Environ. Sci. Technol. 1996, 30, 647-653
Enhanced Photocatalytic Performance of Titania-Based Binary Metal Oxides: TiO2/SiO2 and TiO2/ZrO2 XIANZHI FU, LOUIS A. CLARK,† QING YANG, AND MARC A. ANDERSON* Water Chemistry Program, University of WisconsinsMadison, 660 North Park Street, Madison, Wisconsin 53706
Sol-gel prepared mixtures of silica or zirconia with titania are shown to have significantly higher activities than pure titania for the complete photocatalytic oxidation of ethylene. These higher activities are only apparent when the respective catalysts are stabilized by sintering. The differences become even more pronounced when the catalysts are used in a tubular reactor. Optimum mixture concentrations are found to be 12 wt % zirconia and 16 wt % silica in titania. Both catalyst types exhibit activity maxima with respect to sintering temperature. It is hypothesized that the maxima arise from opposing effects of densification and phase transformation versus beneficial sintering. A comparison of our catalyst compositions with literature in this area suggests that the increase in activity due to the addition of silica or zirconia may be a result of higher surface acidity. However, isoelectric point measurements employing the unsintered and sintered catalysts show no conclusive increase in surface acidity. The effects of sintering temperature on the surface area, porosity, and crystal structure of the catalysts are also presented.
Introduction Photocatalytic purification of water and air has been touted as one of the more promising remediation technologies, particularly for low levels of contaminating organic compounds. A great deal of work has been published in this area, and much of it has been well reviewed (1-7). In addition, a very extensive bibliography has been published containing over 600 references covering many areas of photocatalysis research (8). Of this impressive number of publications, surprisingly few studies have been performed in the gaseous phase (9-15), probably because groundwater contamination is a better known and more extensive problem. The use of this technology to purify air is especially attractive because volatile organic compounds (VOCs) are * Author to whom correspondence should be addressed; telephone: 608-262-2674; fax: 608-262-0454; e-mail address: marc@ coefac.engr.wisc.edu. † Present address: Department of Chemical Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208-3120.
0013-936X/96/0930-0647$12.00/0
1996 American Chemical Society
typically found in lower concentrations in air. Consequently, a more impressive volume of contaminated media can be treated. The reactor can also be operated at higher temperatures without prohibitive heating costs. Even so, slow reaction rates and poor solar efficiency (maximum 5%) have hindered the commercialization of this technology. In hopes of improving reaction rates, we have dedicated much of our research in this area toward improving the photocatalyst. The use of binary metal oxides as photocatalysts is not entirely new. Recently, Do et al. and Papp et al. have published results on TiO2/WO3 and TiO2/MoO3 systems (16, 17). They found that degradation rates of 1,4dichlorobenzene were enhanced by as much as a factor of 3 by ca. 3 mol % addition of WO3 and MoO3 in titania. They have also found a strong correlation between surface acidity and reactivity. In this case, surface acidity is thought to take the form of stronger surface hydroxyl groups. These groups accept holes generated by illumination and, in turn, oxidize adsorbed molecules. Hole traps such as the hydroxyl groups prevent electron-hole recombination and, therefore, increase quantum yield. Thus, a greater number of surface hydroxyl groups may be expected to yield a higher reaction rate. In fact, such a relationship has been observed previously (18, 19). In addition, surface sites with higher acidities may prove to be better adsorption sites or hole traps. The more polarized state of higher aciditity sites may favor hole trapping. Binary metal oxides are well-known in the catalysis field. It has long been observed that most binary mixtures exhibit increased surface acidity. In fact, zirconia- and silicamodified titania display some of the highest acidities among binary metal oxides (20). Further discussion of the mechanisms of doping-induced surface acidity increases are included in the Results section. Much of the recent work in the catalysis field has been motivated by the desire to use binary metal oxides as either solid acids or as supports for other catalysts. Such uses include the combustion of 1,2-dichloroethane (21), the amination of phenol with ammonia (22), and the hydrogenation of CO over supported nickel (23). The purpose of this paper is to expand the use of the silica- and zirconia-modified titania systems into the photocatalysis field. Ethylene was chosen as a probe for its relatively high reactivity and its close structural relationship to some VOCs (e.g., trichloroethylene). Ethylene degradation also has significance in the fruit and flower industry.
