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Co composite: Effect of catalyst precursor on hydrogen generation rate
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Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate O.V. Netskina a, b, *, E.S. Tayban a, I.P. Prosvirin a, O.V. Komova a, V.I. Simagina a a b
Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2019 Received in revised form 13 October 2019 Accepted 8 November 2019 Available online xxx
Tablets on the basis of sodium borohydride and cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) have been studied as hydrogen generation materials. The kinetics of sodium borohydride hydrolysis upon contact of the tablets with water has been investigated. Adsorption and reaction constants have been determined for each of the catalysts using the Langmuir-Hinshelwood model which allowed us to estimate the contribution of BH4 adsorption to the overall rate of hydrogen generation. It was noted that the nature of the catalyst precursor has an influence not only on the specific surface area of the in situ forming catalytically active phase, the particle size of the catalyst, the degree of reduction of cobalt compounds by sodium borohydride but also on the adsorption of BH4 anions from the reaction medium on the catalyst surface. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Sodium borohydride Hydrolysis Cobalt catalyst Langmuir-Hinshelwood model Adsorption
1. Introduction The use of hydrogen as an energy carrier for low-temperature proton-exchange membrane fuel cells (LT-PEM FC) requires compact hydrogen storage systems [1,2]. This is an especially urgent problem to be solved when developing energy sources for unmanned aircraft [3e6] and mobile applications [7,8]. For such applications hydrogen-generating compositions must not only have high hydrogen capacity but also be able to generate hydrogen without a supply of energy with no need of gas purification and humidification. Sodium borohydride containing 10.5 wt% hydrogen most fully meets these requirements [9,10]. Hydrogen evolution from sodium borohydride at environmental temperatures (40 … þ60 С) is brought about by interacting it with water: NaBH4 þ 4H2O / NaB(OH)4 þ 4H2[
(1)
The hydrogen generation rate decreases in the course of the reaction because of growth in pH to 13 as a result of accumulation of sodium tetrahydroxyborate in the reaction medium [11].
* Corresponding author. Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia. E-mail address: [email protected] (O.V. Netskina).
Complete hydride conversion can be achieved either at temperatures above 300 С [12], or by adding inorganic and organic acids [13e16] or in the presence of catalysts [17,18]. However, for the acid hydrolysis a Fourier transform infrared analysis indicated the presence in the forming hydrogen-containing gas of impurities of diborane and vapors of the acids [19]. In the same work in the catalytic hydrolysis hydrogen was found to contain only vapors of water and so it can be fed to the anode space of LT-PEM FC without a purification and humidification. Because of their high activity and a low cost the cobalt compounds are the most studied catalysts for sodium borohydride hydrolysis [20,21]. Cobalt compounds were also added to solidstate hydrogen-generating composites on the basis of sodium borohydride [22e24] proposed as a safe chemically bound form of hydrogen without loss in hydrogen storage capacity. Gas generation starts immediately after the addition of water. The hydrogen generating rate is controllable by using catalysts with different physicochemical properties [25e27]. It has been demonstrated [28] that the smaller the particle size of the active component of the cobalt catalyst the higher the hydrogen generation rate. It was found for solid-state hydrogen-generating composites from sodium borohydride and cobalt oxide [29] that at a high content of defects in the Co3O4 structure there was rapid formation of the active component and a high catalytic activity in sodium borohydride
https://doi.org/10.1016/j.renene.2019.11.031 0960-1481/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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hydrolysis. It should be noted that the kinetics of this reaction can be described with the Langmuir-Hinshelwood model [30e32], where the adsorption of reagents on the catalyst surface plays a key role. However, the effect of the catalyst precursor on the kinetics of sodium borohydride hydrolysis has not yet been discussed. In this work, the kinetics of sodium borohydride hydrolysis upon addition of water to solid-state composites of sodium borohydride with different cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) as catalyst precursors has been analyzed. This allowed us to elucidate the contribution of borohydride anion (BH4) adsorption to the overall rate of the catalytic process.
