Comparative Investigation of Photocatalytic Degradation of Toluene

Feb 8, 2013 - Department of Chemical Engineering and Cooperative Research Centre ... and CO2 reduction on various types of photocatalytic materials. ...
2 downloads 0 Views 482KB Size
Download PDF
Article pubs.acs.org/IECR

Comparative Investigation of Photocatalytic Degradation of Toluene on Nitrogen Doped Ta2O5 and Nb2O5 Nanoparticles Ruh Ullah, Hongqi Sun, Ha Ming Ang, Moses O Tadé, and Shaobin Wang* Department of Chemical Engineering and Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC-CARE), Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia ABSTRACT: Nb2O5 and Ta2O5 nanoparticles were prepared at a moderate sintering temperature of 700 °C using a solution method. Nitrogen doped Nb2O5 and Ta2O5 were obtained by ammonia gas treatment at 700 °C. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and UV−vis diffuse reflectance spectroscopy. All the materials were used for photocatalytic decomposition of gaseous toluene irradiated with artificial solar light and pure visible light. Most of the materials were multiple crystallites with orthorhombic and monoclinic structures. Tantalum based compounds, undoped (Ta2O5 (lab)) and nitrogen doped (Ta2O5:N (lab)), have shown much better performance than niobium based materials. Ta2O5:N (lab) decomposed gaseous toluene with a rate similar to those of Ta2O5 (lab) and Ta2O5:N (com) in the initial 60 min but at a higher rate at extended time under artificial sunlight. Ta2O5:N (lab) decomposed about 70% toluene with artificial sunlight and 30% with pure visible light (λ = 400 nm). The material also exhibited a very stable performance with artificial sunlight and pure visible light.

1. INTRODUCTION Visible light photocatalysis has been intensively investigated in water splitting, oxidation of volatile organic compounds, and CO2 reduction on various types of photocatalytic materials. Recent investigations explored some heavy metal oxides such as Ta2O5 and Nb2O5 that have photophysical properties almost comparable to those of the landmark photocatalyst, i.e., TiO2. These materials and their modified forms may be activated with visible light and can display better performance than the commercial photocatalyst. However, large band gaps (3.9 and 3.4 eV for Ta2O5 and Nb2O5, respectively) are still the major concern which inhibits activation of these materials with pure visible light irradiation. Nevertheless, band gap modification through nitrogen doping has been proven as a successful technique in the red shift of the optical absorption of these materials. The highly mesoporous structure of these materials is advantageous for the efficient transition of photogenerated carriers to the adsorbed species on the surface. Structure porosity and variation in the pore wall thickness, stability, and hydrophobicity of these compounds are strongly dependent on the preparation methods.1 Although the porosity of Ta2O5 prepared with a neutral surfactant templating method (NST) and sintering at 800 °C is weaker than the material sintered at 700 °C and prepared with a ligand assisted templating method (LAT), the NST material has a higher stability and more hydrophobicity. The porosity of Nb2O5 prepared at various sintering temperatures (400, 450, and 500 °C) was reduced at increased calcination temperature,2 indicating pore collapse and particle aggregation at high temperature. Nb2O5 prepared at higher temperature has a lower band gap (3.02 eV) than that prepared at lower temperature; the latter has better photocatalytic activity for H2 production which was mainly attributed to the thinner pore wall. A thicker pore wall will affect electron migration and cause recombination of electron−hole pairs. Kominami et al.3 have reported that amorphous Nb2O5 due to its high surface area possessed better photocatalytic activity for © 2013 American Chemical Society

hydrogen production and mineralization of acetic acid than that of crystalline material and commercial Nb2O5. Although Nb2O5 can be used as an efficient photocatalyst with a better selectivity for aerobic oxidation of various types of amines4 and alcohols4,5 under visible light irradiation, its photocatalytic performance was lower than that of TiO2. Carbon in carbonate species adsorbed on Nb2O5 was successful in modifying the surface and optical absorption of the material which consequently enhanced its photocatalytic activities under visible light irradiation.6 Ta2O5 annealed at 700 °C in an ammonia environment was doped with nitrogen,7 whereas increasing the annealing temperature to 850 °C transformed the material to the TaON phase. UV/visible spectroscopy has shown a red shift in the optical absorption which clearly indicated that doped nitrogen substituted for oxygen and therefore reduced the band gap of the material. Photocatalytic activities of all the materials evaluated with gaseous 2-propanol (IPA) under UV and visible light irradiation revealed that nitrogen doped Ta2O5 decomposed IPA to CO2 and acetone with a faster rate than undoped Ta2O5; however, TaON was not active under visible light irradiation and could not decompose IPA, though the band gap of TaON prepared with sol−gel techniques was observed to be 2.4 eV.8 Sato et al.9 have reported reduction of CO2 to HCOOH via visible light photocatalytic reaction using a p-type N-doped Ta2O5 as a photocatalyst, triethanolamine (TEOA) as an electron donor, and Ru complex as a metal complex. In this system, upon visible light irradiation, electrons were photoexcited to the conduction band of Ta2O5, transferred efficiently to the metal complex adsorbed on the surface of the Received: Revised: Accepted: Published: 3320

August 29, 2012 November 7, 2012 February 8, 2013 February 8, 2013 dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

