Effect of Two-Stage Process on the Preparation and Characterization

Mar 13, 2004 - ... Adyar, Chennai, Tamil Nadu, India, and Department of Chemistry, Loyola ... Citation data is made available by participants in Cross...
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Ind. Eng. Chem. Res. 2004, 43, 1832-1838

MATERIALS AND INTERFACES Effect of Two-Stage Process on the Preparation and Characterization of Porous Carbon Composite from Rice Husk by Phosphoric Acid Activation L. John Kennedy,† J. Judith Vijaya,‡ and G. Sekaran*,† Department of Environmental Technology, Central Leather Research Institute, Adyar, Chennai, Tamil Nadu, India, and Department of Chemistry, Loyola Institute of Frontier Energy, Loyola College, Chennai, Tamil Nadu, India

Activated carbon composite prepared from rice husk using phosphoric acid activation has been studied through precarbonization of the precursor followed by chemical activation. This method can produce carbons with micro- and mesoporous structure. The ratio of chemical activating agent to precarbonized carbon was fixed at 4.2. The surface area, pore volume, and pore size distribution of carbon composite samples activated at three different temperatures (700, 800, and 900 °C) were measured using nitrogen adsorption isotherms at 77 K. The pore-opening and pore-widening effects occurred simultaneously during the process, as evidenced by scanning electron micrographs. The X-ray diffraction curve revealed the evolution of crystallites of carbon and silica during activation at higher temperature. The FTIR spectrum also provided evidence for the presence of silica in the carbon composite. The proper choice of the preparation conditions had an influence on the micropore and mesopore volumes of the activated carbon composite, which were 0.1187 and 0.2684 cm3/g, respectively. The production yield was observed to decrease with increasing activation temperature. Introduction Porous carbon, commonly known as activated carbon, is prepared by physical activation, i.e., gasification of chars in an oxidizing atmosphere, or by chemical activation, i.e., carbonization of carbonaceous materials impregnated with chemical reagents.1 The commonly used precursors are bituminous coal, wood, coconut shell, peat, petroleum, pitch, and polymers.2 Separation and purification of gas or liquid mixtures by adsorption has become a major unit operation in various chemical and petrochemical industries.3 Activated carbons produced from various types of carbonaceous materials have been employed as sorbents in such processes. At the present time, chemical activation is preferred over physical activation for its lowered activation temperature and increased yield. Among the numerous chemical activants such as KOH, ZnCl2, H3PO4, HCl, etc., H3PO4 is widely used for this process as it can be removed easily after activation of the carbon by washing with hot and cold water. In the present investigation, the raw material used is rice husk. Rice husk contains organic matter such as sugars, lignin cellulose, protein, etc., and inorganic matter consisting of silica. Rice husk has a * To whom correspondence should be addressed. Address: Dr. G. Sekaran, Scientist & Sr. Asst. Director, Department of Environmental Technology, Central Leather Research Institute, Adyar, Chennai, India. E-mail: [email protected]. Tel.: 044-24911386 Ext. 341. † Central Leather Research Institute. ‡ Loyola Institute of Frontier Energy.

low calorific value of 3585 kcal/kg.4 The manufacture of activated carbons involves two steps. The first step involves the synchronous process of carbonization of the raw materials in an inert atmosphere, and the second step involves the synchronous process of activation of the carbonized products. The carbonization consists of thermal decomposition of the rice husk materials, eliminating non-carbon species other than silica and producing a fixed carbon mass with a rudimentary pore structure. Very fine and closed pores are created during this step. The purpose of activation is to enlarge the diameter of the pores and to create new pores. According to the International Union of Pure and Applied Chemistry (IUPAC), the pores of porous materials are classified into three groups: micropores (width d < 2 nm), mesopores (2 nm < d < 50 nm), and macropores (d > 50 nm).5 Porous carbon materials play a significant role in adsorption technologies, especially in new applications such as catalytic supports, battery electrodes, capacitors, gas storage, and biomedical engineering. Such applications require the carbon materials to exhibit not only high surface areas but also high ratios of mesopores or macropores to micropores, because many macromolecules and ions exceed the pore diameters of micropores and thus have difficulty entering them. The objective of this study is to produce a low-cost activated carbon with mesoporic ranges using rice husk as the raw material. Several studies6-10 on chemical activation using ZnCl2 have been conducted to maximize the adsorptive capacity and bulk density of activated

