Ind. Eng. Chem. Res. 1999, 38, 3381-3385
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Preparation of Heterogeneous Photocatalyst (TiO2/Alumina) by Metallo-Organic Chemical Vapor Deposition Lecheng Lei, Hiu Ping Chu, Xijun Hu,* and Po-Lock Yue Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
The metallo-organic chemical vapor deposition (MOCVD) technique has been applied to the preparation of the photocatalyst titanium dioxide supported on porous alumina. Titanium tetraisopropoxide (TTIP) was used as the precursor, which was first deposited inside the mesopores of alumina under vacuum and then decomposed under controlled conditions to titanium dioxide. The amount of titanium dioxide deposited was found to be affected by the flow rate of the precursor and the methods of both deposition and decomposition. By using MOCVD, the number of steps in preparing porous media-supported catalyst can be reduced. X-ray diffraction results show that the heterogeneous photocatalyst produced by MOCVD has an anatase crystal structure. Alumina-supported titanium dioxide prepared by MOCVD method has a lower TiO2 content but has a catalytic efficiency similar to the commercial titanium dioxide powder Degussa P-25. The supported catalyst has an advantage in that it can easily be recovered from the treated water by filtration and is then ready for reuse. Introduction Titanium dioxide has widely been used in the pigment industry, catalysis, inorganic membranes, microelectronics, and photocatalysis. In the past decade, the use of titanium dioxide, especially in its anatase crystal structure, as a photocatalyst has received great attention in the treatment of organic-contaminated wastewater. Because titanium dioxide has a small internal surface area, most titanium dioxide particles are usually made in the submicrometer range to maximize the exposition to the wastewater. The photodegradation of organic pollutants in illuminated aqueous dispersions of TiO2 is very effective, but the powder photocatalyst is difficult to separate from the treated water. To overcome the drawback of powder TiO2, the active phase (TiO2) can be dispersed onto a regular support. Matthews coated TiO2 onto a glass surface, thus making the photocatalyst a stationary phase. However, the TiO2 film coated on nonporous glass has a limited contact area with the reactant so that the overall conversion efficiency is low.1 TiO2 can also be attached to silica gel by mixing TiO2 with silica gel in water, sonicating, and rotary evaporating under vacuum at 95 °C.2 TiO2 was also immobilized using polymeric membrane photografted on the surface of microporous cellulose membrane.3 In addition, Dagan and co-workers prepared highly active TiO2 photocatalysts by a sol-gel method. They prepared the photocatalysts by careful mixing of titanium isopropoxide, 2-propanol, water, and nitric acid. After gelation of the sols, the solvents were removed either by freeze-drying, forming cryogels, or by supercritical point drying, forming aerogels.4,5 The surface area of the supported TiO2 prepared by these methods is low; thus the catalytic efficiency is not high. Porous support can be used to enhance the catalytic efficiency of the developed catalyst. The porous solid supported catalyst can be made in the granular form; * Corresponding author. E-mail:
[email protected]. Tel: (852) 2358 7134. Fax: (852) 2358 0054.
hence, it can easily be filtered from the treated water and so recovered. Because the active phase TiO2 is dispersed onto porous support with a very high surface area, the photocatalytic activity of the supported TiO2 can be considerably superior to that of the powdered TiO2 particles. The traditional techniques used to disperse active phase through the porous support are wet impregnation, precipitation, and ion exchange. These methods are not suitable for our photocatalyst preparation technique as the precursor, titanium tetraisopropoxide, is unstable in water. A new technique to equip a support with active material is to use chemical vapor deposition (CVD), whereby the active phase is deposited onto the porous support from gaseous metal precursor. In 1989, Dossi et al.6 attempted to synthesize catalysts using CVD. They prepared Pt/NaY and Pt/NaHY zeolites by ion exchange of [Pt(NH3)4](NO3)2 and then deposited Re onto the zeolite by sublimating Re2(CO)10 at 90 °C in flowing helium, to obtain bimetallic particles on the zeolite. Yoo and coworkers also have applied CVD for catalyst preparation.7-14 They