Environ. Sci. Technol. 2006, 40, 7029-7033
Photocatalytic Degradation of Gaseous Organic Species on Photonic Band-Gap Titania MAOMING REN, R. RAVIKRISHNA, AND KALLIAT T. VALSARAJ* Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803-7303.
The use of photonic band gap (PBG) titania (inverse opal) for the photocatalytic degradation of an organic chemical in air is demonstrated in this study using 1,2-dichlorobenzene. A photonic band gap in the mid-to-high ultraviolet (UV) wavelength range (280-380 nm), normally associated with the optimal photocatalytic activity of anatase titania, is expected to increase the quantum efficiency for the catalyst. To achieve this band gap, porous structures with alternating air and titania spaces with a periodicity of about 150 nm is required. A thin film of porous photonic titania was synthesized in-situ on a quartz glass rod with a sol-gel technique using polystyrene micro-spheres as templates. Scanning electron microscopy images revealed a pore size of about 100 nm and a periodicity of ∼150 nm, necessary for the desired band gap. X-ray diffraction studies of the coating showed the presence of anatase titania, which is known to exhibit photoactivity. The photocatalytic activity of the coated titania film was verified by measuring the degradation of 1,2-dichlorobenzene vapor in a semi-batch mode in the presence of UV radiation (mid-high UV wavelength) PBG titania showed 248% higher photonic efficiency compared to commercially available P25 titania catalyst. Transmission spectra from the thin films showed high absorbance in the UV range, suggesting a band gap in the region of UV illumination.
Introduction Photocatalysis is the use of UV or visible light to activate catalysts such as semiconductors that can potentially oxidize organic and inorganic materials in air or water. The literature is replete with references to this technology in both laboratory and pilot-scale (1-2). In most cases, titania in the powdered form is the preferred catalyst because it is cheap, easily available, and is not easily degraded. The application of UV light excites an electron from its valence band to the conduction band leaving a hole in the valence band. So the positive charged holes are capable of oxidizing adsorbed organic species. Titania can also be doped with substances to make it useful in the visible range of the light spectrum. In the design of photochemical reactors, the choice is to immobilize powdered titania on solid supports so that its recovery and reuse is facilitated. Slurry reactors have the limitation of high cost of catalyst recovery, especially when dealing with ultrafine or nanosize catalyst particles. A number of gaseous organic species have been subject to photooxidation using titania in various reactor types (4-6). However, * Corresponding author phone: 225 578 6522; fax: 225 578 1476; e-mail:
[email protected]. 10.1021/es061045o CCC: $33.50 Published on Web 10/12/2006
2006 American Chemical Society
the problem of efficient light transmission and utilization within the reactor still limits the use of this technology on a large scale (3). Photonic crystals (or photonic band gap (PBG) materials) are inverse opals that have a spatially periodic structure fabricated from materials having different dielectric constants (7). These are capable of influencing electromagnetic waves in a similar manner to electrons in a semiconductor. In other words, photonic crystals exclude the passage of photons of a chosen range of frequencies thereby helping to confine, control, and manipulate photons in three dimensions. According to modified Bragg’s law (eq 1), by controlling the pore size in the photonic crystal, the band gap can be fixed to a given wavelength. The relationship between the band gap and pore size is described by the following equation:
λmax ) 2d111xn2eff - sin 2θ
(1)
neff ) nTiO2f + nair(1 - f)
(2)
where nTiO2 and nair are the refractive index of TiO2 and air respectively, f is TiO2 phase volume percentage. Usually f ) 0.74 for an fcc structure, and d111 is associated with the pore size by eq 3:
d111 )
x23D
(3)
where D is the distance between neighboring air spheres (8). A variety of templating procedures have been used to produce photonic crystals of titania (9-15). A number of investigators have produced systematic periodic structures of TiO2 crystals with air spheres filling the uniform voids using colloidal templates (9, 12, 15). These photonic crystals possess high surface areas, offer a high refractive index contrast, and have optical properties in the UV/visible wavelength region. These properties make them conducive for applications requiring high photonic efficiency. However, their use in photocatalytic applications has not been investigated thus far. In this paper, we report a proof-of-concept study of the application of PBG titania on the photocatalytic degradation of an air pollutant: 1,2- dichlorobenzene (DCB) vapor. The in-situ fabrication of the appropriate periodic structure of titania necessary for the band gap on a quartz glass rod support is described. Experiments to examine the photocatalytic efficiency of the PBG titania were conducted in a tubular reactor with UV illumination. The effects of gas flow rate, temperature, and moisture content on DCB degradation were explored. The degradation efficiency of a photonic band gap titania reactor is compared to that using a thin coat of P25 titania powder to illustrate the effect of the photonic band gap. The objective in this paper is to demonstrate the relatively higher efficiency of using porous titania photonic crystal layer for photocatalysis compared to the traditional coating of dense titania nanoparticles.
