Fluorine and Carbon Codoped Macroporous Titania Microspheres

Nov 13, 2008 - Australian Research Council Centre of Excellence for Functional ... Centre, Brisbane Surface Analysis Facility, Centre for Microscopy a...
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J. Phys. Chem. C 2008, 112, 19655–19661

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Fluorine and Carbon Codoped Macroporous Titania Microspheres: Highly Effective Photocatalyst for the Destruction of Airborne Styrene under Visible Light Melvin Lim,† Yan Zhou,‡ Barry Wood,§ Yanan Guo,| Lianzhou Wang,†,⊥ Victor Rudolph,⊥ and Gaoqing Lu*,† Australian Research Council Centre of Excellence for Functional Nanomaterials, AdVanced Water Management Centre, Brisbane Surface Analysis Facility, Centre for Microscopy and Microanalysis, and DiVision of Chemical Engineering, UniVersity of Queensland, QLD 4072, Brisbane, Australia ReceiVed: September 8, 2008; ReVised Manuscript ReceiVed: October 23, 2008

Airborne styrene is a suspected human carcinogen commonly present in industries where resins are used. Traditional ways of abating this pollutant include the use of various adsorbents (activated carbon or zeolites) or thermal treatment. The earlier method is simply a temporary containment of the pollutant, requiring spentadsorbents to be regenerated frequently, while the latter is energy-intensive. Work involving the photodegradation of airborne styrene is limited in recorded literature. This study uses a well-characterized combined fluorine/carbon codoped macroporous-titania catalyst, capable of destroying airborne styrene using irradiation in the visible range. A fluidized-bed photoreactor, combined with high-flow rates relative to the minimum fluidization velocity (Umf) of the catalysts, is utilized in the photooxidation process. The effects of varying relative humidity (RH, 0 or 20%) are investigated in using both UV (248nm) and visible (400-700nm) irradiation. It is found that RH levels of 20% promote the complete degradation of 300 ppmV airborne styrene under both types of irradiation. Reaction intermediates on the surface of the used catalysts are probed by high-performance liquid chromatography (HPLC). Introduction Styrene (C6H5CHdCH2) is a volatile, aromatic organic compound that is commonly used in industries where resins are employed. It has a human threshold-odor1 of approximately 20 ppmV, and a vapor pressure of 44.6 mmHg (20 °C). The health effects of inhaling styrene have been studied extensively, and it is classified as a suspected carcinogen2 in human beings. Toxicological studies have shown that laboratory mice are exposed to the risk of developing tumors,3 when exposed to relatively higher amounts of styrene continuously. In essence, it is likely that workers in resin-based industries are in the highest risk-group. Traditional methods1 of styrene abatement include the use of adsorbents (activated-carbon, zeolites, etc.), thermal destruction, and biological degradation. In general, these methods temporarily contain the pollutant (in the instance of adsorbents, whereby regeneration required before break-through occurs), require large energy inputs (thermal destruction), or relatively large land-space for the construction of biological ponds. Photocatalysis offers an alternative, potentially sustainable method to the traditional methods of styrene abatment. It involves the use of a photocatalyst (the most commonly reported material being titania), to oxidize substrates in the presence of photonic energy. The desired final products of photocatalysis are water and carbon dioxide. The bandgap energy of anatase * Corresponding author. E-mail: [email protected]. Phone: +61 7 33463883. Fax: +61 7 33656074. Address: Bldg 75, Level 5W, Australian Institute of Bioengineering and Nanotechnology, Cnr Coopers and College Rds, University of Queensland, QLD 4072, Brisbane, Australia. † Australian Research Council Centre of Excellence for Functional Nanomaterials. ‡ Advanced Water Management Centre. § Brisbane Surface Analysis Facility. | Centre for Microscopy and Microanalysis. ⊥ Division of Chemical Engineering.

