i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 8 1 0 – 3 8 1 5

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Porous grape-like spherical silica with hydrogen storagecapability, synthesized using neutral dual surfactants astemplates

Li Dua, Shijun Liaoa,*, Quanbing Liua, Xu Yanga, Huiyu Songa, Zhiyong Fua, Shan Jib

aSchool of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, ChinabSouth Africa Institute for Advanced Materials Chemistry, University of the Western Cape, South Africa

a r t i c l e i n f o

Article history:

Received 12 June 2008

Received in revised form

18 December 2008

Accepted 10 March 2009

Available online 5 April 2009

Keywords:

Super-microporous silica

SMS

Solid sphere

Pd-modified

Hydrogen storage

* Corresponding author. Fax: þ86 20 8711310E-mail address: chsjliao@scut.edu.cn (S. L

0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.03.018

a b s t r a c t

By using two non-ionic surfactants, 1,12-diaminododecane and a triblock copolymer

surfactant F127, as templates, grape-like solid spheres of super-microporous silica (SMS)

with ordered, worm-like pore structures have been successfully synthesized, then char-

acterized by SEM/TEM, XRD, TG/DTA, etc. In a sample synthesized at 70 �C, well-formed

grape-like spheres with worm-like pores were observed by SEM and TEM. The average pore

size is ca. 1.87 nm and the specific surface area of the spheres is 865 m2/g. The hydrogen

storage capacity of the sample Pd/SMS-70-C, prepared by supporting 5 wt% palladium on

SMS (synthesized at 70 �C), is up to 2.56 wt% at 1.2 MPa hydrogen pressure.

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction system, C8TMAB/SDS/F127, as a template to synthesize

Recently, the synthesis of spherical porous materials has

attracted significant attention, due to their potential applica-

tion in aspects of hydrogen storage, and in the controlled

release of medicines. For instance, Buckley et al. [1] reported

that a sample of spherical MCM-41 at 77 K can reversibly

absorb 1.6 wt% of hydrogen at w3.5 MPa and 2.7 wt% at

w4.5 MPa. A representative hydrophobic anticancer drug,

camptothecin (CPT), was loaded into the pores of fluorescent

mesoporous spherical silica nanoparticles, and delivered to

a variety of human cancer cells to induce cell death [2].

Several porous materials with different morphologies and/

or different structures have been reported in recent years,

developed by using mixtures of two or three types of surfac-

tants as templates. Chen et al. [3] used a ternary surfactant

8.iao).ational Association for H

faceted single crystals of mesoporous silica SBA-16. Xiao’s

group [4] demonstrated that a highly ordered mesoporous

silica-based material with unusual hydrothermal stability

could be successfully synthesized using a mixture of fluoro-

carbon surfactant [C3F7O(CFCF3CF2O)2CFCF3CONH(CH2)3-

Nþ(C2H5)2CH3I�, FC-4] and a triblock copolymer surfactant

(EO20PO70EO20, Pluronic P123) as templates. Yang et al. [5]

reported the synthesis of helical mesoporous materials using

C16TAB and a chiral surfactant, perfluorinated carboxylic acid,

as templates. In addition, they demonstrated that the size and

pitch of the helical rods can be controlled by the addition of

perfluorinated carboxylic acid. Accordingly, multi-surfactant

systems provide new methodological pathways for the

synthesis of porous materials and can give rise to new

phenomena in material morphologies and structures.

ydrogen Energy. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 8 1 0 – 3 8 1 5 3811

Hydrogen is one of the most promising future fuels for

transportation and other mobile applications, but storage

presents a major challenge [6]. Carbon nanotubes and modi-

fied carbon nanotubes [7–10], carbon fibers [11], porous

carbons [12,13] and nanocomposite materials [14] were

reported to possess good hydrogen storage properties, and

recently, metal-organic frameworks (MOFs) such as MOF-5

[15], MIL-53 [16], MIL-101 [17], MOF-177 [18] and others were

confirmed to have high hydrogen storage capacities. Micro-

porous or mesoporous silica materials have large surface

areas and pore volumes, and an appropriate pore structure for

hydrogen storage in MOFs, so it is reasonable to believe that

mesoporous structured silica might be good candidates for

storing hydrogen or supporting hydrogen storage materials. It

is interesting that there are, to date, few reports on hydrogen

storage with super-microporous silica materials or modified

super-microporous silica materials.

In the present work, a grape-like silica material with

a worm-like pore structure was synthesized using surfactant

1,12-diaminododecane (DADD) and a tri-block copolymer

surfactant (EO106PO70EO106, Pluronic F127) as templates. It was

found that the synthesized material showed good hydrogen

storage properties when doped with a small amount of

palladium.

