rori

School of Physics and Engineering, Sun Yat-Sen University,

ransmis

stability of its solution under high pH value in air, easily

controlled hydrogen generation (HG) and high purity of

hydrogen from the catalytic hydrolysis of NaBH4 solution,

nonflammability and side product recyclability [35]. Various

catalysts have been developed for catalytic hydrolysis of

[3,6], Pd [7], nickel boride [8] and cobalt boride [9]. The use of

High-surface-area supporting materials provide a potential

route to achieve the well dispersion of a catalyst to increase

the contact area with the reactants sufficiently. Actually,

carbon supported platinum [12] and carbon supported CoB

[13] catalysts show very good activities for the catalytic

* Corresponding author. Tel.: 86 20 84036736; fax: 86 20 84113369.cqu.edu.cn (Z. Wei).

Avai lab le at www.sc iencedi rect .com

w.

i n t e r n a t i on a l j o u r n a l o f h y d r o g e n en e r g y 3 3 ( 2 0 0 8 ) 7 1 1 0 7 1 1 5E-mail addresses: stsspk@mail.sysu.edu.cn (P.K. Shen), zdwei@The supply of hydrogen for portable fuel cells is crucial, but is

still a problem [1]. Among hydrogen storage methods, chem-

ical hydrides have been demonstrated to be promising

hydrogen sources for portable applications [2,3]. Sodium

borohydride (NaBH4) has been intensively investigated due to

its advantages of high H2 storage efficiency (10.8 wt.%),

highly active noble metal catalysts in practical applications is

restricted by their high cost. However, the reported catalysts

used for the decomposition of NaBH4 are readily poisoned by

the borate ion that is generated with hydrogen [10]. Our

motivation is to develop highly active and relative stable

catalysts for the hydrogen generation from NaBH4 [11].Sodium borohydride

Multiwalled carbon nanotubes

CoB catalyst

conditions but also reasonably stable for the continuous hydrolysis of NaBH4 solution.

2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rightsreserved.

1. Introduction NaBH4 solution to generate pure hydrogen, such as Pt [2], RuChongqing 400044, China

a r t i c l e i n f o

Article history:

Received 20 August 2008

Received in revised form

18 September 2008

Accepted 19 September 2008

Available online 7 November 2008

Keywords:

Catalytic hydrolysis

Hydrogen generation0360-3199/$ see front matter 2008 Interndoi:10.1016/j.ijhydene.2008.09.046a b s t r a c t

Multiwalled carbon nanotubes supported cobaltboron catalysts (CoB/MWCNT) were

developed via the chemical reduction of aqueous sodium borohydride with cobalt chloride

for catalytic hydrolysis of alkaline NaBH4 solution. The hydrogen generation (HG) rates

were measured on an improved high-accuracy, low-cost and automatic HG rate

measurement system based on the use of an electronic balance with high accuracy. The HG

of CoB/MWCNT catalyst was investigated as a function of heat treatment, solution

temperature, CoB loading and supporting materials. The catalyst was mesoporous

structured and showed lower activation energy of 40.40 kJ mol1 for the hydrolysis of

NaBH4. The CoB/MWCNT catalyst was not only highly active to achieve the average HG

rate of 5.1 l min1 g1 compared to 3.1 l min1 g1 on CoB/C catalyst under the sameGuangzhou 510275, ChinabThe State Key Laboratory of Power T sion Equipment & System Security and New Technology, Chongqing University,aState Key Laboratory of Optoelectronic Materials and Technologies,Yueqiang Huanga, Yi Wanga, Ruixiong Zhaoa, Pei Kang Shena,*, Zidong Weib,*Accurately measuring the hydhydrolysis of sodium borohydnanotubes/CoB catalysts

journa l homepage : wwational Association for Hgen generation rate forde on multiwalled carbon

e lsev ie r . com/ loca te /heydrogen Energy. Published by Elsevier Ltd. All rights reserved.

hydrolysis of NaBH4 solution. Pore structures of the support-

ing materials affect the performance of catalysts as well.

