SiO2 as catalyst catalyze hydrogen generation

Materials Letters 65 (2011) 3212–3215

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Synthesis and characterization of Co/SiO2 as catalyst catalyze hydrogen generation Chia-Chi Su a, Yu-Jen Shih b, Yao-Hui Huang b, Ming-Chun Lu a,⁎ a b

Department of Environmental Resources Management, Chia Nan University of Pharmacy and Science, Tainan 717, Taiwan Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan

a r t i c l e

i n f o

Article history: Received 26 May 2011 Accepted 3 July 2011 Available online 7 July 2011 Keywords: Catalyst Hydrolysis Hydrogen generation Sodium borohydride Porous SiO2

a b s t r a c t Novel composites of porous SiO2-LP and SiO2-HP supports are synthesized by the silica–polyethylene glycol monooleyl ether surfactant self-assembly method to obtain a large surface area. Cobalt is immobilized in the supports by incipient wetness impregnation. A stable and active Co/SiO2 catalyst is examined using FE-SEM, BET and XRD. Further, the catalyst is tested for catalytic hydrolysis of alkaline sodium borohydride (NaBH4) solution: the rate of hydrogen generation is found to increase with increasing cobalt loading of the Co/SiO2 catalyst. The hydrogen generation rates increase dramatically when the temperature is increased from 17 to 40 °C. The highest hydrogen generation rates of Co/SiO2 catalyst are obtained at 2513 mL min − 1 g− 1 in 20 mL of 5 wt.% NaBH4 solution containing 5 wt.% NaOH at 40 °C. © 2011 Elsevier B.V. All rights reserved.

Hydrogen as an energy source has many advantages, but efficiently controlling the amount of hydrogen generated and storing it safely are two challenges. Chemical hydrides have been developed as potential sources of hydrogen, as hydrogen is generated from the hydrolysis of chemical hydrides such as NaBH4, KBH4 and LiBH4 [1,2]. Among the chemical hydrides, the effects of NaBH4 and NaOH concentrations, as well the effects of temperature, on the generation rate of hydrogen, have been extensively investigated [3]. The hydrolysis of sodium borohydride (NaBH4) occurs according to the following equation:

and reusing of catalyst from alkaline NaBH4 solution. For the practical application of hydrogen generation, the supported catalyst exhibits easier separation from the NaBH4 solution than the various metal powders. Hence, the easy solid/liquid separation and large surface area of the catalyst applied might be an advantage for hydrogen generation from NaBH4 hydrolysis and catalyst recycling. In this study, we prepared a magnetic cobalt/SiO2 catalyst for hydrogen generation from an alkaline NaBH4 solution. The catalysts were prepared by using cobalt immobilized on the porous SiO2 support. Porous supports with small and large surface areas and cobalt loading were used to examine its catalytic effect on the hydrolysis reaction of NaBH4 solution.

NaBH4 + 2H2 O→NaBO2 + 4H2ðgÞ :

ð1Þ

2. Materials and methods

Normally, the rate and amount of hydrogen generated are hastened by the use of a suitable catalyst. Noble and non-noble metals have been coated on the support for hydrogen generation; these include Ru [2,4], Co [5] and Ni [6]. Recently, novel catalysts such as ruthenium nanoparticles immobilized in montmorillonite [7], cobalt-boron (CoB) and NaBH4 implantation in polymers or sodium alginate [8] and porous Fe–Co–B/Ni foam [9] have been prepared and seen to hasten hydrogen generation. Zhao et al. [10] reported that using mesoporous materials, such as MCM-41 and SBA-15 loaded with cobalt, resulted in enhanced hydrogen production during cellulose decomposition. Porous structures with large surface area ensure the access of reactants to the surface active sites of the catalyst [9]. In a previous study [11,12], it was shown that the ferromagnetic property of catalysts certainly offered an advantage to the recycling

2.1. Materials and synthesis

1. Introduction

⁎ Corresponding author. Tel.: + 886 6 2660489; fax: + 886 6 2663411. E-mail address: [email protected] (M.-C. Lu). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.07.005