2. Experimental

2.1. Preparation of SMS

DADD and F127 were dissolved in a solvent mixture of water

and ethanol. The mixture was stirred until homogeneity was

achieved, then tetraethyl orthosilicate (TEOS) was slowly

added to the solution with vigorous stirring for 30 min. The

final mixture, with molar ratios of 1.0 SiO2:0.26 DADD:

(0.0018w0.002) F127:9.44 EtOH:58.7 H2O, was stirred at various

temperatures for 24 h (denoted by SMS-m, where m¼ 30–

90 �C). The solids were recovered by filtration, washed with

distilled water and dried at 80 �C overnight. The templates

were then removed either by calcination in air at 550 �C for 4 h

(SMS-C) or extraction by ethanol at 70 �C, three times (SMS-E).

All synthesized super-microporous silica solid spheres are

denoted as SMS in this work.

2.2. Characterization of SMSs

Scanning electron micrographs (SEMs) were acquired from

Au-coated sample powders using a Philips SEM 505 micro-

scope (Philips, Holland) with a LaB6 filament, operating at

4 keV.

Transmission electron micrographs (TEMs) were obtained

with a Philips CM 300 microscope using an accelerating

voltage of 120 kV. The samples were lightly ground and then

dispersed ultrasonically in ethanol. A drop of the suspension

was evaporated on the holey carbon films, pre-deposited on

150 mesh copper grids.

X-ray diffraction patterns were obtained with a Rigaku X-

ray diffractometer D/MAX-2200/PC (Rigaku, Japan) using Cu

Ka radiation (35 kV, 30 mA) at the rate of 0.5 �/min. The layer

distance (dhkl) and unit cell parameter (a0) were determined

from XRD data by the Bragg equation, dhkl¼ l/(2sin q) and

a0¼ 2dhkl/O3. The wall thickness (Wp) was the difference of

layer distance (d ) and pore size.

N2 adsorption–desorption isotherms were obtained at 77 K

on a Belsorp-mini II using static adsorption procedures. Before

the measurement, samples were degassed at 350 �C for 4 h.

The BET surface area, average pore size and pore volume of

the sample were calculated with software.

Thermal analysis (TG/DTA) was carried out on a Q600SDT

thermal analyzer (TA, U.S.A.) with temperatures ranging from

room temperature to 900 �C at a heating rate of 20 �C/min.

2.3. Preparation of Pd-doped silica

The Pd doped SMS materials were prepared by an impregna-

tion-hydrogen reduction process. The typical procedure for

preparation of Pd/SMS-70-C (5 wt% Pd) was as follows: 1.0 g of

SMS-70-C sample was impregnated for 30 min with a solution

of PdCl2 containing 0.0833 g of PdCl2, dried overnight at 70 �C

in a vacuum oven, then reduced in a tubular furnace at 200 �C

in an H2 flow (20 ml/min) for 2 h.

2.4. Measurement of hydrogen storage capacity

Measurement of the hydrogen storage capacity and recycling

stability of the samples was carried out with a lab-made

hydrogen storage measurement system (Fig. 1) at room

temperature and 0.4–1.2 MPa pressure. High-purity hydrogen

gas (99.999%) was used. The amount of adsorbed and desorbed

hydrogen was measured accurately by two mass flow-meters

with an error of less than 0.1 wt%. Every storage amount was

the average calculated from five cycles of adsorption–

desorption.

3. Results and discussion

3.1. Morphology of SMS and Pd/SMS

Fig. 2(A) shows an SEM image of SMS that was synthesized at

70 �C for 24 h and calcined at 550 �C for 4 h in air. Fig. 2(B) and

(C) shows TEM images of SMS-70-C. The SEM image clearly

shows the sample’s spherical shape, and its worm-like pore

structure is observable in the TEM images, which also reveal

that the spheres are solid and have diameters ranging from

500 nm to 1 mm. Fig. 2(D) and (E) are TEM images of Pd/SMS; it

is clearly shown that the palladium is highly dispersed on the

SMS sphere surfaces.

The addition of F127 plays a crucial role in the formation of

grape-like spheres. If no F127 was added, the product was

a lamellar porous material with a worm-like porous structure,

as reported earlier by Tanev et al. [19,20]. It seems that the

addition of F127 produces a change in shape but not in pore

structure. Based on the above, we have proposed a schematic

model (Fig. 3) for the formation of SMS and the interaction of

the two templates.

