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Role of Platinum Deposited on TiO2 in Phenol Photocatalytic Oxidation Bo Sun,† Alexandre V. Vorontsov,‡ and Panagiotis G. Smirniotis*,† Chemical Engineering Department, University of Cincinnati, Cincinnati, Ohio 45221-0171, and Boreskov Institute of Catalysis, Novosibirsk 630090, Russia Received August 24, 2002. In Final Form: December 18, 2002 Photooxidation of phenol has been studied in aqueous suspensions of titanium dioxide Hombikat UV100 and Degussa P25 loaded with various amounts of Pt. The rates of phenol decomposition and total carbon removal rose by a maximum factor of 1.5 when Hombikat was loaded with 1 wt % Pt; further increase of Pt deposition did not increase the photoactivity any more. The electron movement across the Pt-anatase junction is discussed, and the positive influence of Pt on phenol oxidation over Hombikat is explained by the increase of charge separation in agglomerates of primary particles. The photocatalytic activity of P25 was higher than that of platinized and pure Hombikat. But loading P25 with Pt resulted in a decrease of phenol decomposition and total carbon removal rates. The decrease of Degussa P25 activity after platinization means that Pt cannot further increase the efficiency of charge separation in this TiO2, whose two-phase composition already provides a very efficient suppression of recombination in liquid photocatalytic reaction. The charge separation in P25 is illustrated with the scheme of P25’s band structure. The possible side effects brought by Pt deposition are also discussed.
Introduction Photocatalytic decomposition of organic contaminants is a promising solution in wastewater treatment and polluted air purification. Semiconductor photocatalysis with a primary focus on TiO2 as a durable catalyst has been applied to the photodestruction of hazardous chemical wastes.1 The overall quantum efficiency is expected to be decided by the competition between charge-carrier recombination, trapping, and interfacial charge transfer.2 Oxidant reduction by electrons (milliseconds) is much slower than the oxidation of reductants by holes (100 ns) in photocatalysis with TiO2.1 So the increase of the rate of electron transfer to the oxidant can increase the quantum yield. Combining noble metal(s) with a semiconductor seems to provide one way for increasing the semiconductor photoefficiency.3,4 Many papers have investigated the role played by metals impregnated on titania.5-10 Ohtani et al. found that the photocatalytic dehydrogenation of 2-propanol and transformations of (S)lysine in deaerated aqueous suspensions of P25, which were negligible without Pt loading, increased drastically with the loading up to ca. 0.3 wt % and remained almost constant with a further increase in Pt loadings.5 Chen et * To whom correspondence should be addressed. Tel: 1-513556-1474. Fax: 1-513-556-3473. E-mail: Panagiotis.Smirniotis@ uc.edu. † University of Cincinnati. ‡ Boreskov Institute of Catalysis. (1) Hoffman, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (2) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (3) Scalfani, A.; Mozzanega, M. N.; Herrmann, J. M. J. Catal. 1997, 168, 117. (4) Gerischer, H.; Heller, A. J. Electrochem. Soc. 1992, 139, 113. (5) Ohtani, B.; Iwai, K.; Nishimoto, S. I.; Sato, S. J. Phys. Chem. B 1997, 101, 3349. (6) Dobosz, A.; Sobczynski, A. Monatsh. Chem. 2001, 132, 1037. (7) Chen, J.; Ollis, D. F.; Rulkens, W. H.; Bruning, H. Water Res. 1999, 33, 661. (8) Pichat, P.; Mozzanega, M. N.; Disdier, J.; Herrmamm, J. M. Nouv. J. Chim. 1982, 6, 559. (9) Li, Y. X.; Lu, G. X.; Li, S. B. Appl. Catal., A 2001, 214, 179. (10) Crittenden, J. C.; Liu, J. B.; Hand, D. W.; Perram, D. L. Water Res. 1997, 31, 429.
