Studies of the pore structures of silica gels by electron microscopy and low-temperature nitrogen adsorption

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1 Studies of the pore structures of silica gels by electron microscopy and low-temperature nitrogen adsorption E. ROBIN SON^ Departrnerrt of Cl~emistry, College of Technology, Belfast, Northern Ireland AND R. A. Ross Department of Chemistry, Lakeliead University, Thunder Bay, Orrtario Received October 6, 1969 The pore structures of a number of silica gels have been examined by low-temperature (- 196 "C) nitrogen adsorption isotherms and these data compared with information obtained froni electron photomicrographs. Good general agreement was obtained in the comparison, although the electron photomicrographs reveal more details regarding pore arrangements and dimensional variations. Gel structures were modified by heating and also by treatments with superheated steam or nitric acid. The aggregation of ultimate particles is affected by the steam treatment and this phenomena is compared with the preliminary stages of accepted sintering mechanisms. Canadian Journal of Chemistry, 48,2210 (1970) Introduction Studies of the physical structure of silica gels aimed at deriving data on surface area and porosity are generally made by measurements of low-temperature gas adsorption/desorption isotherms (1, 2). Recently, detailed statistics of such isotherms (N, : "Qhave been reported for silica gels modified by various degrees of heat treatment (3). However, the adsorption method can often lead to uncertainties in interpretation of measurements, for example, in decisions regarding pore shapes (1). Hence it seemed to be valuable to make a direct examination of pore structures by electron microscopy using thin slices of silica gels, and at the same time compare these results with those obtained by nitrogen adsorption. Previous electron microscope studies have confirmed that silica gel has a corpuscular structure (4), while good agreement has been obtained in studies of particle size distributions by various classical methods when compared with results obtained from the electron microscope (5). Experimental Materials Davison '923' chromatographic grade silica gel with a particle size range p was used. After activation at 240" in air for 15 h, the surface area (B.E.T.: N2: - 196") was 660 m2/g and the lump density (6) 1.20 'Address: Lambeg Research Institute, Lisburn, N. Ireland. g/cc. The other thermal treatments were carried out in air for 15 h at the temperatures specified in the text. Modification of the gel by superheated steam was made by treatment in an autoclave for 2 h at 140" with a steam pressure of 38 Ib/in.' followed by heating at 240" for 15 h in air. The nitric acid treatment was similar to that of Thorp and Woolf (7) using 50% v/v nitric acid at 80" for 24 h. Apparatrrs Nitrogen adsorption/desorption isotherms were measured at - 196" in a standard apparatus (8) using purified cylinder nitrogen above 99.99% purity. Prior to the determination of isotherms, samples were outgassed carefully in the apparatus at 10-4 mni for 1 h. The early electron photoniicrographs were taken with an E.M.I. EM6G instrument using a carbon deposition method but this proved suitable only for observations of the edges of particler. Subsequently, gels were electron plioton~icrograpl~ed by Aeon Laboratories, Egham, Surrey, England after applying the specialized technique of vacuum embedding and sectioning. The thickness of sections varied according to the strength and rigidity of samples. Davison '923' gave well-formed wedges of material varying froni about 500 A to less than 100 A at the edges pliotoniicrographed. The section thickness with superheated steam treated gel was lllore uniform at A. Results and Discussion Electron Photomicrographs An examination of the edges of a small fragment of '923' gel, x, showed that the gel globules varied in diameter between 300 and 1000 A. The globules were deeply grown together, indicating that in the bulk material they could not be regarded as separate entities. Part of the sur-

2 ROBINSON AND ROSS: PORE STRUCTURES OF SILICA GELS I face was rough and pitted, suggesting that the large globules around the edges were not ultimate particles but were themselves composed of many smaller particles between 10 and 50 A in diameter, similar to the size noted for ultimate particles earlier (9). In Plate 1, the arrow indicates the direction in which the section was cut. The discontinuities at right angles are compression lines which are always obtained when hard materials, such as silica, are cut. The marked effect of combined heat and steam treatment on the gel structure can be seen clearlv piate with 'late PLA'iE 3 superheated steam treated gel after heating The pores to 5000 fir 15 h. M~croscope magnification, ; circled C in the superheated steam gel range in total magnification, x. size from a~~roximatelv 40 to 100 A and exist A A between discrete globules whichvary in diameter the discrete nature of the gel globules is less I between 150 and 500 A. obvious (Plate 3). The definition on this plate is After modification of the "'perheated excellent and a cross-secfion along the lengths of ' steam treated gel by heating for 15 h at 500, of many pores can be seen. PLATE 1. Davison '923' silica gel, activated at 240". Microscope magnification, x ; total magnification, x. Nitrogen Adsorption The hysteresis loop contained by the nitrogen adsorption-desorption isotherm determined on the 240" gel, (Fig. 1) has a sloping adsorption branch which closes almost vertically. This type of loop is generally accepted (10) as indicating a random distribution in the geometry of packing of the ultimate particles and a narrower distribution of pore openings. Such an explanation is consistent with the structure observed in the electron micrographs of this gel. From similar nitrogen adsorption/desorption isotherms, B.E.T. plots (1 1) were drawn for all of the gels examined. These plots were all good straight lines over the range p/p, = 0.05 to 0.30 and froin their slopes and intercepts, monolayer adsorbed amounts were computed in the usual way using 16.2 A2 as the cross-sectional area of a nitrogen molecule (1 1). Pore size distribution charts were plotted from the adsorption branches by the Cranston and Inkley method (12) after data processing with an Elliott 803 computer. The chart for 240" gel showed that the range of pore diameters was from 100 to below 20 A, with the majority of pores having diameters under 50 A (Fig. 2). The most common pore diameter, given by the mode of the pore size distribution chart, was 25 A. Treatment of gel with nitric acid appeared to PLATE 2. Superheated steam treated gel. Microscope reduce the number pores 25 A. magn~ficat~on, ~OOOO x ; total magnlficat~on, x. treatment shifted the mode of the pore size dis-

