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Efficient Mesoporous Anatase-brookiteTiO2 Photocatalysts for Degradation of Ibuprofen Said M. El-Sheikh*,Tamer M. Khedr, Adel A. Ismial
Waheed A. Badawy
Nanostructured Materials Lab., Advanced Material Department, CMRDI Cairo 11421, Egypt *
[email protected]
Department of Chemistry, Faculty of Science, Cairo University Giza, Egypt
Abstract—Mesoporous anatase-brookite heterojunction TiO2 photocatalysts with tunable brookite/anatase ratios controlled by glycine assistant were successfully synthesized by facile hydrothermal process. In addition, pure brookite phase was prepared in the absence of glycine. The obtained TiO2 catalysts have been characterized by X-ray diffraction, Brunauer– Emmett–Teller analysis, transmission electron microscope and field emission-scanning electron microscopy. The X-ray diffraction revealed the formation of brookite-phase either in pure form in the absence of glycine or in mixing with anatase in the presence of glycine. The field emission-scanning electron microscopy micrograph of pure brookite indicated the formation of spindle-like structures with particle size ~ 25 nm. The transmission electron microscope image of the optimum sample indicated the formation of small quasi-spherical particles and rod-like particles. N2 isotherm measurements confirmed that anatase/brookite TiO2 materials have mesoporous structure. The photocatalytic degradation of ibuprofen with low concentration was investigated using these novel TiO2 materials under UV irradiation light for water treatment. The anatase/brookite TiO2 samples showed a superior photocatalytic activity compared to pure brookite TiO2 sample. Sample containing 74.4% anatase/25.6 brookite displayed the highest photocatalytic activity for ibuprofen degradation (98.9 %) under UV light for 120 min due to the synergistic effect between anatase and brookite phases, high surface area (72.4 m2 g-1) and mesoporous structure.
light stability and environment friendly, it is able to degrade organic pollutants such as IBF into H2O and CO2 under UV light finally [5]. However, the application of such TiO2 catalysts for water treatment experiences a series of technical challenges. The undesired electron–hole recombination creates a strong tendency for catalyst energy loss and decreases the activities of TiO2 in the reaction system. Therefore, the synthesis of anatase-brookite heterojunction TiO2 has been a hot issue in recent years to assist the charge carrier’s separation via the synergistic effect [2]. The main objective of this study is the synthesis mesoporous anatase/brookite heterojunction titania by a facile hydrothermal method in the presence of glycine for degradation of low concentrations of ibuprofen (IBF) under UV irradiation light. Ibuprofen was selected to represent NSAIDs because of its extensive usage. The kinetics of degradation of IBF was also investigated.
Keywords: Anatase-brookite; Ibuprofen; Water treatment
B. Synthesis of TiO2 Photocatalysts The aqueous solution of Ti2(SO4)3 was oxidized via NaNO3 and transformed into transparent colorless solution. Different glycine concentrations (0.00, 0.50, 1.00 and 3.2M) were added to the transparent solution at pH ≈3. The solution pH was then adjusted at pH 10 by NaOH (2.0 M) and 48 mL of the resulting suspension was transferred into a Teflon-lined stainless-steel autoclave (120 mL), followed by heating to 200 oC for 20 h. After cooling to the room temperature, the formed precipitate was separated by centrifugation, washed repeatedly with ethanol and water, and dried at 60 oC overnight. The samples obtained were denoted as I, II, III and IV.
Hydrothermal;
I. EXPERIMENTAL A. Materials Titanium (III) Sulfate (Fisher, 15%), Sodium Nitrate (kochlight laboratories ltd UK, 98%), Glycine (Sigma-Aldrich, 99%), Sodium Hydroxide (Loba Chemie, Pellets, 98%), Ethanol (Sigma-Aldrich, 99.8%), Ibuprofen Sodium (Fluka, 99%), Deionized distilled water (Conductivity > 18 M Ω TOC).
