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Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4
international journal of hydrogen energy xxx (xxxx) xxx
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Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4 b € Gamze Bozkurt a,*, Abdulkadir Ozer , Ays‚e Bayrakc¸eken Yurtcan b a b
Project Coordination Implementation and Research Center, Erzurum Technical University, Erzurum, Turkey Chemical Engineering Department, Atatu¨rk University, Erzurum, Turkey
article info
abstract
Article history:
Hydrogen is an alternative and clean energy carrier, but there are still some production
Received 2 August 2018
related problems. In this aspect, it is crucial to efficiently generate hydrogen from hydrogen
Received in revised form
rich materials such as sodium borohydride. In this study, Co3O4 supported Ni and Co
7 October 2018
catalysts are synthesized via microwave irradiation technique for hydrogen generation
Accepted 12 October 2018
from sodium borohydride. In this context, firstly, Co3O4 support material is synthesized by
Available online xxx
chemical method. Then, Ni and Co catalysts are decorated onto Co3O4 support material by microwave irradiation-polyol method. Prepared catalysts and support material are char-
Keywords:
acterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), trans-
H2 generation
mission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and
Reactor
inductively coupled plasma-mass spectrometer (ICP/MS). A new system is designed by our
Microwave irradiation-polyol
group in order to determine the activity of the prepared catalysts for hydrogen generation.
method
The effects of different initial NaOH concentrations on hydrogen generation rate are investigated. It is observed that the rate of hydrogen generation increased with an increase in initial NaOH concentration. Co-Co3O4 catalyst at 10% NaOH initial concentration shows the highest hydrogen generation rate as 2823 ml/gcat.min. In summary, Co-based catalysts are exhibited more activity than Ni-based catalysts in terms of hydrogen generation. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Alternative energy sources are gaining increasing attention due to the increasing population of the world and the gradual depletion of current fossil based energy sources. Environmental pollution and the increase in greenhouse gases in the atmosphere led the energy authorities to search for different clean energy carriers. In this sense, hydrogen (H2) draws attention for being the cleanest energy carrier. Active utilization of hydrogen depends on solving some problems including practical production and storage. Hydrogen is considered to be
the fuel of the future with its characteristics and environmental friendliness [1e6]. Until now, many materials such as water, natural gas, coal, plants and chemical hydrides were investigated as H2 source. Chemical hydrides and metal hydrides have an impressive volumetric H2 storage density on material basis and are therefore among the most promising H2 storage sources for portable applications. Chemical hydrides are considered as the most appropriate H2 storage and distribution materials [7]. Sodium borohydride (NaBH4) is the most promising one among these hydrides due to its stability, low cost, incombustibility, non-toxicity and high theoretical storage density, up to 10.8 wt% [8,9]. Complete release of H2 at
* Corresponding author. E-mail address: [email protected] (G. Bozkurt). https://doi.org/10.1016/j.ijhydene.2018.10.106 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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ambient temperature without additional heating is possible in the presence of the catalysts that help to achieve high purity hydrogen [10]. The generation of pure H2 by hydrolysis of NaBH4 with the aid of heterogeneous catalysts is important in both industrial and environmental aspects. Another advantage of producing H2 with NaBH4 is that half of the produced hydrogen comes from H2O molecules. In the presence of catalyst, the hydrolysis of NaBH4 is an exothermic reaction and the reaction side product (sodium metaborate (NaBO2)) is harmless to the environment and can be converted back to NaBH4 [11,12]. The NaBH4 hydrolysis reaction at ambient temperature is as follows and requires a suitable catalyst in alkaline environment [13e16]. catalyst
NaBH4 þ2H2 O/NaBO2 þ4H2 þ heat
(1)
Sodium hydroxide (NaOH) is a widely used stabilizer to prevent self-hydrolysis of NaBH4. The stability of the catalyst should be high due to the strong basicity of the solution, the rapid release of hydrogen and the strong exothermic nature of the reaction [17]. Therefore, the catalyst is the most important factor affecting the rate of H2 generation in hydrolysis of NaBH4. Controlled efficient hydrolysis reactions can be provided by using solid-state catalysts such as precious metals (generally functionalized with support) or transition metals and their salts [18]. Recent studies in preparing NaBH4 hydrolysis catalysts showed significant improvements in catalytic activity of transition metal catalysts up to the levels similar to those of noble metal catalysts. Particularly supported non-noble transition metal catalysts can be used as active catalysts due to their easy separation from the solution and reusability of the catalysts [19]. Because precious metals are rare and expensive, the use of catalyst systems based on cheaper transition metals is highly desirable [20]. The doping of the active non-noble metal species on a support can influence the catalytic performance of the catalyst. Generally, transition metals such as Co, Ni, Fe and Cu showed a good level of catalytic activity and selectivity for the hydrolysis of NaBH4 [21]. Especially, nickel (Ni) and cobalt (Co) are more abundant than noble metals and they are the most widely used metals today because of their low cost. Kaufman and Sen have investigated the effect of heterogeneous catalysts such as Co and Ni on BH 4 hydrolysis behaviour and it was reported that the metallic catalysts transfer electrons to the H2O for H2 generation. The effect of these transition metals is characterized by zero-order kinetics [20]. The direct use of nanoparticles as catalyst for the hydrolysis of NaBH4 has various disadvantages. The nanoparticles are severely agglomerated during the reaction due to their high surface energies and therefore the catalytic activity of the nanoparticles is gradually reduced and the reusability is poor. Loading nanoparticles onto a suitable support (TiO2, Al2O3 e.g.) can limit the agglomeration of the nanoparticles [17]. Hydrolysis of NaBH4 was investigated by supporting Co on Al2O3, NiCo on reduced graphene oxide (rGO) and Co-B on TiO2 [22e24]. In addition to, some materials such as Cu sheet [25], Ni foam [26] and carbon cloth [27] have been also used as support materials. Another important factor for an efficient hydrolysis reaction is related with the design of the system. Several reactors
including the catalyst operating in both static and dynamic modes were designed for NaBH4 hydrolysis reaction. In the static system, the catalyst is supported on a powder or pelletized material and placed in the reactor containing NaBH4 solution. Static systems usually exhibit low efficiency due to various phenomena; (a) difficulty in separating the catalyst from the solution, (b) catalyst leaching from the support, (c) deactivation of the catalyst due to the precipitation of NaBO2 (d) mass transport problems. The dynamic systems are based on the flow of the NaBH4-NaOH solution in a tubular reactor containing the suitable catalyst [28] and it is more advantageous than static system. In addition to all of these, many studies in the literature were used the water-displacement method for hydrogen volume measurement [1,29e32]. However, it is important to design a system in order to accurately measure the volume of hydrogen gas generated. In this study, Ni and Co catalysts were loaded on Co3O4 support material with microwave-irradiation polyol technique and H2 generation from NaBH4 were carried out in a dynamic system designed by our group. In the designed system, the above-mentioned mass transfer, removal of the heat generated, and automatic measurement of dry hydrogen volume were significantly improved.
