1276
JAMES HOLMES AKD P. H. EMMETT
(9) HOODAND NORDBERQ’ u. s. patent 2,106,744. (10) JOHNSTONE, H.F.:Report to the National Defense Research Committee, Office of Scientific Research and Development, Washington, D. C. (11) KUBELKA: Kolloid-Z. 66, 129 (1931). (12) PEARSON: Z. physik. Chem. A166, 86 (1931). (13) RITTERA N D DRAKE:Ind. Eng. Chem. 17, 782, 787 (1945). (14) .SCHUCHOWITSKI: Kolloid-Z. 66,139 (1934). (15) SMITH,THORNHILL, A N D BRAY:Ind. Eng. Chem. 33, 1303 (1911); 36, 972 (1943). (16) THOMSOX: Phil. Mag. 42, 448 (1871).
ALTERATION O F THE SIZE AXD DISTRIBUTION O F PORES IN CHARCOALS’ JAMES HOLMES* AND P . H. EMMETT3 Department of Chemical Engineering, The Johns Hopkins University, Baltimore 18, Maryland
Received June 11, 1947 INTRODUCTION
In the course of some of the research work on charcoal conducted by Division 10 of the National Defense Research Committee, it became apparent that charcoal for gas mask use would have to perform the double function of acting as a good adsorbent and as a suitable catalyst or chemical support. Some poison gases could be removed by adsorption, whereas others that might be employed in gas warfare could be removed only by treating the charcoal with certain chemicals which would operate either by direct reaction or by serving as oxidation catalysts. Much was already known about the adsorptive capacity of charcoals and the way of preparing suitable chars for adsorption work. Comparatively little was known as to the structural characteristics and pore distribution that should be possessed by a charcoal that was to serve as a catalyst support. It was rather evident, however, that pore-size distribution would be an important factor. In order to supplement quickly the supply of charcoal which by certain processes of manufacturing seemed suitable for the preparation of chemically treated charcoals or “whetlerites” (8), it seemed necessary to undertake a program of studying ways and means of transforming good adsorbent charcoals into suitable chars for “whetlerization.” Accordingly, the work reported in the present paper was carried out. The detailed account of the relation between pore distribution and the suitability of a charcoal for making whetlerite is restricted information and will not 1 Presented a t the Symposium on the Adsorption of Gases which was held under the awpices of the Division of Colloid Chemistry a t the 110th Meeting of the American Chemical Society, Chicago, Illinois, September 11L12, 1946. 2 Present address: Houdry Process Corporation, Marcus Hook, Pennsylvania. 8 Present address: Mellon Institute, Pittsburgh 13, Pennsylvania.
-4LTERbTION O F PORE SIZE IN CHARCOALS
1277
be included in the present report. Npwtheless, in view of the many and diverse use3 of charcoal in industry, it seems of interest to recount the findings as to the method and extent to which pore-size alteration could be brought about on adsorbent charcoals of the types that are generally available commercially. EXPERIMENTAL
Charcoals used The work reported here was done on three general types of charcoal: ( 1 ) those made by the zinc chloride activation process from wood sawdust, (2) those made by steam activation of coconut shells, and (3) those made by proper calcination and activation of coal. The four charcoals may be described in a little more detail as follows: CWSK 19 mas made by the zinc chloride process; a 0.9 ratio of zinc chloride to wood sawdust was used. It had the unusually low ash content of 0.2 per cent. In the process of manufacture, the sample had been given a final calcination at 850°C. The density of the carbon in the sample as revealed by measurements nith helium v a s 2.09. CWSN S 5 differed from CWSS 19 in that it had an ash content of 0.77 per cent and had not been given a final calcination at 850°C. Furthermore, measurements with helium showed that it had a carbon density of 1.81. The ash content in both of these charcoals was probably mostly zinc oxide or zinc carbonate. The coconut charcoal was a standard product made by the calcination and steam activation of coconut shells. An ash value and carbon density for it are not available. Sample PCI P58 was made from coal by a process of calcining the finely ground and briquetted coal, and then activating it with steam at temperatures up to 850" or 900°C. It had an ash content of 19.4 per cent. Impregnation The base charcoals were dried in an oven at 125°C. before being impregnated. The total volume of the impregnating solution in cubic centimeters was equal to the weight in grams of charcoal used, when the impregnant ma8 added directly in the form of one of its salts. The solution was added dropwise to the charcoal with vigorous stirring to assure uniform mixing. Sufficient weight of the salt was added so that the charcoal would contain the desired percentage of impregnant calculated on the basis of its oxide. In certain cases, the catalyst was added to the charcoal in the form of a solution of one of its salts and subsequently precipitated in the form of the hydroxide by the addition of an excess of ammonium hydroxide. The ammonium hydroxide solution was always added immediately after the addition of the salt solution; 1 cc. of ammonium hydroxide plus salt solution was used for each gram of charcoal. Following the impregnation, the samples were again dried for several hours at 135°C. in an oven. Specific details of preparation for the various samples may be summarized with respect t o the final form of the impregnant as follows: CrtOs: Samples were prepared by ( a ) the direct addition of chromic nitrate
1278
JAMES HOLMES AND P. H. EMMETT
solution; (b) the addition of chromic nitrate solution followed by the addition of concentrated ammonium hydroxide to the charcoal; and (c) the addition of ammonium chromate solution prepared by the addition of two volumes of concentrated ammonium hydroxide solution to one volume of solution of chromic acid. Fe&: Samples were prepared by (a) the direct addition of ferric nitrate solution; and (b) the addition of ferric nitrate solution followed by concentrated ammonium hydroxide. NiO: Samples were prepared by (a) the direct addition of nickel nitrate solution; (b) the addition of nickel nitrate solution followed by concentrated ammonium hydroxide; and (c) the direct addition of ammoniacal nickel chloride solution previously prepared by the addition of excess ammonium hydroxide to a solution of nickelous oxide in hydrochloric acid. MoZOs:The sample was prepared by the addition of a solution of 85 per cent molybdic acid in water which had been made strongly alkaline with ammonium hydroxide. Ka&08: The samples were prepared by the direct addition of a solution of the salt.
