CLAY MINERALS BULLETIN

Transcription

1 CLAY MINERALS BULLETIN DECEMBER, 1959 Vol. 4, NO. 22 A GAS ADSORPTION METHOD FOR RAPID COMPARISON OF SPECIFIC SURFACE AREAS By J. M. HAYNES Research Laboratories, English Clays Lovering Pochin & Co. Ltd., St. Austell, Cornwall. [Read 7th November, 1958] ABSTRACT A single measurement of the adsorption of atmospheric air on a dried, finely-divided solid at - 183~ carried out in a simple apparatus which does not require a high vacuum system, can be related empirically to the specific surface as determined by the Brunauer, Emmett and Teller procedure, with, generally, less than l0 per cent. error. The method is rapid and simple, and therefore suitable for routine use. INTRODUCTION Many of the chemical and physical properties of finely-divided materials which are of practical importance are properties arising at the solid/gas or solid/liquid boundary. For this reason, the determination of the extent of the interface is frequently required, and several methods, depending on a variety of types of measurement, have been devised for the estimation of specific surface. Direct geometrical measurement is possible in a few instances, using either optical or electron microscopy (Heywood, 1933). In other instances, the steady-state resistance to fluid flow in a packed bed of powder (Carman, 1938, 1939; Lea and Nurse, 1939; Rigden, 1943), or the rate at which that steady state is approached (Barrer and Grove, 1951a, 1951b), can be described as a function of surface area. Most other methods rely on adsorption, either from solution--e.g., dyestuff adsorption (Plesch and Robertson, 1948; Ewing and Liu, 1953) and the negative adsorption of anions (Schofield, 1948)--or, more often, fr0m the gas phase (Harkins and Jura, 1943, 1944b). The latter class includes the method devised by Brunauer, Emmett and Teller (1938)--generally known as the B.E.T. method--which has been widely accepted as a standard because of its reproducibility and self-consistency. 69

2 70 J.M. HAYNES It should be pointed out that gas adsorption measurements of this type are only claimed to give relative values of specific surface; however, certain other methods, claimed to be absolute (e.g., Harkins and Jura, 1944a), give results which strongly support the validity of the B.E.T. method. From the point of view of routine surface area measurement, all the methods listed above suffer to some extent from one or more of the disadvantages of elaborate apparatus, slowness, and the need for skilled experimental work, In many applications, some loss of reproducibility can be tolerated if a simple, cheap and rapid method can be devised. The following method, using the low temperature adsorption of air, has been developed to meet these requirements. METHOD The apparatus (Fig. l) consists of a vertical U-tube mercury manometer with arms of length about 60 cm, connected by means of a standard ground-glass joint to a sample-bulb. The bulb can be cooled to - 183~ by immersion to a standard depth in boiling liquid oxygen contained in a Dewar flask. When the bulb, initially containing only air at atmospheric pressure, is cooled, the pressure falls by an amount Apo (measured to the nearest 0.5 mm by means of a scale attached to the manometer) which depends principally on the dimensions of the apparatus. If the bulb contains a weight w of an adsorbent solid, the observed pressure drop Ap will be greater than alp0 due to depletion of the gas phase by adsorption on the solid. For any given material, it is found experimentally that the increased pressure drop is a function of the weight of material taken, and in fact, a plot of Ap against w is a straight line, with an intercept of Apo at w--0 (Fig. 2). It is also to be expected that the pressure drop will depend on the surface area.available for adsorption. A plot was therefore prepared of(dp - Apo)/w, obtained from the slopes of straight lines such as those shown in Fig. 2, against S, the specific surface in square metres per gram. Various adsorbents were used, including clays, aluminas, silica gels and mixed alumina-silica gels. Their specific surfaces, determined by the B.E.T. procedure from measurements of oxygen and nitrogen adsorption made in a conventional volumetric apparatus at -183 and -195~ were in the range from 7 to 250 m2/g. The points were fitted by a straight line through the origin, with a maximum deviation of less than 10 per cent. (Fig. 3). This graph

3 71 COMPARISON OF SURFACE AREAS then serves as a calibration graph for the apparatus, the specific surface being determined from the relationship S=K Ap --Apo (1) w Fie. 1--Apparatus for rapid determination of specific surface by air adsorption. The value of the constant K depends on the dimensions of the apparatus. For comparison purposes, a number of specific surfaces were measured by both the B.E.T. and the rapid methods. The results showed consistent agreement to better than 10 per cent., but it was

