Langmuir 1996, 12, 3173-3179
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Titania Coatings on Monodisperse Silica Spheres (Characterization Using 2-Propanol Dehydration and TEM) A. Hanprasopwattana,† S. Srinivasan,‡,§ A. G. Sault,| and A. K. Datye*,‡ Center for Microengineered Ceramics and Departments of Chemistry and Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131, and Sandia National Laboratories, Albuquerque, New Mexico 87185-0703 Received September 28, 1995. In Final Form: April 4, 1996X Titania coatings ranging from sub-monolayer to ≈7 nm thick have been deposited on silica by hydrolysis of titanium alkoxide precursors. Nonporous model silica particles of spherical shape were used as the support for titania. The ratio of titanium alkoxide to water and the dilution of the reactant mixture in ethanol control the nature of the coating. If the alkoxide concentration is too high, precipitation of secondphase titania particles occurs rather than a uniform coating. On the other hand, higher water concentrations led to aggregated spheres being connected with titania necks. Isolated silica spheres with uniform coatings of titania are obtained when the alkoxide concentration is kept low. The samples were studied using transmission and scanning electron microscopy. 2-Propanol dehydration was used as a probe reaction, since it was found that reactivity for propene formation is directly correlated with anatase surface area. Using this correlation, the effective titania surface area of each sample could be derived. However, it was found that the correlation does not work for titania/silica samples heated to temperatures less than 673 K, since the titania remains amorphous. Hence a 773 K calcination was used to convert the titania to anatase before deriving effective titania surface areas. Agreement between the BET surface area and the effective titania surface area implies that complete coverage of the silica has been achieved in our study.
Introduction Titania is of interest as both a catalyst and a catalyst support.1,2 However, titania when present as a high surface area powder is not thermally stable and loses surface area readily. Therefore, effort has been devoted in recent years to coating titanium oxides on high surface area supports such as silica or alumina.3 The addition of thin film coatings of oxides is also of interest in the synthesis of ceramic materials for structural applications as used by Sacks4 et al. In that particular study, silica was deposited on alumina to achieve the correct stoichiometry for formation of mullite 3Al2O3‚2SiO2. The alumina coated with silica could be transformed into dense mullite at lower temperatures than for the conventional route, which involved solid state reaction between alumina and silica powders. The addition of a second oxide phase such as titania is also of interest as a means to alter the surface chemistry of the oxide substrate. In previous work, we found that deposition of titania improved the coating of boron nitride thin films on the silica.5 The ‘saturation uptake’ was defined in our previous work as the amount of titania that could be deposited by contacting a silica surface with an alkoxide precursor and * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Center for Microengineered Ceramics and Department of Chemistry, University of New Mexico. ‡ Department of Chemical and Nuclear Engineering, University of New Mexico. § Present address: Novellus System, 3970 North First Street, San Jose, CA 95134. | Sandia National Laboratories. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Stafford, U.; Gray, K. A.; Kamat, P. V. J. Phys. Chem. 1994, 98, 6343. (2) Ciambelli, P.; Bagnasco, G.; Lisi, L. Appl. Catal., B 1992, 1, 61. (3) Castillo, R.; Koch, B.; Ruiz, P.; Delmon, B. J. Mater. Chem. 1994, 4, 903. (4) Sacks, M. D.; Bozkjhurt, N.; Scheiffele, G. J. Am. Chem. Soc. 1991, 74, 2428. (5) Borek, T. T.; Qui, X.; Rafuse, L.; Datye, A. K.; Paine, R. T. J. Am. Ceram. Soc. 1991, 74, 2587.