The porous SiO2 support was obtained under conditions in which the silica surfactant occurs simultaneously with condensation of the inorganic species. Sodium silicate (Na2O·2SiO2, mole ratio 2.05 ~ 2.25) was dissolved in distilled water at 40 °C and the pH was controlled to approximately 1.4. Subsequently, the polyethylene glycol monooleyl ether (C18H35 (CH2CH2O)n OH, n = 50) solution was rapidly added to the acidic silicate solution. The resulting solution was then heated at 40 °C for 2 days. After 2 days, the SiO2 was washed and dried overnight, and then calcined in air by heating to 600 °C. Various amounts of surfactant were used to prepare small surface area of porous SiO2 (SiO2-LP) and large surface area of porous SiO2 (SiO2-HP), respectively. Cobalt was introduced to the SiO2 support by aqueous incipient wetness impregnation. The porous SiO2 was mixed with CoCl2·6H2O at room temperature for 24 h, and then dried overnight. The reductive

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Fig. 1. Field emission scanning electron microscope image of (a) SiO2-LP, (b) 24% Co/SiO2-LP, (c) 49% Co/SiO2-LP, (d) SiO2-HP, (e) 24% Co/SiO2-HP and (f) 49% Co/SiO2-HP.

irregular structures (Fig. 1(a) and (d)). The particle size of SiO2-LP and SiO2-HP was more than a micrometer. The micrometer particle of porous SiO2 could easily separate from the solution and be included in the metallic coating process. Fig. 1(b), (c), (e) and (f) reveals some rough morphology on the porous SiO2 surface, in contrast to the smooth morphology observed on the porous SiO2 (Fig. 1(a) and (d)), indicating that cobalt was coated on the porous SiO2 surface. The BET surface areas of SiO 2 -LP and SiO 2 -HP are 223.5 m 2 g − 1 and 443.5 m 2 g − 1, respectively. The result clearly indicates that the BET of SiO2 support depends on the template surfactant concentration. This phenomenon was also reported in the prior study of Ting et al. [13]. The surface compositions of the porous SiO2 and Co/SiO2 catalysts estimated from EDS analysis are listed in Table 1. These results confirmed the existence of Co on the porous SiO2. The cobalt content increased from 20.3 wt.% (or 7.2 at.%) on 24% Co/SiO2-LP to 27.1 wt.% (or 10.6 at.%) on 49% Co/SiO2-LP and from 18.9 wt.% (or 7.3 at.%) on 24% Co/SiO2-HP to 31.0 wt.% (or 12.8 at.%) on 49% Co/SiO2-HP. Fig. 2 shows the XRD pattern of SiO2-LP, SiO2-HP, Co/SiO2-LP and Co/SiO2-HP. The pattern for SiO2-LP and SiO2-HP showed one broad characteristic diffraction peak for SiO2 (2θ = 22°) (Fig. 2(a) and (d)). The diffraction pattern for the Co/SiO2-LP and Co/SiO2-HP catalysts showed two new broad characteristic diffraction peaks at 34.1° and 36.9° (Fig. 2(b), (c), (e) and (f)), and the diffraction peak intensity of SiO2 decreased with increasing cobalt loading. The two new diffraction peaks were matched the JCPDS #75-0419 and #76-1802, which belong to CoO and Co3O4, respectively.

Co/SiO2 catalysts were obtained by reducing the dried powder with 20 mL of 1 wt.% NaBH4 solution. The synthesized black catalysts are abbreviated as 24% Co/SiO2-LP, 49% Co/SiO2-LP, 24% Co/SiO2-HP and 49% Co/SiO2-HP. 2.2. Characterization of catalysts The surface morphologies of the SiO2 support and Co/SiO2 catalysts were examined by a field emission scanning electron microscope (FE-SEM), JEOL JSM-6700F, and the elemental compositions on the catalyst were analyzed with energy dispersive spectroscopy (EDS). X-ray powder diffraction patterns were obtained at room temperature with a DANDONG DX-2500 powder diffractometer using Cu Kα radiation. The specific surface area of catalyst was determined using ASAP 2020. 2.3. Hydrogen generation measurement The hydrogen generation volume of the catalysts was measured using a water-displacement method in which hydrogen was generated by the hydrolysis of sodium borohydride. The 20 mL of NaBH4 solution containing NaOH was placed in a sealed glass column containing catalysts. 3. Results and discussion 3.1. Characterization of Co/SiO2 catalysts

3.2. Hydrogen generation The morphologies of the porous SiO2 and coating cobalt were observed with a FE-SEM; the results are shown in Fig. 1. The FE-SEM images reveal that the porous SiO2 consists of many spherical and

