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Synthesis of LiBH4 from LiBO2 as hydrogen carrier and its catalytic dehydrogenation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of LiBH4 from LiBO2 as hydrogen carrier and its catalytic dehydrogenation Murat Bilen a, Orhan Yılmaz a, Metin Gu¨ru¨ b,* a b
Eti Mine Works, Ayvalı Mah. Halil Sezai Erkut Cad., Afra Sok. No:1/A 06010, Etlik e Kec¸i€oren, Ankara, Turkey Gazi University, Eng. Fac., Chemical Eng. Dep., Maltepe 06570, Ankara, Turkey
article info
abstract
Article history:
This study deals to synthesize LiBH4 by means of mechano-chemical reaction, in which
Received 25 December 2014
LiBO2 and MgH2 were utilized. In order to obtain 90% LiBH4 purity, optimum reaction
Received in revised form
conditions were analyzed at different values of reactant ratio and duration. In addition,
19 February 2015
commercial LiBH4 was hydrolyzed with CoI2 supported on activated carbon (AC). In order to
Accepted 21 February 2015
follow up the intermediate steps of the studies FT-IR, XRD analyses have been used.
Available online 16 March 2015
Activation energy of the dehyrdogeneration reaction was searched at different reaction temperatures and determined as 33.12 kJ/mol. As an optimum operation time for ball-
Keywords:
milling were recorded as 1000 min and reactant ratio (MgH2/LiBO2) were found as 1.0.
LiBH4
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalytic dehydrogenation CoI2 LiBO2
Introduction Carbon based fuels have been creating huge amount of energy demand for long years. There is a widespread idea about the carbon based energy sources which is obviously reflecting they will be exhausted within a few decades. Economic and environment friendly energy sources have to be founded in order to satisfy this huge amount of consumption demand. Combustion of fossil fuels leads to global warming and air pollution due to presence of CO2, SOx and NOx in the emission gasses [1]. Fuel cells using hydrogen as fuel have been successful in producing energy effectively. A proton source is needed to feed the fuel cells for an uninterrupted energy production. There are several methods of safe storage and transportation of hydrogen to the end user. These are the storage of
hydrogen within a pressurized steel tank, as a liquid under the cryogenic conditions, on activated carbon, in carbon nanotube, in graphite nanofiber, or in alloys suitable to adsorption and desorption of hydrogen in their structures [2,3]. Metal borohydrides can adsorb hydrogen and store it in solid form. These compounds release hydrogen by thermal decomposition or catalytic desorption. Theoretically, thermal decomposition yields only 10% (by mass) of the hydrogen in the material used for this purpose. Catalytic approach is advantageous since the hydrogen of the water is also used [4,5]. Hydrogen storage in metal borohydrides due to the excellent properties of boron, is regarded the most promising alternative. For example with sodium borohydride (NaBH4), hydrogen can be produced by the reaction between the water and
* Corresponding author. Fax: þ90 312 2308434. E-mail address: [email protected] (M. Gu¨ru¨). http://dx.doi.org/10.1016/j.ijhydene.2015.02.085 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
NaBH4 in the presence of a catalyst. LiBH4 is also a potential candidate for hydrogen storage, because of its high hydrogen capacity: 18.5% (by mass), 121 kg/m3, respectively [6]. This compound has the highest gravimetric hydrogen densities of 37% (by mass) when catalytically reacted with cycling water [3]. Conventionally, LiBH4 is produced by using NaBH4 in the presence of isopropyl amine as shown below. Different procedures are applied but LiBH4 cannot be synthesized under elevated condition: 150 bars of hydrogen at 923 K [7].
Na(BH4) þ LiCl / Li(BH4) þ NaCl
(1)
Decreasing the operating temperature (600e700 K) during desorption is desirable through the use of additives because of the low melting point (550 K) of LiBH4 [6]. Mg, SiO2, Cu are used for this purpose and promising results are obtained [6,8]. Besides amides, imides and complex hydrides are considered for both decreasing the desorption temperature and for storing hydrogen by themselves [9,10]. However, LiBH4 actually has two desorption temperature. The first one starts at about 473 K and gives a maximum peak at 623 K. At this range the structure gives 3 4 of its hydrogen the rest staying in the LiH structure. The second region starts at about 726 K and in this region no hydrogen is left in the solid [11]. Another way of lowering the desorption temperature of Li-based borohydrides can be proposed by substitution Li with a smaller sized, more electronegative and higher valence cations such as Cu and Mg. This method results in at least a 50 K decrease in the desorption temperature [8e12]. In the current study, LiBO2 was processed in ball-milling reactor in the presence of MgH2 in order to synthesize LiBH4. In addition, CoI2 was supported on AC. Catalyst behavior and kinetic parameters were studied in details.
