React. Kinet. Catal. Lett., Vol. 54, No. i, 29-34 (1995)

RKCL2519

HIGH SURFACE AREA SILICON CARBIDE FROM RICE HUSK:

A SUPPORT MATERIAL FOR CATALYSTS

S.K. Singh, K.M. Parida, B.C. Mohanty and S.B. Rao

Regional Research Laboratory

Bhubanesw~r - 751 013, Orissa, India

Received April 25,1994 Accepted May 18, 1994

Ultrafine 8-SIC with high surface area (150 m 2 g-l)

has been synthesized by inflight processing of charred

rice husk in a r.f. plasma reactor operating at atmos-

pheric pressure. The plasma-synthesized particles were

doped with platinum (1%) and tested as a catalytic

support material. The catalyst (1% Pt doped 8-SIC)

showed 100% conversion of CO to CO 2 at a temperature

as low as 175 ~

INTRODUCTION

In recent years, increasing attention has been given to

prepare new catalysts and catalyst supports [1,2], which have

high thermal and chemical stability. Silicon carbide is con-

sidered as one of the most promising of such materials. The in-

dustrial preparation of SiC by the Acheson process produces a

material of very low specific surface area and therefore is not

suitable as a support in heterogeneous catalysis. This paper

describes a new process for synthesis of high specific surface

area SiC starting from an agricultural waste such as rice husk.

The SiC powder, thus prepared was further used as a catalyst

support.

Akad4miai Kiad6, Budapest

SINGH et al.: SILICON CARBIDE

EXPERIMENTAL

A finer fraction (~i05 ~m) of the semiground rice husk

was charred at 550 ~ for 2 h in nitrogen. The resultant prod-

uct (charred rice husk in the form of a fine black powder) was

subsequently pyrolyzed by plasma processing in a 20 kW r.f.

plasma reactor operating at i atmosphere. The details of the

reactor assembly were given elsewhere [3,4]. The charred rice

husk was injected into the plasma by means of a syringe-pump

driven pneumatic powder feeder. Argon was used as both plasma-

generating gas and carrier gas. Plasma-synthesized powder was

deposited at the inner walls of the reaction chamber. The pow-

der was very loosely attached to the wall, therefore, it could

easily be collected by scrapping.

Platinum (1%) doped silicon carbide was prepared by a-

dopting an impregnation technique. Plasma-synthesized SiC pow-

ders were added to chloroplatinic acid solution and the mix-

ture was evaporated to dryness, while stirring ona hot plate

with a magnetic stirrer. The resulting chloroplatinic acid

soaked silicon carbide was calcined at 600 ~ in a muffle fur-

nace. Figure 1 shows the flow diagram of the process.

Catalytic activity studies for the conversion of CO to

CO 2 for doped and undoped SiC samples were carried out in a stat-

ic bed quartz reactor by taking 100 mg of the sample and pass-

ing a gas mixture of CO (5%) and air (95%) at atmospheric

pressure. The temperature and flow rate were varied from i00 to

700 ~ and from 6 to 10 Lh -I, respectively. The reaction pro-

ducts were analyzed by on-line gas chromatography using Pora-

pak-Q column.

RESULTS AND DISCUSSION

Rice husk consists of silica in hydrated amorphous form

and cellulose [5]. The formation of silicon carbide from rice

husk can be described in two steps. In the first step, the husk

is charred in the absence of air at a relatively low tempera-

ture range (500 to 900 ~ to remove volatiles and to decom-

pose cellulose into amorphous carbon. In the second step, the

30

SINGH et al.: SILICON CARBIDE

! Raw Rice Husk

J

V I round and Sieved

V

I Charring 550~ in Nitrogen

V

I lasma Pyrolysis V

Fig. I.