Treatment of the samples All steam, hydrogen, and high-temperature oxygen treatments were carried out in quartz or other inert combustion tubes. The size of the samples for all the small-scale treatments varied from about 3 g. with steam or hydrogen, to 1 g. with oxygen. Large-scale treatments were limited to about 40-g. charges by the size of the combustion tubes accommodated by the furnace. The latter wm used in a vertical position with the entering gas passing up through the charcoal bed, which was supported upon a fine chrome1 screen. The rate of flow of the hydrogen or the oxygen-nitrogen mixture was measured at the exit, side by a wet-test flowmeter. For the steam runs, rates of steam passage were obtained by using a small, electrically heated boiler in which the liquid level could be read to about 5 cc. In most cases, the flow rate of the entering gases was sufficiently large to keep the sample “jiggling.” This condition was achieved by passing nitrogen as a diluent through the sample during the steam runs. Nitrogen was passed through the sample while the combustion tube was admitted to the furnace, and a very slow flow of nitrogen was passed through while the temperature of the furnace was brought up to the desired value. Nitrogen was again passed through the sample at the end of the run when the tubes were removed and also during cooling outside of the furnace to room temperature. The bed thickness and flow rate seemed to have a marked effect upon the final product only for runs in which free oxygen was the oxidizing agent. For such runs, high rates of flow and thick beds both appeared to have a deleterious effect by producing much less specificity in the removal of carbon. It was accordingly desirable to have very slow flow rates and thin beds during oxidation with free
ALTERATION OF PORE SIZE IN CHARCOALS
1279
oxygen. In order to prepare the large-scale samples by air oxidation, a special furnace having a very large stainless-steel screen was used. The bed depths never exceeded 3 to 2 cm. in thickness; the flow rates were maintained below 0.5 cc.jmin./gram of charcoal. The low-temperature oxygen treatments at 350-500°C.were made with compressed air, whereas the high-temperature oxygen runs were made with tank nitrogen which contained a very small percentage of oxygen as an impurity. In the few runs made at IOOO'C., using prepurified nitrogen as the entering gas, the source of oxygen for the treatment was oxygen present as combined oxygen in the impregnant. Adsorption measurements The change in the pore distribution and size of the charcoal samples was judged by the change in appearance of the nitrogen-adsorption isotherms, as measured in a standard apparatus (3, 5) at - 195OC. The general significance of such adsorption measurements as criteria for pore-size distribution in an adsorbent will be discussed below. RESULTS
In figures 2 to 20 are shown the nitrogen-adsorption isotherms of the samples before and after the specific treatment listed in the legends of the various figures. Figures 2A to 20A give the adsorption results per gram of charcoal, due correction always being made for the weight of impregnating material that may have been added. Whenever density values were determined for the treated sample it became possible to express the results as adsorption per cubic centimeter of sample. The volume in this case is that which one measures by pouring the charcoal sample with appropriate tapping into a graduated cylinder. Figures 2B to 20B give the adsorptions per cubic centimeter of sample. In discussing the result of figures 2 to 20 it will be convenient to subdivide the results into sections relating t o steaming, hydrogenation, and air oxidation without impregnation; and into a section in which the effects of each of the impregnating agents will be considered. First, however, the use of nitrogenadsorption isotherms for estimating pore distribution will be discussed. Also, in connection with figure 1 a few generalizations will be attempted as to the effect of various detailed processes that may reasonably be assumed t o occur during experiments on pore-size alteration. Nitrogen adsorption-the criterion of pore-size changes It is fully realized that nitrogen-adsorption isotherms do not at the present stage of development permit one to calculate quantitatively the exact poresize distribution of an adsorbent such as charcoal. It is equally certain, however, that such nitrogen isotherms are reliable means of giving a qualitative indication as to the pore-size distribution and of showing with certainty the absence of pores in certain size ranges. The isotherms mill be discussed in terms of three relative pressure regions: the
1280
JAMES HOLMES AND P. H. EM.METT
section corresponding t o relative pressure ranges from 0.0 to 0.4 will, for convenience, be called the A B region; that between 0.4 and 0.7 will be referred to as the BC region; and that between 0.7 and 0.99 as the CD region. The selection of these various regions is somewhat arbitrary, though it also has a certain reasonableness based upon the use that was to be made of the charcoals. It is generally agreed, for example, that capillary condensation will not occur at relative pressures lower than that corresponding to four molecular diameters, as calculated by the Kelvin (13) equation
D =
- 4 u ~COS e
RT 2.303 log p / p ,
where u is the surface tension of the adsorbate, 0 is the angle of wetting of the adsorbate on the solid, V is the molal volume of the liquefied adsorbate, p is the pressure at which the adsorption is being measured, and p / p o is the liquefaction pressure of the adsorbate at the temperature of the experiment, T . For nitrogen, this point is at approximately 0.4 relative pressure (actually 0.35 if one uses the value 8.G (7) for the surface tension of liquid nitrogen at -195°C.) It appears, therefore, that the nitrogen adsorption up to 0.4 relative pressure includes that which is present as a monolayer over the entire surface of small and large capillaries plus such multilayer adsorption (equal to or less than about 0.5 layer) as \\auld be expected at a relative pressure of 0.4 (3). Roughly speaking then, the adsorption up to 0.4 relative pressure should be a measure of the adsorption capacity of a charcoal over the range of relative pressures that might be encountered in any gas mask work. The absolute value of this nitrogen adsorption at 0.4 relative pressure rather than the slope of the adsorption curve will be used, then, in discussing the changes in adsorption caoacity brought about by treating the charcoals. If an adsorption isotherm for nitrogen is flat above 0.4 relative pressure, it appears certain (3) that there is no appreciable number of pores present of a size covered by the relative pressure range 0.44.99. The numerical values assigned to pores corresponding to this relative pressure range depend upon assumptions that one makes as to the structure of the charcoal. If one assumes that the capillaries are cylindrical, that the condensed nitrogen has the same density and surface tension as bulk liquid nitrogen, and that the angle of wetting is zero, one finds by equation 1 that the relative pressure range 0.4-0.99 corresponds to pore diameters between about 20 and 1800 A. On the other hand, if the pores are assumed to consist of cracks with plane parallel walls, distances between the plane parallel walls calculated would be one-half as large as the diameter calculated for the cylindrical capillaries. In the present discussion, any slope in the regions BC or CD will be attributed to the presence of capillaries in the size ranges that one would calculate from equation 1 (11). No quantitative estimate of the number and distribution of the pores can be made from the nitrogen isotherms, however, because of the difficulty or impossibility of distinguishing between multilayer adsorption in this relative pressure range and the filling of capillaries by capillary condensation. (See, however, the work of Wheeler on silica gels as reported by Beeck (2)).