prepared CVD Fe/Mo/partially deboronated borosilicate molecular sieve (DBH) catalysts from FeCl3 and MoO2Cl2, with the borosilicate molecular sieve having a surface area of 373 m2/g, in two steps by the CVD technique.7 In general, metallo-organic compounds are usually used as precursors due to their relatively low decomposition temperatures and high volatilities. Hampden-Smith and co-workers synthesized palladium and ZnO thin films using metallo-organic chemical vapor deposition (MOCVD).15,16 This technique was also applied to obtain vapor-grown carbon fibers (VGCF)17,18 and platinum deposited on graphite.19 Recently Hu and co-workers developed a copper/activatedcarbon catalyst for wastewater treatment by MOCVD.20 By applying MOCVD, the catalyst preparation can be simplified compared to the traditional methods such as wet impregnation. By the use of MOCVD, all of the traditional steps in catalyst preparation, such as drying, calcination, and reduction, which critically affect cata-
10.1021/ie980677j CCC: $18.00 © 1999 American Chemical Society Published on Web 07/22/1999
3382 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999 Table 1. Physical Properties of Alumina WHA-204
lattice
bulk density (kg/m3)
total porosity (m3 pore/m3 particle)
macropore porosity (Dp > 6 × 10-9 m) (m3 macropore/m3 particle)
micropore porosity (Dp< 6 × 10-9 m) (m3 micropore/m3 particle)
BET surface area (m2/g)
γ-Al2O3
624
0.44
0.41
0.03
188
lyst performance, can be eliminated. Reduction and activation of the catalyst can be carried out simultaneously in the reactor before the catalytic reaction takes place. MOCVD is a promising technique for catalyst manufacture. The present work aims to prepare the heterogeneous photocatalyst supported on porous alumina using the MOCVD method. A metallo-organic compound, titanium tetraisopropoxide (TTIP), is used as the precursor. Alumina is employed as the support for the photocatalyst due to its high reactivity. The developed catalysts are characterized by using various techniques, such as nitrogen adsorption to measure the Brunauer-Emmett-Teller (BET) surface area and X-ray diffraction (XRD) to determine the crystal structure of TiO2. The catalytic efficiency of the developed catalyst is evaluated in the photocatalytic degradation of phenol solution. The phenol removal rate is represented by the changes of total organic carbon (TOC) and chemical oxygen demand (COD). Experimental Section Alumina WHA-204 was selected as the catalyst support. The properties of which have been characterized previously21 and are listed in Table 1. The pore size is mainly distributed in the range of 6-3000 nm, which is suitable to accommodate the organic compounds to be oxidized. The alumina was crushed into small particles of about 1 mm. The particles were then washed with deionized water until the effluent was clear. The washed particles were soaked in deionized water for 24 h. After removal of the water by filtration, the particles were soaked in a 5% HCl solution with constant shaking for another 24 h to further remove any impurities. The acid solution was then removed by filtration, and the particles were washed again with deionized water. After being dried under vacuum at ambient temperature (20 °C), about 5 g of particles was put into the quartz reactor, which was heated to 300 °C and kept under vacuum overnight to further remove any contaminants. Then, the temperature of the reactor was adjusted to the desired temperature for chemical vapor deposition. Titanium tetraisopropoxide was used as the precursor. The boiling, melting, and flash points, molecular weight, and specific gravity of TTIP are 232, 18-20, and 22 °C, 284.26 and 0.955 g/mL, respectively. To identify the decomposition temperature of the precursor, thermal treatment of the liquid TTIP was carried out on a Netzsch SAT-409C thermo-gravimetric analyzer coupled with a ABB Extrel MS250 mass spectrometer (temperature programmed decompositionmass spectroscopy, TPD-MS) from room temperature to 600 °C with a 10 °C/min temperature ramp. Hydrogen was used as the carrier gas with a flow rate of 10 mL/ min. The effluent from the TGA was introduced into the MS, so that not only the weight loss from but also the composition of the effluent could be detected at the corresponding temperature (as shown in Figure 2). A schematic diagram of the chemical vapor deposition apparatus is shown in Figure 1. Various carrier and
Figure 1. Schematic diagram of the chemical vapor deposition apparatus.
