Journal of Industrial and Engineering Chemistry 18 (2012) 1764–1767 Contents lists available at SciVerse ScienceDirect Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec Preparation, characterization and evaluation of catalytic activity of titania modified with silver and bentonite Dinh Bang Nguyen a, Thi Dieu Cam Nguyen b,*, Thanh Phuong Dao a, Hung Thuan Tran c, Van Noi Nguyen a, Dae Hee Ahn d a Faculty of Chemistry, Hanoi University of Science, Vietnam National University, Hanoi, Viet Nam Faculty of Chemistry, Quy Nhon University, Viet Nam Advanced Materials Technology Center, National Center for Technological Progress, Hanoi, Viet Nam d Department of Environmental Engineering and Biotechnology, Myongji University, Republic of Korea b c A R T I C L E I N F O A B S T R A C T Article history: Received February 2012 Accepted April 2012 Available online 10 April 2012 The aim of this study is to evaluate phenol degradation capability of silver modified titanium dioxide nanomaterial on bentonite support (Ag-TiO2/Bent) The material was synthesized as photocatalyst by adding Ag-TiO2 sol into bentonite suspension The experimental results revealed that photooxidation activity of Ag-TiO2/Bent was greatly higher than that of Ag-TiO2 and TiO2/Bent The phenol removal efficiency was 23.25%, 35.41% and 98.94% for Ag-TiO2, TiO2/Bent and Ag-TiO2/Bent, respectively The dispersion of silver modified TiO2 on bentonite support significantly enhances photocatalytic activity under solar radiation due to surface plasmon resonance formation and prevention of anatase-to-rutile phase transformation ß 2012 The Korean Society of Industrial and Engineering Chemistry Published by Elsevier B.V All rights reserved Keywords: Titanium dioxide Silver Bentonite Photocatalyst Visible light Introduction In recent years, titanium dioxide has been widely applied as a photocatalyst in environmental treatment for treating recalcitrant organic compounds There are some reports on employing TiO2/UV to degrade organic pollutants in aqueous environment [1] Titanium dioxide in anatase form has band gap energy (Ebg) of 3.2 eV, hence it is only active under UV radiation Because there is only about 3–5% of solar radiation that lies in UV region, photocatalytic ability of titania is limited Therefore, there is a need of research on improving titania activity under visible light Many reports on this research were published [2–6] Most of them concentrated on modifying titanium dioxide using transition metals (Fe, Cr, Ni, Ag, and Cu) and non-metals, such as N, S or C Depending on modifying reagent, when doping into TiO2, it can lead to (i) decrease of Ebg [7,8]; (ii) electron transfer from modifying reagent to TiO2 [9,10]; and (iii) surface plasmon resonance formation [11,12] All of these significantly increase photocatalytic activity of nano TiO2 under solar radiation Many studies have been carried out to improve the photocatalytic activity by the insertion of noble metals, and it is found that silver nanoparticles modified TiO2 has been of considerable interest because of its potential applications * Corresponding author Tel.: +84 98 322 2831 E-mail address: nguyendieucam@hus.edu.vn (T.D.C Nguyen) Silver can trap the excited electrons from TiO2 and leave the holes for the degradation reaction of organic species It also results in the extension of their wavelength response toward the visible region [13–15] Moreover, silver particles can facilitate the electron excitation by creating a local electric field [16], and plasmon resonance effect in metallic silver particles shows a reasonable enhancement in this electric field [17] In order to collect and reuse modified nano TiO2, a lot of new synthesis methods are introduced and investigated One of these methods is dispersing on support [18,19] In this study, bentonite was used as support, and Ag-TiO2/Bent was