Mesoporous structures of the supporting materials are bene-

ficial to the gas and water transport. The mesoporous SiCN-

coated multiwalled carbon nanotubes (MWCNTs) paper

supported Pt and Pd showed excellent activities [14]. The

hydrophilicity of the supporting materials contributes to the

well-stable dispersion for a catalyst. MWCNTs have been used

in many areas due to the high specific BET area and unique

testing methods have been developed, such as water

displacement method [3,5], water trap method [2,21], flow

filtered, rinsed thoroughly using distilleddeionized water until

neutral and dried in vacuum at 80 C for 12 h. After the treat-ment, acid treated MWCNTs (1.0 g) were added into 136 ml

CoCl2 solution (0.025 mol l1) in a beaker. Then, the solution

was stirred for 24 h. Subsequently, the alkaline NaBH4 solution

(5.1 ml, 1 mol l1 NaBH4 1 mol l1 NaOH) as a reducing agentwas added to CoCl2 solution drop wise with magnetic stirring.

During the reduction reaction, the reactor was placed in ice

water that was maintained at 0 C in order to prevent

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 3 ( 2 0 0 8 ) 7 1 1 0 7 1 1 5 7111meter method [22], and fuel cell test method [23]. The accu-

racy is limited by using either water displacement method or

water trap method. The electronic gas flow meter gives

accuracy at 1 FS% and is relatively expensive. Besides, the

temperature, pressure and the water vapour in the hydrogen

affect the data accuracy measured by the gas flow meter [23].

The development of a high-accuracy, low-cost and easy-

manipulative method is necessary and important. In this

work, we report an improved testing system for automatically

and accurately measuring the HG rate.

2. Experimental

2.1. Catalyst preparation

All the reagents were of analytical grade and used as received.

In a typical experiment, CoB/MWCNT catalysts were prepared

by the impregnation-chemical reduction method [18].

MWCNTs (3.0 g, Shenzhen Nanotech. Co., Ltd., China) with

nanotube diameters of 1020 nm and purity of >95% were

mixed with 100 ml of the solution of 98% H2SO4 and 65% HNO3(1:1 by volume) under constant stirring for 15 min and then

refluxed at 100 C for 4 h. The acid treated MWCNTs wereelectronic properties. The walls of the MWCNTs can be made

hydrophilic by acid treatment [15]. In the 1950s, Schlesinger

et al. firstly found that cobalt chloride could catalyze the

hydrolysis of NaBH4 [4]. Since then, CoB catalysts were

intensively interested [13,1620] for on-demand HG due to its

good catalytic activity and low cost. In the present work, acid

treated MWCNTs were used to support CoB via the chemical

reduction of aqueous sodium borohydride with cobalt chlo-

ride. HG rate was investigated using the as-prepared CoB/

MWCNT catalysts as a function of heat treatment, CoB

loading, solution temperature and supporting materials.

As we know, the in situ measurement of the HG rate is

important for hydrolysis of NaBH4. Many useful HG rateFig. 1 Schematic diagram of HGa vigorous reaction. The produced black precipitates were

filtered and washed repeatedly using distilleddeionized water.

Five catalyst samples with different CoB loadings (3.26 wt.%,

7.71 wt.%, 17.33 wt.%, 29.36 wt.% and 100 wt.%, respectively)

were prepared by changing the reactants ratios. The CoB/

MWCNT catalysts were then dried in vacuum at 80 C andtreated in Ar (99.99% purity) atmosphere at 300, 500 and 700 C,respectively. CoB on carbon (Vulcan XC-72 C, Carbot, USA)

(CoB/C) catalysts were also prepared for the comparison.

2.2. Catalyst characterization

Phase structures of the as-prepared CoB/MWCNT catalysts

were characterized by powder X-ray diffraction (XRD, Rigaku

D/MAX-III A, Cu Ka1 radiation). The morphologies of the

prepared catalysts were characterized on a scanning electron

microscope (SEM, Quanta 400 FEG). Co and B contents in the

catalysts were determined by inductively coupled plasma-

atomic emission spectrometry (ICP-AES, TJA Iris). Nitrogen

adsorptiondesorption of the catalysts was measured at

196 C on a Nova-1000 surface area analyzer (QuantachromeInstruments).

2.3. HG rate measurement method

We developed a HG rate measurement method based on the

use of an electronic balance. Instead of the measurement of the

gas volume, we measure the weight of the water replaced by

the gas generated during the hydrolysis of NaBH4. High accu-

racy and sensitivity can be achieved using an electronic balance

(0.002 FS% compared to 1 FS% for a gas flow meter) since the

mass density of water is very close to 1.0 g cm3 from 0 C(0.99984 g cm3) to 50 C (0.98804 g cm3). An automatic HG ratetesting system is schematically shown in Fig. 1. An electronic

balance (G&G JJ5000, 0.002 FS% accuracy) was connected to

a computer. The weight data were automatically recorded by

a data acquisition software modified by ourselves (one datum

every two seconds generally). The systematic error and

personal error can be substantially reduced in such a way.rate measurement system.

