1896
Langmuir 2000, 16, 1896-1901
Nitrogen Physisorption on Modified Silica Surfaces L. E. Cascarini de Torre, E. S. Flores, and E. J. Bottani* Instituto de Investigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA)-UNLP-CIC-CONICET, Casilla de Correo 16, Sucursal 4 (1900), La Plata, Argentina Received May 28, 1999. In Final Form: September 13, 1999 The surface of a silica sample (Spherosil) have been modified by means of chemical treatments with sulfuric, phosphoric, and hydrochloric acids and potassium chloride. A sample also has been heated in a furnace at high temperatures. Changes in the degree of hydroxylation have been qualitatively followed by means of IR spectroscopy. Nitrogen, argon, and oxygen physisorption has been employed to study the behavior of the original and treated samples. Monte Carlo computer simulations results have been developed to explain nitrogen adsorption isotherms obtained with the different samples.
Introduction The influence exerted by surface heterogeneity on surface processes and the methods to characterize it have been the object of numerous studies.1,2 Silica coatings are employed to modify surface characteristics of materials, and it is known that the properties of silica surfaces strongly depend on the degree of hydroxylation.3,4 It has been shown that the adsorption properties of hydroxylated oxides depend on the surface OH group density.5 The most commonly employed techniques to study this problem is IR spectroscopy (see for example ref 6) combined with the information provided by gas physisorption. Computer simulations have been employed to study the vibrational spectra of OH groups.7 Computer simulations have also been employed to study the adsorption of polar8 and nonpolar9 gases on model porous silica. In this paper we investigate the effect of surface modifications upon the behavior of several adsorbed gases with emphasis on nitrogen adsorption. The experimental results are compared with computer simulations performed on model surfaces. The paper is organized as follows: First the experimental aspects of the chemical treatments are described including technical details of the computer simulations. Then following the discussion of the experimental results their interpretation based on the simulation results is presented. The experimental adsorption isotherms are compared with the simulated ones. Among the results discussed are the total and lateral interaction energies, nitrogen cross-sectional area, molecular orientations, local * Corresponding author. E-mail:
[email protected]. Fax: 54-221-425-4642. Tel: 54-221-425-7430. (1) Rudzinski, W.; Everett, D. H. Adsorption of Gases on Heterogeneous Surfaces; Academic Press: San Diego, CA, 1992. (2) Jaroniec, M.; Madey, R. Physical Adsorption on Heterogenous Solids; Studies in Physical and Theoretical Chemistry 59. Elsevier: Amsterdam, 1988. (3) Bergna, H. E. In The Colloid Chemistry of Silica; Adv. Chem. Ser. 234; Bergna, H. E., Ed.; American Chemical Society: Washington, DC, 1994, Chapter 28. (4) Bergna, H. E. In The Colloid Chemistry of Silica; Adv. Chem. Ser. 234; Bergna, H. E., Ed.; American Chemical Society: Washington, DC, 1994, Chapter 1. (5) Takeda, S.; Fukawa, M.; Hayshi, Y.; Matsumoto, K. Thin Solid Films 1999, 339, 220. (6) Takei, T.; Meguro, A.; Chikazawa, M. Colloids Surf. A 1999, 150, 77. (7) Ermoshin, V. A.; Smirnov, K. S.; Bougeard, D. Surf. Sci. 1996, 368, 147. (8) Gordon, P. A.; Glandt, E. D. Langmuir 1997, 13, 4659. (9) Kaminsky, R. D.; Monson, P. A. Langmuir 1994, 10, 530.