The SiO2 support without cobalt coating was also used as a catalyst for hydrogen generation from a 5 wt.% NaBH4 solution containing

Table 1 Surface compositions of SiO2 support after calcination at 600 °C and Co/SiO2 catalysts examined with EDS. Elements

O Si Co

SiO2-LP

24% Co/SiO2-LP

49% Co/SiO2-LP

SiO2-HP

24% Co/SiO2-HP

49% Co/SiO2-HP

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

wt.%

at.%

52.4 47.6 –

65.9 34.1 –

58.2 21.5 20.3

76.6 16.2 7.2

48.8 24.1 27.1

69.7 19.7 10.6

50.0 50.0 –

63.8 36.2 –

45.4 35.7 18.9

64.0 28.7 7.3

42.3 26.7 31.0

64.1 23.1 12.8

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and 795 mL min − 1 g − 1 for the 49% Co/SiO2-LP. This result indicated that cobalt loading is a key factor for the hydrolysis of alkaline NaBH4 solution. Additionally, the specific surface area of Co/SiO2-LP and Co/SiO2-HP catalysts does not appear to be the predominant factor for NaBH4 hydrolysis because of the low cobalt loading condition in this study. Increasing the amount of cobalt loading improved the hydrolysis of NaBH4 solution, implying that a surface reaction took place between NaBH4 and the catalyst's surface. Normally, increasing solution temperature can lead to increased rate of chemical reaction. As expected, the hydrogen generation rates increased dramatically when the temperature was increased. As the solution temperature increased from 17 to 40 °C, the steady-state hydrogen generation rate increased from 208 to 1239 mL min − 1 g − 1 for 24% Co/SiO2-LP, from 425 to 2513 mL min − 1 g − 1 for 49% Co/SiO2-LP, from 181 to 1266 mL min − 1 g − 1 for 24% Co/SiO2-HP, and from 375 to 2222 mL min − 1 g − 1 for 49% Co/SiO2-HP catalysts, respectively. Fig. 2. X-ray diffraction patterns: (a) SiO2-LP, (b) 24% Co/SiO2-LP, (c) 49% Co/SiO2-LP, (d) SiO2-HP, (e) 24% Co/SiO2-HP and (f) 49% Co/SiO2-HP.

4. Conclusions

5 wt.% NaOH at 25 °C. The result shows that the hydrogen generation was not found. It indicates that SiO2 support without cobalt has no catalytic activity. Fig. 3 shows the volume of hydrogen generation as a function of time using 30 mg catalyst. It shows that the hydrogen generation volume is almost linearly proportional to the reaction time, suggesting stable catalytic activity of the catalysts. The hydrogen generation rates increased with increasing cobalt loading. The hydrogen generation rates at 25 °C were 402 for 24% Co/SiO2-LP,

By using the silica-surfactant self-assembly method to produce porous silica support, a low- and high-performance porous Co/SiO2 catalyst has been prepared for catalyzing hydrogen generation from an alkaline NaBH2 solution. The experimental result showed that the cobalt loading of the catalyst is a key factor for the hydrolysis of alkaline NaBH4 solution. The hydrogen generation rates increased dramatically when the temperature was increased from 17 to 40 °C. The highest hydrogen generation rates of 49% Co/SiO2-LP catalysts are 2513 mL min − 1 g − 1, in a solution containing 5 wt.% NaBH4 and 5 wt.% NaOH at 40 °C.

Fig. 3. Effect of temperature on volume of hydrogen generated as a function of time in 5 wt.% NaBH4 solution containing 5 wt.% NaOH using (a) 24% Co/SiO2-LP, (b) 49% Co/SiO2-LP, (c) 24% Co/SiO2-HP and (d) 49% Co/SiO2-HP catalyst.

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Acknowledgments The authors would like to thank the National Science Council of Taiwan, for financially supporting this research under Contract No. NSC 99-2221-E-041-012-MY3. References [1] Eigen N, Kunowsky M, Klassen T, Bormann R. J Alloys Compd 2007;430:350–5. [2] Chen CW, Chen CY, Huang YH. Int J Hydrogen Energy 2009;34:2164–73. [3] Liu BH, Li ZP. J Power Sources 2009;187:527–34.

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