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Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
Studies in Surface Science and Catalysis 160
575
P.L. Llewellyn, F. Rodriquez-Reinoso, J. Rouqerol and N. Seaton (Editors)
9 2007 ElsevierB.V. All rights reserved
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry P. Pendleton a, A. Badalyan a, R. Bromball a and W. Skinner b
aCenter for Molecular and Materials Sciences; blan Wark Research Institute; University of South Australia, Mawson Lakes, South Australia, 5095, Australia
FM1/250 and FM1/700 activated carbon cloth samples were chemically treated for 24 hours in 4M N a O H at 373 K. Chemical activation was also carried out in 10 % (w/vol) saturated ammonia peroxydisulfate solution, and in 10 % (w/w) potassium peroxydisulfate solution at room temperatures for 24 hours. Nitrogen adsorption on chemically treated samples was measured at 77 K. All samples showed a classical Type I microporous adsorption isotherm. N a O H treatment caused an increase in total micropore volume for FM1/250, but no statistically significant change for FM1/700. In contrast, a change in primary micropore volume for FM1/700 occurred, but no statistically significant change for FM1/250. The peroxydisulfate treatment decreased primary and total micropore volumes for both samples of activated carbon cloth. Both treatments caused relative increases in oxygen surface structures promoting their selective adsorption potential. 1. INTRODUCTION Activated carbon cloth (ACC) is used for the removal of volatile organic compounds from effluent gas streams [ 1]. Specific micropore structures in ACCs make them real candidates for use in adsorption processes where a high rate of adsorption is accompanied with short contact time between adsorbent and adsorbate. It is very important for manufacturers to produce ACC with desirable pore size distributions, and be able to control the development of various pores from primary micropores to mesopores during activation processes. Adsorption selectivity towards various gases is another very important property, which can be achieved by introducing oxygen complexes onto the surface during activation [2-4]. Our recent surface analyses of powdered activated carbon [5] has been extended to the more regularly defined surfaces exhibited by ACC. These adsorbents often display a "memory" in that their physical and chemical resistance submits to various solvents, albeit to a lesser extent than their parent (un-carbonised) material. We treated two microporous ACC samples with an excessive aqueous oxidizing agent, 4M NaOH, and milder conditions provided by 10 % (w/vol) aqueous ammonium and potassium peroxydisulfates solutions. The oxidation differently affects the distribution of surface carbon-oxygen structures, leading to different adsorption behaviour of ACC samples.
2. EXPERIMENTAL
576
P. Pendleton et al.
2.1. Materials and methods Two samples of activated carbon cloth (ACC), FMI/250 and FM1/700, from Calgon Carbon Corporation were used. These are single, plain weave materials, with the second number designating the value of wetting heat in J/g. The samples were exposed for 24 hours (suffix '24') to 4M N a O H at 373 K at atmospheric pressure. Non-treated samples have suffix '0'. Other samples were suspended via a rotary mixer at room temperatures for 24 hours in 25 mL of 10% (w/vol) ammonia peroxydisulfate (APDS), of analytical grade (suffix '10 %APDS'), in 10% (w/vol) potassium peroxydisulfate (PPDS), of analytical grade (suffix '10 %PPDS'). After treatment, samples were washed in deionised water for 48 hours. These samples were then heated in an atmospheric oven at 398 K for 24 hours for moisture removal. Some moisture still remains in these ACC samples, but subsequently removed when the samples underwent heating in high vacuum before nitrogen adsorption. 