Characterization of the microporosity and surface area of silica aerogels

J. Rouqucrol, F. Rodriguez-Rcinoso, K.S.W. Sing and K.K.Unger (Eds.) Characieriralion of Porous Solids ill Studies in Surfacc Scicncc and Camlysis, Vol. 87 0 1994 Elsevicr Scicncc B.V. All rights rcscrvcd.

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Characterization of the microporosity and surface area of silica aerogels F. Ehrburger-Dollea, J. Dallamanoa, G. M. Pajonkb and E. Elalouib aCentre de Recherches sur la Physico-Chimie des Surfaces Solides, CNRS, 24 Avenue du President Kennedy, F-68200 Mulhouse, France bUniversit6 Claude Bernard, Lyon I, ISM, 43 Boulevard du 11novembre 1918, F-69622 Villeurbanne Cedex, France

Abstract The microporosity, surface area and surface fractal dimension of several silica Aerogels are determined by means of the analysis of N2 (77 K), Ar (77 K) and C02 (273 K) adsorption isotherms. The role of the pH conditions during the preparation and the effect of the rehydroxylation on the microporous texture are evidenced. The value of the molecular area of N2 on hydroxylated surfaces and the mechanism of the secondary micropore filling are discussed.

1. INTRODUCTION Silica Aerogels are extremely porous materials and, therefore, have attracted interest with respect to fundamental research and applications, particularly as insulating materials and catalyst supports 11-41. The high porosity and surface area result from the method of drying (supercritical drying) the silica gel formed by a sol-gel process. It is achieved by increasing the temperature and the pressure of the liquid in the pores above the critical value and replacing the supercritical fluid by air. As a result of the pressure and temperature conditions, the supercritical drying in an alcohol leads to a n esterification reaction that replaces the hydroxyl groups by alkoxy groups confering t o the Aerogel surface an hydrophobic character. Rehydroxylation occurs slowly a t room temperature in contact with atmospheric water vapor and can be accelerated by heating at 250 "C in presence of water vapor. The following typical arrangement of the silica particles in the Aerogel were deduced from the different scattering techniques (SANS, SAXS) 14-71, light scattering 181 and high resolution electron microscopy 191: - the primary particles have sizes between 1 and 1.5 nm. They are arranged in a fractal or compact secondary particle in which internal pores of sizes below 2 nm (micropores) are expected. - the sizes of the secondary particles reach a few tens of nanometers up t o about 100 nm. They are arranged in a more or less chainlike structure in which

716

mesopores a n d macropores a r e expected ( t e r t i a r y p a r t i c l e s ) - the arrangement of the tertiary particle leads t o a powder or a monolithic Aerogel at the macroscopic scale. It was shown that the surface area and the fractal dimensions depend on the nature of the precursor, the pH conditions [lo-111 of the hydrolysis and the conditions of the supercritical drying [12-131. The aim of the present work is to characterize the surface area, the microporosity of the secondary particles and their surface fractal dimension in relation to the method of preparation. It is achieved by the study of the N2 and Ar adsorption isotherms at 77 K, over a very broad range of pressures and C02 adsorption a t 273 K. The comparison of the N2 and Ar isotherms by means of the BET equation is particularly important in the case of hydroxylated surfaces as the question of the specific molecular area still remains open 1141. The comparison of the adsorption mechanism of molecules of similar sizes at two different temperatures (77 and 273 K) leads to informations ( ~ 0 . 3 nm) 5 about the accessibility of the micropores, as narrow constrictions o r throats in a packing of spheres would be only accessible by an activated diffusion process. The adsorption isotherms will be compared by using the Dubinin-Radushkevich plot, largely used for the characterization of the microporosity in carbon materials exhibiting slit shaped pores. The last part will be devoted to the discussion of a possible mechanism for the secondary micropore filling which is suggested from the experimental results obtained with our silica samples. 2. EXPERIMENTAL

