Ind. Eng. Chem. Res. 2006, 45, 3815-3820
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Synthesis of High-Temperature Titania-Alumina Supports† Vaidyanathan Subramanian, Zheng Ni, E. G. Seebauer, and Richard I. Masel* Roger Adams Lab, Box C-3, UniVersity of Illinois at Urbana-Champaign, MC-712, 600 South Mathews, Urbana, Illinois 61801
Catalyst support materials based upon composite metal oxides often incorporate the beneficial aspects of the constituents. For the case of TiO2-Al2O3 composites, however, it has proven difficult to retain a high anatase content and surface area at temperatures of 900 °C and above. This work reports a sol-gel synthesis method that solves these problems. The mixed oxide incorporates 100% anatase regardless of composition and at a Ti:Al-0.4:0.6 weight ratio has a surface area 74 m2/g. Both properties are stable after annealing up to total times of 34 h at 900 °C. Experiments with supported Pd show that metal dispersion on the mixed oxide is observed roughly twice that of a pure Al2O3 support. The action of acetic acid in the preparation mixture during sol formation and calcination, augmented by solid solution formation between TiO2 and Al2O3 during calcinations, seems to account for these results. Introduction Virtually all catalyst support materials are designed to enhance high catalyst dispersion that is stable for long periods of operation. However, in applications such as hydrocarbon reforming to produce hydrogen, operating temperatures exceeding 800 °C, especially in the presence of steam, make catalyst deactivation by sintering particularly difficult to prevent.1,2 Few supports with the required surface area and high-temperature stability exist. One promising approach has focused upon supports consisting of composite materials that incorporate the beneficial aspects of the constituents.3 For example, TiO2 has good catalytic activity due to high surface area at low temperatures but suffers from poor thermal stability due to phase transformation; Al2O3 has high stability and low activity.4 Precursors of these materials can thus be used in sol-gel processing to synthesize nanocomposites that retain the advantages of both TiO2 and Al2O3.5-7 Related mixed oxides of the form Al2O3-X (where X ) TiO2, SiO2, ZrO2) can also be prepared according to this strategy.8,9 The Al2O3-TiO2 composite has found particularly widespread use in a variety of catalytic processes.10-14 A persistent problem for this composite, however, is that TiO2 exists in the following crystalline forms: anatase; rutile; brookite.15 The anatase form is most useful for preventing catalyst aggregation at high temperature.16-18 However, the transformation from anatase to the more stable rutile form occurs in the rather lowtemperature range of 300-700 °C for most sol-gel oxides. This phase transformation lowers the active surface area of the composite. It is therefore essential to find a methodology to prepare supports that retain a high anatase content at high temperature. Sivakumar has recently made an important step in this direction by synthesizing composite Al2O3-TiO2 that retains a high anatase content at 800 °C.7 Hydrocarbons require operation temperatures upward of 800 °C to around 900 °C. Furthermore, the stability of the composite over extended time periods is of * To whom correspondence should be addressed. Tel.: (217) 2442819. Fax: (217) 333-5052. E-mail:
[email protected]. † Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the Department of Defense, the Army Research Office, or the Department of Energy.
