetallic nanoparticles. Nanotechnology 2004;15:269–74. [7] Ritschel M, Uhlemann M, Gutfleisch O, Leonhardt A, Graff A, Taschner C. et al. Hydrogen storage in different carbon nanostructures. Appl Phys Lett 2002;80:2985–7. [8] Rosi NL, Eckert J, Eddaoudi M, Vodak DT, Kim J, O’Keeffe M. et al. Hydrogen storage in microporous metal–organic frameworks. Science 2003;300:1127–9. [9] Rowsell JLC, Spencer EC, Eckert J, Howard JAK, Yaghi OM. Gas adsorption sites in a large-pore metal–organic framework. Science 2005;309(5739):1350–4. [10] Wong-Foy AG, Matzger AJ, Yaghi OM. Exceptional H-2 saturation uptake in microporous metal–organic frameworks. J Am Chem Soc 2006;128(11):3494–5. [11] Kosal ME, Chou JH, Wilson SR, Suslick KS. A functional zeolite analogue assembled from metalloporphyrins. Nat Mater 2002;1(2): 118–21.
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[12] Chapman KW, Chupas PJ, Maxey ER, Richardson JW. Direct observation of adsorbed H-2-framework interactions in the Prussian blue analogue Mn-3(II)[Co-III(CN)(6)](2): the relative importance of accessible coordination sites and van der Waals interactions. Chem Commun 2006;(38):4013–5. [13] Chapman KW, Southon PD, Weeks CL, Kepert CJ. Reversible hydrogen gas uptake in nanoporous Prussian blue analogues. Chem Commun 2005;(26):3322–4. [14] Kaye SS, Long JR. Hydrogen storage in the dehydrated Prussian blue analogues M-3[Co(CN)(6)](2) (M=Mn, Fe, Co, Ni, Cu, Zn). J Am Chem Soc 2005;127(18):6506–7. [15] Kitazawa T, Gomi Y, Takahashi M, Takeda M, Enomoto M, Miyazaki A. et al. J Mater Chem 1996;6(1):119–21. [16] Bonhommeau S, Molnar G, Galet A, Zwick A, Real JA, McGarvey JJ. et al. One shot laser pulse induced reversible spin transition in the spin-crossover complex [Fe(C4 H4 N2 ){Pt(CN)(4)}] at room temperature. Angew Chem Int Ed 2005;44(26):4069–73. [17] Niel V, Martinez-Agudo JM, Munoz MC, Gaspar AB, Real JA. Cooperative spin crossover behavior in cyanide-bridged Fe(II)–M(II) bimetallic 3D Hofmann-like networks (M = Ni, Pd, and Pt). Inorg Chem 2001;40:3838–9. [18] Molnar G, Niel V, Real JA, Dubrovinsky L, Bousseksou A. et al. Raman spectroscopic study of pressure effects on the spin-crossover coordination polymers Fe(pyrazine)[M(CN)(4)]center dot 2H(2)O (M = Ni, Pd, Pt). First observation of a piezo-hysteresis loop at room temperature. J Phys Chem B 2003;107(14):3149–55.
Hydrogen storage properties of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) Yan Li a , Yang Liu a , Yuntao Wang a , Yonghua Leng a , Lei Xie a , Xingguo Li a,b,∗ a Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b College of Engineering, Peking University, Beijing 100871, China Received 14 January 2007; received in revised form 2 March 2007; accepted 2 March 2007 Available online 17 May 2007
Abstract [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) frameworks were synthesized by solution method and their hydrogen storage properties were measured using a PCT measuring system. The series of frameworks were found to adsorb twice hydrogen predicted by monolayer adsorption model. Compared with the hydrogen adsorption property of FeFe(CN)6 , this strange phenomenon is elucidated at a molecular level. The pore diameter of Py series is just enough to contain two hydrogen molecules but only one nitrogen molecule is permitted to get in, which leads to the apparent double-layer adsorption. It is concluded that pores with free diameter smaller than 0.42 nm are not favorable for hydrogen storage because no more than one hydrogen molecule can get into them. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen storage; Framework; Physisorption
1. Introduction Great enthusiasm to hydrogen energy has been stimulated in recent years by the increasingly rapid consumption of fossil fuels due to both economic and environmental reasons. However, the storage of hydrogen is still an unsolved problem which limits its wide application [1,2]. Many materials have been investigated for hydrogen storage, for example, nitrides, borohydrides, metal hydrides, carbon materials [3–7]. In particular, Yaghi et al. reported the hydrogen storage properties of a series of metal organic frameworks consisting of tetrahedral [Zn4 O]6+ units linked by linear aryldicarboxylates [8–10]. In comparison with other nanoporous materials such as zeolites, these materials have lower framework density and higher specific surface areas and attracted a lot of research interests [11]. Thereafter, many porous materials with high specific surface areas were reported to adsorb hydrogen, and became an important branch of hydrogen storage materials [3]. Recently, dehydrated Prussian blue ∗ Corresponding author. Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Tel./fax: +86 010 62765930. E-mail address: [email protected] (X. Li).
