THE USE OF ADSORPTION ISOTHERMS FOR MEASURING THE SURFACE AREAS OF CATALYSTS AND OTHER FINELY DIVIDED MATERIALS.' ABSTRACT.

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1 A paper presented at the Seventy-first General Meeting, held at Philadelphia, Pa., May 1, 1937, Dr. E. L. Kropa presiding. THE USE OF ADSORPTION ISOTHERMS FOR MEASURING THE SURFACE AREAS OF CATALYSTS AND OTHER FINELY DIVIDED MATERIALS.' By P. H. EMMZrT 2 AND STEPHEN BRUNAUgR. 2 ABSTRACT. A method is described for measuring the surface areas of catalysts and of other finely divided substances by using low temperature adsorption isotherms of gases such as N 2, A, CO, 0 2, CO, etc. Representative isotherms for two promoted iron synthetic ammonia catalysts, pumice, Ni on pumice, and NiO on pumice are presented. In the case of the iron catalyst it is shown that, if the beginning of the linear portion of the isotherms is interpreted as the point of completion of a monomolecular layer, surface areas can be calculated that are approximately the same for all gases tried. Furthermore, the surface areas so calculated are large enough to accommodate the observed chemisorption of carbon monoxide at 183 C. and the largest irreversible adsorption of hydrogen observed between 78 and 100 C. Only in the case of oxygen at 183 C. is the volume taken up irreversibly by the catalyst several-fold greater than a monomolecular layer. Presumably oxygen reacts chemically with the outer layers of iron atoms even at 183 C. The method has so far been applied with apparent success to the catalysts and catalyst supports mentioned above and, in addition, to four other synthetic ammonia catalysts, two copper catalysts, glaucosil, two soils and two soil colloids, samples of dry powdered bacteria, chromium oxide gel, and chromium oxide gel that had been "glowed" at 400 C. In pursuing our studies of the various catalysts and catalytic processes pertaining to the fixation of nitrogen, we arrived at a point at which we wished to measure the surface area of a number of iron synthetic ammonia catalysts. A survey of the literature revealed that no method had been suggested that could be used for getting the abso- 1 Manuscript received January 16, ' Bureau of Chemistry and Soils, Washington, D. C. 383

2 384 P. H. EMMETT AND STEPHEN BRUNAUER. lute areas of such pyrophoric metallic catalysts, of various other catalysts, and of catalyst supports such as pumice. A search for some suitable means of measuring these surface areas has culminated in the development of a method, to be briefly described in the present paper, which involves the use of low temperature adsorption isotherms and appears to give accurate relative surface areas and approximate absolute areas. The principle of the method is simple. If one can choose on the low temperature isotherm of such a gas as nitrogen the point that corresponds to a monomolecular layer of adsorbed nitrogen molecules, then a multiplication of the number of molecules required for such a monomolecular layer by the cross-sectional area of each molecule will give the absolute area of the catalyst surface, subject only to the uncertainties in values for the molecular diameters and in the closeness of packing of the adsorbed molecules. In the following pages are presented typical physical and chemical adsorption data that have been obtained on iron synthetic ammonia catalysts, and physical adsorption isotherms for a number of gases on other catalysts and on catalyst supports. They will illustrate the way in which the measurements are carried out and will enable one to assess the value of low temperature isotherms for surface area measurements. RESULTS AND DISCUSSION. The details of experimental procedure have already been described elsewhere. 3 It will suffice to point out that standard adsorption technique has been used, the calibration of the adsorption bulb being made with pure helium. It should also be noted that corrections were applied for deviations of the various gases from perfect gas laws. The work may conveniently be divided into adsorption experiments made upon iron catalysts and those made upon other catalysts and finely divided materials. On the iron catalysts isotherms of some or all of the following gases have been studied at the centigrade temperatures indicated: N 2 and A at ; N2, A, CO and 0 2 at 183 ; CH 4 and NO at 140 ; N2O and CO2 at 78 : NH 3 at 36 ; and C4H 10 at 0. In the cases of CO and 0 2 it was always necessary first to saturate the iron catalysts with respect to irreversible sorptions of these gases at 183 C. before obtaining their physical isotherms. For NO and N 2O the iron catalyst was first saturated with oxygen at 183 C.; otherwise reaction between the NO and iron at 140 C., and N2O and iron at 78 C. prevented the determination of the s Emmett and Brunauer, J. Am. Chem. Soc., February (1937).