ALTERATION OF PORE SIZE IN CHARCOALS
1281
The reason for breaking the region between 0.4 and 0.99 relative pressure into the two sections BC and C D has to do with some of the service tests that are applied to charcoal. Many tests on certain poison gases were made at 80 per cent relative humidity, since charcoals behaved differently toward some gases when moist than when dry, and since 80 per cent relative humidity seemed a reasonable upper limit to any humidity that might be encountered frequently in actual use. The exact pore size that would remain filled with water when the relative humidity is dropped to 80 per cent will, of course, depend upon the angle of wetting of the charcoal by water. In some work carried out by Pierce, Blacet, and Juhola (10) there seems reason to believe that in desorption this angle of wetting may be about 0.6 for most charcoals. The adsorption of nitrogen at 0.7 relative pressure should, accordingly, correspond approximately to the filling of all pores that would remain filled with water if the relative pressure were reduced to 80 per cent. Pores larger than this should remain open and, therefore, available as catalyst supports or as chcmical supports for any whetlerizing agents that might be present. The CD region corresponds to pores which, according to equation 1 applied to cylindrical capillaries, lie between 70 A. and 1800 A. in diameter. Such pores in all probability are very important as channels through which the larger inner surface of the charcoal particles can be reached by adsorbate gases. Here, too, the slope and not the absolute value of the CD region is important, since the slope of this region is a qualitative measure of the volume of pores in this size range. It is now realized that pores larger than 1800 A . in diameter may also be very important in the performance of a charcoal (10). No indication of the pore size or pore-size distribution above 1800 A. was obtained in the present work, though methods are now knonn for making such measurements (4, 6, 12, 14). Generalizations as to separate pore-alteratzon actzons A little reflection will show that the various possible effects of steam, hydrogen, or other activating agents on the pore size and distribution may be divided into about six separate actions. These six cases will now be briefly described in conjunction with figure 1. For the sake of simplicity, we shall assume that the isotherm of the original charcoal is flat like that of CWSS 19 in the N curve of figure 2. We shall also assume that the charcoal with which we start is already activated to such a point that the walls between the capillaries are only about four carbon layers thick and the capillaries are 20 A. or less in diameter. Under these circumstances about one-half of the carbon atoms are actually on the surface of some of the capillaries. This means that on further activating the charcoal, one might strip off as much as a layer of carbon on each capillary without breaking doxm the walls betiyeen the cracks or capillaries. We shall also assume that large pores belong either to the region that would undergo capillary condensation between 0.4 and 0.7 relative pressure ( B C region), between 0.7 and 0.99 relative pressure (CD region), or at relative pressures greater than 0.99. An increase in pores in the BC or CD region will be qualitatively indicated in figures 1A and 1R by a uniform posi-
1
T
r
0
T
CASE I
h
CASE Y
0
rn! o
PIP,
/
P/Po
FIG. 1
Case I: Removing a layer of carbon from the smallest pore and opening up new pores without, however, forming any larger than 20 A. in diameter. The adsorption would increase as indicated in figures 1A and lB, the increase on a weight basis being somewhat larger than,on a volume basis. Figures 1A and 1B are so drawn that the displacement of the isotherms on a weight basis can be
dLTERATION OF PORE SIZE IN CHARCOALS
1283
compared directly with the change on a volume basis. This result becomes clearest if one visualizes the 1B figures in terms of a block of charcoal 1 cc. on a 3
1000 -
-
J
a
3 800 a
a
I V
V
v400
200
0.2
0.6
0.4
0.8
I
P I Po
200
-
.o
4
a 180
N
0
I
0.2
0.4
I
0.6
I
I 0.8
PIP0 FIG.2A (upper), 2B (lower). Charcoal CWSN 19 N, untreated base charcoal 1, nteamed a t 750°C. to43 per cent loss 2, steamed a t 750°C. to 62 per cent loss 3, steamed a t 750°C. t o 94.5 per cent loss
I
I
I.o
side as the starting materia1 and then keeps in mind the effect of the treatment on the weight of the sample.
1284
JAMES HOLMES AND P. H. EMMETT
Case ZZ: Small pores are attacked so vigorously as to convert some of them over at least part of their length into capillaries too large to show capillary condensation at 0.99 relative pressure. The adsorption per gram will remain unchanged (neglecting multilayer adsorption on the relatively small surface associated with the large pores), since it is assumed that the average wall thickness of the small capillaries is not altered but that large groups of them by localized attack are converted into very large pores. On the other hand, the adsorption per cubic centimeter is greatly decreased, since large holes having small area are left unfilled by adsorbate at 0.99 relative pressure. Case ZZZ: The impregnant plugs a certain fraction of the small pores. The adsorption on the weight and volume basis will both decrease by the same percentage. This might also result from a closing of small capillaries by growth of graphite crystals during heat treatment. Case ZV: The particles shrink as a result of increase in density of the carbon samples. The adsorption per gram will then decrease, since all pores will become smaller. Calculations indicate that the adsorption per cubic centimeter will probably remain unchanged, since the pore volume per cubic centimeter will be approximately the same as before the particles were shrunk by heat treatment. Case T’: The small pores are converted into pores in the BC region by a chemical attack which decreases the thickness of some cell walls and completely removes others. As shown in figure lB, the total adsorption per cubic centimeter at 0.99 relative pressure will be a little larger than for the original sample, because some carbon will have been removed in the treatment. The total surface area per cubic centimeter will be increased slightly as a result of enlarging pores of any shape by thinning the walls or of producing cylindrical pores in the BC region by completely removing pore walls. However, removing pore walls from a structure containing either square or rectangular pores would decrease surface area per cubic centimeter. Accordingly, in Case V, figure lB, the B B region is designated as being subject t o either an increase or a decrease when pores less than 20 A. in diameter are transformed into pores in the 20-70 A. range. On a weight basis the treatment indicated for Case V would increase the total pore volume filled with adsorbate at 0.99 t o a greater extent than on the volume basis by the same reasoning applied to Case I. However, the total surface area per gram of sample would, as indicated in figure IA, always increase slightly regardless of the shape of the pores. Case V I : The small pores are converted into pores in the CD region without change in thickness of remaining walls. By the same reasoning applied in Case V, the increase in pore volume will be greater on a weight than on a volume basis. The A B region will be lower on a volume basis as a result of removal of pore walls, regardless of the capillary shape, and nil1 be practically unchanged on a weight basis. By suitable combinations of these effects, it is believed that all of the results reported in the present paper can be explained. Attention will be called in the following discussion to examples in figures 2 t o 20 that apparently illustrate these, though most of the effects are, as would be expected, combinations of several of these separate cases.
