Langmuir 1995,11,2115-2120
2115
Dielectric Properties and Dynamic Simulation of Water Bound to Titanidsilica Surfaces V. M. Gunyko,*V. I. Zarko, V. V. Turov, E. F. Voronin, V. A. Tischenko, and A. A. Chuiko Institute of Surface Chemistry, Kiev, 252022, Ukraine Received August 8, 1994. I n Final Form: February 8, 1995@ Water adsorbed on titaniakilica (TS) surfaces has been studied by dielectric spectroscopy (DS) in the 3-10 MHz frequency range a t T = 120-300 K,thermally stimulated depolarization (TSD) at the same temperatures, lH NMR, and quantum chemical methods. The TSD spectra of TS have only one relaxation maximum after sample heating a t 400 K. The activation energies of depolarization (TSD)and polarization (DS) decrease at Ti02 content increasing in TS. According to the TSD, DS, and lH NMR data, water adsorbed on the TS surfaces essentially in the interface region forms more weakly bound clusters than on the silica surfaces. Simulation of vibrational-rotational relaxation of the water molecules attached to the TS surfaces shows that the activation energy of rotation is a maximum for the molecules having donor-acceptor bonds, Me-OH2 (Me = Si, Ti), in the complexesHO-Me-OH2 mH2O and it is minimum for one water molecule linked to the Ti-O(H)-Si bridge through a hydrogen bond.
+
Introduction The surface OH groups and water molecules adsorbed on oxides are the Hf sources that cause surface conductivity, catalytic activity, and other properties. Increase in the conductivity value is conditioned by growth in the amount of weakly bound proton or of the H+mobility due to water interaction with active acidic surface sites of the Bronsted (B-center) or Lewis (L-center) t y p e ~ . l - Pyro~ genic TS surfaces are strongly heterogeneous for adsorbed water and differ significantly from silica surfaces, due to formation of the individual Ti02 phase and Ti-0-Si or Ti-O(H)-Si bridges in the interface region^.^-^ The amount of the Ti-0-Si bridge in TS nonlinearly increases as T i 0 2 content (nm0J increases and it is a maximum at nnOz FZ 20 wt %.* This value is close to the ,~ maximum solubility of titania in the silica m a t r i ~yet individual crystalline titania phase forms at nnOz 3 5 wt %.lo The adsorption capacity of active sites on the TS surfaces for water is higher than that on Si02.l1 But its increase for TS is not explained only by additive adsorption on the Si02 and Ti02 phases; i.e., there are surface sites with a different nature on TS than on individual Ti02 and Si02 phase. They are the B-sites Si-O(H)-Ti, which have stronger acidic properties than the OH groups on individual Ti02 and SO2.’ Besides, the migratory and orientation mobility of the water molecules are higher on ~ s . 1 2 ~ 3
* Author to whom correspondence should be addressed.
Abstract published in Advance ACS Abstracts, April 15,1995. (1)Morrison, S. R. The Chemical Physics ofSurface; Plenum Press: New York, 1977. (2) Kiselev, V. F.; Krylov, 0.V. Electronic Phenomena in Adsorption and Catalysison Semiconductorsand Dielectrics;Nauka: Moscow, 1979. (3) Kurzaev,A. B.;Kozlov, S. N.;Kiselev, V. F. Dokl. AN USSR 1976, 228, 877. (4) Sushko, R. V.;Voronin, E. F.; Chuiko, A. A. Zh. Fiz. Khim. 1979, 53. 2395. (5) Jabra, R.; Phalippou, J.;Prassa, M. J.Chim. Phys. Phys.-Chim. Biol. 1981, 18, 777. (6) Sushko, R. V.; Gette, A. V.; Mironyuk, I. F.; Chuiko, A. A. Zh. Prikl. Khim. 1983,57, 1229. (7)Zarko, V. I.; Kozub, G. M.; Sivalov, E. G.; Chuiko, A. A. Ukr. Khim. Zh. 1988,54, 1144. (8)Varshal, V. G.; Bobrov, A. V.; Mavrin, B. N. Dokl. AN USSR 1974,216,377. (9) Pakhlov, E. M.; Voronin, E. F.; Sushko, R. V. Zh. Prikl. Spectr. 1987, 47, 311. (10)Kiselev, A. V.; Kuznetsov, A. I.; Lygin,V. I. Kolloid Zh. 1980, 42, 964. (11)Voronin, E. F.; Pakhlov, E. M.;Turutina, N.;Sushko, R. V. Zh. Prikl. Khim. 1989, 63, 399. @
0743-7463/95/2411-2115$09.00/0
The aim of the present work is a n investigation of water adsorbed on titania-silica surfaces and determination of the interface’s influence on characteristic properties of such systems.
