Carbon nanotubes-promoted Co–B catalysts for rapid hydrogen generation via NaBH4 hydrolysis

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Carbon nanotubes-promoted CoeB catalysts for rapid hydrogen generation via NaBH4 hydrolysis Limin Shi a,*, Zheng Chen a, Zengyun Jian a, Fenghai Guo b, Chuanlei Gao a a b

School of Materials and Chemical Engineering, Xi'an Technological University, Xi'an 710021, China State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China

highlights
CNTs-promoted CoeB catalysts for NaBH4 hydrolysis are designed and synthesized.
Catalytic activity is correlated with contents of CNTs promoter.
Remarkably high hydrogen generation rate of 12.00 L$min-1$gcatalyst-1 are achieved.
High electron densities at increased active sites account for superior performance.

article info

abstract

Article history:

In this work, multiwalled carbon nanotubes (MWCNTs) promoted CoeB catalysts for NaBH4

Received 27 March 2019

hydrolysis have been designed and synthesized. The structural features of as-prepared

Received in revised form

catalysts have been investigated and discussed as a function of MWCNTs contents by X-

13 May 2019

ray diffraction, X-ray photoelectron spectra, N2 adsorption/desorption isotherms, scanning

Accepted 24 May 2019

electron microscope. The results show that the catalysts still maintain an amorphous

Available online 20 June 2019

structure with the addition of carbon nanotubes promoter. However, the appropriate amount of MWCNTs promoter in CoeB catalysts leads to large specific surface area, fine

Keywords:

dispersion of active components, increased active sites and high electron density at active

NaBH4 hydrolysis

sites. Moreover, hydrogen spillover on the catalyst is promoted, which contributes to

Carbon nanotubes promoter

regeneration of active sites and accelerating catalytic cycle. Among all the experimental

CoeB catalysts

samples, it is found that the CoeB catalyst promoted by 10 wt% carbon nanotubes exhibits

Hydrogen generation rate

optimal

Hydrogen spillover

12.00 L min1$g1 catalyst and relatively good stability.

catalytic

activity

with

remarkably

high

hydrogen

generation

rate

of

© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction With increasing energy demand and escalation of environmental pollution, hydrogen as a clean and efficient energy has been widely viewed as one of the most promising alternatives of fossil fuels in past decades [1,2]. Considering the storage and transportation problems, chemical hydrides have been

believed to be ideal hydrogen storage materials [2e4]. Among them, sodium borohydride (NaBH4) has been drawn special attention due to its high H2 storage capacity (10.8 wt%), controllable hydrogen generation rate, high purity of hydrogen produced at room temperature, excellent stability in alkaline solution, recycle abilities and nontoxic hydrolysis byproducts [5e10]. Pure hydrogen can be produced via NaBH4 hydrolysis with a controllable rate at room temperature in the

