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Hydrogen generation from alkaline NaBH4 solution using electroless-deposited Co–W–P supported on γ-Al2O3
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Hydrogen generation from alkaline NaBH4 solution using electroless-deposited CoeWeP supported on g-Al2O3 Lina Wang, Zhong Li, Xin Liu, Pingping Zhang, Guangwen Xie* Key Laboratory of Nanomaterials, Qingdao University of Science and Technology, No.53 Zhengzhou Road, Qingdao 266042, PR China
article info
abstract
Article history:
CoeWeP alloy catalysts were prepared on g-Al2O3 supports by electroless deposition.
Received 29 January 2015
Inductively coupled plasma atomic emission spectrometer(ICP-AES), field emission scan-
Received in revised form
ning electron microscope(SEM), energy dispersive X-ray spectrometer(EDS), X-ray dif-
18 April 2015
fraction(XRD), X-ray photoelectron spectroscopy(XPS) and nitrogen adsorption-desorption
Accepted 20 April 2015
isotherm were used to characterize the CoeWeP/g-Al2O3 catalysts. Hydrolysis of sodium borohydride solution to produce hydrogen was used as a probe reaction to evaluate the
Keywords:
catalytic activity of the obtained catalysts. The influences of catalyst preparation condi-
CoeWeP/g-Al2O3 catalyst
tions such as CoSO4/Na2WO4 concentration ratio in electroless bath, electroless deposition
Electroless deposition
time and hydrolysis reaction conditions such as NaOH and NaBH4 concentrations, the
Hydrogen generation
amount of catalysts used and reaction temperature on the hydrogen generation rate were
NaBH4
investigated in the paper. The results show that the obtained CoeWeP/g-Al2O3 catalysts exhibit excellent catalytic activity, the highest hydrogen generation rate can reach 1
11:82 L min g1 catalyst . The CoeWeP/g-Al2O3 catalysts also exhibit favorable cycling perfor-
mance and lower activation energy (49.58 kJ mol1).
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Because of the increasing fossil fuels consumption and the deterioration of the global environment, there is an essential need to seek an alternative energy to replace fossil fuels [1]. Hydrogen fuel cells as a promising power source in response to energy crisis have got much attention due to high efficiency and high power density over the past decade [2,3]. However, hydrogen production and storage are still obstacles to the widespread application and commercialization of hydrogen fuel cells, because traditional methods of hydrogen storage
like physical compress or liquefaction, adsorption on carbon or carbon nanotubes are not suitable for practical application due to the low gravimetric and volumetric storage density [4,5]. Metal hydrides can release H2 rapidly, but the system becomes very heavy and expensive as a heat supply system is indispensable [6]. With the advantages of lower operation pressure and less cost, chemical hydrides like lithium borohydride (LiBH4), potassium borohydride (KBH4) and sodium borohydride (NaBH4) can be employed as hydrogen sources for hydrogen fuel cells [7,8]. Currently, the study of NaBH4 has become more intensively than other chemical hydrides because of its non-flammable and non-toxic nature, excellent
* Corresponding author. Fax: þ86 532 8402 2883. E-mail address: [email protected] (G. Xie). http://dx.doi.org/10.1016/j.ijhydene.2015.04.110 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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stability in alkaline solution at room temperature and high hydrogen storage capability of 10.8 wt.% [9e11]. Moreover, pure hydrogen can be generated with controllable speed by hydrolysis of alkaline NaBH4 solution in the presence of selected catalysts following the Equation (1): NaBH4þ2H2O]NaBO2þ4H2 DH ¼ 217 kJ mol1
(1)
High-performance catalysts used for hydrolysis of NaBH4 have been intensively investigated in the past years. Hydrolysis reaction rate of NaBH4 solution can be effectively enhanced by using transition metals [12]. Though noble metal catalysts like Ru [13], Pt [14] and Pd [15] have been reported to exhibit high catalytic activity for the hydrolysis of NaBH4, non-noble metal catalysts like CoeNieP [16], CoeB [17,18], NieCoeB [19] and CoeNiePeB [20] have been more widely applied to catalytic hydrolysis of NaBH4 solution because of their low cost and good catalytic activity. Among the non-noble metal catalysts, Coebased catalysts show higher catalytic properties than Niebased catalysts [21], and Coebased catalysts prepared with the addition of transition metals such as W, Mo, Cu, Fe and Cr exhibit superior catalytic activity in hydrogen generation by hydrolysis of alkaline NaBH4 solution [22]. CoeCueB [23], CoeWeB [24], CoeMoeB [25], CoeWeP [26] and some other similar catalysts have been synthesized to accelerate the hydrolysis reaction of NaBH4. Additionally, the raw materials for the synthesis of CoeB catalysts are much higher than that for CoeP catalysts. Therefore, it is critical to investigate transition metal-doped CoeP catalysts for the hydrolysis of NaBH4 solution [27]. Recently, Guo YP et al. [26] reported the catalytic property of CoeWeP catalysts supported on Cu substrates by electrodeposition, and the obtained CoeWeP catalysts exhibit high catalytic property and low activation energy. However, some influential parameters such as various reactant concentrations have not been investigated in their report. In addition, the hydrogen generation rate decreases by 49% from 5000 ml 1 1 min1 g1 catalyst to 2533 ml min gcatalyst after 5 cycles. In our previous work, NieCoeP/g-Al2O3 catalysts synthesized by electroless deposition remain high hydrogen generation rate after 10 circles, proving that g-Al2O3 carrier is favorable to enhance the cycling capability of catalysts [28]. Haghtalab A et al. [29], Lu YC et al. [30], Xu DY et al. [31] and Huang YH et al. [32] also reported the use of g-Al2O3 loaded with metal catalysts for hydrogen generation from sodium borohydride solution. Herein, we prepared CoeWeP catalysts supported on the g-Al2O3 catalysts by electroless deposition and studied the influences of preparation conditions and hydrolysis reaction conditions on their catalytic activity systematically. Furthermore, the cycling capability and the kinetic parameters of CoeWeP/g-Al2O3 were also investigated by the hydrogen generation from alkaline NaBH4 solution.
Experimental Preparation of CoeWeP/g-Al2O3 catalysts The g-Al2O3 pellets with the diameter in the range of 2.5e3.5 mm were used as the carrier. They were first washed
in anhydrous ethanol to get rid of the purities and greasy dirt on their surface. Then the pellets were sensitized in 10 g L1 SnCl2 solution and subsequently activated in 0.5 g L1 PdCl2 solution before electroless deposition. The use of ultrasonic agitation can be favor of the removal of dirt and the formation of active Pd on the support surface. The electroless deposition of CoeWeP alloys were performed in an alkaline solution, containing 7.5 g L1 cobalt sulfate heptahydrate, 17.5 g L1 sodium tungstate dehydrate, 50 g L1 sodium hypophosphite monohydrate as reducing agent, 20 g L1 ammonium fluoride as buffers and 20 g L1 trisodium citrate dihydrate as stabilizer. The pH 9 was easily controlled by the addition of ammonia solution and the thickness of the plating was controlled by the deposition time. The process was optimized at 90 C.