adsorption isotherms, and distributions of molecules over their gas-solid and gas-gas energies. Experimental Details Samples taken of one batch of Spherosil silica, aged for more than 10 years, have been employed as starting material. One sample has been heated in a furnace at 1073 K for 12 h in air stream (flow kept constant at 100-105 L/min). Another sample has been treated with concentrated sulfuric acid at 333 K during 31 h and then washed with distilled water until all the acid was removed. The treatment with phosphoric acid was done in a similar way except that the sample has been heated at 373 K. The treatment with concentrated hydrochloric acid was done in three steps; in the first one the silica sample was left in contact with the acid at room temperature for 24 h, then the temperature was raised to 353 K for 8 h, and finally the sample was left again at room temperature for 2 days and then washed with distilled water until the acid was removed. The last sample has been treated with a 4 M potassium chloride solution heated at 373 K during 8 h. Aging effects have not been studied here, and all the adsorption isotherms have been obtained immediately after the sample was considered to be clean as is explained later on. In all cases IR spectra of the solids were obtained using the KBr technique. Silica presents several absorption bands at 3750, 3650, 3500, and 1640 cm-1. Other bands are located at 11101000 cm-1 (Si-O-Si) and 910-830 and 790 cm-1 (Si-OH).10 It is generally accepted that the bands in the region between 3650 and 3750 cm-1 are due to surface hydroxyl groups and that the bands in the region between 3400 and 1640 cm-1 are due to the presence of molecular water on the surface. The band located at 3500 cm-1 is without doubt due to water somehow bound to the surface. Even though there is no agreement if it is chemisorbed or physisorbed water, we will employ this band and the one located at 790 cm-1 since the state of surface water is not relevant for our analysis. The sample treated with phosphoric acid shows a well-defined absorption at 2300 cm-1 that corresponds to P-OH groups and a very small absorption at 3300 cm-1 that has been attributed to the formation of hydrongen-bonded complexes of phosphates on the silica surface.11 Nitrogen, argon, and oxygen adsorption isotherms have been determined at 80.2 K for all samples using conventional volumetric equipment.12 Pressures have been determined with a capacitance transducer system in the range of 1 mTorr up to atmospheric pressure. Temperature was determined with a Pt (Pt-100 DIN) digital thermometer previously calibrated against (10) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: San Diego, CA, 1990. (11) Murashov, V. V.; Leszczynski, J. J. Phys. Chem. A 1999, 103, 1228. (12) Bottani, E. J.; Za´rate, J.; Cascarini de Torre, L. E. Adsorp. Sci. Technol. 1987, 4, 121.
10.1021/la990669n CCC: $19.00 © 2000 American Chemical Society Published on Web 01/05/2000
Nitrogen Physisorption on Modified Silica Surfaces
Langmuir, Vol. 16, No. 4, 2000 1897
Figure 2. Proposed reaction scheme to explain the modification produced by phosphoric acid treatment.
Figure 1. (a) Unmodified oxide solid. Each sphere represents the oxide anions. (b) Modified solid in which 10 surface oxides have been randomly replaced by impurities (light gray spheres). an oxygen vapor pressure thermometer. The BET specific surface areas have been determined by taking 0.162 nm2 for the nitrogen cross-sectional area and using the recommended relative pressure interval (0.05-0.35). All the adsorption isotherms were reversible over the whole pressure range studied indicating that no porosity was generated by the performed chemical treatments. The samples treated with liquids have been dried at temperatures not exceeding 303 K. After that the samples were transferred to a sample holder and degassed at ambient temperature and 10-5 Torr until they were clean; in some cases it took 1 week to achieve this condition. We consider a sample clean when it does not show any color when exposed to a highfrequency discharge after being isolated from the vacuum system for at least 0.5 h. This procedure has been adopted to avoid the possibility of further chemical reactions during a normal degassing procedure at high temperature. Monte Carlo computer simulations of nitrogen adsorption have been performed in the grand canonical ensemble. The untreated oxide solid is described with Bernal’s model,13 which assumes that the oxide structure could be represented by a random packing of spheres of the size of the anion. The contributions of the cations to the interactions are not taken into account following the results obtained by Bakaev et al.14 The solid obtained with this model is an amorphous one with a heterogeneous surface. The unmodified solid (Figure 1a) had a density equal to the real oxide (2.63 g/cm3), and the modified ones (one of them is displayed in Figure 1b as example) presented densities in the range between 2.6 and 2.5 g/cm3. In all cases the solid consisted of 1000 atoms and the simulation cell area was 12.32 nm2. To simulate the modified samples a very simple procedure has been adopted that proved to be capable of reproducing, at least qualitatively, the experimental results. From the original structure the solid atoms (oxide anions) in the most external layer (180 atoms fall in this category) were identified, and then new structures were generated in which those atoms were replaced by other atoms or groups (OH). If the impurities are considered as being OH groups, the resulting silanol numbers ranged between 0.81 and 9.73 OH/nm2. The fraction of replaced atoms ranges from 5.5% to 67%, which correspond to the replacement of 10, 20, 30, 60, 90, and 120 surface atoms. Experimental results quoted in the literature4 indicate that the (13) Bakaev, V. A.; Chelnokova, O. V. Surf. Sci. 1989, 215, 521. (14) Bakaev, V. A.; Steele, W. A. Langmuir 1992, 8, 1372.
