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International Journal of Photoenergy
Volume 2012 (2012), Article ID 874509, 8 pages
Fabrication of Al-Doped TiO2 Visible-Light Photocatalyst for Low-Concentration Mercury Removal
1Graduate Institute of Engineering Science and Technology, National Kaohsiung First University of Science and Technology, No. 2 Jhuoyue Road, Nanzih, Kaohsiung 811, Taiwan
2Department of Environmental, Safety, and Health Engineering, Tungnan University, Section 3, 152, Peishen Road, Shenkeng, New Taipei 222, Taiwan
3Institute of Environmental Engineering and Management, National Taipei University of Technology, Section 3, No. 1, Chung-Hsiao E. Road, Taipei 106, Taiwan
Received 13 September 2011; Revised 30 December 2011; Accepted 31 December 2011
Academic Editor: Gongxuan Lu
Copyright © 2012 Cheng-Yen Tsai et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
High-quality Al-doped TiO2 visible-light photocatalyst was prepared via a single-step direct combination of vaporized Ti, Al, and O2 using a 6 kW thermal plasma system. Results showed that the formed Al-doped TiO2 nanoparticles were a mixture of anatase and rutile phase and had a size between 10 and 105 nm. The absorption spectra of the nanoparticles shifted towards the visible light regions, depending on the Al2O3 addition. Ti4+ and Ti3+ coexisted in the synthesized Al-doped TiO2; the Ti3+ concentration, however, increased with increasing Al2O3 addition due to Al/Ti substitution that caused the occurrence of oxygen vacancy. Hg0 breakthrough tests revealed that the nanoparticles had an appreciable Hg0 removal under visible-light irradiation. Nevertheless, moisture reduced Hg removal by the nanoparticles, especially when visible-light irradiation was applied, suggesting that the competitive adsorption between H2O and Hg species on the active sites of TiO2 surface occurred.
Mercury (Hg) releases from nature and anthropogenic sources have been the major focus of environmental studies owing to the toxicity and bioaccumulative behaviors . Hg species in gaseous phase from emission sources in general exist in three main forms: elemental (Hg0), oxidized (Hg2+), and particle-bound (Hgp). Hg2+ and Hgp can be easily removed by air pollution control devices such as wet flue gas desulfurization and electrostatic precipitators. Nevertheless, Hg0 is highly volatile, insoluble in water, and thus difficult to remove from a gas stream.
Using titanium dioxide (TiO2) photocatalysts as adsorbents and catalysts has been advised as a novel technique to effectively remove Hg0 [2–11]. Wu et al.  used in situ produced TiO2 particles to remove Hg under UV irradiation. Upon irradiation with UV light, active sites become available on the TiO2 particle surface and effectively adsorbed Hg to form a complex with TiO2. The manufacturing procedures strongly influence the purity and surface characteristics of resulting TiO2 nanoparticles, which afterward affect the photocatalytic properties. Sol-gel method has been widely used in the bench-scale TiO2 nanoparticles fabrication due to its simplicity to perform [12–15]. Nevertheless, the processing temperature of sol-gel method syntheses is relatively low. A multistep fabrication procedure, that is, sample synthesis and subsequent calcination, was thus needed to transform the obtained TiO2 into anatase or rutile. Thermal plasma has been shown to possess advantages to develop nanoparticles with clean surface and narrow particle size distribution. Using thermal plasma as a heating source may directly vaporize Ti metal having a high melting point at 1941 K and induce the high-purity TiO2 formation in a single step.
A decisive obstruction in the successful application of TiO2 is the band gap energy of 3.2 eV causing the TiO2 only being activated by UV irradiation. Visible-light (VL) TiO2 photocatalysts have therefore obtained great attention in recent years. Several studies have indicated that an improved TiO2 photocatalyst excited by VL sources can be prepared by substitutional doping with metal atoms, such as Fe [16, 17], Er , and Al [19–23]. Because the ionic radii for Al and Ti are similar (0.053 nm for Al3+ and 0.061 nm for Ti4+), Al can easily fill into a regular cation position and form a substitutional solid solution. Numerous studies have shown that Al-doped TiO2 nanoparticles can be manufactured via vapor-phase. Lee et al.  prepared Al-doped TiO2 with thermal plasma using TiCl4 and AlCl3 as precursors. The absorption band of synthesized Al-doped TiO2 nanoparticles shifted from the UV region to the VL region. Choi et al.  prepared Al-doped TiO2 nanoparticles using a citrate-nitrate autocombustion system with Ti solution and Al(NO3)3 solution as precursors. The authors also demonstrated that Al-doped TiO2 gas sensor was more selective and sensitive to CO and O2 under 600°C.