ALTERATION OF PORE SIZE IN CHARCOALS
1285
P/ Po FIG.3A (upper), 3B (lower). Charcoal CWSN 19 N , untreated base charcoal 1, 0.2 per cent CrzOa(from ammonium chromate), steamed a t 750°C. to 58 per cent loss 2, 1 per cent CrlOs (from ammonium chromate), steamed a t 750°C. to 27 per cent loss 3, 1per cent CrzOa (from ammonium chromate), steamed a t 750°C. to 49 per cent loss 4, 5 per cent CrnOa (from ammonium chromate), steamed at 750°C. t o 46 per cent loss 5, 5 per cent Cr,Ol (from chromic nitrate), steamed a t 750°C. to 46 per cent loss 6, 5 per cent CrzOt (from chiomic nitrate excess ammonium hydroxide), steamed a t 750°C. to 44 per cent loss
+
Eflect of steaming without impregnation The activation of charcoals by steaming has been a standard procedure for many years (9). It is not surprising therefore that, as shown in figures 2A, 12A, 17A, and 19A, the effect of steaming is predominantly to increase the ad-
1286
J A W S HOLMES A N D P. R. EMMETT
550
-
I
I
I
02
I 0.4
I
I
I
0.6
I 0.8
I
I 1.0
PIP, 190
-
A
I
0.2
I
I
I
0.4
I
0.6
I
I 0.8
I
L 1.0
P/ Po FIG.4A (upper), 4B (lower). Charcoal CWSN 19 N , untreated base charcoal 1, 5 per cent FezOi (from ferric nitrate), steamed at 750OC. t o 37 per cent loss 2, 5 per cent Mol08 (from molybdic acid excess ammonium hydroxide), steamed at 760°C. to 41 per cent loss 3, 5 per cent NiO (from nickel chloride excess ammonium hydroxide), steamed at 750" C. t o 43 per cent loss 4, Charcoal CWSN 19 TPE-1(containing cupric oxide), steamed a t 750°C. t o 26 per cent loss
+
+
sorption per gram, as evidenced by the amount of adsorption at 0.4relative pressure (Case I), without altering materially the slope of the rest of the isotherm.
1287
ALTERATION OF PORE SIZE IN CHARCOALS
Such an increase is logically to be expected as resulting from the opening up of new small pores in the charcoal and from slight enlargement of small pores al-
t ,001
I
I
0.2
I
I 0.4
I
I 0.6
I
I
0.8
I
I
I .o
P/ Po
220
c 0.2
0.4
0.6
0.8
PI Po
I.c
FIG.5A (upper), 5B (lower). Charcoal CWSN 19 N , untreated base charcoal 1, treated with hydrogen a t 1ooO"C. t o 26 per cent loss 2, treated with hydrogen a t 1000°C. t o 48 per cent loss 3, treated with hydrogen a t 1000°C. to 74 per cent loss
ready present. It is likewise not unexpected that, as illustrated by runs 1 and 2, figure 17B, steaming might decrease the adsorption per cubic centimeter. This
500
450
J
4
8 400 0
I'
350
0: 0
300 250
0.2
0.4
0.2
0.4
P I Po
P/
0.6
0.8
0.6
0.8
Po
1.0
FIG.6A (upper), 6B (lower). Charcoal CWSN 19
N,untreated base charcoal 1, 1 per cent C n 0 3 (from chromic nitrate), treated with hydrogen at 1000°C. to 21 per cent loss 2, 1 per cent CrzOa (from ammonium chromate), treated with hydrogen at 1000°C. to40 per cent loss 3, 1 per cent CrzOs (from ammonium chromate), treated with hydrogen a t 1000°C. t o 62 per cent loss 4, 5 per cent &foroa (from molybdic acid excess ammonium hydroxide), treated with hydrogen at lOOO'C. to 26 per cent loss 5 , 5 per cent NazO (from sodium carbonate), treated with hydrogen a t 1M)O'C. t o 32 per cent losa
+
1288
1289
ALTERATION OF PORE SIZE IN CHARCO.4LS
c 300 250
210
N 4
-4 -
I 02
I
1
I 0.4
I
I
0.6
I 0.8
I
I
I .o
P/ Po
FIG.7h (upper), 7B (lower). Charcoal CWSN 19 X, untreated base charcoal 1 1 per cent NiO (from nickel nitrate), treated with hydrogen at
1OOO"C.to 15 per cent loss 2, 5 per cent X i 0 (from nickel nitrate), treated with hydrogen at 1000°C.t o 29 per cent loss 3, 5 per cent X i 0 (from nickel nitrate
+
excess ammonium hydroxide), treated with hydrogen at 1OOO"C.t o 27 per cent loss 4, 5 per cent NiO (from nickel chloride excess ammonium hydroxide), treated with hydrogen a t 1oOO"C.t o 25 per cent loss 5 , 5 per cent X i 0 (from nickel chloride excess ammonium hydroxide), treated with hydrogen a t 1oOO"C. to 81 per cent loss
+ +
1290 600
JAMES HOLM7CS AND P. H. EMMETT
-
8J 500 u
0.2
0.8
0.6
0.4
?/Po
I
I
0.2
I
I
I
0.4
I'
0.6
I
I
0.6
I
L 1.0
P/ Po FIG.8A (upper), 8B (lower). Charcoal CWSN 19 N, untreated base charcoal 1, 0.2per cent FesOa (from ferric nitrate excess ammonium hydroxide), treated with hydrogen at looO°C. to 15 per cent loss 2, 1 per cent FezOa (from ferric nitrate), treated with hydrogen a t 1oOO"C. to 15 per cent
+
loss 3, 5 per cent Fe20a (from ferric nitrate), treated with hydrogen at 1ooO"C. to 32 per cent loss 4, 5 per cent FezOa (from ferric nitrate excess ammonium hydroxide), treated with hydrogen a t 100O'C. to 50 per cent loss
+
can result from the removal of pores of all sizes along the sides of major cracks or crevices that are larger than 1800 A. in diameter (Case 11) to an extent more than enough to compensate for the new pores being formed.
