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The chemisorption of oxygen and oxides of carbon on an activated charcoal surface 1937

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THE GHEMISORPTION OF OXYGEN AND OXIDES.OF CARBON ON AN ACTIVATED CHARCOAL SURFACE. by Howard 0. McMahon Under the d i r e c t i o n of Dr. M. J . Marshall A Thesis Submitted i n Part Requirement f o r the Degree of .- MASTER OF ARTS i n the department of , CHEMISTRY. '. THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1937. TABLE OP CONTENTS. Page. Introduction . 1 .A.jp ]p si r* si "t»u.s • * • » « « • « • * • • » • • « « I - Diagram of Apparatus 1 ( l ) Experimental Oxygen as Adsorbate . . . . . . . . . . 3 * Graph I ( t o face) 5 Discussion of r e s u l t s for Oxygen . . . . 6 Carbon Monoxide as Adsorbate 8 Gr 3f SLjOh XX © • .i » i » - e - * - « . * • » • • • » . « » • . • • 9 Carbon Monoxide as Adsorbate Part I I . 9 Graph I I I ( to face) 10 Carbon Dioxide as Adsorbate Part I . . 11 Graph IV . . . . * •«-• . • . . ( to face) 11 Carbon Dioxide as Adsorbate Part I I . . 12 T stbl © X • • • • • • • • * B »« 15 Carbon Dioxide as Adsorbate Part I I I . . 16 T 1© l l o e • • o « s • * * • • a • » X *7 to face) 18 Summary of Results f o r Carbon Dioxide . 18 Bibliography 'dO THE CHEMISORPTION OF OXYGEN AND OXIDES OF CARBON ON AN ACTIVATED CHARCOAL SURFACE. Introduction: For some years i t has been generally conceded that Oxygen adsorbes on a charcoal surface to produce an exceed- i n g l y stable complex. The system has i n fact become recognized as perhaps one of the most convincing arguments f o r chemi- sorptio n . A great deal of work has been published on t h i s subject, a recent survey of which may be found i n the t h e s i s of R. A. Findlayl- The oxides of carbon have always been believed to adsorb i n the ordinary p h y s i c a l manner, i n v o l v i n g only the s o - c a l l e d Van der Waal forces. The heats of adsortion of the oxides, even f o r quite small concentrations are w e l l w i t h i n the l i m i t s u s u a l l y set f o r " p h y s i c a l " processes. The present I n v e s t i g a t i o n w i l l show that despite popular opinion with regard to t h i s p oint, COg and perhaps CO as w e l l , w i l l adsorb on some charcoal surfaces to form a chemical complex having very p e c u l i a r properties. Apparatus; The main features of the apparatus have been described by R. A. Findlay and others, but there are several additions and changes which were found to be necessary throughout t h i s i n v e s t i g a t i o n . Carbon monoxide was prepared by allowing formic a c i d to d r i p through warm concentrated and thoroughly out-gassed , anhydrous sulphuric a c i d . The CO was passed through a l i q u i d a i r trap to remove any condensable m a t e r i a l and was then stored over f r e s h P2O5 f o r future use. Usual precautions were observed such as thoroughly f l u s h i n g out the apparatus before c o l l e c t i n g the f i n a l sample. Carbon dioxide was prepared by successively sublimating and f r e e z i n g out s o l i d CO2 which was obtained i n i t s commercial form from a l o c a l f i r m . This method proved to be very convenient and no evidence of i m p u r i t i e s e x i s t e d a f t e r three or four t r e a t - ments i n the above manner. L i q u i d a i r was used to freeze out the s o l i d COg and the d i f f u s i o n pump was allowed to pump d i r e c t l y on i t f o r about 20 minutes, to ensure the removal of a l l v o l a t i l e i m p u r i t i e s . The C 0 2 was stored over PgO^ and was measured out with the same pi p e t t e as was used f o r the CO and Og Figure I shows the micro-gas an a l y s i s apparatus i n d e t a i l and I I shows the whole gas recovery system. Two major changes were made i n the gas anal y s i s apparatus. The Dekhotinsky j o i n was replaced by a graded pyrex to s o f t - g l a s s s e a l , about 1.5 mm. i n diameter and 1 cm. long. Apeizon-L grease was used through- out the e n t i r e apparatus; and t h i s , together with the graded seal e f f e c t i v e l y eliminated a l l f o r e i g n organic vapors, thus in c r e a s i n g the r e l i a b i l i t y of the analyses. In f i g u r e I I , "M" i s a manometer capable of measuring lar g e pressures and "F" i s a 1000 cc. f l a s k f o r storage purposes  -3- A l i q u i d a i r trap, "L" was introduced to make pos s i b l e l a r g e pressure COg determinations. The f o l l o w i n g c a l i b r a t i o n s of the various volumes were determined i n the accapted manner. Volume "A.C" of the McLeod gauge.. 