Langmuir 1994,10, 1241-1245
1241
Roles of Surface Oxygen Groups on Poly(acrylonitri1e)-Based Active Carbon Fibers in SO2 Adsorption Seiki Kisamori and Isao Mochida' Institute of Advanced Material Study, Department of Molecular Science and Technology, Graduate School of Engineering Science, Kyushu University, 6-1 Kasuga-Koen, Kasuga, Fukuoka 816, Japan
Hiroshi Fujitsu Faculty of General Education, Kyushu Sangyo University, 2-3-1 Shokodai, Higashi-ku Fukuoka, Fukuoka 813, Japan Received June 17,1993. In Final Form: January 21, 1994" SO2 removal activity at 100 "C of three kinds of poly(acry1onitrile)-basedactive carbon fibers (PANACFs) and their heat-treated analogues, FE-1QO-600,FE-200-800,and FE-300-800 (the heat treatment temperatures were 600 and 800 "C),were studied, using a model flue gas containing loo0 ppm SO2. SO2 was removed rapidly and completely for 4.2,7.0, and 5.1 h after SO2 adsorption started, using FE-100-600, FE-200-800, and FE-300-800, respectively. When FE-200-800 and FE-300-800were regenerated in the temperature range of 400-600 "C, SO2 adsorption capacities decreased very drastically. When the regeneration temperature was raised to 800 "C or higher, the SO2 adsorption capacity decreased more slowly, but large weight losses of the ACFs were observed. On the other hand, when FE-100-600 was regenerated at 600 "C, the SO2 adsorption capacity was very stable with a smaller loss of carbon than observedwithFE200-800or FE-300-800. Oxygen functionalities which disturb SO2 adsorption are suggested to be generated during regeneration,because adsorbed HzS04 was decomposed to SO2 reductively, with oxygen remaining on the surface of ACF. These oxygen functionalitieson FE-100-600 were decomposed almost completely to C02 below 600 "C. In the case of FE-200-800, 60% of the surface oxides were decomposed to C02 at 200-400 "C, and 40% were decomposed to CO at 400-900 "C. This is the reason why FE-100-600was regenerated effectively below 600 "C with a small loss of carbon. The adsorption capacities of ACFs for H2SO4 (SO2 in the presence of 0 2 and HzO) were correlated to those for SO2 in the presence of 0 2 and for H2O on the same ACFs to discuss the roles of oxygen groups on the surface. The adsorption of SO2 alone was not corrected, although 60% of the SO2 was estimated to be oxidized on an ACF surface with remaining oxygen.
Introduction A deeper cleaning technology of flue gas by a more efficient method has been searched for to protect the atmospheric environment with less consumption of energy.' A variety of active carbons and active cokes have been applied to solve the pollution pr~blems.~PDry desulfurization through its oxidation by means of the adsorption of S02, using active carbon, is a suggested method.' Juntgen and his groups established the dry process using activated cokes of high mechanical strength. However, the limited adsorption capacity required a large volume of the reactiom6 We have reported that poly(acrylonitrile)-based active carbon fibers (PAN-ACFs) possess the largest capacity for SO2 removal byadsorption.7 A PAN-ACF of large surface area was found to exhibit much larger adsorption ability after heat treatment in a nitrogen flow up to 800 "C, while another PAN-ACF of smaller surface area exhibited a smaller increase by heat treatment at 600 "C.8 Adsorbed SO2 in the form of HzS04 by oxidation and hydration on the carbon is designed to be recovered Abstract published in Advance ACS Abstracts, March 1,1994. (1) Ando, J. Shokubai 1989,31,548. (2) Yn"oto, K.; Seki, M. Kogyo Kagaku Zasshi 1971, 74,1576. (3) Juntgen, H.Carbon 1977,16,273. (4) Komataubara, Y.; Yano,M.; Shiraiehi,I.; Ida, S. J. Fuel SOC.Jpn. 1986,64,256. (6)Knoblauch, K.; Richter, E.; Juntgen, H. Fuel 1981,60, 832. (6) Tsqji, K.; Shiraishi,I.; Tataumoto, K.; Ida, S. R o c . Coal Sci. Conf. Jpn.; T h e Fuel Society of Japan, 1984; p 158. (7) Mochida, I.; Maeumara,Y.; Hirayama, T.; Fujitau, H.; Kawano, S.; Goto, K. Nippon Kagaku Kaishi 1991,269.