2.2. Characterization of Catalysts. Crystalline structures of materials were analyzed by an X-ray diffractometer (Bruker D8 Advance equipped with a Lynx eye detector, Bruker-AXS, Karlsruhe, Germany) operated at 40 kV and 30 mA. The scanning rate was 0.2 s/step with 2θ (10−90°) and step size of 0.02°. Cu Kα (λ = 1.541 78 Å) was used as a X-ray source with a divergent slit of 0.300 and 2.5° primary and secondary Soller slits. The optical absorption of samples was determined by UV−vis absorbance spectroscopy using the diffuse reflectance method (JASCO V-670 Spectrometer). Morphology and chemical composition of the materials were examined by scanning electron microscopy (SEM; ZEISS NEON 40EsB) equipped with an energy dispersive spectrometer (SEM−EDS). Samples were directly coated on an aluminum stub where 3 nm platinum coating was used as a conducting material. The existence of nitrogen in the crystal structure was confirmed by X-ray photoelectron spectroscopy (XPS) analysis using a Kratos Axis ULTRA X-ray machine. Particulars and conditions of the XPS machine were as follows: lens mode, hybrid; resolution, pass energy 160; iris (aperture), slot 3 × 12 mm; (slot) anode, mono (Al (mono)) (150 W); step (meV), 1000.0; dwell (ms), 100; sweeps, 1. High resolution was obtained at a pass energy of 40 eV as the Co level was low. Fourier transform infrared spectroscopic (FTIR) analysis was performed on a Perkin-Elmer Model FTIR-100 with a midinfrared detector. 2.3. Photocatalytic Evaluation. Photocatalytic activities of both undoped and doped samples were evaluated in the decomposition of gaseous toluene with artificial solar light irradiations. A xenon lamp (UXL-306, Ushio) was used as the artificial sunlight source. The average intensities of the solar simulator with a quartz sheet were 26.0 mW/cm2 (at 315−400 nm) and 240.0 mW/cm2 (at 400−1050 nm). However, the intensity of light was reduced to 99.0 mW/cm2 (at 400−1050 nm) with a soda glass filter for visible light. Gaseous toluene was used as a model air contaminant with a concentration of 100 ppm in air. Toluene decomposition was evaluated in a flow reactor connected to an online gas chromatograph (Shimadzu GC17A) fitted with a GS-GSPRO column of 60 m length and 0.32 mm diameter. The oven temperature of the GC was 220 °C, and the flame ionization detector temperature was kept at 250 °C. In a typical experiment, 300 mg of photocatalysts was dispersed in 10 mL of deionized water in a Petri dish of 45 mm diameter and dried in an oven overnight. The Petri dish was kept inside an airtight stainless steel reactor fitted with a removable quartz sheet in order to allow both UV and visible radiation in. A flow of toluene at 100 ppm was continuously passed into the reactor (1.08 L) at a flow rate of 60 mL/min. The temperature of the reactor was controlled by continuously passing cooling water through the reactor and also by blowing fresh air onto the reactor from outside.

semiconductor material, and reduced CO2 to HCOOH. The difference in the potential energy of the conduction band minimum of the semiconductor and the CO2 reduction potential energy of the metal complex was the main reason for the reduction of CO2. It was further detailed10 that, in the case of N-doped Ta2O5, photoexcited electrons were trapped at deep defect sites, after being excited at shallow defect states; however, in the case of Ru loaded N-doped Ta2O5, electronic transition occurred from shallow defect states to the adsorbed Ru complex. Various studies11 have shown that Ta2O5 phase changes to ptype nitrogen doped Ta2O5,12 TaON, and Ta3N5, respectively, by increasing the annealing temperature above 700 °C in nitrogen environment.13 Band gaps and conduction band edges of Ta2O5, TaON, and Ta3N5 determined with the ultraviolet photoelectron spectroscopy (UPS) method were 3.9, 2.4, and 2.1 eV, and −3.93, −4.1, and −4.14, respectively. Valence band energies of TaON (−6.6 eV) and Ta3N5 (−6.02 eV) were much higher than that (−7.93 eV) of Ta2O5. The valence bands of Ta2O5, TaON, and Ta3N5 consisted of O 2p, O 2p + N 2p, and only N 2p orbitals, respectively, whereas conduction bands of these materials are made of Ta 5d orbitals. The latter two compounds have more absorption in the visible region, and might be suitable for photocatalytic reactions. Ho et al.14 have attempted to prepare colloidal Ta3N5 nanoparticles via different techniques for enhanced H2 production via visible light irradiation; however, a very minute increase in the band gap of the nanoparticles was not promising for the enhancement of efficiency. Zhang et al.13 have found that a band gap of Ta3N5 prepared by nitriding amorphous Ta2O5 is particle size dependent with an average value of 2.04 eV. The material has shown better performance than nitrogen doped TiO2 for methylene blue decomposition with visible light irradiation. Herein we report wet-chemical synthesis of Nb2O5, Ta2O5, and nitrogen doped forms. Nitrogen doped Ta2O5 (lab) has shown much better performance than both undoped Ta2O5 (lab) and commercial Ta2O5 and TiO2 for gaseous toluene decomposition with artificial solar light and visible light irradiation.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Polycrystalline Ta2O5 and Nb2O5 materials were prepared by dissolving the precursor salts in ethanol. All chemicals were obtained from Sigma-Aldrich at 99.99% purity and were used as received. In a typical synthesis, 20 mmol of tantalum(V) chloride (TaCl5) or niobium(V) chloride (NbCl5) was dissolved separately in 80 mL of ethanol and mixed with 0.5−1.0 mL of concentrated nitric acid, and the mixture was continuously stirred and heated at 80 °C for 6 h. The final solution with no precipitation was then aged for 24 h at room temperature and was kept in an oven at 60 °C until dried completely. Solid samples were ground and calcined at 700 °C for 40 h in air to obtain undoped Ta2O5 (lab) and Nb2O5 (lab). A commercial Ta2O5 was obtained from SigmaAldrich at 99.99% purity. For synthesis of N-doped Ta2O5 (Ta2O5:N (lab)) and Nb2O5 (Nb2O5:N (lab)), dried Ta2O5 or Nb2O5 was annealed at 700 °C in a tube furnace, where N2 gas was passed through an enclosed flask containing 28% ammonia solution. The mist of ammonia and N2 gas was flowing at a rate of 100 mL/min for 40 h. To maintain the proper quantity of ammonia in nitrogen, the ammonia solution was refreshed after every 10 h.

