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MAS spectroscopy
Journal of Non-Crystalline Solids 134 (1991) 47-57 North-Holland
47
mixed oxide catalysts prepared by sol-gel techniques. Characterization by solid state CP/MAS spectroscopy
TiO2/SiO 2
K.L. W a l t h e r a n d A. W o k a u n Physical Chemistry IL University of Bayreuth, W-8580 Bayreuth, German)'
B.E. H a n d y and A. Baiker Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Ziirich, Switzerland Received 25 October 1990 Revised manuscript received 23 April 1991
The influence of synthesis parameters on the structural properties of mixed oxides derived from silicon tetraethoxide and titanium tetra-isopropoxide precursors has been studied by S1 NMR spectroscopy. A first set of gels, prepared by complete hydrolysis of the individual precursors and subsequent mixing of the hydrosols, is characterized by comparatively narrow lines for the Q2 Q3, and Q4 silica sites. Both the chemical shift values and the time constants of 1H-29Si magnetization transfer are similar to those found in pure silica gels. This indicates that these oxides consist of domains of the individual components. A different appearance of the CP/MAS spectra is noted for the second set of gels, which is prepared by addition of the titanium alkoxide component at a stage where the silicon alkoxide hydrolysis has only proceeded to about 50%, on average. The spectra of these samples are characterized by broad lines for the three types of sites. Larger contributions observed for Q2 and Q3 sites in these samples are attributed to the presence of Si-O-Ti bridges. The influence of preparation parameters such as hydrolysis pH, drying temperature, and pore ripening treatment by redispersion in base, is studied by monitoring the spectral and dynamic parameters derived from the CP/MAS spectra. •
1. Introduction
Properties of support materials and, in particular, their interaction with the dispersed catalytically active metal or compound are important design parameters in catalyst development [1]. Besides testing less common oxides as catalyst cartiers, the use of mixed oxides has recently attracted attention [2], in view of their potential for combining certain desirable properties of the individual oxide supports. For a successful realization of this concept, mechanical mixtures of the components are insufficient: it is required to achieve intimate contact between constituents and, if possible, mixing on a molecular scale. To realize this aim, the use of sol-gel technology [3-5] is particularly promising: molecular hydrosols of the com-
29
.
ponents are mixed prior to their immobilization in a three-dimensional network during the gelation process. The present study has been motivated by recent reports on the preparation of V205 monolayer catalysts supported on the surface of TiO2/SiO 2 mixed oxides [6]. These catalysts exhibit activities in the selective catalytic reduction (SCR) of nitric oxides that are higher than observed with vanadia layers supported on TiO 2 (anatase/rutile, Degussa P25) [7], and are more stable against thermally induced aggregation than systems supported on SiO 2 (Aerosil) [8,9]. The structure of vanadia dispersed on TiOz/SiO 2 has recently been characterized by ESR and Raman spectroscopy [10]. Here we consider the influence of preparation variables on the properties of the TiO2/SiO 2 mixed
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
48
K.L. Walther et al. / Ti02 / Si02 mixed oxide catalysts prepared by sol-gel techniques
oxide supports used in the catalyst systems mentioned above. All catalysts considered contain a weight fraction of 20 mol% TiO 2, and are synthesized starting from silicon and titanium alkoxides, i.e. silicon-tetraethoxide (TEOS) and titanium-tetra-isopropoxide (TIOT). The following preparation variables were tested [11-13]. First, hydrolysis of the components can be performed in either acid or basic medium. In acid, the H+-catalyzed hydrolysis of the alkoxides proceedes rapidly, while the condensation reactions requiring deprotonation are rate determining in the gelation process. As a consequence, linear chains with little cross-linking are generated in acid medium. The adsorption isotherm of the low temperature dried aerogel is of type I [14,15]. Upon high temperature drying (e.g., at 600 ° C), the observed loss of water by condensation reactions is accompanied by a collapse of the structure, and microporous solids result which are less useful for catalytic applications. By contrast, in a basic medium the hydrolysis of the alkoxides is rate limiting, while the OH--catalyzed condensation reactions proceed rapidly. As the hydrolyzed species ('M(OH)4' in a formal notation) are immediately attached to the growing nuclei, comparatively compact and highly cross-linked clusters are formed. Upon drying, these clusters pack into a structure characterized [14] by a type IV adsorption isotherm [15] and by mesopores [16] which are stable up to higher temperatures. The second important preparation parameter [11-13] is the sequence of hydrolysis and mixing. If the two components (TEOS and TIOT) are hydrolyzed to completion (using, for example, a 20 fold molar excess of water) prior to mixing, there is an obvious tendency for the formation of larger clusters of the individual components by non-reversible condensation reactions, in particular, in basic media. The alternative procedure investigated here is partial hydrolysis of TEOS (2 mol of water per mole of alkoxide), followed by addition of the appropriate amount of TIOT, and further addition of the stoichiometric amount of water required for complete hydrolysis (4 mol of water per mole of alkoxide). It is expected that this method should be more suitable for producing solids which are mixed on a molecular scale.
A third variable to be tested [11-13] is the redispersion of the prepared gel in a slightly basic medium. The purpose of this step is to effect changes in pore structure, by catalyzing the breaking of ' M - O - M ' linkages formed during the condensation process, as well as their re-formation. This process should be most effective when applied to wet gels which have not been irreversibly cross-linked by high temperature drying. The use of 29Si C P / M A S N M R as a source of structural information in solid silicates has been extensively described in the literature (cf. Maciel and Sindorf [17]; Sindorf and Maciel [18]; Harris and Newman [19]; Lippmaa et al. [20]; Engelhardt et al. [21]; Grimmer and Radeglia [22]; as well as a more comprehensive list of references contained in the book by Engelhardt [23]). The 29Si nucleus in silicates is found in a tetrahedral oxygen environment. Various types of sites are distinguished by the second-nearest neighbor ligands. The abbreviation Qn denotes a site of the type S i ( - O - S i ) n ( - O - X ) 4 _ n, where X is an ionically bound ligand other than silicon, most frequently a proton. The sites Q1 through Q4 in pure silicates are clearly distinguished by characteristic values of their respective chemical shifts [23]. Recently, pioneering applications of 29Si CP// MAS-NMR to the characterization of sol-gel processes have been published by Wies et al. [2426] where the systems Li20//SiO2 and SiO 2TiO2-ZrO 2 have been investigated. From an evaluation of chemical shift increments, it was found that - O H and - O - T i TM groups have a similar influence on the shift of the central 29Si nucleus [17,26,27]. As a consequence, in a SiO2/ TiO 2 mixed oxide, the Q3 line would comprise contributions from both S i ( - O - S i ) 3 ( - O H ) and S i ( - O - S i ) 3 ( - O - T i TM) structural units, as is discussed below. We have recently described [28] the application of C P / M A S - N M R to investigate the structure of pure silica gels prepared from TEOS using the sol-gel process, whereby some of the parameters mentioned above have been varied. Our present investigation of TiO2/SiO 2 mixed oxide systems will therefore be supported by the information
K.L Waltheret al. / TiO2/ Si02 mixed oxide catalysts prepared by sol-gel techniques [28] o b t a i n e d with the silica gels n o t c o n t a i n i n g titania.
2. Time dependence of the magnetization transfer It is a characteristic feature of the silica gel t y p e s a m p l e s d u r i n g H a r t m a n n - H a h n cross-polarization e x p e r i m e n t s that the m a g n e t i z a t i o n transfer does not follow a simple e x p o n e n t i a l time d e p e n dence [28]. Strong transient oscillations a p p e a r s u p e r i m p o s e d on the overall g r o w t h of 295i m a g n e tization in the r o t a t i n g frame. This behavior, which was characterized for the p u r e silica gels [28], is again very p r o n o u n c e d with the T i O 2 / S i O 2 m i x e d gels, as is exemplified b y the e x p e r i m e n t a l results shown below. The transient oscillations p h e n o m e n o n was first Table 1 Parameters characterizing the
29SiNMR
49
o b s e r v e d b y Mtiller et al. [29], a n d was a n a l y z e d as a c o h e r e n t a n d p e r i o d i c m a g n e t i z a t i o n transfer b e t w e e n the rare S-spin (13C or 29Si, respectively) a n d an i s o l a t e d p r o t o n (I-spin), which is o n l y w e a k l y c o u p l e d to the p r o t o n spin reservoir b y a spin diffusion rate, T~ 1. To a c c o u n t for the superi m p o s e d e x p o n e n t i a l increase of t r a n s f e r r e d magnetization, a n a d d i t i o n a l t e r m d e s c r i b i n g the transfer at a c r o s s - r e l a x a t i o n rate, T ~ ~, a c c o r d i n g to the spin t e m p e r a t u r e c o n c e p t m u s t b e included. This leads to the e x p r e s s i o n for the time d e p e n d e n c e of the m a g n e t i z a t i o n transfer used in our analysis:
M(t) =
M°
( e -t/r',' - ae - t / r ' s -
(1 - a )
x [ ½ e - t / r " + ½ e -3'/2r,, cos(½bt)] }. (1)
spectra of the investigated titania-silica gels
Sample
Struc- Mo (%) tural MAS type
Complete hydrolysis before mixing sols; TEOS with base, TIOT with acid; dried at 120 o C (gel 101)
Q2 Q3
Q4
Mo (%) CP/ MAS
TsiH (ms)
Trtrt (ms)
TloH (ms)
0.4±0.4 2.4±0.8 0.4±0.4 ( ~ ) (oO) 12.2±3.5 20.5±3.0 0.7±0.3 420±420 (o¢) 87.4±14 77.1±12 2.1±1.5 155±150 (or)
a
b (rad/s)
0.5+0.5 70+70 0.5-1-0.1 134+26 0.2+0.1 112+18
Partial hydrolysis of TEOS with acid, Q2 TIOT added, final hydrolysis with H20; Q3 re-dispersed in base, dried at 120°C (gel 102) Q4
15.2±3.3 20.3±8.6 0.3±0.3 9.9±9.9 34.2±6.0 41.8±6.8 0.3±0.2 8.1±4.9 50.6±7.9 37.9±5.1 1.8±0.2 1 ~ ± 1 ~
24.6+4.8 0.5+0.5 32.8+2.3 (0.64) 62.2 ± 9.5 (0.96)
1046+200 491±190 556 ± 59
Partial hydrolysis of TEOS with base, TIOT added, final hydrolysis with H20; dried at 120 o C (gel 103A)
5.2±3.7 13.9±8.1 1.0±0.4 4 ~ ± 4 9 0 28.6+11 0.9±0.3 35.9±11 49.0±11 0.9±0.7 8.9±8.9 34.3 ± 4.1 0.7 ±0.5 58.9±16 37.1±6.9 2.1±0.6 ( ~ ) 222±120 0.8±0.1
195+150 533±340 260±39
Partial hydrolysis of TEOS with acid, TIOT added, final hydrolysis with H20; dried at 600 o C (gel 104B)
Q2
Q3 Q4 Q2
0.8±0.7 2.7±0.6 1.1±1.1 ( ~ ) 12.5±2.0 27.7±2.8 0.6±0.6 23.9±15 86.7±8.4 69.6±6.6 13.8±1.8(~)
(oo) (oo) (oo)
(0.90) 200 ± 200 0.3±0.1 244±52 0.83 ± 0.04 341 +15
Q4
0.8±0.8 2.5±1.3 1.3±1.3 ( ~ ) 7.7±1.6 24.0±3.0 0.5±0.5 30.2±13 91.5±7.9 73.5±11 15.7±4.0(~)
77.5+77 (o¢) (o0)
0.7±0.5 0.3±0.1 0.7±0.1
374±120 350±37 398±14
Complete hydrolysis before mixing sols; TEOS with acid, TIOT with acid; re-dispersed in base, dried at 120 °C (gel 114)
Q2 Q3 Q4
5.1±2.7 9.2±2.3 1.4±0.7 90.2±90 23.2+6.0 0.9+0.2 24.8±6.1 33.6±5.8 1.0±0.3 26.3±19 60.8+11 0.7+0.2 70.1±14 57.2±8.2 1.8±1.5 25.0±6.1 116+25 0.2+0.1
332:1:140 348+65 340+14
Complete hydrolysis before mixing sols; TEOS with acid, TIOT with acid; dried at 600 o C, re-dispersed in base, dried at 120°C (gel 115)
Q2 Q3 Q4
2.5±2.3 5.2±1.2 1.1±0.9 112±110 144+113 0.8+0.2 322+78 22.0±4.9 38.9±4.6 0.8±0.3 25.2±12 251-t-150 0.5+0.1 276+27 75.5±12 55.9±5.2 2.2±1.7 29.3±5.6 694 +640 0.13+0.06 288+7
Partial hydrolysis of TEOS with acid, TIOT added, final hydrolysis with H20; dried at 600 "C, re-dispersed in base (gel 104E)
Q3 Q4 Q2
Q3
Values in parentheses were held constant during the least squares fitting procedure.
