untitled Photocat using a z Venkatesha Satyanaray a Department of C b Department of C *Corresponding a Tel.: +91.080.2360 ARTICLE INFO Received: 26 Nov Received in revis Accepted: 12 Feb Online: 30 June 2 KEYWORDS ZnO‐Ce2O3 Photocatalyst Nanomaterial Methylene blue Gel‐combustion Water treatment 1. Introductio Discharge major source world countri either adsorpt only augment from the liqui treatment and alternative t mediated pho for complete any secondary Use of n dioxide (TiO2 cadmium sulp organic pollut the past two d have attracted many organic TiO2 (3.2 eV) quantum effic 12]. Cerium o (4.1 eV) [13], h Metal oxide suspended me The effect loading on the oxide and zinc various group zinc oxide cat Yellow. A max under optimal talytic deg zinc oxide am Vuppala yana Suggala Chemical Engineerin Chemical Engineerin uthor at: Departme 06934; fax: +91.080 ORMATION vember 2011 ed form: 03 Febru ruary 2012 012 on of industrial of environmen ies [1]. Conven tion or chemic the problem s id to the solid d may result i reatment stra otocatalytic oxid mineralization y pollution. nanoparticle se 2), zinc oxide phide (CdS) in tants in water a decades [1,3‐8] d much attentio pollutants [9, 1 ) as it has sim ciency and high oxide, a semicon has also been c coupling can etal oxide partic t of various pa e degradation o c oxide doped w ps [14‐15]. Cha talyst to study ximum degrada l condition (pH Eu ISSN 2153‐ Europ J gradation ‐cerium o a, Madhu Ga a Venkatab a ng, M.S. Ramaiah In ng, Jawaharlal Neh ent of Chemical Eng 0.23603124. E‐mai ary 2012 dye‐containin ntal problems, ntional treatme al coagulation; ince they mere phase which n in secondary p ategies such dation can be a of organic dy emiconductors (ZnO), iron o the photocatal and air have at ]. In particular on with respec 10]. ZnO is a su milar bandgap her photocataly nducting mater considered as a increase the cles to the visib rameters such of methylene b with other met krabarti and D the degradatio ation of 60% w = 9.7, catalyst uropean Journal Europe 2249 (Print) / IS DOI:10.5155 pean Jo Journal home n of methy oxide cata attumane Mo and Preetham nstitute of Technolo hru Technological U gineering, M.S. Ram il address: madhug ABSTRACT The photocataly source in the p synthesized by electron micro catalyst was fou pH (4.0‐9.2) an batch reactor. experimental p catalyst was cal pH = 9.2. g wastewater especially in nt methods inv but these met ely transfer the necessitates fu pollution [2]. T as semicondu applied success yes without cau such as tita oxide (Fe2O3), lytic degradatio ttracted attenti r, ZnO nanopart ct to degradatio uitable alternati energy [6], la ytic efficiency [ rial with a ban good photocat responsivenes le and UV light. as pH and cat lue (MB) using tals were studie Dutta [16] empl on of MB and E was observed fo loading 1.0 g/L of Chemistry 3 ( ean Journal of Ch SSN 2153‐2257 5/eurjchem.3.2. ournal o epage: www.e ylene blue alyst otappaa,*, m Halugond ogy, Bangalore, 56 University, Anantap maiah Institute of T m_2000@yahoo.co ytic degradation presence of zinc y a gel combusti oscopy, and ene und to be in betw nd dye concentr The degradat arameters. Appr lcined before us is a third volve thods dyes rther Thus, uctor sfully using nium and on of on in ticles on of ive to arger [3,11‐ ndgap talyst. ss of . talyst g zinc ed by loyed Eosin or MB L, dye conc 6.13 palla indu aque Mn‐d [20] visib phot cond In r sem phot al. s TiO2 assis with enha effic the cons degr dest supr whe majo show whe (2) (2012) 191‐1 hemistry (Online)  2012 191‐195.564 of Chem eurjchem.com e danahalli Sad 0054, India pur, 515002, India Technology, Bangal o (M.G. Motappa). n of methylene b c oxide‐cerium o on technique an ergy dispersive ween 45 to 60 n ration (5.0‐20.0 tion rate was reciable degrada e. Best results w centration of 5 31/min). The adium (Pd) dop uced degradati eous solutions doped ZnO [19 were also used ble light. From tocatalytic rea ditions and the recent years i iconductors wi tocatalytic activ studied photoca 2 (CeO2‐TiO2) sted method, w h high recomb ance photocata ciency leads to t catalyst substr sidered the ma radation proces truction of orga rabandgap phot ere OH radicals or role in pho wn in equations → ere h hole a →  195 2 EURJCHEM mistry m dashivaiaha lore, 560054, India blue in aqueous s oxide (ZnO‐Ce2O nd characterized X‐ray spectros nm. The effects o mg/L) on the found to be ation of methyle were observed w 0.0 mg/L, UV l photocatalytic ped ZnO [17] w ion of methan in a photocata ], thulium ion ( d in photocatal m these report action depend power of UV la it has been v ith metals or m vity of these se atalytic degrad nanoparticles which shows t bination rate alytic activity [ the generation o ate interface. T ain species inv ss. The general anic compounds tons, and conti s formed on the otodegradation s 1‐4. and e electro . solution was stu O3) as photocata d by X‐ray diffra copy. The part of catalyst loadin degradation we