3. RESULTS AND DISCUSSION 3.1. Material Structure. Multicrystalline phases of Nb2O5 (lab) containing orthorhombic and monoclinic structures were obtained by sintering the raw mixture at 700 °C for 40 h. The X-ray diffraction (XRD) pattern of Nb2O5 (lab) shown in Figure 1 exactly matched that reported in the literature.15 Lin et al.16 recently observed the phase transformation from amorphous to crystalline at 500 °C and detected major peaks of Nb2O5 at the higher temperature of 650 °C. Nb2O5 crystallizes in various forms (T, B, H, N, and P) which depend 3321

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

structure21 with lattice parameters a = 43.977 Å, b = 3.894 Å, c = 6.209 Å, α = 90.0°, β = 90.0°, and γ = 90.0°, and primitive Pmm2. Most of the XRD patterns of undoped and nitrogen doped materials were similar; however, three weak peaks observed at 2θ = 25.45°, 32.74°, and 42.82° showed the formation of minor TaON.13 Ta2O5 has a distorted structure made of octahedral TaO6 and pentagonal bipyramids TaO7. Each tantalum atom in the structure is attached to seven oxygen atoms with an average tantalum−oxygen bond distance of about 2.04 Å.22 Theoretical and experimental analyses have confirmed the existence of a large number of oxygen vacancies at energy positions (which lie within the visible region) in TaO6 octahedron.24,25 Therefore, we prepared undoped Ta2O5 (lab) and nitrogen doped orthorhombic Ta2O5:N (lab) and use these materials for photocatalytic decomposition of toluene in air. Figure 3 shows SEM images of Ta2O5:N (lab). SEM micrographs reveal that nitrogen doped Ta2O5 crystallizes in various morphologies and sizes. The average size is in the range of 100 nm, which then aggregated in large agglomerates. Gou et al.26 recently observed almost similar morphologies of Ta2O5 but with smaller particle size and annealed at 700 °C for 4 h. Obviously, the longer annealing time in our case promoted the particle aggregation; however, a large number of nanoparticles residing on aggregated structures is highly beneficial for photocatalytic reactions. Figure 3c clearly shows tiny particles on the agglomerated surfaces, which increases the active surface area for the photocatalytic reaction.27 EDS analysis of Ta2O5 (lab) and Ta2O5:N (lab) shown in Figure 4 clearly indicates the existence of Ta and oxygen atoms. Since the structure of Ta2O5 consists of TaO6 octahedra and pentagonal TaO7, more strong peaks for Ta than for O can be observed in the EDS analysis. However, no nitrogen was detected by EDS analysis in the nitrogen doped Ta2O5. XRD has shown some minor peaks for TaON, but due to the very low concentration, nitrogen cannot be detected with this technique. XPS analyses (Figure 5) were employed to confirm the existence of nitrogen in Ta2O5:N (lab). According to XPS spectra, Ta2O5:N (lab) contains tantalum (Ta), oxygen (O), and nitrogen (N) atoms. Ta was observed at binding energies of 23.4−25.1 eV (double peaks), 229.5 eV, and 403.5 eV, which corresponded to Ta 4f, Ta 4d3/2, Ta 4d5/2, and Ta 4p3/2, respectively.28 Oxygen and nitrogen were detected at binding energies of 530.5 and 394 eV, respectively. The large peak (Figure 5, inset) in the N 1s region is from Ta 4p3/2, where the small sharp peak is nitrogen, indicating Ta−N bonding. Murase et al.7 have observed a Ta−N bond at a binding energy of 397 eV, which is at a higher energy level than in our case. The difference in the binding energy of the Ta−N bond in our case arises from the difference in annealing temperature of the sample. We annealed the material at 700 °C, while Murase et al. have prepared materials at lower temperatures. Figure 6 compares UV−vis optical spectra of Ta2O5 (lab), Nb2O5 (lab), and commercial Ta2O5. It is evident that the synthesized Ta2O5 and the commercial Ta2O5 have the same absorption edge of 330 nm, whereas the absorption edge of Nb2O5 (lab) is at 420 nm, suggesting absorption in the visible range. However, the absorption onsets of Nb2O5 prepared at 500 °C and Ta2O5 annealed at 650 °C were 38529 and 315 nm,27 respectively. We annealed samples (Nb2O5 (lab) and Ta2O5 (lab)) at a higher temperature for a longer time; therefore, large number of oxygen vacancies may have been

Figure 1. XRD patterns of Nb2O5 (lab) and Nb2O5:N (lab) calcined at 700 °C.

on the preparation method and sintering temperature.17 The material prepared at low temperatures is metastable, while the one prepared at higher (>1100 °C) temperatures has higher stability. Nb2O5 prepared at lower temperatures (up to 600 °C) for a short time has some forms of oxychloride.18 Therefore, we calcined the raw material at the medium temperature of 700 °C to obtain pure and highly crystalline mesoporous Nb2O5. The crystalline parameters of Nb2O5 (lab) for orthorhombic (a = 6.1750 Å, b = 29.1750 Å, c = 3.930 Å, α = 90.00°, β = 90.00°, and γ = 90.00°) and monoclinic (a = 3.9830 Å, b = 3.8260 Å, c = 12.790 Å, α = 90.00°, β = 90.75°, and γ = 90.00°) structures were similar to those observed for TT phase and T phase, respectively.19 Figure 1 shows the formation of the major phase of Nb4N5 in N-doped Nb2O5 with minor Nb2O5 residuals. Patel et al.20 also obtained a similar diffraction pattern for Nb4N5 by annealing a sample at 765 °C. Figure 2 shows XRD patterns of undoped and nitrogen doped Ta2O5 obtained from either commercial or lab-

