Langmuir 1996,11, 4386-4392
4386
Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments C. Moreno-Castilla," M. A. Ferro-Garcia, J. P. Joly,? I. Bautista-Toledo, F. Carrasco-Marin, and J. Rivera-Utrilla Departamento de Quimica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain Received March 29, 1995. I n Final Form: July 31, 1995@ A series of activated carbons with different degrees of activation was treated with HN03, HzOz, and (NH4)2S208in order to introduce oxygen surface complexes. The effects of the oxidizingtreatments on the surface area, the pore texture, and the surface chemical nature were analyzed by means of N2 and COz adsorption, mercury porosimetry, FTIR, TPD, electrophoretic, and mass titration measurements. Results obtained show that the HN03 treatment affects the surface area and the porosity of the samples to a greater extent than the other treatments. Carboxyl groups were essentially fixed after the three treatments, although ketone and ether groups, as detected by FTIR, were also fixed after the treatments with peroxides. The most important conclusionwas that the stronger acid groups were fixed after the (NH4hSz08treatment rather than after the HN03 treatment, in spite of the fact that this latter treatment fixed the largest number of oxygen complexes that evolved as COz.
Thus, it has been showng-13 that in the adsorption of inorganic compounds on activated carbons from aqueous Oxygen surface complexes are formed on activated solutions the surface chemistry of the adsorbent is, in carbons when they are treated with oxidizing agents either general, more important than the surface area and pore in the gas phase or in solution. The treatments produce texture ofthe adsorbent. Moreover,the presence ofoxygen three types of surface oxides: acidic, basic, a n d n e ~ t r a l . l - ~ surface groups on carbons can also affect the behavior of One of the methods to introduce predominantly acidic supported c a t a l y ~ t s , ~ and ~ J ~ itJ ~is known that acidic surface oxides, i.e., carboxylic, phenolic, and lactonic surface oxides can be impotant in reactions catalyzed by groups, is to treat the activated carbons with different activated carbons, such as the catalytic conversion of oxidizing solutions. In addition to these acidic groups, alcohols and olefin polymerization and racemization.16-19 basic and neutral suface groups that evolve CO after The aim of the present paper is to ascertain the changes heating at high temperature are also introduced. Fixation in pore texture and surface chemitry of a series of activated of the acidic groups on the surface of the activated carbons carbons, prepared with different degrees of activation, makes it more hydrophilic, decreases its pH of the point after their treatments with three oxidizing agents in of zero charge, and increases the negative surface charge aqueous solution: nitric acid, hydrogen peroxide, and d e n ~ i t y . ~ >At~the - l ~same time, the above treatments can ammonium peroxydisulfate. affect the surface area and pore texture of the activated Experimental Section carbons. The above changes in the surface chemistry of the The activatedcarbons were prepared from almond shells. The sample caused by the formation of acidic oxygen surface raw material was pyrolyzed at 1173 K in a Nz flow for 1h and activated in a steam flow at 1123 Kfor different periods of time, complexes will affect the behavior of the activated carbons as described elsewhere.20The final particle size range was when they are used either as adsorbents or as catalysts. between 0.15 and 0.25mm. The sample numberofthe activated carbons, the percentage burn-off (BO) obtained during the * To whom correspondenceshould be addressed. activation step, and the ash content are given in Table 1. Universit6 Lyon 1, Institut des Sciences de la MatiBre, The samples so obtained were oxidized with HN03 and H2Oz Laboratoire d'Application de la Chimie a YEnvironnement,ESCIL, and some of them with (NH&SzOs, all reagent grade supplied 69622 Villeurbanne Cedex, France. by Merck. These oxidized samples will be referred to in the text Abstract published inAdvance ACSAbstracts, October 1,1995. as N, 0, or S, respectively. The procedures were as follows: 1 (1)Boehm, H. P. In Advances in Catalysis; Eley, D. D., Pines, H., g of carbon was treated with 10 mL of concentrated HN03 (13.9 Weisz P. B., Eds.; Academic Press: New York, 1966; Vol. 16, p 179.