titania is approximately 3.2 eV and warrants the need for UV (or irradiation of higher frequency) to excite an electron from the valence band (VB) into the conduction band (CB). Because of this limitation, there has been much concerted effort in trying to decrease the intrinsic bandgap of semiconductor photocatalytic materials. Such recent attempts include doping the original material using a variety of elements, including metal cations (e.g., vanadium,4 copper,5 iron,6 platinum,7,8 and chromium4) or anions (e.g., nitrogen,9,10 sulfur,11,12 carbon,13,14 and iodine15). In many instances, these reports focus much attention on decreasing the bandgap energy of the photocatalytic materials, frequently reporting band-to-band shifts and/or the formation of impurity states.16 Although a particular material’s absorbance spectrum indicates how much it absorbs in a particular wavelength or range of wavelengths of irradiation, it is only a secondary indicator of its potential as a photocatalyst. For instance, Yu and co-workers have found that, although the absorbance spectra of F-doped titania indicate that it absorbs rather weakly in visible light, the material itself is highly responsive in degrading 4-chlorophenol under visible light.17 Park and Choi attribute this to “remote photocatalysis”,18 whereby a suitable medium, for instance moisture, is used to generate a plethora of highly oxidizing and diffusive hydroxyl radicals on the surface of the photocatalyst. It should be emphasized that photocatalysis is predominantly a surface-based process. Consequently, it may be possible that the extent of porosity in a photocatalytic material is of secondary importance, with the consideration of the possibility of deep-oxidation occurring in micro- and mesopores (where present). Generally, the amount of relative humidity (RH) suitable to be used in a photocatalytic reaction to generate hydroxyl radicals decreases with increasing porosity of a chosen photocatalyst. This is attributed to the competitive adsorption between water molecules and targeted substrate molecules.19

10.1021/jp807983a CCC: $40.75  2008 American Chemical Society Published on Web 11/13/2008

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Figure 1. SEM images of (a) F-TiO2 and (b) FC-TiO2.

This study uses good-crystalline macroporous (ca. 5-6m2/ g) fluorine or fluorine and carbon codoped titania microspheres, in a flow reactor operating at very large flow velocities, relative to the minimum-fluidization velocity of the titania particles (ca. 20 µm in size). It is of interest to investigate (i) the ability of F-doped macroporous titania to degrade airborne styrene under various (practical) conditions, and (ii) the effect of simultaneously doping titania with carbon and fluorine (thereby also forming fluorocarbon compounds) on the same photooxidation processes. Experimental Section Materials. Titanium tetrafluoride (TiF4), R-D-glucose (96%, anhydrous), and styrene (RegentPlus, 99%) were purchased from Sigma-Aldrich. Ultrapure Millipore water (18 MΩ · cm) was used in the hydrolysis of TiF4, dissolution of R-D-glucose, and in photocatalytic reactions requiring simulated levels of RH. Catalyst Preparation. In a typical preparation, 0.16 moles of TiF4 were added to 800 mL of water in a plastic beaker. Where required, 0.0016 mol of R-D-glucose was added as a precursor to carbon doping. All solutions were left to stir for 30 min, before transferring into an autoclave and maintained at 180 °C in an oven for 24 h. Resulting samples were quenched to room-temperature under flowing water, subsequently washed to neutral pH with five centrifuge-wash cycles, and then dried in a freeze-drying unit (FTS) for 24 h. The dry samples are then calcined to 550 °C at a ramp rate of 3 °C/min, and held for 6 h before being allowed to cool at 10 °C/min. To prepare F-doped macroporous-titania (F-TiO2), the addition of glucose and the calcination process are omitted. F and C codoped macroporous titania is hereafter named FC-TiO2. Characterization of Catalysts. All catalysts were characterized using various instruments. Unit surface area was measured using a Quantachrome Quadrasorb SI (with a sample degassing period of 20 h at 200 °C), density was measured via a Micromeritics 1340 Pycnometer, and particle-size distribution was measured using dynamic laser-light scattering (Malvern Mastersizer 2000). Crystal phases were analyzed using a Bruker D8 X-ray diffractometer (CuK-R, 40 kV, 30 mA, slit size 0.2 mm, step size 0.01°/min). Light absorption was measured using a UV-vis diffuse reflectance unit (Jasco V-550). Other spectroscopic methods employed include X-ray photoelectron spectroscopy (XPS; Thermo Scientific Escalab220i-XL, monochromated Al KR, charge-referenced using adventitious carbon at 284.8 eV) and Fourier transform infrared spectroscopy (FTIR; Thermo Scientific Nicolet 6700, ATR, diamond window). The