al. observed an increased rate of methanol and ethanol photocatalytic oxidation on metallized TiO2 P25 compared to the pure one, but this platinized titania was less active in degradation of chloroform.7 Pichat et al. found that the highest initial rate of H2 production from primary aliphatic alchohols (CH3OH, C2H5-CH2OH) over Pt/P25 is for a Pt content of 0.1-1 wt %, while a higher Pt content (1-10 wt %) decreased the activity.8 Crittenden et al. observed that the photoactivity of Aldrich TiO2 can be greatly increased by surface modification with 1 wt % Pt or Ag, while that of Degussa P25 is decreased by 0.5-2.0 wt % platinum impregnation.10 Therefore, it seems that there are many contradictory trends for the role played by the metal in M-TiO2 catalysts. Since the metal influence is not clear for TiO2 photocatalysis, the role of the Pt-TiO2 interface for photocatalytic oxidation needs further investigation. In the current study, we chose two very active and morphologically different commercial photocatalysts, Degussa P25 (P25) and Hombikat UV100 (HK), to study the influence of platinum. Photocatalytic oxidation of phenol, an environmentally important and nonbiodegradable pollutant, was studied over the platinized TiO2. Experimental Section Catalyst Syntheses. The catalysts utilized in the present study were made from P25 and HK. They included 0.0, 0.5, 1.0, 1.5, and 3.0 wt % Pt-deposited HK and 0.0, 0.1, 0.25, 0.5, and 1.5 wt % Pt-deposited P25. The Pt loading was performed by the following procedure. First, 0.5 g of P25 or HK powder was mixed with an appropriate volume of 0.25 wt % of Pt solution of H2PtCl6,5,11 and distilled water was added to make the whole suspension volume be 8 mL for adequate mixing; for the 0%Pt/ P25 or 0%Pt/HK, only 8 mL of distilled water was used to mix with 0.5 g of P25 or HK. The mixture was stirred slowly for 20 min. Then, the slurry was heated at 70 °C while stirring until it was dry and became powder. After that, the powder was put into a quartz reactor and was oxidized with oxygen (Wright Brothers, 99.5%) for 1 h and reduced with hydrogen (Matheson, Ultra High Pure) for 1 h at the temperature of 380 °C. (11) Courbon, H.; Herrmann, J. M.; Pichat, P. J. Phy. Chem. 1982, 88, 5210.
10.1021/la0264670 CCC: $25.00 © 2003 American Chemical Society Published on Web 03/05/2003
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Figure 2. BET surface areas of the catalysts.
Figure 1. Schematic drawing of the photocatalytic reactor. Catalyst Characterizations. The Brunauer-EmmettTeller (BET) surface area of the catalysts was measured on a Gemini instrument (Micromeritics) at 77 K with nitrogen. The platinum surface areas of the samples were measured using pulse H2 chemisorption on an Autochem 2910 instrument (Micromeritics) at 50 °C. Prior to the experiment, the catalysts were pretreated with pure H2 at 370 °C for 2 h. Structural investigations of some samples were done using high-resolution transmission electron microscopy (TEM). The Pt and titania particles can be clearly seen from TEM pictures. These experiments were performed on a Philips CM30ST microscope (LaB6 cathode, operated at 300 kV, point resolution of ca. 0.2 nm). For the TEM investigation, ground material was deposited onto a perforated carbon foil supported on a copper grid after each sample was dissolved in ethanol. Photocatalytic Experiments. The suspension for reaction was prepared by dispersing ultrasonically the mixture of 0.200 g of each catalyst, 0.100 g of phenol, and 800 mL of distilled water for half an hour in an ultrasonic bath (Elma, Ultrasonic LC20H) and stirring the suspension for half an hour afterward. Measurements showed that phenol was not removed from suspension during this procedure. The photocatalytic oxidation of phenol (reagent grade, Fisher) in catalyst suspensions was carried out in an annular quartz liquid-phase photocatalytic reactor (Ace Glass, Inc., no. 7840, Figure 1). The immersion-type UV radiation source was a 200 W medium-pressure mercury vapor quartz lamp (Jelight, J05PM1HGC1). A Pyrex filter (Ace Glass, no. 7740) filtered out the far- and mid-UV bands (λ < 320 nm) of the lamp emission spectrum. The wavelength limit of the light for electron-hole creation is 387 nm because the band gap of anatase is about 3.2 eV. Thus, the spectrum of interest with a high peak at the wavelength of 365 nm remained for reaction. The suspension was stirred magnetically, and the reactor was put in a water bath, which maintained the suspension temperature at 12.5 ( 0.5 °C. The lamp was preheated for 5 min before each experiment for obviating the poor irradiation of the lamp in the first several minutes. Oxygen (500 mL/min)12 (Wright Brothers, 99.5%) was
sparged into the solution from a gas distributor near the bottom of the reactor. The pH of the reaction suspension was not adjusted.13 The samples of reaction suspension were taken with a syringe at different intervals and filtered with Cameo 25P polypropylene syringe filters (OSMONICS, DDP02T2550). The sample solutions were analyzed with a GC (HP 6890) and a total organic carbon analyzer (TOC-VCSH, Shimadzu). Several samples were also analyzed on a GC-MS (Shimadzu QP5050A) to identify the stable intermediate products. To detect semivolatiles, derivatization with a trimethylsilyl reagent was used that transformed polar compounds into their volatile trimethylsilyl derivatives. The local light intensities at different locations along the outer wall of the reactor with and without the suspension were measured for every catalyst with a detector (International Light, Inc.; model SED033 no. 3435) connected to a radiometer (International Light, Inc.; model IL 1700). The difference between the photon fluxes detected with and without suspension at the outer wall of the reactor vessel was used to calculate quantum efficiency.