Experimental Materials Sodium borohydride (NaBH4), sodium hydroxide (NaOH), cobalt (II) nitrate hexahydrate (Co(NO3)2$6H2O), nickel (II) chloride hexahydrate (NiCl2$6H2O), cobalt (II) chloride hexahydrate (CoCl2$6H2O) and ethylene glycol were purchased from Merck. All experiments were performed by using deionized water.
Synthesis of support material Firstly, Co(OH)2 was synthesized by 1.0 M Co(NO3)2$6H2O solution and 500 ml of 0.4 M NaOH solution in nitrogen atmosphere. The obtained Co(OH)2 was filtered, washed with distilled water and then dried at 50 C for 24 h [33]. Afterwards cobalt (II, III) oxide (Co3O4) was obtained by the heat treatment of Co(OH)2 at 700 C for 2 h.
Catalysts preparation Co3O4 supported Ni and Co nanoparticles (NPs) were synthesized by microwave-irradiation polyol method. NiCl2$6H2O and CoCl2$6H2O were used as metal precursors, while ethylene glycol was used as the reducing environment. Firstly, 0.1 M solution of Ni or Co salt precursors were prepared by dissolving them separately in deionized water. This solution was then poured into a beaker with 50 ml of ethylene glycol and then the corresponding support material was added to the mixture. The mixture was homogenized for at least half an hour by using an UltraeTurrax Te25 homogenizer and then placed into a domestic microwave oven (800 W, 2.45 GHz) for 90 s. After microwave treatment, the
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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mixture was cooled down with cold water, centrifuged, washed with acetone and deionized water and finally dried in an oven at 100 C for 12 h [34]. Metal loading on the composite supports was set to 20 wt%.
Characterization The prepared catalysts were examined by X-ray diffraction (XRD) performed with PANalytical Empyrean X-ray diffractometer, scanning electron microscope (SEM) performed on Quanta FEG 250 and by transmission electron microscopy (TEM) performed on Jem Jeol 2100F 200 kV HRTEM microscope with high-resolution camera at an accelerating voltage of 200 kV. The surface electronic states of Ni and Co on the catalytic surface of the catalysts were analysed by X-ray photoelectron spectroscopy (XPS) performed with Specs-Flex X-ray photoelectron spectrometer. The loadings of Ni and Co on Co3O4 support was measured by Inductively coupled plasmaMass spectrometer (ICP/MS) (Agilent 7800).
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Measurement of hydrogen generation rate The activities of the catalysts for H2 generation were evaluated using a system designed by our group, shown schematically in Fig. 1. Firstly, the catalyst was placed between the fiberglass and then the tubular reactor was placed. NaBH4/NaOH solution was sent to the reactor by using a pump. The circulation process significantly improves the mass transport between the solution and the catalytic surface. Therefore, the solution was continuously circulated on the surface of the catalyst by means of a peristaltic pump until the end of the reaction. The solution temperature was adjusted with an electric heating system and the cooling was made with the main water system. Before, sending the produced hydrogen gas to the flow meter it was dried in order to remove any water vapour. The hydrogen generation flow was recorded by a Burkert mass flow meter with a maximum flow rate of 1.5 slpm. The data obtained with the help of appropriate software was transferred to the computer. In all experiments, the effect of NaOH
Fig. 1 e Hydrogen generation system (a) schematic diagram (b) photo of designed system by our group. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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(V) versus time (min) in the linear region according to the following equation: r¼
V txm
where: r; hydrogen generation rate (ml/gcat.min), V; volume of generated hydrogen (ml), t; time (min), m; weight of catalyst (g).
Results and discussion Characterization of the catalysts Fig. 2 e XRD patterns for support material and the catalysts.
concentrations were investigated by stabilizing 10 wt% NaBH4 solution with different initial concentrations of NaOH (1, 5, 10 wt%). The hydrogen generation rate (r) was determined from the slope of the plot of the generated hydrogen volume
XRD patterns of the Ni-Co3O4 and Co-Co3O4 catalysts are shown in Fig. 2 which compared with the diffraction patterns of the Co3O4. According to the XRD results Co3O4 with polycrystalline cubic structure was obtained (Joint Committee on Powder Diffraction Standards (JCPDS) No. 74e2120). The crystallite size of the prepared Co3O4 nanoparticles were calculated from the XRD data using the Scherrer equation. It was found that the crystallite size of Co3O4 is about 20.9 nm. Characteristic peaks of Ni corresponding to (111) and (200)
Fig. 3 e SEM images and EDS results of (aeb) Co3O4 (ced) Ni-Co3O4 (eef) Co-Co3O4. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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planes for 2q values of 44.45 and 51.73 may overlap with the (400) and (422) planes of Co3O4, respectively. Similarly, characteristic peak of Co corresponding to (002) plane may overlap with those of (400) plane of Co3O4 for Co-Co3O4 catalyst. Addition of Ni and Co to Co3O4 support material resulted in shifting of XRD peaks approximately 0.1 to the left. In addition, a low-intensity diffraction peak of (533) plane for Co was also observed. SEM images given in Fig. 3a showed the surface morphologies (particle nature) for the Co3O4 support material and the
catalysts. Particle formation was observed in Co3O4 support material with homogeneous dispersion. No significant change in morphology was observed when Ni and Co catalysts were loaded onto the Co3O4 (Fig. 3cee). According to EDS spectrums Co, Ni and O elements were detected for support material and the catalysts. Figs. 4 and 5 illustrate the XPS spectra of general and Co 2p, Ni 2p and O1s level photoemission signals of the catalysts. Co2p and O1s spectra were similar for both Co and Ni catalysts. Therefore, Co2p and O1s spectra were given only for the Co catalyst. XPS peaks located at the binding energies of 787.7 eV and 802.8 eV correspond to Co2p3/2 and Co2p1/2 shake-up (satellite) peaks, respectively (Fig. 4b), which is shifted to lower values due to some CoO features [35]. In this context, it can be considered that Co is usually in the Coþ2 state for the catalysts because the dense shaken Co 2p vacancies originate from unpaired 3d valence orbitals with high spin Coþ2. In addition to this, the peak located at 531.94 eV of the O1s given in Fig. 4c can be attributed to the chemisorb oxygen shaking in the crystals [36]. According to Fig. 5b two peaks at 861 eV for Ni(II) 2p3/2 and 879 eV for Ni(II) 2p1/2 were observed for Ni-Co3O4 catalyst [37]. In order to verify Ni and Co contents in the catalysts ICP-MS analysis was performed. According to the results, the metal loading percentages were found as 5.8% and 82.7% for Ni and Co based catalysts. The
Fig. 4 e XPS spectrums for Co-Co3O4 catalyst (a) general spectrum (b) Co2p (c) O1s electron regions.
