1418
J. Phys. Chem. B 1998, 102, 1418-1423
Photooxidation of Trichloroethylene on Pt/TiO2 M. D. Driessen and V. H. Grassian* Department of Chemistry, UniVersity of Iowa, Iowa City, Iowa 52242 ReceiVed: July 24, 1997; In Final Form: October 14, 1997
The gas-phase photooxidation of trichloroethylene (TCE) on Pt/TiO2 has been investigated. The wavelength dependence of this reaction on Pt/TiO2 shows that the addition of small amounts of platinum (e2%) to TiO2 can enhance the photooxidation of TCE to much longer wavelengths. This result correlates with MaxwellGarnett calculations and experimental absorption curves for Pt/TiO2 samples. Although the spectral response of Pt/TiO2 extends to longer wavelengths, Pt/TiO2 is found to be less effective in the photooxidation of gas-phase TCE upon broad-band irradiation (λ > 300 nm) compared to TiO2. It is proposed that site blocking by the Pt particles of the most active sites, Ti3+ sites, is the cause of the decreased photoactivity. In addition, it has also been determined that the photoproduct distribution changes as a function of platinum loading. Mechanisms to explain the observed effects of Pt loading on the photooxidation of TCE on Pt/TiO2 are discussed.
Introduction The use of semiconductors as photocatalysts for environmental remediation has become increasingly important. The addition of metal particles to semiconductor photocatalysts in order to increase the light-absorbing efficiency and photocatalytic activity of these systems is of great interest.1 In fact, metallized semiconductors have been utilized as solar-absorbing coatings and as photocatalysts in the remediation of groundwater contaminants.1,2 There have been several photocatalytic studies of platinized titanium dioxide. In solution, Pt/TiO2 is found to increase the photodecomposition rate of several organic compounds compared to TiO2.3-7 In contrast to an enhancement in photoactivity, a recent study by Linsebigler et al. showed that the addition of platinum particles to a TiO2 single-crystal surface actually decreased both the rate and total yield of the photooxidation of CO.8 In this study, FT-IR and UV/vis spectroscopy were used to investigate the photooxidation of trichloroethylene (TCE) on Pt/TiO2 photocatalyst. TCE is the most common halogenated contaminant found in groundwater supplies.9 Therefore, a great deal of effort has been put forth in developing methods to degrade or transform this contaminant into more environmentally benign compounds.10-20 Several approaches have been taken; some of the more promising approaches involve the aqueous and gas-phase photooxidation of TCE using semiconductor photocatalysts such as TiO2.10-20 These catalysts are activated by absorption of light with wavelengths contained in the solar spectrum. The optical properties of composite systems, such as metallized semiconductor catalysts, can be simulated using MaxwellGarnett theory in the UV and visible regions of the electromagnetic spectrum. Here we have simulated and measured the optical properties of Pt/TiO2 as a function of platinum loading. Both the simulated and experimental absorption curves of Pt/ TiO2 show an extension of the absorption tail to much longer wavelengths than pure TiO2. Although it was anticipated that * To whom correspondence should be addressed.