134.6 cc. Volume "A.B" of the McLeod gauge... 0.0587 ". Gauge r a t i o 4.35 x 10" 4. "External" volume .» 52.4 " . Volume of storage f l a s k "F" 982 ". Oxygen as Adsorbate. ...... The experimental procedure pursued i n t h i s work was n e c e s s a r i l y determined by the nature of the r e s u l t s obtained; consequently the procedure and r e s u l t s w i l l be treated together i n the f o l l o w i n g sections, i n t h e i r n a t u r a l chronological order as t h i s i s t h e i r best ordered arrangement. A f t e r out-gassing the charcoal f o r a period of 10 hours at approximately 1000°G., the furnace was allowed to cool with the pumps s t i l l operating. The pressure at room temperature was of course unreadable on the 10~ s r a t i o ofx'the McLeod gauge. The d i f f u s i o n pump and c o l l e c t i n g system were then operated f o r an in t e r m i t t a n t period of about 40 hours i n order to determine what quantity of gas could be expected to accumulate from stray sources. During t h i s time, which was approximately one week, only about 1 cu. mm. of gas was accumulated. The an a l y s i s of the gas showed i t to be 33$ CO,.37$ Ng and 30$ of e i t h e r COg or H g0. I t i s l i k e l y that these gases were l i b e r a t e d by the glass w a l l s . Approximately 5 cm. of Og were then measured out on the gas p i p e t t e and allowed to adsorb on the charcoal surface. W i thia a few minutes the pressure had f a l l e n to an immeasurably small value; the surface concentration of the 0 g being 0.66 x 10~ b mols per gram of charcoal ( 0.66 micromols ) . Every attempt to recover gas from the surface by pumping f o r large periods of time withethe d i f f u s i o n pump was completely f u t i l e . The temperature of the system was then r a i s e d slowly, f i r s t i n a thermostated o i l bath and then i n the e l e c t r i c furnace. At i n t e r v a l s of about 50° the temperature was kept constant f o r an hour or more, during which time the pressure v a r i a t i o n was studied. From room temperature to approximately 400°G the general trend seemed to be a more or l e s s continuous r i s e i n pressure, but the magnitudes were so small that the measurements are probably not r e l i a b l e . Thus, at 414°0 the pressure was only about 10 x 10"6mm and i t was quite impossible to recover as l i t t l e as 1 x 10" 3 cc of gas over a period of several hours. I t i s a matter of conjecture whether t h i s small pressure was the r e s u l t of the adsorbed l a y e r or whether i t came from the heated quartz. Upon r a i s i n g the temperature from 414° to 500° and then keeping i t constant, i t was found that the pressure rose from IV x 10"^ to 11V x 10"^mm during the f i r s t 20 minutes and then proceded to f a l l o f f l o g a r i t h m i c a l l y with respect to time, approaching an e q u i l i b r i u m value of 20 x 10~6mm. When the temperature was again r a i s e d from 500° to 542°, the pressure rose to 150 x 10~6mm wi t h i n 15 minutes and upon maintaining the temperature constant, i t again f e l l o f f to 30 x 1 0 ~ 6 m m .  