Degradation;
INTRODUCTION Pharmaceuticals and personal care products (PPCPs) concept was first proposed by in 1999 and subsequently received wide attention as an emerging class of potentially harmful environmental pollutants [1]. Non-steroidal antiinflammatory drugs (NSAIDs) belong to PPCPs, which have been widely used as antibacterial drugs for human and veterinary [2]. These pharmaceuticals cause harmful pollution of surface, ground and drinking water [3]. Ibuprofen (IBF) belongs to this family of medicines, which is an analgetic drug mainly used for the treatment of rheumatoid arthritis, myoskeletal injuries and fever [4]. Thus, the removal of IBF from water represents an emerging environmental concern. TiO2 as one of the most promising photocatalysts has attracted special attention because of its wide band gap, chemical and
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C. Characterization Techniques The X-ray diffraction (XRD, Brukeraxs D8, Germany) using Cu Kα radiation (λ= 0.15405 nm) and a secondary monochromator in the 2θ range from 20 to 70o was used to appreciate the crystalline phase, the phase composition, relative
brookite/anatase ratios have been obtained and the ratio declines as the amount of glycine increases. The characteristic diffraction peaks of the crystal faces of brookite (121) are observed at 2θ (30.8°), which is an evidence of the existence of the brookite crystal form. Similarly, the existence of anatase phase can be confirmed by the “three sharp peaks” [crystal face (103), (004), and (112)] and other diffraction peaks compared with the standard anatase XRD pattern. The weight percentage of each crystal phase can be estimated from individual diffraction peaks on the basis of formulas reported in the literature [9]. The XRD data show that pure brookite (100% brookite) is formed in the absence of glycine, whereas, a bi-phasial anatase and brookite mixture (64.8%/35.2%) is formed at 0.50 M glycine. Furthermore, (74.4%/25.6%) anatase/brookite mixed phase is obtained in the sample III. The increase of the glycine concentration is accompanied with a decrease in the intensity of the peaks assigned to the brookite phase, whereas, the additional anatase phase is formed to reach a anatase/brookite ratio of (86.9%/13.1%) in the IV sample. These results indicate that the phase transformation from brookite to anatase nanoparticles is associated with the increase of the glycine concentration. The average particle size of TiO2 was estimated using Scherrer’s equation. The results indicate that the crystal size of brookite phase decreases with increasing the glycine concentration. The relationship between the phase contents in TiO2 product and the samples is shown in Fig. 1(b).
crystallinity and crystallite diameter (i.e. Scherrer size). The surface morphology of the catalysts are characterized using field emission-scanning electron microscopy (FE-SEM; QUANTAFEG 250, Netherlands) and transmission electron microscope with an acceleration voltage up to 200 kV, magnification power up to 600 kX and resolution power down to 0.2 nm (TEM, JEOL-JEM-1230, Tokyo, Japan). The specific surface area was calculated from the N2 adsorption capacity using the Brunauer-Emmett-Teller (BET) equation. The full isotherms of the samples were determined by the N2 adsorption–desorption method at 77 K, using Quantachrome Instruments (NOVA 2000 series, UK). All the samples were degassed at 453 K overnight before the measurement. The Barrett–Joyner–Halenda (BJH) model with Halsey equation was employed to analyze the sorption data. D. Photocatalytic Activities For all experiments, the initial pH value of IBF was not adjusted. Then 0.5 gL-1 TiO2 catalyst was dispersed into the previously prepared IBF by sonication and shaking in an ultrasonic bath for 15 min. The suspension was first stirred in the dark for ca. 60 min, to ensure establishment of adsorption/desorption equilibrium. Before illumination, an aliquot of the previously equilibrated suspensions was analyzed, being considered as the initial equilibrium concentration [Co]. For degradation using UV light, irradiation experiments were carried out in a Heraeus photoreactor, equipped with TQ-150 high-pressure mercury lamp, keeping a constant power at 150 W, with λmax at 254 nm. The borosilicate glass reactor of 220 mL capacity was made with ports for sampling and gas/air purge. For kinetic studies, samples were taken at regular irradiation time intervals, filtered with 0.22 μm filters to remove catalyst particles and finally analyzed directly using the UV–Vis spectrophotometer. The kinetic rate constants K for all experiments were calculated from the- Langmuir−Hinshelwood first order equation (6). This rate constant is employed to calculate the initial photo-degradation rates for the photo-oxidation of IBF using the formula (7): −d [A]/dt = KCn where K is the rate constant, C the concentration of the IBF, and n the order of the reaction. The HIBF degradation percentage of TiO2 samples were estimated, as follows (8):
Fig. 1(a) XRD patterns of as-synthesized TiO2: I, II, III, and IV; (b) Relationship between different samples and their phase composition .