Introduction
@
(2) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (3)Mattson, J. S.;Mark, H. B., Jr. Activated Carbon: Surface Chemistry and Adsorption from Solution; Marcel Dekker; New York,
1971. (4) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; Wiley: New York, 1988. (5) Bansal, R. C.; Donnet, J. B.; Stoeckli, H. F.Actiue Carbon;Marcel Dekker: New York, 1988. (6) Wen, W. W.; Sun,S. C. Sep. Sci. Technol. 1981, 16, 1492. (7) Lau, A. C.; Furlong, D. N.; Healy, T. W.; Grieser, F. Colloids Surf: 1986, 18, 93. (8) Fuerstenau, D. W.; Rosenbaum, J. M.; You, Y. S. Enegy Fuels 1988,2, 241. (9) Solar, J. M.; Le6n y Le6n, C. A,; Osseo-Asare, K.; Radovic, L. R. Carbon 1990,28,369. (10) Le6n y Le6n, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon;Thrower, P. A., Ed.;Marcel Dekker, Inc.: New York, 1994;Vol. 24, p 213.
(11)Foger, K. In Catalysis. Science and Technology; Springer Verlag: Heidelberg, 1984; Vol. 6, p 227. (12) Rivera-Utrilla, J.; Ferro-Garcia, M. A. Adsorpt. Sci. Technol. 1986, 3, 293. (13)Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A,; Moreno-Castilla, C. Carbon 1994, 32, 93. (14) Moreno-Castilla. C.; Ferro-Garcia. M. A,: Rivera-Utrilla, J.:Jolv, J. P. Energy Fuels 1994, 8 , 1233. (15) Romdn-Martinez, M. C.; Cazorla-Amor6s, D.; Linares-Solano, A,; Salinas-Martinez de Lecea, C.; Yamashita, H.; Anpo, M. Carbon 1996,33, 3. (16) Szymanski, G.S.;Rychlicki, G. Pzrem. Chem. 1988, 67,315. (17) Szymanski, G. S.; Rychlicki, G. Carbon 1991,29, 489. (18) Szymanski, G. S.;Rychlicki, G. Carbon 1993,31,247. (19) F'reiss,H.;Lischke,G.;Eckelt,R.;Miessner, H.;Meyer, K. Carbon 1994,32, 587. (20) Ldpez-Ramh, M. V.; Moreno-Castilla, C.; Rivera-Utrilla, J.; Hidalgo-Alvarez, R. Carbon 1993, 31, 815.
0743-74631951241l-4386$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 12, 1995 4387
Activated Carbon Surface Modifications Table 1. Activation Time, Burn-Off,and Ash Content of Activated Carbons Used sample
activation time (h)
%BO
% ash
A1
1.0 2.5 5.0 8.0
18.6 25.0 42.0 61.4
0.04 0.05
A2 A3 A4
solid in the aqueous slurry increases, its pH takes an asymptotic value which is equivalent to the point of zero net charge of the solid. 10,22,23
Results
0.08
0.10
M) a t 353 K until dryness, and the residue was washed with distilled water until all the nitrates were removed (asdetermined with brucine). The treatment with HzOz was carried out with 1 g of carbon per 10 mL of concentrated H202 (9.8 M) 298 K in a flask placed in a shaking bath for 48 h, and finally, the oxidation with (NH&SzO* was carried out with a saturated solution of this salt in HzS04 1M (1g of carbod10 mL of solution) a t 298 K for 48 h also in a flask placed in a shaking bath. After the treatment, the samples were washed with distilled water until absence of sulfates (as determined with BaC12) was reached. All samples were characterized by Nz and C02 adsorption at 77 and 273 K, respectively. The BET equation was applied to the N2 adsorption isotherms and the Dubinin-Astakhov equation to the COz adsorption isotherms. Mercury porosimetry data were obtained up to a final pressure of 2400 kg.cm-2 using an Autoscan 60. From this technique the surface area in pores greater than 7.5 nm in diameter (S,,t) was obtained, as well as the following pore volumes: VZof pores with a diameter between 7.5 and 50 nm and V3 of pores with a diameter greater than 50 nm. The transmission FTIR spectra of some selected samples were obtained with a Nicolet 20 SXB spectrophotometer using pellets of KBr containing about 0.5% carbon. These pellets were dried overnight a t 393 K before the spectra were recorded. Temperature-programmed desorption (TPD) was carried out by heating the samples up to 1173 K in a He flow at a heating rate of 20 K-min-l and recording the amounts of CO, C02, and HzO with a quadrupole mass spectrometer as a function of temperature as described elsewhere.21 Electrophoretic mobilities were determined with a Zeta-Sizer IIe (from Malvern Instruments, England) as described elsewhere.20All experiments were carried out at 298 Kin acylindrical cell. In each case, 150 mg of fresh sample with a particle size below 0.063 mm was added to plastic bottles containing 65 mL offreshly outgassed distilled water. Each bottle was hand-shaken periodically for 1 day before the electrophoretic mobility measurements were carried out. The pH ofthe slurries was adjusted by adding either HC1 or NaOH. Each pH data point was measured with different portions (150 mg) of the same sample. The electrophoretic mobility values, p,, were converted into [-potential values according to Smoluchowski's equation (1)
The Dubinin-Astakhov equation (DA)has been applied to the COz adsorption isotherms obtained on the activated carbons. The DA equation reads as follow^:^^^^-^^
[
W=W,exp-
(Pi3"I -
In this equation (31,W is the amount adsorbed a t a relative pressure PIPo, WO is the micropore volume, A is the differential molar work given by eq 4, p is the affinity
A = RT I n ( ; )
5 (mV) = 12.8,~~ (108m2/s*V)
(2)
(4)
coefficient, taken28as 0.46, and Eo is the characteristic energy of adsorption in micropores. The CO2 liquid density a t 273 K was taken28,29as 1.03 g/cm3. The DA equation has three unknown parameters: WO, Eo, and n. These were calculated by applying to the experimental results a computer program which uses a n iterative method, as explained e l s e ~ h e r e Values . ~ ~ ~ ~of ~ WO,Eo, and n are compiled in Table 2. The DA equation has been shown30 to fit the COZ adsorption data on activated carbons better than the DR equation ( n = 2 in eq 3) because of the well-accepted fact that5~25-27,32-36 the value of n in eq 3 is not constant for all activated carbons but depends on the percentage of burnoff (BO), Le., on the degree of activation of the sample. Thus, the value of n decreases when the BO increases, indicating that the heterogeneity of the microporosity increases in the same way. Equation 3 has been ~ h o ~ n to~be, based ~ ~ on , a~theoretical ~ , ~ ~ model involving adsorption energies and their distribution. In this approach, n reflects the width of the energy distribution. Dubinin and S t o e ~ k l ifound ~ , ~ ~that there is a linear relationship between n and the characteristic variable A of the Dubinin-Radushkevich-Stoeckli equation when n takes a value between 1 and 2. Thus
n = 2.00 - 1.78 x 106A where D is the dielectric constant and r] is the viscosity of the liquid. For water, a t 298 K, eq 1becomes
(3)
(5)
