Vl. 5 5, N.!, July 19 55 Jurnal f Research f the Natina l Bureau f Standards Resea rch Pape r 26 Adsrptin f Nitrgen n Carbn Adsrbents at Lw Pressures between 69 and 9 K Juan de Dis Lpez-Gnzalez,! Frank G. Carpenter,! and Victr R. Deitz X i t r gen ad s rp ti n i,therm s \\'er e dete rmin ed n t hree car b n ad s r ben t s (tw bn e cha r s and an ac t i vated cc n u t shell ch ar cal) a t t hree te m per atu r es bet w een 69 and 9 K and at r el at i ve pre:isurcs frm 1-7 t.9. A cr.v st a t was uscd t b tain tc m perat u res belw t he nrm al b iling pint f l i q uid ni t r gen. Th e a t tainmen t f steady pressure r eadi ng s a t v cr y lw p ressu r es r equired cnsid er ab l y m r e t im e t han a t h ig her pr essu r es. Cn ect i ns fr t her m m lecul ar diffu si n \\'ere fun d t b e ap p reciab l e at p r essures b el \\' ne m illim eter f mer cury. A t r elati ve p r ess ures b el w 1- ', t he am un t f ni t r gen ad s rb ed d epended upn t he c nd i t i n f t he surf ace in reg ar d t p r ev iu s ad s r pti n f ni trge n. A tec h ni q ue i n v l v i ng a seri es f c nsec u t i \' e ad s r p t in s at l w te mper atures and u tass i n g s a t r m te mperat ur e led t a rep rd ucib le is t herm a t lw p ressures. T hese d ata c u ld b e r ep resen ted b y a F r c u ncl li ch equ ati n. Th e Brun a uer, Emmett, T ell er su r f ace ar eas f t he ca rb n a cl s rbc nt s wer e als dct'e r min ecl. 1. ntrductin 2. Experimental Prcedure Durin g the las t decade the m a in emph as is f l'esea reh in gas adsrpli n has been n th e evalualin f surface area. T he success a Lla in ed h as bee n d ue la rgel y t Lile develpm ent f t ile Brun a uer, E mm et t, T eller (B.E.T.) t\ter.\' f muh imlee ular adsrptin and t the cns t rucl ive eritieal wrk tha t fllwed. '' ll(' fac t LhaL Lltc H.E. T. eq uat in was inappli ca ble t dat a a L rel at ive press ures 2 belw.5 was j' i Ltlc cnscq uence beca use adsrplin measllrcmc nts in this rcg in werc nl need cd in the smfaee-al'ea dete rmin a ti ns. Exp crimental d ifficul ties h avc b ec n an add it inal causc fr t lte scarcily f da ta n phy sical ads rpti n at lw relative press ures. Adsrp tin dat a at vcr.v lw prcss ures ar c necessary fr t he calculatin f t hc1'm cl,v namie fu nctins f the adsrb ed phase. Th ese fun ctins are f spccial in terest in t he ra nge f ver y lw s u d ace-cverage [1].3 The in Lerest in s uch adsrp tin ist herms ha" been furt her st imul ated b y t he pssibili ty f b tain ing detailed infrm atin cncernin g t he hetergeneity f a surface. Th e recen t p apers f Hill [2], H alse,\' [3 ], Rh din [4], Halsey and T aylr [5], Sips [6], and C k, P ack, and Oblad [7] indicate sm e prmisin g a ppraches. t is believed t hat s urface hetergen eit ies a nd im pur it ies ar e t he m r e cmmn cause f t he vari a ble prper ties f cmmercial carbn adsrbents. N i trgen-adsr plin m easurem en ts at relative pressures abve 1-2 were r epr ted by D eit z and Gleysteen [8] fr a numb er f carbn adsrb en ts. As t hese samples wer e still available, the ppr tuni ty was taken t exte nd t he data t relativc pressures f a but 1-7 T hi s col'l'espnds t abslute pressurcs f abu t n e-ten t h micrn f H g. A [ur-s tagc McL ed gagc,4 s hwn in fi gure, was add ed t a c nventina l ad rp ti ll apparatus. The calibratin was m ade gr avimet ricall y wit hm ercll ry a ll d b.\' pressu re-v Lume data, using h el ium. Pressurcs dw n t a bu t.1 mm H g c uld be m cas urcd wi th an acc uracy f abut 1 pcl'cent. An accuracy f t Ac knwledgment is made t \ V. V. Lebcnf' tr in f t he Surface Ch emistr y Sectin fr t he design a nd fabri catin f this gage. HGH VACUU M X 1 X 1 REFR GERANT OR CRYO STAT R esearch Assciate f the Nati na l B ureau f Sta nda rds re presenting the Bne C har Research Prject, nc., R ela ti ve p ressure is th e rat i f t he act ual pressuf e f nitrgen t its eq uilibrium va pr pressur C'. s " igur C's in bracke ts ind icnlc t he li tera t ure references at the end f Lhis paper...-- TO MERCURY LEVE L NG BULBS F[(l U RE 1. Diagrammatic sketch f the gas adsrptin system.