S0743-7463(95)00808-0 CCC: $12.00
washing off the excess in an inert atmosphere.6 These experiments were done without any molecular water being present, and hence the uptake was limited by the concentration of hydroxyls on the silica surface. When the titania loading is less than the saturation uptake, the titania is present in the form of isolated units bound to the oxide support. Therefore, it cannot be readily detected by techniques such as transmission electron microscopy (TEM) or X-ray diffraction, which are better suited to detect crystalline TiO2. However, this titania is chemically different from bulk titania, as seen, for example, in its selectivity for methanol oxidation.6 If a model oxide support consisting of nonporous oxide particles is used, even the dispersed titania can be detected by TEM. This is because the well-defined morphology and uniform contrast of the support make it easier to image the deposited titania. The same advantage (in terms of ease of imaging the titania) is obtained if one is interested in depositing multilayer films of titania. In this paper we have extended our previous work on monolayer titania films to multilayer films of titania on silica microspheres (Sto¨ber spheres). The primary objective here is to develop a method that ensures the titania is uniformly distributed over the support. Such thin film oxide coatings on substrates of differing geometries, such as cubes, spheres, and spindle-shaped particles have been exhaustively studied by Matijevic7-10 and co-workers. In this study, we have followed the fate of these coatings as they are heated to elevated temperatures. Since the deposition of these coatings is of interest as a means to stabilize the titania surface area at higher temperatures, we have used an independent probe, alcohol dehydration reactivity, to measure the exposed surface area of the titania. The extension of this work to coat high surface (6) Srinivasan, S.; Datye, A. K.; Hampden-Smith, M.; Wachs, I. E.; Deo, G.; Jehng, J. M.; Turek, A. M.; Peden, C. H. F. J. Catal. 1991, 131, 260. (7) Gherardi, P.; Matijevic, E. J. Colloid Interface Sci. 1986, 109, 57. (8) Ocana, M.; Hsu, W. P.; Matijevic, E. Langmuir 1991, 7, 2911. (9) Aiken, B.; Matijevic, E. J. Colloid Interface Sci. 1988, 126, 645. (10) Hsu, W. P.; Yu, R.; Matijevic, E. J. Colloid Interface Sci. 1993, 156, 56.
© 1996 American Chemical Society
3174 Langmuir, Vol. 12, No. 13, 1996
area silica substrates is presently underway and will be reported elsewhere.11 Several approaches have been used to deposit titania including grafting, impregnation, precipitation, vapor infiltration, and sol-gel synthesis. Hsu et al.10 worked with monodisperse silica spheres similar to those used in our work and a TiOSO4 precursor. The uniformity of coating was affected by control of the initial and the final pH, the concentration of the silica powder, and the rate of addition of the titanyl sulfate precursor. The coatings were quite rough, with the titania weight loadings ranging from 7 to 30 wt %. To obtain weight loadings greater than 20 wt %, they resorted to a two-step procedure whereby a portion of the titanyl sulfate precursor was allowed to react in one step with the silica, filtered, and resuspended for the second step. The resulting powders have surface properties (electrokinetic mobilities) that approach those of bulk titania sols but remain consistently lower than that of the titania at pH values less than the isoelectric point. Since these results leave some doubt about the coverage of titania, in the present study we show how the use of alcohol dehydration activity can serve as an independent measure of the surface area of the titania. Furthermore, the final pH of the suspension in the study of Hsu et al.10 was 0.7-1.5, a value that may not be suitable for some applications, so in our work we have used titanium alkoxide as a precursor. Most other studies in the literature have used higher surface area silica gel or fumed silica as the substrate. For example, Reichman and Bell12 used incipient wetness impregnation of silica by acidified solutions of TiCl4. They concluded that oligomers of titanium were formed in solution under highly acidic conditions, and these oligomers were deposited on the silica surface. The oligomer transformed into TiO2 when calcined at 823 K in air without forming any intermediates. X-ray diffraction (XRD) of the calcined sample showed peaks corresponding to the anatase phase when the loading was 7 wt % TiO2. No XRD peaks were seen at lower loadings, implying that the titania particles may have been smaller than 4 nm. The coverage of the silica by the titania was not measured in their study. X-ray photoelectron spectroscopy (XPS) represents one approach to measure the dispersion of titania on the support. For example, Fernandez et al.13 used XPS to characterize a 7 wt % TiO2/SiO2 prepared by impregnation of titanium isopropoxide. XPS showed that the titania was well dispersed, and the small particle size of the titania particles was further confirmed by diffuse reflectance UV-vis spectroscopy. When depositing titania on a support, there is always some question about whether the titania deposits preferentially at the surface of the porous particles. This is particularly important for porous samples where titania may deposit near the pore mouth and even plug the pores without effectively covering the oxide support. The distribution of titania can be studied best by SEM or microprobe analysis of a cross section of the support particles. Haukka et al.14 used SEM and energy dispersive X-ray spectroscopy (EDS) to examine the distribution of the TiO2 particles within the high surface area particles. The titania was deposited by vapor phase infiltration of TiCl4, which reacts with surface hydroxyls and deposits on the surface. They found that the silica pores were not blocked by the deposited TiO2 and the largest particles (11) Hannaprasowattana, A.; Sault, A. G.; Datye, A. K. In preparation. (12) Reichmann, M. G.; Bell, A. T. Langmuir 1987, 3, 111. (13) Fernandez, A.; Leyrer, J.; Gonzalez-Elipe, A. R.; Munuera, G.; Kno¨zinger, H. J. Catal. 1988, 112, 489. (14) Haukka, S.; Lakomaa, E.-L.; Jylha¨, O.; Vilhunen, J.; Hornytzkyz, S. Langmuir 1993, 9, 3497.
