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JAMES HOLMES AND P. H. EMMETT
(17) KRAEMER: In A Treatise on Physica2 Chemistry, H. S. Taylor, Editor, Chap. XX, p. 1661. D. Van Nostrand Company, New York (1931). (18) MILLIGAN: Paper presented a t the Symposium on the Adsorption of Gases which was held under the auspices of the Division of Colloid Chemistry a t the 110th Meeting of the American Chemical Society, Chicago, Illinois, September 11-12, 1946. (19) RAO:J . Phys. Chem. 46, 500 (1941). JOHNSON, AND BAUERMEISTER: J. Am. Chem. SOC.67, 1242 (20) RIES,VAN NORDSTRASD, (1945). (21) ZsIGmioNDY: Z. anorg. Chem. 71, 356 (1911).
ISVESTIGATIOS OF LOW-TEMPERATURE NITROGEN ADSORPTIOK AT HIGH RELATIVE PRESSURES' JAMES HOLMES* AND P. H. EMMETTS Department of Chemical Engineering, The Johns Hopkins University, Baltimore 18, Maryland Received June 11, 1947 INTRODUCTION
I n connection with certain phases of the research on gas mask charcoals conducted by Division 10 of the National Defense Research Committee, it became apparent that pore size and pore-size distribution over the size range from 20 t o 160,000 A. diameter might be important factors governing whether a base charcoal mould be suitable or not as a catalyst support. The exact estent t o which the, distribution of large pores proved t o be important in such charcoals is, of course, restricted information and cannot be discussed here. Kevertheless, the present method of measuring large pores and some of the pore-size distributions found may be of interest t o the manufacturers and users of porous catalytic materials and supports. Although it \\'as realized that no entirely satisfactory method existed for obtaining pore-size distribution over this wide range, it was decided that lowtemperature nitrogen-adsorption isotherms could yield a good deal of information on the subject if they could be carried to sufficiently high pressures (1, 11, 14;7, see also paper on another porous glass (8)). The present paper describes the technique used in estending the nitrogen isotherms from a maximum relative pressure of about 0.98 usually employed up t o a relative pressure of 0.9995. The results here cited should, as a whole, be considered as preliminary and illustrative rather than final, since some of them were obtained before the final technique was perfected. Presented a t the Symposium on the Adsorption of Gases which was held under the auspices of the Division of Colloid Chemistry at the 110th Meeting of the American Chemical Society, Chicago, Illinois, September U-12, 1946. 2 Present address: Houdry Process Corporation, h'farcus Hook, Pennsylvania. Present address: Alellon Institute, Pittsburgh 13, Pennsylvania.
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EXPERIMENTAL
Materials investigated The work reported in this paper wm carried out upon a number of different charcoals representative of various commercial processes, upon a number of commercial carbon blacks, upon a special laboratory charcoal prepared by carbonizing an ethylidene chloride polymer, and finally upon samples of both porousglass and sized-glass microspheres. ( a ) Charcoals: The coconut shell charcoal was a standard product that had been prepared by a combination of calcination and steam treatment. P C I P68 is a coal charcoal prepared commercially from finely ground coal which had been briquetted, calcined, and, finally, activated with steam at temperatures up to 850-90O0C. The CFI charcoal is somewhat similar in its preparation to the PCI sample and is likewise a coal charcoal. Saran charcoal was prepared (10) by a suitable extrusion of an ethylidene chloride polymer, followed by a carbonization step. ( b ) Carbon blacks:4 Carbolac N o . 1 is a black of very intense color. It is prepared by the Godfrey L. Cabot Company by an intensive air after-treatment at 800-120O0F. of a fine-particle-size channel black. From electron-microscope pictures the average particle size appears t o be -
0
>
relative
0.95
0.97
0.99
I .OO
P/ Po FIG.3. Nitrogen-adsorption isotherm on Saran charcoal a t -195T. over the relative pressure range 0.95-1.00. Insert shows entire isotherm. Open circles and nniiqres represent adsorption points from t\\o separate runs.
ADSORPTION OF NITROGEN AT L O W TEMPERATURES
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( b ) Saran charcoal: A comparison of the normal adsorption isotherm carried t o 0.95 relative pressure with the Pearson gauge run carried t o nearly saturation, both of which are shown in figure 3, reveals that there is remarkably lktle adsorption above 0.95 p / p o , thus indicating an almost complete lack of pores having diameters in excess of 200 A. This conclusion was further strengthened by the results on mercury penetration6 (14), which indicated that there were virtually no pores having openings as large as 10,000 A. in diameter.