2 percent was attained fr pl'essures dwn t 1-2 mm Hg, 3 percen t fr pressures t 1-3 mm, and 5 percent r mre fr pressures as lw as 1O - mm, the lwest pressure attempted. The accuracy f the high-pressure (abve 1 mm) readings was abut O.5mmHg. Nitrgen frm a cylinder f cmpressed gas (waterpumped) was purified by passage ver cpper at 35 C fr the remval f residual xygen and then thrugh Ascarite and phsphrus pentxide n glass cttn t remve carbn dixide and water vapr. The temperature f the bath surrunding the adsrptin chamber was determined with either a nitrgen r xygen vapof-pressure thermmeter. The temperatures 9. and 77.7 K were btained with liquid xygen and liquid nitrgen, respectively, biling at at,mspheric pressurc. The temperatures 72.2 and 69.5 K were btained with a special crystat described in the fllwing paragraph. n all cases the temperat ure f the adsrptin chamber was cnstant t within abut.5 K. 2.1. Crystat The crystat, see figure 2, cnsisted f a widemuthed -liter Dewar fitted with a rubber stpper and a 3.8-cm inside diameter brass tube clsed at ne end, that reached nearly t the bttm. Liquid nitrgen placed in the annular space between t he brass tube and the Dewar was biled und er a fixed reduced pressure t btain the desired temperature. n peratin, air cnde nsed inside the brass tube t frm all excellen t heat-transfer medium between the adsrptin t ube and brass tube. The vaprized nitrgen was drawn ut f the crystat thrugh tw 8-mm inside diameter tubes t a surge tank fr smthing pressure flu ctuatin s. The pm",ure cntrller was,imil.r t that u"d ij vacuum distillatins and cnsisted f a shrt mercury manmeter with electrdes, as shwn in figure 2. T start peratins th e stpcck n the cntrller was pened t the vacuum. When the desired perating pressure was reached, the stpcck was clsed and the cntrller thereafter maintained this pressure. As a cnsiderable vlume f ni trgen was vaprized, the piping, slenid valve, and the vacuum pump were adjusted t prvide an adequate capacity. n peratin, the slenid valve cycled abut nce every 2 t 5 sees. Each pening f the valve brught a sudden pressure decrease, but with a sufficiencly large surge tank these fluctuatins resulted in variatins f nly abut 5-mm Hg pressure ver the liquid nitrgen in the crystat. The slpe f the vaprpressure curve in the perating regin is abut 5 mm/deg, h ence, the pressure variatin crrespnded t changes f nly abut.1 deg C. These fluctuatins were smthed by the heat capacity f the apparatus, and variatins f nly ±.5 cleg K were indicated by the vapr-pressure thermmeter. As a cnstant temperature was maintained ver a number f hurs, it was necessary t replenish the s upply f liquid nitrgen in the crystat. A valve designed t perate when immersed in liquid nitrgen is shwn in figure 2. t cnsisted f a Pyrex ball jint withut grease, with the tub ing t the ball jint sealed ff and extended ut f the liquid, where it served as a handle. n the ff-psitin the ball was almst disengaged frm the scket. n this psitin the pening in the scket was effectively clsed by the sh ulder f the ball. T pen the valve, the handle was twisted slightly t further disengage the jint. Despite minr leaks and infrequent clgging with ice crystals, th e peratin was quite satisfactry. The entire crystat was lagged except fr a small windw t bserve the level f liquid nitrgen. Abut 3 liters f liquid nitrgen was cnsumed per hur in maintaining a cnstant temperature f abut 7 e. PRESSURE CONTROLLER 2.2. Crrectins fr Thermmlecular Diffusin As there is an appreciable temperature difference between the manmeter and the adsrb en t, there is the pssibility f intrducing appreciable errr frm thermmlecular diffusin when measurements are made at very lw abslute pressures. A careful review f the available literature was made [9,1,11, 12], and the fllwing equatin f Liang [11] was cnsidered the mst apprpriate fr calculating the thermal transpiratin factrs: "'''LV'! HANDLE. :U88ER STOPPE" HtGH CAPACTY VACUUM PUMP CONDENSED f.ter OE... 1t NTltOGEN lollng UNOE" VACUUM...us TUllE SAMPLE OF ADSORBENT FG U RE 2. AR, HEAT (XCHANGE MED UM DEWAR ahe (<gx?