- Email: [email protected]
Improved hydrogen generation from alkaline NaBH4 solution using carbon-supported Co–B as catalysts
International Journal of Hydrogen Energy 32 (2007) 4711 – 4716 www.elsevier.com/locate/ijhydene
Improved hydrogen generation from alkaline NaBH4 solution using carbon-supported Co.B as catalysts Jianzhi Zhao, Hua Ma, Jun Chen ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, PR China Received 1 December 2006; received in revised form 30 April 2007; accepted 1 July 2007 Available online 22 August 2007
Abstract Carbon-supported Co–B catalysts with various loading contents were prepared by impregnation–chemical reduction method. The XRD, ICP, SEM and TEM analyses revealed that the as-prepared Co–B catalysts were in amorphous form with the composition of Co2.0.3.3 B and the carbon-supported Co–B catalysts had a good dispersion and coating condition. The hydrogen generation measurement showed that the average hydrogen generation rate at 25 ◦ C was 1127.2 mL min−1 g−1 for unsupported Co–B catalyst, while it was 1268.1, 1482.1 and 2073.1 mL min−1 g−1 for the carbon-supported catalysts with the Co–B loading of 30.0, 15.6 and 7.44 wt%, respectively. The activation energy of the 30.0 wt% Co–B loading catalyst for the hydrogen generation reaction was measured to be 57.8 kJ mol−1 . Compared with the unsupported Co–B catalyst, the as-prepared carbon-supported catalysts presented higher activity for hydrolysis of NaBH4 aqueous solution, indicating their potential application in mobile hydrogen production for fuel cells. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Co–B catalyst; Hydrogen generation; NaBH4 ; Carbon support
1. Introduction Highly efficient hydrogen release and storage methods have attracted world-wide attention as hydrogen is a potential material to satisfy the increasing demand for efficient and clean energy. In the past few years, complex hydrides have been demonstrated to be promising hydrogen sources for portable application [1–3]. Among the hydrides, sodium borohydride (NaBH4 ) has attracted attention due to its high theoretical hydrogen content of 10.8 wt% and the excellent stability of its solution under high pH value at ambient temperature. Furthermore, high-purity hydrogen can be released controllably from the hydrolysis of NaBH4 alkaline solution in the presence of certain catalysts [4]. Therefore, the catalyst is the key factor that affects the hydrogen generation performance from hydrolysis of NaBH4 . Various catalysts have been developed for hydrogen generation from NaBH4 solutions. The traditional catalysts contain some precious metals such as Pt [2], Ru [5] and PtRu alloy [6], ∗ Corresponding author. Fax: +86 22 2350 9118.
E-mail address: [email protected] (J. Chen).
in which the high cost of the precious catalysts restricts their broad application. Recently, some low-cost metals and alloys, including LaNi4.5 T0.5 (T = Mn, Cr, Co, Fe, Cu, Al) [7], nickel and cobalt [8], nickel boride [9] and cobalt boride [10] have also been introduced to catalyze the hydrolysis of NaBH4 solution to generate pure hydrogen. However, the low-cost catalysts normally show relatively low activities compared with the precious catalysts. It is thus that improving the hydrogen generation activity of the low-cost catalyst is becoming imperative and necessary. As the activity of catalyst is directly related to its particle size and dispersion degree, the small particle size and well dispersion can make the catalysts contact with the reactant sufficiently, which is very important for increasing the reaction rate and saving the dosage of catalyst. Thus, the use of some supporting materials with large surface areas provides a potential route to achieve this point. Indeed, carbonsupported platinum shows superior catalytic activity for the hydrolysis of NaBH4 solution with the maximum hydrogen rate of 2309 mL−1 min−1 and approximately 100% efficiency [11]. In this paper, the carbon (Vulcan XC-72)-supported Co–B catalysts with different loading contents were prepared by the impregnation–chemical reduction method. The structure of the
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.004
4712
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
as-prepared catalysts, the effect of Co–B loading and solution temperature on hydrogen generation rate were discussed in detail. The results show that the carbon-supported Co–B catalysts can effectively increase the hydrogen generation rate. 2. Experimental 2.1. Preparation of Co–B catalysts All the reagents were of analytical grade and used without further purification. In a typical experiment, carbon-supported Co–B catalysts were prepared by the impregnation–chemical reduction method. Forty mL CoCl2 solution was first placed in a sealed three-neck flask. A certain amount of carbon black (Vulcan XC-72) was thrown into the solution and dispersed by ultrasonic vibration. The mixture was stirred vigorously for 30 min. Then, 0.1 M ethylenediamine solution was added into the mixture drop by drop in the ice water bath under stirring to complex with Co2+ ion. The molar ratio of ethylenediamine to CoCl2 was 1:1. After being stirred for another 30 min, a corresponding amount of fresh-prepared NaBH4 aqueous solution was dropped slowly into the mixture under nitrogen atmosphere. After about 1 h, the as-prepared product was collected by centrifugation, washed thoroughly with distilled water and anhydrous ethanol to remove residual ions, and finally dried in vacuum at 60 ◦ C for 12 h. Three different Co–B loading catalysts (30.0, 15.6 and 7.44 wt%) had been prepared by changing the added amount of reactants. The unsupported Co–B catalyst was prepared by a similar process without adding carbon black into the reactant solution. 2.2. Catalyst characterization The as-synthesized catalysts were characterized by powder X-ray diffraction (XRD, Rigaku D/max-2500 X-ray generator, Cu K radiation), inductively coupled plasma emission spectroscopy (ICP-9000, Thermo Jarrell-Ash Corp.), scanning electron microscopy (SEM, JEOL JSM-6700F field emission) and transmission electron microscopy (TEM, Philips Tecnai F20, 200 kV).