FIG.4 . Nitrogen-adsorption isotherm on C F I “CC” charcoal at -195°C. over the relative pressure range 0.95-1.00. Insert shows entire isotherm. Open circles, squares, and triangles represent adsorption points from three separate runs.
( c ) C F I c h ~ r c o a l :This ~ is another case of a sample in which there is an almost complete leveling off of the isotherm, indicative of the absence of large pores. An inspection of the isotherm for this material, shown in figure 4, leads t o the 6 Rlcasurements made by Corville Mace, Jr., as part of the charcoal research project carried out a t The Johns Hopkins University for the National Defense Research Committee. These will be described in a forthcoming publication by one of the authors (P.H.E.),summarizing the adsorption and pore-size studies carried out on charcoal for the National Defense Research Committee.
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JAMES HOLMES AND P. E. EMMETT
0.95
0.99
0.97
1.00
P/Po FIG.5. Kitrogen-adsorption isotherm on P C I P58 charcoal a t -195'C. over the relativc pressure range 0.95-1.00. Insert shows entire isotherm. Open circles and squares represent adsorption points from two separate rum. TABLE 1 VL
V,
SlufpLE
I
VOLUME OF GAS ADSOPBED A I 0.999
V,
VOLUME ADSORBED AT 0.999 5 / 9 0 CAICU*TED AS CC OF LIQUID
CC.
cc.
Pl?'Q I
Glass spheres (7 LA) Glass spheres (3-5 p ) Grade 6 spheronized carbon black Grade 6 agitated carbon black Carbolac No 1
I
cc.ia.
45.7 9.7
0.12 0.23
0.0715 0.01515
Ih Vm
-
__ 380 42
685
23 .O
1.07
30
1271 3350
23.0 230.0
1.99 5.25
55 14.5
54 11.5 0.352
2.36
45 42 44
* Values of apparent density were not taken on the glass spheres. The volume, V.. gvan for glass is calculated on the assumption of close packing of the glass spheres; hence VL/V. in column 8 for the glass spheres represents maximum values. conclusion that this sample has virtually no pores with diameters in the range 150-40,000 A. The lack of appreciable mercury penetrationBat pressures up t o
ADSORPTION OF NITROGEN AT LOW TEMPERATURES
1271
450 lb. per square inch agrees with this by indicating the absence of pores larger than about 4800A. ( d ) PCI P58 charcoal: This sample forms a striking example of a case in which extrapolation of the isotherm obtained up t o a relative pressure of about 0.95 in the conventional way would give a very false indication of large-pore distribution.
P/ Po
FIG.6.
Nitrogen-adsorption isotherm on Spheron Grade 6 carbon black at -195°C. over the relative preseure range 0.9(r1.00. Insert shows entire isotherm. Open circles, squares, trian gles and inverted triangles represent adsorption points from four separate runs.
As can be seen from figure 5 , such an extrapolation would indicate the absence of an appreciable number of large pores, whereas, in actuality, there is considerable porosity in very large pores, as indicated by the increased adsorption with pressure a t the higher relative pressures studied with the Pearson gauge as well as by mercury penetration experiments,' which also showed that there was a consider-
1272
JAMES HOLMES AKD P. H. EMMETT
able pore volume for pores having openings larger than about 10,000 A. in diameter.
i
600
400
$ 0 LL
0
23 J
0
>
200
FIG.7. Nitrogen-adsorption isotherm on Grade 6 carbon black a t -195%. over the relative pressure range 0.95-1 .oO. Insert shows entire isotherm. Open circles, squares, triangles, and inverted triangles represent adsorption points from four separate runs; solid symbols represent desorption points from same runs.
Non-porous adsorbents: 7 p glass spheres, 3-5 p glass spheres, Grade 6' Spheron, and Corbolac KO.1
l h e non-porous adsorbents, covering a size range from 0.005 to 7 microns, are all characterized by a large increase in adsorption at the higher relative pressures. It appears that for all of the following samples, condensation is taking place in the space between the particles. For comparison, the volumes of gas adsorbed at 0.999 relative pressure for the five non-porous solids is summarized in table 1, together with other pertinent data; the detailed isotherms for spheronized Grade G carbon black, unspheronized Grade 6 carbon black, and Carbolac KO.1, are shown in figures 6, 7 , and 8, respectively. ,7
ADSOItPTIOX O F XITROGEN AT LOW TEYPERATVRES
1273
The question naturally arises as t o whether the gas pickup at high relative pressure by these non-porous solids is t o be associated with multilayer adsorption or with capillary condensation between the particles. On the one hand, column 5 shows that the statistical number of layers that are adsorbed at a relative pressure of 0.999 if all of the adsorbate is assumed t o be held as multilayer adsorption ranges from 14.5 for Carbolac t o 380 for the 7 1 glass spheres. This range in the number of layers adsorbed appears a little unlikely. In this connection it should be pointed out that the apparent difference between the adsoiption on the 3-5 fi spheres and that on the 7 1 spheres is open to suspicion. If the adsorption isotherm for the smaller spheres (figure 9) were moved t o the left, 0.001 relative
P/ Po FIG.8. Nitrogen-adsorption isotherm on Carbolac S o . 1 a t -195°C. over the relative pressure range 0.95-1 .oO. Insert shows entire isotherm. Open circles, squares, and triangles represent adsorption points from three separate runs.