In our previous study, high-purity TiO2 nanoparticles using Ti metal as a precursor were successfully manufactured with a transferred plasma torch . Nevertheless, using transferred DC plasma torch as the heating source has defects on low nanoparticles yield. Another plasma system was established in our earlier study using a nontransferred DC plasma torch as the heating source . Our preliminary test has shown that Al-doped TiO2 nanoparticles can be successfully formed in a single step via this non-transferred DC thermal plasma system. However, the anatase/rutile ratio of the formed Al-doped TiO2 was below 58.2 wt%, which may be due to the higher plasma power (i.e., 8 kW). In this study, Al-doped TiO2 photocatalyst with a broad absorption spectrum was developed under a lower plasma power (i.e., 6 kW). The photocatalyst fabricated from this innovative single-step procedure was tested for Hg0 capture under both UV and VL irradiation. We expected that a reducing plasma power can increase the anatase ratio of the formed Al-doped TiO2 crystal. The greater content of anatase phase in TiO2 may enhance the transformation of Hg0 into Hg2+, which could subsequently enhance the removal effectiveness of TiO2. The Hg0 removal effectiveness of Al-doped TiO2 in the presence of O2, H2O, and light irradiation was further discussed. Notably, few studies have examined the VL photocatalytic effects of Al-doped TiO2 on removal of Hg0 at an extreme low concentration, namely, μg Nm−3 level.
2. Experimental Details
2.1. Synthesis of Al-Doped TiO2 Nanoparticles
The DC plasma torch apparatus used for preparing TiO2 nanoparticles is illustrated in Figure 1. The system comprised a non-transferred plasma torch connected to a DC power supply (Model PHS-15C, Taiwan Plasma Corp., Taiwan), a stainless steel reaction chamber (i.d. = 30 cm; length = 100 cm), a stainless steel powder feeder, a powder filter, a buffer tank, and a vacuum pump (GVD-050A, ULVAC) for shifting the particles floating in the exhaust gas. The plasma torch consisted of a water-cooled copper alloy electrode. The system was performed at 30 A and 200 V. Titanium powder (99.8% purity), Al2O3 powder (99.9% purity), and ultrahigh-purity (UHP) O2 were used as precursors. A mixture of UHP Ar and O2 was used as the plasma gas. The flow rate was 60 L min−1 at Ar : O2 = 3 : 1 by volume. UHP Ar with a flow rate of 2 L min−1 was also used as the carrier gas of the Ti and Al2O3 powder feedstock. The Al2O3/Ti mass ratio was controlled at 0, 0.1, 0.3, and 0.5. The powder feeding rate was fixed at 0.2 g min−1. The gas stream containing the formed TiO2 nanoparticles was passed through the stainless steel powder filter and a buffer tank induced by the vacuum pump.
2.2. Al-Doped TiO2 Nanoparticle Characterization
The particle size and morphology of TiO2 nanoparticles were examined with a transmission electron microscope (TEM, Philips CM-200). Powder X-ray diffraction (XRD, Rigaku Rinet 200) with Cu κα radiation (λ = 1.5405 Å) was used for crystal structure identification. The JCPDS database was used for powder crystalline phase identification. The mass fractions of anatase to rutile in formed TiO2 nanoparticles were calculated by , where is the mass fraction of anatase, is the intensity of (101) reflection of anatase, and is the intensity of (110) reflection of rutile. The diffuse reflectance UV-visible spectra (UV-Vis) of TiO2 nanoparticles were measured from 300 to 800 nm using a spectrophotometer (Hitachi U-3010). X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 1600) was used for Ti, O, Al, and Cu bonding patterns identification. The obtained XPS spectra were deconvoluted with the XPSPEAK software.
