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Role of NaCl in NaBH4 production and its hydrolysis
Energy Conversion and Management 72 (2013) 134–140
Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Role of NaCl in NaBH4 production and its hydrolysis Murat Bilen a, Metin Gürü b,⇑, Çetin Çakanyıldırım c a
Eti Mine Works, Tech. Dev. Dep., Fatih S. Mehmet Bul. No: 179, Etimesgut, Ankara, Turkey Gazi University, Eng. Fac., Chemical Eng. Dep., Maltepe 06570, Ankara, Turkey c Hitit University, Eng. Fac., Chemical Eng. Dep., 19030 Çorum, Turkey b
a r t i c l e
i n f o
Article history: Available online 1 April 2013 Keywords: Hydrogen NaBH4 Hydrolysis Catalytic dehydrogenation
a b s t r a c t This study explains completely feasible route to synthesize NaBH4 by means of mechano-chemical reaction, in which NaCl, B2O3 and MgH2 were utilized. Optimum reaction conditions were investigated by different values of reactant ratio and duration to reach 95% NaBH4 purity. In addition, commercial NaBH4 was hydrolyzed with CoI2 supported on activated carbon (AC). Intermediate steps of the studies were traced by means of FT-IR, XRD, SEM and EDX analyses. Activation energy of the catalyst was searched at distinct reaction temperatures and found as 36 kJ/mol. Optimum working time for ball-milling reactor and reactant ratio (MgH2/NaCl) were obtained as 1000 min and 1.0, respectively. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Hydrocarbon containing fossil fuels are the biggest and premier sources to provide the energy demand for many years [1]. All scientists are in consistence that the carbon based energy reservoirs will be expired within a few decades. Thus, economic and environmental friendly energy sources have to be invited to meet that great consumption demand. Energy systems working with hydrogen molecules are very attractive to create comprehensive solutions [2]. Additives doped on Al alloys such as Al–Hg and Al–In in ball-millig can be used to hydrolyze water and liberate hydrogen. NaCl doped mixture can provide 971 mL/min g cat. hydrogen generation rate in pure water [3]. In case higher hydrogen generation rates are required, metal borohydrides are very effective and secure storage mediums to keep hydrogen for long duration. NaBH4 is widely used to reduce reactions and to store hydrogen. It is also the best candidate to feed the fuel cells continuously and deserves to be researched in details. However, some technical problems limit the marketing of NaBH4 and wait for new approaches to raise the feasibility, environmental recyclability and also reduce the production cost of the process [4,5]. Thus regeneration of the hydrolysis spent fuel (NaBO2), is an adequate step to increase the feasibility of the fuel cell systems, which are provided by NaBH4’s hydrogens [6]. There are many methods to synthesis NaBH4 and new ones are being discovered. In the last 60 years, 1000 methods have been suggested for the production of NaBH4 [7]. However it was firstly
⇑ Corresponding author. Tel.: +90 312 5823555; fax: +90 312 2308434. E-mail address: [email protected] (M. Gürü). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2012.08.031
synthesized by Schlesinger et al. [8] according to the following reaction that is the most applied process worldwide.
4NaH þ BðOCH3 Þ3 ¢ NaBH4 þ 3NaOCH3
ð1Þ
Unlike Schlesinger process, mechano-chemical reactions happen by collisions of the particles at high speeds, in other words high kinetic energy. That is beneficial to provide nano-scale working conditions and make some chemical changes possible that are otherwise not easy to achieve or to result limited yields. Syntheses of MgB2 [9], LiBH4 and KBH4 are some of the examples among many applications performed in ball-milling reactors [10,11]. Çakanyıldırım and Gürü have synthesized NaBH4 by mechanochemical reaction, shown in reaction 2. Sodium metaborate (NaBO2) and excess amount of magnesium hydride (MgH2) were utilized and 93% yield has achieved after purification processes [6]. Similarly Li and co-workers synthesized NaBH4 with high purity, in ball milling. Anhydrous borax (Na2B4O7) and MgH2 were used as reactants and MgH2 was also used in 30% excess to benefit its extreme surface conditions, which enhance the conversion [12]. It is also possible to produce NaBH4 under hydrogen atmosphere in the presence of sole Mg addition. In such studies, it was observed that the Ni, Fe and Co elements increase the conversion of NaBO2. However melting point of the Mg element limits the reaction temperature and yield [13].