2.2. Nitrogen adsorption measurements Gaseous nitrogen adsorption measurements were carried out using an automatic manometric gas adsorption apparatus described elsewhere [6]. Before adsorption measurements, all samples were heated in the electric oven at 473 K (much lower than their decomposition temperature of>773 K) and evacuated to a pressure of 0.1 mPa for 8 hours to remove moisture and adsorbed gases. Liquid nitrogen level around the tube with ACC sample was controlled with a precision of_+0.02 mm using a modified liquid nitrogen delivery system [7]. Nitrogen adsorption was carried out at 77 K [6]. Samples reached equilibrium within 1520 minutes. The number of experimental points (58-60) was sufficient for the evaluation of BET specific surface areas (BET-SSA), pore volumes and pore size distribution. Thermal transpiration corrections were made at pressures below 266 Pa [8]. At higher pressures, differences between corrected and non-corrected equilibrium pressures are smaller than the uncertainties in pressure measurements. Relative combined standard uncertainties (RCSUs) of the nitrogen amount adsorbed were calculated for each experimental point [9]. 2.3. XPS analysis X-ray Photoelectron Spectroscopy (XPS) is a widely used procedure for determination of surface functional groups. A Kratos Axis-Ultra X-ray photoelectron spectrometer (Kratos Analytical, UK) was used to determine oxygen-containing functional groups on the surfaces of the treated and untreated ACC samples. The X-ray source was monochromatic Alia. (1486.6 eV) at 150W power. Atomic concentrations were calculated using core-line peak areas from survey spectra and tabulated sensitivity factors for the instrument. Survey spectra were collected in the binding energy range from 0 to 1100 eV at a pass energy of 160 eV. High resolution scans of the CI s were collected at a pass energy of 20 eV. These scans were curve-fitted to determine the presence of various carbon-carbon graphitic character and carbon-oxygen surface functional groups. 3. RESULTS AND DISCUSSION
3.1. Nitrogen adsorption isotherms Fig. 1 presents nitrogen adsorption isotherms for FM1/250 samples. All adsorption isotherms showed classical Type I shape representative of a wholly microporous adsorbent. Since adsorption isotherms are almost parallel over the relative pressure range from 1.E-4 to 1.E-l, the external adsorption surface is unaffected by chemical activation. Chemical activation by
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
577
NaOH changes adsorption behaviour of FM1/250-24 at very low relative pressures p/pO < l0 -4. There is appreciable adsorption at relative pressures below ~ 10 "6, compared to the amount adsorbed at relative pressure of about 10 .6 < p/pO < 10 .5 by FM1/250-0 sample. We interpret this increase as the formation of smaller pores within the micropore structure due to chemical etching. As a reference, for comparison purposes only, the BET-SSA increased by 4.40 % after treatment in NaOH. In Table 1, we intentionally left one additional significant digit in the reported values for indication purposes only. Chemical treatment by 10 %-APDS and 10 %-PPDS reduced the BET SSA by 14.43 and 3.33 %, respectively. 350
300 250 200 150 100 50
o
1.E-07 1.E-06 1.E-05 1.E-04 1.E.03 1.E-02 1.E-01 1.E+00 log(P/P ~
Fig. 1. Nitrogen adsorption isotherms for FM1/250 ACC. (e - FM1/250-0, ~ - FM1/250-24, 9 - FM1/250-10%-APDS, 9 - FM1/250-10%-PPDS). Table 1 BET-SSA and pore volumes for FM1/250 and FM1/700 ACC Sample Name SBET, m2/g Wt,~,,,, , mL/g FM 1/250-0 FM1/250-24 FM 1/250-10%-APDS FM1/250-10%-PPDS FM 1/700-0 FM 1/700-24 FM 1/700-10%-APDS FM 1/700-10%-PPDS
1633.9+10.9 1079.4+5.5 884.7+ 16.3 999.5___10.7 1157.7+6.7 1213.3___9.9 979.9+18.0 1066.3+18.7
_
0.1927+0.0080 0.1883+0.0018 0.1362+0.0043 0.1628+0.0025 0.1640+0.0014 0.1779+0.0035 0.1389+0.0049 0.1609+0.0036
