Studies on Pore Systems in Catalysts

Transcription

1 JOURNAL OF CATALYSIS 4, (1965) Studies on Pore Systems in Catalysts V. The t Method B. C. LIPPENS* AND J. H. DE BOER From the Department of Chemical Technology, Technological University of Delft, The Netherlands Received March 3, 1964 Valuable information about the specific surface area, the size, and the shapes of pores, the setting in of reversible capillary condensation and of complete filling of pores may be obtained by plotting the experimental volumes of adsorbed nitrogen, Vo, aa a function of the statistical thickness, 1, of the adsorbed layer, as given in Paper I of this series. The Vat plot, together with the experimental adsorption and desorption isotherm gives a very good picture of the whole pore system. 1. vat PLOTS Figures 1, 2, and 3 show the V,t plots for In Part I of this series (1) we showed that a number of our samples. These samples are for several wellselected samples of aluminum described in Part II of this series (z?). In hydroxides and oxides in which the multi almost all cases the first part of the curves is molecular layer of adsorbed nitrogen could a straight line through the origin, from which be formed freely on all parts of the surface, St can be calculated; it is in good agreement its statistical thickness t = V,/SBET is with X~ET (Table 1). practically independent of the nature of the At higher relative pressures (higher t sample. values) deviations from a straight line occur. The experimental values of V, (the volume We may distinguish three cases: of nitrogen adsorbed in cm3 STP/g of ad (a) The surface is freely accessible up to sorbent) which are obtained as a function high relative pressures; the multilayer can of the relative pressure, X, may, with the aid form unhindered on all parts of the surface; of Table 1 of Part I of this series (1) be the adsorption branch of the isotherm has transformed to functions of t. By plotting entirely the shape of the t curve; the V,t then V, for an unknown sample as a function plot is one straight line. of the experimental t, we obtain a straight (b) At a certain pressure capillary conline as long as the multilayer is formed un densation will occur in pores of certain hindered. This straight line goes through the shapes and dimensions; the material takes origin and its slope is a measure of surface up more adsorbate than corresponds to the area volume of the multilayer; the adsorption branch lies above the t curve; the slope of the St = 15.47v,/t V,t plot increases. (c) In some types of pores capillary con This quantity St will not always be exactly densation is not possible unless at very high equal to SBET, as instead of the various C relative pressures [slitshaped pores or large values in the BET equation (depending on holes (S)]. As long as the multilayer adsorpthe sample) an average value is used by tion is unhindered, the V,t plot is a straight introducing the t curve. line. If the pressure increases the free space * Present address: Laboratory for Inorganic in the pores becomes smaller owing to the Chemistry and Catalysis, Technological University growth of the adsorbed layer. Large holes Eindhoven, The Netherlands. will only be filled by capillary condensation 319

2 320 LIPPENS AND DE BOER i :.. t A FIG. 1. V,t plot of A and BOW preparations L l,llll.llll,lllllll,l,ii I I I I I I I I I 5.0 7:5 lo:o 125 FIQ. 2. Vbt plot of BOG preparations.

3 PORE SYSTEMS IN CATALYSTS V. 321 FIG. 3. V,t plot of By preparations. TABLE 1 COMPARISON OF SURFACE AREA CALCULATED WITH THE BET EQUATION AND FROM V,& PLOT p/p0 at the beginning of Sample ssm St s, Cap. cond. Hysteresis A A s A A BOW BOW BOG > BOG > BOG > By By By By By By > > > at relative pressures near unity. In a slit by the adsorbed layers on both parallel shaped pore, however, again no capillary walls. The surface area in such pores is no condensation can occur, but at a certain longer accessible above a certain relative moment the pore may be completely filled pressure; the Vat plot will now get a smaller