0743-7463/94/2410-1241$04.50/0
thermally in the form of SO2 to regenerate the adsorption capacity. Such a regeneration must liberate CO and C02, consuming carbon adsorbent or leaving oxygen functional groups on the carbon surface through the oxidation accompanied by the reduction of so3 into SOz. Hence, the carbon loss as well as the adsorption capacities after regeneration h most important for the practical application of PAN-ACFs. We found in a previous paper9 that the heat-treated PAN-ACF of the larger surface area was hardly regenerated to recover its initial adsorption ability. SO2 adsorption on active carbons has been studied for a long time;1° however, so far the oxidation of the carbon by the oxidativelyadsorbed SO2 has not been focused upon in spite of its importance for the practical application of the carbon. In the present study, enhanced adsorption activities of the heat-treated PAN-ACFs of different surface areas and their thermal regeneration after the adsorption and oxidative removal of SO2 were examined to clarify their regeneration ability in relation to their surface properties. Oxygen groups removed by heat treatment and introduced by repeated cycles of SO2 adsorption and desorption for the SOzremoval and recovery were quantified by analyzing CO and CO2 evolved by temperature-programmed de(8) Mochida, I.; Hirayama, T.; Kisamori, s.; Kawano, s.; Fujitau, H. Langmuir 1992.8. 2290. (g) Mochida,'I.;'Kisamori,S.;Kawano, S.;Fujitau, H. Nippon Kagaku Kaishi 1992,1429. (10) Juntgen, H.; Kuhl, H. In Chemistry and Phyaics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, and Baeel, 1989; Vol. 22, p 145.
0 1994 American Chemical Society
Kisamori et al.
1242 Langmuir, Vol. 10, No. 4,1994
sample FE-100 FE-200 FE-300
Table 1. Profiles of PAN-ACFs elemental analysis (wt % ) surface area C H N O(diff) ash (m2g-1) 77.5 75.8 78.1
1.8 1.7 1.4
9.7 5.8 4.5
11.0 16.7 16.0
0.1 0.2 0.3
446 878 1141
'1
FE-200-800
6
composition (TPDE) of ACFs. Their roles are discussed by comparing the adsorption of SO2 with and without oxygen and water. Although the acidJbase nature of surface oxygen groups has been reported to influence the SO2 adsorption,llJ2 the oxidation activity of the ACF surface on the SO2 adsorption is emphasized in the present study. Oxygen and water vapor have been revealed to play important roles in SO2 adsorption on carbon.13J4 Experimental Section The properties of three kinds of PAN-ACFs (Suppliedby Toho Rayon Co.) are shown in Table 1. FE-100 was the most lightly activated ACF of the smallest surface area, and FE-300 was the most heavily activated one of the largest surface area. FE-200 was intermediate between these two. FE-100 was heat-treated in a nitrogen atmosphere at 600 "C, and FE-200 and FE-300were both treated at 800 O C for 1h before SO2 adsorption to obtain their largest adsorption capacity. They are abbreviated as FE100-600,FE-200-800, and FE-300-800, respectively. SO2 adsorption was carried out in a fixed bed flow reactor. The reactant gas contained SO2 (lo00 ppm), 0 2 (5 vol %), and HzO (10vol % ) in N2. The weight of ACF and the total flow rate were 0.5 g and 100 mL min-I, respectively. The adsorption temperature was fixed at 100"C. The SO2 concentrations in the inlet and outlet gases were analyzed by a flame photometric detector (FPD). Adsorption capacities of dry SO2 with and without 02 until SO2 could be detected were also measured by the same method at 100 O C . HzO adsorption capacities of heat-treated and regenerated ACFs at 100"C were measured gravimetrically by using a quartz spring at 10 vol % H2O concentration in nitrogen. Regenerationafter the SO2 adsorptionwas carried out by heattreating the ACF for 1h in a N2 atmosphere in a temperature range of 400-1000 O C . Temperature-programmeddecomposition analysis (TPDE)of the ACFs as-received, heat-treated, and after SO2 adsorption was carried out using a quartz-glass apparatus equipped with a mass spectrometer(TE-600,Nichiden-Anelva Inc.). The sample of 0.1 g was heated in a helium flow up to 1000 "C with 10 "C min-l incrementa, and the evolved gases such as CO, CO2, and SO2 were continuously analyzed by the mass spectrometer. Results Adsorption of SO2 and Regeneration of Adsorption Capacity. SO2 in the model flue gas was removed rapidly and completely by ACFs for several hours after the SO2 adsorption started. The time until SO2 could be detected in the outlet gas was defined as the breakthrough time,
To.
