Carbon, 1972, Vol. 1O, pp. 145-154. Pergamon Press. Printed in Great Britain

THE REACTION OF OZONE W I T H ADSORBENT CHARCOAL

V. R. DEITZ and J. L. B I T N E R Naval Research Laboratory, Washington, D. C. 20390, U.S.A.

(R6ceived 8July 1971)

A b s t r a c t - T h e progressive reaction of ozone and oxygen mixtures with charcoal at 26°C was studied to specified weight-gains and to different subsequent weight-losses when outgassed at various temperatures. Although each ozone-treated adsorbent gave evidence of continuous decomposition when introduced into a vacuum adsorption apparatus, stability was obtained when the sample was cooled to 77"4°K. The degassing is associated with an inherent unstability of the ozone-product formed and not to desorbed gases. The products were characterized by very large decreases in the nitrogen adsorp- tion at 77-4°K. Two explanations are examined to account for the adsorption behavior. First, the etching reactions that prevailed during the formation of the original carbon adsorbent can continue and lead to pore structures of greater diameters (and accordingly lesser specific surface area) than the original charcoal. Second, a blockage of the micro- pore openings to nitrogen molecules can take place due to the chemisorption of oxygen at the critical openings of the porous carbon structure. A sequence of three reactions is suggested to explain both the weight changes upon ozone treatment and the unique behavior of the nitrogen adsorption isotherms determined at 77-4°K.

1. I N T R O D U C T I O N

Almost all commercial carbon adsorbents are f o r me d by the pyrolysis of the organic source material and a 'burn-off" in an oxidiz- ing env i ronment (02, CO, CO2, H20). An elaborate etching o f the carbonaceous residue takes place which results in a large increase o f the surface area. As a consequence of the above process, oxygen is incorpora ted into the network s t ructure of the solid and, in view of the large area, an appreciable frac- tion is in the surface.

This study was unde r t aken to de te rmine the surface proper t ies o f several adsorbents af ter the oxygen content had been signifi- cantly modif ied by react ion with ozone at r oom tempera ture . It was hoped to deduce f rom such informat ion some structural features o f the original charcoal. Quite unexpectedly , the oxygen thus incorpora ted p r o d u c e d quite different surface proper t ies

than that in the original charcoal. T h e ozone react ion will be shown to be progressive and the p roduc t is subject to cont inuous change by thermal t reatments in vacuum. T h e need to unders t and the s t ructure o f the carbon networks in charcoals is fu r the r intensified by the need to unde r s t and the chemical changes that take place in the surface. T h e published investigations on the systems carbon b l a c k - o z o n e [l, 2] and polymer c a r b o n s - a t o m i c oxygen f o r m e d in radio f requency discharge[3,4] furn ished good background for this study.

2. E X P E R I M E N T A L

T h e dry oxygen (60 to 90 ml/min) flowed in sequence th rough a laboratory ozone genera tor , the gas cell o f the spectrophoto- meter , and over the charcoal samples, see Fig. 1. Pyrex glassware, in terchangeable g r o u n d joints, and Tef lon tubing were used

145

146 V. R. DEITZ and J. L. BITNER

HV. Source (A.CJ

~ - ~ Regulated 0 2

, L~Drying tower

| ( Hopcalite bed

~ ' ~ ~ I b -~A,m LV.

' l O ~ I

Adsorption flasks

Fig. 1. Diagramatic sketch for ozone treatment of charcoals at room temperature.

in the construct ion o f the apparatus. T h e concentra t ion o f ozone in oxygen was de t e rmined in a Beckman DU spectrophoto- mete r using the Hart ley band at 2550 ~ and a f low-through quartz cell of 10 mm depth. T h e ozone concentra t ion de te rmined f rom the band ma x im um was quite steady over many hours o f operat ion. T h e con tour o f the band agreed closely with the measurements o f Inn and Ta na ka [5].