Catalyst characterization The morphologies and sizes of the catalysts were characterized using field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F). The atomic ratio of Co, W and P in the coatings was analyzed by energy dispersive X-ray spectroscopy (EDS, Oxford Instrument INCA). The crystal structures of the CoeWeP/g-Al2O3 and g-Al2O3 were observed by X-ray diffraction (XRD, Rigaku D-max-gA XRD with Cu ka radiation, ˚ ) from 5 to 85 . The specific surface area of the l ¼ 1.54178 A sample was measured by BrunauereEmmetteTeller (BET, Micromeritics instrument ASAP-2020) nitrogen adsorptiondesorption method. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a PerkineElmer PHI 550 spectrometer with Al Ka (1486.6 eV) as the X-ray source. The composition and loading of the supported catalysts were confirmed by inductively coupled plasma atomic emission spectrometer (ICP-AES, Leeman PROFILE SPEC). The preparation of samples for ICP analysis was as follows. A number of CoeWeP/g-Al2O3 catalysts were firstly weighted by electronic balance. Then, catalysts were placed into a beaker containing aqua regia to dissolve the alloys deposited on the g-Al2O3. After alloys were completely dissolved, the acid solution was transferred into a volumetric flask. The g-Al2O3 pellets in the beaker were washed by ultra-pure water and the solution was also transferred into the volumetric flask, which was repeated several times. At last, the solution in the volumetric flask was diluted with ultra-pure water to the volume and the element content of Co, W and P in the solution was determined by ICP. If the concentration of element exceeds the measuring range, the solution should be diluted further. The total element content of Co, W and P divided by the amount of CoeWeP/gAl2O3 catalysts equals the CoeWeP loading.
Hydrogen generation measurement Fig. 1 presents the hydrogen generation measurement. A three-necked flask containing 20 ml 4 wt.% NaBH4 with 10 wt.% NaOH was fitted with an outlet tube, which was connected to a water-filled mouth bottle for collecting evolved H2 gas. The catalysts (1.5 g CoeNieW/g-Al2O3) were placed into the reaction flask with a thermostatic bath to initiate hydrolysis reaction of NaBH4 at 45 C. As the reaction proceeded, the volume of water replaced by hydrogen was
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oxygen of atmosphere during the catalyst preparation and storage process. Fig. 1(h) shows the W 4f spectrum of the CoeWeP/g-Al2O3 catalysts. The most intense W 4f7/2 peaks around 31.1 eV is attributed to W metal, while the small W 4f7/ 2 and W 4f5/2 peaks around 35.5 eV and 37.6 eV are attributed to W 6þ [36]. It is interesting to note that the W 4f7/2 peaks around 30.0 eV for the CoeWeP/g-Al2O3 catalysts can also be observed in the appearance of a shoulder in the low energy side of the valence band. This has been observed in other reports [37,38]. Fig. 1(i) shows the XPS spectra of P 2p in the CoeWeP/g-Al2O3 catalysts. The peaks around 134.1 eV, 133.1 eV are assigned to oxidized phosphorus and the peak around 129.1 eV is assigned to phosphorus inter-acting with cobalt and tungsten [39,40]. Fig. 1 e Hydrogen generation measurement system. 1 Thermometer 2 Three-necked flask 3 Thermostatic bath 4 Mouth bottle 5 Graduated cylinder.
measured by a graduated cylinder. After the reaction completed, the catalysts were separated from residual solution by filtration and reserved. The specific hydrogen generation rate (ml min1 g1) for the catalysts was based on the volume of hydrogen generation, the usage time of the reaction and the amount of the catalysts excluding the weight of gAl2O3.
Results and discussion Catalyst characterization Fig. 2 shows the surface morphologies of the as-prepared CoeWeP catalysts on the g-Al2O3 in various electroless deposition time. The products electroless deposited for 240 s present sheet surfaces in Fig. 2(a). With the electroless deposition time increasing, the surface morphologies of the coatings change into granular surfaces in Fig. 2(bed). This may be due to the rapid release of a large amount of hydrogen gas during the electroless deposition process. One can see that the size of particles in Fig. 2(b) is similar with that in Fig. 2(c). But the amount of particles in Fig. 2(c) synthesized in longer electroless deposition time is larger than that in Fig. 2(b). Further increasing the depositing time, the particles in Fig. 2(d) become very large. Fig. 2(e) shows the XRD patterns of the as-prepared products and g-Al2O3 support. For the CoeWeP/g-Al2O3 catalysts, the diffraction peaks around 41.7 (100), 44.8 (002), 47.6 (101) and 75.9 (110) are assigned to the diffraction of Co in hexagonal phase. Fig. 2(f) shows EDS patterns of the products, and cobalt, tungsten and phosphorus could be detected. For further confirming the formation of CoeWeP/g-Al2O3, X-ray photoelectron spectroscopy (XPS) was also applied to the as-prepared product. In the Fig. 1(g), Co 2p3/2 peak at 777.8 eV stands for Co metal [33], which agrees well with the XRD results, as seen in Fig. 1(e). Oxidized Co species could be distinguished from metallic Co based on the higher energy of the 2p3/2 peaks located at 781.3 eV, 785.6 eV and 2p1/2 peaks located at 797.2 eV, 802.9 eV [34,35]. The detectable oxidized Co species could be ascribed to the surface Co combined with
Effect of NaOH concentration on hydrogen generation The influence of the NaOH concentration on hydrogen generation is shown in Fig. 3. It can be seen that the hydrogen generation rate increases with the NaOH concentration increasing, and reaches a maximum value at 10 wt.% NaOH, then decreases with further increasing in NaOH concentration. The results indicate that OH anions play dual roles during the hydrolysis of NaBH4. These findings are consistent with the reports on CoeB [4,17] or Co [11,41] catalysts. However, these results are different from those investigations on CoeMoePdeB [8], Ni-Ru [42] or Ru [43] catalysts. Therefore, the influence of NaOH concentration on NaBH4 hydrolysis greatly depends on the catalyst used in the reaction. Holbrook and Twist [44] were the first to postulate a working mechanism to account for the reaction kinetics observed, which is as follows: 2M þ BH 4 4M BH3 þ M H
(2)
M BH 3 4BH3 þ M þ e
(3)
BH3þOH / BH3(OH)
(4)
M þ e þ H2O / MeH þ OH
(5)
MeH þ MH / H2þ2M
(6)
We can see that OH is involved in the hydrolysis of NaBH4 and appropriate increase of NaOH concentration can be favor of accelerating the hydrolysis reaction of NaBH4 and enhance the hydrogen generation rate. However, it should be noticed that the OH anions and BH 4 anions are competitive adsorption on the surface of catalyst. When the concentration of NaOH increases further, more OH anions will occupy the adsorption sites to reduce BH 4 anions contacting with active species adequately [5]. Furthermore, excessive NaOH concentrations will also lead to more NaBO2 accumulated on the catalyst surface because of low solubility of NaBO2 under alkaline condition [23,45]. Consequently, the active sites on the catalyst would be blocked, the solution viscosity would increase and the mass transfer would be impeded. All of these disadvantageous factors would inhibit the hydrolysis of NaBH4. The inset of ln (HGR) versus ln [NaOH] reveals that the
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Fig. 2 e SEM images of CoeWeP/g-Al2O3 catalysts deposited for (a) 240s (b) 300s (c) 360s (d) 420s; XRD patterns of the asprepared CoeWeP/g-Al2O3 catalysts and g-Al2O3 support (e); EDS patterns of the CoeWeP coatings (f) and XPS spectra of Co 2p (g), W 4f (h), P 2p (i) for CoeWeP/g-Al2O3 catalysts.
slope of the straight line is 0.41, which proves that the increase of the NaOH concentration from 4 wt.% to 10 wt.% has a positive effect on hydrogen generation. Therefore, the optimal concentration of NaOH is chosen as 10 wt.% in the succeed experiments.