1291
A L ~ R A T I O NOF PORE SIZE IN CHARCOALS
Two interesting observations on ateaming were made on charcoal CWSK S5. Steaming the original charcoal at the usual temperature of 750°C. produced
250 1
I 0.2
I
I 0.4
I
I 0.6
I
I
I
I
0.8
1.1
0.8
1.0
P/ Po
0.2
0.4
P/ Po
0.6
FIG.9A (upper), 9B (lower). Charcoal CWSN 19 N, untreated base charcoal 1, treated with oxygen-nitrogen mixture at 450°C. to 30 per cent loss 2, treated with oxygen-nitrogen mixture a t 750°C. to 40 per cent loss
little change in adsorption on either a volume or a weight basis, as is evidenced by curves 3 and 4 in figures 12-4 and 12B. When, however, the charcoal was first heat-treated in a stream of nitrogen to 1200°C., with a resulting shrinkage
1292
JAMES HOLMES AND P. H. EMMETT
0.2
0.6
0.4
0.8
P/ Po FIG.10A (upper), 10B (lower). Charcoal CWSN 19 N , untreated base charcoal
+
1, 5 per cent FezOs (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen at 750'C. t o 14 per cent loss 2, 5 per cent FezOs (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen at 500°C. t o 28 per cent loss 3, 5 per cent FezOs (from ferric nitrate excess ammonium hydroxide), treated with air a t 350°C. to 48 per cent loss 4, 1 per cent FelOs (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen at 750°C. t o 21 per cent loss 5 , 5 per cent CrzOs (from chromic acid excess ammonium hydroxide), treated with tank nitrogen at 10oO°C. t o 19 per cent loss
+ +
+ +
of the particle and increase in the helium density of the carbon from 1.81 to 2.17, the sample became susceptible to further activation by steaming. Curve 5 of figures 12A and 12B shows the adsorption after the 1200°C. sintering (prob-
1293
ALTERATION OF PORE SIZE I N CHARCOALS
3
400
h 2
.#I
350 0
a
I
U I
* 300
s
i
0 250
200 0.2
0.4
1
0.6
0.8
I .(
0.6
0.8
I .(
P/ Po
220
I
a
5 140
::100 60 0.2
0.4
P I Po FIG.114 (upper), 11B (lower). Charcoal CWSN 19 N , untreated base charcoal
+ + +
1, 5 per cent Fei03 (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen a t 1000°C. t o 13 per cent loss 2, 1 per cent Fez03 (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen a t 1000°C. t o 8 per cent loss 3, 5 per cent Fe20a (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen a t 500'C. t o 12 per cent loss
ably Cases I11 and IV), and curves 1 and 2 of these figures show the extent of activation by steaming after the heat-treatment at 1200°C. The second interesting observation on steaming CWSN 55 was the extent to
1294 600
JAMES HOLMES AND P. H. EMMETT
-
P/P* 200
J
le0
3
a
2 160 0
8'
2 140 0
120
P/Pa FIG.12.4 (upper), 12B (lower). Charcoal CWSN S5
N, untreated b a e charcoal
1, heated a t 1200°C. in pure nitrogen, followed by steam treatment a t 750OC. to 44 per cent 108s 2, heated a t 1200°C. i n pure nitrogen, followed by s t e m treatment a t 750°C. to 69 per cent loss 3, steamed at 750'C. to 46 per cent loss 4, stearned a t 750°C. to 55 per cent loss 5, heated for 2 hr. in pure nitrogen a t 1200'C. 6 . steamed at 1000°C. to 72 per cent loss
which a large number of capillaries in the range 20-1800 A. in diameter can be opened up if the steaming is sufficiently severe.. This is illustrated by curve 6 of figures 12A and 12B. Apparently, the pore wdls between the tiny capil-
-4LTER.4TION OF PORE SIZE IN CHARCOALS
1295
P/ Po
200
-
P I Po FIG.13.4 (upper), 13B (lower). CWSN S5
N, untreated base charcoal
+
1, large batch of 0.2 per cent CrzOi (from chromic acid excess ammonium hydroxide), steamed at 750°C. to (a) 35 per cent and (b) 32 per cent loss 2. 0.2 per cent Cr,Ol (from chromic acid excess ammonium hydroxide), steamed at 750°C. to 39 per cent loss 3, 1 per cent Cr20, (from chromic acid excess ammonium hydroxide), steamed at 750" C. to 49 per cent loss 4, 5 per cent CrtOs (from chromic acid excess ammonium hydroxide), steamed a t 750" C. t o 52 per cent loss 5 , CWSN s5 TSW 2697 (containing cupric oxide), steamed at 750'C. to 38 per cent loss
+
+
+
h i e s are eaten away by the strong steaming of the sample at lOOO"C., with a resulting transformation of small pores into those capable of undergoing capillary
1296
.JAMES HOLMES AND P. H. EMKETT
1
450 -
N
4400-
0.2
0.4
0.6
0.8
1.0
P I Po
FIG.14A (upper), 14B (lower). Charcoal CWSN S5 N , untreated base charcoal 1, treated with hydrogen at 1OOO"C. to 37 per cent loss 2, 1 per cent CrzOs (from chromic nitrate), treated with hydrogen at looO°C. t o 18 per cent loss 3, 1 per cent CrlOa (from chromic nitrate), treated with hydrogen a t 1oOO"C. t o 29 per cent loss 4 , 5 per cent X i 0 (from nickel nitrate excess ammonium hydroxide), treated with hydrogen at looO°C. t o 42 per cent losa
+
condensation in the 0.4-0.99 relative pressure range. It will be noticed that the adsorption at 0.4 relative pressure increased 20 per cent on a weight basis
1297
ALTERATION OF PORE SIZE I S CHARCOALS
500 -
P/ Po
200
0.2
0.4
0.6
0.8
P/ Po FIG.15.k (upper), 15B (lower). Charcoal CWSN S5
iX,untreated base charcoal 1, 1 per cent FerOa (from ferric nitrate), treated with hydrogen at 1oOO"C. to 21 per cent loss 2, 1 per cent FelOa (from ferric nitrate), treated with hydrogen a t 1oOO"C. to 34 per cent loss 3, 5 per cent Fez03 (from ferric nitrate excess ammonium hydroxide), treated with hydrogen at 1oOO"C. t o 30 per cent loss 4, 5 per cent Fe20s (from ferric nitrate), treated with hydrogen at loOO°C. t o 37 per cent
+
loss
+
excess ammonium hydroxide), treated with 5, 5 per cent Fez03 (from ferric nitrate hydrogen a t 600°C. to 30 per cent loss
but decreased by about 30 per cent by this very strong steaming at 1000°C. on a volume basis. This is consistent with the transformation of small pores
N
FIG.16A (upper), 16B (lower). Charcoal CWSN 55 N, untreated base charcoal 1, treated a i t h tank nitrogen st 1000°C. to 19 per cent loss 2, 5 per cent FelOs (from ferric nitrate excess ammonium hydroxide), treated with pure nitrogen a t 1000°C. t o 15 per cent loss 3, 5 per cent FelOl (from ferric nitrate excess ammonium hydroxide), treated with tank nitrogen a t 1000°C. to 25 per cent loss
+ +
into those influencing the BC and CD regions (Cases V and VI) together with the formation of some new pores greater than 1800 A. in diameter (Case 11).