Ais, according to Dubinin and S t o e ~ k l i the , ~ ,half-width ~~ of the normalized Gaussian obtained for the distribution of WOwith the structural parameter B of the activated carbon. Once A is known, one can obtain the differential curve for the micropore size distribution by applying eq 6, which was deduced by Dubinin and S t ~ e c k l i . ~ ~ ~ ~
(25)Dubinin, M. M.; Astakhov, V. A.Adv. Chem. Ser. 1970,102,69. Mass titration of aqueous slurries of activated carbons was carried out following a method described e l ~ e w h e r e , which ~ ~ . ~ ~ . ~ ~ (26)Dubinin, M. M.; Stoeckli, H. F. J. Colloid Interface Sci. 1980, 75,34. is a modified version of a method proposed by Schwarz et al.24 (27)Dubinin, M.M.Carbon 1989,27,457. For this purpose, 1 g of carbon was added to 10 mL of CO2-free (28)Ismail, I. M. K. Carbon 1991,29, 119. distilled water kept in a plastic bottle at 298 K and hand-shaken (29)Rodriguez-Reinoso,F.; Linares-Solano, A. In Chemistry and periodically for 1or 2 days until the pH of the slurry was stabilized. Physics ofcarbon;Thrower,P. A,, Ed.; Marcel Dekker, Inc.: New York, 1989;VOl. 21,p 1. After that, a small amount of COz-free distilled water was added (30)Carrasco-Marin, F.: L6~ez-Ram6n.M. V.: Moreno-Castilla, C. and again the pH of the slurry was measured after stabilization. Langmuir 1993,9 , 2758. This procedure was repeated until the pH ofthe slurry approched (31)Salas-Peregrin, M. A.; Carrasco-Marin,F.; L6pez-GarzOn, F. J.; that of the COz-free distilled water. As the concentration of the Moreno-Castilla, C. Energy Fuels 1994,8, 239. (32)Marsh, H.; Rand, B. J. Colloid Interface Sci. 1970,33,101. (21)Ferro-Garcia, M. A.; Utrera-Hidalgo, E.; Rivera-Utrilla, J.; (33)Freeman, E.M.; Siemieniewska,T.; Marsh, H.;Rand, B. Curbon Moreno-Castilla, C.; Joly, J. P. Carbon 1993,31, 857. 1970,8,7. (22)Le6n y Le6n, C. A,;Lizzio, A. A.; Radovic, L. R. In Proceedings (34)Rand, B.J. Colloid Interface Sci. 1976,56,337. of the International Carbon Conference; Paris, France, 1990,p 24. (35)Huber, V.;Stoeckli,H. F.; Hourier, J. P. J . Colloid Interface Sci. (23)Le6n y L e h , C. A.; Solar, J. M.; Calemma, V.; Radovic, L. R. 1978,67,195. Carbon 1992,30,797. (36)Finger, G.; Biilow, M. Carbon 1979,17, 87. (24)Noh, J. S.;Schwarz, J. A. J. Colloid Interface Sci. 1989,130, (37)Stoeckli, H.F. Carbon 1981,19, 325. 157. (38)Stoeckli, H. F. Carbon 1990,28, 1.
4388 Langmuir, Vol. 11, No. 11, 1995
Moreno-Castilla et al.
Table 2. Surface Area and Porosity of the Activated Carbons
EO,
wo
9
WO(COrr),
sample
kJ/mol
n
cm3.g-1
cm3.g-1
A1 A1N A10 A2 A2N A20 A2S A3 A3N A30 A3S A4 A4N A40
22.8 22.6 22.6 20.9 20.9 20.9 20.8 16.3 14.2 16.3 15.8 13.4 12.4 13.5
1.96 1.86 1.95 1.74 1.68 1.78 1.83 1.63 1.12 1.39 1.42 1.41 1.09 1.29
0.270 0.259 0.278 0.330 0.275 0.324 0.368 0.520 0.414 0.580 0.576 0.723 0.561 0.737
0.270 0.259 0.278 0.330 0.275 0.324 0.368 0.509 0.335 0.567 0.562 0.497 0.204 0.503
sco*,
Sext,
SN2r ,2Lg-1
m2.g-l
712 683 733 870 725 854 970 1342 800 1495 1482 1310 538 1326
785 650 750 825 660 800 800 1290 740 1200 1060 1600 820 1400
v3, cm3.g-1
v2 3
m2.g-1
cm3.g-1
18.3 30.7 21.5 22.5 34.6 29.8 21.1 38.2 41.4 42.4 40.5 65.9 34.8 59.1
0.063 0.097 0.071 0.078 0.105 0.089 0.068 0.119 0.128 0.132 0.127 0.225 0.114 0.200
0.068 0.089 0.061 0.078 0.116 0.115 0.074 0.155 0.164 0.166 0.131 0.309 0.194 0.243
-
7.00
6.02 .