T h e ozone concentrat ions were varied f rom 0-2 to 2.5 per cent (2000 to 25,000 ppm). T h e exit gases, af ter contact with the char- coal, were directed th rough a small bed o f Hopcali te which completely des t royed the residual ozone. T h e flasks, which conta ined relatively thin layers o f charcoal, could be r emoved one or more at a time for weighing, analysis, etc. in o rde r to obtain products f o r m e d at different contact times.

Two sources of ash-free charcoal were used, one a steam activated coconut shell charcoal and the o ther a steam activated coal-based charcoal. Both were repeatedly t reated with hydrochlor ic and hydroflor ic acids and then washed witfi a large volume o f distilled water and dried. T h e original steam activated coconut shell charcoal before acid t rea tment had been used in previous investi- gations [6] and five i ndependen t de termina- tions o f the n i t rogen adsorpt ion isotherms at

low tempera tures are shown in Fig. 2. Ref- e rence to these older results will be made in the Discussion.

T h e un t rea ted and ozone- t rea ted char- coals were analyzed for carbon and hydrogen ; in view o f the very small sulfur and n i t rogen content ( < 0.1 per cent), the oxygen could be est imated by difference. T h e vacuum out- gassing o f the sample was in t e r rup ted at regular intervals by isolating the sample container and measur ing the pressure build- up. In addition, the pressures could be de t e rmined in a known par t o f the adsorp- tion volume having a small cold finger which could be maintained at r o o m tempera ture , in liquid ni trogen, or in solid carbon dioxide slush. T h e adsorpt ion isotherms o f n i t rogen for the samples were de t e rmined volumetri- cally at 77.4°K.

o~ 15

E

I 0

5 g

Z

o v •

I I I 1 0.; o.2 0.a 0.4 o.s

P/Po Fig. 2. Nitrogen adsorption of a commercial coconut shell charcoal, (0) original, (×) 40 per cent burn off, (O) 95 per cent burn off in air at 400-

500°C.

3. R E S U L T S

T h e weight changes o f the coconut char- coal du r ing ozone t rea tment at 26 ° are shown in Fig. 3. A similar curve was obtained upon ozonat ion o f the coal-based charcoal. T h e r e was an immediate, significant increase in weight, which passed t h ro u g h a m ax im u m and then slowly decreased. In contrast, previous studies with carbon blacks Ill in ozone r epo r t ed only weight losses. T h e

REACTION OF OZONE W I T H ADSORBENT CHARCOAL 147

2 5

,'~ zo ]

~2 o ID

.~_

i i i

/ .

/ "'. 9260

1365 /~ x ~ 1 8 0 0

I i I

5000 I0000 15000

Contact time, min Fig. 3. Weight changes observed in the ozone treatment of coconut charcoal at 26°C. Samples removed for study at times designated at A, B,

and C.

elementary analyses of the charcoals, how- ever, showed a continuous decrease in carbon and a continuous increase in oxygen. The results given in Table 1 are for the original charcoal and the ozone-treated products sampled at positions A, B and C of Fig. 3. A high oxygen content in the oxone-treated product is compatible with a highly dis- ordered carbon network in the charcoal. This was first pointed out by Watt and Franklin [7, 8] who compared the oxidation of a polyvinylidene chloride char by ozone at room temperature with oxidation by oxygen

Table 1. Elementary analyses of ozone-treated charcoals

C H O Sample (wt. %) (wt. %) (wt. %)

Series II coconut shell charcoal Original 94.1 0" 5 5-4 Point A 78.3 3-5 18.2 Point B 54-2 3.6 42-2 Point C 45.7 3.9 50"3

Series III coal-base charcoal Original 94"2 0"9 4-9 Point A' 88.8 1-8 9.4 Point B' 59.3 2.4 38-3 Point C' 53.0 2.8 44.2

at 300-450°C. The large increase in hydrogen is a topic of further study; water does not appear to be the source since the long expos- ure of the apparatus to dry oxygen and ozone should have minimized its presence. The uncertainty of the source and of the form of binding of the hydrogen to the charcoal prevent any definitive interpretation of the data. It could be due to the volatile products formed by the reaction of ozone on the trace organic vapors in the Teflon materials.