pressure unit, it would superimpose on the curve for the 7 p spheres (figure 10). This makes the results in figure 9 rather doubtful until checked. If, on the other hand, one assumes that the large amount of gas pickup at the higher relative pressure is due to capillary condensation in the space between the particles or groups of particles, one finds that the adsorption at 0.999 p / p , calculated as liquid is equivalent t o 42-54 per cent of the apparent free space except for the 3-5 sample of glass spheres. The apparent correlation between the free space between the particles and the amount of gas pickup lends weight t o the picture of capillary condensation. It must be realized, however, that if, as the Kelvin equation would indicate, the
P/ Po
FIG.9. Nitrogen-adsorption isotherm on 3-5 p glass spheres at -195°C. over the relative pressure range 0.95-1.00. Insert shows entire isotherm. Open circles and squares represent adsorption points from two separate runs; solid symbols represent desorption points rom the same runs.
P/ Po
FIG.10.Nitrogen-adsorption isotherm on 7 p glass spheres a t -195°C. over the relative pressure range 0.95-1.00. Insert shows entire isotherm. Open circles and squares represent adsorption points from two separate runs; solid symbols represent desorption points from the same r u k . 1274
ADSORPTION OF NITROGEN AT LOW TEMPERATURES
1275
space between the particles for condensation at a relative pressure of 0.999 is 18,000A., then for the capillary condensation explanation t o be valid, one has to postulate that about the same percentage of the free space is located in capillaries or regions 18,000 A. or less in diameter for the 35-40 A. particles of Carbolac as well as for the 70,000 A. glass spheres. This too seems a little improbable, even though admittedly the carbon blacks are very loosely packed and might contain many large concavities in the adsorption tube. The authors are inclined to favor the view that the large gas pickup is due t o capillary condensation between the particles but prefer t o defer further discussion until more work has been done. SUMMARY
An adaptation of a differential manometer is described for the measurement of relative pressures close t o unity. Without elaborate thermostating or other precautions, measurements of the adsorption of nitrogen at -195°C. have been carried out up t o relative pressures as high as 0.9995 with an accuracy as great as could be obtained with an ordinary cathetonieter and with greater simplicity. It is pointed out that even higher relative pressures could prpbably be obtained with nearly the same ease, provided the present apparatus were carefully thermostated. Measurements are described on porous glass and a number of charcoals in which there was almost complete absence of any further adsorption of nitrogen above a relative pressure of about 0.95. In terms of capillary adsorption this would indicate the absence of pores in these samples which had diameters of 200 A. or greater. This was further confirmed by mercury-penetration experimentsR in which it was also found that there was an almost complete absence of pores in the range 10,OOO to 100,000 A. Results on a number of carbon black samples indicate that a very large adsorption occurs above a relative pressure of 0.95. Most of this volume of nitrogen is believed t o be condensed in the spaces between the small particles or agglomerates. Measurements upon uniformly sized samples of glass microspheres indicate that, with these samples, all of the space between the spheres has not been filled even a t the highest relative pressures measurable with the present gauge. At a relative pressure of 0.999 between 42 and 54 per cent of the space between the particles appears t o be filled with liquid adsorbate for four out of the five nonporous solids used. REFERENCES (1) ANDERSON: Z.physik. Chem. 88, 191 (1914). (2) BEECK:Rev. Sci. Inatruments 6, 399 (1935). (3) BLOOMQUIST AND CLARK:Ind. Eng. Chem., Anal. Ed. 12, 61-3 (1940). AND TELLER: J. Am. Chem. SOC.60,309 (1938). (4) BRUNAUER, EMMETT, (5) COLUMBIAN CARBON Co : Ind. Eng. Chem , News Ed. 18, 492 (1940). (6) EMMETT: American SOC.Testing Materials, Symposium on New Methods for Particle Size Determination in Subsieve Range, 1941, 95. A N D CINES:J. Phya. Colloid Chem. 61, 1248 (1947). (7) EMMETT (8) EMMETT AND DEWITT:J. Am. Chem. SOC.66, 1253 (1943).
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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 D R A K E 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 t o 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.