Experimental Section Preparation of Catalysts. All catalysts were prepared using sol-gel methods (24). The appropriate metal oxide sols were prepared by acid hydrolysis of their metal alkoxide precursors. The separate sols were stirred until clear and then mixed to the desired composition. Finally, the mixed sols were dialyzed for 4 days (Spectra-Por 3500 MW cutoff) using a sol to water ratio of ca. 1:40. The water (Milli-Q) was changed once a day until the pH of the water was between 3.5 and 4.0 (near the isoelectric point of the metal oxides). The dialyzed sol was then placed in an oven to
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remove water. The resultant solids were sintered, if appropriate to the experiment, for 4 h and then ground and sieved to give particles between 0.5 and 1.4mm in size. In the case of the catalysts used to determine the optimum compositions, the silica-titania catalysts were sintered to 300 °C, and zirconia-titania catalysts were sintered to 400 °C. The precursor titania and zirconia sols were prepared in a similar manner. In both cases, the titanium isopropoxide or zirconium propoxide was slowly added to an aqueous solution of HNO3. The volumetric mixing ratio for the titania sol was 1 HNO3:136.4 H2O:11.36 titanium alkoxide. The corresponding ratio for the zirconia sol was 1 HNO3:50.0 H2O:3.65 zirconium alkoxide. A precipitate forms immediately after adding the alkoxide. This precipitate is peptized during the required 3 days of stirring. The silica sol was prepared in a slightly different manner. Tetraethyl orthosilicate [Si(OC2H5)4] was mixed with ethanol, water, and HCl in a volumetric ratio of 2000:525:625:1. The pure precursor sols were then mixed and processed as described previously. Reactivity Measurements. The catalysts were packed into a 11 cm long 2.4 mm diameter Pyrex tubular reactor. The weights of the catalysts were kept constant at 0.35 g, and in all cases the illuminated length of catalyst was identical. Illumination was provided by four 4-W UV bulbs (GE F4T5-BLB). These bulbs produce a strong peak centered at 365 nm. A photon flux of 1.65 × 10-7 Einstein s-1 was previously measured for this system using a uranyl oxalate actinometer (12). Ethylene in nitrogen (Mattheson, 502 ppm) was mixed with a water-saturated zero air (Liquid Carbonic, 21% oxygen, H2O < 5 ppm, total hydrocarbons < 1 ppm) stream to afford a reactant stream of the following composition: ca. 140 ppm ethylene, 0.14 mol fraction of oxygen, and 0.02 mol fraction of water. The temperature of the reactor under illumination was 107 °C, as measured using a J-type thermocouple. The reactant mixture flow rate was 20 mL/ min. Analysis of the reactor effluent was conducted with a Hewlett Packard 5890 gas chromatograph (Series II) equipped with a flame ionization detector (FID), a thermal conductivity detector (TCD), and a Porapak R column. The GC was calibrated using known concentrations of ethylene and carbon dioxide. The catalysts were allowed to come to adsorption equilibrium with the reactant gas before the reactor was illuminated. Mass balances were closed by measuring ethylene (FID) and carbon dioxide (TCD) concentrations. The necessary 2:1 ratio of carbon dioxide produced to ethylene destroyed was observed at steady state. Steady state was achieved within ca. 3 h after illumination during which no byproducts were observed. The longer than expected approach to steady state was due to the slow heating effect of the bulbs; initially, the reactor was nonisothermal. No decrease in activity was noticed during the runs, which lasted as much as a full day. Fresh catalysts were used for each run, and ethylene was found to be thermally stable in the reactor without illumination. Fractional conversions of ethylene at steady state were used to measure reactivity. In order to better quantify the specific reactivity of these catalysts, a number of power law reaction rate models (1/2 order, first order, 3/2 order, and second order) were fit to the conversion data taken in the tubular reactor. Two
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FIGURE 1. Fit of the TiO2 (sintered at 300 °C) data to a 3/2 order kinetic model. R2 ) 0.9951.