Experimental Section Materials. Highly dispersesilica (AerosilA-300 with specific surface area (S)of 300 m2g-l) and titania-silica (99.5%purity) were produced at PU “Chlorovinil”(Kalush,Ukraine)by pyrogenic synthesis from Sic14 and Tic14 in an air-hydrogen flame at T > 1500 K. The TS samples studied contain 9 (TSg), 14 (TS14), 20 (TSzo),22 (TS22),29 (TS29),and 36 (TS36)wt % of titania with corresponding S values of 215, 137,70, 250,60, and 90 m2g-l. The S values were measured by using nitrogen. Thermally Stimulated Depolarization (TSD) Method.
The samples were polarized by an electrostatic field with intensity F 5 x lo5V/m at room temperature and then cooled to 100 K with the field still applied and reheated without the field with a linear heating rate, ,8 = 0.05 Ws. The measuring sealing cell was filled with nitrogen. The current evolving at depolarization was recorded by an electrometer,which has a range from 10-15 to A. Relative mean errors were 6 ( I ) = f 5 % for current measurement, 6(T) = f 2 K for temperature, and S(B) = f 5 %for the heating rate. Dielectric Spectroscopy (DS)Method. Water adsorption was performed at room temperature with mean errors near f0.01 g of water/g of TS. Dielectric characteristics of the system Aerosil-water were measured by a Q-meter VM-560 (Tesla) in the 3-10 MHz frequency range. The choice of this region of f > 1MHz is conditioneduponneglect ofMaxwell-Wagner effects and the superfluousness of the blocking electrodes. The measurements of dielectric permittivity, 6’ = @(T), and dielectric loss,6‘’ = v(I“), were taken by a thermochamberwith programmed temperature changes. The heating rate was equal to 0.05 Ws with relative mean errors 6(T) = f5%. The 1H NMR spectra were obtained by a high-resolutionWP100SY (Bruker)NMR spectrometer. The TS sampleswith water were kept in air-tight ampoules during 24 h at T = 278 K. The initial adsorbed water content was calculated from increase in the IH NMR signal intensity as additional water is adsorbed.
Results The hydration layers of pyrogenic silica and titania surfaces have essential d i s t i n c t i o n ~ . ~ *Pyrogenic ~J~J~ Ti02 particle surfaces have a n anatase structure (but the cores (12) Zarko, V. I.; Gorlov, Yu.I.; Brey, V. V.; Kozub, G. M. Teoret. Eksperim. Khim. 1986,22,240. (13) Zarko,V.I.; Kozub,G. M.;Pokrovsky,V. A.; Chuiko, A. A. Teoret. Eksperim. Khim. 1991,27, 734.
0 1995 American Chemical Society
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2116 Langmuir, Vol. 11, No. 6, 1995
0
I
I 0 v)
0
0.00
0
10
20
0
40
30
1
1
1 5
10
15
20
25
n(H20), wt%
n(TiOZ),%
Figure 2. Dependence of E' = flnH,o) for the samples si02 (11, TSs (2), TS14 (3), TSzo(4); frequency is equal to 3 MHz.
