硝酸氧化对活性炭吸附性能的影响

活性炭

Carbon Vol. 34, No. 6, pp. 741-746,

1996

Copyright 0 1996 ElsevierScienceLtd Printedin Great Britain.All rightsreserved 0008-6223/96$15.00+ 0.00

80008-6223 (96) 00029-2

INFLUENCE OF ACIDIC SURFACE OXIDES OF ACTIVATED CARBON ON GAS ADSORPTION CHARACTERISTICSDepartment H. TAMON* and M. OKAZAKI of Chemical Engineering, Kyoto University, Kyoto 606-01 Japan

(Received 21 August 1995; accepted in revised form 31 October 1995) Abstract-An activated carbon was oxidized by HNO, at boiling temperature. The influence of acidic surface oxides of the activated carbon was experimentally studied on the adsorption characteristics of eleven different gases or vapors. In the adsorption of cyclohexane, benzene, 2-propanol and 2-butanol, the adsorption capacity decreased greatly with oxidizing the carbon by 13.2 N HNO,. This was because the surface area and micropore volume had decreased by the strong oxidation as suggested by the t-plot analysis of oxidized carbons. When methanol, ethanol, acetone, acetonitrile and sulfur dioxide were adsorbed on the carbons, it was found that the isotherms on the carbon oxidized by 13.2 N HNO, were much lower than those on the original carbon except in the low partial pressure range. On the other hand, the adsorption capacity of ammonia or water increased greatly with increasing surface oxides on the carbon. Especially, ammonia was strongly adsorbed on the surface oxides, and irreversible adsorption appeared. The experimental results suggest that the adsorption sites increase greatly with the surface oxides for the polar molecules whose polarizability is very small. Copyright 0 1996 Elsevier Science Ltd Key Words-Gas adsorption, activated carbon, equation, polarizability,&reversible adsorption. oxidation, acidic surface oxide, micropore volume, DR

1. INTRODUCTION carbons have been used for gas purification, solvent recovery, etc. It is well known that the pore structure (pore volume, surface area and pore-size distribution) plays an important role in the gas adsorption on activated carbons. The chemical nature of carbon surfaces also influence their adsorptive properties. Carbon atoms at the edge of the graphene layers of activated carbon are reactive and form surface functional groups by bonding with heteroatoms such as oxygen, etc. Surface functional groups are mainly divided into acidic groups and basic groups. As for the acidic groups, surface oxides such as carbonyl, carboxyl, phenolic hydroxyl, lactone and quinone groups are representative. On the other hand, the basic groups are much less well characterized compared with the acidic groups. Usually the structures corresponding to chromene and pyrone-like ones belong to the basic groups. The surface functional groups are reviewed in the literature[l-5]. Activated carbons are produced from raw materials that contain relatively much oxygen, e.g. wood or many surface oxides are formed coal. Hence, following the incomplete carbonization of the materials. It is very inte

resting to elucidate an influence of surface oxides on adsorption from the gas or the liquid. Acidic surface oxides of activated carbons seriously influence adsorption equilibria in aqueous solutions[6-121. Adsorption equilibria of many gases and vapors have been reported on activated carbons[13]. It has been reported that the acidic surface oxides increase the adsorption capacity for Activated *To whom correspondence should be addressed. 741

polar molecules such as ammonia, water vapor and alcohol vapors[4,14,15]. However, the role of the chemical nature of the adsorbent surface and adsorptives is not still entirely explained. It is necessary to obtain more systematic adsorption data on oxidized carbons to elucidate the influence of the surface oxides on the adsorption from the gas phase. In this article, we measure adsorption characteristics of eleven gases and vapors on activated carbon and oxidized ones. We show that the adsorption characteristics depend on the pore structure and the acidic surface oxides of the carbons. Then, we elucidate the influence of the acidic surface oxides on the gas adsorption on activated carbons.

