JES-00331; No of Pages 10 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
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Dandan Pang, Lu Qiu, Yunteng Wang, Rongshu Zhu, Feng Ouyang⁎
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Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
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Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation
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AR TIC LE I NFO
ABSTR ACT
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Article history:
The solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide was prepared by a sol–gel 14
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Received 6 October 2014
method using NH4F as nitrogen and fluorine source. The prepared materials were 15
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Revised 20 December 2014
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray 16
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Accepted 14 January 2015
photoelectron spectroscopy (XPS), UV–Visible diffuse reflectance spectroscopy (UV–Vis), 17
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Available online xxxx
ammonia adsorption and temperature-programmed desorption (NH3-TPD), in situ Fourier 18
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Keywords:
photocatalytic activity of the catalyst for acrylonitrile degradation was investigated under 20
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Photocatalytic activity
simulant solar irradiation. The results showed that strong Lewis and Brønsted acid sites 21
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Acrylonitrile
appear on the surface of the sample after N–F doping. Systematic investigation showed that 22
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In situ IR
the highest photocatalytic activity for acrylonitrile degradation was obtained for samples 23
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N–F codoping
calcined at 450°C with molar ratio (NH4F to Ti) of 0.8. The degradation ratio of 71.5% was 24
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transform infrared spectroscopy (FT-IR) and N2 physical adsorption isotherm. The 19
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achieved with the prepared catalyst after a 6-min irradiation, demonstrating the 25
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effectiveness of photocatalytic degradation of acrylonitrile with N–F codoped TiO2/SiO2 26 composite oxide. The photocatalyst is promising for application under solar light 27
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irradiation. Moreover, the intermediates generated after irradiation were verified by gas 28 chromatography–mass spectrometry (GC–MS) analysis and UV–Vis spectroscopy to be 29
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simple organic acids with lower toxicity, and the degradation pathway was also proposed 30 31
for acrylonitrile degradation with the prepared catalyst.
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© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 32
Introduction
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Acrylonitrile (CH2_CH–CN) is considered as a hazardous pollutant since it is mutagenic, carcinogenic and teratogenic to human beings (Léonard et al., 1999). The health authorities of the Federal Republic of Germany have placed acrylonitrile in the category of carcinogenic chemicals for which no threshold limits are established. The US EPA has classified acrylonitrile as a “priority water pollutant” and “hazardous air pollutant”. Traditional control technologies for acrylonitrile include adsorption and desorption (Huang et al., 1999), thermal and
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Published by Elsevier B.V. 33
catalytic incineration at high temperatures (Gervasini and Ragaini, 2000; Hung and Chu, 2006) and biotechnological abatement methods (Zhang and Pierce, 2009). There are many limitations including high operating costs, secondary waste stream problems, etc. Therefore, developing an alternative abatement method such as photocatalytic oxidation attracts increasing interest. To date, TiO2 has been the most widely used photocatalyst due to its ability to convert UV light to chemical energy to decompose most organic pollutants that exist in air and aqueous systems or to generate hydrogen from water (Macwan et al., 2011). The general drawbacks of photocatalytic
⁎ Corresponding author. E-mail: [email protected] (Feng Ouyang).
http://dx.doi.org/10.1016/j.jes.2015.01.017 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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1. Experimental
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1.1. Preparation of photocatalysts
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Silica gel (100–200 mesh) was obtained for TiO2 loading from Shanghai Chemical Company. All other chemicals were of analytical reagent grade quality and were employed without further purification. All aqueous solutions were prepared in deionized water from a Milli-Q water system. The TiO2/SiO2 composite powders were prepared by sol–gel method. Solution A was obtained by mixing 10 mL anhydrous alcohol, 0.5 mL deionized water and 2 mL acetic acid. Solution B was obtained by dissolving the starting material of 5 mL tetrabutyl titanate (Ti(OC4H9)4) in 13 mL anhydrous alcohol. Solution A was added dropwise in solution B with vigorous stirring for 2 hr to obtain the homogeneous transparent sol. 2.0 g silica gel was added to the sol with severe stirring for 1 hr (Chen et al., 2004). The resulting gelatinous solution was aged for 12 hr at room temperature and then dried at 80°C. The dry gel was then crushed and calcined at different temperatures for 2 hr. TiO2/SiO2 composite powders were thus obtained. Ammonium fluoride (NH4F) was used as the doping precursor as the nitrogen–fluorine source (Li et al., 2011) and the molar ratios of NH4F to Ti (RNF) were 0:1, 0.2:1, 0.4:1, 0.8:1 and 1.6:1. NH4F aqueous solution (13.83 mol/L) was added in solution A and the same operation as above was conducted, and N–F codoped TiO2/SiO2 powders were thus obtained.
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100 99
103 Q6
L¼
Kλ B cosθ
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ð1Þ
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where, B2 = B2measured − b2instrumental, L, K, λ and θ are the average crystal size, a shape factor of 0.9 for spherical crystallites, the X-ray wavelength (0.15405 nm) and Bragg diffraction angle, respectively. B, Bmeasured and binstrumental are the breadths of the intrinsic diffraction profile, test sample diffraction integral profile and instrumental diffraction profile, respectively. The value of b for the D/max 2500 Diffractometer is 0.0033 arcs. The morphology was examined using a scanning electron microscope (S4700, Hitachi, Japan). The XPS (PHI-1800, ULVAC-PHI, Japan) measurements were performed with monochromatic Al Kα excitation, and all the bonding energies were calibrated to the C 1s peak at 284.6 eV. UV–Visible diffuse reflectance spectra of the powders were collected with a UV–Vis Spectrophotometer (UV–2540, Shimadzu Corporation, Japan) over the spectral range of 240–800 nm. BaSO4 was used as a reference. The BET specific surface area was measured by nitrogen gas adsorption at 77 K using an adsorption instrument (BELSOROPMINI II, Ankersmid, Holland). Pore volume and pore size distribution were determined by the Barrett–Joyner–Halenda (BJH) method. The nature of acid sites was investigated using pyridine as the probe molecule. Fourier transform infrared spectra were recorded using an infrared spectrometer (Nicolet 380, Thermo Fisher Scientific, USA) with a resolution of 4 cm− 1 and 32 scans in the region of 4000–1000 cm− 1. Self-supported wafers of the samples were prepared by applying 11 ton pressure. The sample was subjected to vacuum (10− 2 mbar) in the sample holder followed by thermal treatment at 400°C for 2 hr to obtain a clean surface. The adsorption of pyridine was done at 180°C for 40 min. The physisorbed pyridine was then desorbed under vacuum at room temperature. The NH3-TPD technique was used for the quantitation of the total surface acidity. The sample was preheated at 400°C for 2 hr to remove organic compounds. Ammonia was adsorbed at 100°C and excess ammonia was purged by helium. The measurement of acid site density was carried out by heating from 100 to 600°C at 15°C/min. The desorbed ammonia was reacted with a 0.05 mol/L HCl solution that was then back-titrated with 0.05 mol/L NaOH solution (Al-Yassir et al., 2005; Le Van Mao et al., 2006). In order to investigate the recombination and lifespan of photogenerated electrons-holes in the semiconductor, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by a Raman spectrometer (inVia, Renishaw, England) illuminated with a 325 nm He–Cd laser.
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The X-ray diffraction patterns of the powders were obtained on an X-ray diffraction analyzer (D/max 2500, Rigaku Corporation, Japan) with Cu Kα X-ray source at a scanning rate of 8°/min in the 2θ range between 10° and 80°. The accelerating voltage and the applied current were 40 kV and 200 mA, respectively. Crystallite size was calculated according to the Scherrer equation (Gu et al., 2004; Tai et al., 2004):
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1.2. Characterization of photocatalysts
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oxidation, such as low reaction rates, require improvement. The photocatalytic activity of TiO2 can be improved by the addition of SiO2, which increases the available surface area of the catalyst (Chen et al., 2004). Another potentially effective way to improve the photocatalyst performance is to increase the number of surface acid sites, because the photocatalytic activity has been shown to increase with catalyst surface acidity. Doping TiO2 with metal oxides was reported to increase the surface acidity and photocatalytic activity of TiO2 (Cui et al., 1995). Sulfated TiO2 can be readily synthesized by the reaction of amorphous TiO2 with sulfur compounds at high temperatures and employed as a photocatalyst (Wang et al., 2006a). However, little work has been done to study the photocatalytic degradation of acrylonitrile on acid catalysts. F-doped silica-supported TiO2 was used for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation (Pang et al., 2014). Gas-phase photocatalytic oxidation of acrylonitrile on sulfated TiO2 was also reported (Jõks et al., 2011). In this study, solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide was synthesized by sol–gel method. The objective of the present research was to determine the effectiveness of photocatalytic degradation of acrylonitrile with N–F codoped TiO2/SiO2 composite oxides. The declared objective was achieved by experimental research on effects of two variables – calcination temperature and NH4F ratio in the starting material – on the photocatalytic activity of the prepared catalysts for acrylonitrile degradation. The identification of the intermediates of photocatalytic oxidation of acrylonitrile was carried out and mechanisms for the degradation process were also proposed.