The measured values, corrected f o r "thermal t r a n s p i r a t i o n " are recorded i n Graphl. The diagonally shaded areas are periods of heating and the spaces i n between represent constant temp- erature periods. These curves must be taken as i n d i v i d u a l and q u a l i t a t i v e examples only. For a more d e t a i n e d account of the work, the o r i g i n a l laboratory record must be consulted,. There must n e c e s s a r i l y be a large time l a g between the pressure over the charcoal and that recorded on the gauge; hence the r e s u l t s are probably r e l i a b l e only as a q u a l i t a t i v e d e s c r i p t i o n , When the furnace was cooled down to room temperature and was again r a i s e d slowly i n steps of 50° w i t h the object of . t r y i n g to reproduce the e f f e c t ; i t was found that the phenom- enon could not he repeated throughout the temperature range previously studi ed» Upon reaching the upper l i m i t of 618°the pressure, however, rose to the e q u i l i b r i u m value to which i t had l a s t f a l l e n and a f u r t h e r r i s e i n temperature caused a d u p l i c a t i o n of the r i s i n g and fat l i n g e f f e c t . This procedure was continued up to 800°C» The temperature was then allowed to drop to 20° j> and the whole process was repeated, the r e s u l t s of which are graphed i n I (b). I t i s i n t e r e s t i n g to note t h a t even at 1000° the gas showed a tendency to readsorb back on the charcoal. Small q u a n t i t i e s of gas were drawn o f f the charcoal from time to time and i n every case the a n a l y s i s proved to be 100f0 CO with no trace of CO^ or 0%,  As added confirmation as to the i d e n t i t y of the gas which was exerting these pressures, small q u a n t i t i e s of GO were gen- erated and added at various temperatures. In every case the .pressure f e l l r a p i d l y to the corresponding e q u i l i b r i u m value. Discussion of Results fd?Oxygen. T The f o l l o w i n g conclusions are a d i r e c t r e s u l t of the above observations. Small q u a n t i t i e s of O2 adsorb on a c t i v a t e d char- coal at room temperature to form an exceedingly stable complex whose decomposition pressure i s w e l l below 10~6mm. This complex i s s i m i l a r i n nature to one which would be formed by allowing CO to adsorb on charcoal at a temperature i n the region of 400 - 500° C , so we may regard i t as p o t e n t i a l l y adsorbed CO. Upon r a i s i n g the temperature, a point i s at length reached where the l e a s t tenaciously held CO escapes from i t s seatings and the pressure b u i l d s up by v i r t u e of the Clapeyron-Clausius r e l a t i o n . Upon holding the temperature constant the pressure decreases. The readsorbed CO must c e r t a i n l y have found more act i v e spaces than those which i t l e f t and which are now impoverished of CO. As the temperature i s again r a i s e d the e f f e c t i s repeated and we must assume that the CO Is expelled from a s l i g h t l y more stable range ( again by v i r t u e of the Clapeyron-Clausius equation ) and i s readsorbed by a more stable range of spaces than those which were responsible f o r the previous readsorption. The e f f e c t may be repeated again and again and each increase i n temperature causes a rearrangement of the surface complexes n e c e s s a r i l y r e s u l t i n g i n the formation of a product of greater s t a b i l i t y than any preceding one. A question which immediately suggests i t s e l f i s t h i s : why does the CO not form i t s more 'stable complexes at the lower temperatures; why should i t refuse to adsorb on the more act i v e centres i n preference to the l e s s a c t i v e ones 1 The answer must evidently be that the rate of adsorption on the more a c t i v e centres i s extremely slow at moderate temperatures and moreover; we may say that the rate of adsorption seems to vary i n an inverse manner with respect to the s t a b i l i t y of the complex produced. I t would then appear to be reasonable to assume that the greater the s t a b i l i t y of an adsorbed GO molecule, the greater i s the heat of adsorption and also the heat of a c t i v a t i o n . Burwell and T a y l o r 2 were d i r e c t e d towards t h i s same conclusion when studying rates of adsorption of H 2 on m e t a l l i c oxides. "The data points to a lower heat of adsorption f o r the process with the lower energy of a c t i v a t i o