+f3he (<gx) + R", ahe (lpgx)2 + f3he (<gx) +1 ' VAPO't PRESSUrtE THERMOMfTffl -;_w;j where P 2 is the bserved pressure, P the crrected pressure, and R the crrectin factr. X is the prduct f P 2 by d, where d is the diameter f the tube cnnecting the cld t the warm part f the apparatus. Rm = (TdT2)1 /2, where T=temperaturef the Diagrammatic sketch f the liquid-nitrgen crystat and cntrls. 12 '---.--- (1)
(' ld end f the c nn ective tube (adsrbent temperature) and T 2 = temperature f th e warm end f the c n neetive tu be (rm temperature). The terms a H e and /3He arc empirical cnstants: (1) ll!he= 2.52 independent f temp erat ure a nd (2) /3He = 7.68 (J - R m ), the press ure being m easured in millimeters f mercury and the tube diameter in millimeters. The factr <Pg is th e empirical relative "pressureshifting factol"" and is dependent upn the gas bu t nt the temperature. t is related t the cllisin diameter f the gas mlecules. With the abve listed values f ll! and /3, <P He= 1 and <PN2 = 3.28. t is ev ident that as the pressure appraches zer (X--? O); then R appraches its theretical minimum valu e, Rm- Als, fr high pressures (large values f X ), R ppraches unity. n t he present wrk, the cnnectmg tube was l.o-mm diam, and nitrgen was the gas used at lw pressures. n figure 3 are SbO'VJl the variatins f B with pressure fr th e present case in. t.he in terval frm.1 t 1 mm Hg fr t he fur dfferent temperatures. The values f R fr pressures abve 1 mm Hg are very clse t unity. mm..,_loo p....j 2.3. Materials Th e t hree carbn adsrbents were amng thse previusly investigated [8J. C har 2 was a n ew bne char btained frm the manufacturer. C har 27 had b.een empl?yed in a cane-sugar refinery fr a c nsderable tme and had been remved frm t h e cbar stck because f its high specific density (1.3 gjml cmpared t.63 gjml f char 2). The sample f char 27 used in this wrk differed frm that previusly reprted [8] in tha.t freign substances visually detectable were mechamcally remved. C har 1 an activated ccnut-sh ell charcal, was identical,ith that previusly reprted [8J. 3. Experimental Prcedure The three adsrb en ts were simultan eusly u tgassed at 4 C fr 16 h' at the b eginnin g f the experimen ts. Th e pressure ver the freshly utgassed adsrbent befre any additin f nitrgen was always f t he rder f 1-6 mm Hg. n all cases, t he first add itin f nitrgen was very small s that equilibrium was attained at a very l, pressure. fhen, mre mtrge n was added, and the lum es adsrbed w er btained at increasing equilbrium pressures. After sever al pints had be en determined, t be samples were utgassed fr J 6 h1' at rm temperature. This prccss f adsrptin and utgas in g at rm temperature was repeated several tmes fr each sample. B ecause f the areat experimcntal diffi cul ty in btaining desrpti n bda ta in the lw press ure ran ge, all f the res ul ts reprted here arc fr adsrptin. <n 4. Results C...J 4.1. Attainment f Steady-State Pressure z :E ::...J lop r-----t-. 7-ft------r---------+---- :: '" :E '"ex::: <n <n '" ex: a..... ex:... p 69.,-1( <n CD " " ci '"...,..:....1 P ''-;.4:----...- :_l: ------'L,-----L..