* Corresponding author. E-mail address: [email protected] (L. Shi). https://doi.org/10.1016/j.ijhydene.2019.05.206 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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presence of appropriate catalysts. Efficient catalysts always play a key role in acceleration of NaBH4 hydrolysis. It has been proven that noble-metal catalysts such as Pt [11], Pd [12], Ru [13] and Rh [14] exhibit superior catalytic activity towards NaBH4 hydrolysis with rapid hydrogen generation. However, their expensive cost restrains the widespread applications of noble-metal catalysts. Currently, non-noble metal CoeB based catalysts have been received considerable attention and extensively investigated for NaBH4 hydrolysis owing to their high catalytic activity, low cost and good cycling stability [15e17]. CoeB catalysts can be effortlessly obtained by means of chemical reduction methods. However, the exothermic nature during the synthesis leads to the agglomeration of catalyst particles due to the high surface energy involved. The agglomeration phenomenon causes inevitably decrease of the effective surface area and number of active sites, which limits catalytic performance of CoeB based catalysts for hydrogen generation from NaBH4 hydrolysis [18,19]. In order to further improve the catalytic activity and increase the hydrogen generation rate, active components are often supported over some materials with larger surface areas, which contributes to high dispersion of active components, increase of active surface areas and active sites [17,20e22]. In addition, it has been found that introducing suitable transition metal promoters into CoeB catalysts can also effectively prevent particles from agglomeration and significantly improve the catalytic activity [18,23e26]. As reported, the presence of moderate Cu in CoeB sample avoids agglomeration of the catalyst particles and significantly increases the specific surface area, which provides an excellent condition for adsorption of BH 4 ions [26]. With the addition of Fe promoter, the hydrogen generation rate of CoeFeeB catalyst increases notably to 4.3 L min1$g1 catalyst in comparison with that (2.8 L min1$g1 catalyst) of CoeB sample, and the enhanced activity is believed to result from its large active surface area, high dispersion of CoeB and electron transfer from B and Fe to active Co sites [27]. It has been also proven that the metals Cr, Mo, and W promoters in oxidized state inhibit CoeB agglomeration and increase the active surface area, respectively. Moreover, Cr3þ, Mo4þ and W4þ species on the surface can act as Lewis acid sites for OH adsorption, which can account for their superior activity [18,24,26]. Meanwhile, the promoting effect of Ni in the CoeNieB catalyst has been investigated and the results indicate that the Ni-doped CoeB sample shows excellent catalytic activity due to large active surface area, electron enrichment on Co active sites and the synergetic effect of Co and Ni atoms [23]. The work of Hou et al. has further suggested that the presence of hydrogen spillover as well as large surface area and increase of electron density at active sites in the Ni-promoted CoeB catalyst can be responsible for its high activity with a hydrogen generation rates of 6.4 L min1$g1 catalyst and excellent stability [19]. It is generally accepted that MWCNTs are ideal carriers due to their high specific surface area, excellent adsorption properties and unique electronic properties [28]. It has been demonstrated that MWCNTs supported CoeB catalyst achieves remarkably higher hydrogen generation rate (5.1 L min1$g1CoeB) in comparison with those of unsupported catalyst (0.5 L min1$g1CoeB) and carbon supported one (3.1 L min1$g1CoeB) under the same conditions due to

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the better dispersion and more active surface area [29]. The work of Li et al. [5] has indicated that CoB nanoparticles supported carbon nanotubes with open tips (o-CNTs) exhibits higher catalytic activity comparing with that of the catalyst supported carbon nanotubes with closed tips (c-CNTs). The enhanced activity is attributed to fine dispersion and high loading amount [5]. It has been reported that LiBH4 dispersed on MWCNTs preserves significantly improved hydrolysis due to the increase of contact areas between reactants and enhanced hydrogen diffusion with the addition of MWCNTs [30]. In addition, the nanocomposite (Fe3O4-CNTs) prepared with a co-precipitation has been selected to support Co catalyst for NaBH4 hydrolysis [31]. The results show that the Co/ Fe3O4-CNTs catalyst preserves high activity, good stability and easy separation due to the strong magnetic properties of support [31]. It has been found that Ru nanoparticles deposited on MWCNTs exhibits high reaction rate during the ammonia borane hydrolysis due to the hydrogen spillover [32]. It has been proven that MWCNTs supported active species exhibit significantly improved catalytic performance due to large active surface area, high dispersion and hydrogen spillover. Considering the fact that MWCNTs itself has no catalytic activity towards NaBH4 hydrolysis, improvement in catalytic performance is still limited due to low relatively content of active species for MWCNTs supported catalysts. For CoeB based catalysts, the increase in CoeB content and decrease of MWCNTs are expected to obtain efficient catalysts for NaBH4 hydrolysis. According to the previous literature [33e35], a small amount of MWCNTs can significantly enhance the catalytic performance via promoted hydrogen spillover and MWCNTs have been proven to be excellent promoters in CO/CO2 hydrogenation. Considering that large specific surface area of MWCNTs itself and the ability to promote hydrogen spillover are beneficial for improvement of catalytic activity, MWCNTs has been selected as promoters and MWCNTs-promoted CoeB catalysts for hydrogen generation via NaBH4 hydrolysis have been designed and synthesized in this work. Structural characteristic and catalytic performance of MWCNTs-promoted CoeB catalysts have been carefully investigated and discussed as a function of MWCNTs contents. Meanwhile, the reusability and the reasons for constant loss in activity have been also analyzed. It is expected to reveal promoting effect of MWCNTs in CoeB catalysts for hydrogen generation from NaBH4 hydrolysis. The obtains in this work can be expected to provide experimental guidelines for design and development of efficient non-precious metal catalysts for hydrogen generation from NaBH4 hydrolysis.