maximum hydroxylation obtained corresponds to an average silanol number equal to 4.6 OH/nm2. Technical details of the algorithm employed to perform the computer simulations have been described elsewhere.15 The interaction potential between a gas molecule and the surface has been described by means of Lennard-Jones (6,12) functions with different well depths to distinguish between oxide anions and hydroxyl groups. In a first approximation the size of both species were considered equal (σ ) 0.316 nm). The well depths employed were the following: (N-O2-) ) 120 K;16 (N-impurity) ) 50 K. Lateral interactions between adsorbed molecules were taken into account including the quadrupolequadrupole interaction. To calculate each point of the simulated isotherms 2 × 106 Monte Carlo attempts have been generated except for the first point where 6 × 106 were generated. Each consisted of a displacement and rotation or creation/destruction attempts. The displacement-rotation acceptance ratio was kept approximately at a 40-60%, and creation-destruction acceptance ratio was 1-2%. In all cases equilibrated configurations were stored in files to be processed later. During the simulations the equilibrium condition was achieved and verified through the energy and number of adsorbed molecules control charts described elsewhere.15
Results and Discussion If one accepts that the IR band at 3500 cm-1 corresponds to molecular water somehow bonded to the surface and that the absorption at 790 cm-1 is due to Si-OH groups,4,17 the obtained spectra show that the treatments with sulfuric and hydrochloric acids and potassium chloride increase the hydroxylation degree of the surface. As could be expected, heating the sample at high temperature produces a decrease in the hydroxylation degree. The IR spectra also show the presence of P-OH groups (absorption at 2300 and 3300 cm-1)10,11 introduced by the treatment with phosphoric acid as well as a decrease in the hydroxylation degree. Both facts could be explained by a series of reactions on the surface of the oxide starting with the reaction between the acid and a surface OH group followed by further dehydration with a neighbor OH surface group. The reaction scheme is displayed in Figure 2. According to this reaction scheme, the result is that the surface is not altered to a great extent since two OH groups are replaced by one OH and one oxide. It also explains why the adsorption isotherms of the different gases studied do not show appreciable differences with respect to the other samples. The adsorption isotherms of all gases do not show any unusual characteristics. The BET specific surface areas and the corresponding C values obtained with nitrogen are quoted in Table 1, and the adsorption isotherms are (15) Bottani, E. J.; Bakaev, V. A. Langmuir 1994, 10, 1550. (16) Cascarini de Torre, L. E.; Flores, E. S.; Llanos, J. L.; Bottani, E. J. Langmuir 1995, 11, 221. (17) Pelmenschikov, A. G.; Morosi, G.; Gamba, A. J. Phys. Chem. A 1997, 101, 1178.
1898
Langmuir, Vol. 16, No. 4, 2000
Cascarini de Torre et al.