W,o] , mL/g 0.4595• 0.5007• 0.3992• 0.4035• 0.5775• 0.5927• 0.4646• 0.5004•
P. Pendleton et al.
578
Nitrogen adsorption isotherms for FM 1/700 samples are also Type I, but again shown in Fig. 2 as Vad, VS log(P/P ~
to emphasize the low pressure amount adsorbed. In
contrast with FM1/250-24, the NaOH-treated FM1/700-24 sample does not show an increased amount adsorbed, however, it does show a modest 4.80% increase in BET-SSA over FM1/700-0. Treatment with APDS and/or PPDS results in 15.36 and 7.90% reduction in BET-SSA. 400 350 300 250
~ 200 ] 150 100 50 0 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 log(P/P ~
Fig. 2. Nitrogen adsorption isotherms for FMI/700 ACC. (o - FM1/700-0, ~ - FM1/700-24, 9 FMI/700-10%-APDS, 9 FMI/700-10%-PPDS). 3.2. Micropore volumes and pore size distribution Primary and total micropore volumes were determined after a s -uncertainty analyses [9].
A non-graphitized, non-porous carbon black [7] was used as the adsorbent standard. The increased amount adsorbed by FM1/250-24 at low pressures (in Fig.l) are equated to an increase in total micropore volume, however, the primary volume remains statistically constant. The similar treatment of FM1/700 saw no statistical change in total micropore volume, but an 8.48% increase in primary micropore volume. Treatment with PDS solutions results in a widening of the pore volume distributions as suggested by the increase in uncertainty in each primary and total micropore volume. Changes in micropore structure are further borne out by the Horwath-Kawazoe defined micropore-size distribution data in Fig. 3 and 4. Chemical activation of FM1/250 by NaOH shifted the maxima of the pore size distributions from ~ 0.50 nm to ~. 0.40 nm, thus indicating the opening of smaller micropores due to possible leaching effects. Similar treatment of FM1/700 did not significantly change pore size distribution. Treatment with either of the PDS solutions shifts the pore size distribution maxima to ~ 0.46 nm for the both FM1/250 and FM1/700.
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
579
14.0
12.0
10.0
8.0
"~ 6.0 <1 4.0
2.0
0.0
...... 0.3
'~-.~: ~0.4
,7
I
I
I
I
I
I
I
I l l
It
I l l
I
|
|
|
I
|
|
|
_
0.5 Pore width, d R nm
0.6
0.7
Fig. 3. Pore size distribution for F M 1 / 2 5 0 ACC. ( o - FM1/250-0, O - FM1/250-24, 9 FM1/250-10%-APDS, 9 FM1/250-10%-PPDS). 7.0
6.0
5.0
4.0
3.0 <1 2.0
1.0
0.0
r-,
0.30
0.40
0.50
0.60
0.70
P o r e w i d t h , dp, n m
Fig. 4. Pore size distribution for F M 1 / 7 0 0 ACC. ( o - FM1/700-0, O - FM1/700-24, 9 - FM1/700-10%-APDS, 9 - FM1/700-10%-PPDS).
P. Pendleton et al.
580
3.3. XPS results According to Calgon, the nomenclature 250 and 700 refers to the heat of wetting (presumably water) with units of J/g [ 10]. These relatively high enthalpies indicate the pores are of similar dimensions to water and/or due to strong interactions with the surface functional groups. The nitrogen adsorption isotherms for both materials show a condensation over the same relative pressure ranges, indicating both materials contain pores of similar dimensions. Thus, the difference in heat of wetting is primarily due to strong water-surface functional group interactions. The type of oxygen functional groups defined via XPS are: ether (C-O-C) at ~284.5 eV, carbonyl (C=O) at ~286.5 eV, carboxylic (O-C-O) at ~288.2 eV, and anhydrate (O-(C=O)-O) at -~290.2 eV (see Fig. 5-8). Both FM1/250 and FM1/700 samples show similar relative concentrations of C-C carbon, regardless of differences during their activation manufacturing process. Slight difference in relative concentrations of ether and carbonyl groups in both non-treated ACC samples are probably due to the above activation differences. On exposure to NaOH, FM 1/250 exhibits relatively greater susceptibility to surface ether structure formation, a more non-polar oxygen structure than the carbonyl oxygen. In this case, surface oxidation is associated with leaching giving enhanced adsorption in the lower pressure range. A small decrease in the relative concentration of the carboxylic group is due to specific reaction with NaOH. In contrast, the more activated material FM 1/700 experiences a relative increase in carbonyl structure and a nominal relative increase in acid and anhydride structures. These relative increases are structural oxidation effects leading to pore widening. Treatment with APDS and PPDS solutions reduces apparent relative concentration of CC graphitic structure, with the effect of PPDS being more pronounced. APDS solution gave a relative increase in carbonyl content for both FM1/250 and 700, with associated nominal widening of the micropores and overall reduction in pore volume. The 10 % PPDS solution gave no appreciable relative change in oxygen content in FM1/250, but affected the FM1/700 in a manner analogous to the 10 % APDS treatment. 12000 10000
C-O-C 8ooo
C=O
6000
~
~
O-C=O
-C
4000 2000 i
296
294
292 290 288 286 284 Binding Energy, eV
282
280
Fig. 5. Analysis of Ci s XPS spectrum for FM1/250-0 showing types of C-O surface groups.