When DADD and F127 were dissolved in the mixture of

ethanol and de-ionized water, a type of micelle was formed

due to the interaction of the two types of templates. The

hydrophilic groups of DADD and F127 extended to the outer

Gas outlet

Three way valve

AutoclaveVacuum pump

Pressure reducing valve

Mass flow meter

Pressure meter Vacuum pressure meter

Mass flow meter

Hydrogen source

Fig. 1 – Schematic diagram of the hydrogen storage measurement system.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 8 1 0 – 3 8 1 53812

interface with the solvent molecules. The spherical structure

was formed by the self-assembly process of various amounts

of micelles. After the addition of TEOS, the spherical porous

material was formed by the combination of partially hydro-

lyzed Si(OC2H5)4�xOHx species with the hydrophilic group of

the surfactants in the micelles.

3.2. Structure of SMS by XRD and N2 adsorption–desorption isotherms method analysis

For the synthesized samples, the templates could be removed

by either calcination or extraction. A shift in the diffraction

Fig. 2 – SEM and TEM images of calcined SMS-70-C sample: (A)

images of Pd/SMS-70 sample.

peak could be observed when the template was removed by

calcination, shown in Fig. 4A(b). The diffraction peak of the

sample shifted ca. 0.4�, compared with no shift when the

template was removed by extraction [Fig. 4A(c)].

Fig. 4B and C shows the XRD patterns of a series of samples

synthesized at various temperatures. The result illustrates

that the diffraction angle of the samples shifted toward the

small angle domain with increasing synthesis temperature,

indicating that higher synthesis temperature results in larger

pore size. Table 1 provides more detailed information for

samples synthesized at different temperatures, and shows

that the average pore size always increased with increasing

SEM image; (B) and (C) HRTEM images; (D) and (E) HRTEM

Si

Mesoporous framework

Addition

ethanol

and H2O

TEOS

additionCalcination

+

F127DADD

Self-assembly

Solid silica sphereMicella Partial structure

Fig. 3 – Proposed mechanism for the formation of solid sphere super-microporous silica.

2 3 4 5 6 7 8 9 10

3.543.95

A

cb

aIn

ten

sity / a.u

.

2 theta / deg

2 3 4 5 6 7 8 9 102 theta / deg

2 3 4 5 6 7 8 9 102 theta / deg

B

d

cb

a

In

ten

sity / a.u

.

C

d

cb a

In

ten

sity / a.u

.

Fig. 4 – Low angle XRD patterns of SMS samples. (A) SMS samples synthesized at 30 8C: a, as-synthesized; b, templates

removed by calcination; c, templates removed by extraction. (B) SMS samples synthesized at: a, 30 8C; b, 50 8C; c, 70 8C; d,

90 8C, and templates removal by calcination. (C) SMS samples synthesized at: a, 30 8C; b, 50 8C; c, 70 8C; d, 90 8C with

templates removed by extraction.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 8 1 0 – 3 8 1 5 3813

synthesis temperature, regardless of whether the template

was removed by calcination or extraction.

However, different results were observed for specific

surface areas using BET theory, as shown in Table 1. Firstly,

Table 1 – Characteristics of the crystalline structure andpore structure of SMS materials synthesized at differenttemperature.

Sample dhkla

(nm)a0

b

(nm)Averagepore sizedp (nm)

WallthicknessWp

c (nm)

Vt

(cm3 g�1)BET

surfacearea

(m2 g�1)

SMS-30-

E

2.23 2.58 1.76 0.82 0.4392 723

SMS-50-

E

2.45 2.83 1.80 1.03 0.4192 931

SMS-70-

E

2.85 3.29 1.86 1.42 0.3487 814

SMS-90-

E

3.31 3.82 1.94 1.88 0.3202 645

SMS-30-

C

2.49 2.88 1.63 1.25 0.4634 946

SMS-50-

C

2.57 2.97 1.69 1.28 0.4343 1028

SMS-70-

C

2.95 3.41 1.87 1.55 0.3788 865

SMS-90-

C

3.53 4.08 1.97 2.11 0.3691 749

a dhkl¼ l/(2sin q).

b a0¼ 2dhkl/O3.

c Wp¼ a0� dp.

all samples in which the templates were removed by calci-

nation exhibited higher surface areas than those synthesized

at the same temperature but whose templates were removed

by extraction. Secondly, the specific surface areas did not

change linearly with the synthesis temperature, but maximal

values were observed for samples synthesized at 50 �C. For

samples SMS-50-E and SMS-50-C, the surface areas were

931 m2/g and 1028 m2/g, respectively. Thirdly, the pore

volume and wall thickness exhibited linear changes,

decreasing and increasing, respectively, with the increases in

temperature. All BET surface areas, average pore sizes and

pore volumes were calculated from N2 adsorption–desorption

isotherms, shown in Fig. 5. As can be seen there, no distinct

hysteresis loop exists between the adsorption and desorption

isotherms, implying that the samples possess a mean pore

distribution.