Figure 1 illustrates the influences of regeneration temperature on the adsorption capacities of FE-100-600, FE-200-800, and FE-300-800 in their repeated uses. The breakthrough times, TO, observed in the first runs with FE-100-600, FE-200-800, and FE-300-800 were 4.2, 7.0, and 5.1h, respectively, which were significantlylarger than those of their as-received forms reported previously? When FE-200-800 and FE-300-800 were regenerated below 600 (11)Voll, M.;Boehm, H. P. Carbon 1970,8,741. (12)Davini, P.Carbon 1990,28,56. (13)Dratwa, H.; Juntgen, H.; Peters, W. Chem.-Zng.-Tech.1967,39,
949.
(14)Yamamoto,K.; Seki, M.; Kawazoe, K. Nippon Kagaku Kaishi 1972,1046.
FE-300-800
--
'tu 2
4
6
8
2 4 6 8 SOr adsorption cycle
2
4
6
8
Figure 1. Changes of breakthrough time TO in the SO2 adsorption-regeneration cycles of PAN-ACFs. SO2 adsorption conditions: SOZ, lo00 ppm; 02,5 vol %; HzO, 10 vol %; Nz, balance; W/F= 5.0 X 109g min d - l ; reaction temperature,100 O C . Regeneration conditions: heating for 1hat ( 0 )400 O C , (A) 600 O C , and ( 0 )800 "C under NZflow.
F
'I
s"
s
100
200
300
400
500
60
Temperature / 'c
Figure 2. SO2 desorption profiles of heat-treated ACFs after SO2 adsorption: (1) FE-100-600; (2) FE-200-800; (3) FE-300800; sample weight, 100 mg; carrier gas, He; flow rate, 100 mL min-1.
"C, TOdecreased very rapidly in the successive runs. When the regeneration temperature was raised to 800 "C or higher, TOdecreased more slowly, but still distinctly. When FE-100-600 was regenerated at 600 "C,the value of TOwas very stable up to the fifth run as shown in Figure 1,while on regeneration at 400 "C it decreased very slowly to 80% of the first run at the seventh cycle. Desorption Profiles of SO2 in the Regeneration. Figure 2 illustrates the desorption profiles of SO2 adsorbed on ACFs in the regeneration by heating. SO2 started to be evolved at 150-180 "C, exhibiting a maximum of liberation at 250 "C, and ceasing at 400 "C (FE-100-600) and 560 "C (FE-200-800 and FE-300-800), although the amounts of SO2 evolved from the ACFs were different, reflecting the adsorbed quantities. TemperatureBrogra"ed Decompositionof PANACFs. Figure 3 shows the TPDE profiles for CO and C02
Roles of Surface Oxygen Groups on PAN-ACFs -"""
I
3
Langmuir, Vol. 10, No. 4, 1994 1243
I
1
0
"0
200
400
600
800
200
600
400
loo0
Temperature I C (a) FE-100-600
800
loo0
Temperature /
Figure 3. TPDE spectra of CO and COZevolution from raw PAN-ACFs: sample weight, 100 mg; carrier gas, He; flow rate, 100mL min-1; heating rate, 10 OC min-l; (1)CO from FE-100, (2) CO from FE-200; (3) CO from FE-300; (4)CO2 from FE-100, (5) COz from FE-200; (6) COz from FE-300.