n " . We may then p i c t u r e the charcoal surface as being a continuum of spaces of varying degrees of a c t i v i t y each having a heat of a c t i v a t i o n which i s higher f o r the more a c t i v e spaces. This p i c t u r e explains the experimental r e s u l t s of t h i s paper, moreover; i t o f f e r s an explanation f o r the manner i n which the d i f f e r e n t i a l heat of adsorption of Og on charcoal was found to increase with increasing temperature as shown by Garner . Evidence has been obtained i n t h i s i n v e s t i g a t i o n , as w e l l as many others conducted on adsorption systems 4, that an adsorbed l a y e r undergoes a very slow "secondary r e a c t i o n " known as " d r i f t "  This might we l l be taken as an i n d i c a t i o n that the adsorbate . i s f i n d i n g more and more stable seatings but that the rate of r e a c t i o n at these more stable spaces i s small due to the higher • a c t i v a t i o n energy. Thus i t i s seen that the surface concept which o f f e r s the most reasonable explanation to the data presented, i s also i n accordance with the work of many other i n v e s t i g a t o r s . Carbon Monoxide As Adsorbate Part I. In view of the i n t e r e s t i n g r o l e played by. GO In the previous s e c t i o n , i t was believed that pertinant information could be by adsorbing CO d i r e c t l y on the charcoal surface at room temp- erature. About 10 cm of the gas was measured out on the p i p e t t e and allowed to adsorb on the previously out-gassed charcoal. A volume double that of Og was used i n order that the e f f e c t i v e oxygen concentration would be the same as before ( 0.67 micro . mo'ls per gram of charcoal ). The adsorption of the GO was of course much slower than the 0.2 and the e q u i l i b r i u m pressure was 13 x 10"6mm. That an e q u i l i b r i u m a c t u a l l y e x i s t e d was demonstrated by pumping small q u a n t i t i e s of gas o f f the charcoal and noting that the pressure b u i l t up almost instantaneously to i t s o r i g i n a l value. This e q u i l i b r i u m underwent a very gradual " d r i f t " and i t was noticed that the rate of l i b e r a t i o n of gas into the d i f f u s i o n pump decreased slowly with time even though the surface concentration was not changed appreciably. A f t e r two or three days no gas could be recovered. • - • ; i l t u t : . i t\. : m • o —: t-H T — M • S S 3 O a a ' o - 1 1 o 1 , m o i ; -̂—̂  < ; i i 0 3 O M : M' ; - • H « 0 3 W - EH !=> - M o a •' .i •( ' ; ̂ ' * ? ! ' i H I a M !-4 E H O ^ - » ^ * CO O 1 ' ' ! 0 O o - - 4C 0 : 300 PR ESSURE , IN 0 UNITS OF 1 100 1 ,0" 5 MM. i c j ; " ; • . 1 , ; ( - ( , , . ; • • - . • > ) • . : • • • • , • The temperature of the system was now r a i s e d as i n the previous section and the pressure was p l o t t e d against time, Thus at 52°C the pressure rose from 12.5 x 10" 5 to 8VxlO~ 5 over a period of about one hour and then f e l l slowly, r e q u i r - ing a period of about 4 hours to f a l l to 34 x 10~° mm. ( see graph Il(.a) . ) . Prom 52° to 104° the pressure rose r a p i d l y to 345 x I0~5ram w i t h i n 30 minutes and i n 2 hours i t had f a l l e n to 36 x 10"5inm as shown i n graph I I (b). Upon cool i n g the system to room temperature, the pressure decreased to zero and no gas could be recovered by pumping f o r several hours. When the temperature was again r a i s e d , no GO was produced u n t i l 104° was approached and at t h i s temperature the pressure rose to i t s e q u i l i b r i u m value once again. A f u r t h e r r i s e In temperature caused the pressure to behave i n the same manner as o u t l i n e d above. This proeedure was c a r r i e d on to approximately 400°0-, and above t h i s temperature the system behaved j u s t as the 0g complex did. Carbon Monoxide as Adsorbate - Part I I The f i r s t experiment with CO offered no new information regarding the charcoal surface other than a confirmation of the hypothesis already presented. I t d i d , however suggest another method of attacking