Adsorption at Interfaces

15 Effect o f Surface O x y g e n Complexes o n Surface Behavior o f Carbons BALWANT RAI PURI

Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0008.ch015

Department of Chemistry, Panjab University, Chandigarh, 160014, India

Introduction It is now well know that microcrystalline carbons contain appreciable amounts of combined oxygen which gives rise to stable carbon-oxygen surface complexes (1). The work reported from our laboratories (2,3) and from elsewhere (4,5) indicates that there are definite surface groups or complexes which evolve carbon dioxide and, similarly, there are distinct surface entities which evolve carbon monoxide on evacuation at increasing temperatures. The effect of combined oxygen on various surface properties, such as wettability (6), heats of immersion in various liquids (7-9), adsorbability of water and other polar vapours (10-12), selectivity in adsorption from binary mixtures (13,14) has been reported. However, the effect of the individual oxygen complexes such as acidic CO2-complex (15), nonacidic CO2-complex (16) and CO-complex (3) on some of the surface properties mentioned above has not received adequate attention. An attempt has been made in the present paper to spell out the effect of each complex on (i) selective adsorption from mixtures of methanol and benzene, and (ii) adsorption of (a) benzene vapour, (b) dry ammonia, and (c) phenol from dilute aqueous solutions. Experimental Mogul (a colour black), Spheron-6 (a channel black) and a charcoal prepared by the carbonisation of recrystallised cane sugar (17) were used as such as well as after (i) outgassing at different temperature upto 1000°C, and (ii) treatment with aqueous hydrogen peroxide or potassium persulphate (l6) so as to get materials associated with different amounts of surface oxygen complexes. The amount of combined oxygen and the form of its disposition was obtained by evacuating 0.5 g portion in a resistance tube furnace, raising the temperature to 1200°C and analysing the gases evolved in the usual manner (17). 212 Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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Surface Oxygen Complexes

213

The base a d s o r p t i o n c a p a c i t y , estimated by t i t r a t i n g against aqueous barium hadroxide (17), f i x e d the amount o f a c i d i c CO2complex. This when subtracted from the amount o f carbon d i o x i d e evolved on evacuation gave the amount o f n o n - a c i d i c C0 -complex (l6) which a r i s e s through the f i x a t i o n o f oxygen at unsaturated s i t e s (17). The amount o f CO-complex was obtained from the amount o f carbon monoxide evolved on evacuation. The n i t r o g e n surface areas and the t h r e e main types o f surface oxygen com plexes of the v a r i o u s samples are recorded i n Table I . S e l e c t i v e a d s o r p t i o n from methanol-benezene s o l u t i o n s was s t u d i e d by mixing 0.25 g carbon w i t h a known weight (5-6 g) o f the s o l u t i o n i n a s m a l l g l a s s tube drawn out a t one end. The l a t t e r was s e a l e d , a f t e r c o o l i n g i n a f r e e z i n g m i x t u r e , t o min imise evaporation. Several such tubes c o n t a i n i n g 0.25 g carbon mixed w i t h s o l u t i o n s o f d i f f e r e n t c o n c e n t r a t i o n s were p l a c e d i n a shaker (12 rev/min) h e l d i n a thermostat maintained at 35 + 0.05°C f o r ho hours. The change i n composition of the l i q u i d was determined i n t e r f e r o m e t r i c a l l y . Adsorption isotherms o f benzene as w e l l as d r y ammonia were determined at 35 + 0.05°C by u s i n g McBain's t o r s i o n balance technique. A d s o r p t i o n isotherms of phenol from aqueous s o l u t i o n s were determined by mixing 0.5 g p o r t i o n s w i t h a known weight ( ~ 5 g) o f s o l u t i o n s of v a r i o u s c o n c e n t r a t i o n s , shaking the suspensions i n a r e v o l v i n g wheel h e l d i n a thermostat a t 35 + 0.05°C f o r 2k hours. The change i n c o n c e n t r a t i o n o f the s o l u t i o n was determined i n t e r f e r o m e t r i c a l l y . The experiments were conducted i n the low c o n c e n t r a t i o n range upto 20 m m o l e s / l i t r e .