Figure 2. XRD patterns of (a) undoped commercial Ta2O5, (b) undoped Ta2O5 (lab), (c) commercial Ta2O5 treated in NH3/nitrogen (Ta2O5:N (com)), and (d) Ta2O5:N (lab).

synthesized Ta2O5. All the major peaks of the lab-prepared materials observed at 2θ = 22.70, 28.19, 36.58, 46.68, and 55.54° corresponding to (001), (100), (101), (002), and (102)21 were matched with those of the commercial Ta2O5. Unlike high temperature sintered materials, H-Ta2O522 and LTa2O5,23 Ta2O5 (lab), and Ta2O5:N (lab) have orthorhombic 3322

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

Figure 4. Energy dispersive spectroscopy of Ta2O5 (lab) and Ta2O5:N (lab).

gaps30,31 of all the materials are given in Table 1. It must be noted that two absorption edges for Ta2O5:N (com) have been recently observed by Ho et al.8 The two absorption edges obviously originated from the two main sources, i.e., (i) electronic excitation from the conduction band to the valence band of the material and (ii) electronic excitation at the various defect sites such as oxygen vacancies and reduced Ta metal defects.8 Various studies have shown that the conduction band of pristine Ta2O5 is composed of Ta 5d orbitals with an energy position of −7.3 eV and the valence band consists of O 2p orbitals with an energy position of −3.4 eV, thus resulting in a wide band gap of 3.9 eV.11 Even though the pristine Ta2O5 has a wide band gap, it still has very prominent photoluminescence at visible light. This huge photoluminescence of pristine Ta2O5 originates both from shallow (350−450 nm)32 and deep (500− 650 nm) defect states25 which are mainly attributed to the existence of oxygen vacancies, and Ta defect sites. Heat treatment of Ta2O5 in the reduced environment further increases the defect states by producing new oxygen vacancies at shallow energy levels (350−500 nm).25 Definitely the new defect states at the shallow energy levels resulted in a significant optical absorption as observed on Ta2O5:N (com). An almost similar shoulder in the optical absorption from 310 to 550 nm of commercial Ta2O5 treated in ammonia has been reported,33 which was referred to the formation of TaON and Ta3N5. Unlike treated commercial material, the laboratory made Ta2O5:N (lab) has very smooth optical absorption starting from 545 nm. This indicates that the defect states in Ta2O5:N (lab) are extended to broad energy bands34 rather than the discrete energy levels originating from particular defect states. As shown in Figure 6, undoped Ta2O5’s (both laboratory made and commercial) have exactly the same optical absorption, where no evidence of longer wavelength absorption can be seen. This indicates that a limited number of defect states in pristine Ta2O5 can give rise to photoluminescence,35 but they are not enough to absorb a significant amount of visible light. However, nitrogen doping in Ta2O5 contributes to the optical absorption of the material by two ways, i.e., increasing the number of defect states and producing an energy level above the valence band originated from the N 2p orbital. Chun et al.11 have calculated the conduction and valence band positions of the undoped and nitrogen doped Ta2O5’s and suggested that the contribution of N 2p orbitals shifted the valence band to the higher energy level and reduced the effective band gap of Ta2O5 to 2.4 eV.

Figure 3. SEM micrographs of Ta2O5:N (lab).

introduced into these materials, which resulted in a red shift in the optical absorption.12 Figure 7 shows UV−vis diffuse reflectance spectroscopy of Nb2O5:N (lab), Ta2O5 (lab), Ta2O5:N (lab), and Ta2O5:N (com), which clearly indicates the effect of doping on the optical absorption of the materials.12 Nitrogen doped Nb2O5 has a huge absorption which is extended to the longer wavelengths. Nitrogen doped commercial Ta2O5 has two absorption onsets at 330 and 550 nm, respectively, while Ta2O5:N (lab) has a single absorption starting at 545 nm. Ta2O5:N (lab) absorbs more visible light than Ta2O5:N (com). The absorption edges and the corresponding calculated band 3323

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

Figure 5. XPS spectra of Ta2O5:N (lab) showing existence of N 1s peaks at energy levels 394−403 eV. N peaks are further detailed in the inset at the top right.

Table 1. Absorption Edges and Band Gaps of Materials

a

material

abs edge (nm)

band gapa (eV)

Ta2O5 (com) Ta2O5 (lab) Ta2O5:N (com) Ta2O5:N (lab) Nb2O5

330 330 330, 550 545 420

3.76 3.76 3.76, 2.25 2.28 2.97

Eg = 1240/λ.

Theoretical36 and experimental37 investigations have confirmed that pristine Ta2O5 has defect states originating mainly from oxygen vacancies. These defect states are the main reasons for the photoluminescence38 and photocatalytic performance of this material. Other studies have also shown that the number of these defect states can be further enhanced by doping Ta2O5 with nitrogen. Ta2O5:N (lab) has significant optical absorption, confirming the contributions of both the large number of defect states and the N 2p orbitals to the optical and electronic properties of the material.36 3.2. Photocatalytic Decomposition of Gaseous Toluene. The photocatalytic performances of all the materials [Nb2O5, Nb2O5:N, Ta2O5 (lab), Ta2O5:N (com), and Ta2O5:N (lab)] were examined by decomposition of gaseous toluene and compared with that of commercial Ta2O5. Figure 8 shows photocatalytic decomposition of toluene on these materials irradiated with artificial solar light. Nb-based compounds (Nb2O5 and Nb2O5:N) were almost inactive under solar light because they only decompose 5−10% of toluene within the first few hours of irradiation. The commercial Ta 2O5 was comparatively better than the Nb-based materials and decomposed about 30% of toluene within a similar time of irradiation. However, N-doped commercial Ta2O 5 was considerably more effective, giving almost 60% toluene removal within the initial 2 h of solar light irradiation. Ta2O5 (lab) was also efficient, giving toluene removal similar to that of Ta2O5:N (com). Ta2O5:N (lab) was found to be the most efficient since it could decompose more than 70% of toluene within the initial 1.5 h under artificial solar light irradiation.