2.3. Hg Removal Experiments
Al-doped TiO2 synthesized at the Al2O3/Ti mass ratio of 0.5 was evaluated for the removal effectiveness of low-concentration gaseous Hg0. Gaseous Hg0 was generated with a certificated Hg0 permeation tube (VICI Metronics) which heated at 70 ± 0.1°C to ensure a constant Hg0 diffusion rate. Gaseous Hg0 with a known concentration was mixed with N2, O2, and water vapor which was generated by streaming N2 passed through a water bubbler. All gas mixing and Hg0 injection occurred within a temperature-controlled chamber and heated tubes/lines to prevent water condensation. The generated Hg0-containing gas with a concentration of 10–15 μg Hg0 Nm−3 and a flow rate of 1.5 L min−1 flowed through the photochemical reactor with 30 mg Al-doped TiO2 were irradiated with UV or VL light. The photochemical reactor was a cylindrical quartz tube with an i.d. = 25 mm and a length = 150 mm. The nanoparticles were uniformly coated onto a glass slide, which was placed horizontally in the tube. The UV and VL light sets was located 1 cm above the tube. The photochemical tube reactor was operated at 25°C and atmospheric pressure. The effluent gas from the photochemical reactor flowed through a moisture trap (i.e., a nefion tube) to remove H2O from the gas stream and thus to minimize the interference in Hg detection. The tail gas then flowed through a gold amalgamation column held by a heating coil (Brooks Rand model AC-01) where the Hg0 in the gas was adsorbed. The Hg0 that was concentrated on the gold was then thermally desorbed and sent as a concentrated Hg stream to a cold-vapor atomic fluorescence spectrophotometer (Model III, Brooks Rand Lab) for analysis. Six minutes were needed to complete a test run; ten runs were performed for each test condition. Finally, the exhaust was passed through a carbon trap before it was effluent into the fume hood.
3. Results and Discussion
Figure 2 illustrates the TEM images of the nanoparticles produced at various Al2O3/Ti mass ratios. TEM results indicated that the synthesized Al-doped TiO2 was homogeneous, without significant phase separation or coating on the surface. It was also noticed that the synthesized nanoparticles was in hexagonal or spherical shapes; adding Al2O3 powder had insignificant effects on the shape of the formed nanoparticles. The darker areas in the TEM micrographs showed the agglomeration of Al-doped TiO2 nanoparticles. The powder size of the feedstock Ti and Al2O3 powder were about 5–15 μm. However, nanoparticles formed at Al2O3/Ti mass ratios of 0 to 0.5 were approximately between 10 and 105 nm. These observation results indicate that the injected Ti and Al2O3 powders successfully vaporized at the high flame temperature and subsequently synthesized Al-doped TiO2 nanoparticles via the recombination of vaporized Ti, O and Al atoms in the thermal plasma environment.
XRD powder patterns for the nanoparticles fabricated at various Al2O3/Ti mass ratios are presented in Figure 3. All the peak intensity of powder diffraction was normalized by anatase (101). The experimental results suggested that most of the diffraction peaks could be designated as the presence of anatase and rutile phases. However, the diffraction peaks standing for Al2O3 appeared as Al2O3/Ti = 0.5, based on 2θ = 35.15°, 43.35°, 52.5°, and 57.5° that are indexed to the Al2O3 diffraction pattern. These observations are consistent with our previous work  and suggest that at Al2O3/Ti = 0.5 loading, the thermal plasma was less effective to vaporize the Al2O3 powders and to induce the interactions between Ti, O, and Al atoms. Moreover, the relative content of anatase, depicted with the value (Table 1), noticeably reduced with increasing Al2O3/Ti due to consequent transformation into rutile at an elevating temperature . Notably, the TiO2 was fabricated at a fixed plasma power of 6 kW. The plasma temperature change at various Al2O3/Ti ratios should be small and may not markedly influence the transformation of anatase into rutile. The enhancement in transformation of anatase into rutile may also attribute to the increase in Al doping [19, 21, 22, 28].
Figure 4 demonstrates the UV-visible spectra of the Al-doped TiO2 nanoparticles synthesized at various Al2O3/Ti mass ratios over the wavelength range of 300–800 nm. The commercial Degussa P-25 photocatalyst was also tested for comparison. The experimental results showed that the absorption edge was at ~390 nm for P-25 photocatalyst. The TiO2 nanoparticles synthesized in thermal plasma at the Al2O3/Ti mass ratio of 0 to 0.3 possessed an absorption edge at ~400 nm. The absorption spectra of Al-doped TiO2 slightly shifted from UV to VL region with reference to an increase in Al2O3/Ti mass ration can be assigned to the band gap narrowing relation to the interstitial Al species in the TiO2 crystal [19, 29, 30]. Especially, TiO2 synthesized at Al2O3/Ti = 0 also had band gap absorption at ~400 nm, which was comparable to that of Al-doped TiO2 formed at Al2O3/Ti = 0.1. The extent of red shift and band broadening may be attributed to the presence of oxygen vacancies in TiO2 crystal formed in the high-temperature plasma flame. Numerous studies have shown the appearance of the visible-light activity was attributed to the newly formed oxygen vacancy state in the TiO2 band structure [24, 31]. Further evidences and explanation are presented based on XPS analysis. In contrast, the Al-doped TiO2 synthesized at Al2O3/Ti = 0.5 showed strong absorption in the VL range, suggesting that the extent of red shift and broadening was greatly dependent on Al2O3 concentrations in the plasma environment.
