Materials Science Communication

The study of thermal stability of the SiO2 powderswith high speci®c surface area

Liwei Wang, Zichen Wang*, Hua Yang, Guangli YangDepartment of Chemistry, Jilin University, Changchun 130023, China

Received 20 May 1998; received in revised form 20 August 1998; accepted 28 August 1998

Abstract

The SiO2 powders with high speci®c surface area were synthesized by the precipitation method. The effects of calcined temperature on

the structure of the SiO2 powders were characterized by infrared spectrum, X-ray diffraction, scan electron microscopy. It is shown that the

SiO2 powders with high speci®c surface area were stable at high temperatures. When heated at different temperatures below 6508C, the

speci®c surface area of the SiO2 powders hardly changed, therefore the SiO2 powders have better thermal stability. # 1999 Elsevier

Science S.A. All rights reserved.

Keywords: High speci®c surface area; SiO2; Thermal stability; Porous

1. Introduction

With the development of materials, many scienti®c

researchers have paid much attention to the porous materi-

als. Porous silica combines a lot of excellent physical and

chemical properties [1,2] which makes it possible for a wide

range of materials to be accommodated. The structure [3,4]

of porous silica has many advantages for chemical-sensor

applications [5] and catalyst supporters such as the high

speci®c surface area which can enhance the interfacial

reaction and improve the selectivity, etc [6,7]. But porous

materials must have better thermal stability when they are

applied [8,9], so it is important that the thermal stability of

porous materials is studied. Thermal stability of silica with

high speci®c surface area has not been systematically studied

even though silica has been widely used in various ®elds.

There are many papers about synthesis of the porous silica

which are prepared via the sol±gel route [10±13]. This paper

brie¯y describes our synthesis and characterization of the

silica powders with high speci®c surface area. The thermal

stability of silica was primarily studied.

2. Experimental

2.1. Preparation of the SiO2 powders

There are water glass (mode number is 3.3), hydrochloric

acid and an amount of the surfactant in the reactive system.

The reactive temperature is kept at 508C to form precursor.

The stable agent was added under vigorous stirring when the

pH is equal to 8. Then the formed precursors were washed,

dried, and calcined at 4708C for 1 h and then the SiO2

powders with high speci®c surface area were produced.

2.2. Measurement of thermal stability

The SiO2 powders with high speci®c surface area were

heated at different temperatures. The structure of the sample

was measured by infrared spectrum. The phase character-

izations of the sample were researched by X-ray diffraction

(XRD). The speci®c surface area and pore diameter of the

samples were measured by BET autocontrol physical adsor-

bent instrument (ASAP2400). The morphology and grain

size of the powders were determined by a scanning electron

microscope (HITACHI X-650).

3. Results and discussion

3.1. X-ray analysis

Two SiO2 samples with different speci®c surface areas

were calcined at different temperatures. Fig. 1(a) shows that

the structure of the SiO2 powder with lower speci®c surface

area ((a) 660 m2 gÿ1) calcined from 5008C to 9008C is

amorphous; the structure of the SiO2 powder calcined at

9508C is the cubic christobalite. From Fig. 1(b) it is

observed that the structure of the SiO2 powder with higher

Materials Chemistry and Physics 57 (1999) 260±263

*Corresponding author. E-mail: wangzc@mail.jlu.edu.cn

0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

P I I : S 0 2 5 4 - 0 5 8 4 ( 9 8 ) 0 0 2 2 6 - 0

speci®c surface area ((b) 1046 m2 gÿ1) is amorphous cal-

cined from 5008C to 10008C; the structure of the powder

calcined at 11008C is cubic. So the sample with higher

speci®c surface area has better thermal stability.

3.2. IR measurement

In order to study the effect of the calcined temperature on

the structure of the SiO2 powders, the speci®c surface area

and IR spectra of the samples heated at various temperatures

are shown in Table 1 and Fig. 2, respectively.

With an increase of the temperature, the absorbent peaks

at 3451, 1629 and 969 cmÿ1 decreased gradually. It is shown

that structural water, capillary pore water and surface

absorptive water decreased at high temperatures. The peak

at 969 cmÿ1 resulted from the bending vibration of Si±OH

[14], its disappearance shows that the structure of meta-

silicic acid gel (SiO2�XH2O) were destroyed. In the gel, the

reaction may be followed:

SiÿOH � HOÿSi!ÿSiÿOÿSiÿ�H2O (1)

Fig. 2(a) and (b) shows that the SiO2 powders with

different speci®c surface area were calcined at the same

temperature, but the structures of the SiO2 were destroyed at

different temperatures. The peak at 969 cmÿ1 of the sample

with smaller speci®c surface area disappeared at 7008C, but

the peak of the sample with larger speci®c surface area

disappeared at 8008C. Because there is Si±OH group on the

surface of the SiO2 with higher speci®c surface area, the Si±

OH of the sample is dishydrated to form Si±O±Si at higher

temperature; as for the sample with lower speci®c surface

area, the Si±OH is dishydrated at lower temperature. When

the samples with higher speci®c surface area were heated at

9508C, the peaks at 3472 and 1637 cmÿ1 are weak due to the

smaller grain size, higher activity of the SiO2 and absorbing

water at air.

