Fabrication of Al-Doped TiO<sub>2</sub> Visible-Light Photocatalyst for Low-Concentration Mercury Removal

Tsai, Cheng-Yen; Kuo, Tien-Ho; Hsi, Hsing-Cheng

doi:https://doi.org/10.1155/2012/874509

International Journal of Photoenergy

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TiO<sub>2</sub> Photocatalytic Materials

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Research Article | Open Access

Volume 2012 | Article ID 874509 | https://doi.org/10.1155/2012/874509

Fabrication of Al-Doped TiO₂ Visible-Light Photocatalyst for Low-Concentration Mercury Removal

Cheng-Yen Tsai,¹Tien-Ho Kuo,²and Hsing-Cheng Hsi³

Academic Editor: Gongxuan Lu

Received13 Sept 2011

Revised30 Dec 2011

Accepted31 Dec 2011

Published14 Mar 2012

Abstract

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.

1. Introduction

Mercury (Hg) releases from nature and anthropogenic sources have been the major focus of environmental studies owing to the toxicity and bioaccumulative behaviors [1]. Hg species in gaseous phase from emission sources in general exist in three main forms: elemental (Hg⁰), oxidized (Hg²⁺), and particle-bound (Hg_p). Hg²⁺ and Hg_p 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. [2] 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 [18], 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. [19] 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. [23] 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 [24]. 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 [25]. 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 [26], 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 [25] 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 [27]. 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 Ti_2p for the synthesized TiO₂ nanoparticles are shown in Figure 5. The Al_2p 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 Ti_2p 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⁺_2p1/2and Ti⁴⁺_2p3/2) and Ti₂O₃ (Ti³⁺_2p1/2and Ti³⁺_2p3/2), respectively [32]. For the Al-doped TiO₂, the Al_2p 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. [33] and our previous work [25]. In addition, the formal charge generated by the substitution of Ti⁴⁺ with Al³⁺ can also be compensated via the formation of O^- from O^2-, which resulted in the oxygen vacancies found in Al-doped TiO₂ [21, 33, 34].

(a)

(b)

(c)

(d)

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 Ti_2p binding energy of XPS examinations [35]. 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 [6]. 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.

4. Conclusion

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 NO_x, 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.

Acknowledgments

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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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.

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