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Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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Identification of products was verified using gas chromatography–mass spectrometry (GC–MS) (Thermo 183 Q13 trace GC-DSQ Tune, Thermo Fisher Scientific, USA). The 184 solution after irradiation was extracted using chloroform. 185 Then the extract was filtered through a 0.45 μm filter and 186 stored at 4°C until instrumental analysis was conducted. 187 The gas chromatography–mass spectrometry (GC–MS) was 188 equipped with a DB-WAX capillary column (30 m, 0.25 mm 189 internal diameter, 0.25 μm film thickness) and electron 190 ionization (EI) detector. The temperature of the injector 191 and the MS-transfer were both held at 200°C. Mass spectra 192 were compared with those in the GC–MS database for 193 compound identification.
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1.4. Analytical methods
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The acrylonitrile concentration in the solution was quantified 210 using an HPLC (LC2000, Shanghai TianMei Scientific Instru211 Q15 ments Co., Ltd., China) equipped with a LC-2030 UV detector 212 and a Sunfire™ C18 column (150 mm, 4.6 mm id.). The eluent 213 was composed of methanol (30%) and water (70%). The flow rate 214 was kept at 1.0 mL min−1. The absorption spectra showed 215 absorption maxima at 197 nm for acrylonitrile, but the detec216 tion wavelength selected for acrylonitrile was 210 nm to 217 remove the noise from methanol with the cutoff wavelength 218 of 205 nm. 219 In general, at low substrate concentration, the kinetics of 220 the photocatalytic oxidation process is described well by a 221 pseudo-first-order equation (Chen et al., 2004).
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ð2Þ
where Co, Ct, t and kapp are the reactant concentration after the system reaches adsorption equilibrium (mg/L), the reactant concentration at time t (mg/L), the irradiation time (min) and the apparent pseudo-first-order rate constant (min−1), respectively. The degradation ratio of the reactant was also used to evaluate the photocatalytic activity of a sample: Co − Ct 100% D¼ Co
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Fig. 1 presents the SEM images of pure TiO2, TiO2/SiO2, and N–F codoped TiO2/SiO2 calcined at 450°C. Pure TiO2 was irregularly agglomerated from primary particles. The TiO2/SiO2 possessed a rough and porous surface, resulting in higher surface area than pure TiO2. Additionally, pure TiO2 and TiO2/SiO2 showed the formation of secondary particles by the agglomeration of primary particles. However, N–F codoped TiO2/SiO2, consisting of solid microspheres, showed little agglomeration and did not collapse. The agglomerate size of N–F codoped TiO2/SiO2 was smaller than that of TiO2/SiO2. Guo et al. reported that ammonium fluoride could modify the zeta potential of spherical poly-condensed titania species generated at the initial stage of the hydrolysis reaction (Guo et al., 2003). The results show that NH4F as N–F doping precursor can inhibit the agglomeration of TiO2 powder effectively, which is beneficial for higher activity.
235
2.2. XPS analysis
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where D is the degradation ratio of the reactant.
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Fig. 2 shows XPS spectra of N–F codoped TiO2/SiO2 with RNF of 0.8 calcined at 450°C. The binding energies of Ti 2p3/2 and Ti 2p1/2 are equal to 458.7 and 464.5 eV, respectively (Fig. 2a), indicating that Ti exists in the Ti4+ form (Wu et al., 2010; Xiang et al., 2010; Xie et al., 2007). The O 1s signal is shown in Fig. 2b. The peak at 529.9 eV corresponds to lattice oxygen of TiO2, and a strong peak centered at 533.2 eV is assigned to surface hydroxides (Xie et al., 2007). Fig. 2c presents the F 1s XPS spectra. The F 1s region is composed of two contributions. One peak located around 687.7 eV is attributed to the substitute F atoms that occupied oxygen sites in the TiO2 crystal lattice (Huang et al., 2006; Wu et al., 2010; Xie et al., 2007). This indicates that F atoms are incorporated into the crystal lattice of TiO2 prepared by the sol–gel method using NH4F as precursor. Another peak located at 684.2 eV is originated from F− ions physically adsorbed on the surface of TiO2 (`Ti–F) (Wu et al., 2010; Xiang et al., 2010). The surface `Ti–F group may act as an electron-trapping site and reduce interfacial electron transfer rates via tightly holding trapped electrons due to the strong electronegativity of the fluorine (Park and Choi, 2004; Xiang et al., 2010). Fig. 2d shows the XPS spectra for the N 1s. The peak appearing at 400.0 eV is greater than the typical binding energy of 396.9 eV in TiN (Saha and Tomkins, 1992). It should be assigned to the formation of a N–Ti–O linkage (Jang et al., 2006; Wang et al., 2006b; Xie et al., 2007). When nitrogen replaces oxygen in the O–Ti–O structure, the electron density around N is reduced. Thus, the N 1s binding energy in an O–Ti–N environment is higher than that in a N–Ti–N environment (Wang et al., 2006b). The actual amounts of F and N in the sample are 1.9% and 0.5%, respectively.
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2.3. Nitrogen physical adsorption
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Photocatalytic reaction was carried out in a self-made 196 cylindrical glass reactor using a 350 W xenon arc lamp 197 (AHD350, Shenzhen AnHongDa Opto Technology Co., Ltd., 198 Q14 China) as simulant solar light. A typical degradation experi199 ment was carried out as follows: the catalyst was added to 200 180 mL of aqueous solution of acrylonitrile with an initial 201 concentration of 10 mg/L. The reactor was sealed. The 202 reaction mixture was stirred for 60 min in the dark to reach 203 adsorption equilibrium. Then the cooling water system and 204 the xenon lamp were turned on. A small amount of solution 205 was periodically withdrawn from the reaction mixture with a 206 syringe and filtered through a 0.45 μm filter for high perfor207 mance liquid chromatography (HPLC) analysis.
Co ¼ kapp t ln Ct
2.1. SEM images
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2. Results and discussion
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Fig. 3 presents Nitrogen adsorption and desorption iso- 283 therms of N–F codoped TiO2 without SiO2 and N–F codoped 284 TiO2/SiO2 nanoparticles calcined at 450°C. According to 285
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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2.4. UV–Vis analysis
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Fig. 4 shows the UV–Vis diffuse reflection spectra of TiO2/SiO2 and N–F codoped TiO2/SiO2 calcined at 450°C, respectively. The undoped TiO2/SiO2 obtained was a white powder. However, a slightly yellow color appeared for the N–F codoped catalyst. Generally, the color of a solid is determined by the
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are both 8.0 nm. However, the specific surface area of the 297 former (214 m2/g) is significantly higher than that of the 298 latter (60 m2/g). 299
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IUPAC classification, N–F codoped TiO2 and N–F codoped TiO2/SiO2 nanoparticles display type IV isotherms and H2 hysteresis, which indicate the presence of mesoporous materials (Gregg and Sing, 1997). The plot of the pore size distribution (inset in Fig. 3) was determined by using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherm. It shows that both clearly have a mesoporous structure. The mesoporous structure is probably formed by agglomeration of primary particles (interparticle pores) (Wu et al., 2010). The average pore diameters of N–F codoped TiO2/SiO2 and N–F codoped TiO2 nanoparticles
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Fig. 1 – SEM images of (a) pure TiO2, (b) TiO2/SiO2 and (c) N–F codoped TiO2/SiO2 with molar ratio (NH4F to Ti) of 0.8.
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Fig. 2 – Ti 2p (a), O 1 (b), F 1s (c) and N 1s (d) high-resolution XPS spectra of N–F codoped TiO2/SiO2. Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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dVp/drp
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Eg ¼ 1239:8=λ
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1549 Transmittance (a.u.)
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In situ characterization of surface species and surface active sites during heterogeneous photocatalytic processes is one of the most important experimental approaches. Many spectroscopic techniques have been employed to study photocatalytic reactions on the surface. Among these, FT-IR is probably the best method for in situ photocatalytic studies because it is inexpensive, sensitive, and capable of determining chemical species, and can be carried out at atmospheric pressure and room temperature. The adsorption of pyridine combined with
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ð4Þ
where Eg and λ are the band gap and wavelength of the absorption edge in the spectrum, respectively. The absorption edge of the sample was shifted to longer wavelength after N–F doping. The absorption edge of the undoped TiO2/SiO2 occurred at around 388 nm. After N–F doping, the absorption edge of the sample occurred at around 414 nm and the band gap was changed to about 2.99 eV. The band gap narrowing of the doped sample should be attributed to doping N from ammonium fluoride, since F doping did not result in any significant shift in the fundamental absorption of TiO2 (Huang et al., 2007; Li et al., 2005).
Absorbance (a.u.)
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2.5. Effect of NH4F ratio in starting material on physical and 321 photocatalytic properties 322
position of its absorption edge. The band gaps of the samples are determined by the following equation (O'Regan and Grätzel, 1991):
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Fig. 3 – Nitrogen adsorption and desorption isotherms of (1) N–F codoped TiO2 without SiO2 and (2) N–F codoped TiO2/SiO2 nanoparticles. Inset shows Barrett–Joyner–Halenda (BJH) pore size distributions of the corresponding samples.
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Fig. 4 – UV–Vis absorption spectra of (1) TiO2/SiO2 and (2) N–F codoped TiO2/SiO2.