50
K.L, Walther et al. / TiO 2/ SiO 2 mixed oxide catalysts prepared by sol-gel techniques
In this equation, the parameter a (0 < a < 1) represents the weight of the exponential part of magnetization transfer. The oscillatory part (relative weight ( 1 - a)) is characterized by the oscillation frequency, b, and the spin diffusion rate T~ 1. M0 denotes the maximum transferred magnetization that would correspond to spin temperature equilibrium in the absence of spin lattice relaxation in the rotating frame (Tip ~ oo). The prefactor (1 X)-1, with X = Tin~Tip, accounts for the effects of a finite spin lattice relaxation time. This model has been discussed more fully in our previous communication [28]. Given a set of integrated cross-polarization intensities, as obtained from deconvolution [30] of the transfer-time dependent spectra, the non-linear least squares fit to the data for each individual Qn site involves the adjustment of six parameters, i.e. the times T , , T~s, and T~p, the oscillation frequency, b, the relative amplitude, M 0, and the weighting factor, a. The results of the fits, as summarized in table 1, are discussed below.
temperature of 600 o C, then re-dispersed in base for 4 days, and finally dried at 120°C. Synthesis of gel 102 was initiated by partial hydrolysis (2 molecules of water per metal alkoxide) of TEOS with acid, followed by addition of TIOT, and completion of hydrolysis with deionized water, up to a final molar ratio of (HEO)/(TEOS + TIOT) equal to 4. The mixed wet gel was re-dispersed in base for 4 days, and dried at 120° C. Gel 103A was obtained by partial hydrolysis of TEOS with base. The completion of hydrolysis was performed in the same way as with gel 102; the wet gel was dried at 120 o C. For gel 104B, the mixed oxide was prepared as with gel 102 (hydrolysis step), but the wet gel was dried at 600 ° C. Gel 104E, finally, was synthesized in the same manner as gel 104B, except that after drying at 600 ° C, the gel was re-dispersed in base for 4 days, and subsequently dried at 600 o C.
3.2. N M R measurements
3. Experimental 3.1. Preparation of the TiO2/ SiO 2 mixed oxides Important preparation parameters which distinguish the gels investigated in this study are briefly indicated. Details of the preparation procedure have been given elsewhere [31]. All samples contained 20 mol% Z i O 2 and 80 mol% SiO 2. Gel 101 was prepared by complete hydrolysis of tetraethoxy-silicate (TEOS) and tetra-isopropoxy-titanate (TIOT) (addition of 20 molecules of water per metal alkoxide) prior to the mixing of the corresponding sols. TEOS was hydrolyzed with ammonia water at pH = 9; T I O T was hydrolyzed in 1 N HC1. After mixing, the wet gel was dried at 120°C. The preparation of gel 114 also involved the complete hydrolysis of the respective alkoxides, but TEOS was hydrolyzed in acid, as was TIOT. The mixed wet gel was re-dispersed in base for 4 days, and dried at 120 ° C. Gel 115 was hydrolyzed in the same way as gel 114, but the mixed wet gel was dried at the higher
Experiments were performed with a high resolution solid state N M R spectrometer (Briiker, model MSL 300). The 29Si spectra were recorded
~
~
k
~
Gel 101
I
I
I
-50
-100
-150
-200
5 / ppm Fig. 1. CP/MAS spectra of two TiO2 (20 mol%)/SiO2 mixed gels. A Hartmann-Hahn contact time of 30 ms was used. Gel 101 was obtained by completehydrolysisof the alkoxidesprior to mixing; gel 102 was prepared by partial hydrolysisof TEOS with acid, followed by addition of TIOT, and completion of hydrolysiswith deionizedwater. Details are given in the text.
K.L. Walther et aL / TiO: / SiO: mixed oxide catalysts prepared by sol-gel techniques
at room temperature at a frequency of 59.6 MHz. Samples were spun at the magic angle in Kel-F rotors, using a frequency of 4 kHz. The on-resonance nutation frequency for l H (as determined from the pre-set length of 10 ~s for a "~ pulse) corresponded to a value of wl = 3 x 105 rad s-1. The H a r t m a n n - H a h n condition was adjusted by 29 . . . . 29 varying the St rf power, and maximizing the S1 C P / M A S signal for a known sample {QsMs, Si80~2[OSi(CH3)3] 8 } at its optimum contact time. C P / M A S spectra were obtained in single-contact experiments by using variable contact times between 1 and 50 ms, and a recycle delay of 10 s. 500 scans have been accumulated for each spectrum. All chemical shifts are reported with respect to TMS. A typical spectrum for a sample prepared by complete hydrolysis (gel 101) and by partial hydrolysis of the components (gel 102), respectively, are compared in fig. 1. •
51
Ti02/Si02 - Gel 114
40.0
!
I
30.0 ~
/
~~ ao.o
•
4. Results
The observed spectra are composed of three lines which are assigned to the structural units Q2, Ti02/Si02
Gel 101
40.0
30.0
.-~ 10.0
0.0 0
10
20 contact
30 40 time / ms
50
Fig. 3. Dependence of 29Si C P / M A S signal intensities on contact time for gel 114. The alkoxides were completely hydrolyzed (both in acid) prior to mixing. The mixed wet gel was redispersed in base for four days, and then dried at 120°C. Notation and symbols are the same as in fig. 2.
Q3, and Q4 of the SiO 2 network. For gels 101, 114, and 115, the chemical shifts are typically - 9 2 . 3 ppm for Q2, - 1 0 1 . 5 ppm for Q3, and - 1 1 1 . 0 ppm for Q4. For gels 102 and 103A, the corresponding shifts are obtained as - 91.7 ppm, - 99.9 ppm, and - 1 0 8 . 1 ppm, respectively. Chemical shifts for gels 104B and 104E are - 9 2 . 9 ppm, - 1 0 1 . 2 ppm, and - 1 1 0 . 1 ppm, for the sites Q2, Q3, and Q4, respectively. Ti02/Si % - Gel 115
40.0
,d ao.o
30.0
•~ l o . o '
~ eo.o
/ 0.0
0
10
20 contact
30 40 time / ms
3' 50
Fig. 2. Dependence of 29Si C P / M A S signal intensities on contact time for gel 101• The gel consists of 20 mol% TiO 2 and 80 mol% SiO2; this composition is the same for all the gels investigated in this study• The alkoxides were completely hydrolyzed (TEOS in base, TIOT with acid) prior to mixing; the gel was dried at 120°C. The dependence of the integrated signal intensities, as obtained from deconvohition of the spectra, is plotted against the H a r t m a n n - H a h n contact time for Q2 sites (circles), Q3 sites (triangles), and Q4 sites (squares). The solid line represents a least squares fit according to eq. (1).
e~ '~ 10.0
o.o ft - - " 0
T T ~ T,-7-~ 10
20 contact
i
30 40 time / ms
,
i
50
Fig. 4. Dependence of 29Si C P / M A S signal intensities on contact time for gel 115. The alkoxides were completely hydrolyzed (both in acid) prior to mixing. The gel was dried at 600 ° C, then redispersed in base for four days, and finally dried at 120°C. Notation and symbols are the same as in fig. 2.
K.L. Walther et aL / TiO2/ SiO 2 mixed oxide catalysts prepared by sol-gel techniques
52
Ti02/Si02 - Gel 102
Ti02/Si02 - Gel 104 B
40.0 60.0 30.0
d 40.0
~o.o
20.0
.~ l o . o
,
0.0
0
10
~
20 contact
,
30 40 time / ms
50
Fig. 5. Dependence of 29Si C P / M A S signal intensities on contact time for gel 102. The preparation involved partial hydrolysis of TEOS in acid (2 molecules of water per alkoxide), followed by addition of TIOT, completion of hydrolysis up to a water/alkoxide molar ratio of four, redispersion in base, and a drying step at 120 o C. Notation and symbols ar the same as in fig. 2.
