1995 Surface and Adsorption Characterisics of

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1995 Surface and Adsorption Characterisics of Powered By Docstoc


                        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[3]. 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)[2].  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[4].

   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
                                                                                                                                                                                .""                                 "".
                                                                                                                                                                                                                               - 0.60

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-
                                                                                                                                                                                                                               - 0.45
drying at 150"(] for 24t~ show weight-loss of                                                                                                              ...'"                                       •       SBE-
                                                                                                                                                                                                                  r                          <
                                                                                                      oo               15oo
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~                                  •
                                                                                                                                                      SeEr                                                     .~.
currently used methods.                                                                                                                               Acidity                                      .........                        0.65
                                                                                                                                               ...... Acidity                             ."'"

                       REFERENCES                                                                         g                                                                                                                          0.60

                                                                                                           m                2000                                                                                                               E
                                                                                                                                                                                                                                     0.55     ._Z2
 1. J.B. Donnet and R.C. Bansal, Carbon                                                                                     1800           _

                                                                                                                                           _                           '"
     Fibers, Marcel Dekker, Inc., New York                                                                                  1600
                                                                                                          o0                                                                                                                                   8
         (1984)                                                                                           o
                                                                                                                                                                                                                                     0.50     ~:
 2. H.P. Boehm, Adv. in Catalysis, 16, 198                                                                 o.
         (1966)                                                                                           o0                1200
                                                                                                                                                                                                                                   - 0.45
 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


"••ted              Time
                                          1             I

                 Ssz.r(m~/g)     697
                                          I    1337
                                                        I    3o

                                                            2654     2641


                                                                                                                             105 -

                                                                                                                     ~: lOO            -

                 total pore                                                                                          f:::
                  volume        0.278          0.567        1.772    1.146       1.706   0A1
                   (cc/g)                                                                                        ~
                                                                                                                 •            95
                 macropors                                                                                           8
                                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
 properties        (cclg)

                 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
                 Ave. pore
                   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.