NaBO2 þ 2MgH2 ! NaBH4 þ 2MgO
ð2Þ
Mainly two ways applied to liberate the hydrogen of NaBH4; (i) thermal dehydrogenation and (ii) catalytic dehydrogenation. Thermal applications are limited by the melting temperature of the reactants. Moreover, it is difficult to obtain all of the hydrogen and the reaction residue is composed of some complex boron compounds, which are not easy to recycle. On the other hand, catalytic
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process makes it possible to gain 2.5 L of hydrogen per gram of NaBH4. This amount equals twice that of solely NaBH4 has and indicates hydrolysis of water molecules happens during the process [5,14]. NaOH generally used to suppress the sudden hydrogen release during the catalytic hydrolysis. There are many studies discussing the effect of NaOH amount on the reaction. However there is no common idea about the effect of NaOH. Prevalent idea on that 3–5% of NaOH is adequate to control the reaction [15,16]. Alike NaOH, amount of the water utilized in the catalytic hydrolysis changes the weight of the system and also feasibility. Minimum water amount is limited by the solubility of NaBH4 and NaBO2 those are equal to 0.55 and 0.28 g per mL of water. Water requirement of the hydrolysis system must be fifteen times less than the current systems to apply this technology on board engines [14]. In catalytic applications, high surface area is necessary to provide enough active sites for the molecules to react. Active sites must be homogeneously distributed over the support and catalyst preparation conditions and technique has great effect on the homogeneity and the particle size of the catalyst. Also support material must not be broken or dissolved in NaBH4 alkaline solution. In that manner, activated carbon, alumina and silica could be considered as suitable support materials. Salts of 3d transition metals have extreme desorption behaviors. The platinum family metals can be applied for the catalytic hydrolysis of NaBH4. They are effective at room temperature, following the order Ru, Rh > Pt > Co > Ni > Os > Ir > Fe Pd. Among these metals Co and Ni are likely to be widely used in catalytic processes because of their advantageous costs [7]. PtRu–LiCoO2 catalyst efficiency was found to be twice as high as that of either Ru–LiCoO2 or Pt–LiCoO2 [15]. Zhang et al. studied in detail the kinetics of NaBH4 hydrolysis on Ru-on-carbon catalyst. At low temperatures zero-order and at high temperatures, first-order kinetic behaviors were obtained. Results prove that the reaction consists of two steps: first, NaBH4 adsorption on the surface, and second, reaction of the adsorbed species on the catalyst surface. The Langmuir– Hinshelwood model led to the conclusion that the adsorption step was responsible for kinetic order variation [16]. Besides, exothermic character of the reaction may shape the reaction order. Thus changes in reaction temperature cause order to shift from zero to first. In the current study, naturally available NaCl salt was processed in ball-milling reactor in the presence of MgH2 and B2O3 in order to synthesize NaBH4. In addition, CoI2 was supported on AC and utilized for catalytic decomposition NaBH4. Catalyst behavior and kinetic parameters were studied in details.
Table 1 Thermodynamic data of reaction 3. 2NaCl + 4MgH2 + B2O3 ? 2NaBH4 + 3MgO + MgCl2
DHf (kJ/mol) DSf (J/mol K) DGf (kJ/mol) Kp LnKp
367.823 66.819 388.738 1.161 0.149
MgH2/NaCl mol ratios have arranged as 0.65, 1, and 1.3. Reactants were handed and all studies take place under inert atmosphere otherwise indicated. 300, 600, 800 and 1000 min have considered for the ball-milling reaction durations. In order to purify the reaction product (NaBH4) filtration method was applied by means of ethylene diamine (EDA) whose solubility is 22.0 g NaBH4/ 100 g EDA at 75 °C [17]. Filtrate was dried at room temperature, under vacuum. Evaporated EDA held in cold trap immersed in liquid nitrogen.
2.1. Catalyst preparation CoI2 has supported on activated carbon in the presence of 25 wt% stearic acid by milling for one hour. The mixture pressed to give cylindrical shape (13 mm diameter and 3.6 mm high) under 590 MPa for the hydrolysis reaction of NaBH4. Two features were taken in the account to decide the optimum catalyst composition; first the physical strength, second the hydrogen generation rate of the catalyst. Catalyst pellets containing 20%, 30%, 40% and 45% CoI2 were 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.
2.2. Dehydrogenation tests Catalyst pellets were put in hydrolysis reactor. 100 mg (2.632 mmol) of NaBH4 and 70 mg (1.75 mmol) of NaOH were dissolved in deionized water and introduced into reactor. Produced hydrogen was measured in inverse burette system depicted in Fig. 1 in details. After completion of the hydrolysis, catalyst rapidly rinsed with deionized water and next hydrogenation takes place.
2. Experimental NaBH4 was provided from Merck for the hydrolysis experiments at 98% purity. MgH2 and NaCl were provided by Merck and CarloErba with 99.9% purity. Eti Mine Works supports B2O3 with 99% purity. NaBH4 was synthesized according to the reaction 3, in Spex 8000M type ball-milling. XRD experiments were performed to control the phase composition and crystalline state of the catalysts by Shimadzu powder diffractometer (Cu Ka radiation, k = 1.5406 Å). Jasco 480 model FT-IR was used simultaneously to determine the chemical chances during mechano-chemical process. Joel 6360 LV SEM equipped with EDX detector and Quantachrome Nova 2200 sorptometer were utilized to investigate the surface and pore morphology of the catalyst. The thermodynamic values of this reaction were summarized in Table 1. Negative value of the Gibbs free energy indicates the possibility of the desired reaction.
2NaCl þ 4MgH2 þ B2 O3 ! 2NaBH4 þ 3MgO þ MgCl2
ð3Þ
Fig. 1. Hydrolysis setup (1) catalyst, (2) NaBH4–NaOH solution, (3) reactor, (4) water reservoir, and (5) burette.
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Fig. 2. FT-IR analyses that show the effect of the working time (a) 300 min, (b) 600 min, (c) 800 min, and (d) 1000 min.
2.3. Kinetic studies Eight distinct experiments (at 20, 30, 40, 50, 55, 60, 65 ve 70 °C) were performed with 40% CoI2 loaded on AC support. All of the kinetic trials were aimed to find out the reaction rate order and activation energy of the reaction. During the experiments reactor was kept in water bath whose accuracy is 0.1 °C.