3.1. Outgassing behavior When the ozonized charcoal was evacuated

in preparation for gas adsorption studies, the pressure built-up when the sample was iso- lated from the pumps was too rapid even at room temperature to permit volumetric adsorption measurements. The evolution of gas corresponded to a weight loss of the sample large enough to raise uncertainties in the amount and composition of the adsorbent sample. In order to establish the magnitude of the weight loss at different temperatures, ozonized charcoal was followed by T.G.A. in a flow of nitrogen (83 ml/min) with a constant temperature gradient of l°C/min. The weight-loss was continuous during the time of observation and there was no indication that a steady-state weight might be realized. After 6 hr the weight had decreased by 33 per cent.

The gases evolved were mainly carbon dioxide, carbon monoxide and smaller amounts of water vapor. An approximate analysis could be made by isolating a fraction in a known volume having the cold finger at various temperatures. When the finger was cooled at 77-4°K, the residual gas was princip- ally carbon monoxide. Table 2 summarizes the results and these show that the evolved gas varied with time, temperature, and the extent of ozone treatment. The two charcoals behaved similarly in this respect.

Fortunately, the evolution of gases from an ozone-treated charcoal decreased to prac- tically zero when the sample was cooled with

148 V. R. DEITZ and J. L. BITNER

Table 2. Gases (noncondensible at 77"4°K) evolved from ozone-treated charcoals

Temp. Time Accumulated gas Sample (°C) (min) (/~ moles)

Position A

Position B

Position C

Coconut-shellcharcoal(SeriesII) 26"8 4020 44 98 1020 42 26"7 93 1-5

213 3-2 125 90 464

210 486 230 90 436

210 438 119 60 390

145 458 205 482

230 79 291 167 299 285 3O2

Coal-based charcoal Original 110 31 0"9 Position A' 110 139 7-9 Position B' 109 120 202

227 585 26O 690

Position C' 113 113 200

35

30

2 5 o~

~-" 2o

o " J IO

I . . . . , Cooling

i i

. . ~ Constant . . / " ternperotun

/ - f

1 / / / °

Heat ing / I ° p e r m in

I I

/ /

/ /

/ /

/

I00 200 300

Temperature, °C

Fig. 4. Loss in weight upon heating ozone-treated coconut charcoal in nitrogen (83 ml/min) in a

constant gradient of l°/min.

liquid ni trogen. T h e following exper imenta l p rocedure was thus feasible: (1) a given charcoal can be reacted with ozone to increase the oxygen content by a specified amount , (2) the sample can then be evacuated for a specified time and t empera tu re to modify fu r the r the composit ion o f the char- coal, (3) the sample can then be cooled to 77.4°K with liquid ni trogen. At this tempera- ture the adsorpt ion o f one o f several possible adsorbates (N2, 02, CH4, CO, Kr), can be de te rmined as a funct ion o f the residual composit ion o f the charcoal. In this paper the ni t rogen adsorpt ion isotherms are r epo r t ed for charcoals at selected stages o f ozone t rea tment and at certain intervals in the subsequent outgassing. In each case the weight o f the sample and the content o f carbon and hydrogen were de t e rmined af ter the n i t rogen adsorpt ion measurements had

REACTION OF OZONE WITH ADSORBENT CHARCOAL 149

been completed and the samples returned to room temperature in nitrogen.

3.2. Nitrogen adsorption at 77"4°K

3.2.1. Coconut shell charcoal. The adsorption of nitrogen on the untreated steam activated coconut shell charcoal is shown at the top of Fig. 5. The BET plots were not linear and an estimate of monolayer coverage, determined by the point B-method [9], was 16,000/.~ moles/ g of outgassed material.

