Photocatalytic degradation of imazapyr using mesoporous Al2O3–TiO2 nanocomposites

Photocatalytic degradation of imazapyr using mesoporous Al2O3–TiO2 nanocomposites

Separation and Purification Technology 145 (2015) 147–153 Contents lists available at ScienceDirect Separation and Purification Technology journal hom...

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Separation and Purification Technology 145 (2015) 147–153

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Photocatalytic degradation of imazapyr using mesoporous Al2O3–TiO2 nanocomposites Adel A. Ismail a,b,⇑, Ibrahim Abdelfattah c,d, M. Faycal Atitar d, L. Robben e, Houcine Bouzid b, S.A. Al-Sayari a,f, D.W. Bahnemann d,g,⇑ a

Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, 11421 Cairo, Egypt Advanced Materials and NanoResearch Center, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia Water Pollution Research Dept., National Research Centre, 33 EL Bohouth St. (Former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt d Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, 30167 Hannover, Germany e Chemische Kristallographie fester Stoffe, Universität Bremen, Germany f Chemistry Department, Science and Art at Sharurah, Najran University, Saudi Arabia g Laboratory ‘‘Photoactive Nanocomposite Materials’’, Saint-Petersburg State University, Ulyanovskaya str. 1 Peterhof, Saint-Petersburg 198504, Russia b c

a r t i c l e

i n f o

Article history: Received 11 January 2015 Received in revised form 12 March 2015 Accepted 16 March 2015 Available online 20 March 2015 Keywords: Mesoporous Al2O3–TiO2 Photocatalyst Degradation Imazapyr

a b s t r a c t Mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents (0–5 wt.%) have been synthesized through a facile sol–gel method using tetrabutyl orthotitanateand and aluminum-tri-sec-butoxide as a precursors for TiO2 and Al2O3 sources. XRD and Raman spectra confirmed that highly crystalline anatase TiO2 phase was formed at low Al2O3 content; however, the produced Al2O3–TiO2 nanocomposites are amorphous phase at 3 and 5 wt.% Al2O3. TEM images show TiO2 particles are quite uniform with 10 ± 2 nm sizes with mesoporous structure. The surface area of mesoporous TiO2 was increased from 174 m2/g to 325 m2/g by increasing Al2O3 contents from 0% to 5%. The photocatalytic efficiencies of Al2O3–TiO2 nanocomposites were evaluated by photodegradation of herbicide imazapyr compared with UV-100 under UV illumination. The photocatalytic efficiency of 2 wt.% Al2O3–TiO2 nanocomposite is significantly 2 and 3-fold higher than that of mesoporous TiO2 and UV-100, respectively. 2 wt.% Al2O3–TiO2 is considered to be the optimum photocatalyst for the highest photonic efficiency 4%. The superiority of 2 wt.% Al2O3–TiO2 nanocomposite is explained by the highly crystalline and small particles sizes with mesoporous structure. The proposed mechanism of this system and the role of amorphous Al2O3 are explained by details. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Mesoporous TiO2 network is known to be important to improve the accessibility and diffusion of target molecules for photocatalysis applications [1–4]. The effect of surface areas, crystallinity, and mesostructure of TiO2 nanoparticles are the key factors to improve their photoactivity and hence the photonic efficiency [5–10]. Researchers have done a lot of efforts to improve the photocatalytic efficiency of the TiO2 by doping metal oxides such as RuO2, Fe2O3, ZnO and V2O5 [11–15]. On the other hand, Al2O3 is a highly attractive material for different applications such as ⇑ Corresponding authors at: Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, 11421 Cairo, Egypt (A.A. Ismail). Institut für Technische Chemie, Leibniz Universität Hannover, Callinstrasse 3, 30167 Hannover, Germany (D.W. Bahnemann). E-mail addresses: [email protected] (A.A. Ismail), [email protected] uni-hannover.de (D.W. Bahnemann). http://dx.doi.org/10.1016/j.seppur.2015.03.012 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

passive-matrix electrochromic, heterogeneous catalysis supports for petroleum refinement and adsorbents owing to its unique physical properties such as thermal stability, amphoteric character, hydrolytic stability, and capability to convert catalytic activity [16–19]. Al2O3–TiO2 nanocomposites were prepared by sol–gel method to obtain highly dispersed and small particle sizes of Al2O3. [20–25] Highly ordered mesoporous Al2O3/TiO2 was employed for solar cells applications and highly efficient photocatalyst [26,27]. Nonmetal elements (C, N and S) and metal elements such as Ru, Si, and Te were doped TiO2/Al2O3 composite films for environment applications [28]. TiO2–Al2O3 nanocomposites either membrane or film were employed for photodegradation of different pollutants such as dyes, NO oxidation, 2,4 dichlorophenoxiacetic acid, 4-nitrophenol, dioxin and acetaldehyde gas under UV and visible light [20–25,28,29]. All the prepared TiO2/ Al2O3 nanocomposites photocatalysts exhibited their the superiority as efficient photocatalysts more than pure TiO2 [20–25,28,29].