1299
ALTERATION OF PORE SIZE M CBARCOALS
700
3
0.2
0.4
0.6
0.8
1.r
P/Po
FIG.17A (upper), 17B (lower). Coconut charcoal X, untreated base charcoal
1, steamed at 750°C.to 24 per cent loss 2, steamed at 750°C.to 40 per cent loss 3, steamed a t 750°C. to 64 per cent loss
4, 0.2 per cent Cr103(from chromic acid 750OC.to 33 per cent loss 5, 5 per cent CrlOI(from chromic acid 750°C.t o 39 per cent loss
+ excess ammonium hydroxide), steamed a t + excess ammonium hydroxide), steamed at
It is not known with certainty, at present, whether the radical change in the pore distribution illustrated by curve 6 of figure 12 is due to a peculiarity of
1300
JAMES HOLMES AND P. H. EMMETT
500
2
N
2501
I
I 0.2
I
I 0.4
I P/
Po
I
0.6
I
I 0.8
I
I
1.0
FIG.18A (upper), 18B (lower). Coconut charcoal N, untreated base charcoal 1, treated with hydrogen a t 1000°C.to 31 per cent loss 2. 0.2 per cent CrzOa (from chromic acid excess ammonium hydroxide), treated with hydrogen at 1000°C.t o 35 per cent loss 3, 5 per cent CrZOs (from chromic acid excess ammonium hydroxide), treated with hydrogen at 1000°C.to 38 per cent loss
+ +
CWSN 55 or whether it would have been shown by the other charcoals had they been steamed at 1000°C. instead of the usual 750°C. Usually, steaming any one
ALTERATION O F PORE SIZE IN CHARCOALS
1301
450-
400 -
-1
P/P, FIG.19A (upper), 19B (lower). Charcoal PCI P58 S , untreated base charcoal 1, steamed at 750°C. to 31 per cent loss 2, 0.2 per cent CrzOa (from chromic acid
750°C. t o 39 per cent loss 3, 5 per cent CrzOs (from chromic acid C. to 36 per cent loss
+ excess
ammonium hydroxide), steamed at
+ exces8 ammonium hydroxide), steamed at 750"
of the four charcoals to as much as 50 per cent weight loss at 750°C. produced practically no alteration in the upper part of the adsorption isotherm, but merely
1302
JAMES HOLMES AND P. E. EIICME1TT
500 -
a 400 8 a -
1
I
0.2
I
I
I
0.4
I 0.6
I
I
0.8
I
I IS
P/ Po
200 -
-
FIG.20A (upper), 20B (lower). Charcoal PCI P5S N, untreated base charcoal 1, treated with hydrogen at 1OOO"C. t o 48 per cent loss
+
2, 0.2 per cent Cr203(from chromic acid excess ammonium hydroxide), treated with hydrogen at 1000°C. t o 31 per cent lose 3, 5 per cent Cr& (from chromic acid exes8 ammonium hydroxide), treated with hydrogen a t 1000°C. t o 43 per cent 1086 4, activated for 150 min., extracted with hydrofluoric acid, treated with hydrogen at 1000°C. t o 28 per cent loss
+
a shift of the total adsorption to higher adsorption values per gram and either higher or lower adsorptions per cubic centimeter of charcoal.
ALTERATION OF PORE SIZE IN CHARCOALS
1303
Hydrogenation without impregnation
It does not seem to have been emphasized in the literature that it is possible to hydrogenate charcoal at temperatures of 1000°C. and t o thereby remove as much of the carbon content of the sample as one may wish to do. Actually, however, this behavior might be expected since, m pointed out by Bahr and Jessen (l), carbon deposited on iron carbide can be removed by hydrogen a t temperatures greater than 400°C. even though the carbon is present in the free form and not as iron carbide. Mild hydrogenation, like mild steaming, either increases or leaves unchanged the value of the adsorption per gram at 0.4 relative pressure. Furthermore, the slope of the isotherm in the BC region is usually unaltered. However, at relative pressures higher than 0.7 and especially a t pressures between 0.9 and 0.99, hydrogenated samples have adsorption isotherms that rise very sharply. These facts are illustrated by the curves for hydrogenation without impregnation in figures 5, 14, 18, and 20 (Cases I, 11, and VI). Without exception, all of the charcoals that were hydrogenated sufficiently to cause the sharp rise between 0.9 and 0.99 relative pressure showed lower adsorption in the A B region per cubic centimeter than did the original charcoal. The results are consistent with the view that the hydrogen attacks the small capillaries and converts them preferentially into capillaries in the range that would have capillary condensation between 0.9 and 0.99, or even into capillaries permitting capillary condensation only at relative pressures higher than 0.99. This latter is strongly suggested by a rather large drop in adsorption per cubic centimeter in the A B region induced in some of the samples by the hydrogenation. Effect of mild oxidation with free oxygen Mild oxidation with an oxygen-nitrogen mixture was tried on only one of the four charcoals, CWSN 19. On this sample the mild oxidation a t either 450' or 750°C. had very much the same effect as steaming at 750'C. in that it increased the low-pressure adsorption ( A B region) without materially affecting the slope of the isotherm above 0.4 relative pressure (Case I). The single experiment a t 1000°C. on CWSIL'.S5 with tank nitrogen (containing its usual small quota of oxygen as an impurity) to a total of 19 per cent loss is confusing, because of the sintering effect that apparently always occurs on this charcoal when it is heated to 1000°C. or 1200°C. The behavior was much the same (curve 1, figure 16) as was observed on heating this charcoal in pure nitrogen for 2 hr. at 1200°C. (curve 5, figure 12). It will be noted, incidentally, in both of these runs, that the percentage decrease in the adsorption per cubic centimeter is less than the per cent decrease per gram (Cases I11 and Iy). This should be true for all those instances in which the particle shrinks in siw as a result of the true carbon density increasing and probably loses a few of the smallest pores by virtue of partial graphitization. Effect of impregnation on heating charcoal in a stream of steam, hydrogen, nitrogen, OT a nitrogen-oxygen mixture It should be realized a t the outset that the specificity of the various impreg-
1304
JAMES HOLMES AND P. H. EMMETT