Effect of NaBH4 concentration on hydrogen generation
Fig. 3 e Effect of NaOH concentration on hydrogen generation (4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 4 gives the influence of NaBH4 concentration on the practical hydrogen generation performance. It is observed that the hydrogen generation rate drastically increases with increase in NaBH4 concentration and subsequently decreases with further increase in NaBH4 concentration. Many researchers [11,41] had found that the hydrogen generation rate decreases with higher NaBH4 concentration. It is because that BH-4 can be in good adsorption and desorption in the surface of catalysts at low concentration of sodium borohydride. But when the concentration of NaBH4 is higher than 4 wt.%, the solution viscosity rises. The formation of the reaction byproduct NaBO2 could significantly affect the mass transfer of NaBH4 to catalyst and lead to blockage of the catalytic sites
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specific surface area, which is 256 m2 g1 detected by nitrogen adsorption-desorption isotherm. This is mainly due to the difference in surface morphology and particle size of the asprepared CoeWeP/g-Al2O3 catalysts with different electroless deposition time in Fig. 2(aed). As determined by ICP-AES measurement, the CoeWeP loading is measured to be 0.81%, which contains 72.66 wt.% Co, 18.27 wt.% W and 8.27 wt.% P, corresponding to a formula of Co12.4W1P2.7. Therefore, the highest hydrogen generation rate can reach 1182 L min1 g1 catalyst .
Effect of CoSO4/Na2WO4 concentration ratio on hydrogen generation
Fig. 4 e Effect of NaBH4 concentration on hydrogen generation (10 wt.% NaOH, 45 C, 1.5 g catalyst).
during the reaction. The plot of ln (HGR) versus ln [NaBH4] shown in the inset of Fig. 4 reveals the negative effect of NaBH4 on hydrogen generation with concentrations higher than 4 wt.%, because the slope of the straight line is 0.55. Niu WL et al. [11] reported that the reaction order of NaBH4 hydrolysis catalyzed by the carbon supported cobalt catalyst is 0.30 with regard to the NaBH4 concentration from 0.25 wt.% to 1.00 wt.%. Vernekar AA et al. [46] found that hydrolysis reaction of NaBH4 with CoeCo2B nanocomposites or NieNi2B nanocomposites is zero order with respect to the concentration of NaBH4 from 0.05 M to 0.4 M. Herein, the kinetics of NaBH4 hydrolysis is not only dependent on NaBH4 concentration, but also on the type of the catalysts used for hydrogen generation.
Fig. 6 shows the effect of CoSO4/Na2WO4 concentration ratio in electroless bath on the hydrogen generation rate of the CoeWeP/g-Al2O3 catalysts. The corresponding element percentage of Co, W and P at three different regions on the catalyst surface has been tested and the average results are presented in Table 1 considering the measuring errors of EDS analysis. Interestingly, with decreasing CoSO4/Na2WO4 concentration ratio, the Co content in the CoeWeP alloy increases while the P content decreases. But the content of W appears to be less sensitive to the changing of CoSO4/Na2WO4 concentration ratio. According to Younes et al. [47], the electroless bath solution may contain only low concentration of [(Co) (WO4) (Cit) (H)]2, so the influence of CoSO4/Na2WO4 concentration ratio on the W content is very little. It is clearly seen from Fig. 6 that the hydrogen generation rate promotes apparently with decrease in CoSO4/Na2WO4 concentration ratio, subsequently decreases with further decrease in CoSO4/Na2WO4 concentration ratio. This may be due to that the P content in the catalysts decreases with decreasing CoSO4/Na2WO4 concentration ratio while Co/W atomic ratio increases.
Effect of electroless deposition time on hydrogen generation Effect of the amount of CoeWeP/g-Al2O3 catalysts used on hydrogen generation
The effect of electroless deposition time from 240 se420 s on hydrogen generation is presented in Fig. 5. It is found that the CoeWeP/g-Al2O3 catalysts electroless deposited in 360 s exhibit the highest hydrogen generation rate and have a large
The effect of the amount of CoeWeP/g-Al2O3 catalysts used on hydrogen generation performance was investigated by
Fig. 5 e Effect of electroless deposition time on hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 6 e Effect of different CoSO4/Na2WO4 concentration ratio in electroless bath on hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
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Table 1 e Composition of different CoeWeP coatings by EDS. CoSO4: Na2WO4 1:1 2:3 3:7 1:4
Co
W
P
Co/W
83.99 85.85 87.24 88.32
5.81 5.89 5.80 5.82
10.20 8.26 6.96 5.86
14.46 14.58 15.04 15.18
carrying out a series of experiments with initial amount of CoeWeP/g-Al2O3 from 0.5 g to 2.5 g and the result is shown in Fig. 7. It can be seen that the hydrogen generation rate increases apparently with increase in the amount of the catalysts used. Possibly, the more the CoeWeP/g-Al2O3 catalysts used, the more BH 4 ions were adsorbed [11]. The inset in Fig. 7 shows the plot of ln (HGR) versus ln (amount of catalysts) for the same reaction. The slope of the straight line is 0.32, which is between zero order kinetics and first order kinetics. Herein, the amount of catalysts has a positive effect on hydrogen generation.
Hydrogen generation kinetics of CoeWeP/g-Al2O3 catalysts Hydrolysis of sodium borohydride solution is sensitive to reaction temperature. As expected, the hydrogen generation rate increases with the elevating reaction temperature significantly, which can be seen in Fig. 8. The hydrogen generation rates were used to determine the activation energy (Ea) for the hydrolysis of NaBH4 in the presence of the CoeWeP/g-Al2O3 catalysts by the following Ea , where k is the reaction Arrhenius equation: lnk ¼ lnA e RT 1 1 gcatalyst ), R is the gas constant constant (mol min (8.314 J mol1 K1), T is the hydrolysis temperature and A is the pre-exponential factor. The Arrhenius plot of lnk versus 1/T is shown in Fig. 9 and the activation energy was calculated to be 49.58 kJ mol1, within the experimental errors. Compared with the activation energy values of different Co-based metal catalysts summarized in Table 2, the Ea of CoeWeP/g-Al2O3
Fig. 7 e Effect of the amount of CoeWeP/g-Al2O3 catalysts used on the hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 8 e Effect of reaction temperature on the hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 1.5 g catalyst). (49.58 kJ mol1) is similar with that of reported CoeRueMoeB (48.8 kJ mol1) [5], CoeCueB (49.6 kJ mol1) [23], slightly higher than that of catalysts like: CoeNiePeB (29 kJ mol1) [20], CoeMoeB (39 kJ mol1) [22] and lower than that of catalysts like: CoeP (60 kJ mol1) [48], Raney Co (53.7 kJ mol1) [49]. The favorable activation energy together with the low cost make the CoeWeP/g-Al2O3 catalysts seem to be very preferable in practical application.