with as-received FE-100, -200, and -300. The active carbon fibers released H2O below 300 "C in addition to CO andCO2. C02 began to be evolved at ca. 200 "C, showed a maximum evolution at 300 "C, and then gradually decreased its amount to become null at 900 "C regardless of the extent of activation of the fibers. The amount of evolved C02 increased with the increasing extent of activationor surface area, being 0.18, 0.30, and 0.59 mmol g1from FE-100, -200, and -300, respectively. Although CO started to be evolved at 300 "C, its amount became significant above 500 "C to show a maximum at 790-810 "C. After a minimum evolution at 900 "C, its evolution increased again. Such profiles were common to all ACFs. The evolved amounts up to 900 "C were 1.2,1.6, and 2.5 mmol g1from FE-100, -200, and -300,respectively. Again the fiber of larger surface area provided more CO. Figure 4 illustrates TPDE profiles for C02 and CO evolving from FE-100-600 and FE-200-800 before and after the SO2 adsoroption. FE-100-600 before the SO2 adsorption released CO above 600 "C, providing the first peak at 800 "C and initiation of the second one at 900 "C as observed with as-received FE-100. C02 was hardly observable in the temperature range as expected because of the heat treatment at 600 "C before the SO2 adsorption. The same ACF after the SO2 adsorption released a significant amount of C02 in a temperature range of 200450 "C, with a long tail extending to 800 "C, while the amount of evolved CO was much the same as that before the SO2 adsorption, although the evolution temperature appeared to shift slightly to a lower temperature range. The amount of oxygen left on the ACF surface by the desorption of S02, which should be adsorbed in the form of H2SO4, was comparable to that contained in evolved C02, indicating that the SO2 recovery by heating the ACF to 600 "C will not leave any oxygen on the surface of this particular ACF. All such TPDE profiles were very reproducible unless the sample was left in the air for long periods of time. FE-200-800 before the SO2 adsorption evolved principally CO above 800 "C before the SO2 adsorption, the profile being very similar to that of the original FE-200 above 800 "C shown in Figure 3. Some evolution of CO was also observable below 600 "C even though the fiber had been heat-treated at 800 "C. A small amount of C02 was found in the evolved gas. The oxidation of FE-200800 appears to take place during ita long storage in the air at room temperature. FE-200-800 after the SO2adsorption
TemperatureI c (b) FE-200-800
Figure 4. TPDE spectra of CO and CO2 evolution from ACFs before and after SO2 adsorption: sample weight, 100 mg;carrier gas,He; flow rate, 100mL min-l; (- - -1 COZbefore SO2adsorption; (-1 COZafter SO2 adsorption; (- - -) CO before SO2 adsorption; (- - -) CO after SO2 adsorption. Table 2. Quantity of Adsorbed Molecules of Heat-Treated and Regenerated ACFs
FE-100-600 400 (7) FE-200-800
m(5) 800 (3)
400 (7) 600 (7) 800 (7) FE-300-800
0.10 0.17 0.16 0.13 0.18 0.09 0.14 0.12 0.13
400 (7)
0.06
lo00 800(9) (7)
0.17 0.16
0.34 0.24 0.20 0.40 0.56 0.16 0.27 0.36 0.26 0.11 0.34 0.23
1.1
1.6 1.7 1.6 1.0 2.1 1.3 0.5 0.6 1.4 0.8 0.7
2.3 2.0 2.8 1.5 3.8 0.7 1.7 4.0 2.7 0.6 2.0 2.0
'Adsorption of SO2 (lo00 ppm) without 0 2 and H2O at 100 "C. SO2 (lo00 ppm) with 02 (5 vol %) without HnO at 100 "C. Adsorption of SO2 (lo00ppm) with 02 (5 vol 7%) and Ha0 (10 vol %) at 100 OC. d Adsorption of HzO (10 vol %) at 100 "C.