reactant gases are supplied from the gas cylinders. Moisture traps are mounted after the needle valves to remove the impurities in the gas before it enters the evaporator. The evaporator is placed in a water bath and the CVD reactor is placed in a furnace to control their temperatures. They are made of quartz glass to resist the high temperature used. Vacuum is supplied to increase the volatility of the precursor. A cold trap made of stainless steel is placed inside a water bath to condense the unreacted precursor and to protect the vacuum pump. All the containers are sealed by using O-ring fittings clamped between the grooved flanges of the body and the cover. The complete apparatus is located within a fume cupboard. The liquid TTIP was stored in the glass evaporator. Nitrogen was used as the carrier gas, which was flowing through the evaporator to bring the precursor to the alumina support located inside the reactor. A calcination temperature of 650 °C was used by Versteeg et al.22 to decompose TTIP deposited on sapphire to TiO2. There are two approaches to synthesizing the heterogeneous titanium dioxide supported on alumina during the deposition and calcination steps. One is to combine the two steps into one, i.e., simultaneous deposition and calcination at 600 °C for 12 h. The other method is to deposit TTIP onto alumina first at 25 °C for 12 h and then calcine the particles at 600 °C for another 3 h to decompose TTIP to titanium dioxide. In both processes, the evaporation temperature of TTIP was kept at 80 °C and the carrier gas (nitrogen) flow rate was 6.6 mL/s. Vacuum (3 mmHg) was employed during the deposition process. After the CVD experiment, the final particle size remains 1 mm. Powder X-ray diffraction (XRD) analysis by a Philips PW1825 diffractometer using Cu KR radiation (wavelength 1.540562 Å) was used to identify the crystal structure of TiO2 supported on alumina and the relative amount of TiO2. The TiO2 and copper catalyst diffraction analyses were performed over angular ranges of 2θ ) 24°-28° and 25°-75°, respectively. All of the samples were scanned at a speed of 0.01°/s and with a step of 0.01°; the equipment was operated at 40 kV and 50 mA. The amount of TiO2 loading on the support was measured using an Optima 3000XL inductively coupled plasma (ICP) spectroscope. The catalysts were soaked in 65% HNO3 for 24 h to dissolve the metal oxide into
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Figure 2. TPD-Mass spectra of liquid TTIP.
Figure 3. TPD-Mass spectra of adsorbed TTIP on alumina.
the acid. The titanium concentration of the solution was determined by ICP. The effectiveness of the developed TiO2/alumina catalyst was evaluated by the photodegradation of phenol solution. The experiment was performed in a 2 L glass reactor, with a ultraviolet (UV) lamp inserted. The wavelength and intensity of the lamp were 365 nm and 1.2 mW/cm2, respectively. Initially, 1.5 L of 100 ppm phenol solution, together with 5 g of TiO2/alumina catalyst, were introduced into the reactor and the solution pH value was adjusted to between 3.00 and 3.05 by adding sulfuric acid. The reaction temperature was maintained at 30 °C. Oxygen and air were circulated from the bottom of the reactor through the solution at flow rates of 1 and 30 L/min, respectively. When the temperature reached the desired value and the system was stable, the UV-lamp was turned on and the reaction was assumed to start. This was time t ) 0. The oxygen acted as the oxidant and the air acted to stir and mix the solution. Liquid samples were taken from the reactor every 15 min and analyzed for COD and TOC. The reaction took approximately 150 min, and 11 samples were collected for each experiment. The UV lamp was switched off and the gases were turned off when the experiment was finished. The treated wastewater was discharged, and the reactor was cleaned by filling and rinsing it with deionized water three times. Degussa P25 titanium dioxide (anatase) powder was also used as a photocatalyst for comparison with the heterogeneous titanium dioxide catalyst prepared by MOCVD. The amount of P25 titanium dioxide powder added was 0.5 g for each experiment. The other experimental conditions were the same as above.