10.1021/ie034093f CCC: $27.50 © 2004 American Chemical Society Published on Web 03/13/2004

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carbons produced from lignocellulosic materials such as peach stones.11 However, phosphoric acid is preferred because it does not entail the problems with corrosion, inefficient chemical recovery, and other environmental disadvantages that are associated with ZnCl2.12 Phosphoric acid activation has been applied to coconut shell;13 peach stones;14 coals and hard woods;15 and shells of nuts such as almonds, pecans, and English walnuts. These precursors are of cellulosic or lignocellulosic origin. There has been very little or perhaps no study focusing on the use of H3PO4 as a chemical activating agent for rice husk. Experimental Section Materials. Rice husk as the precursor material obtained from the agricultural industry was well washed with H2O several times for the removal of dust and used after oven drying at 110 °C for 6 h. The dried samples were then sieved to about 600-µm in size, and this fraction was used for the preparation of carbon. Methods. Preparation of Activated Carbons. Porous carbons were prepared in two sequential steps: precarbonization and chemical activation. In the precarbonization process, the rice husk was heated at 400 °C at the rate of 10 °C/min for about 4 h under N2 atmosphere and cooled to room temperature at the same rate. The resulting material is labeled as precarbonized carbon (PCC). The precarbonized carbon is subjected to chemical activation. In the chemical activation process, 50 g of the precarbonized carbon was agitated with 250 g of aqueous solution containing 85% H3PO4 by weight. The ratio of chemical activating agent to precarbonized carbon was fixed at 4.2. The chemical activant and precarbonized carbon were homogeneously mixed at 85 °C for 4 h. After being mixed, the precarbonized carbon slurry was dried under vacuum at 110 °C for 24 h. The resulting samples were then activated in a vertical cylindrical furnace under N2 atmosphere at a flow rate of 100 mL/min. This was followed by heating to one of three different temperatures, namely, 700, 800, and 900 °C, at a heating rate of 5 °C/min using a programmer and maintained at the final temperature for 1 h before cooling. After being cooled, the activated carbon was washed successively several times with hot water until the pH became neutral and finally washed with cold water to remove the excess phosphorus compounds. The washed samples were dried at 110 °C to obtain the final product. The samples heated at activation temperatures of 700, 800, and 900 °C were labeled C700, C800, and C900, respectively. N2 Adsorption-Desorption. N2 adsorption-desorption isotherms of the activated carbon silica composites were measured using an automatic adsorption instrument (Quantachrome Corp., Nova-1000 gas sorption analyzer) for the determination of the surface area and the total pore volumes. Prior to measurement, the carbon samples were degassed at 150 °C overnight. The nitrogen adsorption-desorption data were recorded at liquid nitrogen temperature, 77 K. The surface areas were calculated using the BET equation, which is the most widely used model for determining the specific surface area. In addition, the t-plot method16 was applied to calculate the micropore volume and external surface area (mesoporous surface area). The total pore volume was estimated to be the liquid volume of adsorbate at a relative pressure of 0.99. All surface area measurements were calculated from the nitrogen ad-