an increase in spectral response may be beneficial in photocatalysis because an extended region of the solar spectrum could be absorbed by the catalyst, it was determined that the addition of platinum to TiO2 actually decreased the rate and yield of gas-phase TCE photooxidation upon broad-band irradiation, in agreement with the recent work by Linsebigler et al.8 A mechanism to explain these contrasting results is proposed. Experimental Section The IR cell used in these experiments has been described previously.27-29 The cell consists of a 23/4 in. stainless steel cube with two differentially pumped barium fluoride windows and a sample holder through which thermocouple and power feedthroughs are connected to a photoetched tungsten sample grid. The sample holder design is such that the sample may be cooled to near liquid nitrogen temperatures and heated resistively up to 1300 K. The temperature is monitored using a thermocouple wire spot-welded to the top of the sample grid. The cell is attached to an all stainless steel vacuum chamber through a 2 ft bellows hose. The vacuum system is pumped by an 80 L/s ion pump after being rough pumped with a turbomolecular pump. Platinized titanium dioxide samples were made using hexachloroplatinic acid (Johnson-Matthey, 99.9%) as the platinum precursor and TiO2 (Degussa, P-25). A slurry of the platinum salt and TiO2 in acetone and water was sprayed onto one-half of a photoetched tungsten grid until the desired sample weight was obtained (∼60 mg). The sample was then placed in the infrared sample cell and evacuated for several hours at 673 K. The sample was then oxidized to remove carbon-containing impurities by introducing 1 Torr of O2 while the sample was maintained at 473 K for 30 min. The sample cell was then evacuated for 15 min, and the platinum salt was reduced using hydrogen. Approximately 400 Torr of hydrogen was introduced into the sample cell for 15 min while the sample remained at 473 K, followed by evacuation for 15 min. This process was repeated for 30, 60, and then 120 min, each followed by a 15 min evacuation. The sample was allowed to cool to room temperature after the final reduction. The processing conditions
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Photooxidation of Trichloroethylene on Pt/TiO2 used here result in samples with isolated Pt particles deposited on the TiO2 particles, and the strong metal-support interaction (SMSI) does not occur.30 An Hitachi H-600 transmission electron microscope operating at an acceleration voltage of 100 kV was used to obtain electron micrographs for 0.5, 1, and 2% Pt/TiO2 samples. The Pt particle number density increased with metal loading. The average particle size for 1 and 2% samples was measured to be 2.8 nm; the particle diameters ranged from 1.0 to 3.4 nm. For 0.5% Pt/TiO2, the particle size was measured to be near 1.6 nm; the diameters ranged from 1.2 to 1.7 nm. It should be noted that smaller particles less than 1 nm in diameter are difficult to detect in TEM so the size distribution measured this way is biased toward particles with larger diameters. After processing the samples, the IR cell was then placed on a linear translator inside the sample compartment of a Mattson RS-1 FT-IR spectrometer equipped with a narrow-band MCT detector. The linear translator allows each half of the sample grid to be reproducibly translated into the infrared beam. This permits the investigation of gas-phase and adsorbed species on the photocatalyst surface under identical reaction conditions. Each spectrum was recorded by averaging 1000 scans at an instrument resolution of 4 cm-1. Each absorbance spectrum shown represents a single beam scan referenced to the appropriate single beam scan of the clean photocatalyst or the blank grid prior to adsorption, unless otherwise noted. The UV/vis spectra were recorded using a Perkin-Elmer Lambda 20 UV/vis spectrophotometer equipped with an internal diffuse reflectance attachment (LabSphere). A BaSO4 standard was used as the reference spectrum. For the broad-band photooxidation of TCE, a 300 nm long pass filter (% T ) 0 at 300 nm) was placed in front of a 500 W mercury lamp (Oriel Corp.) equipped with a water filter. The broad-band light was then reflected off of an aluminum-coated mirror and turned by a 1 in. quartz prism onto the sample. The quartz prism is mounted inside of the FT-IR sample compartment so that the N2 purge was not broken during irradiation. The power at the sample was measured before each experiment and was typically 190 mW/cm2. The temperature of the sample did not exceed 325 K during these experiments. Wavelength-dependent studies were performed using longpass filters from Oriel Corp. These filters were placed in the beam path starting with the longest wavelengths first. Successively shorter wavelengths were allowed to pass through to the sample. The following Oriel filters with the stated cutoff (0% transmission below the stated wavelength), and the corresponding fraction of the full arc power that the filter delivered were used: #59460, λ > 330 nm, 0.82; #59472, λ > 385 nm, 0.70; #59484, λ > 440 nm, 0.54; #59492, λ > 475 nm, 0.51. TCE (Aldrich, 99+%) was transferred to a glass sample bulb and was subjected to several freeze-pump-thaw cycles prior to use. Hydrogen (Air Products, Research Grade) and oxygen (Air Products, 99.6%) were used as received. Gas pressures were initially measured in a volume of 823 mL and then expanded into the infrared cell, which is 320 mL in volume. A valve between these two portions of the vacuum chamber was then closed before irradiation so that the total amount of reactants available in the infrared cell are the pressures specified in the text contained in the 320 mL volume of the infrared cell. Results and Discussion Maxwell-Garnet Simulations of Pt/TiO2 and TiO2. The absorption properties of platinized titanium dioxide have been modeled using Maxwell-Garnett theory. Maxwell-Garnett
J. Phys. Chem. B, Vol. 102, No. 8, 1998 1419 theory is an effective medium theory that has been successfully used to determine the absorption properties of inhomogeneous composites.21-26 Effective medium theories are applicable to metal/insulator and metal/semiconductor composites systems.21-26 In Maxwell-Garnett theory, the composite is assumed to be composed of two components: inclusions and a host matrix. For Pt/TiO2 samples, the inclusions are the Pt particles embedded in a TiO2 matrix. The absorption profile depends on the optical properties of the metal, matrix, and the volume fraction of the metal in the composite mixture. The particles are large enough to apply Maxwell’s equations, so that the metal grains can be described by a wavelength-dependent dielectric constant. The particles are not large enough to approach the wavelength of light, so that we may assume a dielectric constant for a collection of metal particles. In the calculations, it is assumed that the metal particles are spherical. An average dielectric constant, av, for the composite is then calculated using eq 1.