Hanprasopwattana et al.
seemed to form in the larger pores. At temperatures below 573 K they found amorphous titania, but with increasing temperature, the titania formed crystalline anatase or rutile particles. At the 773 K reaction temperature, large rutile particles were found, suggesting that the partially hydroxylated titanium precursor is quite mobile. The distribution of the titania within the silica particles was also studied by Castillo et al.,3 who used electron microprobe analysis of cross-sectioned samples. They concluded that precipitation and impregnation both gave higher titania concentrations at the surface of the oxide support particles. Only grafting (reaction of the surface hydroxyls with the precursor) gave a uniform concentration of titania. After calcination at 773 K, even the lowest loaded sample (≈9 wt %) exhibited faint peaks in the XRD spectrum, suggesting that small particles of crystalline titania were present. These authors also used the isoelectric point of their sample as a measure of the apparent surface coverage (ASC) of silica by titania. Their results showed that even the sample that had the most uniform coating of titania (obtained by grafting) still had an apparent surface coverage of only 80%. This review of previous literature on titania coatings shows that none of these studies provide a direct measure of the coverage of silica by the titania or the specific titania surface area, an aspect that is important if the titania is used as a catalyst or catalyst support. Therefore, in this paper, we have used 2-propanol dehydration as a reactive probe for measuring the titania surface area. Since the reactivity of titania is almost two orders of magnitude greater than that of the silica support, this method provides a sensitive measure of the coverage of silica by titania. We have previously shown15 that the propene evolved during 2-propanol dehydration carried out during temperature-programmed reaction serves as a good probe of titania surface area in titania/silica samples. When working with porous materials as supports, it is not always easy to determine the uniformity of the titania coating and its thickness. Therefore, in order to better understand the parameters that control coating thickness and morphology, we have used nonporous silica microspheres (Sto¨ber spheres) as support. Our results show that we are able to obtain sub-monolayer to multilayer coatings of titania on the silica microspheres. These coatings conform to the curvature of the silica spheres and are stable at temperatures up to 773 K, transforming from amorphous titania into crystalline anatase at these temperatures. Experimental Section Sto¨ber silica spheres16 (0.3 g), 270 nm in diameter, were sonicated in 100 mL of ethanol (Spectrophotometric grade, Aldrich) for 15 min. A stock solution containing 4 mL of titanium n-butoxide (TBOT) and 100 mL of ethanol was used for the coating. The total amount of ethanol was maintained at 250 mL while the amount of water and TBOT was varied as shown in Table 1. The final mixture was refluxed for 1.5 h and then vacuum filtered. The residue was washed with 20 mL of ethanol three times and then dried under vacuum. In one experiment, titanium isopropoxide was used instead of TBOT. In this case, the solution of 4.5 mL of water and 200 mL of ethanol was added to a solution of 2.4 mL of titanium isopropoxide and 250 mL of ethanol. The mixture was added to 0.9 g of Sto¨ber silica spheres which had been previously sonicated in 300 mL of ethanol for 15 min. The final mixture was refluxed for 1.5 h and then filtered and washed as described above. (15) Biaglow, A. I.; Gorte, R. J.; Srinivasan, S.; Datye, A. K. Catal. Lett. 1992, 13, 313. (16) Sto¨ber, W.; Fink, A.; Bohn, E. J. J. Colloid Interface Sci. 1968, 26, 62.