Table 1. BET Surface Areas of the Different Samplesa sample original heated at high temp sulfuric acid
S BET C (m2/g) BET 92.9 42.1
208 238
157.1
24
sample
S BET C (m2/g) BET
hydrochloric acid potassium chloride
96.9 62.6
301 333
phosphoric acid
42.1
112
a
The nitrogen cross-sectional area employed was 0.162 nm2, and the standard relative pressure range was considered.
shown in Figure 3a as examples. Microscopic observation of all samples reveals that there are no morphological differences between them and the original one. Moreover, the average particle size was not modified by the chemical or thermal treatments performed. In consequence, the differences observed in the adsorption isotherms must be explained on a different basis. When the adsorption isotherms are represented as V/Vm, where Vm is the BET monolayer capacity, as a function of the equilibrium pressure, argon and oxygen isotherms do not show any difference between modified and unmodified samples (see Figure 3b,c). Nitrogen isotherms are all coincident except the one corresponding to the sulfuric acid treated sample that adsorbs less than the others do. The different behavior of the studied gases could be explained by taking into consideration the nature of the gas-solid interactions present in these systems. Neither oxygen nor argon has a quadrupole moment and cannot present preferential interactions with surface groups. On the other hand nitrogen has a large quadrupole moment that explains its ability to interact with certain surface groups. In a previous study performed by House et al.,18 silicas chemically modified with sodium and potassium hydroxides were studied using nitrogen physisorption; they mention the lack of sensitivity of argon to detect any difference between the modified samples. Computer simulations were performed to study nitrogen adsorption in more detail. The replacement of 10 (5%) up to 60 (33%) surface atoms produced surfaces with silanol numbers compatible with real solids; two additional surfaces have been generated by replacement of 90 (50%) and 120 (67%) surface atoms which present silanol numbers (7.3 and 9.73, respectively) that are larger than the maximum experimentally observed. The idea was to cover the full range comprised between a pure oxide surface and a pure ideal hydroxylated surface. Some of the simulated isotherms are compared in Figure 4 with the experimental isotherms corresponding to the untreated, high-temperature heated, and sulfuric acid treated samples. The adsorption has been expressed in terms of molecules/Å2 using the BET areas obtained from each isotherm. It can be observed that the simulated isotherm corresponding to the pure oxide reproduces the isotherms of the original and high-temperature-heated samples. The sample with 90 impure atoms reproduces the isotherm obtained for the sulfuric acid treated sample. The disagreement between experimental and simulated isotherms at high pressures is explained on the basis of the inaccuracy of the representation of lateral interactions at high densities of the adsorbed phase.15 We will now discuss the results obtained from the computer simulations. The specific surface area of the modified samples monotonically decreases with the increase of the silanol number in the same fashion as observed by House et al.18 except that the BET area of the (18) House, W. A.; Born, G.; Brauer, P.; Franke, S.; Henneberg, K. H.; Hofer, P.; Jaroniec, M. J. Colloid Interface Sci. 1984, 99, 493.
Figure 3. (a) Nitrogen adsorption isotherms corresponding to all the samples studied at 80.2 K: (4) SO4H2; (2) ClH; (O) original; (b) high temperature; (]) PO4H3; (1) ClK; (0) ClH-2. (b) Argon adsorption isotherms obtained at 80.2 K: (4) SO4H2; (O) original; (b) high temperature. (c) Oxygen adsorption isotherms obtained at 80.2 K: (4) SO4H2; (O) original; (b) high temperature.
Nitrogen Physisorption on Modified Silica Surfaces
Langmuir, Vol. 16, No. 4, 2000 1899
Figure 4. Nitrogen simulated and experimental adsorption isotherms at 80.2 K. BET specific surface areas have been calculated from each isotherm to express the adsorbed quantity as molecules/Å2: (~) SO4H2; (0) N ) 10; (O) N ) 20; ([) N ) 30; (3) N ) 60; (2) N ) 90; ($) original; (]) N ) 0; (4) high temperature.
Figure 5. Total potential energy of the adsorbed molecules: (b) N ) 0; (O) N ) 10; (1) N ) 20; (3) N ) 30; (9) N ) 60; (0) N ) 90; ([) N ) 120.