the problem. I f i t i s true that the spaces of higher a c t i v i t y become a v a i l a b l e f o r adsorption only at higher temperatures, and i f t h i s i s the reason f o r the readsorption of the gas expelled from the l e s s active spaces; then i f these more active seats  ~I0~ could be f i l l e d i n some manner, and thus eliminated, the pressure-decrease phenomenon could be stopped. "With t h i s point of view i n mind the f o l l o w i n g experiment was conducted. The charcoal was heated to 380° and was then allowed to cool quite slowly i n the presence of an excess of CO. Within about 5 hours the furnace had cooled to room temperature and a t o t a l of 20 cm on the pipette had been added. This corresp- onds to a surface concentration of 1.2 micromols of oxygen per gram of charcoal. This value i s about twice as much as that used i n the preceding experiments, yet the eq u i l i b r i u m pressure was just as small as before, thus i n d i c a t i n g that the " e f f e c t i v e " surface.had been increased by 100%. The temperature was now r a i s e d slowly from 23° to 100° where i t was held constant f o r several hours. The pressure rose f a i r l y r a p i d l y and i n 45 minutes, had attained a value of 90 x 10~6mm. During the fo l l o w i n g 30 minutes there was a s l i g h t decrease,the f i n a l being 85 x 10"6mm. See graph I I I ( a ) . Prom 100° to 126°, the pressure rose continuously to 63 x 10 °mm and showed no tendency to f a l l o f f . Upon cooling the charcoal and repeating the measurements again and again, the curves were found to be very nearly reproducer- able, i n marked contrast with a l l previous r e s u l t s . Even at 170° the pressure rose to 600 x 10~5mm and showed absolutely no signs of any decrease a f t e r standing f o r 4 hours. At higher temperatures the pressure was f a r too great to be read on the gauge, but as soon as I380^5;was exceeded the gas began to readsorb at a great 1 ' 1 • * • • . : j • j . '. • i < 1 - I ! •• I • i ? • r : • c /-y • * i • ' C •WWi9-0I, . i l 0' 10 SIINQ N 5 , , 1 ' _ . 0 2 C i?, • o< ! 0 9 l^-J— CO • ' ^ i ' ' o ^ — i «E IN MINI 1 1 , ! . i i ! j I \ \ i • '! ; [ ! ' ! j ! ' : 1 , . ; > ' i . i i ' w «• • • CD '• .. ! i 1- i \. • r : i ~< 1. s J i - GRAPH i 1 ;; :. - 1 . r-J O i - v - - | - - j •' | • i- •:  /• 1 . : ;• i i i • ; . . ; ; i i s r ; -; i ! , O • 1 O H • , 7 ' / W' ;o - M. ' O ' ' , . ' >< ' t 1 I !''\':, ? ; . : i • i • s 1 > i : • i j • < 1 •Tl r!'-Ti -•• ? rate and was soon down to normal pressures again. Above t h i s temperature the system behaved just as though Og had been adsorbed at room temperature. The r e s u l t s o u t l i n e d i n t h i s section present a powerful argument i n favor of the hypothesis i n question, p a r t i c u l a r l y i n view of the f a c t that t h i s behavior was a c t u a l l y predicted p r i o r to doing the experimental work. Carbon Dioxide as Adsorbate - Part I The object of the f o l l o w i n g i n v e s t i g a t i o n was to study i n more d e t a i l the nature of the charcoal surface and to t r y to obtain some information with regard to the degree of activit3>- of the " a c t i v e centres" a v a i l a b l e f o r room-temperature ad- sorpt i o n , and also the r e l a t i v e abundance of such centres. An amount of COg equivalent to a concentration of 0.66 micromoles was measured out on the gas pipette and admitted to the charcoal. The adsorption was f a i r l y r a p i d and i n 1 hour the pressure had f a l l e n to 6.4 x 10~5mm. Pumping with the d i f f u s i o n pump f o r several hours served to remove only a very small quantity of gas - too small to analyse; although there was an i n d i c a t i o n that the gas was l a r g e l y COg. The temperature was r a i s e d as usual to 54° and was then held constant. The pressure rose from 17 x 10~ 6 to 60 x 10~"6mm. i n a period of 40 minutes and then f e l l slowly to 27 x 10~&mm as shown i n graph IV (a) . A small quantity of gas was pumped of f at the end of 1 hour and i t proved to be 100% CO with no