Hydrogen generation from sodium borohydride solution using a ruthenium supported on graphite catalyst

Transcription

1 international journal of hydrogen energy 35 (2010) Available at journal homepage: Hydrogen generation from sodium borohydride solution using a ruthenium supported on graphite catalyst Yan Liang, Hong-Bin Dai, Lai-Peng Ma, Ping Wang*, Hui-Ming Cheng Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang , PR China article info Article history: Received 30 April 2009 Accepted 6 July 2009 Available online 31 July 2009 Keywords: Sodium borohydride Hydrogen generation Ruthenium/graphite catalyst Modified impregnation method abstract The catalyst with high activity and durability plays a crucial role in the hydrogen generation systems for the portable fuel cell generators. In the present study, a ruthenium supported on graphite catalyst (Ru/G) for hydrogen generation from sodium borohydride (NaBH 4 ) solution is prepared by a modified impregnation method. This is done by surface pretreatment with NH 2 functionalization via silanization, followed by adsorption of Ru (III) ion onto the surface, and then reduced by a reducing agent. The obtained catalyst is characterized by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Very uniform Ru nanoparticles with sizes of about 10 nm are chemically bonded on the graphite surface. The hydrolysis kinetics measurements show that the concentrations of NaBH 4 and NaOH all exert considerable influence on the catalytic activity of Ru/G catalyst towards the hydrolysis reaction of NaBH 4. A hydrogen generation rate of 32.3 L min 1 g 1 (Ru) in a 10 wt.% NaBH 4 þ 5 wt.% NaOH solution has been achieved, which is comparable to other noble catalysts that have been reported. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. 1. Introduction As a green energy, hydrogen has been recognized as an attractive alternative to fossil fuels. However, it has not become widely available and economical because of some critical fundamental challenges on hydrogen storage and transport. The hydrogen storage has been recognized as a key technical challenge in commercialization of the portable fuel cell generators [1]. In comparison with the various reversible hydrogen storage materials, irreversible hydrogen storage via hydrolysis of chemical hydrides is gaining an increasing attention. Among the chemical hydrides of interest, sodium borohydride (NaBH 4 ) receives the most extensive studies owing to its combined advantages of high hydrogen capacity (with a theoretical value of 10.8 wt.%), good storability and reaction controllability, low reaction-initiating temperature and the environmentally benign hydrolysis product (borax, NaBO 2 ) [2]. Base-stabilized NaBH 4 solution can controllably hydrolyze to generate hydrogen when contacting with appropriate catalysts, therefore, an effective catalyst is the heart of hydrolysis reaction of NaBH 4 solution. Besides acid accelerators, a number of transition metals, their alloys or their salts have been identified to be effective for accelerating the hydrolysis of NaBH 4, including noble and non-noble transition metals, such as Ru, Pt, Pd, Pt Ru, Pt Pd alloys, Co and Ni borides [3 17]. Among these catalysts, Ru has been widely used for hydrogen generation (HG) from the catalytic hydrolysis of NaBH 4 due to its excellent catalytic activity. Compared to metal powdered catalysts and their salts, the supported catalysts are highly appreciated in the practical applications owing to preventing the aggregation of catalyst particles into * Corresponding author. Tel.: þ ; fax: þ address: pingwang@imr.ac.cn (P. Wang) /$ see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi: /j.ijhydene