581
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
12000 f .~
10000 C-O-C 8000 C:
.~
.~ 6000
'
4000
'
0
C-C
O-C=O
-
2000 0
l....t.
294
296
292
290
288
286
284
282
280
B i n d i n g Energy, eV Fig. 6. Analysis of C1S XPS spectrum for FM 1/250-24 showing types of C-O surface groups.
12000 10000
i C-O-C
,m
8000 C-C ~" 6000
oH
4000 O
r~ 2000 _ ,
296
l
,
I
l
294
,
l
I
l
292
,
l
I
l
290
l
l
I
l
288
i
,
I
l
-----~-
|
286
284
282
280
Binding Energy, eV Fig. 7. Analysis of C~s XPS spectrum for FM 1/700-0 showing types of C-O surface groups.
582
P. Pendleton et al.
12000 ,~ 10000 C-O-C 8000
C-O
6000 O-
C
~4000 r,.) 2000
296
294
292 290 288 286 284 Binding Energy, eV
282
280
Fig. 8. Analysis of C~s XPS spectrum for FM 1/700-24 showing types of C-O surface groups. 4. CONCLUSIONS ACC derived from polymer-based starting materials are succeptible to oxidation and leaching on contact with 4M NaOH. Basic peroxydisulfate solutions also oxidise these materials, resulting in wider micropore widths and reduced pore volumes. Each oxidising agent introduces a relatively greater oxygen content, sites which may subsequently be used for selective adsorption and/or catalyst support or binding. ACKNOWLEDGEMENT We thank the Center for Molecular and Materials Sciences and the University of South Australia Research Office for their generous support of this work. REFERENCES [ 1] M.P. Cal, M.J. Rood and S.M. Larson, J. Gas Sep. Purif., 10 (1995) 117. [2] S.S. Barton and J.E. Koresh, JCS, Faraday Trans., 73 (1983) 1173. [3] K. Bh. Pradhan and N.K. Sandle, Carbon, 37 (1999) 1323. [4] C.L. Mangun, K.R. Benak, J. Economy and K.L. Foster, Carbon, 39 (2001) 1809. [5] S.H. Wu and P. Pendleton, J. Colloid Interface Sci., 243 (2001) 306. [6] A. Badalyan., P. Pendleton and H. Wu, Rev. Sci. Instrum., 72 (2001) 3038. [7] A. Badalyan and P. Pendleton, Langmuir, 19 (2003) 7919. [8] T. Takaishi and Y. Sensui, Trans. Faraday Soc., 59 (1963) 2503. [9] A. Badalyan and P. Pendleton. In 7th International Symposium on the Characterisation o f Porous Solids, COPS-7. 26-28 May 2005. Aix-en-Provence, France. Poster 69. [ 10] G. Palmgren, Calgon Carbon Corporation. Private Communication, (2003).