2

R e s u l t s and D i s c u s s i o n The composite a d s o r p t i o n isotherms, reproduced i n F i g u r e 1 from a recent r e p o r t from the author's l a b o r a t o r y (l8), show how the preference o f Mogul f o r a d s o r p t i o n from methanol-benzene mix t u r e s of v a r i o u s c o n c e n t r a t i o n s i s a l t e r e d i n the presence o f v a r i o u s surface oxygen complexes. I t i s seen t h a t 1000°-outgassed Mogul, which i s e s s e n t i a l l y f r e e o f combined oxygen, shows strong preference f o r benzene at a l l c o n c e n t r a t i o n s , g i v i n g a t y p i c a l l y U-shaped isotherm (marked A ) . The 7 0 0 ° - outgassed Mogul, which i s f r e e o f COg-complex but r e t a i n s over 2.5 per cent oxygen capable o f e v o l v i n g carbon monoxide (C0-complex), shows, s u r p r i s i n g l y enough, even g r e a t e r preference f o r benzene, the l e s s p o l a r component of the m i x t u r e , a l l along the c o n c e n t r a t i o n range. This i s c o n t r a r y t o the view g e n e r a l l y h e l d (l3 lh) t h a t combined oxygen imparts g r e a t e r preference f o r the more p o l a r component o f the mixture. I t appears t h a t the presence o f q u i nonic groups which form a p a r t of the C0-complex (l£) promotes preference f o r benzene due t o p o s s i b i l i t y of i n t e r a c t i o n o f e l e c t r o n s of benzene r i n g w i t h the p a r t i a l p o s i t i v e charge on the carbonyl carbon atom (20). The l*00°-outgassed and the o r i g i n a l samples of Mogul, which c o n t a i n i n c r e a s i n g amounts o f CO2 com9

Mittal; Adsorption at Interfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

ADSORPTION A T INTERFACES

214

Table I . Surface Area and Surface Oxygen Complexes o f the Carbons used i n t h e Present Work.

D e s c r i p t i o n o f the sample

Surface area m /g 2

Acidic CO2complex moles/ 100 g

NonC0acidic complex CO2mmoles/ complex 100 g mmoles/


100g

Mogul Original 308 306 Outgassed a t 1*00°C 3lk Outgassed at 600°C 325 Outgassed at T00°C 318 Outgassed at 850°C Outgassed a t 1000°C 326 Original, treated 310 w i t h aq.H 02 Original, treated with K S 08 312 Outgassed at 1000°, t r e a t e d w i t h aq. H 02328 2

2

2

2

68.5

275 269 198 163 52

1*2 nil nil nil nil

nil nil nil nil nil nil

nil

1*5

nil

281*

189

nil

315

nil

120

20

15.5

nil nil nil nil

122.5 108 nil

nil

135

Spheron-6 Original Outgassed at 600°C Outgassed at 850°C Outgassed a t 1000°C Original, treated w i t h aq.H202 Outgassed a t 1000°, t r e a t e d w i t h aq.H202

110 112 109

116 11^

2.5 nil nil

61

51

106

nil

10

1*

1*12 51* 502

355 51 nil

nil nil nil

1*91

1*1*3

*53

nil

56Ο

488

nil

165

20

Sugar Charcoal Original Outgassed a t 600°C Outgassed a t 1000°C Original, treated w i t h aq.H202 Outgassed a t 1000° t r e a t e d w i t h aq.H 02 2