av - s m - s ) fm av + κs m + κs
(1)
where s is the dielectric constant of support, m the dielectric constant of the metal, av the average dielectric, κ the screening parameter (2 for spheres), and fm the volume fraction of metal. The average dielectric constant can be separated into real (av′) and imaginary parts (av′′) as given in eq 2.
av ) av′ + iav′′
(2)
where
av′ ) nav2 - kav2 and
av′′ ) 2navkav The average refractive index, nav, and the extinction coefficient, kav, can be extracted from the real and imaginary parts of the dielectric function. The absorptivity can be determined from the extinction coefficient, eq 3, and finally, the absorbance of the material can be calculated (eq 4).
R ) 4πkav/λ
(3)
absorbance ) Rd
(4)
where d is the film thickness and λ is the wavelength of light. The above equations were solved, and the spectra were simulated; optical constants for Pt and TiO2 were taken from ref 31. Figure 1 displays the simulated UV/vis absorption spectra of TiO2 as a function of platinum loading. The simulated spectra show that TiO2 begins to absorb light near 420 nm and is strongly absorbing by 300 nm. This absorption is attributed to the excitation of electrons from the valence band to the conduction band in TiO2. This correlates well with the band gap of TiO2 (∼390 nm). The absorption of TiO2 is extended to wavelengths as long as 650 nm with the addition of even small amounts of platinum (0.1%) to the TiO2 powder. The absorption at longer wavelengths increases as a function of platinum volume fraction. The increased absorption of Pt/TiO2
1420 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Driessen and Grassian
Figure 3. Wavelength dependence of the photooxidation of TCE on TiO2 and Pt/TiO2. The integrated area of the infrared absorption band for gas-phase CO2 following photooxidation of TCE is plotted as a function of irradiation wavelength. Figure 1. Simulated absorption spectra for Pt/TiO2 as a function of platinum loading. (See text for further details.)
Figure 2. Experimental UV-vis spectra taken of Pt/TiO2 as a function of platinum loading.
compared to TiO2 at wavelengths between 400 and 650 nm is more easily seen in the inset of Figure 1. To test the validity of the theoretical curves, TiO2 and several powdered Pt/TiO2 samples were made using different platinum loadings, and the UV/vis spectrum was recorded for each sample. Figure 2 displays the UV/vis spectra for Pt/TiO2 as a function of platinum loading between 0 and 2%.32 The TiO2 shows an absorption edge near 420 nm and strong absorption by 300 nm. Again this is due to the band gap absorption of TiO2. As platinum is added, there is a large increase in the absorption of Pt/TiO2 at much longer wavelengths compared to pure TiO2, as predicted by Maxwell-Garnett theory. The details of the experimental UV/vis spectra shown in Figure 2 differ from the simulated spectra shown in Figure 1. The simulated spectra show a slight maximum near 480 nm for 2% Pt/TiO2 whereas the experimental curves do not. In addition, the experimental curves show that, by adding 2% platinum to a TiO2 sample, the absorption intensity below the band gap is nearly half as much as the intensity at the maximum (near 320 nm) due to band gap absorption. Problems with the prediction of absorption intensities have been reported in the application of this theory to supported metal catalysts.25 Maxwell-Garnett theory (Figure 1) overpredicts the absorption intensity near the band gap when compared to the UV/vis spectra shown in Figure 2. This overprediction may be related to the