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Table 1. Sample Morphology and Titania Loading of the Samples Investigated sample code 1 2
alkoxidesa (104 mol) 2.92 8.76
water (mol)
effective TiO2b (m2/g)
0.06 0.09
3.3 (400) 14 (RT) 12 (400) 8 (500) 161 (RT) 35 (400) 14 (500) 73 (400) 68 (500) 585 (RT) 150 (400) 68 (500) 407 (RT) 44 (400) 23 (500) 117 (400) 60 (500)
3
23.0
0.08
4
29.0
0.08
5
27.1 (P)
0.08
6
23.0
0.12
7
29.0
0.14
BETb,c (m2/g)
wt % TiO2 by EDS
Ti/nm2 from EDS
0.5% 1.1% 56 (RT) 31 (400) 20 (500)
36.9%
3.0 6.6 222
20.2% 178 (RT) 69 (500) 73 (RT) 37 (400) 32 (500)
sample morphology isolated spheres isolated spheres isolated spheres necked spheres second-phase TiO2 necked spheres second-phase TiO2
40.6% 40.7%
necked spheres second-phase TiO2
42.3%
necked spheres second-phase TiO2
50 (500)
a All samples were prepared using titanium butoxide except for sample (P), which was prepared from titanium isopropoxide. The amounts of alkoxide and water are based on 250 mL of ethanol solvent. b The pretreatment temperature is in parentheses. RT implies no pretreatment; the as-prepared sample was used directly for alcohol dehydration at 523 K. c The BET surface area of the uncoated spheres is 12 m2/g.
The effective titania surface areas of the coatings on silica spheres were determined by measuring 2-propanol dehydration activity. To determine the relationship between titania BET surface area and activity, we prepared a series of titania samples with differing surface area. These samples were prepared by hydrolysis of titanium isopropoxide using the method suggested by Nishiwaki et al.17 While we were unable to obtain the wide variation in surface area obtained by this group, the samples we obtained covered a large enough range to serve as a useful calibration series. The calibration curve includes samples prepared by hydrolysis of titanium isopropoxide and a high surface area commercially available sample (Tioxide) containing sulfate impurities which were removed by washing in hot water repeatedly before use. For the coated spheres, the pretreatment temperature was varied in order to find the conditions that would yield the best agreement between BET surface area and effective titania surface area for samples that we expect to have 100% surface coverage of silica by titania. Therefore the alcohol dehydration activity of the samples was measured without pretreatment and then after treatment in flowing helium at 673 or 773 K overnight. Alcohol was introduced into a 20.2 sccm stream of flowing helium using a saturator at room temperature. Dehydration reactivity was measured between 448 and 648 K by using a gas chromatograph equipped with a DB-wax capillary column. The nature of coating was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). In addition, the amount of titania coated on silica spheres was analyzed by energy dispersive X-ray spectroscopy (EDS), atomic absorption spectroscopy (AA) and X-ray fluorescence (XRF). The sample solution for titania determination by AA was prepared by two methods. In the first method, about 0.05 g of each sample was fused with 5 g of NaOH in a nickel crucible for 30 min. Concentrated HCl (20 mL) was added to the crucible to dissolve the fused mixture, and the mixture was filtered and diluted to 100 mL in a volumetric flask. The second method used a mixture of 5 mL of HF and 5 mL of HNO3 to dissolve about 0.05 g of each sample in a Teflon beaker. The Teflon beaker was heated at 353 K for 2 h. The solution was then transferred to a 100 mL volumetric flask containing 5 mL of KNO3 and was diluted to 100 mL with deionized water. For XRF, 1 g of the sample was fused with lithium tetraborate around 1273 K for 15 min. The melt mixture was poured into a mold and analyzed by XRF. Table 2 shows the elemental analysis of two of the multilayer samples and sample 8, which is a titania coating on a silica gel. Since EDS typically involves the analysis of a few silica spheres, one does not obtain a good average, since only a few spheres get analyzed. Moreover, there is decided bias in favor of spheres that have a well defined coating of titania. Since (17) Nishiwaki, K.; Kakuta, N.; Ueno, A.; Nakabayashi, H. J. Catal. 1989, 118, 498.
