- Email: [email protected]
A new reactivation method towards deactivation of honeycomb ceramic monolith supported cobalt–molybdenum–boron catalyst in hydrolysis of sodium borohydride
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
A new reactivation method towards deactivation of honeycomb ceramic monolith supported cobaltemolybdenumeboron catalyst in hydrolysis of sodium borohydride Da-Wei Zhuang b, Hong-Bin Dai a,*, Yu-Jie Zhong b, Li-Xian Sun c, Ping Wang a,* a School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, PR China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China c Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guangxi Key Laboratory of Information Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China
article info
abstract
Article history:
Sodium borohydride (NaBH4) is considered as being a promising hydrogen carrier.
Received 12 April 2015
The monolith supported-catalyst is highly prized in the practical NaBH4-based hydrogen
Received in revised form
generation system due to simple design of hydrogen generator, controllability of reaction
27 May 2015
and easy separation from the spent solution. In the present study, honeycomb ceramic
Accepted 28 May 2015
monolith supported cobaltemolybdenumeboron catalyst was prepared using a modified
Available online 19 June 2015
electroless plating method. The resultant catalyst exhibits an impressive activity towards
Keywords:
solution with fast kinetics and 100% fuel conversion, but its catalytic activity gradually
Sodium borohydride
declines over cycles. To recover the initial reactivity of catalyst, low temperature calcina-
Hydrogen generation
tion treatment at 120 C in air was found to be an effective reactivation method towards the
Honeycomb ceramic monolith
deactivation of catalyst in hydrolysis of NaBH4.
Reactivation
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
hydrolysis of NaBH4 solution, which can release 4.2 wt% H2 in 20 wt% stabilized NaBH4
reserved.
Introduction The widespread use of hydrogen as a clean alternative to fossil fuels is severely hampered by the lack of safe and efficient means for hydrogen storage. Over the past decade,
considerable efforts have been directed towards the development of hydrogen-rich chemical hydrides as potential hydrogen source medium [1e3], including sodium borohydride (NaBH4), ammonia borane (NH3BH3), hydrous hydrazine (N2H4$H2O), formic acid (HCOOH), and so on. Among them, NaBH4 attracts the most extensive attention due to its
* Corresponding authors. Tel.: þ86 20 3938 0583. E-mail addresses: [email protected] (H.-B. Dai), [email protected] (P. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.05.177 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
9374
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
combined advantages of the stability in alkaline solutions, moderate operation temperature, non-flammability, nontoxicity, environmental safety and high theoretical hydrogen capacity. NaBH4 þ (2 þ x) H2O / NaBO2 xH2O þ 4H2[, △H ¼ 210 kJ mol1
(1)
NaBH4 stores atomic hydrogen (10.6 wt%) and can generate molecular hydrogen (10.8 wt%, x ¼ 0) by its hydrolysis following Eqn. (1), where x denotes the excess of water, and x is strongly dependent on reaction temperature [4]. In 2000, Amendola et al. [5,6] published two classic papers dealing with hydrogen generation (HG) from the stabilized NaBH4 solution. Since then, numerous efforts have been dedicated to developing NaBH4-based HG system, which resulted in significant progress in synthesis of highperformance catalyst [7e21], device design [22,23], and regeneration chemistry [24]. However, even with the aid of these encouraging advances, the NaBH4-based HG system is still limited in most practical applications [25]. Besides the low effective hydrogen capacity of the system and the prohibitively high hydrogen cost, this is primarily due to lack of a high activity and good durability monolith-supported catalyst. Catalyst study is a central issue in developing NaBH4based HG system. The Co-based alloys have been identified to be the most cost-effective catalysts, which can greatly accelerate the hydrolysis reaction and reach 100% fuel conversion at ambient conditions. Most of them are as in the form of the powder, however, their activity and durability need to be improved to meet the on-demand hydrogen generator. By comparison with the powdery catalysts, the monolith supported catalysts are highly appreciated in the practical applications owing to their easy separation from fuel solution, simple device design, and consequently ready controllability of the hydrolysis reaction. Several methods have been developed for preparation of monolith supported catalysts, e.g. dipping-coating [15], pulsed laser deposition [16,17], electroplating [18,19] and electroless plating [20,21]. Among these methods, electroless plating is clearly one of the most popular and effective methods, which is an important industrial technique for metalizing insulators and objects with geometries that are difficult to coat by other methods. It is expected that the catalyst activity can be further improved with advanced catalyst preparation method. However, very few papers studied the catalyst durability and this is a mistake from an application point of view [25]. A costeffective method is therefore needed to overcome deterioration of the activity, which will cause severe engineering problems. Herein, we report synthesis of honeycomb ceramic monolith (HCM) supported cobaltemolybdenumeboron (CoeMoeB) catalyst (denoted as CoeMoeB/HCM catalyst) using a modified electroless plating method. The developed CoeMoeB/HCM catalyst exhibits a high activity towards hydrolysis of NaBH4 solution, but it gradually deactivates over cycles. To regain the initial reactivity of catalyst, low temperature calcination treatment at 120 C in air was found to be an effective reactivation method towards the deactivation of catalyst in hydrolysis of NaBH4.
Experimental Chemicals NaBH4 (96% purity), sodium hydroxide (NaOH, 96%), cobalt chloride hexahydrate (CoCl2$6H2O, 99%) sodium molybdate dihydrated (Na2MoO4$2H2O, 99%) and ethylene glycol (EG, 99%) were purchased from Sinopharm. All reagents were used as received. Deionized water was used in preparation of all the solutions.
Preparation of the catalyst The HCM was selected as the catalyst support material for its high porosity, straight channel structure, and its low pressure-drop, high thermal and chemical stability under the hydrolysis conditions. In the present study, the selected HCM, which is composed of Al2O3$2MgO$5SiO2, has about 400 channels per square inch. Before usage, a piece of HCM of 25 mm in diameter and 10 mm in length was etched via acidic and alkaline treatments according to the procedure described by Galindo et al. [26]. The appropriate acidic/alkaline treatments induced a selective leaching of a portion of Mg, Al and Si oxides from the support surface, building a rough and porous surface, and promoting the stability of ceramic support in alkaline medium. The CoeMoeB nanoparticles were selected as the catalyst based on our previous results [11]. The CoeMoeB/HCM catalyst was prepared by a modified electroless plating method. The plating method involved activation and plating processes. In the activation process, the etched-pores HCM was immerged in 0.25 M CoCl2 solution for 3 h at room temperature, and the remaining slurry in the channels of the isolated HCM was removed with the pressurized air. Finally, the HCM was freeze-dried to obtain the support with an even distribution of CoCl2. In the plating process, the activated HCM was transferred in 20 mL of electroless plating bath at room temperature, which was composed of 1 M of NaBH4, 0.25 M of NaOH and 0e0.02 M of Na2MoO4 using 50 wt% EG and 50 wt% H2O mixture as solvent. It took about 15 min until the gas bubbling ceased, and then the isolated HCM was washed thoroughly with deionized water and ethanol to remove the residual Naþ, BH4 , BO2 , MoO4 2 and Cl ions. As-prepared catalyst sample was finally dried at 50 C for 12 h. The loading of catalyst was determined according to the weight change of HCM before and after plating.
Characterization and HG performance testing The catalyst samples were characterized by scanning electron microscopy (SEM, FEI Nova Nano SEM 430), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka X-ray source) and synchronous thermogravimetry/mass spectroscopy analyses (TG/MS, Netzsch 449C Jupiter/QMS 403C). In the XPS measurements, high-resolution scans of elemental lines were recorded at 50 eV pass energy of the analyzer. All the binding energies were calibrated using the C 1s peak (at 284.6 eV) of the adventitious carbon as an internal standard. The curve fitting was performed using XPS PEAK 4.1 software. Element
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
analyses of the catalyst samples were conducted in an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Intrepid). The HG performance testing was conducted in a 100 mL flask. Unless specified otherwise, the flask was placed in a thermostat that was equipped with a water circulating system to maintain the reaction temperature, typically within ±0.5 C. In a typical measurement run, the NaBH4 aqueous solution was preheated and held at the designated temperature, and then a piece of catalyst was dropped into the fuel solution to initiate the hydrolysis reaction. The generated hydrogen gas passed through a trap/heat exchanger to cool to room temperature followed by contacting with silica drier to remove water vapor. The HG rate was measured using an online mass flow meter (Seven Star Huachuang, MFM D07e7BM, with an accuracy of ±2%) that was equipped with a computer. The HG volume was calculated by integrating the measured HG rate over time.