synthesized by adding Ag-TiO2 sol into bentonite suspension The removal of phenol was investigated to evaluate the relative photocatalytic activity of the prepared photocatalyst samples The potential routs for mechanisms of phenol photooxidation were proposed and discussed, based on experimental results of phenol degradation using AgTiO2/Bent under natural solar light, solar simulator and in the dark Materials and methods 2.1 Synthesis of Ag-TiO2, Ag-TiO2/Bent and TiO2/Bent Ag-TiO2 was synthesized using sol–gel method A mixture of 22 mL isopropylic alcohol and mL of tetraisopropyl orthotitanate solution was added to the beaker The solution was stirred and kept at 65 8C in 30 AgNO3 dissolved in 80% CH3COOH solution (the 1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry Published by Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.jiec.2012.04.004 D.B Nguyen et al / Journal of Industrial and Engineering Chemistry 18 (2012) 1764–1767 1765 amount of Ag equals 2.5 wt.% in comparison with (Ag + TiO2)) was added dropwisely After that, solution was kept at 65 8C in h The obtained Ag-TiO2 sol was dried at 90 8C The material was then calcined at 700 8C in h with temperature increasing rate of 8C/ Synthesis of Ag-TiO2/Bent: Ag-TiO2 sol was added dropwisely into 2% clay suspension (adjusted to pH 6.5) It was stirred for 48 h, and then dried at 90 8C After that it was then calcined at 700 8C in h with temperature increasing rate of 8C/min TiO2/Bent was synthesized using the Ag-TiO2/Bent synthesis procedure but without AgNO3 2.2 Phenol degradation experimental set-up Fig UV–vis absorption spectra of TiO2, Ag-TiO2 and Ag-TiO2/Bent Take 300 mL of phenol solution (100 mg/L) in 500 mL beaker For each test, 0.50 g catalyst (Ag-TiO2/Bent or TiO2/Bent) was added The solution was stirred and left 15 in order to achieve phenol adsorption equilibrium Light source in this experiment is natural solar light or solar simulator (Newport, USA) 2.3 Analytical methods Phase composition of TiO2 was determined by X-ray diffraction (XRD) method (D8-Advance 5005) Material surfaces were characterized by scanning electronic microscopy (SEM) (JEOL JSM-6500F) Light absorption capability was evaluated by UV–vis absorption spectroscopy (3101PC Shimadzu) Oxidation state of elements (Ti and Ag) was revealed using X-ray photoelectron spectroscopy (XPS) (Kratos Axis ULTRA) Phenol concentration was measured by spectrometric method, using 4-aminoantipyrin as coloring agent at 510 nm COD value was determined by dichromate method using UV–vis Novaspec II Results and discussion 3.1 Material characterization The XRD patterns of the materials were shown in Fig 1, for 2u diffraction angles between 58 and 708 The XRD pattern of Ag-TiO2 shows peaks at 27.338 (1 0), 36.128 (1 1), 41.558 (1 1) and 55.028 (2 1) which can be attributed to different diffraction planes of rutile TiO2 The peaks at 38.058 (1 1), 44.348 (2 0) and 64.588 (2 0) can be attributed to Ag (0) When dispersing Ag-TiO2 on bentonite support, there are no peaks of rutile phase and Ag (0) on the XRD pattern This proves that the use of bentonite support has effects on phase transformation of TiO2 and the distribution of Ag on TiO2 From UV–vis absorption spectra (Fig 2), it can be seen that after being modified with Ag, TiO2 can absorb radiation in visible region SEM images (Figs and 4) show that Ag-TiO2/Bent consists of TiO2 particles (size is about 30 nm) dispersed on bentonite surface, while Ag-TiO2 is composed of TiO2 particles in bigger size and Ag particles on TiO2 surface, which causes surface plasmon resonance Fig XRD pattern of Ag-TiO2 and Ag-TiO2/Bent To determine chemical composition of Ag-TiO2/Bent and oxidation state of elements, the catalyst was characterized by XPS Results obtained from XPS spectra in Fig show that Ag-TiO2/ Bent contains Ti, Ag, O, Si, and Al elements Peaks of Ti 