575
P.L. Llewellyn, F. Rodriquez-Reinoso, J. Rouqerol and N. Seaton (Editors)
9 2007 ElsevierB.V. All rights reserved
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry P. Pendleton a, A. Badalyan a, R. Bromball a and W. Skinner b
aCenter for Molecular and Materials Sciences; blan Wark Research Institute; University of South Australia, Mawson Lakes, South Australia, 5095, Australia
FM1/250 and FM1/700 activated carbon cloth samples were chemically treated for 24 hours in 4M N a O H at 373 K. Chemical activation was also carried out in 10 % (w/vol) saturated ammonia peroxydisulfate solution, and in 10 % (w/w) potassium peroxydisulfate solution at room temperatures for 24 hours. Nitrogen adsorption on chemically treated samples was measured at 77 K. All samples showed a classical Type I microporous adsorption isotherm. N a O H treatment caused an increase in total micropore volume for FM1/250, but no statistically significant change for FM1/700. In contrast, a change in primary micropore volume for FM1/700 occurred, but no statistically significant change for FM1/250. The peroxydisulfate treatment decreased primary and total micropore volumes for both samples of activated carbon cloth. Both treatments caused relative increases in oxygen surface structures promoting their selective adsorption potential. 1. INTRODUCTION Activated carbon cloth (ACC) is used for the removal of volatile organic compounds from effluent gas streams [ 1]. Specific micropore structures in ACCs make them real candidates for use in adsorption processes where a high rate of adsorption is accompanied with short contact time between adsorbent and adsorbate. It is very important for manufacturers to produce ACC with desirable pore size distributions, and be able to control the development of various pores from primary micropores to mesopores during activation processes. Adsorption selectivity towards various gases is another very important property, which can be achieved by introducing oxygen complexes onto the surface during activation [2-4]. Our recent surface analyses of powdered activated carbon [5] has been extended to the more regularly defined surfaces exhibited by ACC. These adsorbents often display a "memory" in that their physical and chemical resistance submits to various solvents, albeit to a lesser extent than their parent (un-carbonised) material. We treated two microporous ACC samples with an excessive aqueous oxidizing agent, 4M NaOH, and milder conditions provided by 10 % (w/vol) aqueous ammonium and potassium peroxydisulfates solutions. The oxidation differently affects the distribution of surface carbon-oxygen structures, leading to different adsorption behaviour of ACC samples.
2. EXPERIMENTAL
576
P. Pendleton et al.
2.1. Materials and methods Two samples of activated carbon cloth (ACC), FMI/250 and FM1/700, from Calgon Carbon Corporation were used. These are single, plain weave materials, with the second number designating the value of wetting heat in J/g. The samples were exposed for 24 hours (suffix '24') to 4M N a O H at 373 K at atmospheric pressure. Non-treated samples have suffix '0'. Other samples were suspended via a rotary mixer at room temperatures for 24 hours in 25 mL of 10% (w/vol) ammonia peroxydisulfate (APDS), of analytical grade (suffix '10 %APDS'), in 10% (w/vol) potassium peroxydisulfate (PPDS), of analytical grade (suffix '10 %PPDS'). After treatment, samples were washed in deionised water for 48 hours. These samples were then heated in an atmospheric oven at 398 K for 24 hours for moisture removal. Some moisture still remains in these ACC samples, but subsequently removed when the samples underwent heating in high vacuum before nitrogen adsorption. 