Experimental Fabrication of Titania Photonic Crystals on a Glass Rod Substrate. The methodology used for the production of photonic band gap titania on a glass rod substrate was that employed by Colvin and co-workers (9). The process began with roughening the surface of a quartz rod (3 mm diameter and 70 mm length), which was subsequently washed repeatedly with distilled water and finally wiped with a soft cloth to remove any attached particles. Next, the rod was dipped VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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in a 5% (v/v) solution of polystyrene (PS) latex microspheres (120 nm diameter, Duke Scientific Corporation, Palo Alto, CA) for 5 min and dried in an oven at 50-60 °C for about 20 min. The process was repeated three times. A glove box was prepared with a nitrogen blanket and the coated glass rod was transferred to it. Inside the glove box a solution of 288 µL distilled, deionized water was mixed with 50 mL of ethyl alcohol and stirred. 136 µL of titanium ethoxide in ethyl alcohol was added into the liquid with constant stirring. When the liquid became cloudy, the polystyrene-coated glass rod was dipped into it and maintained in the liquid for 8h. The rod was then removed and oven dried at 60 °C for 2 h. Thereupon the rod was dipped in pure methylene chloride overnight to remove the polystyrene template. The resulting PBG titania coated glass rod was air-dried and in the oven at 450 °C for 12 h. Characterization of Photonic Titania. A 3 cm section of the coated glass rod was glued on to a glass slide and coated with a thin layer of gold to obtain secondary electron images using a scanning electron microscope (SEM). Images of both coated polystyrene and the photonic crystalline TiO2 on the rod were obtained. A Bruker-Siemens D5000 automated powder X-ray diffractometer with a Psi solid-state detector and Rietvald analysis software was used to obtain the XRD spectra of the sample. SEM photographs were obtained on a Hitachi Instruments model S-3600N. Transmission spectra of the coated titania samples were collected to ascertain the wavelength of UV absorption by the photonic crystal titania coating using a HACH 4000 UV/vis spectrophotometer. A quartz glass sheet coated with titania in the method described above was exposed to UV light in a specially fitted sample holder. UV transmission was measured under scan mode between the wavelengths of 500 and 190 nm. Background scans were obtained from uncoated quartz glass sheet. The organic test contaminant in air was chosen to be 1,2-dichlorobenzene (DCB). It is a prevalent air pollutant used in many day-to-day applications such as acid nonhousehold metal cleaners (liquid), automobile body polish and cleaners, building and construction plastic foam insulation, deodorants/air fresheners (non-personal, non-aerosol), drain pipe solvents, and other polishing preparations and related products. It is a suspected endocrine, immuno-, and neuro- toxicant. It has a molecular weight of 147, an aqueous solubility of 156 mg/L and a vapor pressure of 1.5 mmHg at 298K. Description of the Reactor Setup. The reactor system consisted of a glass quartz tube (11 cm length, 1.2 cm diameter) into which the PBG titania-coated glass rod was placed. The reactor was designed to be operated in a semibatch (recirculating) mode. A known volume of DCB vapor at a selected initial concentration was recirculated through the reactor for a given length of time using a mini air-pump. Pure DCB vapor was generated outside the reactor from a small volume (∼10 mL) of pure liquid DCB in a gas bubbler, through which air was recirculated. An empty bubbler was placed immediately after the bubbler vapor exit in order to minimize any entrained liquid DCB from entering the reactor tube. DCB vapor was sampled through a gas sampling port and analyzed using a gas chromatograph/ mass spectrometer. When the desired saturation of DCB in the vapor phase was achieved, the DCB vapor was switched to a loop that bypassed the vapor generator tube with the pure DCB liquid. Two UV lamps (model XX-15L UVP Inc., Upland, CA) placed on either side of the reactor supplied the necessary UV light (with 300 < λ < 380 nm) for the reaction. The lateral position of the UV lamps on either side of the reactor tube could be varied to obtain different light intensities incident on the reactor. The light intensity was measured using a UVX-36 radiometer (UVP Inc., Upland, CA). The entire reactor assembly was placed inside a temperature-controlled box so that the 7030
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FIGURE 1. Schematic of the reactor set up used for photocatalytic degradation of 1,3-dichlorobenzene vapor.