Experimental procedures Catalyst preparation The commercial multiwalled carbon nanotubes with outer diameter 10e20 nm were supplied by Chengdu Organic Chemical Industries Ltd., China and marked as CNTs. CNTs-promoted CoeB catalysts were prepared by a simple chemical reduction. Typically, appropriate CoCl2$6H2O was completely dissolved in 40 mL distilled water. The calculated CNTs was added into the above solution and then treated under ultrasonic condition for

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1 h. Consequently, 30 mL solution containing 5 wt% NaBH4 and 5 wt% NaOH as a reducing agent was added dropwise under stirring at room temperature. After filtration and thoroughly washing with distilled water and ethanol, respectively, the obtained solid was dried in a vacuum oven at 60  C overnight. The contents of CNTs are 5, 10 and 20 wt% and corresponding samples are designed as CoeB-5CNTs, CoeB-10CNTs and CoeB-20CNTs, respectively. In order to facilitate comparison, the CoeB catalyst without CNTs was also synthesized by the same method and conditions as mentioned above.

Catalyst characterization The crystalline structure was characterized by means of X-ray powder diffraction (XRD) on a Shimadzu 7000 diffractometer A). The XRD patterns were with Cu Ka radiation (l ¼ 0.154178
obtained in the range of 2q between 10 and 80 at a rate of 8 / min. The textural characteristics of the catalysts were analyzed according to N2 adsorption/desorption isotherms obtained on a TriStar II Plus micromeritics at 196  C. Both samples were firstly degassed in vacuum at 200  C for 6 h prior to measurements. BET and BJH methods have been used to determine the specific surface area and pore size distribution, respectively [36]. The surface morphology of the samples was investigated via a JSM-7500F scanning electron microscope (SEM) with 30 kV of accelerating voltage. Element compositions and chemical states on catalyst surface were analyzed according to X-ray photoelectron spectra (XPS) obtained on the Escalab 250Xi spectrometer with an Al Ka radiation. All the binding energies were referenced to C1s at 284.8 eV.

Hydrogen generation testing For catalytic activity tests, 10 mL alkaline solution containing 5 wt% NaBH4 and 5 wt% NaOH was confined into a roundbottom flask. Meanwhile, the flask was immersed in a water bath to maintain 25  C of the reaction temperature without any stirring. Then, 0.1 g of the catalyst was added into the solution. The hydrogen generation was measured by a conventional water displacement method [37]. The change in volume of hydrogen gas was recorded at intervals of 10 s. The hydrogen generation rate was calculated on the basis of the amount of catalyst. The reusability of the catalyst was investigated via hydrogen generation from NaBH4 hydrolysis for multi-cycle tests. Each test conditions are the same as the above. When the hydrolysis reaction stopped, the catalyst was collected, rinsed with distilled water for three times, dried in a vacuum oven at 60  C for 10 h and then weighed. The recycled catalyst was added to new 10 mL alkaline solution to catalyze the above hydrolysis reaction.