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Herbicides have been employed on a large scale for agricultural applications. Imazapyr herbicides are considered to a nonselective for weeds control and it exhibits no aquatic use; however, it could get in the surface water by spray drift. It persists in soil for over 6–12 months, and it causes a destroy plants even at very low concentrations [29]. Photocatalytic oxidation of herbicide imazapyr was widely investigated by commercial TiO2 photocatalysts and mesoporous TiO2 [30–32]. In the present work, Al2O3 was selected to improve the photocatalytic performance of TiO2 due to its insulating characteristic and it is employed as an energy barrier to reduce the recombination rate of charge carriers in photocatalysis application [22,33–35]. Additionally, Al3+ surface has acid sites for O2 photoadsorption and hence TiO2 can be induced for the photoactivation of O2 [36]. Therefore, in the present work, one-step synthesis of mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents (0–5 wt.%) through sol gel process using tetrabutyl orthotitanate and aluminum-tri-sec-butoxide as a precursors for TiO2 and Al2O3 sources in presence triblock copolymer surfactant (F-127) as structure directing agent was studied and investigated. It confirms that complexation of metal-alkoxide precursors with the functional segment of the block copolymer is extremely important to attain smaller particle size, its narrow distribution and phase specific dispersion. Our prepared photocatalysts have been compared with the commercial photocatalysts Sachtleben Hombikat UV-100 for photodegradation of herbicide imazapyr in aqueous suspensions under UV illumination to calculate the corresponding photonic efficiencies. The research was focused on the photocatalytic efficiency of mesoporous Al2O3–TiO2 nanocomposites and photocatalytic reaction mechanism for the imazapyr degradation under UV. The reason behind the enhancement of the photocatalytic activity of Al2O3–TiO2 nanocomposites was investigated by details.

The block copolymer surfactant EO106–PO70EO106(F-127, EO = –CH2CH2O–, PO = –CH2(CH3)CHO–), MW 12600 g/mol), tetrabutyl orthotitanate (TBOT)Ti[OC(CH3)3]4, Aluminum-tri-sec-butoxide (ASB), Al[OCH(CH3)C2H5]3, HCl, H3PO4, CH3OH, C2H5OH, CH3COOH and imazapyr (C13H15N3O3 > 99%) were purchased from Sigma– Aldrich. Water was purified in a Millipore Mill-Q system (resistivity P 18 MX cm).

secondary Ni-filter and a X’Celerator multi-stripe detector. All measurements were carried out in a range between 10° and 80° 2h, a step width of 0.0170° 2h and a measuring time of 203 s each step. The obtained data were fitted using the Rietveld method (TOPAS V4.2, Bruker AXS). During the refinements, general parameters like the scale factors, one linear and one 1/x background parameter, the sample height error and the surface roughness were optimized. Profile shape calculations were carried out on the basis of standard instrumental parameters using the fundamental parameter approach implemented in the program. For TiO2 the crystal structures of Anatase (ICSD 93098) and b-TiO2 (ICSD 41056) had to be included in the refinements. To fit all occurring residual intensities in the diffraction patterns a second Anatase phase was included. The lattice parameters of this phase were constrained to be equal to the first one and the crystallite size was constrained to a maximum of 3 nm. During the refinements the lattice parameters of all phases as well as the average crystal size (LVol(IB)) were varied. Raman spectra of the samples were recorded with a Horiba LabRAM Aramis spectrometer equipped with a 460 mm spectrograph, an Olympus BX41 confocal microscope and a laser of 785 nm wavelength. Data were collected using an 1800 grating in the Raman shift range from 0 to 1300 cm1 in two parts with the accumulation of 3 spectra measured for 10 s each. Transmission electron microscopy (TEM) was conducted at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument equipped with a Gatan GIF 2001 energy filter and a 1k-CCD camera in order to obtain EEL spectra. The nitrogen adsorption and desorption isotherms at 77 K were measured using a Quantachrome Autosorb at 3B. The samples were vacuum-dried at 200 °C overnight. The sorption data were analyzed using the Barrett–Joyner–Halenda (BJH) model with Halsey equation [38]. The bandgap energy of the prepared photocatalysts was determined using diffuse reflectance spectroscopy (DRS). The reflectance spectra of the prepared samples were recorded over a range of 200–700 nm with a Perkin ElmerLambda 950-UV–visible spectrometer equipped with a Labsphere integrating sphere diffuse reflectance accessory and using BaSO4 as reference material [39]. UV–vis spectra were performed in the diffuse reflectance mode (R) and transformed to the Kubelka– Munk function F(R) to separate the extent of light absorption from scattering. Furthermore the band gap values were obtained from the plot of the modified Kubelka–Munk function (F(R)E1/2) versus the energy of the absorbed light E [40].

