Activated carbons prepared from rice hull by one-step phosphoric acid activation

Activated carbons were prepared from rice hull by one-step phosphoric acid activation in this work. The evolution of pore structure and surface chemistry in the activation temperature range of 170-450 degrees C was investigated through various characterization techniques. The results showed that the development of porosity (extent of activation) was negligible at activation temperature below 300 degrees C, and rapid evolution occurred in 300-400 degrees C. Porous activated carbon with bimodel pore structure (pore < 1 nm and pore > 1 nm) and BET surface area as high as 1295 m(2)/g was obtained at 450 degrees C. The ash contents of samples prepared in this study were in the range of 5-21 %. The ash contents of carbons prepared in this study initially decreased from 21.03% to 4.89% with the change of temperature from 170 to 300 degrees C, then increased to 8.72% at 450 degrees C. Boehm titration results suggested that low activation temperature (<= 300 degrees C) benefits the formation of acidic surface groups. With the increase of activation temperature from 300 to 350 degrees C, the concentrations of strong, intermediate and weak acidic surface groups decreased from 2.23, 1.87, and 2.73 to 1.66, 1.32, and 2.16 mmol H+/g, respectively. Over 350 degrees C, the change of these groups were insignificant. FTIR results revealed the existence of carbonyt-containing, phosphorus-containing groups, and groups containing Si-O bond. The relative concentration of carbonyl-containing groups decreases with an increase in activation temperature, while that of phosphorus-containing groups follows the reverse trend. The content of Si-O decreased first, then slowly increased with the increase of activation temperature. Boehm titration and FTIR (Fourier transform infrared spectroscopy) results indicated that the surfaces of these carbons contain both temperature-sensitive and temperature-insensitive groups. The temperature-sensitive part consists mainly of carbonyl-containing groups, such as carboxylic groups, while the temperature-in sensitive part is primarily phosphorus-containing groups and groups containing Si-O bond. This study demonstrated that carbon products with relative low ash content and high activation degree can be prepared from rice hull by H3PO4 activation at suitable temperature. (c) 2006 Elsevier Inc. All rights reserved.