----..L-----.J.5.6.7.8.9 1. R= P CORRECTED POBSERVE FGU RE 3. Th ermal-transpiratin fa ctrs fr nitrgen in a 1.-mm t1.be. Th e time required t reach a steady state was fund t b e a functin f tbe final pressure f the adsrbed nitrgen, and d e pend ed als UpOl1 the stage f utgassing peratins. After t be first utaassin a b b at. 49 C, t J1e s.tea d. 1. state at t he lwest pressure readmg was attamed 111 1 t 2 h', but in the m easm'ements after the final utgassings at rm temperature the attainm ent f a steady state required a much ln ger time, as is illustrated in figure 4. n all cases, t he attainment f a steady state after the intrductin f the first prtin f nitrgen required th e ln gest time. Curve A in figure 4 was bserved fr the lwest pint f the istherm fr char 27 at 9 K (see fig. 6) and a steady-state pressure f.4 J.L was realized after 1 hr. n several ther similar cases it was bserved that such a steady state persisted after 13 additinal hr. Curve B' f fiaure 4 illustrates that nly 3 h' was required whn the steady-state pressure was abut 2 J.L. When the pressure was in excess f 1 mm, the steady state was reached in abut 2 min, in agreement with earlier exp erien ce [13J (curves C, D, E, and F, fig. 4). The. influen.ce f cnsecutive utgassings n the quantlty f mtrgen adsrbed was f particular interest. Figure 5 cn tains the lw-pressure adsrp13 -------- --
L 7mm '" ":: -" <.) OOmm Omm - :: mm <.) UJ "'. UJ :: <f 1 i: 8 UJ <f CD 1 \xx,,..! [< E c!, 1 B \ \ j. A.- j TARLE 1. Adsrptin,.,f nitrgen n char 2 9. K n,7 K 69.5 K pl' F pl' V pl' 11 ------------ --- 1. 63 X lo- 7 rnl rnl ml 3. ()(). 28 X 1-7 3. 6 6. 72X lo--6 9. 5. 23X JO-6 4. 25 2, 2 X - 6 6. 23 5. 33XO-'. 76 5. 63 X JO--6 5.5 2.5X O-' 8. 69 3. 27 X 1-' 14.8 6. 33 X 1-' 7.44 2. 67XlO- ' ll, 16 5. 26X lo-' 21. 48 6, 22X lo- '. 76 3.9X -' 12. 6. 87X lo-2 24. 63 3. 25 X O-' 15. n 6. 97 X O- ' 14. 71 4, 18XO-2 27. ()(). 157 2.33 8. 3X 1-' 15.1 5. 72 X O-' 27.96, 5 19 24.15 2. 54X 1-' 17. 96 9. 63 X O-2 3.22. 937 26.53 6. 72X 1-' 2. 39. 1215 31. 69. 1319 28. 24 1. 43 X 1-2 22. 41. 1634 33.4. 1984 3.83 3. 63 X 1-2 24. 92. 2197 35.95. 2811 34.2 5. 9 X -' 26.,2725 38. 36. 3131 35. 15 6. 93 X 1-2 27.3.3493 42. 15 --- -- ------. ----- 9. 14X l-2 29. 1. 441 47.5 ------------ --.- -.1323 3. 41.529 53. 54 ------------ -----. 1785 32.38. 6847 73. 9 ---------- -----. 3942 41. 7 ---- - ----- - ----- --- ---.---- -----.5822 53. 68 ---------- - ----- 1',,,= 25.54 ',,= 27.33 V'/= 28.65 1'=2,698 mm R g 1'= 796.5 mll H g 1'= 27.3 mm Hg 11 expressed at 273.16 K and 1 atm.1 1 12 13 TME, HOURS Attainment f steady state at varius final pressw"es. FGURE 4. Curyes A and B: char 27 at 9 K ; C, D, E, and F: char 27 at 72.2 K. 12 r---t_----_i--------+_------_+------ 4... 1 6,-------,--------,--------=---------, $ 'E! 8--f_--------_i--------+_------_+------ 4! :- 4 H ---+----+----+-----:l,.l=---- 4 O L_--- FGURE 5. XO-:3 2XO-3 3 XO- 3 4 XO- 3 5XO-3 RELATVE PRESSURE, p/ p Lw-pressure istherms f nitrgen n char 2 at 9 K. T he infiucnce f cnsecutive utgassings with the adsrbent at rm tern perat.ure is shwn; init ial utgassin g a t 4 C. tin data fr char 2 at 9 K, and it m ay be seen frm this example that t he amun t adsrbed increased with cnsecutive utgassings. By in terplatin g each curve t an equilibrium relative pressure f 2 X 1- " the vlume f nitrgen (milliliter STP) adsrbed was.4 ml aftcr the initial utgassing at 4 C and.65, 1.6, 4.3, 9.1, 9.1, and 9.1 ml, respectively, after each f six cnsecutive lltgassings TARLE 2. Adsrpti n f nitl'gen n cha?o 27 9. K 7i.i K i2.2 K: 1'11' T pi p F 1'11' V ml ml m l 8. 5 X 1-8. 553 1. 68X O-7. 9 3. 83 X lo... 4. 66 X 1-' 1. 28 8. 31 X O- 7 1. 28 5,53X O-' 1. 76 2, 47 1. 