reduction. As shown in Fig. 1, a weak and broad diffraction peak around 2q 45.0 is observed for the CoeB sample, corresponding to the typical amorphous phase of CoeB alloy [5]. Meanwhile, Bragg peaks with low intensities appearing around 2q 33.7 and 59.6 are attributed to CoO phase [7,8]. With the addition of 5 wt% CNTs promoters, the amorphous phase of CoeB alloy and CoO phase are still identified in the CoeB-5CNTs sample, while no characteristic diffraction peak assigned to graphite phase of CNTs is observed due to low content. With further increasing CNTs content, the graphite phase of CNTs around 2q 26.0 is identified except for the amorphous structure in the CoeB-10CNTs and CoeB-20CNTs catalysts [36]. The results of XRD analysis suggest that the addition of CNTs promoter does not change the amorphous structure of CoeB alloy. An amorphous structure is always believed to be short-range order and long-range disorder, unsaturated surface coordination sites and lack of crystal defects, which has been proven to improve catalytic activity for hydrolysis of NaBH4 [9,38]. Surface compositions, elemental states and electronic interaction between atoms in CoeB and CoeB-10CNTs samples have been analyzed by XPS measurements. Fig. 2 displays XPS survey spectra of the samples and high resolution XPS spectra of Co 2p and B 1s in the CoeB and CoeB-10CNTs catalysts, respectively. The existence of Co, B, O and C elements is further confirmed in the samples according to Fig. 2(a). It is generally believed that Co 2p spectra contains two types of spin-orbital states of Co 2p3/2 and Co 2p1/2 as shown in Fig. 2(b). From XPS spectra of Co 2p3/2 in the CoeB sample, the observed dominant peaks at binding energy (BE) of 780.9 eV and 782.6 eV with a satellite peak around 787.0 eV indicate that Co species exist in both metallic (Co0) and oxidized (Co2þ) states [39,40]. Similarly, peaks at BE of 780.4 eV and 782.1 eV are corresponded to Co0 and Co2þ species in the CoeB-10CNTs catalyst, respectively. The BE value of Co0 in the CoeB-10CNTs sample has a negative shift of 0.5eV comparing with that of CoeB catalyst, suggesting the existence of higher electron density on cobalt sites [19]. Meanwhile, the contents of Co0 in total Co species for CoeB and CoeB-10CNTs catalysts

Results and discussion Microstructure characterization of the as-synthesized catalysts Fig. 1 presents the XRD patterns of pure CoeB and CNTspromoted CoeB catalysts prepared by a simple chemical

Fig. 1 e XRD patterns of pure CoeB and CNTs promoted CoeB samples.

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Fig. 2 e XPS survey spectra (a) and XPS spectra of (b) Co 2p and (c) B 1s level for the CoeB and CoeB-10CNTs catalysts.

calculated from XPS spectra are 53.2% and 58.0%, respectively. Metal Co species have been proven to be active sites during catalytic hydrolysis reaction of NaBH4, on which BH 4 ions are firstly chemisorbed [18,19,41]. The presence of high electron density at the catalytically active sites is crucial for an efficient catalyst, which can enhance the hydrolysis of NaBH4 [42]. According to the XPS analysis, it is obvious that the addition of CNTs promoter increases not only active sites but electron density at Co sites. The CoeB-10CNTs catalyst is expected to possess high catalytic activity towards hydrolysis of NaBH4. For the B1s level in Fig. 2(c), the B1s signals of CoeB and CoeB10CNTs samples appear around 192.1 eV and 191.9 eV, respectively, which can be ascribed to boron oxide species [43]. The N2 adsorption/desorption isotherms of representative samples CoeB and CoeB-10CNTs are shown in Fig. 3 and the corresponding specific surface area, pore volume and average pore diameter are summarized in Table 1. Both samples exhibit IV-type isotherms with distinct hysteresis loops according to IUPAC classification, indicating mesoporous structure [44]. Meanwhile, it can be observed from BJH pore size distribution shown in the inset that the pore size distribution is centralized and the CoeB and CoeB-10CNTs catalysts possess ~3.5 nm of the most probable value. In addition, the specific surface area of CoeB-10CNTs catalyst (72.6 m2/g) significantly increases comparing with that of CoeB sample (32.8 m2/g) and the increasing amplitude reaches as high as ~122%, suggesting that the addition of 10 wt% CNTs is beneficial to the dispersion of active components, and higher