4 322 LIPPENS AND DE BOER slope, corresponding accessible. to the surface area still 2. CONCLUSIONS, DRAWN FROM V,t PLOTS Some conclusions which can be drawn from the I,t plots of our samples are in agreement with the conclusions we drew in Parts II (2) and III (4). Samples obtained from gelatinous boehmite (A 120, etc., Fig. 1 and Table 1) generally show capillary condensation at a relative pressure lower than that corresponding to the beginning of the hysteresis loop (Table 1). de Boer (2) has already pointed out that this reversible capillary condensation can take place in the cones and wedges formed, e.g. by the planes of crystals touching each other. For Sample A 750 the beginning of the capillary condensation almost coincides with that of the hysteresis loop; at a temperature of 750 C a noticeable sintering has already occurred; the sharp edges are apparently rounded by this sintering. The same conclusion has already been drawn from the changes of the shape of the desorption isotherms [Part II (2)J. Sample BOW 120, consisting of small flat, sixsided, crystals of boehmite shows a straight line up to a relative pressure of about 0.70, somewhat below the beginning of the hysteresis loop. Sample BOW 450, obtained by heating BOW 120 above the decomposition temperature of boehmite, shows a v,t curve with two breaks, one at a relative pressure of about 0.26, the other at The first break must apparently be explained by pores being closed by the adsorbed layer. Assuming that this will occur when the thickness of the adsorbed layer is equal to half the width of the narrow pores, we $ind for this width a value of = 9.5A. From the slope of the second part of the V,t curve we can calculate that the remaining surface area of the wide pores amounts to 68.0 m /g Al203 (indicated as S, in Table l), approximately the same as the SBET or St value of BOW 120. The extra surface area of about 25 m /g, which BOW 450 shows, is apparently present in narrow slitshaped pores formed during the heating at 45O C. Capillary condensation in BOW 450 occurs only at a relative pressure of 0.7, just as with BOW 120, showing that apparently nothing has changed in the outer shape of these particles. The hysteresis loop of BOW 450, however, closes at a relative pressure of 0.4. Comparing the hysteresis shapes in Fig. 2 of Part II (2) shows that BOW 450 shows a small extra Btype hysteresis curve superimposed on the curve shown already by BOW 120. The heating at 450 C has, apparently, also produced some wider slitshaped pores of about 25A width and a surface area of about 5 m2/g, as can be calculated from the distribution curve of Fig. 2 of Article III (4). During the heating of the wellcrystallized hydroxides (BOG and By) also narrow slitshaped pores are formed, as follows from the V,t curves. These narrow pores disappear again on heating at still higher temperatures (Figs. 2 and 3, Table 1). For samples having a hysteresis loop of type B, capillary condensation starts again at a relative pressure much higher than that at which the hysteresis loop is closed. The cumulative surface area calculated from the desorption branch of the isotherm, down to z = 0.3 [Part III (4)] is, in all these cases, greater than the surface area S, of the wide pores calculated from the second part of the V,t curve. This may indicate that in these cases we are dealing with relatively wide pcres with narrower openings [of about 25A, as described in Part IV of this series (5)]. If this conclusion is right, the complete absence of capillary condensation at relative pressures up to 0.86 on the adsorption branch indicates that either the volume of these openings of the wide pores is negligibly small, or that the openings themselves are formed by slitshaped pores. In view of the crystallographic properties (6, 7) of these materials the last supposition is the most probable one. Samples By 580 and By 750 apparently have a completely different pore structure. The wide pores are still present but other pores have a shape in which capillary condensation on adsorption is possible. Consequently severe sintering must have occurred. This could also be shown by electron microscope observations, as we have published elsewhere (7 ). Whereas at low temperature the 7 alumina shows a strict pseudomorphosis

5 PORE SYSTEMS IN CATALYSTS V. 323 to the original bayerite particles, recrystallization occurs at higher temperatures giving small rodshaped crystals, which still show some preferred orientation, but no strict pseudomorphism. As already observed in Part II (9) the hysteresis loops of By 580 and By 750 are combinations of the Btype loop of By 200 (and other By preparations up to By 450) and an Etype loop. This latter loop and the Vat curves of these preparations show that the narrow slitshaped pores have disappeared; they seem to have been replaced by pores with rectangular cross sections with varying diameters. In this connection the work of de Boer, Steggerda, and Zwietering (8) should be mentioned. From measurements of the optical birefringence of heated gibbsite crystals they arrived at the conclusion that at lower temperature slitshaped pores are formed, which are separated by platelike particles parallel to each other and perpendicular to the c axis of the original gibbsite. At higher temperatures, however, rodlike particles are formed which are perpendicular to the c axis of the original gibbsite and parallel to each other. They also observed a marked change in the shape of the adsorption isotherm of their samples. These optical observations give a picture analogous to that we developed for the dehydration products of bayerite. REFERENCES 1. LIPPENS, B. C., LINSEN, B. G., AND DE BOER, J. H., J. Catalysis 3, 32 (1964). 2. DE BOER, J. H., AND LIPPENS, B. C., J. Catalysis 3, 38 (19644). 3. DE BOER, J. H., The Shape of Capillaries, in Evereth, D. H., and Stone, F. S., The Structure and Properties of Porous Materials. Butterworth, London, $. LIPPENS, B. C., ASD DE BOER, J. H., J. Catalysis 3, 44 (1964). 5. DE BOER, J. H., VAN DEN HEUVEL, A., AND LIN SEN, B. G., J. Cutalysis 3, 268 (1964). 6. LIPPENS, B. C., Thesis, Delft, The Netherlands, LIPPENS, B. C., AND DE BOER, J. H., Acta Cryst. 17, 1312 (1964). 8. DE BOER, J. H., STECGERDA, J. J., AND ZWIETERINC, P., Proc. Koninkl. Ned. Akad. Wetenschap. B59, 435 (1956).