81 X 1-' 1. 48 3. 36X O-6 1. 6 5. 3 X lo- ' 3. 42 2. 54X 1-' 1. 59 2. 9XO-' 2. 8. 39 X 1-3 5, 1. llx O- ' 1. 91 3. 19X 1O- ' 2.88.272 5. 81 3. 2 X -' 2. 19 4.67X -' 3. 1, 77 6. 69 6. 73 X lo-' 2.65 1. 46X -' 3. 68, 92 7.3 9. 87 X -' 2. 95 4, 23X-' 4, 31, 1396 7.63 3. 34 X 1-' 3.68. 153 5, 16. 194 8. 27 7. 51X l-' 4.23, 362 5,8.3496 1, 21.116 4. 54. 748 6.48. 3982 1.82. 17 4. 83. 1481 7.4, 553 1 13.23. 232 5.8. 225 7.8. 6162 14.47.283 5.25. 2945 8.57.678 16.25. 33 1 5.38. ;:1515 9.2. 7774 2. 39, 383 5.51.4194 9. 96, 8474 26.63. 445 5.65. 5654. 88, 9212 39.53. 597 5.94. 7934 17. 69, 9574 53.54, 923 6. 43.8724 22.52, 1147 6. 69. 1482 7.7. 1979 7.6. 272 8. 57. 375 8. 79 - ----- ------ ",,=6. 28 ',,=6.6D V ",= 6.95 P= 2,698 mm H g 1'= 796,5 mll Hg P=397,3 mm H g at rm temperature. The istherms becm e cinciden t abve a r elative pressure f abut 1-2, in which regin this effect was nt eviden t. n the first adsrptin experimen ts with chars 2 and 27 after the first utgassing at 4 C, it small increase in pressure was bserved i.n the system when t he first prtin f nitrgen at very lw pressure was intrduced int t he adsrptin chamber. This increase was bserved nly during the first few minutes 14
, 1 TABLE 3. Adsrptin j nitrgen n char 1 9. K pip, pip, ' ----------------------------1 7.23X - ' 1. 5XO-' 5.7X -' 1.74X- 3.5 X - 5.33X 1- ' u.8x O- ' 2.7 X -' 5.2X O-' 7.9X 1-'.126.155. 235. 36 1. 543. 6S7. 976 11 29. 1253. 1389 1525 1823. 292. 2338. 2493. 2571. 2592. 2627.3387.3699 _.-----------. -------------- -------------- -------- ------ -----------.-- -------------. -------------- -------------- -.-.---------- ---.-.---.---- -----.----_._- rnl 3 1. 37 56.48 11. 6 135.59 161. 7 174.41 18 1. 64 22.81 242.34 252.34 26 1. 15 268.6 279.5 292.5 1 36. 76 316.79 327.62 333.46 :33(j.94 34.8 :]4:1. 348.52 :352.56 :)54.87 :],\6.:3 356.51 356.87 357.29 :364. 2 :173. 16 3.46X - ' 2.95X 1-' 9.26X 1-' 2.3X - ' 4.J9X 1-' 8.67X 1- ' 1.5X 1-' 2.4X 1-' 2.79X 1-' 7.12 X - '.173.254.4 12.482. 585.744. 835. 1:1:32 ii:].243 1.3219.3552.497. 465 1.584.53 16 (i317. 6897.7136. 7662 rnl 3 1. 37 JOG 36 141. 88 178.5 21. 22 223.6 235.86 246.65 251. 5 27.59 293. 73 34.91 32.4 325. J6 33 1. 4 339. 6 342. 73 354.35 36.39 364. 88 37. 68 372. 24 374.98 376.95 378. 78 379.58 383. 84 386.5 387. 2 387.44 7672 39. 61 8241 393. 46 829 394. 59 8377 395. 58.87,13 399. 78.9282 42. M 9328 45. 47. 9687 41.45.981 42.91.988 43 1. 76 9996 471. 56 ------ - ---- -- ---------------- \ '.. =378 P= 2,698 mm f g 1'.. =397 P= i96.5 mm H -'! f cntact, and then the pressure decreascd in a nrmal way tward equilibrium. When t his effect was bserved, the final values f the relative pressures were ntably high. A pssible explanatin is presented in a later paragraph. The Limes required t realize a steady state fr char 27 were nly abut half thse required fr chars 2 and 1. at the sam e final pressure. Perids f abu t 5 hl' were necessary t attain equilibrium in chars 1 and 2 at t he lwest pressures btained. The vlumes f nitrgen adsrbed fr char 2 (69.5 77.7, 9. K ), char 27 (72.2, 77.7, 9. K ), a d char 1 (77.7, 9. K ) are given in tables ], 2, and 3, respectively. The data recrded are th final steady-state values bserved after a number f cnsecutive utgassings at rm tempera Lures sufficien t t yield reprducible values fr t he vlumes f nitrgen adsrbed. 