Solid-state NaBH4/Co composites were prepared from a mechanical mixture of sodium borohydride (0.0465 g) and cobalt compounds. The following cobalt compounds were used as catalyst precursor: CoCl2$6H2O (CAS 7791-13-1; Sigma-Aldrich 255599), Co(CH3COO)2$4H2O (CAS 6147-53-1; Sigma-Aldrich 403024), Co3O4 (CAS 1308-06-1; Sigma-Aldrich 221643) and anhydrous CoSO4 prepared by annealing CoSO4$7H2O (CAS 10026-24-1; Sigma-Aldrich C6768) at 500 С under argon. Samples of the composites were ground in a mortar grinder (PULVERISETTE 2, Germany) and pressed into tablets using a manual tablet press machine (TDP-0, China). The molar ratio NaBH4:Co was 16. A tablet of NaBH4/Co composite was loaded into a temperaturecontrolled glass reactor to which was added 5 mL of water and the kinetic measurements were performed at 40 С. The volume of the evolving gas was measured with a gas burette. Each experiment was repeated three times and average values were used to represent the kinetic plots. The relative error did not exceed 2%. For the physicochemical studies the catalysts were separated from the reaction medium with a magnet, washed three times with distilled water and three times with acetone. Then the catalysts were evacuated at room temperature for 24 h after which they were stored in desiccator under an inert medium (argon) to prevent sample oxidation by air oxygen. The specific surface areas of the catalysts were determined from thermal desorption of argon using Sorbtometr-M (Russia). The relative error of determination was ±3%. The reduced catalysts were studied by transmission electron microscopy (TEM). The electron microscopic images of the catalysts were obtained on a JEM-2010 electron microscope with an accelerating voltage of 200 kV and a resolving power of 1.4 Å. The samples to be analyzed were applied to a holey carbon film fixed on a standard copper grid. Size distribution diagrams were constructed using measured diameters (di) of at least 300 particles (ni). The mean Co particle sizes are quoted [33] as a number average diameter (d):
i
,
3. Results and discussion The objects of the study were solid-state composites containing sodium borohydride and cobalt compounds e CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4. According to Refs. [35,36], the catalytic phase of the cobalt catalysts forms in the reaction medium under the action of sodium borohydride: 2Co2þ þ 4BH4 þ 8H2O / (4Coe2B)catalyst þ 2B(OH)4 þ 12H2[ (3)
2. Experimental
X d ¼ ni di
structure and to calculate the ratio of the oxidized to reduced form of sulfur on the sample surface, taking into account the element sensitivity coefficients [34].
X ni
The catalytic phase represents a black ferromagnetic precipitate. From Fig. 1 it is seen that when a magnet is brought to the reactor wall, particles of the cobalt catalyst easily separate from the reaction medium with an intense evolution of hydrogen bubbles from their surfaces. No gas formation was observed in the bulk of the solution. The rate of hydrogen generation rate varied depending on the precursor of the catalytic active phase (Fig. 2). In the case of watersoluble salts CoCl2$6H2O, Co(CH3COO)2$4H2O the active phase forms instantaneously and hydrogen evolution starts at a high rate immediately after the addition of water. In the case of the slowly dissolving anhydrous cobalt sulfate (CoSO4) or insoluble cobalt oxide (Co3O4) there was an induction period with a very slow hydrogen generation rate when the formation of the active component took place [37]. The rate of gas evolution sharply increased at the end of this process [38]. The kinetic data were analyzed using the Van’t Hoff kinetics equations (Fig. 3) [39]. The method is based on the dependence of dC the natural logarithm of reaction rate (ln reagent ) on the natural dt logarithm of reagent concentration (lnCreagent ). The slope of such a plot yields the reaction order for a selected reagent. Using this approach, it was shown that in the presence of CoCl2$6H2O, Co(CH3COO)2$4H2O CoSO4 and Co3O4 the reaction order for sodium borohydride was fractional (Table 1) indicating a multi-stage character of the catalysis process where the adsorption of reagents onto the catalytically active surface played a key role [40]. Because of the limited quantity of the catalyst in the reaction volume the reaction rate is strongly dependent on catalyst surface
(2)
i
The XPS (X-ray photoelectron spectroscopy) spectra were taken with a SPECS photoelectron spectrometer (Germany) using a PHOIBOS-150-MCD-9 hemispheric analyzer and a FOCUS-500 monochromator (AlKa, hn ¼ 1486.74 eV, 150 W). The binding energy (BE) scale of the spectrometer was pre-calibrated using the Au4f7/2 (84.0 eV) and Cu2p3/2 (932.6 eV) core level peaks. The binding energies were determined with an accuracy of ±0.1 eV. The samples were applied onto a conducting scotch tape and studied without pretreatment. The charge of the sample was taken into account using C1s lines (284.8 eV). Analysis of the individual spectra of the elements allowed us to determine their electronic
Fig. 1. Formation of hydrogen bubbles on the surface of cobalt catalyst separated from the reaction medium by a magnet.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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С 0;NaBH4 CNaBH4 þ
Fig. 2. The effect of the cobalt salt on the hydrogen generation rate. NaBH4:Co ¼ 16. Water volume is 5 mL.
Fig. 3. Analysis of kinetic data (Fig. 1) using Van’t Hoof method for sodium borohydride hydrolysis over catalysts from different cobalt salts. NaBH4:Co ¼ 16. Water volume is 5 mL.