sorption isotherms by assuming the area of the nitrogen molecule to be 0.162 nm2. X-ray Diffraction Technique. X-ray diffraction experiments were performed with a Philips X’pert diffractometer for 2θ values from 10 to 80° using Cu KR radiation at a wavelength of λ ) 1.54060 Å. The other experimental conditions included 1/2° divergence slits, a 5-s residence time at each step, and intensity measured in counts. FT-IR Studies. A Perkin-Elmer infrared spectrometer was used for the investigation of the surface functional groups. The carbon samples were mixed with KBr of spectroscopic grade and made into pellets at a pressure of about 1 MPa. The pellets were about 10 mm in diameter and 1 mm in thickness. The samples were scanned in the spectral range of 4000-400 cm-1. Surface Morphology. Surface morphology measurements were carried out on the samples using a LeoJEOL scanning electron microscope. The carbon composite samples were coated with gold by a gold sputtering device for clear visibility of the surface morphology. Production Yield. The yield of the activated carbon is defined as the ratio of the weight of the resulting activated carbon to that of the original rice husk, with both weights being measured on a dry basis.17

yield (%) ) W2/W0 × 100 where W0 is the original mass of the precursor on a dry basis and W2 is the mass of the carbon after activation, washing, and drying. Results and Discussion The precursor rice husk has a very complicated composition. The main components of rice husk are listed in Table 1.18 The process of preparing activated carbon consists of precarbonizing the precursor at 400 °C for 4 h under nitrogen atmosphere; during this step, dehydration of cellulose, lignin, and other components takes place. In the chemical activation process, the concentration of the activation agent was maintained, and care was taken to achieve a considerable surface area, pore volume, and pore size distribution at different activation temperatures. Nitrogen Isotherms. Figure 1 presents nitrogen adsorption/desorption isotherms at 77 K of precarbonized carbon PCC and carbons chemically activated at temperatures of 700, 800, and 900 °C, labeled C700, C800, and C900, respectively. The isotherms of the precarbonized carbon (PCC) are of type I as the adsorption and desorption branches remain nearly horizontal and parallel over a wide range of relative pressures, which is the characteristic behavior of microporous materials. However, in the isotherms for the samples chemically activated at activation temperatures of 700, 800, and 900 °C, the knee becomes rounded. The hysteresis effect and the slope of the plateau increased to yield type IV isotherms with a significant increase in the nitrogen uptake through the entire pressure range, indicating the presence of mesopores. This increase in uptake of nitrogen in the samples with increasing temperature is a result of the major increase in porosity created from the carbon and silica components. Thus, the uptake of the nitrogen increases as the heat treatment temperature is increased. The carbon activated at 900 °C exhibited the most prominent hysteresis effect,

1834 Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004 Table 3. Sample Identification, Average Pore Diameter, Mesoporosity,a and Production Yield of Rice Husk Derived Carbons Composite sample

average pore diameter (Å)

Vmeso/Vtot (%)

production yield of carbon (%)

C700 C800 C900

38.82 39.36 35.28

66.78 68.41 69.33

40.66 39.19 37.69

a Mesoporosity ) mesopore volume (V meso)/total volume (Vtot) × 100%.

Figure 1. Adsorption-desorption isotherms of nitrogen at 77 K for (a) precarbonized carbon, (b) C700, (c) C800, (d) C900. Table 1. Main Components and Contents of Rice Husks component

content (wt %)

SiO2 lignin cellulose protein fat nutrients after full digestion

18.8-22.3 9.0-20.0 28.0-38.0 1.9-3.0 0.3-0.8 9.3-9.5

Table 2. Surface Area Parameters of the Activated Carbons

c

sample

SBETa (m2/g)

Smicb (m2/g)

Smesoc (m2/g)

C700 C800 C900

344.7 379.4 438.9

202.7 214.6 214.9

142.0 164.8 224.0

a S b BET ) BET surface area. Smic ) micropore surface area. Smeso ) mesopore surface area.