difficulty in the determination of optical constants near the absorption edge of TiO2.31 There are some modifications to the Maxwell-Garnett equations that may bring the simulated spectra in better agreement with the actual absorption spectra of Pt/TiO2. The shape of the particles in the simulation are assumed to be perfect spheres of all the same size. Different particle shapes and sizes may broaden the long wavelength absorption to more closely resemble the absorption spectra shown in Figure 2. Confinement effects in these particles will reduce the mean free path of the electrons, resulting in a deviation of the optical constants from that of the bulk data used in the simulations.26 Although the simulated and actual absorption spectra of Pt/TiO2 are not identical, they give similar behavior in that both show an increase in absorption at much longer wavelengths upon addition of platinum. Therefore, Pt/ TiO2 will have an increase in spectral response of the solar spectrum compared to TiO2. Wavelength Dependence of TCE Photooxidation on Pt/ TiO2. The wavelength dependence of the photooxidation of TCE on TiO2 and Pt/TiO2 was investigated to determine whether Maxwell-Garnett calculations can be used to predict the wavelength dependence of the photocatalytic activity of metallized semiconductors. First, the wavelength dependence of TCE on TiO2 was investigated using the procedure and filters described in the Experimental Section. As discussed below, TCE photooxidation results in the formation of gaseous CO2. Therefore, an action spectrum of gas-phase CO2 production versus wavelength can be used as a measure of the photocatalytic activity of TiO2 and Pt/TiO2. Starting with the longest wavelength first, TiO2 was irradiated for 1 min in the presence of 2.9 Torr of TCE and 7.2 Torr of O2. The next longest wavelength filter was then put in place, and irradiation was continued for an additional 1 min. This procedure was followed for all six filters used in this experiment. The increase in integrated area of the CO2 infrared band after irradiation with each filter was then corrected for differences in light output between filters. The wavelength dependence of the photooxidation of TCE on Pt/TiO2 was then investigated. Pt/TiO2 was irradiated in the presence of 2.9 Torr of TCE and 7.2 Torr of O2. Irradiation times of at least 2 min were used. Longer times were necessary when using the longer wavelength filters because of the lower power output of the lamp at these wavelengths. All integrated CO2 absorptions were then corrected for differences in irradiation time and light output between filters. The results of these experiments are shown in Figure 3. For TiO2, the onset of photooxidation is between 385 and 440 nm. This correlates with the absorption edge of TiO2 as determined by the simulated and experimental curves shown in Figures 1
Photooxidation of Trichloroethylene on Pt/TiO2
Figure 4. Difference infrared spectra showing gas-phase product formation and the loss of gas-phase TCE following photooxidation on TiO2 and Pt/TiO2 (0.1, 1, and 2%). The positive features are due to gas-phase products, and the negative features are due to the consumption of TCE during photooxidation. The inset shows a plot of the relative ratio of CO2:COCl2 as a function of Pt loading. The plot shows that there is an increase in the product ratio of CO2:COCl2 with Pt loading.
and 2, respectively. The wavelength dependence of the photooxidation of TCE on TiO2 indicates that the photooxidation of TCE is initiated by excitation of the photocatalyst and not by direct absorption of TCE, which does not absorb UV light of λ > 250 nm33 or undergo oxidation at 298 K in the absence of TiO2, as discussed previously.16,34 On 2% Pt/TiO2, as predicted from the simulated and experimental absorption curves, the photooxidation of TCE occurs at longer wavelengths compared to TiO2. The wavelength dependence of TCE photooxidation on Pt/TiO2 and TiO2 has been repeated three times each, and the activity of Pt/TiO2 at longer wavelengths, above 400 nm, is always observed. The data show that for platinized titanium dioxide there is an increased spectral response in the solar spectrum compared to TiO2. Product Distribution and Rate of Broad-Band Photooxidation of TCE on Pt/TiO2. The photooxidation of TCE on Pt/TiO2 and TiO2 was monitored by FT-IR spectroscopy; spectra of both the gas phase and the photocatalyst were recorded. Figure 4 displays the difference infrared spectra of the gas phase following 40% conversion of 2.9 Torr of TCE upon irradiation of TiO2 and Pt/TiO2 (0.1, 1, and 2% platinum loading) in the presence of 7.9 Torr of O2. As these are difference infrared