2.2. Preparation of mesoporous Al2O3–TiO2 nanocomposites

FðRÞE1=2 ¼

2. Experimental 2.1. Materials

Mesoporous Al2O3–TiO2 nanocomposites were synthesized by sol–gel method in the presence of the F127 triblock copolymer as structure directing agent. In typical, 1.6 g of F127 was dissolved in 30 mL of ethanol to obtain clear solution and then 2.3 mL of CH3COOH, 0.74 mL of HCl and 3.5 mL of TBOT were added to F127 solution with magnetic stirring for 60 min [9,37]. The calculated amount of ASB was added to (F127–TBOT–CH3COOH) mesophase with vigorously stirring for 60 min to obtain 0.5, 1, 2, 3 and 5 wt.% Al2O3–TiO2 nanocomposites. The produced transparent solution was put in a Petri dish to evaporate ethanol and form gel in humidity chamber 40% at 40 °C for 12 h. The produced gel was aged at 65 °C for 24 h. The as-prepared hybrid materials were calcined at 450 °C in air for 4 h at a heating rate of 1 °C/min and a cooling rate of 2 °C/min to remove the surfactant and obtain mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents. 2.3. Characterization X-ray powder diffraction data were collected on a Panalytical X’Pert PW3373 diffractometer, using Cu Ka1,2 radiation, a

!1=2 ð1  RÞ2  ht 2R

2.4. Photocatalytic activity tests Mesoporous Al2O3–TiO2 nanocomposites were dispersed in 50 mL of water (Ccat = 1 g/L) in addition to 10 mM KNO3, by sonication and shaking in an ultrasonic cleaning bath for 15 min. KNO3 was added to keep the ionic strength of the solution constant throughout the experiment and to inhibit the effect of HCl addition on the photodegradation rate. The values of pH was adjusted to be 4 using HCl. An aliquot of a stock solution of imazapyr (7.65 mmol l1) was further added in order to obtain the desired initial concentration (0.08 mmol l1). The reactor was covered with foil and the mixture was stirred at 300 rpm overnight in order to reach adsorption equilibrium. Afterwards, irradiation experiments were carried out under top illumination of a borosilicate glass beaker with a beam turning assembly with dichroic mirror from Newport Technology and the photonic flow was q = 2 mW cm2. Before illumination, 1 mL of the previously equilibrated suspensions was analyzed, being considered as the initial equilibrium

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concentrations. The temperature of the liquid phase, periodically monitored during the experiment, was 25 ± 1 °C. The suspensions were homogenized under continuous stirring. For analysis, samples were taken at regular time intervals. The analysis of imazapyr concentrations was performed in a high performance liquid chromatography (HPLC) system from Agilent Technologies 1260 Infinity composed of a G1311C-1260 Quat pump and a G1365D1260 MWD UV Detector adjusted to 254 nm. An Agilent Eclipse plus C18 column (100 mm Long.  4.6 mm i.d., 3.5 lm particles) working at room temperature was employed as stationary phase, and a mixture of methanol and water (30:70%v/v) adjusted to pH 3 by adding H3PO4 as mobile phase. The flow rate was kept constant at 0.8 mL/min, the retention time at 4.60 min. A calibration curve (R2 = 0.9996) was obtained from the analysis of patrons of 6 different concentration in a range from 0 to 0.08 mmol l1. The photodegradation rate of imazapyr was measured during the first 30 min of UV illumination for each sample. The photonic efficiency was determined for each sample as the ratio of the photodegradation rate of imazapyr and the incident light intensity as given in the following equation [41].