analogues were reported to have higher adsorption enthalpies [12–14]. This was partly attributed to the cyanides in the frameworks. Therefore, in this work, we built another framework with cyanides and studied its hydrogen storage property. Generally, physisorption is counted for the hydrogen storage in these porous materials [14]. Storage in this way has many advantages, such as the fast storage speed and excellent circulation properties, but the mass storage density is relatively smaller. Therefore, adsorbing more hydrogen is one of the most important aims in hydrogen storage via physisorption. In this work, the hydrogen adsorptions of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) (Py = pyrazine) were investigated (for short, they are named FePy, CoPy, NiPy, respectively). The 3D coordinating compound [Fe(Py){Ni(CN)4 }] was first reported by Kitazawa et al. [15]. Thereafter, its analogues were widely studied due to their superior spin crossover properties [16–18]. To our knowledge, this series of materials have not been investigated for their gas adsorption properties. In this research, it was found that the series of frameworks adsorb much more hydrogen in terms of mass storage per 1000 m2 /g specific surface area, which is twice hydrogen predicted by monolayer adsorption. This strange phenomenon is elucidated at a molecular level and some useful conclusions are drawn for further design of framework for hydrogen storage.
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.03.006
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2. Experimental Synthesis: The series of frameworks were prepared by a solution method. The compound deposited when a 0.01 mol/l stoichiometric M nitrate (M = Fe, Co, Ni) and pyrazine solution (H2 O: CH3 OH = 1: 1) was added dropwise to potassium tetracyanonickelate (II) solution (H2 O: CH3 OH = 1: 1) under constant stirring. The color of the precipitate varied with the added ions, FePy orange, CoPy gray, NiPy light purple. The obtained precipitate was then filtered and washed by 100 ml water for three times and by 100 ml C2 H5 OH for one time after aging in the mother liquid for 3 days. Then, the precipitate was dried in the air for 24 h at room temperature before other tests. Some other bivalent transition metal ions, such as Mn2+ , Cu2+ , Zn2+ , Cd2+ , were also used in the synthesis. The compounds of these ions are hard to form due to their weak coordinating properties. Structure determination: Powder X-ray diffraction (XRD) was used to determine the structure of the samples. The tests were taken on Rigaku D/Max2500 VB2 + PC from 10◦ to 60◦ at a scanning rate of 6◦ / min. The XRD of the samples were tested again after gas adsorption test to monitor whether the compounds were destroyed during the dehydration process at high temperature. Thermogravimetric analysis (TGA) was used to determine the stability of the samples on thermal analysis SDT2960, which is useful in the determination of the proper temperature for dehydration. Gas adsorption: After dehydration at 493 K for 4 h, nitrogen adsorptions of the samples were tested to get the information about pore structure of the samples on a COULTER SA 3100 apparatus at 77 K. After the same dehydration process, hydrogen storage properties of the samples were taken with pressure-content-temperature (PCT) measuring system based on volumetric method. Before each hydrogen storage test, hydrogen was charged into the equipment to the pressure of 7 MPa and the pressure changed less than 0.003 MPa during 12 h, which demonstrated that there is no hydrogen leaking in the equipment. In the experiment, the powder sample was put into the reaction container, and the container was immersed in liquid nitrogen in a Dewar vessel to get the adsorption at 77 K. 3. Results and discussion In the series of [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni), coordinating planes are formed by Ni(CN)4 2− ions tetragonally coordinated with M2+ [18]. At each side of the plane, there are coordinating vacancies of M2+ ions. In this research, the vacancies are occupied by the nitrogen atoms of pyrazines. The coordinating polymers form 3D pores with tetragonal crystal structure. The coordinating ability of M2+ ions to the ligands increases in the following order: Fe2+ < Co2+ < Ni2+ . This difference determines the crystallization and stability of the frameworks. The conclusion is confirmed by the XRD patterns of the as-prepared series, which are shown in Fig. 1. The patterns are in good accordance with the ones that are reported by other researchers, which demonstrates the success of synthesis [17]. The peaks of each curve move right in the order: FePy < CoPy < NiPy. This fact is in accordance with the
Fig. 1. XRD patterns of the as-prepared samples FePy, CoPy and NiPy. The marked by arrows: some unknown impurities.