Results and discussion Preparation and property of the CoeMoeB/HCM catalyst Electroless plating is an autocatalytic redox reaction process which provides a functional coating on all surface area of the support regardless of its configuration or geometry. A series of HCM-supported CoeMoeB catalysts were prepared using a modified electroless plating method, as schemed in Fig. 1. This method mainly involved three steps. Firstly, an etching step of the acidic and alkaline treatments was performed to make sure the adhesion between the coating and the substrate; Secondly, an activation step for the etched-pores HCM support was carried out using CoCl2 solution; Thirdly, the activated HCM contained Co2þ ions was placed into the electroless plating solution to prepare the CoeMoeB supported catalyst. Upon adding the treated HCM to the strongly reductive NaBH4 solution, the in-situ reduced Co and B in the etched-pores can induce the co-deposition of Mo [11]. The deposited Co atoms themselves acted as self-catalysts for further CoeMoeB deposition via this electroless plating method. The loading of a piece of HCM was measured to be about 140 mg (CoeMoeB) per plating time. The as-prepared Co-based alloy catalyst presents a high activity towards hydrolysis of NaBH4 solution, but the catalytic activity is closely related to the Mo content in the
9375
CoeMoeB catalyst. As shown in Fig. 2, the CoeMoeB/HCM catalyst with a Mo/Co molar ratio of 0.15 shows the maximum catalytic activity, which has an actual composition of Co63.4Mo6.2B30.4 according to the ICP-AES analysis. By using this catalyst, the 5 wt% NaBH4 þ 5 wt% NaOH solution can yield H2 at an HG rate of 18.8 L min1 g1 (CoeMoeB) at 30 C, corresponding to our previous developed CoeMoeB powdery catalyst [11]. This catalytic activity is quite promising as compared with the top level of catalysts reported up to date, e.g. CoeB/Ni foam (11 L min1 g1 at 30 C) [20], CoeWeB/Ni foam (15 L min1 g1 at 30 C) [21], Ru2Pt1/TiO2 (15.2 L min1 g1 at 20 C) [27], Ru60Co40 (17.5 L min1 g1 at 25 C) [28] catalysts, and so on. It is well-documented that the catalytic hydrolysis reaction of NaBH4 over the powdery catalyst follows zero or first-order kinetics, dependent on NaBH4 concentration and reaction temperature [29,30]. As compared with the powdery catalyst, the kinetic curve here shows a serious deviance from the linear relationship with the reaction time with proceeding hydrolysis reaction (Fig. 3). Fitting the experimental data indicates the catalytic hydrolysis reaction over the monolith supported-catalyst follows first-order kinetics with respect to the NaBH4 concentration. Fig. 4 gives the HG kinetics curves of alkaline NaBH4 solution using the CoeMoeB/HCM catalyst in the temperature ranging
Fig. 2 e Effect of Mo/Co molar ratio on HG rate of the system composed of 20 g of 5 wt% NaBH4 þ 5 wt% NaOH solution at 30 C using a piece of CoeMoeB/HCM catalyst.
Fig. 1 e Schematic diagram of preparation of the CoeMoeB/HCM catalyst.
9376
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 3 e Fitting the experimental data obtained from the system composed of 20 g of 5 wt% NaBH4 þ 5 wt% NaOH solution at 30 C using a piece of CoeMoeB/HCM catalyst.
from 30 to 50 C. According to the first-order rate constants at varied temperatures, the apparent activation energy of the hydrolysis reaction using this catalyst was found to be 35.7 kJ mol1. This value compares favorably with the literature results of Ru/IRA (47 kJ mol1) [5], CoeB (45 kJ mol1) [8], CoeWeB (41 kJ mol1) [8] and CoeB/Ni foam (45 kJ mol1) [15]. Low effective hydrogen capacity is one of the major problems that limit the practical application of NaBH4-based HG system, which should be mainly ascribed to the solubility limitation of by-product (NaBO2) [25]. With the highly exothermic hydrolysis reaction proceeding, the fuel solution temperature increases, resulting in a high solubility of NaBO2. No attempt was therefore made to control the reaction temperature in the following experiments. Fig. 5 gives the effect of NaBH4 concentration on the HG system. It was observed that the systems with a low NaBH4 concentration shows a fast HG kinetics and higher fuel conversion, and increasing the NaBH4 concentration results in a degradation of the HG performance, e.g. when the concentration of NaBH4 reaches 25 wt%, the HG system can fulfill only a 79% conversion. One possible reason for this is that the precipitated and accumulated borate will cover in the surface of catalyst, and block the channels of the HCM, resulting in a degradation of the HG performance. In consideration of the combined demands for high hydrogen density and fast kinetics in the practical application, we selected the 20 wt% NaBH4 solution for further investigation, which possesses a materialbased hydrogen capacity of 4.2 wt%.
A reactivation method towards the deactivation of CoeMoeB/HCM catalyst Although the newly developed CoeMoeB/HCM catalyst exhibits an impressing catalytic activity, its activity gradually declines over cycles. As shown in Fig. 6, the as-prepared catalyst can initiate the hydrolysis reaction immediately upon contracting with NaBH4 solution with no appreciable lag time. The maximum initial HG rate can reach 7.2 L min1 per piece at the fuel solution temperature up to 85 C,
Fig. 4 e HG kinetics curves of the hydrolysis reactions of NaBH4 at varied temperatures using a piece of CoeMoeB/ HCM catalyst (top). The system is composed of 20 g of 5 wt % NaBH4 þ 5 wt% NaOH solution. The Arrhenius plots of the temperature-dependent first-order rate constant (bottom), which determined the apparent activation energy of the hydrolysis reactions of NaBH4.
Fig. 5 e Effect of NaBH4 concentration on the HG performance of system composed of 20 g of x wt% NaBH4 þ 5 wt% NaOH (x ¼ 5, 10, 15, 20 and 25) solution using one piece of CoeMoeB/HCM catalyst.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 6 e HG kinetics of the CoeMoeB/HMC catalyst over 3 cycles, the dipped NaBO2, post-calcined and acid-washed catalysts. The system was composed of 20 g of 20 wt% NaBH4 þ 5 wt% NaOH solution.
corresponding to 51.4 L min1 g1 (CoeMoeB). By contrast, the used catalyst even in the second cycle shows a distinct lag time of about 4.5 min, and only reaches its first maximum rate of about 54%, indicating a significant deterioration of the catalyst activity at the initial stage despite reaching 100% fuel conversion. As demonstrated [31], surface oxidation exerts quite limited influence on the catalytic activity. For example, the air-exposed catalyst even for 6 months showed no lag time, indicating that surface oxidation is not the reason for reduced activity of catalyst over the cycles. The reason for this is that the Co and Mo oxides can be readily reduced back to the corresponding metallic states upon contacting with the reductive NaBH4 solution. In addition, the initial reactivity was also not restored over the first used catalyst after several thorough washing operations with deionized water and ethanol. ICP analysis of the supernatant after each cycle and washing demonstrated that neither Co nor Mo was detected, indicating no leaching of active components over the course of reaction or washing. Based on the results reported by Akdim et al. in their study of the Co-based catalyzed NaBH4 system [26], the reactivity deterioration could be attributed to the precipitation/adsorption of borate species on the surface of catalyst. The deactivation also occurs when dipping an asprepared catalyst in a 5 wt% NaBO2 solution for 30 s. This may be seen as an evidence of adsorption of the boron-contained species on the catalyst surface.
