CVD Germania on Pyrogenic Silica - Langmuir (ACS Publications)

Jan 22, 1997 - Germania on a pyrogenic silica surface (GS) has been synthesized by chemical vapor deposition (CVD) and investigated by infrared (IR), ...
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Langmuir 1997, 13, 250-258

CVD Germania on Pyrogenic Silica V. M. Gun’ko,* E. F. Voronin, V. I. Zarko, and E. M. Pakhlov Institute of Surface Chemistry, 31 Prospect Nauki, Kiev 252022, Ukraine Received May 6, 1996. In Final Form: September 27, 1996X Germania on a pyrogenic silica surface (GS) has been synthesized by chemical vapor deposition (CVD) and investigated by infrared (IR), optical, and dielectric relaxation (DRS) spectroscopies and quantum chemical ab initio and semiempirical MNDO, AM1, and PM3 methods. The GS synthesis was carried out via GeCl4 vapor chemisorption and subsequent hydrolysis of the GesCl bonds in the OsGeCl3 groups bound to the surface. In the IR spectra of GS, absorption bands of tSiOsH at 3750 cm-1, tGeOsH at 3682 cm-1, and the strained rings dGeOOSid at 885 cm-1 were observed. According to the dielectric relaxation and optical spectra of water and n-dimethylaminoazobenzene (DMAAB) adsorbed on GS, the new surface sites are generated on GS having more acidic properties compared with those for the parent silica. Their concentration depends on the germania content, GS hydration degree, and pretreatment temperature. Structures of these sites and adsorption complexes have been simulated by quantum chemical ab initio and semiempirical methods.

Introduction Synthesis of oxides on highly disperse pyrogenic silica makes possible the production of new inorganic materials, and synthetic conditions can be varied to control mixed oxide properties.1-4 Germania/silica (GS) glasses and other materials involving GeO2/SiO2, possessing, for example, interesting optical properties, have attracted attention in the last few years.5-9 The multicomponent oxide systems including GS may also be useful if the active surface sites should have acidic properties weaker than those for alumina/silica but stronger than those on silica. Materials with GS can have unique optical characteristics depending on the synthesis method, but in so doing highly disperse oxides synthesized on the pyrogenic silica substrates can have peculiar surface structures and properties. Therefore, the aim of this study is CVD synthesis of germania onto highly disperse silica and an investigation of the nature of the active surface sites of GS by experimental and theoretical methods. Materials and Techniques The pyrogenic silica samples (Aerosil, 99.5% purity, specific surface area of 300 m2 g-1, “Chlorovinyl”, Kalush) were placed in a glass reactor equipped with a mixer. The loaded reactor was purged with dry air at 473 K for 1 h and then cooled to 423 K; subsequent stages of GS synthesis were performed at 423 K, with intensive stirring. Thereupon the corresponding amount of GeCl4 (99.5%) vapor was bled into the reactor through a leak, and chemisorption of GeCl4 onto silica occurred for 1 h. Thereafter * Author to whom correspondence should be addressed. Fax: 380 044 264 0446. Telephone: 380 044 265 6731. E-mail: lena%[email protected]. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Tertykh, V. A.; Belyakova, L. A. Chemical Reaction Involving Silica Surface; Naukova Dumka: Kiev, 1991. (2) Chuiko, A. A.; Gorlov, Yu.I. Chemistry of Silica Surface; Naukova Dumka: Kiev, 1992. (3) Pakhlov, E. M.; Zarko, V. I.; Voronin, E. F. Ukr. Khim. Zh. 1993, 59, 376. (4) Voronin, E. F.; Zarko, V. I.; Kozub, G. M.; Pakhlov, E. M. Zh. Fiz. Khim. 1993, 67, 123. (5) Keskar, N. R.; Chelikowsky, J. R.; Wentzcovitch, R. M. Phys. Rev. B: Condens. Mater 1994, 50, 9072. (6) Dong, L.; Pinkstone, J.; Russell, P. S.; Payne, D. N. J. Opt. Soc. Am. B: Opt. Phys. 1994, 11, 2106. (7) Paine, D. C.; Kim, T. Y.; Caragianis, C.; Shigesato, Y. Z. J. Electron. Mater. 1994, 23, 901. (8) Sakaguchi, S. J. Non-Cryst. Solids 1994, 171, 228. (9) Barrio, R. A.; Galeener, F. L.; Martinez, E.; Elliott, R. J. Phys. Rev. B: Condens. Mater 1993, 48, 15672.

the corresponding amount of water was leaked into the reactor, and hydrolysis of the Ge-Cl bonds occurred for 1 h; then the reactor with a sample was purged with dry air at 473 K for 1 h for removal of HCl. Five species of germania/silica GSi, i ) 1-5, containing 1.5, 3, 6, 11, and 20 wt % germania, respectively, have been synthesized using several cycles of GeCl4 chemisorption onto silica only for GS5 synthesis. Highly disperse pyrogenic alumina/silica (8 wt % alumina, specific surface area 180 m2 g-1) was used for comparison with GS. The pressed samples (8 × 28 mm2) weighing 15 mg were used for documentation of the IR spectra by a UR-20 (Germany) spectrophotometer. A portable cell with optical CaF2 windows was used for pretreatment of the samples in vacuum and for reaction with trimethylchlorosilane (TMCS). The dielectric characteristics of hydrated GS were examined by a BM-560 (“Tesla”) Q-meter in the 0.1-10 MHz range for the samples (30 mm diameter, 1.3-3.0 mm thickness) pressed at 3 MPa. Measurements were performed in a standard cell for dielectric property examination, and the temperature was maintained to (2 °C. The acidic surface sites of GS and alumina/silica were investigated by an Hammet color indicator, (dimethylamino)azobenzene (DMAAB) (Chemapol, 99.9%) using its diffuse reflection spectra recorded on a SF-18 (Russia) spectrophotometer. Thereupon these data were converted to the absorption spectra using the Kubelka-Munk formula. DMAAB adsorption onto the samples thermoevacuated previously was carried out from the gas phase at 338 ( 5 K for 2 h. Assignment of the DMAAB absorption bands was done by comparison with the spectra of this indicator in neutral and acidic solutions and based upon previous literature reports.10-12 According to them, DMAAB can be adsorbed on oxides in four states with the corresponding bands: I, 430-470 nm, physisorption via dispersion interaction or weak hydrogen bonds (d-DMAAB); II, 480-500 nm, hydrogenbonded complexes (H-DMAAB); III, 520-550 nm, H+ transfer from the Brønsted acid sites (B-site) as tM1sO(H)sM2t (e.g., M1 ) Al, Ge; M2 ) Si) or adsorbed proton-donor molecules (e.g., tM1(OH)rOH2) to DMAAB (H+DMAAB) as follows Ph