3 MEASURING THE SURFACE AREAS OF CATALYSTS. 385 respective physical adsorption isotherms. In the case of carbon dioxide, pre-saturation with carbon dioxide had to be carried out at 78 C. if the catalysts contained an alkaline oxide. The iron catalysts have all been prepared by the standard fusion process used in the preparation of iron synthetic ammonia catalysts. 4 Six such iron catalysts thus far investigated include one "pure" iron (unpromoted) catalyst, two doubly 251 NZ at Aat-183 4and 6 i^^ -CO at -183 (total) I7 ä 5 i^ 1/ Iand7-183 ( CO2z at -78 Z^^ at v E ö 7 Figure I. Catalyst 954 (10.2%AI20j) Series I IVU CUU iuu YVU ^VV OVV IvV Pressure mm Fin. 1. Low temperature adsorption isotherms for 44 g. of singly promoted iron catalyst 954. promoted catalysts containing both Al203 and K20, two catalysts promoted with Al20 3 and one containing only K 20 as promoter. The other materials on which isotherms have so far been run include two metallic copper catalysts, pumice, nickel oxide supported on pumice, nickel supported on pumice, glaucosil, several soils and soil colloids, 5 4 Larson and Richardson, Ind. Eng. Chem., 17, 971 (1925). 5 The work on two soils and two soil colloids will soon be submitted to Soil Science for publication by the authors and Miss Katherine Love; that on the powdered bacteria is described briefly by Dean Burk in a forthcoming article in the Bach-Festschrift number of Biochimia (Moscow). It will be presented in more detail in a separate article soon to be published by, Hans Lineweaver. Isotherms of all other gases and solids mentioned in the present paper but not shown here in detail are being submitted as a series of detailed articles to the J. Am. Chem. Soc.

4 386 P. H. EMMETT AND STEPHEN BRUNAUER. dry powdered bacteria, chromium oxide gel and chromium gel that had been "glowed" by being heated above 400 C. In this second group of materials the isotherms are for the most part those of nitrogen and argon at 183 C. The results obtained on iron catalysts can be illustrated by Fig. 1 for singly promoted iron catalyst 954 containing 10.2 per cent Al203 and by Fig. 2 for doubly promoted catalyst 931 containing 1.3 per cent 9( 91 mi 71 ä 61 I- 5 SI c ä41 E > 3 pressure mm FIG. 2. Low temperature adsorption isotherms for 45 g. of doubly promoted iron catalyst 931. Al203 and 1.59 per cent K 20. The solid lines in the two figures show the physical isotherms of various gases at temperatures at or near the normal boiling points of the gases. The isotherms drawn with dotted lines represent the total adsorption of CO and CO2 (chemisorption plus physical adsorption). Let us now consider the shapes of the isotherms and the possibility of choosing a point on them that may reasonably be interpreted as a monomolecular layer. In general, the isotherms may be regarded as S-shaped curves in which the central portion connecting the two curved extremities is, over I