nating agents as regards their influence on the action of the various gases is much less marked and definite than is the general character of steaming, hydrogenating, or oxidizing the samples in the absence of impregnating agents. Frequently, a given impregnating agent has a very different effect on one charcoal than on another; still more eonfusing is the fact that a given impregnating agent differs in its activity on a particular process as a function of the particular chemical reaction by which it was deposited on the charcoal. Examples of this will appear in the following detailed discussion : ( I ) Fen03 was used as an impregnant only in runs with samples CWSX 19 and CWSN S5, the two charcoals prepared by zinc chloride activation. Of the nineteen runs made, one was with steaming, nine with hydrogenation, eight with either air or tank nitrogen, and one with pure nitrogen. A few generalizations appear possible for the runs, though much more work would be required to make clear all of the details of the manner in which and the mechanism by which iron oxide alters the effects produced by steaming, by hydrogenation, and by the other gas treatments used. Three characteristics of the use of ferric oxide seem rather definite. In the first place, it, seems certain that ferric oside catalyzes the hydrogenation of CWS S 19 and CWSS 55. For esample, curve 4 of figure 8, and curve 5 of figure 15 clearly show a very drastic pore alkration produced by hydrogen at 600°C. in the presence of 5 per cent ferric oxide. Without this impregnant, hydrogen has practically no action on these charcoals at OOO°C., whereas with the impregnant a 30 per cent weight, loss can be brought about in less than an hour. Secondly, n.ith one exception (curve 3, figure 11), all runs using ferric oxide as impregnant resulted in an increase in slope in the C D region corresponding to the building up of pores in the size range 70-1800 A. in diameter (figures 4,8, 10, 11, 15, and 1G). Finally, in a run with tank nitrogen at 1000°C. to a 13 per cent weight loss, there was obserT7ed a 20-40 per cent decrease in adsorption on both a weight and a volume basis (curve 1, figures 11A and 11R), together with the development of a marked slope in the BC and CD section of the isotherm. This is one of several examples (curve 3, figure 20) of such an effect in the work on implegnation. The effect is consistent with 'the conclusion that in some way iron blocks off a number of the small pores (Case 111) and at the same time converts ot,her small pores into large ones (Cases V and VI). In a general way, it may be said that ferric oxide tended to drop the adsorption on both a per gram and per volume basis at relative pressures loiver than 0.4. As a matter of fact, practically the only treatment that would cause the A B region of the adsorption to decrease on CWRS 19 on a gram basis was impregnation with ferric oxide followed by treatment vith steam, hydrogen, nitrogen, or a nit,rogen-oxygen mixture. Concentrations of impregnant over t,he entire range from 0.2 to 5.0 per cent were effective, though the catalytic effect of the iron on the reaction of gases with the charcoal increased as a rule with the amount present. (2) CrfOa was tried as an impregnating agent for all four charcoals. Its influence is rather different for the various charcoals, so that the results can most effectively be discussed for each charcoal separately.
ALTERATION OF PORE SIZE IN CHARCOALS
1305
On CWSN 19, chromic oxide produced comparatively little effect on steaming.
A comparison of curves 2 and 6 of figure 3 with curve 1 of figure 2 illustrates the similarity between steaming results with and without chromic oxide. In the other four curves in figure 3, however, it is evident that under some conditions of impregnation, the chromic oxide induces an increase in slope in the isotherms in addition to increasing the absolute adsorption per gram of charcoal. On hydrogenation, chromic oxide appears to eliminate the small pore development that characterized hydrogenation in the absence of impregnant and instead catalyzes the conversion of small pores into those showing adsorption in the CD region, as well as to those that are too large to permit capillary condensation even a t 0.99 relative pressure (figure 6, curves 2 and 3.) On charcoal CWSS 55, chromic oxide appears to catalyze the conversion of small pores into larger ones by steaming (Cases V and VI). This is indicated by the decided slope that is given to the isotherms, in contrast to the comparatively flat isotherms obtained on samples that were steamed without impregnation. (Compare curves in figure 13 with curves 3 and 4, figure 12.) The effect of chromic oxide on the hydrogenation of CWSS S5 (curves 2 and 3, figure 14) can perhaps best be described by saying that it appears to catalyze the same t,ype of particle sint.ering and pore plugging (Cases I11 and IV) that occurs when a sample is heated in a,n inert gas to 1200°C. (curve 5 , figure 12). In addition to this effect, chromic oxide apparently promotes the attack by the hydrogen on small pores with their conversion into those having condensation in the CD region, and a t even higher relative pressures (Cases I1 and VI). On coconut charcoal, impregnation with 0.2 per cent chromic oxide caused, on steaming, nearly a 100 per cent increase in the adsorption up to 0.4 relative pressure on a weight basis, a 30 per cent increase on a volume basis, and a 15 per cent slope increase in the CLI region (figures 17A and 17B, curve 4). Strangely enough, steaming a sample impregnated with 5 per cent chromic oxide caused very little change in the amount of adsorption from that, of straight stearning, although the slope in the BC and C D regions was 10-20 per cent greater after steaming in the prescnce of the chromic oxide (figure 17, curve 5 ) than in its absence. Possibly this contrast between the behavior of 5 per cent and 0.2 per cent chromic oxide could result from the larger chromic oxide impregnation superimposing a 40 per cent plugging effect (Case 111) on top of an initial increase in pore area and volume analogous to that caused by 0.2 per cent chromic oxide. Hydrogenation of coconut charcoal impregmted with 0.2 or 5 per cent chromic oxide (figure 18, curves 2 and 3) caused none of the slope change characterizing similar runs on @WSN19 or CWSN 55,the slope of the isotherms being identical with that of the original charcoal; in fact, the marked upturn of the isotherm between 0.9 and 0.99 produced by straight hydrogenation is absent when chromic oxide is present. It should be noted, however, that (curves 2 and 3, figure 18R) the chromic oxide actually was influencing the hydrogenation. For example, as a result of hydrogenation in the presence of 5 per cent chromic oxide, the adsorption decreased about 5 per cent per gram and 20 per cent per cubic centimeter. (Case 11). It would seem, accordingly, that considerable attack on the
1306
JAMES HOLMES AiiD P. R. EMMETT