2.2. Nitrogen adsorption measurements Gaseous nitrogen adsorption measurements were carried out using an automatic manometric gas adsorption apparatus described elsewhere [6]. Before adsorption measurements, all samples were heated in the electric oven at 473 K (much lower than their decomposition temperature of>773 K) and evacuated to a pressure of 0.1 mPa for 8 hours to remove moisture and adsorbed gases. Liquid nitrogen level around the tube with ACC sample was controlled with a precision of_+0.02 mm using a modified liquid nitrogen delivery system [7]. Nitrogen adsorption was carried out at 77 K [6]. Samples reached equilibrium within 1520 minutes. The number of experimental points (58-60) was sufficient for the evaluation of BET specific surface areas (BET-SSA), pore volumes and pore size distribution. Thermal transpiration corrections were made at pressures below 266 Pa [8]. At higher pressures, differences between corrected and non-corrected equilibrium pressures are smaller than the uncertainties in pressure measurements. Relative combined standard uncertainties (RCSUs) of the nitrogen amount adsorbed were calculated for each experimental point [9]. 2.3. XPS analysis X-ray Photoelectron Spectroscopy (XPS) is a widely used procedure for determination of surface functional groups. A Kratos Axis-Ultra X-ray photoelectron spectrometer (Kratos Analytical, UK) was used to determine oxygen-containing functional groups on the surfaces of the treated and untreated ACC samples. The X-ray source was monochromatic Alia. (1486.6 eV) at 150W power. Atomic concentrations were calculated using core-line peak areas from survey spectra and tabulated sensitivity factors for the instrument. Survey spectra were collected in the binding energy range from 0 to 1100 eV at a pass energy of 160 eV. High resolution scans of the CI s were collected at a pass energy of 20 eV. These scans were curve-fitted to determine the presence of various carbon-carbon graphitic character and carbon-oxygen surface functional groups. 3. RESULTS AND DISCUSSION
3.1. Nitrogen adsorption isotherms Fig. 1 presents nitrogen adsorption isotherms for FM1/250 samples. All adsorption isotherms showed classical Type I shape representative of a wholly microporous adsorbent. Since adsorption isotherms are almost parallel over the relative pressure range from 1.E-4 to 1.E-l, the external adsorption surface is unaffected by chemical activation. Chemical activation by
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
577
NaOH changes adsorption behaviour of FM1/250-24 at very low relative pressures p/pO < l0 -4. There is appreciable adsorption at relative pressures below ~ 10 "6, compared to the amount adsorbed at relative pressure of about 10 .6 < p/pO < 10 .5 by FM1/250-0 sample. We interpret this increase as the formation of smaller pores within the micropore structure due to chemical etching. As a reference, for comparison purposes only, the BET-SSA increased by 4.40 % after treatment in NaOH. In Table 1, we intentionally left one additional significant digit in the reported values for indication purposes only. Chemical treatment by 10 %-APDS and 10 %-PPDS reduced the BET SSA by 14.43 and 3.33 %, respectively. 350
300 250 200 150 100 50
o
1.E-07 1.E-06 1.E-05 1.E-04 1.E.03 1.E-02 1.E-01 1.E+00 log(P/P ~
Fig. 1. Nitrogen adsorption isotherms for FM1/250 ACC. (e - FM1/250-0, ~ - FM1/250-24, 9 - FM1/250-10%-APDS, 9 - FM1/250-10%-PPDS). Table 1 BET-SSA and pore volumes for FM1/250 and FM1/700 ACC Sample Name SBET, m2/g Wt,~,,,, , mL/g FM 1/250-0 FM1/250-24 FM 1/250-10%-APDS FM1/250-10%-PPDS FM 1/700-0 FM 1/700-24 FM 1/700-10%-APDS FM 1/700-10%-PPDS
1633.9+10.9 1079.4+5.5 884.7+ 16.3 999.5___10.7 1157.7+6.7 1213.3___9.9 979.9+18.0 1066.3+18.7
_
0.1927+0.0080 0.1883+0.0018 0.1362+0.0043 0.1628+0.0025 0.1640+0.0014 0.1779+0.0035 0.1389+0.0049 0.1609+0.0036