I
I
,
I
60
80
L,
I
O ~ , , , , i , , , , i , , , , i , , , , i , , , , l 0
20
40
100
n(TiOZ),% Figure 1. Dependencies of the Si-0-Ti bridge content on nTiOz in TS according to the ratio of intensity of the band of 950 cm-l (a) and of water content on nnOZ(b).
I 8
are rutile)14 and their coverage by water is better than for pyrogenic silica surfaces.15 According to the precedential works,11J6the Si-O(H)-Ti and Si-0-Ti bridges may be the most active sites for water adsorption on TS. These bridges are formed a t high temperature (31500 K) and they are structurally stable sites for adsorbed water. Yet, a part of the surface Si-0-Ti bridges can be decomposed through hydrolysis that leads to a sorption capacity increase a t the interface regions. Hydrolysis of the Si0-Ti bridges is more probable for mixed oxides produced by the chemical vapor deposition method or in the liquid phase in small amount a t the new phase (Ti02 on Si02 or vice versa). The amount of the Si-0-Ti bridges increases fairly linearly a t nmOz < 20 wt %, but a t nnOz 3 20 wt %, it is nearly a constant according to the change in the ratio of the intensities of the bands vsion and vsioS:J7 a t 950 cm-l (Figure la). Close inspection ofthe DS data for pyrogenicTi02 shows that continuous water layer formation occurs a t a coverage of 0 2 that accords with water content of nHzO 2.7 wt %.l8J9 The nHzO magnitude for TS is higher than for individual Si02 and Ti02 under the same conditions (temperature and water vapor pressure);ll i.e., the Ti02 phase of TS may be fully hydrated with continuous water (14) Gette, A. V.; Zarko, V. I.; Kozub, G. M.; Chuiko, A. A. Ukr.Khim. Zh. 1988,54, 653. (15)Yatsvuk. S.: Brev. V.: Chuiko, A. Dokl. AN Ukr., Ser. B 1987, N l , 60. (16)Yurchenko, G. R.; Morev, A. V.; Pakhlov, E. M.; Golovko, L. V. Ukr. Khim. Zh. 1988,54, 476. (17)Pakhlov, E. M.; Zarko, V. I.;Voronin, E. F. Ukr.Khim. Zh. 1993, 59, 373. (18)Iwaki, T.; Morimoto, T. J . Chem. SOC.,Faraday Trans. 1 1987, 83. 957. (19)Zarko, V. I.; Gulko, 0.V.; Kozub, G. M.; Chuiko, A. A. DokZ.AN Ukr. 1992,N8, 143.
I
16
24
n(Ti02),% Figure 3. Dependence of e' on nnOzfor TS at water content of 4 (1)and 6 wt % (2).
layer formation. However, the water adsorption amount decreases a t n H z O > 29 wt % (Figure lb). The dielectric permittivity of TS depends on n H z O more strongly than for silica (Figure 2, curves 2-4). A characteristic of the function E' = f(nH,o) allows us to explain the obtained data within the scope of the layered model,20according to which the molecules adsorbed in the first layer on the most active sites have lower orientational mobility in the external electric field than those in the second layer, which contribute more to the orientational polarization of adsorbate. This polarization increases as a consequence of the action of the Ti02 phase on the adsorbed water (especially in the interface regions), and as a result the magnitude of c'grows, although the values for dehydrated pyrogenic silica and TS are closely equal.21 The E' value change depends not only on nHzO but also on the Si-O(H)-Ti bridge content nearly linearly (Figures 2 and 3); i.e., we can assume that these sites are the most important for water adsorption and 6' is proportional to the nmOz value and Si-O(H)-Ti bridge amount. These sites presumably can promote fast growth of the water clusters bound to them as nHzO increases. A kink of the function 6' = ~ ( T Z H , ~ which ), is observed for TS14 and TSzo (Figure 2, curves 3 and 4) (at such nnOzvalues, individual titania phase forms into TS), attests that a continuous water layer forms on the Ti02 phase. (20) Chelidze, T. L.; Derevyanko, A. N.; Kurilenko, 0. D. EZectrical Spectroscopy ofHeterogeneous Systems;Naukova Dumka: Kiev, 1977. (21) Zarko, V. I.; Gette, A. V.; Kozub, G. M.; Sushko, R. V.; Chuiko, A. A. Izu. AN USSR, Neorgan. Mater. 1983,19, 239.