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Fig. 5 – Infrared spectra of chemisorbed pyridine molecules adsorbed on the N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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t1:7 t1:8 t1:9 t1:10 t1:11 t1:12
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I, II, IV, and V denote as the number of the peak.
ratios. The surface acid site densities of N–F codoped TiO2/SiO2 and undoped sample were calculated, as shown in Table 1. With the increase of NH4F/Ti molar ratio, an increase for the amount of acid sites was observed. However, excess N–F doping results in the decrease in surface acid sites (Fig. 5d and Table 1). Essentially, the acidity profile of the N–F codoped samples exhibits three or four separated peaks, while that of the undoped sample shows only two desorption peaks. The last two peaks at about 450 and 510°C for N–F codoped samples (0.8:1 and 1.6:1) corresponding to stronger acid sites, possibly play a more important role in improving the photocatalytic activity. PL emission spectra are widely used to determine the efficiency of charge carrier trapping, migration, transfer and separation, and the recombination of photogenerated electrons and holes in semiconductors (Xiang et al., 2010; Zhang et al., 2000). Fig. 7 shows a comparison of the PL spectra of undoped and N–F codoped TiO2/SiO2 samples prepared with varying RNF values in the wavelength range of 450–650 nm. It is found that the PL emission intensity decreases with the increase of RNF. However, the sample with excessive N–F doping exhibits an increase in emission intensity. This suggests that an appropriate amount of N–F doping will slow the radiative recombination rate of photogenerated electrons
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Fig. 6 – NH3-TPD profile of N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1.
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(a) (b) (c) (d)
Peak temperature (°C)
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Samples
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Table 1 – Acid sites density of N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1 calculated by using NH3-TPD.
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in situ FT-IR technique is well known for characterizing the surface Brønsted and Lewis acidic sites (Al-Yassir et al., 2005; Jin et al., 2009; Wang et al., 2006a). Fig. 5 shows the IR spectra of pyridine chemisorbed on the N–F codoped TiO2/SiO2 with different NH4F/Ti molar ratios. The IR spectrum of Fig. 5c clearly displays the characteristic bands of the chemisorbed pyridine. The band observed at 1597 cm−1 is assigned to hydrogen-bonded pyridine (Al-Yassir et al., 2005). The bands at 1445 and 1579 cm− 1 are due to the interaction of pyridine with strong and weak Lewis acid sites, respectively (Al-Yassir et al., 2005; Le Van Mao et al., 1999; Wang et al., 2006a). The peak at 1549 cm−1 is due to the vibration of pyridine adsorbed on Brønsted acid sites (Le Van Mao et al., 1999). The peak at 1491 cm−1 was assigned to pyridine adsorbed on both Lewis and Brønsted acid sites (Jin et al., 2009). Most notably, only one small peak at 1446 cm−1 corresponding to pyridine adsorbed on Lewis acid site was observed for the undoped sample (Fig. 5a). With the increase of NH4F/Ti molar ratio, a significant increase in the intensity of the characteristic bands of pyridine was observed. This suggests that the surface acid sites can become stronger and react with more pyridine molecules after N–F doping. These strong Lewis and Brønsted acid sites provided better adsorption centers for oxygen and reactant molecules and converted adsorbed water into active hydroxyl groups, which improved the photocatalytic activity (Pelaez et al., 2012; Wang et al., 2006a). The Lewis acid sites are strong adsorption centers and can accept electron pairs. Acrylonitrile, with lone-pair electrons, can be easily adsorbed onto the Lewis acid sites and oxidized by UOH. The Brønsted acid sites can donate protons (H+) and generate UOOH, a powerful oxidant. The UOH and UOOH formations are then involved in acrylonitrile oxidation reactions, leading to the decrease in the re-combination ratio of electrons and holes, as shown in Fig. 7. On the other hand, from the viewpoint of microscopic reaction kinetics, the stronger the adsorption ability of Lewis acid sites is, the longer the residence time of reactant molecules adsorbed on the surface and the higher the reaction activity. Fig. 6 shows the distribution curves of the acid strength of the N–F codoped TiO2/SiO2 with different NH4F/Ti molar
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TCD signal
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Fig. 7 – Photoluminescence emission spectra of N–F codoped TiO2/SiO2 composition oxides with different NH4F/Ti molar ratio (RNF).
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433
0.15 0.12 0.09
(204) (116) (220) (215) (301)
550°C
F
450°C
350°C
10
20
30
O
409
C
408
E
407
R
406
R
405
N C O
404
U
403
Apparent rate constant (min-1)
402
(200) (105) (211)
40
50 2θ (°)
60
70
80
R O
Fig. 9 shows the XRD patterns of N–F codoped TiO2/SiO2 powders prepared at different calcination temperatures. All N–F codoped TiO2/SiO2 powders contained only anatase phase TiO2, and no phase transformation from anatase to rutile was observed, even after calcination at 650°C for 2 hr. Moreover, the peak intensity of anatase increased with the increase of calcination temperature and the peaks became sharper, suggesting that the relative crystallinity and crystalline size significantly increased. The average crystallite sizes of samples can be calculated by the Scherrer equation using the full-width at half-maximum of the X-ray diffraction peaks at 2θ = 25.2°. The average crystallite size increased with the increase of calcination temperature. However, the surface area decreased as the calcination temperature increased (Table 2). Fig. 10 shows the effect of calcination temperature on the photocatalytic activity of N–F co-doped TiO2/SiO2 for 6 min irradiation. It can be observed that the photocatalytic activity of N–F codoped TiO2/SiO2 for acrylonitrile photodegradation increased with the increase of calcination temperature, and reached a maximum at the calcination temperature of 450°C.
401
(004) (103) (112)
650°C
Fig. 9 – XRD patterns of N–F codoped TiO2/SiO2 samples calcined at 350, 450, 550 and 650°C.
It appears that the increase in photocatalytic activity at higher calcination temperature is mainly due to the higher crystallinity of the powder (Fig. 9), which results in a decrease in the defect density on the catalyst surface (Maeda et al., 2005). Meanwhile, with further increasing calcination temperature, the average crystal size of the N–F codoped TiO2/SiO2 catalyst increased. However, the photocatalytic activity decreased. This implies that besides the higher crystallinity, suitable defect density is also essential for high photocatalytic activity. This is consistent with the nature of the surface acid sites (Fig. 5). We consider that surface acid sites are related to surface defects. The surface defects are generated by the coordination of different surface atoms. The difference in surface charge distribution may cause the formation of different types of surface acid sites (Brønsted and Lewis acid sites).
P
413
400
Anatase
D
2.6. Effect of calcination temperature on photocatalytic activity
399
Intensity (a.u.)
412
398
(101)
T
411
and holes in the samples. Moreover, the lower the intensity of PL spectra, the higher the photocatalytic activity, which is consistent with earlier published reports (Xiang et al., 2010; Yu et al., 2002). Fig. 8 shows the dependence of the photocatalytic activity of acrylonitrile degradation for 6 min irradiation over N–F codoped TiO2/SiO2 on the NH4F/Ti molar ratio of the starting materials. The apparent rate constant increases with the NH4F/Ti molar ratio to reach a maximum at NH4F/Ti = 0.8, which is due to the increase of the surface acid sites on the sample after N–F doping (Fig. 5). However, excessive N–F doping results in the decrease in the photocatalytic activity, which is attributed to the introduction of new recombination centers (defect sites) of photogenerated electrons and holes (Fig. 7). There is an optimal dopant concentration, which is similar to the previous studies (Li et al., 2011; Maeda et al., 2005).
397
E
396
Q2
434 435 436 437 438 439 440 441 442 443 444 445 446 447 448
2.7. Influence on the photocatalytic activities of light absorption 449 properties of N–F codoped TiO2/SiO2 450 Fig. 11 shows the degradation of acrylonitrile in the presence of undoped and N–F codoped samples under simulant solar light and UV irradiation (λ ≤ 400 nm), respectively. The N–F codoped TiO2/SiO2 photocatalyst exhibits higher activity than the undoped sample, and 71.5% and 64.3% acrylonitrile could be removed after 6 min under simulant solar light and UV irradiation, respectively. The above details suggest that the photocatalytic performance of TiO2/SiO2 composite oxide is greatly improved by N–F doping under simulant solar light and UV light irradiation, which demonstrates that high
451
Table 2 – Specific surface area and crystal size of N–F codoped TiO2/SiO2 powders at different calcination temperatures.
t2:1 t2:2 t2:3
452 453 454 455 456 457 458 459 460
0.06 0.03 0.00
0.0
0.5 1.0 1.5 NH4F/Ti molar ratio of the starting material
Fig. 8 – Dependence of the photocatalytic activity of acrylonitrile degradation over N–F codoped TiO2/SiO2 calcined at 450°C on the NH4F/Ti molar ratio of starting materials.
Calcination temperature (°C)
Specific surface area (m2/g)
Crystal size (nm)
350 450 550 650
219 214 206 196
24.8 26.6 28.2 29.1
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
t2:4
t2:5 t2:6 t2:7 t2:8 t2:9
8
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
0.6
0.4
1
2
3 4 Irradiation time (min)
5
6
Fig. 10 – Effect of calcination temperature on photocatalytic activity of N–F codoped TiO2/SiO2.