spectra, there are negative features near 1590, 1560, 1250, and 942 cm-1 which are due to the loss of parent TCE. Under the conditions used in this study, infrared absorption bands are observed for gas-phase CO2 (2349 cm-1), CO (2144 cm-1), COCl2 (1827 cm-1), DCAC (1794 cm-1), and CHCl3 (1220 cm-1) after approximately 40% conversion of TCE on TiO2. HCl is also observed as a photoproduct for TCE photooxidation at high TCE conversion.16,34 The complete oxidation of TCE does not occur on TiO2 under these conditions. Undesirable byproducts such as phosgene are formed, thereby making TiO2 a less than desirable catalyst for the photooxidation of TCE. In a previous study, we have examined the factors that influence the gas-phase product distribution for photooxidation of TCE on TiO2.34 It was found that the gas-phase product distribution depends on TCE pressure, O2 pressure, and the photocatalyst surface. In this study, it is shown that the same gas-phase products are observed following photooxidation of TCE on Pt/ TiO2 (see Figure 4). However, there is a change in the product
J. Phys. Chem. B, Vol. 102, No. 8, 1998 1421
Figure 5. Infrared spectra of the photocatalyst surface, in the presence of the gas phase, following the photooxidation of TCE on TiO2 and Pt/TiO2 (0.1, 1, and 2%). Several adsorbed products are identified: dichloroacetate, Cl2HCCO2-TiO2 (1569, 1420, and 1230 cm-1), water, H2O-TiO2 (1657 cm-1), and carbon monoxide, CO-Pt (2133 cm-1).
distribution. The amount of CO2 formed relative to COCl2 differs for each sample. The ratio of the integrated absorbances of CO2 to COCl2 is plotted in the inset of Figure 4 and is found to increase as a function of Pt loading. There is a near linear dependence observed for the CO2:COCl2 ratio with Pt loading. The infrared spectra recorded of the catalyst surface in the presence of the gas phase are shown in Figure 5. Absorptions due to both gas-phase and surface-bound photoproducts are seen in the spectra. The assignment of surface-bound products on the photocatalyst surface is more complicated than that of the gas phase. The frequencies of the infrared bands (1000-1700 cm-1) are in the region where adsorbed carbonate and adsorbed water absorptions occur.35-38 Each band in the spectrum is quite broad and may be comprised of several absorption bands. One of the predominant surface-bound species has been previously identified by FT-IR34 and solid-state NMR39 spectroscopy as dichloroacetate. Other adsorbates on the TiO2 surface include adsorbed water. An absorption band at 2133 cm-1 is due to CO adsorbed on the platinum surface. With the exception of the band due to CO adsorbed on the platinum surface, a comparison of the adsorbed species shows that similar intensity patterns and frequencies are observed on all TiO2 and Pt/TiO2 samples following the photooxidation of approximately the same amount of TCE. The effect of platinum on the rate of TCE photooxidation over TiO2 was also investigated. Irradiation of 2.9 Torr of TCE in the presence of 7.2 Torr of O2 on TiO2 and Pt/TiO2 samples (0.1, 1, and 2%) with differing amounts of platinum was monitored as a function of time. Figure 6 shows the loss of TCE as a function of broad-band irradiation. In all cases, the rate of TCE conversion is decreased on Pt/TiO2 compared to TiO2. The initial rate for TCE photooxidation on all three Pt/ TiO2 samples is nearly an order of magnitude slower than on TiO2. Role of Pt in the Photooxidation of TCE on Pt/TiO2. The major results of this study are as follows: (i) Pt/TiO2 absorbs light at longer wavelengths in the solar spectrum compared to TiO2; (ii) TCE can be photooxidized at longer wavelengths on Pt/TiO2 compared to TiO2; (iii) upon broad-band irradiation (λ > 300 nm), there is a decrease in the photooxidation rate of TCE on Pt/TiO2 compared to TiO2; (iv) there is a change in
1422 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Figure 6. Plot of the normalized integrated area of the 942 cm-1 band of gas-phase TCE as a function of irradiation. The plot shows that the rate of TCE photooxidation is slower on Pt/TiO2 compared to TiO2.