3. Results and discussion NaBH4 usage as a hydrogen storage material is directly depends on the feasibility of its production. Thus in economic point of view NaCl, which is one of the cheapest substances on the markets, has been chosen as one of the reactant to perform NaBH4 synthesis. Working time and reactants ratio are two main parameters investigated in this study to determine the optimum conditions for both economy and high yield. Studied reaction times indicate that the longer duration results better yields as given by the FT-IR spectrums in Fig. 2. It is evident that the duration enhances the results without dependence to reactant ratios. Peaks between 2400 and 2200 cm1 clearly prove this fact. Micro peaks at 1461 and 1465 cm1 give MgO existence and the success of the oxygen remove from B2O3 structure similar to the previous studies [1,5,9,18,19]. Furthermore Fig. 3 was created to show the areas covered by of FT-IR peaks at which 2400– 2200 cm1 region. These integrations depict to 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 NaBH4 in the reaction product can be raised by further purification processes to provide the energy economy of the process.
XRD analysis performed for the different working times are given in Fig. 4. Results indicate that the products are in crystalline phase and the particles are in nano scale according to the software of the XRD device that makes calculation by Scherrer equation. Mechano-chemical devices generally provide ⁄⁄nano scale reactants and (or) product, which is necessary to obtain high yields in short duration [20,21]. However, in Fig. 4 any peak was detected belonging to NaBH4. May be, this is simply caused by the amorphous phase of NaBH4. Moreover, it is possible to realize the diffraction peaks of the starting reactants. That indicates uncompleted transaction and the necessity of purification process. XRD spectrum also indicates that the MgH2 peak at 28 2h became weaker proportional to working time meanwhile MgO peak at 43 2h is more visible for longer durations. These changes simply indicate the occurrence of the reaction 3. Purification process is indispensible to obtain high yield and remove the some side reaction products. EDA is a suitable solvent to separate the NaBH4. It does not decompose NaBH4 or cause any reaction with the residue. XRD determination of the purified sample (Fig. 5a) shows great difference if compared with the side product’s (Fig. 5b). Long and sharp peaks prove that the NaBH4 becomes in crystal phase during dissolving, filtration and drying processes. XRD analyses of the solid filtration product show that it contains unreacted MgH2, NaCl and side product MgO at 28, 32 and 43 2h, respectively (Fig. 5b). Commercial and synthesized NaBH4 were compared by FT-IR analysis in Fig. 6. Results depict that the two samples have similar peaks at 2400–2200 cm1, however residue EDA makes it hard to determine the peaks between 1700 and 900 cm1.
3.1. Kinetic studies In catalytic dehydrogenation, hydrogen containing substances need catalysts to rapidly release hydrogen. While the decomposition, hydrogen atoms both come from NaBH4 and solvent water. Oxygen of the water adheres to sodium and boron and forms NaBO2. This compound is easy to recycle and that raise the feasible usage of NaBH4. CoI2 was utilized to synthesize 4 different concentration (20, 30, 40 and 45 wt%) AC supported catalysts. 20 wt% CoI2 including catalyst could only be used for 28 times and lost its activity completely. Low solubility and blockage of the side product NaBO2 and removal of Co particles during the process should result that activation loose. Moreover, pH chances because of the formation of BðOHÞ 4 anion can limit the hydrogen generation rate [22]. Two hydrogen generation rates, minimum and maximum, were measured for 20 wt% CoI2/AC catalyst as 2.78 and 31.92 mL/min g cat.
Fig. 3. Stoichiometry and working time effect on FT-IR peak areas that indicates NaBH4 existence.
M. Bilen et al. / Energy Conversion and Management 72 (2013) 134–140
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Fig. 4. XRD results for reaction 3 at different durations (MgH2/NaCl ratio is 1.3).
Fig. 5. XRD diffractions of NaBH4 filtration (a) synthesized NaBH4 and (b) filtration spent fuel (MgH2/NaCl ratio is 1, 1000 min).
Despite of 20, 30 and 40 wt% CoI2/AC catalysts could achieve more than 300 hydrolysis without deactivation. In Fig. 7 some of the hydrolysis results were summarized for 40 wt% CoI2/AC. Hydrogen generation rate is always slow for the first trials since
CoI2 is reduced by NaBH4 and actually Co and (or) CoBx (x = 0.5, 1 or 2) starts to play an important role for the hydrogen generation [23,24]. Although, the long working life of the catalysts, their maximum generation rates were measured for 30 and 40 wt% CoI2/AC
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Fig. 6. FT-IR graphs of commercial and produced NaBH4 (1) original NaBH4 and (2) synthesized NaBH4 (1000 min, MgH2/NaCl ratio is 1.0).
Fig. 7. Hydrolysis test for 40 wt% CoI2/AC (100 mg NaBH4, 70 mg NaOH, 0.55 g catalyst at 20 °C).