Fig. 5 e XPS spectrums for Ni-Co3O4 catalyst (a) general spectrum (b) Ni2p electron regions.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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Fig. 7 e Time-dependent volumes of hydrogen generated at different NaOH initial concentrations for Ni-Co3O4 catalyst.
and 10 wt% NaBH4 by changing the initial NaOH concentrations. NaOH is used as stabilizing agent in order to prevent the self-hydrolysis of NaBH4 during the addition of water. The rate of self-hydrolysis reaction decreases above pH 13. The mechanism of self-hydrolysis is defined as follows;
Fig. 6 e TEM images of (a) Co3O4 (b) Ni-Co3O4 (c) Co-Co3O4.
higher content of Co in Co based catalyst can be attributed to the Co content coming from the support material. TEM images of the support material-Co3O4 are given in Fig. 6a. The particles showed regular shape, and their structures were hexagonal. Particle sizes were in between 40 and 50 nm. According to the TEM images (Fig. 6 b-c) taken for the catalysts, there was a non-homogeneous distribution on the support material, the particles were overlapped and the particle boundaries were not clearly visible. It was observed that the nanoparticles formed rectangular structure on the support material.
NaBH4 ðkÞ4Naþ ðaqÞ þBH4 ðaqÞ
(2)
þ BH 4 ðaqÞ þHðaqÞ 4BH3ðaqÞ þH2ðgÞ
(3)
BH3ðaqÞ þ3H2 OðsÞ 4BðOHÞ3ðaqÞ þ3H2ðgÞ
(4)
þ BðOHÞ3ðaqÞ þH2 OðsÞ 4BðOHÞ 4ðaqÞ þ HðaqÞ
(5)
þ 2 4BðOHÞ 4ðaqÞ þ2HðaqÞ 4B4 O7ðaqÞ þ9H2 OðsÞ
(6)
Step 1 corresponds to salt decomposition of NaBH4 and the equilibrium constant can be easily obtained from the general data tables. Steps 2 and 3 correspond to H2 generation. Step 3 corresponds to the complex reaction paths followed by borane complexes being formed depending on the pH of the solution [38]. Step 4 shows the borate formation step in alkaline
Hydrogen generation of the catalyst Figs. 7 and 8 showed the amounts of hydrogen generation from NaBH4 hydrolysis with Ni and Co based catalysts at different NaOH initial concentrations. All the experiments were performed at 25οC with 50 mg of corresponding catalysts
Fig. 8 e Time-dependent volumes of hydrogen generated at different NaOH initial concentrations for Co-Co3O4 catalyst.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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environment. According to the mechanism, the reduction of H2 ions in the solution does not occur directly in Step 2 reaction and causes the Step 2 in the alkaline environment to become restrictive. However, this explains the increase in the experimentally confirmed pH as a function of the reaction time. There is an explanation for this situation; reaction byproducts, especially tetrahydroxyborate (B(OH)-4), can be self-reactant to produce the tetraborate (B4O2 7 ) ion. This molecule can be formed by side reaction in Step 5 where 2 protons are used and the reaction solution becomes more and more alkaline as experimentally observed [39,40]. Three different mechanisms for the hydrolysis of NaBH4 in basic medium were proposed. In the first mechanism, firstly an activated complex is formed Hþ- BH 4 - H2O and then it decomposes to BH3 and H2. Then, BH3 reacts with water and produces H2 in the subsequent step [38]; slow
þ þ BH 4 þH3 O 4 ½H BH4 H2O/H2 þBH3 þH2 O
(7)
fast
BH3 þ3H2 O/H3 BO3 þ3H2
(8)
Second mechanism involves the formation of an intermediate BH3OH, which reacts with water in the other steps and releases H2; slow
þ BH 4 þH3 O 4 ½H þ BH4 H2 O/H2 þBH3 þH2 O
(9)
fast
BH3 þ 2H2 O/ BH3 OH þH3 Oþ
(10)
BH3 OH þH3 Oþ /½Hþ BH3 OH H2 Oþ2H2 O/ H3 BO3 þ3H2 þH2 O fast
(11) In the third mechanism, a short-lived reaction intermediate of BH5 is formed, subsequently decomposed into BH3 and H2 according to the following reaction; slow
þ BH 4 þH / BH5 /BH3 þH2
(12)
NaOH is generally used to prevent immediate hydrogen release during the catalytic hydrolysis. Although there are many studies discussing the effect of the amount of NaOH on the reaction, there is no common idea about the effect of NaOH concentration, and the widespread opinion is that 3e5% NaOH is sufficient to control the reaction [41]. Table 1 shows the measured pH values before and after the hydrolysis reaction. According to the table, an increase in pH was observed after the reaction. The resulting NaBO2 solution increases the pH of the solution. The hydrogen generation rates of the catalysts according to different NaOH initial concentrations are given in Table 2. Increasing the initial NaOH concentration for both catalysts caused an increase in the hydrogen generation rate. It was
Table 1 e Measured pH values before and after the hydrolysis reaction. pH
Before After
1% NaOH
5% NaOH
10% NaOH
13,33 14,16
13,62 14,60
13,74 14,81
Table 2 e H2 generation rate (HGR) of catalysts at different NaOH initial concentrations. Catalyst
Ni-Co3O4 Co-Co3O4
HGR, ml/gcat.min 1% NaOH
5% NaOH
10% NaOH
1375 2238
1466 2446
1925 2823