coverage by BH4 anions whose concentration is much smaller than the amount of water. Taking into account that the BH4 ions adsorption is described by the Langmuir adsorption isotherm the equation for the reaction rate becomes as follows:
WNaBH4 ¼
dCNaBH4 dt
¼ k
К ads ,С NaBH4 1 þ К ads ,С NaBH4
(4)
Its integration gives the following concentration dependence on reaction time:
3
C0;NaBH4 ¼ k,t , ln К ads CNaBH4 1
(5)
where k is the observed reaction constant, К ads ¼ kk1 is adsorp1 tion constant, k1 is the reaction constant of BH4 interaction with the active sites of the catalyst, k-1 is the constant of BH4 desorption from the catalyst surface. From equation (5) it is seen that when the К ads is 1 M1, the reaction rate can be described by zero-order kinetic equation. In this case the reaction rate is limited only by the interaction of reagents on the catalyst surface. When the К ads are «1 M1, the NaBH4 concentration in the course of the reaction will be changing according to the first-order kinetic equation. Optimizing the value of the adsorption constant from 0 to ∞ (Fig. 4a,c,e,g) the observed reaction constants of sodium borohydride hydrolysis have been determined for catalysts formed in the reaction medium from different cobalt compounds (Fig. 4b,d,f,h). According to the results given in Fig. 4a and c the adsorption constants for the catalysts from CoCl2$6H2O and Co(CH3COO)2$4H2O are close to 1 M1, indicating almost equal rates of adsorption and desorption of the BH4 anions. From this it follows that the hydrogen generation rate is determined by a specific surface area of the catalyst. The larger is the specific surface area, the greater is the amount of the adsorbed BH4 anions on the catalyst and, hence, the higher is the rate of the reaction. Indeed, the catalyst from cobalt chloride with a specific surface area of 22 m2 g1 and an average particle size of 21 nm (Fig. 5a and b) was more active than the catalyst from cobalt acetate with a specific surface area of 19 m2 g1 and an average particle size of 27 nm (Fig. 5c and d). For cobalt oxide the value of К ads is higher than unity apparently because of the slow desorption rate of BH4 ions. In spite of the longer time of BH4retention on the catalytically active surface the hydrogen generation rate did not reach the values shown by cobalt chloride and cobalt acetate (Fig. 2). The slow sodium borohydride conversion over Co3O4 is confirmed by its lowest reaction constant among the studied catalytic systems (Fig. 4f). The reason for such a behavior may be a small content of the active phase because of the incomplete reduction of cobalt oxide in reaction medium, as it was observed in Ref. [37]. A TEM study of cobalt oxide after the reaction confirmed that a portion of cobalt oxide did not undergo any transformation in the reaction medium and served as a support for localization of the metal-like particles of active phase (Fig. 6). Hence, the slow rate of hydrogen generation over Co3O4 is a result of a small size of the catalytically active surface accessible for the adsorption of BH4 ions. In the case of anhydrous cobalt sulfate the hydrolysis reaction proceeds very slowly (Fig. 2). However, taking into account the observed reaction constants (k ¼ 0.07233 M s1) determined with the Langmuir-Hinshelwood kinetic model (Fig. 4h), the sodium borohydride conversion over this catalyst should proceed with a higher rate than in the presence of cobalt oxide with k ¼ 0.000689 M s1. Besides, the average size of the catalyst particles forming from anhydrous cobalt sulfate is 18 nm (Fig. 7). This is
Table 1 Kinetic parameters of hydrogen generation over catalysts from different cobalt salts derived by the Van’t Hoff method. NaBH4:Co ¼ 16. Water volume 5 mL. Cobalt compound
S*BET, m2$g1
Average particle diameter (d), nm*
Reaction order
R2
CoCl2$6H2O Co(CH3COO)2$4H2O Co3O4 CoSO4 anhydrous Without catalyst
22 19 18 23 e
20 27 e 18 e
0.65 0.61 0.65 1.39 1.10
0.997 0.996 0.998 0.982 0.962
* - determined after the reaction.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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Fig. 4. Optimization of the adsorption constants and linear regression for Langmuir-Hinshelwood model: (a, b) CoCl2$6H2O, (c, d) Co(CH3COO)2$4H2O, (e, f) Co3O4 and (g, h) anhydrous CoSO4.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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Fig. 5. TEM images of catalysts and their particle size distribution: (a, b) catalyst forming from CoCl2$6H2O and (c, d) catalyst forming from Co(CH3COO)2$4H2O.
