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Fluorine promoted and silica supported TiO2 for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation
Chemical Engineering Journal 258 (2014) 43–50
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Fluorine promoted and silica supported TiO2 for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation Dandan Pang, Yunteng Wang, Xiaodong Ma, Feng Ouyang ⇑ Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, PR China
h i g h l i g h t s F doped samples were obtained by a simple sol–gel method. Bronsted and Lewis sites are observed on the surface of the samples by in situ IR. The number and strength of surface acid sites are increased due to F doping. The F doped samples show superior photocatalytic activity.
a r t i c l e
i n f o
Article history: Received 28 March 2014 Received in revised form 2 July 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: Photocatalytic activity Acrylonitrile F doping Silica In situ IR
a b s t r a c t The F doped TiO2/SiO2 composition oxides were prepared by sol–gel method using HF solution as fluorine source. The prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), ultraviolet–visible absorption spectroscopy (UV–Vis), photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FT-IR) and ammonia adsorption and temperature-programmed desorption (NH3-TPD). The photocatalytic activities of the powders for acrylonitrile degradation have been inspected. The results show that the presence of SiO2 as a support for TiO2 loading benefits the formation of the nanoparticles with a large surface. In situ IR and NH3-TPD investigation shows that the higher photocatalytic activity for F doped samples is attributed to the increase of number and strength of surface acid sites. The highest photcatalytic activity for acrylonitrile degradation is obtained for a sample calcined at 450 °C with molar ratio (HF to Ti) of 1.1 and 36% TiO2 loading. The degradation ratio of acrylonitrile can reach to 66% under simulant solar light for 6 min, indicating the effectiveness of photocatalytic degradation acrylonitrile using F doped TiO2/SiO2 composite oxide. Ó 2014 Published by Elsevier B.V.
1. Introduction Many studies demonstrated that acrylonitrile may cause gene mutations, chromosome aberrations, unscheduled DNA synthesis and cell transformation [1]. Exposure to acrylonitrile can occur from residual acrylonitrile in commercial fibrous polymeric material and in styrene–acrylonitrile resins [1]. In acrylonitrile production, a large amount of acrylonitrile wastewater is generated. The US EPA classified acrylonitrile as a ‘‘water priority pollutant’’ and a ‘‘hazardous air pollutant’’. Traditional removal techniques including absorption methods [2], thermal-catalytic incineration [3,4] and biotechnological abatement methods [5], have disadvantages like high costs, insufficient abatement problems and harsh operation conditions. ⇑ Corresponding author. E-mail address: [email protected] (F. Ouyang). http://dx.doi.org/10.1016/j.cej.2014.07.068 1385-8947/Ó 2014 Published by Elsevier B.V.
Photocatalytic oxidation using semiconductor photocatalyst TiO2 to mineralize most of organic pollutants should be an effective alternative remediation technology due to the thermal stability, facile synthesis, low cost and the low toxicity of TiO2 [6,7]. It was well known that the photocatalytic performance of the TiO2 strongly depended on its size, shape, composition and crystallinity [8]. Among various morphologies, mesoporous TiO2 spheres are especially important since a spherical morphology has been demonstrated to present excellent stability, mono-disperse nature, and enhanced light harvesting property [9]. After doping F, mono-disperse mesoprous F-TiO2 spheres showed enhanced light harvesting which resulted in superior photocatalytic activity in dye degradation [10]. The high photocatalytic of fluorine doped TiO2 is also related to smaller crystalline size, well crystalline phase narrow band gap, intense absorption in the visible light region and higher amount of surface hydroxyl groups [11,12]. The photocatalytic activity is shown to increase with catalyst
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surface acidity [13]. The number of surface acid sites may be increased by F doping into TiO2 due to the stronger acidity of HF. On the other hand, large surface area and high adsorption capacity are very important to increase the photocatalytic activity of TiO2 particles. To take silica with large surface as a support material for TiO2 loading is a feasible approach [14]. Krichevskaya et al. reported photocatalytic degradation of acrylonitrile in gas–solid systems by using Degussa P25 under UV irradiation [15]. Jõks et al. reported that gas-phase photocatalytic oxidation of acrylonitrile on sulphated TiO2 showed improved performance at higher temperature and longer retention times under UV irradiation [16]. Although the photocatalytic degradation of acrylonitrile in solution attracts some attentions, such works are little so far. In this study, F doped TiO2/SiO2 composite oxide was synthesized by sol–gel method using HF solution as fluorine source. We investigated the influences of silica, HF/Ti ratio and calcination temperature on the physical properties, such as microstructures, morphology, special surface area, compositions and the nature of the surface active sites. The effects of TiO2 loading, HF ratio in starting material, calcination temperature on the photocatalytic activities of the prepared composite oxides for acrylonitrile degradation were also estimated. 2. Material and methods 2.1. Preparation of photocatalysts Tetrabutyl titanate was used as a starting material and HF as a fluorine source. All chemicals used were of analytical grade. Firstly, 5 mL tetrabutyl titanate (Ti(OC4H9)4) was dissolved in 13 mL anhydrous alcohol to form solution A. 10 mL anhydrous alcohol, 0.5 mL deionized water, 2 mL acetic acid and hydrofluoric acid solution (with a concentration 40 wt.%) were mixed with stirring for 20 min to form solution B. Secondly, Solution B was added dropwise into solution A with vigorous stirring for 2 h to get the homogeneous transparent sol. A certain amount of silica gel (100–200 mesh) [14] was added to the sol with severe agitation for 1 h. The resulted gelatinous solution was aged for 12 h at room temperature and then was dried at 80 °C. The dry gel was then crushed and calcined at different temperatures for 2 h. F doped TiO2/SiO2 composition powders were thus gained. In this paper, the prepared particles were denoted as F doped wt.% TiO2/SiO2, where wt.% was the amount of TiO2 loading by weight. The molar ratio of HF to Ti (RF) was 0.0:1, 0.3:1, 0.7:1, 1.1:1 and 2.0:1. For comparison, pure TiO2, undoped 36% TiO2/ SiO2, F doped TiO2 without SiO2 nanoparticles were also synthesized by the same method.