3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the XRD patterns of the carbon-supported catalysts with different Co–B loadings. All the XRD patterns always present two broad peaks at 2 = 25◦ and 45◦ . The peak at about 2 = 25◦ can be indexed to the Vulcan carbon, and the broad peak around 2 = 45◦ is caused by the amorphous form of the as-prepared Co–B [12]. This result indicates that both Co–B and carbon exist in the catalyst. The remaining broad peak around 2 = 33◦ may be attributed to cobalt oxide. Furthermore, it can be seen that the difference of peak intensity between carbon and Co–B decreases with increasing the Co–B content from 7.44 to 30.0 wt%. ICP analysis reveals that the atomic ratio of Co to B in the as-synthesized Co–B catalysts is in the range of 2.0–3.3. Fig. 2 shows the SEM images of the carbon black (Vulcan XC-72), as-synthesized unsupported catalyst and carbonsupported catalysts. All the samples are composed of small particles. It can be seen that the carbon-black particles with the size less than 100 nm dispersed in a large range (Fig. 2a). Compared with the morphology of carbon black, the unsupported Co–B catalyst contains larger, irregular and more aggregate particles with the size of 100.200 nm (Fig. 2b). Figs. 2c, d and e show the SEM images of the as-synthesized carbon-supported Co–B samples with the loading of 7.44, 15.6 and 30.0 wt%, respectively. It is noted that the degree of the particle size and aggregation of the carbon-supported Co–B increase with increasing the Co–B loading from 7.44 to 30.0 wt%. However, the large aggregate of Co–B particles have not been found in the carbon-supported samples. This means that the amorphous Co–B particles have been successfully coated on the carbon black and the particle size is decreased.
2.3. Hydrogen generation measurement In a typical experiment, 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH was placed in a sealed flask fitted with an outlet tube for collecting evolved H2 gas. The outlet tube exhaust was placed under an inverted, water-filled gas burette which was situated in a water-filled vessel. A certain amount of catalyst was added into the solution under mild stirring. The hydrolysis reaction was carried out at controlled temperatures. The volume of generated H2 was measured by the water displacement method. After the hydrolysis reaction completed, the residual solution was filtered and the catalyst was reserved. The specific hydrogen generation rate (mL min−1 g−1 ) for the assynthesized catalysts was based on the amount of Co–B, excluding the carbon black.
Fig. 1. XRD patterns of as-prepared carbon-supported catalysts with different Co–B loadings: (a) 7.44 wt%, (b) 15.6 wt%, (c) 30.0 wt%.
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
4713
Fig. 2. SEM images of (a) carbon black, (b) unsupported Co–B catalyst and carbon-supported Co–B catalysts with different loadings: (c) 7.44 wt%, (d) 15.6 wt%, (e) 30.0 wt%.
Fig. 3. TEM images of carbon-supported catalyst with 15.6 wt% Co–B loading.
TEM has been employed to take a further examination on the Co–B coating conditions of the as-prepared supported catalyst. Fig. 3 shows the TEM images of the 15.6 wt% Co–B/C catalyst in different magnification. As shown in Fig. 3a, the sample displays a good uniformity and dispersed morphology. The amorphous Co–B alloys are identified as the dark part on the light carbon supports (Fig. 3b). The size of the carbonsupported catalyst particles is around 40.50 nm. The brightness difference between the center and the edge of the nanoparticles indicates that the amorphous Co–B particles have a successful and excellent coating on the carbon supports. 3.2. Catalytic activities To test the catalytic activities of the as-prepared carbonsupported Co–B catalysts for hydrogen generation of NaBH4 solution, a series of experiments were carried out. In the first experiment, by using different amounts of the carbon-supported catalysts, the activities of catalysts with different Co–B loadings (7.44, 15.6 and 30.0 wt%) containing the same Co–B content (5 mg) were tested and compared with 5 mg unsupported amorphous Co–B catalyst at 25 ◦ C. Fig. 4 shows
Fig. 4. H2 volume generated as a function of time with carbon-supported Co–B catalysts and unsupported amorphous Co–B catalyst. The reactant is 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. The Co–B loading: (a) 7.44 wt%, (b) 15.6 wt%, (c) 30.0 wt%, (d) unsupported Co–B. The Co–B content: 5 mg.