which can be characterized by the formation of slitshaped pores and pores of any kind as well.16 Surface Area and Pore Volume. The specific surface area was calculated using the BET model. The mesopore surface area was calculated by the t-plot method. The nitrogen BET surface area increased with increasing activation temperature. (See Table 2.) The precarbonized carbon had a very low surface area, 55 m2/g. The BET surface area increased considerably after impregnation and activation at higher temperatures. The increase in surface area can be attributed to the release of certain volatile components as a result of the acid treatment on the precursor material containing organic and inorganic materials over the temperature range of 700-900 °C. The pores could be generated on both the components, the carbon matrix and the silica matrix. Consequently, pores of different dimensions are produced. Silica, one of the major components, remains as such in the material as it cannot be evaporated to generate porous silica and contribute to the surface area at the temperatures employed. Silica, if removed from the precursor material, would have resulted in a still greater increase in surface area and pore volume. The

mesoporous surface area increases steeply, especially for samples C800 and C900. The micropore surface area was obtained by subtracting the mesopore surface area from the corresponding BET surface area. Only a small increase in the micropore surface area occurs for sample C800 compared to sample C700. Compared to sample C900, however, the micropore surface area becomes almost constant with a significant increase in the mesopores. These results suggest that, at higher activation temperatures, the new pores created were enlarged or widened to fall into this mesoporic range. The heat treatment temperature has a pronounced effect on the pore volume profile. Total pore volumes were estimated from nitrogen adsorption at a relative pressure of 0.99. Micropore volumes were obtained by the t-plot method. The mesopore volume was calculated by subtracting the micropore volume from the total pore volume. The incremental increase in total pore volume is only within a small range. The percentage of micropore volume was observed to be nearly constant with the rise in temperature. There was a considerable linear increase in mesopore volume from 0.2234 to 0.2684 cm3/g from the heat treatment temperature of 700 °C to that of 900 °C. The increase in mesopore volume can be attributed to simultaneous pore opening and pore widening by the activating agents, but at the higher temperature 900 °C, the pore-widening effect dominates the pore-opening effect.19 The mesoporosity values (percentage of mesopore compared to total pore volume Vmeso/Vtot) of the carbon samples were in the range between 66.78 and 69.33% corresponding to C700 and C900, respectively, as shown in the Table 3. Figure 2 reveals that the mesopore volumes of samples C700, C800, and C900 were 0.2234, 0.2554, and 0.2684 cm3, respectively. These data suggest that the increase in temperature created new pores that were widened immediately into the mesoporic range. This result is in good agreement with the results of the total surface area measurements, which increased with increasing temperature. These results suggest that 900 °C can be considered as the optimum temperature for the production of mesoporous activated carbons. Pore Size and Pore Distribution. Figure 3 shows the pore size distributions of activated carbon composite samples. The average pore size distribution is dependent mainly on the concentration of impregnation chemical and the heat treatment temperature.16 The average pore diameter of 38.8 Å obtained at 700 °C is from the average values of the pore diameters in the carbon and silica portions. The pore diameter increased with increasing activation temperature up to 800 °C and decreased at still higher temperature. The activated carbon composite obtained at 700 °C, characterized by an average pore diameter of 38.8 Å, is seen to exhibit a marginal increase in pore diameter to 39.36 Å at 800 °C. The small increase in pore diameter at 800 °C can be attributed to certain unorganized carbons or residual

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Figure 2. Pore volume as a function of temperature.

Figure 4. X-ray diffraction curves for precarbonized carbon (PCC) and C900 carbon composite samples

Figure 3. Pore size distribution of activated carbon.

tar materials being expelled by the opening of closed pores20 and existing pores widening into larger pores of small magnitude through gasification of carbons in the pore walls having labile carbon structures.17 However, this kind of effect cannot be attributed to the silica component. If this effect had proceeded in the silica component, then there would have been a sharp increase in the average pore diameter, and also the gasification of silica is highly improbable. The decrease in the pore diameter of C900 is due to the suppression of the pore widening of stable carbon structures21 formed during the precarbonization process and coalesence of pores from the silica component. X-ray Studies. It is a general fact that the creation of porosity is considerably influenced by various factors such as active-site clustering or fusion, carbon structure, inorganic impurities, and gas diffusion.22 Pastor et al. indicated that the internal structure of the carbon is considered to be the most important among these factors.23 The X-ray diffractograms of the two samples PCC and C900 are shown in Figure 4. Sample PCC indicates the presence of completely amorphous silica24 by the appearance of a broad peak centered at the 2θ angle of 22°. Sample C900, in contrast, contains a mixture of amorphous and crystalline phases of silica. The crystalline phases of silica are identified as crystobalite and tridymite.25 In addition, a sharp peak at 2θ ≈ 26° and a very small one at 2θ ≈ 44° are also found,