1800 system (ULVAC-PHI, Japan) with monochromatic Al ka excitation. The binding energy of the C 1s line (284.6 eV) was taken for calibration the obtained spectra. The general morphology was characterized by a Hitachi S4700 scanning electron microscope. UV–Vis diffuse reflectance spectra were recorded using Shimadzu Corporation UV-2540 over the spectral range 240–800 nm. BaSO4 was used as a reference. The BET specific surface area was measured by nitrogen gas adsorption at 77 K using BELSOROP-MINI II adsorption instrument. Pore size distribution was determined by Barrett–Joyner–Halenda (BJH) method. The PL spectra were obtained on a Renishaw inVia Raman spectrometer at room temperature with a He–Cd laser (325 nm) as the light source. The powder sample on a slide was detected by a CCD array detector. Fourier transform infrared spectra were collected using Nicolet 380 with a resolution of 4 cm1 and 32 scans in the region of 4000– 1000 cm1. The nature of the surface acid sites was investigated by chemical adsorption of pyridine onto clean self-bonded sample wafers after an outgassing under vacuum (102 mbar) at 673 K for 2 h. The adsorption of pyridine was done at 180 °C for 40 min. The physisorbed pyridine was then desorbed under vacuum at room temperature. Ammonia adsorption and temperature-programmed desorption (NH3-TPD) technique was used for the study of the surface acidity. The sample was preheated at 400 °C for 2 h in flowing helium at a flow rate of 40.0 mL/min. After cooling to 100 °C, the sample was exposed to a stream of ammonia for 1 h at a flow rate of 20.0 mL/min. Then, the sample was left in flowing helium at the same temperature for 3 h in order to purge any excess of ammonia. Finally, the TPD operation was performed by heating from 100 to 600 °C at 15.0 °C/min. The acid density was measured using a 0.05 mol/L HCl solution which was then back-titrated with a 0.05 mol/L NaOH solution in accordance with the method reported in literatures [19–21]. 2.3. Photocatalytic experiments The photocatalytic activities of the prepared photocatalysts were evaluated by measuring the degradation rate of acrylonitrile. In this test, a 350 W xenon short arc lamp (ShenZhen AnHongDa Opto Technology Co., Led.) with the similar characteristic spectrum with sun light was used as a light source of simulant solar light (Fig. 1). The catalyst was suspended in 180 mL aqueous solution of acrylonitrile with an initial concentration of 10 mg/L and the reactor was sealed. Prior to light illumination, the suspension
2.2. Characterization of photocatalysts The phase composition and crystallite size of the samples were obtained on a Rigaku D/max 2500 X-ray diffraction analyzer with Cu Ka X-ray source at a scanning rate of 8°/min in the 2h 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 Scherrer equation [17,18]:
L¼
Kk 2 ; B2 ¼ B2measured binstrumental B cos h
ð1Þ
where L, K, k and h are the average crystal size, the shape factor 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 intrinsic diffraction profile, the test sample diffraction integral profile and instrumental diffraction profile, respectively. The XPS measurements were carried out using an ULVAC-PHI
Fig. 1. The emission spectra of the simulant solar light and sun light.
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was magnetically stirred for 60 min in the dark for adsorption/ desorption equilibrium. Then the xenon lamp was turned on and the temperature of suspension was maintained at 25 °C by circulation of water through an external cooling coil. At given intervals of illumination, a small amount of suspension was taken out and filtered through 0.45 lm filter for HPLC analysis (LC2000, ShangHai Echcomp Co). The detection wavelength selected for acrylonitrile was 210 nm. 2.4. Analytical methods At low substrate concentration, the kinetics of the photocatalytic oxidation process has been described by a pseudo-first-order equation [14,22].
ln
Co C
¼ kapp t
ð2Þ
where Co is the acrylonitrile concentration after the system got to adsorption equilibrium (mg/L), C is the acrylonitrile concentration at time t (mg/L), t is the irradiation time and kapp is the apparent pseudo-first-order rate constant (min1). The degradation ratio of acrylonitrile was also used to evaluate the photocatalytic activity of a sample: D = (Co C) 100%/Co, where D is the degradation ratio of the reactant. 3. Results
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Fig. 2(c) shows the SEM micrograph for the F doped TiO2 without SiO2. As shown in the images, F doped TiO2 is composed of numerous small crystallites with about 250 nm in diameter due to the chemical etching of HF. However, a poor porosity is present on the samples. Different from F doped TiO2, F doped 36% TiO2/SiO2 spheres consist of numerous well-packed nano-crystallites without heavy aggregation and possess a rough surface (Fig. 2(d)). The result is consistent with the result of BET specific surface area measurement for F doped TiO2 (47 m2/g) and F doped 36% TiO2/ SiO2 (221 m2/g) (Table 1). 3.2. Nitrogen physical adsorption Fig. 3(a) presents nitrogen adsorption and desorption isotherms of F doped TiO2 without SiO2 and F doped 36% TiO2/SiO2 nanoparticles calcined at 450 °C. According to IUPAC classification, F doped TiO2 and F doped 36% TiO2/SiO2 nanoparticles display type IV isotherm and H2 hysteresis, which indicate the presence of mesoporous materials [23]. Fig. 3 (b) shows the pore size distribution curve calculated by the BJH method from the adsorption branch of nitrogen isotherm. It could be found that both have mesoporous structure. The average pore diameters of F doped 36% TiO2/SiO2 and F doped TiO2 nanoparticles are both 7.0 nm. However, the specific surface area of the former (221 m2/g) is significantly higher than that of the later (47 m2/g). Moreover, compared to the former, the later has poor porosity, which is consistent with SEM results as shown in Fig. 2(c) and (d).
3.1. SEM images 3.3. XPS analysis Fig. 2(a) and (b) shows the SEM micrograph for pure TiO2 and 36% TiO2/SiO2 calcined at 450 °C. Pure TiO2 are irregularly agglomerated by primary particles. The 36% TiO2/SiO2 possesses a rough and porous surface, resulting in higher surface area than pure TiO2. Additionally, pure TiO2 and 36% TiO2/SiO2 show the formation of secondary particles by the agglomeration of primary particles.
XPS survey spectra of F doped 36% TiO2/SiO2 show that the F doped sample contains predominantly Ti, O, F, Si elements and a trace amount of carbon (not shown). The F 1s spectrum gives two peaks centered at 684.0 and 687.5 eV, respectively (Fig. 4). The peak centered at 687.5 eV is attributed to the doped F atoms
Fig. 2. SEM images for pure TiO2 (a); 36% TiO2/SiO2 (b); F doped TiO2 (c) and F doped 36% TiO2/SiO2 (d).