Table 2 BET analysis of the catalyst before and after application. SBET (m2/g)
Catalyst 20 wt% CoI2/AC
Fresh Used
451.7 501.3
30 wt% CoI2/AC
Fresh Used
487.4 666.7
40 wt% CoI2/AC
Fresh Used
479.6 684.6
34.83 and 46.60 mL/min g cat, respectively. It is obvious that these rates are not satisfactory for high hydrogen consuming systems but reasonable for small size devices. Experiments performed with 45 wt% CoI2/AC could not performed more than 1 or 2 trials because of the physical weakness of the catalyst pellets. Thus, 40 wt% CoI2/AC catalyst was decided to has the best performance. Fig. 7 explains that the catalyst can keep its activity for many attempts, which take 1000 h approximately. On the other hand, it is evident by the 20 wt% CoI2/AC performance, the catalyst activity always decrease by time. Therefore, it may be assumed that if optimum catalyst and support ratio is provided decrease of activa-
tion can be compensated by something different. Performed BET, EDX and SEM analysis make this phenomenon easy to understand. Surface areas of the catalyst before and after the experiments are given in Table 2. During the process, all catalysts become rich in surface area. That means more interaction possibility between catalyst and BH4- anion and higher hydrogen generation values. Thus we may conclude that the physical changes on the catalyst surface compensate the deactivation and provide the solution to reach deeper. SEM micrographs given in Fig. 8 clearly demonstrate that the physical changes occurred on the surface. Moreover EDX results (not reported) prove that the surface is covered by NaBO2 even the subsequently rinsing of the catalysts. This coverage starts to decrease the activation of the catalyst pellet at the further applications. 3.2. Kinetic behaviors of the catalyst 40 wt% CoI2/AC catalyst was studied at eight different temperatures (20, 30, 40, 50, 55, 60, 65 and 70 °C) and reaction rate order was investigated. Experimental data fit zero order kinetic (Fig. 9) that means reaction depends on catalyst used instead of concentration. Activation energy computation results 36.63 kJ/mol. This low activation energy value is very advantageous if compared with the values given in other studies.
M. Bilen et al. / Energy Conversion and Management 72 (2013) 134–140
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Fig. 8. SEM micrographs of 40 wt% CoI2/AC catalyst before and after 300 tests.
Fig. 9. (a) Concentration changes with time at different temperatures. (b) Changes of reaction rate constants with the inverse of temperature.
4. Conclusion Widespread usage of the fuel cell systems are strongly depend on the feasibility of the hydrogen providing chemicals and the economy of utilized catalysts. This study discusses the synthesis of NaBH4 by means of NaCl in ball-milling for the first time. Results prove that the produced NaBH4 needs to be purified and it reaches 95% purity and becomes in crystalline phase after purification. It was observed that the yield of the products raise with working time. Thus 1000 min has been decided to be the optimum reaction time. Moreover, studies with different ratios of MgH2/NaCl 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 NaBH4. 40 wt% CoI2 on AC can achieve 45.6 mL/min g cat. hydrogen generation rate at room temperature for 300 cycle (approximately 1000 h) long. We believe this value is adequate to feed the fuel cell systems unless high consumption rates are necessary. Hydrolysis experiments obey zero order kinetic and its activation energy is 36.63 kJ/mol. Extra studies concerning, nano-
size catalyst synthesis and additives that increase the activity and catalyst life, should be performed to improve the hydrogen generation rate and the efficiency of the hydrolysis process. Acknowledgments We are indebted to the Eti Mining Works of Turkey for continuing support. Also this work was supported by the Gazi University BAP Project No. 06/2010-57. References [1] Pisßkin MB. Investigation of sodium borohydride production process: ulexite mineral as a boron source. Int J Hydrogen Energy 2009;34:4773–9. [2] Amendola SC, Shaip-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25(10):969–75. [3] Fan M, Sun L, Xu F. Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation. Energy Convers Manage 2010;51(3):594–9.
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[4] Demirci UB, Akdim O, Miele P. Ten-year effort sand a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage. Int J Hydrogen Energy 2009;34(6):2638–45. [5] Çakanyıldırım Ç, Gürü M. Production of NaBH4 and hydrogen release with catalyst. Renew Energy 2009;34(11):2362–5. [6] Çakanyıldırım Ç, Gürü M. Processing of NaBH4 from NaBO2 with MgH2 by ballmilling and usage as hydrogen carrier. Renew Energy 2010;35:1895–9. [7] Santos DMF, Sequeira CAC. Sodium borohydride as a fuel for the future. Renew Sustain Energy Rev 2011;15:3980–4001. [8] Schlesinger HI, Brown HC, Finholt AE. The preparation of sodium borohydride by the high temperature reaction of sodium hydride with borate esters. J Am Chem Soc 1953;75:205–9. [9] Akyüz Y, Gürü M, Tekeli S. Synthesis of MgB2 from anhydrous B2O3 by ballmilling method and determination of super conductivity properties. Key Eng Mater 2007;336–338:723–5. [10] Çakanyıldırım Ç, Gürü M. Processing of lithium borohydride from its elements by ball milling method. Renew Energy 2008;33:2388–92. [11] Li ZP, Liu BH, Morigasaki N, Suda S. Preparation of potassium borohydride by a mechano-chemical reaction of saline hydrides with dehydratedborate through ball-milling. J Alloys Compd 2003;354:243–7. [12] Li ZP, Morigazaki N, Liu BH, Suda S. Preparation of sodium borohydride by the reaction of MgH2 with dehydrated borax through ball milling at room temperature. J Alloys Compd 2003;349:252–6. [13] Liu BH, Li ZP, Morigasaka N, Suda S. Kinetic characteristics of sodium borohydride formation when sodium meta-borate reacts with magnesium. Int J Hydrogen Energy 2008;33:1323–8. [14] Çakanyıldırım Ç, Gürü M. Hydrogen cycle with sodium borohydride. Int J Hydrogen Energy 2008;33:4634–9.