N

PhNR2 + H+

N A

Ph

NH+

N

B Ph

N

PhNR2 + H+

N A

Ph

C

C

C

C

C

N

C

N

NR2

PhNH+R2

(1)

(2)

B

where R ) CH3; IV, 555-560 nm, DMAAB interaction with the Lewis acid sites13 (L-sites) via the donor-acceptor bonds with incompletely O-coordinated metal atoms tMrNDMAAB (LDMAAB).

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Theoretical investigations were carried out by quantum chemical ab initio14 and semiempirical MNDO,15 AM1,16 and PM317 methods and by molecular mechanics (MM).18 The ab initio calculations were carried out by the Gaussian 92 program package14 with the Los Alamos ECPs (effective core potentials) and the Dunning/Huzinaga valence double-ζ basis set for H (2s), O (3s6p), Si (2s6p), and Ge (2s6p) (LANLDZ).19-22 For the calculations of the simple silica models the 6-31G(d) and 6-31G(d,p) basis sets have been applied. Full geometry optimization has been performed for all ab initio, semiempirical, and MM calculations. The use of ab initio and a few semiempirical methods allows us to elucidate possible errors, which can appear especially in the semiempirical calculations. However, large oxide clusters can be studied only by the semiempirical methods; therefore, the ab initio and semiempirical calculations complement each other.

Results and Discussion The absorption bands of the tSiOH groups at 3750 cm-1, adsorbed water at 3500 cm-1, and a band at 3682 cm-1 are observed in the IR spectra of GS (Figure 1). From examination of these spectra (Figure 1, curves 2-5) and in agreement with literature data,23 the band at 3682 cm-1 may be assigned to the tGeOsH stretching vibrations. These groups are thermally stable to 673-723 K (Figure 1). On further heating the tGeOsH band, intensity falls off noticeably. After pretreatment of GS3 at 973 K and sequential rehydration in air at room temperature, the intensity of νOH for tGeOsH increases substantially (Figure 1). A similar dependence was observed for all GSi samples. When the germania content (CG) in GS increases to 10 wt %, a monotonic decrease of the tSiOH concentration is detected, which eventually levels off (Figure 2). The tGeOH concentration increases as CG grows to 6 wt %, after which it remains practically constant (Figure 2). When the germania content increases to 20 wt %, the GeO2 phase can cover more and more area of the silica surface, but the intensity of νOH for the tSiOsH and tGeOsH groups does not change practically for CG g 11 wt % (Figure 2). Consequently, a shell of germania on silica is not continuous; i.e., the GeO2 phase can have a cluster, patch structure. If previously thermoevacuated GSi species interact with water vapors that the intensity of νOH for tSiOsH does not change but the tGeOsH band intensity increases approximately by a factor of two (Figure 2), that may be connected with structural changes of the germania phase, which is more sensitive to dehydration/rehydration processes than pyrogenic silica. Absorbance in the IR range 800-1000 cm-1 for GS and parent silica differs noticeably (Figure 3). A band with low intensity at 885 cm-1 appears on GS heating, but at (10) Zubareva, N. A.; Kiselev, A. V.; Lygin, V. I. Kinet. Katal. 1974, 15, 483. (11) Sivalov, E. G.; Tarasevich, Yu.I. Adsorpts. Adsorbenty, 1982, N10, 49. (12) Brazdil, J. F.; Yeager, E. B. J. Phys. Chem. 1981, 85, 1005. (13) Tarasevich, Yu.I.; Sivalov, E. G. Dokl. Akad. Nauk Ukr. SSR, Ser. B 1978, N3, 252. (14) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, Revision G.3; Gaussian, Inc.: Pittsburgh, PA, 1992. (15) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (16) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. E.; Stewart, J. J. J. Am. Chem. Soc. 1985, 107, 3902. (17) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. (18) Burket, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982. (19) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (20) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (21) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (22) Dunning, T. H.; Hay, P. J. Modern Theretical Chemistry; Plenum: New York, 1976; Chapter 1, pp 1-28. (23) Fink, P.; Camara, J. B.; Welz, E. Z. Chem. 1971, 2, 473.

Figure 1. IR spectra of GS3 in air (1); after evacuation at 293 (2), 473 (3), 723 (4), and 973 K (5); after air bleed-in at 293 K (6); for 17 h of exposure (7).