461
472
2.8. Photocatalytic decomposition pathway of acrylonitrile
473
Many organic pollutants can be decomposed by photocatalytic oxidation process with low cost. However, the intermediates
Simulant solar light 70 60 50 40 30 20 10 0
475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512
UV light (λ≤400 nm) 100
Relative abundance (%)
474
C
470
E
469
R
468
R
467
O
466
C
465
N
464
U
463
Degradation ratio (%)
462
T
471
activity is not only dependent on surface active sites but also related to the light absorption properties (Fig. 4). Though the shift of absorption edge is small, corresponding energy levels are suitable for an electron transition to generate UOH; moreover, the interval of shift falls on the UV light region of the simulant solar light. This implies that the prepared catalyst may have the highest efficiency under irradiation of simulant solar light in the region. The catalyst may absorb enough strong light and excite electron transition, then cause a series of photochemical reactions, leading to degradation of a large amount of acrylonitrile.
O
0
R O
0.0
F
0.2
may be more toxic and unacceptable for human heath than their precursors. Therefore, it is necessary to identify the intermediates formed during the photocatalytic process. Krichevskaya et al. reported that the main products of gas-phase acrylonitrile photocatalytic oxidation included nitrogen dioxide, nitrous oxide, carbon dioxide, water, hydrogen cyanide and carbon monoxide by using Degussa P25 under UV irradiation (Krichevskaya et al., 2009). Jõks et al. reported that the gaseous products of photocatalytic oxidation of acrylonitrile on sulfated P25 were CO2, HCN and HNCO (Jõks et al., 2011). However, there are a few reports of the intermediates of photocatalytic oxidation of acrylonitrile in solution under simulant solar light irradiation. Fig. 12 illustrates the GC–MS analysis of the decomposition products of acrylonitrile in the presence of N–F codoped TiO2/SiO2. Six dominant peaks at 2.55, 2.75, 11.29, 11.68, 12.46 and 14.50 min were detected, and the mass spectral peaks identified the corresponding intermediate compounds as acrylonitrile, acetonitrile, acetic acid, formic acid (or oxalic acid), acroleic acid and acrylamide, respectively. Further confirmation was made by comparing the retention times of pure samples of the intermediates. Most of the intermediates are simple organic acids and less toxic than acrylonitrile. Although several intermediates in the solution were detected, no by-products were detected by GC–MS in the gas phase in the upper reactor during any of the experiments. The importance of the derivative spectroscopy for interpretation of UV–Vis spectra is well known and documented (Askal et al., 2008). A derivative spectrum can enhance small peaks and shoulders compared to the absorbance spectrum (zero-order band). Derivative spectra can be used to enhance differences among spectra, to resolve overlapping bands in qualitative analysis and, most importantly, to reduce or eliminate background interference from a wide range of sources (Owen, 2000). The most distinctive feature of the second-order derivative is a negative band with minimum at the same wavelength as the maximum on the absorbance band. Perhaps more importantly, derivatives discriminate
P
ln(C0 /C)
0.8
R2 = 0.9984 R2 = 0.9997 R2 = 0.9990 R2 = 0.9991
D
350°C 450°C 550°C 650°C
E
1.0
3 (1) 2.55 min acrylonitrile (2) 2.75 min acetonitrile 80 (3) 11.29 min acetic acid (4) 11.68 min formic acid (or oxalic acid) (5) 12.46 min acroleic acid (6) 14.90 min acrylamide 60 4 40
20 N-F codoped Undoped TiO2/SiO2 TiO2/SiO2
N-F codoped Undoped TiO2/SiO2 TiO2/SiO2
Fig. 11 – Photocatalytic activity of acrylonitrile photodegradation over (a) N–F codoped TiO2/SiO2 and (b) undoped TiO2/SiO2 samples under simulant solar light and UV light (λ ≤ 400 nm) irradiation, respectively.
5
12 0
0
2
4
6
8 10 Time (min)
12
6 14
16
Fig. 12 – GC chromatogram of extract liquid after the photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
Q3
9
J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
CH2=CH-CN
0.05
H
·O H
·O
(c)
0.04 207.5 0.02
OH
(b)
CH2 =CHC
198.5
CH3CN + HCOOH oxidation and deamination
d2A/d2λ (nm)
0.03
NH2
OH
0.01 (a)
0.00 198.0 -0.01
210
220 230 Wavelength (nm) 2
240
250
2
3. Conclusions
541
The solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide, has been prepared using NH4F solution as nitrogen and fluorine source. N–F codoped TiO2/SiO2 composite oxides show the mesoporous structure. The presence of N–F causes a red shift of the absorption edge for TiO2/SiO2 composite oxide. NH4F concentration and calcination temperature have significant effects on the BET surface area, surface acid sites,
521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537
542 543 544 545 546 547
C
520
E
519
R
518
R
517
N C O
516
U
515
P CH3COOH or HOOCCOOH or HCOOH
D
540 539
514
CH2 =CHC OH
Fig. 14 – Proposed degradation process of acrylonitrile.
T
538
against broad absorbance bands relative to narrow absorbance bands. The second order derivative spectra of the acrylonitrile solution before adding N–F codoped TiO2/SiO2, reaching adsorption equilibrium before irradiation and after exposure to the simulant solar light for 6 min are shown in Fig. 13, respectively. The acrylonitrile solution shows only one peak at 197.0 nm, which is the same as the characteristic absorbance band for acrylonitrile (Fig. 13a). The band for acrylonitrile shifts to longer wavelength after introduction of catalyst and irradiation, which may be due to the presence of the catalyst (Fig. 13b and c). After irradiation, a new strong peak appearing at 207.5 nm is characteristic of the compounds containing an amide group (Owen, 2000). It should belong to acrylamide. Based on the above results, the study suggests that two distinct pathways might occur during the photocatalytic degradation of acrylonitrile. In the first pathway, some acrylonitrile molecules are converted to acrylamide, when the triple-bonded nitrogen is attacked by UOH, and then acroleic acid by deamination. Acroleic acid is further oxidized to acetic acid and formic acid (or oxalic acid). In the secondary pathway, some acrylonitrile molecules are oxidized to formic acid and acetonitrile, when the vinyl group is attacked by UOH. Acetonitrile is further oxidized to acetic acid. The pathways are depicted in Fig. 14.
O
E
513
CH3COOH
R O
Fig. 13 – Second derivative spectra (d A/dλ ) of the acrylonitrile solution before adding N–F codoped TiO2/SiO2 (a), the solution reaching adsorption equilibrium before irradiation (b) and after exposure to simulant solar light for 6 min (c). A: Absorbance; λ: wavelength.
O
200
NH2
deamination
190
CH2 =CHC
197.0
F
O
-0.02
crystallinity and the photocatalytic activity of the composite oxides. The in situ IR shows that Brønsted and Lewis acid sites appear on the surface of the sample after N–F codoping and they play a crucial role in improving photocatalytic activity. The degradation ratio for acrylonitrile degradation can reach 71.5% after reaction for 6 min under simulant solar light, which indicates that our catalyst is promising for application under solar light. Moreover, the reaction intermediates were verified, and the tentative degradation pathway is proposed for acrylonitrile degradation.
548
Acknowledgments
559 558
549 550 551 552 553 554 555 556 557
This research was financially supported by the Science and 560 Technology Innovation Commission of Shenzhen Municipal- 561 ity, China (No. JCYJ20120613154128107). 562
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Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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3Q5
Dandan Pang, Lu Qiu, Yunteng Wang, Rongshu Zhu, Feng Ouyang⁎
4
Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
O
R O
5
F
2
Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation
1
AR TIC LE I NFO
ABSTR ACT
9
Article history:
The solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide was prepared by a sol–gel 14
10
Received 6 October 2014
method using NH4F as nitrogen and fluorine source. The prepared materials were 15
11
Revised 20 December 2014
characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray 16
12
Accepted 14 January 2015
photoelectron spectroscopy (XPS), UV–Visible diffuse reflectance spectroscopy (UV–Vis), 17
13
Available online xxxx
ammonia adsorption and temperature-programmed desorption (NH3-TPD), in situ Fourier 18
34
Keywords:
photocatalytic activity of the catalyst for acrylonitrile degradation was investigated under 20
35
Photocatalytic activity
simulant solar irradiation. The results showed that strong Lewis and Brønsted acid sites 21
36
Acrylonitrile
appear on the surface of the sample after N–F doping. Systematic investigation showed that 22
37
In situ IR
the highest photocatalytic activity for acrylonitrile degradation was obtained for samples 23
38
N–F codoping
calcined at 450°C with molar ratio (NH4F to Ti) of 0.8. The degradation ratio of 71.5% was 24
D
P
8
C
T
E
transform infrared spectroscopy (FT-IR) and N2 physical adsorption isotherm. The 19
39
achieved with the prepared catalyst after a 6-min irradiation, demonstrating the 25
E
effectiveness of photocatalytic degradation of acrylonitrile with N–F codoped TiO2/SiO2 26 composite oxide. The photocatalyst is promising for application under solar light 27
R
irradiation. Moreover, the intermediates generated after irradiation were verified by gas 28 chromatography–mass spectrometry (GC–MS) analysis and UV–Vis spectroscopy to be 29
R
simple organic acids with lower toxicity, and the degradation pathway was also proposed 30 31
for acrylonitrile degradation with the prepared catalyst.