Fig. 2. TGA curves of sample FePy, CoPy and NiPy.
prediction because the unit cell parameters decrease with the radii of M2+ ions, and the diffraction peaks move right according to Bragg equation. The crystallization of NiPy is the best one of the three frameworks. The adjacent peaks in the curves, such as the diffractions at about 24 and 37, are recognizable. The crystallization of FePy is the worst. The peaks of some unknown impurity are found in the curve, which are marked with arrows in Fig. 1. The TGA tests in nitrogen flow give a further proof to the prediction about stability of the series, and the curves are shown in Fig. 2. The samples have good stability at high temperature. The framework of FePy does not collapse up to 523 K. Due to the increased coordinating abilities, the decomposing temperatures of CoPy and NiPy are even higher, up to 600 and 620 K, respectively. The series were prepared in aqueous solution and the pores of the framework are filled with solution molecules, water molecules. Therefore, the series must be dehydrated to free
Y. Li et al. / International Journal of Hydrogen Energy 32 (2007) 3411 – 3415
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Fig. 4. BET curves of samples FePy, CoPy and NiPy.
Fig. 3. XRD patterns of sample FePy, CoPy and NiPy after dehydration. The marked by arrows: the unknown impurities.
the pores for gas adsorption tests. According to TGA results, the dehydration of Py series was carried out at 493 K, which is 30 K below the decomposition temperature. In this condition, the frameworks will not be damaged and the solution molecules will be taken out. The colors of Py series change obviously after dehydration, FePy from orange to dark red, CoPy from gray to dark green and NiPy from light purple to light pink, which indicates the impact of adsorbates. It is noticeable that the colors of Py series changed back after the compounds were exposed to the air, and if steam is accessible, this process finishes immediately. The reversibility of the color change indicates that the series keeps the structure during the dehydration at high temperature. After the dehydration and hydrogen storage test, the XRD patterns of the series are tested again to monitor the structure change. The XRD patterns are shown in Fig. 3. The patterns are almost identical to that of the as-prepared ones. However, the peaks of the impurity in FePy move left, which are marked by arrows in Fig. 3. Despite this little change, the series keeps the structure, which confirms the stability of Py series. To get more information about the surface states of Py series, nitrogen adsorptions were tested at liquid nitrogen temperature and the curves are shown in Fig. 4. According to BET equation, the specific surface areas are calculated and the data are listed in Table 1. The specific surface areas of the series increase in the order of FePy < CoPy < NiPy. This result conflicts with the prediction because NiPy have smaller unit cells and larger molecular weight. However, as mentioned above, the series have increasingly better crystallization, which is the main factor that accounts for the specific surface area changes. The large reduce of FePy is attributed both to the imperfectness of the crystalline and the little amount of unknown impurity in the sample. The hydrogen storage properties of the series are tested at liquid nitrogen temperature and the curves are shown in
Fig. 5. The max storages of Py series at 7.7 MPa are 1.46, 2.19, 2.34 wt% respectively, which are listed in Table 1. All the samples have high storage ability at low pressure, which is typical for the gas adsorption process in microporous materials. The storage abilities of the series increase with hydrogen pressure and are not saturated even up to 7.7 MPa, which indicates the frameworks will adsorb more hydrogen at higher pressure. As mentioned above, physisorption is counted for the hydrogen storage in porous materials. According to BET theory, gas molecules are adsorbed layer by layer on the adsorbent. The adsorption energy of the first layer is determined by the interaction between gas and adsorbent, while that of the second and after layers is similar to the latent heat of vaporization of the adsorbate. Therefore, only a single monolayer is formed in the situation where the adsorbate cannot be liquefied, for example, the supercritical adsorption. The hydrogen storage properties of Py series are tested at 77 K. This is much higher than hydrogen’s critical temperature, 33 K, which means only monolayer adsorption happens in this research. Therefore, based on the monolayer hypothesis, the hydrogen adsorption amount on unit specific surface area is estimated to be 2.27 wt% per 1000 m2 /g [2]. However, this is a saturated amount and cannot be accomplished in the situation far above critical temperature. In terms of hydrogen storage amount on unit specific surface area, the capacity of Py series is very high and the storages at 7.7 MPa are 4.41% for FePy, 4.41% for CoPy, 4.52% for NiPy per 1000 m2 /g, respectively, which are almost twice as much as the monolayer adsorption. Double-layer adsorption seems to occur in Py series but this can hardly happen. To get further information about this abnormal phenomenon, the hydrogen storage properties of a Prussian blue analogue, FeFe(CN)6 , is measured and the curve is shown in Fig. 5, and the corresponding information is listed in Table 1. The storage capacity of FeFe(CN)6 is 2.25 per 1000 m2 /g, only half of NiPy, but it is almost the same as monolayer prediction. In the gas adsorption of the Py series and FeFe(CN)6 , the entire surface exposed to gas is the electron cloud of the K-bands (including cyanides and pyrazine) and there is only a few of the C–H bands in Py series. The interaction of K-bands with hydrogen molecules contributes most to the adsorption which determines the intensity of adsorption.