3 2212 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 FIG. 1. Nitrogen adsorption/desorption isotherm on 240" gel at - 196". Inset, B.E.T. plot. FIG. 2. Pore Diameter /Ao/ Pore size distribution chart for 240" gel. tribution to 40 A with the majority of pores lying between 38 and 180 A. Steam treatment followed by heating at 500" shifted the mode still further to 45 A. Heat treatment (3) of normal '923' gel up to 900" causes little change ir, the pore size distribution, although the total pore volume diminishes rapidly above 500". The principal surface parameters of the gels examined by gas adsorption are show11 in the Table 1. TABLE 1 Surface parameters of gels determined from nitrogen adsorption/desorption isotherms at - 196" Mode.of Activation Surface pore slze temperature area distribution Treatment ("(3 (mz/g) (A) None None Superheated steam Superheated steam Nitric Acid t~ns/in.~ The surface areas of two samples were also determined by the more recently introduced t-method (13), in which the volume of gas adsorbed/g of adsorbent at standard temperature and pressure, V,, is plotted against the statistical thickness, t, of the adsorbed layer. So long as the multilayer forms unhindered, a straight line is obtained passing through the origin and its slope is a measure of the surface area, S, = V,/t. For many well-selected samples, Lippens and de Boer (14) found that the first parts of such curves were straight lines from which St could be calculated. Surface areas determined from the t-plots in Fig. 3 were 620 and 325 m2/g for 240" and superheated steam treated gels, respectively. When t = 4.5 A the curve for 240" gel starts to bend downwards. At this value (13) some of the pores are completely filled by the adsorbed layers of nitrogen and the surface in them is no longer available for further adsorption which indicates a minimum pore diameter around 9 A. The separate effect of pressure on the surface parameters of '923' gel was determined after compression of the material at 10 ton~/in.~ for 3 min. This treatment appeared to result in an increase in the number of pores < 20 A diameter but it was apparent from photomicrographs,

4 ROBINSON AND ROSS: PORE STRUCTURES OF SILICA GELS 2213 A B FIG. 3. V,--t plot for (A) 240' and (B) superheated steam treated gel. 50 x, that compression had caused fragmentation of gel particles. The lump density measured by mercury displacement (6) was reduced to 0.99 g/cc, which indicates that the gel particles had been so reduced in size that the mercurv was unable to penetrate the interstices between particles. Confirmation of this theory was obtained by reducing the particle size of the original gel to the same size range (< 5 p) by light grinding. As a result the lump density was reduced to 0.76 from 1.20 g/cc. General Treatment of gel with superheated steam causes a severe loss in surface area, Table 1, while the pores increase in size to the 40 to 100A range. The electron micrographs indicate that treatment with superheated steam may cause the ultimate particles to aggregate and the globules to become denser and more discrete. The consequent loss of pore volume from the interior of the globules would explain the diminution in their size, the loss of surface area of the gel and the increase in its lump density to 1.48 g/cc. It has been suggested (5, 9) that the increase in pore diameter and decrease in surface area is the result of a regular growth of the individual ultimate particles of the solid at the expense of others, large particles become larger and smaller ' SUPERHEATED STEAM FIG. 4. Suggested effect of superheated steam treatment. (A) Silica gel globules, (B) ultimate particles. ones disappear. Previous steam treatments have been more severe, e.g. up to 215" for 401 h (15), and it may be that the first step in the suggested mechanism is the aggregation of ultimate particles so that within the globules the mechanism is analogous to that of sintering (16) where adhesion of ultimate particles is involved as a preliminary stage. However, this process must be confined to the interior of the globules since treatment with superheated steam does not reduce the total pore volume (15). This suggested effect of superheated steam is shown in F ~ 4. ~ I After consideration of 15 shape groups of pores, de Boer (1) analyzed the types of hysteresis loops which would result from capillary condensation in them. He proposed 5 main types of loops of which 2, Types A and E, are found here and in our previous studies on the adsorption of various gases on silica gels. For example, the type A loop is given by the isotherm for ethylene oxide at OK on superheated steam treated gel (17). The adsorption/desorption branches of type A isotherms are steep, pores are cylindrical, radius r,, and contain spherical cavities, r,, where r, 1 2rn. The type E loop is found with cylindrical pores, r,, which contain large cavities of variable radius, r,, while r, > 2r,. The adsorption branch has a sloping character and the desorption branch is steep. If there is a variation in r,, the desorption branch will have a sloping character also, but the broadening of the loop still exists (13). This describes the loop shown by dimethyl ether isotherms (18) and indeed by most isotherms obtained on 240" '923' gel. After treatment with superheated steam the hysteresis loops change, and satisfy the pressure relationship (1) shown by Type A loops which