Results and Discussion BET Surface Area of the Samples. Figure 2 shows that the BET surface area of the platinized Hombikat decreases with increasing deposited platinum amount. The sample 0%Pt/HK has the highest surface area (196 m2/g). The BET surface area of the starting commercial Hombikat UV100 is 334 m2/g. So there must be agglomeration and clustering during the preparation of the Pt/HK. It is interesting to note that the increase in H2PtCl6 content caused stronger agglomeration. The pH of the 0.25 wt % Pt solution of H2PtCl6 is 3.52, and that of the pure water pH is 6.94. The difference in pHs during HK’s mixing with Pt precursors and suspension drying causes the different extent of hydrolyzation, and then aggregation and agglomeration.14 In contrast to Hombikat UV100, the BET surface area of platinized Degussa P25 (Figure 2) does not change much after platinum deposition and is equal to 50 ( 1.4 m2/g. This TiO2 is synthesized by hightemperature flame pyrolysis. Obviously, the energy barrier between large particles is high, which makes the particles resistant to agglomeration during the preparation.15 Pt Surface Area and Particle Size. When the platinum surface area was measured, there was no H2 adsorption observed on pure Hombikat or Degussa P25, (12) Davydov, L.; Pratsinis, S. E.; Smirniotis, P. G. Environ. Sci. Technol. 2000, 34, 3435. (13) Augugliaro, V.; Palmisano, L.; Sclafani, A. Toxicol. Environ. Chem. 1988, 16, 89. (14) Pierre, A. C. Introduction to Sol-Gel Processing; Kluwer: Boston, 1998; P183-186 and P229-232. (15) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990; Chapter 4.
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Figure 3. Platinum surface areas of the catalysts.
indicating that the adsorption was caused only by the platinum loaded on the samples. Figure 3 demonstrates the surface area of platinum on both Pt/HK and Pt/P25 catalysts as a function of Pt content. The Pt surface area of Pt/HK increases linearly with the increase of the platinum amount loading and reaches a value of about 1.6 m2/g at the maximum platinum content of 3 wt %. This means that the metal surface area constitutes less than 2% of the total sample surface area, so the decrease of the catalyst BET surface area due to the Pt sites covering the support is negligible. The Pt surface area of Pt/P25 also increases linearly with the increase of the platinum amount loading and reaches 1.7 m2/g at the maximum platinum content of 1.5 wt %. Figure 4a,b shows high-resolution TEM images of 3%Pt/ HK and 1%Pt/P25. The diameter of Pt particles in 3%Pt/ HK is about 1 nm. The scanning transmission electron microscopy (STEM) images (not shown) of 3%Pt/HK reveal platinum particles with a diameter of 0.4-0.8 nm. The size of most Pt particles in the 1%Pt/P25 sample is also about 1 nm. The STEM images provide a Pt particle diameter of 0.7-0.9 nm for this catalyst. The TEM images show that TiO2 particles are in good contact with platinum particles with several Pt particles residing on each TiO2 particle. Photocatalytic Oxidation of Phenol by Pt/HK. The photocatalytic oxidation of phenol in the present study was followed by measuring the phenol concentration. Detected intermediate products of phenol oxidation are shown in Table 1. The overall mineralization process was traced using measurements of the total carbon (TC) concentration in solution. Figure 5a,b shows phenol concentration profiles and TC profiles over the series of Pt/HK catalysts. Several runs were done to double-check the degradation curves. The curves of phenol concentration are relatively well fitted by a first-order decay kinetics. The corresponding rate coefficients and quantum efficiencies are listed in Table 2. The TC degradation plots are almost linear for all the samples, and the rate of TC removal and the corresponding quantum efficiency are contained in Table 2. One can see that the lowest rates and quantum efficiencies indexed by both phenol and TC are observed over the sample 0%Pt/HK, and the highest are observed over the 1%Pt/HK sample; Pt deposited can increase the photoreactivity of HK in the range of 0-1 wt % Pt although BET surface area decreases. However, the photodestruction efficiencies of the samples with Pt loading above 1% are not much different from that of the latter sample.