examples of the catalyst were used: a TiO2 catalyst sintered at 300 °C and a 16 wt % SiO2/TiO2 catalyst sintered at 350 °C. The best fit to the pure TiO2 data was achieved using the 3/2 order power law model:
r ) kC 3/2
(1)
where r is the surface area-based reaction rate (mol of C2H4 min-1 m2); k is the reaction rate constant (mol-1/2 min-1 m5/2); and C is the concentration of reactant in feed (mol m-3). A mass balance (eq 2) can be used in conjunction with this reaction rate model to derive an expression relating reactor space time to fractional conversion (eq 3):
WS VC
f
)
∫ 0
df
(2)
r
(
2 WS 1/2 1 ) -2 C V k x1 - f
)
(3)
where W is the weight of catalyst (g); V is the inlet volumetric flow (m3/min); f is the fractional conversion of ethylene (dimensionless); and S is the specific surface area of catalyst (m2 g-1). Equation 3 can be used to fit kinetic data by picking appropriate abscissa and ordinate values to generate a plot that is expected to be linear. It can then be solved for the rate constant and substituted back into eq 1 to yield a final expression for rate as a function of fractional conversion (eq 4):
r)
VC(2 - 2x1 - f ) SWx1 - f
(4)
Sets of equations similar to eqs 3 and 4 can also be generated for a first-order model (eqs 5 and6):
1 WS 1 ) ln V k 1-f
( ) 1 VC ln r) WS (1 - f )
(5) (6)
Figures 1 and 2 show that the 3/2-order and first-order power law models fit the TiO2 and TiO2/SiO2 data closely (R2 values are 0.9951 and 0.9960, respectively). There is presently insufficient data to postulate a model for the TiO2/ ZrO2 catalyst. For the purpose of generating reaction rates plots, it will be assumed that the TiO2/ZrO2 catalyst follows the same kinetic model as the TiO2/SiO2 catalyst.
FIGURE 2. Fit of the TiO2/SiO2 (sintered at 350 °C) data to a first-order kinetic model. R2 ) 0.9960.
FIGURE 3. Optimization of the catalyst activity as a function of additive weight percent. Initial reaction rates were calculated using the kinetic models. (2) TiO2/SiO2; (b) TiO2/ZrO2.
Physicochemical Analysis. Surface areas were calculated using nitrogen adsorption data at 77 K and BET analysis. Porosities were determined from the adsorption maxima. X-ray diffraction analysis was performed using a Scintag diffractometer and CuKR radiation. Crystal size determinations were done using the Schrerrer equation for peak broadening. Percent composition calculations were made using eq 7:
% rutile )
(
1 × 100 A × 0.884 + 1 R
)
(7)
where A is the area of anatase peak; and R is the area of rutile peak. The number 0.884 is a scattering coefficient. Isoelectric point measurements were conducted using a Penkem System 3000 electrophoretic mobility apparatus.
Results and Discussion Sol-gel prepared mixtures of titanium dioxide with silicon dioxide or zirconium dioxide display higher porosity, higher specific surface area, and an improved thermal stability over the corresponding pure titanium dioxide samples. In addition, these systems show higher conversions through a single pass plug flow (tubular) reactor. Reactivity studies were used to determine the optimum composition of the two catalyst types. Figure 3 shows the relative activity of the two catalysts versus weight percent SiO2 or ZrO2. The curves are not directly comparable because the titania/zirconia catalyst was sintered at 400 °C while the titania/silica catalyst was sintered at 300 °C. Optimums near 8 mol % (12 wt %) and 20 mol % (16 wt %) were found for the zirconia- and silica-derivatized titania, respectively. It is interesting to note that the optimum
FIGURE 4. Effect of sintering temperature on the surface area of the catalysts. The mixed oxide composition was that previously determined to have achieved the optimum reaction rate. (9) TiO2; (2) TiO2/SiO2; (b) TiO2/ZrO2.