Fig. 6. TEM images of cobalt oxide (a) before and (b) after the reaction with EDX-analysis of selected areas.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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the lowest value among the catalysts studied in this work (Table 1). This discrepancy may be explained by a hindered BH4 adsorption on the catalytically active surface since the adsorption constant is considerably smaller than 1 M1 (Fig. 4g). For the catalyst from anhydrous CoSO4 the XPS data (Fig. 8) indicated the presence on the catalyst surface of sulfur in a reduced and oxidized state. The lowest value of the binding energy of 192 eV corresponds to elemental sulfur in an amorphous state [41,42] while the binding energy of 198 eV is characteristic of an unreduced cobalt sulfate [43]. Thus with XPS it was established for the first time that in the sodium borohydride interaction with cobalt sulfate there was reduction not only of the metal but also of sulfur. The forming sulfur and unreduced cobalt sulfate strongly adsorb on the surface of the catalytically active particles and cannot be removed by a multiple washing of the catalyst. In this catalyst the molar ratio Co:S is 7.7. It can be suggested that blocking of a portion of the active surface by sulfur-containing impurities is the main reasons for the low activity of the catalyst forming from the anhydrous CoSO4, since this substantially reduces the number of adsorption centers accessible for the BH4 ions. Treating the obtained results within the formal kinetics approach [44], it was found that changes in hydride concentration in the presence of the catalyst from anhydrous CoSO4 take place according to the second-order kinetic equation (Fig. 9a). This dependence indicates an important role of water in this catalytic process as is in the case in the spontaneous sodium borohydride hydrolysis in the absence of catalyst (Fig. 9b). This becomes especially pronounced when hydroxide ions (OH) get accumulated in the reaction medium as a result of dissociation of the forming sodium tetrahydroxyborate [45]: BðOHÞ 4 þ H2 O 4 BðH2 OÞðOHÞ3 þ OH ; pKb ¼ 4; 76
(6) The liberated OH ions can be coordinated by 2e5 water molecules [46e48]. It appears that the coordination of the hydrolysis products by water molecules substantially confines the interaction of the water with sodium borohydride on the catalytically active surface. The observed second order of the reaction may also be due to a large contribution of spontaneous sodium borohydride hydrolysis (without a catalyst) to the overall kinetics of the process (Fig. 2). 4. Conclusions
Fig. 8. XPS spectra of the S2p core level for cobalt catalyst forming from the anhydrous CoSO4.
water was added to solid-state NaBH4 composites containing cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) which form a catalytically active phase in the reaction medium under the action of hydride. It was found that in the presence of cobalt catalysts the reaction order of sodium borohydride hydrolysis had fractional values as a result of the multi-stage character of the catalytic process in which the adsorption of reagents on a catalytically active surface plays a key role. Using the Langmuir-Hinshelwood model the adsorption and reaction constants have been determined for each catalyst. In the case of CoCl2$6H2O and Co(CH3COO)2$4H2O the value of the adsorption constant was close to 1 M1 and the hydrogen generation rate was therefore determined by the specific surface area of the catalyst. Thus the reason for the high activity of the catalyst from cobalt chloride is its larger specific surface area as compared with the less active catalyst formed from cobalt acetate. For Co3O4 the adsorption constant is equal to 2.95 M-1 which indicates a slow rate of desorption of the BH4 anions. Nevertheless, the longer time of BH4 retention on the catalytically active surface of this catalyst does not lead to an increased hydrogen generation rate because of smaller content of the active phase as a result of incomplete reduction of the cobalt oxide. Anhydrous CoSO4 showed the lowest value of the
The kinetics of hydrogen generation has been studied when
Fig. 7. (a) TEM image and (b) particle size distribution for catalyst forming from anhydrous CoSO4.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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Fig. 9. Model predictions for zero-order, first-order and second-order with the experimental data: (a) with catalyst e anhydrous CoSO4 (NaBH4:Co ¼ 16:1) and (b) without catalyst. NaBH4 mass e 0.0465 g. Water volume is 5 mL.
adsorption constant (0.0095 M-1) which indicates that the gasgeneration rate was limited by insufficient adsorption of reagents on the catalytically active surface decorated by elemental sulfur and unreduced cobalt sulfate. Herewith, the amount of water in the reaction medium also plays an important role as in the case of the non-catalytic sodium borohydride hydrolysis. Thus, the nature of the catalyst precursor has an influence not only on the specific surface area of the forming catalytically active phase, the particle size of the catalyst, the degree of reduction of cobalt compounds by sodium borohydride but also on the BH4 adsorption from reaction medium. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Ministry of Science and Higher Education of the Russian Federation (BIC SB RAS: Project ААААА17-117041710089-7). The authors are grateful to PhD. Ishcenko A.V. for investigation of catalysts by TEM. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.11.031. References [1] T. Sinigaglia, F. Lewiski, M.E. Santos Martins, J.C. Mairesse Siluk, Production, storage, fuel stations of hydrogen and its utilization in automotive applications-a review, Int. J. Hydrogen Energy 42 (2017) 24597e24611, https://doi.org/10.1016/j.ijhydene.2017.08.063. [2] P. Preuster, A. Alekseev, P. Wasserscheid, Hydrogen storage technologies for future energy systems, Annu. Rev. Chem. Biomol. Eng. 8 (2017) 445e471, https://doi.org/10.1146/annurev-chembioeng-060816-101334. [3] E.S. Jung, H. Kim, S. Kwon, T.H. Oh, Fuel cell system with sodium borohydride hydrogen generator for small unmanned aerial vehicles, Int. J. Green Energy 15 (2018) 385e392, https://doi.org/10.1080/15435075.2018.1464924. [4] A. Gong, D. Verstraete, Fuel cell propulsion in small fixed-wing unmanned aerial vehicles: current status and research needs, Int. J. Hydrogen Energy 42