also found that the Co-Co3O4 is more active than Ni-Co3O4 catalyst. The highest hydrogen generation rate was obtained for the Co-Co3O4 catalyst at 10 wt% NaOH initial concentration as 2823 ml/gcat.min. According to Fig. 7 hydrolysis reaction with 1 wt% NaOH initial concentration continues for 150 min, while the hydrolysis reaction with 10 wt% NaOH initial concentration continues for 75 min for Ni-Co3O4 catalyst. Similarly, the reaction time was reduced with increasing NaOH initial concentration for the Co-Co3O4 catalyst (Fig. 8). It shows that hydroxide ions affect the activation of Ni and Co catalysts. In general, the reason for this positive effect for Ni and Co based catalysts is attributed to the complex surface reaction on the hydroxide ion. In the literature, it was reported that the hydrogen generation rate remained almost constant over 10 wt% initial concentrations when using nickel catalyst [42]. S‚ahiner et al. reported that the rate of hydrogen generation increased linearly with increasing NaOH concentrations for the Co-based catalyst [43]. Metal acceptance of hydride and the ability of basic character types to accept a proton are important factors for the hydrolysis reaction. The ligands bound to the central atomic metal increase the electron density of the metal. The H ions attached to the metal react with water molecules to form H2 and OH. Thus, the increase in electron density of the central atomic metal can provide a faster reaction [44]. However, as the NaOH concentration increased, a decrease in the total volume of hydrogen was observed. This is attributed to the reduced amount of free water required for the reaction at high NaOH concentrations and hence the low solubility of the reaction by-product NaBO2. Therefore, the initial concentration of NaOH should not exceed 10%. BH 4 anions originated from the NaBH4 solution are available on catalyst active sites in order to generate hydrogen. In an excessive amount of NaOH solution case, higher amounts of OH anions are transferred to the active sites of the catalysts that hinder the transfer of BH 4 anions which will decrease the hydrogen generation rate [27,45]. Table 3 shows the summary of the reported hydrogen generation rates for hydrolysis of NaBH4 catalyzed by various catalysts reported in the literature [22e24,46e49]. The catalysts synthesized in this study showed promising activity with high hydrogen generation rate than most of the Ni and Co based catalysts reported in the literature previously. In the study of Walter et al. the activity of the Ni-based catalyst was investigated at 60 C for 5% NaOH initial concentration and the hydrogen generation rate was found as 1300 ml/gcat.dk [46]. In our study, the hydrogen generation rate is found as 1400 ml/ gcat.dk at ambient conditions and 5% NaOH initial concentration. In this study, better hydrogen generation rates were obtained for our catalysts than Co-based catalysts given in the literature at 30 C as given in Table 3.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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Table 3 e Comparison of HGR of various catalysts for the hydrolysis of sodium borohydride. Catalysts
Co/Al2O3 Ni-Co/rGO Co-B/TiO2 Ni3B Co3B Ni complex Raney Ni Co/Cu Co-P/Cu Ni/Co3O4 Co/Co3O4
Method
impregnation-chemical reduction electroless coating in-situ reduction chemical reduction chemical reduction chemical chemical electroplating electroplating microwave irradiation-polyol microwave irradiation-polyol
Operating conditions wt% NaBH4
wt% NaOH
Temp.
10 10 1.5 5 5 2 10 20 20 10 10
1 5 5 5 5 7 10 5 5 10 10
30 40 30 60 60 30 25 30 30 25 25
HGR, ml/gcat.min
Ref
220 1280 1980 1300 6000 2240 228.5 52 954 1925 2823
[22] [23] [24] [46] [46] [47] [48] [49] [49] This work This work
rGO: reduced graphene oxide, MWNT: multi-walled nanotube.
By comparing our catalytsts with the ones reported in the literature, the new findings from the outcome of this work can be summarized as follows:
for doctoral research (Grant No. 1649B031502644) and financial support by Atatu¨rk University BAP project through grant number 2015/360.
1. Ni/Co3O4 and Co/Co3O4 were used as catalysts for hydrogen production from NaBH4 for the first time. 2. The catalysts prepared in our study exhibited higher hydrogen generation rates at ambient conditions than some hydrogen generation rates given in the literature carried out at higher temperature conditions. 3. In order to investigate the catalytic activities of the catalysts, a system is design that provides better removal of heat generated during the reaction, better mass transfer between the solution and the catalyst and obtaining dry hydrogen gas.
references
Conclusions Co3O4 supported Ni and Co catalysts were prepared by microwave irradiation-polyol method. A system was designed in order to determine the hydrogen generation activities of the catalysts. A dynamic reactor was used in the designed system and the NaBH4 solution was circulated onto the catalysts in order to increase the mass transport. The activity of the synthesized catalysts were tested in this system. The effects of different initial concentrations of NaOH solutions on hydrogen generation rate were investigated. Increase in initial concentrations of NaOH for both Ni and Co based catalysts were increased the hydrogen generation rate. However, as the NaOH initial concentration was increased, a decrease in the total volume of hydrogen was observed. In general, it was observed that Co-based catalysts are exhibited more activity than Ni-based catalysts in terms of hydrogen generation. On the other side, Ni/Co3O4 and Co/Co3O4 catalysts showed good catalytic activity. It can be concluded that the Ni/Co3O4 and Co/Co3O4 catalysts can be used for hydrogen generation by hydrolysis of NaBH4.