2. EXPERIMENTAL Adsorbent Activated carbon (CAL) was supplied by Calgon Co. Ltd, USA, and its BET surface area was 9422.1

m’/g. The activated carbon was oxidized using the following method: oxidation was carried out by stirring 50 g of the activated carbons in 0.5 1 of 1.1 or 6.6 N HNO, solutions at the boiling temperature for 1 hour[ 123. Brown gas, which was considered to be N02, was produced during the wet oxidation. Then, we prepared the oxidized carbon by impregnating the activated carbon in 13.2 N HNO, at the boiling temperature until brown gas stopped to evolve (about 5 hours). The oxidized carbons were washed by distilled water until the pH of the effluent water became around 5.0, and then were vacuum-dried at 383 K. Of many techniques to characterize the acidic surface oxides of carbon, the method proposed by

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H

TAMON and M OKAZAKI

Boehm et ul.[ 161 was adopted in the present work as follows. The carbon sample was added to the excess standard base (0.001 N NaHC03, 0.001 N Na,CO,, 0.01 N NaOH or 0.02 N NaOC,H,) solution, and the acidic surface oxides of the carbon surface were determined by back-titration with HCl after reaching the equilibrium. We determined the micropore structure of the prepared carbons by N, adsorption at 17 K. An adsorption (BEL Inc.; apparatus Japan, BELSORP28) was used for this measurement. We adopted the adsorption isotherm of N, on Sphron 6, which had been measured by BEL Japan, Inc., as the standard isotherm for the t-plot method[ 171. 2.2 Adsorption experiment Fixed-bed runs were carried out with the prepared carbons to determine breakthrough curves of eleven different gases or vapors. The experimental apparatus is shown in Fig. 1. A stainless steel column (6.0 mm I D and 10 cm long) was filled with 1.0 g of the prepared carbon particles. Before starting adsorption r

uns, the carbon particles were pretreated in He at 423 K for 3 hours. The adsorptive gases (NH, or SO,) were diluted by He and were introduced to the adsorption column. The organic or water vapor from the saturator was fed to the column after dilution by He. The temperature of the saturator was controlled at 283 K. The outlet concentration of the column was continually measured by a TCD detector of a gas chromatograph (Shimadzu Co. Ltd, GC-SA). When the outlet concentration became equal to the inlet one, the equilibrium amount adsorbed was calculated from the breakthrough curve. We used water (H,O), methanol (CH,OH), ethanol (C,H,OH), 2-propanol (2-C,H,OH), 2-butanol

(2-C,H,OH), benzene (C,H,), cyclohexane (C,H,,), acetone((CH,),CO), acetonitrile (CH,CN), sulfur dioxide (SO,) and ammonia (NH,) as adsorptives in this work. The breakthrough curve was obtained under the following conditions. The temperatures of adsorption were 323 K, and the gas flow rate was 0.833 cm”/s.AND DISCUSSION

3. RESULTS

3.1 Churacterization of the oxidized carbon The acidic surface oxides measured by Boehm’s method[ 161 are shown in Table 1. It is found that the phenolic hydroxyl group and the total acidity greatly increase due to the wet oxidation. The total acidity of the carbon oxidized by 13.2 N HNO, (AC-0X3) is nine times that of the original carbon (AC). N, adsorption isotherms were measured on the carbons at 77 K. The t-plot analysis is available to estimate a micropore structure of the carbons from the isotherm. We used the standard isotherm of N, Table 1. Acidic surface oxides of activatedcarbon Total acidity@q/kg) 0.356 0.572 1.055 3.290

Adsorbent AC AC-OX1 AC-OX2 AC-OX3

I@q/kg) 0.032 0.064 0.126 0.601

II

III

IV

(eqikg)0.008 0.015 0.018 0.114

(eqikg)0.249 0.371 0.741 2.413

(eq/kg)0.07 1 0.123 0.169 0.162

I: strongly acidic carboxyl group; II: weakly acidic carboxy1 group; III: phenolic hydroxyl group; IV: carbonyl group, AC: untreated activated carbon, AC-0X1: AC oxidized by 1.1 N HNO,, AC-0X2: AC oxidized by 6.6 N HNO,, AC-0X3: AC oxidized by 13.2 N HNO,.