additive concentration, in terms of mole percent, is slightly lower in the zirconia-modified system though the levels are close. One way to explain this phenomena is to suggest that some degree of thermal-induced doping is necessary to enhance the activity. Since the zirconia-modified system was sintered at a higher temperature, it should have a correspondingly greater amount of doping at a given composition. If this theory is correct, then the optimum reactivity composition will decrease with increases in sintering temperature. In both cases, the optimization curves increase to maxima at relatively low concentration levels and then decay less rapidly. It is well-known that the addition of a second metal oxide such as zirconia (25) increases the specific surface area of the catalyst, but this cannot explain the increased activity since the reaction rate has been normalized with respect to surface area. The increase in activity is likely due to a chemical change on the catalyst surface. Possible changes in surface chemistry will be discussed later. The obvious detriment to photoactivity is the presence of ZrO2 and SiO2; both have bandgaps greater than 5 eV (26) and, therefore, cannot act as photocatalysts under the illumination used. Thus, it is no surprise that the activity decreases at higher silica and zirconia concentrations. The thermal treatment process that these catalyst undergo is necessary to stabilize their microstructures and adhere them to supports. A pure titania sample sintered at 200 °C loses ca. 30% of its surface area after 2 weeks of use at 100 °C (a decrease from 266 to 194 m2/g). In addition, it is likely that if these catalysts are used commercially they will be coated onto silica fibers or other light-transmitting supports. Some thermal treatment is necessary to bond the catalyst well to a silica or other type of support. When these catalysts are in the form of pellets, very little of their weight is utilized photocatalytically, so much expense could be avoided. At elevated temperatures, the silica- and zirconiamodified titania systems display significantly higher surface areas than the pure titania. Figure 4 shows that the modified systems displaying optimum reactivity retain their high surface areas better than pure titania after thermal treatment. It can be seen that the silica-modified titania has a larger surface area at low temperatures and retains its surface area better than the others at higher temperatures. The zirconia system falls in between the two other catalysts in terms of surface area and stability. The surface area of the pure titania is smaller and decreases more quickly with
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FIGURE 5. Percent porosity as a function of sintering temperature as determined by nitrogen adsorption maximums. (9) TiO2; (2) TiO2/ SiO2; (b) TiO2/ZrO2.
FIGURE 7. Effect of sintering temperature on phase transformation from anatase to rutile in pure titania.
a
b FIGURE 6. XRD patterns of TiO2, TiO2/SiO2, and TiO2/ZrO2 sintered at 500 °C. All the pure TiO2 peaks except the farthest to the left are the rutile form of titania. The remainder of the peaks in the figure correspond to the anatase form.
increasing temperature. It has been hypothesized that the presence of nearby silica or zirconia inhibits densificaton and crystallite growth by providing dissimilar boundaries. The modified systems also retain the high porosities of the xerogel systems better after thermal treatment. Figure 5 displays the porosities of the various optimized samples as a function of sintering temperature. The porosity of the pure titania sample begins to decrease after the sintering temperature is increased to 300 °C from its initial value of ca. 50%. The silica- and zirconia-modified titania samples retain their porosities of 47% and 55% nearly irrespective of sintering temperature. Modifying the titania also inhibits phase transformations in the solid. At low sintering temperatures, all of the titania in the three systems is in the anatase form. Figure 6 shows typical XRD spectra for the three systems at a high sintering temperature (500 °C). There were no identifiable trends in crystallite size with respect to firing temperature or composition. Overall, crystallite sizes determined from peak broadening suggest that the anatase crystallites are10 ( 2 nm in diameter. These titania samples undergo a phase transformation from anatase to rutile beginning at 200 °C. A steady transformation is observed until the catalyst is 100% rutile after sintering at 600 °C. Figure 7 shows this trend graphically. Neither the silica- nor zirconia-modified systems contain titania in its rutile form except the zirconiamodified sample sintered at 600 °C, which was 24.9% rutile. These results are promising since rutile has long been considered the less photocatalytically active form of titania (1).