Properties of Water Bound to Titania ISilica
O”O
Langmuir, Vol. 11, No. 6, 1995 2117
11
1
U 0.05
A
> d 4 3
100
150
2 50
200
300
T, K Figure 4. Dependence of 6’‘ on temperature for TS9 at frequencies of 3.2 (1)and 9.4 (2) MHz. 1
- 12.0 -
0-12.5
i
\
0
\
1,
I ,
I
r r r ) I
175
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A-300
41 45 32 22 11
Si
\
/
0
I
200
v
I n
r ( r 1
I t t
225
1)
I
r r r I
250
I
It1
275
kJ/mol
50,
11‘
Tm,K
35 29 25
236 293 235 238 229
Ib 1.3 x 1.6 x 1.0 x
1.7 x 1.0
11‘ 2.38 x 1.47 x 8.80 x 10-5
These samples were obtained by chemical vapor deposition method at interactionof Tic4 with Si02 (A-300)then by hydrolysis of the Ti-C1 bonds as T FZ 470 K. I values are the initial slopes of the TSD curves. I1 values are the results obtained according to eq 6 for the entire temperature range. a
The dependence on temperature ofthe value of dielectric loss (Figure 4) for TSg differs from E”(T)for pyrogenic silica, and the 6‘‘ intensity for TS in the 220-300 Krange is well above that for silica. However, the nHzOvalue for TSg is higher than for silica a t water adsorption in air only at 2-3 wt %: for TSg nHzO 7-8 wt % and for Si02 nHzO FZ 4-6 wt %. Hence, this distinction of E”(T)for TS and Si02 is not only connected with a different amount of water a t the Si02 and TSg surfaces. It may be caused by a variation of the size distributions of the adsorbed water clusters and by changing of the proton transfer contributions in the relaxation processes for these oxides. At temperatures, below 220 K such variations of E” are characteristic of a surface containing many water clusters, but it is observed for pure Si02 only a t nHzO 3 25 wt %. A minimum of two relaxation maxima are seen for TSg in the 160-220 Krange, a range in which only one maximum
(11
/I\
/ Si
/ \
Calculation of the polarization activation energy (E,) may be performed through a shift of the relaxation maximum for two different frequencies 0 2 0
E,
s
0
\
/ \
Table 1. The TSD Spectra Parameters of Hydrated Aerosil (A-300)and TS after Heating at T = 400 K (1 h) samplesa
Si
0
/
-Ti-
a d
T, K Figure 5. TSD data for TSlo (11, TS33 (21, TS19 (3).
7333
H
Si
- 13.0
TSio TSis
Figure 6. The IH NMR spectra of hydrated TS (1,2) and Si02 (3,4)on air (1,3) and in CDC13 (2, 4).
/I \
150
0
is exhibited for silica, Le., on the TS surfaces a few types of water adsorption sites exist, e g .