5 MEASURING THE SURFACE AREAS '0 CATALYSTS. 387 a considerable pressure range, linear within experimental error. The curves at 183 C. for nitrogen and for carbon monoxide are linear throughout a very large pressure range; they do not show the curved region at the high pressure end because the curvature does not begin at as low a pressure as 760 mm., the maximum used for the isotherms in the two figures. On the experimental adsorption isotherms there appear to be five points that can be picked with accuracy and reproducibility and that might be useful for surface measurements in the event that a correspondence to a monomolecular layer or definite multiple thereof could be shown. They are designated as points A, B, C, D and E on the curve for nitrogen in Fig. 4; the surface-measuring methods based on the use of these points will, for convenience, be referred to as methods A, B, C, D and E. A sixth possible point would be the one at which the actual isotherm meets the saturation pressure Ps of the gas being adsorbed; it is omitted from consideration because of the almost insurmountable experimental difficulties involved in measuring it and our conviction that it corresponds to much more than a monomolecular layer. Points A, B, C and E appear to have at least some semblance of theoretical significance in connection with choosing the point on the isotherms corresponding to a monomolecular layer. If, as postulated in a previous paper, the linear portion of an isotherm corresponds to the building up of a second layer of molecules, then A, the zero pressure intercept, and B, the actual beginning of the linear portion, represent possible logical choices of points at which the first layer might be just completed. C might conceivably represent such a point if the isotherm were regarded as a smooth curve with no straight-line portion, C then being the point of inflection between the two curved ends of the S- shaped curve. E is the value of the linear portion of the curve extrapolated to the saturation pressure P. It would seem to be an approximate representation of the volume of gas in both the first and the second completed layers if it could be interpreted as representing the point at which the second layer would be complete if the formation of the latter would remain linear throughout, and if no third or higher layers would form at pressures below P. D marks the upper extremity of the linear portion of the curve. Let us now direct our attention to possible criteria by which we may differentiate between the various methods A, B, C, D and E for interpreting the low temperature isotherms in terms of surface areas.

6 388 P. H. EMMETT AND STEPHEN BRUNAU)rR. In the absence of any theoretical arguments that could be adduced for judging the point of formation of a monolayer, it is necessary to resort entirely to experimental correlations between the physical adsorption isotherms of various gases and certain chemisorptions that were encountered in the course of the present work. In particular it seems that two experimental criteria may be used. In the first place, the surface area obtained by choosing the monomolecular layer point on the isotherm of one gas by any particular method must be, in general, the same as that obtained by choosing a similar point on the isotherm of any other gas on the same catalyst. In the second place, the surface area calculated from the low temperature isotherms must be ample to account for all activated adsorptions or chemisorptions that do not appear to be complicated by chemical reaction with the underlying layers of the catalyst. A comparison of methods A, B, C, D and E with a view to ascertaining which of them seems the most reliable measure of the catalyst surface 6 indicates the general superiority of method B. For example, when method B is applied to the isotherms in Fig. 1, one finds that the maximum deviation of the surface area calculated and derived from any one isotherm is about 7 per cent from the average area determined by all the isotherms of this figure. For the curves in Fig. 2 the corresponding maximum deviation is about 8 per cent; for the isotherms on all six of the iron catalysts the largest maximum deviation from the average on a given catalyst was about 12 per cent by this method. Method A gives decidedly worse agreement, as a whole, than any of the other four methods, the maximum deviation on a given catalyst ranging from 7 to 28 per cent. Method B has a maximum deviation from the mean of as much as 27 per cent in some cases, though it compares favorably with method B if the isotherms for carbon dioxide at 78 C. are omitted. Neither method C nor D has a maximum deviation from the mean greater than 20 per cent for any of the isotherms, though the agreement is, as a whole, decidedly poorer than that of method B. All comparisons mentioned so far have been on the basis of molecular areas from the solidified gases. Molecular areas from the liquefied gases also give similar indication of the superiority of method B, but In calculating numerical values for the surface areas, it is necessary to have a value fot the area occupied by each physically adsorbed molecule. Such areas might be obtained by using molecular diameters calculated from the density of either the liquefied gas or the solidified gas. Assuming close packing among the adsorbed molecules one obtains the following areas per molecule in square angstroms from the densities of the liquefied (L) and the solidified (S) gases respectively: N (S), 17.0 (I, at 183 C.), 16.2 (I, at 195 C.); OZ (S), 14.1 (L); A-12.8 (S), 14.4 (L); CO-13.7 (S), 16.8 (L); CO (S), 17.0 (I,); and CiH (S), 32.1 (I,).