531 nil



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215

p l e x , though n e a r l y equal amounts o f CO-complex ( c f . Table I ) , show, on the other hand, p r e f e r e n t i a l a d s o r p t i o n o f methanol over a p p r e c i a b l e range o f c o n c e n t r a t i o n ( c f . isotherms marked C and D). This remarkable change i n the preference may be a s c r i b e d t o the presence o f a c i d i c C02-complex. This view r e c e i v e s support from the f a c t t h a t when the amount of the a c i d i c complex i s r a i s e d from 6 8 . 5 mmoles t o 1*5 mmoles on treatment w i t h aqueous hydrogen peroxide and 189 mmoles on treatment w i t h potassium per s u l p h a t e , the preference o f the s u r f a c e i s s h i f t e d i n c r e a s i n g l y i n favour of methanol ( c f . isotherms Ε and F ) . The comparison o f the isotherms Β and F shows c l e a r l y how the presence o f a l a r g e amount o f a c i d i c C02-complex has brought about almost complete r e v e r s a l of the preference o f the carbon s u r f a c e . I t i s a l s o i n t e r e s t i n g t o note t h a t the 1000°-outgassed Mogul y i e l d s almost i d e n t i c a l isotherm a f t e r f i x a t i o n o f about * per cent o f oxygen, as non-acidic C02-complex on treatment w i t h aqueous hydrogen p e r o x i d e , as before f i x a t i o n of any such oxygen (cf. isotherms marked G and A ) . The presence of n o n - a c i d i c com p l e x , e v i d e n t l y , produces l i t t l e o r no e f f e c t on the preference o f the s u r f a c e . Thus the view h e l d by p r e v i o u s workers ( 1 3 » 1 * ) t h a t the e n t i r e combined oxygen imparts p o l a r i t y t o the surface and p r o motes preference f o r the more p o l a r component o f a b i n a r y mixture needs r e v i s i o n i n the l i g h t o f the observations d i s c u s s e d above. A d s o r p t i o n o f Benzene Vapour. The above c o n c l u s i o n s were checked by s t u d i n g a d s o r p t i o n of benzene vapour d i r e c t l y on a number o f carbon b l a c k s (21). The s o r p t i o n - d e s o r p t i o n isotherms (35±0.05°C) on some of the samples o f Mogul ( F i g u r e 2 ) almost superimpose showing almost complete r e v e r s i b i l i t y o f the process. The amount o f s o r p t i o n a t each r e l a t i v e vapour pressure i s seen to i n c r e a s e a p p r e c i a b l y as Mogul i s outgassed at i n c r e a s i n g tem p e r a t u r e s . The maximum e f f e c t i s produced when the b l a c k i s out gassed at 600°C. I t appears t h a t w i t h the e l i m i n a t i o n o f the p o l a r C02-complex and the emergence o f CO-complex as the o n l y predominant s u r f a c e oxygen complex, the s o r p t i o n o f benzene i n creases a p p r e c i a b l y on account of reasons advanced i n the p r e v i o u s paragraph. The outgassing o f Mogul a t 850°C lowers the amount o f CO-complex c o n s i d e r a b l y and t h e r e i s s i g n i f i c a n t f a l l i n the s o r p t i o n v a l u e at a l l r e l a t i v e vapour pressures. With the complete e l i m i n a t i o n o f the complex at 1000°C, t h e r e i s a f u r t h e r f a l l i n the s o r p t i o n of benzene. A d s o r p t i o n of Dry Ammonia. A d s o r p t i o n o f dry ammonia on m i c r o c r y s t a l l i n e carbons at d i f f e r e n t temperatures has been i n v e s t i g a t e d by a number o f workers amongst which mention may be made of the work o f Anderson and Emmett ( 2 2 ) , V o s k r e s e n s k i i (23.), Holmes and Beebe ( 2 * ) , Studebaker ( 2 5 ) and Dupupet e t a l ( 2 6 . There i s a g e n e r a l agreement t h a t a d s o r p t i o n i s enhanced appre c i a b l y i n the presence o f combined oxygen. However, the exact


216


1-2

0·8

OA


Ε

OO

•fraction

Methanol->-

8

c° - 0 - 4

-1-2

Figure 1. Composite adsorption isotherms of methanolbenzene mixtures on Mogul before and after various treat ments. A = Mogul outgassed at 1000° C, Β = Mogul out gassed at 700°C, C = Mogul outgassed at 400°C, D = Mogul original, Ε = Mogul original, treated with aq. H 0 , F = Mogul original treated with acidified K S 0 , G = Mogul outgassed at 1000°C, treated with aq. H 0 . 2

2

2

8

2

~ΟΛ

0·2 0·3 0 4 0 5 0 6 0·7 Ο θ 0·9 Relative vapour pressure—*:

2

PÔ"

Figure 2. Adsorption isotherms of benzene on Mogul. The solid points denote desorption data. Ο = original, Δ = outgassed at 600°C, • = outgassed at 850°C, · = oxygen-free.