Acknowledgments The authors are gratefully acknowledge for scholarship from the Scientific and Technological Research Council of Turkey
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Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4 b € Gamze Bozkurt a,*, Abdulkadir Ozer , Ays‚e Bayrakc¸eken Yurtcan b a b
Project Coordination Implementation and Research Center, Erzurum Technical University, Erzurum, Turkey Chemical Engineering Department, Atatu¨rk University, Erzurum, Turkey
article info
abstract
Article history:
Hydrogen is an alternative and clean energy carrier, but there are still some production
Received 2 August 2018
related problems. In this aspect, it is crucial to efficiently generate hydrogen from hydrogen
Received in revised form
rich materials such as sodium borohydride. In this study, Co3O4 supported Ni and Co
7 October 2018
catalysts are synthesized via microwave irradiation technique for hydrogen generation
Accepted 12 October 2018
from sodium borohydride. In this context, firstly, Co3O4 support material is synthesized by
Available online xxx
chemical method. Then, Ni and Co catalysts are decorated onto Co3O4 support material by microwave irradiation-polyol method. Prepared catalysts and support material are char-
Keywords:
acterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), trans-
H2 generation
mission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and
Reactor
inductively coupled plasma-mass spectrometer (ICP/MS). A new system is designed by our
Microwave irradiation-polyol
group in order to determine the activity of the prepared catalysts for hydrogen generation.
method
The effects of different initial NaOH concentrations on hydrogen generation rate are investigated. It is observed that the rate of hydrogen generation increased with an increase in initial NaOH concentration. Co-Co3O4 catalyst at 10% NaOH initial concentration shows the highest hydrogen generation rate as 2823 ml/gcat.min. In summary, Co-based catalysts are exhibited more activity than Ni-based catalysts in terms of hydrogen generation. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Alternative energy sources are gaining increasing attention due to the increasing population of the world and the gradual depletion of current fossil based energy sources. Environmental pollution and the increase in greenhouse gases in the atmosphere led the energy authorities to search for different clean energy carriers. In this sense, hydrogen (H2) draws attention for being the cleanest energy carrier. Active utilization of hydrogen depends on solving some problems including practical production and storage. Hydrogen is considered to be
the fuel of the future with its characteristics and environmental friendliness [1e6]. Until now, many materials such as water, natural gas, coal, plants and chemical hydrides were investigated as H2 source. Chemical hydrides and metal hydrides have an impressive volumetric H2 storage density on material basis and are therefore among the most promising H2 storage sources for portable applications. Chemical hydrides are considered as the most appropriate H2 storage and distribution materials [7]. Sodium borohydride (NaBH4) is the most promising one among these hydrides due to its stability, low cost, incombustibility, non-toxicity and high theoretical storage density, up to 10.8 wt% [8,9]. Complete release of H2 at
* Corresponding author. E-mail address: [email protected] (G. Bozkurt). https://doi.org/10.1016/j.ijhydene.2018.10.106 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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ambient temperature without additional heating is possible in the presence of the catalysts that help to achieve high purity hydrogen [10]. The generation of pure H2 by hydrolysis of NaBH4 with the aid of heterogeneous catalysts is important in both industrial and environmental aspects. Another advantage of producing H2 with NaBH4 is that half of the produced hydrogen comes from H2O molecules. In the presence of catalyst, the hydrolysis of NaBH4 is an exothermic reaction and the reaction side product (sodium metaborate (NaBO2)) is harmless to the environment and can be converted back to NaBH4 [11,12]. The NaBH4 hydrolysis reaction at ambient temperature is as follows and requires a suitable catalyst in alkaline environment [13e16]. catalyst
NaBH4 þ2H2 O/NaBO2 þ4H2 þ heat
(1)
Sodium hydroxide (NaOH) is a widely used stabilizer to prevent self-hydrolysis of NaBH4. The stability of the catalyst should be high due to the strong basicity of the solution, the rapid release of hydrogen and the strong exothermic nature of the reaction [17]. Therefore, the catalyst is the most important factor affecting the rate of H2 generation in hydrolysis of NaBH4. Controlled efficient hydrolysis reactions can be provided by using solid-state catalysts such as precious metals (generally functionalized with support) or transition metals and their salts [18]. Recent studies in preparing NaBH4 hydrolysis catalysts showed significant improvements in catalytic activity of transition metal catalysts up to the levels similar to those of noble metal catalysts. Particularly supported non-noble transition metal catalysts can be used as active catalysts due to their easy separation from the solution and reusability of the catalysts [19]. Because precious metals are rare and expensive, the use of catalyst systems based on cheaper transition metals is highly desirable [20]. The doping of the active non-noble metal species on a support can influence the catalytic performance of the catalyst. Generally, transition metals such as Co, Ni, Fe and Cu showed a good level of catalytic activity and selectivity for the hydrolysis of NaBH4 [21]. Especially, nickel (Ni) and cobalt (Co) are more abundant than noble metals and they are the most widely used metals today because of their low cost. Kaufman and Sen have investigated the effect of heterogeneous catalysts such as Co and Ni on BH 4 hydrolysis behaviour and it was reported that the metallic catalysts transfer electrons to the H2O for H2 generation. The effect of these transition metals is characterized by zero-order kinetics [20]. The direct use of nanoparticles as catalyst for the hydrolysis of NaBH4 has various disadvantages. The nanoparticles are severely agglomerated during the reaction due to their high surface energies and therefore the catalytic activity of the nanoparticles is gradually reduced and the reusability is poor. Loading nanoparticles onto a suitable support (TiO2, Al2O3 e.g.) can limit the agglomeration of the nanoparticles [17]. Hydrolysis of NaBH4 was investigated by supporting Co on Al2O3, NiCo on reduced graphene oxide (rGO) and Co-B on TiO2 [22e24]. In addition to, some materials such as Cu sheet [25], Ni foam [26] and carbon cloth [27] have been also used as support materials. Another important factor for an efficient hydrolysis reaction is related with the design of the system. Several reactors
including the catalyst operating in both static and dynamic modes were designed for NaBH4 hydrolysis reaction. In the static system, the catalyst is supported on a powder or pelletized material and placed in the reactor containing NaBH4 solution. Static systems usually exhibit low efficiency due to various phenomena; (a) difficulty in separating the catalyst from the solution, (b) catalyst leaching from the support, (c) deactivation of the catalyst due to the precipitation of NaBO2 (d) mass transport problems. The dynamic systems are based on the flow of the NaBH4-NaOH solution in a tubular reactor containing the suitable catalyst [28] and it is more advantageous than static system. In addition to all of these, many studies in the literature were used the water-displacement method for hydrogen volume measurement [1,29e32]. However, it is important to design a system in order to accurately measure the volume of hydrogen gas generated. In this study, Ni and Co catalysts were loaded on Co3O4 support material with microwave-irradiation polyol technique and H2 generation from NaBH4 were carried out in a dynamic system designed by our group. In the designed system, the above-mentioned mass transfer, removal of the heat generated, and automatic measurement of dry hydrogen volume were significantly improved.