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Table 1 BET surface area and total pore volume of the samples with different TiO2 loading. Samplea
BET (m2/g)
Total pore volume (cm3/g)
Silica gel 16% TiO2/SiO2 22% TiO2/SiO2 36% TiO2/SiO2 53% TiO2/SiO2 70% TiO2/SiO2 TiO2
359 281 279 221 208 170 47
0.91 0.87 0.85 0.65 0.58 0.45 0.08
a All sample were calcined at 450 °C for 2 h and the molar ratio of HF to Ti was 1.1, except for silica gel.
Fig. 4. F 1s high-resolution XPS spectra of F doped 36% TiO2/SiO2 powders.
Fig. 5. The acrylonitrile degradation ratio of F doped TiO2/SiO2 samples calcined at 450 °C as a function of the TiO2 loading.
Fig. 3. (a) Nitrogen adsorption and desorption isotherms, and (b) BJH pore size distributions of (1) F doped TiO2 without SiO2 and (2) F doped 36% TiO2/SiO2 nanoparticles.
in TiO2 crystal lattice, i.e. the substitute F atoms that occupied oxygen sites in the TiO2 crystal lattice [24,25]. The peak centered at 684.0 eV is a typical value for surface fluoride („TiAF) species on the TiO2 crystal surface [24–26].
TiO2/SiO2 particles retained relatively large surface area and mesoporous structures, compared with the sample without SiO2 (TiO2). Fig. 5 shows the degradation ratio of F doped TiO2/SiO2 samples for acrylonitrile change with the TiO2 loading. Acrylonitrile cannot be degraded under irradiation with silica gel. The degradation ratio is increased rapidly with the increase of TiO2 loading until 36%. The increase of the TiO2 loading increases the surface coverage of TiO2 on the silica gel, which results in the enhancement of the rate of acrylonitrile photodegradation, since the photodegradation of acrylonitrile only takes place on the TiO2 particles surface. However, the degradation ratio decreases gradually when the loading is more than 36%. Under high TiO2 loading, lower the surface area and smaller pore volume decrease the photocatalytic activity of TiO2/SiO2 mixed oxides (Table 1) [27].
3.4. Effect of TiO2 loading on physical and photocatalytic properties The BET surface area and total pore volume of the samples with different TiO2 loading are shown in Table 1. With the TiO2 loading increasing, the BET surface area and total pore volume of TiO2/SiO2 particles decrease gradually, which indicates that a part of TiO2 particles dispersed inside the pore of silica gel. Moreover, the
3.5. Effect of HF ratio in starting material on physical and photocatalytic properties The nature of the surface active sites in solid acids is defined by coordinately unsaturated cationic centers giving Lewis acidity, and by the presence of protons, generating Bronsted acidic sites. The
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Fig. 6. FT-IR spectroscopy of chemisorbed pyridine adsorbed on F doped 36% TiO2/ SiO2 with different HF/Ti molar ratio. (a) 0:1, (b) 0.3:1, (c) 1.1:1, (d) 2.0:1.
adsorption of base molecules (e.g. pyridine or ammonia) combined with vibration spectroscopic techniques is well known for characterizing these surface active sites [13]. Fig. 6 shows the IR spectra of chemisorbed pyridine adsorbed on the F doped 36% TiO2/SiO2 with different HF/Ti molar ratio. As shown in Fig. 6(c), F doped 36% TiO2/SiO2 (RF = 1.1) exhibits bands due to hydrogen bonded pyridine (1597 cm1), strong Lewis bound pyridine (1446 cm1), weak Lewis bound pyridine (1579 cm1), pyridine ion ring vibration due to pyridine bound to Bronsted acid sites (1549 cm1) and a band at 1491 cm1 which can be assigned to pyridine associated with both Bronsted and Lewis sites [19,20,28,29]. There is little pyridine adsorbed on the undoped sample, except the band at 1446 cm1 (Fig. 6(a)). With the increase of HF/Ti molar ratio, a marked increase for all the bands is observed. However, excess F doping results in the decrease of surface acid sites (Fig. 6(d)). (Fig. 7) shows the distribution curves of the acid strength of F doped 36% TiO2/SiO2 and undoped sample. The desorbed amount of NH3 from these acid sites was calculated and is summarized in
Fig. 7. NH3-TPD profile of (a) undoped 36% TiO2/SiO2 and (b) F-doped 36% TiO2/ SiO2.
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Table 2. While the undoped sample exhibits two desorption peaks (Fig. 7(a)), the F doped sample shows four desorption peaks (Fig. 7(b)). Moreover, the total acidity of F doped sample is much higher (Table 2). Moreover, Fig. 7 reveals that the first and second desorption peaks for F doped sample shift to higher temperature compared to undoped sample, which indicates the acid sites with enhanced strength. The new two desorption peaks at 381 and 452 °C for F doped sample probably make the sample surface reach more favorable ‘reactant adsorption’ situation and improve the photocatalytic activity. And the strong-strength acid sites are possibly more important than weak–strength acid sites for photocatalytic activity. Fig. 8 shows the photoluminescence spectra for F doped and udoped samples. The PL emission of the anantase TiO2 is governed by the recombination of electron–hole pairs via the localized levels within the forbidden gap of some defect-related centers which presumably reside in the surface region of TiO2 [26,30]. It could be observed that a significant decrease in emission intensity between the undoped sample and F doped sample with RF of 1.1. This indicates that an appropriate amount of F doping can slow the recombination rate of photo-generated electrons and holes in TiO2. Samples with excessive F doping with RF of 2.0:1, however, exhibit an increase in emission intensity. This may be due to the introduction of new recombination centers that enhances the recombination of photo-generated electrons and holes [26,30,31]. The above results show that the sample with lower emission intensity of PL spectra presumably exhibits higher photocatalytic activity. The relationship between the apparent rate constant of F doped 36% TiO2/SiO2 for photodegradation acrylonitrile and HF/Ti molar ratio of the starting material was shown in Fig. 9. With the increase of HF/Ti molar ratio of the starting material, the apparent rate constant increase rapidly and reached the maximum at HF/Ti = 1.1, since the improved photocatalytic activity by doping F is due to the enhancement of surface acidity (Figs. 6 and 7, Table 2). However, excessive F doping decreases photocatalytic activity, which is ascribed to the enhancement of the recombination between photo-generated electrons and holes (Fig. 8). 3.6. Effect of calcination temperature on photocatalytic activity The phase composition of F doped 36% TiO2/SiO2 samples with different calcination temperatures was determined by XRD test, and the results are shown in Fig. 10. The distinctive peaks at 2h = 25.24°, 37.80°, 48.00°, 53.88°, 55.02°, 62.68°, 68.80°, 70.26°, 75.08°, corresponding to the anatase (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0), (2 1 5) crystal planes (JCPDS No. 211272) are observed in all samples. No characteristic peaks of other impurities and no phase transformation from anatase to rutile were observed. It is generally believed that the crystal phase of TiO2 is a critical factor and the anatase phase usually shows a better photoactivity than the rutile phase [32]. Compared with pure TiO2, the doped samples show a little shift of the (1 0 1) peaks, indicating a lattice distortion of the doped samples [33]. Moreover, with the increase of calcination temperature, the peak intensity of anatase increases and the peaks are getting sharper, suggesting that the relative crystallinity and crystalline size significantly increase. According to Scherrer equation, the average crystallite sizes of F doped 36% TiO2/SiO2 powders at different calcination temperatures were calculated using the full-width at half-maximum of the X-ray diffraction peaks at 2h = 25.2° (Table 3). The average crystallite size increases as the calcinations temperature increases, while the specific surface area only starts to decrease at 650 °C. (Fig. 11) shows the effect of calcination temperature on the photocatalytic activity of F doped 36% TiO2/SiO2 under simulant solar light for 6 min. The photocatalytic activity of F doped 36%
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Table 2 Results of the measurement of the acid sites density by using the NH3-TPD. Samples
F doped Undoped
Peak temperature (°C)
Amount of desorbed NH3 (mmol/g)
I
II
IV
V
I
II
IV
V
Total
200 155
331 327
381
452
0.25 0.11
0.01 0.04
0.15
0.09
0.50 0.15
Fig. 8. PL spectra of undoped and F doped 36% TiO2/SiO2 samples with different HF/ Ti molar ratio.