[15] Kriskan P, Yang TH, Lee WY, Kim CS. PtRu–LiCoO2 an efficient catalyst for hydrogen generation from sodium borohydride. J Power Sources 2005;143:17–23. [16] Zhang JS, Delgass WN, Fisher TS, Gore JP. Kinetic of Ru-catalyzed sodium borohydride hydrolysis. J Power Sources 2007;164:772–81. [17] Fessenden RJ, Fessenden JS. Organic chemistry. fifth ed. California: Brooks/Cole Publishing Company; 1994. [18] Kojima Y, Haga T. Recycling of sodium metaborate to sodium borohydride. Int J Hydrogen Energy 2003;28:989–93. [19] Kantürk A, Pisßkin S. Innovation in sodium borohydride production process from borosilicate glass with sodium under hydrogen atmosphere: high pressure process. Int J Hydrogen Energy 2007;32:3981–6. [20] Çakanyıldırım Ç, Gürü M. Production of NaBH4 from its elements by mechanochemical reaction and usage in hydrogen recycle. Energy Sources Part A 2011;33:1912–20. _ Synthesis of magnesium borohydride from its elements [21] Kaya S, Gürü M, Ar I. and usage in hydrogen recycle. Energy Sources Part A 2011;33(23):2157–70. [22] Andrieux J, Demirci UB, Hannauer J, Gervais C, Goutaudier C, Miele P. Spontaneous hydrolysis of sodium borohydride in harsh conditions. Int J Hydrogen Energy 2011;36:224–33. [23] Patel N, Fernandes R, Miotello A. Hydrogen generation by hydrolysis of NaBH4 with efficient Co–P–B catalyst: a kinetic study. J Power Sources 2008;188:411–20. [24] Fernandes R, Patel N, Miotello A, Filippi M. Studies on catalytic behavior of Co– Ni–B in hydrogen production by hydrolysis of NaBH4. J Mol Catal A Chem 2009;298:1–6.
Contents lists available at SciVerse ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Role of NaCl in NaBH4 production and its hydrolysis Murat Bilen a, Metin Gürü b,⇑, Çetin Çakanyıldırım c a
Eti Mine Works, Tech. Dev. Dep., Fatih S. Mehmet Bul. No: 179, Etimesgut, Ankara, Turkey Gazi University, Eng. Fac., Chemical Eng. Dep., Maltepe 06570, Ankara, Turkey c Hitit University, Eng. Fac., Chemical Eng. Dep., 19030 Çorum, Turkey b
a r t i c l e
i n f o
Article history: Available online 1 April 2013 Keywords: Hydrogen NaBH4 Hydrolysis Catalytic dehydrogenation
a b s t r a c t This study explains completely feasible route to synthesize NaBH4 by means of mechano-chemical reaction, in which NaCl, B2O3 and MgH2 were utilized. Optimum reaction conditions were investigated by different values of reactant ratio and duration to reach 95% NaBH4 purity. In addition, commercial NaBH4 was hydrolyzed with CoI2 supported on activated carbon (AC). Intermediate steps of the studies were traced by means of FT-IR, XRD, SEM and EDX analyses. Activation energy of the catalyst was searched at distinct reaction temperatures and found as 36 kJ/mol. Optimum working time for ball-milling reactor and reactant ratio (MgH2/NaCl) were obtained as 1000 min and 1.0, respectively. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Hydrocarbon containing fossil fuels are the biggest and premier sources to provide the energy demand for many years [1]. All scientists are in consistence that the carbon based energy reservoirs will be expired within a few decades. Thus, economic and environmental friendly energy sources have to be invited to meet that great consumption demand. Energy systems working with hydrogen molecules are very attractive to create comprehensive solutions [2]. Additives doped on Al alloys such as Al–Hg and Al–In in ball-millig can be used to hydrolyze water and liberate hydrogen. NaCl doped mixture can provide 971 mL/min g cat. hydrogen generation rate in pure water [3]. In case higher hydrogen generation rates are required, metal borohydrides are very effective and secure storage mediums to keep hydrogen for long duration. NaBH4 is widely used to reduce reactions and to store hydrogen. It is also the best candidate to feed the fuel cells continuously and deserves to be researched in details. However, some technical problems limit the marketing of NaBH4 and wait for new approaches to raise the feasibility, environmental recyclability and also reduce the production cost of the process [4,5]. Thus regeneration of the hydrolysis spent fuel (NaBO2), is an adequate step to increase the feasibility of the fuel cell systems, which are provided by NaBH4’s hydrogens [6]. There are many methods to synthesis NaBH4 and new ones are being discovered. In the last 60 years, 1000 methods have been suggested for the production of NaBH4 [7]. However it was firstly
⇑ Corresponding author. Tel.: +90 312 5823555; fax: +90 312 2308434. E-mail address: [email protected] (M. Gürü). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2012.08.031
synthesized by Schlesinger et al. [8] according to the following reaction that is the most applied process worldwide.