He

so2 NH3

1. constant 4. adsorption

flow valve column bath

2. rotameter 5. TCD 8. vapor saturator

3. four-way 6. soap-tilm

valve meter

7. temperature-controlled

Fig. 1. Experimental

apparatus.

活性炭

Influence

of acidic surface oxides of activated

carbon

on gas adsorption

characteristics

143

on Sphron 6 and applied the t-plot method to the measured isotherm. Then, we determined the surface area S, and the micropore volume VP, and estimated the average slit width of micropores 6 by a geometrical consideration. The values of S,, V, and 6 of the prepared carbons are presented in Table 2. The value of S, is almost equal to the BET surface area SBET listed in this table. It is very interesting to elucidate the position of the surface oxides on the activated carbon. It can be seen that S,, and VP slightly decrease with increasing total acidity for AC, AC-OX1 and AC-0X2. It was foun

d that the micropore width 6 is independent of the total acidity for AC, AC-OX1 and AC-OX2 as shown in Table 2. As for AC-0X3, however, S, and VP decrease greatly with the oxidation, and 6 becomes wider by the oxidation. Consequently, we suppose that surface oxides are formed at the entrances of micropores blocking a part of the micropores under present conditions. The reason the micropore width becomes wider on AC-OX3 cannot be explained at the present stage of investigation. A detailed analysis will be needed in the future.

decrease greatly on AC-0X3. The results are attributed to the decrease of surface area S, and micropore volume VP by the strong oxidation as shown in Table 2. Similar results were obtained for C,H,, 2-C3H,0H and 2-C,H,OH. Figures 3 and 4 show the adsorption isotherms of C,H,OH and (CH,),CO on the activated carbon and the oxidized carbons. It can be seen that the isotherms are almost identical on AC, AC-OX1 and AC-0X2. The adsorption on AC-OX3 is smaller than on AC, AC-OX1 and AC-OX2 except in the low pressure range. In this range, the isotherm on AC-OX3 is almost equal to those on AC, AC-OX1 and AC-0X2. Similar results were obtained for CH,OH, SO, and CH,CN. Although alcohols, (CH,),CO, SO1 and C’H,CN are polar molecules, the surface oxides of carbon do not increase the adsorption capacity. On the contrary, the capacity decreases with the oxidation because of the change of surface area and micropore volume. We have obtained quite different results for H,O and NH, compared to the above adsorptivcs. The

3.2 Adsorption

equilibria4

-

DR

Figure 2 shows adsorption isotherms of CbH12 on the prepared carbons. The amounts of C,H,, adsorbed are almost the same on AC, AC-OX1 and AC-0X2. On the other hand. the amounts adsorbedTable 2. Pore structure S HET (m’/g) 942 918 907 181 of prepared carbons

equation

Adsorbent AC AC-OX AC-OX2 AC-OX3

(m’/g) 954 902 891 149

SP3.78 x 3.60x 3.54 x 6.33 x 10-l IO-’ 10-l lo-’ 0.79 0.79 0.79 0.85 3.93 3.82 3.78 7.48 x x x x 10-l 10-l 10-l lo-’

I

AC, AC-0X1, AC-OX2 and AC-0X3: in Table 1; SsET: BET surface area; V,: micropore volume; S,: surface area; 6: average slit width of micropore; W,: limiting volume of micro-adsorption space estimated by DR equation.

Partial pressure of ethanol p

[kPa 11:arbon

Fig. 3. Adsorption equilibria of ethanol on activated and oxidized carbons at 323 K.

-

DR

equation

5

a14% b. B s$j AC-OX3‘: g d 0”

-

DR

equation

3

2L

’

2

4

6

0

2

4

6

8

10

12

14

Partial pressure of cyclohexanep

[kPa]

Partial pressure of acetonep

[kPaJcarbon

Fig. 2. Adsorption equilibria of cyclohexane on activated carbon and oxidized carbons at 323 K.

Fig. 4. Adsorption equilibria of acetone on activated and oxidized carbons at 323 K.