surface area of the CoeB-10CNTs catalyst can provide more active sites for the hydrolysis of NaBH4 [45]. Correspondingly, the CoeB-10CNTs sample exhibits larger pore volume and smaller average pore diameter comparing with those of CoeB catalyst. SEM measurements have been performed to investigate the morphology of pure CoeB and CNTs promoted CoeB

Fig. 3 e N2 adsorption/desorption isotherms of CoeB and CoeB-10CNTs samples and inset is the corresponding BJH pore size distribution plots.

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Table 1 e Textural parameters of CoeB and CoeB-10CNTs catalysts. Sample CoeB CoeB-10CNTs

Specific surface Pore volume Average Pore area (m2/g) (cm3/g) diameter (nm) 32.8 72.6

0.097 0.109

9.4 5.1

catalysts. SEM images of the typical CoeB and CoeB-10CNTs samples are displayed in Fig. 4. From Fig. 4(a), the agglomeration of CoeB particles can be clearly observed for the CoeB catalyst without CNTs. By contrast, the dispersion of CoeB particles for the CoeB-10CNTs sample is enhanced with the addition of 10 wt% promoter though CNTs itself has serious agglomeration and twining phenomenon, which means that the CoeB-10CNTs catalyst can expose more active surface for catalytic hydrolysis of NaBH4. It is expected that the CoeB10CNTs sample exhibits superior activity due to the important role of CNTs in the improvement of catalytic performance [46].

Catalytic activity of CNTs-promoted CoeB samples The effects of different CNTs contents on activity of CoeB catalysts have been investigated and the experimental results are illustrated in Fig. 5. Obviously, the CNTs promoter in itself has no catalytic activity towards the hydrolysis reaction of NaBH4. In contrast, pure CoeB and CNTs promoted CoeB catalysts exhibit significantly high catalytic activity, suggesting that efficient CoeB based catalysts for hydrogen generation via NaBH4 hydrolysis can be obtained by means of the simple preparation method in this work. It is necessary to point out that hydrogen releases immediately with the addition of the catalysts into the hydrolysis solution without an induction time in this work. According to Fig. 5(a), the cumulative hydrogen volumes almost maintain 1230 mL for pure CoeB and CNTs-promoted CoeB catalysts, while the H2 generation rates are significantly different with various CNTs contents as shown in Fig. 5(b). The H2 generation rate is 4.54 L min1$g1 catalyst for the CoeB sample. With the addition of 5 wt% CNTs, the H2 generation rate of CoeB-5CNTs sample increases by 80.6% in comparing with that of CoeB catalyst. Enhanced activity can