Experimental Materials Sodium borohydride (NaBH4), sodium hydroxide (NaOH), cobalt (II) nitrate hexahydrate (Co(NO3)2$6H2O), nickel (II) chloride hexahydrate (NiCl2$6H2O), cobalt (II) chloride hexahydrate (CoCl2$6H2O) and ethylene glycol were purchased from Merck. All experiments were performed by using deionized water.
Synthesis of support material Firstly, Co(OH)2 was synthesized by 1.0 M Co(NO3)2$6H2O solution and 500 ml of 0.4 M NaOH solution in nitrogen atmosphere. The obtained Co(OH)2 was filtered, washed with distilled water and then dried at 50 C for 24 h [33]. Afterwards cobalt (II, III) oxide (Co3O4) was obtained by the heat treatment of Co(OH)2 at 700 C for 2 h.
Catalysts preparation Co3O4 supported Ni and Co nanoparticles (NPs) were synthesized by microwave-irradiation polyol method. NiCl2$6H2O and CoCl2$6H2O were used as metal precursors, while ethylene glycol was used as the reducing environment. Firstly, 0.1 M solution of Ni or Co salt precursors were prepared by dissolving them separately in deionized water. This solution was then poured into a beaker with 50 ml of ethylene glycol and then the corresponding support material was added to the mixture. The mixture was homogenized for at least half an hour by using an UltraeTurrax Te25 homogenizer and then placed into a domestic microwave oven (800 W, 2.45 GHz) for 90 s. After microwave treatment, the
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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mixture was cooled down with cold water, centrifuged, washed with acetone and deionized water and finally dried in an oven at 100 C for 12 h [34]. Metal loading on the composite supports was set to 20 wt%.
Characterization The prepared catalysts were examined by X-ray diffraction (XRD) performed with PANalytical Empyrean X-ray diffractometer, scanning electron microscope (SEM) performed on Quanta FEG 250 and by transmission electron microscopy (TEM) performed on Jem Jeol 2100F 200 kV HRTEM microscope with high-resolution camera at an accelerating voltage of 200 kV. The surface electronic states of Ni and Co on the catalytic surface of the catalysts were analysed by X-ray photoelectron spectroscopy (XPS) performed with Specs-Flex X-ray photoelectron spectrometer. The loadings of Ni and Co on Co3O4 support was measured by Inductively coupled plasmaMass spectrometer (ICP/MS) (Agilent 7800).
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Measurement of hydrogen generation rate The activities of the catalysts for H2 generation were evaluated using a system designed by our group, shown schematically in Fig. 1. Firstly, the catalyst was placed between the fiberglass and then the tubular reactor was placed. NaBH4/NaOH solution was sent to the reactor by using a pump. The circulation process significantly improves the mass transport between the solution and the catalytic surface. Therefore, the solution was continuously circulated on the surface of the catalyst by means of a peristaltic pump until the end of the reaction. The solution temperature was adjusted with an electric heating system and the cooling was made with the main water system. Before, sending the produced hydrogen gas to the flow meter it was dried in order to remove any water vapour. The hydrogen generation flow was recorded by a Burkert mass flow meter with a maximum flow rate of 1.5 slpm. The data obtained with the help of appropriate software was transferred to the computer. In all experiments, the effect of NaOH
Fig. 1 e Hydrogen generation system (a) schematic diagram (b) photo of designed system by our group. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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(V) versus time (min) in the linear region according to the following equation: r¼
V txm
where: r; hydrogen generation rate (ml/gcat.min), V; volume of generated hydrogen (ml), t; time (min), m; weight of catalyst (g).
Results and discussion Characterization of the catalysts Fig. 2 e XRD patterns for support material and the catalysts.
concentrations were investigated by stabilizing 10 wt% NaBH4 solution with different initial concentrations of NaOH (1, 5, 10 wt%). The hydrogen generation rate (r) was determined from the slope of the plot of the generated hydrogen volume
XRD patterns of the Ni-Co3O4 and Co-Co3O4 catalysts are shown in Fig. 2 which compared with the diffraction patterns of the Co3O4. According to the XRD results Co3O4 with polycrystalline cubic structure was obtained (Joint Committee on Powder Diffraction Standards (JCPDS) No. 74e2120). The crystallite size of the prepared Co3O4 nanoparticles were calculated from the XRD data using the Scherrer equation. It was found that the crystallite size of Co3O4 is about 20.9 nm. Characteristic peaks of Ni corresponding to (111) and (200)
Fig. 3 e SEM images and EDS results of (aeb) Co3O4 (ced) Ni-Co3O4 (eef) Co-Co3O4. Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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planes for 2q values of 44.45 and 51.73 may overlap with the (400) and (422) planes of Co3O4, respectively. Similarly, characteristic peak of Co corresponding to (002) plane may overlap with those of (400) plane of Co3O4 for Co-Co3O4 catalyst. Addition of Ni and Co to Co3O4 support material resulted in shifting of XRD peaks approximately 0.1 to the left. In addition, a low-intensity diffraction peak of (533) plane for Co was also observed. SEM images given in Fig. 3a showed the surface morphologies (particle nature) for the Co3O4 support material and the
catalysts. Particle formation was observed in Co3O4 support material with homogeneous dispersion. No significant change in morphology was observed when Ni and Co catalysts were loaded onto the Co3O4 (Fig. 3cee). According to EDS spectrums Co, Ni and O elements were detected for support material and the catalysts. Figs. 4 and 5 illustrate the XPS spectra of general and Co 2p, Ni 2p and O1s level photoemission signals of the catalysts. Co2p and O1s spectra were similar for both Co and Ni catalysts. Therefore, Co2p and O1s spectra were given only for the Co catalyst. XPS peaks located at the binding energies of 787.7 eV and 802.8 eV correspond to Co2p3/2 and Co2p1/2 shake-up (satellite) peaks, respectively (Fig. 4b), which is shifted to lower values due to some CoO features [35]. In this context, it can be considered that Co is usually in the Coþ2 state for the catalysts because the dense shaken Co 2p vacancies originate from unpaired 3d valence orbitals with high spin Coþ2. In addition to this, the peak located at 531.94 eV of the O1s given in Fig. 4c can be attributed to the chemisorb oxygen shaking in the crystals [36]. According to Fig. 5b two peaks at 861 eV for Ni(II) 2p3/2 and 879 eV for Ni(II) 2p1/2 were observed for Ni-Co3O4 catalyst [37]. In order to verify Ni and Co contents in the catalysts ICP-MS analysis was performed. According to the results, the metal loading percentages were found as 5.8% and 82.7% for Ni and Co based catalysts. The
Fig. 4 e XPS spectrums for Co-Co3O4 catalyst (a) general spectrum (b) Co2p (c) O1s electron regions.