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H. TAMON

andM.

OKAZAKI

adsorption isotherms of H,O are shown in Fig. 5. This figure suggests that the amount adsorbed increases with the increase of acidic surface oxides by the wet oxidation. Figur

e 6 shows the isotherms of NH, on the prepared carbons. It is found that the amount of NH, adsorbed on AC-OX3 is about ten times that on AC. One can see that the surface oxides increase the adsorption capacity for the polar adsorptives such as H,O and NH,, whose polarizability is very small. 3.3 DR equation The DubininRadushkevich (DR) equation is given by eqn (1) and applied to adsorption equilibria on the carbons. I+‘= W,ev-k{RWp,lp))21

(1)

Wis the volume adsorbed (=(IM/P,), where A4 is the molecular weight, p,_ is the density in the liquid state at the adsorption temperature 7; and R is the gas constant. W, is the volume adsorbed in the case

Tz

1.0

DR

equation

$

2f

cc

0.8

0.6 AC-OX2

s: 0 * 0.4 E g E 0.2

on,.~0.0

0.5

1.0

1.5

Partial pressure of waterp

[kPa]carbon

Fig. 5. Adsorption equilibria of water on activated and oxidized carbons at 323 K.

that p is equal to the saturated vapor pressure ps. The constant k is related to the adsorptive interaction. If k is small. the amount adsorbed is large in the region of low partial pressure of adsorptive. The value of ps has been estimated by the Antoine equation[IS], and the constants W, and k have been determined by a method of least squares to correlate measured adsorption isotherms. Figure 7 shows a few examples of DR plots. W, and k determined are presented in Table 3. The curves correlated by eqn ( 1) are shown in Figs 2-6. One can see that eqn ( 1) can be applied to the adsorption equilibria on the prepared carbons. Equation (1) has also been applied to N, adsorption on the carbons, and the values of W, are listed in Table 2. tt can be seen that W, corresponds roughly to the micropore volume V,. Table 3 shows that k values for C,H,, and C,H, increase with oxidation, and that the adsorptive interaction becomes weak. On the other hand, k values decrease with the oxidation in the adsorption of other gases and vapors, and the interaction becomes strong. In the adsorption of alcohols, (CH,)2C0. CH,CN, S02, H,O and NH,. these molecules and the surface oxides possess nonbonding electron pairs, and hydrogen bonding takes place. Hydrogen bonding has a strong interaction energy on the oxidized carbons because a part of the surface of carbon becomes hydrophilic. It is well known that the adsorption of the molecules on activated carbons is due to micropore filling. A comparison of Tables 2 and 3 shows that k& values correspond roughly to the micropore volumes V’, estimated from the t-plots except for H,O and NH,. One can see that W, values decrease with the oxidation from Table 3. In the adsorption of H,O and NH,, the micropore filling does not occur because the carbon surface is hydrophobic. When activated carbons are oxidized. the surface becomes hydrophilic and the pore tilling may take place. The W,, values for H,O and NH,j increase greatly with

4

$ 2 g 5 0 0 I 2 3 4 5 6 1

0.001 0 2x10s 4x1$ 6x10* 8x108 Partial pressure of ammoniap[kPa] (RT ln@$ Ip)] 2[J2/mo121

Fig. 6.

Adsorption equilibria carbon and oxidized

of ammonia on activated carbons at 323 K.

Fig. 7. DR plots of adsorption equilibrium data of ammonia on activated carbon and oxidized carbons at 323 K.