The gels prepared by complete hydrolysis were characterized by well-resolved lines. Representative linewidths were 360 Hz for the structural type Q2, 410 Hz for Q3, and 520 Hz for Q4. For the samples prepared by partial hydrolysis, significantly broader and unresolved lines were observed, which had to be decomposed into the Q2, Q3, and Q4 components by deconvolution [30].
¢
0,0
,
¢
~,
,
0
10
(} ,
20 contact
|
¢
¢
,
,
30 40 time / ms
¢ ,
50
Fig. 7. Dependence of 29Si C P / M A S signal intensities on contact time for gel 104B. The preparation was the same as used for gel 102 (fig. 5) except for redispersion; but the gel was dried at 600 o C. Notation and symbols are the same as in fig. 2.
Linewidths, obtained from the fit, are 430 Hz, 480 Hz, and 630 Hz, respectively. The contact time dependence of the transferred 29Si magnetization for the Q2, Q3, and Q4 sites in gel 101 is presented in fig. 2. The corresponding data for gels 114 and 115 are shown in fig. 3 and 4. Figures 5-8 represent the cross-polarization dynamics of gels 102, 103A, 104B, and 104E, respectively. The quantity TsiH (table 1) corre-
Ti02/Si02 - Gel 103 h
TiO2/Si02 - Gel 104 E
40.0 60.0 30.0
d 40.0
20.0 2e~ •-
20.0
10.0
0.0
,
0
10
~
20 contact
,
,
30 40 time / ms
,
50
Fig. 6. Dependence of 2 9 S i C P / / M A S signal intensities on contact time for gel 103A. The preparation involved the same steps as used for gel 102 (fig. 5), but the hydrolysis of TEOS was performed in base. Notation and symbols are the same as in fig. 2.
o.o
,
O
10
; ~ 20 contact
t
¢
30 40 time / ms
50
Fig. 8. Dependence of 29Si C P / M A S signal intensities on contact time for gel 104E. The preparation was the same as used for gel 102 (fig. 5), but the gel was dried at 600 o C, then redispersed in base for four days, and dried again at 600 o C. Notation and symbols are the same as in fig. 2.
K.L. Walther et al. / TiO2/ S i O 2 mixed oxide catalysts prepared by sol-gel techniques
sponds to the cross-relaxation time, Tis, of exponential magnetization transfer (eq. (1)). The parameters of the oscillatory magnetization transfer are also indicated in table 1, whereby THH corresponds to the spin diffusion time (T H in eq. (1)). The solid lines in figs. 2-8 represent leastsquares fits to the data based on eq. (1); the resulting parameters are summarized in the table. The fractional amplitude, M 0, of the three 28Si N M R signals (Q2, Q3, and Q4) obtained from MAS not employing cross-polarization (cf. table 1) will be compared with the corresponding contributions determined by CP/MAS, i.e. the relative amplitudes of the curves describing the time dependence of magnetization transfer obtained for each site from the C P / M A S fit.
5. Discussion
5.1. Gels prepared by mixing after complete hydrolysis of the components Gel 101 is characterized by comparatively narrow lines and a high percentage of Q4 units. This indicates a high degree of cross-linking, which is promoted by the basic hydrolysis medium. In the C P / M A S spectra, the relative amplitude of the Q3 sites (20.5%) is considerably larger than in the MAS-spectrum without cross polarization (12.2%). This enhancement points to the fact that for most of the Q3 centers, the single ligand different from -OSi is a hydroxyl group, rather than -OTi TM. The cross-polarization times, TsiH, are comparatively short, which is typical for all the gels prepared by complete hydrolysis prior to mixing; these time constants increase in the sequence from Q2 to Q4. Small values of TSi H indicate that protons are present in spatial vicinity of the silicon nuclei. By contrast, the spin diffusion time, THH, is comparatively long. Such a behavior could be expected if the sample consist of silica clusters with proton-covered surfaces and a comparatively weak interaction between the clusters. The observation of a large oscillatory fraction (1 - a) of the magnetization transfer, i.e. 0.5 for Q2 and Q3, and 0.8 for Qa, is consistent with this picture. The oscillation frequency, b, of the coherent magneti-
53
zation transfer is small, both as compared with pure silica gels [28] and with the other samples investigated in this study. No long-time decrease of the transferred magnetization was observed, i.e. T,~ was set to infinity. Gels 114 and 115 prepared by acid hydrolysis again exhibit comparatively narrow, well-resolved lines for the three types of sites. This fact points to the existence of small TiO 2 and SiO2 domains, which is typical for all samples prepared by complete hydrolysis of the respective alkoxides prior to mixing. The percentage of Q4 sites is not as high as for gel 101, which has been hydrolyzed under basic conditions. The cross-polarization times TsiH are on the same order of magnitude, the spin diffusion times THH somewhat smaller than for gel 101. Long-time decay of the magnetization is most striking for sample 114 (Tlo = 23 ms for Q2). For the Q2 sites, the magnetization transfer dynamics are predominantly exponential (a = 0.8-0.9), i.e. any fast coherent oscillations are not resolved. By contrast, for the Q4 lines the oscillations are clearly discernible (a < 0.2). The oscillation frequencies, b ~ 300 rad/s, are on the lower limit of the interval observed for pure silica gels [281.
5.2. Gelsprepared by mixing after partial hydrolysis of the components In gel 102, the percentage of Q2 and Q3 units is significantly enhanced, as compared to gel 101. This is due the fact that a Si-O-Ti TM bond gives rise to a similar chemical shift increment as S i - O - H [17,26,27], as mentioned above. As a consequence, the 0 2 line comprises contributions from environments of the three types (-SiO-)2Si(-OH)2, (-SiO-)2Si(-OH)(-OTi-), and (-SiO-)2Si(-OTi-) 2, and correspondingly for Q3. From the increased fraction of Q2 and Q3 sites in gel 102, we suggest that the sample contains SiO-Ti bridges, i.e. SiO2 and TiO 2 are - at least partly - mixed on a molecular scale. This mixing is confirmed by the considerably larger width of the Q2, Q3, and Q4 lines, i.e., for each type of site there is a chemical and structural diversity of environments.
54
K.L. Walther et al. / Ti02 / SiO 2 mixed oxide catalysts prepared by sol-gel techniques
The cross-relaxation times, TSiH, are similar to those of the gels discussed above. However, the spin diffusion time Trt H is very small for Q2 and Q3. This smallness appears to indicate that the diffusion of proton magnetization is unrestricted across the entire sample, by contrast with the cluster model discussed above for the gels prepared by complete hydrolysis of the respective alkoxides. (The large value of THH for Q4 sites is not reliable because, for this line, the exponential magnetization transfer is dominant, a = 0.96.) There is a pronounced long-time decrease of the silicon magnetization, as expressed by short relaxation times Tip in the rotating frame. Small values of Tip are usually interpreted [18] in terms of exchange processes between the abundant proton spins. The re-dispersion treatment in base that influences the concentration of surface OH groups could be responsible for this observation. From table 1 we see that a > 0.5, and that the relative weight of the exponential contribution increases from Q2 to Q4. The fact that the oscillatory fraction (1 - a) of the magnetization transfer is small suggests that there are few isolated protons; rather, each silicon nucleus appears to be interacting with a large proton spin reservoir. For this reason, the values of the oscillation frequencies derived for this sample are not significant. Sample 103A has parameters similar to those of gel 102. The following differences are noted. The site distribution is shifted towards higher-coordinated sites. This shift is a result of the different preparation procedure: in gel 103A, the TEOS precursor was partially hydrolyzed in base, which results in a higher degree of cross-linking, as reflected by the increased Q4 fraction. However, the Q2 percentage is still significantly higher than for gels 101, 114 and 115. This implies that also for sample 103A, the Q2 line contains contributions from (-SiO-)2Si(-OH)2_~(-OTi-)n (n = 1, 2) sites. As with gel 102, the cross-relaxation and spin diffusion times are short; the relaxation times, T~p, are comparable as well (except for Q4). Little oscillatory transfer is observed, as reflected by high values of a. Small but significant chemical shift differences are noted for the line positions, as deduced from
the least squares fit. In samples 102 and 103A, Q3 (~ = -99.9 ppm) is less shielded by = 1.5 ppm, as compared with gels 101,114, and 115 (~ = - 101.5 ppm). Similarly, the chemical shift for Q2 (~ = -91.7 ppm for gels 102, 103A) differs from the average of gels 101, 114, and 115 ( ~ = - 9 2 . 3 ppm). This again points to the fact that in the gels ~9repared by partial hydrolysis, the environment of Si on Qz and Q3 sites comprises Ti(IV) nearest neighbors. In comparison to sample 102, the gels 104B and 104E have been calcined at a higher temperature (600 o C). This treatment results in further crosslinking by condensation reactions of Si-OH groups, formation of siloxane bridges and release of water, as evident from the significantly increased percentage of Q4 sites and a small fraction of Q2 sites. The higher degree of cross-linking also leads to an increase in linewidths. As a further consequence of the loss of water, the cross-polarization time TsiH for Q4 is greater, by almost an order of magnitude, as compared with the previously discussed gels. Large values of THH are also consistent with this picture. Note that there is no large time decrease of the magnetization (Tip --+ 00). Oscillatory magnetization transfer is significant only for Q3 sites ( a = 0 . 3 ) ; the value of b is consistent with oscillatory magnetization exchange with a second nearest neighbor proton (cf. the discussion in ref. [28]). The overall similarity in the NMR parameters of gels 104B and 104E is not unexpected, as the preparation differs only by the additional ageing in base employed for gel 104E.