Table 2 name
EDS
XRF
AA (NaOH + HCl)
3 6 8
36.9 40.7 16.4
24.5 31.9 17.1
23.8 23.7 16.2
AA (HF + HNO3)
Ti/nm2 from XRF
20.2 29.9 14.8
147.5 8
not all spheres are coated to the same degree, the XRF and AA provide more reliable elemental analyses and show that the average titania loading is actually lower than that of the few spheres analyzed by EDS. However, the EDS analysis as reported in Table 1 provides a measure of the titania loading on the silica spheres whose images are reported in this paper. When a sample is truly uniform, such as the silica gel sample 8 (from ref 11) that is reported in Table 2, the EDS results happen to agree well with those of AA and XRF. Also reported in Tables 1 and 2 is the loading of TiO2 in terms of Ti/nm2 based on the surface area of the uncoated spheres. This surface loading is not reported in cases where second-phase particles of titania are seen, since the number of Ti/nm2 has no physical significance in this case.
Results Alcohol Dehydration as a Test for Titania Specific Surface Area. In a previous study6 where we coated silica spheres with a ‘saturation uptake’ of titania, we found that the activity for propene formation was consistent with that of a titania powder having the same BET surface area as the uncoated silica spheres. However, the effect of pretreatment temperature had not been studied. Therefore, in order to establish the validity of this approach over a range of experimental conditions, we prepared a series of titania samples of differing surface area and measured the activity after pretreatment at 673 and 773 K for 8 h. The samples marked “no pretreatment” imply heating to 523 K in flowing helium (the reaction temperature) followed by activity measurement at that temperature. As shown in Figure 1, the activity for propene formation scales linearly with the BET surface area of the titania over the range of surface areas encountered in this study, independent of treatment temperature. All of the titania samples contain anatase as the primary phase. One commercial sample, Tioxide, is also included in this plot. The Tioxide sample contains sulfate impurities which tend to increase the alcohol dehydration activity; however, washing with hot water appears to get rid of the sulfate and bring the activity in line with those of the other samples. In Figure 1, we have
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Figure 1. Specific activity (moles of propene formed/(g s)) on titania samples as a function of surface area. There is no effect of pretreatment temperature.
Hanprasopwattana et al.
Figure 3. Transmission electron micrograph of sample 2.
Figure 4. Specific activity (moles of propene formed/(g s)) on sample 2 reported as an Arrhenius plot.
Figure 2. Scanning electron micrograph of TiO2 on SiO2 (sample 2).
excluded samples that contain mixtures of anatase and rutile phases, since the specific activities of these phases may differ. Sub-monolayer Coatings of Titania on Silica. We will first describe titania films whose thicknesses are below that of a monolayer. A monolayer loading based on the density of the (101) plane of anatase would be 11.3 Ti/ nm2. In these samples, the presence of titania is evident at most as a surface texturing of the silica surface. Figure 2 shows a scanning electron micrograph of such a silica sample (sample 2). The silica spheres look smooth and are unagglomerated. The silica surface of sample 2 is shown at a higher magnification in the TEM micrograph in Figure 3, after use in alcohol dehydration. The only noticeable feature is a slight texturing of the silica surface, consistent with the low loading of the titania. Figure 4 compares the alcohol dehydration activity of sample 2 and uncoated silica spheres. As seen from this figure, the activity of titania-coated silica is over an order of magnitude greater than that of the silica spheres. Using the calibration curve in Figure 1, we can deduce the effective titania surface area of the coated spheres. Table 1 shows that the effective titania surface area is comparable to the BET surface area of the uncoated spheres. Multilayer Coatings of TiO2/Silica. The amount of titania deposited on the silica was determined by both the amount of precursor alkoxide used and the amount of water added before refluxing. Multilayer coatings were obtained by increasing the amount of alkoxide precursor
Figure 5. SEM micrograph of multilayer TiO2 on SiO2 (sample 3).
while keeping the amount of water constant. As shown in the SEM image in Figure 5, sample 3 had isolated spheres that contained about 36.9 wt % TiO2, as determined by EDS spectroscopy in the electron microscope. Since only a few spheres are analyzed at one time, this method does not provide a good average, as explained in the Experimental Section. The titania loading as obtained from averaging the results of the XRF and AA suggests that the weight loading is 22.1 wt %. Despite the large amount of TiO2, there was no significant necking between the spheres, and the titania has deposited as a conformal coating and not as second-phase particles. This is clearly seen in the TEM images in Figure 6. The low-magnifica-
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Langmuir, Vol. 12, No. 13, 1996 3177
Figure 7. TEM micrograph of multilayer TiO2 on SiO2 (sample 3), after 773 K pretreatment.