critical importance and is yet to be examined. In general, however, several studies have shown that alumina retards the transformation of anatase to rutile19-21 by inhibiting initial crystallization of TiO2.16,17 Experimental Section Titanium butoxide (Aldrich, >99%), glacial acetic acid, 2-propanol, and deionized water were used for the synthesis of TiO2 colloids. The butoxide precursor was employed because Bacsa has reported minimal rutile formation with this precursor.22 Al2O3 (obtained from Degussa) was used without further treatment. The data provided by Degussa are as follows: specific surface area, 100 ( 15 m2/g; average primary particle size, 13 nm; Al2O3 content, g99.6 wt %. No phase details were available, so this information was obtained by X-ray powder diffraction (XRPD) as discussed later. The sol-gel synthesis technique was adapted to prepare colloidal TiO2. An 80 mL volume of deionized water was taken in a 250 mL round-bottom flask. An 8.5 mL volume of glacial acetic acid was added to the water. The flask was placed in an ice bath, and the mixture was stirred for a few minutes to allow the solution to cool. A 1 mL volume of 2-propanol was added to a dropping funnel followed by 4 mL of titanium precursor. The Ti was added at a drip rate of approximately one drop/s over the course of 3040 min. The water/acetic acid solution was vigorously stirred during the addition process. The contents of the flask were poured into a 200 mL beaker and then placed in a water bath and heated with vigorous stirring. As the solution warmed, the white precipitate present began to disappear, and the viscosity of the solution increased. Eventually this solution became gellike. It was allowed to cool gradually and then reheated until it formed a sol again. This procedure was repeated 3-4 times to form uniform colloidal particles. The purpose of the multiple heating cycles was to reduce overall volume of solution to ∼6080 mL. Once the heating was finished, the colloidal solution was loaded into a specially designed thick-walled glass cell (∼150 mL volume) having a narrow long opening. Care was taken to ensure that the volume of the solution was less than half the volume of the glass bulb. The sealed bulb was placed in an oven set to a maximum temperature of 260 °C, and the temperature was maintained for 12 h. After 12 h the oven was switched off and the contents were allowed to cool to room
10.1021/ie051175q CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006
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Figure 1. Process diagram for sol-gel synthesis of the mixed oxide TiO2Al2O3.
temperature over a period of 18-24 h. To make the solid component soluble again, the solution was sonicated. After this procedure the solution remained stable for many days.23 Mixtures of the oxides were prepared on a weight fraction basis. Figure 1 shows the synthesis scheme. The composite powders were made by preparing colloidal TiO2 and mixing the resulting gel with commercial Al2O3 to form slurries having different weight ratios. The slurry was heated from 25 to 130 °C over 1 h, from 130 to 900 °C over 6 h, and at a constant temperature of 900 °C for 6 h. The resulting material was then cooled from 900 to 25 °C over 6 h. The oxide composites were characterized by several methods. A general-area detector diffraction system (Bruker) was used to measure the X-ray powder diffraction (XRPD) pattern. In this instrument, a four-circle diffractometer and HiStar multiwire area detector collected data in a wide-angle format. Cu KR radiation (0.154 18 nm) was used as the X-ray source, and the scan rate was 0.6 deg/min. The particle size t was estimated using the Scherrer equation:
t)
Cλ B cos θ
Figure 2. (a) XRPD pattern of the mixed oxide with different compositions (a) Ti(0), (b) Ti(0.25), (c) Ti(0.43), (d) Ti(0.64), and (e) Ti(1). The peaks labeled with “A” correspond to anatase. (b) Slow-scan XRPD pattern of TiO2-Al2O3 composites with different ratios (100% Al2O3 f Ti(0.25) f Ti(0.43) f Ti(0.64) f 100%TiO2). The peaks labeled with “A” correspond to anatase.
Results and Discussion
(1)
where C ) 0.9, λ is the X-ray wavelength, B is the full width at half-maximum corrected for instrumental broadening, and θ is measured Bragg angle. Instrumental broadening was estimated by taking a diffraction pattern of a standard lanthanum hexafluoride sample obtained from NIST. The Brunauer-Emmett-Teller (BET) method was used to estimate the total surface area (SBET). Nitrogen BET was performed with a Micromeritics Ltd [Pulsed Chemisrob (Chemisorb) model no. 2705] multipurpose instrument. The same instrument was also used to measure the dispersion of Pd on the mixed oxide using hydrogen chemisorption. A solution of H2PdCl4 (99% Aldrich) in water (equivalent to 3 wt % with respect to the oxide composite support) was used. The Pd was loaded by the wet impregnation method. The impregnation was followed by oxidation at 450 οC in air for 3 h and reduction at 900 °C for 3 h. Hydrogen chemisorption for surface area determination was performed at 35 °C. Thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA7 instrument. Differential scanning calorimetry (DSC) measurements were conducted with a Mettler-Toledo DSC821e. TGA and DSC were performed under a nitrogen atmosphere at a heating rate of 10 °C/min using aluminum pans. Scanning electron microscopy (SEM) was performed with a Hitachi S4700, and transmission electron microscopy (TEM) was performed using a JEOL 2010.