7 MEASURING THE SURFACE AREAS OF CATALYSTS. 389 the maximum deviation from the mean is decidedly larger than with solidified gas areas, being for Fig. 1 and 2, respectively, 17 and 11 per cent, rather than 7 and 8 per cent. Turning now to a consideration of the various chemisorptions, we find that method B appears to lead to a surface area ample to accommodate both the activated adsorption of hydrogen and chemisorption of carbon monoxide that have been noted. In several recent publications we have pointed out a very peculiar and, from our point of view, very fortunate chemisorption of carbon monoxide on iron catalysts at 183 C. Within a few minutes after exposing a completely reduced and carefully evacuated iron catalyst to carbon monoxide at 183 C. and at a few mm. pressure, a chemisorption of carbon monoxide on the iron completes itself. Additional irreversible adsorption does not occur at pressures up to 760 mm. over a period of several hours. Furthermore, the chemisorbed carbon monoxide does not pump off at either 183 or 78 C. These characteristics of the CO chemisorption have been interpreted 7 as indicating that CO forms a complete monomolecular layer of chemisorbed gas on iron at 183 C. practically instantaneously. It, therefore, becomes possible in the case of pure iron catalysts to compare the volume of the chemisorbed CO with the volume of CO required to form a monomolecular layer of physical adsorption. Such comparisons indicate that on a sample of a pure iron catalyst the maximum 183 C. chemisorption of CO was 13 per cent greater than that required according to method B. However, at 78 C. some additional chemisorption of CO occurred. If this is assumed still to be a part of the monomolecular layer, then the chemisorption at 78 C. is 33 per cent greater than the physical adsorption corresponding to point B on the physical isotherms. It is difficult to obtain a reliable estimate of the density of packing of CO molecules that are chemically bound to the surface iron atoms. However, if one makes the reasonable assumptions that one such CO molecule is adsorbed on each surface iron atom and that the faces of the iron crystallites are 100 planes, then the average area per CO molecule would be 8.18A. 2 Therefore, one might expect the carbon monoxide chemisorption to be as much as 68 per cent greater than the volume of carbon monoxide required for a monomolecular layer of physical adsorption, whereas actually it was only 33 per cent greater. By the same reasoning, if the activated adsorption of hydrogen is molecular and occurs on the same iron atoms that are capable of chemisorbing carbon monoxide, +Emmett and Brunauer, J. Am. Chem. Soc., February (1937).

8 390 P. H. EMMETT AND STEPHEN BRUNAUER. the activated adsorption of hydrogen might exceed by 68 per cent the volume of physically adsorbed carbon monoxide required for a monolayer; actually it is approximately the same on the pure iron catalysts as the volume of carbon monoxide that can be chemisorbed at 78 C. or below. Furthermore, if the small size of the hydrogen molecule and the possible, detailed spacings of the surface iron atoms are taken into consideration, it would not be surprising if the maximum volume of activated adsorption of hydrogen were even twice as large as the chemisorption of carbon monoxide. Apparently, then, method B leads to a surface area ample to accommodate the irreversible adsorptions of both hydrogen and of carbon monoxide. It should be emphasized at this point that only in the case of the "pure" iron catalysts (unpromoted) have we been able to make use of the low temperature (- 183 C.) chemisorption of carbon monoxide to confirm conclusions drawn from the physical adsorption isotherm as to the volume of gas needed for a monomolecular layer. All promoted catalysts chemisorb considerably smaller quantities of CO than are equivalent to the formation of a monomolecular layer. Thus in the case of catalysts 954 and 931, whose isotherms are shown in Fig. 1 and 2, the chemisorption of CO at 183 C. amounts to 10.2 cc. and 50 cc. respectively, though the volumes of physically adsorbed CO necessary for a monomolecular layer according to method B are 38.4 cc. and 130 cc. For those catalysts promoted with K 20, either with or without Al2O3i an explanation for this has already been advanced.' The alkali presumably accumulates on and covers a large fraction of the catalyst surface. The chemisorption of carbon monoxide is, therefore, small since only a small fraction of the total surface consists of iron atoms; in agreement with this the chemisorption of carbon dioxide on these alkali-containing catalysts is large, corresponding to the covering of a large fraction of the surface by alkali. Furthermore, on the catalysts containing 10.2 and 1.03 per cent Al203, the chemisorption of CO is smaller by 55 and 35 per cent respectively than the volume of CO needed for a monomolecular layer of physical adsorption. This is interpreted as indicating that Al2O3 promoters likewise accumulate on the surface of iron synthetic ammonia catalysts; in the two cases mentioned, at least 55 and 35 per cent of the surfaces are covered with Al2O3, or some compound of the latter with iron that is not capable of chemisorbing carbon monoxide. 8 Emmett and Brunauer, J. Am. Chem. Soc., February (1937).