Figure 6. (a) Low-magnification TEM micrograph of multilayer TiO2 on SiO2 (sample 3, as prepared). (b) Higher magnification TEM micrograph of multilayer TiO2 on SiO2 (sample 3, as prepared).
tion view shows that the coating is uniform, and the higher magnification view in Figure 6b shows that the coating is amorphous, even after reaction, and ranges in thickness from 12 to 15 nm. The EDS analysis of spheres such as the one shown in Figure 6b yielded a loading of 36.9 wt % TiO2 or 222.1 Ti/nm2 using the BET surface area of the uncoated spheres. On the basis of the density of anatase titania (11.3 Ti/nm2) and a 3.55 Å interlayer spacing, this loading should corespond to a titania film about 7 nm. When this sample was heated to 673 K and used for alcohol dehydration, the titania coating crystallized to form the anatase phase. The TEM image in Figure 7 shows the sample after the 773 K treatment, and the contrast suggests the presence of randomly oriented crystallites within the film. The change in density upon crystallization may also cause some coalescence of the titania film, and it may not completely cover the silica spheres. The surface coating therefore starts to look more patchy, but the film thickness is consistent with the prediction based on titania loading above. The alcohol dehydration activity of this multilayer sample is shown in Figure 8. The activity of the amorphous titania film is significantly greater than that after the higher temperature pretreatments. If one uses the calibration curve in Figure 1, an effective surface area
Figure 8. Effect of heat pretreatment on specific activity (moles of propene formed/(g s)) of multilayer titania on SiO2 (sample 3) in an Arrhenius plot.
of 161 m2/g can be derived. This is considerably greater than the N2 adsorption (BET) surface area of 56 m2/g. However, as the titania becomes crystalline, there is better agreement between the BET surface area and the effective surface area derived from alcohol dehydration activity. Experimental Conditions that Favor Aggregation of the Spheres. In order to increase the loading of the titania, we studied the effect of increasing the amount of precursor as well as the amount of water during refluxing. However, in every instance this procedure led to necking of the spheres and formation of agglomerates. There was also a significant amount of second-phase titania precipitate in addition to the coating. This led to larger BET surface areas as well as effective titania surface area, as seen in Table 1. Furthermore, the coating must be porous as deposited, leading to higher BET surface areas in the as-deposited state, with a falloff in surface area as the coating becomes more crystalline. Figure 9 shows an SEM image of sample 7 where the spheres appear to be fused together. The low magnification TEM view of sample 5 in Figure 10a clearly shows the necks between the silica spheres. The titania coating is amorphous as prepared. When the sample is heated to 673 K in air, the titania crystallizes to form the anatase phase. Within the film, the anatase crystallites are randomly oriented, as seen from the lattice fringes in Figure10b. Despite the crystalline nature of the film, the film conforms to the surface of the silica spheres, as seen in Figure 10a.
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Hanprasopwattana et al.