3.3. TG analysis and thermal stability

The TG/DTA profiles of the as-synthesized SMS-70 samples

are shown in Fig. 6. Three weight loss regions are distin-

guishable on the TG curve. The first weight loss of ca. 5 wt%

occurred at 25–100 �C and can be attributed to the elimina-

tion of water and ethanol. The second weight loss of 14 wt%

in the range of 100–400 �C can be ascribed to the decompo-

sition of the DADD and F127 organic template. The third

weight loss of 12 wt% above 400 �C can be assigned to the

calcination of residual coke generated by decomposition of

the templates, and the partial loss of silanol groups [21,22].

The total weight loss from 25 �C to 550 �C was approximately

0

50

100

150

200

250

300

350V

a / cm

3 (S

TP

) g

-1

p / p0

d

c

a

b

0.0 0.2 0.4 0.6 0.8 1.0

Fig. 5 – N2 adsorption–desorption isotherms for calcined

SMS materials synthesized at different temperatures: a,

30 8C; b, 50 8C; c, 70 8C; d, 90 8C. The closed symbols

represent the adsorption isotherm and the open symbols

represent the desorption isotherm.

Table 2 – Hydrogen storage capacity versus hydrogenpressure for samples.

Sample Mass flowa

(sccm)Pressure

(MPa)Hydrogen storage

(wt%)

SMS-70-C 200 0.4 –

Pd/SMS-

70-C

200 0.4 1.78

Pd/SMS-

70-C

200 0.8 2.02

Pd/SMS-

70-C

200 1.2 2.56

a sccm, Standard-state cubic centimeter per minute.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 4 ( 2 0 0 9 ) 3 8 1 0 – 3 8 1 53814

34 wt%, which coincides with the molar amount of template

used to prepare the material, indicating that the formation of

super-microporous silica with the DADD/F127 templates

involved the assembly of the organic templates and the TEOS

hydrolysis products into solid silica sphere aggregates.

According to TG and XRD analysis of the SMS samples, all

samples calcined in air at 550 �C were stable, which implies

that the synthesized spherical porous materials possess good

thermal stability.

3.4. Hydrogen storage performance of SMS material

As shown in Table 2, when the synthesized grape like spher-

ical porous materials were modified by supporting 5 wt%

80.8101

365.547

430.44

499.981

0 200 400 600 80050

60

70

80

90

100

110

120

b

a

Temperature / oC

Weig

ht / %

-20

-10

0

10

20

30

40

Heat F

lo

w / (m

W)

Fig. 6 – TG-DTA profiles of as-synthesized SMS synthesized

at 70 8C.

palladium, they exhibited good hydrogen storage capacities.

At room temperature and 0.4 MPa, the hydrogen storage

capacity of Pd/SMS-70-C was 1.78 wt%, and when the pressure

of hydrogen was raised to 1.2 MPa, its hydrogen capacity could

be high up to 2.56 wt%, which is higher than that of the Pd-Ni/

CNTs sample we reported earlier [9]. We propose that the

hydrogen storage capacity of Pd/SMS-70-C results from its

very high surface area, spherical shape, and appropriate

worm-like pore structure. The detailed mechanism of

hydrogen storage in spherical porous materials will be further

investigated and reported.

4. Conclusion

By using F127 as the co-template and DADD as the main

template, a porous material with worm-like pore structure

and beautiful grape-like spherical morphology has been

successfully synthesized. For the sample synthesized at 70 �C,

the pore size is ca. 1.87 nm and the surface area is up to

865 m2/g. It was verified that the addition of F127 is crucial for

the spherical morphology. Of additional importance was the

finding that when this material was modified by depositing

a small amount of palladium, it exhibited good hydrogen

storage properties at room temperature and low pressure,

with a storage capacity of up to 2.56 wt%, which might make it

a promising hydrogen storage candidate.

Acknowledgments

The authors thank the National Scientific Foundations of

China (NSFC, Project Nos. 20467034, 20673040) and the

Guangdong Provincial Scientific Foundation (Project No.

36055) for financial support, and the Ankersmid China

Corporation for N2 adsorption–desorption isotherms analysis

support.

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