41 42 40 43
N C O
© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 32
Introduction
46
Acrylonitrile (CH2_CH–CN) is considered as a hazardous pollutant since it is mutagenic, carcinogenic and teratogenic to human beings (Léonard et al., 1999). The health authorities of the Federal Republic of Germany have placed acrylonitrile in the category of carcinogenic chemicals for which no threshold limits are established. The US EPA has classified acrylonitrile as a “priority water pollutant” and “hazardous air pollutant”. Traditional control technologies for acrylonitrile include adsorption and desorption (Huang et al., 1999), thermal and
47 48 49 50 51 52 53 54 55
U
45
Published by Elsevier B.V. 33
catalytic incineration at high temperatures (Gervasini and Ragaini, 2000; Hung and Chu, 2006) and biotechnological abatement methods (Zhang and Pierce, 2009). There are many limitations including high operating costs, secondary waste stream problems, etc. Therefore, developing an alternative abatement method such as photocatalytic oxidation attracts increasing interest. To date, TiO2 has been the most widely used photocatalyst due to its ability to convert UV light to chemical energy to decompose most organic pollutants that exist in air and aqueous systems or to generate hydrogen from water (Macwan et al., 2011). The general drawbacks of photocatalytic
⁎ Corresponding author. E-mail: [email protected] (Feng Ouyang).
http://dx.doi.org/10.1016/j.jes.2015.01.017 1001-0742/© 2015 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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2
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1. Experimental
101
1.1. Preparation of photocatalysts
102
Silica gel (100–200 mesh) was obtained for TiO2 loading from Shanghai Chemical Company. All other chemicals were of analytical reagent grade quality and were employed without further purification. All aqueous solutions were prepared in deionized water from a Milli-Q water system. The TiO2/SiO2 composite powders were prepared by sol–gel method. Solution A was obtained by mixing 10 mL anhydrous alcohol, 0.5 mL deionized water and 2 mL acetic acid. Solution B was obtained by dissolving the starting material of 5 mL tetrabutyl titanate (Ti(OC4H9)4) in 13 mL anhydrous alcohol. Solution A was added dropwise in solution B with vigorous stirring for 2 hr to obtain the homogeneous transparent sol. 2.0 g silica gel was added to the sol with severe stirring for 1 hr (Chen et al., 2004). The resulting gelatinous solution was aged for 12 hr at room temperature and then dried at 80°C. The dry gel was then crushed and calcined at different temperatures for 2 hr. TiO2/SiO2 composite powders were thus obtained. Ammonium fluoride (NH4F) was used as the doping precursor as the nitrogen–fluorine source (Li et al., 2011) and the molar ratios of NH4F to Ti (RNF) were 0:1, 0.2:1, 0.4:1, 0.8:1 and 1.6:1. NH4F aqueous solution (13.83 mol/L) was added in solution A and the same operation as above was conducted, and N–F codoped TiO2/SiO2 powders were thus obtained.
107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124
R
O
C
106
N
105
U
104
R
100 99
103 Q6
L¼
Kλ B cosθ
127 128 Q7 129 130 131 132
ð1Þ
F
76
where, B2 = B2measured − b2instrumental, L, K, λ and θ are the average crystal size, a shape factor of 0.9 for spherical crystallites, the X-ray wavelength (0.15405 nm) and Bragg diffraction angle, respectively. B, Bmeasured and binstrumental are the breadths of the intrinsic diffraction profile, test sample diffraction integral profile and instrumental diffraction profile, respectively. The value of b for the D/max 2500 Diffractometer is 0.0033 arcs. The morphology was examined using a scanning electron microscope (S4700, Hitachi, Japan). The XPS (PHI-1800, ULVAC-PHI, Japan) measurements were performed with monochromatic Al Kα excitation, and all the bonding energies were calibrated to the C 1s peak at 284.6 eV. UV–Visible diffuse reflectance spectra of the powders were collected with a UV–Vis Spectrophotometer (UV–2540, Shimadzu Corporation, Japan) over the spectral range of 240–800 nm. BaSO4 was used as a reference. The BET specific surface area was measured by nitrogen gas adsorption at 77 K using an adsorption instrument (BELSOROPMINI II, Ankersmid, Holland). Pore volume and pore size distribution were determined by the Barrett–Joyner–Halenda (BJH) method. The nature of acid sites was investigated using pyridine as the probe molecule. Fourier transform infrared spectra were recorded using an infrared spectrometer (Nicolet 380, Thermo Fisher Scientific, USA) with a resolution of 4 cm− 1 and 32 scans in the region of 4000–1000 cm− 1. Self-supported wafers of the samples were prepared by applying 11 ton pressure. The sample was subjected to vacuum (10− 2 mbar) in the sample holder followed by thermal treatment at 400°C for 2 hr to obtain a clean surface. The adsorption of pyridine was done at 180°C for 40 min. The physisorbed pyridine was then desorbed under vacuum at room temperature. The NH3-TPD technique was used for the quantitation of the total surface acidity. The sample was preheated at 400°C for 2 hr to remove organic compounds. Ammonia was adsorbed at 100°C and excess ammonia was purged by helium. The measurement of acid site density was carried out by heating from 100 to 600°C at 15°C/min. The desorbed ammonia was reacted with a 0.05 mol/L HCl solution that was then back-titrated with 0.05 mol/L NaOH solution (Al-Yassir et al., 2005; Le Van Mao et al., 2006). In order to investigate the recombination and lifespan of photogenerated electrons-holes in the semiconductor, the photoluminescence (PL) emission spectra of the samples were measured at room temperature by a Raman spectrometer (inVia, Renishaw, England) illuminated with a 325 nm He–Cd laser.
O
75
R O
74
126
P
73
The X-ray diffraction patterns of the powders were obtained on an X-ray diffraction analyzer (D/max 2500, Rigaku Corporation, Japan) with Cu Kα X-ray source at a scanning rate of 8°/min in the 2θ range between 10° and 80°. The accelerating voltage and the applied current were 40 kV and 200 mA, respectively. Crystallite size was calculated according to the Scherrer equation (Gu et al., 2004; Tai et al., 2004):
D
72
125
E
71
1.2. Characterization of photocatalysts
T
70
oxidation, such as low reaction rates, require improvement. The photocatalytic activity of TiO2 can be improved by the addition of SiO2, which increases the available surface area of the catalyst (Chen et al., 2004). Another potentially effective way to improve the photocatalyst performance is to increase the number of surface acid sites, because the photocatalytic activity has been shown to increase with catalyst surface acidity. Doping TiO2 with metal oxides was reported to increase the surface acidity and photocatalytic activity of TiO2 (Cui et al., 1995). Sulfated TiO2 can be readily synthesized by the reaction of amorphous TiO2 with sulfur compounds at high temperatures and employed as a photocatalyst (Wang et al., 2006a). However, little work has been done to study the photocatalytic degradation of acrylonitrile on acid catalysts. F-doped silica-supported TiO2 was used for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation (Pang et al., 2014). Gas-phase photocatalytic oxidation of acrylonitrile on sulfated TiO2 was also reported (Jõks et al., 2011). In this study, solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide was synthesized by sol–gel method. The objective of the present research was to determine the effectiveness of photocatalytic degradation of acrylonitrile with N–F codoped TiO2/SiO2 composite oxides. The declared objective was achieved by experimental research on effects of two variables – calcination temperature and NH4F ratio in the starting material – on the photocatalytic activity of the prepared catalysts for acrylonitrile degradation. The identification of the intermediates of photocatalytic oxidation of acrylonitrile was carried out and mechanisms for the degradation process were also proposed.
C
69
E
68
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
134 133 135 136 137 138 139 140 141 142 Q8 143 Q9 144 145 146 147 Q10 148 149 150 151 Q11 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 Q12 180
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181
Identification of products was verified using gas chromatography–mass spectrometry (GC–MS) (Thermo 183 Q13 trace GC-DSQ Tune, Thermo Fisher Scientific, USA). The 184 solution after irradiation was extracted using chloroform. 185 Then the extract was filtered through a 0.45 μm filter and 186 stored at 4°C until instrumental analysis was conducted. 187 The gas chromatography–mass spectrometry (GC–MS) was 188 equipped with a DB-WAX capillary column (30 m, 0.25 mm 189 internal diameter, 0.25 μm film thickness) and electron 190 ionization (EI) detector. The temperature of the injector 191 and the MS-transfer were both held at 200°C. Mass spectra 192 were compared with those in the GC–MS database for 193 compound identification.
C
1.4. Analytical methods
209
U
N C O
R
R
E
The acrylonitrile concentration in the solution was quantified 210 using an HPLC (LC2000, Shanghai TianMei Scientific Instru211 Q15 ments Co., Ltd., China) equipped with a LC-2030 UV detector 212 and a Sunfire™ C18 column (150 mm, 4.6 mm id.). The eluent 213 was composed of methanol (30%) and water (70%). The flow rate 214 was kept at 1.0 mL min−1. The absorption spectra showed 215 absorption maxima at 197 nm for acrylonitrile, but the detec216 tion wavelength selected for acrylonitrile was 210 nm to 217 remove the noise from methanol with the cutoff wavelength 218 of 205 nm. 219 In general, at low substrate concentration, the kinetics of 220 the photocatalytic oxidation process is described well by a 221 pseudo-first-order equation (Chen et al., 2004).