From above we conclude that the temperature at which

the structure of the SiO2 with higher speci®c surface area

was destroyed is higher than that of the SiO2 with low

speci®c surface area. It can been seen from IR spectra and

Fig. 1. XRD patterns of the SiO2 powders calcined at different

temperatures (a) 660 m2 gÿ1, (b) 1046 m2 gÿ1.

Table 1

The specific surface area of SiO2 calcined at different temperatures

Line

No.

Temperature

(8C)

Specific surface

area (m2 gÿ1)

L1 500 660

L2 700 400

L3 800 67

L4 900 27

L5 950 7

M1 470 1046

M2 600 982

M3 700 814

M4 800 184

M5 950 8

Fig. 2. Infrared spectra of SiO2 calcined at different temperatures.

L. Wang et al. / Materials Chemistry and Physics 57 (1999) 260±263 261

XRD that the structure of the sample with lower speci®c

surface area is transmitted from amorphous to crystalline at

9508C, with higher speci®c surface area at 11008C.

3.3. Measurement of the specific surface area

The speci®c surface areas of samples being measured by

BET were shown in Fig. 3.

With the rise of temperature, the grains of the samples

were agglutinated, the gel structure of the sample was

destroyed, the number of the micropores of the samples

decreased, so its speci®c surface areas depressed. This

process was made of two stages: the speci®c surface area

decreased gradually from 5008C to 7008C and decreased

steeply after 7008C. The speci®c surface area was below

200 m2 gÿ1 at 8008C. It is shown that its micropores were

agglutinated and only outer surface existed at high tem-

perature, therefore the speci®c surface area decreased. With

the rise of temperature the outer surface area still decreased.

The speci®c surface area of crystalline SiO2 had the mini-

mum at 9508C.

So we conclude that the effect of calcined temperature on

the structure of the SiO2 is related to the speci®c surface area

of the samples. The effect of calcined temperature on the

sample with larger speci®c surface area is more obvious

than that of the samples with smaller speci®c surface area.

The temperature at which the structure of the sample with

larger speci®c surface area was changed is higher than that

of the sample with smaller speci®c surface area.

The thermal stability of SiO2 with the speci®c surface

area 982 m2 gÿ1 which had been calcined at 6508C was

tested at different temperatures for 10 h. The speci®c sur-

face areas were listed in Table 2. It is shown that the porous

SiO2 is stable below 6508C and its speci®c surface area does

not change.

By controlling the calcined temperature from 4708C to

5008C, purer SiO2 powders with higher speci®c surface

areas were prepared. Because there are many micropores in

SiO2, SiO2 as the carrier of the catalyst, will apply to the

®eld of the catalytic reaction. There is better thermal

stability for the catalyst below 7008C.

3.4. SEM analysis

The morphology and grain size of the SiO2 calcined at

different temperatures were observed by SEM in Fig. 4. It is

shown that the grain size of the sample did not change below

9008C, but at 11008C the grains were sintered and the

structure of the sample was changed from amorphous to

crystalline structure.

The grain size and outer-surface of SiO2 did not change

obviously from 5008C to 9008C. It is concluded that at high

temperature the speci®c surface area of the sample

decreases because of the sintered internal pore.

4. Conclusion

1. The porous SiO2 with high speci®c surface area was

prepared by chemical precipitation. The materials have

uniform pore diameter of about 25 AÊ .

2. The larger the specific surface area of the sample is, the

better the thermal stability of the sample will be.

Acknowledgements

The project was supported by the National Natural

Science Foundation of China.

Fig. 3. The effect of the calcined temperature on the specific surface area.

Table 2

The specific surface area of SiO2 heated at different temperature

T (8C) 100 200 300 400 500 600

S (m2 gÿ1) 982 985 983 980 982 984

Fig. 4. SEM images of SiO2 powders calcined at different temperatures.

262 L. Wang et al. / Materials Chemistry and Physics 57 (1999) 260±263

References

[1] J. Shen, Y. Wang, J. Inorg. Mater. 10(1) (1995) 69.

[2] A. Venkateswara Rao, G.M. Pajonk, J. Mater. Sci. 29 (1994) 1807.

[3] R. Birringer, H. Gleiter, Phys. Lett. 102A (1984) 365.

[4] B.I. Lee, Kuotung Chou, J. Mater. Sci. 31 (1996) 1367.

[5] X. Yao, L. Zhang, S. Wang, Sensors and Actuators B 24±25 (1995)

347.

[6] S. Henning, L. Svensson, Phys. Sci. 23 (1981) 697.

[7] M. Granauer, J. Fricke, Acustica 59 (1986) 177.

[8] J. Rupp, R. Birringer, Phys. Rev. B 36 (1987) 7888.

[9] E.W. Bittner, B.C. Bockrath, J. Catal. 149 (1994) 206.

[10] Sridhar Komarneni, Rustum Roy, J. Mater. Res. 8(12) (1993) 3163.

[11] M. Inooe, H. Kominami, J. Mater. Sci. 29 (1994) 2459.

[12] Dong Jin Suh, Tae-Jin Park, Chem. Mater. 8 (1996) 509.

[13] Akiko Hirose, Hiroshi Yamashita, Anal. Sci. 10 (1994) 737.

[14] Cuidi Le, Lanfen Huang, The field of the Chemistry 4 (1991) 149.

L. Wang et al. / Materials Chemistry and Physics 57 (1999) 260±263 263