5 2214 CANADIAN JOURNAL OF CHEMISTRY. VOL. 48, 1970 SUPERHEATED FIG. 5. Suggested effect of superheated steam on pore dimensions. also persist on subsequent thermal treatment of the steam treated gel at 500" (19). This implies that the pore dimensions have changed, r, > 2r, becoming r, 5 2r,, which is consistent (1) with a widening of the constrictions between spheroidal cavities, Fig. 5. Comparison of the cross-sections of pores in the untreated gel, Plate 1, with those in the steam treated gel activated at 500, Plate 3, shows that such a change probably occurs in the pore structure. In addition to the pores in the 40 to 100 A range, electron micrographs, x, indicate that pores < 10 A may be present in the superheated steam treated gel. This bidisperse pore structure has been reported by Kiselev and co-workers (20) who proposed that the small pores were formed by water trapped as the gel structure changes under steam treatment. When driven out on drying the water molecules form the fine channels in the gel. Conclusions The deductions made from the adsorption isotherm analyses regarding the pore characteristics of these gels agree in general with the electron microscope observations, although the latter method is more discerning. Davison '923' silica gel has been examined, a highly pure, readily available, chromatographic-grade material, to provide the possible added advantage of links with earlier work on adsorbate/adsorbent and adsorbate/adsorbate interactions involving mainly reactive gases on hydrous oxides. All of these materials have been described (3) but past attention was largely focussed on '923' gel. In computing the pore data of the normal and modified '923' gels from the nitrogen adsorption/ desorption isotherms, extensive use was made of the Cranston and Inkley method of analysis (12) in which the critical pore radius is equated to the Kelvin radius. Now, when the molecular diameter of the adsorbed gas approaches a value of a few times the Kelvin radius and it is about three times here for the majority of pores in normal '923' gel, then the applicability of analytical schemes based on the classical Kelvin equation for macro-system capillaries remains unresolved (21). Clearly there is a need for additional theoretical study of this particular problem. However, in the present investigation the electron microscope observations have provided an independant model for the pore structures which is internally consistent with the other deductions. We thank the British Petroleum Company for the award of a research scholarship (E.R.). 1. J. H. DE BOER. Itz Colston papers. Vol. 10. Butterworths, London. 68, (1958). 2. H. A. BENESI, R. V. BONAR, and C. F. LEE. J. Phys. Chem. 58,95 (1954); R. A. BEEBE and D. M. YOUNG. Anal. Chem. 27, 1963 (1955). 3. A. H. TAYLOR and R. A. Ross. J. Chem. Eng. Data, 14, (3), 347 (1969). 4. I. SUGAR and F. GUBA. Acta Chim. Acad. Sci. Hung. 7, 233 (1955). 5. A. V. KISELEV. In Colston DaDers. Vol. 10. Butterworths, London. 195 (1958) W. B. JEPSON. J. Sci. Inst. 36, 319 (1959). 7. J. M. THORP and J. B. WOOLF. Trans. Faraday SOC. 63, 2068 (1967). 8. A. S. JOY. Vacuum, 111, No. 3, (1953). 9. C. R. ADAMS and H. H. VOGUE. J. Phys. Chem. 61, 722 (1967). 10. R. VENA~E and W. H. WADE. J. Phys. Chem. 69, 1395 (1965). 11. S. BRUNAUER, P. H. EMMETT, and E. TELLER. J. Amer. Chem. Soc. 60, 309 (1938). 12. R. W. CRANSTON and F. A. INKLEY. Advan. Catal. 9, 143 (1957). 13. B. G. LINSEN and A. VAN DER HEUVAL. The solidgas interface. Edited by E. A. Flood. Edward Arnold London. 2, 1025 (1967). 14. B. C. LIPPENS and J. H. DE BOER. J. Catal. 4, 319 (1965). 15. V. M. CHERTOV, D. B. DZAMBAEVA, and I. E. NEIMARK. Kolloid. Zh. 27, (2), 279 (1965). 16. G. F. HUTTIC. Kolloid. Zh. 98, 263 (1942). 17. E. RonrNsoN and R. A. Ross. J. Chem. Soc. (A), 2521 (1969). 18. E. ROBINSON and R. A. Ross. Can. J. Chem., 48, 13 (1970). 19. E. ROBINSON and R. A. Ross. J. Chem. Soc. (A). In press. 20. V. YA DAVYDOV, A. V. KISLEV, and L. T. ZHORA- VELEV. Trans. Faraday Soc. 60, 2254 (1964). 21. D. H. EVERETT. The solid-gas interface. Edited by E. A. Flood. Edward Arnold, London. 2, 1077 (1967).