b Adsorption of
released C02 principally in a range of 200-400 "C with a tail up to 900 "C. A much higher evolution of CO above 400 "C than before the SO2 adsorption was characteristic for this ACF. The heat treatment up to 900 "C appeared necessary to remove all oxygen left by the desorption of SO2 on this ACF heat-treated at 800 "C before the SO2 adsorption. Adsorption of SO2 on ACFs and Their Regenerated Forms. Table 2 summarizes the amounts of adsorbed SO2on as-received,as-heat-treated, and regenerated ACFs in the presence of both 0 2 and H2O when SO2 may be adsorbed in the form of HzSO4 (see H2SO4 in the fourth column of adsorbed amounts). First of all, heat treatment
Kisamori et al.
1244 Langmuir, Vol. 10,No. 4,1994 5 1
Table 3. Carbon Loss in Regeneration sample
FE-200-800
regeneration temp ("C) (cycle no.)
weight loss (%) 14 22 25
400 (7)
-P .-
FE-300-800 (7)
0
7 4 M
.E
8 20 35 15 33 35
600 (7)
1
O A
2
4 2 -
8
0
* 1 -
increased the adsorption of SO2 with both 0 2 and H2O as reported* in part in a previous paper. SO2 alone was also found to be adsorbed (see column 3 in Table 2) on ACF, although the amount was much smaller than that with 0 2 and H20 on the same ACF. Oxygen increased the adsorption of dry SO2 (see column 4 in Table 2) on ACF by 1.3-3.4 times that without it. Thus, combination of oxygen and water further increased the amount by 3.8-11 times as compared to dry SO2 with 0 2 , indicating Scheme 1of SO2 adsorption on ACF where
"
0
-
0.1
O
0.2 0.3 0.4 SO1 adsorption / m o l g'
-
3.0
S02ad
-
A 0
H2S04ad
SO2 is oxidized and hydrated regardless of the ACFs. H20 in the absence of 0 2 hardly increased the adsorption of
so2.
2.5
0
SO2ad + 1/202 SO, ad
SO, ad + H,O
0.6
n
Scheme 1 SO2
0.5
It is noted that heat treatment increased the adsorption of SO2 with 0 2 regardless of H20 presence. Adsorption of SO2 on ACF was also influenced by the heat treatment at 800 "C. When SO2 alone was adsorbed on ACFs at 100 "C, 40% of the adsorbed SO2 was liberated in a nitrogen flow at the same temperature. SO2 of this amount is probably physically adsorbed in the form of SO2 molecules. The rest of the adsorbed SO2 was desorbed up to 400 "C, being oxidativelyadsorbed even if no oxygen was supplied during the adsorption although strong adsorption on basic sites has been reported.12 The remaining oxygen on the ACF surface may oxidize SO2 into SO3. In marked contrast, a very small amount of SO2 was found in the liberated gas on heating in a nitrogen flow below 150 "C when the adsorption of SO2 was performed with 0 2 or 0 2 plus H20, indicating that the major part of the SO2 was oxidized to SO3 and hydrated to H2S04 after its adsorption. Weight Loss of ACFs by Cycles of SO2 Adsorption and Desorption. As TPDE profiles in Figure 4 indicate, ACFs, which had adsorbed SOz, released large amounts of CO and C02 at the recovery of SO2 to regenerate their adsorption capacity. The amounts of oxygen released as CO and CO2 corresponded to the oxygen left by the reduction of adsorbed H2S04 into SO2. Such releases led to the significant weight loss of ACFs in the repeated cycles of SO2 adsorption and desorption. Table 3 summarizes the weight loss of ACFs after several cycles. Both FE200-800and FE-300-800 lost large fractions of their weight up to 30 5% , during the repeated cycles when the regeneration temperature was up to 800 "C, necessary for the regeneration of the adsorption capacity. Regeneration at 400 "C allowed a much smaller weight loss of &15%; however,the adsorption ability was unacceptablyreduced.