4714
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
Table 1 Hydrogen generation performance of the unsupported Co–B and carbon-supported Co–B catalysts in the first experiment at 25 ◦ C Sample
H2 generation rate (mL min−1 g−1 )a
Total H2 generation volume (mL)
Reaction time (min)
Generation efficiency
Unsupported 7.44 wt% 15.6 wt% 30.0 wt%
1127.2 2073.1 1482.1 1268.1
155.3 180.0 170.2 158.8
33 19 25 27
80.7 92.0 87.0 81.1
b
(%)
a Mass b The
specific activity per 1 g Co–B in this paper. theoretic total generation volume of H2 is 195.7 mL under the experimental condition.
the hydrogen generation performances of the carbon-supported catalysts and unsupported catalyst. As presented in Fig. 4, for all the catalysts, the generated hydrogen volume is almost linearly proportional to the reaction time, indicating the stable catalytic activity of the catalysts. In the last minutes before the reaction finish, the rate of H2 generation decreases with time due to the extremely low concentration of NaBH4 . It is clear that the carbon-supported Co–B catalysts have higher catalytic activities for hydrogen generation than the unsupported one despite containing the same Co–B content. The average hydrogen generation rate was calculated to be 1268.1, 1482.1, 2073.1 and 1127.2 mL min−1 g−1 for 30.0, 15.6 and 7.44 wt% Co–B loading catalysts and unsupported Co–B catalyst. The reason is that the supporting carbon can improve the dispersion of the catalysts efficiently to make more active sites contact with the reactant sufficiently. As to the carbon-supported catalysts, the rate of hydrogen generation is accelerated with decreasing Co–B loading, while the hydrogen generation efficiency has the same tendency. For achieving the same Co–B content, the amount of the supported materials used for hydrogen generation reaction will be more. It means that, as the Co–B loading decreases from 30.0 to 7.44 wt%, the same amount of catalyst will be dispersed on a larger supporting surface due to the increasing content of carbon. Thus, more and more catalysts will become active sites for the reaction. As seen from Fig. 4 the catalyst owning higher activity can also improve the reaction efficiency and promote the total H2 generation volume. The hydrogen generation rate, total H2 generation volume, reaction time and production efficiency are listed in Table 1. Fig. 5 compares the hydrogen generation performances of 20 mg carbon-supported catalysts with different Co–B loadings (7.44, 15.6 and 30.0 wt%) in initial 12 min. Opposite to the result of the first experiment, the rate of hydrogen generation is accelerated with increasing Co–B loading. As the used amount of catalyst is identical, less Co–B disperse on a large supporting surface for the low-Co–B-loading catalyst. Since the Co–B loading is relatively low, the active sites are not enough to drive the reaction finish in a short time, and result in a low hydrogen generation rate. Thus, the Co–B content is the main influential fact of the hydrogen generation rate in this experiment. It is noticed that, for the catalyst with 30.0 wt% Co–B loading (6 mg Co–B in 20 mg Co–B/C), the hydrogen generation rate increases from 1268.1 to 2319.0 mL min−1 g−1 compared with the result of the first experiment. The probable reason may be that increasing the amount of catalyst without changing the
Fig. 5. H2 volume generated as a function of time with 20 mg carbon-supported Co–B catalysts at 25 ◦ C. The reactant is 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. The Co–B loadings: (a) 30.0 wt%, (b) 15.6 wt%, (c) 7.44 wt%.
Fig. 6. Temperature effect on hydrogen generation rate using 20 mg carbon-supported catalyst with 30.0 wt% Co–B loading in 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. (a) 25 ◦ C, (b) 30 ◦ C, (c) 35 ◦ C, (d) 40 ◦ C.
volume of reactant makes more Co–B become the active sites well touching with the reactants, and improves the utilization of catalyst. For the same reason, the reduction amount of Co–B
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
4715
Table 2 Rate constants and H2 generation rate for the hydrolysis of NaBH4 catalyzed by 20 mg carbon-supported catalyst with 30.0 wt% Co–B loading in 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH at different temperatures Temperature (◦ C)
H2 generation rate (mL min−1 g−1 )
Rate constant (mol min−1 g−1 )
25 30 35 40
2319.1 3903.2 5762.0 7020.0
0.096 0.162 0.240 0.292
as reaction process prolonged, indicating the zero order reaction kinetics of NaBH4 hydrolysis. The hydrogen generation rates and rate constant k at various temperatures are listed in Table 2. Fig. 7 shows the Arrhenius plot, ln k versus the reciprocal absolute temperature (1/T ). The slope of the straight line gives activation energy of 57.8 kJ mol−1 , which is lower than the previously reported value, 68.9 kJ mol−1 , for Co–B catalyst [13] and a little higher than the value of precious catalyst, 56 kJ mol−1 , for Ru supported on IRA-400 [3]. Compared with the activation energies found for NaBH4 hydrolysis catalyzed with other catalysts (75 kJ mol−1 for Co, 71 kJ mol−1 for Ni, 63 kJ mol−1 for Raney Ni [14]), the as-prepared Co–B/C catalyst shows a lower value, indicating higher catalytic activities for hydrogen generation from hydrolysis of NaBH4 . Fig. 7. The Arrhenius plot, ln k versus the reciprocal absolute temperature 1/T , in the temperature range of 298.313 K.