4NaH þ BðOCH3 Þ3 ¢ NaBH4 þ 3NaOCH3
ð1Þ
Unlike Schlesinger process, mechano-chemical reactions happen by collisions of the particles at high speeds, in other words high kinetic energy. That is beneficial to provide nano-scale working conditions and make some chemical changes possible that are otherwise not easy to achieve or to result limited yields. Syntheses of MgB2 [9], LiBH4 and KBH4 are some of the examples among many applications performed in ball-milling reactors [10,11]. Çakanyıldırım and Gürü have synthesized NaBH4 by mechanochemical reaction, shown in reaction 2. Sodium metaborate (NaBO2) and excess amount of magnesium hydride (MgH2) were utilized and 93% yield has achieved after purification processes [6]. Similarly Li and co-workers synthesized NaBH4 with high purity, in ball milling. Anhydrous borax (Na2B4O7) and MgH2 were used as reactants and MgH2 was also used in 30% excess to benefit its extreme surface conditions, which enhance the conversion [12]. It is also possible to produce NaBH4 under hydrogen atmosphere in the presence of sole Mg addition. In such studies, it was observed that the Ni, Fe and Co elements increase the conversion of NaBO2. However melting point of the Mg element limits the reaction temperature and yield [13].
NaBO2 þ 2MgH2 ! NaBH4 þ 2MgO
ð2Þ
Mainly two ways applied to liberate the hydrogen of NaBH4; (i) thermal dehydrogenation and (ii) catalytic dehydrogenation. Thermal applications are limited by the melting temperature of the reactants. Moreover, it is difficult to obtain all of the hydrogen and the reaction residue is composed of some complex boron compounds, which are not easy to recycle. On the other hand, catalytic
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M. Bilen et al. / Energy Conversion and Management 72 (2013) 134–140
process makes it possible to gain 2.5 L of hydrogen per gram of NaBH4. This amount equals twice that of solely NaBH4 has and indicates hydrolysis of water molecules happens during the process [5,14]. NaOH generally used to suppress the sudden hydrogen release during the catalytic hydrolysis. There are many studies discussing the effect of NaOH amount on the reaction. However there is no common idea about the effect of NaOH. Prevalent idea on that 3–5% of NaOH is adequate to control the reaction [15,16]. Alike NaOH, amount of the water utilized in the catalytic hydrolysis changes the weight of the system and also feasibility. Minimum water amount is limited by the solubility of NaBH4 and NaBO2 those are equal to 0.55 and 0.28 g per mL of water. Water requirement of the hydrolysis system must be fifteen times less than the current systems to apply this technology on board engines [14]. In catalytic applications, high surface area is necessary to provide enough active sites for the molecules to react. Active sites must be homogeneously distributed over the support and catalyst preparation conditions and technique has great effect on the homogeneity and the particle size of the catalyst. Also support material must not be broken or dissolved in NaBH4 alkaline solution. In that manner, activated carbon, alumina and silica could be considered as suitable support materials. Salts of 3d transition metals have extreme desorption behaviors. The platinum family metals can be applied for the catalytic hydrolysis of NaBH4. They are effective at room temperature, following the order Ru, Rh > Pt > Co > Ni > Os > Ir > Fe Pd. Among these metals Co and Ni are likely to be widely used in catalytic processes because of their advantageous costs [7]. PtRu–LiCoO2 catalyst efficiency was found to be twice as high as that of either Ru–LiCoO2 or Pt–LiCoO2 [15]. Zhang et al. studied in detail the kinetics of NaBH4 hydrolysis on Ru-on-carbon catalyst. At low temperatures zero-order and at high temperatures, first-order kinetic behaviors were obtained. Results prove that the reaction consists of two steps: first, NaBH4 adsorption on the surface, and second, reaction of the adsorbed species on the catalyst surface. The Langmuir– Hinshelwood model led to the conclusion that the adsorption step was responsible for kinetic order variation [16]. Besides, exothermic character of the reaction may shape the reaction order. Thus changes in reaction temperature cause order to shift from zero to first. In the current study, naturally available NaCl salt was processed in ball-milling reactor in the presence of MgH2 and B2O3 in order to synthesize NaBH4. In addition, CoI2 was supported on AC and utilized for catalytic decomposition NaBH4. Catalyst behavior and kinetic parameters were studied in details.
Table 1 Thermodynamic data of reaction 3. 2NaCl + 4MgH2 + B2O3 ? 2NaBH4 + 3MgO + MgCl2
DHf (kJ/mol) DSf (J/mol K) DGf (kJ/mol) Kp LnKp
367.823 66.819 388.738 1.161 0.149
MgH2/NaCl mol ratios have arranged as 0.65, 1, and 1.3. Reactants were handed and all studies take place under inert atmosphere otherwise indicated. 300, 600, 800 and 1000 min have considered for the ball-milling reaction durations. In order to purify the reaction product (NaBH4) filtration method was applied by means of ethylene diamine (EDA) whose solubility is 22.0 g NaBH4/ 100 g EDA at 75 °C [17]. Filtrate was dried at room temperature, under vacuum. Evaporated EDA held in cold trap immersed in liquid nitrogen.
2.1. Catalyst preparation CoI2 has supported on activated carbon in the presence of 25 wt% stearic acid by milling for one hour. The mixture pressed to give cylindrical shape (13 mm diameter and 3.6 mm high) under 590 MPa for the hydrolysis reaction of NaBH4. Two features were taken in the account to decide the optimum catalyst composition; first the physical strength, second the hydrogen generation rate of the catalyst. Catalyst pellets containing 20%, 30%, 40% and 45% CoI2 were 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.