trace of C0 o. -12- Again, upon r a i s i n g the temperature there was the usual r i s e and f a l l i n pressure . . u n t i l , above 400° the system be- haved j u s t as though Og or GO had been adsorbed. There was never any i n d i c a t i o n that anything but CO ever l e f t the COg complex; thus d e f i n i t e l y e s t a b l i s h i n g the fa c t that the adsorption of QO2 involves chemical forces. ©arbon Dioxide as Adsorbate - Part I I Many i n v e s t i g a t o r s have studied the adsorption of COg on charcoal and i n no case on record has the system showed any tendency to reduce the COg. P o s s i b l y the refinements of method and apparatus are responsible f o r i t s detection i n t h i s l a b - oratory, but i t seems more l i k e l y that the charcoal used i n t h i s work i s unique. In view of the r e s u l t s obtained i n the previous s e c t i o n ; i t became a matter of great i n t e r e s t to know jus t how l a r g e an amount of COg could be reduced i n t h i s manner. The charcoal was out-gassed as before and fresh COg was generated and p u r i f i e d , s p e c i a l precautions being taken to obtain a very pure product. The temperature of the system was maintained at 23.5°C by means of a s p e c i a l l y designed therm- ostat. Carbon dioxide was then added to the system i n increments of about 0.66 micromols per gram of charcoal. The pressure decreased a f t e r each ad d i t i o n comparitiveijr slowly and several hours was u s u a l l y necessary f o r the pressure to become sensibly constant, although a slow d r i f t p e r s i s t e d f o r several days i n many cases, p a r t i c u l a r l y at the lower concentrations. Small q u a n t i t i e s were pumped, offy" rusually at i n t e r v a l s of 10, 30 and 60 minutes, and i n some cases 12 Or more hours a f t e r a d d i t i o n . The rate of desorption and analysis were noted i n •each case. Between COg concentrations of zero and 13.2 micromois per gram, a t o t a l Of 0.112 micromois per gram of CO was recovered and analysed. The COg was added i n 14 increments and during t h i s time 30 samples of gas were withdrawn from the charcoal. In every case the sample proved to be 100^ GO with only an occasional trace of COg. The COg was found to go onto the char- coal at a s u r p r i s i n g l y high r a t e and i n many cases, one minute was s u f f i c i e n t to completely deplete the gas phase of COg and replace i t with a high pressure of CO which was i n turn slowly readsorbed. By removing a comparitively small quantity of CO the e q u i l i b r i u m pressure could be decreased almost to zero; and t h i s e f f e c t was found to be even more pronounced at the higher concentrations of COg. When a surface concentration of 13.2 had been reached, the e q u i l i b r i u m pressure of CO had r i s e n to 4. 100 x 10 "mm. I t was shown that the e q u i l i b r i u m pressure of COg at t h i s concentration was below the vapor pressure of s o l i d COg at l i q u i d oxygen temperature ( about 5 x 10~6mm), by cooling a po r t i o n of the apparatus and noting that no COg was frozen out even a f t e r 12 hours. For concentrations above 13.2 micromois per gram of char- coal i t was found necessary to modify the procedure s l i g h t l y on account of the unweildly pressures of CO which developed. -14- The procedure followed i n the present section was as follows. The McLeod gauge was cut o f f from the charcoal and evacuated, aftec which a measured quantity of COg was admitted to the adsorbent. At a c e r t a i n time l a t e r , ( u s u a l l y about 10 min. ) the gauge was opened and the gas was allowed to expand o f f the charcoal i n t o i t ? Then, a f t e r another period of about 10 min- utes the gauge-system was cut o f f from the charcoal and the C0 2 content of the gas was determined. This procedure of f i r s t evacuating the gauge makes c e r t a i n that the sample drawn o f f i s r e a l l y representative of of the true e q u i l i b r i u m mixture and i s not d i l u t e d with gas which would otherwise