content in the used catalysts makes the catalytic activities of the carbon-supported catalysts with 15.6 and 7.44 wt% Co–B loading have a little reduction. Though the promotion of Co–B catalyst concentration will increase the hydrogen generation rate and volume, there is no directly proportional relation in them according to the results obtained in the two experiments. Further effect of the catalyst dispersion and coating condition on the hydrogen generation is needed to be studied in the future. From the results of Figs. 4 and 5 we can conclude that: (1) if the amount of the active material Co–B is fixed, the catalyst with the low Co–B loading shows the high catalytic efficiency because increasing the dispersion degree of Co–B can lead to more active sites to react with the reactant. (2) If the total amount of the carbon-supported Co–B catalyst used for the reaction is fixed, the actual content of Co–B will be the key influential fact of the reaction rate. The catalyst with the high Co–B loading exhibits an improved catalytic activity. Temperature effect on hydrogen generation rate using 20 mg as-prepared catalyst with 30.0 wt% Co–B loading is given in Fig. 6. As expected, the rate of hydrogen generation rises dramatically with the increase in temperature and shows a nearly linear relationship with reaction time. The hydrogen generated from hydrolysis of NaBH4 in different temperatures has the same total volume (about 172 mL). The reaction rate was found to be almost unchanged when NaBH4 concentration decreased
4. Conclusion In summary, carbon-supported Co–B catalysts with different loadings have been synthesized via impregnation–chemical reduction method. The as-synthesized carbon-supported Co–B catalysts show higher catalytic activity for hydrogen generation compared with the unsupported Co–B catalyst due to their reduced particle size and the well-dispersed characteristic, which make more active sites well touch the reactants. The improved hydrogen generation rate, hydrogen generation efficiency and the lower activation energy indicate that the carbon-supported Co–B is a promising catalyst for hydrolysis of NaBH4 . Acknowledgment This work was supported by the National NSFC (50631020), 863 (2006AA05Z130) and 973 (2005CB623607) project. References [1] Züttle A, Wenger P, Rentsch S, Sudan P, Mauron P, Emmenegger C. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1. [2] Kojima Y, Suzuki K, Fukumoto K, Sakaki M, Yamamoto T, Kawai Y. et al. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27:1029. [3] 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;25:969. [4] Schlesinger HI, Brown HC, Finbolr AE, Gilbreath JR, Hockstra HR, Hyde EK. Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen. J Am Chem Soc 1953;75:215.
4716
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
[5] Özkar S, Zahmakıran M. Hydrogen generation from hydrolysis of sodium borohydride using Ru(0) nanoclusters as catalyst. J Alloys Compd 2005;404–406:728. [6] Krishnan P, Yang T-H, Lee W-Y, Kim C-S. PtRu-LiCoO2 —an efficient catalyst for hydrogen generation from sodium borohydride solutions. J Power Sources 2005;143:17. [7] Korobov II, Mozgina NG, Blinova LN. Effect of intermetallic LaNi[4.5] T[0.5] (T = Mn, Cr, Co, Fe, Cu, Al) compounds on the hydrolysis of sodium borohydride in alkaline solutions. Kinet Catal 1995;48:380. [8] Liu B, Li Z, Suda S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloys Compd 2006;415:288. [9] Dong H, Yang H, Ai X, Cha C. Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. Int J Hydrogen Energy 2003;28:1095.
[10] Wu C, Wu F, Bai Y, Yi B, Zhang H. Cobalt boride catalysts for hydrogen generation from alkaline NaBH4 solution. Mater Lett 2005;59:1748. [11] Wu C, Zhang H, Yi B. Hydrogen generation from catalytic hydrolysis of sodium borohydride for proton exchange membrane fuel cells. Catal Today 2004;93–95:477. [12] Glavee GN, Klabunde KJ, Sorensen CM, Hadjapanayis GC. Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates. Langmuir 1992;8:771. [13] Jeong SU, Kim RK, Cho EA, Kim H-J, Nam S-W, Oh I-H. et al. A study on hydrogen generation from NaBH4 solution using the highperformance Co-B catalyst. J Power Sources 2005;144:129. [14] Kaufman CM, Sen B. Hydrogen generation by hydrolysis of sodium tetrahydroborate: effects of acids and transition metals and their salts. J Chem Soc Dalton Trans 1985; 307.