The ozone-treated coconut shell charcoal sampled at position A (7 weight p e r cent increase) of Fig. 3 was evacuated at 98°C for 1000 min, cooled to 77-4°K, and the nitrogen isotherm determined. The sampl e was subsequently heated for 1000 min at 228°C, cooled to 77.4°K, and the nitrogen adsorp- tion again determined. These two isotherms, given in Fig. 5 are slightly below that for the

15oo

~,. IOOOO

5000

I. , I A i t J

I I I ! I o.1 0.2 0.3 0.4 0.5

Relat ive pressure, P/Po

Fig. 5. Nitrogen adsorption (77.4°K) by ozone- treated coconut charcoal sampled at point A of Fig. 3 (3 per cent weight loss upon evacuation at 98°C, 6 per cent accumulated weight loss upon

evacuation at 228°C).

untreated charcoal; all three adsorption isotherms have the same general shape and magnitude. Although the charcoal sampled at position A had increased in weight by about 7 per cent, it lost this amount on out- gassing at 228 °. Thus, it might appear that the adsorption characteristics of the original charcoal had been recovered. However, a chemical analysis of the outgassed ozone- treated charcoal showed that the oxygen content had only decreased from 18.2 (see Table 1) to 14.4 per cent by weight. This is still considerably above the oxygen content of the original charcoal and demonstrates the permanent change brought about by ozone.

Using an area for monolayer coverage of 1500m2/g and a cross-sectional area of 10 A s for a chemisorbed oxygen atom, it is estimated that a monolayer coverage of oxygen would correspond to a weight increase of 25 weight per cent. Therefore, approximately one third of the available charcoal surface is statistically involved when the sample is ozonized up to position A.

The product sampled at position B (22 weight per cent increase) of Fig. 3 was evacuated progressively at 26.7 °, 125 °, and 230°C and after each heating, the nitrogen adsorption was determined at 77.4°K. The isotherms are given in Fig. 6. As with the charcoal sampled at position A, a progressive loss in weight occurred with an increase in the temperature of outgassing, and the nitro- gen adsorbed per unit weight of outgassed material decreased in the same sequence. Moreover, hysteresis was observed, as seen in Fig. 6 for all three outgassed samples, with no indication of loop closure at the lowest ob- served desorption pressure. The significant change is the decrease in magnitude of the nitrogen adsorption to approximately one- third of that determined for the untreated charcoal sample; a less-sharp knee is also evident in the adsorption isotherms.

The sample obtained at position C (7 weight per cent increase), had a completely different adsorption behavior, see Fig. 7.

150 V . R . DE IT Z a n d J. L. B I T N E R

6000

5000

o 4000

=t

oC" 3000

2000

IOOC

t I i J I

B . . ~ 3-4°/= loss.

. ~ " ~ . . . . . . . . . . . ~ I0"l% lOSS

~// ~ - - ~ - ~ 2,.6-/o loss-

. . . . J I I I I o.l o.2 0.3 0.4 0.5

Relative pressure, P/Po

Fig. 6. Nitrogen adsorption (77"4°K) by ozone- treated charcoal sampled at point B of Fig. 3. (3-4 per cent weight loss upon evacuation at 26"7 ° , 10.1 per cent accumulated weight loss upon evacua- tion at 125 ° , and 21.6 per cent accumulated

weight loss upon evacuation at 230°C.

T h e sequential evacuation was at 26-8 °, 119 °, and 230°C, and the cor responding weight losses dur ing desorpt ion accumulated to 52 weight per cent at the highest t empera ture . T h e magni tude o f the n i t rogen adsorpt ion was now about 1/20th that o f the original charcoal and the p r o n o u n c e d hysteresis showed no indication of closure at the lowest desorpt ion pressure. In contrast to the sam- ples obtained at positions A and B, the greatly reduced ni t rogen adsorpt ion iso- therms now increased in the same sequence as weight losses. It may be no ted in Fig. 7 that the adsorpt ion isotherm af ter degassing at 230°C is nearly linear, not a c o m m o n behavior in physical adsorpt ion o f n i t rogen at 77.4°K, but one which must be fu r the r studied for possible time dependence .