1. Composition and Structure. Figure 2a shows XRPD pattern of the composite oxide for varying Al:Ti ratios with Ti weight fraction, x, defined as Ti/(Ti + Al). It is worthwhile to mention here that Al2O3 was added to the TiO2 sol to increase the surface area of the composite since (1) Al2O3 demonstrates high surface area compared to TiO2 at higher temperatures and (2) Al2O3 is sinter resistant. The scan range of 2θ ) 20-65° was covered in a split pattern (2θ ) 20-45 and 40-65°). Peaks corresponding to only TiO2 and Al2O3 could be observed for the mixed oxides, indicating that no new Ti-Al compound forms during calcination. The Inorganic Crystal Structure Database (ICSD) was used to identify the peaks. The alumina peaks matched the θ phase (ICSD No. 82604). The TiO2 peaks matched exclusively the anatase phase (ICSD No. 96946). To confirm that no other TiO2 phases were present, additional patterns were acquired at 2θ ) 21-32° using slow scanning at 0.15°/min. Other phases of TiO2 generally show peaks in this range. However, Figure 2b shows that no such additional peaks could be detected other than that corresponding to anatase at 25.4°. Importantly, the X-ray patterns showed no evidence of brookite, which induces the transformation of anatase into rutile.15 Brookite typically appears as a prominent peak at ∼31°.15 No such peak could be observed in our samples. To examine if the TiO2 and Al2O3 were well mixed, TEM was performed. Figure 3 shows micrographs and diffraction patterns collected when a holey carbon grid loaded with the mixed oxide was examined with the tightly focused electron
Ind. Eng. Chem. Res., Vol. 45, No. 11, 2006 3817 Table 1. BET Areas following 6 h of Calcination BET surface area (m2/g)
Figure 3. TEM image of (a) TiO2, (b) Al2O3, and (c) TiO2-Al2O3 with Ti(0.4) following calcination at 900 °C for 6 h. (d)-(f) show the electron diffraction patterns of the TiO2, Al2O3, and the TiO2-Al2O3 composite with Ti(0.4), respectively.
Figure 4. BET surface area vs Ti weight fraction of the composite oxide calcined at 900 °C for 6 h.
beam. Figure 3a,b shows that the TiO2 powder and the Al2O3 has a particle size of ∼10-15 nm. (Note that this particle size measured also confirms with the Al2O3 size reported by Degussa.) Figure 3c shows that the particle size of the composite formed is ∼10-15 nm. Further, the diffraction patterns corresponding to the TiO2 (dotted lines) and Al2O3 (continuous lines) are shown in Figure 3d,e. The most interesting observation is noted in Figure 3f. The diffraction peaks for both Al2O3 and TiO2 noted in Figure 3d,e are present in Figure 3f. This observation clearly indicates that the two oxides are well mixed. This verification was important because sol-gel synthesis of composite oxides typically involves mixing dissolved precursors at the sol stage. By contrast, the present experiments employed solid alumina mixed into a titania sol. Therefore there was a higher probability of strong phase segregation. Figure 4 shows the changes in surface area (SBET) for single and mixed oxides. Table 1 shows similar data at different temperatures. The areas measured for the single oxides were ∼90 m2/g for Al2O3 and 2.7 m2/g for TiO2. The area decreased
composition (wt fraction of TiO2)
300 °C
600 °C
900 °C
0 0.33 0.735 0.81 1
78.7 88 82.6 79.1 74.5
76.3 84.4 79.9 77.2 58.1
90 80 54.1 45.7 2.7
monotonically as x increased. However, at modest values of x between 0.3 and 0.5, SBET remained quite highsabout 70m2/g. Even small amounts of alumina dramatically increased SBET compared to pure anatase. For example, only 10% alumina content increased SBET by nearly 1 order of magnitude from the pure anatase value. The extent of metal dispersion was examined after Pd deposition on the mixed oxide substrate. The substrates were wet impregnated with Pd salt solution, oxidized at 450 °C, and reduced at 900 °C for 3 h. The dispersion (D) was estimated using hydrogen chemisorption measurements (refer to the Supporting Information). D was 0.2 and 0.39 for pure Al2O3 and the TiO2-Al2O3 (0.4) composite, respectively. This result indicates that TiO2 enhances dispersion of metal. Similar observations of higher dispersion in the presence of TiO2 have also been reported by Wang and co-workers, over a mixed substrate containing Pd reduced with hydrogen at 150 °C.24 This shows that the mixed oxide are promising substrates to enhance Pd dispersion. Propane oxidation studies are underway to examine the catalytic activity over these supports. 