Fig. 10. XRD patterns of different samples: F doped 36% TiO2/SiO2 samples calcined at 350, 450, 550 and 650 °C.
Table 3 The specific surface area and crystal size of F doped 36% TiO2/SiO2 powders at different calcination temperatures. Samples
Specific surface area (m2/g)
Crystal size (nm)
350 °C 450 °C 550 °C 650 °C
222 221 217 212
11.1 12.8 14.9 16.9
Fig. 9. Apparent rate constant of F doped 36% TiO2/SiO2 particles calcined at 450 °C for the degradation acrylonitrile as a function of the HF/Ti molar ratio of starting material.
TiO2/SiO2 for the photodegradation of acrylonitrile is increased with an increase in calcination temperature, and reached a maximum at calcination temperature of 450 °C. It appears that the increase in photocatalytic activity at higher calcination temperature is mainly due to the higher crystallinity of the powder from 350 to 450 °C (Fig. 10), which results in a decrease in the defect density on the catalyst surface [34]. With calcination temperature further increasing, the average crystal size of the F doped 36% TiO2/ SiO2 catalyst increases (Table 3). However, its photocatalytic activity decreases (Fig. 10).
Fig. 11. Effect of calcination temperature on photocatalytic activity of F doped 36% TiO2/SiO2.
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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. 5. Conclusions
Fig. 12. Repeated runs for acrylonitrile photodegradation with F doped 36% TiO2/ SiO2 under simulant solar light.
F doped 36% TiO2/SiO2 mixed oxides have been prepared by the sol–gel method. F doped TiO2/SiO2 composite oxides exhibit the spherical shape and mesoporous structure. The TiO2 loading, HF concentration and calcination temperature have a great influence on the BET surface area, crystallinity, crystalline size and the photocatalytic activity of the composite oxides. In situ IR and NH3-TPD show that the higher photocatalytic performance for F doped sample can be explained by the introduction of the surface acid sites and the strong-strength acid sites possibly play more important role in improving the photocatalytic activity than weak–strength acid sites. The optimum preparation conditions are calcination temperature at 450 °C, molar ratio (RF) of 1.1 and 36% TiO2 loading. A degradation ratio of 66% for acrylonitrile can be achieved with 1.0 g F doped 36% TiO2/SiO2 for 6 min irradiation.
3.7. Recycle performances of F doped 36% TiO2/SiO2
Acknowledgment
The F doped 36% TiO2/SiO2 sample has been found to be active in the successive cycles with similar rates of acrylonitrile decomposition under simulant solar light irradiation (Fig. 12). This shows that the F doped sample is stable and it is not easily to lose activity in the reaction.
This research was financially supported by ‘the Double Hundred Plan’.
4. Discussion Compared to the undoped sample, F doping does not cause an obvious shift in the fundamental absorption edge of TiO2 (The UV–vis absorption spectra of 36% TiO2/SiO2 and F doped 36% TiO2/SiO2 are not shown). It means that the doped F in TiO2 particles could not affect significantly the optical absorption property. This conclusion is consistent with the previous report in literature [8,35]. However, The F doped 36% TiO2/SiO2 photocatalyst exhibits higher activity. The degradation ratio of acrylonitrile can reach to 66% and 6% with 1.0 g F doped sample under simulant solar light and visible light (k > 400 nm) irradiation for 6 min, compared to only 2.8% and 0.7% with the undoped sample, respectively. This indicates that the photocatalytic activity of 36% TiO2/SiO2 composite oxide is greatly enhanced by F doping, either under simulant solar light or visible light irradiation. The XPS analysis shows that surface fluorides („TiAF) appear on the TiO2 crystal surface. It is generally known that the fluorination on the surface of TiO2 may accelerate the photocatalytic degradation organic pollutants since the OH radicals generated on the surface of F-TiO2 are more mobile than those generated on pure TiO2 [9,26]. Moreover, the in situ IR and NH3-TPD indicate that the acid sites appear on the external surface of the samples after F doping. The surface acid sites favored the adsorption of both reactant and oxygen molecules and converted of adsorbed water into the active hydroxyl groups, which contributed to the higher photocatalytic performance [13]. Obviously, F doping plays a pivotal role in the enhancement of photocatalytic activity due to the introduction of the strong Lewis and Bronsted acid sites on the surface of the photocatalyst. Figs. 10 and 11 show that higher crystallinity of the powder is one of important factors for high photocatalytic activity; however, suitable defect density is also required for reactant molecules activation on catalyst surface. This is consistent with the Bronsted and Lewis acid sites observed on the surface of the samples (Fig. 6). We regard as that surface acid sites may be relevant with surface
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Fluorine promoted and silica supported TiO2 for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation Dandan Pang, Yunteng Wang, Xiaodong Ma, Feng Ouyang ⇑ Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, PR China
h i g h l i g h t s F doped samples were obtained by a simple sol–gel method. Bronsted and Lewis sites are observed on the surface of the samples by in situ IR. The number and strength of surface acid sites are increased due to F doping. The F doped samples show superior photocatalytic activity.