r  100 I

where n is the photonic efficiency (%), r the photodegradation rate of imazapyr (mol L1 s1), and I the incident photon flux (7.03  106 Ein L1 s1). The UV-A incident photon flow was determined by ferrioxalate actinometry [42]. 3. Results and discussion 3.1. Materials investigations It is clearly seen that the hydrolysis of Al and Ti of alkoxide precursors to obtain a uniform hydrated oxides of Al and Ti was occurred. And then, the as-prepared gel was heat treated, the amorphous TiO2 was crystallized at 400 °C and amorphous Al2O3 needs more than 600 °C for beginning crystallization, therefore, the crystallization of TiO2 put out Al species to the edges of TiO2 crystallites [16]. The Al2O3 species were consolidated on the surface of mesoporous TiO2 crystallites, lastly forming a thin layer was covered TiO2 phase through Ti–O–Al bonds [16,22]. Fig. 1 shows the X-ray powder diffraction patterns of the mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents calcined at 450 °C. Mesoporous TiO2, 0.5%, 1% and 2% Al2O3–TiO2

β-TiO2

normalized intensity / AU

ANATASE

(d) (e) (d) (c) (b) (a) 10

20

30

40

50

60

70

80

diffraction angle Cukα1,2 / °2Θ Fig. 1. Powder X-ray diffraction data of the mesoporous TiO2 (a) and Al2O3–TiO2 nanocomposites at different Al2O3 contents 0.5% (b), 1% (c), 2% (d), 3% (e) and 5% (f), along with the reflection positions of anatase and b-TiO2. The samples containing a high amount of Al2O3 (samples e and f) are showing only amorphous scattering.

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nanocomposites show reflections from anatase phase with peaks characteristic for the (1 0 1), (0 0 4), (2 0 0), (2 1 1) and (2 1 3) lattice planes (powder diffraction file No. 86-1157, ICCD) evincing that TiO2 phase easily nucleates during heating and forms into nanocrystals at low Al2O3 content (Fig. 1). Concerning the three TiO2 phases in the samples a rac parameter was calculated, giving the ratio of well crystalline anatase to the low crystalline TiO2 phases (Table 1). Al2O3 inhibits the TiO2 crystallization even at low concentrations, giving rac values less than 1 (Table 1). Rietveld refinements show the crystallite sizes are distinctively smaller (Anatase: 8.2–10.3 nm, b-TiO2: 1.3–1.8 nm). The peak position of anatase phase of TiO2, 0.5%, 1% and 2% Al2O3–TiO2 nanocomposites are similar, thereby implying both contain the crystallized anatase phase. However, the broadness of the peak became larger as more Al was incorporated. In addition, the average crystallite size was measured to be about 8.7, 7.6, and 7.0 nm for mesoporous TiO2, 1% Al2O3, and 2% Al2O3–TiO2, respectively. The formation of low crystalline TiO2 is more pronounced for 3% and 5% Al2O3 samples and they do not produce any crystalline material at all as previously published papers [16,43]. Interestingly no hints for Al2O3 can be found in the XRD data for all prepared samples. This is attributed to the retardation of crystallite growth upon addition of Al. Because of the relatively mismatch between Ti+4 (0.605 Å) and Al+3 (0.535 Å) cations size, Al+3 are not expected to occupy the Ti+4 sites in the lattice of anatase [43]. However, at high amount of Al2O3, it is covered TiO2 surface and suppress the crystallite growth of anatase [16,22,44]. The crystallinity of the TiO2 phase obtained in Al2O3– TiO2 nanocomposites at different Al2O3 contents was further confirmed by Raman spectroscopy (Fig. 2). Raman spectra show only distinct peaks of anatase at 148 cm1 for mesoporous TiO2, 0.5% Al2O3, 1% Al2O3 and 2% Al2O3–TiO2 nanocomposites, however, there are no peaks at all for 3% and 5% Al2O3–TiO2 nanocomposites. The spectral features (especially the peak positions) comply well with reference data [44], and show within the standard deviation no correlation with the amount of Al2O3. The Raman spectra are consistent with the result obtained from the XRD analysis. N2 adsorption isotherms of mesoporous TiO2, 3%Al2O3 and 5%Al2O3–TiO2 nanocomposites were depicted in Fig. 3. For all prepared samples, type-IV isotherms were assigned which are characteristic a mesoporous solids. The pore sizes were amounted to be ranged 6.8–9.1 nm (Table 1). Additionally, the average pore volumes and pore diameters were slightly increased with the increase Al2O3 contents (Table 1). It clearly seen that the addition of Al2O3 led to produce larger pore sizes than that of mesoporous TiO2 causing so much damage into the channel TiO2 walls. These results are similar and in agreement of our pervious published work [45]. The surface area of mesoporous TiO2 was increased from 174 m2/g to 325 m2/g by increasing Al2O3 contents from 0% to 5% Al2O3–TiO2 nanocomposites. The surface area and pore volume of the produced photocatalysts were increased by addition Al2O3, particularly at high Al2O3 content (Table 1). This is explained by the inhibition of TiO2 crystallite growth, which are in agreement with XRD results and hence both surface areas and pore volumes of Al2O3–TiO2 nanocomposites at high Al2O3 content were improved [26]. Fig. 4 shows the TEM and HR-TEM images of 0.5%, 2% and 5% Al2O3–TiO2 nanocomposites. TEM images of 0.5% Al2O3–TiO2 and 2% Al2O3–TiO2 nanocomposites (Fig. 4a and c) show porosities with disordered structure. TiO2 particles are quite uniform in size and shape (Fig. 4a and c) and the particle size of TiO2 nanoparticles was amounted to be 10 ± 2 nm. The HRTEM images and the SAED measurements of both 0.5% Al2O3 and 2% Al2O3–TiO2 nanocomposites show only TiO2 crystalline (Fig. 4b and d). HR-TEM image of 2% Al2O3–TiO2 nanocomposite shows highly anatase crystalline with