Figure 5. FTIR spectra of (a) PCC, (b) C700, (c) C800, and (d) C900 porous carbon composite samples.

indicating the slight formation of the (002) and (100/ 101) planes, respectively, of the graphitic structure.19 The temperature employed for the activities of the precursor material can be cited as the reason for the formation of the small graphitic structures26 and for the crystallization of silica. The one interesting fact is that the peak at 2θ ≈ 44° is very small, indicating that the pores are also created by the decomposition of carbon structures along the a direction of the graphitic structures. It also clearly suggests that precarbonization of the raw material before chemical activation produces relatively well organized aromatic carbons with sp2 bonding character that are more stable than the amorphous-like carbons of sp3 bonding character.19

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Figure 6. Scanning electron micrographs of (a) precarbonized carbon (PCC) and (b) porous carbon composite (C900).

FTIR Spectra. Infrared spectroscopy provides information on the chemical structure of these materials. All of the carbon samples show a wide band at about 33503425 cm-1 (Figure 5). The O-H stretching mode of hexagonal groups and adsorbed water can be assigned to this band. The position and asymmetry of this band at lower wavenumbers indicate the presence of strong hydrogen bonds.27 A weak band at 3780-3786 cm-1 can be assigned to isolated O-H groups. Samples C700, C800, and C900 showed absorption bands due to aliphatic C-H at 2920 cm-1, and this band was found to be low in the precarbonized sample (PCC). A very small peak near 1700 cm-1 is assigned to CdO stretching vibrations of ketones, aldehydes, lactones, or carboxyl groups. The weak intensity of this peak for all of the carbons indicates that the precarbonized and phosphoric acid activated carbons contain a small amount of carboxyl group. The band near 1615 cm-1 in PCC is due to aromatic stretching vibrations (CdC) enhanced by a polar functional group, but this band is not found in any

of the high-temperature-treated carbons. A broad band between 1250 and 1000 cm-1 is observed. The broad peak at about 1095 cm-1 in sample PCC and a broad peak shouldered at 1180 cm-1 for C700, C800, and C900 indicates the presence of phosphorus in the samples. The bands appearing in these regions can also be assigned to C-O stretching in acids, alcohols, phenols, ethers, and esters, but these bands cannot be assumed to be due to C-O stretching because the bands in these regions can disappear at higher temperatures. The appearance of these bands is only a characteristic for phosphorus and phosphocarbonaceous compounds.26-32 The appearance of a peak at 1190 cm-1 is mainly assigned to phosphates arising as a result of phosphoric acid activation, and the band at 1203 cm-1 is due to phosphoric acid esters, as has been reported previously.27,28 Hence, the previously reported data and as reported by Puziy et al.,34 the bands at 900-1300 cm-1 could be due to phosphorus species generated by the phosphoric acid activation. The peak at 1180 cm-1 can