9 MEASURING THE SURVACE AREAS or CATALYSTS. 391 The one remaining irreversible gas sorption on the iron catalysts that should be mentioned is that of oxygen. In this instance it is well to bear in mind, of course, that chemical combination throughout the catalyst might well be occurring. Experiment indicates that such is the case, for at 183 C. the iron catalysts take up instantaneously and irreversibly a volume of oxygen that is five to ten times as great as the volume of carbon monoxide that they are capable of chemisorbing at 183 C. Such an amount of oxygen can hardly be accounted for on any other basis than that of reaction with several layers of iron 3! nnnnnnn 3( Fp! N nn 21 a.ö ö It Bn_n_nn_ I( IL91^1^IYl:i wu Luu avu 41)5 auv buv (us 055 Pressure mm. Fin. 3. Low temperature adsorption isotherms for nitrogen and argon on 11 g. of pumice, 11 g. of pumice supporting 1.24 g. of NiO, and 11 g. of pumice supporting 0.98 g. of Ni. atoms, for it is several-fold too large to be chemisorbed in a monomolecular layer, even if the latter corresponded to point D on the isotherms. So far as the iron catalysts are concerned, the combined physical and chemical adsorption data may be said, therefore, to point to the following conclusions: (1) the interpretation of point B on the low temperature adsorption isotherms as corresponding to a monomolecular layer leads to a more nearly constant value for the catalyst surface area as determined by various gases than if points A, C, D or E are chosen; (2) the chemisorptions of carbon monoxide and of hydrogen are both consistent with the areas calculated from point B of the low temperature isotherms, provided reasonable assumptions are made as to

10 392 P. H. EMMETT AND STEPHEN BRUNAUER. the closeness of packing of the irreversibly adsorbed H 2 and CO. The irreversible taking up of oxygen appears definitely to be a chemical reaction with several of the outer layers of iron atoms. (3) The use of molecular diameters calculated from the solidified gases gives somewhat closer agreements among the calculated surface areas than the use of molecular diameters from the liquefied gas. Let us now consider the results obtained on attempting to apply the low temperature isotherm method to materials other than iron synthetic TABLE I. The Surface Areas of a Number of Typical Catalysts and Catalyst Supports as Determined from the Adsorption Isotherm for Nitrogen at 183 C. Substance Surface Area in Sq. Meters per g. Iron catalyst 973 (0.15% Al2O3) Iron catalyst 931 (1.59% K20, 1.3% Al203) Iron catalyst 958 (0.35% A1sO3, 0.08% K20) Iron catalyst 424 (1.03% AlO3) Iron catalyst 954 (10.2% A1303) Iron catalyst 930 (1.07% K.20) Pumice Pumice supporting NiO (36.6) Pumice supporting Ni (12.3) * Copper catalyst (from fused CuO) Copper catalyst (from commercial CuO) Glaucosil Chromium oxide gel (before glowing) Chromium oxide gel (after glowing) ' The value in parentheses is the surface area per gram of NiO in the one case and per gram of Ni in the next. ammonia catalysts. In Fig. 3 are shown isotherms for the adsorption of nitrogen and of argon on pumice, pumice-nio, and pumice -Ni. They suffice to illustrate the type of result obtained on non-ferrous materials. It is evident that the isotherms are identical in appearance with those on iron. If, therefore, one assumes that the beginning of the straightline portion of these isotherms (point B) corresponds to the completion of a monomolecular layer, just as it appears to do in the case of iron catalysts, we may calculate the surface areas of the various materials in square meters per gram. The values so calculated for these samples, together with those for six iron synthetic ammonia catalysts and for a number of other materials that have been studied, are shown in Table I.