the product distribution for TCE photooxidation on Pt/TiO2 that depends on platinum loading. These results, some of which are contrasting, suggest that the role of platinum is complex. A discussion of the effect of platinum on the photocatalytic activity of Pt/TiO2 is presented below. At long wavelengths, Pt/TiO2 exhibits photocatalytic activity. The simulated and experimental absorption curves clearly show that Pt/TiO2 absorbs light at longer wavelengths compared to TiO2. However, the mechanism for TCE photooxidation at long wavelengths is unclear. One possibility is that photooxidation is occurring on the Pt particles at long wavelengths, and TiO2 is not involved in the process. It has been shown on singlecrystal Pt(111) surfaces that O2 adsorbed on Pt(111) can undergo photodissociation at long wavelengths.40 To test this hypothesis, the photooxidation of TCE on Pt/SiO2 was investigated. Silica, an insulator, does not absorb light of λ > 300 nm nor does TCE undergo photooxidation at λ > 300 nm on silica. Therefore, experiments on Pt/SiO2 could in principle provide information concerning the role of the Pt particles and determine whether Pt particles alone are active in TCE photooxidation. It was found that TCE did not undergo photooxidation on Pt/SiO2 upon broad-band irradiation (λ > 300 nm). This result shows that Pt particles alone are not involved in the photooxidation of TCE on Pt/TiO2. Another possibility is that the long wavelength behavior is due to the presence of new electronic states, from the addition of platinum, which lie in the band gap of TiO2. A study by Schierbaum et al. has shown that in the case of Pt/TiO2(110) there are newly derived electronic states below the band gap due to the presence of Pt.41 The observed photooxidation at long wavelengths (λ > 400 nm) can be due to these Pt-derived states. One possibility is that an electron from the valence band of TiO2 is excited to the intermediate electronic state formed due to the addition of Pt to the TiO2. This electron can then react with O2 to form O2- which can go on to oxidize TCE.5 Although the addition of platinum to TiO2 extends the photooxidation of TCE to longer wavelengths (λ > 400 nm) than TiO2, there is an overall decrease in the photooxidation rate of TCE on Pt/TiO2. Linsebigler et al. have shown that platinum particles can cover active sites on the TiO2 surface.8 In those experiments, the photooxidation of CO to CO2 was followed as a function of fractional platinum coverage on the TiO2(110) single-crystal surface. It was demonstrated that defect sites on the TiO2 surface were necessary for the photooxidation of CO to CO2. These defect sites were identified as Ti3+ on
Driessen and Grassian the TiO2 surface and are necessary for the adsorption and photoactivation of oxygen. The data demonstrated that as Ti3+ sites were titrated with platinum particles, the photoyield of CO2 decreased drastically. It was concluded that the platinum particles were detrimental in the photooxidation of CO to CO2 due to their Ti3+ site blocking activity. A similar site-blocking mechanism would explain the results on the powdered surfaces studied here. Although it is shown that Pt promotes long wavelength photooxidation, Pt covers the most active sites, i.e., Ti3+ sites; therefore, there is a net decrease in activity upon broad-band excitation. Besides a difference in the rate of photooxidation, the infrared data show that there are differences in the product distribution between TiO2 and Pt/TiO2. In particular, there is an increase in the amount of CO2 produced as compared to COCl2 as the amount of platinum in the sample is increased. Platinum can serve as a reactive surface that promotes thermal reactions, as suggested previously.8 One of the possible roles of the platinum surface is shown in reaction 5. The changing product distribution in this case could be due, at least in part, to the reaction of COCl2 adsorbed on the surface of the platinum particles.