Figure 4. (a) High-resolution TEM of the 3%Pt/HK sample; (b) TEM of the 1%Pt/P25 sample.
Because an absorption length of the semiconductor materials is at least 100 nm,16 which is much larger than the diameter of the HK particles, light traverses the particles and produces electron-hole pairs throughout the particles. The number of electron-hole pairs created is limited by the flux of photons with energy that is high enough and the amount of photons absorbed by the particles. The created electrons and holes in the colloidal TiO2 go from the interior to the interphase and then undergo an interfacial electron transfer14 to create the active oxygen species including hydroxyl radicals.17,18 An equilibrium between carriers’ recombination, trapping, and creation is reached inside unmodified anatase particles in a specific photochemical system. The difference between electron and hole densities, which is caused by the quicker oxidation of reductants by holes than oxidant reduction by electrons,1 provides the potential gradient according to Poisson’s equation19 as well as the carrier concentration gradient. These two (16) Gratzel, M.; Frank, A. J. J. Phys. Chem. 1982, 86, 2964. (17) Okamato, K. I.; Yamamoto, Y.; Tanaka, H.; Itaya, A. Bull. Chem. Soc. Jpn. 1985, 58, 2023. (18) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (19) Warner, R. M.; Grung, B. L. Transistors: Fundamentals for the Integrated-Circuit Engineer; John Wiley & Sons: New York, 1983.
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Table 1. Products Detected in Photocatalytic Oxidation of Phenol over TiO2 P25
gradients are the drives for the carriers to drift and diffuse, which can be seen from hole and electron transport equations.19 The Pt particles deposited on anatase particle surfaces change the original equilibrium by drawing the electrons out of the bulk TiO2 through the Pt-TiO2 contact. The amount of electrons passing through the contact increases at higher Pt-TiO2 contact area (Figure 6), which should be proportional to the Pt surface area; the electron density in TiO2 particles decreases, and the recombination rate decreases because the rate depends on the carriers’ densities in TiO2 particles. However, at the same time, the electrical potential gradient becomes lower and the rate of electron diffusion decreases due to the decreasing of the electron density gradient. When the two gradients are too small to increase the electron flux through the Pt-TiO2 contact further, a new equilibrium is reached and further platinum deposition cannot increase the charge separation further; then the decomposition rate stops growing. What is more, the electron transfer from anatase particles into Pt particles can deform the potential field in anatase particles and draw a part of holes near the Pt-TiO2 junction, which can increase the electronhole recombination rate. The increasing of Pt-TiO2 contact area can bring more probability for the recombination and reduces the overall photoactivity. Pt deposition can also decrease photocatalytic activity by reducing the light exposed to HK particles. Therefore, there should be an optimal Pt amount. Photocatalytic Oxidation of Phenol by Pt/P25. The behavior of Pt/P25 catalysts was different from the behavior of Pt/HK in photocatalytic degradation of phenol. Figure 7a,b shows phenol concentration profiles and TC profiles over the series of Pt/P25 samples, and the corresponding kinetic reaction parameters and quantum efficiencies are listed in Table 2. One can see that the highest phenol degradation and TC removal rates are observed at the vicinity of 0.1 wt % Pt loadings. Other samples showed longer degradation times. The observation conveys that platinum should not necessarily be present for photocatalytic phenol oxidation with Degussa P25. Degussa P25 consists of anatase and rutile phases,20 and good interparticle contacts are formed between
Figure 5. Time course of the degradation of (a) phenol and (b) total carbon with the Pt/HK catalysts. Catalyst concentration, 0.25 g/L; pH, not adjusted; reaction temperature, 12.5 ( 0.5 °C.