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be assigned to the stretching mode of hydrogen-bonded PdO, to O-C stretching vibrations in a P-O-C (aromatic) linkage,30-35 and to PdOOH.35 The shoulder at 1080-1070 cm-1 can be ascribed to the ionized linkage P+-O- in acid phosphate esters30,31 and to symmetrical vibration in a chain of P-O-P (polyphosphate)35,36 The broad peak shouldered at about 1115 cm-1 and sharp peaks at 805 and 475 cm-1 in PCC and C700, C800, and C900, respectively, indicate the presence of silica, in parallel with peaks from commercial-grade silica.37 However, all of these data indicate that, at temperatures of 700, 800, and 900 °C, the silica components remain as such in the carbon material, framing a major component in the porous carbon matrix. Surface Morphology. Figure 6a and b shows the morphology of the samples before and after phosphoric acid activation. As the sample activated at 900 °C showed better results when compared with those activated at the other activation temperatures, the surface morphology was studied for that particular sample alone. In the precarbonized sample PCC (Figure 6a), there is no evidence of pore formation in the rice husk where only carbonization of the raw material takes place without creating pores. In contrast, Figure 6b indicates the formation of pores for the sample C900 due to chemical activation. It is made very clear that the opening of the pores in the surface of the rice husk should be due to the extraction of some materials, e.g., dissolution of lignins and other mineral components from the husk during the impregnation process, so as to create, upon activation, micro- and mesopores in the carbon and silica components. As a result of the creation of pores, there is an increase in both the surface area and the pore volume, which are stably created in the carbon composite.20 Production Yield. The precarbonized carbon yield was 53%. However, after H3PO4 impregnation, activation, and washing, the carbon yields corresponding to samples C700, C800, and C900 were in the range 41-38%, as presented in the Table 3. The difference in the carbon yield between the precarbonization and activation stages was due to further removal of volatiles through opening of the closed pores at high temperatures. A decrease in the yield of the carbon with increasing activation temperature can also be explained by the gasification of carbons during the process.38 Conclusion Activated carbon produced from rice husk can be tailored to contain desired surface areas and pore sizes depending on the application for which it is intended. The inclusion process prior to chemical activation makes the process simpler and cheaper. Also, it was found that chemical activation at 900 °C is optimum within the experimental range of temperatures tested for the preparation of better carbons from rice husk. The use of phosphoric acid has the effect of producing considerable quantities of micropores and mesopores with phosphorus compounds being bound to the carbon lattice, as evidenced from infrared spectroscopy. Poreopening and pore-widening effects were evidenced from scanning electron micrographs. At all of the heat treatment temperatures, it was found that the silica remained in the carbon matrix. Also, these activation temperatures induced the formation of slight crystallinity in the carbon structure and crystalline silica, which was noted through XRD curves. The presence of