2.1. Preparation of the silica Aerogel samples From a stock solution of tetramethoxysilane (TMOS) dissolved in anhydrous methanol with a volume percentage of 12 % of TMOS, three alcogels were first made in acidic (pH=4, acetic acid), neutral (pH=6) and basic (pH=9, ammonia) conditions respectively. All alcogels were obtained from the reaction at room temperature between TMOS and water with a molecular TMOS/H20 ratio equal to 4. The three alcogels were dried supercritically with respect to methanol at T=250 "C in an autoclave [ E l . The Aerogels were obtained under the form of a fine powder when prepared in acidic (A12) or neutral (N12) conditions whereas a monolith was formed in basic (B12) conditions. The apparent density of B12 (monolith) was 0.076 g/cm3, whereas that of the powdery A12 and N12 were respectively 0.051 and 0.040 g/cm3. 2.2. Gas adsorption A classical volumetric device was used for the determination of the adsorption isotherms. Nitrogen and argon adsorption isotherms were measured a t 77 K, carbon dioxide adsorption, a t 273 K. The pressure was measured by three different pressure sensors (BAROCEL 1,100 and 1000 Torr) in order to cover the whole range of pressure with a better accuracy. The samples were outgassed during 12 hours a t 200 "C in vacuum (10-6 Torr) before the adsorption measurements. The time allowed for equilibrium was generally 45 minutes or slightly longer in the very low pressure range.

717 3. METHODS OF ANALYSIS OF THE EXPERIMENTAL DATA

3.1. Determination of the BET surface area The surface area of the samples will be determined by the classical BET method leading to the number of molecules forming the monolayer. The question which arises is that of the value of the molecular surface CT.This point was discussed recently by Ismail [14]. For argon, there is a n overall agreement for 0=0.138nm2. Therefore this value will be used here as a reference for the measurement of the BET surface areas. The molecular area of nitrogen, calculated as above for argon, from the value of the molar volume in the liquid state [14], is 0=0.162 nm2 and it decreases to 0=0.138 nm2 in the solid state 1141. We have shown 1161 previously that the BET N2 surface area of a carbon black, calculated with 0=0.162 nm2, agrees well with the BET Ar surface area, calculated with 0=0.138 nm2. Rouquerol et a1.1171 have shown that, due t o its high quadrupole moment, the nitrogen molecules adsorbed on silica surfaces interact [181 with the hydroyl groups . Therefore, because of the most probable mean orientation of the nitrogen molecules, their cross-sectional area is smaller than 0.162 nm2 [17]. The smallest calculated value is 0=0.112 nm2 and leads to a good agreement for the Aerosil 200 BET surface area determined by nitrogen and argon adsorption [161. 3.2. Characterization of the microporosity One of the methods used t o characterize the microporosity is the fit of the data t o the Dubinin-Radushkevich (DR) equation [191: In w = In WO - ( u E O P

WPP

with A = RT In (pdp)

(1)

In this equation, W is the volume of the micropores filled a t the relative pressure plpo and WOis the total volume of the micropores. For carbon materials, exhibiting slit shaped micropores, it was shown [201 that Eo, called characteristic energy of the solid, is related to the micropore width L by the following relation : Eo=k/L

(2)

As a first approximation, k is considered as a constant (k=26 kJImole nm) for values of Eo below about 25 Wlmole [20], i.e. for pores able to accomodate more than about 2-3 molecular layers (in the case of the most commonly used molecules of sizes ranging between about 0.3 and 0.5 nm). However, to our knowledge, there is no evidence that relation (2) is also valid for the more or less spherical pores present in a packing of spheres as expected in silica materials. The coefficient p is called coefficient of affinity and was introduced by Dubinin in order to obtain a relation which is characteristic of the solid and independent of the adsorbate. The value p=1 was chosen for benzene. Its determination is still under debate, as shown recently by Wood [21]. The question is to know t o which molar characteristics (the molecular parachors, the molar polarization or the