1
-13.5
a 6 4 2 Chem. shift ppm
io
= const ln(Af)lAT,,
(2)
where ATma is peak temperature shift. According to eq 2, the E, magnitude for TSg is equal to 28 kJ/mol(220300 K) and 21 kJ/mol(l20-200 K). An estimation of the E, value may be made via a maximum of E” measured a t a certain frequency (fmax2n)-1 = r
GZ
(h/kT)exp(AFlRT)
(3)
where AF is the free energy of activation. We measure the dependence of E”(T),but not E”O,such that
A F = E, = RT,,
ln(T,JT,)
(4)
where To= h/kr. An estimation of the activation energies for three DS peaks of TSg, according to eq 4,gives values of 18,21, and 25 kJImol for the peaks in Figure 4 (curve 2). Additional information about water adsorbed at the oxide surfaces can be gleaned from the TSD measurement, in which in contradistinction to the DS method the dipole disordering is observed a t heating. The depolarization activation energy (Ed,) was calculated from the initid slope of the TSD curve (i.e. T In I vs 1/T, where I is the depolarization current (Figure 5). The relaxation time was found from a temperature maximum e q ~ a t i o n ~ ~ , ~ ~
T , = PEdprlk where B is the heating rate and r = r,, exp(EddkT) (Table 1).The TSD data can be handled for the full temperature (22) Hippel. A. R. Dielectric and Waves; Wiley: New York, 1954.
(23) Bucci, C.; Fiechi, R. Phys. Reu. 1966,148, 816.
Gun’ko et al.
2118 Langmuir, Vol. 11, No. 6, 1995
a
b C
A’
4.7
,.
I
-
L
I
-
U
A
10 8 6 4
.
L
2
-
u
-
I
u
.
~
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0 -2 -4-6
Chsm. shift ppm
1.7
2
8
6
4
2
0
Chem. shift ppm
8
7
6
6
4
3
2
1
0
Chem. shift ppm
Figure 7. The lH NMR spectra of hydrated Si02 (a), TSm (b), and TSzz (c) at different temperatures: (a) 1, - 225 K, 2, 235 K; 3, 250 K 4, 265 K 5, 285 K (b) 1, - 260 K; 2, 270 K 3, 290 K; 4, 315 K, 5, 325 K, (c) 1, - 260 K, 2, 270 K 3, 290 K, 4, 305 K 5, 325 K.
water molecule participating in four hydrogen bonds, contrary to what occurs for adsorbed water molecules, which have fewer such bonds. But the thickness of the water layer on silica is nearly equal to 0.6 of the value of I = S,ooTPoexp(-E/kT+(o,k/EP)(T? exp(-E/kTo) a statistical monolayer and the fact that the chemical exp(-E/kT))) (6) shift is similar to that of liquid water makes it possible to conclude that each adsorbed molecule involves formawhere S, is surface area of the electrodes, Pois the frozen tion of a few hydrogen bonds, which is possible upon polarization, E = Edp,Tois initial temperature, and oois formation of a large cluster of adsorbed water. the temperature-independent part of the transition probWater layers on the TS and silica surfaces have been ability ofthe dipoles. The E d p magnitude tends to diminish studied by lH NMR spectroscopy in association with the with a buildup of Ti02 content and therewith the tovalue method of freezing-out of liquid waterz6 to study the increases, but the magnitudes obtained for the initial slope interaction of hydrated oxides with air, water, and CDC13 of the TSD curve are different than the results found for (CDCl3 is used for elimination of noise signals from lH of the full temperature range because of the high relative CHC13). errors for the initial part of the TSD spectra. The relative The ‘H NMR spectra for water/TS essentially differ from number of the water molecules strongly bound to the those of water/SiOz (Figure 6). Aside from a n adsorbed surfaces decreases as the amount of adsorbed water water signal, we observe one due t o water molecules that increases for TS. These effects may result from a stronger have sluiced off and dissolved in CDC4 (6 = 1.7 ppm) and local fields and more heterogeneity of the TS surfaces by its intensity is higher for TS. Upon lowering the temcomparison with silica and from water molecule localizaperature the signal width of the lH NMR for hydrated tion in the TiOdSiOz interface regions in large clusters. samples placed in CDCl increases (Figure 7). This effect Consequently, the DS and TSD data indicate that the size is due to a decline in the adsorbed molecules’ mobility. of the adsorbed water