catalysts were also observed under both scanning electron microscopy (SEM; JEOL 890) and transmission electron microscopy (TEM; Phillips Technai-F20). Concentration of Styrene and Quantification. Concentrations of styrene in air were measured using a GC-8A (Shimadzu) with a flame ionization detector (FID), and a packed column (Supelco 5% SP-1200, 1.75% Bentone 34 on 100/120, 6′ × 1/8′′ × 2.1 mm). The retention time for styrene is ca. 7.2 min. Fluidized-Bed Photocatalytic Destruction of Styrene. All photooxidation reactions were carried out in a custom-designed fluidized-bed photoreactor. Two grams of catalyst is used in each run, under either UV-C (248nm, Heraeus TNN 12/20, 12 W) or visible (400-750 nm, NEC, FL15BR, 15 W) irradiation. Photolysis of styrene is observed to be negligible under both UV-C and visible irradiation. Levels of RH were controlled by varying the flow rate of a moisture-saturated air stream entering the reactor. Additional information regarding the setup is detailed elsewhere.19 In a typical experimental run, styrene (carried by air, and made up to 300 ppmV), moisture, and makeup air were introduced into the photoreactor in the dark, and effluent steady-state concentrations of approximately 300 ppmV styrene were allowed to be reached. The catalyst (prewarmed to 50 °C) was then introduced into the photoreactor, and the irradiation source was switched on. The internal temperature of the reactor was measured to be an average of about 55 °C, as a result of the heating effect from the photonic sources. Reaction Intermediates. The possibility of intermediate species occurring in the photooxidation processes were probed by washing used catalysts in water, followed by passing the leachates through 0.45 µm syringe membrane-filters (Millipore). High-performance liquid chromatography (HPLC; Agilent, 1100 Series, Zorbax C8 150 mm × 4.6 mm × 5 µm, water and 0.1% formic acid/acetonitrile (25:75) as the mobile-phase, UV detector 254 nm, 1 mL/min) was used in detecting the presence of possible organic compounds in the leachates. Results and Discussion SEM Observations. Figure 1 depicts SEM images of both F-TiO2 and FC-TiO2. Spherical colloidosomes, largely representative of FC-TiO2 and F-TiO2, are observed in both samples. Each individual sphere is about 5.5-7 µm, and tends to aggregate, forming larger particles. It is also evident, from the images, that the external faces of the spherical particles exhibit different textures. In F-TiO2, thinner flake-like particles aggregrate to form a sphere, while, in FC-TiO2, these particles appear

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Figure 4. Particle size-distribution of catalysts.

Figure 2. X-ray diffractograms and corresponding crystal faces of FCTiO2 and F-TiO2.

Figure 3. (a) HRTEM observation of an ultrathin cross-sectional slice of FC-TiO2. (Note arrows depicting smaller anatase crystals.) (b) SAED of [420] plane from similar region.

to be chunkier. It is likely that the presence of glucose (and its hydrolysis) competitively reduced the amount of water present for the hydrolysis of TiF4 in the process where FC-TiO2 was prepared. The presence of a few, random, anatase single crystals on the scale of several microns is noted in F-TiO2, but is not a subject of consideration in the current report. X-ray Diffraction (XRD) Analysis. XRD analyses of the catalysts reveal that the bulk of both F-TiO2 and FC-TiO2 predominantly consists of highly crystalline anatase TiO2, as judged from the sharp and narrow peaks obtained (Figure 2). It is also observed that the strength of the anatase peaks (in counts per second) in FC-TiO2 are generally weaker compared to that of F-TiO2. This hints to the heterogeneous presence of carbon and fluorine atoms within FC-TiO2, possibly creating amorphous regions among the crystal domains of anatase titania. High-Resolution TEM (HRTEM) Observations. HRTEM observations on ultrathin cross-sections (Figure 3a) of the spherical aggregates reveal smaller anatase crystals (10-15nm), as evidenced by selected-area electron diffraction (SAED) studies (Figure 3b) depicting the [420] plane (lattice-spacing of 0.846 Å) of anatase TiO2. Together with the earlier SEM observations, it is observed that the catalysts are solid, and consist of relatively larger anatase crystals near or at the surface of the spherical colliodosomes. This is different from an earlier study20 reporting the development of hollow ZnO spheres, where a relatively larger amount of glucose was added to the starting mix. In FC-TiO2, it is likely that the relatively smaller amount of R-D-glucose present (as compared to the amount of Ti4+)