Materials and method LiBH4 was provided from Merck for the dehydrogenation experiments at 98% purity. MgH2 and LiBO2 were provided by Merck and Carlo-Erba respectively with 99.9% purity. LiBH4 was synthesized by means of the Reaction 2, in Spex 8000 M type ball-milling. In order to check the phase composition and crystalline state of the catalysts, XRD experiments were performed by Shimadzu powder diffrac˚ ). Jasco 480 model FTtometer (Cu Ka radiation, k ¼ 1.5406 A IR was used simultaneously to determine the chemical chances during mechano-chemical process. The thermodynamic values of this reaction were summarized in Table 1. A negative Gibbs free energy value indicates the possibility of the desired reaction.
LiBO2 þ 2MgH2 / LiBH4 þ 2MgO
Table 1 e Thermodynamic data of Reaction 2. LiBO2 þ 2MgH2 / LiBH4 þ 2MgO DHf, kJ/mol DSf, J/mol.K DGf, kJ/mol Kp lnKp
216.77 23.797 224.22 1.09 0.086
atmosphere otherwise indicated. The selected durations for ball-milling reaction are 300, 600, 800 and 1000 min. The product (LiBH4) of reaction was purified by means of filtration with ethylene diamine (EDA). Filtrate was dried at room temperature, under vacuum. Evaporated EDA held in cold trap immersed in liquid nitrogen.
Catalyst preparation CoI2 has supported on activated carbon in the presence of 25 wt % stearic acid by milling for one hour. The mixture pressed and shaped as cylinder (13 mm diameter and 3.6 mm high) under 590 MPa for the dehydrogenaration reaction of LiBH4. The optimum amount of CoI2 catalyst, which is 40%, has been used [1]. Catalyst pellets containing 40% CoI2 was kept at 200 C for 1 h to remove the stearic acid and create macro pores. Measurements showed that the weights of the catalyst pellets are at about 0.55 ± 0.05 g.
Dehydrogenation tests Catalyst pellets were placed in dehydrogenation reactor. 100 mg (2.632 mmol) of LiBH4 and 70 mg (1.75 mmol) of LiOH were dissolved in deionized water and introduced into reactor. Produced hydrogen was measured in inverse burette system shown in Fig. 1. After completion of the dehydrogenation, catalyst rapidly rinsed with deionized water and next hydrogenation takes place.
(2)
MgH2/LiBO2 mole ratios were arranged as 0.65, 1.0 and 1.3. All studies performed and reactants were handled under inert
Fig. 1 e Hydrolysis setup (1) catalyst, (2) LiBH4eLiOH solution, (3) reactor, (4) water reservoir, and (5) burette.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
Fig. 2 e FT-IR analyses that show the effect of the working time (1) 600 min, (2) 300 min, (3) 800 min, and (4) 1000 min.
Kinetic studies There were eight distinct experiments (at 20, 30, 40, 50, 55, 60, 65 ve 70 C) which performed with 40% CoI2 loaded on AC support. All of these kinetic experiments were performed to find out the reaction rate order and activation energy of the reaction. During the experiments reactor was kept in water bath which has 0.1 C accuracy.
Results and discussion As a hydrogen storage material, LiBH4 usage is directly depends on the feasibility of production. In this study, working time and reactants ratio are analyzed in order to determine the optimum conditions for both economy and high efficiency. According to studied reaction times, the longer duration the better yields as it is shown by the FT-IR spectrums in Fig. 2. It is obvious that the duration improves the results without any dependence to reactant ratios. As it can be followed from Fig. 2, the peaks between 2400 cm-1 and 2200 cm-1 clearly prove this fact. Micro peaks at 1461 and 1465 cm-1 give MgO existence and the success of the oxygen removal from B2O3 structure like in the case of previous studies [1,13,14]. Additionally, Fig. 3 was created to show the areas covered by of FTIR peaks at which 2400 cm-1e2200 cm-1 region. These integrations are showing the positive effect of working time for the experiments. However, in order to reduce the working time, process might be stopped at a certain time and the content of the LiBH4 in the reaction product can be increased by further purification processes to get economic process in terms of energy. XRD analysis performed for the different working times are shown in Fig. 4. According to the software of the XRD device that makes calculation by means of Scherrer equation, the results indicate that the products are in crystalline phase and the particles are in nano-scale. Mechanochemical devices generally provide nanoscale reactants and/or products, which is necessary to obtain high yields in short duration [15,7]. However, in Fig. 4, the peak was detected belonging to LiBH4. This is simply may be caused by the
15215
Fig. 3 e The effect of stoichiometry and working time on FT-IR peak areas indicating LiBH4 existence.