9377
Catalyst activity is closely associated with the strength of chemical interaction between the active sites and the adsorbates including stable intermediates, relevant transition states and by-products, but the catalyst that binds the adsorbates too strongly will be poisoned [33e36]. Therefore, the highly activate Co-based catalysts in hydrolysis of NaBH4 will adsorb strongly the boron-contained species from the intermediates and by-products of NaBH4 hydrolysis, leading to block the active sites. To confirm this, we performed the analyses of SEM and XPS for the catalyst samples. As seen in Fig. 7, the used CoeMoeB/HCM catalyst was examined by the SEM and compared with the as-prepared catalyst. The asprepared CoeMoeB/HMC catalyst presents a rough surface with many small particles uniformly distributed on the support, whereas the used catalyst loses its initial roughness, implying the adsorption of some compounds on the surface of catalyst. Fig. 7d presents a representative EDX result of the used catalyst samples. It was observed that the presence of Co, Mo and O elements as well as the Si, Mg and Al elements from the HMC support, but the signals related to Na element are not detected for all the catalyst samples, indicating no Nacontained compound. This was further confirmed by XPS result, as shown in Fig. 8 (left). In contrast, Kim et al. [37] claimed that the deactivation was caused by a film of Na and B-contained species. It was observed that there are subtle differences for the state changes of Co, Mo and B elements in all the samples, but the changes of Co and Mo here should exert no significant influence on the catalyst deactivation [31]. Fig. 8 (right) gives the B 1s core level XPS spectra of the asprepared and used catalysts. The binding energy (BE) of 193.6, 192.1 and 189.8 eV could be assigned to B(OH) 4 or B(OH)3, B2O3 and B (0), respectively [38]. Careful comparison of the XPS results between two catalysts found the presence of B(OH) 4 or B(OH)3 on the surface of used catalyst, which should origin from the intermediates and/or by-products in terms of the classic catalytic hydrolysis mechanism of NaBH4 [39]. The deactivation could be therefore attributed to the adsorbed Bcontained species, certainly the borates denoted as BaOb(OH)g hereinafter [32,40e43], which consists of a 2D surface network via the CoeOeB and the BeOeB bonds. It is to say that the BaOb(OH)g or BxOy$nH2O [44] compounds block BHe 4 ions and H2O molecules from contacting with the active sites of the catalyst, resulting in the deterioration of activity. The deactivation seems to be inevitable due to the strong adsorption of BxOy$nH2O compounds on the Co-based catalyst via the CoeOeB bond. To regain the initial reactivity of the catalyst, it was proposed to wash it with a diluted acid solution, which can be done after each hydrolysis [32,40e42]. Unfortunately, the initial activity of our developed catalyst was not recovered over the acidic washing method (Fig. 6). This could be caused by alloying of Co and Mo, leading to increase the strength of the CoeOeB bond. In our effort to solve this problem, we fortuitously found that appropriate calcination treatment for the used catalyst provides a simple but effective method for recovering the initial activity. For example, the initial HG rate was almost regained over the calcination of the used catalyst at 120 C for 2 h in air (Fig. 6). Some irregular micro-pores and cracks were observed on the surface of post-calcined catalyst (Fig. 7c), while the BE of 193.6 eV of B(OH)e 4 or B(OH)3 disappeared, the BE of 192.1 eV of
9378
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 7 e SEM morphologies of the as-prepared (a), used (b), post-calcined (c) CoeMoeB/HCM catalysts and a representative EDX result of the used CoeMoeB/HCM catalysts (d).
Fig. 8 e Na 1s and B 1s XPS spectra of the as-prepared (a), the used (b) and post-calcined (c) CoeMoeB/HCM catalysts.
B2O3 was only observed (Fig. 8). This suggests that the dehydration of the BxOy$nH2O compounds occurs during the low temperature calcination treatment. This was directly evidenced by the synchronous TG/MS result of the used catalyst, as presented in Fig. 9. A H2O-desorption peak was observed at about 110 C with a negligible mass loss of 0.3e0.5 wt% up to 400 C under Ar atmosphere. The micro-pores and cracks may be caused by the shrinkage and breakage of the adsorption layer during the course of the dehydration. The produced micro-pores and cracks can provide good accessibility for the BHe 4 and H2O to the active sites of the catalyst. Once restarting the exothermic hydrolysis reaction, the bubbling of H2 with a concomitant reaction heat can effectively repel the remained adsorption layer/viscous intermediates or by-products off the catalyst surface and meanwhile, promote the mass transfer of reactants to the active sites. The post-calcined CoeMoeB/HCM catalyst is expected to contribute to improve its catalytic activity, the durability of the reactivated catalyst was tested and compared with unactivated catalyst over 10 cycles. As shown in Fig. 10, the reactivated catalyst exhibits a satisfactory reusability. It can initiate the hydrolysis reaction immediately upon contracting with NaBH4 solution with no
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
9379
35%. It was observed that the continuous HG NaBH4-based system can enlarge the catalysts durability because of the continuous borates elimination at elevated temperatures [45]. However, the deactivation of reactivated catalyst recurs at the time of a new utilization in hydrolysis. Further investigation is still needed in order to minimize the adsorption of borates on the catalyst surface.
Conclusions By using a modified electroless plating method, a CoeMoeB supported on HCM catalyst was prepared for catalyzing HG from alkaline NaBH4 solution. The newly-developed catalyst shows an impressive activity, which can release 4.2 wt% H2 in 20 wt% stabilized NaBH4 solution with fast kinetics and 100% fuel conversion. However, also as most of the Co-based catalysts, the resultant catalyst deactivates over cycles due to adsorption/precipitation of borate species on the catalyst surface. To regain the initial reactivity of catalyst, low temperature calcination treatment at 120 C in air was found to be an effective reactivation method towards the deactivation of catalyst in hydrolysis of NaBH4. Our work revealed the importance of calcination treatment in making the catalyst durable and would have great implication for the development of advanced catalysts in NaBH4-base HG system. Fig. 9 e TG/MS result of the used CoeMoeB/HCM catalyst dried at 50 C for 12 h.
Acknowledgments
appreciable lag time, and can steadily retain its initial activity of 88% even at its 10 time usage. In contrast, the unactivated catalyst at its 10 time cycle shows a distinct lag time of about 10 min, and can only reach its initial activity of
The financial supports for this research from the National Outstanding Youth Science Foundation of China (Grant No. 51125003), National Natural Science Foundation of China (Grant No. 51471168) and 985 Project of South China University of Technology are gratefully acknowledged.
Fig. 10 e Cyclic performance of the reactivated and unactivated CoeMoeB/HCM catalysts in catalyzing the hydrolysis reaction of NaBH4 employing 20 g of 20 wt% NaBH4 þ 5 wt% NaOH solution.