Fig. 5 e XPS spectrums for Ni-Co3O4 catalyst (a) general spectrum (b) Ni2p electron regions.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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Fig. 7 e Time-dependent volumes of hydrogen generated at different NaOH initial concentrations for Ni-Co3O4 catalyst.
and 10 wt% NaBH4 by changing the initial NaOH concentrations. NaOH is used as stabilizing agent in order to prevent the self-hydrolysis of NaBH4 during the addition of water. The rate of self-hydrolysis reaction decreases above pH 13. The mechanism of self-hydrolysis is defined as follows;
Fig. 6 e TEM images of (a) Co3O4 (b) Ni-Co3O4 (c) Co-Co3O4.
higher content of Co in Co based catalyst can be attributed to the Co content coming from the support material. TEM images of the support material-Co3O4 are given in Fig. 6a. The particles showed regular shape, and their structures were hexagonal. Particle sizes were in between 40 and 50 nm. According to the TEM images (Fig. 6 b-c) taken for the catalysts, there was a non-homogeneous distribution on the support material, the particles were overlapped and the particle boundaries were not clearly visible. It was observed that the nanoparticles formed rectangular structure on the support material.
NaBH4 ðkÞ4Naþ ðaqÞ þBH4 ðaqÞ
(2)
þ BH 4 ðaqÞ þHðaqÞ 4BH3ðaqÞ þH2ðgÞ
(3)
BH3ðaqÞ þ3H2 OðsÞ 4BðOHÞ3ðaqÞ þ3H2ðgÞ
(4)
þ BðOHÞ3ðaqÞ þH2 OðsÞ 4BðOHÞ 4ðaqÞ þ HðaqÞ
(5)
þ 2 4BðOHÞ 4ðaqÞ þ2HðaqÞ 4B4 O7ðaqÞ þ9H2 OðsÞ
(6)
Step 1 corresponds to salt decomposition of NaBH4 and the equilibrium constant can be easily obtained from the general data tables. Steps 2 and 3 correspond to H2 generation. Step 3 corresponds to the complex reaction paths followed by borane complexes being formed depending on the pH of the solution [38]. Step 4 shows the borate formation step in alkaline
Hydrogen generation of the catalyst Figs. 7 and 8 showed the amounts of hydrogen generation from NaBH4 hydrolysis with Ni and Co based catalysts at different NaOH initial concentrations. All the experiments were performed at 25οC with 50 mg of corresponding catalysts
Fig. 8 e Time-dependent volumes of hydrogen generated at different NaOH initial concentrations for Co-Co3O4 catalyst.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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environment. According to the mechanism, the reduction of H2 ions in the solution does not occur directly in Step 2 reaction and causes the Step 2 in the alkaline environment to become restrictive. However, this explains the increase in the experimentally confirmed pH as a function of the reaction time. There is an explanation for this situation; reaction byproducts, especially tetrahydroxyborate (B(OH)-4), can be self-reactant to produce the tetraborate (B4O2 7 ) ion. This molecule can be formed by side reaction in Step 5 where 2 protons are used and the reaction solution becomes more and more alkaline as experimentally observed [39,40]. Three different mechanisms for the hydrolysis of NaBH4 in basic medium were proposed. In the first mechanism, firstly an activated complex is formed Hþ- BH 4 - H2O and then it decomposes to BH3 and H2. Then, BH3 reacts with water and produces H2 in the subsequent step [38]; slow
þ þ BH 4 þH3 O 4 ½H BH4 H2O/H2 þBH3 þH2 O
(7)
fast
BH3 þ3H2 O/H3 BO3 þ3H2
(8)
Second mechanism involves the formation of an intermediate BH3OH, which reacts with water in the other steps and releases H2; slow
þ BH 4 þH3 O 4 ½H þ BH4 H2 O/H2 þBH3 þH2 O
(9)
fast
BH3 þ 2H2 O/ BH3 OH þH3 Oþ
(10)
BH3 OH þH3 Oþ /½Hþ BH3 OH H2 Oþ2H2 O/ H3 BO3 þ3H2 þH2 O fast
(11) In the third mechanism, a short-lived reaction intermediate of BH5 is formed, subsequently decomposed into BH3 and H2 according to the following reaction; slow
þ BH 4 þH / BH5 /BH3 þH2
(12)
NaOH is generally used to prevent immediate hydrogen release during the catalytic hydrolysis. Although there are many studies discussing the effect of the amount of NaOH on the reaction, there is no common idea about the effect of NaOH concentration, and the widespread opinion is that 3e5% NaOH is sufficient to control the reaction [41]. Table 1 shows the measured pH values before and after the hydrolysis reaction. According to the table, an increase in pH was observed after the reaction. The resulting NaBO2 solution increases the pH of the solution. The hydrogen generation rates of the catalysts according to different NaOH initial concentrations are given in Table 2. Increasing the initial NaOH concentration for both catalysts caused an increase in the hydrogen generation rate. It was
Table 1 e Measured pH values before and after the hydrolysis reaction. pH
Before After
1% NaOH
5% NaOH
10% NaOH
13,33 14,16
13,62 14,60
13,74 14,81
Table 2 e H2 generation rate (HGR) of catalysts at different NaOH initial concentrations. Catalyst
Ni-Co3O4 Co-Co3O4
HGR, ml/gcat.min 1% NaOH
5% NaOH
10% NaOH
1375 2238
1466 2446
1925 2823