2.2. Dehydrogenation tests Catalyst pellets were put in hydrolysis reactor. 100 mg (2.632 mmol) of NaBH4 and 70 mg (1.75 mmol) of NaOH were dissolved in deionized water and introduced into reactor. Produced hydrogen was measured in inverse burette system depicted in Fig. 1 in details. After completion of the hydrolysis, catalyst rapidly rinsed with deionized water and next hydrogenation takes place.
2. Experimental NaBH4 was provided from Merck for the hydrolysis experiments at 98% purity. MgH2 and NaCl were provided by Merck and CarloErba with 99.9% purity. Eti Mine Works supports B2O3 with 99% purity. NaBH4 was synthesized according to the reaction 3, in Spex 8000M type ball-milling. XRD experiments were performed to control the phase composition and crystalline state of the catalysts by Shimadzu powder diffractometer (Cu Ka radiation, k = 1.5406 Å). Jasco 480 model FT-IR was used simultaneously to determine the chemical chances during mechano-chemical process. Joel 6360 LV SEM equipped with EDX detector and Quantachrome Nova 2200 sorptometer were utilized to investigate the surface and pore morphology of the catalyst. The thermodynamic values of this reaction were summarized in Table 1. Negative value of the Gibbs free energy indicates the possibility of the desired reaction.
2NaCl þ 4MgH2 þ B2 O3 ! 2NaBH4 þ 3MgO þ MgCl2
ð3Þ
Fig. 1. Hydrolysis setup (1) catalyst, (2) NaBH4–NaOH solution, (3) reactor, (4) water reservoir, and (5) burette.
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Fig. 2. FT-IR analyses that show the effect of the working time (a) 300 min, (b) 600 min, (c) 800 min, and (d) 1000 min.
2.3. Kinetic studies Eight distinct experiments (at 20, 30, 40, 50, 55, 60, 65 ve 70 °C) were performed with 40% CoI2 loaded on AC support. All of the kinetic trials were aimed to find out the reaction rate order and activation energy of the reaction. During the experiments reactor was kept in water bath whose accuracy is 0.1 °C.
3. Results and discussion NaBH4 usage as a hydrogen storage material is directly depends on the feasibility of its production. Thus in economic point of view NaCl, which is one of the cheapest substances on the markets, has been chosen as one of the reactant to perform NaBH4 synthesis. Working time and reactants ratio are two main parameters investigated in this study to determine the optimum conditions for both economy and high yield. Studied reaction times indicate that the longer duration results better yields as given by the FT-IR spectrums in Fig. 2. It is evident that the duration enhances the results without dependence to reactant ratios. Peaks between 2400 and 2200 cm1 clearly prove this fact. Micro peaks at 1461 and 1465 cm1 give MgO existence and the success of the oxygen remove from B2O3 structure similar to the previous studies [1,5,9,18,19]. Furthermore Fig. 3 was created to show the areas covered by of FT-IR peaks at which 2400– 2200 cm1 region. These integrations depict to 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 NaBH4 in the reaction product can be raised by further purification processes to provide the energy economy of the process.
XRD analysis performed for the different working times are given in Fig. 4. Results indicate that the products are in crystalline phase and the particles are in nano scale according to the software of the XRD device that makes calculation by Scherrer equation. Mechano-chemical devices generally provide ⁄⁄nano scale reactants and (or) product, which is necessary to obtain high yields in short duration [20,21]. However, in Fig. 4 any peak was detected belonging to NaBH4. May be, this is simply caused by the amorphous phase of NaBH4. Moreover, it is possible to realize the diffraction peaks of the starting reactants. That indicates uncompleted transaction and the necessity of purification process. XRD spectrum also indicates that the MgH2 peak at 28 2h became weaker proportional to working time meanwhile MgO peak at 43 2h is more visible for longer durations. These changes simply indicate the occurrence of the reaction 3. Purification process is indispensible to obtain high yield and remove the some side reaction products. EDA is a suitable solvent to separate the NaBH4. It does not decompose NaBH4 or cause any reaction with the residue. XRD determination of the purified sample (Fig. 5a) shows great difference if compared with the side product’s (Fig. 5b). Long and sharp peaks prove that the NaBH4 becomes in crystal phase during dissolving, filtration and drying processes. XRD analyses of the solid filtration product show that it contains unreacted MgH2, NaCl and side product MgO at 28, 32 and 43 2h, respectively (Fig. 5b). Commercial and synthesized NaBH4 were compared by FT-IR analysis in Fig. 6. Results depict that the two samples have similar peaks at 2400–2200 cm1, however residue EDA makes it hard to determine the peaks between 1700 and 900 cm1.
3.1. Kinetic studies In catalytic dehydrogenation, hydrogen containing substances need catalysts to rapidly release hydrogen. While the decomposition, hydrogen atoms both come from NaBH4 and solvent water. Oxygen of the water adheres to sodium and boron and forms NaBO2. This compound is easy to recycle and that raise the feasible usage of NaBH4. CoI2 was utilized to synthesize 4 different concentration (20, 30, 40 and 45 wt%) AC supported catalysts. 20 wt% CoI2 including catalyst could only be used for 28 times and lost its activity completely. Low solubility and blockage of the side product NaBO2 and removal of Co particles during the process should result that activation loose. Moreover, pH chances because of the formation of BðOHÞ 4 anion can limit the hydrogen generation rate [22]. Two hydrogen generation rates, minimum and maximum, were measured for 20 wt% CoI2/AC catalyst as 2.78 and 31.92 mL/min g cat.