be s t i l l i n the gauge. Using the methods o u t l i n e d above, the surface concentrat- ion was extended from 13.2 to 16.6 micromols of COg per gram. The e q u i l i b r i u m pressure of CO rose from 100 x 10""* to 440 x ' -4 10 mm with s t i l l no trace of COg ; and since t h i s pressure was near to the readable l i m i t of the gauge, i t was decided to pump o f f a l l the CO which could be recovered. Pumping f o r <.5 hours served to recover only 3.25 micro- mols of CO per gram of charcoal and pumping i n excess of t h i s time was t o t a l l y useless as the rate of desorption becomes very small even a f t e r the second hour. Between the concentrations of 16.6 and 43.3 m-m / .gm , G0 o was added i n 3.3 m-m / gm corresponding to 25 cm on the gas p i p e t t e . A f t e r about an hour the r e s u l t i n g CO was pumped o f f and measured. Table I contains the record of the r e s u l t s of t h i s s e c t i o n The f i r s t column l i s t s the t o t a l i n i t i a l concentration of C0 2 expressed i n micromois of C0 2 per gram of charcoal; (m-m / gm ) •The second column contains the amount of C0 2 added i n the same u n i t s as the f i r s t . Column three shows the pressure of CO a f t e r a period of 20 minutes. The f i f t h column l i s t s the quantity of CO which could be recovered by pumping f o r the length of time recorded i n the fourth column. The quantity of gas recovered was measured with a cathetometer and involved the measurement of two mercury columns whose height d i f f e r e d by about 1 - 2 cm. For t h i s reason, these r e s u l t s are to be looked upon as having an inherent error of about 5 - 10 f0 due to s t i c t i o n of the Hg. TABLE I I n i t i a l cone of C0 2 m-m / gram Amount of CO 2 added m-m / gr. Pressure a f t e r 20 min xl0-4 Time of pumping hours Amount of CO recovered m-m / gram 1 ; 16.6 3.30 120 •7 1.96 2 19.9 3.30 151 6 1.87 3 23.2 3 9 31 220 .7 ' 1.80 a : 26.5 3. 38 346 8 1.92 5 29.9 3.38 p>400 8 1.81 6 33.3 3. 30 p>500 6.5 •1.77 7.; 36.6 3.35 p<1000 6 1.82 * 8 40.0 3.44 p»1000 7 1.89 * At the concentration of 36.6 m-m / gm , the system was exerting an e q u i l i b r i u m pressure of C0 2 of approx. 5 x 10~6mn. This was shown by c o o l i n g the 60g t i p and n o t i c i n g that even a f t e r 18 hours i n contact with l i q u i d oxygen, only a very small, amount of G0 2 was frozen out ( j u s t large enough to .register on the McLeod gauge.) The above table brings to l i g h t a very b a f f l i n g property of the system. Apparently each new a d d i t i o n of C0 2 which i s adsorbed on the surface causes the formation, or at l e a s t the l i b e r a t i o n , o f a l i t t l e more than one h a l f as many mols of CO , and t h i s appears to be independent of the t o t a l concentration of C0 2 between 16.6 and 43.4 m-m / gm. The pressures of these equal q u a n t i t i e s of CO which are produced, Increase very r a p i d l y with respect to the C0 2 concentration. Carbon Dioxide as Adsorbate - Fart When the surface concentration of C0 2 was increased beyond the value of 43.4 m~m / gm , the r a t e of disappearance of C0 2 became measureably small and a f t e r many hours the composition of the gas phase became constant. The experimental procedure used was e s s e n t i a l l y the same as that i n that of the preceding s e c t i o n , with the exception that the gauge was evacuated and allowed to r e f i l l several times during one run i n order to obtain data r e l a t i v e to the rate of conversion of Q0 2 to CO. Table I I i s compiled from the r e s u l t s of t h i s section. Column I as before, shows the t o t a l 8 0 2 added up to the time i n question i n m-m / gram. Column Ho l i s t s the quantity of COg added for' each run and I I I , the pressure a f t e r 20 minutes. -17- The fourth column was obtained by p l o t t i n g percent composition of the gas phase against time, zero time being taken when the COg was admitted to the charcoal. The time required f o r the 'composition to f a l l to 50$ C0 2 was then measured o f f the graph and t h i s i s the value which appears i n column IV . The columns V and VI contain the amounts of COg and CO r e s p e c t i v e l y which could be recovered by pumping f o r approximately 6 hours. Table I I I I I I I I IV V VI Cone of COg mm / gr . c o 2 added m-ffl/g. Press a f t e r 20 min x IGr 4 Time f o r 50$ conv- min. COg recovery m-m /gr. CO recovery m-m /gr. 1, .asst. ' 1 069 ,.516 11 0.53? 