clusters strongly depends on the But the signal width increase a t T = 285 K is caused by surface site characteristics. The nature of these phea higher rate of the proton exchange between the water nomena may be perceived from a n analysis of the lH NMR molecules and the surface OH groups. data and quantum chemical modeling of water adsorption Concurrent detection of two signal of the water molon the TS surfaces. ecules, which form varied complexes for the TS/Hz0/CDC13 The chemical shift (6) in the lH NMR spectra of water system (Figure 7), is possible when the time of molecular adsorbed on a surface is averaged for the molecules which exchange between different states is considerably longer interact with the primary and secondary adsorption sites. than time of cross molecular spin-spin r e l a x a t i ~ n . ~ ~ The primary complexes of the water molecules adsorbed Chloroform-d (or other inert solvents without H atoms) on Si02 from the gas phase were covered a d e q ~ a t e l y . ~ ~ inhibits ~ ~ , ~ ~molecular exchange between the water molecules The 6 value for the water molecule adsorbed on the primary from neighboring adsorption complexes. The higher sites of silica is equal to 2.7 ppmZ6a t small coverages. temperature dependence of the second signal (Figure 7) This value is 2 ppm less than for liquid water. A shift in suggests that corresponding surface complexes have strong magnetic fields is seen despite the fact that the stronger intermolecular bonds.2s As a verification of this hydrogen bond energy for HzO a t the silica surfaces is suggestion, the signal increase found for water in chlohigher than for liquid water. This results from each liquid roform is basically a result of decrease of the first signal intensity. Therefore, the second signal can be assigned range according Reichle et al.24and the following equation for a current peak (I)of the dipole orientation
(24) Reichle, M.; Nedetzka, T.; Mayer, A,; Vogel, H.J.Phys. Chem. 1970,74, 2659. (25)Chuiko,A.A.;Gorlov,Yu. I. Surface ChemistryofSilica,Surface Structure, Active Sites, Sorption Mechanisms; Naukova Dumka: Kiev, 1992. (26) Turov, V.V.React. Kinet. Catal. Lett. 1993,50, 243.
(27) Zhu, S. B.;Fillingim, T. G.; Robinson, G. W. J. Phys. Chem. 1991,95,1002. (28) Emsly, J.; Finey, J.; Satcliff, L. Nuclear Magnetic Resonance Spectroscopy; Mir: Moscow, 1968.
Properties of Water Bound to Titania JSilica
Langmuir, Vol. 11, No. 6, 1995 2119
to the water clusters on the Ti-O(H)-Si bridge regions for the Ti and Si atoms. As for the titania surface when the first signal is caused by water adsorbed a t the fragments, the clusters, including from three to six hexahedrons Ti0613with anatase structure, have been silica phase surfaces. calculated. The TiOz/SiOzsurface models contained from Initial nHzomagnitudes in air and into CDC13 are about one to three Ti06,3 hexahedrons and 7-10 SiOq2 tetra0.7 the value of a monolayer for silica and five layers for hedrons. The H3*SiO and OH groups were used on the TSZO.From the data analysis (Figure 8) we can assume cluster boundaries. The T i 0 2 and TiO$SiOz clusters had that actions of the silica and TS surfaces on adsorbed the Ti-O(H)-Me (Me = Ti and Si) bridges and terminal water appreciably differ as for silica two statistic monoOH groups. layers of water are not frozen a t T = 270 K, and for TS The activation energies of water moleculerotations (E?) this effect is observed closely to 25 molecular layers, Le., were calculated for H-bonded and donor-acceptor comthe influence of the TS surface on adsorbate is long range. plexes including from one to a few water molecules. We Water freezing a t interfaces at T < 273 K occurs when assume that relaxation mechanisms in DS and TSD are the free energy of the adsorbed molecules is equal to the cause mainly by the water molecule’s rotations. The E: free energy of ice.29 Therefore, this function can be value is a minimum (for molecules bound directly to a transformed to the free energy change function30 of the surface) in the H-bonded complex of only one molecule average