To get information about the surface morphology of the samples, some selected powders have been investigated by TEM and FE-SEM (cf. Fig. 2). The typical FE-SEM image of the sample I (pure brookite) shows spindle-like particles with diameters up to 25 nm and lengths up to 108 nm (cf. Fig. 2(a)). The brookite spindle-like morphology in the SEM micrograph disclosed a high uniformity and a clean surface without any contamination. Fig. 2(b) presents typical micrograph of the sample III obtained in the presence of 1.00 M glycine. It shows that the morphology of the sample III consists, mainly, of rod-like particles, accompanied by some small particles and the spindle-like structure disappears. These results indicate that the controlled addition of glycine has great influence on the crystal structure and morphologies of the products. From the XRD data, we can conclude that the small particles are the anatase phase, and the rod-like particles are the brookite phase.
DP % = [(Co - Ct) / Co] x 100 Where Co and Ct are the concentrations of IBF before and after irradiation, respectively. II. RESULTS & DISCUSSION A. Physicochemical Characterization of the Synthesized TiO2 Catalysts The XRD patterns of all as-synthesized TiO2 with (JCPDS); file No. (15-0875, Brookite) and file No. (84-1286, Anatase) are presented in Fig. 1(a). The XRD results indicated that in the absence of glycine, pure brookite TiO2 with an essential millar index (121) is obtained. When the different concentrations of glycine controlled between 0.50 and 3.2 M are added, mixed phase titania samples with tunable
the pure brookite sample (sample I) is 84.5% (cf. Fig. 3(c) and Table I). Whereas 97.8%, 98.9% and 98.5% of the photodegradation have been reached in presence of the II, III and IV samples, respectively (cf. Fig. 3(c) and Table I). With the help of a linear regression, the rate constants for the degradation of IBF were calculated based on the natural logarithm (ln) of the IBF concentration and the illumination time, i.e., first-order decomposition kinetics under UV (cf. Table I). The calculated rate constants for IBF degradation under UV light (cf. Fig. 3(b)) with I, II, III and IV samples are 0.0158, 0.0317, 0.0377 and 0.0351 min-1, respectively. The initial rates of IBF degradation under UV light using I, II, III and IV samples (cf. Table I) were calculated to be 137.6, 268.1, 315.5, 295.3 μM min-1 x 10-2, respectively.
According to the inset of Fig. 2(b), the pore walls, as revealed by the electron diffraction (ED) patterns, consist of aggregated nano-crystalline brookite/anatase with an average crystallite size of 5 nm. The electron diffraction (ED) ring patterns are in a good agreement with the XRD patterns. Fig. 2. (a) FE-SEM of sample I and (b) TEM image of sample III.
N2 adsorption–desorption isotherms of the I, II, III and IV samples were investigated. Results revealed that the III and IV samples show the type IV isotherm according to IUPAC classification, indicative of a mesoporous structure [10]. While, the I and II samples show the type III isotherm. The adsorption-desorption isotherms of III and IV samples show a large hysteresis loop due to capillary condensation in mesoporous channels and/or cages. The low-pressure portion of isotherm indicated the existence of microspores [11]. The corresponding Barrett-Joyner-Halenda (BJH) pore-size distribution of the I, II, III and IV indicated that all samples have a mesoporous structure with diameter 2.5−8.8 nm. The total pore volume, and BET surface area of heterojunction samples (II, III and IV) are larger than those of pure brookite sample (I), and show the uptrend with further increase in the anatase content in the heterojunction samples. The total pore volume of the sample I is 0.047 cm3g-1, and of hetrojunction samples increases from 0.084 to 0.271 cm3g-1. The surface area values of the samples I, II, III and IV are 21.74, 30.2, 72.4, and 65.5 m2 g-1, respectively. B. Photocatalytic Activity of TiO2 Photocatalysts
TABLE I. The kinetics, R2 and efficiencies for IBF degradation under UV for 120 minwith I, II, III and IV photocatalysts. Samples
K (min-1)
ro(μM min-1x10-2)
R2
DP (%)
I
0.0158
137.6
0.994
84.5
II
0.0317
268.1
0.999
97.8
III
0.0377
315.5
0.999
98.9
IV
0.0351
295.3
0.999
98.5
Fig. 3. (a) Plot of Ct/Co vs irradiation time, (b) The corresponding degradation efficiency and (c) Plot of ln (Ct/Co) vs irradiation time for degradation of IBF under UV irradiation (TQ-150, λmax = 254 nm) with I, II, III and IV photocatalysts.