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Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate O.V. Netskina a, b, *, E.S. Tayban a, I.P. Prosvirin a, O.V. Komova a, V.I. Simagina a a b
Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia Novosibirsk State University, Pirogova Str. 2, Novosibirsk, 630090, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2019 Received in revised form 13 October 2019 Accepted 8 November 2019 Available online xxx
Tablets on the basis of sodium borohydride and cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) have been studied as hydrogen generation materials. The kinetics of sodium borohydride hydrolysis upon contact of the tablets with water has been investigated. Adsorption and reaction constants have been determined for each of the catalysts using the Langmuir-Hinshelwood model which allowed us to estimate the contribution of BH4 adsorption to the overall rate of hydrogen generation. It was noted that the nature of the catalyst precursor has an influence not only on the specific surface area of the in situ forming catalytically active phase, the particle size of the catalyst, the degree of reduction of cobalt compounds by sodium borohydride but also on the adsorption of BH4 anions from the reaction medium on the catalyst surface. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Hydrogen Sodium borohydride Hydrolysis Cobalt catalyst Langmuir-Hinshelwood model Adsorption
1. Introduction The use of hydrogen as an energy carrier for low-temperature proton-exchange membrane fuel cells (LT-PEM FC) requires compact hydrogen storage systems [1,2]. This is an especially urgent problem to be solved when developing energy sources for unmanned aircraft [3e6] and mobile applications [7,8]. For such applications hydrogen-generating compositions must not only have high hydrogen capacity but also be able to generate hydrogen without a supply of energy with no need of gas purification and humidification. Sodium borohydride containing 10.5 wt% hydrogen most fully meets these requirements [9,10]. Hydrogen evolution from sodium borohydride at environmental temperatures (40 … þ60 С) is brought about by interacting it with water: NaBH4 þ 4H2O / NaB(OH)4 þ 4H2[
(1)
The hydrogen generation rate decreases in the course of the reaction because of growth in pH to 13 as a result of accumulation of sodium tetrahydroxyborate in the reaction medium [11].
* Corresponding author. Boreskov Institute of Catalysis SB RAS, Pr. Akademika Lavrentieva 5, Novosibirsk, 630090, Russia. E-mail address: [email protected] (O.V. Netskina).
Complete hydride conversion can be achieved either at temperatures above 300 С [12], or by adding inorganic and organic acids [13e16] or in the presence of catalysts [17,18]. However, for the acid hydrolysis a Fourier transform infrared analysis indicated the presence in the forming hydrogen-containing gas of impurities of diborane and vapors of the acids [19]. In the same work in the catalytic hydrolysis hydrogen was found to contain only vapors of water and so it can be fed to the anode space of LT-PEM FC without a purification and humidification. Because of their high activity and a low cost the cobalt compounds are the most studied catalysts for sodium borohydride hydrolysis [20,21]. Cobalt compounds were also added to solidstate hydrogen-generating composites on the basis of sodium borohydride [22e24] proposed as a safe chemically bound form of hydrogen without loss in hydrogen storage capacity. Gas generation starts immediately after the addition of water. The hydrogen generating rate is controllable by using catalysts with different physicochemical properties [25e27]. It has been demonstrated [28] that the smaller the particle size of the active component of the cobalt catalyst the higher the hydrogen generation rate. It was found for solid-state hydrogen-generating composites from sodium borohydride and cobalt oxide [29] that at a high content of defects in the Co3O4 structure there was rapid formation of the active component and a high catalytic activity in sodium borohydride
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hydrolysis. It should be noted that the kinetics of this reaction can be described with the Langmuir-Hinshelwood model [30e32], where the adsorption of reagents on the catalyst surface plays a key role. However, the effect of the catalyst precursor on the kinetics of sodium borohydride hydrolysis has not yet been discussed. In this work, the kinetics of sodium borohydride hydrolysis upon addition of water to solid-state composites of sodium borohydride with different cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) as catalyst precursors has been analyzed. This allowed us to elucidate the contribution of borohydride anion (BH4) adsorption to the overall rate of the catalytic process.