a r t i c l e
i n f o
Article history: Received 28 March 2014 Received in revised form 2 July 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: Photocatalytic activity Acrylonitrile F doping Silica In situ IR
a b s t r a c t The F doped TiO2/SiO2 composition oxides were prepared by sol–gel method using HF solution as fluorine source. The prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET), ultraviolet–visible absorption spectroscopy (UV–Vis), photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FT-IR) and ammonia adsorption and temperature-programmed desorption (NH3-TPD). The photocatalytic activities of the powders for acrylonitrile degradation have been inspected. The results show that the presence of SiO2 as a support for TiO2 loading benefits the formation of the nanoparticles with a large surface. In situ IR and NH3-TPD investigation shows that the higher photocatalytic activity for F doped samples is attributed to the increase of number and strength of surface acid sites. The highest photcatalytic activity for acrylonitrile degradation is obtained for a sample calcined at 450 °C with molar ratio (HF to Ti) of 1.1 and 36% TiO2 loading. The degradation ratio of acrylonitrile can reach to 66% under simulant solar light for 6 min, indicating the effectiveness of photocatalytic degradation acrylonitrile using F doped TiO2/SiO2 composite oxide. Ó 2014 Published by Elsevier B.V.
1. Introduction Many studies demonstrated that acrylonitrile may cause gene mutations, chromosome aberrations, unscheduled DNA synthesis and cell transformation [1]. Exposure to acrylonitrile can occur from residual acrylonitrile in commercial fibrous polymeric material and in styrene–acrylonitrile resins [1]. In acrylonitrile production, a large amount of acrylonitrile wastewater is generated. The US EPA classified acrylonitrile as a ‘‘water priority pollutant’’ and a ‘‘hazardous air pollutant’’. Traditional removal techniques including absorption methods [2], thermal-catalytic incineration [3,4] and biotechnological abatement methods [5], have disadvantages like high costs, insufficient abatement problems and harsh operation conditions. ⇑ Corresponding author. E-mail address: [email protected] (F. Ouyang). http://dx.doi.org/10.1016/j.cej.2014.07.068 1385-8947/Ó 2014 Published by Elsevier B.V.
Photocatalytic oxidation using semiconductor photocatalyst TiO2 to mineralize most of organic pollutants should be an effective alternative remediation technology due to the thermal stability, facile synthesis, low cost and the low toxicity of TiO2 [6,7]. It was well known that the photocatalytic performance of the TiO2 strongly depended on its size, shape, composition and crystallinity [8]. Among various morphologies, mesoporous TiO2 spheres are especially important since a spherical morphology has been demonstrated to present excellent stability, mono-disperse nature, and enhanced light harvesting property [9]. After doping F, mono-disperse mesoprous F-TiO2 spheres showed enhanced light harvesting which resulted in superior photocatalytic activity in dye degradation [10]. The high photocatalytic of fluorine doped TiO2 is also related to smaller crystalline size, well crystalline phase narrow band gap, intense absorption in the visible light region and higher amount of surface hydroxyl groups [11,12]. The photocatalytic activity is shown to increase with catalyst
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surface acidity [13]. The number of surface acid sites may be increased by F doping into TiO2 due to the stronger acidity of HF. On the other hand, large surface area and high adsorption capacity are very important to increase the photocatalytic activity of TiO2 particles. To take silica with large surface as a support material for TiO2 loading is a feasible approach [14]. Krichevskaya et al. reported photocatalytic degradation of acrylonitrile in gas–solid systems by using Degussa P25 under UV irradiation [15]. Jõks et al. reported that gas-phase photocatalytic oxidation of acrylonitrile on sulphated TiO2 showed improved performance at higher temperature and longer retention times under UV irradiation [16]. Although the photocatalytic degradation of acrylonitrile in solution attracts some attentions, such works are little so far. In this study, F doped TiO2/SiO2 composite oxide was synthesized by sol–gel method using HF solution as fluorine source. We investigated the influences of silica, HF/Ti ratio and calcination temperature on the physical properties, such as microstructures, morphology, special surface area, compositions and the nature of the surface active sites. The effects of TiO2 loading, HF ratio in starting material, calcination temperature on the photocatalytic activities of the prepared composite oxides for acrylonitrile degradation were also estimated. 2. Material and methods 2.1. Preparation of photocatalysts Tetrabutyl titanate was used as a starting material and HF as a fluorine source. All chemicals used were of analytical grade. Firstly, 5 mL tetrabutyl titanate (Ti(OC4H9)4) was dissolved in 13 mL anhydrous alcohol to form solution A. 10 mL anhydrous alcohol, 0.5 mL deionized water, 2 mL acetic acid and hydrofluoric acid solution (with a concentration 40 wt.%) were mixed with stirring for 20 min to form solution B. Secondly, Solution B was added dropwise into solution A with vigorous stirring for 2 h to get the homogeneous transparent sol. A certain amount of silica gel (100–200 mesh) [14] was added to the sol with severe agitation for 1 h. The resulted gelatinous solution was aged for 12 h at room temperature and then was dried at 80 °C. The dry gel was then crushed and calcined at different temperatures for 2 h. F doped TiO2/SiO2 composition powders were thus gained. In this paper, the prepared particles were denoted as F doped wt.% TiO2/SiO2, where wt.% was the amount of TiO2 loading by weight. The molar ratio of HF to Ti (RF) was 0.0:1, 0.3:1, 0.7:1, 1.1:1 and 2.0:1. For comparison, pure TiO2, undoped 36% TiO2/ SiO2, F doped TiO2 without SiO2 nanoparticles were also synthesized by the same method.