hydrogen was used as the carrier gas. Signals at m/e ) 16, 29, 30 and 60, which correspond to O*, C2H5*, C2H6, and C3H8O*, could not be detected. Figure 3 shows the TPD-MS spectra of TTIP adsorbed on alumina at 25 °C for 12 h under vacuum. Observations similar to liquid TTIP are found, but the intensity is lower than that of liquid TTIP because the adsorbed TTIP concentration is small compared with the pure liquid TTIP. The TPD-MS results demonstrate that TTIP decomposition is complete when the temperature reaches 300 °C. After calcination at high temperature (>300 °C), the organic ligand is removed and the catalytic active component, TiO2, is formed. A simplified reaction mechanism could be used to describe the sequential and surface reaction steps:23
Results and Discussion The TPD-MS spectra of liquid TTIP is shown in Figure 2. It can be seen that the major decomposition happens at about 230 °C. The desorption products between 10 < m/e < 50 were monitored by a quadrupole mass spectrometer with computer-controlled mass scanning and signal acquisition. Peaks at m/e ) 18, 28, 41, 42, 43, and 44 were observed at about 220-240 °C, corresponding to water, nitrogen, and the major cracking fragments of sec-propenyl (C3H5*), propylene (C3H6), sec-propyl (C3H7*), and propane (C3H8), respectively. The data for m/e ) 1 and 2 were not collected because
TTIP(g) + M(g) f I(g)
(1)
I(g) + S f I(a)
(2)
I(a) f P(g) + TiO2(s)
(3)
In eq 1, gas-phase TTIP is activated by a two-body collisional process to form a more reactive intermediate species (I). The collision partner, M, can be either a N2 carrier or a second TTIP molecule. The intermediate diffuses to the substrate and is adsorbed at a vacant surface site (S). The remaining alkoxide groups in the adsorbed intermediate then decompose to form gasphase products (P), and a new TiO2 unit is left on the surface. XRD analysis was done to identify the crystal structure of TiO2 supported on alumina and the relative amount of TiO2. The results for the two deposition and calcination methods are shown in Figure 4. In the figure, cvd1 refers to the TiO2 catalyst prepared by simultaneous deposition and calcination at 600 °C for 12 h. In addition, cvd2 is the catalyst prepared by two steps; i.e., the TTIP was first adsorbed onto alumina at 25 °C for 12 h and then calcined at 600 °C for 3 h. In both cases, the evaporation of the TTIP precursor was set at 80 °C, and the carrier gas flow rate was 6.6 mL/ s. From the XRD results it can be surmised that the TiO2 catalyst obtained by CVD has a crystal structure of anatase, which has a better catalytic efficiency in the photooxidation of organic compounds than rutile TiO2.
3384 Ind. Eng. Chem. Res., Vol. 38, No. 9, 1999
Figure 4. XRD of alumina and catalysts. cvd1: catalyst prepared with simultaneous deposition and calcination; cvd2: catalyst prepared with calcination after deposition.
Figure 6. Photocatalytic degradation of phenol solution in the presence of three catalysts. P-25: Degussa commercial TiO2; cvd1: TiO2/alumina prepared by simultaneous deposition and calcination; cvd2: TiO2/alumina prepared by two steps of deposition and calcination.
Figure 5. XRD of alumina and catalysts. cvd1: FN2 ) 6.6 mL/s; cvd3: FN2 ) 14.3 mL/s; cvd4: FN2 ) 1.5 mL/s Table 2. TiO2 Loading on the Alumina Support catalyst
cvd1
cvd2
cvd3
cvd4
TiO2 loading (wt %)
10.2
1.3
3.2
0.03
The simultaneous deposition and calcination method produces much more of the anatase TiO2 catalyst deposited on alumina. The flow rate of carrier gas also plays an important role in the TiO2 amount deposited on alumina. Figure 5 shows the XRD results of the three developed catalysts using nitrogen flow rates of 14.3, 6.6, and 1.5 mL/s, respectively. Simultaneous deposition and calcination was applied here since it is a better way to prepare TiO2 catalyst. The TiO2 loading on the alumina support was measured by ICP, and the results are shown in Table 2. A flow rate of carrier gas at 6.6 mL/s promotes the most deposition among the three flow rates. The amount of TiO2 deposited on alumina decreases if the flow rate is too high or too low. This may be explained as below. The vapor pressure of TTIP is a function of both temperature and flow rate and depends on the evaporation rate of the precursor. For a given temperature, the vapor pressure of precursor increases to a maximum and then decreases as the flow rate of carrier gas increases. If the flow rate of carrier gas is too low, the