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FIGURE 8. Activities of the three catalysts as a function of sintering temperature. (a) Percent conversion in the tubular reactor. (b) Calculated initial reaction rates based on the kinetic models. (9) TiO2; (2) TiO2/SiO2; (b) TiO2/ZrO2.
Once the optimum catalyst compositions were known for ethylene oxidization, the effect of changing the sintering temperature could be best investigated. Figure 8b depicts reaction rates normalized with respect to surface area for ethylene oxidation on the three catalysts as a function of sintering temperature. In Figure 8a, the original percent conversion of ethylene data has also been included to emphasize the usefulness of these catalysts in a plug flow reactor. If this technology becomes useful commercially, then plug flow reactors of some type will certainly be used since they usually provide the highest conversions for a given amount of catalyst. The substantial difference between the pure titania and the modified catalysts in the two plots arise from their differences in reaction order and surface area. The lower orders of the modified catalysts are beneficial because they imply that the rate decreases less with decreasing reactant concentration. The modified catalysts will be particularly active compared to the pure titania at the lower concentrations typically found in indoor
air. For comparison, a conversion of 27.6% was measured using Degussa P-25. It can be seen that the activity of the pure titania catalyst is initially quite high and decays quickly with increasing sintering temperature. This effect is explained using hydroxyl group density effects throughout the range and crystalline phase effects beyond 200 °C. FTIR measurements (25) indicate that the surface hydroxyl group band (3260 cm-1) diminishes with increasing sintering temperatures and nearly disappears at 500 °C. As previously noted, the rutile content increases with sintering temperature after 200 °C and so may contribute to the decrease in activity. The behavior of the modified titania systems is more complicated. Both systems exhibit peaks in reactivity with respect to sintering temperature. The peaks are probably due to the opposing contributions of dehydration and beneficial sintering. As sintering temperatures increase, pairs of hydroxyl surface groups will combine to lose a water molecule and form a metal-oxygen-metal link. Beneficial sintering is the thermal-induced doping of the silica or zirconia by the titanium. The sintering may be necessary to form favorable surface groups, to favorably transform the ones already present, or to establish dissimilar oxide interfaces. It seems possible that these new or transformed surface groups are more acidic because binary metal oxide systems often display increased acidity over their pure counterparts. At least two models have been proposed to explain this increase in acidity (27-29). The model proposed by Tanabe et al. (27) assumes that the dopant oxide’s cation enters the lattice of its host oxide and retains its original coordination number. Since the dopant cation is still bonded to the same number of oxygens even though the oxygen atoms are now of a new coordination, a charge imbalance is created. The charge imbalance must be satiated, so Bro¨nsted sites (extra protons) are expected to form when the charge imbalance is negative. Lewis sites are expected to form when the charge is positive. The charge imbalance is calculated for each individual bond to the dopant cation and multiplied by the number of bonds to the cation. Silica is tetrahedrally coordinated with each oxygen bonded to two silicon atoms, and zirconia is 8-fold coordinated with each oxygen bonded to four zirconium atoms. Finally, titania is octahedrally coordinated with each oxygen bonded to three titanium atoms. So, if a titanium atom enters a silica lattice, then each of its six bonds are now attached to an oxygen having only one other cation bond. Four valence electrons (on Ti) divided by six bonds minus two available electrons on oxygen divided by two gives a charge imbalance of -1/3 per bond. Since the imbalance is negative, protons are expected to associate with the nearby oxygen atoms. There is now more surface hydroxyl groups then would be initially expected. Figure 9 illustrates the four possible doping configurations and the calculations required to predict the acid site type. The presence of the maxima are not likely to be related to the formation of new compounds or to phase transitions. The thermodynamically stable zirconium-titanium phase (30) at this composition and these temperatures is a mix of ZrTiO4 and tetragonal TiO2. ZrTiO4 has been shown to have low photoactivity (31) and so is an unlikely contributor to high activity. The XRD spectra also show no peaks other than those attributable to TiO2 in its various forms. Recently, characterization work has been done on silicatitania systems for use as conventional catalysts. One might expect that in situations where there is more titania than
..