The XPS spectra of Ti2p for the synthesized TiO2 nanoparticles are shown in Figure 5. The Al2p spectra for the synthesized TiO2 nanoparticles at a Al2O3/Ti mass ratio of 0.1, 0.3, and 0.5 are shown in Figure 6. The Ti2p XPS spectra acquired from TiO2 fabricated at various Al2O3/Ti mass ratios were deconvoluted into four peaks within 456.4–464.5 eV, including Ti4+2p1/2, Ti3+2p1/2, Ti4+2p3/2, and Ti3+2p3/2. These peaks are indications to the presence of TiO2 (Ti4+2p1/2 and Ti4+2p3/2) and Ti2O3 (Ti3+2p1/2 and Ti3+2p3/2), respectively . For the Al-doped TiO2, the Al2p peaks at a binding energy of 75.5 eV can be attributed to the presence of Al3+. The results suggested that the Al3+ content enhanced with an increase in Al2O3 addition. The calculated Ti3+/(Ti3+ + Ti4+) ratios for Al-doped TiO2 at Al2O3/Ti = 0, 0.1, 0.3, and 0.5 were 3.1, 17.1, 17.5, and 33.2%, respectively, based on the deconvoluted peak area. These data indicated that Ti3+ concentration greatly enhanced with increasing Al2O3 addition owing to the transformation of TiO2 into Ti2O3. We suspected that Ti4+/Al3+ ionic substitution may take place during the Al doping. If this assumption held, when Ti4+ (ionic radius = 0.061 nm) is substituted by Al3+ (ionic radius = 0.053 nm) from TiO2 crystal, the lattice mismatch occurs as a result of that the ionic radius of Ti4+ is larger than that of Al3+. To atone for the smaller ionic radius of Al3+, a Ti species having an ionic radius larger than Ti4+ is needed. Consequently, Ti4+ is reduced to Ti3+ (ionic radius = 0.067 nm). The observed Ti3+ peaks in the present study are consistent with Steveson et al.  and our previous work . In addition, the formal charge generated by the substitution of Ti4+ with Al3+ can also be compensated via the formation of O- from O2-, which resulted in the oxygen vacancies found in Al-doped TiO2 [21, 33, 34].
Notably, about 1.95–4.62 at.% Cu was found in the formed Al-doped TiO2 based on the XPS analysis (Table 2). The Cu impurity was from the vaporization of plasma torch made of Cu alloy under the high-temperature plasma environment. Associated with the results from the UV-visible analysis, the doped Cu and the oxygen vacancy may synergistically contribute to the observed red shift in the UV-visible absorption spectrum for the non-Al-doped TiO2 (i.e., Al2O3/Ti = 0). Nevertheless, Zhang et al. suggested that Cu content < 38 at.% in TiO2 had negligible effects on the Ti2p binding energy of XPS examinations . Therefore, isomorphous substitution due to Al doping into TiO2 crystal should be the major contribution of the increasing Ti3+. The red shift in the absorption spectra of TiO2 nanoparticles thus primarily was attributed to the Al doping and generated oxygen vacancy (Figure 4).