2p are at 464.2 and 458.8 eV This confirms that Ti is only present in Ti4+ form [20,21] Peaks of O 1s is at 530.6 eV, Si 2p is at 103.0 eV and Al 2p is at 75.0 eV Peak of Ag (3d) is at 367.7 eV, which means that Ag is existed in Ag (0) form There are no peaks of ion Ag+ Thus, it can be said that Ag ions at the surface are reduced to silver metal Results obtained from this method agree with reports of other authors [22,23] 3.2 Tests on photocatalytic activity of Ag-TiO2 and Ag-TiO2/Bent Data in Table show that dispersing silver modified TiO2 on bentonite support greatly enhances photocatalytic activity under solar radiation This can be explained that the material dispersion on bentonite support prevents anatase-rutile phase transformation, i.e., form of titania in Ag-TiO2 sample is rutile while in AgTiO2/Bent sample is anatase (Fig 1) Moreover, particle size of TiO2 Fig SEM image of Ag-TiO2 Fig SEM image of Ag-TiO2/Bent D.B Nguyen et al / Journal of Industrial and Engineering Chemistry 18 (2012) 1764–1767 1766 Table Phenol degradation using Ag-TiO2/Bent with solar simulator as light source Time (min) Removal efficiency (%) 20 40 60 90 Solar simulator In the dark 16.18 22.51 27.01 31.14 4.28 6.12 6.82 7.05 In order to improve the reaction rate to be sufficiently high, the oxidizing agent H2O2 was used for further investigation To prove that in presence of H2O2 the decomposition of phenol using AgTiO2/Bent is still followed photocatalytic mechanism, experiments were carried out with and without light source Data show that the phenol conversion decreased significantly for experiments in the dark (Table 4) Based on experimental results of phenol degradation using AgTiO2/Bent under natural solar light, solar simulator and in the dark, it can be assured that Ag-TiO2/Bent acts as photocatalyst with and without oxidizing agent H2O2 3.3 Discussion about role of Ag on TiO2/Bent in phenol decomposition Fig XPS spectra of Ag-TiO2/Bent is significantly reduced, leading to the increase of surface area and number of active centers Another factor is that phenol adsorption capacity is improved by using bentonite support Data in Table show that after 90 min, removal efficiency reaches 31.14% when using solar simulator as light source, while it is only 7.05% if experiments were carried out in the dark The decrease of phenol concentration in the dark is explained by the adsorption of phenol on the material This leads to the conclusion that Ag-TiO2/Bent has photocatalytic activity Effect of oxidizing agent H2O2 on phenol conversion was also investigated Experimental results in Table indicate that phenol decomposition rate increases significantly when H2O2 is present This can be explained that the presence of H2O2 increases ability of forming photogenerated electrons and decreases the recombination rate of electrons and holes, hence enhancing phenol decomposition To reassure this observation, experiments on phenol decomposition with H2O2 but without Ag-TiO2/Bent were carried out (Table 3) Experimental results reveal that phenol removal efficiency decreases significantly (only 14.84% after 90 min) if no Ag-TiO2/Bent is used Table Phenol degradation using Ag-TiO2 and Ag-TiO2/Bent with solar simulator as light source and H2O2 addition Time (min) 10 20 30 60 90 Removal efficiency (%) Ag-TiO2 Ag-TiO2/Bent 10.34 16.82 20.96 23.25 25.18 33.19 69.41 86.92 98.94 – Basing on characterization of Ag-TiO2, Ag-TiO2/Bent and test on catalytic activity of these materials, as well as reference materials about oxidation of organic compounds using Ag-TiO2 catalyst, it can be suggested that modifying TiO2 using Ag cannot decrease band gap energy because Ag cannot substitute Ti in TiO2 lattice due to unsuitability in size of ion Ag+ (128 pm) and Ti4+ (68 pm), but agglomerate on TiO2 surface [24] Theoretically, if band gap energy is not reduced, silver