Improved hydrogen generation from alkaline NaBH4 solution using carbon-supported Co.B as catalysts Jianzhi Zhao, Hua Ma, Jun Chen ∗ Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, PR China Received 1 December 2006; received in revised form 30 April 2007; accepted 1 July 2007 Available online 22 August 2007
Abstract Carbon-supported Co–B catalysts with various loading contents were prepared by impregnation–chemical reduction method. The XRD, ICP, SEM and TEM analyses revealed that the as-prepared Co–B catalysts were in amorphous form with the composition of Co2.0.3.3 B and the carbon-supported Co–B catalysts had a good dispersion and coating condition. The hydrogen generation measurement showed that the average hydrogen generation rate at 25 ◦ C was 1127.2 mL min−1 g−1 for unsupported Co–B catalyst, while it was 1268.1, 1482.1 and 2073.1 mL min−1 g−1 for the carbon-supported catalysts with the Co–B loading of 30.0, 15.6 and 7.44 wt%, respectively. The activation energy of the 30.0 wt% Co–B loading catalyst for the hydrogen generation reaction was measured to be 57.8 kJ mol−1 . Compared with the unsupported Co–B catalyst, the as-prepared carbon-supported catalysts presented higher activity for hydrolysis of NaBH4 aqueous solution, indicating their potential application in mobile hydrogen production for fuel cells. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Co–B catalyst; Hydrogen generation; NaBH4 ; Carbon support
1. Introduction Highly efficient hydrogen release and storage methods have attracted world-wide attention as hydrogen is a potential material to satisfy the increasing demand for efficient and clean energy. In the past few years, complex hydrides have been demonstrated to be promising hydrogen sources for portable application [1–3]. Among the hydrides, sodium borohydride (NaBH4 ) has attracted attention due to its high theoretical hydrogen content of 10.8 wt% and the excellent stability of its solution under high pH value at ambient temperature. Furthermore, high-purity hydrogen can be released controllably from the hydrolysis of NaBH4 alkaline solution in the presence of certain catalysts [4]. Therefore, the catalyst is the key factor that affects the hydrogen generation performance from hydrolysis of NaBH4 . Various catalysts have been developed for hydrogen generation from NaBH4 solutions. The traditional catalysts contain some precious metals such as Pt [2], Ru [5] and PtRu alloy [6], ∗ Corresponding author. Fax: +86 22 2350 9118.
E-mail address: [email protected] (J. Chen).
in which the high cost of the precious catalysts restricts their broad application. Recently, some low-cost metals and alloys, including LaNi4.5 T0.5 (T = Mn, Cr, Co, Fe, Cu, Al) [7], nickel and cobalt [8], nickel boride [9] and cobalt boride [10] have also been introduced to catalyze the hydrolysis of NaBH4 solution to generate pure hydrogen. However, the low-cost catalysts normally show relatively low activities compared with the precious catalysts. It is thus that improving the hydrogen generation activity of the low-cost catalyst is becoming imperative and necessary. As the activity of catalyst is directly related to its particle size and dispersion degree, the small particle size and well dispersion can make the catalysts contact with the reactant sufficiently, which is very important for increasing the reaction rate and saving the dosage of catalyst. Thus, the use of some supporting materials with large surface areas provides a potential route to achieve this point. Indeed, carbonsupported platinum shows superior catalytic activity for the hydrolysis of NaBH4 solution with the maximum hydrogen rate of 2309 mL−1 min−1 and approximately 100% efficiency [11]. In this paper, the carbon (Vulcan XC-72)-supported Co–B catalysts with different loading contents were prepared by the impregnation–chemical reduction method. The structure of the
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.004
4712
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
as-prepared catalysts, the effect of Co–B loading and solution temperature on hydrogen generation rate were discussed in detail. The results show that the carbon-supported Co–B catalysts can effectively increase the hydrogen generation rate. 2. Experimental 2.1. Preparation of Co–B catalysts All the reagents were of analytical grade and used without further purification. In a typical experiment, carbon-supported Co–B catalysts were prepared by the impregnation–chemical reduction method. Forty mL CoCl2 solution was first placed in a sealed three-neck flask. A certain amount of carbon black (Vulcan XC-72) was thrown into the solution and dispersed by ultrasonic vibration. The mixture was stirred vigorously for 30 min. Then, 0.1 M ethylenediamine solution was added into the mixture drop by drop in the ice water bath under stirring to complex with Co2+ ion. The molar ratio of ethylenediamine to CoCl2 was 1:1. After being stirred for another 30 min, a corresponding amount of fresh-prepared NaBH4 aqueous solution was dropped slowly into the mixture under nitrogen atmosphere. After about 1 h, the as-prepared product was collected by centrifugation, washed thoroughly with distilled water and anhydrous ethanol to remove residual ions, and finally dried in vacuum at 60 ◦ C for 12 h. Three different Co–B loading catalysts (30.0, 15.6 and 7.44 wt%) had been prepared by changing the added amount of reactants. The unsupported Co–B catalyst was prepared by a similar process without adding carbon black into the reactant solution. 2.2. Catalyst characterization The as-synthesized catalysts were characterized by powder X-ray diffraction (XRD, Rigaku D/max-2500 X-ray generator, Cu K radiation), inductively coupled plasma emission spectroscopy (ICP-9000, Thermo Jarrell-Ash Corp.), scanning electron microscopy (SEM, JEOL JSM-6700F field emission) and transmission electron microscopy (TEM, Philips Tecnai F20, 200 kV).