silica highly affects the porosity of the carbon material. Silica, if removed from the precursor, would lead to still higher values of surface area and pore volume. Acknowledgment The authors highly thank the Council of Scientific and Industrial Research (CSIR), India, for providing financial assistance to carry out this work. Literature Cited (1) Rodriguez-Reinoso, F. Activated carbon, structure, characterization, preparation and applications. In Introduction to Carbon Technologies; Marsh, H., Heintz, E. A., Rodriguez-Reinoso, F., Eds.; University of Alicante; Alicante, Spain, 1997; Chapter2. (2) Salame, I. I.; Bandoz, T. J. Comparison of the surface features of two wood based activated carbons. Ind. Eng. Chem. Res. 2000, 39, 301-306. (3) Sircar, S.; Golden, T. C.; Rao, M. B. Activated carbon for gas separation and storage. Carbon 1996, 34, 1. (4) Yalegin, N.; Seving, V. Studies of the surface area and porosity of activated carbons prepared from rice husks. Carbon 2000, 38, 1943-1945. (5) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W. Pure Appl. Chem. 1985, 57, 603. (6) Teng, H.; Yeh, T.-S. Preparation of activated carbons from bituminous coals with zinc chloride activation. Ind. Eng. Chem. Res. 1998, 37, 58-65. (7) Ahmadpour, A.; Do, D. D. The preparation of active carbons from coal by chemical and physical activation. Carbon 1996, 34, 471. (8) Caturla, F.; Molina-Sabio, M.; Rodriguez-Reinoso, F. Preparation of acrivated carbon by chemical activation with ZnCl2. Carbon 1991, 29, 999. (9) Ruiz Bevia, F.; Prats Rico, D.; Marcilla Gomis, A. F. Activated carbon from almond shells. Chemical activation. 2. ZnCl2 activation temperature influence. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 269. (10) Zhonghua, H.; Srinivasa, M. P.; Yaming, N. Novel activation process for preparing highly microporous and mesoporous activated carbons. Carbon 2000, 39, 877-886. (11) Molina-Sabio, M.; Rodriguez-Reinoso, F.; Caturla, F.; Selles, M. J. Porosity in granular carbons activated with phosphoric acid. Carbon 1995, 33, 1105-1113. (12) Hsisheng, T.; Tien-Sheng, Y.; Li-Yeh, H. Preparation of activated carbons from bituminous coal with phosphoric acid activation. Carbon 1998, 36, 1387-1395. (13) Laine, J.; Calafat, A.; Labady, M. Preparation and characterization of activated carbons from coconut shell impregnated with phosphoric acid. Carbon 1989, 27, 191. (14) Jagtoyen, M.; Derbyshire, F. Activated carbons from yellow poplar and white oak by H3PO4 activation. Carbon 1998, 36, 10851097. (15) Toles, C. A.; Marshall, W. E.; Johns, M. M. Phosphoric acid activation of nut shells for metals and organic remediation, process optimization. J. Chem. Technol. Biotechnol. 1998, 72, 255263. (16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (17) Yulu, D.; Walawender, W. P.; Fan, L. T. Activated carbons prepared from phosphoric acid activation of grain sorghum. Bioresour. Technol. 2002, 81, 45-52. (18) Yupeng, G.; Shaofeng, Y.; Yu, K.; Jingzhe, Z. The preparation and mechanism studies of rice husk based porous carbon. Mater. Chem. Phys. 2002, 74, 320-323. (19) Chang Hun, Y.; Park, Y. H.; Park, C. R. Effect of precarbonization on porosity development of activated carbons from rice straw. Carbon 2001, 39, 559. (20) Gyu Hwan, O.; Park, C. R. Preparation and characteristics of rice straw based porous carbons with high adsorption capacity. Fuel 2002, 81, 327-336. (21) Marsh, H.; Kuo, K. Kinetics and catalysis of carbon gasification In Introduction to Carbon Science; Marsh, H., Ed.; Butterworth: London, 1989, pp107-51.

1838 Ind. Eng. Chem. Res., Vol. 43, No. 8, 2004 (22) Rodriguez-Reinoso, F. Controlled Gasification of Carbon and Pore Structure Development; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; pp 533-565. (23) Pastor, A. C.; Rodriguez-Reinoso, F.; Marsh, H.; Martinez, M. A. Preparation of activated carbon cloths from viscous rayon. Part I. Carbonization procedures. Carbon 1999, 37, 1275-83. (24) Kalapathy, U.; Proctor, A.; Shultz, J. An improved method for production of silica from rice hull ash. Bioresour. Technol. 2002, 85, 285-289. (25) Yalcin, N.; Sevinc, V. Studies on silica obtained from rice husk. Ceram. Int. 2001, 27, 219-224. (26) Manivannan, A.; Chirila, M.; Giles, N. C.; Seehra, M. S. Microstructure, dangling bonds and impurities in activated carbons. Carbon 1999, 37, 1741-1747. (27) Solum, M. S.; Pugmire, R. J.; Jagtoyen, M.; Derbyshire, F. Evolution of carbon structure in chemically activated wood. Carbon 1995, 33, 1247-54. (28) Jagtoyen, M.; Thwaites, M.; Stencel, J.; McEnaney, B.; Derbyshire, F. Adsorbent carbon synthesis from coals by phosphoric acid activation. Carbon 1992, 30, 1089. (29) Zawadzki, J. Infrared Spectroscopy in Surface Chemistry of Carbons. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; pp 147-386. (30) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1954. (31) Corbridge, D. E. C. Infrared analysis of phosphorous compounds. J. Appl. Chem. 1956, 6, 456-65.

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Received for review August 27, 2003 Revised manuscript received January 7, 2004 Accepted January 30, 2004 IE034093F