718

molar volumes Vm) P correlates better. By definition, P is introduced in order to normalize the filling of the micropore volume, calculated by multiplying the measured value of the number of moles n adsorbed a t a given p/po by the molar volume Vm (W=nVm) as a function of the adsorption potential A=RT ln(pdp) (kJ/mole). Figure 1 shows some reported values of p collected by Wood [21] for the most common adsorbates 2 plotted against their molar volume (calculated by the ratio of the molecular mass M and the liquid 1.5 density d). The slope of the line obtained by a least square method is 1.08 10-2. As, by definition p=1 for 1 benzene (Vm=88.91cm3/mo 1) one obtains: P=1.125 10-2 Vm. Both the scattering of the data and the small 0.5 difference between the two slopes justifies that the ratio of the molar volumes will be used t o calculate P. 0 The values of p obtained for the 0 50 loo 150 200 adsorbates used in the present work Figure Relation between some are summarized in Table 1, along experimental p values [211 and vm. with the other parameters used for the calculations. Table 1. Summary of the parameters of the adsorbates used for the analysis of the adsorption measurements. Adsorbate PO (Torr) dliq Vm (cm31g) P 0.808 [221 34.67 0.39 760 N2 (77 K)

Ar (77 K)

217

[23]

1.457 [23]

27.42

0.31

C02 (273 K)

26142 [241

1.023 [241

43.01

0.48

It is, however, important to note that the characterization of the molecular volume by using a macroscopic data leads only to a first approximation, as only a few molecules are able to accomodate within micropores. Obviously one should characterize the molecular volumes and sizes, with respect to the pore width. As only an integer number of molecules will fit within the micropores, one has to take into account the fluctuations of the density resulting from differences in the compacity of the adsorbed molecules as a function of the relative pore and molecule sizes [25-27], leading thus to variations of 0. The interest of our method of normalization by the molar volume is that such effects could be taken into account similarly on both coordinate axes.

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3.3. Determination of the surface fractal dimension Pfeifer and Cole [281 have shown that the adsorption isotherm on a fractal surface, in the multilayer range, depends on the surface fractal dimension. When the number of adsorbed molecules n, depends only on the surface potential, the adsorption isotherm follows the fractal FHH equation: n = Pn (pdp)l-l/m

with m=3/(3-D)

(3)

As 2cDc3, m is larger than 3. For smooth, non fractal surfaces, D=2 and the classical FFH equation, with m=3, is recovered. However, most experimental results obtained from the nitrogen adsorption on different surfaces indicate that m is generally smaller than 3 [29]. This result can be explained [28] by the effect of a cross over between the BET regime, governed by the surface potential and the capillary condensation (CC) one, governed by the surface tension. In the CC regime, the exponent in equation [3] becomes m=1/(3D). Physically, this situation correspond to surfaces where the rapid increase of the number of adsorbed molecules n, due to capillary condensation, is partly compensated by the decrease of n in regard to the first layer, due to the fractal character of the surface. It follows that for m23, the real value of D will depend on the existence or not of mesopores in which capillary condensation would occur. 4. EXPERIMENTAL RESULTS AND DISCUSSION

4.1. BET surface areas Table 2 shows the values of the BET surface areas for the initial and rehydroxylated A12 and B12 Aerogels. It appears that the Ar BET surface area Table 2. Results of the BET analysis of the isotherms. * Indicates that the value of (J was calculated by assuming SBET(A~)= SBET(N2). The uncertainty on the value of nm is estimated to about f0.4 mmole/g. A12init. A12hydrox. B12init. B12hydrox. nm (mmoYg) N2

Ar

8.3

8.8

6.3

7.8

0.146*

0.112

0.131*

0.112

SBET (m2/g)

(731)

593

(498)

527

nm (mmoYg)

8.8

8.1

6

6.35

SBET 6%) 731

673

498

527

(J

(nrn2)

of the rexydroxylated samples is slightly smaller for A12 and slightly larger (or more probably similar within the experimental errors +5%)for B12. The values of o(N2) for the initial samples suggests that the surface is not completely methoxylated. The results obtained for the hydroxylated samples also justify the value of 0=0.112 nm2 for N2 as already shown 1161 for Aerosil 200 for which the

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N2 BET surface area (S=146 m%g) is in good agreement with the Ar one (S=143 m2/g) calculated with 0=0.138 nm2. 4.2. Analysis of the DR plots