1800 system (ULVAC-PHI, Japan) with monochromatic Al ka excitation. The binding energy of the C 1s line (284.6 eV) was taken for calibration the obtained spectra. The general morphology was characterized by a Hitachi S4700 scanning electron microscope. UV–Vis diffuse reflectance spectra were recorded using Shimadzu Corporation UV-2540 over the spectral range 240–800 nm. BaSO4 was used as a reference. The BET specific surface area was measured by nitrogen gas adsorption at 77 K using BELSOROP-MINI II adsorption instrument. Pore size distribution was determined by Barrett–Joyner–Halenda (BJH) method. The PL spectra were obtained on a Renishaw inVia Raman spectrometer at room temperature with a He–Cd laser (325 nm) as the light source. The powder sample on a slide was detected by a CCD array detector. Fourier transform infrared spectra were collected using Nicolet 380 with a resolution of 4 cm1 and 32 scans in the region of 4000– 1000 cm1. The nature of the surface acid sites was investigated by chemical adsorption of pyridine onto clean self-bonded sample wafers after an outgassing under vacuum (102 mbar) at 673 K for 2 h. The adsorption of pyridine was done at 180 °C for 40 min. The physisorbed pyridine was then desorbed under vacuum at room temperature. Ammonia adsorption and temperature-programmed desorption (NH3-TPD) technique was used for the study of the surface acidity. The sample was preheated at 400 °C for 2 h in flowing helium at a flow rate of 40.0 mL/min. After cooling to 100 °C, the sample was exposed to a stream of ammonia for 1 h at a flow rate of 20.0 mL/min. Then, the sample was left in flowing helium at the same temperature for 3 h in order to purge any excess of ammonia. Finally, the TPD operation was performed by heating from 100 to 600 °C at 15.0 °C/min. The acid density was measured using a 0.05 mol/L HCl solution which was then back-titrated with a 0.05 mol/L NaOH solution in accordance with the method reported in literatures [19–21]. 2.3. Photocatalytic experiments The photocatalytic activities of the prepared photocatalysts were evaluated by measuring the degradation rate of acrylonitrile. In this test, a 350 W xenon short arc lamp (ShenZhen AnHongDa Opto Technology Co., Led.) with the similar characteristic spectrum with sun light was used as a light source of simulant solar light (Fig. 1). The catalyst was suspended in 180 mL aqueous solution of acrylonitrile with an initial concentration of 10 mg/L and the reactor was sealed. Prior to light illumination, the suspension
2.2. Characterization of photocatalysts The phase composition and crystallite size of the samples were obtained on a Rigaku D/max 2500 X-ray diffraction analyzer with Cu Ka X-ray source at a scanning rate of 8°/min in the 2h 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 Scherrer equation [17,18]:
L¼
Kk 2 ; B2 ¼ B2measured binstrumental B cos h
ð1Þ
where L, K, k and h are the average crystal size, the shape factor 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 intrinsic diffraction profile, the test sample diffraction integral profile and instrumental diffraction profile, respectively. The XPS measurements were carried out using an ULVAC-PHI
Fig. 1. The emission spectra of the simulant solar light and sun light.
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was magnetically stirred for 60 min in the dark for adsorption/ desorption equilibrium. Then the xenon lamp was turned on and the temperature of suspension was maintained at 25 °C by circulation of water through an external cooling coil. At given intervals of illumination, a small amount of suspension was taken out and filtered through 0.45 lm filter for HPLC analysis (LC2000, ShangHai Echcomp Co). The detection wavelength selected for acrylonitrile was 210 nm. 2.4. Analytical methods At low substrate concentration, the kinetics of the photocatalytic oxidation process has been described by a pseudo-first-order equation [14,22].
ln
Co C
¼ kapp t
ð2Þ
where Co is the acrylonitrile concentration after the system got to adsorption equilibrium (mg/L), C is the acrylonitrile concentration at time t (mg/L), t is the irradiation time and kapp is the apparent pseudo-first-order rate constant (min1). The degradation ratio of acrylonitrile was also used to evaluate the photocatalytic activity of a sample: D = (Co C) 100%/Co, where D is the degradation ratio of the reactant. 3. Results
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Fig. 2(c) shows the SEM micrograph for the F doped TiO2 without SiO2. As shown in the images, F doped TiO2 is composed of numerous small crystallites with about 250 nm in diameter due to the chemical etching of HF. However, a poor porosity is present on the samples. Different from F doped TiO2, F doped 36% TiO2/SiO2 spheres consist of numerous well-packed nano-crystallites without heavy aggregation and possess a rough surface (Fig. 2(d)). The result is consistent with the result of BET specific surface area measurement for F doped TiO2 (47 m2/g) and F doped 36% TiO2/ SiO2 (221 m2/g) (Table 1). 3.2. Nitrogen physical adsorption Fig. 3(a) presents nitrogen adsorption and desorption isotherms of F doped TiO2 without SiO2 and F doped 36% TiO2/SiO2 nanoparticles calcined at 450 °C. According to IUPAC classification, F doped TiO2 and F doped 36% TiO2/SiO2 nanoparticles display type IV isotherm and H2 hysteresis, which indicate the presence of mesoporous materials [23]. Fig. 3 (b) shows the pore size distribution curve calculated by the BJH method from the adsorption branch of nitrogen isotherm. It could be found that both have mesoporous structure. The average pore diameters of F doped 36% TiO2/SiO2 and F doped TiO2 nanoparticles are both 7.0 nm. However, the specific surface area of the former (221 m2/g) is significantly higher than that of the later (47 m2/g). Moreover, compared to the former, the later has poor porosity, which is consistent with SEM results as shown in Fig. 2(c) and (d).
3.1. SEM images 3.3. XPS analysis Fig. 2(a) and (b) shows the SEM micrograph for pure TiO2 and 36% TiO2/SiO2 calcined at 450 °C. Pure TiO2 are irregularly agglomerated by primary particles. The 36% TiO2/SiO2 possesses a rough and porous surface, resulting in higher surface area than pure TiO2. Additionally, pure TiO2 and 36% TiO2/SiO2 show the formation of secondary particles by the agglomeration of primary particles.
XPS survey spectra of F doped 36% TiO2/SiO2 show that the F doped sample contains predominantly Ti, O, F, Si elements and a trace amount of carbon (not shown). The F 1s spectrum gives two peaks centered at 684.0 and 687.5 eV, respectively (Fig. 4). The peak centered at 687.5 eV is attributed to the doped F atoms
Fig. 2. SEM images for pure TiO2 (a); 36% TiO2/SiO2 (b); F doped TiO2 (c) and F doped 36% TiO2/SiO2 (d).