4 72 J.M. HAYNES noticed that those of the rapid method tended to be lower. This was found to be due to the presence of pre-adsorbed water on the surface of some samples, this apparently inhibiting the subsequent lowtemperature adsorption of permanent gases in proportion to the amount of water present. The effect, which was confirmed by measurements in a volumetric apparatus, varied in magnitude from one sample to another and is presumably due to blocking of very small pores, since the reduction in accessible surface can exceed the area actually covered by water. This source of error was removed completely by adequate drying of samples and by minimizing contact with a humid atmosphere prior to making measurements. It is not necessary to dry the air in the apparatus, since its volume is such that its water vapour content cannot even at high relative humidities exceed a small fraction of 1 per cent. of the normal sample weight used J ~" 300 ~ ~o 7" J ~--3~ ~,~',,g ~ 200 "q 2C / ~ ' 0 0" w~ g adsorbent FIG. 2- Relationship between pressure drop, Ap, and weight of adsorbent, w, for a powder with a specific surface, S, of 33 '5 mz/g. f f / Y v O ( Ap--Apo)/W, r FIG. 3--Relationship between specific surface, S, of various powders, as determined by the B.E.T. method, and the slope of the Ap-w curves for these powders. / With materials of low specific surface, for which a sample weight of I g or more may be necessary, it is advisable to correct Ap for variations in the volume of air in the sample bulb due to the volume occupied by the sample. For a small range of variation of effective bulb volume, it is a sufficiently good approximation to assume that plots of Ap against w for any one solid are parallel, but displaced vertically by the amount Apo varies as the bulb volume is changed. Apo was therefore measured as a function of effective bulb volume, by filling the bulb with various weights of non-adsorbing solids of known density, such as glass beads or coarse sand. The results

5 COMPARISON OF SURFACE AREAS 73 were used to prepare a graph from which the appropriate correction to Ap could be read off for any weight of sample of known true density. For materials of specific surface greater than about 10m 2/g the corrections are usually negligible compared with Ap. DISCUSSION The theory of the method is complicated by the use of a mixed adsorbate. However, an approximate analysis yields the following conclusions. (a) The plots of Ap against w and of (Ap-Apo)/w against S are approximately linear, irrespective of the exact shape of the adsorption isotherm, provided that this is of Type I, II, or IV of the classification of Brunauer (1945)--but it is highly unlikely that physical adsorption of the permanent gases at the temperature of liquid oxygen is of any other type. (b) The sensitivity of the apparatus is controlled mainly by the bore of the manometer tube, D, and by the volume, 1I, of the bulb and connecting tubing. Calculation shows that there is apparently no objection to extending the range of the apparatus to specific surfaces of 1 m2/g, or even lower, simply by making the apparatus smaller. The effect of varying D and V is shown by the following values of K (the constant in eqn. (1)) obtained in an apparatus with the approximate dimensions shown in Table 1. TABLE 1--Influence of dimensions* on sensitivity. D (mm) V(cmZ) P0 (cm Hg) K(m2/cm Hg) ' *Volume of bulb immersed in refrigerant =3cm :~ in eachcase. The smaller apparatus has been used successfully in the measurement of specific surface below 1 m Z/g. Any further improvement of the sensitivity would be difficult to achieve, since the correction for the sample volume becomes too large in comparison with Ap. CONCLUSIONS Comparison of specific surface areas by adsorption of air has the advantages of rapidity, simplicity, and cheapness, in applications where the high degree of reproducibility of the B.E.T. and other methods is not essential. It can be applied to materials of quite low

6 74 J.M. HAYNES specific surface, where the B.E.T. method requires special precautions. Since the method is empirical, calibration with materials of known specific surface is essential. Aeknowledgment.--The author is grateful to English Ciays Lovering Pochin & Co. Ltd., St. Austell, Cornwall, for permission to publish this paper, the work for which was carried out in their Research Laboratories. REFERENCES BARRER, R. M.,and GROvE, D. M., 1951a. Trans. Faraday Soe., 47, 837. Trans. Faraday Soe., 47, b. BRUNAUER, S., EMMETT, P. H., and TELLER, E., , 309. J. Amer. chem. Soe., BRUNA~ER, S., Adsorption of Gases and Vapours. Press, vol. l, p Oxford University CARMAN, P. C., J. Soc. chem. hld., Lond., 57, 225. CARMAN, P. C., J. Soc. chem. Ind., Lond., 58, 1. EWING, W. W., and LIu, F. W, J., J. Colloid Sci., 8, 204. HARKENS, W. D., and JURA, G., J. chem. Phys., 11,431. HARKINS, W. D., and JURA, G., 1944a. J. Amer. chem. Soc., 66, HARKENS, W. D., and JURA, G., 1944b. J. Amer. chem. Soc., 66, HEYWOOD, H., Proc. Instn. mech. Engrs., Lond., 125, 383, LEA, F. M., and NURSE, R. W., J. Soc. chem. Ind., Lond., 58, 277. PLESeH, P. H., and ROBERTSON, R. H. S., Nature, Lond., 161, RIGDEN, P. J., J. Soc. chem. Ind., Lond., 62, 1. SC,OFIELD, R. K., Trans. Brit. Ceram. Soc., 48, 207.