5.3. Comparison of the gels The NMR parameters will now be compared across the series of samples investigated. For the Q2 and Q3 sites, the fact that their relative contribution in CP/MAS experiments is always higher than in MAS reference experiments (without cross-polarization) indicates that at least a fraction of the nearest neighbors are protons in all the samples. Hydrolysis of TEOS under basic conditions leads to a larger degree of cross-linking and to the formation of highly branched cluster (higher Q4 percentage, as compared with the acid-hydro-
K.L. Walther et al. / Ti02 // Si02 mixed oxide catalysts prepared by sol-gel techniques
lyzed samples). Re-dispersion in base of wet gels tends to increase the Q2 percentage, as seen from a comparison of gels 103A and 102. If one compares TsiH values for the corresponding Qn sites, generally similar values are found with two striking exceptions: Very slow cross-relaxation is noted for the Q4 lines of gels 104 prepared by partial hydrolysis and high-temperature drying. The spin diffusion time THH, a parameter of the oscillatory magnetization transfer component, should be discussed only in those cases where the latter contribution is significant (a _< 0.5). THH is large in sample 101 where TEOS was hydrolyzed in base (compact clusters), and small in samples 114, 115 where the hydrolysis was performed in an acid medium. Next, this parameter will be compared for the four gels prepared by partial hydrolysis of the precursors; we focus on the Q3 sites where the oscillatory contribution is significant. Spin diffusion is very fast (8-9 ms) for gels 102 and 103A, and somewhat slower (24-30 ms) for the high temperature dried gels 104B and 104E. Small Tip values are often associated [18] with exchange processes within the proton spin reservoir. In this respect, the Q2 and Q3 sites of gels 102, 103A, and 114, are distinct from the other samples investigated. The origins of these observations, i.e. the influence of drying and redispersion treatments, will be the subject of further investigations. As stated earlier [28], the frequency calculated for coherent magnetization transfer between a 295i nucleus and a directly bound OH proton is too high (--- 800 rad/s) to be resolved with the mixing time increments used in our experiments. Further, in a powder sample the oscillations are averaged over a multitude of geometries and orientations. Frequencies, b, around 400 r a d / s are typical for OH protons attached to nearest neighbor (and further removed) protons [28]. The fact that sample 101 exhibits particularly low frequencies might be associated with the fact that the complete hydrolysis of TEOS in base results in very compact silica clusters. Hydroxyl groups, present mainly on the surface of these clusters, would then be remote from the silicon nuclei in the interior. On the other hand, gel 102 appears to be a prototype of a
55
molecularly mixed network with comparatively little cross-linking. Here, for any 29Si center the distance to the nearest hydroxyl proton is smaller. The quantity (1 - a) is a measure of the oscillatory fraction of the magnetization transfer. For the gels prepared by complete hydrolysis, this contribution is larger than previously found for the pure silica gels. In the partially hydrolyzed gels 104B and 104E, only the Q3 site exhibits significant oscillations. With gels 102 and 103A, the cross-polarization dynamics are predominantly exponential in nature. The lack of an oscillatory component with these samples might also be associated with the existence of S i - O - T i bridges. Chemical shift values obtained from the spectral deconvolution provide valuable information on the presence or absence of domains consisting of the individual components. For a given type of site, a more negative chemical shift value indicates a lower number of S i - O - T i bridges [23,26]. First we compare the group of mixed oxides prepared by complete hydrolysis (gels 101, 114, and 115). According to the mentioned criterion, domain formation is most pronounced in sample 101, as judged, for example, from the chemical shifts of Q3 sites: ~ = -101.7 ppm for gel 101, as compared with -101.4 ppm for gels 114 and 115. The conclusion that S i - O - T i bridges are largely absent in sample 101 is supported by the observation that the percentage of Q4 sites (87%) is significantly higher than in gels 114 and 115 (70 and 76%, respectively). Further, the linewidth of the N M R signals is narrowest for gel 101, which is explained by the same fact. The gels prepared by partial hydrolysis show a complementary behavior, as their N M R parameters indicate that TiO 2 and SiO 2 are at least partly mixed on a molecular scale. S i - O - T i bridges are most abundant when the partial hydrolysis of TEOS is performed in acid. This conclusion is reached from a comparison of several N M R parameters of gels 102 and 103A. First, the silicon nuclei are less shielded in gel 102, as a consequence of the presence of Ti TM neighbors for Q2 and Q3 sites ( ~ j ( O 2 ) = - 9 1 . 2 and -92.1 ppm, ~(Q3) = _ 99.6 and -100.1 ppm, for gels 102 and 103A, respectively). Second, the linewidths are consistently larger in gel 102 (FWHM(Q 2) = 500
56
K.L Walther et al. / TiO: / SiO2 mixed oxide catalysts prepared by sol-gel techniques
and 414 Hz, F W H M ( Q 3) = 489 and 453 Hz, for gels 102 and 103A, respectively), which means that there is a higher chemical and structural diversity of environments in gel 102. Third, the percentage of Q2 sites, which includes the species featuring one or two Ti TM nearest neighbors, is largest among all samples (15%) in gel 102. The redispersion treatment in base is only effective when applied to a wet gel, i.e. before drying. Due to the desired ripening process, any micropores present after preparation are converted to mesopores, as has been shown [31] for gels 102 and 114 where the micropore volume after redispersion is zero within the accuracy of the measurement. For these two gels, the Q2/Q3 ratio (both with and without cross-polarization) is largest among all samples investigated. This observation may indicate the presence of Si(OH)2 groups on the walls of the ripened mesopores.
6. Conclusions The results presented above demonstrate that pronounced structural differences exist between the mixed oxides prepared by partial and complete hydrolysis of the precursors prior to mixing. Domain formation is quite extensive when the respective alkoxides are completely hydrolyzed in excess of water before mixing. Within the series of mixed oxides prepared by complete hydrolysis of the components, the hydrolysis p H of TEOS is the decisive factor that determines the extent of domain formation. The most pronounced development of silica domains is observed in samples produced by hydrolysis of TEOS in base. (The titania precursor is always hydrolyzed in acid.) The cross-polarization parameters indicate that compact silica clusters are thereby formed. For m a n y 29Si nuclei within such a cluster, the distance to the nearest surface proton is small. Molecularly mixed solids are approached only when the T I O T component is added after partial hydrolysis (50% on average) of TEOS. Spectral and dynamic N M R parameters demonstrate that the molecular mixing of the components is most pro-
nounced when the partial hydrolysis of TEOS is performed in acid, which results in a reduced tendency of aggregation and cross-linking. Gels prepared by partial hydrolysis in acid are, however, not very resistant to high temperature drying. U p o n treatment at 600 o C, the pore volume shows a drastic decrease, and the dried gel exhibits only the catalytically less useful micropores [31]. Subsequent redispersion of these oxides is not capable of reversing the pore collapse [31]. In work in progress in our laboratory, further information on the structural properties of mixed oxide gels prepared by partial and complete hydrolysis of the precursors is being derived from F T I R and R a m a n investigations.
Sincere thanks are due to A. Sebald and L. Merwin for performing the C P / M A S experiments. Financial support by the Deutsche Forschungsgemeinschaft (SFB 213) and by the Swiss Federal Institute of Technology (ETH) is gratefully acknowledged.
References [1] A.B. Stiles, Catalyst Supports and Supported Catalysts (Butterworths, Boston, MA, 1987). [2] A. Baiker and A. Wokaun, Z. Naturwiss. 76 (1989) 168. [3] D. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [4] P.A. Haas, Chem. Eng. Prog. 85 (1989) 44. [5] B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. [6] A. Baiker, A. Dollenmeier, M. Glinski and A. Relier, Appl. Catal. 35 (1987) 365. [7] A. Balker, A. Dollenmeier, M. Glinski and A. Relier, Appl. Catal. 35 (1987) 351; M. Schraml, W. Fluhr, A. Wokaun, and A. Baiker, Ber. Bunsenges. Phys. Chem. 93 (1989) 852. [8] F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J. Medema, V.H.J. de Beer and P.J. Gellings, J. Phys. Chem. 84 (1980) 2783. [9] M. Tagaki, M. Soma, T. Onishi and K. Tamaru, Can. J. Chem. 58 (1980) 2132. [10] K.L. Walther, M. Schraml-Marth, A. Wokaun and A. Baiker, Catal. Lett. 4 (1990) 327. [11] C.J. Brinker, J. Non-Cryst. Solids 100 (1988) 31. [12] M. Guglielmi and G. Carturan, J. Non-Cryst. Solids 100 (1988) 16. [13] C.J. Brinker and G.W. Scherer, in: Ultrastructure Processing of Ceramics, Glasses, and Composites, eds.
K.L. Walther et al. / TiO 2/ S i O 2 mixed oxide catalysts prepared by sol-gel techniques
[14]
[15]
[16] [17] [18]
[19] [20]
[21]
L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) ch. 5. B.E. Handy, K.L. Walther, A. Wokaun, and A. Baiker, in: Preparation of Catalysts V, eds. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Elsevier, Amsterdam, 1990) p. 239. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. G.E. Maciel and D.W. Sindorf, J. Am. Chem. Soc. 102 (1980) 7606. D.W. Sindorf and G.E. Maciel, J. Am. Chem. Soc. 103 (1981) 4263; D.W. Sindorf and G.E. Maciel, J. Phys. Chem. 86 (1982) 5208; D.W. Sindorf and G.E. Maciel, J. Am. Chem. Soc. 105 (1983) 1487. R.K. Harris and R.H. Newman, J. Chem. Soc., Faraday Trans. II 73 (1977) 1204. E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A.R. Grimmer, J. Am. Chem. Soc. 102 (1980) 4889; E. Lippmaa, M, Magi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. Soc. 103 (1981) 4992. G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z. Anorg. Allg. Chem. 482 (1981) 49.
57
[22] A.R. Grimmer and R. Radeglia, Chem. Phys. Lett. 106 (1984) 262. [23] G. Engeihardt, High Resolution Solid State NMR of Silicates and Zeolites (Wiley, New York, 1987). [24] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. G~Sktas and G.H, Frischat, Ber. Bunsenges. Phys. Chem. 92 (1988) 689. [25] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. GOktas and G.H. Frischat, J. Non-Cryst. Solids 116 (1990) 161. [26] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. GiSktas and G.H. Frischat, Phys. Chem. Glasses 31 (1990) 138. [27] G.A. Fyfe, G.G. Gobbi and G.J. Kennedy, J. Phys. Chem. 89 (1985) 277. [28] K.L. Walther, A. Wokaun and A. Baiker, Mol. Phys. 71 (1990) 769. [29] L. Mialler, A. Kumar, Th. Baumann and R.R. Ernst, Phys. Rev. Lett. 32 (1974) 1402. [30] Deconvolution of the spectra was performed using the GLINFIT software by Briiker Inc., Karlsrnhe. [31] B.E. Handy, A. Baiker, K.L. Walther and A. Wokaun, in: Synthesis and Properties of New Catalysts, eds. E.W. Corcoran and M.J. Ledoux (Materials Research Society, Pittsburgh, PA, 1990) p. 107; B.E. Handy, A. Baiker and A. Wokaun. submitted to J. Mater. Sci.