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Table 1 Textural properties of mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents calcined at 450 °C and commercial Hombikat UV100 and their photocatalytic properties. Samples

Content (wt.%)

Meso-TiO2 0.5%Al2O3 1% Al2O3 2% Al2O3 3% Al2O3 5% Al2O3 UV-100

rac

Anatase

Anatase

70.8(4) 33.6(3) 18.8(3) 44.5(3) n/a n/a 100

18.4(3) 13.4(3) 7.9(3) 16.0(2) n/a n/a –

a

LvolIB (nm)

b-TiO2 10.8(2) 53.0(3) 73.2(4) 39.5(3) n/a n/a –

2.42 0.51 0.23 0.8 n/a n/a –

Anatase

b-TiO2

13.24(6) 9.7(1) 8.2(1) 10.3(1) n/a n/a –

–/– 1.5(1) 1.3(1) 1.8(1) n/a n/a –

Bandgap (eV)

SBET (m2 g1)

Dp (nm)

Vp (cm3/g)

r  107 (mol L1 s1)

Photonic efficiency (%)

Removal efficiency (%)

3.30 3.30 3.21 3.10 3.33 3.36 3.20

174 179 187 196 242 325 232

6.53 6.62 6.93 7.42 8.22 9.01 –

0.267 0.265 0.276 0.279 0.310 0.380 –

1.252 1.911 2.264 2.83 0.475 0.333 0.86

1.78 2.71 3.22 4.02 0.67 0.47 1.22

82.01 83.10 86.01 97.90 40.21 35.11 46.03

The parameter rac gives the ratio of the weight percentages of Anatase to the sum of the low crystalline anatase and the b-TiO2, SBET – Surface area, Vp – pore volume, Dp – pore diameter. a Low crystallinity.

normalized intensity / AU

(f)

(e) (d)

(c) (b) (a) 100

200

300

400

500

600

Raman shift / cm

700

800

900

-1

Fig. 2. Raman spectra of the mesoporous TiO2 (a) and Al2O3–TiO2 nanocomposites at different Al2O3 contents 0.5% (b), 1% (c), 2% (d), 3% (e) and 5% (f), samples (e) and (f) do not show any distinct peaks, samples (a)–(d) shows only peaks that can be attributed to anatase.

