전기 화학적 표면처리에 의한 활성탄소의 흡착특성

박 수진, 김 기동

한국화학연구소 화학소재연구부

Adsorption Characteristics of Activated Carbons modified by

Electrochemical Surface Treatment

Soo-Jin PARK and Ki-Dong KIM

Advanced Material Division, Korea Research Institute of Chemical Technology

P.O. Box 107, Yusong, Taejon 305-600, Korea.

(E-mail: psjin@pado.krict.re.kr)

1. Introduction

Activated carbons (ACs) are considerable interest from the high adsorbent materials community, since they have been initially developed for the removal of impurity captures and purification of gases and liquids. The porous structures, shapes and surface areas of carbon adsorbents are dominant parameters in the superior adsorption behavior.

It is well known that activation process is essential to high performance activated carbon, and the activation conditions are largely considered by two different courses, depending on the nature of the raw material used: the physical or thermal activation process carried out at high temperature between 800 and 1,000℃ in the presence of suitable oxidizing gases, and the chemical activation process impregnated in the concentrated acidic solution such as phosphoric acid, sulfuric acid and nitric acid, and consecutively pyrolyzed at the temperature between 400 and 600℃ in the absence of oxygen[1, 2]. During activation, the enlargement of pore diameters and development of microporous structures are therefore introduced in the carbonized carbon, resulting in increase of extreme large specific surface areas. It is also noted that activated carbons have been widely used for the separation of nonpolar organic molecules from gas or liquid matters, due to its large surface area and porosity. However, the limitation of the activated carbons shows in the adsorption behavior for polar molecules, such as ammonia, nitric oxide complex and sulfuric oxide complex, because of decomposition of oxide functional groups on carbon surfaces resulted in pyrolysis at high temperature[3, 4].

The objective of the present work is to study the effect of anodic surface treatment on activated carbons, in a very short procedure, and to investigate the role of surface functional groups created after anodic treatment with a viewpoint of adsorption surface chemistry.

2. Experimental

2. 1. Materials: The porous activated carbon (8×12mesh size) from coconut shell obtained from Dong-Yang Carbon Co. of Korea was used in this work. ACs were subjected to electrolytic reaction in an aqueous solution of 35 wt.% NaOH whereby negative ions were attracted to the surface of the ACs acting as an anode, thereby modifying the ACs surfaces. Electrolytic oxidation was performed at following system which ACs on graphite bath filled with electrolytes were fixed over graphite anode roller. And a cathod graphite plate was also submerged in the solution of electrolytes.

The experimental conditions are listed in Table 1. Prior to use, the residual solution of electrolyte on treated-AC was removed by boiling method on soxhlet extractor with acetone at 80℃ for 2 hrs. And the ACs treated studied were washed with distilled water and dried in convection oven at 85℃ for 24 hrs.

2. 2. Surface Properties: The pH of ACs was measured with ASTM. About 1.0g of each dried samples was added to 20ml standard buffer solution (pH=7.0 at 25℃) and shaken over 12 hrs. Then the pH of ACs was measured the solution after filtration. The surface acidity and surface basicity of the ACs were defined on the basis of adsorption of 0.1N NaOH and 0.1N HCl standard solution. For about 0.5g of a sample was immersed in 100ml of these solutions, sealed and shaking it for over 48 hrs at 25℃. A portion of the ambient solution was then separated and titrated with 0.1N HCl and 0.1N NaOH solution.

2. 3. Adsorption Characteristics: Specific surface area and pore structure of the ACs were measured by BET-method[5] at 77K with N2-gas (Micromeritics, ASAP 2400). And the mesopore ratio has been calculated from the relative pressure (P/P0) range of adsorption isotherm by BJH method[6]. Prior to analysis, the ACs were outgassed at 298K for 6 hrs to obtained a residual pressure of less than 10-3 torr.

3. Results and Discussion

3. 1. Modification of the surface properties: The result of surface properties for the samples modified with electrochemical treatments are given in Table 2. Taking into account that NaOH titrated with carboxyl, lactone and phenolic groups, and acid values could obtain the amount of the different oxygen groups present on the carbon surface. The acidic surface oxides measured by Boehm's method[7] were shown that the surface acidity based on carboxylic and phenolic hydroxyl groups increases in increasing the oxidation current intensity. Following the increase in surface acidity, the pH of samples was markedly decreases. These results indicate that the acid and base values are greatly influenced by the surface oxygen groups of ACs measured.

3. 2. Effect on adsorption characteristics with modified surface properties: The adsorption surface characteristics of the samples studied are listed in Table 3. The results of N2 adsorption isotherm at 77K of both untreated and anodic oxidized ACs show that well-developed porous structure were shown in Fig 1. From which, the results of isothermal N2-adsorbed amounts of the ACs, it tends to slightly decrease the adsorption amount of N2-adsorbate with increasing the current intensity. It may be considered that the wide presence of surface functional groups due to electrochemical surface treatments on the AC can also be attributed to the diminution of adsorption between polar adsorbent and non-polar N2 adsorbate. This result obviously showed  that the surface modification with anodic oxidation of ACs slightly changes their surface areas and pore structures, but strongly influences on their polar surface chemistry. Although the presence of acidic and basic surface functional groups decreases in slight the adsorption capacity, the polar surface characteristics of activated carbons increases after anodic oxidation. Therefore, the surface acidic and basic properties of activated carbons may be significantly influenced the adsorption on                                                                             adsorbates being opposite surface polarity. On this basis, it can be concluded that the electrochemical surface treatment of activated carbon can be used to develop more selective and effective adsorbents for gas and liquid pollution control.

<Fig. 1.> Isothermal N2 adsorption of ACs

References

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2. Mattson, J. S. and Mark, Jr. H. B., "Activated Carbon", Marcel Dekker, New       York, 1971.

3. Park, S. J. and Lee, J. R., J. Mater. Sci., 33, 647 (1998).

4. Kaneko, K., Ozeki, S. and Inouye, K., Colloid Polym. Sci., 265, 1018 (1987).

5. Brunauer, S., Emmett, P. H. and Teller, E., J. Am. Chem. Soc., 60, 309 (1938).

6. Barrett, E. P., Joyner, L. G. and Halenda, P. P., J. Am. Chem. Soc., 73, 373       (1951).

7. Boehm, H. P., Diehl, E., Heck, W. and Sappok, R., Angew. Chem. Int. Ed. Engl.,    10, 669 (1964).