2 3024 international journal of hydrogen energy 35 (2010) larger particles, which leads to a decrease in catalytic activity and lifetime. The general method used in preparation of the supported catalysts is the surface impregnation followed by reduction treatment using either reducing solvents or hydrogen [4,10]. Although this method is well established, a durable strong adhesion between the catalyst and the support still remains a challenge. The ability of aminofunctional silane to improve adhesion on inorganic materials is well known [18,19]. It is typically used as a pretreatment technique for the modification of the adhesion between the adsorbent and the support [20,21]. In the present study, a Ru supported on graphite powder (Ru/G) catalyst is synthesized by a modified impregnation method. This is done by surface pretreatment with NH 2 functionalization via silanization, followed by adsorption of Ru (III) ion onto the surface, and then reduced by a reducing agent. This chemically bonded catalyst exhibits highly catalytic performance and better durability towards the hydrolysis reaction of NaBH Experimental 2.1. Preparation of Ru/G catalyst A graphite powder was selected as the catalyst support material for its low density, high thermal and chemical stability under the hydrolysis conditions. The graphite powder (99.9% purity) had an average particle size of 30 mm and a density of 2.10 g cm 3. The silane coupling agent was g- aminopropyltriethoxysilane (APTES, NH 2 (CH 2 ) 3 Si(OC 2 H 5 ) 3 ). The chemical reagents used for preparation the Ru/G catalyst were all of analytical grade and used as received. The Ru/G catalyst was prepared by a modified impregnation method as presented in Scheme 1. To increase the number of OH groups on the graphite powder, it was firstly treated with a freshly prepared solution of 98 wt.% H 2 SO 4 and 30 wt.% H 2 O 2 (3:1, v/v) at 85 C for 30 min and then laid aside to cool to room temperature. The hydroxylated graphite powder was immersed in a mixture of APTES and ethanol (1:12, v/v), and heated to reflux for 4 h, and then aged overnight, separated by vacuum filtration, washed thoroughly with distilled water, and dried at 80 C for 1 h. Five grams of the APTES-covered graphite powder were exposed to a 100 ml of 0.1 M RuCl 3 solution (ph ¼ 1, adjusted with 10 wt.% HCl solution), stirred magnetically for 30 min at room temperature, and then the separated graphite powder was placed in a 100 ml of 0.4 M NaBH 4 solution. The mixture was kept undisturbed under the magnetic stirring until the bubble generation ceased, which generally took about 20 min. The obtained Ru/G catalyst was separated from the solution by vacuum filtration, followed by washing with deionized water thoroughly until the ph value of the washed water reached 7, then was washed with ethanol. The samples were finally dried in vacuum at 40 C for 48 h, and ready for use. For comparison, a Ru/G catalyst with Ru loading of 3.0 wt.% was synthesized by a conventional impregnation method following the literature procedure [10] Characterization of catalyst The Ru/G catalyst was characterized by transmission electron microscope (TEM, JEM 2010) equipped with an energy dispersive X-ray (EDX) analysis unit (Oxford). The surface element electronic states of the catalyst were analyzed by using X-ray photoelectron spectroscopy (XPS) (ESCABLAB250 spectrometer). All the binding energy values were calibrated by using C1s ¼ ev as a reference. The curve fitting was performed by using XPS PEAK 4.1 software. The composition of the catalyst was analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Iris Intrepid) Catalyst performance testing The hydrogen generation amount was measured by using a classic water-displacement method to characterize the catalytic effectiveness of the Ru/G catalyst. In a typical measurement, the reaction solution containing NaBH 4 and NaOH was thermostated in a sealed flask fitted with an outlet for collection of evolved H 2 gas, and then the Ru/G catalyst was dropped into the designated temperature solution to initiate hydrolysis reaction. As the reaction proceeded, the water displaced from a graduated cylinder connected to the reaction flask was continually monitored. 