活性炭

Influence

of acidic surface oxides of activated carbon on gas adsorption characteristics Table 3. Constants W, and k in DR equation k t0noJ/J)2)

745

ACC,H,, C,H, CH,OH C,H,OH 2-C,H70H 2-C,H90H (CH,),CO CH,CN SD* Hz0 NH, 2.85 x 3.01 x 1.54 x 8.99 x 6.33 x 4.03 x 5.73 x 1.03 x 6.26 x 1.08 x 3.35 x lo-9 1o-9 10-a 1o-9 1O-9 1o-9 1o-9 lo-* 1O-9 lo-” 1oe9

AC-OX 2.70 x 3.41 x 1.33 x 9.51 x 5.39 x 3.98 x 5.34 x 9.85 x 6.29 x 1.41 x 3.22 x

1

AC-OX2 3.18 x 10-g 3.71 x 1o-9 1.15 x lo-* 6.28 x 1O-g 5.11 x10-9 4.12 x 1O-9 5.42x 1O-9 8.33 x 1O-9 5.99x 10-9 1.56 x lo-’ 3.05 x 10-Q

AC-OX3 4.37 x 4.65 x 5.11 x 5.00 x 3.42 x 3.24 x 4.99 x 4.50 x 4.47 x 1.39 x 1.00 x 1o-9 1O-9 1o-9 1o-9 10m9 10m9 10-g 10-g 10-g 10-a 1om9

1O-9 1o-9 10-s 1o-9 10-g 1o-9 1o-9 1O-9 1O-9 lo-* 1O-9

W, (cm3/g) AC CsHlz GH, CH,OH C2H,0H 2-C,HTOH 2-C,H90H (CH,),CO CH,CN SO, Hz0 NH, AC, AC-0X1, 3.94 x 10-l 3.73 x 10-l 2.36 x 10-l 3.50 x 10-r 2.68 x10-l 4.22 x 10-l 2.74x 10-l 3.57 x 10-r 1.35 x 10-l 7.10 x 1o-3 3.63 x 1O-2 AC-OX 1 3.96 x 4.03 x 2.43 x 3.91 x 2.44 x 4.45 x 2.72 x 3.65 x 1.66 x 7.80 x 4.82 x 10-l 10-r 10-l 10-l lo- 1 10-l 10-l 10-l 10-l 1o-3 lo-’ AC-OX2 3.79 x 3.88 x 2.07 x 3.53 x 2.23 x 3.96 x 2.63 x 3.59 x 1.53 x 1.28 x 8.08 x 10-l 10-l 10-r 10-l 10-l 10-l IO-’ 10-l lo- 1 lo-’ 10-Z AC-OX3 5.48 x 9.08 x 7.10 x 1.08 x 3.80 x 8.08 x 1.97 x 8.61 x 7.22 x 2.87 x 1.22 x IO-* 1o-2 1o-2 10-l 1om2 lo-’ 10-z 1O-z lo-’ lo-* 10-l

AC-OX2 and AC-0X3:

as in Table 1.

increasing

the surface oxides as shown in Table 3. Especially, W, for NH3 is larger than V, on AC-0X3. Consequently, the surface oxides act as adsorption sites, and W, increases with the oxides.

3r----a%

3.4 Irreversible adsorption of NH, on oxidized carbon The carbons loaded with NH, have been regenerated by desorption in a He stream at 423 K. Then, we have measured the amount of NH, adsorbed on the regenerated carbons at 323 K under the following conditions; p= 1.2 kPa for AC, AC-OX1 and AC-0X2, and 0.73 kPa for AC-0X3. Figure 8 shows the change of amount adsorbed with the regeneration cycle number. In the adsorption of NH, on AC-0X3, the amount adsorbed decreases substantially after the first regeneration, and the amount drops asymptotically to ultimately around 35% of that of the virgin carbon. It is found that irreversible adsorption appears on AC-0X3. This figure also shows that the decrease of the amount adsorbed with cyclic regeneration is not so large for AC-OX1 and AC-0X2, and that perfect regeneration is achieved with AC. Hence, one can see that the adsorption irreversibility ofNH3 We NH3 is attributed have to the acidic surface oxides of of the carbon. measured the adsorption isotherms

.s

2A

0

0

1 Regeneration

2

3[-)

4

cycle number n

Fig. 8. Repeating regeneration of carbons ammo

nia.

loaded

with

after the fifth regeneration. The isotherm is shown in Fig. 9. The curves are the results correlated by eqn ( 1). The constants determined on

on AC-OX3

AC-OX3 after fifth regeneration are W,=0.0740 cm3/g and k=2.37 x 10e9 mo12/JZ. The difference in the adsorbed quantity between the virgin AC-OX3 and the five times-regenerated one is regarded as irreversible quantity. The difference is about 1.75 mol/kg, and is almost independent of the partial pressure of NH,. This figure indicates the appearance of irreversible adsorption on AC-0X3.