Ultrafinz [~- SiC

V

Platinum doping I V

High Surfacr Area SiC:Pt(1% ) Catalyst

Flow diagram of the process

charred husk is fired at high temperature (>1500~ in an inert

or reducing atmosphere. The presence of high surface area sili-

ca in intimate association with active carbon in charred rice

husk makes it amenable for ready conversion to SiC during high

temperature pyrolysis. The possible reaction was given by Lee

and Cutler [6] as

SiO 2 (amorphous) + 3C (amorphous) § SiC + 2C0 (I)

The reaction time can be reduced greatly by increasing the

temperature. The effect of CO from the reaction may be suffi-

ciently significant to decrease the reaction rate. Thus, CO

needs to be constantly flushed out with argon. Both of the a-

bove processes can easily be attained in thermal plasma reac-

31

SINGH et al.: SILICON CARBIDE

.--

L.

c m

Fig. 2.

~-SiC

I~-sic 5-sic (b)

,,,,[,,,,l,,,,l,,,,i,,,,l,,,, i,,,, l,,,,l,,,,l,,,, ,,,,+l,,,,,,,,,i,,,,

20 40 60 BO 100 120 140

28 ---m,-

X-ray diffraction patterns (Cu K s radiation)

of (a) charred rice husk (550 ~ for 2 h in

nitrogen) and (b) plasma-treated rice husk

tors. Moreover, very high temperatures (104 K), steep tempera-

ture gradients (106 Ks -1 ) and high quench rates (106 Km -I) as-

sociated with thermal plasma can be a unique route for the

preparation of ultrafine SiC from rice husk.

The X-ray diffraction (XRD) (Fig. 2a) of the charred husk

represents a curve with a maximum at 20=22 ~ , characteristic of

silica in the amorphous form. The formation of E-SiC is clearly

evident from the XRD pattern (Fig. 2b) of the plasma-treated

powder. The XRD peaks are rather broad, indicating the ultra-

fine nature of the powder. This is confirmed by field emission

scanning electron microscopy (FESEM), (Fig. 3). The particles

are in the range of 20-30 nm.

Platinum loaded SiC samples were found to be very active

and showed 100% conversion of CO to CO 2 at a temperature as

low as 175 ~ On the contrary, undoped SiC samples were found

to be inactive although the powders have high specific surface

area (150 m 2 g-l). The activity of the platinum doped samples

did not change with change in flow rate or increase in temper-

32

SINGH et al.: SILICON CARBIDE

Fig. 3. FESEM micrograph of the plasma-synthesized

SiC powder

ature up to 700 ~ The samples did not deactivate even after

30 h of reaction. The high catalytic performance of the plati-

num doped samples is perhaps due to the uniform distribution

of platinum on the surface of SiC particles. Detailed inves-

tigation is in progress and likely to provide a clear picture

on the catalytic performance of the samples.

CONCLUSION

Thermal plasma processing appears to be an attractive

route for converting rice husk, an agricultural waste, to high

surface area SiC, which is suitable for use as a catalytic

support.

33

SINGH et al.: SILICON CARBIDE

Acknowledgements. The authors thank Dr. S.L. Girshick, Prof. E.

Pfender and Dr. L. Stachowicz, University of Minnesota, Minnea-

polis, USA, for their support and help. The authors also wish

to thank the Director, Regional Research Laboratory, Bhubanes-

war, for granting permission to publish this paper.

REFERENCES

i. P.W. Lednor: Catalysis Today, 15, 243 (1992).

2. M.J. Ledoux, C. Pham-Huu: Catalysis Today, 15, 263 (1992).

3. S.L. Girshick, C.P. Chiu, R. Muno, C.Y. Wu, L. Yang, S.K.

Singh, P.H. McMurry: 24, 367 (1993).

4. C.Y. Wu: M.S. Thesis, University of Minnesota, Minneapolis,

1990.

5. N.K. Sharma, W.S. Williams, A. Zangvil: J. Am. Ceram. Soc.,

67, 715 (1984).

6. J.G. Lee, I.B. Cutler: Amer. Ceram. Soc. Bull., 5_44, 195

(1975).

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