4.2. Calculatin f stherm Equatin An istherm equatin was sught, valid fr t he regin f the lwest measurable pres ures, which culd then be used with sm e cnfidence t extraplate t the vlume adsrbed at pressures t lw t measul'e. Such an extraplatin is necessary in t he calculatin f thermdynamic functins f adsrbed mlecules [14]. Plts f the vlume adsrbed as a functin f relative pressure fr chars 2, 27, and 1 yielded curves that became sleeper as the pressure was reduced, as shwn in figure 6 fr char 27. A lin ear behavir was fund, hwever, n a Freundlich plt fr all temperatures and all chars frm the lwest relative pressul'es up t abut 1-4 Typical plts are shwn in fi gure 7. The istherms bserved at the lwer temperatures were linear up t a higher value f relative press ure. Fr example, in char 27 at 72.2 K a straight line was bserved up t p /P 3 X 1-2 ; at 77.7 K the straight line was valid up L 5 X lo- \ and at 9. K up t 4 X O- 4 A similar behavir was bserved in chars 1 a nd 2. Parameters n f Lhe Freundlich equatin w('re determined, and these are given in Lablc 4. T AH LB 4. C har 'T'C'lll penlture stherm cnstants Jr adsrptin j nil?'gen Freundlich cnstant, fl B.E.T. parameters \ '.. c /,'. Surface area --- ------- - { / ( ml m'l(f 77.7 3.92 97 b 4() L. 4.3S3 1.744 { { 9..82 378 b 65 4. 61 1. 741 ()9.5 7. i 5 28.65 194 4.255 121. 9 2 77.7 7.39 27.33 18,) 4.38 \19.8 9. 7.31 25.54 183 4. 61 117.5 72.2 7.3 6.95 11 4.295 29.85 27 _.. 77.7 7. 3 6.6 123 4. 383 28.93 9. 7.3 6. 28 17 4.61 28. R9 11 Packing cnstant based n density f liquid nitrgen fr cnverti ng V", t surface area. b Calculated frm t he Lang muir istherm. 8 c gj <f) w ":l "', c; '. XQ 5 1 72.2 K 77.7 K 9. K 1 2XK) 3XO - 4 XO - RELATVE PRESSURE, p/p 1 l i 5XO Fw URE 6. Nitrgen adsrptin istherms n char 27 at 72.2, 77.6, and 9 K. 15
... '; 1 :: f-!!!- = E - - "''" '" Ul 72.Z 1(. 77.7 1( 9. "1( ( «"' : :..J.1 1-1 1-4 1-5 RELATVE PRESSURE. p/p 7. Freundlich istherms fr the nitrgen adsrptin data at lw pressure n char 27 at 72.2, 77.6, and 9 K. FG URE Bth scales arc lgarithmic. 4.3. Surface Area Calculatins CO 2, CO, and Hz by cntinued thermal treatment. Sme f this cmplex culd be remved during the utgassing at 4 C, but an appreciable prtin always remained in the adsrbent. The presence f this residual cmplex culd have a majr influen ce n the bserved steady-state pressure in the range f very lw pressures. The experimental results suggest that ne f the factrs respnsible fr the behavir bserved at lw pressures may be directly related t the residual cmplex. The first prtin f nitrgen intrduced was able t displace sme cnstituent f the cmplex, and, as a result, the liberated decmpsitin prducts, such as CO, CO 2, and H 2, cntributed t the bserved pressure. Unfrtunately, means were nt available t analyze the prducts f utgassing at the lw pressures emplyed, but this wuld be a desirable future experiment. The cnsecutive utgassings at rm temperature, tgether with intrductin f mre nitrgen, served t remve the cmplex until a stage f r elative stability was reached, as shwn by the final reprducible values. The particular prgram f utgassing is bviusly ne f a pssible large number. Any alteratin in the technique f utgassing, such as a secnd heating t 4 C, wuld n dubt have had an additinal influence n the number f adsrptin sites shared between the surface cmplex and the adsrbed nitrg-en. Anther factr f influence is assciated with the diffusin f gas mlecules at lw pressures. Because f the intricate structure f the prus material, mre time is required fr the nitrgen t lcate the adsrptin sites f minimum ptential energy. A similar argument may b e valid in cnnectin with surfacediffusin phenmena. Sme such cnsideratin is required t explain the fact that even relatively well utgassed adsrbents (4 C fr 16 h') required lng perids t attain equilibrium in the lw-pressure range [15]. Other things b eing equal, relatively shrter perids were required t attain a steady