2


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217

r o l e o f the v a r i o u s surface oxygen complexes, which c o n s t i t u t e t h i s oxygen, has not "been e l u c i d a t e d . Adsorption isotherms o f ammonia on sugar c h a r c o a l , Mogul and Spheron-6 "before and a f t e r v a r i o u s treatments are shown i n Figures 3» h and 5, r e s p e c t i v e l y . I t i s seen t h a t , i n each case, there i s a considerable f a l l i n the s o r p t i o n o f ammonia a t each pressure on outgassing and t h a t the e f f e c t i s r e l a t i v e l y more when a carbon i s outgassed a t 600°C, causing e l i m i n a t i o n o f most of C02-complex ( c f . Table I ) , than t h a t when i t i s outgassed i n 600-1000° temperature range, causing e l i m i n a t i o n o f CO-complex. This shows t h a t the s o r p t i o n o f ammonia i s i n f l u e n c e d more by the former than by the l a t t e r s u r f a c e oxygen complex. The treatment of o r i g i n a l samples w i t h aqueous hydrogen peroxide which r e s u l t s i n an a p p r e c i a b l e r i s e i n the amount o f the a c i d i c complex w i t h out a f f e c t i n g much the value o f C0-complex ( c f . Table I ) , i s seen t o cause increase i n the s o r p t i o n o f ammonia a t each vapour p r e s sure, as can be seen on comparison o f isotherms a and d i n each of the F i g u r e s . The treatment o f 1000° - outgassed carbons w i t h aqueous hydrogen peroxide r e s u l t s i n a p p r e c i a b l e f i x a t i o n o f oxygen which, however, gives r i s e mostly t o non-acidic C02-comp l e x ( c f . Table I ) . The isotherm on t h i s sample i s seen t o be almost i d e n t i c a l w i t h t h a t on the corresponding oxygen-free sam p l e . These observations show c l e a r l y t h a t the e n t i r e combined oxygen does not have a uniform e f f e c t on the s o r p t i o n o f dry ammonia by carbon. The oxygen present as a c i d i c C02-complex i n f l u e n c e s the s o r p t i o n o f ammonia t o a r e l a t i v e l y l a r g e r extent than t h a t present as C0-complex w h i l e the oxygen-present as nona c i d i c C02-complex has h a r d l y any e f f e c t a t a l l . Adsorption isotherms on sugar c h a r c o a l , Mogul and Spheron-6, a l l p r e v i o u s l y outgassed a t 1000° and t h e r e f o r e e s s e n t i a l l y f r e e of oxygen, are p l o t t e d i n F i g u r e 6. I t i s seen t h a t the v a r i o u s p o i n t s f i t around a s i n g l e curve showing t h a t the extent o f sorpt i o n / m o f carbon surface when f r e e o f oxygen i s about the same i n every case i r r e s p e c t i v e o f d i f f e r e n c e s i n p o r o s i t y o f these m a t e r i a l s . I t appears t h a t ammonia, being a s m a l l molecule w i t h t h i c k n e s s equal t o 2.36 A, becomes e a s i l y a c c e s s i b l e t o the inner surface o f carbons as w e l l . 2

Adsorption o f Phenol from Aqueous S o l u t i o n s . Adsorption o f phenol from aqueous s o l u t i o n , b e i n g o f i n t e r e s t from the standpoint o f water treatment,was s t u d i e d u s i n g Mogul,Spheron-6,Graphon(ahighl y g r a p h i t i s e d carbon b l a c k ) and sugar c h a r c o a l . The isotherms (35° C) p l o t t e d on the b a s i s o f amounts adsorbed ( μ moles)per m o f surface i n the v a r i o u s carbons i n the o r i g i n a l s t a t e are pre sented i n Figure 7. I t i s seen t h a t the extent o f a d s o r p t i o n a t a given c o n c e n t r a t i o n i s maximum i n the case o f Graphon which i s f r e e o f oxygen and decreases i n the order Graphon>Spheron-6 > Mogul > Sugar c h a r c o a l . This i s a l s o the order o f decreasing oxy gen content o f these m a t e r i a l s . The r o l e o f chemisorbed oxygen i n adversely a f f e c t i n g the amount o f a d s o r p t i o n i s , t h e r e f o r e , q u i t e evident. 2




Figure 3. Adsorption isotherms of ammonia on various samples of sugar charcoal. Ο = original, Δ = 600°-out gassed, • = 1000°-outgassed, · = original, treated with aq. H 0 , A = 1000°-outgassed, treated with aq. H 0 . 2

2

2

2

Figure 4. Adsorption isotherms of ammonia on various samples of Mogul. Ο = original, Δ = 600°-outgassed, • = 1000°-outgassed, · = original, treated with aq. H 0 , A = 1000°-outgassed, treated with aq. H 0 . 2

2

2

2



15. puni


219

5δθ Pressure ^Torr )—

Figure 5. Adsorption isotherms of ammonia on vanous samples of Spheron-6. Q = original, Δ = 600°outgassed, • = 1000°-outgassed, · = original, treated with aq. H O , A = 1000°-outgassed, treated with aq. H O . 2

z

2

g

Pressure ^ T o r r ) — • ·

Figure 6. Adsorption isotherms of ammonia on oxy gen-free (1000°-outgassed) samples of sugar charcoal, Mogul and Spheron-6. Q = 1000°-outgassed sugar charcoal, Δ = 1000°-outgassed Mogul, • = 1000°outgassed Spheron-6.