modified TiO2 is not capable of absorbing visible radiation However, experimental data show that under solar simulator, Ag-TiO2/Bent is more efficient in degrading phenol than TiO2/Bent (Table 5), which means that TiO2 modified by Ag can improve catalytic activity of TiO2 under solar radiation To find out a reasonable explanation of role of Ag-TiO2/Bent, experiments on phenol degradation were carried out under two light sources: solar simulator and visible light simulator (l > 420 nm) Data in Table show that Ag-TiO2/Bent is active in the visible region However, the activity shows much higher under solar simulator compared to visible light simulator It would be logically suggested that the low photocatalytic activity in latter case is mostly due to surface plasmon resonance formed only from electrons of Ag, because silver modified TiO2 cannot decrease band gap energy, and visible light simulator cannot excite electrons from valence band to conduction band In contrast, if solar simulator or solar light is employed, electrons can absorb UV radiation emitted from this light source to jump from valence band to conduction band Therefore electron density in Ag particles is higher [25] Now, surface plasmon resonance is formed on catalyst surface, resulting in the increase of photocatalytic activity Surface plasmon resonance deviates photon directions, making them rebound and come back to the material This enhances the ability of Table Phenol degradation with and without catalyst in presence of H2O2 Time (min) 20 40 60 90 Removal efficiency (%) H2O2 (4.72 Â 10À2M)/Ag-TiO2/Bent H2O2 (4.72 Â 10À2M) 58.75 75.87 96.67 99.96 6.78 11.78 13.21 14.84 D.B Nguyen et al / Journal of Industrial and Engineering Chemistry 18 (2012) 1764–1767 1767 Table Phenol and COD removal efficiency under different irradiation with H2O2 addition Time (min) Removal efficiency (%) In the dark 10 20 30 60 90 120 150 Natural solar light COD Phenol COD Phenol COD 8.71 9.08 9.82 9.82 9.82 9.82 10.12 1.87 1.92 1.92 1.92 1.92 1.92 1.93 25.23 51.71 63.96 93.42 98.20 – – 4.32 20.34 31.12 50.13 63.24 92.02 – 33.19 69.41 86.92 98.94 – – – 6.07 26.31 44.53 73.88 99.00 – – Table Catalytic activity of TiO2/Bent and Ag-TiO2/Bent on phenol degradation Time (min) 10 20 30 60 Removal efficiency (%) TiO2/Bent Ag-TiO2/Bent 18.05 27.24 32.48 35.41 33.19 69.41 86.92 98.94 Table Phenol degradation with different light sources Time (min) 10 20 30 60 Solar simulator Phenol Conversion (%) Vis simulator Solar simulator 9.30 12.90 29.96 30.02 33.19 69.41 86.92 98.94 absorbing radiation in visible region Therefore, surface plasmon resonance is the reason why photocatalytic efficiency of TiO2 under solar light increases TiO2 modified by Ag not only increases light absorption ability but also (i) enhances the formation of free radicals, and (ii) lowers recombination rate of electrons and holes due to the agglomeration of Agn on TiO2 Although there are many researches on silver modified titania [26–29], the exact mechanism of catalysis and role of Ag is still in debate We hope that our idea can reasonably give a hand to elucidate the role of the catalyst in degrading organic compounds, in general, and phenol, in particular Conclusions Ag-TiO2/Bent 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Basing on characterization of Ag-TiO2, Ag-TiO2/Bent and test on catalytic activity of these materials, as well as reference materials about oxidation of organic compounds using Ag-TiO2 catalyst,... recombination rate of electrons and holes due to the agglomeration of Agn on TiO2 Although there are many researches on silver modified titania [26–29], the exact mechanism of catalysis and role of. .. of photocatalytic activity Surface plasmon resonance deviates photon directions, making them rebound and come back to the material This enhances the ability of Table Phenol degradation with and