amorphous phase of LiBH4. Moreover, the realization of diffraction peaks of the starting reactants can be possible. This refers to uncompleted transaction and requirement for the purification process. XRD spectrum also indicates that the MgH2 peak at 28 2 h became weaker proportional to working time meanwhile MgO peak at 43 2 h is more visible for longer durations. These changes simply indicate the occurrence of the Reaction 2. Purification process is inevitable to obtain high yield and remove the some side products. EDA is a suitable solvent to separate the LiBH4 as well as it does not decompose LiBH4 or cause any reaction with the residue. If it is compared with the side product's (Fig. 4b), XRD of the purified sample (Fig. 4a) shows great difference. It can be understand from long and sharp peaks that the LiBH4 becomes in crystal phase during dissolving, filtration and drying processes. XRD analyses of the solid filtration product indicates that it contains unreacted MgH2, LiB(OH)2 and side product MgO at 28, 32 and 43 2 h, respectively (Fig. 4b). Commercial and synthesized LiBH4 were compared by means of FT-IR analysis in Fig. 5. Results indicate that the two samples have similar peaks at 2400 cm1 e2200 cm-1, but residue EDA makes it hard to determine the peaks between 1700 cm-1 and 900 cm-1.
Kinetic studies In order to rapid release of hydrogen, in catalytic dehydrogenation, hydrogen containing substances need catalysts. Hydrogen atoms both come from LiBH4 and solvent water while the decomposition. Oxygen of the water adheres to lithium and boron and forms LiBO2. This compound is easy to recycle and that increase the feasible utility of LiBH4. CoI2 was utilized to synthesize 40% CoI2 concentration AC supported catalysts [1]. CoI2 is reduced by LiBH4 and actually Co and (or) CoBx (x ¼ 0.5, 1 or 2) starts to play an important role for the hydrogen generation [16e18].
Kinetic behaviors of the catalyst 40wt % CoI2/AC catalyst was studied at eight different temperatures (20, 30, 40, 50, 55, 60, 65 and 70 C) (Fig. 6) and reaction rate order was investigated. Experimental data fit zero
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
Fig. 4 e XRD diffractions of (a) synthesized LiBH4 from solution and (b) filtration cake (MgH2/LiBO2 ratio is 1, 1000 min).
order kinetic (Fig. 7). This means that reaction depends on catalyst used instead of concentration. Activation energy computation results 33.12 kJ/mol. If compared with the values given in other studies, activation energy value is very advantageous as it is lower.
Conclusion This study attends to analyze the synthesis of LiBH4 by means of LiBO2 in ball-milling for the first time. Results of
the study show that the produced LiBH4 needs to be purified. By purification it reaches to 90% purity and becomes in crystalline phase after purification. It was observed that the yield of the products increases with working time. Thus 1000 min has been determined to be the optimum reaction time. Furthermore, studies with different ratios of MgH2/ LiBO2 result in that the stoichiometric ratio (1.0) is the best value. CoI2 was supported on AC in the presence of stearic acid to decompose LiBH4. 40 wt % CoI2 on AC can achieve 148 mL/min g-cat at 20 C, 1275.06 mL/min g-cat at 70 C. We believe this
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
15217
Acknowledgments We are grateful to the Eti Mine Works of Turkey for absolute supports. Also this work was supported by the Gazi University BAP Project No. 06/2010-57.
references
Fig. 5 e FT-IR graphs of commercial and produced LiBH4 (1) original LiBH4 and (2) synthesized LiBH4 (1000 min, MgH2/ LiBO2 ratio is 1.0).
Fig. 6 e Concentration changes through with time at different temperatures, CaOeCa-time graph.