9380
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
references
[1] Yadav M, Xu Q. Liquid-phase chemical hydrogen storage materials. Energy Environ Sci 2012;5:9698e725. [2] Liu BH, Li ZP. A review: hydrogen generation from borohydride hydrolysis reaction. J Power Sources 2009;187:527e34. [3] Demirci UB. The hydrogen cycle with the hydrolysis of sodium borohydride: a statistical approach for highlighting the scientific/technical issues to prioritize in the field. Int J Hydrogen Energy 2015;40:2673e91. [4] Marrero-Alfonso EY, Gray JR, Davis TA, Matthews MA. Minimizing water utilization in hydrolysis of sodium borohydride: the role of sodium metaborate hydrates. Int J Hydrogen Energy 2007;32:4723e30. [5] Amendola SC, Sharp-Goldman SL, Janjua MS, Kelly MT, Petillo PJ, Binder M. An ultrasafe hydrogen generator: aqueous, alkaline borohydride solutions and Ru catalyst. J Power Sources 2000;85:186e9. [6] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969e75. [7] Liu HB, Li Q. A highly active CoeB catalyst for hydrogen generation from sodium borohydride hydrolysis. Int J Hydrogen Energy 2008;33:7385e91. [8] Patel N, Fernandes R, Miotello A. Promoting effect of transition metal-doped CoeB alloy catalysts for hydrogen production by hydrolysis of alkaline NaBH4 solution. J Catal 2010;271:315e24. [9] Patel N, Fernandes R, Edla R, Lihitkar PB, Kothari DC, Miotello A. Superior hydrogen production rate by catalytic hydrolysis of ammonia borane using CoeB nanoparticles supported over mesoporous silica particles. Catal Commun 2012;23:39e42. [10] Li ZP, Ma SL, Li GR, Liu BH. Hydrogen generation from borohydride hydrolysis on surface-alloyed Ni foam. J Power Sources 2013;242:621e6. [11] Zhuang DW, Kang Q, Muir SS, Yao XD, Dai HB, Ma GL, et al. Evaluation of a cobaltemolybdenumeboron catalyst for hydrogen generation of alkaline sodium borohydride solutionealuminum powder system. J Power Sources 2013;224:304e11. € [12] Baydaroglu F, Ozdemir E, Hasimoglu A. An effective synthesis route for improving the catalytic activity of carbon-supported CoeB catalystfor hydrogen generation through hydrolysis of NaBH4. Int J Hydrogen Energy 2014;39:1516e22. [13] Sankir M, Semiz L, Serin RB, Sankir ND. Hydrogen generation from nanoflower platinum films. Int J Hydrogen Energy 2015;40:8522e9. [14] Pfeil TL, Pourpoint TL, Groven LJ. Effects of crystallinity and morphology of solution combustion synthesized Co3O4 as a catalyst precursor in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2014;39:2149e59. [15] Lee J, Kong KY, Jung CR, Cho E, Yoon SP, Han J, et al. A structured CoeB catalyst for hydrogen extraction from NaBH4 solution. Catal Today 2007;120:305e10. [16] Patel N, Guella G, Kale A, Miotello A, Patton B, Zanchetta C, et al. Thin films of CoeB prepared by pulsed laser deposition as efficient catalysts in hydrogen producing reactions. Appl Catal A Gen 2007;323:18e24. [17] Patel N, Miotello A, Bello V. Pulsed laser deposition of Co-nanoparticles embedded on B-thin film: a very efficient catalyst produced in a single-step process. Appl Catal B-Environ 2011;103:31e8.
[18] Krishnan P, Advani SG, Prasad AK. Thin-film CoB catalyst templates for the hydrolysis of NaBH4 solution for hydrogen generation. Appl Catal B-Environ 2009;86:137e44. [19] Li H, Liao JY, Zhang XB, Liao WW, Wen LL, Yang JB, et al. Controlled synthesis of nanostructured Co film catalysts with high performance for hydrogen generation from sodium borohydride solution. J Power Sources 2013;239:277e83. [20] Dai HB, Liang Y, Wang P, Cheng HM. Amorphous cobaltboron/nickel foam as an effective catalyst for hydrogen generation from alkaline sodium borohydride solution. J Power Sources 2008;177:17e23. [21] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, et al. Highperformance cobalt-tungsten-boron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33:4405e12. [22] Zhang Q, Smith G, Wu Y, Mohring R. Catalytic hydrolysis of sodium borohydride in an auto-thermal fixed-bed reactor. Int J Hydrogen Energy 2006;31:961e5. [23] Kim T, Kwon S. Design and development of a fuel cellpowered small unmanned aircraft. Int J Hydrogen Energy 2012;37:615e22. [24] Li ZP, Liu BH, Zhu JK, Morigasaki N. NaBH4 formation mechanism by reaction of sodium borate with Mg and H2. J Alloys Compd 2007;437:311e6. [25] Demirci UB, Akdim O, Miele P. Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage. Int J Hydrogen Energy 2009;34:2638e45. [26] Galindo H, Carvajal Y, Suib SL. Sulfonation of the surface of cordierite monoliths through a novel multi-step wet chemical process. Microporous Mesoporous Mater 2010;135:37e44. [27] Demirci UB, Garin F. Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate. J Alloys Compd 2008;463:107e11. [28] Park JH, Shakkthivel P, Kim HJ, Han MK, Jang JH, Kim YR, et al. Investigation of metal alloy catalyst for hydrogen release from sodium borohydride for polymer electrolyte membrane fuel cell application. Int J Hydrogen Energy 2008;33:1845e52. [29] Dai HB, Liang Y, Ma LP, Wang P. New insights into catalytic hydrolysis kinetics of sodium borohydride from MichaelisMenten model. J Phys Chem C 2008;112:15886e92. [30] Zhang JS, Delgass WN, Fisher TS, Gore JP. Kinetics of Ru-catalyzed sodium borohydride hydrolysis. J Power Sources 2007;164:772e81. [31] Dai HB, Liang Y, Wang P. Effect of trapped hydrogen on the induction period of cobaltetungsteneboron/nickel foam catalyst in catalytic hydrolysis reaction of sodium borohydride. Catal Today 2011;170:27e32. [32] Akdim O, Demirci UB, Miele P. Deactivation and reactivation of cobalt in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2011;36:13669e75. [33] Nørskov JK, Bligaard T, Rossmeisl J, Christensen CH. Towards the computational design of solid catalysts. Nat Chem 2009;1:37e46. [34] Xin HL, Holewinski A, Schweitzer N, Nikolla E, Linic S. Electronic structure engineering in heterogeneous catalysis: identifying novel alloy catalysts based on rapid screening for materials with desired electronic properties. Top Catal 2012;55:376e90. [35] Holewinski A, Xin HL, Nikolla E, Linic S. Identifying optimal active sites for heterogeneous catalysis by metal alloys based on molecular descriptors and electronic structure engineering. Curr Opin Chem Eng 2013;2:312e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
[36] Vojvodic A, Nørskov JK, Abild-Pedersen F. Electronic structure effects in transition metal surface chemistry. Top Catal 2014;57:25e32. [37] Kim JH, Kim KT, Kang YM, Kim HS, Song MS, Lee YJ, et al. Study on degradation of filamentary Ni catalyst on hydrolysis of sodium borohydride. J Alloys Compd 2004;379:222e7. [38] Wagner CD, Riggs WM, Davis LE, Moulder JF, Mulenberg GE. Handbook of X-ray photoelectron spectroscopy. 1st ed. Perkin Elmer Corporation; 1979. [39] Holbrook KA, Pwist PJ. Hydrolysis of the borohydride ion catalysed by metaleboron alloys. J Chem Soc A 1971:890e4. [40] Akdim O, Demirci UB, Miele P. Highly efficient acid-treated cobalt catalyst for hydrogen generation from NaBH4 hydrolysis. Int J Hydrogen Energy 2009;34:4780e7. [41] Akdim O, Chamoun R, Demirci UB, Zaatar Y, Khoury A, Miele P. Anchored cobalt film as stable supported catalyst for hydrolysis of sodium borohydride for
[42]
[43] [44]
[45]
9381
chemical hydrogen storage. Int J Hydrogen Energy 2011;36:14527e33. Demirci UB, Miele P. Reaction mechanisms of the hydrolysis of sodium borohydride: a discussion focusing on cobalt-based catalysts. C R Chimie 2014;17:707e16. Demirci UB, Miele P. Cobalt-based catalysts for the hydrolysis of NaBH4 and NH3BH3. Phys Chem Chem Phys 2014;16:6872e85. Ozerova AM, Simagina VI, Komova OV, Netskina OV, Odegova GV, Bulavchenko OA, et al. Cobalt borate catalysts for hydrogen production via hydrolysis of sodium borohydride. J Alloys Compd 2012;513:266e72. nez De Haro MC, Ferna ndez A, Arzac GM, Hufschmidt D, Jime nez MA, et al. Deactivation, reactivation Sarmiento B, Jime and memory effect on CoeB catalyst for sodium borohydride hydrolysis operating in high conversion conditions. Int J Hydrogen Energy 2012;37:14373e81.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
A new reactivation method towards deactivation of honeycomb ceramic monolith supported cobaltemolybdenumeboron catalyst in hydrolysis of sodium borohydride Da-Wei Zhuang b, Hong-Bin Dai a,*, Yu-Jie Zhong b, Li-Xian Sun c, Ping Wang a,* a School of Materials Science and Engineering, Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, PR China b Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China c Guangxi Collaborative Innovation Center of Structure and Property for New Energy and Materials, Guangxi Key Laboratory of Information Materials, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China
article info
abstract
Article history:
Sodium borohydride (NaBH4) is considered as being a promising hydrogen carrier.