be attributed to the increased specific surface area and more exposed active sites due to the addition of small amount of CNTs. Moreover, CNTs itself have the capability of physically adsorbing H molecule [1], which can in principle stabilize the H atoms existed in an intermediate during the hydrolysis reaction. The H2 generation rate further improves obviously and achieves as high as 12.00 L min1$g1 catalyst over the CoeB10CNTs catalyst when CNTs content increases to 10 wt%. In combination with its large surface area, excellent adsorption ability and increase of electron density at active sites, appropriate amount of CNTs in CoeB catalyst can promote hydrogen spillover and regenerate Co active sites, which can be responsible for its high activity. However, considering the fact that CNTs itself has no activity for NaBH4 hydrolysis, the addition of excessive CNTs in the catalyst means the decrease of active components, which leads to the decrease of catalytic activity. As a result, the H2 generation rate decreases significantly to 7.08 L min1$g1 catalyst for the CoeB-20CNTs sample when CNTs content further increases to 20 wt%. As shown in Fig. 5(b), the H2 generation rate follows a trend of increase first and then decrease with the increase of CNTs contents from 0 to 20 wt%. The CoeB catalyst promoted by 10 wt% CNTs preserves the highest activity with a H2 generation rate of 12.00 L min1$g1 catalyst among the experimental samples. Recently reported CoeB based catalysts with high activity are summarized in Table 2. Compared with the catalysts listed in Table 2, the CoeB-10CNTs sample in this work exhibits obviously or slightly high H2 generation rate, while the H2 generation rate is only lower than those of FeeCoeB/Ni foam [47] and CoeNieB/Cu sheet (22.00 L min1$g1 catalyst) (14.78 L min1$g1 catalyst) [21]. In order to further understand the origin of superior catalytic activity, it is important to consider the mechanism of NaBH4 hydrolysis catalyzed by the CoeB-10CNTs sample. Currently, the dissociative chemisorption of BH 4 ions on the catalyst surface is generally accepted as the first kinetic step [18,19,41,48]. According to the mechanism proposed by Holbrook and Twist [49], hydrogen is generated from both NaBH4 and H2O. In view of CoeB-10CNTs catalyst in this work, BH 4 ions are firstly chemisorbed on electron-enriched Co active sites of CoeB-10CNTs and further dissociated to form CoeBH-3 and CoeH intermediates. CoeBH-3 subsequently reacts with

Fig. 4 e SEM images of CoeB and CoeB-10CNTs catalysts.

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Fig. 5 e Effects of CNTs contents on activity (a) and H2 generation rate of the CoeB catalysts (b) (reaction conditions: 10 mL solution containing 5 wt% NaBH4 and 5 wt% NaOH, 0.1 g catalyst and reaction temperature of 25  C). H2O, possibly via a BH3 intermediate, to generate BH3(OH)- and CoeH species. Then, BH3(OH)- species undergo stepwise replacement of BeH bonds by BeOH- bonds and finally yield CoeB(OH)-4. Meanwhile, CoeH species combine another CoeH to afford H2 and the active sites are regenerated. The presence of CNTs promoter in CoeB catalyst results in the increase of specific surface area, and more active sites are exposed, which can enhance the adsorption of BH 4 and increase the reaction rate. Meanwhile, the adsorption of H2 is also promoted with increasing reaction rate. During the hydrolysis reaction of NaBH4, H2 agglomeration and coverage of the active sites could hinder the formation of new active sites for catalytic cycle. Timely desorption of H2 and regeneration of active sites are considered to be crucial for catalytic cycle. The addition of CNTs promoter can also enhance desorption of H2 and regeneration of active sites through promoted hydrogen spillover in this work. Based on the mechanism of NaBH4 hydrolysis catalyzed by the CoeB-10CNTs sample, it can be considered that the promoting effect of CNTs in CoeB catalysts is mainly reflected in increased surface area, more active sites, enhanced desorption of H2 and regeneration of active sites due to the hydrogen spillover.

Kinetics of NaBH4 hydrolysis Considering the fact that reaction temperature is an important factor influencing the hydrolysis kinetics of NaBH4, the effect of reaction temperature on hydrolysis of NaBH4 has

been investigated at different temperatures from 298 to 313 K. The hydrolysis kinetics catalyzed by the CoeB-10CNTs sample at different temperatures are shown in Fig. 6. It is observed from Fig. 6(a) that the catalytic activity increases with the increase of reaction temperatures. Fig. 6(b) illustrates the Arrhenius plot of ln k versus 1/T for the CoeB-10CNTs catalyst. The activation energy of NaBH4 hydrolysis reaction can be obtained from the Arrhenius equation as followed. ln k ¼ ln A - (Ea/RT) Where k is the initial hydrogen generation rate (mol$min1$g1) at corresponding temperature, A is preexponential factor, R is molar gas constant (8.314 J K1 mol1), Ea is the activation energy (kJ$mol1), and T is the reaction temperature (K). According to the linear slope in Fig. 6(b), Ea for NaBH4 hydrolysis catalyzed by CoeB-10CNTs sample has been calculated to be 23.5 kJ mol1. Low activation energy in this work indicates that the CoeB-10CNTs catalyst preserves high reaction rate.