Fig. 7 e Changes of reaction rate constants with the inverse of temperature, lnk-1/T graph.
value is sufficient to feed the fuel cell systems if there is no high consumption rates are necessary. Dehydrogeneration experiments allow zero order kinetic and its activation energy is 33,12 kJ/mol. In order to improve the hydrogen generation rate and the efficiency of the dehydrogneration process, additional studies about nano-size catalyst synthesis and additives that increase the activity and catalyst life should be performed.
[1] Bilen Murat, Gu¨ru¨ Metin, C ¸ akanyıldırım C¸etin. Role of NaCl in NaBH4 production and its hydrolysis. Energy Convers Manag 2013;72:134e40. [2] Kojima Y, Kawai Y, Kimbara M, Nakanishi H, Matsumuto S. Hydrogen generation by hydrolysis reaction of lithium borohydride. Int J Hydrogen Energy 2004;29:1213e7. [3] Kojima Y, Suzuki K, Kawai Y. Hydrogen generation from lithium borohydride solution over nano-sized platinum dispersed on LiCoO2. J Power Sources 2006;155:325e8. [4] Kaya S, Gu¨ru¨ M, Ar I. Synthesis of magnesium borohydride from its elements and usage in hydrogen recycle. Energy Sources Part A 2011;33:2157e70. [5] Matsunaga T, Buchter F, Miwa K, Towata S, Orimo S, Zuttel A. Magnesium borohydride: a new hydrogen storage material. Renew Energy 2008;33:193e6. [6] Orimo S, Nakamori Y, Kitahara G, Miwa K, Ohba N, Towata S, et al. Dehydriding and rehydriding reactions of LiBH4. J Alloy Compd 2005;404e406:427e30. [7] C¸akanyıldırım C¸, Gu¨ru¨ M. Processing of LiBH4 from its elements by ball milling method. Renew Energy 2008;33:2388e92. [8] Miwa K, Ohba N, Towata S, Nakamori Y, Orimo S. Firstprinciples study on cupper-substituted lithium borohydride, (Li1_xCux)BH4. J Alloy Compd 2005;404e406:140e3. [9] Ichikawa T, Hanada N, Isobe S, Leng HY, Fujii H. Hydrogen storage properties in Ti catalyzed LieNeH system. J Alloy Compd 2005;404e406:435e8. [10] Nakamori Y, Orimo S. Destabilization of Li-based complex hydrides. J Alloy Compd 2004;370:271e5. [11] Kojima Y, Haga T. Recycling of sodium metaborate to sodium borohydride. Int J Hydrogen Energy 2003;28:989e93. [12] Zuttel A, Wenger P, Rentsch S, Sudan P, Ph Mauron, Ch Emmenegger. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1e7. [13] Pis‚kin MB. Investigation of sodium borohydride production process: ulexite mineral as a boron source. Int J Hydrogen Energy 2009;34:4773e9. [14] Kantu¨rk A, Piskin S. Innovation in sodium borohydride production process from borosilicate glass with sodium under hydrogen atmosphere: high pressure process. Int J Hydrogen Energy 2007;32:3981e6. [15] C¸akanyıldırım C¸, Gu¨ru¨ M. Production of NaBH4 from its elements by mechano-chemical reaction and usage in hydrogen recycle. Energy Sources Part A 2011;33:1912e20. [16] Patel N, Fernandes R, Miotello A. Hydrogen generation by hydrolysis of LiBH4 with efficient CoePeB catalyst: a kinetic study. J Power Sources 2008;188:411e20. [17] Fernandes R, Patel N, Miotello A, Filippi M. Studies on catalytic behavior of CoeNieB in hydrogen production by hydrolysis of LiBH4. J Mol Catal A Chem 2009;298:1e6. [18] C¸akanyıldırım C¸, Gu¨ru¨ M. Production of NaBH4 and hydrogen release with catalyst. Renew Energy 2009;34:2362e5.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis of LiBH4 from LiBO2 as hydrogen carrier and its catalytic dehydrogenation Murat Bilen a, Orhan Yılmaz a, Metin Gu¨ru¨ b,* a b
Eti Mine Works, Ayvalı Mah. Halil Sezai Erkut Cad., Afra Sok. No:1/A 06010, Etlik e Kec¸i€oren, Ankara, Turkey Gazi University, Eng. Fac., Chemical Eng. Dep., Maltepe 06570, Ankara, Turkey
article info
abstract
Article history:
This study deals to synthesize LiBH4 by means of mechano-chemical reaction, in which
Received 25 December 2014
LiBO2 and MgH2 were utilized. In order to obtain 90% LiBH4 purity, optimum reaction
Received in revised form
conditions were analyzed at different values of reactant ratio and duration. In addition,
19 February 2015
commercial LiBH4 was hydrolyzed with CoI2 supported on activated carbon (AC). In order to
Accepted 21 February 2015
follow up the intermediate steps of the studies FT-IR, XRD analyses have been used.