Discussion
to propene was used as a test reaction to measure the specific surface area of the titania in these titania/silica samples. To establish the validity of this probe reaction for measuring surface area, we prepared a series of titania samples ranging in surface area from 40 to 230 m2/g and measured their alcohol dehydration activity. The propene formation activity varied linearly with titania BET surface area, and activity was found to be independent of pretreatment temperature, up to 773 K. On the basis of these results, we obtained a specific titania reactivity of 3.24 × 10-8 moles of propene formed/(s m2) at 523 K. Since the correlation in Figure 1 was obtained with anatase titania, in principle it cannot be used to deduce effective titania surface areas for samples whose loading is below the limit at which crystalline titania starts to form. In previous work,6 we found no crystalline titania could be detected (by Raman spectroscopy) on fumed silica at loadings up to 1 Ti/nm2 while on silica spheres the loading could be as high as 7 Ti/nm2 without any crystalline TiO2 being detected. The ‘saturation uptake’ of titania was dependent on the types of hydroxyls present on silica; Sto¨ber spheres are extensively hydroxylated and hence show higher saturation uptakes than fumed silica.18 The loading of 1.1 wt % by EDS on sample 2, assuming the BET surface area was similar to that of the uncoated spheres, would correspond to about 6.6 Ti/nm2. A monolayer loading based on the density of the (101) plane of anatase would be about 11.3 Ti/nm2. While the titania loading on sample 2 is clearly less than a monolayer, the reactivity is comparable to that of anatase titania (in terms of moles of alcohol reacted per square meter). We have seen a similar agreement (in alcohol reactivity/m2) in our previous work between anatase titania and coatings that correspond to the saturation uptake of titania on silica spheres6 and silica gels.11 It appears that, despite the differences in structure and density, anatase titania and the thin films of titania with a loading equal to the ‘saturation uptake’ have similar activity for alcohol dehydration (per m2). TiO2-SiO2 mixed oxides have been reported to produce stronger acid sites than titania or silica single-phase oxides. Reactivity for 1-butene isomerization and 2-propanol TPD was also reported to be enhanced on the mixed oxides.19,20 We suspect that the higher reactivity of this dispersed titania in silica may be responsible for our observation that the reactivity per square meter of a sub-monolayer of TiO2 is comparable to that of bulk anatase having a much higher Ti areal density. On the basis of the titania loadings reported in Table 1, we can classify the first two samples as representing sub-monolayer coatings of titania on silica. The sample morphology seen by TEM (Figure 3) is also consistent with this interpretation, since the actual thickness of the titania is below the detection limits of TEM, and what is evident is at most a surface texturing of the silica due to the presence of the titania coating. The third sample in Table 1 (sample 3) represents the highest loading we were able to obtain and still retain isolated spheres. When the specific reactivity from the correlation in Figure 1 was used to infer titania surface area in sample 3, there were major discrepancies. On this sample as-prepared (RT), the effective titania surface area was 161 m2/g while the BET surface area of this sample was only 56 m2/g. When the same sample was treated in flowing helium at 773 K, the effective titania surface area dropped to 14 m2/g while the BET surface area dropped to 20 m2/gm. This trend was seen in all of the titania/silica samples in that heat
In this paper, we have demonstrated that titania coatings ranging in thickness from sub-monolayer to ≈7 nm can be deposited on silica spheres (270 nm diameter) by using alkoxide precursors. Dehydration of 2-propanol
(18) Srinivasan, S.; Datye, A. K.; Smith, M. H.; Peden, C. H. F. J. Catal. 1994, 145, 565. (19) Liu, Z.; Tabora, J.; Davis, R. J. J. Catal. 1994, 117, 149. (20) Itoh, M.; Hattori, H.; Tanabe, K. J. Catal. 1974, 35, 225.
Figure 9. SEM micrograph of sample 4 showing aggregation of coated spheres.
Figure 10. (a) Low-magnification TEM image of aggregation of coated spheres in sample 5. (b) High-magnification TEM image of sample 5 where aggregation of coated spheres is evident.
Titania Coatings on Monodisperse Silica Spheres
treatment at 773 K was necessary to bring the effective titania surface areas into agreement with the BET surface area. When these samples were examined by high-resolution TEM, we found that the titania coating was amorphous, as prepared. The coating remained amorphous after reaction at 523 K. After 673 K heat treatment, the hydrous titanium oxide on silica became crystalline, as seen by TEM; however, the effective surface area is still somewhat higher than the BET surface area, perhaps because of incomplete crystallization. It is only after heat treatment at 773 K that the effective titania surface area becomes comparable to the BET surface area. Therefore, in subsequent work, we have used the effective surface area after 773 K treatment as an indication of the true titania surface area. Samples tested at lower temperatures always have BET surface areas less than the effective titania surface areas, but we think this is simply an indication that specific titania reactivity derived from anatase titania (Figure 1) cannot be applied to the amorphous titania films. We conclude that the amorphous titania multilayer films