Solid-state NaBH4/Co composites were prepared from a mechanical mixture of sodium borohydride (0.0465 g) and cobalt compounds. The following cobalt compounds were used as catalyst precursor: CoCl2$6H2O (CAS 7791-13-1; Sigma-Aldrich 255599), Co(CH3COO)2$4H2O (CAS 6147-53-1; Sigma-Aldrich 403024), Co3O4 (CAS 1308-06-1; Sigma-Aldrich 221643) and anhydrous CoSO4 prepared by annealing CoSO4$7H2O (CAS 10026-24-1; Sigma-Aldrich C6768) at 500 С under argon. Samples of the composites were ground in a mortar grinder (PULVERISETTE 2, Germany) and pressed into tablets using a manual tablet press machine (TDP-0, China). The molar ratio NaBH4:Co was 16. A tablet of NaBH4/Co composite was loaded into a temperaturecontrolled glass reactor to which was added 5 mL of water and the kinetic measurements were performed at 40 С. The volume of the evolving gas was measured with a gas burette. Each experiment was repeated three times and average values were used to represent the kinetic plots. The relative error did not exceed 2%. For the physicochemical studies the catalysts were separated from the reaction medium with a magnet, washed three times with distilled water and three times with acetone. Then the catalysts were evacuated at room temperature for 24 h after which they were stored in desiccator under an inert medium (argon) to prevent sample oxidation by air oxygen. The specific surface areas of the catalysts were determined from thermal desorption of argon using Sorbtometr-M (Russia). The relative error of determination was ±3%. The reduced catalysts were studied by transmission electron microscopy (TEM). The electron microscopic images of the catalysts were obtained on a JEM-2010 electron microscope with an accelerating voltage of 200 kV and a resolving power of 1.4 Å. The samples to be analyzed were applied to a holey carbon film fixed on a standard copper grid. Size distribution diagrams were constructed using measured diameters (di) of at least 300 particles (ni). The mean Co particle sizes are quoted [33] as a number average diameter (d):
i
,
3. Results and discussion The objects of the study were solid-state composites containing sodium borohydride and cobalt compounds e CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4. According to Refs. [35,36], the catalytic phase of the cobalt catalysts forms in the reaction medium under the action of sodium borohydride: 2Co2þ þ 4BH4 þ 8H2O / (4Coe2B)catalyst þ 2B(OH)4 þ 12H2[ (3)
2. Experimental
X d ¼ ni di
structure and to calculate the ratio of the oxidized to reduced form of sulfur on the sample surface, taking into account the element sensitivity coefficients [34].
X ni
The catalytic phase represents a black ferromagnetic precipitate. From Fig. 1 it is seen that when a magnet is brought to the reactor wall, particles of the cobalt catalyst easily separate from the reaction medium with an intense evolution of hydrogen bubbles from their surfaces. No gas formation was observed in the bulk of the solution. The rate of hydrogen generation rate varied depending on the precursor of the catalytic active phase (Fig. 2). In the case of watersoluble salts CoCl2$6H2O, Co(CH3COO)2$4H2O the active phase forms instantaneously and hydrogen evolution starts at a high rate immediately after the addition of water. In the case of the slowly dissolving anhydrous cobalt sulfate (CoSO4) or insoluble cobalt oxide (Co3O4) there was an induction period with a very slow hydrogen generation rate when the formation of the active component took place [37]. The rate of gas evolution sharply increased at the end of this process [38]. The kinetic data were analyzed using the Van’t Hoff kinetics equations (Fig. 3) [39]. The method is based on the dependence of dC the natural logarithm of reaction rate (ln reagent ) on the natural dt logarithm of reagent concentration (lnCreagent ). The slope of such a plot yields the reaction order for a selected reagent. Using this approach, it was shown that in the presence of CoCl2$6H2O, Co(CH3COO)2$4H2O CoSO4 and Co3O4 the reaction order for sodium borohydride was fractional (Table 1) indicating a multi-stage character of the catalysis process where the adsorption of reagents onto the catalytically active surface played a key role [40]. Because of the limited quantity of the catalyst in the reaction volume the reaction rate is strongly dependent on catalyst surface
(2)
i
The XPS (X-ray photoelectron spectroscopy) spectra were taken with a SPECS photoelectron spectrometer (Germany) using a PHOIBOS-150-MCD-9 hemispheric analyzer and a FOCUS-500 monochromator (AlKa, hn ¼ 1486.74 eV, 150 W). The binding energy (BE) scale of the spectrometer was pre-calibrated using the Au4f7/2 (84.0 eV) and Cu2p3/2 (932.6 eV) core level peaks. The binding energies were determined with an accuracy of ±0.1 eV. The samples were applied onto a conducting scotch tape and studied without pretreatment. The charge of the sample was taken into account using C1s lines (284.8 eV). Analysis of the individual spectra of the elements allowed us to determine their electronic
Fig. 1. Formation of hydrogen bubbles on the surface of cobalt catalyst separated from the reaction medium by a magnet.
Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031
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С 0;NaBH4 CNaBH4 þ
Fig. 2. The effect of the cobalt salt on the hydrogen generation rate. NaBH4:Co ¼ 16. Water volume is 5 mL.
Fig. 3. Analysis of kinetic data (Fig. 1) using Van’t Hoof method for sodium borohydride hydrolysis over catalysts from different cobalt salts. NaBH4:Co ¼ 16. Water volume is 5 mL.