Available online 16 March 2015
Activation energy of the dehyrdogeneration reaction was searched at different reaction temperatures and determined as 33.12 kJ/mol. As an optimum operation time for ball-
Keywords:
milling were recorded as 1000 min and reactant ratio (MgH2/LiBO2) were found as 1.0.
LiBH4
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Catalytic dehydrogenation CoI2 LiBO2
Introduction Carbon based fuels have been creating huge amount of energy demand for long years. There is a widespread idea about the carbon based energy sources which is obviously reflecting they will be exhausted within a few decades. Economic and environment friendly energy sources have to be founded in order to satisfy this huge amount of consumption demand. Combustion of fossil fuels leads to global warming and air pollution due to presence of CO2, SOx and NOx in the emission gasses [1]. Fuel cells using hydrogen as fuel have been successful in producing energy effectively. A proton source is needed to feed the fuel cells for an uninterrupted energy production. There are several methods of safe storage and transportation of hydrogen to the end user. These are the storage of
hydrogen within a pressurized steel tank, as a liquid under the cryogenic conditions, on activated carbon, in carbon nanotube, in graphite nanofiber, or in alloys suitable to adsorption and desorption of hydrogen in their structures [2,3]. Metal borohydrides can adsorb hydrogen and store it in solid form. These compounds release hydrogen by thermal decomposition or catalytic desorption. Theoretically, thermal decomposition yields only 10% (by mass) of the hydrogen in the material used for this purpose. Catalytic approach is advantageous since the hydrogen of the water is also used [4,5]. Hydrogen storage in metal borohydrides due to the excellent properties of boron, is regarded the most promising alternative. For example with sodium borohydride (NaBH4), hydrogen can be produced by the reaction between the water and
* Corresponding author. Fax: þ90 312 2308434. E-mail address: [email protected] (M. Gu¨ru¨). http://dx.doi.org/10.1016/j.ijhydene.2015.02.085 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
15214
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
NaBH4 in the presence of a catalyst. LiBH4 is also a potential candidate for hydrogen storage, because of its high hydrogen capacity: 18.5% (by mass), 121 kg/m3, respectively [6]. This compound has the highest gravimetric hydrogen densities of 37% (by mass) when catalytically reacted with cycling water [3]. Conventionally, LiBH4 is produced by using NaBH4 in the presence of isopropyl amine as shown below. Different procedures are applied but LiBH4 cannot be synthesized under elevated condition: 150 bars of hydrogen at 923 K [7].
Na(BH4) þ LiCl / Li(BH4) þ NaCl
(1)
Decreasing the operating temperature (600e700 K) during desorption is desirable through the use of additives because of the low melting point (550 K) of LiBH4 [6]. Mg, SiO2, Cu are used for this purpose and promising results are obtained [6,8]. Besides amides, imides and complex hydrides are considered for both decreasing the desorption temperature and for storing hydrogen by themselves [9,10]. However, LiBH4 actually has two desorption temperature. The first one starts at about 473 K and gives a maximum peak at 623 K. At this range the structure gives 3 4 of its hydrogen the rest staying in the LiH structure. The second region starts at about 726 K and in this region no hydrogen is left in the solid [11]. Another way of lowering the desorption temperature of Li-based borohydrides can be proposed by substitution Li with a smaller sized, more electronegative and higher valence cations such as Cu and Mg. This method results in at least a 50 K decrease in the desorption temperature [8e12]. In the current study, LiBO2 was processed in ball-milling reactor in the presence of MgH2 in order to synthesize LiBH4. In addition, CoI2 was supported on AC. Catalyst behavior and kinetic parameters were studied in details.