Figure 2. Dependence of the optical density of the absorption bands for tSiOH (3750 cm-1) (O, b) and tGeOH (3682 cm-1) (0 ,9) as a function of germania content in GS solid (b, 9) after rehydration in air.

higher temperature it disappears (Figure 3a). This band may be due to the strained rings O Ge

Si O

which are analogous to the dSi(O)(O)Sid rings for silica having the νSiO bands at 888 and 908 cm-1 on heating at

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Figure 3. IR spectra in the range 800-1000 cm-1. Treatment temperatures for GS5 (a): 290 (curve 1), 380 (2), 470 (3), 570 (4), 670 (5), 770 (6), 920 (7), 1020 K (8). For parent silica (b): 290 (curve 1), 470 (2), 720 (3), and 920 K (4). Dashed line (7a) corresponds to GS5.

high temperatures.24 At the same time the IR spectra of GSi do not have other new separated bands corresponding to the MO stretching vibrations (M ) Si, Ge) in comparison with those for parent silica. However, some broadening of the bands in the range 800-1000 cm-1 and a slower lowering of the corresponding intensities are observed with thermoevacuation of GS in comparison with parent silica (Figure 3), which may be caused not only by formation of the separated GeO2 phase but also by the GeO2-SiO2 interfaces. The availability of these interfaces can influence adsorption of polar molecules (water and others) as new surface active sites can form there. On the basis of the data obtained, the chemisorption and hydrolysis of GeCl4 on the silica surface can occur via two pathways as follows SiOH + GeCl4

Si –HCl

A

O

+nH2O

GeCl3

–mHCl

B (

SiO)3GeOH

(3)

SiOH∗GeO2

(4)

C

i.e., the tSisOsGet bridges can form via eq 3 as well as the separated GeO2 phase via eq 4. The tSisOsGet bridges were detected in GeO2/SiO2 glasses,25 and the Ge atoms remained 4-fold O-coordinated for pressures up to 1.8 × 103 MPa.26 In the case of alkali germano-silicate glasses not only the GeO4 units were found but also GeO6.27 Interaction of the tSiOH and tGeOH groups of GS species with trimethylchlorosilane, TMCS, SiOH

SiOSi(CH3)3 + HCl

(5)

GeOSi(CH3)3 + HCl

(6)

+ ClSi(CH3)3 GeOH

has been studied for comparison of their reactivities. These groups readily react with TMCS vapor at 653 K (Figures 4 and 5). A study of the tSiOH group reactivity for GS3 and parent silica upon interaction with TMCS shows a (24) Morrow, B. A.; Gay, I. D. J. Phys. Chem. 1988, 92, 5569. (25) Sharma, S. K.; Matson, D. W.; Philpotts, J. A.; Roush, T. L. J. Non-Cryst. Solids 1984, 68, 99. (26) Kushiro, I.; Sharma, S. K.; Matson, D. W. J. Non-Cryst. Solids 1985, 71, 429.

Figure 4. IR spectra of GS3 (a) and parent silica (b) after thermoevacuation at 870 K (1) and after reaction with TMCS at 650 K for 45 min and subsequent degassing (2).

weak influence of the GeO2 phase. For the sake of simplicity we compared the degree of substitution (χ) of OH (in tSiOH, 3750 cm-1) for OSi(CH3)3 at 653 K for 45 min. The χ value was calculated as follows

χ ) 1 - Dt/D0

(7)

where D0 and Dt are the optical density of the tSiOsH band at 3750 cm-1 pre- and postinteracting with TMCS (Figures 4 and 5). We obtained χ ) 0.768 for parent silica and χ ) 0.748 for GS3; i.e., the influence of the GeO2 phase on the tSiOH group reactivity is really weak. The tSisOsGet bridges forming via the tGeOH group reaction with TMCS can be rather easily hydrolyzed by water adsorbed on the surface from air even at ambient temperature (Figure 5, curve 5). These results agree closely with data for the interaction of organosilicon compounds with the tSiOH groups and their hydrolizability on other mixed oxides as titania/silica and alumina/ silica.28 Consequently, adsorbed water can have a dramatic effect on the properties of the parent or modified GS surfaces. Further information about water adsorbed on the GS surface can be obtained by the dielectric relaxation spectroscopy (DRS) method.29,30 Relaxation processes observed by the DRS method for hydrated oxides are mainly due to adsorbed water. Both for the silica and GS4 samples the relaxation maxima for dielectric loss, ′′(T), can be observed in two temperature ranges: I, 240320 K; II, T < 230 K (Figures 6 and 7). According to our (27) Osaka, A.; Ariyoshi, K.; Takahashi, K. J. Non-Cryst. Solids 1986, 83, 335. (28) Pakhlov, E. M.; Voronin, E. F.; Bogillo, V. I.; Chuiko, A. A. Dokl. Akad. Nauk Ukr. SSR, Ser. B 1989, N8, 50. (29) McIntosh, R. L. In The Solid-Gas Interface; Flood, E. A., Ed.; Marcel Dekker: New York, 1967. (30) Morimoto, T.; Iwaki, T. J. Chem. Soc., Faraday Trans. 1 1987, 83, 943.

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Figure 7. Dielectric loss ′′ of GS4 for the water amounts 0.01 g/g (1), 0.07 g/g (2), and 0.18 g/g (3); f ) 8 MHz. Table 1. Activation Energy of Polarization of Water Adsorbed on Silica and GS at 150 < T < 230 K NN 1a 2b 3b 4 5 6 7 8

species SiO2 SiO2 SiO2 SiO2 GS1 GS4 GS4 GS4

CW, wt %

Ep, kJ mol-1

5.0 10.0 25.0 45.0 2.4 1.0 7.0 18.0

12.6

a Result from our previous work.32 K is not observed.

Figure 5. IR spectra of GS3 after evacuation at 873 K (1), after bleed-in of saturated TMCS vapor at 293 K (2), after reaction with TMCS at 653 K for 45 min (3), after leak-in of air at 293 K (4), after exposure for 17 h (5), and after heating in air at 673 K (6).