Received 12 April 2015
The monolith supported-catalyst is highly prized in the practical NaBH4-based hydrogen
Received in revised form
generation system due to simple design of hydrogen generator, controllability of reaction
27 May 2015
and easy separation from the spent solution. In the present study, honeycomb ceramic
Accepted 28 May 2015
monolith supported cobaltemolybdenumeboron catalyst was prepared using a modified
Available online 19 June 2015
electroless plating method. The resultant catalyst exhibits an impressive activity towards
Keywords:
solution with fast kinetics and 100% fuel conversion, but its catalytic activity gradually
Sodium borohydride
declines over cycles. To recover the initial reactivity of catalyst, low temperature calcina-
Hydrogen generation
tion treatment at 120 C in air was found to be an effective reactivation method towards the
Honeycomb ceramic monolith
deactivation of catalyst in hydrolysis of NaBH4.
Reactivation
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
hydrolysis of NaBH4 solution, which can release 4.2 wt% H2 in 20 wt% stabilized NaBH4
reserved.
Introduction The widespread use of hydrogen as a clean alternative to fossil fuels is severely hampered by the lack of safe and efficient means for hydrogen storage. Over the past decade,
considerable efforts have been directed towards the development of hydrogen-rich chemical hydrides as potential hydrogen source medium [1e3], including sodium borohydride (NaBH4), ammonia borane (NH3BH3), hydrous hydrazine (N2H4$H2O), formic acid (HCOOH), and so on. Among them, NaBH4 attracts the most extensive attention due to its
* Corresponding authors. Tel.: þ86 20 3938 0583. E-mail addresses: [email protected] (H.-B. Dai), [email protected] (P. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.05.177 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
9374
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
combined advantages of the stability in alkaline solutions, moderate operation temperature, non-flammability, nontoxicity, environmental safety and high theoretical hydrogen capacity. NaBH4 þ (2 þ x) H2O / NaBO2 xH2O þ 4H2[, △H ¼ 210 kJ mol1
(1)
NaBH4 stores atomic hydrogen (10.6 wt%) and can generate molecular hydrogen (10.8 wt%, x ¼ 0) by its hydrolysis following Eqn. (1), where x denotes the excess of water, and x is strongly dependent on reaction temperature [4]. In 2000, Amendola et al. [5,6] published two classic papers dealing with hydrogen generation (HG) from the stabilized NaBH4 solution. Since then, numerous efforts have been dedicated to developing NaBH4-based HG system, which resulted in significant progress in synthesis of highperformance catalyst [7e21], device design [22,23], and regeneration chemistry [24]. However, even with the aid of these encouraging advances, the NaBH4-based HG system is still limited in most practical applications [25]. Besides the low effective hydrogen capacity of the system and the prohibitively high hydrogen cost, this is primarily due to lack of a high activity and good durability monolith-supported catalyst. Catalyst study is a central issue in developing NaBH4based HG system. The Co-based alloys have been identified to be the most cost-effective catalysts, which can greatly accelerate the hydrolysis reaction and reach 100% fuel conversion at ambient conditions. Most of them are as in the form of the powder, however, their activity and durability need to be improved to meet the on-demand hydrogen generator. By comparison with the powdery catalysts, the monolith supported catalysts are highly appreciated in the practical applications owing to their easy separation from fuel solution, simple device design, and consequently ready controllability of the hydrolysis reaction. Several methods have been developed for preparation of monolith supported catalysts, e.g. dipping-coating [15], pulsed laser deposition [16,17], electroplating [18,19] and electroless plating [20,21]. Among these methods, electroless plating is clearly one of the most popular and effective methods, which is an important industrial technique for metalizing insulators and objects with geometries that are difficult to coat by other methods. It is expected that the catalyst activity can be further improved with advanced catalyst preparation method. However, very few papers studied the catalyst durability and this is a mistake from an application point of view [25]. A costeffective method is therefore needed to overcome deterioration of the activity, which will cause severe engineering problems. Herein, we report synthesis of honeycomb ceramic monolith (HCM) supported cobaltemolybdenumeboron (CoeMoeB) catalyst (denoted as CoeMoeB/HCM catalyst) using a modified electroless plating method. The developed CoeMoeB/HCM catalyst exhibits a high activity towards hydrolysis of NaBH4 solution, but it gradually deactivates over cycles. To regain the initial reactivity of catalyst, low temperature calcination treatment at 120 C in air was found to be an effective reactivation method towards the deactivation of catalyst in hydrolysis of NaBH4.
Experimental Chemicals NaBH4 (96% purity), sodium hydroxide (NaOH, 96%), cobalt chloride hexahydrate (CoCl2$6H2O, 99%) sodium molybdate dihydrated (Na2MoO4$2H2O, 99%) and ethylene glycol (EG, 99%) were purchased from Sinopharm. All reagents were used as received. Deionized water was used in preparation of all the solutions.
Preparation of the catalyst The HCM was selected as the catalyst support material for its high porosity, straight channel structure, and its low pressure-drop, high thermal and chemical stability under the hydrolysis conditions. In the present study, the selected HCM, which is composed of Al2O3$2MgO$5SiO2, has about 400 channels per square inch. Before usage, a piece of HCM of 25 mm in diameter and 10 mm in length was etched via acidic and alkaline treatments according to the procedure described by Galindo et al. [26]. The appropriate acidic/alkaline treatments induced a selective leaching of a portion of Mg, Al and Si oxides from the support surface, building a rough and porous surface, and promoting the stability of ceramic support in alkaline medium. The CoeMoeB nanoparticles were selected as the catalyst based on our previous results [11]. The CoeMoeB/HCM catalyst was prepared by a modified electroless plating method. The plating method involved activation and plating processes. In the activation process, the etched-pores HCM was immerged in 0.25 M CoCl2 solution for 3 h at room temperature, and the remaining slurry in the channels of the isolated HCM was removed with the pressurized air. Finally, the HCM was freeze-dried to obtain the support with an even distribution of CoCl2. In the plating process, the activated HCM was transferred in 20 mL of electroless plating bath at room temperature, which was composed of 1 M of NaBH4, 0.25 M of NaOH and 0e0.02 M of Na2MoO4 using 50 wt% EG and 50 wt% H2O mixture as solvent. It took about 15 min until the gas bubbling ceased, and then the isolated HCM was washed thoroughly with deionized water and ethanol to remove the residual Naþ, BH4 , BO2 , MoO4 2 and Cl ions. As-prepared catalyst sample was finally dried at 50 C for 12 h. The loading of catalyst was determined according to the weight change of HCM before and after plating.
Characterization and HG performance testing The catalyst samples were characterized by scanning electron microscopy (SEM, FEI Nova Nano SEM 430), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka X-ray source) and synchronous thermogravimetry/mass spectroscopy analyses (TG/MS, Netzsch 449C Jupiter/QMS 403C). In the XPS measurements, high-resolution scans of elemental lines were recorded at 50 eV pass energy of the analyzer. All the binding energies were calibrated using the C 1s peak (at 284.6 eV) of the adventitious carbon as an internal standard. The curve fitting was performed using XPS PEAK 4.1 software. Element
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
analyses of the catalyst samples were conducted in an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Intrepid). The HG performance testing was conducted in a 100 mL flask. Unless specified otherwise, the flask was placed in a thermostat that was equipped with a water circulating system to maintain the reaction temperature, typically within ±0.5 C. In a typical measurement run, the NaBH4 aqueous solution was preheated and held at the designated temperature, and then a piece of catalyst was dropped into the fuel solution to initiate the hydrolysis reaction. The generated hydrogen gas passed through a trap/heat exchanger to cool to room temperature followed by contacting with silica drier to remove water vapor. The HG rate was measured using an online mass flow meter (Seven Star Huachuang, MFM D07e7BM, with an accuracy of ±2%) that was equipped with a computer. The HG volume was calculated by integrating the measured HG rate over time.