Reusability of CoeWeP/g-Al2O3 catalysts in the hydrolysis of sodium borohydride The cycling ability of the catalysts is crucial to the practical hydrogen generation application. In the present work, the CoeWeP catalysts supported on g-Al2O3 by electroless deposition were tested with respect to the reusability in the hydrolysis of sodium borohydride solution in the presence of 10 wt.% NaOH and 4 wt.% NaBH4 at 45 C. After each catalytic reaction, the used CoeWeP/g-Al2O3 catalysts were separated
Fig. 9 e Arrhenius plot obtained from the data in Fig. 8 for hydrogen generation from NaBH4 solution.
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Table 2 e Comparison of the key parameters for the NaBH4 hydrolysis reaction catalyzed by different Co-based metal catalysts. Co-based metal catalysts
Activation energy (kJ mol1)
Hydrogen generation rate ( mL min1 g1 catalyst )
Hydrolysis temperature ( C)
Ref.
CoeRueMoeB CoeMoePdeB NieCoeB CoeNiePeB CoeCreB CoeMoeB CoeCueB CoeNieB CoeFeeB CoeWeB CoeCueB CoeWeP CoeNieP/g-Al2O3 CoeCo2B CoeP Raney Co CoeWeB/Ni film CoeMneB CoeLaeZreB CoeWeP/g-Al2O3
48.8 36.36 62 29 37 39 30 34 31 41 49.6 22.8 52.05 35.245 60 53.7 29 52.1 60.06 49.58
8075 6023 2608 2400 3400 2875 2210 1175 1130 2570 4800 5000 6599.6 4300 3300 267.5 15,000 2500 1252 4000
25 25 28 25 25 25 25 25 25 25 30 30 55 30 30 20 30 30 50 30
[5] [8] [19] [20] [22] [22] [22] [22] [22] [22] [23] [26] [28] [46] [48] [49] [50] [51] [52] This work
from the solution and washed thoroughly with deionized water, then dried 10 h at 60 C in the oven. Fig. 10(a) presents the cycling performance of the CoeWeP/g-Al2O3 catalysts. The catalysts show good durability in cyclic usage. Even after six cycles, the hydrogen generation rate still reaches 7.90 L min1 g1 catalyst , which is slightly inferior to that in the previous five cycles. As determined by ICP-AES measurement, the element percentage of Co, W and P in the catalysts after 6 cycles was 85.41 wt.%, 12.38 wt.% and 2.21 wt.%, respectively. The element percentage of the catalysts before and after changed a little. Fig. 10(b) shows that the used CoeWeP/g-Al2O3 catalysts consist of two-dimensional nanosheets with a thickness of 5 nm. It can be concluded that the slight decrease in catalytic activity may be due to the physical changes and the decrease of W percentage on the CoeWeP/g-Al2O3 catalysts surface. Recently, the reported Co/Ni foam catalysts lose most of its catalytic activity after 3 cycles [53] and the CoeWeP catalysts supported on Cu
substrates lose 49% activity after 5 cycles [26]. Compared with those Co-based metal catalysts, the CoeWeP/g-Al2O3 catalysts exhibit significantly improved durability and reusability.
Conclusions In summary, CoeWeP/g-Al2O3 catalysts were prepared by electroless deposition and their catalytic activities in the hydrolysis of NaBH4 solution were investigated. The preparation and the hydrolysis of NaBH4 solution conditions have obvious effect on the hydrogen generation performance. The catalysts electroless deposited for 360 s with 3:7 CoSO4/Na2WO4 concentration ratios in electroless bath exhibit fine catalytic property, which have a great specific surface area of 256 m2 g1. The highest hydrogen generation rate can reach 11:82 L min1 g1 catalyst in the hydrolysis conditions of 10 wt.%
Fig. 10 e Reusability tests of the CoeWeP/g-Al2O3 catalysts in the NaBH4 hydrolysis at 45 C (a), SEM image of CoeWeP/gAl2O3 catalyst after 6 cycles (b) (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
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NaOH, 4 wt.% NaBH4, 1.5 g catalyst, 45 C. The CoeWeP/gAl2O3 catalysts also exhibit favorable cycling performance and lower activation energy (49.58 kJ mol1) in comparison with that of other reported Co-based catalysts.
[16]
[17]
Acknowledgments The work was financially supported by the Natural Science Foundation of Shandong Province (Contracted No.ZR2011EMM005) and the key project of the National Natural Science Foundation of China (Contracted No.21136008).
[18]
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[33] Bera P, Seenivasan H, Rajam KS, WilliamGrips VK. XRD, FESEM and XPS studies on heat treated Co-W electrodeposits. Mater Lett 2012;76:103e5. [34] Ai LH, Gao XY, Jiang J. In situ synthesis of cobalt stabilized on macroscopic biopolymer hydrogel as economical and recyclable catalyst for hydrogen generation from sodium borohydride hydrolysis. J Power Sources 2014;257:213e20. [35] Yang P, Xu AD, Xia J, He J, Xing HL, Zhang XM, et al. Facile synthesis of highly catalytic activity Ni-Co-Pd-P composite for reduction of the p-Nitrophenol. Appl Catal A: Gen 2014;470:89e96. [36] Wang WY, Yang SJ, Qiao ZQ, Liu PL, Wu K, Yang YQ. Preparation of NieWePeB amorphous catalyst for the hydrodeoxygenation of p-cresol. Catal Commun 2015;60:50e4. [37] Katrib A, Hemming F, Hilaire L, Wehrer P, Maire G. XPS studies of supported tungsten carbide(s). J Electron Spectrosc Relat Phenom 1994;68:589e95. V. [38] OrlovskajaU L, Matulionis E, Timinskas A, Sukien e Electrocrystallization of magnetic Co-W-Mn films. Surf Coat Technol 2000;135:34e41. [39] Guo YP, Feng QH, Dong ZP, Ma JT. Electrodeposited amorphous CoeP catalyst for hydrogen generation from hydrolysis of alkaline sodium borohydride solution. J Mol Catal A: Chem 2013;378:273e8. [40] Xiao MS, Miao YQ, Tian YP, Yan YH. Synthesizing nanoparticles of Co-p-Se compounds as electrocatalysts for the hydrogen evolution reaction. Electrochim Acta 2015;165:206e10. [41] Zhu J, Li R, Niu WL, Wu YJ, Gou XL. Fast hydrogen generation from NaBH4 hydrolysis catalyzed by carbon aerogels supported cobalt nanoparticles. Int J Hydrogen Energy 2013;38:10864e70. [42] Liu CH, Chen BH, Hsueh CL, Ku JR, Jeng MS, Tsau F. Hydrogen generation from hydrolysis of sodium borohydride using NieRu nanocomposite as catalysts. Int J Hydrogen Energy 2009;34:2153e63. [43] Hsueh CL, Chen CY, Ku JR, Tsai ST, Hsu YY, Tsau F, et al. Simple and fast fabrication of polymer template-Ru composite as a catalyst for hydrogen generation from alkaline NaBH4 solution. J Power Sources 2008;177:485e92.