The catalytic activities of the synthesized catalysts for the

hydrolysis of alkaline NaBH4 solution were studied in a batch

reactor. In a typical experiment, 50.0 g of the alkaline NaBH4solution and 10.0 mg catalyst were introduced into a round

bottom flask which was water-bathed in a thermostatic

apparatus. The NaBH4 solution was maintained at a tempera-

ture of 30 C. In order to reduce the pressure variation, thegenerated gas was cooled down to 25 C by a thermostaticapparatus. After the test, the corresponding hydrogen volume

was calculated according to the weight of water displaced by

hydrogen. The specific HG rate (l min1 g1 CoB) was used tocompare the activities of the catalysts.

3. Results and discussion

3.1. Effect of heat-treatment temperature

Fig. 2 is a SEM micrograph which shows the surface

morphology of the CoB/MWCNT catalyst treated at 300 C.The tube diameter of the original MWCNT is 2040 nm. After

related to the heat-treatment temperature. Fig. 4 shows the

real-time HG rate of CoB/MWCNT catalysts treated at different

20 30 40 50 60 70 80

(d)

(c)

(b)

In

ten

sity / a.u

.

2 Theta / deg.

(a)

MWCNTMetallic CoCoB Co2B

Fig. 3 XRD patterns of CoB/MWCNT catalysts heat-

treated at (a) 80 8C, (b) 300 8C, (c) 500 8C and (d) 700 8C,

respectively.

0 5 10 15 20 25 30

0

1

2

3

4

5

6

7

HG

rate / l m

in

-1 g

-1 C

o-B

Time / min

80 oC 300 oC500 oC700 oC

Fig. 4 Effect of the heat temperature on the HG rate

i n t e r n a t i on a l j o u r n a l o f h y d r o g e n en e r g y 3 3 ( 2 0 0 8 ) 7 1 1 0 7 1 1 57112loaded catalyst, each MWCNT was uniformly coated with

a layer of CoB and the tube diameter was expanded to 90

120 nm.

It was reported that the freshly prepared CoB catalysts

were amorphous in structure, while crystallization and

decomposition of the CoB amorphous occurred at tempera-

ture above 300 C [9,24]. The dependence of the productcomposition on the temperature is shown in Fig. 3. It is clear

that the products were amorphous until the heat treatment

over 300 C. In fact, it is a mixture of different CoBcompounds. The diffraction peak around 44 is overlapped bythe peaks of CoB compounds and metallic Co when the

product was treated over 500 C. Scherrer equation wasadopted to calculate the average crystalline size of the CoB

catalysts, as described by Eq. (1):Fig. 2 SEM micrograph of a 17.33 wt.% CoB/MWCNT

catalyst treated at 300 8C.L 0:9lKa1B2q cos qmax

(1)

where lKa1 is the wavelength of Cu Ka1 radiation (1.54056 nm),

B2q is the half-width of the diffraction peak. Another param-

eter qmax is the 2q degree of the diffraction peak. The value of

the average crystalline size L can be calculated through Eq. (1).

The peak at 44.2 was used to calculate the average crystalline

sizes of the catalysts and the values are 15 nm, 23 nm and

52 nm at the heat temperature of 300 C, 500 C and 700 C,

respectively. The effect of the heat temperature of CoB/

MWCNT catalysts on their catalytic activities is described in

the following sections.

It is known that the activity of catalyst is proportional to its

active surface area or inversely proportional to its particle size.

The XRD results showed that the crystalline size of CoB ismeasured in 20 wt.% NaBH4D 3 wt.% NaOH solution

containing 10 mg of 17.33 wt.% CoB/MWCNT at 30 8C.

could be contributed to the size effect that the catalyst treated at

25 30 35 40 45 50 55 600

5

10

15

20

25A

verag

e H

G rate / l m

in

-1 g

-1 C

o-B

Temperature / oC

a

3.0 3.1 3.2 3.3 3.4

-2.0

-1.5

-1.0

-0.5

0.0

E=40.40 kJ mol-1

lnr=lnk0+(-E/R)*T-1

ln

rreact

T-1

/ [1000*K

-1]

b

Fig. 5 (a) Effect of solution temperature on the average HG

rate measured after 20 min in 20 wt.% NaBH4D 3 wt.%

NaOH solution containing 10.0 mg of 17.33 wt.% CoB/

MWCNT and (b) Plot of ln r versus 1/T according to the

original data as shown in Fig. 5a.