Figure 6. Diffuse reflectance spectroscopy of synthesized Ta2O5 (lab), Nb2O5 (lab), and commercial Ta2O5.

Figure 7. Diffuse reflectance spectroscopy of undoped Ta2O5 (lab), nitrogen doped Ta2O5:N (lab), Ta2O5:N (com), and Nb2O5:N.

3324

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

400 nm); however, the nitrogen doped commercial material, Ta2O5:N (com), was capable of decomposing up to 10% of toluene with visible light irradiation. Unlike Ta2O5:N (com), with solar light activity, laboratory made Ta2O5 showed much better performance than Ta2O5:N (com), at about 15% toluene removal. Ta2O5:N (lab) has shown up to 30% toluene decomposition. The higher photocatalytic activities of Ta2O5:N (lab) are mainly attributed to the smaller band gap of the material, the existence of large number of defect states, and oxygen vacancies on the material surface. Nitrogen doped Ta2O5 has been recently investigated for photocatalytic oxidation of organic compounds,7 water splitting into hydrogen/oxygen,40 and CO2 reduction to methanol9 with UV and solar light irradiation. The higher photocatalytic hydrogen production of a mesoporous Ta2O5 was mainly attributed to the efficient charge transfer due to the thinner pore wall of the material.41 In addition, the photocatalytic hydrogen production with visible light was also attributed to the combined effect of NiO coating on Ta2O5 surface.42,43 However, various other studies have mentioned the presence of defect states and oxygen vacancies,24 which lie within the range of visible light; therefore, the material experiences photoluminescence.44 Although the band gap of Ta2O5 is higher and even larger than that of the landmark TiO2, Ta2O5 possess excellent photoluminescence due to the existence of defect states at lower energy levels. Additionally, the number of these defect states was increased with nitrogen doping,25 which also enhanced the photocatalytic activity. Photoluminescence arises from the recombination of photogenerated electron−hole pairs, which means that nitrogen doping further enhances the recombination of carriers. However, we suggest that the electron−hole pairs after generation by incident light quickly react with surface species before undergoing recombination and produce the hydroxyl radicals. The recombination of electron− hole pairs requires more energy than what is needed for formation of hydroxyl radicals. Although the photogenerated carriers have enough energy to recombine, photogenerated holes participate immediately in hydroxyl radical formation due to the lower energy for the process. The photoexcited electrons also leave a hole at the trap center of a cap oxygen vacancy between TaO6 octahedra and TaO7 bipyramids.45 Both of these processes depend upon the defect states and oxygen vacancies which are increasing with nitrogen doping. Thus the photocatalytic activities of nitrogen doped Ta2O5 are enhanced. Intermediate adsorption, photocatalyst stability, and reusability are important factors affecting the photocatalytic performances of various materials. Figure 10 shows the performances of commercial Ta2O5, Ta2O5:N (com), and Ta2O5:N (lab) in repeated tests. The photocatalytic activities of Ta2O5 (com) and Ta2O5:N (com) significantly reduced after they were used repeatedly. The activity of Ta2O5:N (com) reduced toluene removal from 60 to 40% when used for a second time. Ta2O5(com)also exhibited similar activity reduction when used for a second run. However, Ta2O5:N (lab) has shown much more stable performance and its activity remained the same even after it was used four times. The XRD analysis of Ta2O5:N (lab) before and after photocatalytic reaction is shown in Figure 11, which clearly indicates that the structure of the material remains unchanged. This revealed that Ta2O5:N (lab) has a much stronger stability under solar light irradiation where no photocorrosion or instability of crystalline structure occurred.

Figure 8. Photocatalytic decomposition of toluene with various materials irradiated with solar simulator.

Furukawa et al.4 have reported selective oxidation of various amines to imines using Nb2O5 as a heterogeneous photocatalyst irradiated with UV light (330−390 nm). NiO and Pt coated Nb2O52,16 have also been used for hydrogen production with UV light irradiation (λ = 360 nm). The use of acidic Nb2O5 has been attempted for the degradation of 1-pentanol with UV light.19 It was found that Nb2O5 exhibited good photocatalytic performance only with UV light irradiation in all the above studies.39 However, in our case neither Nb2O5 nor Nb2O5:N was capable to decompose gaseous toluene. The malfunctioning of these materials for toluene decomposition was due to the lower energy of UVA radiation. Band gap and surface defect states are two important factors influencing the photocatalytic activities of materials. Neither the band gap of Nb2O5 is suitable for visible light excitation nor have the materials enough defect states to generate electron−hole pairs with visible light.4 Figure 6 shows that Nb2O5 cannot absorb light above 390 nm due to the larger band gap. In this investigation, the lamp used produced much less UV intensity than previous reports. Therefore, Nb2O5 and Nb2O5:N are not capable of photocatalytic reaction with visible light irradiation. Therefore, we concentrated on Ta2O5 and its modified forms, i.e., nitrogen doped materials. Figure 9 shows the photocatalytic performance of commercial Ta2O5, Ta2O5:N (com), Ta2O5 (lab), and Ta2O5:N (lab) under visible light. The commercial Ta2O5 was unable to convert toluene with pure visible light (λ >

Figure 9. Photocatalytic decomposition of toluene with undoped and nitrogen doped Ta2O5 (lab) irradiated with visible light (λ > 400 nm). 3325

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

Figure 10. Repeated use of materials for photocatalytic decomposition of toluene with solar light. Figure 12. FTIR analysis of Ta2O5:N (lab) before and after photocatalytic reaction.

acid,51 and benzyl alcohol.14 However, the concentration of these byproducts in the gas phase was very small,14 where weak evidence of CO was also observed.52,53 CO2 has been also observed as one of the major byproducts;54 however, its formation is dependent on the humidity level. These and some other byproducts have been observed in the case of high toluene concentration (few hundred parts per million), whereas at lower parts per million toluene,47,53 and even at parts per billion level,55 it was difficult to detect the byproducts.56 In our case we used GC−MS to analyze gas samples and found that the concentrations of CO and other byproducts were beyond the detection limit of the GC−MS.