Figure 6. Dielectric loss ′′ of hydrated parent silica for the following water amounts: 0.10 g/g (1), 0.25 g/g (2), and 0.45 g/g (3); f ) 8 MHz.

previous reports,31-34 we can assume that the first is connected with the relaxation of the molecules inside the large tridimensional clusters of liquid water adsorbed on the GS surface (freezing of interfacial water for titania/ (31) Tishchenko, V. A.; Gun’ko, V. M. Colloids Surf. A 1995, 101, 287. (32) Zarko, V. I.; Gun’ko, V. M. Funct. Mater. 1995, 2, 110. (33) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Voronin, E. F.; Tischenko, V. A.; Chuiko, A. A. Langmuir 1995, 11, 2115. (34) Gun’ko, V. M.; Turov, V. V.; Zarko, V. I.; Voronin, E. F.; Tischenko, V. A.; Dudnik, V. V.; Pakhlov, E. M.; Chuiko, A. A. Langmuir, submitted.

b

21.0 18.5 19.3 22.3 15.2 A ′′ maximum at 150-230

silica or alumina/silica occurs at 210 K < T < 273 K, and the stronger is water interaction with a surface, the lower is a freezing temperature, and the larger is unfrozen water clusters (in air) or an interfacial layer (in aqueous suspension)33,34). Formation of such large clusters is connected with the absence of a continuous layer of water adsorbed on silica (or germania) even for a high amount of adsorbed water (up to 30 wt % for silica);32,33 therewith, these clusters form near the tSiOH groups. It is known32 that the ′′ value linearly depends on the water amount (i.e., on cluster size) at T ≈ 300 K for pyrogenic silica that corresponds to the absence of the continuous water layer; i.e., the increase of the drop-shaped water clusters on the surface can occur without their integration, as the distance between neighboring tSiOH groups on pyrogenic silica is near 0.7 nm.1,2 The second temperature range (T < 230 K) of relaxation is proper for the adsorbed water clusters possessing mainly the two-dimensional structure with smaller size than tridimensional ones when a major part of these water molecules has direct contact with the oxide surface and each molecule has only one or two hydrogen bonds (i.e., this water is neither liquid water nor ice).31-34 Therefore, this water should be more sensitive to the nature of the active surface sites which produce strong local electrostatic fields, changing the hydrogen bond network structure in interfacial water and lowering its freezing temperature.33,34 That has an influence on the ′′(T) function behavior and on the change of the structure of the adsorbed water clusters on GS compared with those on the parent silica surfaces; e.g., the ′′ value for T < 230 K is higher for GS than that for silica even for smaller adsorbed water content for GS (Figures 6 and 7); consequently, the amount of two-dimensional water in the clusters strongly ordered by the surface is higher for GS than for silica. It can be seen that the activation energy of the Debye type polarization (Ep)29 grows as the germania content increases (Table 1) (i.e., the average number of hydrogen bonds per one water molecule increases as Ep grows;

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consequently, the adsorbed water clusters increase in size). The calculation of Ep may be performed via a relaxation maximum shift for two frequencies fi

Ep ) RT1T2 ln(f2/f1)/(T2 - T1)

(8)

where Ti is the temperature of a maximum of ′′(fi). The Ep value for GS4 with a different content of adsorbed water (CW) exhibits a maximum value for CW near 7 wt % (Table 1). The Ep value dependence on CW for silica differs from that for GS at T < 220 K (Table 1). Such behavior of the Ep value can testify to the different nature of the twodimensional water clusters growing on the silica and GS surfaces. This may be caused by formation of the main part of the adsorbed water clusters at the interfaces of GS, which have different spatial and electronic structures with higher polarizability in comparison with the parent silica surface; i.e., precisely the interfaces have a strong influence on the structure of the adsorbed water clusters. Adsorption phenomena observed upon interaction of polar molecules with GS may be different in comparison with those on parent silica and germania. The structure of the water clusters growing on the GS surface, as indicated by increasing the ′′max value (Figures 6 and 7), can result from peculiarities of the GS interfaces as more active adsorption regions on the surface. The characteristics for these interfaces and active surface sites can be studied via adsorption of the indicator molecules and analysis of their optical spectra depending on the nature of the adsorption sites. The optical spectra of DMAAB adsorbed on GS have new bands (Figure 8) in comparison with the analogous spectra for parent pyrogenic silica, for which only one or two (on heating) bands are detected (Figure 9). The latter bands are due to formation of adsorption complexes via the dispersive and hydrogen bonds to the surface. We may assume that a weak absorbance at 525 nm observed for silica (Figure 9b) is connected with formation of the H+DMAAB complexes. Consequently, a very small quantity of weak B-sites may be found even on parent silica. It should be noted that the DMAAB spectra for GS1 (Figure 8a) are close to the spectra for silica (Figure 9) and do not have practically the H+-DMAAB band in contrast to those of GS5 (Figure 8b, curve 2); i.e., the germania content plays an important role in acid site generation, but increasing the acidic properties of GSi, as CG grows, can be due to the change of the concentration of the acid sites (as an increase of the content of alumina in zeolites leads to some decrease of the acidity of the B-sites35,36). DMAAB adsorbed on GS5 has the bands corresponding to appearance of the acid sites with different nature: B-sites (Figure 8b, curve 2) and L-sites (curve 3). The intensity of the H+DMAAB band at 530 nm is higher than that for L-DMAAB at 555 nm; i.e., the B-site content on GS surfaces can be higher at such a pretreatment temperature. The spectrum of GS5 looks like the DMAAB spectrum of alumina/silica (Figure 8c). It is known35-38 that alumina/silicas as crystalline (e.g., zeolites) or amorphous (e.g., pyrogenic) modifications have the B- and L-sites. The Al2O3/SiO2 sample has a more complicated spectrum for adsorbed DMAAB, as it is a more complex oxide than GS and Al atoms in pyrogenic Al2O3/SiO2 can have coordination numbers 4 or 6,34 but Ge is only 4-fold O-coordinated in GS and incompletely O-coordinated Al (35) Kazansky, V. B. Zh. Fiz. Khim. 1985, 59, 1057. (36) Kazansky, V. B. Usp. Khim. 1988, 57, 1937. (37) Tanabe, K. Solid Acids and Bases; Kodansha: Tokyo, 1970. (38) Brey, V. V.; Guba, G. Ya.; Gulyanitskaya, N. E. Zh. Prikl. Khim. 1994, 67, 377.