300

Volume adsorbed/ (cc/g)

250

200

150

100

Meso-TiO2 3% Al2 O3 -TiO2

50

however at 2% Al2O3–TiO2 nanocomposites, it is slightly smaller than TiO2 particles (Fig. 4b and d). At high 5% Al2O3 content (Fig. 4e and f), mesostructure was disappeared and amorphous phase was formed (Fig. 4e). At HR-TEM, we could not detect the lattice fringe of anatase phase and its diffraction rings exactly correspond to the amorphous phase (Fig. 4f). SAED pattern became ringlike with low intensity (i.e., TiO2 crystallization was progressively decreased). Energy dispersive X-ray (EDX) element mapping measurements of the samples were investigated. EDX analysis shows uniform intensity of Ti and Al signals throughout the particles, revealing the homogeneous distribution of both components within the inorganic framework. Also, the final content of atomic percent of Al, Ti and O of the prepared samples 0.5% and 5% Al2O3–TiO2 confirmed that Al and Ti were detected and the final are consistent the starting sol precursors of the Al2O3 and TiO2 (Fig. 4g and h). To explore the relationship between the electronic structure and the photonic efficiencies of Al2O3–TiO2 nanocomposites, diffuse reflectance UV–visible spectra of Al2O3–TiO2 nanocomposites at different Al2O3 contents are depicted in Fig. 5. It is clearly seen that for all prepared samples, they have a broad band absorption from 250 to 350 nm, due to the transition from the O2 antibonding orbital to the lowest empty orbital of Ti4+ [8]. Also, this is attributed to the electronic structure of the isolated TiO4 species and the dispersed tetrahedral AlO4 species [46]. The band gaps of the mesoporous TiO2, 0.5%, 1%, 2%, 3% and 5% Al2O3–TiO2 photocatalysts estimated from the tangent lines are 3.30, 3.30, 3.21, 3.10, 3.33 and 3.36 eV respectively. These band gap values are listed in Table 1. We concluded that 3% and 5% Al2O3–TiO2 nanocomposites in amorphous phase have higher band gap values 3.33 and 3.36 eV than Al2O3–TiO2 nanocomposites at low Al2O3 content with crystalline phase. These results are in good agreement of previous published work [23]. The light reflectance of 2% Al2O3–TiO2 photocatalyst is obviously lower than that of the all prepared samples at the range 250–400 nm. And hence this sample is promising for photocatalysis application with high photonic efficiency as you see in photocatalytic activity section.

5% Al2 O3 -TiO2 0.0

0.2

0.4

0.6

0.8

3.2. Photocatalytic activity evaluation

Relative pressure p/po Fig. 3. N2 adsorption–desorption isotherms of mesoporous TiO2, 3% Al2O3–iO2 and 5% Al2O3–TiO2 nanocomposites.

pore walls (Fig. 4d). The selective electron diffraction (SAED) pattern of 0.5% Al2O3 and 2% Al2O3–TiO2 samples, indicating its diffraction rings exactly correspond to the anatase phase (1 0 1) lattice plane (fringe spacing  0.352 nm) (Fig. 4b and d inset). The pore walls consist of TiO2 particles with a size of 10 ± 2 nm,

It is well known that TiO2 is the more photoactive for photocatalysis reactions and the addition of amorphous Al2O3 is led to increase the separation efficiency of charge carriers [46]. The photocatalytic activity of mesoporous TiO2 and Al2O3–TiO2 nanocomposites was evaluated by decomposing herbicide imazapyr compared with Hombikat UV-100 under UV illumination. The photolysis experiment revealed that there was no change in imazapyr concentration after 6 h illumination. However, only 5–9% imazpyr concentration was adsorbed even after overnight. It is

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1400

TiKa

3600 3200

TiKb

TiLl

1200

CuKb

CuKa

400

2000 1600

TiKb

600

AlKa

800

2400

AlKa

Counts

1000

OKa TiLa

2800

1200 TiLl TiLa CuLl CuLa

Counts

4000

TiKa

1600

002 4400

CKa

1800

001

OKa

2000

800

200

400 0

0 0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

keV

10.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

10.00

keV

Fig. 4. TEM images of mesoporous Al2O3–TiO2 nanocomposites at 0.5% Al2O3 (a), 2% Al2O3 (c) and 5% Al2O3 (e). HRTEM image of Al2O3–TiO2 nanocomposites at 0.5% Al2O3 (b), 2% Al2O3 (d) and 5% Al2O3 (f). The insets at (b) and (d) show the SAED patterns for the anatase phase and (f) shows the amorphous phase, Energy dispersive X-ray spectra (EDX) of 0.5% and 5% Al2O3–TiO2 (g and h).