0 20 40 60 80 1000

1

2

3

4

5

6

Averag

e H

G rate / l m

in

-1 g

-1 C

o-B

Co-B loading / wt.%

Fig. 6 Relationship between CoB loading and the average

HG rate measured after 1 h running in 20 wt.%

NaBH4D 3 wt.% NaOH solution containing 10 mg of CoB/

MWCNT.

0.0 0.2 0.4 0.6 0.8 1.0

40

80

120

160

0 5 10 15 20 25 300.00.20.40.60.81.01.2

dV

(lo

gd

)/cm

3

g-1

Pore Diameter/nm

Co-B/MWCNT

Ad

so

rb

ed

N

itro

gen

V

olu

me/cm

3 g

-1

Relative Pressure/P Po-1

Co-B/MWCNT Co-B/C

Fig. 7 Nitrogen adsorption/desorption isotherm curves for

CoB/MWCNT and CoB/C. The inset is the pore size

distribution of the CoB/MWCNT.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 3 ( 2 0 0 8 ) 7 1 1 0 7 1 1 5 7113300 C gave the smallest particle size.

3.2. Effect of solution temperature

The dependence of the HG rate on the temperature (2560 C)was measured in 20 wt.% NaBH4 3 wt.% NaOH solutioncontaining 10.0 mg of the as-prepared CoB/MWCNT catalysttemperatures. It can be clearly seen from Fig. 4 that the catalyst

treated at 300 C exhibited the best catalytic activity for thehydrolysis of NaBH4 and achieved an average HG rate of

4.78 l min1 g1, which is much higher than those treated at500 C (1.25 l min1 g1) and 700 C (0.246 l min1 g1). Thisand the results are presented in Fig. 5a. It is clear that the

average HG rate increases from 3.35 to 22.18 l min1 g1 with

0 10 20 30 40 50 60

0

1

2

3

4

5

6

7

Real-tim

e H

G rate / l m

in

-1g

-1C

o-B

Time / min

Co-B/MWCNTCo-B/CCo-B

Fig. 8 Steady-state performance of CoB supported on

different materials for hydrolysis of NaBH4 in 20 wt.%

NaBH4D 3 wt.% NaOH solution containing 10 mg of

catalysts.

t.%

w0.08 3.9 [18]

i n t e r n a t i on a l j o u r n a l o f h y d r o g e n en e r g y 3 3 ( 2 0 0 8 ) 7 1 1 0 7 1 1 57114the solution temperature increasing from 25 to 60 C. Thefollowing equation can be used to calculate the reaction rate:

r k0 expERT

(2)

where r is the reaction rate (mol min1 g1), k0 is the reactionconstant (mol min1 g1), E is the activation energy for thereaction, R is the gas constant and T is the solution tempera-

ture. By plotting ln r versus 1/T based on the data in Fig. 5a,

a linear relationship is observed as shown in Fig. 5b. The

activation energy for the HG reaction was calculated from the

slope of the linear curve in Fig. 5b to be 40.40 kJ mol1, which islower than the reported value of 64.87 kJ mol1 on other

catalyst [16].

3.3. Effect of catalyst loading

Fig. 6 shows the relationship between the CoB loading and

the HG rate. It is obvious that the HG rate increases with the

increase in the CoB loading up to 10 wt.%. However, the HG

rate tends to decrease with the further increase in the CoB

loading. The highest activity achieved is 5.12 l min1 g1 CoB.

The unsupported CoB exhibits the lowest activity due to

lowest dispersion degree.

3.4. Effect of supporting material

MWCNTs and carbon as supporting materials were compared.