FIGURE 2. Scanning electron microscope images of (A) 100 nm PS microspheres templates (B) ×30 000 magnification of the PBG titania and (C) ×45 000 magnification of the PBG titania synthesized using PS template (scale: 1 µm). reaction can be carried out at different temperatures. Figure 1 is a schematic of the reactor set up. Experimental Procedure. After ensuring a leak-free system, DCB vapor was introduced into the system by recirculating air through the vapor generation loop with the pure DCB liquid. When DCB concentration in the vapor reached 4000-5000 µg/L the vapor flow path was switched to the reactor mode bypassing the generator tube and the DCB vapor was recirculated through the reactor tube in a semi-batch (recirculating) mode. The DCB vapor was recirculated within the reactor system for a certain period of time without UV radiation, until a steady state in the concentration was reached. At this point, the UV lamps were turned on and the photocatalytic reaction was allowed to occur. Samples of DCB were taken from the recirculation loop using a gastight syringe (Hamilton) at regular intervals. The gas samples were absorbed in 1 mL of hexane and injected into a gas chromatograph (Agilent 6890, Agilent Technologies, CA) with an electron capture detector to analyze DCB in the sample.
Results and Discussion Properties of Photonic Band Gap Titania. Figure 2 shows SEM images of the PS templates and the resulting macroporous samples generated on the glass rod. It is clear from the image that a highly ordered macroporous structure is
FIGURE 3. X-ray Diffraction (XRD) spectrum of the PBG titania templated on the quartz substrate showing anatase phase
FIGURE 4. UV Transmission spectra of photonic band gap titania and powdered titania. obtained with pores of diameter about 100-150 nm. The top surface of the structure was very smooth and, therefore, improved the optical transmission through the sample. The porous structure of the material was clear and the air voids occupied those spaces previously occupied by the PS template. The next lower layer of air spheres is visible in some of the SEM images indicating an interconnected threedimensional network. Figure 3 is the XRD spectrum of the sample of titania photonic crystal obtained from our work. The peak at 2θ ) 25° is representative of the (101) crystalline anatase phase. Thus, the spectral features clearly reveal that the form of titania obtained is indeed of the anatase form which has a refractive index of ∼2.5. According to Wijnhoven and Vos (11), such a refractive index is necessary to achieve band gaps for the UV-visible spectrum. Figure 4 shows the UV transmission spectra of the different coatings of titania on a quartz slide. The transmission data was collected in the range of 190-500 nm since our objective is to observe the band gap behavior in the UV wavelengths (190-380 nm). According to eq 1, the band gap of a photonic crystal with a pore size of 120 nm is in the UV range. The spectra from the PBG titania coating clearly shows a drop in
transmission with a decrease in wavelength around 350 nm. Figure 4 also shows the transmission spectra from other two non-photonic crystal configurations: (a) titania layer obtained using the sol-gel reaction without the polystyrene (PS) template and (b) P25 titania powder. These both show low transmission for all wavelengths and do not exhibit a characteristic jump in the transmission as shown by the PBG sample. It is known that an inverse opal (PBG titania) has a transparent window in the visible range, but in the anatase form it exhibits a broad stop band, not a full band gap (18). This clearly illustrates the band gap phenomenon by the photonic crystal layer in the UV range, thus causing UV light to be trapped within the PBG catalyst layer. The transmission data shown is not normalized to the thickness of the three samples because the primary objective was to look for the band gap and absolute transmission values of the samples were not a matter of concern. Analysis of Reactor Data. As mentioned earlier, the reactor was run in a semi-batch mode where the DCB stream was recirculated through the reactor multiple times and the concentration change in the exit stream measured with time. For a semi-batch reactor the slope of the plot of concentration with time gives the first-order rate constant for DCB degradation. Thus, the overall change in concentration in the reactor is given by