Fig. 2 shows the DR plots for the initial and rehydroxylated B12 samples; similar curves were obtained for A12. The results of the DR analysis are reported in Table 3. On fig. 3, we have plotted the DR isotherms obtained for a n Aerogel silica obtained in neutral conditions (N12) and for the non microporous (C02 is almost not adsorbed) Aerosil200. The comparison of the two figures leads to the following informations: -1 -2

-3 -4

-5 -6

-7

0

250

500

750 lo00 (kJ/mol)2

Figure 2. DR plots obtained for the base catalyzed Aerogel B12. (open symbols: initial sample; closed symbols: rehydroxylated sample)

0

250

500

750 lo00 (kJ/mol)2

Figure 3. DR plots obtained for the N12 Aerogel and the pyrogenic silica Aerosil200.

- The values of Eo (i.e.the slope of the DR lines) obtained for A12 and B12 are very similar t o the one obtained for the non microporous Aerosil 200. It follows that the low temperature adsorption of N2 and Ar is limited to the external surface. Similar results have already been obtained for other base catalyzed Aerogels and for precipitated silicas [30]. This also suggests that the diameter of the throats in the primary particle packing, is close to that of the adsorbed molecules (0.35-0.40 nm). Therefore, the classical BET N2 does not take into account the internal, microporous surface accessible a t higher temperature. - C02 is adsorbed into the micropores by a volume filling process, as suggested by the results obtained for N12, for which the internal microporosity is accessible to Ar and N2 a t 77 K. - The linearity of the DR plots describing the adsorption on a non microporous silica, also evidenced by Carrott et al. [311 suggests that adsorption occurs on surface geometrical heterogeneities [321. Such heterogeneities could be located a t the contact between two or three primary particles packed in a more open way than the internal core, and forming more or less chain like aggregates at the

72 1

surface. As the adsorption of Ar and N2 is a surface adsorption the values of Wo indicated in Table 3 have no physical meaning. The value of the surface, deduced from the corresponding no are smaller than that obtained by the BET method as it takes into account only the adsorption process on one fraction of the total surface available. Table 3. Results of the DR analysis of the different isotherms of adsorption on A12 and B12 samples. The values of the mean adsorption potential A=dn/2 PEo are also indicated €or N2 and Ar. The uncertainty on the value of Wo is estimated to about f0.02 cm3/g. A 12init . A 12hydrox . B 12init . €3 12hydrox. C02 (273 K) WO (cm31g) Eo (kJ/mol) no (mmol/g>

0.04 18.8 0.9

0.15 18.2 3.5

0.08 16 1.9

0.13 17.7 3.0

N2 (77K) WO (cm3/g> Eo (kJ/mol) nm (mmoVg) d d 2 PEo

0.24 12.7 7.1 4.4

0.29 14.1 8.4 4.9

0.15 11.9 4.3 4.1

0.22 13.2 6.3 4.6

Ar (77 K) WO (cm3/g) Eo (kJ/mol) nm (mmoVg) d d 2 PEo s (m2k)

0.22 12.7 8.0 3.5 666

0.20 13.2 7.3 3.6 606

0.095 12.9 3.5 3.5 288

0.12 13.4 4.4 3.7 364

- The increase of the micropore volume after rehydroxylation, evidenced by C02 adsorption on A12 would evidence a partial collapse of this surface microporosity, leading to more closed micropores in which C 0 2 would now be able to condense. This new microporosity will no longer (or a t least only partly in the case of Ar) be accessible a t low temperature. This effect is also in agreement with the slight decrease of the BET surface area. It could be related to the tenuous character of acid Aerogels, exhibiting properties of a polymeric material 1111. In B12 samples, such a collapse seems to be less pronounced, as expected from the more colloidal character 1111 of basic Aerogels. From the above discussion, one may also conclude that the BET surface area depends mainly on the size of the primary particles. As the micropore volume (not accessible t o the Ar or N2 molecules a t low temperature) and the BET surface area of A12 samples are larger than that of the B12 Aerogel, the size of the A12 primary particles is probably smaller than that of the basic Aerogel. This is also in agreement with SANS results 1331.