anatase and rutile particles in water.21 Band bending22 happens in both anatase and rutile through Fermi level lineup (Figure 8) when they contact each other. Their Fermi levels are about at the middle of their respective band gap because they are undoped.22,23 The band gaps of anatase and rutile are 3.2 and 3.0 eV, respectively.24,25 The positions of their valence bands mainly consisting of O2p orbitals are situated at 3.0 eV (vs standard hydrogen electrode (SHE)).25,26 The average static dielectric constant (20) Datye, A. K.; Riegel, G.; Bolton, J. R.; Huang, M.; Prairie, M. R. J. Solid State Chem. 1995, 115, 236. (21) Ohno, T.; Jarukawa, K.; Tokieda, K.; Matsumura, M. J. Catal. 2001, 203, 82. (22) Sze, S. M. Physics of Semiconductor Devices; John Wiley & Sons: New York, 1981. (23) Bickley, R. I.; Gonzalez-Carreno, T.; Lees, J. S.; Palmisano, L.; Tilley, R. J. D. J. Solid State Chem. 1991, 92, 178. (24) Sclafani, A.; Palmisano, L.; Schiavello, M. J. Phys. Chem. 1990, 94, 829. (25) Rao, M. V.; Rajeshwar, K.; Verneker, V. R. P.; Dubow, J. J. Phys. Chem. 1980, 84, 1987. (26) Kawahara, T.; Konishi, Y.; Tada, H.; Tohge, N.; Nishii, J.; Ito, S. Angew. Chem., Int. Ed. 2002, 41, 2811.
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Table 2. Reaction Rate Coefficients and Estimated Quantum Efficiency of the Catalysts total carbon removal
phenol decomposition
catalyst
rate, mg carbon/(L min)
quantum efficiencya
rate coefficient, 1/min
initial quantum efficiencyb
0.0%Pt/HK 0.5%Pt/HK 1.0%Pt/HK 1.5%Pt/HK 2.0%Pt/HK 3.0%Pt/HK 0.0%Pt/P25 0.1%Pt/P25 0.25%Pt/P25 0.5%Pt/P25 1.5%Pt/P25
0.143 ( 0.001 0.185 ( 0.004 0.214 ( 0.003 0.210 ( 0.002 0.194 ( 0.002 0.204 ( 0.003 0.333 ( 0.002 0.345 ( 0.005 0.292 ( 0.006 0.292 ( 0.010 0.232 ( 0.005
0.0068 0.0086 0.0097 0.0096 0.0087 0.0092 0.0162 0.0165 0.0141 0.0139 0.0104
0.002 37 ( 0.000 07 0.005 41 ( 0.000 16 0.009 24 ( 0.000 21 0.007 80 ( 0.000 36 0.006 75 ( 0.000 19 0.007 39 ( 0.000 23 0.017 74 ( 0.000 26 0.016 27 ( 0.000 47 0.012 77 ( 0.000 18 0.012 15 ( 0.000 29 0.010 56 ( 0.000 15
0.0012 0.0026 0.0043 0.0036 0.0031 0.0034 0.0088 0.0079 0.0063 0.0059 0.0048
a Assume one photon for each carbon atom removed. b Initial quantum efficiency is calculated with the slope at t ) 0. Assume one photon for each phenol molecule removal.
Figure 6. Schematic drawing of photocatalytic events in the Pt/HK particles.
of rutile is about 100, while that of anatase is 31.27,28 If we assume that the density of electrons created in anatase particles upon UV irradiation with a certain intensity is 1.0 × 1019/cm3,27 then the space charge layer width29 in the anatase particles is about 2 nm with the total band bending of 0.1 eV. This width changes with the electronhole density which is influenced by the light intensity in turn, but no correlation between the light intensity and electron-hole density is reported yet. One can see from Figure 8 that the conduction band energy increase in the space charge layer of anatase stops the electrons going from anatase to rutile, but the holes in anatase particles can be transferred to rutile particles through the valence band bending. These concepts can explain P25’s intrinsic charge separation and high photoactivity in liquid reaction, which should not apply for gas reaction. The structure of P25’s conduction band cannot offer any more help in the electron-hole separation than the function shown above. On the contrary, tunneling can occur when the electron density in anatase is very high. Then, the question is how much negative effect tunneling brings if it exists. The electron effective mass m* in rutile is about 20m0 and that in anatase is about 1m0 (m0 is electron rest mass);27
Figure 7. (a) Time course of the degradation of (a) phenol and (b) total carbon with the Pt/P25 catalysts. Catalyst concentration, 0.25 g/L; pH, not adjusted; reaction temperature, 12.5 ( 0.5 °C.