3.2.2. Coal-base charcoal. T h e weight changes dur ing ozone t rea tment of the coal-based charcoal were qualitatively similar to that given in Fig. 3. T h e products were

C ///~ 52-0% loss

1500 / /

I= / J

. /7 . fOOC

g 30% loss o / z / /

. ~ -" 0.9% loss

o o

I I I I I I 0.1 0"2 0-3 0-4 0-5 0-6

Relative pressure, P/Po Fig. 7. Nitrogen adsorption (77"4°K) by ozone- treated charcoal sampled at point C of Fig. 3. (0-9 per cent weight loss upon evacuation at 26.8°C, 30 per cent accumulated weight loss upon evacuation at 119°C, and 52 per cent accumulated

weight loss upon evacuation at 230°C).

sampled at the same relative positions in the weight per cent increase vs. t ime plot, namely A' 3.67 wt-%, B' 17.6 wt-%, and C' 7-8 wt-%.

T h e ni t rogen adsorpt ion isotherms for the un t rea ted and the three ozone-modif ied charcoals are shown in Fig. 8. In each case the outgassing t empera tu re was l l0°C and the time o f heat ing 900 min. T h e ni t rogen adsorpt ion decreased with the extent o f ozone t reatment . T h e hysteresis was ra the r small relative to that observed for the products using coconut shell charcoal.

T h e new findings of the ozone reaction with charcoals may be summar ized as follows:

(1) T h e r e is a sequence o f weight gains followed by weight losses dur ing the ozone t rea tment which makes any in terpre ta t ion o f adsorptive proper t ies on a unit-weight basis quite incomplete.

(2) T h e r e are changes in the composit ion o f the t reated charcoals that progressively

REACTION OF OZONE WITH ADSORBENT CHARCOAL 151

I0000

8000

::1l. 6000

I

.~ 4000 ~I -

2000 I / C' /

/

I l I I o oq 0.2 0.3 0 4 0 5

Relotive pressure, P/Po Fig. 8. Nitrogen adsorption (77.4°K) by the coal- base ozone-treated charcoals sampled at the A', B' and C' positions in weight-increase. All samples

were evacuated at 110°C for 900 rain.

increase the oxygen content and, unexpect- ently, the hydrogen content.

(3) There are complicated changes in nitrogen adsorption at 77"4°K that vary with the extent of the ozone treatment and with the subsequent outgassing treatment (time and temperature) given to each sample before the adsorption measurements at 77-4°K. These include (a) only a small change in adsorption when sampled at stage A for coconut charcoal, but a significant change for the coal-based charcoal, (b) substantial decreases for both charcoals when sampled at stage B, and (c) very large decreases at the C stage of ozonization for both charcoals. There is some recovery of nitrogen adsorp- tion by the stage C sample of coconut charcoal after outgassing at progressively higher temperatures.

(4) The surprisingly large increase in the hydrogen content of the charcoal during ozone treatment is a subject for further study. The increase was too large to be due to the accumulation of the initial hydrogen as the carbon was selectively gasified. Also, it

could be due only in a minor part to adsorbed water, since the long exposure of the appara- tus to dry oxygen and ozone minimized the presence of water vapor. It could be due to the reaction products formed from ozone and the low concentration of organic vapors. that originated in the Teflon grease and'. perhaps in the commercial grade of Teflon tubing that was used.

4 . D I S C U S S I O N

One characteristic property of a gas adsorbent charcoal is the extensive micropore structure. It has been proposed [10] that the adsorbed gas has to enter into the porous structure through slit-like openings not much larger than the molecular dimensions of the adsorbed molecule.

The reaction of ozone with such a struc- ture can be considered to occur in several stages. At the beginning the charcoal surface is pitted or etched in a manner similar to what happened in the 'burn-oW reactions in the formation of the carbon adsorbent. The etching process would maintain or increase the surface area, i.e. the nitrogen adsorption. A second stage of oxidation can then gasify the walls that are a part of the porous struc- ture and, when this occurs, the surface area should decrease. An alternate course of ozone reactivity could lead to the chemisorp- tion of an oxygen atom at the narrow open- ings of the microporous structure of the charcoal. Thus, the access of adsorbate molecules into the micropore would be blocked. The observed decrease in nitrogen adsorption, there{ore, could be due either to the progressive removal of the wall structure and/or the micropore plugging by chemi- sorbed oxygen.