Fig. 3. Stoichiometry and working time effect on FT-IR peak areas that indicates NaBH4 existence.
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Fig. 4. XRD results for reaction 3 at different durations (MgH2/NaCl ratio is 1.3).
Fig. 5. XRD diffractions of NaBH4 filtration (a) synthesized NaBH4 and (b) filtration spent fuel (MgH2/NaCl ratio is 1, 1000 min).
Despite of 20, 30 and 40 wt% CoI2/AC catalysts could achieve more than 300 hydrolysis without deactivation. In Fig. 7 some of the hydrolysis results were summarized for 40 wt% CoI2/AC. Hydrogen generation rate is always slow for the first trials since
CoI2 is reduced by NaBH4 and actually Co and (or) CoBx (x = 0.5, 1 or 2) starts to play an important role for the hydrogen generation [23,24]. Although, the long working life of the catalysts, their maximum generation rates were measured for 30 and 40 wt% CoI2/AC
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Fig. 6. FT-IR graphs of commercial and produced NaBH4 (1) original NaBH4 and (2) synthesized NaBH4 (1000 min, MgH2/NaCl ratio is 1.0).
Fig. 7. Hydrolysis test for 40 wt% CoI2/AC (100 mg NaBH4, 70 mg NaOH, 0.55 g catalyst at 20 °C).
Table 2 BET analysis of the catalyst before and after application. SBET (m2/g)
Catalyst 20 wt% CoI2/AC
Fresh Used
451.7 501.3
30 wt% CoI2/AC
Fresh Used
487.4 666.7
40 wt% CoI2/AC
Fresh Used
479.6 684.6
34.83 and 46.60 mL/min g cat, respectively. It is obvious that these rates are not satisfactory for high hydrogen consuming systems but reasonable for small size devices. Experiments performed with 45 wt% CoI2/AC could not performed more than 1 or 2 trials because of the physical weakness of the catalyst pellets. Thus, 40 wt% CoI2/AC catalyst was decided to has the best performance. Fig. 7 explains that the catalyst can keep its activity for many attempts, which take 1000 h approximately. On the other hand, it is evident by the 20 wt% CoI2/AC performance, the catalyst activity always decrease by time. Therefore, it may be assumed that if optimum catalyst and support ratio is provided decrease of activa-
tion can be compensated by something different. Performed BET, EDX and SEM analysis make this phenomenon easy to understand. Surface areas of the catalyst before and after the experiments are given in Table 2. During the process, all catalysts become rich in surface area. That means more interaction possibility between catalyst and BH4- anion and higher hydrogen generation values. Thus we may conclude that the physical changes on the catalyst surface compensate the deactivation and provide the solution to reach deeper. SEM micrographs given in Fig. 8 clearly demonstrate that the physical changes occurred on the surface. Moreover EDX results (not reported) prove that the surface is covered by NaBO2 even the subsequently rinsing of the catalysts. This coverage starts to decrease the activation of the catalyst pellet at the further applications. 3.2. Kinetic behaviors of the catalyst 40 wt% CoI2/AC catalyst was studied at eight different temperatures (20, 30, 40, 50, 55, 60, 65 and 70 °C) and reaction rate order was investigated. Experimental data fit zero order kinetic (Fig. 9) that means reaction depends on catalyst used instead of concentration. Activation energy computation results 36.63 kJ/mol. This low activation energy value is very advantageous if compared with the values given in other studies.
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Fig. 8. SEM micrographs of 40 wt% CoI2/AC catalyst before and after 300 tests.
Fig. 9. (a) Concentration changes with time at different temperatures. (b) Changes of reaction rate constants with the inverse of temperature.
4. Conclusion Widespread usage of the fuel cell systems are strongly depend on the feasibility of the hydrogen providing chemicals and the economy of utilized catalysts. This study discusses the synthesis of NaBH4 by means of NaCl in ball-milling for the first time. Results prove that the produced NaBH4 needs to be purified and it reaches 95% purity and becomes in crystalline phase after purification. It was observed that the yield of the products raise with working time. Thus 1000 min has been decided to be the optimum reaction time. Moreover, studies with different ratios of MgH2/NaCl 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 NaBH4. 40 wt% CoI2 on AC can achieve 45.6 mL/min g cat. hydrogen generation rate at room temperature for 300 cycle (approximately 1000 h) long. We believe this value is adequate to feed the fuel cell systems unless high consumption rates are necessary. Hydrolysis experiments obey zero order kinetic and its activation energy is 36.63 kJ/mol. Extra studies concerning, nano-
size catalyst synthesis and additives that increase the activity and catalyst life, should be performed to improve the hydrogen generation rate and the efficiency of the hydrolysis process. Acknowledgments We are indebted to the Eti Mining Works of Turkey for continuing support. Also this work was supported by the Gazi University BAP Project No. 06/2010-57. References [1] Pisßkin MB. Investigation of sodium borohydride production process: ulexite mineral as a boron source. Int J Hydrogen Energy 2009;34:4773–9. [2] Amendola SC, Shaip-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25(10):969–75. [3] Fan M, Sun L, Xu F. Study of the controllable reactivity of aluminum alloys and their promising application for hydrogen generation. Energy Convers Manage 2010;51(3):594–9.
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