3. Results and discussion 3.1. Preparation and characterization of Ru/G catalyst Scheme 1 Schematic representation of the preparation procedure for the Ru/G catalyst. It is well known that silane coupling agent can undergo a reaction with the hydroxyl (OH) groups of the support to form a self-assembled monolayer (SAM) on the surface of the support in both aqueous and non-aqueous solutions [22,23]. During the preparation of the Ru/G catalyst, the hydroxylated graphite powder could react with APTES dissolved in ethanol to form a SAM which was covalently bonded to the graphite surface. The amine group ( NH 2 ) of APTES is a strong electron donor and possesses great ligand capacity to metal ions due to lone pair electrons of nitrogen atom. On the other hand, Ru (III) ion whose outer electron structure is 4d 6 5s 0 possesses vacant orbits for accepting electrons; therefore, the Ru ion is

3 international journal of hydrogen energy 35 (2010) able to form stronger coordination bonds with nitrogen atom [22]. When RuCl 3 is exposed to the silane-modified graphite powder, the APTES can react with Ru (III) ions, giving rise to formation of the complex of the Ru (III) APTES via the coordination bond of N / Ru(III). After the Ru (III) APTES-covered graphite powder was placed in the NaBH 4 solution, the metallic Ru clusters reduced in situ by the reducing agent of NaBH 4 were chemically bonded on the graphite surface. The as-prepared Ru/G catalyst was characterized by TEM and XPS. Fig. 1 gives a typical TEM image of the as-prepared Ru/G catalyst. As observed in Fig. 1, the Ru particles (<10 nm) are identified as dark part on the light graphite support, which shows a remarkable uniform and dispersion of metal particles on the graphite surface. Based on the TEM results, the modified impregnation method facilitates the formation of smaller and more uniform dispersion of Ru particles on the graphite surface. In addition, the EDX examination shows that the Ru/G catalyst was composed of Ru, C, Si and a small amount of O (the result is not shown here), this agrees well with the following XPS examination. The electronic state of the Ru element on the graphite powder was analyzed by using XPS. As seen in Fig. 2, the XPS spectrum for Ru 3d shows the presence of Ru species in the form of metallic and oxidized states, as evidenced by the identification of metallic Ru (280.2 ev) and Ru O (281.2 ev) species [24]. The single peak centered at ev is attributed to C 1s. As a whole, XPS studies show that the Ru/G catalyst is mostly composed of metallic Ru, with a trace of Ru O compound. According to the ICP-AES analysis result, the asprepared catalyst possesses Ru loading of 3.0 wt.%. Fig. 2 XPS spectrum of Ru 3d for the as-prepared Ru/G catalyst Catalytic activity for sodium borohydride hydrolysis The concentrations of NaBH 4 and NaOH exert important influences on the practical performance of on-demand HG system. Higher NaBH 4 concentration is highly wanted for achieving high hydrogen capacity, but gets restricted by the solubility limitation of NaBH 4 itself and hydrolysis product NaBO 2 in water. Certain amount of NaOH is required for stabilizing the NaBH 4 fuel solution, but causes capacity loss and may pose a problem for rapid HG and instant response for HG requirement. In the present study, we examined the influences of these two key factors on the HG process using the Ru/G catalyst. Fig. 3 shows the influence of NaBH 4 concentration on the HG rate in the present amount of 0.30 g of Ru/G catalyst at 30 C (the weight percent NaOH was held constant at 5 wt.%). It has been found that the HG rate increases with the concentration of NaBH 4 up to 10 wt.%, and reaches the maximum of 32.3 L min 1 g 1 (Ru) at a NaBH 4 concentration of Fig. 1 TEM image of the as-prepared Ru/G, showing Ru nanoparticles uniformly assembled on the graphite support. Fig. 3 Influence of NaBH 4 concentration on the HG rate at 30 8C, fixed NaOH concentration of 5 wt.% and the Ru/G catalyst amount of 0.30 g.