Materials and method LiBH4 was provided from Merck for the dehydrogenation experiments at 98% purity. MgH2 and LiBO2 were provided by Merck and Carlo-Erba respectively with 99.9% purity. LiBH4 was synthesized by means of the Reaction 2, in Spex 8000 M type ball-milling. In order to check the phase composition and crystalline state of the catalysts, XRD experiments were performed by Shimadzu powder diffrac˚ ). Jasco 480 model FTtometer (Cu Ka radiation, k ¼ 1.5406 A IR was used simultaneously to determine the chemical chances during mechano-chemical process. The thermodynamic values of this reaction were summarized in Table 1. A negative Gibbs free energy value indicates the possibility of the desired reaction.
LiBO2 þ 2MgH2 / LiBH4 þ 2MgO
Table 1 e Thermodynamic data of Reaction 2. LiBO2 þ 2MgH2 / LiBH4 þ 2MgO DHf, kJ/mol DSf, J/mol.K DGf, kJ/mol Kp lnKp
216.77 23.797 224.22 1.09 0.086
atmosphere otherwise indicated. The selected durations for ball-milling reaction are 300, 600, 800 and 1000 min. The product (LiBH4) of reaction was purified by means of filtration with ethylene diamine (EDA). Filtrate was dried at room temperature, under vacuum. Evaporated EDA held in cold trap immersed in liquid nitrogen.
Catalyst preparation CoI2 has supported on activated carbon in the presence of 25 wt % stearic acid by milling for one hour. The mixture pressed and shaped as cylinder (13 mm diameter and 3.6 mm high) under 590 MPa for the dehydrogenaration reaction of LiBH4. The optimum amount of CoI2 catalyst, which is 40%, has been used [1]. Catalyst pellets containing 40% CoI2 was kept at 200 C for 1 h to remove the stearic acid and create macro pores. Measurements showed that the weights of the catalyst pellets are at about 0.55 ± 0.05 g.
Dehydrogenation tests Catalyst pellets were placed in dehydrogenation reactor. 100 mg (2.632 mmol) of LiBH4 and 70 mg (1.75 mmol) of LiOH were dissolved in deionized water and introduced into reactor. Produced hydrogen was measured in inverse burette system shown in Fig. 1. After completion of the dehydrogenation, catalyst rapidly rinsed with deionized water and next hydrogenation takes place.
(2)
MgH2/LiBO2 mole ratios were arranged as 0.65, 1.0 and 1.3. All studies performed and reactants were handled under inert
Fig. 1 e Hydrolysis setup (1) catalyst, (2) LiBH4eLiOH solution, (3) reactor, (4) water reservoir, and (5) burette.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
Fig. 2 e FT-IR analyses that show the effect of the working time (1) 600 min, (2) 300 min, (3) 800 min, and (4) 1000 min.
Kinetic studies There were eight distinct experiments (at 20, 30, 40, 50, 55, 60, 65 ve 70 C) which performed with 40% CoI2 loaded on AC support. All of these kinetic experiments were performed to find out the reaction rate order and activation energy of the reaction. During the experiments reactor was kept in water bath which has 0.1 C accuracy.
Results and discussion As a hydrogen storage material, LiBH4 usage is directly depends on the feasibility of production. In this study, working time and reactants ratio are analyzed in order to determine the optimum conditions for both economy and high efficiency. According to studied reaction times, the longer duration the better yields as it is shown by the FT-IR spectrums in Fig. 2. It is obvious that the duration improves the results without any dependence to reactant ratios. As it can be followed from Fig. 2, the peaks between 2400 cm-1 and 2200 cm-1 clearly prove this fact. Micro peaks at 1461 and 1465 cm-1 give MgO existence and the success of the oxygen removal from B2O3 structure like in the case of previous studies [1,13,14]. Additionally, Fig. 3 was created to show the areas covered by of FTIR peaks at which 2400 cm-1e2200 cm-1 region. These integrations are showing the positive effect of working time for the experiments. However, in order to reduce the working time, process might be stopped at a certain time and the content of the LiBH4 in the reaction product can be increased by further purification processes to get economic process in terms of energy. XRD analysis performed for the different working times are shown in Fig. 4. According to the software of the XRD device that makes calculation by means of Scherrer equation, the results indicate that the products are in crystalline phase and the particles are in nano-scale. Mechanochemical devices generally provide nanoscale reactants and/or products, which is necessary to obtain high yields in short duration [15,7]. However, in Fig. 4, the peak was detected belonging to LiBH4. This is simply may be caused by the
15215
Fig. 3 e The effect of stoichiometry and working time on FT-IR peak areas indicating LiBH4 existence.