4. CONCLUSIONS Ta2O5 and Nb2O5 were successfully prepared by a solution method and were doped with nitrogen using an ammonia gas treatment at 700 °C. A commercial Ta2O5 was also doped with nitrogen by a similar annealing process. Undoped Ta2O5, commercial Ta2O5, Ta2O5:N (com), and Ta2O5:N (lab) decomposed toluene with artificial solar light. Ta2O5:N (lab) was the most efficient among all the prepared Ta- and Nbbased materials, converting more than 70% of toluene with artificial solar light. This material was also capable of removing 30% of toluene with pure visible light irradiation. Ta2O5:N (lab) has also shown a very stable and long-life performance after being used several times. The higher photocatalytic performance of Ta2O5:N (lab) under artificial sunlight and pure visible light was attributed mainly to the large number of defect states and oxygen vacancies.

Figure 11. XRD analysis of Ta2O5:N (lab) before and after use for photocatalytic reaction.

Intermediate adsorption on the photocatalytic surfaces plays a major role in deactivation of photocatalytic materials. Along with carbon dioxide and water, toluene decomposition through photocatalytic oxidation resulted in formation of other hydrocarbons such as benzaldehyde,46 benzoic acid, and benzyl alcohol. In the case of TiO247 these intermediates and water molecules48 were strongly adsorbed on the photocatalyst surface, which consequently covered the active sites and reduced the performance of the material. Regeneration through UV irradiation and washing with water was not successful in recovering the performance of TiO2 material. Figure 12 shows FTIR analyses of fresh and used Ta2O5:N (lab) materials, where no intermediate adsorption was detected on Ta2O5:N (lab). However, a very weak peak at 1632 cm−1 was observed on Ta2O5:N (lab), indicating water adsorption.49 Water vapors from the atmosphere may be attached to the material, which is insufficient to significantly affect the performance of the material. Contrary to Ta2O5:N (lab), large amounts of water and intermediates were adsorbed on the TiO2 surface, which obviously reduced its photocatalytic activity.49 Numerous studies have shown that decomposition and/or oxidation of toluene through photocatalytic reaction13 resulted in different types of byproducts.50 The most common byproducts from gaseous toluene are benzaldehyde, benzoic

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+61) 8 92663776. Fax: (+61) 8 92662681. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Kondo, J. N.; Takahara, Y.; Lee, B.; Lu, D.; Domen, K. Synthesis and property of mesoporous tantalum oxides. Top. Catal. 2002, 19, 171−177. 3326