3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the XRD patterns of the carbon-supported catalysts with different Co–B loadings. All the XRD patterns always present two broad peaks at 2 = 25◦ and 45◦ . The peak at about 2 = 25◦ can be indexed to the Vulcan carbon, and the broad peak around 2 = 45◦ is caused by the amorphous form of the as-prepared Co–B [12]. This result indicates that both Co–B and carbon exist in the catalyst. The remaining broad peak around 2 = 33◦ may be attributed to cobalt oxide. Furthermore, it can be seen that the difference of peak intensity between carbon and Co–B decreases with increasing the Co–B content from 7.44 to 30.0 wt%. ICP analysis reveals that the atomic ratio of Co to B in the as-synthesized Co–B catalysts is in the range of 2.0–3.3. Fig. 2 shows the SEM images of the carbon black (Vulcan XC-72), as-synthesized unsupported catalyst and carbonsupported catalysts. All the samples are composed of small particles. It can be seen that the carbon-black particles with the size less than 100 nm dispersed in a large range (Fig. 2a). Compared with the morphology of carbon black, the unsupported Co–B catalyst contains larger, irregular and more aggregate particles with the size of 100.200 nm (Fig. 2b). Figs. 2c, d and e show the SEM images of the as-synthesized carbon-supported Co–B samples with the loading of 7.44, 15.6 and 30.0 wt%, respectively. It is noted that the degree of the particle size and aggregation of the carbon-supported Co–B increase with increasing the Co–B loading from 7.44 to 30.0 wt%. However, the large aggregate of Co–B particles have not been found in the carbon-supported samples. This means that the amorphous Co–B particles have been successfully coated on the carbon black and the particle size is decreased.
2.3. Hydrogen generation measurement In a typical experiment, 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH was placed in a sealed flask fitted with an outlet tube for collecting evolved H2 gas. The outlet tube exhaust was placed under an inverted, water-filled gas burette which was situated in a water-filled vessel. A certain amount of catalyst was added into the solution under mild stirring. The hydrolysis reaction was carried out at controlled temperatures. The volume of generated H2 was measured by the water displacement method. After the hydrolysis reaction completed, the residual solution was filtered and the catalyst was reserved. The specific hydrogen generation rate (mL min−1 g−1 ) for the assynthesized catalysts was based on the amount of Co–B, excluding the carbon black.
Fig. 1. XRD patterns of as-prepared carbon-supported catalysts with different Co–B loadings: (a) 7.44 wt%, (b) 15.6 wt%, (c) 30.0 wt%.
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
4713
Fig. 2. SEM images of (a) carbon black, (b) unsupported Co–B catalyst and carbon-supported Co–B catalysts with different loadings: (c) 7.44 wt%, (d) 15.6 wt%, (e) 30.0 wt%.
Fig. 3. TEM images of carbon-supported catalyst with 15.6 wt% Co–B loading.
TEM has been employed to take a further examination on the Co–B coating conditions of the as-prepared supported catalyst. Fig. 3 shows the TEM images of the 15.6 wt% Co–B/C catalyst in different magnification. As shown in Fig. 3a, the sample displays a good uniformity and dispersed morphology. The amorphous Co–B alloys are identified as the dark part on the light carbon supports (Fig. 3b). The size of the carbonsupported catalyst particles is around 40.50 nm. The brightness difference between the center and the edge of the nanoparticles indicates that the amorphous Co–B particles have a successful and excellent coating on the carbon supports. 3.2. Catalytic activities To test the catalytic activities of the as-prepared carbonsupported Co–B catalysts for hydrogen generation of NaBH4 solution, a series of experiments were carried out. In the first experiment, by using different amounts of the carbon-supported catalysts, the activities of catalysts with different Co–B loadings (7.44, 15.6 and 30.0 wt%) containing the same Co–B content (5 mg) were tested and compared with 5 mg unsupported amorphous Co–B catalyst at 25 ◦ C. Fig. 4 shows
Fig. 4. H2 volume generated as a function of time with carbon-supported Co–B catalysts and unsupported amorphous Co–B catalyst. The reactant is 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. The Co–B loading: (a) 7.44 wt%, (b) 15.6 wt%, (c) 30.0 wt%, (d) unsupported Co–B. The Co–B content: 5 mg.