Figure 5. Absorbance spectra of catalysts.

was well-dispersed (with the aid of relatively larger volumes of water), and had undergone the processes of aromatization/ carbonization21 among the deposition and redissolution (due to the presence of hydrofluoric acid (HF) as a byproduct) of TiO2 particles. Particle-Size Distribution/Density. Figure 4 shows the particle size-distribution of the catalysts. The mean aggregatedparticle size of F-TiO2 (17.1 µm) is smaller than that of FCTiO2 (22.2 µm), with densities being 3860 and 4430 kg/m3, respectively. On the basis of these values, and invoking the Wen and Yu22 correlation, the Umf values of F-TiO2 and FC-TiO2 are calculated to be 1.5 × 10-4 and 1.8 × 10-4 ms-1, correspondingly. These are relatively small, but expected values, given the low d0.5 (mean-particle diameter) values of the catalysts. Surprisingly, when fluidized to more than 100 times the Umf, these catalyst-beds remain well-behaved, and can generally be classified as Geldart Class B.23 It is possible that frictional wall-effects, coupled with the relatively rough textures of the surfaces of the catalysts, act in the fluidization process. UV-vis Spectroscopy. Figure 5 shows the absorbance spectra of the catalysts. Commercial Degussa P-25 is included as a reference. The absorption edges of FC-TiO2 and F-TiO2 are observed to red-shift from the P25 reference. To the naked eye, FC-TiO2 is white in color, while F-TiO2 exhibits a slight tinge of grayish-blue. Asahi and co-workers carried out calculations9 of the resultant density of states for fluorine-doped anatase titania, and concluded that the fundamental adsorption edge of the doped titania does not shift with respect to the neat material. On the basis of Shapiro’s method,24 the bandgap energies of F-TiO2 and FC-TiO2 are found to be close, at 3.05 and 3.07 eV, respectively. These values are lower than that of

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Figure 6. XPS Spectra of F-TiO2 (a,c,e,g) and FC-TiO2 (b,d,f,h). Peak envelopes and fitted peaks (marked in the panels) are included as solid and broken lines, respectively.

P25-TiO2 (a mixture of 25% rutile and 75% anatase crystals), estimated at about 3.2 eV, and are consistent with various reports25,26 on fluorine-doped titania. Even though carbon-doping has been experimentally27,28 and computationally9 found to narrow the bandgap energy of titania, it is likely that the carbon atoms in FC-TiO2 are present as surface-bound, fluorocarbonbased entities, and in amounts small enough (since the atomic

ratio of F in FC-TiO2, via XPS, is ca. 1.0 atom %) to not affect the intrinsic (bulk) bandgap of the material. XPS/FTIR Analysis. The overall chemical compositions of F-TiO2 and FC-TiO2 are investigated using XPS. Figure 6 shows results from high-resolution XPS analyses of the catalysts, depicting F1s, C1s, O1s, and Ti2p electronic emissions. All peaks are fitted (Casa XPS Ver. 2.3.12, Casa Software, Ltd.)

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Figure 8. Nitrogen adsorption-desorption isotherms of catalysts.