Figure 8. Optical spectra of DMAAB adsorbed on GS1 (a), GS5 (b), and Al2O3/SiO2 (8 wt % of alumina) (c) after dehydration in vacuum at 973 K (a and b) and 623 K (c). Bands: d-DMAAB (curve 1a and c); H-DMAAB (curves 2a and c and 1b), H+DMAAB (curves 2b and 3c); L-DMAAB (curves 3b and 4c).

Figure 9. Optical spectra of DMAAB adsorbed on parent silica after dehydration in vacuum at 373 K (a) and 773 K (b). Bands: d-DMAAB (curves a and 1b); H-DMAAB (curve 2b).

can be a stronger L-site in alumina/silica than Ge in germania. Some similarity of the spectra of DMAAB adsorbed on GS and alumina/silica (Figure 8b and c) confirms our conclusion about the nature of the active surface sites on GS as the B- and L-sites.

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a water molecule to form the t(OH)GerOH2 complex than for GeCl4 to form tSiO(H)fGeCl4. As is clear from the experimental studies, there are a few types of active acidic sites at the GS surface. We can assume that they are free tGeOH and tSiOH groups or generated via molecular and dissociative adsorption of water as follows Ge(OH)

OH2,

OH2,

Si(OH)

Ge

O

Si

,

Si

,

OH2

A

B

C

OHδ–

Ge

O

Si

,

Ge

O

OH2

D

Si

,

OH

Hδ+

Ge

O

OHδ–

Hδ+ E

F (10)

Figure 10. GS clusters (O*3SiO)2Si(OH)OGe(OH)(OSiO*3)2 (a) and (O*3SiO)3SiOGe(OSiO*3)3 (b) used for semiempirical calculation: initial structure of the clusters (a and b); complex with donor-acceptor bond GerOH2 (c); complex with dissociatively adsorbed water molecule on the tGesOsSit bridge (d).

The spectral studies give results in agreement with the pH measurements of aqueous suspensions of silica and GS (1 wt %): for silica pH ) 5.15 at room temperature, for GS1 pH ) 5.17, and for GS5 pH ) 5.00; i.e., GS1 is close to silica, but GS5 gives a more acidic suspension as a result of the increase in the content of the acidic sites. Experimental studies by IR, optical, and DRS methods give but a rough idea of the nature and structure of the active sites at the germania/silica surface, which is distinct from the free tGeOH and tSiOH groups. Theoretical modeling of the surface sites can close this gap. Theoretical Simulation Quantum chemical calculations were carried out in a cluster approach. For modeling by semiempirical MNDO, AM1, and PM3 methods, a few types of the germania/ silica clusters were used (Figure 10, Table 2), where O* is a pseudoatom of oxygen. Only one or two polyhedron models were used in the ab initio calculations (Table 3). A GeCl4 molecule has an electron-acceptor level (the lowest unoccupied MO, Tables 2 and 3, ELUMO), which is lower than the bottom of the conduction zone of silica and germania, and a significant polarizability (one order of magnitude higher than that for a water molecule), which is connected with lone electron pairs of Cl (the highest occupied MO, EHOMO). Besides, the Ge charge is high (Tables 2 and 3, qM). Therefore, the complex H • • • Cl (9) SiO

Ge Cl3

can form, and reaction 3 occurs according to electrophilic substitution SEi(OSi) of an H atom linked with OSi. The change in total energy for reaction 3A f 3B equals -20 kJ/mol. The subsequent hydrolysis of the GesCl bonds is exothermic too, but for the tSiOGeCl3 group ELUMO is higher than that for GeCl4; however, qGe(Cl3) > qGe(Cl4) (Table 2). The ELUMO level for tGeOH is still higher than that for tSiOGeCl3; i.e., the electron-acceptor properties of the tGeOH groups are lower, and it is more difficult for

If the amount of water adsorbed on GS is high enough, the complexes 10C and 10E can transform with hydrolysis of the tGesOsSit bridges and formation of new tMOH groups; e.g., the increase in the content of tGeOH is observed after rehydration of GS (Figure 2). The strained rings can form on GS heating as a result of associative desorption of the water molecules from the neighboring tGeOH and tSiOH groups. These rings can transform to charged or radical centers as follows O Ge

Si O A

Geδ+(•)

O

Si(Oδ–(•))

(11)

B

The rings in 11A can interact with DMAAB as L-sites, and they are more active than tMOH groups (Table 2, ELUMO). Analogous rings in heated pure silica have IR bands at 888 and 908 cm-1.24 The absorption band at 885 cm-1, which may be caused by formation of the strained rings in 11A, is observed for heated GS (Figure 3). The AM1 calculations of the IR spectra in an harmonic approach scaling the force constants according to data for the strained rings in silica (888 and 908 cm-1) give νMO at 735 and 883 cm-1 for such rings of GS and at 672 and 722 cm-1 for pure germania (i.e., for dGeOOGed). Therefore, the observed band at 885 cm-1 (Figure 3) may be assigned to the SiO stretching vibrations for the strained rings in 11A but not for the GesO in 11A, as νGeO ) 735 cm-1. According to the near linear correlation between the acidic properties of the B-sites of oxides and the νOH value for the OH stretching vibrations,39 the tGeOH groups can have higher acidic properties (as the B-sites) than those for tSiOH, as νOH for tGeOsH is lower than that for tSiOsH (Figure 1). Therefore, we can assume that the band at 530 nm for H+DMAAB (Figure 8) can be partially (left side of the corresponding curve) due to interaction of the indicator molecules with terminal tGeOH groups. The right side of this band may be due to DMAAB interactions with adsorbed water molecules in the complexes such as hydrogen-bonded, donoracceptor, and dissociative interactions (i.e., forming the B-sites). The MNDO calculations of the t(HO)GerOH2 complexes do not give a stable donor-acceptor bond GerOH2 (Table 2), which can be caused by the errors in the corecore interaction in MNDO (corrected in AM116), since by (39) Trokhimets, A. I. Zh. Prikl. Spectrosk. 1985, 41, 987.