4714
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
Table 1 Hydrogen generation performance of the unsupported Co–B and carbon-supported Co–B catalysts in the first experiment at 25 ◦ C Sample
H2 generation rate (mL min−1 g−1 )a
Total H2 generation volume (mL)
Reaction time (min)
Generation efficiency
Unsupported 7.44 wt% 15.6 wt% 30.0 wt%
1127.2 2073.1 1482.1 1268.1
155.3 180.0 170.2 158.8
33 19 25 27
80.7 92.0 87.0 81.1
b
(%)
a Mass b The
specific activity per 1 g Co–B in this paper. theoretic total generation volume of H2 is 195.7 mL under the experimental condition.
the hydrogen generation performances of the carbon-supported catalysts and unsupported catalyst. As presented in Fig. 4, for all the catalysts, the generated hydrogen volume is almost linearly proportional to the reaction time, indicating the stable catalytic activity of the catalysts. In the last minutes before the reaction finish, the rate of H2 generation decreases with time due to the extremely low concentration of NaBH4 . It is clear that the carbon-supported Co–B catalysts have higher catalytic activities for hydrogen generation than the unsupported one despite containing the same Co–B content. The average hydrogen generation rate was calculated to be 1268.1, 1482.1, 2073.1 and 1127.2 mL min−1 g−1 for 30.0, 15.6 and 7.44 wt% Co–B loading catalysts and unsupported Co–B catalyst. The reason is that the supporting carbon can improve the dispersion of the catalysts efficiently to make more active sites contact with the reactant sufficiently. As to the carbon-supported catalysts, the rate of hydrogen generation is accelerated with decreasing Co–B loading, while the hydrogen generation efficiency has the same tendency. For achieving the same Co–B content, the amount of the supported materials used for hydrogen generation reaction will be more. It means that, as the Co–B loading decreases from 30.0 to 7.44 wt%, the same amount of catalyst will be dispersed on a larger supporting surface due to the increasing content of carbon. Thus, more and more catalysts will become active sites for the reaction. As seen from Fig. 4 the catalyst owning higher activity can also improve the reaction efficiency and promote the total H2 generation volume. The hydrogen generation rate, total H2 generation volume, reaction time and production efficiency are listed in Table 1. Fig. 5 compares the hydrogen generation performances of 20 mg carbon-supported catalysts with different Co–B loadings (7.44, 15.6 and 30.0 wt%) in initial 12 min. Opposite to the result of the first experiment, the rate of hydrogen generation is accelerated with increasing Co–B loading. As the used amount of catalyst is identical, less Co–B disperse on a large supporting surface for the low-Co–B-loading catalyst. Since the Co–B loading is relatively low, the active sites are not enough to drive the reaction finish in a short time, and result in a low hydrogen generation rate. Thus, the Co–B content is the main influential fact of the hydrogen generation rate in this experiment. It is noticed that, for the catalyst with 30.0 wt% Co–B loading (6 mg Co–B in 20 mg Co–B/C), the hydrogen generation rate increases from 1268.1 to 2319.0 mL min−1 g−1 compared with the result of the first experiment. The probable reason may be that increasing the amount of catalyst without changing the
Fig. 5. H2 volume generated as a function of time with 20 mg carbon-supported Co–B catalysts at 25 ◦ C. The reactant is 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. The Co–B loadings: (a) 30.0 wt%, (b) 15.6 wt%, (c) 7.44 wt%.
Fig. 6. Temperature effect on hydrogen generation rate using 20 mg carbon-supported catalyst with 30.0 wt% Co–B loading in 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH. (a) 25 ◦ C, (b) 30 ◦ C, (c) 35 ◦ C, (d) 40 ◦ C.
volume of reactant makes more Co–B become the active sites well touching with the reactants, and improves the utilization of catalyst. For the same reason, the reduction amount of Co–B
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
4715
Table 2 Rate constants and H2 generation rate for the hydrolysis of NaBH4 catalyzed by 20 mg carbon-supported catalyst with 30.0 wt% Co–B loading in 10 mL 0.2 M NaBH4 solution containing 20 mmol NaOH at different temperatures Temperature (◦ C)
H2 generation rate (mL min−1 g−1 )
Rate constant (mol min−1 g−1 )
25 30 35 40
2319.1 3903.2 5762.0 7020.0
0.096 0.162 0.240 0.292
as reaction process prolonged, indicating the zero order reaction kinetics of NaBH4 hydrolysis. The hydrogen generation rates and rate constant k at various temperatures are listed in Table 2. Fig. 7 shows the Arrhenius plot, ln k versus the reciprocal absolute temperature (1/T ). The slope of the straight line gives activation energy of 57.8 kJ mol−1 , which is lower than the previously reported value, 68.9 kJ mol−1 , for Co–B catalyst [13] and a little higher than the value of precious catalyst, 56 kJ mol−1 , for Ru supported on IRA-400 [3]. Compared with the activation energies found for NaBH4 hydrolysis catalyzed with other catalysts (75 kJ mol−1 for Co, 71 kJ mol−1 for Ni, 63 kJ mol−1 for Raney Ni [14]), the as-prepared Co–B/C catalyst shows a lower value, indicating higher catalytic activities for hydrogen generation from hydrolysis of NaBH4 . Fig. 7. The Arrhenius plot, ln k versus the reciprocal absolute temperature 1/T , in the temperature range of 298.313 K.