Reusability and stability The reusability of catalysts under reaction conditions is also an important issue, which has to be considered for practical applications. In the present work, the reusability of CoeB10CNTs catalyst with optimal activity has been investigated through five consecutive cycles for the hydrolysis reaction of

Table 2 e Comparison of the catalytic activity between CoeB-10CNTs and recently reported CoeB based catalysts. Catalyst CoeMoeB hollow NieCoeB CoeBeO CoB/o-CNTs CoeB/Cu sheet CoeSneB/GP CoeB/C film FeeCoeB/Ni foam CoeNieB/Cu sheet CoeB-10CNTs

Reaction medium (wt.%)

Temperature ( C)

H2 generation rate (L$min1$g1 catalyst)

1.8%NaBH4þ2%NaOH 1%NaBH4þ2%NaOH 5%NaBH4þ2%NaOH 1%NaBH4þ3.75%NaOH 1%NaBH4þ5%NaOH 5%NaBH4þ5%NaOH 0.1%NaBH4þNaOH 15%NaBH4þ5%NaOH 5%NaBH4þ1%NaOH

30 30 25 25 30 30 25 30 25

4.20 6.40 7.45 3.04 10.09 11.27 8.10 22.00 14.78

[24] [19] [50] [5] [20] [51] [52] [47] [21]

5%NaBH4þ5%NaOH

25

12.00

This work

Ref.

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Fig. 6 e Hydrogen generation kinetics from the hydrolysis of NaBH4 catalyzed by the CoeB-10CNTs sample (a) and the corresponding Arrhenius plot of ln k versus 1/T (b).

NaBH4 at 25  C and the results are displayed in Fig. 7. It can be observed from Fig. 7(a) that the total hydrogen volume almost maintains around 1230 mL for every cycling test. However, the H2 generation rate shown in Fig. 7(b) gradually decreases with constant cycling tests. The H2 generation rate of the CoeB10CNTs catalyst reaches as high as 11.80 L min1$g1 catalyst for the first test, while decreases to 10.29 L min1$g1 catalyst after 2 cycles, and the catalyst loses 12.8% of its initial activity. It is believed that the catalyst is more stable than the reported Co/ PCM catalyst and Co/Ni catalyst after 2 cycles which lose about 32.0% [53] and most of the initial activity [54], respectively. After 5 catalytic cycles, the experimental sample still shows superior activity with a high H2 generation rate of 7.55 L min1$g1 catalyst, exceeding those of most of Co-based catalysts shown in Table 2, and can maintain 64.0% of its initial activity. In contrast, after 5 cycles, the recently reported NieCoeB hollow nanospheres catalyst retains 88.3% of the initial activity [19], CoeB/Cu sheet catalyst preserves 84.0% of its initial activity [20], and CoeNieB/Cu sheet sample retains 87.9% of the initial activity [21]. However, the activity of CoeWeP catalyst only reaches 49.0% after 5 cycles [55] and the CoeBeW/AgeTiO2 catalyst only maintains 50.0% of its initial activity [56]. Therefore, the CoeB-10CNTs catalyst in this work can be believed to possess superior activity for the NaBH4 hydrolysis and relatively good stability.