Table 2. BET Surface Areas and C Values Obtained from the Simulated Isothermsa no. of replaced no. of replaced atoms/ S BET atoms/ S BET 2 percentage (Å ) C BET percentage (Å2) C BET 0/0 10/5.5 20/11 30/17
939.1 1218.2 1152.3 1112.9
145 75 84 82
60/33 90/50 120/67
958.8 900.4 833.6
64 22 10
a The percentage of replaced atoms has been calculated with respect to the maximum number or replaceable atoms (180).
unmodified surface is slightly lower and falls off the general curve. Table 2 summarizes the surface area and C parameter values obtained from the simulated isotherms. It can be noted that the C values obtained from the simulated isotherms also decrease with the number of impure atoms. The C value of the model surface with 0% of replaced atoms is close to the experimental ones obtained for the original and high-temperature samples. The value obtained for the sulfuric acid treated sample is similar to the one obtained for the model surface with 50%. The experimental C values obtained for samples treated with HCl and KCl are the highest of the series. This could probably be due to the presence of other impurities (ions) on the surface. The total energy (gas-solid plus gas-gas energies) of the adsorbed molecules is showed in Figure 5. The profiles have the same shape corresponding to a heterogeneous surface. It can be noticed that replacing surface atoms decreases the total energy as could be expected for large and moderate coverages. Nevertheless an unusual order, which will be explained later on, is found at very low surface coverages. This effect is larger than the statistical error of the simulations for silanol numbers larger than 2.4 (equivalent to 20% of impure atoms). When the lateral interaction energy is plotted against the number of adsorbed molecules, all the points fall over the same curve, see Figure 6, indicating that the differences observed in the total energy profiles are related to
Figure 6. Average lateral interaction energy obtained from the simulations. The differences observed between the points corresponding to the unmodified surface with respect to the others is less then the statistical error of the simulations. Key: (b) N ) 0; (O) N ) 10; (1) N ) 20; (3) N ) 30; (9) N ) 60; (0) N ) 90; ([) N ) 120.
the gas-solid interactions. The distributions of molecules with respect to their lateral interaction energy calculated during the simulation are all coincident and in agreement with the results displayed in Figure 6. From the simulated isotherms it is possible to calculate the adsorption energy distribution functions as has been described elsewhere.19 In Figure 7 the obtained distributions are presented which are essentially Gaussian in shape slightly distorted showing nonzero values at higher energies. For the unmodified surface the distribution is the narrowest of the set indicating that this surface is the (19) Cascarini de Torre, L. E.; Bottani, E. J. Colloids Surf. A 1996, 116, 285.
1900
Langmuir, Vol. 16, No. 4, 2000
Figure 7. Adsorption energy distribution functions obtained from the simulated isotherms.
Figure 8. Nitrogen cross-sectional area for molecules in contact with the surface: (b) N ) 0; (3) N ) 10; (4) N ) 30; (0) N ) 60; (O) N ) 90.
most homogeneous of the group. Moreover, the replacement of surface atoms with impurities even though they are less attractive increases the heterogeneity of the surface. This effect probably could be more evident if the impurities are also of different size. The energy maxima of the distributions that show an unusual order will be explained later on. The introduced heterogeneity affects the adsorptive properties of the surface in a subtle way since it does not alter neither the average cross-sectional area of the molecules adsorbed in direct contact with the surface (see Figure 8) nor the general shape of the density profiles (see Figure 9). The profiles shown in Figure 9 have been calculated at the same equilibrium pressure, and the differences observed are due to differences in the number of adsorbed molecules. More information could be obtained from the analysis of the local isotherms. From the configurations stored
Cascarini de Torre et al.
Figure 9. Density profiles as a function of the distance to the surface. All the profiles have been calculated for the same equilibrium pressure, and the differences observed are due to differences in the average number of adsorbed molecules.