220


I n order t o study the e f f e c t of CO2- and CO-complexes, sep a r a t e l y , the isotherms were a l s o determined on 6 0 0 ° - and 1 0 0 0 ° outgassed samples. The r e s u l t s are p l o t t e d i n F i g u r e s 8, 9 and 10 f o r Spheron-6, Mogul and Sugar Charcoal. I t i s h i g h l y s i g n i f i c a n t t o note t h a t when the a c i d i c CO2- complex i s e l i m i n a t e d s u b s t a n t i a l l y on outgassing a t 600° and CO-complex becomes the predominant surface complex, the extent of a d s o r p t i o n a t each c o n c e n t r a t i o n i n c r e a s e s a p p r e c i a b l y i n the case o f each carbon and even surpasses the corresponding values f o r the oxygen-free ( i . e . , 1000°-outgassed) samples. This shows t h a t the adverse e f f e c t o f combined oxygen, as r e p o r t e d by previous workers (27, 28), can not be t r u e f o r the whole o f the oxygen. A p a r t o f the oxygen disposed o f as carbon monoxide, i n f a c t , enhances adsorb a b i l i t y o f phenol t o an a p p r e c i a b l e extent. The p o s i t i v e e f f e c t of a p a r t o f the combined oxygen i n enhancing the s o r p t i o n o f phenol i s being r e p o r t e d , probably, f o r the f i r s t time. The r e s u l t s c l e a r l y i n d i c a t e t h a t when the a c i d i c C02~complex i s pre dominant, the surface p r e f e r s the s t r o n g l y p o l a r molecule o f water (the s o l v e n t ) , which a d v e r s e l y a f f e c t s the a d s o r b a b i l i t y o f phenol. However, w i t h the e l i m i n a t i o n o f t h i s complex and w i t h the emergence o f CO-complex, as the predominant surface complex, the preference o f the surface f o r phenol r i s e s a p p r e c i a b l y . This appears t o be due t o i n t e r a c t i o n o f OH groups o f phenol w i t h p h e n o l i c and quinonic oxygens a s s o c i a t e d w i t h CO-complex. Summary The e f f e c t o f carbon-oxygen surface complexes on s e l e c t i v e a d s o r p t i o n from methanol-benzene mixtures as w e l l as adsorb a b i l i t y of benzene vapour, dry ammonia and phenol from d i l u t e aqueous s o l u t i o n s by a few samples o f m a c r o c r y s t a l l i n e carbons has been i n v e s t i g a t e d . The view o f previous workers t h a t the combined oxygen a f f e c t s these p r o p e r t i e s more or l e s s u n i f o r m l y has not been s u b s t a n t i a t e d . Thus, w h i l e the presence o f a c i d i c C02-complex enhances preference of the surface f o r methanol, the more p o l a r component o f methanol-benzene s o l u t i o n s , t h a t o f COcomplex enhances preference f o r benzene, the l e s s p o l a r component of these s o l u t i o n s . The presence o f non a c i d i c C02-complex has h a r d l y any e f f e c t . Again, w h i l e the a c i d i c C02-complex supresses the s o r p t i o n o f pure benzene from vapour phase, t h a t of CO-com p l e x enhances i t a p p r e c i a b l y and t h a t o f non a c i d i c complex has h a r d l y any e f f e c t at a l l . Adsorption o f d r y ammonia, which f o r oxygen-free carbons i s l a r g e l y a f u n c t i o n o f surface a r e a , i s enhanced c o n s i d e r a b l y by a c i d i c C02-complex, t o a much smaller extent by CO-complex and not a l a l l by non a c i d i c C02-complex. Adsorption o f phenol from d i l u t e aqueous s o l u t i o n s i s i n f l u e n c e d a d v e r s e l y by a c i d i c C02-complex, f a v o u r a b l y by CO-complex but not by non a c i d i c complex. S u i t a b l e explanations have been o f f e r e d f o r the apparent anamolies.



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Equilibrium concentration^ moles/litre J —>-

Figure 7. Adsorption isotherms of phenol on Graphon, sugar charcoal, Mogul, and Spheron-6. X = Graphon, • = sugar charcoal, Δ = Mogul, • = Spheron-6.

Equilibrium concentration (m moles/litre) —

Figure 8. Adsorption isotherms of phenol on Spheron-6 before and after outgassing at 600 and 1000° C. · = original Spheron-6, Δ = 600°-outgassed Spheron-6, • = 1000°-outgassed Spheron-6.


221



Figure 9. Adsorption isotherms of phenol on Mogul before and after outgassing at 600 and 1000°C. · = original Mogul, Δ = 600°-outgassed Mogul, • = 1000°-outgassed Mogul.

τ

Adsorption at Interfaces

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