5.m .
with LO as the pore width a t the maximum micropore distribution curve. The results obtained from application of eq 6 to C02 adsorption on the activated carbons are depicted in Figures 1-5. In the cases where the micropore distributions extended beyond a micropore width of 2 nm, the limit of the micropores, the distribution curves were integrated up to this limit (2 nm) and the WOvalues so obtained will be referred to in the text as WO(~,,~). This method was used in a previous paper.31 Wo(corr)therefore represents the micropore volume of the activated carbon, which has been convertedto an apparent surface area, Sco2given in Table2, using a valuez9of 0.195 nm2 for the cross-sectional area of the COZmolecule. Finally, the nitrogen surface area, S N obtained ~, from the BET equation (the cross-sectional area of N2 a t 77 K was taken29as 0.162 nm2),the values of Sext,Vz, and V3 are also shown in Table 2. The FTIR spectra of samples A3, A3N, A30, and A3S are shown in Figure 6, as a n example. Similar results were obtained with the other oxidized samples. The band assignments made in this work were based on previous assignments in the l i t e r a t ~ r e . ~ > ~ ~ - ~ ~ It is well-known that when a carbonaceous material is subjected to a program of increasing temperature, the oxygen complexes desorb primarily as C02 and C0.3-5,43,44 Thus, COZresults from the decomposition of carboxyl, anhydride (acidic), and lactonic groups, whereas CO results from the decomposition of phenolic, carbonyl, quinone, pyrone, and anhydride (acidic) groups. TPD experiments were carried out with all samples, and the amounts ofCO and C02 evolvedup to 1173Kare compiled in Table 3. The C02, CO, and H2O desorption profiles are depicted,in Figure 7 for samples A3N, A30, and A3S, as an example. Desorption profiles with similar shapes were obtained for the other oxidized samples. The nature of the oxidizing agent can be seen to influence the shape of these profiles.
6403.
B
.
3.m
A3
1.m .
0.m 0.75
Infrared Spectroscopy; Academic Press: New York, 1985;Vol. 4,p 169. (40)Colthup, N. B.; Daly, L. H.; Wiberlay, S. E. Introduction to Infrared Raman Spectroscopy, 3rd ed.; Academic Press: New York, 1989. (41) Zawadzki, J.In Chemistry and Physics of Carbon; Thrower, P. A,, Ed.; Marcel Dekker, Inc.: New York, 1989; Vol. 21, p 147. (42) Fanning, P. E.; Vannice, M. A. Carbon 1993,31, 721. (43) Tremblay, G.;Vastola, F.J.;Walker, P. L., Jr. Carbon 1978,I6, 35. (44) Calo, J. M.; Hall, P. J. In Fundamental Issues in Control of Carbon GasiFcationReactiuity;Lahaye, J.,Ehrburger, P., Eds.; NATO/ AS1 Series E192; Kluwer Academic: New York, 1991; p 329.
095
115
135
155
1.75
195
215
235
L (nm)
Figure 1. Micropore size distribution for activated carbons Al-A4. 703
1
6.m
.
5.m
I
6 4.m $3.03
2.m
1.m .
-
0.m 0 75
0 85
0 95
I 05
I15
I 25
I 35
t (nm)
Figure 2. Micropore size distribution for activated carbons from the A1 series. 250
1
200
6 150
s
100 050
om 075
(39) Painter, P.; Starsinic, M.; Coleman, M. In Fourier Transform
A4
2.m
085
095
105
115
125
135
145
155
165
175
L (nm)
Figure 3. Micropore size distribution for activated carbons from the A2 series.
The net surface charge of the activated carbons was determined by both electrophoresis and mass titration measurements. According to Corapcioglu and H ~ a n g , ~ ~ the electrophoretic mobility measurements detect the (45) Corapcioglu, M. 0.;Huang, C. P. Carbon 1987,25,569.