Figure 10. Local adsorption isotherms corresponding to cell no. 1. The total numbers of impure atoms/actual number of impure atoms are the following: N ) 0/0; (O); N ) 10/3 (3); N ) 30/6 (0); N ) 60/14 (]); N ) 90/18 (4).
during the simulation it is possible to determine the average number of adsorbed molecules on arbitrary chosen regions of the surface. If the surface is divided into 16 cells, each one has an area of 0.771 nm2. The average value obtained by employing the BET method for the local isotherms is 0.761 nm2, and the monolayer capacity of each cell is between 4 and 5 molecules. The local isotherms have been calculated for surfaces having 0, 10, 30, 60, and 90 impure atoms, and the actual number of impure atoms in cell no. 1 are 0, 3, 6, 14, and 18, respectively. The calculated local isotherms for this cell are shown in Figure 10. It is expected that as the number of impure atoms increases the surface becomes less attractive and in consequence the adsorbed quantity should decrease. In Figure 10 it can be noticed that the isotherms corre-
Nitrogen Physisorption on Modified Silica Surfaces
sponding to the surfaces with 10 and 30 impure atoms adsorb more than the untreated surface at the low- and high-pressure limits. At intermediate pressures where the most energetic regions of the cell are already occupied it is verified that an increase in the number of impure atoms corresponds to smaller adsorbed quantities. The order of the isotherms at low pressures can be explained on the basis of what could be defined as a local heterogeneity effect. In fact, a molecule adsorbed on a given cell of the untreated surface will be located where the adsorption energy is the largest within the cell. At this point the molecule will adopt the most favorable orientation at a certain distance from the solid atoms within its interaction range. When one impure atom replaces a “normal” surface atom in the cell, the adsorbed molecule should experience a decrease in the gas-solid energy unless it changes its orientation and/or the distance to the surface. In our simulations “normal” atoms are 2.4 times more attractive than the impurities. A simple calculation based on the Lennard-Jones potential (disregarding lateral interactions since we are considering the low-pressure region of the isotherms) shows that to experience the same attractive energy the adsorbed molecule should be a 16% closer to the impure atom than to the normal atom; to experience the same repulsion energy it should be an 8% closer. The net effect is that the molecule tends to be closer to the surface due to a net increasing in interaction energy with all the atoms in the cell. This situation will continue until there are so many impure atoms in the cell that their less attractive potential prevails upon the normal atoms. As examples in Figure 11a,b, the energy maps of cell no. 1 for the untreated surface and the one that has three impure atoms are shown. These maps represent the potential energy of the molecule averaged over all the orientations. It must be pointed out that the topography of the cell is not changed when impure atoms replace the normal ones since they are located at the same coordinates and have the same size. The net increase in potential energy is approximately 41%, which is consistent with an average 12% shortening in the molecule-to-surface distance estimated with the Lennard-Jones potential. These arguments also explain the energy maxima position of the adsorption energy distribution functions shown in Figure 7 as well as the obtained heterogeneity degree. The same argument can explain the trend in the specific surface area of the different samples. The fact that the adsorbed molecules could be closer to the surface and in consequence form a denser film explains the increase in surface area (through the increase in the monolayer capacity) of the sample with 10 impure atoms. When the number of impurities increases, their less attractive character begins to compensate the larger attraction of normal atoms and the specific surface area decreases. Conclusions The modifications of silica surfaces produced by thermal and chemical treatments performed in this work, as indicated by IR spectra and optical microscopy, are basically changes in the hydroxylation degree of the surface. Nitrogen adsorption is interpreted with the aid of Monte Carlo computer simulations. The specific surface area values obtained with the BET method are explained with simple arguments based on the gas-solid potential
Langmuir, Vol. 16, No. 4, 2000 1901
Figure 11. (a) Energy map of cell no. 1 corresponding to the untreated surface. (b) Energy map corresponding to the same cell in which three surface atoms have been replaced. The approximate locations of these atoms are indicated by the lines labeled 7, 8, and 10.
behavior. What we called a local heterogeneity effect could be classified as energetic heterogeneity. This kind of heterogeneity affects nitrogen adsorption through the gas-solid potential. Further studies should be pursued to know if these conclusions could be extended to more complex adsorbates. Acknowledgment. This research project is supported by grants from the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas-CONICET (PIP 448/98), Universidad Nacional de La Plata-UNLP (11/X223), and Comisio´n de Investigaciones Cientı´ficas de la Provincia de Buenos Aires-CIC (individual grants). L.E.C.d.T. and E.J.B. are researchers of CIC, and E.J.B. is an Associate Professor of the Engineering Faculty, UNLP. LA990669N