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Table 2. Parameters of Molecules and Complexesa ∆Et, -EHOMO, ELUMO, kJ/mol eV eV µ, D

structure GeCl4 R3SiOGeCl3 R3SiOGe(OH)3 R3GeOH X3GeOH R3GeOSiR3

-20b

dGe(OH)OSi(OH)d R3SiOGe(Cl3)rOH2 R3Ge(OH)rOH2 R3Si(OH)rOH2 d(OH)Ge(rOH2)OSid tGe(rOH2)OSit tSi(rOH2)OSit tGe(rOH-)(OH+)Sit dHOGerOH-(OH+)Sit tSi(rOH-)O(H+)Sit X3Ge(OH)rOH2 tGe(rOH2)OGet tGe(rOH-)O(H+)Get tGe(O)OSit tGe(O)OGet

18 0 -37 -73 -83 -21 -6 -165 -53 -46 -82 0 -15 -10

13.78 11.42 11.06 10.59 11.01 10.79 10.73 10.52 11.27 10.70 10.61 10.26 10.40 10.42 10.58 10.64 8.54 10.24 10.65 10.86 9.44 9.05 10.96 10.99

-3.26 -2.20 -0.48 -0.30 1.61 -0.24 -0.44 -0.52 -1.51 0.31 0.82 -0.02 0.00 0.52 -0.09 -0.27 1.05 -0.46 0.38 1.71 -0.34 -1.03 -0.62 -1.23

0.0 5.04 2.56 2.31 1.18 0.11 0.60 2.06 8.18 1.30 1.29 1.91 2.98 2.44 0.56 0.47 0.87 5.83 0.30 1.40 1.78 6.78

qM

-qC

1.677 1.874 2.026 2.163 2.177 2.211 2.124 2.010 1.968 2.182 1.897 2.023 2.128 1.941 2.207 2.089 0.986 1.941 1.937 2.177 1.794 1.778 1.889 1.842

0.896 0.938 0.729 0.734 0.936 1.012 0.664 0.907 0.730 0.715 0.733 1.041 0.966 0.735 0.712 0.377 0.682 0.683 0.731 0.963 0.724 0.908 0.891

qH

0.203 0.196 0.196 0.215 0.192 0.233 0.213 0.191 0.203 0.192

rMO, nm rMrO, nm -qO(W) qH(W) rOH(W), nm

-qCl

0.169 85 0.181 24 0.180 85 0.181 01 0.175 70 0.182 12 0.186 46 0.175 03 0.181 52 0.174 88 0.187 82 0.185 88 0.171 05 0.181 52 0.195 20 0.186 30 0.197 14 0.185 49 0.181 28 0.189 38 0.198 25 0.186 57 0.186 63

0.419 MNDO 0.485 MNDO MNDO MNDO MNDO MNDO AM1 AM1 0.547 MNDO MNDO AM1 AM1 AM1 AM1 MNDO AM1 PM3 AM1 AM1 MNDO AM1 AM1 AM1 AM1

0.707

0.198 69 0.306 39 0.214 82 0.215 32 0.207 23 0.207 80 0.203 07 0.198 04 0.189 95 0.186 86 0.189 24 0.444 21 0.207 32 0.190 40

0.362 0.332 0.351 0.357 0.394 0.354 0.604 0.691 0.390 0.672 0.662 0.334 0.400 0.714

0.240 0.174 0.247 0.253 0.274 0.254 0.243 0.296 0.259 0.311 0.292 0.166 0.287 0.262

0.094 70 0.092 83 0.096 92 0.096 85 0.097 34 0.097 04 0.093 51

0.092 95 0.097 68

method

a Note: R ) O *SiO; X ) O *GeO. b ∆E for the reaction tSiOH + GeCl f tSiOGeCl + HCl; (W) is a parameter for an adsorbed water 3 3 t 4 3 molecule.

Table 3. Parameters of Molecules and Complexes (ab Initio) structure

∆Et, -EHOMO, ELUMO -EHF, au kJ/mol eV µ, D eV

590.891 69 Si(OH)4 590.920 55 Si(OH)4 305.665 72 Si(OH)4 305.506 84 Ge(OH)4 (OH)4SirOH2 667.904 90 (OH)4SirOH2 666.894 17 (OH)4SirOH2 666.881 66 (OH)4SirOH2 666.866 27 (OH)4SirOH- 666.343 37 (OH)4SirOH2 381.689 32 (OH)4GerOH2 381.532 66 (OH)4GerOH2 381.522 42 (OH)4GerOH- 380.994 18 R4GeOHSiR3 611.196 09 R3GeOSiR3

535.176 956 459.073 06

O R2Ge

-48 22 35 95 -329 -33 -39 -12 -422 -62

13.49 13.48 13.57 13.58 13.26 12.31 12.03 11.85 6.83 13.40 13.22 12.21 7.35 12.85 13.41

244

12.90

SiR2

qM

-qO

qH

rMO, nm

4.73 0.02 1.488 0.848 0.476 0.16289 4.79 0.01 1.492 0.737 0.364 0.16265 5.04 0.01 2.192 1.015 0.467 0.16423 4.21 0.85 2.144 1.002 0.457 0.17290 4.41 1.14 1.521 0.870 0.476 0.16583 4.96 2.92 1.533 0.892 0.474 0.16652 4.80 3.63 1.506 0.903 0.475 0.16739 4.68 4.09 1.473 0.912 0.476 0.16806 10.61 1.13 1.524 0.893 0.430 0.16965 4.82 2.70 2.187 1.039 0.472 0.16529 4.51 1.65 2.120 0.995 0.455 0.17377 4.73 4.06 2.069 1.015 0.432 0.17751 10.01 1.17 2.002 1.002 0.402 0.18024 4.28 2.36 2.121 1.006 0.456 0.17564 2.240 (Si) 0.16730 (Si) 4.24 0.05 2.243 0.995 0.457 0.17204 2.314 1.022 0.468 0.16404 (Si) 0.17092 3.22 0.22 2.034 0.989 0.470 0.17891 2.205 1.010 0.480

rHrO, nm

0.30743 0.195a 0.18a 0.17a 0.17904 0.31613 0.24517 0.185a 0.18705 0.22058

-qO(W)