Figure 7. FTIR spectra of catalysts.

using either linear or Shirley-type backgrounds, together with suitable Gaussian-Lorentzian line shapes. These peaks, including their relative compositions, are listed (see Supporting Information). Titanium is present as Ti(IV) in both catalysts, as deduced from the difference in binding energy between the Ti2p3/2-Ti2p1/2 doublet split (5.7 eV), and that between Ti2p and O1s (71 eV in both catalysts, from wide XPS surveys not shown). The fitted F1s peak of F-TiO2 reveals the presence of monovalent F- entities physisorbed on the surface. This is absent in FC-TiO2, as calcination at 550 °C would have removed such excess surface ions. TiOF2, substitutional F atoms (TiO2-xFx), and fluorocarbon entities (CFy, where y ) 1-3) are assigned to the F1s spectrum in FC-TiO2. Although XRD patterns do not reveal specific peaks for TiOF2 (instead, indicating only wellcrystallized TiO2 in both F-TiO2 and FC-TiO2), it must be emphasized that XPS probes only the surface (i.e., ca. 50-100 nm) of the catalysts in the current technique utilized. In addition, with the relatively small total amounts of fluorine present and detected via XPS in F-TiO2 (1.0 atom %) and FC-TiO2 (2.8 atom %), it is not surprising that the diffraction peaks of TiOF2 are not observed using XRD, a bulk-analysis method. In addition, FTIR spectroscopy reveals the presence of -CF3 groups on the surface of FC-TiO2. This is depicted in Figure 7, where a broad transmittance shoulder at 798 cm-1 corresponds to the out-of-plane (OPLA) vibrations of these -CF3 entities on FC-TiO2. The s-stretching mode of -CF2 (s-stretching, 954 cm-1) is also observed in the spectrum of FC-TiO2. For F-TiO2, the C-F stretch (914 cm-1) is recorded weakly. Specific Surface Area and Density Measurements. Pcynometric studies carried out on the catalysts reveal the densities of F-TiO2 and FC-TiO2 to be 3860 and 4430 kg/m3, respectively, as described earlier. In employing the Barrett-Joyner-Halenda (BJH) model29 on the desorption branches of the isotherms presented in Figure 8, it is deduced that F-TiO2 has a specific surface area of 6.0 m2/g, while that of FC-TiO2 is 5.1 m2/g. The adsorption branches of F-TiO2 and FC-TiO2 are classified as type II,29 and this is indicative of the macroporous nature of the catalysts. The monolayer adsorption of nitrogen on FC-TiO2 and F-TiO2 is estimated to be completed at partial pressures of 0.04 and 0.05, respectively. In addition, hysteresis loops for both materials are classified as type B, and indicate the presence of

slit-shaped pores.29 Such slit-shaped pores can also be inferred by SEM observations (see earlier section) of both catalysts, with stacked-layers observed at the macroscopic level. Photocatalytic Degradation of Airborne Styrene. Figure 9 depicts the destruction of styrene using both F-TiO2 and FCTiO2, under both UV irradiation and visible light. The use of FC-TiO2 in a moist environment of 20% RH enables the degradation to proceed to completion in approximately 3.5 (UV) or 4 h (visible light). In contrast, the presence of FC-TiO2 in a dry environment enables only a partial degradation of styrene, and this is more pronounced under visible light than UV irradiation. While the availability of moisture seems to lower the degradation-efficiency of F-TiO2 under UV irradiation, the opposite occurs in the use of visible light. This is possibly due to the presence of more reaction intermediates forming in the presence of UV, as discussed in a later section. Table 1 lists the overall degradation efficiency of F-TiO2 and FC-TiO2 in the photocatalytic experiments. Intermediates of Reaction. Benzaldehyde, benzene, benzyl alcohol, benzoic acid, hydroquinone, and phenol are found to be present as reaction intermediates on the surfaces of both catalysts in all the UV photooxidation processes. In contrast, only benzaldehyde was found on catalysts used under visible light. Mechanistic Implications. The introduction of F-atoms as an in situ process in the hydrolysis of TiF4 creates a charge imbalance when F-1 substitutes for O2- in the TiO2 lattice. It has been suggested that this charge-imbalance is possibly neutralized17 by an OH- radical, when available. The presence of substitutional F atoms (TiO2-xFx), together with TiOF2, encourages the desorption of strongly oxidizing OH radicals on the surface of FC-TiO2 by the following mechanism:18 + tTi-F + H2Oads + hVB f tTi-F + •OH(airborne) + H+