W,o] , mL/g 0.4595• 0.5007• 0.3992• 0.4035• 0.5775• 0.5927• 0.4646• 0.5004•
P. Pendleton et al.
578
Nitrogen adsorption isotherms for FM 1/700 samples are also Type I, but again shown in Fig. 2 as Vad, VS log(P/P ~
to emphasize the low pressure amount adsorbed. In
contrast with FM1/250-24, the NaOH-treated FM1/700-24 sample does not show an increased amount adsorbed, however, it does show a modest 4.80% increase in BET-SSA over FM1/700-0. Treatment with APDS and/or PPDS results in 15.36 and 7.90% reduction in BET-SSA. 400 350 300 250
~ 200 ] 150 100 50 0 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 log(P/P ~
Fig. 2. Nitrogen adsorption isotherms for FMI/700 ACC. (o - FM1/700-0, ~ - FM1/700-24, 9 FMI/700-10%-APDS, 9 FMI/700-10%-PPDS). 3.2. Micropore volumes and pore size distribution Primary and total micropore volumes were determined after a s -uncertainty analyses [9].
A non-graphitized, non-porous carbon black [7] was used as the adsorbent standard. The increased amount adsorbed by FM1/250-24 at low pressures (in Fig.l) are equated to an increase in total micropore volume, however, the primary volume remains statistically constant. The similar treatment of FM1/700 saw no statistical change in total micropore volume, but an 8.48% increase in primary micropore volume. Treatment with PDS solutions results in a widening of the pore volume distributions as suggested by the increase in uncertainty in each primary and total micropore volume. Changes in micropore structure are further borne out by the Horwath-Kawazoe defined micropore-size distribution data in Fig. 3 and 4. Chemical activation of FM1/250 by NaOH shifted the maxima of the pore size distributions from ~ 0.50 nm to ~. 0.40 nm, thus indicating the opening of smaller micropores due to possible leaching effects. Similar treatment of FM1/700 did not significantly change pore size distribution. Treatment with either of the PDS solutions shifts the pore size distribution maxima to ~ 0.46 nm for the both FM1/250 and FM1/700.
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
579
14.0
12.0
10.0
8.0
"~ 6.0 <1 4.0
2.0
0.0
...... 0.3
'~-.~: ~0.4
,7
I
I
I
I
I
I
I
I l l
It
I l l
I
|
|
|
I
|
|
|
_
0.5 Pore width, d R nm
0.6
0.7
Fig. 3. Pore size distribution for F M 1 / 2 5 0 ACC. ( o - FM1/250-0, O - FM1/250-24, 9 FM1/250-10%-APDS, 9 FM1/250-10%-PPDS). 7.0
6.0
5.0
4.0
3.0 <1 2.0
1.0
0.0
r-,
0.30
0.40
0.50
0.60
0.70
P o r e w i d t h , dp, n m
Fig. 4. Pore size distribution for F M 1 / 7 0 0 ACC. ( o - FM1/700-0, O - FM1/700-24, 9 - FM1/700-10%-APDS, 9 - FM1/700-10%-PPDS).
P. Pendleton et al.
580
3.3. XPS results According to Calgon, the nomenclature 250 and 700 refers to the heat of wetting (presumably water) with units of J/g [ 10]. These relatively high enthalpies indicate the pores are of similar dimensions to water and/or due to strong interactions with the surface functional groups. The nitrogen adsorption isotherms for both materials show a condensation over the same relative pressure ranges, indicating both materials contain pores of similar dimensions. Thus, the difference in heat of wetting is primarily due to strong water-surface functional group interactions. The type of oxygen functional groups defined via XPS are: ether (C-O-C) at ~284.5 eV, carbonyl (C=O) at ~286.5 eV, carboxylic (O-C-O) at ~288.2 eV, and anhydrate (O-(C=O)-O) at -~290.2 eV (see Fig. 5-8). Both FM1/250 and FM1/700 samples show similar relative concentrations of C-C carbon, regardless of differences during their activation manufacturing process. Slight difference in relative concentrations of ether and carbonyl groups in both non-treated ACC samples are probably due to the above activation differences. On exposure to NaOH, FM 1/250 exhibits relatively greater susceptibility to surface ether structure formation, a more non-polar oxygen structure than the carbonyl oxygen. In this case, surface oxidation is associated with leaching giving enhanced adsorption in the lower pressure range. A small decrease in the relative concentration of the carboxylic group is due to specific reaction with NaOH. In contrast, the more activated material FM 1/700 experiences a relative increase in carbonyl structure and a nominal relative increase in acid and anhydride structures. These relative increases are structural oxidation effects leading to pore widening. Treatment with APDS and PPDS solutions reduces apparent relative concentration of CC graphitic structure, with the effect of PPDS being more pronounced. APDS solution gave a relative increase in carbonyl content for both FM1/250 and 700, with associated nominal widening of the micropores and overall reduction in pore volume. The 10 % PPDS solution gave no appreciable relative change in oxygen content in FM1/250, but affected the FM1/700 in a manner analogous to the 10 % APDS treatment. 12000 10000
C-O-C 8ooo
C=O
6000
~
~
O-C=O
-C
4000 2000 i
296
294
292 290 288 286 284 Binding Energy, eV
282
280
Fig. 5. Analysis of Ci s XPS spectrum for FM1/250-0 showing types of C-O surface groups.