Results and discussion Preparation and property of the CoeMoeB/HCM catalyst Electroless plating is an autocatalytic redox reaction process which provides a functional coating on all surface area of the support regardless of its configuration or geometry. A series of HCM-supported CoeMoeB catalysts were prepared using a modified electroless plating method, as schemed in Fig. 1. This method mainly involved three steps. Firstly, an etching step of the acidic and alkaline treatments was performed to make sure the adhesion between the coating and the substrate; Secondly, an activation step for the etched-pores HCM support was carried out using CoCl2 solution; Thirdly, the activated HCM contained Co2þ ions was placed into the electroless plating solution to prepare the CoeMoeB supported catalyst. Upon adding the treated HCM to the strongly reductive NaBH4 solution, the in-situ reduced Co and B in the etched-pores can induce the co-deposition of Mo [11]. The deposited Co atoms themselves acted as self-catalysts for further CoeMoeB deposition via this electroless plating method. The loading of a piece of HCM was measured to be about 140 mg (CoeMoeB) per plating time. The as-prepared Co-based alloy catalyst presents a high activity towards hydrolysis of NaBH4 solution, but the catalytic activity is closely related to the Mo content in the
9375
CoeMoeB catalyst. As shown in Fig. 2, the CoeMoeB/HCM catalyst with a Mo/Co molar ratio of 0.15 shows the maximum catalytic activity, which has an actual composition of Co63.4Mo6.2B30.4 according to the ICP-AES analysis. By using this catalyst, the 5 wt% NaBH4 þ 5 wt% NaOH solution can yield H2 at an HG rate of 18.8 L min1 g1 (CoeMoeB) at 30 C, corresponding to our previous developed CoeMoeB powdery catalyst [11]. This catalytic activity is quite promising as compared with the top level of catalysts reported up to date, e.g. CoeB/Ni foam (11 L min1 g1 at 30 C) [20], CoeWeB/Ni foam (15 L min1 g1 at 30 C) [21], Ru2Pt1/TiO2 (15.2 L min1 g1 at 20 C) [27], Ru60Co40 (17.5 L min1 g1 at 25 C) [28] catalysts, and so on. It is well-documented that the catalytic hydrolysis reaction of NaBH4 over the powdery catalyst follows zero or first-order kinetics, dependent on NaBH4 concentration and reaction temperature [29,30]. As compared with the powdery catalyst, the kinetic curve here shows a serious deviance from the linear relationship with the reaction time with proceeding hydrolysis reaction (Fig. 3). Fitting the experimental data indicates the catalytic hydrolysis reaction over the monolith supported-catalyst follows first-order kinetics with respect to the NaBH4 concentration. Fig. 4 gives the HG kinetics curves of alkaline NaBH4 solution using the CoeMoeB/HCM catalyst in the temperature ranging
Fig. 2 e Effect of Mo/Co molar ratio on HG rate of the system composed of 20 g of 5 wt% NaBH4 þ 5 wt% NaOH solution at 30 C using a piece of CoeMoeB/HCM catalyst.
Fig. 1 e Schematic diagram of preparation of the CoeMoeB/HCM catalyst.
9376
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 3 e Fitting the experimental data obtained from the system composed of 20 g of 5 wt% NaBH4 þ 5 wt% NaOH solution at 30 C using a piece of CoeMoeB/HCM catalyst.
from 30 to 50 C. According to the first-order rate constants at varied temperatures, the apparent activation energy of the hydrolysis reaction using this catalyst was found to be 35.7 kJ mol1. This value compares favorably with the literature results of Ru/IRA (47 kJ mol1) [5], CoeB (45 kJ mol1) [8], CoeWeB (41 kJ mol1) [8] and CoeB/Ni foam (45 kJ mol1) [15]. Low effective hydrogen capacity is one of the major problems that limit the practical application of NaBH4-based HG system, which should be mainly ascribed to the solubility limitation of by-product (NaBO2) [25]. With the highly exothermic hydrolysis reaction proceeding, the fuel solution temperature increases, resulting in a high solubility of NaBO2. No attempt was therefore made to control the reaction temperature in the following experiments. Fig. 5 gives the effect of NaBH4 concentration on the HG system. It was observed that the systems with a low NaBH4 concentration shows a fast HG kinetics and higher fuel conversion, and increasing the NaBH4 concentration results in a degradation of the HG performance, e.g. when the concentration of NaBH4 reaches 25 wt%, the HG system can fulfill only a 79% conversion. One possible reason for this is that the precipitated and accumulated borate will cover in the surface of catalyst, and block the channels of the HCM, resulting in a degradation of the HG performance. In consideration of the combined demands for high hydrogen density and fast kinetics in the practical application, we selected the 20 wt% NaBH4 solution for further investigation, which possesses a materialbased hydrogen capacity of 4.2 wt%.
A reactivation method towards the deactivation of CoeMoeB/HCM catalyst Although the newly developed CoeMoeB/HCM catalyst exhibits an impressing catalytic activity, its activity gradually declines over cycles. As shown in Fig. 6, the as-prepared catalyst can initiate the hydrolysis reaction immediately upon contracting with NaBH4 solution with no appreciable lag time. The maximum initial HG rate can reach 7.2 L min1 per piece at the fuel solution temperature up to 85 C,
Fig. 4 e HG kinetics curves of the hydrolysis reactions of NaBH4 at varied temperatures using a piece of CoeMoeB/ HCM catalyst (top). The system is composed of 20 g of 5 wt % NaBH4 þ 5 wt% NaOH solution. The Arrhenius plots of the temperature-dependent first-order rate constant (bottom), which determined the apparent activation energy of the hydrolysis reactions of NaBH4.
Fig. 5 e Effect of NaBH4 concentration on the HG performance of system composed of 20 g of x wt% NaBH4 þ 5 wt% NaOH (x ¼ 5, 10, 15, 20 and 25) solution using one piece of CoeMoeB/HCM catalyst.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 6 e HG kinetics of the CoeMoeB/HMC catalyst over 3 cycles, the dipped NaBO2, post-calcined and acid-washed catalysts. The system was composed of 20 g of 20 wt% NaBH4 þ 5 wt% NaOH solution.
corresponding to 51.4 L min1 g1 (CoeMoeB). By contrast, the used catalyst even in the second cycle shows a distinct lag time of about 4.5 min, and only reaches its first maximum rate of about 54%, indicating a significant deterioration of the catalyst activity at the initial stage despite reaching 100% fuel conversion. As demonstrated [31], surface oxidation exerts quite limited influence on the catalytic activity. For example, the air-exposed catalyst even for 6 months showed no lag time, indicating that surface oxidation is not the reason for reduced activity of catalyst over the cycles. The reason for this is that the Co and Mo oxides can be readily reduced back to the corresponding metallic states upon contacting with the reductive NaBH4 solution. In addition, the initial reactivity was also not restored over the first used catalyst after several thorough washing operations with deionized water and ethanol. ICP analysis of the supernatant after each cycle and washing demonstrated that neither Co nor Mo was detected, indicating no leaching of active components over the course of reaction or washing. Based on the results reported by Akdim et al. in their study of the Co-based catalyzed NaBH4 system [26], the reactivity deterioration could be attributed to the precipitation/adsorption of borate species on the surface of catalyst. The deactivation also occurs when dipping an asprepared catalyst in a 5 wt% NaBO2 solution for 30 s. This may be seen as an evidence of adsorption of the boron-contained species on the catalyst surface.