evaporation will be slow so the vapor pressure of TTIP is low. However, if the flow rate of carrier gas is too high, the evaporation rate of precursor cannot catch up to the rate of the carrier gas. A carrier gas flow rate of 6.6 mL/s generates a maximum precursor concentration in the vapor stream. More experiments around the flow rate of 6.6 mL/s should be carried out to further identify the best flow rate of carrier gas. The information on the vapor pressure of TTIP within the system is quite important as it affects the amount of TiO2 deposited onto the support, but unfortunately, it is very difficult to do this measurement under vacuum and with the carrier gas flowing through. In addition, an experimental phenomenon was found that vacuum deposition was much better than atmospheric deposition according to the XRD results. This is because the volatility of liquid TTIP is increased under vacuum. The BET surface area of the developed photocatalyst using a nitrogen flow rate of 6.6 mL/s and simultaneous deposition and calcination is 165 m2/g. Compared with the surface area of the original alumina (188 m2/g), the reduction of surface area after CVD is quite small (12%), which means that the TiO2 coated onto the alumina support is a thin layer. The surface area of the developed TiO2/alumina catalyst is much higher than that of the commercial Degussa P-25 TiO2 powder, which was typically in the range of 40-60 m2/g. To evaluate the photocatalytic activity of supported TiO2 on alumina, the photocatalytic degradation of phenol was examined. Three photocatalysts: Degussa P25, cvd1, and cvd2, were tested to compare their catalytic efficiency. The results are shown in Figure 6. The reductions in the TOC and COD are used to measure the extent of phenol mineralization (conversion
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to carbon dioxide and water). After a reaction of 2 h, the TOC removal rates using catalysts of P-25, cvd1, and cvd2 are 66.3%, 57.4%, and 49.3%, respectively. The catalyst cvd1 is better than cvd2 in terms of both COD and TOC removals. This is consistent with the XRD result. Because cvd1 has a higher TiO2 loading, its photocatalytic activity is better. The catalytic efficiencies of cvd1 and cvd2 are not as good as but are close to that of P25. The commercial Degussa P-25 is the best catalyst; however, it should be noted that P-25 is a commercially available material and has been developed and improved over several years. The preparation conditions of supported TiO2/alumina by MOCVD, however, have to be further optimized. The catalytic efficiency of TiO2/alumina will be improved as the preparation conditions are optimized. Furthermore, the alumina-supported TiO2 catalyst has an industrial advantage in that it is in the granular form, so the catalyst can be easily recovered by filtration from the treated wastewater and recycled. Conclusions MOCVD is a simple and effective method to prepare heterogeneous catalyst supported on porous solid. The amount of active phase deposited onto the support is affected by the flow rate of carrier gas. Simultaneous deposition and calcination is a better method in preparing TiO2 supported on alumina. In comparison with the commercial titanium dioxide powder P-25, the titanium dioxide supported on alumina prepared by the MOCVD method, although it has less TiO2 amount, can obtain nearly the same catalytic efficiency. Alumina-supported TiO2 catalyst has an industrial advantage in that it can easily be separated from the treated water by filtration and so recovered. Acknowledgment This work was supported by the Research Grants Council, and the Industry and Technology Development Council of Hong Kong. Literature Cited (1) Matthews, R. W. Photooxidation of Organic Impurities in Water Using Thin Films of Titanium Oxide. J. Phys. Chem. 1987, 91, 3328-3333. (2) Zhang, Y.; Crittenden, J. C.; Hand, D. W.; Perram, D. L. Fixed-Bed Photocatalysts for Solar Decontamination. Environ. Sci. Technol. 