47
mixed oxide catalysts prepared by sol-gel techniques. Characterization by solid state CP/MAS spectroscopy
TiO2/SiO 2
K.L. W a l t h e r a n d A. W o k a u n Physical Chemistry IL University of Bayreuth, W-8580 Bayreuth, German)'
B.E. H a n d y and A. Baiker Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH Zentrum, CH-8092 Ziirich, Switzerland Received 25 October 1990 Revised manuscript received 23 April 1991
The influence of synthesis parameters on the structural properties of mixed oxides derived from silicon tetraethoxide and titanium tetra-isopropoxide precursors has been studied by S1 NMR spectroscopy. A first set of gels, prepared by complete hydrolysis of the individual precursors and subsequent mixing of the hydrosols, is characterized by comparatively narrow lines for the Q2 Q3, and Q4 silica sites. Both the chemical shift values and the time constants of 1H-29Si magnetization transfer are similar to those found in pure silica gels. This indicates that these oxides consist of domains of the individual components. A different appearance of the CP/MAS spectra is noted for the second set of gels, which is prepared by addition of the titanium alkoxide component at a stage where the silicon alkoxide hydrolysis has only proceeded to about 50%, on average. The spectra of these samples are characterized by broad lines for the three types of sites. Larger contributions observed for Q2 and Q3 sites in these samples are attributed to the presence of Si-O-Ti bridges. The influence of preparation parameters such as hydrolysis pH, drying temperature, and pore ripening treatment by redispersion in base, is studied by monitoring the spectral and dynamic parameters derived from the CP/MAS spectra. •
1. Introduction
Properties of support materials and, in particular, their interaction with the dispersed catalytically active metal or compound are important design parameters in catalyst development [1]. Besides testing less common oxides as catalyst cartiers, the use of mixed oxides has recently attracted attention [2], in view of their potential for combining certain desirable properties of the individual oxide supports. For a successful realization of this concept, mechanical mixtures of the components are insufficient: it is required to achieve intimate contact between constituents and, if possible, mixing on a molecular scale. To realize this aim, the use of sol-gel technology [3-5] is particularly promising: molecular hydrosols of the com-
29
.
ponents are mixed prior to their immobilization in a three-dimensional network during the gelation process. The present study has been motivated by recent reports on the preparation of V205 monolayer catalysts supported on the surface of TiO2/SiO 2 mixed oxides [6]. These catalysts exhibit activities in the selective catalytic reduction (SCR) of nitric oxides that are higher than observed with vanadia layers supported on TiO 2 (anatase/rutile, Degussa P25) [7], and are more stable against thermally induced aggregation than systems supported on SiO 2 (Aerosil) [8,9]. The structure of vanadia dispersed on TiOz/SiO 2 has recently been characterized by ESR and Raman spectroscopy [10]. Here we consider the influence of preparation variables on the properties of the TiO2/SiO 2 mixed
0022-3093/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
48
K.L. Walther et al. / Ti02 / Si02 mixed oxide catalysts prepared by sol-gel techniques
oxide supports used in the catalyst systems mentioned above. All catalysts considered contain a weight fraction of 20 mol% TiO 2, and are synthesized starting from silicon and titanium alkoxides, i.e. silicon-tetraethoxide (TEOS) and titanium-tetra-isopropoxide (TIOT). The following preparation variables were tested [11-13]. First, hydrolysis of the components can be performed in either acid or basic medium. In acid, the H+-catalyzed hydrolysis of the alkoxides proceedes rapidly, while the condensation reactions requiring deprotonation are rate determining in the gelation process. As a consequence, linear chains with little cross-linking are generated in acid medium. The adsorption isotherm of the low temperature dried aerogel is of type I [14,15]. Upon high temperature drying (e.g., at 600 ° C), the observed loss of water by condensation reactions is accompanied by a collapse of the structure, and microporous solids result which are less useful for catalytic applications. By contrast, in a basic medium the hydrolysis of the alkoxides is rate limiting, while the OH--catalyzed condensation reactions proceed rapidly. As the hydrolyzed species ('M(OH)4' in a formal notation) are immediately attached to the growing nuclei, comparatively compact and highly cross-linked clusters are formed. Upon drying, these clusters pack into a structure characterized [14] by a type IV adsorption isotherm [15] and by mesopores [16] which are stable up to higher temperatures. The second important preparation parameter [11-13] is the sequence of hydrolysis and mixing. If the two components (TEOS and TIOT) are hydrolyzed to completion (using, for example, a 20 fold molar excess of water) prior to mixing, there is an obvious tendency for the formation of larger clusters of the individual components by non-reversible condensation reactions, in particular, in basic media. The alternative procedure investigated here is partial hydrolysis of TEOS (2 mol of water per mole of alkoxide), followed by addition of the appropriate amount of TIOT, and further addition of the stoichiometric amount of water required for complete hydrolysis (4 mol of water per mole of alkoxide). It is expected that this method should be more suitable for producing solids which are mixed on a molecular scale.
A third variable to be tested [11-13] is the redispersion of the prepared gel in a slightly basic medium. The purpose of this step is to effect changes in pore structure, by catalyzing the breaking of ' M - O - M ' linkages formed during the condensation process, as well as their re-formation. This process should be most effective when applied to wet gels which have not been irreversibly cross-linked by high temperature drying. The use of 29Si C P / M A S N M R as a source of structural information in solid silicates has been extensively described in the literature (cf. Maciel and Sindorf [17]; Sindorf and Maciel [18]; Harris and Newman [19]; Lippmaa et al. [20]; Engelhardt et al. [21]; Grimmer and Radeglia [22]; as well as a more comprehensive list of references contained in the book by Engelhardt [23]). The 29Si nucleus in silicates is found in a tetrahedral oxygen environment. Various types of sites are distinguished by the second-nearest neighbor ligands. The abbreviation Qn denotes a site of the type S i ( - O - S i ) n ( - O - X ) 4 _ n, where X is an ionically bound ligand other than silicon, most frequently a proton. The sites Q1 through Q4 in pure silicates are clearly distinguished by characteristic values of their respective chemical shifts [23]. Recently, pioneering applications of 29Si CP// MAS-NMR to the characterization of sol-gel processes have been published by Wies et al. [2426] where the systems Li20//SiO2 and SiO 2TiO2-ZrO 2 have been investigated. From an evaluation of chemical shift increments, it was found that - O H and - O - T i TM groups have a similar influence on the shift of the central 29Si nucleus [17,26,27]. As a consequence, in a SiO2/ TiO 2 mixed oxide, the Q3 line would comprise contributions from both S i ( - O - S i ) 3 ( - O H ) and S i ( - O - S i ) 3 ( - O - T i TM) structural units, as is discussed below. We have recently described [28] the application of C P / M A S - N M R to investigate the structure of pure silica gels prepared from TEOS using the sol-gel process, whereby some of the parameters mentioned above have been varied. Our present investigation of TiO2/SiO 2 mixed oxide systems will therefore be supported by the information
K.L Waltheret al. / TiO2/ Si02 mixed oxide catalysts prepared by sol-gel techniques [28] o b t a i n e d with the silica gels n o t c o n t a i n i n g titania.
2. Time dependence of the magnetization transfer It is a characteristic feature of the silica gel t y p e s a m p l e s d u r i n g H a r t m a n n - H a h n cross-polarization e x p e r i m e n t s that the m a g n e t i z a t i o n transfer does not follow a simple e x p o n e n t i a l time d e p e n dence [28]. Strong transient oscillations a p p e a r s u p e r i m p o s e d on the overall g r o w t h of 295i m a g n e tization in the r o t a t i n g frame. This behavior, which was characterized for the p u r e silica gels [28], is again very p r o n o u n c e d with the T i O 2 / S i O 2 m i x e d gels, as is exemplified b y the e x p e r i m e n t a l results shown below. The transient oscillations p h e n o m e n o n was first Table 1 Parameters characterizing the
29SiNMR
49
o b s e r v e d b y Mtiller et al. [29], a n d was a n a l y z e d as a c o h e r e n t a n d p e r i o d i c m a g n e t i z a t i o n transfer b e t w e e n the rare S-spin (13C or 29Si, respectively) a n d an i s o l a t e d p r o t o n (I-spin), which is o n l y w e a k l y c o u p l e d to the p r o t o n spin reservoir b y a spin diffusion rate, T~ 1. To a c c o u n t for the superi m p o s e d e x p o n e n t i a l increase of t r a n s f e r r e d magnetization, a n a d d i t i o n a l t e r m d e s c r i b i n g the transfer at a c r o s s - r e l a x a t i o n rate, T ~ ~, a c c o r d i n g to the spin t e m p e r a t u r e c o n c e p t m u s t b e included. This leads to the e x p r e s s i o n for the time d e p e n d e n c e of the m a g n e t i z a t i o n transfer used in our analysis:
M(t) =
M°
( e -t/r',' - ae - t / r ' s -
(1 - a )
x [ ½ e - t / r " + ½ e -3'/2r,, cos(½bt)] }. (1)
spectra of the investigated titania-silica gels
Sample
Struc- Mo (%) tural MAS type
Complete hydrolysis before mixing sols; TEOS with base, TIOT with acid; dried at 120 o C (gel 101)
Q2 Q3
Q4
Mo (%) CP/ MAS
TsiH (ms)
Trtrt (ms)
TloH (ms)
0.4±0.4 2.4±0.8 0.4±0.4 ( ~ ) (oO) 12.2±3.5 20.5±3.0 0.7±0.3 420±420 (o¢) 87.4±14 77.1±12 2.1±1.5 155±150 (or)
a
b (rad/s)
0.5+0.5 70+70 0.5-1-0.1 134+26 0.2+0.1 112+18
Partial hydrolysis of TEOS with acid, Q2 TIOT added, final hydrolysis with H20; Q3 re-dispersed in base, dried at 120°C (gel 102) Q4
15.2±3.3 20.3±8.6 0.3±0.3 9.9±9.9 34.2±6.0 41.8±6.8 0.3±0.2 8.1±4.9 50.6±7.9 37.9±5.1 1.8±0.2 1 ~ ± 1 ~
24.6+4.8 0.5+0.5 32.8+2.3 (0.64) 62.2 ± 9.5 (0.96)
1046+200 491±190 556 ± 59
Partial hydrolysis of TEOS with base, TIOT added, final hydrolysis with H20; dried at 120 o C (gel 103A)
5.2±3.7 13.9±8.1 1.0±0.4 4 ~ ± 4 9 0 28.6+11 0.9±0.3 35.9±11 49.0±11 0.9±0.7 8.9±8.9 34.3 ± 4.1 0.7 ±0.5 58.9±16 37.1±6.9 2.1±0.6 ( ~ ) 222±120 0.8±0.1
195+150 533±340 260±39
Partial hydrolysis of TEOS with acid, TIOT added, final hydrolysis with H20; dried at 600 o C (gel 104B)
Q2
Q3 Q4 Q2
0.8±0.7 2.7±0.6 1.1±1.1 ( ~ ) 12.5±2.0 27.7±2.8 0.6±0.6 23.9±15 86.7±8.4 69.6±6.6 13.8±1.8(~)
(oo) (oo) (oo)
(0.90) 200 ± 200 0.3±0.1 244±52 0.83 ± 0.04 341 +15
Q4
0.8±0.8 2.5±1.3 1.3±1.3 ( ~ ) 7.7±1.6 24.0±3.0 0.5±0.5 30.2±13 91.5±7.9 73.5±11 15.7±4.0(~)
77.5+77 (o¢) (o0)
0.7±0.5 0.3±0.1 0.7±0.1
374±120 350±37 398±14
Complete hydrolysis before mixing sols; TEOS with acid, TIOT with acid; re-dispersed in base, dried at 120 °C (gel 114)
Q2 Q3 Q4
5.1±2.7 9.2±2.3 1.4±0.7 90.2±90 23.2+6.0 0.9+0.2 24.8±6.1 33.6±5.8 1.0±0.3 26.3±19 60.8+11 0.7+0.2 70.1±14 57.2±8.2 1.8±1.5 25.0±6.1 116+25 0.2+0.1
332:1:140 348+65 340+14
Complete hydrolysis before mixing sols; TEOS with acid, TIOT with acid; dried at 600 o C, re-dispersed in base, dried at 120°C (gel 115)
Q2 Q3 Q4
2.5±2.3 5.2±1.2 1.1±0.9 112±110 144+113 0.8+0.2 322+78 22.0±4.9 38.9±4.6 0.8±0.3 25.2±12 251-t-150 0.5+0.1 276+27 75.5±12 55.9±5.2 2.2±1.7 29.3±5.6 694 +640 0.13+0.06 288+7
Partial hydrolysis of TEOS with acid, TIOT added, final hydrolysis with H20; dried at 600 "C, re-dispersed in base (gel 104E)
Q3 Q4 Q2
Q3
Values in parentheses were held constant during the least squares fitting procedure.