Analysis about constant loss in activity In order to further understand the reasons for constant loss in the activity of CoeB-10CNTs catalyst, XPS measurements have been carried out for the sample after 5 cycles. Fig. 8 displays XPS spectra of Co2p, B1s, C1s and O1s in the CoeB10CNTs samples before cycling and after 5 cycles, respectively. From XPS spectra of Co 2p3/2 shown in Fig. 8(a), two peaks at binding energy of 781.3 eV and 783.2 eV appear for the CoeB-10CNTs sample after 5 cycles, corresponding to Co0 and Co2þ species, respectively [39], which is close to those of sample before cycling. By comparison, the peak intensity significantly decreases after 5 cycles. Moreover, the content of metallic Co decreases from 58.0% to 51.3% according to the results calculated on basis of XPS spectra. For the B1s level in Fig. 8(b), the B1s signals of both samples appear around 191.9 eV and 192.6 eV, respectively, which can be ascribed to boron oxide species [43] and the peak intensity is weaken after 5 cycles. From Fig. 8(c), it is observed that both C1s spectra are deconvoluted into four contributions at binding energy of about 284.7, 285.2, 286.3 and 290.0 eV corresponding to CeC bond, -C-OH, eC]O and-COOH groups, respectively [57]. There is no obvious change in intensity of the four peaks before cycling and after 5 cycles. The XPS spectra of O1s shown in Fig. 8(d) have been deconvoluted into two peaks at

Fig. 7 e Reusability tests of the CoeB-10CNTs catalyst for the hydrolysis of NaBH4 at 25  C (a) and the histogram of H2 generation rate versus number of cycles (b).

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Fig. 8 e XPS spectra of (a) Co 2p, (b) B 1s, (c) C 1s and (d) O 1s level for the CoeB-10CNTs samples before cycling and after 5 cycles.

around 530.6 eV and 531.7 eV, which can be attributed to lattice oxygen and surface oxygen species, respectively [40]. Based on the above XPS results, the decrease in relative contents of Co, B elements and metallic Co species for the CoeB10CNTs catalysts before cycling and after 5 cycles probably result in the part loss of catalytic activity. The used CoeB-10CNTs sample was firstly separated, washed, dried and then weighted after each cycling test. The ratios of the sample weight following each cycle to the initial

weight (W/W1st) is plotted in Fig. 9. Obviously, the amounts of CoeB-10CNTs catalyst gradually decrease with each cycle. The 0.1000 g of catalyst is used for the first cycle, the mass of catalyst decreases significantly and the augment is as high as 33.5% after 5 cycles. Increasing amounts of catalysts has been demonstrated to have a positive effect on hydrogen generation rates [19,58]. It can be confirmed that the notable decrease in the weight of CoeB-10CNTs catalyst with each catalytic cycle probably accounts for its loss in activity to a great degree. In addition, the enrichment of byproducts on the catalyst surface can lower catalytic activity due to decrease of active sites [59]. However, the byproducts on the surface are easily carried away by hydrogen due to a rapid H2 generation rate in the present work [60], which scarcely influences its catalytic activity.

Conclusions

Fig. 9 e Curves of the weight change of CoeB-10CNTs catalyst vs number of cycles.

In the present work, carbon nanotubes-promoted CoeB catalysts with superior activity have been successfully synthesized by the simple chemical reduction. It is found that the addition of CNTs promoter with various contents has no obvious effect on the amorphous structure of CoeB catalysts, while significantly influences the catalytic performance. The presence of small amount of CNTs in the CoeB catalyst results in the increase of catalytic activity through improving surface area and stabilizing the H atoms. Nevertheless, the addition of excessive CNTs promoter leads to the decrease of active

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species in the catalyst, which lowers the catalytic activity. Among the as-prepared samples, 10 wt% CNTs promoted CoeB catalyst possesses optimal catalytic activity with remarkably high hydrogen generation rate of 12.00 L min1$g1 catalyst. Moreover, the sample has been proven to be a reusable catalyst and maintains 64.0% of its initial activity after 5 cycles in hydrolysis of NaBH4 at room temperature. With the existence of appropriate CNTs, large active area, fine dispersion of active components, high electron density at active sites and promoted hydrogen spillover are achieved, which can account for its superior performance.

Acknowledgements

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The authors gratefully acknowledge the National Natural Science Foundation of China (No. 21406174).

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