The Hg0 adsorption breakthrough results for Al-doped TiO2 nanoparticles synthesized at Al2O3/Ti = 0.5 are shown in Figure 7. The experimental parameters included O2 concentration, humidity, and the type of light sources. These parameters were tested alternately by gradually increasing the O2 concentration from 0% to 12% combined with introducing H2O (20% relative humidity) and UV/VL irradiation to the photocatalytic reactor. The experimental results showed that Hg0 capture was very small with rapid breakthrough at the 0% O2, dry (H2O < 0.1 vol%), and dark condition (runs 0–10 in Figure 7), manifesting that the synthesized Al-doped TiO2 (Al2O3/Ti = 0.5) was less effective in removal of Hg0 under the test condition. This result was anticipated because Hg0 appeared to the main Hg species at the 0% O2, dry, and dark condition and was not easy to form a strong binding with the surface of TiO2. However, an increase in the Hg0 capture to 25% was apparently observed when UV irradiation was applied (breakthrough down to approximately 0.75; runs 11–20). The significant enhancement in Hg0 removal for Al-doped TiO2 nanoparticles under UV irradiation is a strong indication that this sample had a good photocatalytic potential to transform Hg0 into Hg2+ that enhanced the adsorption onto Al-doped TiO2. It is also noteworthy that without the presence of O2, VL was less effective in photocatalytic oxidation than UV (runs 21–30). In addition, a significant decrease in Hg0 capture was observed at the humid condition (runs 31–60). This result suggest that H2O competitively adsorbs onto the TiO2 active sites, causing the reemission of adsorbed Hg species, which can be Hg0 or Hg2+ needed to be further examined. The experimental results presented here are in agreement with those found in earlier studies [6, 7, 24, 36]. Li and Wu reported that the physically adsorbed Hg0 can be desorbed from the surface of a SiO2-TiO2 composite by water vapor at high concentration, which suggested that Hg0 is only weakly adsorbed on the sorbent surface . Dissimilar to H2O, O2 notably improved the adsorption of Hg0; increasing O2 concentration strongly enhanced the Hg0 capture of Al-doped TiO2 to up to 40% (equivalent to breakthrough of 0.6; Figure 7). It is noteworthy that when O2 was > 6%, Hg0 capture was similar for Al-doped TiO2 under either UV or VL irradiation (runs 131–150 and 221–240). These data not only revealed the importance of O2 in enhancing Hg0 capture, but also verified the visible-light activity of the synthesized Al-doped TiO2 nanoparticles on Hg0 oxidation/adsorption.
Al-doped TiO2 nanoparticles were successfully synthesized in a single step using Ti powders, Al2O3 powders, and O2 by a nontransferred plasma torch system. TiO2 nanoparticles formed at Al2O3/Ti mass ratios 0 to 0.5 were approximately between 10 and 105 nm. The crystal phases of the formed TiO2 nanoparticles were mainly in anatase and rutile forms. However, increasing the Al2O3 addition caused the ratio of anatase to rutile decreased. The presence of oxygen vacancy and the substitution of Ti4+ with Al3+ were suspected to cause the slight red shift in the absorption edge to lower energy due to band gap narrowing. Al doping and oxygen vacancy in the TiO2 crystal may also result in the phase transformation from TiO2 to Ti2O3. Hg breakthrough results showed that the Hg0 removal with formed Al-doped TiO2 in a dry condition was greater than that in a humid condition when light irradiation was applied. Hg capture was also found to be markedly enhanced by increasing O2 concentration. Nevertheless, H2O showed deteriorating effects on the adsorption of Hg through competition for active sites on the Al-doped TiO2 surface. Results presented here suggest that Hg0 removal using synthesized Al-doped TiO2 nanoparticles may be greatly affected by the extent of catalytic transformation of Hg0 into Hg2+ and amphoteric (hydrophilic-hydrophobic) surface properties of Al-doped TiO2.
We envision that the photocatalyst can be successfully used to capture Hg from coal-derived flue gas. Nevertheless, it is imperative to note that this study employed gas streams consisting of Hg0 in a moisture-oxygen-nitrogen mix. Coal-derived flue gas is a complex mixture also containing fly ash particles, moisture, CO, and many acid gases. For example, a typical untreated flue gas derived from the combustion of a US low sulfur eastern bituminous coal can contain 5–7% H2O, 3-4% O2, 15-16% CO2, 1 ppbv total Hg, 20 ppm CO, 10 ppm hydrocarbons, 100 ppm HCl, 800 ppm SO2, 10 ppm SO3, 500 ppm NOx, and balance N2 [9–11]. The influences of the flue gas components on Hg removal using TiO2 photocatalysts are thus highly needed to be further investigated.
This paper was supported by National Science Council of Taiwan (NSC95-2622-E-327-002-CC3) and Taiwan Plasma Corp., Kaohsiung, Taiwan. The authors acknowledge Mr. Chin-Cheng Ho, Taiwan Plasma Corp. and Dr. Hsunling Bai, National Chiao Tung University, Taiwan, for their technical assistance. The authors also thank the two anonymous reviewers for their valuable comments and suggestions to improve the quality of the paper.
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