223 222 224 225 226 227 228 229
ð2Þ
where Co, Ct, t and kapp are the reactant concentration after the system reaches adsorption equilibrium (mg/L), the reactant concentration at time t (mg/L), the irradiation time (min) and the apparent pseudo-first-order rate constant (min−1), respectively. The degradation ratio of the reactant was also used to evaluate the photocatalytic activity of a sample: Co − Ct 100% D¼ Co
231 230
234
Fig. 1 presents the SEM images of pure TiO2, TiO2/SiO2, and N–F codoped TiO2/SiO2 calcined at 450°C. Pure TiO2 was irregularly agglomerated from primary particles. The TiO2/SiO2 possessed a rough and porous surface, resulting in higher surface area than pure TiO2. Additionally, pure TiO2 and TiO2/SiO2 showed the formation of secondary particles by the agglomeration of primary particles. However, N–F codoped TiO2/SiO2, consisting of solid microspheres, showed little agglomeration and did not collapse. The agglomerate size of N–F codoped TiO2/SiO2 was smaller than that of TiO2/SiO2. Guo et al. reported that ammonium fluoride could modify the zeta potential of spherical poly-condensed titania species generated at the initial stage of the hydrolysis reaction (Guo et al., 2003). The results show that NH4F as N–F doping precursor can inhibit the agglomeration of TiO2 powder effectively, which is beneficial for higher activity.
235
2.2. XPS analysis
251
where D is the degradation ratio of the reactant.
ð3Þ
P
D
236 237 238 239 240 241 242 243 244 245 246 247 248 249 250
Fig. 2 shows XPS spectra of N–F codoped TiO2/SiO2 with RNF of 0.8 calcined at 450°C. The binding energies of Ti 2p3/2 and Ti 2p1/2 are equal to 458.7 and 464.5 eV, respectively (Fig. 2a), indicating that Ti exists in the Ti4+ form (Wu et al., 2010; Xiang et al., 2010; Xie et al., 2007). The O 1s signal is shown in Fig. 2b. The peak at 529.9 eV corresponds to lattice oxygen of TiO2, and a strong peak centered at 533.2 eV is assigned to surface hydroxides (Xie et al., 2007). Fig. 2c presents the F 1s XPS spectra. The F 1s region is composed of two contributions. One peak located around 687.7 eV is attributed to the substitute F atoms that occupied oxygen sites in the TiO2 crystal lattice (Huang et al., 2006; Wu et al., 2010; Xie et al., 2007). This indicates that F atoms are incorporated into the crystal lattice of TiO2 prepared by the sol–gel method using NH4F as precursor. Another peak located at 684.2 eV is originated from F− ions physically adsorbed on the surface of TiO2 (`Ti–F) (Wu et al., 2010; Xiang et al., 2010). The surface `Ti–F group may act as an electron-trapping site and reduce interfacial electron transfer rates via tightly holding trapped electrons due to the strong electronegativity of the fluorine (Park and Choi, 2004; Xiang et al., 2010). Fig. 2d shows the XPS spectra for the N 1s. The peak appearing at 400.0 eV is greater than the typical binding energy of 396.9 eV in TiN (Saha and Tomkins, 1992). It should be assigned to the formation of a N–Ti–O linkage (Jang et al., 2006; Wang et al., 2006b; Xie et al., 2007). When nitrogen replaces oxygen in the O–Ti–O structure, the electron density around N is reduced. Thus, the N 1s binding energy in an O–Ti–N environment is higher than that in a N–Ti–N environment (Wang et al., 2006b). The actual amounts of F and N in the sample are 1.9% and 0.5%, respectively.
252
2.3. Nitrogen physical adsorption
282
E
T
Photocatalytic reaction was carried out in a self-made 196 cylindrical glass reactor using a 350 W xenon arc lamp 197 (AHD350, Shenzhen AnHongDa Opto Technology Co., Ltd., 198 Q14 China) as simulant solar light. A typical degradation experi199 ment was carried out as follows: the catalyst was added to 200 180 mL of aqueous solution of acrylonitrile with an initial 201 concentration of 10 mg/L. The reactor was sealed. The 202 reaction mixture was stirred for 60 min in the dark to reach 203 adsorption equilibrium. Then the cooling water system and 204 the xenon lamp were turned on. A small amount of solution 205 was periodically withdrawn from the reaction mixture with a 206 syringe and filtered through a 0.45 μm filter for high perfor207 mance liquid chromatography (HPLC) analysis.
Co ¼ kapp t ln Ct
2.1. SEM images
O
1.3. Photocatalytic experiments
195
208
233 232
R O
194
2. Results and discussion
F
182
253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281
Fig. 3 presents Nitrogen adsorption and desorption iso- 283 therms of N–F codoped TiO2 without SiO2 and N–F codoped 284 TiO2/SiO2 nanoparticles calcined at 450°C. According to 285
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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292 293 294 295 296
458.7
a
E
Relative intensity
C
Ti 2p
R O
291
2.4. UV–Vis analysis
300
Fig. 4 shows the UV–Vis diffuse reflection spectra of TiO2/SiO2 and N–F codoped TiO2/SiO2 calcined at 450°C, respectively. The undoped TiO2/SiO2 obtained was a white powder. However, a slightly yellow color appeared for the N–F codoped catalyst. Generally, the color of a solid is determined by the
301
P
290
D
289
are both 8.0 nm. However, the specific surface area of the 297 former (214 m2/g) is significantly higher than that of the 298 latter (60 m2/g). 299
E
288
IUPAC classification, N–F codoped TiO2 and N–F codoped TiO2/SiO2 nanoparticles display type IV isotherms and H2 hysteresis, which indicate the presence of mesoporous materials (Gregg and Sing, 1997). The plot of the pore size distribution (inset in Fig. 3) was determined by using the Barrett–Joyner–Halenda (BJH) method from the adsorption branch of the isotherm. It shows that both clearly have a mesoporous structure. The mesoporous structure is probably formed by agglomeration of primary particles (interparticle pores) (Wu et al., 2010). The average pore diameters of N–F codoped TiO2/SiO2 and N–F codoped TiO2 nanoparticles
T
287
b
529.9
454
Q1
458 460 462 464 Binding energy (eV)
N
684.2
466
687.7
468
682
526
c
528
530 532 534 Binding energy (eV)
536
400.0
N 1s
538
d
Relative intensity
U
Relative intensity
F 1s
680
456
C
452
O
R
R
464.5
533.2
O 1s
Relative intensity
286
O
F
Fig. 1 – SEM images of (a) pure TiO2, (b) TiO2/SiO2 and (c) N–F codoped TiO2/SiO2 with molar ratio (NH4F to Ti) of 0.8.
684
686 688 Binding energy (eV)
690
692
394
396
398
400 402 404 Binding energy (eV)
406
408
410
Fig. 2 – Ti 2p (a), O 1 (b), F 1s (c) and N 1s (d) high-resolution XPS spectra of N–F codoped TiO2/SiO2. Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
302 303 304 305
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500
0.12
400
dVp/drp
300
0.08 2
0.06 0.04 1 0.02
200
0.00 0
5
10
15 20 25 Pore diameter (nm)
30
35
2
F
Adsorbed volume (cm3/g, STP)
0.10
100
00.0
0.4
0.6 Relative pressure (P/P0)
0.8
1.0
R O
0.2
O
1
308
Eg ¼ 1239:8=λ
317 318 319 320
d
T
C
c
1549 Transmittance (a.u.)
316
E
315
R
314
In situ characterization of surface species and surface active sites during heterogeneous photocatalytic processes is one of the most important experimental approaches. Many spectroscopic techniques have been employed to study photocatalytic reactions on the surface. Among these, FT-IR is probably the best method for in situ photocatalytic studies because it is inexpensive, sensitive, and capable of determining chemical species, and can be carried out at atmospheric pressure and room temperature. The adsorption of pyridine combined with
R
313
N C O
312
U
311
ð4Þ
where Eg and λ are the band gap and wavelength of the absorption edge in the spectrum, respectively. The absorption edge of the sample was shifted to longer wavelength after N–F doping. The absorption edge of the undoped TiO2/SiO2 occurred at around 388 nm. After N–F doping, the absorption edge of the sample occurred at around 414 nm and the band gap was changed to about 2.99 eV. The band gap narrowing of the doped sample should be attributed to doping N from ammonium fluoride, since F doping did not result in any significant shift in the fundamental absorption of TiO2 (Huang et al., 2007; Li et al., 2005).
Absorbance (a.u.)
310 309
2.5. Effect of NH4F ratio in starting material on physical and 321 photocatalytic properties 322
position of its absorption edge. The band gaps of the samples are determined by the following equation (O'Regan and Grätzel, 1991):
D
307
E
306
P
Fig. 3 – Nitrogen adsorption and desorption isotherms of (1) N–F codoped TiO2 without SiO2 and (2) N–F codoped TiO2/SiO2 nanoparticles. Inset shows Barrett–Joyner–Halenda (BJH) pore size distributions of the corresponding samples.