more than 90% of the initial imazpyr molecules still retained in the solution. Fig. 6 shows the relation between imazapyr concentration and illumination time through 120 min. The initial

photodegradation rates for all prepared samples are summarized in Table 1. The results indicated that the imazapyr degradation rate was increased from 1.25  107 to 2.83  107 mol L1 s1 with

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O 0.5% Al2O 2 33 O 1% Al2O 2 33 O 2% Al2O 2 33 O 3% Al2O 2 33 O 5% Al2O 2 33

2.5

1.2 [F(R)E]1/2/ eV1/2

Diffuse reflectance

1.6

4

Photonic efficiency ζ / %

2.0

0.8 0.4

2.0 1.5 1.0 0.5

0.0

0.0

-0.4 200

300

2

1 2.2

400

2.4

2.6

500

2.8

3.0 3.2 E/ eV

600

3.4

3.6

3.8

700

0 U

5%

0 10 V

l O2

lO A 2

A

3

iO 2 -T

3

iO 2 -T

iO 2 -3T

iO 2

iO 2 -3T

-3T

l O2

l O2

A

A

lO A 2

3%

2%

1%

5%

iO 2

the increase Al2O3 content from 0 to 2 wt.% and then decreased to 3.3  108 mol L1 s1 at 5 wt.% Al2O3 content whereas the imazapyr degradation rate using UV-100 was 8.6  108 mol L1 s1. These results indicated that the photodegradation rate of imazapyr using 2% Al2O3–TiO2 nanocomposite is nearly 2 and 3-fold higher than that mesoporous TiO2 and UV-100, respectively. Additionally, the removal efficiency of imazapyr using mesoporous TiO2 and 2 wt.% Al2O3-TiO2 nanocomposite was 82% and 98%, respectively within 120 min, however, the removal efficiency of 5 wt.% Al2O3–TiO2 nanocomposite was 35% (Fig. 6 and Table 1). Fig. 7 shows the photonic efficiency of Al2O3–TiO2 nanocomposites and UV-100 for imazapyr photodegradation. The results revealed that the photonic efficiency of mesoporous TiO2 is 1.78% which increases with the increase Al2O3 content up to 2 wt.% Al2O3–TiO2 nanocomposite with the maximum photonic efficiency being 4%. Subsequently, the photonic efficiency gradually decreases with increasing Al2O3 content to reach 0.46% at 5 wt.% Al2O3–TiO2 nanocomposite, however, the photonic efficiency of UV-100 photocatalyst is 1.22%. Such high photonic efficiency of the mesoporous TiO2 as compared with UV-100 can be attributed

0.

Fig. 5. Diffuse reflectance spectra for the Al2O3–TiO2 nanocomposites at different Al2O3 contents 0.5%, 1%, 2%, 3% and 5%. Inset, plot of transferred Kubelka–Munk versus energy of the light absorbed of the mesoporous of 2% Al2O3–TiO2 nanocomposites.

T oes M

Wavelength/nm

100

3

Fig. 7. Comparison of Photooxidation of imazapyr over mesoporous TiO2 (a) and Al2O3–TiO2 nanocomposites at different Al2O3 contents and UV-100. Catalyst loading, 1 g/L; (0.08 mmol l1) imazapyr (O 2 saturated, pH = 4; T = 25 ± 1 °C); volume, 50 mL; Io = 2 mW/cm2 (ca. > 320 nm).

to a facile diffusion of imazapyr to reach the active sites and and low light scattering of photons [8]. On the other hand, the addition of Al2O3 more than 2% leads to decrease of the photodegradation rates and hence the photonic efficiency. This may be ascribed to the inhibition effect of Al2O3 doping at high content on the crystallization of anatase phase and/or the decrease of the transparency caused by the inclusion of amorphous Al2O3 [26]. From the above results, we can suggest the following mechanism: The enhancement of mesoporous Al2O3–TiO2 nanocomposites can be explained by TiO2 is the more photoactive than Al2O3 for photocatalysis reactions, however, the Al and Ti species in the active Al2O3–TiO2 photocatalysts are the tetrahedral AlO4 and TiO4 species [46]. The addition of amorphous Al2O3 at low contents onto TiO2 particles led to the synergistic effect (Scheme 1) and hence increased the separation efficiency of charge carriers [46,47]. Imazapyr was easily reached to the active sites (the entity of dispersed tetrahedral both AlO4 and TiO4 [46]). In the previous study, they found that photogenerated electrons of TiO2 could be transferred to the defect levels of Al2O3, although the CB potential of Al2O3 is higher than that of TiO2 [48,49]. In our system, it behaves the same