50
K.L, Walther et al. / TiO 2/ SiO 2 mixed oxide catalysts prepared by sol-gel techniques
In this equation, the parameter a (0 < a < 1) represents the weight of the exponential part of magnetization transfer. The oscillatory part (relative weight ( 1 - a)) is characterized by the oscillation frequency, b, and the spin diffusion rate T~ 1. M0 denotes the maximum transferred magnetization that would correspond to spin temperature equilibrium in the absence of spin lattice relaxation in the rotating frame (Tip ~ oo). The prefactor (1 X)-1, with X = Tin~Tip, accounts for the effects of a finite spin lattice relaxation time. This model has been discussed more fully in our previous communication [28]. Given a set of integrated cross-polarization intensities, as obtained from deconvolution [30] of the transfer-time dependent spectra, the non-linear least squares fit to the data for each individual Qn site involves the adjustment of six parameters, i.e. the times T , , T~s, and T~p, the oscillation frequency, b, the relative amplitude, M 0, and the weighting factor, a. The results of the fits, as summarized in table 1, are discussed below.
temperature of 600 o C, then re-dispersed in base for 4 days, and finally dried at 120°C. Synthesis of gel 102 was initiated by partial hydrolysis (2 molecules of water per metal alkoxide) of TEOS with acid, followed by addition of TIOT, and completion of hydrolysis with deionized water, up to a final molar ratio of (HEO)/(TEOS + TIOT) equal to 4. The mixed wet gel was re-dispersed in base for 4 days, and dried at 120° C. Gel 103A was obtained by partial hydrolysis of TEOS with base. The completion of hydrolysis was performed in the same way as with gel 102; the wet gel was dried at 120 o C. For gel 104B, the mixed oxide was prepared as with gel 102 (hydrolysis step), but the wet gel was dried at 600 ° C. Gel 104E, finally, was synthesized in the same manner as gel 104B, except that after drying at 600 ° C, the gel was re-dispersed in base for 4 days, and subsequently dried at 600 o C.
3.2. N M R measurements
3. Experimental 3.1. Preparation of the TiO2/ SiO 2 mixed oxides Important preparation parameters which distinguish the gels investigated in this study are briefly indicated. Details of the preparation procedure have been given elsewhere [31]. All samples contained 20 mol% Z i O 2 and 80 mol% SiO 2. Gel 101 was prepared by complete hydrolysis of tetraethoxy-silicate (TEOS) and tetra-isopropoxy-titanate (TIOT) (addition of 20 molecules of water per metal alkoxide) prior to the mixing of the corresponding sols. TEOS was hydrolyzed with ammonia water at pH = 9; T I O T was hydrolyzed in 1 N HC1. After mixing, the wet gel was dried at 120°C. The preparation of gel 114 also involved the complete hydrolysis of the respective alkoxides, but TEOS was hydrolyzed in acid, as was TIOT. The mixed wet gel was re-dispersed in base for 4 days, and dried at 120 ° C. Gel 115 was hydrolyzed in the same way as gel 114, but the mixed wet gel was dried at the higher
Experiments were performed with a high resolution solid state N M R spectrometer (Briiker, model MSL 300). The 29Si spectra were recorded
~
~
k
~
Gel 101
I
I
I
-50
-100
-150
-200
5 / ppm Fig. 1. CP/MAS spectra of two TiO2 (20 mol%)/SiO2 mixed gels. A Hartmann-Hahn contact time of 30 ms was used. Gel 101 was obtained by completehydrolysisof the alkoxidesprior to mixing; gel 102 was prepared by partial hydrolysisof TEOS with acid, followed by addition of TIOT, and completion of hydrolysiswith deionizedwater. Details are given in the text.
K.L. Walther et aL / TiO: / SiO: mixed oxide catalysts prepared by sol-gel techniques
at room temperature at a frequency of 59.6 MHz. Samples were spun at the magic angle in Kel-F rotors, using a frequency of 4 kHz. The on-resonance nutation frequency for l H (as determined from the pre-set length of 10 ~s for a "~ pulse) corresponded to a value of wl = 3 x 105 rad s-1. The H a r t m a n n - H a h n condition was adjusted by 29 . . . . 29 varying the St rf power, and maximizing the S1 C P / M A S signal for a known sample {QsMs, Si80~2[OSi(CH3)3] 8 } at its optimum contact time. C P / M A S spectra were obtained in single-contact experiments by using variable contact times between 1 and 50 ms, and a recycle delay of 10 s. 500 scans have been accumulated for each spectrum. All chemical shifts are reported with respect to TMS. A typical spectrum for a sample prepared by complete hydrolysis (gel 101) and by partial hydrolysis of the components (gel 102), respectively, are compared in fig. 1. •
51
Ti02/Si02 - Gel 114
40.0
!
I
30.0 ~
/
~~ ao.o
•
4. Results
The observed spectra are composed of three lines which are assigned to the structural units Q2, Ti02/Si02
Gel 101
40.0
30.0
.-~ 10.0
0.0 0
10
20 contact
30 40 time / ms
50
Fig. 3. Dependence of 29Si C P / M A S signal intensities on contact time for gel 114. The alkoxides were completely hydrolyzed (both in acid) prior to mixing. The mixed wet gel was redispersed in base for four days, and then dried at 120°C. Notation and symbols are the same as in fig. 2.
Q3, and Q4 of the SiO 2 network. For gels 101, 114, and 115, the chemical shifts are typically - 9 2 . 3 ppm for Q2, - 1 0 1 . 5 ppm for Q3, and - 1 1 1 . 0 ppm for Q4. For gels 102 and 103A, the corresponding shifts are obtained as - 91.7 ppm, - 99.9 ppm, and - 1 0 8 . 1 ppm, respectively. Chemical shifts for gels 104B and 104E are - 9 2 . 9 ppm, - 1 0 1 . 2 ppm, and - 1 1 0 . 1 ppm, for the sites Q2, Q3, and Q4, respectively. Ti02/Si % - Gel 115
40.0
,d ao.o
30.0
•~ l o . o '
~ eo.o
/ 0.0
0
10
20 contact
30 40 time / ms
3' 50
Fig. 2. Dependence of 29Si C P / M A S signal intensities on contact time for gel 101• The gel consists of 20 mol% TiO 2 and 80 mol% SiO2; this composition is the same for all the gels investigated in this study• The alkoxides were completely hydrolyzed (TEOS in base, TIOT with acid) prior to mixing; the gel was dried at 120°C. The dependence of the integrated signal intensities, as obtained from deconvohition of the spectra, is plotted against the H a r t m a n n - H a h n contact time for Q2 sites (circles), Q3 sites (triangles), and Q4 sites (squares). The solid line represents a least squares fit according to eq. (1).
e~ '~ 10.0
o.o ft - - " 0
T T ~ T,-7-~ 10
20 contact
i
30 40 time / ms
,
i
50
Fig. 4. Dependence of 29Si C P / M A S signal intensities on contact time for gel 115. The alkoxides were completely hydrolyzed (both in acid) prior to mixing. The gel was dried at 600 ° C, then redispersed in base for four days, and finally dried at 120°C. Notation and symbols are the same as in fig. 2.
K.L. Walther et aL / TiO2/ SiO 2 mixed oxide catalysts prepared by sol-gel techniques
52
Ti02/Si02 - Gel 102
Ti02/Si02 - Gel 104 B
40.0 60.0 30.0
d 40.0
~o.o
20.0
.~ l o . o
,
0.0
0
10
~
20 contact
,
30 40 time / ms
50
Fig. 5. Dependence of 29Si C P / M A S signal intensities on contact time for gel 102. The preparation involved partial hydrolysis of TEOS in acid (2 molecules of water per alkoxide), followed by addition of TIOT, completion of hydrolysis up to a water/alkoxide molar ratio of four, redispersion in base, and a drying step at 120 o C. Notation and symbols ar the same as in fig. 2.
The gels prepared by complete hydrolysis were characterized by well-resolved lines. Representative linewidths were 360 Hz for the structural type Q2, 410 Hz for Q3, and 520 Hz for Q4. For the samples prepared by partial hydrolysis, significantly broader and unresolved lines were observed, which had to be decomposed into the Q2, Q3, and Q4 components by deconvolution [30].