2. Effects of Heating. Figure 5 shows the TGA of TiO2 and the mixed oxides. The gels were dried at 80 °C overnight prior to analysis. The thermal degradation trend over 100-900 °C was similar for TiO2 and the mixed oxides. There were three primary weight loss regimes, with most of the weight loss occurring below 600 °C. The total loss at 900 °C was 4% for the TiO2 and ∼5.5% for the composite oxides. Heating first starts with the removal of volatile organics followed by combustion intermediates.TGA shows several distinct loss peaks. For the sol-gel processed TiO2 precursor, the loss below 200 °C is typically attributed to the removal of water and low-boiling alcohols. Further, the steep decrease at 350-450 °C is attributed to the removal of various adsorbed groups and organic byproducts of hydrolysis and decomposition by pyrolysis of the alkoxide precursor.25 Most literature studies report higher weight losses than those observed here, typically between 10 and 50%.4,25,26 Weight loss depends on the method adopted for synthesis. Lower weight losses have been reported in cases where the mixed oxides are formed using two different precursors, along with solvents.11,19,20,26 The use of preformed oxides (such as Al2O3 powder employed here) can lead to lower
Figure 5. Thermogravimetric analysis of (a) Ti(1), (b) Ti(0.4), (c) Ti(0.2), and (d) Ti(0.15). The samples were heated overnight at 80 °C before TGA analysis.
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Figure 6. Differential scanning calorimetry (DSC) data for (a) Ti(0.4) and (b) Ti(1). The samples were heated overnight at 80 °C before DSC analysis.
Figure 8. XRPD pattern of the TiO2-Al2O3 composite Ti(0.4) following calcination at 300, 600, and 900 °C for 6 h. The peaks labeled with “A” correspond to anatase. Table 2. Cell Parameters and Particle Size of the TiO2 and Mixed Oxides Figure 7. BET surface area of composite oxides calcined at 900 °C in stages up to 6 h for different Ti weight fractions.
weight loss since the usage of volatiles in the synthesis is minimal. It is also possible that alumina accelerates the removal of the acetate and related organics during heating, although such effects need further investigation. Thus, the TGA analysis implies that the weight decrease can be attributed to the removal of volatile organics used in the sol formation. Figure 6 shows the DSC curves for TiO2 and the mixed oxides. All materials show endothermic peaks, which appear at roughly similar intervals. The peak at ∼100 °C for TiO2 corresponds to the removal of physisorbed water from the dried gel. This result corroborates the TGA results indicating water removal at ∼100 °C. The peak at ∼300 °C probably corresponds to dehydroxylation reactions, and so does the TGA peak at 300400 °C. The peaks at 300-400 °C observed in TGA and DSC correspond to the removal of other residual organics (such as acetates and alcohols added during the synthesis stage). Peaks from the mixed oxides are generally shifted to somewhat lower temperatures than the corresponding peaks from pure TiO2. The exact calcination procedure did not affect SBET appreciably. Calcination in stages (2 h, 450 °C f 2 h, 700 °C f 2 h, 900 °C) yielded SBET only 5% lower than those oxides calcined at 900 °C directly. The effects of calcination time and temperature on surface area were examined.The temperature was ramped up linearly at ∼0.6 °C/min to the calcination temperature (300, 600, or 900 °C), and calcined for up to 6 h (in increments of 1 h) followed by cooling. Figure 7 shows data for 900 °C for several oxide compositions. The mixed oxides all lose a fraction (