coverage by BH4 anions whose concentration is much smaller than the amount of water. Taking into account that the BH4 ions adsorption is described by the Langmuir adsorption isotherm the equation for the reaction rate becomes as follows:
WNaBH4 ¼
dCNaBH4 dt
¼ k
К ads ,С NaBH4 1 þ К ads ,С NaBH4
(4)
Its integration gives the following concentration dependence on reaction time:
3
C0;NaBH4 ¼ k,t , ln К ads CNaBH4 1
(5)
where k is the observed reaction constant, К ads ¼ kk1 is adsorp1 tion constant, k1 is the reaction constant of BH4 interaction with the active sites of the catalyst, k-1 is the constant of BH4 desorption from the catalyst surface. From equation (5) it is seen that when the К ads is 1 M1, the reaction rate can be described by zero-order kinetic equation. In this case the reaction rate is limited only by the interaction of reagents on the catalyst surface. When the К ads are «1 M1, the NaBH4 concentration in the course of the reaction will be changing according to the first-order kinetic equation. Optimizing the value of the adsorption constant from 0 to ∞ (Fig. 4a,c,e,g) the observed reaction constants of sodium borohydride hydrolysis have been determined for catalysts formed in the reaction medium from different cobalt compounds (Fig. 4b,d,f,h). According to the results given in Fig. 4a and c the adsorption constants for the catalysts from CoCl2$6H2O and Co(CH3COO)2$4H2O are close to 1 M1, indicating almost equal rates of adsorption and desorption of the BH4 anions. From this it follows that the hydrogen generation rate is determined by a specific surface area of the catalyst. The larger is the specific surface area, the greater is the amount of the adsorbed BH4 anions on the catalyst and, hence, the higher is the rate of the reaction. Indeed, the catalyst from cobalt chloride with a specific surface area of 22 m2 g1 and an average particle size of 21 nm (Fig. 5a and b) was more active than the catalyst from cobalt acetate with a specific surface area of 19 m2 g1 and an average particle size of 27 nm (Fig. 5c and d). For cobalt oxide the value of К ads is higher than unity apparently because of the slow desorption rate of BH4 ions. In spite of the longer time of BH4retention on the catalytically active surface the hydrogen generation rate did not reach the values shown by cobalt chloride and cobalt acetate (Fig. 2). The slow sodium borohydride conversion over Co3O4 is confirmed by its lowest reaction constant among the studied catalytic systems (Fig. 4f). The reason for such a behavior may be a small content of the active phase because of the incomplete reduction of cobalt oxide in reaction medium, as it was observed in Ref. [37]. A TEM study of cobalt oxide after the reaction confirmed that a portion of cobalt oxide did not undergo any transformation in the reaction medium and served as a support for localization of the metal-like particles of active phase (Fig. 6). Hence, the slow rate of hydrogen generation over Co3O4 is a result of a small size of the catalytically active surface accessible for the adsorption of BH4 ions. In the case of anhydrous cobalt sulfate the hydrolysis reaction proceeds very slowly (Fig. 2). However, taking into account the observed reaction constants (k ¼ 0.07233 M s1) determined with the Langmuir-Hinshelwood kinetic model (Fig. 4h), the sodium borohydride conversion over this catalyst should proceed with a higher rate than in the presence of cobalt oxide with k ¼ 0.000689 M s1. Besides, the average size of the catalyst particles forming from anhydrous cobalt sulfate is 18 nm (Fig. 7). This is
Table 1 Kinetic parameters of hydrogen generation over catalysts from different cobalt salts derived by the Van’t Hoff method. NaBH4:Co ¼ 16. Water volume 5 mL. Cobalt compound
S*BET, m2$g1
Average particle diameter (d), nm*
Reaction order
R2
CoCl2$6H2O Co(CH3COO)2$4H2O Co3O4 CoSO4 anhydrous Without catalyst
22 19 18 23 e
20 27 e 18 e
0.65 0.61 0.65 1.39 1.10
0.997 0.996 0.998 0.982 0.962
* - determined after the reaction.
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Fig. 4. Optimization of the adsorption constants and linear regression for Langmuir-Hinshelwood model: (a, b) CoCl2$6H2O, (c, d) Co(CH3COO)2$4H2O, (e, f) Co3O4 and (g, h) anhydrous CoSO4.
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Fig. 5. TEM images of catalysts and their particle size distribution: (a, b) catalyst forming from CoCl2$6H2O and (c, d) catalyst forming from Co(CH3COO)2$4H2O.
Fig. 6. TEM images of cobalt oxide (a) before and (b) after the reaction with EDX-analysis of selected areas.