活性炭

146

H. TAMON and M. OKAZAKI

:

-

DRequaiion 0

Adsorption sites of the carbon increase with the surface oxides for the polar molecules whose polarizability is very small.

Acknowledgements-TheHirano, Ichiro Okada experimental works.

authors are grateful to Makotoand Eiji Miyamoto for their help in

REFERENCES1. H. P. Boehm, In Adtxznces in Cutalysis (Edited by D. D. Eley, H. Pines, and P. B. We&), Vol. 16, pp. 179-274.

ail

I

2

3

4

5

6

Academic Press, New York (1966). 2. J. B. Donnet, Curhon 6, 161 11968). 3. J. S. Mattson and H. B. Mark. Jr, InSucfirce Chemistry and Adsorprioiz,from

Partial pressure of ammoniap

[kPa]

Actiouted Carbon: Solution, pp. 25

-86. Marcel

Fig. 9. Adsorption equilibria of ammonia on virgin AC-OX3 and after fifth regeneration at 323 K. 4. CONCLUSIONS We oxidized an activated carbon by HNO, at the boiling temperature, and measured adsorption equilibria of eleven adsorptives on the prepared carbons. We studied the influence of acidic surface oxides on the adsorption from the gas phase using the DR equation. The following conclusions were obtained: Although the acidic surface oxides of the carbon are increased by the wet oxidation, their surface areas and micropore volumes become small only after strong oxidation In the adsorption of H,O and NH,, the adsorption capacity increases greatly with the oxidation of the carbon. Especially, NH, is strongly adsorbed on the surface oxides of carbons, and irreversible adsorption appears. In the adsorption of other gases or vapors, the adsorption capacity decreases with the oxidation of the carbon because its surface area and micropore volume are decreased even if the adsorptive interaction is strong on the oxidized carbon.

4. H. Jankowska,Actioe

Dekker, Inc., New York (1971). H. A. Swiatkowski and J. Choma, In Carbon, pp. 81-104. Ellis Horwood. New York

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(1991). H. P. Boehm, Curbon 32, 759 (1994). R. W. Coughlin and F. S. Erza, J. Enoironmentul Sci. Technol. 2, 291 (1968). R. W. Coughlin, F. S. Erza and R. N. Tan, J. Co/l.[nret$ce Sci. 28, 386 (1968). J. Zawadzki, Carbon 19, 19 (1981). K. Urano, Y. Koichi and Y. Nakazawa. J. Golf. I~wyfirce Sci. 81, 477 (1981). T. Asakawa and K. Ogino, J. Colloid Interface Sri. 102, 348 (1984). T. Asakawa, K. Ogino and K. Yamabe, Bull. Chum. Sor Jpn. 58, 2009 (1985). H. Tamon and M. Okaraki, In Fundarnentuls of Adsorpuon (Edited by

M. Suzuki), pp. 663-669. Kodansha, Tokyo (1993). D. P. Valenzuela and A. L. Myers, In Adsorption Equilihrirtnt D&I Handbook, pp. 17-205. Prentice-Hall Inc., Englewood Cliffs, New Jersey (1989). Y. Matsumura, J. Appl. Chem. Biotechnol 25, 39 (1975 ). S. S. Barton, M. J. B. Evans and J. A. F. MacDonald, Langmuir 10, 4250 (1994). H. P. Boehm, E. Diehl. W. Neck and R. Sappok, Angw.Chem. Intern. Ed. Engl. 3, 669 (1964).

4, 17. B. C. Lippens and J. H. de Boer, J. Catch. 319 (1965). (Kiso18. The Chemical Society of Japan, In Kugaku-Binran Hrn), 4th Ed., Vol. 2, pp. 124-135. Maruzen, Tokyo (1993).

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