0.911 0.873 0.832 0.791 1.004 0.847 0.861 0.880 1.044 1.124

qH(W) rOH(W), nm

0.474 0.515 0.529 0.540 0.418 0.445 0.464 0.530 0.393 0.549

0.097 47 0.095 43 0.095 64 0.095 81 0.094 47 0.095 50 0.095 52 0.095 59 0.095 03 0.095 61

basis 6-31G* 6-31G** LANLDZ LANLDZ MP2/6-31G* 6-31G* 6-31G* 6-31G* 6-31G* LANLDZ LANLDZ LANLDZ LANLDZ LANLDZ LANLDZ LANLDZ

0.17128 0.16206 (Si-OH)

O

GeCl4 a

62.579 38

13.75

-0.57 0.00 1.060

0.265

0.21864 (Cl)

LANLDZ

rMrO is fixed; R ) OH.

ab initio (Table 3) and AM1 (Table 2) methods the complexes with the GerOH2 bonds are stable. However, according to the ab initio calculations, the GerOH2 bond length (rGerOH2) is 0.07 nm longer than that for an ordinary GesO bond in germania as well as those calculated by AM1 or MNDO methods (Table 2, rMrO). Therefore, we can assume that a complex with rGerOH2 ≈ rGesO (with high polarity of the OsH bonds (Table 3, forced rGerOH2, qH(W))) is hardly probable as a generator of the Brønsted acid sites, as all calculations show a relatively weak donor-acceptor bond MrOH2 and qH(W) is close to qH in tMOH (Tables 2 and 3); i.e., the nature of the B-sites for GS differs from MrOH2. It is possible to form complexes like 10E or 10F with the participation of one or a few water molecules and one DMAAB molecule per one active site, when OH- and H+ generation is more probable. The energy of formation of the 10E complex is more than that for complexes of molecular type 10A-10D (Tables 2 and 3, ∆Et), but ∆Et

is higher for dissociative water adsorption on the germanosiloxane bond (10E) than that for the 10F complex (when Ge already has one OH group) in contrary to analogous complexes on the tAlsOsSit bridge34 (i.e., the process of degradation of the GS interfaces by hydrolysis of tSisOsGet can occur more slowly than that for alumina/silica, tSisOsAlt); i.e., the GS interfaces can be stable. Upon OH- interaction with the tGeOH or tGesOsSit groups (for OH- alone or with dissociation of H2O), the GerOH- bond length is close to the ordinary rGesO in germania and the energy of complex formation is high (Tables 2 and 3), which corresponds to a high solubility of silica or germania in basic solution relative to aqueous suspension. In the complexes (OH)4MrOH2 (M ) Si, Ge) with a fixed (forced) short MrOH2 bond (0.175-0.190 nm), the qH(W) values for H from H2O are higher than those for complexes with an optimized MrOH2 bond (Table 3, qH(W)). Therefore, with donor-acceptor bond formation upon

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Langmuir, Vol. 13, No. 2, 1997 257

Figure 11. PES section for dissociative adsorption of one water molecule on tSisOsMt (a); with addition of the second water molecule to the adsorption complex (b): M ) Si (1), Ge (2), and Al (3).

inelastic collision of a water molecule (having high kinetic energy) with a surface40 (when rMrOH2 can be shorter than that in the relaxed state), the polarization of the OsH bonds grows, which can promote water molecule dissociation as in 10E. DMAAB as a base can increase OsH polarization in complexes 10A-10D to the point of dissociation of a water molecule (even in the relaxed state at room temperature), and the H+DMAAB/MrOHcomplex forms as stable (i.e., complex 10E may result from simultaneous adsorption of the water and DMAAB molecules on one active site). However, we may assume according to the ab initio (Table 3) or AM1 and PM3 (Table 2) calculations that the stable complexes with dissociation of the water molecules can form without any influence of adsorbed DMAAB. The calculations of potential energy surface (PES) sections along the reaction pathway for reaction

tSisOsMt + H2O f tSisO(Hδ+)sM(OHδ-)t (12) (Figure 11) show that the activation energy for eq 12 and M ) Ge is lower than that for M ) Si and closer to that for M ) Al, especially if the second water molecule takes part in dissociative chemisorption of the first H2O molecule (with the H+ transfer from the first water molecule to the second and then to O from tSisOsGet). The possibility of stabilization of complexes such as 10E or 10F with mobile H+ is confirmed by the optical spectra of adsorbed DMAAB possessing the H+DMAAB band (Figure 8). Consequently, in the large water clusters adsorbed at the GS interfaces dissociation of the water molecules can occur with the activation energies Eq < 70 kJ/mol (Figure 11b), which do not correspond to a high temperature of reaction if we also take into consideration a high energy of ion solvation (Es) in large water clusters,34 as a part of Es can be used for overcoming the activation barrier for water molecule dissociation. The structural changes upon DMAAB interaction with the B-sites have been studied by AM1 and molecular mechanics. A proton can be linked to amino or azo groups (40) Gun’ko, V. M. Colloids Surf. A 1995, 101, 279.

Figure 12. DMAAB in initial state (a); H+DMAAB with H+ added to N from (b) the amino group as in eq 2b, (c) the azo group as in eq 1b, (d) the azo group but the other N.

of DMAAB. The AM1 calculations show (Table 4) that the probability of H+ bonding to the N from the azo group as in eq 1 is higher than that for H+ bonding to the N from the amino group as in eq 2 (Figure 12). But the difference in the energies of formation for these complexes is small (Table 4). The lowest energy of formation for H+DMAAB corresponds to H+ addition to the first N atom in the 1,4place relative to the amino group (Figure 12, Table 4). The AM1 results agree with the MM calculations, which suggest that the H+DMAAB 1B structure is 16 kJ/mol more stable than that in Figure 12b. A small distinction in the energies of formation of H+DMAAB with H+ bonding to amino or azo groups agrees with the probability of the generation of two types of H+DMAAB complexes. Formation of these two complexes leads to some broadening of the optical spectra of adsorbed DMAAB, as the acceptor (ELUMO) and donor (EHOMO) levels for these complexes are different (Table 4). Besides, a surface heterogeneity gives additional broadening of the optical spectra of adsorbed DMAAB, as its electronic structure is sensitive to the electronic states of the active surface sites and local electrostatic fields near these sites.