The remarkable photocatalytic performance of the heterojunction samples (samples II, III and IV), compared to pure brookite (sample I) may be arise from the junction (synergistic) effect between two crystalline phases and surface area effect. The synergistic effect between anatase and brookite is because: upon irradiation, the electron-hole pairs are formed by photo-excitation. In the single phase TiO2 (brookite TiO2), lifetime separation of electron-hole pairs is very small so it has low activity. Taking advantage of conduction band, and valence band energy edge difference between anatase and brookite in heterojunction samples, electrons that are in the CB of brookite can readily transfer to the CB of anatase, leaving behind the holes in the VB of the brookite phase. Consequently, electron-hole pairs are well separated. Furthermore, the results indicate that as the surface area of the samples increases, the photocatalytic activity increases and the photocatalytic activity of the samples follow the order: sample III (72.4 m2g-1) > sample IV (65.5 m2g-1) > sample II (30.2 m2g-1) > sample I (21.74 m2g-1). The large surface area may also facilitate the contact probability of
The photocatalytic performance of the TiO2 photocatalysts for the photodegradation of IBF under UV light for 120 min was evaluated as shown in Figs. 3. At first, the photodegradation of IBF was performed in an aqueous solution by photolysis using UV light without photocatalyst. The results indicate that the IBF concentration was almost remained at the initial concentration of 20 mg L-1, indicating that the degradation of IBF is negligible without photocatalyst, which indicates that IBF cannot easily be degraded either by UV illumination. The initial IBF concentration has also been calculated before and after the dark adsorption (for 60 min) in the case of UV. Fig. 3(a) shows the change of IBF concentration as a function of irradiation time for IBF degradation under UV light for 2h. The results reveal that the photocatalytic IBF degradation over doped mixed phase TiO2 samples (II, III, and IV) is higher than that over un-doped pure brookite phase. The sample III gives the highest photocatalytic activity for IBF degradation under UV irradiation. It is obvious that the IBF’s percentage of photo-degradation using
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catalyst surface and IBF, enhance the active site of the response, and accelerate the photocatalytic decomposition reaction of IBF aqueous solution. In addition to the above reasons, the high crystallinity and the mesoporosity A/B TiO2 photocatalysts can help in the enhancement of the photocatalytic activity. This is explained by the high adsorption capacity as a result of faster and facile diffusion of the target IBF molecule to the active sites through the porous TiO2 network. As well as, the energy band matching of anatase and brookite facilitates the interfacial migration of photo-induced accumulated electrons from brookite to anatase depicted on the basis of the larger band gap of brookite compared to anatase [8,12]. Consequently, these electrons transferred to oxygen adsorbed on the TiO2 surface, producing the O2•− which may help in IBF degradation.
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IV. CONCLUSIONS In conclusion, mixed phase titania, A/B TiO2, and single brookite phase, were synthesized, conveniently and economically via novel hydrothermal treatment of Ti2(SO4)3 in the presence of different glycine concentrations (0.00 M − 3.2 M) at 200oC for 20 h. The obtained powders were characterized by XRD, BETSA, FE-SEM and TEM. The results revealed that, the phase structure and composition were controlled by the concentration of glycine. In absence of glycine (0.00 M), pure brookite was formed. Increasing glycine concentration from 0.5 M to 3.2 M, leads to the formation of the mixed phase, A/B TiO2 (with different brookite/anatase ratios and nano-rod-like structure). The total pore volume and BETSA has its maximum at the mixed phase ratio 74.4% anatase and 25.6 brookite (sample III), which represent the optimum sample for the photo-catalytic degradation of IBF. The photo-catalytic activity of all samples was assessed by the degradation of IBF in aqueous solution under UV irradiation. The photo-catalytic efficiency of A/BTiO2 samples towards IBF degradation was higher than that of single-phase brookite sample. The optimized TiO2 sample (sample III) with its high BETSA and pore volume gives the highest efficiency (98.9%) for IBF photodegradation.
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