1.05 ? 2 45*1 1 663 590 39 0. 31 0.46 3 46-.B 1.22 '367 • 112 : 0.43 0.60 4 : 48.0 1.48 538 209 0.72 0. 48 An examination of the data brings to l i g h t a number of very i n t e r e s t i n g p oints. F i r s t l y i t w i l l be noticed that between the comparitively close l i m i t s of 43.4 and 48.0 , the rate of conversion of COg undergoes a very d r a s t i c change. This f a c t i s perhaps shown more c l e a r l y i n graph V. , i n which the time taken f o r the gas phase to become 50$ CO i s p l o t t e d against the t o t a l quantity of COg on the charcoal. I t i s quite obvious that the number of spaces capable of accept- ing COg chemically at 23.5°C cannot be greater than about 50 m-m / gram , as the rate of conversion becomes zero . -• - • > i 1 i1 , 1— rt : • ..' ( 1 i 1 ! ' SSI "' . a O . M l O : - - -• ... -y, - —•— : ': ~TT -, « •P 1 : S ; • i P O M __y [ i i'' - ! ' | ' _ _ _ _ _ — i 1 i ! • ! < M X! ' o M P B pq CA RB ON  Di : • 3 : tI TI TY  O F C " i • 1 w .0 2(  TO TA L QU A • i .t-; -oVS.-: i r' •!- '•ll-r'W' 2 DO TIME TAKl W F( 1 )R COM MIN 00 POSITION C •JTES. )F GAS1 TO I M 0 . , 5ALL TO 50 1 i o ;J c o 2 - . ; : . • • . ! • ' i J. . 1 ' I i . • •  • ; 1 . i: - } "i. i t .•; -•; t j : -18- Th i s l i m i t i s very i n t e r e s t i n g , p a r t i c u l a r l y i n view of the fa c t that t h i s i s very nearly the same range over which the d i f f e r e n t i a l heat of adsorption of Og on charcoal has a con- 5 stant value of 72.0 K c a l . I t i s possible that a fundamental e x i s t s relation/between these two systems and an i n v e s t i g a t i o n conducted with the object of determining t h i s r e l a t i o n would probably be very productive. Summary of Results f o r Carbon Dioxide. The experimental f a c t s recorded i n the previous sections may be b r i e f l y summarized as f o l l o w s : - (1) COg adsorbes on ac t i v a t e d charcoal chemically and d i s s o c i a t e s i n part at l e a s t to produce two molecules of CO which may or may not be held with equal t e n a c i t y . (2) Some of the CO which i s produced i s held so l o o s e l y that i t i s allowed to escape and i t may subsequently be readsorbed or i t may be pumped o f f . The number of mols of t h i s type of CO seems to be approximately one h a l f of the number of mols of COg adsorbed, and t h i s f a c t seems to be independent of the t o t a l quantity of COg which has been added. (3) There are a d e f i n i t e and l i m i t e d number of spaces able to accept COg chemically and t h i s i s roughly the same as that number which w i l l accept Og chemically. (4) The c o e f f i c i e n t r e l a t i n g pressure to surface concen- t r a t i o n , i . e . (|[E) T f o r p h y s i c a l l y adsorbed CO increases f o r higher concentrations of chemically adsorbed COg. -19- This i n d i c a t e s that some of the spaces which w i l l hold GO p h y s i c a l l y are able to accept COg chemically, thus accounting f o r the fa c t that the COg has the e f f e c t of decreasing the a v a i l a b l e surface f o r the CO. Much more data i s neceasary before i t w i l l be possible to come to any conclusions regarding the mechanism of t h i s obviously complex system. •20- -"R. A. Fi n d l a y , Thesis , U n i v e r s i t y of B r i t i s h Golumhia ( 1935 ) . Burwell and Taylor , J . Am. Chem. Soc. 58 , 697 , 1936. Blench and Garner , J . Ghem. S o c , 125, 1288, 1924. also ¥. E. Garner , Nature., 128 , 583 , 19 31 B u l l , H a l l and Garner , J . Ghem. S o c , 837, 1931. Marshall and Brampston-Gook, J . Am. Chem. Soc. 1929

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