The choice among these alternatives can be based on the determination of the adsorp- tion of molecules of different molecular cross-sections. Charcoals are known [11] to show a molecular-sieve effect by the exclusion of large adsorbate molecules by micropore openings. Progressive etching by ozone

152 V.R. DEITZ and J. L. BITNER

could enlarge the openings and thus reduce the steric influence o f the modif ied openings on the adsorpt ion process. However , if micropore plugging had taken place to an appreciable extent , the molecular-sieve effect would become more critical than in the original charcoal.

In o rde r to gain a perspective of the overall influence o f ozone on charcoal, the amounts o f ni t rogen adsorbed at P[Po = 0.1 for the various samples are plot ted in Fig. 9. It is striking that the s t ructure can contain about 20 weight per cent oxygen before the nitro- gen adsorpt ion is appreciably reduced. Moreover , bo th coconut shell (points ©) and coal-based (points ×) charcoals behave simil- arly in this respect. T h e r e appears to be a cont inuous in terconnect ing porous s t ructure in both charcoals that permits the adsorbate to en ter the internal s t ructure f rom any part o f the boundary o f a particle.

An almost l inear behavior was found relat- ing the weight increase with the weight per cent of oxygen in the products. This is shown in Fig. 10 to include all points obta ined on the weight-increasing par t o f Fig. 3.

u

o-

5_0

-o

i o

0.5

I FO

Weight %,

X

20 50 40 50

oxygen in product

Fig. 9. The adsorption of nitrogen (77"4°K) at P/Po = 0- l as a function of the oxygen contained in ozone-treated charcoals. The ordinate is the frac- tion of nitrogen adsorbed relative to the untreated charcoals. [(O) coconut charcoal source; (×) coal

base charcoal source].

2O

X

._= I0

~ 5

I0 20 .30 40 50

Weight %, oxygen in product

Fig. 10. Relationship between weight increases and the oxygen contents of ozone-treated charcoals. The points with an arrow attached are from the

weight-decreasing part of Fig. 3.

However , the relat ionship is no longer valid af ter the max imum weight increase had been realized; the points with an arrow at tached are f rom the weight-decreasing par t o f Fig. 3. T h r e e model reactions o f ozone with charcoal may serve for a partial explana- tion of the results. T h e chemisorpt ion o f oxygen atoms that originate in the ozone, see reaction I, would increase the oxygen content o f the charcoal without loss in weight. This react ion can lead to micropore plugging that was ment ioned above. Gasification o f the carbon results f rom reactions II and I I I and these are involved when progressive etching takes place. Reaction II leads to minor weight changes and reaction III to major weight changes.

CxOy(solid) + 03 ~ CxOy+l(solid) + 02 I CxOy(solid) + 03 ~ Cx_lOy+1(solid) + CO2 II CxOy(solid) + 03 ~ Cx_2Oy(solid) + CO2 + CO

II I

T h e weight changes shown in Fig. 3 are readily explained by the relative extents to which the above reactions take place. In particular, the n i t rogen adsorpt ion behavior p resen ted in Fig. 9 can be expla ined by reac-

REACTION OF OZONE WITH ADSORBENT CHARCOAL 153

tion II where one C atom is lost and one oxygen atom gained by the solid without change in extent of surface. The overall result is the atom exchange in the surface structure of the charcoal. Just how this may be accomplished in the confines of a so-called micropore system is not clear. In the reaction of ozone with carbon black (Spheron 6) Papirer, Donnet, and Schutz[1] reported carbon dioxide as the volatile product. The gases evolved on heating samples of ozonized charcoal also include both carbon monoxide and water vapor, the former in quantities much larger than anticipated. Marsh, O'Hair, and Wynne-Jones[4] also reported both gases in heating polyvinylidene chloride carbon progressively from room temperature to 900°C; their carbon had been subjected to reaction with atomic oxygen in a microwave discharge.