(1) This, in effect, increases the availability of such radicals in the proximity of the surfaces of FC-TiO2, and accounts for remote photooxidation of styrene molecules. Physisorbed, monovalent F- ions on the surfaces of F-TiO2 are deemed responsible for the relatively lower photocatalytic activities of F-TiO2 by reducing the number of active sites available to airborne styrene. This is different in a separate investigation18 by Park and Choi, where these physisorbed F- entities (although no specific amount of F on the surfaces of the titania catalyst used was reported)

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Figure 9. Photocatalytic destruction of styrene under (a) UV irradiation and (b) visible light. Note that the presence of 20% RH is indicated by “/H2O”; all other runs were at 0% RH.

TABLE 1: Efficiencies of Catalysts in the Photocatalytic Degradation of 300 ppmV Styrene

catalyst

RH (%)

F-TiO2 FC-TiO2 F-TiO2 FC-TiO2

0 20

degradation efficiency (%) under UV

degradation efficiency (%) under visible light

39.2 73.3 30.1 100

24.8 38.5 48.9 100

are concluded to be the main reason for remote photocatalysis. The authors propose two other main reasons for the effectiveness of FC-TiO2 in degrading styrene: (i) Hydrophobic CFy groups present on the surfaces of FCTiO2 prevent surfaces to be flooded with water molecules, thereby reserving more active sites for the absorption and photooxidation of styrene molecules. The static contact angles of water with pelletized F-TiO2 and FC-TiO2 are depicted in Figure 10, with FC-TiO2 showing a larger angle of contact with water and air compared to F-TiO2. This indicates the relatively more water-repellent surface of FC-TiO2. (ii) These fluorocarbon entities present are known to be electron-withdrawing groups,30 because of the highly electronegative nature of fluorine atom(s). It is likely that any conduction-band electrons formed, in the process of FC-TiO2 being exposed to photons in the photoreactor, are continuously scavenged by surface CFy groups, therefore effectively reducing the recombination between holes and electrons. This results in an increased survival rate and hence availability of surface holes, for the photodegradation process to occur efficiently, even under visible light. A summary of the mechanisms discussed, and occurring on the surface of FC-TiO2, in the presence of 20% RH and UV or

Figure 10. Wetting angle of (a) FC-TiO2 (30.2 o) and (b) F-TiO2 (19.8°).

SCHEME 1: Fundamental Processes in the Photooxidation of Styrene occurring on FC-TiO2 with 20% RHa

a

Note: bold, double-arrowed lines represent the oxidation of styrene.

visible light is depicted in Scheme 1. The possibility of the formation of hydroxyl radicals (capable of oxidizing styrene) via the direct photolysis of water molecules under UV irradiation is also included in the scheme. Conclusions Both fluorine-doped (F-TiO2) and fluorine/carbon codoped (FC-TiO2) titania (consisting of highly crystalline anatase as observed under XRD) catalysts were incorporated into a fluidized-bed photoreactor in attempting to degrade 300 ppmV airborne styrene under both UV irradiation and visible light. Experimental runs reveal that FC-TiO2 is a highly effective photocatalyst capable of degrading styrene under both types of photonic energy in 20% RH (moisture). The presence of hydrophobic, electron-withdrawing CFy groups on the surface of the catalyst is believed to contribute to the effectiveness of the photocatalytic process, while the process of remote photocatalysis in the presence of both substitutional F atoms (TiO2-xFx) and TiOF2 occur simultaneously. These two factors enable the photooxidation of airborne styrene molecules to occur both on the surface and also the near-surface of the photocatalysts. To our knowledge, this is the first reported instance of codoping fluorine and carbon in titana, and is strongly believed to contribute in paving the direction for studying synergisticdoping mechanisms in the development of visible-light responsive photocatalysts. Acknowledgment. The Australian Research Council and the University of Queensland Graduate School are acknowledged

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