small pores took place with a conversion of many of them to pores too large to be detected by adsorption to 0.99 relative pressure (Case 11). PCI P58 impregnated with chromic oxide behaved little differently on steaming from samples not impregnated as regards the slope of the isotherms; however, the larger ( 5 per cent) chromic oxide content caused a drop in the absolute adsorption of 15 or 20 per cent on both the weight and volume basis (curve 3, figures 19.4 and 19B, compared to curve 1) (Case 111). Hydrogenation after impregnation with chromic oxide produced the same slope increase in the C D region that was obtained without any impregnation; the total pore volume of this charcoal, honrever, was decreased by the impregnation both on the weight and volume basis (figures 20A and 20B,curve 3). As a matter of fact, the 60 per cent decrease in adsorption on both a weight and a volume basis and the slope change caused by 5 per cent chromic oxide in the hydrogenation experiments suggest that considerable pore plugging (Case 111) by the impregnant occurs, together with conversion of small pores into those in the CD region. (3)NiO on CWSN 19 produces (curve 3, figure d), on steaming, no change in slope of the isotherms, just as was true of straight steaming; however, it appears t o drop the total adsorption on both a volume and a weight basis by about 35 per cent, compared to the volume of adsorption after steaming in the absence of nickelous oxide. This behavior might be produced by an extensive pore plugging (Case 111)superimposed on the usual new pore formation that is characteristic of straight steaming (Case I). It also might be considered as an esample of Case 11. For hydrogenation, the nickel results are especially noteworthy in that they show a decided specificity depending on the particular nickel salt used in impregnation. For example, if the nitrate is used and precipitated by ammonium hydroxide, hydrogenation to a weight loss of 27 per cent results in a nitrogen isotherm that is a straight line from 0.4 to 1.0 relative pressure and that has a slope corresponding to a 20 per cent increase in the volume of adsorption over this pressure range (curve 3 in figures SA and 5B). In contrast to this, the same per cent nickelous oxide produced from nickel chloride plus ammonium hydroxide caused no change in slope and only a few per cent decrease in total adsorption compared to the untreated CWSN 19 (curve 4, figures SA and 7B). Actually, the nickelous oxide from nickel chloride appears to have very little effect other than to cause a slight pore plugging. On CWSN S5 in the single experiment in which nickelous oxide was precipitated from the nitrate by an excess of ammonium hydroxide and the samplr then hydrogenated at 1000°C. to a 42 per cent loss, there seems to have been little effect other than a decrease of about 15 per cent in the adsorption on both volume and a weight basis. Again the results are consistent with the nickelous oxide producing a pore plugging (Case 111)and at the same time eliminating the usual sharp upturn of the isotherm in the CD region that is characteristic of hydrogenation without an impregnant. (4) A few experiments were made using molybdenum sesquioside, sodium carbonate, or cupric oxide as impregnants. Xeither 5 per cent molybdenum sesquioxide nor 5 per cent sodium carbonate produced any change in slope on the isotherms of CWSN 19 on hydrogenation to a 26 per cent weight loss at 1000°C.;
ALTERATION ON PORE SIZE IN CHARCOALS
1307
this was also true of hydrogenation to a similar weight loss in the absence of an impregnant. However, the absolute volume of adsorption in the sample hydrogenated in the presence of the molybdenum sesquioxide was about 20 per cent less than it lvould have been in the absence of the impregnant; the absolute adsorption after the sodium carbonate treatment was almost exactly the same as it would have been without the sodium carbonate (figures 6A and 6B). The two experiments on steaming a charcoal containing cupric oxide produced markedly different results. On a CWSN 19 sampk containing cupric oxide steaming at 750%. to a 26 per cent loss produced no change in the slope of the isotherm but an absolute adsorption about 35 per cent smaller on both a weight and a volume basis than it would have been if steamed to a 43 per cent loss in the absence of cupric oxide (curve 4 of figure 4 compared to curve 1 of figure 2). In contrast to this, CWSS 55 containing cupric oxide on being steamed to a 38 per cent loss produced a marked change of its isotherm, the adsorption increasing 20 per cent between 0.4 and 0.99 relative pressure (curve 5, figure 13). CONCLUSIONS
On the basis of the work here reported, it seems that the following general conclusions are evident: 1. It is possible to tailor-make the pore distributions and pore sizes of charcoals almost at will by suitable combinations of steaming, hydrogen treating, partial oxidation, sintering, and impregnating with ferric oxide, chromic oxide, nickelous oxide, cupric oxide, molybdenum sesquioxide, or sodium carbonate. 2. Much specificity exists as to the action of the impregnating agents, as regards both the particular chemicals from which the oxide is produced and the behavior of one charcoal compared to another. 3. There is some indication that a mere change in the absolute density of cnsbon in a charcoal as a result of sintering it t o 1200°C. in an inert atmosphere may actually increase the ease of pore alteration by steaming at 750OC. REFERENCES B A H R A NJESSEN: D Ber. MB,1238 (1933). BEECK:Rev. Modern Phys. 17, 61 (1945). BRUNAUER, EMMETT, AND TELLER: J . Am. Chem. Sac. 60,309 (1938). DRAKE AND RITTER:Ind. Eng. Chem., Anal. Ed. 17,787 (1945). (5) EMMETT: Am. SOC.Testing Materials, Symposium on New Methods for Particle Size Determination in Subsieve Range l941,95. (6) HOLMES AND EMMETT: J. Phys. Colloid Chem. 61, 1262 (1947). (7) International Critical Tables, Vol. IV, p. 441. McGraw-Hill Book Company, Inc., New York (1928). (8) LAMB,WILSON,AND CHANEY: Ind. Eng. Chem. 11,420 (1919). (9) MCBAIN:Sorption of Gases and Vapours by Solids, pp. 58-79. G. Routledge and Sons, Ltd.,London (1932). (10) PIERCE, BLACET, AND JUHOLA: Part of the research on charcoal done for the National Defenae Research Committee; t o be published. AND BAUERMEISTER: J. Am. Chem. SOC. 6'7, 1242 (11) RIES, VAN NORDSTRAND, JOHNSON, (1945). (12) RITTERAND DRAKE:Ind. Eng. Chem., Anal. Ed. 17,782 (1945). (13) THOMSON: Phil. Mag. 141 4!& 448 (1871). (14) WABHBURN: Phyu. Rev. 17,273 (1921); Ptoc. Natl. Acsd. Sei. U. 6.7; 115 (1921). (1) (2) (3) (4)