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Table 1 Properties of the samples Sample (m2 /g)
Specific surface area Hydrogen storage at 7.7 MPa (wt%) Hydrogen storage per 1000 m2 /g (wt%) Nitrogen molecules per unit cell (P /P0 = 0.2) Hydrogen molecules per unit cell (P = 7.7 MPa) Ratio of hydrogen and nitrogen molecules
FePy
CoPy
NiPy
FeFe(CN)6
331 1.46 4.41 1.28 2.16 1.69
497 2.19 4.41 1.90 3.28 1.73
518 2.34 4.52 1.95 3.5 1.79
405 0.91 2.25 5.54 4.84 0.87
FeFe(CN)6 can only absorb about half of the hydrogen that Py series absorb on per 1000 m2 /g. It is concluded that pores with free diameter smaller than 0.42 nm are not favorable for hydrogen storage because no more than one hydrogen molecule can get into them. On the other hand, pore with too large diameter is not favorable for hydrogen storagebecause the enhancement of adsorbing ability in the pore will be weak, and the adsorption of hydrogen will not be very strong [10]. It is noticeable that the hydrogen/nitrogen ratio adsorbed in FeFe(CN)6 is not 1 but 0.87. Although the ratio of Py series is twice of FeFe(CN)6 ’s, it is about 1.7 not 2. The weak interaction between hydrogen and adsorbent is accounted for this phenomenon. Fig. 5. Hydrogen adsorption curves of samples FePy, CoPy, NiPy and FeFe(CN)6 at liquid nitrogen temperature.
Therefore, the storage capacity of Py series and FeFe(CN)6 should be similar. The experimental results are inconsistent with the theoretical prediction. To solve this problem, the gas molecules adsorbed into the unit cell of the samples are calculated and the data are listed in Table 1. Nitrogen adsorption values at P /P0 = 0.2 are used in the calculation of nitrogen adsorbed, because multilayer adsorption does not occur at this relative pressure and the adsorption can be considered occurring mainly in the pores. According to the results, about two hydrogen molecules get into the unit cell of Py series as one nitrogen molecule gets in. However, in FeFe(CN)6 , only one hydrogen molecule can get in at the same condition. The comparison of gas molecule sizes and pore diameters leads to further understanding of this problem. The van der Waals diameter of nitrogen and hydrogen molecules is about 0.29 and 0.24 nm, respectively. In FeFe(CN)6 , the pore diameter is about 0.23 nm, which is determined by four tetragonally coordinated cyanides. Because the pore diameter is small, the pore that permits one nitrogen molecule to get in allows only one hydrogen molecule to get in. However, in Py series the pore diameter is determined by two adjacent pyrazines, which is about 0.42 nm, nearly twice as big as that of FeFe(CN)6 . Here, still only one nitrogen molecule can get into the hole, while two hydrogen molecules can get into the same pore. The pore that contains one nitrogen molecule in FeFe(CN)6 can contain only one hydrogen molecule, while the pore that contains one nitrogen molecule in Py series can contain two hydrogen molecules. This phenomenon may be considered as another kind of molecule-sieve effect. That is the reason why
4. Conclusion [M(Py){Ni(CN)4 }] (M = Fe, Co, Ni) coordinating frameworks were synthesized by solution method. The series have strong hydrogen storage of 4.52% per 1000 m2 /g, which is nearly twice the amount predicted by monolayer adsorption, 2.27% per 1000 m2 /g. Compared with hydrogen adsorption property of FeFe(CN)6 (2.25% per 1000 m2 /g), it is found that this abnormal strong adsorption is caused by the proper pore size of the frameworks. The pore that contains one nitrogen molecule in FeFe(CN)6 can hold only one hydrogen molecule, while the pore that contains one nitrogen molecule in Py series can contain two hydrogen molecules. It is concluded that the free pore diameters of hydrogen storage materials should not be smaller than about 0.42 nm because no more than one hydrogen molecule can get into pores of that size. Acknowledgment The authors acknowledge the National Natural Science Foundation of China (Nos. 20221101, 10335040 and 20671004) and MOST of China (No. 2006AA02Z130). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ijhydene.2007.03.006. References [1] Schlapbach L, Züttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414(6861):353–8. [2] Züttel A. Materials for hydrogen storage. Mater Today 2003;6:24–33.
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