state fr char 27 cmpared t char 2. t has been shwn elsewhere [16] that the crystalline structure f basic calcium phsphate (abut 9 percent in these materials) was The adsrptin data fr chars 2 and 27 were fund t give lineal' B. E. T. plts in the range f relative pressures between.5 and.35. The values f Vm and c were evaluated frm the B. E. T. equatin fr a free surface (2) where V (reprted at STP) is the vlume adsrbed at pressure p, and P is the saturatin pressure. The values f the cnstants V rn and c are given in table 4. The crrespnding surface areas were based n a packing factr calculated frm the density f liquid nitrgen [8]. The values given in table 4 were calculated frm the adsrptin data recrded in tables 1, 2, and 3. V rn, als calculated frm the data btained in the first adsrptin run after utgassing at 4 C, was fund t be 26.5 cmpared t 25.5 fr the steady-state value. Despi te the large difference between the first prtin f this istherm and the crrespnding part f the istherm btained after several utgassings at rm temperature, there was clse agreement in the values f V m btained frm the tw sets f data. The adsrptin data fr char 1 did nt give a linear B. E. T. plt. The crrect values f Vm fr char 1 are given by Langmuir plts, which were fund valid in the relative pressure range between.2 and _8. The values f Vm at 77.7 and 9. K are als given in table 4. 5. Discussin 5.1. Attainment f Steady-State Pressures As the carbn adsrbents investigated were essentially pyrlysis residues f bne r ccnut shell and nt subj ect t a defined state f decmpsitin, it is highly prbable that sme surface cmplex cntaining carbn, hydrgen, and xygen remained with the adsrbent. This culd decmpse int 16 '---- ---
;, "w, mre highly develped in char 27. n this case the cntributin f the vids in t he prus structure t the ttal adsrbing surface was less. n additin, the carbn aceus r esidue in char 27 bad a gr eater stability as a r esul t f many heatings t which it had been subj ected in t he cmmercial r ev ivificatin prcess. 5.3. Surface Area When th e rati V /V m was pltted (see fig. 8) as a fun ctin f p/p frm 1-7 t 1- \ i t, was fund t hat the curves fr chars 2 and 27 were in cincidence at 9 K. n t hi s r egin n mre t han 4 pet'ce n t f the surface was cvered. This agreed with previus wrk in th e range.1 t. 3 [8], where t he entire surface was cvered. Hwever, at t he lwer temperatures, t he curves fr differen t adsrben ts were separated t a sign ifi cant exten t, which implies a different interactin f t he nitrgen ml ecules with t he surface. Fr t he case f a given adsrbent at 2 r 3 temperatures, t he curv es were separated t greater exte nts than indicated by the crres pnding chan ges in t he densities f liquid nitrgen. The values f V /V m ar e largest at the lwest temperature fr a given valu e f p/p' The surface areas given in t able 4 ind icate an in crease with decrease in t he temp er ature. t is nt knwn wh eth er r nt this is within t he experim ental errr f t he Vm determinat in. t may als be related either with unknwn changes in density f t he adsrbed nitrgen p hase r with t he pssibili ty t hat t he effective van del' Waal's diametcr f the adsrb ed nitrgen mlecule at t he differ ent temperatu res is cri t ical with t he dimensins f the v id. Therefrc, t he s urface a vailable t a ni trge n mlecule, especially in prus m ateri a ls, mi ght be c ttespnd i n gl' greater. 