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[44] Holbrook KA, Twist PJ. Hydrolysis of the borohydride ion catalysed by metaleboron alloys. J Chem Soc A 1971;15:890e4. € zkar S. Cobalt-nickel-phosphorus [45] Rakap M, Kalu EE, o supported on Pd-activated TiO2 (Co-Ni-P/Pd-TiO2) as costeffective and reusable catalyst for hydrogen generation from hydrolysis of alkaline sodium borohydride solution. J Alloys Compd 2011;509:7016e21. [46] Vernekar AA, Bugde ST, Tilve S. Sustainable hydrogen production by catalytic hydrolysis of alkaline sodium borohydride solution using recyclable Co-Co2B and Ni-Ni2B nanocomposites. Int J Hydrogen Energy 2012;37:327e34. [47] Younes-Metzler O, Zhu L, Glieadi E. The anomalous codeposition of tungsten in the presence of nickel. Electrochim Acta 2003;48:2551e62. [48] Eom KS, Cho KW, Kwon HS. Effects of electroless deposition conditions on microstructures of cobalt-phosphorous catalysts and their hydrogen generation properties in alkaline sodium borohydride solution. J Power Sources 2008;180:484e90. [49] Liu BH, Li ZP, Suda S. Nickel-and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloys Compd 2006;415:288e93. [50] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, et al. Highperformance cobaltetungsteneboron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33:4405e12. [51] Yuan XX, Jia C, Ding XL, Ma ZF. Effects of heat-treatment temperature on properties of CobalteManganeseeBoride as efficient catalyst toward hydrolysis of alkaline sodium borohydride solution. Int J Hydrogen Energy 2012;37:995e1001. [52] Loghmani MH, Shojaei AF. Hydrogen generation from hydrolysis of sodium borohydride by cubic Co-La-Zr-B nanoparticles as novel catalyst. Int J Hydrogen Energy 2013;38:10470e8. [53] Akdim O, Demirci UB, Miele P. Deactivation and reactivation of cobalt in hydrolysis of sodium borohydride. Int J Hydrogen Energy 2011;36:13669e75.
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Hydrogen generation from alkaline NaBH4 solution using electroless-deposited CoeWeP supported on g-Al2O3 Lina Wang, Zhong Li, Xin Liu, Pingping Zhang, Guangwen Xie* Key Laboratory of Nanomaterials, Qingdao University of Science and Technology, No.53 Zhengzhou Road, Qingdao 266042, PR China
article info
abstract
Article history:
CoeWeP alloy catalysts were prepared on g-Al2O3 supports by electroless deposition.
Received 29 January 2015
Inductively coupled plasma atomic emission spectrometer(ICP-AES), field emission scan-
Received in revised form
ning electron microscope(SEM), energy dispersive X-ray spectrometer(EDS), X-ray dif-
18 April 2015
fraction(XRD), X-ray photoelectron spectroscopy(XPS) and nitrogen adsorption-desorption
Accepted 20 April 2015
isotherm were used to characterize the CoeWeP/g-Al2O3 catalysts. Hydrolysis of sodium borohydride solution to produce hydrogen was used as a probe reaction to evaluate the
Keywords:
catalytic activity of the obtained catalysts. The influences of catalyst preparation condi-
CoeWeP/g-Al2O3 catalyst
tions such as CoSO4/Na2WO4 concentration ratio in electroless bath, electroless deposition
Electroless deposition
time and hydrolysis reaction conditions such as NaOH and NaBH4 concentrations, the
Hydrogen generation
amount of catalysts used and reaction temperature on the hydrogen generation rate were
NaBH4
investigated in the paper. The results show that the obtained CoeWeP/g-Al2O3 catalysts exhibit excellent catalytic activity, the highest hydrogen generation rate can reach 1
11:82 L min g1 catalyst . The CoeWeP/g-Al2O3 catalysts also exhibit favorable cycling perfor-
mance and lower activation energy (49.58 kJ mol1).
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Because of the increasing fossil fuels consumption and the deterioration of the global environment, there is an essential need to seek an alternative energy to replace fossil fuels [1]. Hydrogen fuel cells as a promising power source in response to energy crisis have got much attention due to high efficiency and high power density over the past decade [2,3]. However, hydrogen production and storage are still obstacles to the widespread application and commercialization of hydrogen fuel cells, because traditional methods of hydrogen storage
like physical compress or liquefaction, adsorption on carbon or carbon nanotubes are not suitable for practical application due to the low gravimetric and volumetric storage density [4,5]. Metal hydrides can release H2 rapidly, but the system becomes very heavy and expensive as a heat supply system is indispensable [6]. With the advantages of lower operation pressure and less cost, chemical hydrides like lithium borohydride (LiBH4), potassium borohydride (KBH4) and sodium borohydride (NaBH4) can be employed as hydrogen sources for hydrogen fuel cells [7,8]. Currently, the study of NaBH4 has become more intensively than other chemical hydrides because of its non-flammable and non-toxic nature, excellent
* Corresponding author. Fax: þ86 532 8402 2883. E-mail address: [email protected] (G. Xie). http://dx.doi.org/10.1016/j.ijhydene.2015.04.110 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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stability in alkaline solution at room temperature and high hydrogen storage capability of 10.8 wt.% [9e11]. Moreover, pure hydrogen can be generated with controllable speed by hydrolysis of alkaline NaBH4 solution in the presence of selected catalysts following the Equation (1): NaBH4þ2H2O]NaBO2þ4H2 DH ¼ 217 kJ mol1
(1)
High-performance catalysts used for hydrolysis of NaBH4 have been intensively investigated in the past years. Hydrolysis reaction rate of NaBH4 solution can be effectively enhanced by using transition metals [12]. Though noble metal catalysts like Ru [13], Pt [14] and Pd [15] have been reported to exhibit high catalytic activity for the hydrolysis of NaBH4, non-noble metal catalysts like CoeNieP [16], CoeB [17,18], NieCoeB [19] and CoeNiePeB [20] have been more widely applied to catalytic hydrolysis of NaBH4 solution because of their low cost and good catalytic activity. Among the non-noble metal catalysts, Coebased catalysts show higher catalytic properties than Niebased catalysts [21], and Coebased catalysts prepared with the addition of transition metals such as W, Mo, Cu, Fe and Cr exhibit superior catalytic activity in hydrogen generation by hydrolysis of alkaline NaBH4 solution [22]. CoeCueB [23], CoeWeB [24], CoeMoeB [25], CoeWeP [26] and some other similar catalysts have been synthesized to accelerate the hydrolysis reaction of NaBH4. Additionally, the raw materials for the synthesis of CoeB catalysts are much higher than that for CoeP catalysts. Therefore, it is critical to investigate transition metal-doped CoeP catalysts for the hydrolysis of NaBH4 solution [27]. Recently, Guo YP et al. [26] reported the catalytic property of CoeWeP catalysts supported on Cu substrates by electrodeposition, and the obtained CoeWeP catalysts exhibit high catalytic property and low activation energy. However, some influential parameters such as various reactant concentrations have not been investigated in their report. In addition, the hydrogen generation rate decreases by 49% from 5000 ml 1 1 min1 g1 catalyst to 2533 ml min gcatalyst after 5 cycles. In our previous work, NieCoeP/g-Al2O3 catalysts synthesized by electroless deposition remain high hydrogen generation rate after 10 circles, proving that g-Al2O3 carrier is favorable to enhance the cycling capability of catalysts [28]. Haghtalab A et al. [29], Lu YC et al. [30], Xu DY et al. [31] and Huang YH et al. [32] also reported the use of g-Al2O3 loaded with metal catalysts for hydrogen generation from sodium borohydride solution. Herein, we prepared CoeWeP catalysts supported on the g-Al2O3 catalysts by electroless deposition and studied the influences of preparation conditions and hydrolysis reaction conditions on their catalytic activity systematically. Furthermore, the cycling capability and the kinetic parameters of CoeWeP/g-Al2O3 were also investigated by the hydrogen generation from alkaline NaBH4 solution.