9377
Catalyst activity is closely associated with the strength of chemical interaction between the active sites and the adsorbates including stable intermediates, relevant transition states and by-products, but the catalyst that binds the adsorbates too strongly will be poisoned [33e36]. Therefore, the highly activate Co-based catalysts in hydrolysis of NaBH4 will adsorb strongly the boron-contained species from the intermediates and by-products of NaBH4 hydrolysis, leading to block the active sites. To confirm this, we performed the analyses of SEM and XPS for the catalyst samples. As seen in Fig. 7, the used CoeMoeB/HCM catalyst was examined by the SEM and compared with the as-prepared catalyst. The asprepared CoeMoeB/HMC catalyst presents a rough surface with many small particles uniformly distributed on the support, whereas the used catalyst loses its initial roughness, implying the adsorption of some compounds on the surface of catalyst. Fig. 7d presents a representative EDX result of the used catalyst samples. It was observed that the presence of Co, Mo and O elements as well as the Si, Mg and Al elements from the HMC support, but the signals related to Na element are not detected for all the catalyst samples, indicating no Nacontained compound. This was further confirmed by XPS result, as shown in Fig. 8 (left). In contrast, Kim et al. [37] claimed that the deactivation was caused by a film of Na and B-contained species. It was observed that there are subtle differences for the state changes of Co, Mo and B elements in all the samples, but the changes of Co and Mo here should exert no significant influence on the catalyst deactivation [31]. Fig. 8 (right) gives the B 1s core level XPS spectra of the asprepared and used catalysts. The binding energy (BE) of 193.6, 192.1 and 189.8 eV could be assigned to B(OH) 4 or B(OH)3, B2O3 and B (0), respectively [38]. Careful comparison of the XPS results between two catalysts found the presence of B(OH) 4 or B(OH)3 on the surface of used catalyst, which should origin from the intermediates and/or by-products in terms of the classic catalytic hydrolysis mechanism of NaBH4 [39]. The deactivation could be therefore attributed to the adsorbed Bcontained species, certainly the borates denoted as BaOb(OH)g hereinafter [32,40e43], which consists of a 2D surface network via the CoeOeB and the BeOeB bonds. It is to say that the BaOb(OH)g or BxOy$nH2O [44] compounds block BHe 4 ions and H2O molecules from contacting with the active sites of the catalyst, resulting in the deterioration of activity. The deactivation seems to be inevitable due to the strong adsorption of BxOy$nH2O compounds on the Co-based catalyst via the CoeOeB bond. To regain the initial reactivity of the catalyst, it was proposed to wash it with a diluted acid solution, which can be done after each hydrolysis [32,40e42]. Unfortunately, the initial activity of our developed catalyst was not recovered over the acidic washing method (Fig. 6). This could be caused by alloying of Co and Mo, leading to increase the strength of the CoeOeB bond. In our effort to solve this problem, we fortuitously found that appropriate calcination treatment for the used catalyst provides a simple but effective method for recovering the initial activity. For example, the initial HG rate was almost regained over the calcination of the used catalyst at 120 C for 2 h in air (Fig. 6). Some irregular micro-pores and cracks were observed on the surface of post-calcined catalyst (Fig. 7c), while the BE of 193.6 eV of B(OH)e 4 or B(OH)3 disappeared, the BE of 192.1 eV of
9378
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
Fig. 7 e SEM morphologies of the as-prepared (a), used (b), post-calcined (c) CoeMoeB/HCM catalysts and a representative EDX result of the used CoeMoeB/HCM catalysts (d).
Fig. 8 e Na 1s and B 1s XPS spectra of the as-prepared (a), the used (b) and post-calcined (c) CoeMoeB/HCM catalysts.
B2O3 was only observed (Fig. 8). This suggests that the dehydration of the BxOy$nH2O compounds occurs during the low temperature calcination treatment. This was directly evidenced by the synchronous TG/MS result of the used catalyst, as presented in Fig. 9. A H2O-desorption peak was observed at about 110 C with a negligible mass loss of 0.3e0.5 wt% up to 400 C under Ar atmosphere. The micro-pores and cracks may be caused by the shrinkage and breakage of the adsorption layer during the course of the dehydration. The produced micro-pores and cracks can provide good accessibility for the BHe 4 and H2O to the active sites of the catalyst. Once restarting the exothermic hydrolysis reaction, the bubbling of H2 with a concomitant reaction heat can effectively repel the remained adsorption layer/viscous intermediates or by-products off the catalyst surface and meanwhile, promote the mass transfer of reactants to the active sites. The post-calcined CoeMoeB/HCM catalyst is expected to contribute to improve its catalytic activity, the durability of the reactivated catalyst was tested and compared with unactivated catalyst over 10 cycles. As shown in Fig. 10, the reactivated catalyst exhibits a satisfactory reusability. It can initiate the hydrolysis reaction immediately upon contracting with NaBH4 solution with no
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
9379
35%. It was observed that the continuous HG NaBH4-based system can enlarge the catalysts durability because of the continuous borates elimination at elevated temperatures [45]. However, the deactivation of reactivated catalyst recurs at the time of a new utilization in hydrolysis. Further investigation is still needed in order to minimize the adsorption of borates on the catalyst surface.
Conclusions By using a modified electroless plating method, a CoeMoeB supported on HCM catalyst was prepared for catalyzing HG from alkaline NaBH4 solution. The newly-developed catalyst shows an impressive activity, which can release 4.2 wt% H2 in 20 wt% stabilized NaBH4 solution with fast kinetics and 100% fuel conversion. However, also as most of the Co-based catalysts, the resultant catalyst deactivates over cycles due to adsorption/precipitation of borate species on the catalyst surface. To regain the initial reactivity of catalyst, low temperature calcination treatment at 120 C in air was found to be an effective reactivation method towards the deactivation of catalyst in hydrolysis of NaBH4. Our work revealed the importance of calcination treatment in making the catalyst durable and would have great implication for the development of advanced catalysts in NaBH4-base HG system. Fig. 9 e TG/MS result of the used CoeMoeB/HCM catalyst dried at 50 C for 12 h.
Acknowledgments
appreciable lag time, and can steadily retain its initial activity of 88% even at its 10 time usage. In contrast, the unactivated catalyst at its 10 time cycle shows a distinct lag time of about 10 min, and can only reach its initial activity of
The financial supports for this research from the National Outstanding Youth Science Foundation of China (Grant No. 51125003), National Natural Science Foundation of China (Grant No. 51471168) and 985 Project of South China University of Technology are gratefully acknowledged.
Fig. 10 e Cyclic performance of the reactivated and unactivated CoeMoeB/HCM catalysts in catalyzing the hydrolysis reaction of NaBH4 employing 20 g of 20 wt% NaBH4 þ 5 wt% NaOH solution.