(27) Tang, H.; Prasad, K.; Sanjines, R.; Schmid, P. E.; Levy, F. J. Appl. Phys. 1994, 75, 2042. (28) Frederikse, H. P. R. J. Appl. Phys. 1961, 32, 2211. (29) Serpone, N.; Pelizzetti, E. Photocatalysis: Fundamentals and Applications; John Wiley & Sons: New York, 1989; P139.
then, the mobility of the electrons in rutile is about 89 times lower than that in anatase according to µ ∼ (m*)-3/2T1/2 for polar semiconductors.22 And according to the Einstein relationship D ) (kT/q)µ, the diffusivity of
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TiO2 surface can only divert some photogenerated electrons into the metal but probably cannot increase the charge separation further. Moreover, platinum can decrease the activity of TiO2 in photocatalytic oxidation because of possible water reduction on platinum particles that cannot take place on pure TiO2. Water reduction to hydrogen will consume a part of photogenerated electrons that otherwise could take part in oxygen reduction forming reactive oxygen species. The TC curves for 0%Pt/P25 and 0.1%Pt/P25 (Figure 7b) nearly overlap with each other in the first 150 min. The TC removal with 0%Pt/P25 slowed more afterward. The kinetic parameters for these two catalysts convey that Pt deposited on P25 cannot increase the phenol reaction rate but can increase the rate of intermediates decomposition. As shown in Table 1, alcohols and acids are the intermediates formed. The acceleration of photocatalytic oxidation of alcohols and acids by Pt deposition on P25 was addressed by others.7,8 Conclusions
Figure 8. Proposed mechanism of electron-hole separation in P25 during photocatalysis (the electron-hole recombination process is not shown here).
the electrons in rutile is also about 89 times smaller than that in anatase. Therefore, we get that the electron flux in rutile is about 2 orders of magnitude lower than that in anatase from the electron transport equation.22 The electrons flux going from anatase to rutile by tunneling is negligible even if it happens. Therefore, holes are concentrated in rutile and electrons are left in the anatase particles, which indicates that oxidation happens mostly on rutile and reduction mostly on anatase, that is, sites are separated. All these reduce not only electron-hole volume recombination but also surface recombination. The slow movement of the electrons in rutile provides more chances for electron-hole recombination and may be the fatal reason for rutile’s low activity. So it is speculated that carriers created in the rutile phase of P25 do not help the photoactivity a lot as do those in pure rutile, and the rutile phase in P25 only plays a role of charge separation and provides sites for oxidation. The electron-hole transfer process already reaches an equilibrium in P25 with the highest transfer rate and the lowest recombination rate possible under the conditions in our experiment. In this case, platinum particles on the
Deposition of platinum particles on HK resulted in acceleration of phenol photocatalytic oxidation when Pt content increased in the vicinity of 1 wt %; further increase of Pt content did not increase the photoactivity. The photooxidation of phenol with P25 was not accelerated by modifying P25 with Pt. The photoefficiency decreased by about 50% when platinum content increased from 0 to 1.5 wt %. Pt deposition on anatase particles can reduce the electron-hole recombination in anatase. But the charge separation played by Pt deposition is limited and Pt can also bring some bad effects. There should be an optimal Pt amount for Pt deposition on anatase. The band bending near anatase and rutile particles’ contact formed in liquid makes it possible for the holes created in anatase particles to transfer to rutile particles, but stops the electrons going from anatase to rutile. The mobility of electrons in rutile is 2 orders lower than that in anatase, which helps deter the bad effect brought by the possible electron tunneling. The good charge-separation characteristic of P25 illustrated implies that Pt deposition cannot increase its photoefficiency for phenol oxidation in water. Acknowledgment. The authors acknowledge the National Science Foundation (NSF) and the U.S. Department of Army for partial support for this work through Grants CTS-0097347 and DAAD 19-00-1-0399, respectively. We also acknowledge funding from the Ohio Board of Regents (OBR) that provided matching funds for equipment to the NSF CTS-9619392 Grant through the OBR Action Fund No. 333. LA0264670