Experimental Preparation of CoeWeP/g-Al2O3 catalysts The g-Al2O3 pellets with the diameter in the range of 2.5e3.5 mm were used as the carrier. They were first washed
in anhydrous ethanol to get rid of the purities and greasy dirt on their surface. Then the pellets were sensitized in 10 g L1 SnCl2 solution and subsequently activated in 0.5 g L1 PdCl2 solution before electroless deposition. The use of ultrasonic agitation can be favor of the removal of dirt and the formation of active Pd on the support surface. The electroless deposition of CoeWeP alloys were performed in an alkaline solution, containing 7.5 g L1 cobalt sulfate heptahydrate, 17.5 g L1 sodium tungstate dehydrate, 50 g L1 sodium hypophosphite monohydrate as reducing agent, 20 g L1 ammonium fluoride as buffers and 20 g L1 trisodium citrate dihydrate as stabilizer. The pH 9 was easily controlled by the addition of ammonia solution and the thickness of the plating was controlled by the deposition time. The process was optimized at 90 C.
Catalyst characterization The morphologies and sizes of the catalysts were characterized using field-emission scanning electron microscopy (FESEM, JEOL JSM-6700F). The atomic ratio of Co, W and P in the coatings was analyzed by energy dispersive X-ray spectroscopy (EDS, Oxford Instrument INCA). The crystal structures of the CoeWeP/g-Al2O3 and g-Al2O3 were observed by X-ray diffraction (XRD, Rigaku D-max-gA XRD with Cu ka radiation, ˚ ) from 5 to 85 . The specific surface area of the l ¼ 1.54178 A sample was measured by BrunauereEmmetteTeller (BET, Micromeritics instrument ASAP-2020) nitrogen adsorptiondesorption method. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a PerkineElmer PHI 550 spectrometer with Al Ka (1486.6 eV) as the X-ray source. The composition and loading of the supported catalysts were confirmed by inductively coupled plasma atomic emission spectrometer (ICP-AES, Leeman PROFILE SPEC). The preparation of samples for ICP analysis was as follows. A number of CoeWeP/g-Al2O3 catalysts were firstly weighted by electronic balance. Then, catalysts were placed into a beaker containing aqua regia to dissolve the alloys deposited on the g-Al2O3. After alloys were completely dissolved, the acid solution was transferred into a volumetric flask. The g-Al2O3 pellets in the beaker were washed by ultra-pure water and the solution was also transferred into the volumetric flask, which was repeated several times. At last, the solution in the volumetric flask was diluted with ultra-pure water to the volume and the element content of Co, W and P in the solution was determined by ICP. If the concentration of element exceeds the measuring range, the solution should be diluted further. The total element content of Co, W and P divided by the amount of CoeWeP/gAl2O3 catalysts equals the CoeWeP loading.
Hydrogen generation measurement Fig. 1 presents the hydrogen generation measurement. A three-necked flask containing 20 ml 4 wt.% NaBH4 with 10 wt.% NaOH was fitted with an outlet tube, which was connected to a water-filled mouth bottle for collecting evolved H2 gas. The catalysts (1.5 g CoeNieW/g-Al2O3) were placed into the reaction flask with a thermostatic bath to initiate hydrolysis reaction of NaBH4 at 45 C. As the reaction proceeded, the volume of water replaced by hydrogen was
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oxygen of atmosphere during the catalyst preparation and storage process. Fig. 1(h) shows the W 4f spectrum of the CoeWeP/g-Al2O3 catalysts. The most intense W 4f7/2 peaks around 31.1 eV is attributed to W metal, while the small W 4f7/ 2 and W 4f5/2 peaks around 35.5 eV and 37.6 eV are attributed to W 6þ [36]. It is interesting to note that the W 4f7/2 peaks around 30.0 eV for the CoeWeP/g-Al2O3 catalysts can also be observed in the appearance of a shoulder in the low energy side of the valence band. This has been observed in other reports [37,38]. Fig. 1(i) shows the XPS spectra of P 2p in the CoeWeP/g-Al2O3 catalysts. The peaks around 134.1 eV, 133.1 eV are assigned to oxidized phosphorus and the peak around 129.1 eV is assigned to phosphorus inter-acting with cobalt and tungsten [39,40]. Fig. 1 e Hydrogen generation measurement system. 1 Thermometer 2 Three-necked flask 3 Thermostatic bath 4 Mouth bottle 5 Graduated cylinder.
measured by a graduated cylinder. After the reaction completed, the catalysts were separated from residual solution by filtration and reserved. The specific hydrogen generation rate (ml min1 g1) for the catalysts was based on the volume of hydrogen generation, the usage time of the reaction and the amount of the catalysts excluding the weight of gAl2O3.