must have higher specific reactivity than bulk, crystalline titania. The amorphous titania is stabilized only on the silica surface and is not encountered when bulk hydrous titanium oxide precipitated from the alkoxide is used for alcohol dehydration. This study shows that the optimal conditions for obtaining a thin film coating on silica spheres involve a dilute solution of the alkoxide in the solvent and controlled amounts of water to perform the hydrolysis and condensation reactions. The amount of water necessary is greater than the stoichiometric requirement for the hydrolysis reactions. For example, in sample 3, 0.08 moles of water (0.32 mol/L) and 2.3 × 10-3 moles of TBOT (0.0091 mol/L) were used. Only 32% of the titanium in the precursor was deposited as a hydrous thin film of titania; the rest stayed behind in solution. The concentrations deduced by Ocana et al.8 to ensure that second-phase titania particles did not form were 0.015 mol/L of TBOT and 0.270.54 mol/L of water. These authors suggested that concentrations of TBOT in excess of 0.025 mol/L would result in aggregation of the particles being coated. However, in our work, we were not able to exceed the concentration of 0.0091 mol/L without causing aggregation of the support particles. The smoothness of the coating seen here leads us to believe that the process involves controlled hydrolysis of the TBOT followed by heterogeneous nucleation of the hydrous titanium oxide on the silica sphere. This is in agreement with the observations of Ocana et al.8 for titania coated on ZnO. If the hydrolysis and condensation reactions caused the formation of finely dispersed hydrous titanium oxide particles before depositing on the silica, one might have expected a more rough coating such as that seen when TiOSO4 was used as the precursor to deposit titania on silica by Hsu et al.10 One of the objectives of this study was to investigate whether the deposited titania would retain the morphology of the underlying support as it was heated to elevated temperatures. The results show that indeed, even after the titania crystallizes into polycrystalline anatase, the coating remains conformal with the support. After 773 K heat treatment, the multilayer sample 3 has a BET surface area of 20 m2/g while the uncoated silica spheres have a surface area of 12 m2/g, implying that, after coating,
Langmuir, Vol. 12, No. 13, 1996 3179
there must be some intergranular surface accessible to N2 adsorption. The TEM image in Figure 10b is consistent with this interpretation, since the coating is polycrystalline and may have cracks and voids around the grain boundaries that may provide accessible surface in addition to that of the external surface. Samples that contain secondphase particles of TiO2 (for example 5) show effective titania surface areas (after treatment at 773 K) that significantly exceed the BET surface area of the uncoated silica spheres. Sample 2, which contains a titania loading equal to the saturation uptake of titania on silica spheres, shows an effective titania surface area equal to the BET surface area of the uncoated spheres. If similar coatings could be applied to high surface area silica samples, they may allow a thermally stable support such as silica to stabilize a high surface area of titania. This could be a potential advantage, since the poor mechanical properties of titania often limit its use in high-temperature catalytic processes. Further work is also necessary to investigate the use of alternate titania precursors that would eliminate the need for solvents such as ethanol. Conclusions Titania coatings ranging from sub-monolayer to approximately 7 nm thick have been deposited on monodisperse silica spheres 270 nm in diameter. The silica spheres provide a model support that facilitates the study of the titania coatings. The titania coatings were deposited by the controlled hydrolysis of TBOT in an ethanol solvent while being heated under reflux. The highest titania coating we were able to obtain without causing aggregation of the silica spheres was ≈22.3 wt %. It was found that if the TBOT or water concentration was increased, secondphase particles of TiO2 were deposited and aggregation of the spheres occurred. The titania coatings are amorphous as deposited and remained amorphous when used for alcohol dehydration at 523 K. The amorphous titania coatings exhibit 2-propanol dehydration activities per square meter that are several times greater than that of crystalline anatase. Heating to 673 K transforms these coatings into polycrystalline TiO2. However, we found it necessary to heat the samples to 773 K for the alcohol dehydration activity per square meter to equal that of crystalline anatase TiO2. Hence, we used the 2-propanol dehydration activity of the 773 K heated samples to infer the actual TiO2 surface area. The results show that effective TiO2 surface areas equal to the BET surface area of the uncoated silica can be obtained by this method, implying that the silica surface is completely or nearly completely covered by TiO2 even after 773 K calcination. Acknowledgment. This work was supported by contracts AM3520 and AK5119 from Sandia National Laboratories, which is supported by U.S. Department of Energy contract DE-ACO4-94AL85000. Transmission electron microscopy was performed at the electron microbeam analysis laboratory located in the department of Earth and Planetary Sciences, University of New Mexico. We thank John Hussler for assistance with the XRF and AA measurements and Elaine Boesflug for assistance with BET surface area measurements. LA950808A