COCl2 + Pt/TiO2 f CO(g or a) + 2Pt-Cl
(5)
Zhu and White have shown that COCl2 can undergo photochemistry on Pt(111) upon irradiation with light of wavelengths shorter than 370 nm.42 Several products were observed upon irradiation of adsorbed phosgene including gas-phase CO and adsorbed Cl and CO. The data presented here correlate well with the results of COCl2 irradiation on single-crystal platinum surfaces, in that CO is adsorbed on the platinum surface following photooxidation of TCE. As discussed, CO photooxidizes to CO2 on TiO2 and Pt/TiO2 surfaces.8 Therefore, gas-phase CO formed via reaction 5 can go on to form CO2. Due to this side reaction and to the differing amounts of CO adsorbed on the platinum surface and in the gas phase for each sample, it is difficult to compare the amount of CO formed in comparison to the other products. What is most clear, as presented in the inset of Figure 4, is the decrease in COCl2 product formation relative to more complete oxidation products as the amount of platinum in the samples is increased. To summarize, there are many examples which show that solution-phase photocatalytic processes are enhanced when platinized titanium dioxide is used compared to titanium dioxide. As shown here, TCE photooxidation is suppressed when platinized titanium dioxide is used. There are other examples of suppressed photoactivity for gas-phase reactions on metallized titanium dioxide, including a decrease in the rate and yield of CO photooxidation on single-crystal Pt/TiO2(110)8 and the rate of photooxidation of chloroethane on Pt/TiO2 powdered samples.43 A general conclusion that can be drawn from these studies is that the role of Pt is complex, and mechanisms for photocatalysis in solution on metallized TiO2 are different from that in the gas phase. The exact nature of the difference in solution versus gas-phase photocatalysis on metallized TiO2 needs to be further explored. Conclusions Maxwell-Garnett theory has been used to simulate the absorption characteristics of Pt/TiO2 photocatalysts. Both simulated and experimental curves show that the absorption tail of Pt/TiO2 is extended to longer wavelengths compared to TiO2. The wavelength dependence of TCE photooxidation on Pt/TiO2 was measured and showed enhanced decomposition at longer
Photooxidation of Trichloroethylene on Pt/TiO2 wavelengths, in agreement with the simulated and experimental absorption curves. However, addition of platinum to the TiO2 photocatalyst caused a significant decrease in the rate of conversion of TCE as compared to pure TiO2 when broad-band irradiation, λ > 300 nm, was used. To explain these seemingly contrasting results, it is proposed that the long wavelength activity is due to absorption and electron-hole pair formation at defect sites caused by the Pt particles; however, the decrease in the rate of conversion of TCE on Pt/TiO2 is due to the blocking of the most active sites, Ti3+ sites, on TiO2. In addition to a decrease in rate, there is also a change in the gas-phase product distribution for this reaction as the platinum loading was increased; in particular, the amount of CO2 compared to COCl2 increased with platinum loading. These results suggest that the chemistry of photoproducts, in this case phosgene, on the surface of the Pt particles can lead to a change in the gasphase product distribution for Pt/TiO2 compared to TiO2. Acknowledgment. The authors gratefully acknowledge the support of the National Science Foundation (Grant CHE9614134). M.D.D. also thanks the General Electric Foundation for support in the form of a Fellowship. The authors also wish to thank Mr. Saleem Farooqui for his assistance in preparing the Pt/TiO2 samples for the experimental UV/vis measurements. References and Notes (1) Nicklasson, G. A. Sol. Energy Mater. 1988, 17, 217. (2) Blake, D. M. Bibliography of Work on the Heterogeneous Photocatalytic RemoVal of Hazardous Compounds from Water and Air Update Number 1 to June 1995; NREL/TP-473-20300; Golden, CO, National Renewable Energy Laboratory, 1995. (3) Izumi, I.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1981, 85, 218. (4) St. John, M. R.; Furgala, A. J.; Sammells, A. F. J. Phys. Chem. 1983, 87, 801. (5) Izumi, I.; Dunn, W. W.; Wilbourn, K. O.; Fan, F.-R. F.; Bard, A. J. J. Phys. Chem. 1980, 84, 3207. (6) Fox, M. A.; Dulay, M. T. Chem. ReV. (Washington, D.C.) 1993, 93, 341 and references therein. (7) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. (Washington, D.C.) 1995, 95, 735 and references therein. (8) Linsebigler, A.; Rusu, C.; Yates, J. T., Jr. J. Am. Chem. Soc. 1996, 118, 5284. (9) Dyksen, J. E.; Hess, A. F., III J. Am. Water Works Assoc. 1982, 74, 394. (10) Yamazaki-Nishida, S.; Nagano, K. J.; Phillips, L. A.; CerveraMarch, S.; Anderson, M. A. J. Photochem. Photobiol. A: Chem. 1993, 70, 95. (11) Dibble, L. A.; Raupp, G. B. Catal. Lett. 1990, 4, 345. (12) Pruden, A. L.; Ollis, D. F. J. Catal. 1983, 82, 404.
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