0.5
1.0
1.5
0 2.0
Madsorption mmolg'
Figure 5. Comparison of SO2 adsorption capacities in the presence of 02 and HzO vs SO2 adsorption capacities in the presence of 02or HzO adsorption capacities of ACFs heat-treated and regenerated after SO2 adsorption: (0)FE-100-600,( 0 )FE200-800; (A) FE-300-800;SOz,loo0ppm; 02,s ~ 0 1 9 %;No, balance; W/F= 5.0 x 10-3 g min mL-1; reaction temperature, 100 "C.The adsorbed quantities of each apecies on ACFs are given in Table 2.
Insufficient regenerations of FE200-800 and FE-300800 at 400 and 600 "C reduced significantlythe adsorption capacity of SO2 alone and more markedly in the presence of 0 2 . Hence, the amounts of adsorbed SO2 in the presence of 02 only and of HzO with 02 on all ACFs were related to each other as shown in Figure 5, although the latter amounts were always much larger than those without 02 and H2O. Some of the oxygen functional groups, which were introduced by the adsorption of SO2 and ita desorption and left by the insufficient thermal regeneration, may commonly hinder the oxidative adsorption of SO2 with 0 2 on ACFs regardless of the presence of H2O. Adsorption of HzO on ACFs and Their Regenerated Forms. Table 2 also includes the amounts of adsorbed H2O on ACFs at 100 "C. The heat treatment of ACFs of large surface area at 800 "C significantly reduced HzO adsorption. In contrast, insufficientregeneration at lower temperatures increased its adsorption markedly. Fibers of moderate surface area exhibited much the same amount of H2O adsorption regardless of their heat treatment and regeneration. Oxygen functional groups on ACFs appear to influence strongly the adsorption of H2O as reported in the literature.13J4 Discussion PAN-ACF FE-100 of a moderate surface area was found in the present study to exhibit an excellent capacity for SO2 removal by adsorption and complete regeneration at
Roles of Surface Oxygen Groups on PAN-ACFs
Langmuir, Vol. 10, No. 4, 1994 1245
400-600 OC with aminimum weight loss. In contrast, PANACFs of larger surfacearea, especiallyafter heat treatment at 800 O C , possessed much higher adsorption capacities in their fiist run; however, their regeneration required heating to 800 OC to recover the initial capacity, and a larger fraction of their mass was lost. The weight loss of FE-100-600 corresponds to that of the carbon evolved as COz in the regeneration, and the quantity of oxygen in the evolved COZagreed with that left on the surface after the reduction of SO3 to SO2 on desorption of oxidatively adsorbed SOZ. Hence, such a loss of carbon is inevitable in the present kind of SO2 removal and recovery. In contrast, other ACFs after heat treatment lose mass by evolving CO as well as COz in the regeneration stages, suffering a larger loss at the same SO2 removal. It is not solved in the present study why oxidatively adsorbed SO2 produces almost exclusively CO2-generatinggroups on the ACF of moderate surface area while both CO2 and CO were produced on the ACF of large surface area. Different carbon species are suggested to be present on the surfaces of ACFs according to the extent of activation. The oxidative adsorption of SO2 and ita reductive desorption have been reported to increase the surface area and porosity to a certain extent.ls A major change was noted in the numbers of surface oxygen groups, which may play major roles in the adsorption. The adsorption of SO2 in the presence of 02 and H20 on ACFs appears to follow the steps of adsorption of SO2 molecules and their oxidation to SO3,followed by hydration. The last two steps are found to increase significantly the saturation amount of adsorbed SOz. Hence, the sites for the respective steps may be different. Thus, SO3 produced from SO2 on the oxidative sites of ACF is transferred by the hydration to the sites which can hold HzS04. Adsorption of SO2 in the presence of both 02 and H2O continues until the H2SO4 holding sites are saturated. Such a mechanism is consistent with the influences of 0 2 and HzO on the adsorption of SO2 which increases with their concentrations below