Langmuir, Vol. 21, No. 11, 1995 4389
Activated Carbon Surface Modifications
Table 3. Amount of CO and C02 Evolved after Heating up to 1173 K in He Flow
1
2m
A3
075
115
095
A
155
135
175
215
I95
235
t (nm)
Figure 4. Micropore size distribution for activated carbons from the A3 series. 2.m 1
A4
150
co,
coz,
sample
mmol-g-l
mmol-g-l
CO/COZ
%Oa
A1 A1N A10
0.41 5.43 1.47
0.04 3.18 0.49
10.3 1.7 3.0
0.8 18.9 3.9
A2 A2N A20 A2S
0.32 5.74 1.38 3.38
0.02 3.56 0.40 1.27
16.0 1.6 3.5 2.7
0.6 20.6 3.5 9.5
A3 A3N A30 A3S
0.24 5.93 1.62 3.40
0.01 3.34 0.45 1.14
24.0 1.8 3.6 3.0
0.4 20.2 4.0 9.1
A4 A4N A40
0.26 6.22 2.02
0.00 3.27 0.38
1.9 5.3
0.4 20.4 4.4
"From the amount of CO and COZ.
6
5 100
51
U
A
A
J
A
050
000 1
12
I6
16
I4
2
22
24
3
28
26
L (nm)
Figure 5. Micropore size distribution for activated carbons from the A4 series. 157q 1430
291:
I
34po
1720, I 'I
I ,
I1
I
,1260
1
300
400
500
600
700
800
900
lo00
1100
1200
900
lo00
1100
1200
T (K) I'
I
51
...._.,.-...-... 0 300
400
500
600
700
800
T (K) 3600
3200
2SOo
2400
2000
1600
izoo
aoo
cm-l Figure 6. FTIR spectra of activated carbonsfrom the A3 series. potential a t the shear plane which is adjacent to the external surface. Therefore these measurements yield false information on the overall surface characteristics of granular activated carbons, whereas mass titration measurements involve the transfer of Hf and OH- ions between the bulk phase and the surface. These two types of measurements have been used in carbon surface
Figure 7. TPD spectra for oxidized samples of the A3 series. (-)A3N, (- . ) A 3 0 , and (-- -)A3S. (A)C02 desorptionprofiles, (B)CO desorption profiles, and ( C ) H20 desorption profiles.
chemistry research to assess the surface charge distribution.9J0,22,23 Thus, the pH of the wetted activated carbon particles, determined by mass titration, can be taken as the point of zero charge (pzc), i.e., the pH value below which all the surface of the carbon accessible to water will be, on average, positively charged. The isoelectric point (iep)is determined by electrophoresismeasurements, and it is the pH value below which the external surface of the
Moreno-Castilla et al.
4390 Langmuir, Vol. 11, No. 11, 1995
activated carbon will be, on average, positively charged. Therefore, the difference between the pzc and the iep has been taken as a measurement of the surface charge d i s t r i b ~ t i o n . Thus, ~ ~ ~ ~when ~ * ~the ~ pzc value is higher than the iep value, the internal surface of the carbon particles will be more positively charged than the external surface, and vice versa.10,2z,23 The 5-potential-pH curves ofthe activated carbons A l A4 and of the oxidized samples from activated carbons A2 and A3 are shown in Figure 8. From these curves, the pHiepvalues were obtained, which are compiled in Table 4, together with those of the pH,,,.
Discussion Surface areas and pore textures of all samples used are given in Table 2 and Figures 1-5, which show the calculated micropore size distributions. From the results obtained on the surface characterization the following points can be deduced. First, for the original activated carbons (from A1 to A4), the increase in the degree of activation (%BO in Table 1) brings about an increase in the S Nand ~ Sextvalues. Sco2also increases but only up to about 40%BO (sample A3). The mean micropore width, Lo,is shifted to higher values (Figure 11, and there is a n increase in Vz and V3 values. All these results indicate that an increase in %BO produces an increase in micropore volume up to 40% BO, a widening of the microporosity, and also a n increase in the meso- and macroporosity of the samples. At higher BO, sample A4, the microporosity is centered in the supermicropores (higher limit of micropores) and in the lower limit ofthe mesopores which makes this the only sample in which S N>~Sco2,due to capillary condensation of Nz into the supermicropores. In the other cases, Sco2 is always similar or quite close to
."I
>ti
sc02
SN2.