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

(23) Grey, I. E.; Mumme, W. G.; Roth, R. S. The crystal chemistry of L-Ta2O5 and related structures. J. Solid State Chem. 2005, 178, 3308− 3314. (24) Ramprasad, R. First principles study of oxygen vacancy defects in tantalum pentoxide. J. Appl. Phys. 2003, 94, 5609−5612. (25) Shin, H.; Park, S. Y.; Bae, S.-T.; Lee, S.; Hong, K. S.; Jung, H. S. Defect energy levels in Ta2O5 and nitrogen-doped Ta2O5. J. Appl. Phys. 2008, 104, 116108. (26) Guo, G.; Huang, J. Preparation of mesoporous tantalum oxide and its enhanced photocatalytic activity. Mater. Lett. 2011, 65, 64−66. (27) Sreethawong, T.; Ngamsinlapasathian, S.; Suzuki, Y.; Yoshikawa, S. Nanocrystalline mesoporous Ta2O5-based photocatalysts prepared by surfactant-assisted templating sol-gel process for photocatalytic H2 evolution. J. Mol. Catal. A: Chem. 2005, 235, 1−11. (28) Zhu, S.; Fu, H.; Zhang, S.; Zhang, L.; Zhu, Y. Two-step synthesis of a novel visible-light-driven K2Ta2O6-xNx catalyst for the pollutant decomposition. J. Photochem. Photobiol. A: Chem. 2008, 193, 33−41. (29) Chen, X.; Yu, T.; Fan, X.; Zhang, H.; Li, H.; Ye, J.; Zou, Z. Enhanced activity of mesoporous Nb2O5 for photoctalytic hydrogen prodcution. Appl. Surf. Sci. 2007, 253, 8500−8506. (30) Ye, J.; Zou, Z. Visible light sensitive photocatalysts In1−xMxTaO4 (M=3d transition-metal) and their activity controlling factors. J. Phys. Chem. Solids 2005, 66, 266−273. (31) Kudo, A.; Sayama, K.; Tanaka, A.; Asakura, K.; Domen, K.; Maruya, K.; Onishi, T. Nickel-loaded K4Nb6O17 photocatalyst in the decomposition of H2O into H2 and O2: structure and reaction mechanism. J. Catal. 1989, 120, 337−352. (32) Lau, W. S.; Leong, L. L.; Han, T.; Sandler, N. P. Detection of oxygen vacancy defect states in capacitors with ultrathin Ta2O5 films by zero-bias thermally stimulated current spectroscopy. Appl. Phys. Lett. 2003, 83, 2835−2837. (33) Bae, S.-T.; Shin, H.; Lee, S.; Kim, D.; Jung, H.; Hong, K. Visiblelight photocatalytic activity of NH3-heat-treated Ta2O5 to decompose rhodamine B in aqueous solution. React. Kinet., Mech. Catal. 2012, 106, 67−81. (34) Alers, G. B.; Fleming, R. M.; Wong, Y. H.; Dennis, B.; Pinczuk, A.; Redinbo, G.; Urdahl, R.; Ong, E.; Hasan, Z. Nitrogen plasma annealing for low temperature Ta2O5 films. Appl. Phys. Lett. 1998, 72, 1308−1310. (35) Devan, R. S.; Lin, C. L.; Gao, S. Y.; Cheng, C. L.; Liou, Y.; Ma, Y. R. Enhancement of green-light photoluminescence of Ta2O5 nanoblock stacks. Phys. Chem. Chem. Phys. 2011, 13, 13441−13446. (36) Sawada, H.; Kawakami, K. Electronic structure of oxygen vacancy in Ta2O5. J. Appl. Phys. 1999, 86, 956−959. (37) Ivanov, M. V.; Perevalov, T. V.; Aliev, V. S.; Gritsenko, V. A.; Kaichev, V. V. Electronic structure of delta-Ta2O5 with oxygen vacancy: ab initio calculations and comparison with experiment. J. Appl. Phys. 2011, 110, 024115. (38) Di Franco, F.; Santamaria, M.; Di Quarto, F.; Tsuji, E.; Habazaki, H. The influence of nitrogen incorporation on the optical properties of anodic Ta2O5. Electrochim. Acta 2012, 59, 382−386. (39) Furukawa, S.; Shishido, T.; Teramura, K.; Tanaka, T. Reaction mechanism of selective photooxidation of hydrocarbons over Nb2O5. J. Phys. Chem. C 2011, 115, 19320−19327. (40) Arakawa, H.; Sayama, K. Solar hydrogen production. Significant effect of Na2CO3; addition on water splitting using simple oxide semiconductor photocatalysts. Catal. Surv. Jpn. 2000, 4, 75−80. (41) Nakajima, K.; Lu, D.; Hara, M.; Domen, K.; Kondo, J. N. Synthesis and application of thermally stable mesoporous Ta2O5 photocatalyst for overall water decomposition. Stud. Surf. Sci. Catal. 2005, 158B, 1477−1484. (42) Zou, J.-J.; Liu, C.-J.; Zhang, Y.-P. Control of the metal-support interface of NiO-loaded photocatalysts via cold plasma treatment. Langmuir 2006, 22, 2334−2339. (43) Parida, K. M.; Mahanta, S. K.; Martha, S.; Nashim, A. Fabrication of NiO/Ta2O5 composite photocatalyst for hydrogen production under visible light. Int. J. Energy Res. 2013, 37, 161−170, DOI: 10.1002/er.1924.

(2) Chen, X.; Yu, T.; Fan, X.; Zhang, H.; Li, Z.; Ye, J.; Zou, Z. Enhanced activity of mesoporous Nb2O5 for photocatalytic hydrogen production. Appl. Surf. Sci. 2007, 253, 8500−8506. (3) Kominami, H.; Oki, K.; Kohno, M.; Onoue, S.-i.; Kera, Y.; Ohtani, B. Novel solvothermal synthesis of niobium oxide powders and their photocatalytic activity in aqueous suspensions. J. Mater. Chem. 2001, 11, 604−609. (4) Furukawa, S.; Ohno, Y.; Shishido, T.; Teramura, K.; Tanaka, T. Selective amine oxidation using Nb2O5 photocatalyst and O2. ACS Catal. 2011, 1, 1150−1153. (5) Shishido, T.; Miyatake, T.; Teramura, K.; Hitomi, Y.; Yamashita, H.; Tanaka, T. Mechanism of photooxidation of alcohol over Nb2O5. J. Phys. Chem. C 2009, 113, 18713−18718. (6) Ge, S.; Jia, H.; Zhao, H.; Zheng, Z.; Zhang, L. First observation of visible light photocatalytic activity of carbon modified Nb2O5 nanostructures. J. Mater. Chem. 2010, 20, 3052−3058. (7) Murase, T.; Irie, H.; Hashimoto, K. Visible light sensitive photocatalysts, nitrogen-doped Ta2O5 powders. J. Phys. Chem. B 2004, 108, 15803−15807. (8) Ho, C.-T.; Low, K.-B.; Jash, P.; Shen, H.; Snee, P. T.; Meyer, R. J. Formation of sol-gel-derived TaOxNy Photocatalysts. Chem. Mater. 2011, 23, 4721−4725. (9) Sato, S.; Morikawa, T.; Saeki, S.; Kajino, T.; Motohiro, T. Visiblelight-induced selective CO2 reduction utilizing a ruthenium complex electrocatalyst linked to a p-type nitrogen-doped Ta2O5 semiconductor. Angew. Chem., Int. Ed. 2010, 49, 5101−5105. (10) Yamanaka, K.-i.; Sato, S.; Iwaki, M.; Kajino, T.; Morikawa, T. Photoinduced electron transfer from nitrogen-doped tantalum oxide to adsorbed ruthenium complex. J. Phys. Chem. C 2011, 115, 18348− 18353. (11) Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. Conduction and valence band positions of Ta2O5, TaON, and Ta3N5 by UPS and electrochemical methods. J. Phys. Chem. B 2003, 107, 1798−1803. (12) Morikawa, T. Dual functional modification by N doping of Ta2O5: p-type conduction in visible-light-activated N-doped Ta2O5. Appl. Phys. Lett. 2010, 96, 142111. (13) Zhang, Q.; Gao, L. Ta3N5 nanoparticles with enhanced photocatalytic efficiency under visible light irradiation. Langmuir 2004, 20, 9821−9827. (14) Ho, C.-T.; Low, K.-B.; Klie, R. F.; Maeda, K.; Domen, K.; Meyer, R. J.; Snee, P. T. Synthesis and characterization of semiconductor tantalum nitride nanoparticles. J. Phys. Chem. C 2010, 115, 647−652. (15) Luo, H.; Wei, M.; Wei, K. Synthesis of Nb2O5 nanorods by a soft chemical process. J. Nanomater. 2009, No. 758353. (16) Lin, H.-Y.; Yang, H.-C.; Wang, W.-L. Synthesis of mesoporous Nb2O5 photocatalysts with Pt, Au, Cu and NiO cocatalyst for water splitting. Catal. Today 2011, 174, 106−113. (17) Schäfer, H.; Gruehn, R.; Schulte, F. The modifications of niobium pentoxide. Angew. Chem., Int. Ed. 1966, 5, 40−52. (18) Filonenko, V. P.; Zibrov, I. P. High-Pressure Phase Transitions of M2O5(M = V, Nb, Ta) and thermal stability of new polymorphs. Inorg. Mater. 2001, 37, 953−959. (19) Ohuchi, T.; Miyatake, T.; Hitomi, Y.; Tanaka, T. Liquid phase photooxidation of alcohol over niobium oxide without solvents. Catal. Today 2007, 120, 233−239. (20) Patel, U.; Avci, S.; Xiao, Z. L.; Hua, J.; Yu, S. H.; Ito, Y.; Divan, R.; Ocola, L. E.; Zheng, C.; Claus, H.; Hiller, J.; Welp, U.; Miller, D. J.; Kwok, W. K. Synthesis and superconducting properties of niobium nitride nanowires and nanoribbons. Appl. Phys. Lett. 2007, 91, 162508−3. (21) Huang, H.-C.; Hsieh, T.-E. Preparation and characterizations of tantalum pentoxide (Ta2O5) nanoparticles and UV-curable Ta2O5acrylic nanocomposites. J. Appl. Polym. Sci. 2010, 117, 1252−1259. (22) Stephenson, N. C.; Roth, R. S. The crystal structure of the high temperature form of Ta2O5. J. Solid State Chem. 1971, 3, 145−153. 3327