their respective saturation levels. All H2S04 produced on the ACF surface stays on the sites, and no elution has been observed under the present conditions of temperature and humidity. Lower temperature and higher humidity may allow the elution of H2SO4.16 The SO2 adsorption properties of carbon surfaces have been discussed in relation to the acidlbase nature of surface oxygen g r o u p ~ . ~However, ~J~ the oxidation activity appears more influential in the presence of both 02 and HzO since more SO2 was adsorbed than is equivalent to the basic sites. The reason why FE-100-600 is regenerated almost completely at 600 "C while FE-200-800 and FE-300-800 require 800 "C is apparently clear as indicated by their TPDE profiles. Complete regeneration requires the complete removal of the oxygen left by decomposition of oxidatively adsorbed SO2 and ita recovery as SOz. CO-
generating groups produced on the latter ACFs are eliminated completely after the regeneration above 900 OC, while CO2-generating groups are decomposed below 600 OC. In marked contrast, only CO2-generating groups are introduced on the surface of FE-100-600, and they are completely eliminated at 600 "C to regenerate the surface without any extra oxygen functional groups. Figure 5 correlates the adsorption capacities of SO2 in the presence of 02 and HzO with HzO adsorption on the same ACFs. This approximate correlation indicates that larger adsorption of HzO suppresses the adsorption capacity of SO2. As also illustrated in Figure 5, the SO2 removal capacity of ACFs is governed by the adsorption of SO2 in the presence of oxygen. The adsorption of SO2 without oxygen does not correlate well with that in the presence of 0 2 and H20, indicative of the important role of oxidation activity. Hence, it is concluded that adsorption of HzO may hinder the oxidative adsorption of SO2 probably because of covering the oxidative sites heavily, physically blocking the access of SO2 to the ACF surface, although H20 is very essential to hydrate SO3 to HzS04 and increases the capacity of SO2 removal. ACFs of larger surface area insufficiently regenerated at lower temperatures around 600 "C are still covered by many COgenerating groups on their surface which may adsorb too much HzO. Another explanation may also be possible. The insufficient regeneration also decreased the adsorption of dry SO2 in the presence of 0 2 . The oxidation activity may also be reduced by CO-generating groups, or removal of such groups may create vacancies in the active sites for oxidation on the ACF surface. Thus, the regeneration at lower temperatures, which fails to completely remove such unwanted groups, decreases the adsorption capacity for SO2 in the repeated use of ACFs. The heat treatment of ACFs before SO2 adsorption increased the SO2 adsorption capacity in the presence of 0 2 and both 0 2 and H20, the extent of the increase depending upon the extent of activation of the surface area. The ACFs of large surface area showed a larger enhancement of SO2 adsorption by the heat treatment at the most appropriate temperature. The heat treatment before the adsorption removed the CO2-evolving groups completely and CO-evolvinggroups partly from as-received ACFs as shown by TPDE. A major portion of oxygen functional groups appears to disturb the SO2 adsorption as described above. Hence, their removal by the heat treatment increases the adsorption capacity of S02. The same explanation given above agrees also with this observation. So far, the roles of oxygen functional groups or their vacancies in SO2 adsorption and oxidation,HzO adsorption and hydration, and 0 2 activationare not yet clearly defined. The oxidizing sites are now examined at room temperature to reveal how they can oxidize SO2 and NO17 in the presence of 02 and HzO.
(15) Mochida, I.; Kawano, S.;Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1992,275. (16) Kisamori, 5.;Kawano, S.; Mochida, I. Chem. Lett. 1993, 1899.
(17) Mochida, I.; Kisamori, S.; Kawano, S. The 207th ACF National Meeting Abstract, San Diego, CA; to be published.