1579
b
1597
1446
2 a
1
1650 300
1491
400
500 600 Wavelength (nm)
700
Fig. 4 – UV–Vis absorption spectra of (1) TiO2/SiO2 and (2) N–F codoped TiO2/SiO2.
800
1600
1550 1500 Wavenumber (cm-1)
1450
1400
Fig. 5 – Infrared spectra of chemisorbed pyridine molecules adsorbed on the N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
323 324 325 326 327 328 329 330 331
6
348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371
II
438 468
498 505 520
0.11 0.07 0.25 0.13
0.04 0.08 0.09 0.14
d
t1:6
0.13 0.08
0.12 0.24 0.16
0.15 0.27 0.71 0.51
t1:7 t1:8 t1:9 t1:10 t1:11 t1:12
F
I, II, IV, and V denote as the number of the peak.
ratios. The surface acid site densities of N–F codoped TiO2/SiO2 and undoped sample were calculated, as shown in Table 1. With the increase of NH4F/Ti molar ratio, an increase for the amount of acid sites was observed. However, excess N–F doping results in the decrease in surface acid sites (Fig. 5d and Table 1). Essentially, the acidity profile of the N–F codoped samples exhibits three or four separated peaks, while that of the undoped sample shows only two desorption peaks. The last two peaks at about 450 and 510°C for N–F codoped samples (0.8:1 and 1.6:1) corresponding to stronger acid sites, possibly play a more important role in improving the photocatalytic activity. PL emission spectra are widely used to determine the efficiency of charge carrier trapping, migration, transfer and separation, and the recombination of photogenerated electrons and holes in semiconductors (Xiang et al., 2010; Zhang et al., 2000). Fig. 7 shows a comparison of the PL spectra of undoped and N–F codoped TiO2/SiO2 samples prepared with varying RNF values in the wavelength range of 450–650 nm. It is found that the PL emission intensity decreases with the increase of RNF. However, the sample with excessive N–F doping exhibits an increase in emission intensity. This suggests that an appropriate amount of N–F doping will slow the radiative recombination rate of photogenerated electrons
b
450 200
300 400 Temperature (°C)
500
600
Fig. 6 – NH3-TPD profile of N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1.
500
t1:5
Total
RNF = 0:1 RNF = 1.6:1 RNF = 0.2:1 RNF = 0.8:1
a
t1:4
V
c
100
t1:1 t1:2 t1:3 Q4
IV
O
347
I
R O
346
327 268 302 290
V
P
345
155 199 205 208
IV
D
344
II
E
343
I
Amount of desorbed NH3 (mmol/g)
Intensity (a.u.)
342
(a) (b) (c) (d)
Peak temperature (°C)
T
341
C
340
E
339
R
338
R
337
Samples
O
336
Table 1 – Acid sites density of N–F codoped TiO2/SiO2 with NH4F/Ti molar ratio of (a) 0:1, (b) 0.2:1, (c) 0.8:1 and (d) 1.6:1 calculated by using NH3-TPD.
C
335
N
334
in situ FT-IR technique is well known for characterizing the surface Brønsted and Lewis acidic sites (Al-Yassir et al., 2005; Jin et al., 2009; Wang et al., 2006a). Fig. 5 shows the IR spectra of pyridine chemisorbed on the N–F codoped TiO2/SiO2 with different NH4F/Ti molar ratios. The IR spectrum of Fig. 5c clearly displays the characteristic bands of the chemisorbed pyridine. The band observed at 1597 cm−1 is assigned to hydrogen-bonded pyridine (Al-Yassir et al., 2005). The bands at 1445 and 1579 cm− 1 are due to the interaction of pyridine with strong and weak Lewis acid sites, respectively (Al-Yassir et al., 2005; Le Van Mao et al., 1999; Wang et al., 2006a). The peak at 1549 cm−1 is due to the vibration of pyridine adsorbed on Brønsted acid sites (Le Van Mao et al., 1999). The peak at 1491 cm−1 was assigned to pyridine adsorbed on both Lewis and Brønsted acid sites (Jin et al., 2009). Most notably, only one small peak at 1446 cm−1 corresponding to pyridine adsorbed on Lewis acid site was observed for the undoped sample (Fig. 5a). With the increase of NH4F/Ti molar ratio, a significant increase in the intensity of the characteristic bands of pyridine was observed. This suggests that the surface acid sites can become stronger and react with more pyridine molecules after N–F doping. These strong Lewis and Brønsted acid sites provided better adsorption centers for oxygen and reactant molecules and converted adsorbed water into active hydroxyl groups, which improved the photocatalytic activity (Pelaez et al., 2012; Wang et al., 2006a). The Lewis acid sites are strong adsorption centers and can accept electron pairs. Acrylonitrile, with lone-pair electrons, can be easily adsorbed onto the Lewis acid sites and oxidized by UOH. The Brønsted acid sites can donate protons (H+) and generate UOOH, a powerful oxidant. The UOH and UOOH formations are then involved in acrylonitrile oxidation reactions, leading to the decrease in the re-combination ratio of electrons and holes, as shown in Fig. 7. On the other hand, from the viewpoint of microscopic reaction kinetics, the stronger the adsorption ability of Lewis acid sites is, the longer the residence time of reactant molecules adsorbed on the surface and the higher the reaction activity. Fig. 6 shows the distribution curves of the acid strength of the N–F codoped TiO2/SiO2 with different NH4F/Ti molar
U
333
TCD signal
332
J O U RN A L OF E N V I RO N ME N TA L S CIE N CE S X X (2 0 1 5 ) XXX –XXX
550 600 Wavelength (nm)
650
Fig. 7 – Photoluminescence emission spectra of N–F codoped TiO2/SiO2 composition oxides with different NH4F/Ti molar ratio (RNF).
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395
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410
414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433
0.15 0.12 0.09
(204) (116) (220) (215) (301)
550°C
F
450°C
350°C
10
20
30
O
409
C
408
E
407
R
406
R
405
N C O
404
U
403
Apparent rate constant (min-1)
402
(200) (105) (211)
40
50 2θ (°)
60
70
80
R O
Fig. 9 shows the XRD patterns of N–F codoped TiO2/SiO2 powders prepared at different calcination temperatures. All N–F codoped TiO2/SiO2 powders contained only anatase phase TiO2, and no phase transformation from anatase to rutile was observed, even after calcination at 650°C for 2 hr. Moreover, the peak intensity of anatase increased with the increase of calcination temperature and the peaks became sharper, suggesting that the relative crystallinity and crystalline size significantly increased. The average crystallite sizes of samples can be calculated by the Scherrer equation using the full-width at half-maximum of the X-ray diffraction peaks at 2θ = 25.2°. The average crystallite size increased with the increase of calcination temperature. However, the surface area decreased as the calcination temperature increased (Table 2). Fig. 10 shows the effect of calcination temperature on the photocatalytic activity of N–F co-doped TiO2/SiO2 for 6 min irradiation. It can be observed that the photocatalytic activity of N–F codoped TiO2/SiO2 for acrylonitrile photodegradation increased with the increase of calcination temperature, and reached a maximum at the calcination temperature of 450°C.
401
(004) (103) (112)
650°C
Fig. 9 – XRD patterns of N–F codoped TiO2/SiO2 samples calcined at 350, 450, 550 and 650°C.
It appears that the increase in photocatalytic activity at higher calcination temperature is mainly due to the higher crystallinity of the powder (Fig. 9), which results in a decrease in the defect density on the catalyst surface (Maeda et al., 2005). Meanwhile, with further increasing calcination temperature, the average crystal size of the N–F codoped TiO2/SiO2 catalyst increased. However, the photocatalytic activity decreased. This implies that besides the higher crystallinity, suitable defect density is also essential for high photocatalytic activity. This is consistent with the nature of the surface acid sites (Fig. 5). We consider that surface acid sites are related to surface defects. The surface defects are generated by the coordination of different surface atoms. The difference in surface charge distribution may cause the formation of different types of surface acid sites (Brønsted and Lewis acid sites).
P
413
400
Anatase
D
2.6. Effect of calcination temperature on photocatalytic activity
399
Intensity (a.u.)
412
398
(101)
T
411
and holes in the samples. Moreover, the lower the intensity of PL spectra, the higher the photocatalytic activity, which is consistent with earlier published reports (Xiang et al., 2010; Yu et al., 2002). Fig. 8 shows the dependence of the photocatalytic activity of acrylonitrile degradation for 6 min irradiation over N–F codoped TiO2/SiO2 on the NH4F/Ti molar ratio of the starting materials. The apparent rate constant increases with the NH4F/Ti molar ratio to reach a maximum at NH4F/Ti = 0.8, which is due to the increase of the surface acid sites on the sample after N–F doping (Fig. 5). However, excessive N–F doping results in the decrease in the photocatalytic activity, which is attributed to the introduction of new recombination centers (defect sites) of photogenerated electrons and holes (Fig. 7). There is an optimal dopant concentration, which is similar to the previous studies (Li et al., 2011; Maeda et al., 2005).