4 3026 international journal of hydrogen energy 35 (2010) wt.%. However, NaBH 4 concentration above 10 wt.% results in a reduced HG rate. This phenomenon is consistent with those of the previous researches [5,9,11]. It is well recognized that the solution viscosity increases with the increasing of NaBH 4 concentration, which may cause mass transport limitations of NaBH 4 and water to contact the catalyst surface [5]. The influence of NaOH concentration on the HG rate at 30 C in the present amount of 0.30 g of catalyst (the weight percent NaBH 4 was held constant at 5 wt.%) is shown in Fig. 4. It is clearly shown that the HG rates dramatically decrease with increasing NaOH concentration. Similar conclusions were also reached by Amendola et al. [5] and Liu et al. [11], which should be attributed to the inhibiting effect of hydroxide ions. Although the mechanism of the hydrolysis kinetics of NaBH 4 in the presence of catalyst has not been well established, Amendola et al. believed that the possible reason is the reduced activity of water at higher NaOH concentrations [5]. This occurs primarily because the ions, especially OH, strongly complex water, thus decreasing the available free water needed for NaBH 4 hydrolysis. However, these results are opposite to the hydrolysis reactions using non-noble transition metals/alloys as catalysts, in which the NaOH was found to act as accelerator to promote the hydrolysis reaction, at least in a certain concentration range [15 17]. The NaBH 4 reaction kinetics was further investigated at varied temperatures. Taking into account the observed influences of NaBH 4 and NaOH concentrations on the hydrogen generation rates, this set of experiments was carried out using a 5 wt% NaBH 4 þ 5 wt% NaOH solution. To minimize the effect of temperature changes due to the exothermic hydrolysis reaction, we checked and carefully controlled the solution temperature by adding ice-water blend. In addition, the amount of catalyst used was reduced to 0.10 g. These additional measures allow the solution temperature to be controlled to within 2 C during the reaction. The obtained kinetic curves are presented in Fig. 5. As expected, the rate of HG rises with elevating solution temperature. In the present study, the initial HG rates at varied solution temperature were used to determine the activation energy. The rate equation can be written as follows: Fig. 5 HG kinetic curves of the 5 wt.% NaBH 4 D 5 wt.% NaOH solution employing the Ru/G catalyst of 0.1 g at a solution temperature ranging from 25 to 45 8C. r ¼ k 0 exp E a RT where r is the reaction rate (ml min 1 g 1 ), k 0 the reaction constant (ml min 1 g 1 ), E a the activation energy for the reaction (kj mol 1 ), R the gas constant (8.314 kj mol 1 K 1 ) and T the reaction temperature (K). An Arrhenius plot, in which ln r is plotted against the reciprocal of absolute temperature (1/T ), is shown in Fig. 6. From the slope of the straight line, the activation energy is calculated to be kj mol 1 for the Ru/G catalyzed hydrolysis of sodium borohydride. This value was higher than the previously reported values, 47 and 56 kj mol 1 for Ru catalyst supported on IRA-400 [4,5], but was lower than the values, 66.9 and 83.5 kj mol 1 for Ru catalyst supported on carbon [7]. Catalyst durability is crucial in the practical HG apparatus application. In the present study, the Ru/G catalysts prepared by using the modified and conventional impregnation (1) Fig. 4 Influence of NaOH concentration on the HG rate at 30 8C, fixed NaBH 4 concentration of 5 wt.% and the Ru/G catalyst amount of 0.30 g. Fig. 6 Arrhenius plot for the determination of the activation energy employing a 5 wt.% NaBH 4 D 5 wt.% NaOH solution.

5 international journal of hydrogen energy 35 (2010) High-Tech Research and Development Program of China (863 Program, Grant No. 2006AA05Z104) and the National Basic Research Program of China (973 Program, Grant No. 2010CB631305) are gratefully acknowledged. references Fig. 7 A comparison of the durability for the Ru/G catalysts prepared by the modified and conventional impregnation methods employing a 5 wt.% NaBH 4 D 5 wt.