..
FIGURE 9. Illustration of the Ti-O-Zr and Ti-O-Si linkages proposed in Tanabe’s model to describe the increased surface acidity of binary metal oxides. (a) Silicon in titania, (b) zirconium in titania, (c) titanium in silica, and (d) titanium in zirconia.
silica that silicon atoms might be more likely to migrate into the titania, this is not the case. It has been found that the new acidic sites created by lattice substitution have Bro¨nsted acid character instead of Lewis acid character (32). This behavior can be explained by experiments indicating that, contrary to expectations, the titanium migrates into silicon sites (34, 35). Doolin et al. have shown that with fully prehydrolyzed mixtures calcined at 350 °C the highest Bro¨nsted acidity is achieved at ca. 10 mol % silica in titania (32). They also provide an excellent comparison of preparation techniques. Itoh et al. also report a similar maxima (33). This is fully consistent with the hypothesis regarding our fully prehydrolyzed catalyst, acidity maxima have been found at compositions close to where our activity maxima are found. In another study, increasing acidity with increasing titania content up to nearly pure titania (99.5 mol %) was observed for coprecipitated silica-titania catalysts (34). The authors suggest that their catalyst was predominantly a mechanical mixture of the two oxides. While this paper was in review, Anderson et al. (36) reported increased reaction rates over an optimum mixture of 30:70 TiO2/SiO2. Their preparation technique was similar except that the precursor metal alcoxides were mixed prior to hydrolysis and the synthesis was carried out in an organic (2-propanol) with HCl as the hydrolysis agent. The substantial differences between the optimum found in this work and theirs may be due to a difference in microstructure. Since titanium alcoxides hydrolyze more quickly than silica alcoxides, it is possible that their oxide mixture was less homogeneous and, therefore, required a greater amount of silica to establish the same number of dissimilar oxide interfaces. Additionally, they were testing for photocatalytic activity with a large compound that was known to adsorb preferentially on silica. Electrophoretic mobility studies of the three catalysts were performed on unsintered (xerogel) samples as well as ones that had been sintered at 300 °C. Since the mobility of the catalyst particles are changed by adjusting the pH of the solution, the surface hydroxyl groups are assumed to lose and gain protons accordingly. Consequently, only the Bro¨nsted type of surface acidity can be measured using this technique. In general, there appears to be no decrease in the isoelectric point of the catalysts after sintering and, therefore, no conclusive increases in Bro¨nsted acidity.
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FIGURE 10. Electrophoretic mobility studies for xerogel and 300 °C sintered TiO2. (() TiO2 fired to 300 °C; (2) TiO2 xerogel.
FIGURE 11. Electrophoretic mobility studies for xerogel and 300 °C sintered TiO2/SiO2. (2) TiO2/SiO2 xerogel; (() TiO2/SiO2 fired to 300 °C.