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Table 1 BET surface area and total pore volume of the samples with different TiO2 loading. Samplea
BET (m2/g)
Total pore volume (cm3/g)
Silica gel 16% TiO2/SiO2 22% TiO2/SiO2 36% TiO2/SiO2 53% TiO2/SiO2 70% TiO2/SiO2 TiO2
359 281 279 221 208 170 47
0.91 0.87 0.85 0.65 0.58 0.45 0.08
a All sample were calcined at 450 °C for 2 h and the molar ratio of HF to Ti was 1.1, except for silica gel.
Fig. 4. F 1s high-resolution XPS spectra of F doped 36% TiO2/SiO2 powders.
Fig. 5. The acrylonitrile degradation ratio of F doped TiO2/SiO2 samples calcined at 450 °C as a function of the TiO2 loading.
Fig. 3. (a) Nitrogen adsorption and desorption isotherms, and (b) BJH pore size distributions of (1) F doped TiO2 without SiO2 and (2) F doped 36% TiO2/SiO2 nanoparticles.
in TiO2 crystal lattice, i.e. the substitute F atoms that occupied oxygen sites in the TiO2 crystal lattice [24,25]. The peak centered at 684.0 eV is a typical value for surface fluoride („TiAF) species on the TiO2 crystal surface [24–26].
TiO2/SiO2 particles retained relatively large surface area and mesoporous structures, compared with the sample without SiO2 (TiO2). Fig. 5 shows the degradation ratio of F doped TiO2/SiO2 samples for acrylonitrile change with the TiO2 loading. Acrylonitrile cannot be degraded under irradiation with silica gel. The degradation ratio is increased rapidly with the increase of TiO2 loading until 36%. The increase of the TiO2 loading increases the surface coverage of TiO2 on the silica gel, which results in the enhancement of the rate of acrylonitrile photodegradation, since the photodegradation of acrylonitrile only takes place on the TiO2 particles surface. However, the degradation ratio decreases gradually when the loading is more than 36%. Under high TiO2 loading, lower the surface area and smaller pore volume decrease the photocatalytic activity of TiO2/SiO2 mixed oxides (Table 1) [27].
3.4. Effect of TiO2 loading on physical and photocatalytic properties The BET surface area and total pore volume of the samples with different TiO2 loading are shown in Table 1. With the TiO2 loading increasing, the BET surface area and total pore volume of TiO2/SiO2 particles decrease gradually, which indicates that a part of TiO2 particles dispersed inside the pore of silica gel. Moreover, the
3.5. Effect of HF ratio in starting material on physical and photocatalytic properties The nature of the surface active sites in solid acids is defined by coordinately unsaturated cationic centers giving Lewis acidity, and by the presence of protons, generating Bronsted acidic sites. The
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Fig. 6. FT-IR spectroscopy of chemisorbed pyridine adsorbed on F doped 36% TiO2/ SiO2 with different HF/Ti molar ratio. (a) 0:1, (b) 0.3:1, (c) 1.1:1, (d) 2.0:1.
adsorption of base molecules (e.g. pyridine or ammonia) combined with vibration spectroscopic techniques is well known for characterizing these surface active sites [13]. Fig. 6 shows the IR spectra of chemisorbed pyridine adsorbed on the F doped 36% TiO2/SiO2 with different HF/Ti molar ratio. As shown in Fig. 6(c), F doped 36% TiO2/SiO2 (RF = 1.1) exhibits bands due to hydrogen bonded pyridine (1597 cm1), strong Lewis bound pyridine (1446 cm1), weak Lewis bound pyridine (1579 cm1), pyridine ion ring vibration due to pyridine bound to Bronsted acid sites (1549 cm1) and a band at 1491 cm1 which can be assigned to pyridine associated with both Bronsted and Lewis sites [19,20,28,29]. There is little pyridine adsorbed on the undoped sample, except the band at 1446 cm1 (Fig. 6(a)). With the increase of HF/Ti molar ratio, a marked increase for all the bands is observed. However, excess F doping results in the decrease of surface acid sites (Fig. 6(d)). (Fig. 7) shows the distribution curves of the acid strength of F doped 36% TiO2/SiO2 and undoped sample. The desorbed amount of NH3 from these acid sites was calculated and is summarized in
Fig. 7. NH3-TPD profile of (a) undoped 36% TiO2/SiO2 and (b) F-doped 36% TiO2/ SiO2.
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Table 2. While the undoped sample exhibits two desorption peaks (Fig. 7(a)), the F doped sample shows four desorption peaks (Fig. 7(b)). Moreover, the total acidity of F doped sample is much higher (Table 2). Moreover, Fig. 7 reveals that the first and second desorption peaks for F doped sample shift to higher temperature compared to undoped sample, which indicates the acid sites with enhanced strength. The new two desorption peaks at 381 and 452 °C for F doped sample probably make the sample surface reach more favorable ‘reactant adsorption’ situation and improve the photocatalytic activity. And the strong-strength acid sites are possibly more important than weak–strength acid sites for photocatalytic activity. Fig. 8 shows the photoluminescence spectra for F doped and udoped samples. The PL emission of the anantase TiO2 is governed by the recombination of electron–hole pairs via the localized levels within the forbidden gap of some defect-related centers which presumably reside in the surface region of TiO2 [26,30]. It could be observed that a significant decrease in emission intensity between the undoped sample and F doped sample with RF of 1.1. This indicates that an appropriate amount of F doping can slow the recombination rate of photo-generated electrons and holes in TiO2. Samples with excessive F doping with RF of 2.0:1, however, exhibit an increase in emission intensity. This may be due to the introduction of new recombination centers that enhances the recombination of photo-generated electrons and holes [26,30,31]. The above results show that the sample with lower emission intensity of PL spectra presumably exhibits higher photocatalytic activity. The relationship between the apparent rate constant of F doped 36% TiO2/SiO2 for photodegradation acrylonitrile and HF/Ti molar ratio of the starting material was shown in Fig. 9. With the increase of HF/Ti molar ratio of the starting material, the apparent rate constant increase rapidly and reached the maximum at HF/Ti = 1.1, since the improved photocatalytic activity by doping F is due to the enhancement of surface acidity (Figs. 6 and 7, Table 2). However, excessive F doping decreases photocatalytic activity, which is ascribed to the enhancement of the recombination between photo-generated electrons and holes (Fig. 8). 3.6. Effect of calcination temperature on photocatalytic activity The phase composition of F doped 36% TiO2/SiO2 samples with different calcination temperatures was determined by XRD test, and the results are shown in Fig. 10. The distinctive peaks at 2h = 25.24°, 37.80°, 48.00°, 53.88°, 55.02°, 62.68°, 68.80°, 70.26°, 75.08°, corresponding to the anatase (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1), (2 0 4), (1 1 6), (2 2 0), (2 1 5) crystal planes (JCPDS No. 211272) are observed in all samples. No characteristic peaks of other impurities and no phase transformation from anatase to rutile were observed. It is generally believed that the crystal phase of TiO2 is a critical factor and the anatase phase usually shows a better photoactivity than the rutile phase [32]. Compared with pure TiO2, the doped samples show a little shift of the (1 0 1) peaks, indicating a lattice distortion of the doped samples [33]. Moreover, with the increase of calcination temperature, the peak intensity of anatase increases and the peaks are getting sharper, suggesting that the relative crystallinity and crystalline size significantly increase. According to Scherrer equation, the average crystallite sizes of F doped 36% TiO2/SiO2 powders at different calcination temperatures were calculated using the full-width at half-maximum of the X-ray diffraction peaks at 2h = 25.2° (Table 3). The average crystallite size increases as the calcinations temperature increases, while the specific surface area only starts to decrease at 650 °C. (Fig. 11) shows the effect of calcination temperature on the photocatalytic activity of F doped 36% TiO2/SiO2 under simulant solar light for 6 min. The photocatalytic activity of F doped 36%
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Table 2 Results of the measurement of the acid sites density by using the NH3-TPD. Samples
F doped Undoped
Peak temperature (°C)
Amount of desorbed NH3 (mmol/g)
I
II
IV
V
I
II
IV
V
Total
200 155
331 327
381
452
0.25 0.11
0.01 0.04
0.15
0.09
0.50 0.15
Fig. 8. PL spectra of undoped and F doped 36% TiO2/SiO2 samples with different HF/ Ti molar ratio.