5.2. Validity f stherm at Very Lw Pressures Th e exp erimental data shw that a Freundlich ist herm is valid in the m easurable range at very lw press ures. When a Langmuir istherm is valid at lw pressures, H enry's law, V = ax, is autmatically btain ed as the limiting case. Hill [17] has shwn frm statistical mechanical-tb ermdyn amic cnsideratins t hat all adsrp tin istherms must apprach H enry 's law. Hence, in t he present case wher e Freundlich istherms a re bserved at the lwest pressures, it is n ecessary t pstulate a tran si. tin range belw which ne mu st r eali ze t he lin eal' depend ence f adsrbed vlume n pressure. t is nt knwn wh ere t hi s range exists fr the adso l'bents investigated. Fr H enry's law t b e beyed, the slpe f the plts in figure 7 must be unity. t m ay b e seen frm these gr aphs that t he first bserved pints at t he lwest pressure fr the isth erms at 9. and 77.7 K are displaced in the crrect directin. Hwever, it will be n ecessary t btain additinal data at even lwer pressure t be sure f t his behavir. 5.4. Cnclusins.5 The attainment f equilibrium at very lw press ures is a cmplica ted prcess, in whieh t he nat ure f the s urface and the impurit ies asscia ted with t he slid surface pl ay a n imprlan t rl e. Variatins in the utgassing techniques can chan ge this par t f the istherm t a nta ble ex tent. As th e nat ure f the pred min a nt frces respnsible fr adsrptin cba nges signifi cantly with s urface cverage, n knwn is t herm equatin ean be expected t be valid frm x = O t x = 1. The Freu ndlich equatin appears t be valid at very lw pressures in a grea t many cases, but it is nt valid when the surface cverage appraches a mnlayer wher e x is apprximately.1. The B.E.T. equatin appears t be frequently valid betwee n the pin t wh ere th e Freundli ch r elatinship fails and relative press ures f abut.35. n this regin (.5 t. 35) the factrs that arc nt taken int accunt in the B.E.T. mdel wuld nt cn tribute appreciably t the vlum e adsrbed. The beh avir abve.35 in relative press ure has at present n adequate th eretical basis. n general, ne can say th at the nature f the predminant frces invlved in an y adsrptin prcess is a func t in n t nly f the nature f th e adsrbate, adsrbent, a nd temperature, but als f the pressure range, whi ch determines the fractin f t he surface cvered. r - - - -, - - - -, - - - -, - - - -, - - ----, G9C."" 2.4.3 E ' 9C.",,ft \.2.1 lf--- -- - + - - - + - - - - j - - - - + - -----j F rcu RE 8. rl1he ra ti, (l1, Lw-pressure adsrpti n data f r char 27 at 72.2, 77.7, and 9 K. is shwn a.s a fun ctin f. t where x= rclativc pressure, a nd V= vlum e f nitrgen adsrbed (S'rp ). 17
---- 6. References {] T. L. Hill, P. H. Emmett, and L. G. Jyn er, J. Am. Chem Sc. 73, 512 (1951). [2] T. L. Hill, J. Ch ern. Phys. 17, 762 (1949). [3] G. D. Halsey, J. Am. Chem. Sc. 73, 2693 (1951). [4] T. N. Rhdin, J. Am. Chem. Sc. 72, 5692 (195). [5] G. D. Halsey and H. S. Taylr, J. Chern. Phys. 15, 624 (1947). [6] R. Sips, J. Chern. Phys. 16, 49 (1948); 18, 124 (195). [7] M. H. Ck, E. H. Pack, a nd A. G. Oblad, J. Chem. Phys. 19, 267 (1951 ). [8] V. R. Deitz and L. F. GJeysteen, J. R esearch N BS 29, 191 (1942) RP1496. [9] F. C. Thmpkins and D. E. Wheeler, Trans. Faraday Sc. 29, 1248 (1933). [1] S. Weber, W. H. <eesm, and G. Schmidt, R ep. V Cngres nt. Frid 2, 45, 61, 74 (1936). [11] S. Chu Liang, J. App!. Phys. 22, 148 (1951); J. Phys. Chem. 56, 66 (1952); 57, 91 (1953). [12] W. V. Lbenst ein a nd V. R. D eitz, J. R esearch NBS <i6, 51 (1951) RP2174. [13] V. R. Deitz and L. F. GJ eyst een, J. R esearch N BS 35 285 (1945) RP1674. ' [14] T. L. Hill, J. Chem. Phys. 17, 52 (1 949). [15] S. Chu Liang, J. Phys. Chem. 57, 84 (1953). [16] R eprt fr the Five Y ear P erid 1939-44 f the Bne Char R esearch Prject. nc. [17] T. L. Hill, J. Ch em. Phys. 18, 246 (195). 'VASHNG'l'ON, D ecember 3, 1954. 18