Figure 10 shows the mobility data for the pure titania system. The isoelectric point appears to change little between the xerogel and the 300 °C sintered samples. The isoelectric points of the xerogel and sintered samples are 6.2 and 6.3 pH units, respectively. The mobility maxima seen in the tails of the two curves do change with sintering temperature. The xerogel sample reaches its maxima at 3.0 × 10-8 and -2.8 × 10-8 m2 V-1 s-1, and the sintered sample reaches its maxima at 2.6 × 10-8 and -2.3 × 10-8 m2 V-1 s-1. The difference probably arises from different particle charge to mass ratios, which correspond to different hydroxyl group surface densities. The modified titania samples show dissimilar changes between their xerogels and 300 °C sintered states. The characteristic curves for the silica-modified system are coincident with a common isoelectric point of 5.2 (Figure 11). The corresponding curves for the zirconia-modified system show a clear divergence at their centers, resulting in different isoelectric points. The isoelectric points of the xerogel and sintered samples are 5.9 and 6.3 pH units, respectively. In general, the mobility data provide no conclusive evidence for a change in surface acidity with sintering temperature. The zirconia-modified system clearly displays a decrease in surface acidity corresponding to the increase in isoelectric point (Figure 12). This could be explained by noting that Tanabe’s model predicts Lewis acidity in the case where titanium migrates into the zirconia. However, it can only be hypothesized, at this point, that the titanium enters the zirconia in a fashion analogous to the silicatitania system instead of the reverse. Lewis sites on the sintered mixture could be beneficial by providing extra or
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FIGURE 12. Electrophoretic mobility studies for xerogel and 300 °C sintered TiO2/ZrO2. (() TiO2/ZrO2 fired to 300 °C; (2) TiO2/ZrO2 xerogel.
stronger adsorption sites, or they could act as electron traps since they are positive in charge. When the mobility data for the two systems are contrasted, the silica-modified system becomes more interesting. Since the zirconia-modified system shows a decrease in surface acidity with sintering temperature, it would be reasonable to expect the silica-modified system to show the same behavior. This is not the case. An increase in surface acidity due to sintering-induced doping could be masked by a decrease due to dehydration. Alternatively, if there is no formation of new acid sites or change of sites on the titania, then the activity increase may simply be due to the addition of the silica sites, as suggested by Anderson et al. (36). The silica provides more acidic sites that may be beneficial. Silica prepared in a fashion analogous to the silica sol used in the catalyst has an isoelectric point of ca. 4 pH units. Unfortunately, this does not explain the effect that sintering temperature has on activity. The dotted line in Figure 11 suggests that the 300 °C sintered mobility data are a combination of two separate silica and titania curves. It is not clear how these data relate to surface properties. It is also possible that the increase in activity with sintering temperature could be the result of other electronic effects that are unique to photocatalysts. It may be that the sintering allows the two oxides to come into more intimate contact. The interface between the two particles could act as a sort of photoelectrochemical diode. A difference in electron or hole energy levels between the two oxides could lead to enhanced charge separation, which would result in an increase in reaction rates. These types of systems were studied by Nozik as enhancements for water photosplitting (37, 38). It is hoped that diffuse reflectance FTIR studies in our lab will provide more insight into the types and populations of surface groups in these mixed oxide photocatalysts. Further surface chemistry studies appear to be necessary to identify the rate enhancement mechanism with certainty.
Conclusion The modification of titania with silica and zirconia has been shown to produce a better photocatalyst for the oxidation of ethylene. Conversions of ethylene in a single pass tubular reactor can be increased by as much as a factor of 3 using these catalysts. This substantial increase in activity is in part due to increased surface area. Increased thermal stability with respect to both densification and phase transformation also plays a significant role. The modification maintains the higher surface areas of the new systems even after sintering and also inhibits the anatase to rutile
phase transformation. There is also some evidence to suggest that changes in surface chemistry, particularly acidity, have been beneficial.
Acknowledgments We are grateful to Akawat Sirisuk and Ruhaidah Md. Hassan for their invaluable help with the kinetic and mobility data. Additionally, Dr. Walt Zeltner’s assistance is always appreciated.
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Received for review June 7, 1995. Revised manuscript received September 22, 1995. Accepted September 25, 1995.X ES950391V X
Abstract published in Advance ACS Abstracts, December 1, 1995.
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