C(t) ) exp(-k*t) C0
(4)
where k* is the pseudo first-order rate constant (min-1) and C0 is the initial gas-phase concentration of DCB. The overall rate constant is composed of two factors, viz., the reaction at the surface and the mass transfer of the vapor toward the catalyst surface (6).
1 1 1 ) + k* kmav kK
(5)
where k is the intrinsic surface reaction rate constant (mol‚L-1 min-1), K is the Langmuir-Hinshelwood adsorption constant (L‚mol-1), km is the mass transfer coefficient (cm/min), and VOL. 40, NO. 22, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Effect of UV radiant flux intensity on the degradation of DCB on PBG titania
TABLE 1. Pseudo First-Order Rate Constants for the Degradation of 1,2-DCB on Photonic Crystals of Titaniaa parameter UV intensity/mW‚cm-2 66 114 flow rate/mL‚min-1 186 436 temperature/K 303 316 324 moisture dry air/DCB mixture saturated air/DCB mixture titania characteristic P25 titania powder coated on glass rod PBG titania photonic crystal on glass rod
k/min-1
r2
0.0023 ( 0.00012 0.0038 ( 0.00041
0.983 0.926
0.0018 ( 0.0003 0.0023 ( 0.0002
0.937 0.852
0.0017 ( 0.0001 0.0028 ( 0.0006 0.0031 ( 0.0002
0.987 0.975 0.960
0.0031 ( 0.0003 0.0023 ( 0.0003
0.946 0.982
0.0013 ( 0.0001
0.926
0.0037 ( 0.0003
0.943
a All data are at 324K unless otherwise noted. r2 is the correlation coefficient of the data for the fit to first-order kinetics expression.
av is the surface area of PBG titania per unit volume of the reactor (cm2/cm3). Effect of Radiant Flux Intensity. Figure 5 shows the effects of radiant flux intensity at the reactor wall upon the overall degradation of DCB vapor from the air stream. The radiant flux at the reactor was varied by changing the position of the UV lamps from the reactor. The concentration decline with time is shown. It is clear that for the control experiment in the absence of the UV light the reaction is negligible. This clearly demonstrates that the photonic band gap titania is photoactive toward the degradation of DCB in the UV wavelength range. The radiant flux corresponds to approximately 20% of the electrical power consumed in the reaction. For an n-type semiconductor (e.g., titania) the photoinduced holes are much less numerous than electrons and hence holes are the limiting active species. Thus, it can be concluded that the rate of a photochemical reaction is generally proportional to radiant flux at low values up to ≈25 mW‚cm-2, beyond which it is only proportional to the square root of the radiant flux (16) and has been noted for powdered titania. In our study, the PBG titania shows a discrepancy in this respect and the rate constant is proportional to the radiant flux intensity even at intensities higher than 25 mW‚cm-2. The exact mechanism causing this behavior is yet 7032
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unknown and should be studied in detail. The observation of a linear relationship between the rate and radiant flux might be due to the effects enhancing charge separation or suppressing recombination. To further demonstrate the photocatalytic activity of PBG titania, we conducted a series of tests on the effects of various parameters that affect the photocatalytic behavior. The rate constants for these runs are shown in Table 1. As mentioned earlier (eq 5), in a flow-reactor the mass transfer limitations can sometimes mask the surface reaction rates. The mass transfer limitation can be overcome by increasing the gas flow velocity over the immobilized catalyst surface and decreasing the gas-film diffusion resistance given by the first term on the right-hand side of eq 2. In the present case, gas flow rate was found to have minimal effect on the overall rate coefficient beyond 436 mL‚min-1 (Table 1). For all subsequent experiments including those in Figure 5, the flow rate was set at this value. The effect of temperature was to increase the rate of DCB degradation as indicated by the increase in rate coefficient from 0.0017 min-1 at 303 K to 0.0031 min-1 at 324 K. A plot of ln k* versus 1/T gives the following equation for the temperature dependence:
ln(k*/min-1) ) -
2950 K + 3.24 T
with (r2 ) 0.956) (6)
Thus an activation energy of 20 kJ‚mol-1 was calculated for the photocatalytic conversion of DCB on PBG titania. It should be, however, remembered that there are two competing effects of temperature on a photocatalytic reaction. Photocatalysis occurs via two stages, viz., the reactant adsorbs on the PBG surface and subsequently reacts. Increasing temperature will increase the reaction step, but may decrease the adsorption process. Further delineation of these two steps was not attempted in this work. Moisture in air competes effectively with DCB for adsorption sites on the PBG crystal surface. Therefore, it can lead to decreased photocatalytic degradation. For example, the rate constant for DCB degradation from a water-saturated air stream was decreased by 26% from that in a dry air-DCB mixture (Table 1). Comparison with P25 Titania Powder. P-25 titania is a commercially available form of anatase titania (from Degussa Corporation) that has been frequently used in fixed film titania photocatalysis applications and also as a reference sample to compare the relative photonic efficiency of any synthesized titania sample. In the present work, a P-25 titania coating on the glass rod was achieved by a dip-coating method described previously from our laboratory (17) and used in the reactor setup described in this work. The comparison was done for similar UV light intensity, vapor flow rate and temperature. Figure 6 is the rate of change of DCB in the air in the two cases as a function of time. Significant improvement in the overall rate constant was observed with PBG titania. The intial photonic efficiencies were calculated as
ξ)
0 rDCB (mol‚m-3s-1)
I0(Einstein‚m-3s-1)
(7)
where r0DCB is the initial reaction rate and I0 is the incident photon flow per unit volume of reactor (I0 ) 0.16 Einstein‚m-3s-1) determined by radiometric measurements as described in the experimental section. The value of ξ obtained were 1.6 × 10-5 mole.Einstein-1 for PBG titania and 4.6 × 10-6 mole‚Einstein-1 for P25 titania. Thus, a 248% increase in the photonic efficiency was observed for the PBG crystal in this work. From SEM images of the cross-sections of the coated quartz rods, the average thickness of the
FIGURE 6. Comparison of the first-order degradation rate of DCB in the semi-batch reactor using P25 titania powder and PBG titania on the quartz glass rod. photonic crystal titania layer on the quartz rod was estimated to be 2 µm, while that of the P25 titania layer was 4 µm. Since the photonic crystals stop part of the UV propagation into the catalyst, the outer surface is the one where the reaction mainly takes place. The increase in the outer surface area due to the porous surface is insignificant compared to the observed increase in the rate (248%). This implied that on the basis of either film thickness or catalyst mass, the PBG titania exhibits higher photonic efficiency and is, therefore, a more efficient photocatalyst. The effectiveness of the photonic crystal titania in a larger scale multichannel monolith reactor is currently being investigated in our laboratory.
Acknowledgments This work was supported by a grant from the National Science Foundation (CTS-0520524).
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Received for review May 2, 2006. Revised manuscript received July 31, 2006. Accepted September 5, 2006. ES061045O
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