thickness of the disperse particle’s water coating (Figure 8). Assuming that the free energy of frozen water Ti, weakly depends on the availability of the disperse oxides, 0 OH2 (Me = Si, Ti) (7) we conclude that obtained curves (Figure 8, curves 3) are / Me determinated through distinctions between the surface potentials for silica and TS. and it is about 10 kJ/mol. In this case, if such molecules The adsorbed water signal with the greatest intensity interact with other water molecules,the EPvalue amounts may be attributed to the molecules bound to the Ti-O(H)to 25 kJ/mol. For donor-acceptor complexes (DAC) Si bridges; i.e., the long-range surface potential of TS is Ti-OH2, the E: magnitude is higher ( ~ 4 kJ/mol, 0 which essentially dictated by the interface regions, e.g. a t is nearly equal to E,S for DAC on the silica surfaces, 42 formation of separated ion pairs Ti-O--Si-mHzO kJ/mol). Upon increasing the number of water molecules H30+nH20. However, the signals of the water clusters in a complex when the first adsorbed molecule forms a adsorbed on the silica phase and TiOz/SiOzphase boundary DAC, the E? value rises to 55-60 kJ/mol. If a molecule’s region were not separated for the samples with relatively surroundings in a DAC are modeled as fully rigid, then small specific area ( T S ~ O and ) high n H z O (Figure 7). At a EPis equal to 85kJ/mol. However,formation of a Ti--OH2 higher S (TS22) and lower nH20 the signal separation is DAC on pyrogenic TS surfaces is unlikely because the Ti seen (Figure 7). Differences of the lH NMR data for Si02 atom’s surroundings at the surfaces usually have hexaand TiOz/SiOz are in good agreement with the DS and hedron structures, TiOG13; i.e., the main part of the water TSD spectra in relation to the influence ofnTiOzon adsorbed molecules, which are directly linked with the TS surfaces, water characteristics. forms the complexes as shown in the eq 7 or complexes Thus, the influence ofmedium on water layer structures with terminal OH groups. in the interface regions can be studied through layer From a comparison of experimental data with theoretithickness dependencies of unfrozen water on temperature. cal simulation results we can conclude that the following The closer the water molecule is to a surface, the lower type complexes are most likely formed the temperature a t which the free energy of a water molecule adsorbed corresponds to ice. If in a given H H H temperature range this energy is lower than for ice, that I ,oI .H-0 /H \ O - H ... o, H,\.oi,/H adsorbed water is unfrozen and the intensity of the proton NMR signal of bound water is a constant. Preferential adsorption of water in the TiOz/SiO2 interface region leads to sizable distinctions in its thermal desorption processes for the TS and silica s ~ r f a c e s . ~ ~ ? ~ ~ 0-H . .O‘ ;O.. .H-O 4+ The temperature-programmed desorption (TPD) spectra .H 10in eq 8) participate in this process.32 Formation of the ion pairs primarily occurs in the TiOdSiOz interface region and then some are produced in the T i 0 2 phase, which has more weak B-sites.
Conclusion Analysis of the DS, TSD, TPD, NMR spectra favors a view that the most probable sites for water molecule adsorption are the B-sites a t the TiOZ/Si02phase boundary (32) Gun’ko, V. M. Zh. Fiz. Khim.1991,65, 398.
regions and that the water molecules adsorbed on the Ti-O(H)-Si bridges lose fewer degrees of freedom than in donor-acceptor complexes (Me-OH2). These effects have influence on the decrease of the activation barriers of the polarization and depolarization of water clusters adsorbed on the mixed oxide surfaces. The TS interface regions have essential long-range influence on adsorbed water characteristics, e g . on free energy of the molecules. With the same content of water adsorbed a t the TS and Si02 surfaces the water amount a t the Si02 phase in TS is lower than for pure Si02; i.e.,the water molecules localize in the interface regions of the Ti02 phase.
Acknowledgment. We are grateful to the ShenvinWilliams Co. (U.S.A.)for support of this work. LA940622W