Fig. 10. XRD patterns of different samples: F doped 36% TiO2/SiO2 samples calcined at 350, 450, 550 and 650 °C.
Table 3 The specific surface area and crystal size of F doped 36% TiO2/SiO2 powders at different calcination temperatures. Samples
Specific surface area (m2/g)
Crystal size (nm)
350 °C 450 °C 550 °C 650 °C
222 221 217 212
11.1 12.8 14.9 16.9
Fig. 9. Apparent rate constant of F doped 36% TiO2/SiO2 particles calcined at 450 °C for the degradation acrylonitrile as a function of the HF/Ti molar ratio of starting material.
TiO2/SiO2 for the photodegradation of acrylonitrile is increased with an increase in calcination temperature, and reached a maximum at calcination temperature of 450 °C. It appears that the increase in photocatalytic activity at higher calcination temperature is mainly due to the higher crystallinity of the powder from 350 to 450 °C (Fig. 10), which results in a decrease in the defect density on the catalyst surface [34]. With calcination temperature further increasing, the average crystal size of the F doped 36% TiO2/ SiO2 catalyst increases (Table 3). However, its photocatalytic activity decreases (Fig. 10).
Fig. 11. Effect of calcination temperature on photocatalytic activity of F doped 36% TiO2/SiO2.
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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. 5. Conclusions
Fig. 12. Repeated runs for acrylonitrile photodegradation with F doped 36% TiO2/ SiO2 under simulant solar light.
F doped 36% TiO2/SiO2 mixed oxides have been prepared by the sol–gel method. F doped TiO2/SiO2 composite oxides exhibit the spherical shape and mesoporous structure. The TiO2 loading, HF concentration and calcination temperature have a great influence on the BET surface area, crystallinity, crystalline size and the photocatalytic activity of the composite oxides. In situ IR and NH3-TPD show that the higher photocatalytic performance for F doped sample can be explained by the introduction of the surface acid sites and the strong-strength acid sites possibly play more important role in improving the photocatalytic activity than weak–strength acid sites. The optimum preparation conditions are calcination temperature at 450 °C, molar ratio (RF) of 1.1 and 36% TiO2 loading. A degradation ratio of 66% for acrylonitrile can be achieved with 1.0 g F doped 36% TiO2/SiO2 for 6 min irradiation.
3.7. Recycle performances of F doped 36% TiO2/SiO2
Acknowledgment
The F doped 36% TiO2/SiO2 sample has been found to be active in the successive cycles with similar rates of acrylonitrile decomposition under simulant solar light irradiation (Fig. 12). This shows that the F doped sample is stable and it is not easily to lose activity in the reaction.
This research was financially supported by ‘the Double Hundred Plan’.
4. Discussion Compared to the undoped sample, F doping does not cause an obvious shift in the fundamental absorption edge of TiO2 (The UV–vis absorption spectra of 36% TiO2/SiO2 and F doped 36% TiO2/SiO2 are not shown). It means that the doped F in TiO2 particles could not affect significantly the optical absorption property. This conclusion is consistent with the previous report in literature [8,35]. However, The F doped 36% TiO2/SiO2 photocatalyst exhibits higher activity. The degradation ratio of acrylonitrile can reach to 66% and 6% with 1.0 g F doped sample under simulant solar light and visible light (k > 400 nm) irradiation for 6 min, compared to only 2.8% and 0.7% with the undoped sample, respectively. This indicates that the photocatalytic activity of 36% TiO2/SiO2 composite oxide is greatly enhanced by F doping, either under simulant solar light or visible light irradiation. The XPS analysis shows that surface fluorides („TiAF) appear on the TiO2 crystal surface. It is generally known that the fluorination on the surface of TiO2 may accelerate the photocatalytic degradation organic pollutants since the OH radicals generated on the surface of F-TiO2 are more mobile than those generated on pure TiO2 [9,26]. Moreover, the in situ IR and NH3-TPD indicate that the acid sites appear on the external surface of the samples after F doping. The surface acid sites favored the adsorption of both reactant and oxygen molecules and converted of adsorbed water into the active hydroxyl groups, which contributed to the higher photocatalytic performance [13]. Obviously, F doping plays a pivotal role in the enhancement of photocatalytic activity due to the introduction of the strong Lewis and Bronsted acid sites on the surface of the photocatalyst. Figs. 10 and 11 show that higher crystallinity of the powder is one of important factors for high photocatalytic activity; however, suitable defect density is also required for reactant molecules activation on catalyst surface. This is consistent with the Bronsted and Lewis acid sites observed on the surface of the samples (Fig. 6). We regard as that surface acid sites may be relevant with surface
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