content in the used catalysts makes the catalytic activities of the carbon-supported catalysts with 15.6 and 7.44 wt% Co–B loading have a little reduction. Though the promotion of Co–B catalyst concentration will increase the hydrogen generation rate and volume, there is no directly proportional relation in them according to the results obtained in the two experiments. Further effect of the catalyst dispersion and coating condition on the hydrogen generation is needed to be studied in the future. From the results of Figs. 4 and 5 we can conclude that: (1) if the amount of the active material Co–B is fixed, the catalyst with the low Co–B loading shows the high catalytic efficiency because increasing the dispersion degree of Co–B can lead to more active sites to react with the reactant. (2) If the total amount of the carbon-supported Co–B catalyst used for the reaction is fixed, the actual content of Co–B will be the key influential fact of the reaction rate. The catalyst with the high Co–B loading exhibits an improved catalytic activity. Temperature effect on hydrogen generation rate using 20 mg as-prepared catalyst with 30.0 wt% Co–B loading is given in Fig. 6. As expected, the rate of hydrogen generation rises dramatically with the increase in temperature and shows a nearly linear relationship with reaction time. The hydrogen generated from hydrolysis of NaBH4 in different temperatures has the same total volume (about 172 mL). The reaction rate was found to be almost unchanged when NaBH4 concentration decreased
4. Conclusion In summary, carbon-supported Co–B catalysts with different loadings have been synthesized via impregnation–chemical reduction method. The as-synthesized carbon-supported Co–B catalysts show higher catalytic activity for hydrogen generation compared with the unsupported Co–B catalyst due to their reduced particle size and the well-dispersed characteristic, which make more active sites well touch the reactants. The improved hydrogen generation rate, hydrogen generation efficiency and the lower activation energy indicate that the carbon-supported Co–B is a promising catalyst for hydrolysis of NaBH4 . Acknowledgment This work was supported by the National NSFC (50631020), 863 (2006AA05Z130) and 973 (2005CB623607) project. References [1] Züttle A, Wenger P, Rentsch S, Sudan P, Mauron P, Emmenegger C. LiBH4 a new hydrogen storage material. J Power Sources 2003;118:1. [2] Kojima Y, Suzuki K, Fukumoto K, Sakaki M, Yamamoto T, Kawai Y. et al. Hydrogen generation using sodium borohydride solution and metal catalyst coated on metal oxide. Int J Hydrogen Energy 2002;27:1029. [3] 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;25:969. [4] Schlesinger HI, Brown HC, Finbolr AE, Gilbreath JR, Hockstra HR, Hyde EK. Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen. J Am Chem Soc 1953;75:215.
4716
J. Zhao et al. / International Journal of Hydrogen Energy 32 (2007) 4711 – 4716
[5] Özkar S, Zahmakıran M. Hydrogen generation from hydrolysis of sodium borohydride using Ru(0) nanoclusters as catalyst. J Alloys Compd 2005;404–406:728. [6] Krishnan P, Yang T-H, Lee W-Y, Kim C-S. PtRu-LiCoO2 —an efficient catalyst for hydrogen generation from sodium borohydride solutions. J Power Sources 2005;143:17. [7] Korobov II, Mozgina NG, Blinova LN. Effect of intermetallic LaNi[4.5] T[0.5] (T = Mn, Cr, Co, Fe, Cu, Al) compounds on the hydrolysis of sodium borohydride in alkaline solutions. Kinet Catal 1995;48:380. [8] Liu B, Li Z, Suda S. Nickel- and cobalt-based catalysts for hydrogen generation by hydrolysis of borohydride. J Alloys Compd 2006;415:288. [9] Dong H, Yang H, Ai X, Cha C. Hydrogen production from catalytic hydrolysis of sodium borohydride solution using nickel boride catalyst. Int J Hydrogen Energy 2003;28:1095.
[10] Wu C, Wu F, Bai Y, Yi B, Zhang H. Cobalt boride catalysts for hydrogen generation from alkaline NaBH4 solution. Mater Lett 2005;59:1748. [11] Wu C, Zhang H, Yi B. Hydrogen generation from catalytic hydrolysis of sodium borohydride for proton exchange membrane fuel cells. Catal Today 2004;93–95:477. [12] Glavee GN, Klabunde KJ, Sorensen CM, Hadjapanayis GC. Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates. Langmuir 1992;8:771. [13] Jeong SU, Kim RK, Cho EA, Kim H-J, Nam S-W, Oh I-H. et al. A study on hydrogen generation from NaBH4 solution using the highperformance Co-B catalyst. J Power Sources 2005;144:129. [14] Kaufman CM, Sen B. Hydrogen generation by hydrolysis of sodium tetrahydroborate: effects of acids and transition metals and their salts. J Chem Soc Dalton Trans 1985; 307.