It was mentioned above that an oxidative etching reaction on charcoal should maintain or increase the surface area. An example is shown in Fig. 2 where the nitrogen adsorp- tion is plotted for the original coconut shell charcoal and for the products after 40 and 95 per cent burn-off in air at 400-500°C. There is very little change in adsorption per unit weight of product. Coconut charcoal appears to be quite uniform and the oxidative removal of the outer layer of the charcoal granule leaves the remainder of the granule untouched. In agreement with the density data of Kipling and McEnaney [12] charcoals from this source appear to possess a complete system of interconnecting pores.

The oxygen incorporated in the structure of an adsorbent charcoal during its formation is quite different than that introduced by the ozone reaction. The oxygen in the former case is contained in a thermally stable residue produced in the pyrolysis and burn-off reactions; in the latter case it is chemisorbed since there is involved a sharing of elec- trons between two categories, namely, the adsorbate molecule and the adsorbent sur- face. Unpublished observations from this

Laboratory show pronounced changes in the electron spin resonance of adsorbent charcoals after exposure to ozone at room temperature. The ESR lines enlarge and sharpen in proportion to the increase of the oxygen content of the charcoal.

Much work remains to be done in under- standing the surface behavior of ozone- treated charcoal. As a result of the present investigation it is now possible to study the nature and properties of chemisorbed oxygen in a temperature range where many of the thermal decomposition reactions can be minimized. An important fundamental problem is the reversibility of the oxidation reactions. Loebenstein and Deitz [13] showed that oxygen adsorbed at 200°C by charcoal could be in part desorbed as O, by heating to 400°C. There is evidence from current researches that residual ozone is present when a mixture of ozone and oxygen are circulated in a closed system through charcoal for long periods. Thus, it would appear possible to establish chemisorption isotherms for ozone on charcoal.

The development of hysteresis in the ad- sorption of nitrogen by ozone-treated coconut charcoals is compatible with the model of micropore plugging mentioned above. Carmon and Raal showed in 1951 [14] that porous structures having hysteresis behavior could be formed by pelleting fine particles of carbon; more recently, Bailey and Everett [15] were able to eliminate hysteresis in several polymer carbons by grinding to a particle size passing the 400 British Standard Sieve. The present work demonstrates that hysteresis in adsorbent carbon can be con- trolled by chemical r eac t ion - the hysteresis can be introduced by ozone treatment at room temperature or it may be restricted, or at least not influenced, by oxidation at 400-500°C.

R E F E R E N C E S 1. Papirer E., Donnet J. B . and Schutz A.,

Carbon, 5, 113 (1967).

CARBON Vo|. 10. No. 2 - C

154 V. R. DEITZ and J. L. BITNER

2. Donnet J. B. and Ehrburger P., Carbon, 8, 697 (1970).

3. Marsh H., O 'Ha i r T. E. and Wynne-Jones W. F. K., Trans. Faraday Soc. 61,274 (1965).

4. Marsh, H., O 'Ha i r T. E. and Lord Wynne- Jones, Carbon, 7,555 (1969).

5. Inn E. C. Y. and Tanaka Y., Adv. Chem. 21, 263 (1959).

6. Deitz V. R. and Gleysteen L. F., J. Res. Natl. Bureau of Standards 29, 191 (1942).

7. Watt .]. D. and Franklin, Rosalind E., Nature, 180, 1190 (1957).

8. Watt J. D. and Franklin, R. E., 1957 Conf. Industrial Carbon and Graphite, pp. 321-325 (1958).

9. Brunauer S. and Emmet t P. H., J. Am. Chem. Soc. 57, 1754 (1935).

10. Dubinin M. M., Quart. Rev. 9, 101 (1955). 11. Spencer D. H. T., In Adsorption, Surface Area

and Porosity, (Edited by R. L. Bond, Chapter 3, pp. 87-154, Academic Press, London (1967).

12. Kipling J. J. and McEnaney B., 2nd Conf. Industrial Carbon and Graphite, April, 1965, pp. 380 -389.

13. Loebenstein W. V. and Deitz V. R., J. Phys. Chem. 59, 481 (1955).

14. Carman P. C. and Raal F. A., Proc. Roy. Soc., A209, 59 (1951).

15. Bailey A. and Everett D. H., Nature, 211, 1082 (1966).