also found that the Co-Co3O4 is more active than Ni-Co3O4 catalyst. The highest hydrogen generation rate was obtained for the Co-Co3O4 catalyst at 10 wt% NaOH initial concentration as 2823 ml/gcat.min. According to Fig. 7 hydrolysis reaction with 1 wt% NaOH initial concentration continues for 150 min, while the hydrolysis reaction with 10 wt% NaOH initial concentration continues for 75 min for Ni-Co3O4 catalyst. Similarly, the reaction time was reduced with increasing NaOH initial concentration for the Co-Co3O4 catalyst (Fig. 8). It shows that hydroxide ions affect the activation of Ni and Co catalysts. In general, the reason for this positive effect for Ni and Co based catalysts is attributed to the complex surface reaction on the hydroxide ion. In the literature, it was reported that the hydrogen generation rate remained almost constant over 10 wt% initial concentrations when using nickel catalyst [42]. S‚ahiner et al. reported that the rate of hydrogen generation increased linearly with increasing NaOH concentrations for the Co-based catalyst [43]. Metal acceptance of hydride and the ability of basic character types to accept a proton are important factors for the hydrolysis reaction. The ligands bound to the central atomic metal increase the electron density of the metal. The H ions attached to the metal react with water molecules to form H2 and OH. Thus, the increase in electron density of the central atomic metal can provide a faster reaction [44]. However, as the NaOH concentration increased, a decrease in the total volume of hydrogen was observed. This is attributed to the reduced amount of free water required for the reaction at high NaOH concentrations and hence the low solubility of the reaction by-product NaBO2. Therefore, the initial concentration of NaOH should not exceed 10%. BH 4 anions originated from the NaBH4 solution are available on catalyst active sites in order to generate hydrogen. In an excessive amount of NaOH solution case, higher amounts of OH anions are transferred to the active sites of the catalysts that hinder the transfer of BH 4 anions which will decrease the hydrogen generation rate [27,45]. Table 3 shows the summary of the reported hydrogen generation rates for hydrolysis of NaBH4 catalyzed by various catalysts reported in the literature [22e24,46e49]. The catalysts synthesized in this study showed promising activity with high hydrogen generation rate than most of the Ni and Co based catalysts reported in the literature previously. In the study of Walter et al. the activity of the Ni-based catalyst was investigated at 60 C for 5% NaOH initial concentration and the hydrogen generation rate was found as 1300 ml/gcat.dk [46]. In our study, the hydrogen generation rate is found as 1400 ml/ gcat.dk at ambient conditions and 5% NaOH initial concentration. In this study, better hydrogen generation rates were obtained for our catalysts than Co-based catalysts given in the literature at 30 C as given in Table 3.
Please cite this article as: Bozkurt G et al., Hydrogen generation from sodium borohydride with Ni and Co based catalysts supported on Co3O4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.10.106
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Table 3 e Comparison of HGR of various catalysts for the hydrolysis of sodium borohydride. Catalysts
Co/Al2O3 Ni-Co/rGO Co-B/TiO2 Ni3B Co3B Ni complex Raney Ni Co/Cu Co-P/Cu Ni/Co3O4 Co/Co3O4
Method
impregnation-chemical reduction electroless coating in-situ reduction chemical reduction chemical reduction chemical chemical electroplating electroplating microwave irradiation-polyol microwave irradiation-polyol
Operating conditions wt% NaBH4
wt% NaOH
Temp.
10 10 1.5 5 5 2 10 20 20 10 10
1 5 5 5 5 7 10 5 5 10 10
30 40 30 60 60 30 25 30 30 25 25
HGR, ml/gcat.min
Ref
220 1280 1980 1300 6000 2240 228.5 52 954 1925 2823
[22] [23] [24] [46] [46] [47] [48] [49] [49] This work This work
rGO: reduced graphene oxide, MWNT: multi-walled nanotube.
By comparing our catalytsts with the ones reported in the literature, the new findings from the outcome of this work can be summarized as follows:
for doctoral research (Grant No. 1649B031502644) and financial support by Atatu¨rk University BAP project through grant number 2015/360.
1. Ni/Co3O4 and Co/Co3O4 were used as catalysts for hydrogen production from NaBH4 for the first time. 2. The catalysts prepared in our study exhibited higher hydrogen generation rates at ambient conditions than some hydrogen generation rates given in the literature carried out at higher temperature conditions. 3. In order to investigate the catalytic activities of the catalysts, a system is design that provides better removal of heat generated during the reaction, better mass transfer between the solution and the catalyst and obtaining dry hydrogen gas.
references
Conclusions Co3O4 supported Ni and Co catalysts were prepared by microwave irradiation-polyol method. A system was designed in order to determine the hydrogen generation activities of the catalysts. A dynamic reactor was used in the designed system and the NaBH4 solution was circulated onto the catalysts in order to increase the mass transport. The activity of the synthesized catalysts were tested in this system. The effects of different initial concentrations of NaOH solutions on hydrogen generation rate were investigated. Increase in initial concentrations of NaOH for both Ni and Co based catalysts were increased the hydrogen generation rate. However, as the NaOH initial concentration was increased, a decrease in the total volume of hydrogen was observed. In general, it was observed that Co-based catalysts are exhibited more activity than Ni-based catalysts in terms of hydrogen generation. On the other side, Ni/Co3O4 and Co/Co3O4 catalysts showed good catalytic activity. It can be concluded that the Ni/Co3O4 and Co/Co3O4 catalysts can be used for hydrogen generation by hydrolysis of NaBH4.
Acknowledgments The authors are gratefully acknowledge for scholarship from the Scientific and Technological Research Council of Turkey
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