581
Effect of oxidizing agent on activated carbon cloth porosity and surface chemistry
12000 f .~
10000 C-O-C 8000 C:
.~
.~ 6000
'
4000
'
0
C-C
O-C=O
-
2000 0
l....t.
294
296
292
290
288
286
284
282
280
B i n d i n g Energy, eV Fig. 6. Analysis of C1S XPS spectrum for FM 1/250-24 showing types of C-O surface groups.
12000 10000
i C-O-C
,m
8000 C-C ~" 6000
oH
4000 O
r~ 2000 _ ,
296
l
,
I
l
294
,
l
I
l
292
,
l
I
l
290
l
l
I
l
288
i
,
I
l
-----~-
|
286
284
282
280
Binding Energy, eV Fig. 7. Analysis of C~s XPS spectrum for FM 1/700-0 showing types of C-O surface groups.
582
P. Pendleton et al.
12000 ,~ 10000 C-O-C 8000
C-O
6000 O-
C
~4000 r,.) 2000
296
294
292 290 288 286 284 Binding Energy, eV
282
280
Fig. 8. Analysis of C~s XPS spectrum for FM 1/700-24 showing types of C-O surface groups. 4. CONCLUSIONS ACC derived from polymer-based starting materials are succeptible to oxidation and leaching on contact with 4M NaOH. Basic peroxydisulfate solutions also oxidise these materials, resulting in wider micropore widths and reduced pore volumes. Each oxidising agent introduces a relatively greater oxygen content, sites which may subsequently be used for selective adsorption and/or catalyst support or binding. ACKNOWLEDGEMENT We thank the Center for Molecular and Materials Sciences and the University of South Australia Research Office for their generous support of this work. REFERENCES [ 1] M.P. Cal, M.J. Rood and S.M. Larson, J. Gas Sep. Purif., 10 (1995) 117. [2] S.S. Barton and J.E. Koresh, JCS, Faraday Trans., 73 (1983) 1173. [3] K. Bh. Pradhan and N.K. Sandle, Carbon, 37 (1999) 1323. [4] C.L. Mangun, K.R. Benak, J. Economy and K.L. Foster, Carbon, 39 (2001) 1809. [5] S.H. Wu and P. Pendleton, J. Colloid Interface Sci., 243 (2001) 306. [6] A. Badalyan., P. Pendleton and H. Wu, Rev. Sci. Instrum., 72 (2001) 3038. [7] A. Badalyan and P. Pendleton, Langmuir, 19 (2003) 7919. [8] T. Takaishi and Y. Sensui, Trans. Faraday Soc., 59 (1963) 2503. [9] A. Badalyan and P. Pendleton. In 7th International Symposium on the Characterisation o f Porous Solids, COPS-7. 26-28 May 2005. Aix-en-Provence, France. Poster 69. [ 10] G. Palmgren, Calgon Carbon Corporation. Private Communication, (2003).