Values of Sex,,V Z and , V3 increase with respect to the original activated carbon up to sample A3N (40% BO). Therefore, up to this degree of activation the destruction and widening of the micropores by oxidation causes an and the volume increase in the external surface area (SeXt) ofthe meso- and macropores. However, for a higher degree of activation (sample A4 with 61.4%BO) the nitric acid treatment (sample A4N) causes a decrease in the Sext, VZ, and V, values; so in this case, it appears that the mesoand macroporous network is also partially destroyed, since as explained before a n increase in the degree of activation makes the pore walls thinner. Finally, the treatments with hydrogen peroxide and ammonium peroxydisulfate yielded fairly similar results but their effects on the pore texture and surface area of the original samples were quite different from those observed with nitric acid. The samples obtained after both treatments have practically the same surface area
r
A2
-30
v
4r -40
S N Z*
Second, when the original activated carbons were oxidized with HN03 there was a decrease of both S Nand ~ Sco2 with regard to the respective values of the original samples, and the decrease in these values became more pronounced with increasing degree of activation of the original activated carbon, i.e., from A1 to A4. Since a n increase in the degree of activation makes the pore walls thinner and, thus, more easily destroyed by the HN03 treatment, this results in a widening of the microporosity and consequently a diminishing ofthe Sco2and S Nvalues. ~ This is especially significant with sample A4N, which presents the largest decrease in the Sco2 value, with all the micropore volume corresponding to the upper limit of the micropores, Le., the supermicropores (see Figure 51, showing again capillary condensation of N2 at 77 Kmaking
Y
- 10 o
-801 -
0
A2S
2 7
4
60
7
8
PH 2o 10
I
0
- 10 -20
5
n
-30
W
+
A3
-40 -4Oi I
-50
-- 560 01
A3N
-70
A3S A30
-701
Figure 8. HzOz, and for a given oxidizing agent, the amount of oxygen fixed does not seem to depend on the degree of activation of the sample. (ii) As deduced from the CO/COz ratio values, the three oxidizing treatments do not fix the same proportion of oxygen groups evolving as CO and COZ;followingthis ratio the contrary sequence given above. This could be in agreement with the appearance ofketone and ether groups on the FTIRspectra of the oxidized samples with hydrogen peroxide and ammonium peroxydisulfate. Results obtained from electrophoretic and mass titration measurements are given in Figure 8 and Table 4 and indicate that. (i)For activated carbons Al-A4 both pH,,, and pH,,, increase with the degree of activation or percentage of burn-off of the activated carbon, which means, as reported elsewhere,20 that the increase in activation also increases the surface basicity of the samples making it less negatively charged. In these cases, as pH,,, > pHi,,, the internal surface is always more basic than the external surface. However, the difference between these two values decreases from sample AI to A4 due to the opening of the porosity of the samples. After the three oxidizing treatments of samples A2 and A3, the pH,,, values decreased to a greater extent than the pHiepvalues, although pH,,, was still higher than pHi,,. This means that the external surface was somewhat more acidic than the internal surface, but the fixation of the oxygen groups with an acid character essentially took place on the internal surface. (ii) The three oxidizing treatments decreased the pH,,, of the activated carbons to the acid range, and for the same treatment pH,,, seems to be independent ofthe degree of activation of the sample. The pH,,, values after the three oxidizing treatments follow the order HZOZ> HN03 > (NH4)zSzOs. This sequence indicates that, after treatment with ammonium peroxydisulfate, the surface was more acidic than after the treatment with HN03, in spite of the fact that the latter treatment (samples A2N and A3N) fured a larger number of oxygen complexes that evolved COz than the former (samples A2S and A3S). Moreover, the