1994, 28, 435-442. (3) Bellobono, I. R.; Carrara, A.; Barni, B.; Gazzotti, A. Laboratory and Pilot Plant Scale Photodegradation of Chloroaliphatics in Aqueous Solutions by Photocatalytic Membranes Immobilizing Titanium Dioxide. J. Photochem. Photobiol. A: Chem. 1994, 84, 83-90. (4) Dagan, G.; Tomkiewicz, M. TiO2 Aerogels for Photocatalytic Decontamination of Aquatic Environments. J. Phys. Chem. 1993, 97, 12651-12655. (5) Tomkiewicz, M.; Dagan, G.; Zhu, Z. Morphology and Photocatalytic Activity of TiO2 Aerogels. Res. Chem. Intermed. 1994, 20, 701-710. (6) Dossi, C.; Schaefer, J.; Sachtler, W. M. H. Mechanism of Particle Formation in Decomposing Re2(CO)10 on NaY and NaHY
Zeolites: Effect of Prereduced Pt Clusters in the Supercages. J. Mol. Catal. 1989, 52, 193-209. (7) Yoo, J. S.; Donohue, J. A.; Kleefisch, M. S.; Lin, P. S.; Elfline, S. D. Gas-phase Oxygen Oxidations of Alkylaromatics over Chemical Vapor Deposited Fe/Mo/Borosilicate Molecular Sieve, Fe/Mo/ DBH. I. The Selective Synthesis of Terephthaldehyde from paraXylene. Appl. Catal., A 1993, 105, 83-105. (8) Yoo, J. S.; Lin, P. S.; Elfline, D. Gas-phase Oxygen Oxidation of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve. II. The Role of Carbon Dioxide as a Co-oxidant. Appl. Catal. A 1993, 106, 259-273. (9) Yoo, J. S.; Donohue, J. A.; Kleefisch, M. S. Gas-phase Oxygen Oxidation of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve Fe/Mo/DBH. III. Selective Oxidation of p-Xylene from Isomer Mixture Containing Ethylbenzene. Appl. Catal. A 1994, 110, 75-86. (10) Yoo, J. S.; Chin, C. F.; Donohue, J. A. Gas-phase Oxygen Oxidation of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve Fe/Mo/DBH. IV. Effect of Supporting Matrix on pXylene Oxidation. Appl. Catal. A 1994, 118, 87-101. (11) Yoo, J. S.; Sohail, A. R.; Grimmer, S. S.; Chin, C. F. OneStep Hydroxylation of Benzene to Phenol. II. Gas-Phase N2O Oxidation over Mo/Fe/Borosilicate Molecular Sieve. Catalysis Lett. 1994, 29, 299-310. (12) Yoo, J. S. Gas-phase Oxygen Oxidation of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve Fe/Mo/DBH. VII. Oxidative Dehydrogenation of Alkylaromatics. Appl. Catal. A 1996, 142, 19-29. (13) Yoo, J. S. The CVD Fe/Mo/DBH (Deboronated Borosilicate Molecular Sieve)-Catalyzed Oxidation Reactions. Appl. Catal. A 1996, 143, 29-51. (14) Zajac, G. W.; Chin, C. F.; Faber, J.; Yoo, J. S.; Patel, R.; Hochst, H. Characterization and Oxidation Catalysis of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve: Fe/Mo/DBH. J. Catal. 1995, 151, 338-348. (15) Bhaskaran, V.; Hampden-Smith, M. J.; Kodas, T. T. Palladium Thin Films Grown by CVD from (1,1,1,5,5,5-hexafluoro2,4-pentanedionato) Palladium(II). Chem. Vapor Deposition, 1998, 4, 51-59. (16) Jain, S.; Kodas, T. T. and Hampden-Smith, M. J. Synthesis of ZnO Thin Films by Metal-organic CVD of Zn(CH3COO)2. Chem. Vapor Deposit. 1998, 4, 51-59. (17) Serp, Ph.; Figueiredo, J. L. A Microstructural Investigation of Vapor-Grown Carbon Fibers. Carbon 1996, 34, 1452-1454. (18) Serp, Ph.; Figueiredo, J. L. An Investigation of VaporGrown Carbon Fiber Behavior towards Air Oxidation. Carbon 1997, 35, 675-683. (19) Ngo, T.; Brandt, L.; Williams, R. S.; Kaesz, H. D. Scanning Tunneling Microscopy Study of Platinum Deposited on Graphite by Metalorganic Chemical Vapor Deposition. Surf. Sci. 1993, 291, 411-417. (20) Chu, H. P.; Lei, L.; Hu, X.; Yue, P. L. Metallo-Organic Chemical Vapor Deposition (MOCVD) for the Development of Heterogeneous Catalysts. Energy Fuels 1998, 12, 1108-1113. (21) Lei, L.; Hu, X.; Chu, H. P.; Chen, G.; Yue, P. L. Catalytic Wet Air Oxidation of Dyeing and Printing Wastewater. Water Sci. Technol. 1997, 35, 311-319. (22) Versteeg, V. A.; Avedisian, C. T.; Raj, R. Metalorganic Chemical Vapor Deposition by Pulsed Liquid Injection Using an Ultrasonic Nozzle: Titanium Dioxide on Sapphire from Titanium Isopropoxide. J. Am. Ceram. Soc. 1995, 78, 2763-2768. (23) Zhang, Q.; Griffin, G. L. Gas-Phase Kinetics for TiO2 CVD: Hot-Wall Reactor Results. Thin Solid Films 1995, 263, 6571.
Received for review October 27, 1998 Revised manuscript received June 6, 1999 Accepted June 6, 1999 IE980677J
