SURFACE AND ADSORPTION CHARACTERISTICS OF ACTIVATED CARBON
FIBERS TREATED WITH OZONE IN AQUEOUS SOLUTIONS
Chul G. Kim, Jae K. Lee*, Seung K. Ryu*, Oh S. Yoon
Dept. of Environ. Eng., Taejon National University of Technology
Taejon 300-717, KOREA
*Dept. of Chem. Eng., Chungnam National University
Taejon 305-764, KOREA
I T O U TO
NR D CI N RESULTS AND DISCUSSION
Until now, treatment with ozone on carbon Nitrogen adsorption isotherms are used to
fiber has been used in gas-phase oxidative determine specific surface area, and pore
treatments for surface modification. The structure of activated carbon fibers treated
gaseous treatment is, however, generally very with ozone in different solutions.
drastic and causes severe degradation and Specific surface area and total pore volume
excessive pitting of the carbon fiber surface. of ACFs are apparently increased without
Most liquid-phase oxidative treatments are considerable weight loss with increasing
milder and generally do not cause excessive treatment time and concentration. This result
pitting and degradation of the carbon fiber, but is in contrast with results of common liquid-
less effective compared with the gaseous phase treatment, which could be explained by
methods[l]. blocking of the narrow pores by surface
The present work is concerned with surface complexes introduced.
treatment on activated carbon fibers with ozone In the pore size distributions, the micropore
in aqueous solutions to demonstrate the new % of the total pore volume is slightly
method, which is milder and more efficient for decreased with increasing treatment time and
the developing of their porous structure and concentration. On the other hand, macropore
surface properties. volume is increased by about 7 percent.
Average pore radius is also increased slightly.
EXPERIMENTS This means that mild reaction of hydroxyl
radicals with the surface is taking place
The pitch-based ACF(A-7, AD'ALL Co. homogeneously on carbon fibers. This could
Japan) is used as a base material. The ACFs be explained by indirect reaction of ozone on
are subjected to treatment with ozone in the surface. The reaction of molecular
different conditions. N2 adsorption isotherms ozone (standard oxidation-reduction potential,
were measured at 77K by volumetric method E ° : + 2.07V), which is one of powerful
(Autosorb-1, Quantachrome). Surface acidifies oxidizing agents, is particularly selective to
are determined by Boehm's method (Metxohm unsaturated bonds and somewhat slower.
titroprocesser Model 602). Qualitative Whereas, in alkaline solutions, the ozone
analysis of surface functional groups is made decomposes to form hydroxyl radical, .OH (E °
by the F T - I R spectrophotometer (Model IFS88, • + 3,06V), which then reacts more rapidly
Burker). TGA/DSC and SEM analyses are also and much less selectively. The comparison of
measured. Decolorizafions of methylene blue are SEM photographs with directly treated surface
conducted at 30°C, and analyzed by UV-Vis and indirectly treated surface with ozone in
spectxophotometer (Perkin Elmer Model ll00B) aqueous media could support this explanation.
at 665nm. Surface acidity of ACFs is also dramatically
increased with increasing treatment time and
concentration. Methylene blue as cationic
organics is preferentially adsorbed on
negatively charged acidic surfaces.
From the analyse of F T - I R spectra, 3500 0.70
predominant surface functional groups on ACFs . - 0.65
treated with ozone in alkali solutions, however, 3000 - L~ "
are different from non- treated ACF. It seem ..-.. •"0
that most carboxylic groups are transformed to ~E 2500 - - 0.55 ..-.
-O- and CO2 by the reaction o f - C O 0 - with v
hydroxyl radicals. The results from TGA/DSC - 0.50 E
analyses of treated and non-treated ACFs after
< 2000 - ._z-
drying at 150"(] for 24t~ show weight-loss of ...'" • SBE-
adsorbed H20 of 1.58% and 5.16%, and .." -~ S~mT 0.40
desorption temperature of 36.05°C and 64.4°C, o. 0.35 (/)
respectively. This is in good agreement with lOOO I ...... Ac~i~ 030
surface acidity results.
500- ' ~---______ I ~ i 0.25
0 20 40 60 80 100
CONCLUSION Treatment Time (min.)
New oxidative surface treatment method of Fig. 1 Development of specific s u r f a c e a r e a (SeE. r) a n d
carbon fibers has been developed by reaction of surface acidity on treatment of activated carbon fiber
ozone in different alkali solutions. This method with ozone in 1M NaOH s o l u t i o n a t 20 ° C
is milder and more effective for the develop- 2800 0.70
ment of texture and surface properties than 2600
• Seer z~ •
currently used methods. Acidity ......... 0.65
...... Acidity ."'"
REFERENCES g 0.60
m 2000 E
1. J.B. Donnet and R.C. Bansal, Carbon 1800 _
Fibers, Marcel Dekker, Inc., New York 1600
2. H.P. Boehm, Adv. in Catalysis, 16, 198 o.
(1966) o0 1200
3. H.Tomiyasu, H.Fukutomi, and G. Gordon, 1000
Inorg. Chem. 24, 2962 (1985)
800 L .. I I 0.40
4. S.S. Barton, Carbon, 25, 3, 343 (1987) -0.5 0.0 0.5 1.0 1.5
Conc. of NaOH (moles)
Fig. 2 Development of specific surface area (SBET) a n d
Table 1. Textural and Chemical Characteristics of ACFs surface acidity on treatment of activated carbon fiber
treated with ozone and AC w i t h ozone in different concentration of NaOH solution
~: lOO -
total pore f:::
volume 0.278 0.567 1.772 1.146 1.706 0A1
0.019 0.059 0 231 0.159 0.200 0.04 tD
Textural volume cl
(6.95%) (10.34%) (13.04%) (13.86%) (11.73%) (9.76%) 90
micropore • / / @ ~ Adsorption Condition •
0.258 0.508 1.541 O587 1505 037 - time • 30 min
(93.05%) (89.66%) (86.96%) (86.14%) (88.27%) (90.24%) 85
• - temp " 30°C
radius 7.97 8.474 I 8599 8.674 8,544 11.1 80t. i i l
(A) -0.5 0.0 0.5 1.0
I Conc. of NaOH (moles)
acidity 0.31 0.379 0.625 0.656 ! 0.576 I -
property (,meq./g) •
Fig. 3 Decolorization of methylene blue v.s. concentration
i 1 i
of NaOH treated on activated carbon fiber.