11 MEASURING THE SURRACE AREAS OP CATALYSTS. 393 It is not possible at the present time to offer some reassuring theoretical considerations that would support the postulates that we have made in order to choose the point on the low temperature isotherms corresponding to monomolecular layers, though work is now in progress to develop such a theory.' We believe, however, that the selfconsistency among the surface areas calculated with the help of physical isotherms of different gases, and the agreement between the areas calculated from the physical adsorptions and those required by the aoo U, 300 ä I- U, E a zoo Aso A ioo ' Figure-4. N0 on Fe-AI =O Catalyst at -195.ß C. C CCI4onHgat11C. accordin to Caasel) V.2^ v v_ 200 ö a E E 100 ti 50 IS CU 30 ao so ba 7o eo so i0 r/rs *Luu Fin. 4. A comparison between an adsorption isotherm for nitrogen at C. obtained in the present work on iron catalyst 954 and the adsorption isotherm for CC14 on liquid Hg calculated by Cassel [Trans. Faraday Soc., 28, 177 (1932)] from his measurements of the effect of CC14 vapor on the surface tension of mercury. chemisorptions, will suffice to make the method appear reasonable. One precedent for our interpretation of the linear part of the isotherm as the building up of a second layer of molecules may, however, be cited. Cassel9 published several years ago data from which the number of molecules of CC14 adsorbed per square centimeter on a liquid mercury surface could be calculated as a function of pressure. The data were obtained by applying the Gibbs equation to measurements of the decrease in surface tension of the mercury with an increase in the partial pressure of the surrounding CC14 atmosphere. On assigning to each adsorbed CC1 4 molecule an area calculated from the density of liquid CC1 4, he obtained the curve shown in Fig. 4. Included in the same figure is an isotherm that we have obtained for nitrogen on an Cassel, Trans. Faraday Soc., 28, 177 (1932).

12 394 DISCUSSION. iron synthetic ammonia catalyst at C. The similarity between the two curves is striking. The point of special interest for the present discussion, however, is the fact that the beginning of the linear portion of the CC14 isotherms coincides with the completion of a monomolecular layer of adsorbed molecules. In conclusion, we would like to reiterate that method B suggested in the present paper for measuring the surface areas of various finely divided materials appears to be a logical and consistent explanation of all of the adsorption data that we have obtained over the course of several years' work. A great deal more experimental work is needed, of course, before one can be certain that this particular method that we have chosen for interpreting the isotherms is necessarily the one giving the closest approximation to the absolute surface areas for solid adsorbents in general. From the results obtained so far, we do feel very confident that the use of low temperature adsorption isotherms of gases such as nitrogen, argon, etc., interpreted by method B affords an easy and reliable way of measuring catalyst surface areas and is destined to become increasingly useful in measuring the total inner and outer surfaces of a wide variety of finely divided substances. DISCUSSION. A. T. LARSONi : Dr. Emmett, have you made a direct comparison between the same catalyst prepared in different ways? For example, most of your work has been on the iron-ammonia catalyst; have you compared precipitated catalysts with fused catalysts containing the same promoters? P. H. EMMETT: That is a very good question, Dr. Larson, but I regret to say that we have not as yet made the comparison. The ample supply of synthetic ammonia catalysts that you so kindly left with us at the Fixed Nitrogen Research Laboratory was a very tempting source of material on which to start these experiments, so we have not tried precipitated catalysts up to the present time. N. K. CHANEv 11 : Dr. Emmett, have you applied your method to determine the surfaces of highly activated carbons? P. H. EMMETT: Of all the materials we have tried, the only one on which isotherms differed from those here described was charcoal. COLIN G. FINK' 2 : That is not anything even approximately definite, is it? P. H. EMMETT: In the case of charcoal one obtains isotherms that do not have a long linear portion that has characterized the isotherms of all other adsorbents tried. The particular sample of charcoal that we tried gave perfectly smooth isotherms that were not S-shaped and that agreed quite well with the Langmuir isotherm equation. Isotherms on one commercial purifying carbon that we have studied, however, are of the usual S-shape. D. B. KEYEs 13 : Have you described your apparatus and technique since 1934? P. H. EMMETT: The apparatus and technique are not essentially different from the descriptions given in the 1934 paper. We have therefore not presented any new, detailed description of either of them. 10 Ammonia Dept., B. I. dupont de Nemours & Co., Inc., Wilmington, Del. li Director of Research, United Gas Improvement Company, Philadelphia, Pa. ' Head, Division of Electrochemistry, Columbia University, New York City. 13 Dept. of Chemistry, University of Illinois, Urbana, Ill.