9380
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
references
[1] Yadav M, Xu Q. Liquid-phase chemical hydrogen storage materials. Energy Environ Sci 2012;5:9698e725. [2] Liu BH, Li ZP. A review: hydrogen generation from borohydride hydrolysis reaction. J Power Sources 2009;187:527e34. [3] Demirci UB. The hydrogen cycle with the hydrolysis of sodium borohydride: a statistical approach for highlighting the scientific/technical issues to prioritize in the field. Int J Hydrogen Energy 2015;40:2673e91. [4] Marrero-Alfonso EY, Gray JR, Davis TA, Matthews MA. Minimizing water utilization in hydrolysis of sodium borohydride: the role of sodium metaborate hydrates. Int J Hydrogen Energy 2007;32:4723e30. [5] Amendola SC, Sharp-Goldman SL, Janjua MS, Kelly MT, Petillo PJ, Binder M. An ultrasafe hydrogen generator: aqueous, alkaline borohydride solutions and Ru catalyst. J Power Sources 2000;85:186e9. [6] Amendola SC, Sharp-Goldman SL, Janjua MS, Spencer NC, Kelly MT, Petillo PJ, et al. A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst. Int J Hydrogen Energy 2000;25:969e75. [7] Liu HB, Li Q. A highly active CoeB catalyst for hydrogen generation from sodium borohydride hydrolysis. Int J Hydrogen Energy 2008;33:7385e91. [8] Patel N, Fernandes R, Miotello A. Promoting effect of transition metal-doped CoeB alloy catalysts for hydrogen production by hydrolysis of alkaline NaBH4 solution. J Catal 2010;271:315e24. [9] Patel N, Fernandes R, Edla R, Lihitkar PB, Kothari DC, Miotello A. Superior hydrogen production rate by catalytic hydrolysis of ammonia borane using CoeB nanoparticles supported over mesoporous silica particles. Catal Commun 2012;23:39e42. [10] Li ZP, Ma SL, Li GR, Liu BH. Hydrogen generation from borohydride hydrolysis on surface-alloyed Ni foam. J Power Sources 2013;242:621e6. [11] Zhuang DW, Kang Q, Muir SS, Yao XD, Dai HB, Ma GL, et al. Evaluation of a cobaltemolybdenumeboron catalyst for hydrogen generation of alkaline sodium borohydride solutionealuminum powder system. J Power Sources 2013;224:304e11. € [12] Baydaroglu F, Ozdemir E, Hasimoglu A. An effective synthesis route for improving the catalytic activity of carbon-supported CoeB catalystfor hydrogen generation through hydrolysis of NaBH4. Int J Hydrogen Energy 2014;39:1516e22. [13] Sankir M, Semiz L, Serin RB, Sankir ND. Hydrogen generation from nanoflower platinum films. Int J Hydrogen Energy 2015;40:8522e9. [14] Pfeil TL, Pourpoint TL, Groven LJ. Effects of crystallinity and morphology of solution combustion synthesized Co3O4 as a catalyst precursor in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2014;39:2149e59. [15] Lee J, Kong KY, Jung CR, Cho E, Yoon SP, Han J, et al. A structured CoeB catalyst for hydrogen extraction from NaBH4 solution. Catal Today 2007;120:305e10. [16] Patel N, Guella G, Kale A, Miotello A, Patton B, Zanchetta C, et al. Thin films of CoeB prepared by pulsed laser deposition as efficient catalysts in hydrogen producing reactions. Appl Catal A Gen 2007;323:18e24. [17] Patel N, Miotello A, Bello V. Pulsed laser deposition of Co-nanoparticles embedded on B-thin film: a very efficient catalyst produced in a single-step process. Appl Catal B-Environ 2011;103:31e8.
[18] Krishnan P, Advani SG, Prasad AK. Thin-film CoB catalyst templates for the hydrolysis of NaBH4 solution for hydrogen generation. Appl Catal B-Environ 2009;86:137e44. [19] Li H, Liao JY, Zhang XB, Liao WW, Wen LL, Yang JB, et al. Controlled synthesis of nanostructured Co film catalysts with high performance for hydrogen generation from sodium borohydride solution. J Power Sources 2013;239:277e83. [20] Dai HB, Liang Y, Wang P, Cheng HM. Amorphous cobaltboron/nickel foam as an effective catalyst for hydrogen generation from alkaline sodium borohydride solution. J Power Sources 2008;177:17e23. [21] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, et al. Highperformance cobalt-tungsten-boron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33:4405e12. [22] Zhang Q, Smith G, Wu Y, Mohring R. Catalytic hydrolysis of sodium borohydride in an auto-thermal fixed-bed reactor. Int J Hydrogen Energy 2006;31:961e5. [23] Kim T, Kwon S. Design and development of a fuel cellpowered small unmanned aircraft. Int J Hydrogen Energy 2012;37:615e22. [24] Li ZP, Liu BH, Zhu JK, Morigasaki N. NaBH4 formation mechanism by reaction of sodium borate with Mg and H2. J Alloys Compd 2007;437:311e6. [25] Demirci UB, Akdim O, Miele P. Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage. Int J Hydrogen Energy 2009;34:2638e45. [26] Galindo H, Carvajal Y, Suib SL. Sulfonation of the surface of cordierite monoliths through a novel multi-step wet chemical process. Microporous Mesoporous Mater 2010;135:37e44. [27] Demirci UB, Garin F. Ru-based bimetallic alloys for hydrogen generation by hydrolysis of sodium tetrahydroborate. J Alloys Compd 2008;463:107e11. [28] Park JH, Shakkthivel P, Kim HJ, Han MK, Jang JH, Kim YR, et al. Investigation of metal alloy catalyst for hydrogen release from sodium borohydride for polymer electrolyte membrane fuel cell application. Int J Hydrogen Energy 2008;33:1845e52. [29] Dai HB, Liang Y, Ma LP, Wang P. New insights into catalytic hydrolysis kinetics of sodium borohydride from MichaelisMenten model. J Phys Chem C 2008;112:15886e92. [30] Zhang JS, Delgass WN, Fisher TS, Gore JP. Kinetics of Ru-catalyzed sodium borohydride hydrolysis. J Power Sources 2007;164:772e81. [31] Dai HB, Liang Y, Wang P. Effect of trapped hydrogen on the induction period of cobaltetungsteneboron/nickel foam catalyst in catalytic hydrolysis reaction of sodium borohydride. Catal Today 2011;170:27e32. [32] Akdim O, Demirci UB, Miele P. Deactivation and reactivation of cobalt in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2011;36:13669e75. [33] Nørskov JK, Bligaard T, Rossmeisl J, Christensen CH. Towards the computational design of solid catalysts. Nat Chem 2009;1:37e46. [34] Xin HL, Holewinski A, Schweitzer N, Nikolla E, Linic S. Electronic structure engineering in heterogeneous catalysis: identifying novel alloy catalysts based on rapid screening for materials with desired electronic properties. Top Catal 2012;55:376e90. [35] Holewinski A, Xin HL, Nikolla E, Linic S. Identifying optimal active sites for heterogeneous catalysis by metal alloys based on molecular descriptors and electronic structure engineering. Curr Opin Chem Eng 2013;2:312e9.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 3 7 3 e9 3 8 1
[36] Vojvodic A, Nørskov JK, Abild-Pedersen F. Electronic structure effects in transition metal surface chemistry. Top Catal 2014;57:25e32. [37] Kim JH, Kim KT, Kang YM, Kim HS, Song MS, Lee YJ, et al. Study on degradation of filamentary Ni catalyst on hydrolysis of sodium borohydride. J Alloys Compd 2004;379:222e7. [38] Wagner CD, Riggs WM, Davis LE, Moulder JF, Mulenberg GE. Handbook of X-ray photoelectron spectroscopy. 1st ed. Perkin Elmer Corporation; 1979. [39] Holbrook KA, Pwist PJ. Hydrolysis of the borohydride ion catalysed by metaleboron alloys. J Chem Soc A 1971:890e4. [40] Akdim O, Demirci UB, Miele P. Highly efficient acid-treated cobalt catalyst for hydrogen generation from NaBH4 hydrolysis. Int J Hydrogen Energy 2009;34:4780e7. [41] Akdim O, Chamoun R, Demirci UB, Zaatar Y, Khoury A, Miele P. Anchored cobalt film as stable supported catalyst for hydrolysis of sodium borohydride for
[42]
[43] [44]
[45]
9381
chemical hydrogen storage. Int J Hydrogen Energy 2011;36:14527e33. Demirci UB, Miele P. Reaction mechanisms of the hydrolysis of sodium borohydride: a discussion focusing on cobalt-based catalysts. C R Chimie 2014;17:707e16. Demirci UB, Miele P. Cobalt-based catalysts for the hydrolysis of NaBH4 and NH3BH3. Phys Chem Chem Phys 2014;16:6872e85. Ozerova AM, Simagina VI, Komova OV, Netskina OV, Odegova GV, Bulavchenko OA, et al. Cobalt borate catalysts for hydrogen production via hydrolysis of sodium borohydride. J Alloys Compd 2012;513:266e72. nez De Haro MC, Ferna ndez A, Arzac GM, Hufschmidt D, Jime nez MA, et al. Deactivation, reactivation Sarmiento B, Jime and memory effect on CoeB catalyst for sodium borohydride hydrolysis operating in high conversion conditions. Int J Hydrogen Energy 2012;37:14373e81.