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the lowest value among the catalysts studied in this work (Table 1). This discrepancy may be explained by a hindered BH4 adsorption on the catalytically active surface since the adsorption constant is considerably smaller than 1 M1 (Fig. 4g). For the catalyst from anhydrous CoSO4 the XPS data (Fig. 8) indicated the presence on the catalyst surface of sulfur in a reduced and oxidized state. The lowest value of the binding energy of 192 eV corresponds to elemental sulfur in an amorphous state [41,42] while the binding energy of 198 eV is characteristic of an unreduced cobalt sulfate [43]. Thus with XPS it was established for the first time that in the sodium borohydride interaction with cobalt sulfate there was reduction not only of the metal but also of sulfur. The forming sulfur and unreduced cobalt sulfate strongly adsorb on the surface of the catalytically active particles and cannot be removed by a multiple washing of the catalyst. In this catalyst the molar ratio Co:S is 7.7. It can be suggested that blocking of a portion of the active surface by sulfur-containing impurities is the main reasons for the low activity of the catalyst forming from the anhydrous CoSO4, since this substantially reduces the number of adsorption centers accessible for the BH4 ions. Treating the obtained results within the formal kinetics approach [44], it was found that changes in hydride concentration in the presence of the catalyst from anhydrous CoSO4 take place according to the second-order kinetic equation (Fig. 9a). This dependence indicates an important role of water in this catalytic process as is in the case in the spontaneous sodium borohydride hydrolysis in the absence of catalyst (Fig. 9b). This becomes especially pronounced when hydroxide ions (OH) get accumulated in the reaction medium as a result of dissociation of the forming sodium tetrahydroxyborate [45]: BðOHÞ 4 þ H2 O 4 BðH2 OÞðOHÞ3 þ OH ; pKb ¼ 4; 76
(6) The liberated OH ions can be coordinated by 2e5 water molecules [46e48]. It appears that the coordination of the hydrolysis products by water molecules substantially confines the interaction of the water with sodium borohydride on the catalytically active surface. The observed second order of the reaction may also be due to a large contribution of spontaneous sodium borohydride hydrolysis (without a catalyst) to the overall kinetics of the process (Fig. 2). 4. Conclusions
Fig. 8. XPS spectra of the S2p core level for cobalt catalyst forming from the anhydrous CoSO4.
water was added to solid-state NaBH4 composites containing cobalt compounds (CoCl2$6H2O, Co(CH3COO)2$4H2O, Co3O4 and anhydrous CoSO4) which form a catalytically active phase in the reaction medium under the action of hydride. It was found that in the presence of cobalt catalysts the reaction order of sodium borohydride hydrolysis had fractional values as a result of the multi-stage character of the catalytic process in which the adsorption of reagents on a catalytically active surface plays a key role. Using the Langmuir-Hinshelwood model the adsorption and reaction constants have been determined for each catalyst. In the case of CoCl2$6H2O and Co(CH3COO)2$4H2O the value of the adsorption constant was close to 1 M1 and the hydrogen generation rate was therefore determined by the specific surface area of the catalyst. Thus the reason for the high activity of the catalyst from cobalt chloride is its larger specific surface area as compared with the less active catalyst formed from cobalt acetate. For Co3O4 the adsorption constant is equal to 2.95 M-1 which indicates a slow rate of desorption of the BH4 anions. Nevertheless, the longer time of BH4 retention on the catalytically active surface of this catalyst does not lead to an increased hydrogen generation rate because of smaller content of the active phase as a result of incomplete reduction of the cobalt oxide. Anhydrous CoSO4 showed the lowest value of the
The kinetics of hydrogen generation has been studied when
Fig. 7. (a) TEM image and (b) particle size distribution for catalyst forming from anhydrous CoSO4.
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7
Fig. 9. Model predictions for zero-order, first-order and second-order with the experimental data: (a) with catalyst e anhydrous CoSO4 (NaBH4:Co ¼ 16:1) and (b) without catalyst. NaBH4 mass e 0.0465 g. Water volume is 5 mL.
adsorption constant (0.0095 M-1) which indicates that the gasgeneration rate was limited by insufficient adsorption of reagents on the catalytically active surface decorated by elemental sulfur and unreduced cobalt sulfate. Herewith, the amount of water in the reaction medium also plays an important role as in the case of the non-catalytic sodium borohydride hydrolysis. Thus, the nature of the catalyst precursor has an influence not only on the specific surface area of the forming catalytically active phase, the particle size of the catalyst, the degree of reduction of cobalt compounds by sodium borohydride but also on the BH4 adsorption from reaction medium. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by Ministry of Science and Higher Education of the Russian Federation (BIC SB RAS: Project ААААА17-117041710089-7). The authors are grateful to PhD. Ishcenko A.V. for investigation of catalysts by TEM. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.11.031. References [1] T. Sinigaglia, F. Lewiski, M.E. Santos Martins, J.C. Mairesse Siluk, Production, storage, fuel stations of hydrogen and its utilization in automotive applications-a review, Int. J. Hydrogen Energy 42 (2017) 24597e24611, https://doi.org/10.1016/j.ijhydene.2017.08.063. [2] P. Preuster, A. Alekseev, P. Wasserscheid, Hydrogen storage technologies for future energy systems, Annu. Rev. Chem. Biomol. Eng. 8 (2017) 445e471, https://doi.org/10.1146/annurev-chembioeng-060816-101334. [3] E.S. Jung, H. Kim, S. Kwon, T.H. Oh, Fuel cell system with sodium borohydride hydrogen generator for small unmanned aerial vehicles, Int. J. Green Energy 15 (2018) 385e392, https://doi.org/10.1080/15435075.2018.1464924. [4] A. Gong, D. Verstraete, Fuel cell propulsion in small fixed-wing unmanned aerial vehicles: current status and research needs, Int. J. Hydrogen Energy 42
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Please cite this article as: O.V. Netskina et al., Hydrogen storage systems based on solid-state NaBH4/Co composite: Effect of catalyst precursor on hydrogen generation rate, Renewable Energy, https://doi.org/10.1016/j.renene.2019.11.031