13 MEASURING THE SURFACE AREAS OF' CATALYSTS. 395 N. K. CUANEY : I am rather interested in that statement about the characteristics of charcoal as compared with other materials, because the method which we used to determine what we call the retentivity of charcoal, that is, the amount of a given vapor which will be held at low pressure as compared with the larger quantities of vapor which can be held in the same charcoal by capillary absorption, is based upon the rate of desorption with time at a specified pressure of, say, 2 mm. of mercury. These curves are indicative of what happens in the early period of adsorption. That is, with a vacuum apparatus of sufficiently large capacity, the desorption curves would straighten out and become linear after complete removal of the vapors loosely held by capillary forces. The measure of "retentivity" or surface adsorption was obtained by simply extrapolating this linear curve of the residual vapor content vs. time back to zero time. Whether or not the values so obtained represent a monomolecular layer, or a series of layers, their behavior indicates that they are a measure of specific surface forces rather than capillary phenomena. This is further confirmed by technical experience which shows that these retentivity values have an industrial significance not possessed by total or capillary absorption values. E. C. WILLIAMS 14 : I realize that this is only one phase of the authors' work intended to show the physical arrangement of the catalyst and that equilibrium measurements are a valuable guide. Have the authors any additional information yet on the correlation between equilibrium measurements under static conditions and the performance of the catalyst under conditions of flowing gas streams? Such correlation would make these determinations of particular practical interest. COLIN G. FINK: From an industrial point of view. E. C. WILLIAMS: Yes. P. H. EMMEr'r: I cannot yet say what fraction of the inner surface of a particular catalyst particle is actually utilized when a gaseous mixture passes over it. We are planning experiments to throw additional light on this question. A partial answer to your question can perhaps be given. A comparison of the surface areas and the known activities of the catalysts used in the present experiments is given in Table I. The presence of the alkali in the doubly promoted catalyst unquestionably enhances the activity per unit area of iron. For, although a catalyst promoted with both potassium oxide and aluminum oxide had a surface area only about one-third as large as one promoted with aluminum oxide alone, it was much superior in activity to the catalyst promoted with aluminum oxide. To this rather incomplete answer to your question I might add one additional bit of information. It is evident that the geometric surface of a 40-mesh sample of a catalyst will be about four times as great as that of a 10-mesh sample. The question arises as to whether the activity, also, will be four times as great. The answer is very definitely "No!" The activity is not four times as great as on the 40-mesh sample; in fact, there is no appreciable difference in the activity of the 40- as compared to the 10-mesh sample. This seems to indicate that either the entire inner surface of the catalyst or else a definite fraction of that surface is utilized in a catalytic reaction. A. T. LARSON: On that last point, Dr. Emmett, I should like to ask if, in comparing different mesh sizes, the difference in the surface area which you just mentioned is essentially a calculated difference. The point I am trying to make is that it is quite conceivable that, when you take a tube of four-mesh material and a tube of 20-mesh material, actually the surface of the free space is essentially the same. Consequently your deduction, if that should prove to be the case, is not necessarily correct. Personally, I tend to agree with you, but not for the reason you have given. P. H. EMMETT: I neglected to say that the above experiment was carried out in conjunction with measurements of the total areas of the 40- and 10-mesh samples. The total "inner" surface areas were found to be the same on the two samples even though the geometric or outer surface of the 40-mesh sample is presumably four times as great as that of the 10-mesh sample. 14 Shell Development Company, Emeryville, Calif.