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328

Industrial & Engineering Chemistry Research

Article

(44) Zhu, M.; Zhang, Z.; Miao, W. Intense photoluminescence from amorphous tantalum oxide films. Appl. Phys. Lett. 2006, 89, 021915-3. (45) Devan, R. S.; Lin, C.-L.; Gao, S.-Y.; Cheng, C.-L.; Liou, Y.; Ma, Y.-R. Enhancement of green-light photoluminescence of Ta2O5 nanoblock stacks. Phys. Chem. Chem. Phys. 2011, 13, 13441−13446. (46) Maira, A. J.; Yeung, K. L.; Soria, J.; Coronado, J. M.; Belver, C.; Lee, C. Y.; Augugliaro, V. Gas-phase photo-oxidation of toluene using nanometer-size TiO2 catalysts. Appl. Catal., B: Environ. 2001, 29, 327− 336. (47) d’Hennezel, O.; Pichat, P.; Ollis, D. F. Benzene and toluene gasphase photocatalytic degradation over H2O and HCl pretreated TiO2: by-products and mechanisms. J. Photochem. Photobiol., A: Chem. 1998, 118, 197−204. (48) Ullah, R.; Sun, H.; Wang, S.; Ang, H. M.; Tadé, M. O. Wetchemical synthesis of InTaO4 for photocatalytic decomposition of organic contaminants in air and water with UV-vis light. Ind. Eng. Chem. Res. 2012, 51, 1563−1569. (49) Wei, Z.; Sun, J.; Xie, Z.; Liang, M.; Chen, S. Removal of gaseous toluene by the combination of photocatalytic oxidation under complex light irradiation of UV and visible light and biological process. J. Hazard. Mater. 2010, 177, 814−821. (50) Larson, S. A.; Falconer, J. L. Initial reaction steps in photocatalytic oxidation of aromatics. Catal. Lett. 1997, 44, 57−65. (51) Einaga, H.; Futamura, S.; Ibusuki, T. Heterogeneous photocatalytic oxidation of benzene, toluene, cyclohexene and cyclohexane in humidified air: comparison of decomposition behavior on photoirradiated TiO2 catalyst. Appl. Catal., B: Environ. 2002, 38, 215−225. (52) Zhu, Z.; Li, X.; Zhao, Q.; Li, H.; Shen, Y.; Chen, G. Porous “brick-like” NiFe2O4 nanocrystals loaded with Ag species towards effective degradation of toluene. Chem. Eng. J. 2010, 165, 64−70. (53) Ibusuki, T.; Takeuchi, K. Toluene oxidation on UV-irradiated titanium dioxide with and without O2, NO2 or H2O at ambient temperature. Atmos. Environ. 1986, 20, 1711−1715. (54) Mo, J.; Zhang, Y.; Xu, Q.; Zhu, Y.; Lamson, J. J.; Zhao, R. Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene. Appl. Catal., B: Environ. 2009, 89, 570−576. (55) Debono, O.; Thevenet, F.; Gravejat, P.; Hequet, V.; Raillard, C.; Lecoq, L.; Locoge, N. Toluene photocatalytic oxidation at ppbv levels: kinetic investigation and carbon balance determination. Appl. Catal., B: Environ. 2011, 106, 600−608. (56) Sleiman, M.; Conchon, P.; Ferronato, C.; Chovelon, J.-M. Photocatalytic oxidation of toluene at indoor air levels (ppbv): towards a better assessment of conversion, reaction intermediates and mineralization. Appl. Catal., B: Environ. 2009, 86, 159−165.

3328

dx.doi.org/10.1021/ie302326h | Ind. Eng. Chem. Res. 2013, 52, 3320−3328