Changes Of Porous Structure And Surface Nature
CHANGES OF POROUS STRUCTURE AND SURFACE NATURE
OF ACTIVE CARBON AFTER OXIDATION PROCESS
B. Buczek
Cracow University of Economics, Faculty of Commodity Science,
31-510 Cracow, Poland
Activated carbons are widely used as adsorbents to remove impurities from gas and water streams. Properties of activated carbons are strongly influenced by oxygen groups involved in the modification of surface with oxidizing gases or solutions. Depending on the formation conditions, affected by the nature of carbonaceous precursor, porous structure and ash content, surface oxides may impart basic or acidic properties to the carbon surface [1].
The subject of the work was to study the changes of texture and oxidation degree of active carbons obtained as a result of different procedures (liquid or gas phase treatment).
Materials
The following carbons were prepared by these treatments: calcinated under nitrogen at 973K (CC) and subsequently oxidized with air as oxidant at 623-673K (C21). Oxidation in liquid phase was carried out starting from demineralized and carbonized sample CC using the concentrated (68 %) nitric acid at 363K. After treatment with HNO3 the sample was purified by washing and vacuum distillation at 473K (CN/O). In order to enhance the content of oxygen-containing groups on the surface of active carbon, potassium permanganate was also chosen. CC carbon underwent oxidation by boiling in 1.42 % KmnO4 solutions with or without the addition of H2SО4. After such treatments, samples were washed with a solution of HCl and dried at 293K (CM2/O and CM1/O, respectively).
Experimental and Results
The porous structure of active carbon samples was determined from low-temperature argon adsorption-desorption measurements in a standard volumetric equipment. Texture parameters: specific surface area (SBET), micropore volume (Wo) and mesopore volume (Vme) and surface (Sme) were calculated [2-4]. The analysis of texture of all carbons indicates their well developed micro- and
mesoporous structure. The oxidation process leads to a decrease in volume of micropores present in the carbons. In the case of oxidation with HNO3 (CN/O) the observed changes in micropores appeared to be smaller.
The concentration of oxygen-containing surface groups was investigated by:
– water vapor adsorption at 298K using a microburette-type apparatus,
– thermogravimetric method with the analysis of the decomposition products.
The adsorption isotherms of water vapour were analyzed by equation proposed by Dubinin and Serpinsky [5]. The equation was used to find the number of surface oxygen groups n·10-17 per 1 m2. For the series of carbons, it was: CC, 3.16; C21, 6.80; CN/O, 12.14; CM1/O 14.38; CM2/O, 13.93.
Thermal analysis of active carbons was carried out within the temperature range of 298-1273K simultaneously with the analysis of decomposition products. On the basis of our results, 453K was assumed to be the final temperature where H2O desorption ceases. Using DTA curves the temperature of 973K was adopted as the initial point of CO formation and the end of CO2 formation [1, 4]. The mass losses found within the discussed temperature range show that more gases form in the case of oxidized carbons then for CC sample. For the gas-phase oxidized carbon, the total observed mass loss is much lower than for those oxidized in liquid phase. In the case of CC and C21, the amount of formed CO is higher than that of CO2, while an opposite effect was found for liquid-phase oxidized carbons.
Conclusions
The oxidation process in liquid phase leads to a higher oxidation degree of carbons than the use of air as oxidant. Using two independent methods, the differences in surface properties of oxidized active carbons were confirmed. It is necessary for further characterization of carbonaceous surface to determine directly the oxygen groups and their population.
References
1. Bandosz T. J., Buczek B., Grzybek T., Jagiełło J., Fuel, 76, 1409 (1997).
2. Brunauer S., Emmett P. H., Teller E., J. Am. Chem. Soc., 60, 309 (1938).
3. Dubinin M. M., Izw. Ak. Nauk SSSR, Seria Chim., 9, 1 (1991).
4. Bansel R. C., Donnet J. B., and Stoeckli , Active Carbon, Marcel Dekker, New York (1991).
5. Dubinin, M. M., Sierpinsky V. V., Dokl. A. N. SSSR, 99, 1033 (1954).
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