Results and discussion Catalyst characterization Fig. 2 shows the surface morphologies of the as-prepared CoeWeP catalysts on the g-Al2O3 in various electroless deposition time. The products electroless deposited for 240 s present sheet surfaces in Fig. 2(a). With the electroless deposition time increasing, the surface morphologies of the coatings change into granular surfaces in Fig. 2(bed). This may be due to the rapid release of a large amount of hydrogen gas during the electroless deposition process. One can see that the size of particles in Fig. 2(b) is similar with that in Fig. 2(c). But the amount of particles in Fig. 2(c) synthesized in longer electroless deposition time is larger than that in Fig. 2(b). Further increasing the depositing time, the particles in Fig. 2(d) become very large. Fig. 2(e) shows the XRD patterns of the as-prepared products and g-Al2O3 support. For the CoeWeP/g-Al2O3 catalysts, the diffraction peaks around 41.7 (100), 44.8 (002), 47.6 (101) and 75.9 (110) are assigned to the diffraction of Co in hexagonal phase. Fig. 2(f) shows EDS patterns of the products, and cobalt, tungsten and phosphorus could be detected. For further confirming the formation of CoeWeP/g-Al2O3, X-ray photoelectron spectroscopy (XPS) was also applied to the as-prepared product. In the Fig. 1(g), Co 2p3/2 peak at 777.8 eV stands for Co metal [33], which agrees well with the XRD results, as seen in Fig. 1(e). Oxidized Co species could be distinguished from metallic Co based on the higher energy of the 2p3/2 peaks located at 781.3 eV, 785.6 eV and 2p1/2 peaks located at 797.2 eV, 802.9 eV [34,35]. The detectable oxidized Co species could be ascribed to the surface Co combined with
Effect of NaOH concentration on hydrogen generation The influence of the NaOH concentration on hydrogen generation is shown in Fig. 3. It can be seen that the hydrogen generation rate increases with the NaOH concentration increasing, and reaches a maximum value at 10 wt.% NaOH, then decreases with further increasing in NaOH concentration. The results indicate that OH anions play dual roles during the hydrolysis of NaBH4. These findings are consistent with the reports on CoeB [4,17] or Co [11,41] catalysts. However, these results are different from those investigations on CoeMoePdeB [8], Ni-Ru [42] or Ru [43] catalysts. Therefore, the influence of NaOH concentration on NaBH4 hydrolysis greatly depends on the catalyst used in the reaction. Holbrook and Twist [44] were the first to postulate a working mechanism to account for the reaction kinetics observed, which is as follows: 2M þ BH 4 4M BH3 þ M H
(2)
M BH 3 4BH3 þ M þ e
(3)
BH3þOH / BH3(OH)
(4)
M þ e þ H2O / MeH þ OH
(5)
MeH þ MH / H2þ2M
(6)
We can see that OH is involved in the hydrolysis of NaBH4 and appropriate increase of NaOH concentration can be favor of accelerating the hydrolysis reaction of NaBH4 and enhance the hydrogen generation rate. However, it should be noticed that the OH anions and BH 4 anions are competitive adsorption on the surface of catalyst. When the concentration of NaOH increases further, more OH anions will occupy the adsorption sites to reduce BH 4 anions contacting with active species adequately [5]. Furthermore, excessive NaOH concentrations will also lead to more NaBO2 accumulated on the catalyst surface because of low solubility of NaBO2 under alkaline condition [23,45]. Consequently, the active sites on the catalyst would be blocked, the solution viscosity would increase and the mass transfer would be impeded. All of these disadvantageous factors would inhibit the hydrolysis of NaBH4. The inset of ln (HGR) versus ln [NaOH] reveals that the
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Fig. 2 e SEM images of CoeWeP/g-Al2O3 catalysts deposited for (a) 240s (b) 300s (c) 360s (d) 420s; XRD patterns of the asprepared CoeWeP/g-Al2O3 catalysts and g-Al2O3 support (e); EDS patterns of the CoeWeP coatings (f) and XPS spectra of Co 2p (g), W 4f (h), P 2p (i) for CoeWeP/g-Al2O3 catalysts.
slope of the straight line is 0.41, which proves that the increase of the NaOH concentration from 4 wt.% to 10 wt.% has a positive effect on hydrogen generation. Therefore, the optimal concentration of NaOH is chosen as 10 wt.% in the succeed experiments.
Effect of NaBH4 concentration on hydrogen generation
Fig. 3 e Effect of NaOH concentration on hydrogen generation (4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 4 gives the influence of NaBH4 concentration on the practical hydrogen generation performance. It is observed that the hydrogen generation rate drastically increases with increase in NaBH4 concentration and subsequently decreases with further increase in NaBH4 concentration. Many researchers [11,41] had found that the hydrogen generation rate decreases with higher NaBH4 concentration. It is because that BH-4 can be in good adsorption and desorption in the surface of catalysts at low concentration of sodium borohydride. But when the concentration of NaBH4 is higher than 4 wt.%, the solution viscosity rises. The formation of the reaction byproduct NaBO2 could significantly affect the mass transfer of NaBH4 to catalyst and lead to blockage of the catalytic sites
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specific surface area, which is 256 m2 g1 detected by nitrogen adsorption-desorption isotherm. This is mainly due to the difference in surface morphology and particle size of the asprepared CoeWeP/g-Al2O3 catalysts with different electroless deposition time in Fig. 2(aed). As determined by ICP-AES measurement, the CoeWeP loading is measured to be 0.81%, which contains 72.66 wt.% Co, 18.27 wt.% W and 8.27 wt.% P, corresponding to a formula of Co12.4W1P2.7. Therefore, the highest hydrogen generation rate can reach 1182 L min1 g1 catalyst .
Effect of CoSO4/Na2WO4 concentration ratio on hydrogen generation
Fig. 4 e Effect of NaBH4 concentration on hydrogen generation (10 wt.% NaOH, 45 C, 1.5 g catalyst).
during the reaction. The plot of ln (HGR) versus ln [NaBH4] shown in the inset of Fig. 4 reveals the negative effect of NaBH4 on hydrogen generation with concentrations higher than 4 wt.%, because the slope of the straight line is 0.55. Niu WL et al. [11] reported that the reaction order of NaBH4 hydrolysis catalyzed by the carbon supported cobalt catalyst is 0.30 with regard to the NaBH4 concentration from 0.25 wt.% to 1.00 wt.%. Vernekar AA et al. [46] found that hydrolysis reaction of NaBH4 with CoeCo2B nanocomposites or NieNi2B nanocomposites is zero order with respect to the concentration of NaBH4 from 0.05 M to 0.4 M. Herein, the kinetics of NaBH4 hydrolysis is not only dependent on NaBH4 concentration, but also on the type of the catalysts used for hydrogen generation.
Fig. 6 shows the effect of CoSO4/Na2WO4 concentration ratio in electroless bath on the hydrogen generation rate of the CoeWeP/g-Al2O3 catalysts. The corresponding element percentage of Co, W and P at three different regions on the catalyst surface has been tested and the average results are presented in Table 1 considering the measuring errors of EDS analysis. Interestingly, with decreasing CoSO4/Na2WO4 concentration ratio, the Co content in the CoeWeP alloy increases while the P content decreases. But the content of W appears to be less sensitive to the changing of CoSO4/Na2WO4 concentration ratio. According to Younes et al. [47], the electroless bath solution may contain only low concentration of [(Co) (WO4) (Cit) (H)]2, so the influence of CoSO4/Na2WO4 concentration ratio on the W content is very little. It is clearly seen from Fig. 6 that the hydrogen generation rate promotes apparently with decrease in CoSO4/Na2WO4 concentration ratio, subsequently decreases with further decrease in CoSO4/Na2WO4 concentration ratio. This may be due to that the P content in the catalysts decreases with decreasing CoSO4/Na2WO4 concentration ratio while Co/W atomic ratio increases.