Meso-TiO2

% Photocatalytic efficiency

0.5% Al2O3 1% Al2O3

80

2% Al2O3 3% Al2O3 5% Al2O3

60

UV-100

40

20

0 0

20

40

60

80

100

120

Illumination time/ min Fig. 6. Change of the imazapyr concentration as a function of the illumination time using mesoporous TiO2 and Al2O3–TiO2 nanocomposites and UV-100. Photocatalyst loading, 1 g/L; (0.08 mmol l1) of imazapyr (O 2 saturated, pH = 4; T = 25 ± 1 °C); volume, 50 mL; Io = 2 mW/cm2 (ca. > 320 nm).

Scheme 1. Schematic illustration of the proposed mechanism to explain the enhancement photonic efficiency of mesoporous Al2O3–TiO2 nanocomposites for photodegradation of imazapyr.

A.A. Ismail et al. / Separation and Purification Technology 145 (2015) 147–153

phenomena that the holes on TiO2 surface react with OH ions or H2O molecules, yielding highly oxidative hydroxyl (OH) radicals, which are considered to be the key oxidants in the photocatalytic oxidation process. The photogenerated electrons are reduced adsorbed O2 molecules to form O2 and than H2O2 is formed [50], which also contribute to the oxidation of imazapyr by the intermediate formation of OH radicals (Scheme 1). Amorphous Al2O3 is playing an essential role for minimizing the recombination of electrons and holes. It has high electron-transfer ability from TiO2 because amorphous Al2O3 contains more defect sites than crystals (Scheme 1) and also the bond length of Ti–O–Al and Al– O–Al O atoms in the amorphous phase is shorter than crystalline one, which improves its photoabsorption and photonic efficiency [51]. For the second reason, a thin Al2O3 overlayer was formed onto mesoporous TiO2 surface and this Al2O3 layer intervened between adsorbed imazapyr and TiO2 would certainly effect on the charge carries transfer under illumination [22]. In general, the addition of Al2O3 does not prohibit the capture of conduction band electron by the absorbed O2 molecules onto the surface of photocatalyst (Scheme 1), and hence the active radical species, such as O2 and HO were generated [22]. On the other hand, at 3 and 5 wt.% Al2O3–TiO2 nanocomposites, the amorphous structure of the produced nanocomposites act as recombination centres for electrons and holes, which decrease the photocatalytic activity and reduce the photonic efficiencies. One can say that the decrease of the photocatalytic activity in the high Al2O3 contents is owing to the aggregation of TiO4 and AlO4 species.

4. Conclusions Preparation of mesoporous Al2O3–TiO2 nanocomposites at different Al2O3 contents (0–5 wt.%) through a facile sol–gel reaction have been investigated. XRD revealed that highly crystalline anatase phase at low Al2O3 contents (0–2%) was confirmed. However, at high Al2O3 contents 3% and 5%, they do not produce any crystalline material at all. TiO2 particles are quite uniform in size and shape and the particle size of TiO2 nanocrystals were amounted to be 10 ± 2 nm. It is evident that mesoporous TiO2 revealed higher photonic efficiency for photodegradation of herbicide imazapyr than commercial UV-100 photocatalyst. Such high photonic efficiencies of the mesoporous TiO2 as compared with UV-100 can be attributed to a facile diffusion of imazapyr to reach the active sites of mesoporous Al2O3–TiO2 nanocomposites. 2% Al2O3–TiO2 nanocomposite is considered to be the optimum photocatalyst for the highest photonic efficiency 4%. The overall photocatalytic activity of 2% Al2O3–TiO2 nanocomposite for photodegradation of imazapyr are significantly 2 and 3-fold higher than that of mesoporous TiO2 and UV-100, respectively. At 3 and 5 wt.% Al2O3–TiO2 nanocomposites, the amorphous structure of the produced nanocomposites act as recombination centres for electrons and holes, which decrease the photocatalytic activity and reduce the photonic efficiencies.

Acknowledgements I.A. thanks the Egyptian Ministry of Higher Education for providing him a Postdoctoral fellowship. Advanced Materials and NanoResearch Centre, Najran University, Najran, Saudi Arabia has been gratefully acknowledged for instrumental facility. A.A. I. acknowledges the Alexander von Humboldt (AvH) Foundation for granting him a research fellowship.

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