¢
0,0
,
¢
~,
,
0
10
(} ,
20 contact
|
¢
¢
,
,
30 40 time / ms
¢ ,
50
Fig. 7. Dependence of 29Si C P / M A S signal intensities on contact time for gel 104B. The preparation was the same as used for gel 102 (fig. 5) except for redispersion; but the gel was dried at 600 o C. Notation and symbols are the same as in fig. 2.
Linewidths, obtained from the fit, are 430 Hz, 480 Hz, and 630 Hz, respectively. The contact time dependence of the transferred 29Si magnetization for the Q2, Q3, and Q4 sites in gel 101 is presented in fig. 2. The corresponding data for gels 114 and 115 are shown in fig. 3 and 4. Figures 5-8 represent the cross-polarization dynamics of gels 102, 103A, 104B, and 104E, respectively. The quantity TsiH (table 1) corre-
Ti02/Si02 - Gel 103 h
TiO2/Si02 - Gel 104 E
40.0 60.0 30.0
d 40.0
20.0 2e~ •-
20.0
10.0
0.0
,
0
10
~
20 contact
,
,
30 40 time / ms
,
50
Fig. 6. Dependence of 2 9 S i C P / / M A S signal intensities on contact time for gel 103A. The preparation involved the same steps as used for gel 102 (fig. 5), but the hydrolysis of TEOS was performed in base. Notation and symbols are the same as in fig. 2.
o.o
,
O
10
; ~ 20 contact
t
¢
30 40 time / ms
50
Fig. 8. Dependence of 29Si C P / M A S signal intensities on contact time for gel 104E. The preparation was the same as used for gel 102 (fig. 5), but the gel was dried at 600 o C, then redispersed in base for four days, and dried again at 600 o C. Notation and symbols are the same as in fig. 2.
K.L. Walther et al. / TiO2/ S i O 2 mixed oxide catalysts prepared by sol-gel techniques
sponds to the cross-relaxation time, Tis, of exponential magnetization transfer (eq. (1)). The parameters of the oscillatory magnetization transfer are also indicated in table 1, whereby THH corresponds to the spin diffusion time (T H in eq. (1)). The solid lines in figs. 2-8 represent leastsquares fits to the data based on eq. (1); the resulting parameters are summarized in the table. The fractional amplitude, M 0, of the three 28Si N M R signals (Q2, Q3, and Q4) obtained from MAS not employing cross-polarization (cf. table 1) will be compared with the corresponding contributions determined by CP/MAS, i.e. the relative amplitudes of the curves describing the time dependence of magnetization transfer obtained for each site from the C P / M A S fit.
5. Discussion
5.1. Gels prepared by mixing after complete hydrolysis of the components Gel 101 is characterized by comparatively narrow lines and a high percentage of Q4 units. This indicates a high degree of cross-linking, which is promoted by the basic hydrolysis medium. In the C P / M A S spectra, the relative amplitude of the Q3 sites (20.5%) is considerably larger than in the MAS-spectrum without cross polarization (12.2%). This enhancement points to the fact that for most of the Q3 centers, the single ligand different from -OSi is a hydroxyl group, rather than -OTi TM. The cross-polarization times, TsiH, are comparatively short, which is typical for all the gels prepared by complete hydrolysis prior to mixing; these time constants increase in the sequence from Q2 to Q4. Small values of TSi H indicate that protons are present in spatial vicinity of the silicon nuclei. By contrast, the spin diffusion time, THH, is comparatively long. Such a behavior could be expected if the sample consist of silica clusters with proton-covered surfaces and a comparatively weak interaction between the clusters. The observation of a large oscillatory fraction (1 - a) of the magnetization transfer, i.e. 0.5 for Q2 and Q3, and 0.8 for Qa, is consistent with this picture. The oscillation frequency, b, of the coherent magneti-
53
zation transfer is small, both as compared with pure silica gels [28] and with the other samples investigated in this study. No long-time decrease of the transferred magnetization was observed, i.e. T,~ was set to infinity. Gels 114 and 115 prepared by acid hydrolysis again exhibit comparatively narrow, well-resolved lines for the three types of sites. This fact points to the existence of small TiO 2 and SiO2 domains, which is typical for all samples prepared by complete hydrolysis of the respective alkoxides prior to mixing. The percentage of Q4 sites is not as high as for gel 101, which has been hydrolyzed under basic conditions. The cross-polarization times TsiH are on the same order of magnitude, the spin diffusion times THH somewhat smaller than for gel 101. Long-time decay of the magnetization is most striking for sample 114 (Tlo = 23 ms for Q2). For the Q2 sites, the magnetization transfer dynamics are predominantly exponential (a = 0.8-0.9), i.e. any fast coherent oscillations are not resolved. By contrast, for the Q4 lines the oscillations are clearly discernible (a < 0.2). The oscillation frequencies, b ~ 300 rad/s, are on the lower limit of the interval observed for pure silica gels [281.
5.2. Gelsprepared by mixing after partial hydrolysis of the components In gel 102, the percentage of Q2 and Q3 units is significantly enhanced, as compared to gel 101. This is due the fact that a Si-O-Ti TM bond gives rise to a similar chemical shift increment as S i - O - H [17,26,27], as mentioned above. As a consequence, the 0 2 line comprises contributions from environments of the three types (-SiO-)2Si(-OH)2, (-SiO-)2Si(-OH)(-OTi-), and (-SiO-)2Si(-OTi-) 2, and correspondingly for Q3. From the increased fraction of Q2 and Q3 sites in gel 102, we suggest that the sample contains SiO-Ti bridges, i.e. SiO2 and TiO 2 are - at least partly - mixed on a molecular scale. This mixing is confirmed by the considerably larger width of the Q2, Q3, and Q4 lines, i.e., for each type of site there is a chemical and structural diversity of environments.
54
K.L. Walther et al. / Ti02 / SiO 2 mixed oxide catalysts prepared by sol-gel techniques
The cross-relaxation times, TSiH, are similar to those of the gels discussed above. However, the spin diffusion time Trt H is very small for Q2 and Q3. This smallness appears to indicate that the diffusion of proton magnetization is unrestricted across the entire sample, by contrast with the cluster model discussed above for the gels prepared by complete hydrolysis of the respective alkoxides. (The large value of THH for Q4 sites is not reliable because, for this line, the exponential magnetization transfer is dominant, a = 0.96.) There is a pronounced long-time decrease of the silicon magnetization, as expressed by short relaxation times Tip in the rotating frame. Small values of Tip are usually interpreted [18] in terms of exchange processes between the abundant proton spins. The re-dispersion treatment in base that influences the concentration of surface OH groups could be responsible for this observation. From table 1 we see that a > 0.5, and that the relative weight of the exponential contribution increases from Q2 to Q4. The fact that the oscillatory fraction (1 - a) of the magnetization transfer is small suggests that there are few isolated protons; rather, each silicon nucleus appears to be interacting with a large proton spin reservoir. For this reason, the values of the oscillation frequencies derived for this sample are not significant. Sample 103A has parameters similar to those of gel 102. The following differences are noted. The site distribution is shifted towards higher-coordinated sites. This shift is a result of the different preparation procedure: in gel 103A, the TEOS precursor was partially hydrolyzed in base, which results in a higher degree of cross-linking, as reflected by the increased Q4 fraction. However, the Q2 percentage is still significantly higher than for gels 101, 114 and 115. This implies that also for sample 103A, the Q2 line contains contributions from (-SiO-)2Si(-OH)2_~(-OTi-)n (n = 1, 2) sites. As with gel 102, the cross-relaxation and spin diffusion times are short; the relaxation times, T~p, are comparable as well (except for Q4). Little oscillatory transfer is observed, as reflected by high values of a. Small but significant chemical shift differences are noted for the line positions, as deduced from
the least squares fit. In samples 102 and 103A, Q3 (~ = -99.9 ppm) is less shielded by = 1.5 ppm, as compared with gels 101,114, and 115 (~ = - 101.5 ppm). Similarly, the chemical shift for Q2 (~ = -91.7 ppm for gels 102, 103A) differs from the average of gels 101, 114, and 115 ( ~ = - 9 2 . 3 ppm). This again points to the fact that in the gels ~9repared by partial hydrolysis, the environment of Si on Qz and Q3 sites comprises Ti(IV) nearest neighbors. In comparison to sample 102, the gels 104B and 104E have been calcined at a higher temperature (600 o C). This treatment results in further crosslinking by condensation reactions of Si-OH groups, formation of siloxane bridges and release of water, as evident from the significantly increased percentage of Q4 sites and a small fraction of Q2 sites. The higher degree of cross-linking also leads to an increase in linewidths. As a further consequence of the loss of water, the cross-polarization time TsiH for Q4 is greater, by almost an order of magnitude, as compared with the previously discussed gels. Large values of THH are also consistent with this picture. Note that there is no large time decrease of the magnetization (Tip --+ 00). Oscillatory magnetization transfer is significant only for Q3 sites ( a = 0 . 3 ) ; the value of b is consistent with oscillatory magnetization exchange with a second nearest neighbor proton (cf. the discussion in ref. [28]). The overall similarity in the NMR parameters of gels 104B and 104E is not unexpected, as the preparation differs only by the additional ageing in base employed for gel 104E.