397
E
396
Q2
434 435 436 437 438 439 440 441 442 443 444 445 446 447 448
2.7. Influence on the photocatalytic activities of light absorption 449 properties of N–F codoped TiO2/SiO2 450 Fig. 11 shows the degradation of acrylonitrile in the presence of undoped and N–F codoped samples under simulant solar light and UV irradiation (λ ≤ 400 nm), respectively. The N–F codoped TiO2/SiO2 photocatalyst exhibits higher activity than the undoped sample, and 71.5% and 64.3% acrylonitrile could be removed after 6 min under simulant solar light and UV irradiation, respectively. The above details suggest that the photocatalytic performance of TiO2/SiO2 composite oxide is greatly improved by N–F doping under simulant solar light and UV light irradiation, which demonstrates that high
451
Table 2 – Specific surface area and crystal size of N–F codoped TiO2/SiO2 powders at different calcination temperatures.
t2:1 t2:2 t2:3
452 453 454 455 456 457 458 459 460
0.06 0.03 0.00
0.0
0.5 1.0 1.5 NH4F/Ti molar ratio of the starting material
Fig. 8 – Dependence of the photocatalytic activity of acrylonitrile degradation over N–F codoped TiO2/SiO2 calcined at 450°C on the NH4F/Ti molar ratio of starting materials.
Calcination temperature (°C)
Specific surface area (m2/g)
Crystal size (nm)
350 450 550 650
219 214 206 196
24.8 26.6 28.2 29.1
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
t2:4
t2:5 t2:6 t2:7 t2:8 t2:9
8
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0.6
0.4
1
2
3 4 Irradiation time (min)
5
6
Fig. 10 – Effect of calcination temperature on photocatalytic activity of N–F codoped TiO2/SiO2.
461
472
2.8. Photocatalytic decomposition pathway of acrylonitrile
473
Many organic pollutants can be decomposed by photocatalytic oxidation process with low cost. However, the intermediates
Simulant solar light 70 60 50 40 30 20 10 0
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UV light (λ≤400 nm) 100
Relative abundance (%)
474
C
470
E
469
R
468
R
467
O
466
C
465
N
464
U
463
Degradation ratio (%)
462
T
471
activity is not only dependent on surface active sites but also related to the light absorption properties (Fig. 4). Though the shift of absorption edge is small, corresponding energy levels are suitable for an electron transition to generate UOH; moreover, the interval of shift falls on the UV light region of the simulant solar light. This implies that the prepared catalyst may have the highest efficiency under irradiation of simulant solar light in the region. The catalyst may absorb enough strong light and excite electron transition, then cause a series of photochemical reactions, leading to degradation of a large amount of acrylonitrile.
O
0
R O
0.0
F
0.2
may be more toxic and unacceptable for human heath than their precursors. Therefore, it is necessary to identify the intermediates formed during the photocatalytic process. Krichevskaya et al. reported that the main products of gas-phase acrylonitrile photocatalytic oxidation included nitrogen dioxide, nitrous oxide, carbon dioxide, water, hydrogen cyanide and carbon monoxide by using Degussa P25 under UV irradiation (Krichevskaya et al., 2009). Jõks et al. reported that the gaseous products of photocatalytic oxidation of acrylonitrile on sulfated P25 were CO2, HCN and HNCO (Jõks et al., 2011). However, there are a few reports of the intermediates of photocatalytic oxidation of acrylonitrile in solution under simulant solar light irradiation. Fig. 12 illustrates the GC–MS analysis of the decomposition products of acrylonitrile in the presence of N–F codoped TiO2/SiO2. Six dominant peaks at 2.55, 2.75, 11.29, 11.68, 12.46 and 14.50 min were detected, and the mass spectral peaks identified the corresponding intermediate compounds as acrylonitrile, acetonitrile, acetic acid, formic acid (or oxalic acid), acroleic acid and acrylamide, respectively. Further confirmation was made by comparing the retention times of pure samples of the intermediates. Most of the intermediates are simple organic acids and less toxic than acrylonitrile. Although several intermediates in the solution were detected, no by-products were detected by GC–MS in the gas phase in the upper reactor during any of the experiments. The importance of the derivative spectroscopy for interpretation of UV–Vis spectra is well known and documented (Askal et al., 2008). A derivative spectrum can enhance small peaks and shoulders compared to the absorbance spectrum (zero-order band). Derivative spectra can be used to enhance differences among spectra, to resolve overlapping bands in qualitative analysis and, most importantly, to reduce or eliminate background interference from a wide range of sources (Owen, 2000). The most distinctive feature of the second-order derivative is a negative band with minimum at the same wavelength as the maximum on the absorbance band. Perhaps more importantly, derivatives discriminate
P
ln(C0 /C)
0.8
R2 = 0.9984 R2 = 0.9997 R2 = 0.9990 R2 = 0.9991
D
350°C 450°C 550°C 650°C
E
1.0
3 (1) 2.55 min acrylonitrile (2) 2.75 min acetonitrile 80 (3) 11.29 min acetic acid (4) 11.68 min formic acid (or oxalic acid) (5) 12.46 min acroleic acid (6) 14.90 min acrylamide 60 4 40
20 N-F codoped Undoped TiO2/SiO2 TiO2/SiO2
N-F codoped Undoped TiO2/SiO2 TiO2/SiO2
Fig. 11 – Photocatalytic activity of acrylonitrile photodegradation over (a) N–F codoped TiO2/SiO2 and (b) undoped TiO2/SiO2 samples under simulant solar light and UV light (λ ≤ 400 nm) irradiation, respectively.
5
12 0
0
2
4
6
8 10 Time (min)
12
6 14
16
Fig. 12 – GC chromatogram of extract liquid after the photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2.
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
Q3
9
J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 5 ) XX X–XXX
CH2=CH-CN
0.05
H
·O H
·O
(c)
0.04 207.5 0.02
OH
(b)
CH2 =CHC
198.5
CH3CN + HCOOH oxidation and deamination
d2A/d2λ (nm)
0.03
NH2
OH
0.01 (a)
0.00 198.0 -0.01
210
220 230 Wavelength (nm) 2
240
250
2
3. Conclusions
541
The solid acid catalyst, N–F codoped TiO2/SiO2 composite oxide, has been prepared using NH4F solution as nitrogen and fluorine source. N–F codoped TiO2/SiO2 composite oxides show the mesoporous structure. The presence of N–F causes a red shift of the absorption edge for TiO2/SiO2 composite oxide. NH4F concentration and calcination temperature have significant effects on the BET surface area, surface acid sites,
521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537
542 543 544 545 546 547
C
520
E
519
R
518
R
517
N C O
516
U
515
P CH3COOH or HOOCCOOH or HCOOH
D
540 539
514
CH2 =CHC OH
Fig. 14 – Proposed degradation process of acrylonitrile.
T
538
against broad absorbance bands relative to narrow absorbance bands. The second order derivative spectra of the acrylonitrile solution before adding N–F codoped TiO2/SiO2, reaching adsorption equilibrium before irradiation and after exposure to the simulant solar light for 6 min are shown in Fig. 13, respectively. The acrylonitrile solution shows only one peak at 197.0 nm, which is the same as the characteristic absorbance band for acrylonitrile (Fig. 13a). The band for acrylonitrile shifts to longer wavelength after introduction of catalyst and irradiation, which may be due to the presence of the catalyst (Fig. 13b and c). After irradiation, a new strong peak appearing at 207.5 nm is characteristic of the compounds containing an amide group (Owen, 2000). It should belong to acrylamide. Based on the above results, the study suggests that two distinct pathways might occur during the photocatalytic degradation of acrylonitrile. In the first pathway, some acrylonitrile molecules are converted to acrylamide, when the triple-bonded nitrogen is attacked by UOH, and then acroleic acid by deamination. Acroleic acid is further oxidized to acetic acid and formic acid (or oxalic acid). In the secondary pathway, some acrylonitrile molecules are oxidized to formic acid and acetonitrile, when the vinyl group is attacked by UOH. Acetonitrile is further oxidized to acetic acid. The pathways are depicted in Fig. 14.
O
E
513
CH3COOH
R O
Fig. 13 – Second derivative spectra (d A/dλ ) of the acrylonitrile solution before adding N–F codoped TiO2/SiO2 (a), the solution reaching adsorption equilibrium before irradiation (b) and after exposure to simulant solar light for 6 min (c). A: Absorbance; λ: wavelength.
O
200
NH2
deamination
190
CH2 =CHC
197.0
F
O
-0.02
crystallinity and the photocatalytic activity of the composite oxides. The in situ IR shows that Brønsted and Lewis acid sites appear on the surface of the sample after N–F codoping and they play a crucial role in improving photocatalytic activity. The degradation ratio for acrylonitrile degradation can reach 71.5% after reaction for 6 min under simulant solar light, which indicates that our catalyst is promising for application under solar light. Moreover, the reaction intermediates were verified, and the tentative degradation pathway is proposed for acrylonitrile degradation.
548
Acknowledgments
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This research was financially supported by the Science and 560 Technology Innovation Commission of Shenzhen Municipal- 561 ity, China (No. JCYJ20120613154128107). 562
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U
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693
Please cite this article as: Pang, D., et al., Photocatalytic decomposition of acrylonitrile with N–F codoped TiO2/SiO2 under simulant solar light irradiation, J. Environ. Sci. (2015), http://dx.doi.org/10.1016/j.jes.2015.01.017
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