The acid treated MWCNTs or carbon supported CoB catalysts

were developed via chemical reduction of aqueous sodium

borohydride with cobalt chloride. Fig. 7 shows nitrogen

adsorption/desorption isotherm curves for CoB/MWCNT and

CoB/C. The pore size distribution of the CoB/MWCNT is

shown in the inset of the figure. The specific surface areas of

Table 1 Comparison of HG rates on various catalysts

Catalyst Initial solutiontemperature for HG (C)

NaBH4concentration (w

Ru/IRA-400 32.5 7.5

Pt/XC-72 C 30 5

Pt/Al2O3 30 5

PtRuLiCoO2 25 5

Co/gAl2O3 30 5

CoB/C 30 w0.8

CoB/C 30 20

CoB/MWCNTs 30 20the CoB/MWCNT and CoB/C catalysts, calculated by the

BrunauerEmmettTeller (BET) equation, were 118 and

176 cm2 g1, respectively. The hysteresis on isotherm curve ofCoB/MWCNT is the evidence of the mesoporous structure

even the BET area is lower. The mesopore distribution peak is

located at 4.8 nm. In fact, the support with mesopores is more

useful than that of micropores used in liquid media due to the

liquid-sealing effect.

Fig. 8 compares the steady-state catalytic activities for the

NaBH4 hydrolysis on CoB/MWCNT, CoB/C and pure CoB

without support. The supported catalysts showed higher

activity than that of unsupported one due to the better

dispersion and higher surface area. It is further demonstrated

that the catalytic activity of mesoporous CoB/MWCNT is4. Conclusions

CoB/MWCNT catalysts were prepared via the chemical

reduction of aqueous sodium borohydride with cobalt chlo-

ride for catalytic hydrolysis of NaBH4 solution. It is proved that

the use of MWCNTs as support is beneficial to the dispersion

of the CoB to increase the active surface area, resulting in

enhanced catalytic activity. The catalyst was mesoporous

structured and showed lower activation energy of

40.40 kJ mol1 for the hydrolysis of NaBH4. A high-accuracy,low-cost and automatic HG rate measurement system was

developed based on the use of an electronic balance, which

has high accuracy of 0.002 FS%. The CoB/MWCNT catalyst

treated at 300 C exhibited the highest catalytic activity andachieved the average HG rate of 5.1 l min1 g1 compared to3.1 l min1 g1 on CoB/C catalyst in 20 wt.% NaBH4 3 wt.%higher than that of CoB/C since the mesoporous structure of

MWCNT allows to increase the active surface area. The

comparison of HG rate of CoB/MWCNT catalyst with other

catalysts is given in Table 1. Amendola et al. [3] reported that

IRA-400 supported Ru catalysts produced an average HG rate

of 0.6 min1 g1 catalyst. Ye et al. [25] found that the carbonsupported Pt and Al2O3 supported Pt gave HG rates of 3.7 and

3.0 l min1 g1 catalyst in 5 wt.% NaBH4 5 wt.% NaOH solu-tion. It is also reported that the HG rates of 2.4 and

0.15 l min1 g1 on PtRuLiCoO2 and Co/gAl2O3 catalysts[26,27]. Carbon supported CoB catalyst gave a HG rate of

3.9 l min1 g1 at very low concentration of 0.8 wt.% NaBH4containing 0.08 wt.% NaOH [20]. In the present work, CoB/

MWCNT catalyst exhibits a higher average HG rate of

5.1 l min1 g1.

3 3.1 This paper

3 5.1 This paper)NaOH

concentration (wt.%)Average HG

rate (l min1 g1)Reference

1 0.6 [3]

5 3.7 [25]

5 3.0 [25]

5 2.4 [26]

5 0.15 [27]NaOH solution containing 10 mg catalyst at 30 C.

Acknowledgements

The work was financially supported by the Guangdong Sci. &

Tech. Key Projects (2007A010700001, 2007B090400032),

Guangzhou Sci. & Tech. Key Projects (2007Z1-D0051,

SKT[2007]17-11), Dongguan Sci. & Tech. Project (2005d029), the

NNSF of China (20676156), China National 863 Program

(2006AA11A141, 2007AA05Z124), the Chinese Ministry of

Education (307021) and the Chongqing Sci. & Tech. Key Project

(CSTC2007AB6012).

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Accurately measuring the hydrogen generation rate for hydrolysis of sodium borohydride on multiwalled carbon nanotubes/Co-B catalystsIntroductionExperimentalCatalyst preparationCatalyst characterizationHG rate measurement method

Results and discussionEffect of heat-treatment temperatureEffect of solution temperatureEffect of catalyst loadingEffect of supporting material

ConclusionsAcknowledgementsReferences