5.3. Comparison of the gels The NMR parameters will now be compared across the series of samples investigated. For the Q2 and Q3 sites, the fact that their relative contribution in CP/MAS experiments is always higher than in MAS reference experiments (without cross-polarization) indicates that at least a fraction of the nearest neighbors are protons in all the samples. Hydrolysis of TEOS under basic conditions leads to a larger degree of cross-linking and to the formation of highly branched cluster (higher Q4 percentage, as compared with the acid-hydro-
K.L. Walther et al. / Ti02 // Si02 mixed oxide catalysts prepared by sol-gel techniques
lyzed samples). Re-dispersion in base of wet gels tends to increase the Q2 percentage, as seen from a comparison of gels 103A and 102. If one compares TsiH values for the corresponding Qn sites, generally similar values are found with two striking exceptions: Very slow cross-relaxation is noted for the Q4 lines of gels 104 prepared by partial hydrolysis and high-temperature drying. The spin diffusion time THH, a parameter of the oscillatory magnetization transfer component, should be discussed only in those cases where the latter contribution is significant (a _< 0.5). THH is large in sample 101 where TEOS was hydrolyzed in base (compact clusters), and small in samples 114, 115 where the hydrolysis was performed in an acid medium. Next, this parameter will be compared for the four gels prepared by partial hydrolysis of the precursors; we focus on the Q3 sites where the oscillatory contribution is significant. Spin diffusion is very fast (8-9 ms) for gels 102 and 103A, and somewhat slower (24-30 ms) for the high temperature dried gels 104B and 104E. Small Tip values are often associated [18] with exchange processes within the proton spin reservoir. In this respect, the Q2 and Q3 sites of gels 102, 103A, and 114, are distinct from the other samples investigated. The origins of these observations, i.e. the influence of drying and redispersion treatments, will be the subject of further investigations. As stated earlier [28], the frequency calculated for coherent magnetization transfer between a 295i nucleus and a directly bound OH proton is too high (--- 800 rad/s) to be resolved with the mixing time increments used in our experiments. Further, in a powder sample the oscillations are averaged over a multitude of geometries and orientations. Frequencies, b, around 400 r a d / s are typical for OH protons attached to nearest neighbor (and further removed) protons [28]. The fact that sample 101 exhibits particularly low frequencies might be associated with the fact that the complete hydrolysis of TEOS in base results in very compact silica clusters. Hydroxyl groups, present mainly on the surface of these clusters, would then be remote from the silicon nuclei in the interior. On the other hand, gel 102 appears to be a prototype of a
55
molecularly mixed network with comparatively little cross-linking. Here, for any 29Si center the distance to the nearest hydroxyl proton is smaller. The quantity (1 - a) is a measure of the oscillatory fraction of the magnetization transfer. For the gels prepared by complete hydrolysis, this contribution is larger than previously found for the pure silica gels. In the partially hydrolyzed gels 104B and 104E, only the Q3 site exhibits significant oscillations. With gels 102 and 103A, the cross-polarization dynamics are predominantly exponential in nature. The lack of an oscillatory component with these samples might also be associated with the existence of S i - O - T i bridges. Chemical shift values obtained from the spectral deconvolution provide valuable information on the presence or absence of domains consisting of the individual components. For a given type of site, a more negative chemical shift value indicates a lower number of S i - O - T i bridges [23,26]. First we compare the group of mixed oxides prepared by complete hydrolysis (gels 101, 114, and 115). According to the mentioned criterion, domain formation is most pronounced in sample 101, as judged, for example, from the chemical shifts of Q3 sites: ~ = -101.7 ppm for gel 101, as compared with -101.4 ppm for gels 114 and 115. The conclusion that S i - O - T i bridges are largely absent in sample 101 is supported by the observation that the percentage of Q4 sites (87%) is significantly higher than in gels 114 and 115 (70 and 76%, respectively). Further, the linewidth of the N M R signals is narrowest for gel 101, which is explained by the same fact. The gels prepared by partial hydrolysis show a complementary behavior, as their N M R parameters indicate that TiO 2 and SiO 2 are at least partly mixed on a molecular scale. S i - O - T i bridges are most abundant when the partial hydrolysis of TEOS is performed in acid. This conclusion is reached from a comparison of several N M R parameters of gels 102 and 103A. First, the silicon nuclei are less shielded in gel 102, as a consequence of the presence of Ti TM neighbors for Q2 and Q3 sites ( ~ j ( O 2 ) = - 9 1 . 2 and -92.1 ppm, ~(Q3) = _ 99.6 and -100.1 ppm, for gels 102 and 103A, respectively). Second, the linewidths are consistently larger in gel 102 (FWHM(Q 2) = 500
56
K.L Walther et al. / TiO: / SiO2 mixed oxide catalysts prepared by sol-gel techniques
and 414 Hz, F W H M ( Q 3) = 489 and 453 Hz, for gels 102 and 103A, respectively), which means that there is a higher chemical and structural diversity of environments in gel 102. Third, the percentage of Q2 sites, which includes the species featuring one or two Ti TM nearest neighbors, is largest among all samples (15%) in gel 102. The redispersion treatment in base is only effective when applied to a wet gel, i.e. before drying. Due to the desired ripening process, any micropores present after preparation are converted to mesopores, as has been shown [31] for gels 102 and 114 where the micropore volume after redispersion is zero within the accuracy of the measurement. For these two gels, the Q2/Q3 ratio (both with and without cross-polarization) is largest among all samples investigated. This observation may indicate the presence of Si(OH)2 groups on the walls of the ripened mesopores.
6. Conclusions The results presented above demonstrate that pronounced structural differences exist between the mixed oxides prepared by partial and complete hydrolysis of the precursors prior to mixing. Domain formation is quite extensive when the respective alkoxides are completely hydrolyzed in excess of water before mixing. Within the series of mixed oxides prepared by complete hydrolysis of the components, the hydrolysis p H of TEOS is the decisive factor that determines the extent of domain formation. The most pronounced development of silica domains is observed in samples produced by hydrolysis of TEOS in base. (The titania precursor is always hydrolyzed in acid.) The cross-polarization parameters indicate that compact silica clusters are thereby formed. For m a n y 29Si nuclei within such a cluster, the distance to the nearest surface proton is small. Molecularly mixed solids are approached only when the T I O T component is added after partial hydrolysis (50% on average) of TEOS. Spectral and dynamic N M R parameters demonstrate that the molecular mixing of the components is most pro-
nounced when the partial hydrolysis of TEOS is performed in acid, which results in a reduced tendency of aggregation and cross-linking. Gels prepared by partial hydrolysis in acid are, however, not very resistant to high temperature drying. U p o n treatment at 600 o C, the pore volume shows a drastic decrease, and the dried gel exhibits only the catalytically less useful micropores [31]. Subsequent redispersion of these oxides is not capable of reversing the pore collapse [31]. In work in progress in our laboratory, further information on the structural properties of mixed oxide gels prepared by partial and complete hydrolysis of the precursors is being derived from F T I R and R a m a n investigations.
Sincere thanks are due to A. Sebald and L. Merwin for performing the C P / M A S experiments. Financial support by the Deutsche Forschungsgemeinschaft (SFB 213) and by the Swiss Federal Institute of Technology (ETH) is gratefully acknowledged.
References [1] A.B. Stiles, Catalyst Supports and Supported Catalysts (Butterworths, Boston, MA, 1987). [2] A. Baiker and A. Wokaun, Z. Naturwiss. 76 (1989) 168. [3] D. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [4] P.A. Haas, Chem. Eng. Prog. 85 (1989) 44. [5] B.E. Yoldas, J. Mater. Sci. 14 (1979) 1843. [6] A. Baiker, A. Dollenmeier, M. Glinski and A. Relier, Appl. Catal. 35 (1987) 365. [7] A. Balker, A. Dollenmeier, M. Glinski and A. Relier, Appl. Catal. 35 (1987) 351; M. Schraml, W. Fluhr, A. Wokaun, and A. Baiker, Ber. Bunsenges. Phys. Chem. 93 (1989) 852. [8] F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijn, J. Medema, V.H.J. de Beer and P.J. Gellings, J. Phys. Chem. 84 (1980) 2783. [9] M. Tagaki, M. Soma, T. Onishi and K. Tamaru, Can. J. Chem. 58 (1980) 2132. [10] K.L. Walther, M. Schraml-Marth, A. Wokaun and A. Baiker, Catal. Lett. 4 (1990) 327. [11] C.J. Brinker, J. Non-Cryst. Solids 100 (1988) 31. [12] M. Guglielmi and G. Carturan, J. Non-Cryst. Solids 100 (1988) 16. [13] C.J. Brinker and G.W. Scherer, in: Ultrastructure Processing of Ceramics, Glasses, and Composites, eds.
K.L. Walther et al. / TiO 2/ S i O 2 mixed oxide catalysts prepared by sol-gel techniques
[14]
[15]
[16] [17] [18]
[19] [20]
[21]
L.L. Hench and D.R. Ulrich (Wiley, New York, 1984) ch. 5. B.E. Handy, K.L. Walther, A. Wokaun, and A. Baiker, in: Preparation of Catalysts V, eds. G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon (Elsevier, Amsterdam, 1990) p. 239. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. G.E. Maciel and D.W. Sindorf, J. Am. Chem. Soc. 102 (1980) 7606. D.W. Sindorf and G.E. Maciel, J. Am. Chem. Soc. 103 (1981) 4263; D.W. Sindorf and G.E. Maciel, J. Phys. Chem. 86 (1982) 5208; D.W. Sindorf and G.E. Maciel, J. Am. Chem. Soc. 105 (1983) 1487. R.K. Harris and R.H. Newman, J. Chem. Soc., Faraday Trans. II 73 (1977) 1204. E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A.R. Grimmer, J. Am. Chem. Soc. 102 (1980) 4889; E. Lippmaa, M, Magi, A. Samoson, M. Tarmak and G. Engelhardt, J. Am. Chem. Soc. 103 (1981) 4992. G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak and M. Magi, Z. Anorg. Allg. Chem. 482 (1981) 49.
57
[22] A.R. Grimmer and R. Radeglia, Chem. Phys. Lett. 106 (1984) 262. [23] G. Engeihardt, High Resolution Solid State NMR of Silicates and Zeolites (Wiley, New York, 1987). [24] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. G~Sktas and G.H, Frischat, Ber. Bunsenges. Phys. Chem. 92 (1988) 689. [25] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. GOktas and G.H. Frischat, J. Non-Cryst. Solids 116 (1990) 161. [26] Ch. Wies, K. Meise-Gresch, W. Miiller-Warmuth, W. Beier, A.A. GiSktas and G.H. Frischat, Phys. Chem. Glasses 31 (1990) 138. [27] G.A. Fyfe, G.G. Gobbi and G.J. Kennedy, J. Phys. Chem. 89 (1985) 277. [28] K.L. Walther, A. Wokaun and A. Baiker, Mol. Phys. 71 (1990) 769. [29] L. Mialler, A. Kumar, Th. Baumann and R.R. Ernst, Phys. Rev. Lett. 32 (1974) 1402. [30] Deconvolution of the spectra was performed using the GLINFIT software by Briiker Inc., Karlsrnhe. [31] B.E. Handy, A. Baiker, K.L. Walther and A. Wokaun, in: Synthesis and Properties of New Catalysts, eds. E.W. Corcoran and M.J. Ledoux (Materials Research Society, Pittsburgh, PA, 1990) p. 107; B.E. Handy, A. Baiker and A. Wokaun. submitted to J. Mater. Sci.