Effect of electroless deposition time on hydrogen generation Effect of the amount of CoeWeP/g-Al2O3 catalysts used on hydrogen generation
The effect of electroless deposition time from 240 se420 s on hydrogen generation is presented in Fig. 5. It is found that the CoeWeP/g-Al2O3 catalysts electroless deposited in 360 s exhibit the highest hydrogen generation rate and have a large
The effect of the amount of CoeWeP/g-Al2O3 catalysts used on hydrogen generation performance was investigated by
Fig. 5 e Effect of electroless deposition time on hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 6 e Effect of different CoSO4/Na2WO4 concentration ratio in electroless bath on hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
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Table 1 e Composition of different CoeWeP coatings by EDS. CoSO4: Na2WO4 1:1 2:3 3:7 1:4
Co
W
P
Co/W
83.99 85.85 87.24 88.32
5.81 5.89 5.80 5.82
10.20 8.26 6.96 5.86
14.46 14.58 15.04 15.18
carrying out a series of experiments with initial amount of CoeWeP/g-Al2O3 from 0.5 g to 2.5 g and the result is shown in Fig. 7. It can be seen that the hydrogen generation rate increases apparently with increase in the amount of the catalysts used. Possibly, the more the CoeWeP/g-Al2O3 catalysts used, the more BH 4 ions were adsorbed [11]. The inset in Fig. 7 shows the plot of ln (HGR) versus ln (amount of catalysts) for the same reaction. The slope of the straight line is 0.32, which is between zero order kinetics and first order kinetics. Herein, the amount of catalysts has a positive effect on hydrogen generation.
Hydrogen generation kinetics of CoeWeP/g-Al2O3 catalysts Hydrolysis of sodium borohydride solution is sensitive to reaction temperature. As expected, the hydrogen generation rate increases with the elevating reaction temperature significantly, which can be seen in Fig. 8. The hydrogen generation rates were used to determine the activation energy (Ea) for the hydrolysis of NaBH4 in the presence of the CoeWeP/g-Al2O3 catalysts by the following Ea , where k is the reaction Arrhenius equation: lnk ¼ lnA e RT 1 1 gcatalyst ), R is the gas constant constant (mol min (8.314 J mol1 K1), T is the hydrolysis temperature and A is the pre-exponential factor. The Arrhenius plot of lnk versus 1/T is shown in Fig. 9 and the activation energy was calculated to be 49.58 kJ mol1, within the experimental errors. Compared with the activation energy values of different Co-based metal catalysts summarized in Table 2, the Ea of CoeWeP/g-Al2O3
Fig. 7 e Effect of the amount of CoeWeP/g-Al2O3 catalysts used on the hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
Fig. 8 e Effect of reaction temperature on the hydrogen generation (10 wt.% NaOH, 4 wt.% NaBH4, 1.5 g catalyst). (49.58 kJ mol1) is similar with that of reported CoeRueMoeB (48.8 kJ mol1) [5], CoeCueB (49.6 kJ mol1) [23], slightly higher than that of catalysts like: CoeNiePeB (29 kJ mol1) [20], CoeMoeB (39 kJ mol1) [22] and lower than that of catalysts like: CoeP (60 kJ mol1) [48], Raney Co (53.7 kJ mol1) [49]. The favorable activation energy together with the low cost make the CoeWeP/g-Al2O3 catalysts seem to be very preferable in practical application.
Reusability of CoeWeP/g-Al2O3 catalysts in the hydrolysis of sodium borohydride The cycling ability of the catalysts is crucial to the practical hydrogen generation application. In the present work, the CoeWeP catalysts supported on g-Al2O3 by electroless deposition were tested with respect to the reusability in the hydrolysis of sodium borohydride solution in the presence of 10 wt.% NaOH and 4 wt.% NaBH4 at 45 C. After each catalytic reaction, the used CoeWeP/g-Al2O3 catalysts were separated
Fig. 9 e Arrhenius plot obtained from the data in Fig. 8 for hydrogen generation from NaBH4 solution.
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Table 2 e Comparison of the key parameters for the NaBH4 hydrolysis reaction catalyzed by different Co-based metal catalysts. Co-based metal catalysts
Activation energy (kJ mol1)
Hydrogen generation rate ( mL min1 g1 catalyst )
Hydrolysis temperature ( C)
Ref.
CoeRueMoeB CoeMoePdeB NieCoeB CoeNiePeB CoeCreB CoeMoeB CoeCueB CoeNieB CoeFeeB CoeWeB CoeCueB CoeWeP CoeNieP/g-Al2O3 CoeCo2B CoeP Raney Co CoeWeB/Ni film CoeMneB CoeLaeZreB CoeWeP/g-Al2O3
48.8 36.36 62 29 37 39 30 34 31 41 49.6 22.8 52.05 35.245 60 53.7 29 52.1 60.06 49.58
8075 6023 2608 2400 3400 2875 2210 1175 1130 2570 4800 5000 6599.6 4300 3300 267.5 15,000 2500 1252 4000
25 25 28 25 25 25 25 25 25 25 30 30 55 30 30 20 30 30 50 30
[5] [8] [19] [20] [22] [22] [22] [22] [22] [22] [23] [26] [28] [46] [48] [49] [50] [51] [52] This work
from the solution and washed thoroughly with deionized water, then dried 10 h at 60 C in the oven. Fig. 10(a) presents the cycling performance of the CoeWeP/g-Al2O3 catalysts. The catalysts show good durability in cyclic usage. Even after six cycles, the hydrogen generation rate still reaches 7.90 L min1 g1 catalyst , which is slightly inferior to that in the previous five cycles. As determined by ICP-AES measurement, the element percentage of Co, W and P in the catalysts after 6 cycles was 85.41 wt.%, 12.38 wt.% and 2.21 wt.%, respectively. The element percentage of the catalysts before and after changed a little. Fig. 10(b) shows that the used CoeWeP/g-Al2O3 catalysts consist of two-dimensional nanosheets with a thickness of 5 nm. It can be concluded that the slight decrease in catalytic activity may be due to the physical changes and the decrease of W percentage on the CoeWeP/g-Al2O3 catalysts surface. Recently, the reported Co/Ni foam catalysts lose most of its catalytic activity after 3 cycles [53] and the CoeWeP catalysts supported on Cu
substrates lose 49% activity after 5 cycles [26]. Compared with those Co-based metal catalysts, the CoeWeP/g-Al2O3 catalysts exhibit significantly improved durability and reusability.
Conclusions In summary, CoeWeP/g-Al2O3 catalysts were prepared by electroless deposition and their catalytic activities in the hydrolysis of NaBH4 solution were investigated. The preparation and the hydrolysis of NaBH4 solution conditions have obvious effect on the hydrogen generation performance. The catalysts electroless deposited for 360 s with 3:7 CoSO4/Na2WO4 concentration ratios in electroless bath exhibit fine catalytic property, which have a great specific surface area of 256 m2 g1. The highest hydrogen generation rate can reach 11:82 L min1 g1 catalyst in the hydrolysis conditions of 10 wt.%
Fig. 10 e Reusability tests of the CoeWeP/g-Al2O3 catalysts in the NaBH4 hydrolysis at 45 C (a), SEM image of CoeWeP/gAl2O3 catalyst after 6 cycles (b) (10 wt.% NaOH, 4 wt.% NaBH4, 45 C, 1.5 g catalyst).
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NaOH, 4 wt.% NaBH4, 1.5 g catalyst, 45 C. The CoeWeP/gAl2O3 catalysts also exhibit favorable cycling performance and lower activation energy (49.58 kJ mol1) in comparison with that of other reported Co-based catalysts.
[16]
[17]
Acknowledgments The work was financially supported by the Natural Science Foundation of Shandong Province (Contracted No.ZR2011EMM005) and the key project of the National Natural Science Foundation of China (Contracted No.21136008).
[18]
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