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Table 4. Parameters of DMAAB and H+DMAAB (AM1 Method) structure

-∆Et, kJ/mol

Me2NPhNdNPh

EHOMO, eV

ELUMO, eV

qN,amino

8.41

-0.16

-0.268

Me2NH+PhNdNPh

667

11.92

-4.36

-0.055

Me2NdPhdNsNH+Ph

690

12.10

-5.12

-0.195

Me2NsPhNH+dNPh

660

11.62

-5.42

-0.240

qN,azo -0.011 0.017 0.058 0.002 -0.097 0.118 0.070 0.073

qH

rHN, nm

0.248

0.10314

0.300

0.10180

0.302

0.10281

The structures of the (OH)4MrOH2 (M ) Si, Ge) complexes have some distinctions (Table 3); e.g., rMrOH2 is shorter for M ) Ge and ∆Et is higher in this case than those for M ) Si. However, the structure of (OH)4SirOH2 has some dependence on the basis set (Table 3). According to ab initio calculation,34,41 the energy of the complex tSiO(H)‚‚‚HOH equals nearly 20 kJ/mol, and for tSiOH‚‚‚OH2 it equals ≈30 kJ/mol, and experimental data42 correspond to 25-33 kJ/mol for the first complex and 3342 kJ/mol for the second. The calculations pertaining to electron correlation (6-31G(d)/MP2) give shorter rSirOH2 than that by LANLDZ, giving rSirOH2 shorter than that by 6-31G(d) (Table 3). But all ab initio calculations give a rSirOH2 value considerably longer than rSisO in silica (Table 3), but its energy is higher than that for simple hydrogen bonds between H2O and tSiOH groups. Sequential transformations of such complexes give the structures as in 10E or 10F. It should be noted that the MrOH2 bond for silica is more stable in complex 10B than that for tSisOsSit (however, rMrOH2 is longer for 10B), but for the GeO2/SiO2 interfaces complex 10C is more stable than 10A (Table 2). In the case of pure germania complexes, 10C and 10E (M1 ) M2 ) Ge) are less stable than those for GS interfaces (Table 2); i.e., the tM1sOsM2t bonds can be the most active sites for molecular and dissociative adsorption of water molecules if M1 * M2. The atomic charge of H (qH(W)) in 10E or 10F is only a trifle larger (0.04-0.05) than qH in the tMOH groups; i.e., charge is delocalized, and the effect of other water molecules for stabilization of complexes 10E or 10F is less than that for ion solvation in aqueous media.34 For example, addition of two H2O to H3+O gives ∆Et ≈ -130 kJ/mol, but addition of two H2O to tSi(rOH)O(H)Sit gives only ∆Et ≈ -30 kJ/mol (AM1). However, in the case of the complex 10E addition of two water molecules forming the hydrogen bonds GerOHδ-‚‚‚OH2 and >O-Hδ+‚‚‚OH2 gives ∆Et ) -90 kJ/mol (total ∆Et ) -254 kJ/mol relative to the pure GS cluster). Upon interaction of two water molecules with complex 10C, ∆Et is equal to -43 kJ/mol (total ∆Et )

-126 kJ/mol). Consequently, formation of the adsorbed water clusters is more probable at the GS interfaces, which agrees with the DRS data for the ′′(T) behavior of water adsorbed on GS in comparison with parent silica. Thus, complexes 10E and 10F, according to the ab initio and semiempirical calculations, are stable (Tables 2 and 3, ∆Et); i.e., one water molecule can dissociate on the GS interfaces and form the B-sites; however, the content of these sites may be small. In this case rGerOH(δ-) (where the remaining charge δ- ≈ -0.4) is less than rGerOH2 and close to rGesO in GeO2. This process can be written as follows

(41) Garrone, E.; Ugliengo, P. Mater. Chem. Phys. 1991, 29, 287. (42) Zhuravlev, L. T. Extened Abstracts of the International Conference on Oxide Surface Chemistry and Reaction Mechanisms, Kiev, September 13-19, 1992; Institute of Surface Chemistry: Kiev, 1992; p 4.

Acknowledgment. We are grateful to the SherwinWilliams Co. (U.S.A.) for financial support of this work.

OH Ge

O

Si

+ H2O

Ge

H O

Si

(13)

This change of the tGesOsSit bridge nature emphasizes an essential effect of adsorbed water on the interface properties for biphase oxides, as the tM1sOsM2t bonds can be more active adsorption sites for water molecules than tMOH groups. Conclusions The tGeOH groups are generated with germania synthesis on the silica surface via GeCl4 chemisorption, subsequent hydrolysis of the GesCl bonds, and formation of the GeO2 phase. The reactivity of the tGeOH groups in reaction with trimethylchlorosilane is comparable with that for tSiOH, and the germania phase in GS has a weak influence on the silica phase reactivity. The GS interfaces play an important role in adsorption of water molecules and vice versa. These molecules can form donor-acceptor complexes with Si or Ge atoms from the tSiOH, tGeOH, and tGesOsSit groups and generate the Brønsted acid sites on the GS interfaces via dissociation of H2O and formation of GerOHδ- and >OsHδ+ bonds, which are stabilized under adsorption of water or basic probe molecules. On sample heating, it is possible to form the strained rings dGeOOSid at the GS interfaces, which give the IR band at 885 cm-1 corresponding to νSiO.

LA960442H