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4.3. Determination of the surface fractal dimension In Fig. 4, we have plotted the variation of the surface coverage n/nm on rehydroxylated Aerogels, A12 and B12, as a function of In (pdp) in logarithmic coordinates, following equation 131. Thermoporometry measurements (not reported here), indicate the presence of mesopores of radii ranging between a few nm up to about 15 nm, in both types of Aerogels although their volume is larger in the basic one. It follows that one has to take into account a capillary condensation mechanism leading to l/m=(3-D). The surface fractal dimension obtained for both initial and rehydroxylated A12 Aerogels is close to 3 (D=2.95), suggesting that the surface of the secondary particles is almost volume filling and, therefore, that the surface chain like aggregates are crumpled and close to each other. The surface fractal dimension obtained for the B12 samples nln, (D=2.64) is in agreement with that 5 obtained by SANS [34]. This value also confirms t h a t the surface aggregates are in a more expanded state than in the acid Aerogels and t h a t t h e collapse d u r i n g a rehydroxylation process could be less important. On t h e same figure are also plotted the results obtained for the nonmicroporous Aerosil200 in which 1 no capillary condensation occurs. The slope is now equal to (3-D)/3 and D=2.1. It indicates that the surface is 0.5 almost smooth a t the molecular level 0.01 0.1 1 10 as expected for isolated chain like ln(p&) aggregates made of primary particles which are much larger than that of Figure 4. FHH plots (adsorption of the Aerogel samples. N2).

4.4. Further comments concerning the physical meaning of the adsorption in the low pressure range The methodology we have used here is unusual for silica samples but traditional for carbon materials, particularly active carbons, exhibiting slit shaped micropores. It explains that the DR equation became limited to the characterization of the micropore volume filling as a function of the micropore geometry. However, formally, the DR equation describes an adsorption isotherm resulting from a distribution of surface heterogeneities [321, leading to a mean surface potential A=./d2 PEo,which is proportional to the excess of adsorption energy with respect t o a flat homogeneous surface as graphite [351. An excess of adsorption energy could arise from geometrical defects, at the molecular scale, onto which the adsorbed molecule would have more than one contact with the underlying surface as, for example, in the region of contact between 2 or 3 silica primary particles. We have shown that C02 a t 273 K is not adsorbed on such geometrical heterogeneities, although they are strong adsorption sites. It

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becomes now likely that the mechanism of adsorption into micropores of sizes larger than about twice that of the molecules (in which there is no enhancement of the adsorption potential [231), i.e. the secondary micropore filling introduced by Sing et al. [311, is completely different. It would take place only in a confined medium into which the molecules would be in a metastable, liquidlike phase. It follows t h a t the secondary micropore filling would be rather a n entropic (configurational) effect, as already suggested by Carrot et al. [251 than a n enthalpic one. Such a n hypothesis would also be consistent with the molecular theory of adsorption proposed by Quirke et al. I361 in order to describe the continuous filling of pores of sizes below a critical width, which is not accounted for in the classical thermodynamic approach. Particularly, the use of a non local mean field theory [37] provides a more accurate interpretation of the micropore size distribution and could explain the difficulties to find a general relation between Eo and the pore width in the classical potential theory approach. Moreover, the above comments and hypothesis arise further questions concerning the relation between the heat of immersion of microporous solids in liquids and the micropore size [38]. 5. CONCLUSION

The analysis of the isotherms of adsorption of N2 and Ar a t 77 K over a very broad range of pressures and that of C o g at 273 K allows the characterization of the surface and microporosity of silica Aerogels and leads to more precise informations, a t the nanometric scale, than the scattering methods. It is also evidenced that the classical BET N2 method alone is not adapted to the determination of the surface area of microporous silicas. By ignoring the interaction of the quadrupole moment of the nitrogen molecule with the hydroxyls groups and assuming ~ 2 = 0 . 1 6 2nm2, the BET surface area is overestimated. Moreover, as the internal surface is, in some cases, not accessible t o small molecules at low temperature, t h e overall surface area is underestimated. Furthermore, the analysis of the adsorption isotherms of N2 and Ar in the low pressure range and the comparison with the results obtained on a non microporous silica, suggests different insights in the mechanism of the secondary micropore filling.

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