% NaOH solution. methods were tested with respect to the durability in the cyclic use. After the catalytic hydrolysis reaction, the used catalyst was separated from the by-product solution by the vacuum filtration, washed thoroughly with deionized water and reused. As seen in Fig. 7, the Ru/G catalyst prepared by the modified impregnation method shows a good durability in cyclic usage. Even after five times of usage, the exhibited catalytic activity is only slightly inferior to that in the first cycle, whereas the Ru/G catalyst prepared by the conventional impregnation method can only remain 85% of its initial activity. The activity of the catalyst prepared by the present impregnation method does not decrease significantly, which may be attributed to a durable strong adhesion between the Ru nanoparticles and the support. 4. Conclusions A Ru/G catalyst for hydrolysis of alkaline NaBH 4 solution was prepared by a modified impregnation method. Remarkable uniform and dispersion nano-structure Ru particles with sizes of about 10 nm were chemically bonded on the graphite surface. Increasing the concentrations of NaBH 4 (more than 10 wt.%) and NaOH results in negative effect on hydrogen generation rate. Employing thus-prepared catalyst in a 10 wt.% NaBH 4 þ 5 wt.% NaOH solution has produced a hydrogen generation rate of 32.3 L min 1 g 1 (Ru). This catalyst with high activity and durability shows a potential for the portable fuel cell generators. Acknowledgements The financial supports for this research from the Hundred Talents Project of Chinese Academy of Sciences, the National [1] Schlapbach L, Züttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414: [2] Kong VCY, Foulkes FR, Kirk DW, Hinatsu JT. Development of hydrogen storage for fuel cell generators. I: hydrogen generation using hydrolysishydrides. Int J Hydrogen Energy 1999;24: [3] Brown HC, Brown CA. New highly active metal catalysts for the hydrolysis of borohydride. J Am Chem Soc 1962;84: [4] Amendola SC, Sharp-Goldman SL, Janjua MS, Kelly MT, Petillo PJ, Binder M. An ultrasafe hydrogen generator: aqueous alkaline borohydride solutions and Ru catalyst. J Power Sources 2000;85: [5] 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: [6] Kojima Y, Suzuki K, Fukumoto K, Sasaki 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: [7] Zhang JS, Delgass WN, Fisher TS, Gore JP. Kinetics of Rucatalyzed sodium borohydride hydrolysis. J Power Sources 2007;164: [8] Guella G, Zanchetta C, Patton B, Miotello A. New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from 11B NMR measurements. J Phys Chem B 2006;110: [9] Krishnan P, Yang TH, Lee WY, Kim CS. PtRu LiCoO 2 an efficient catalyst for hydrogen generation from sodium borohydride solutions. J Power Sources 2005;143: [10] 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: [11] Liu Z, Guo B, Chan SH, Tang EH, Hong L. Pt and Ru dispersed on LiCoO 2 for hydrogen generation from sodium borohydride solutions. J Power Sources 2008;176: [12] Dai HB, Liang Y, Wang P, Cheng HM. Amorphous cobalt boron/nickel foam as an efficient catalyst for hydrogen generation from alkaline sodium borohydride solution. J Power Sources 2008;177: [13] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, et al. High-performance cobalt tungsten boron catalyst supported on nickel foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33: [14] Mitov M, Rashkov R, Atanassov N, Zielonka A. Effects of nickel foam dimensions on catalytic activity of supported Co Mn B nanocomposites for hydrogen generation from stabilized borohydride solutions. J Mater Sci 2007;42: [15] 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: [16] Jeong SU, Kim RK, Cho EA, Kim HJ, Nam SW, Oh IH, et al. A study on hydrogen generation from NaBH 4 solution using the high-performance Co B catalyst. J Power Sources 2005;144:

6 3028 international journal of hydrogen energy 35 (2010) [17] Ingersoll JC, Mani N, Thenmozhiyal JC, Muthaiah A. Catalytic hydrolysis of sodium borohydride by a novel nickel cobalt boride catalyst. J Power Sources 2007;173: [18] Yuan W, Ooij WJ. Characterization of organofunctional silane films on zinc substrates. J Colloid Interface Sci 1997; 185: [19] Demjén Z, Pukánszky B, Földes E, Nagy J. Interaction of silane coupling agents with CaCO 3. J Colloid Interface Sci 1997;190: [20] Williams M, Pineda-Vargas CA, Khataibe EV, Bladergroen BJ, Nechaev AN, Linkov VM. Surface functionalization of porous ZrO 2 TiO 2 membranes using g-aminopropyltriethoxysilane in palladium electroless deposition. Appl Surf Sci 2008;254: [21] Holtzman A, Richter S. Electroless plating of silicon nitride using (3-aminopropyl) triethoxysilane. J Electrochem Soc 2008;155:D [22] Dai H, Li H, Wang F. Electroless Ni P coating preparation of conductive mica powder by a modified activation process. Appl Surf Sci 2006;253: [23] Moberg P, McCarley RL. Electroless deposition of metals onto organosilane monolayers. J Eletrochem Soc 1997;144: [24]