amorphous phase of LiBH4. Moreover, the realization of diffraction peaks of the starting reactants can be possible. This refers to uncompleted transaction and requirement for the purification process. XRD spectrum also indicates that the MgH2 peak at 28 2 h became weaker proportional to working time meanwhile MgO peak at 43 2 h is more visible for longer durations. These changes simply indicate the occurrence of the Reaction 2. Purification process is inevitable to obtain high yield and remove the some side products. EDA is a suitable solvent to separate the LiBH4 as well as it does not decompose LiBH4 or cause any reaction with the residue. If it is compared with the side product's (Fig. 4b), XRD of the purified sample (Fig. 4a) shows great difference. It can be understand from long and sharp peaks that the LiBH4 becomes in crystal phase during dissolving, filtration and drying processes. XRD analyses of the solid filtration product indicates that it contains unreacted MgH2, LiB(OH)2 and side product MgO at 28, 32 and 43 2 h, respectively (Fig. 4b). Commercial and synthesized LiBH4 were compared by means of FT-IR analysis in Fig. 5. Results indicate that the two samples have similar peaks at 2400 cm1 e2200 cm-1, but residue EDA makes it hard to determine the peaks between 1700 cm-1 and 900 cm-1.
Kinetic studies In order to rapid release of hydrogen, in catalytic dehydrogenation, hydrogen containing substances need catalysts. Hydrogen atoms both come from LiBH4 and solvent water while the decomposition. Oxygen of the water adheres to lithium and boron and forms LiBO2. This compound is easy to recycle and that increase the feasible utility of LiBH4. CoI2 was utilized to synthesize 40% CoI2 concentration AC supported catalysts [1]. CoI2 is reduced by LiBH4 and actually Co and (or) CoBx (x ¼ 0.5, 1 or 2) starts to play an important role for the hydrogen generation [16e18].
Kinetic behaviors of the catalyst 40wt % CoI2/AC catalyst was studied at eight different temperatures (20, 30, 40, 50, 55, 60, 65 and 70 C) (Fig. 6) and reaction rate order was investigated. Experimental data fit zero
15216
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
Fig. 4 e XRD diffractions of (a) synthesized LiBH4 from solution and (b) filtration cake (MgH2/LiBO2 ratio is 1, 1000 min).
order kinetic (Fig. 7). This means that reaction depends on catalyst used instead of concentration. Activation energy computation results 33.12 kJ/mol. If compared with the values given in other studies, activation energy value is very advantageous as it is lower.
Conclusion This study attends to analyze the synthesis of LiBH4 by means of LiBO2 in ball-milling for the first time. Results of
the study show that the produced LiBH4 needs to be purified. By purification it reaches to 90% purity and becomes in crystalline phase after purification. It was observed that the yield of the products increases with working time. Thus 1000 min has been determined to be the optimum reaction time. Furthermore, studies with different ratios of MgH2/ LiBO2 result in that the stoichiometric ratio (1.0) is the best value. CoI2 was supported on AC in the presence of stearic acid to decompose LiBH4. 40 wt % CoI2 on AC can achieve 148 mL/min g-cat at 20 C, 1275.06 mL/min g-cat at 70 C. We believe this
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 5 2 1 3 e1 5 2 1 7
15217
Acknowledgments We are grateful to the Eti Mine Works of Turkey for absolute supports. Also this work was supported by the Gazi University BAP Project No. 06/2010-57.
references
Fig. 5 e FT-IR graphs of commercial and produced LiBH4 (1) original LiBH4 and (2) synthesized LiBH4 (1000 min, MgH2/ LiBO2 ratio is 1.0).
Fig. 6 e Concentration changes through with time at different temperatures, CaOeCa-time graph.
Fig. 7 e Changes of reaction rate constants with the inverse of temperature, lnk-1/T graph.
value is sufficient to feed the fuel cell systems if there is no high consumption rates are necessary. Dehydrogeneration experiments allow zero order kinetic and its activation energy is 33,12 kJ/mol. In order to improve the hydrogen generation rate and the efficiency of the dehydrogneration process, additional studies about nano-size catalyst synthesis and additives that increase the activity and catalyst life should be performed.
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