strongly depen ene blue was ach with a catalyst loa lamp 16 W an degradation was applied as c nol, glucose an alytic reactor. (Tm3+)‐doped n lytic degradatio ts, it can be c ds on the de amp and the nat verified that t metallic oxides miconductors [ dation MB usin s synthesized that coupling a to a suitable 22]. The high of more electro These electrons volved in the scheme of the s begins with it nues through r e photocatalyst n [23]. The m on udied using a UV alyst, which was action, scanning icle size of the ng (1.0‐8.0 g/L), ere studied in a ndent on these hieved when the ading of 5 g/L at d air flow rate of dye using catalyst for UV‐ nd sucrose in ZnO/CdO [18], nanometer ZnO on of MB under concluded that esign, working ture of catalyst. the doping of s increases the [21]. Magesh et ng Ce modified by polymer a photocatalyst material will UV absorption ons and holes at s and holes are photocatalytic e photocatalytic ts excitation by redox reactions t surface, play a echanisms are (1) (2) V s g e , a e e t e g ‐ n , O r t g . f e t d r t l n t e c c y s a e 192 Vuppala et al. / European Journal of Chemistry 3 (2) (2012) 191‐195 →  (3) →  (4) The objective of the present study involves the preparation, characterization and application of zinc oxide coupled cerium oxide catalyst for the degradation of MB. Synthesis of the catalyst was carried out by self‐ignition gel combustion method and its application in photo catalytic degradation of MB dye was explored. The effect of catalyst loading, concentration of the dye and pH on the photo catalytic reaction was studied in detail. 2. Experimental 2.1. Instrumentation To study the surface morphology and elemental analysis of the photocatalyst, scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDAX) were carried out (ESEM Quanta 200, FEI, USA). X‐ray diffraction study of the catalyst was carried out (Bruker D2 Phaser, USA) at 2θ = 5‐60 o, step size of 0.02 o with a time step 0.5 sec. Methylene blue concentration was estimated using UV‐visible (Systronics‐117, India) spectrophotometer at wavelength of 663.2 nm. Quartz cuvette of 10 mm path length was used. 2.2. Synthesis of ZnO‐Ce2O3 and characterization Synthesis of ZnO‐Ce2O3 was carried out by gel combustion process. Zinc nitrate hexahydrate (Merck, >96.0%) and cerium (III) nitrate (Himedia, >99.0%) were used with glycine as fuel. Suitable quantities of zinc nitrate, cerium nitrate and glycine were dissolved in 125 mL distilled water, so that the fuel to nitrate molar ratio was maintained as 0.24. The reaction mixture was placed on a hot plate. As the heating progressed, water vapour and nitrates (nitric gases) were released during heating resulting in the formation of gel. The reaction was completed by self‐ignition (combustion), minutes after the formation of the gel, leaving behind golden yellow powder. The catalyst was then calcined at 400 oC. The synthesized powder (ZnO‐Ce2O3) was characterized by SEM, X‐ray diffraction (XRD), and (EDAX) to study the surface morphology, size, and composition. 2.3. Evaluation of photocatalytic activity The reaction set up consisted of a static batch photo reactor of 800 mL capacity, (cylindrical glass flask) open to air. The glass flask was kept on a magnetic stirrer which ensured oxygenation, and uniform mixing of the reaction mixture. Irradiation was assured by artificial light, using two UV lamps of 8 W (Philips), positioned directly above the liquid surface. The distance between the lamp and base of the beaker was 26 cm and the depth of liquid in the beaker was 2.5 cm. Reaction temperature was maintained at ambient conditions (27 ± 3 oC) for all experimental trials. Each experimental run was carried out for 120 min. The concentration of the dye in the reaction mixture was measured at regular interval by measuring the absorbance of the aliquot solution using the UV‐visible (Systronics‐117) spectrophotometer (at 663.2 nm) with de‐ ionized water as reference. 3. Results and discussion 3.1. SEM and XRD analysis Figure 1 shows the SEM pictures of ZnO‐Ce2O3 particles produced by the gel combustion technique. It shows two different size of particles agglomerated to a certain extent. The small sized spherical particles of cerium oxide were well‐ dispersed on the pellet type structured zinc oxide. XRD of as‐ prepared ZnO‐Ce2O3 is represented in Figure 2a and XRD of 400 oC calcined ZnO‐Ce2O3 represented in Figure 2b. The peaks in the Figure 2b match with the ZnO and Ce2O3 standards. From JCPDS data, Powder Diffraction File. Card no: 89‐8435 [24] and Powder Diffraction File. Card no: 36‐1451 [25], we can confirm the presence of Ce2O3 and ZnO (Wurtzite phase). As‐prepared ZnO‐Ce2O3 XRD shows amorphous morphology and calcined at 400 oC showed crystalline morphology having broad peaks. A crystal size of the sample was calculated by Scherrer’s formula (d = 0.94 λ/β cosθ) and was found to be 45‐60 nm. Figure 1. SEM image of ZnO‐Ce2O3 (magnification 100000X). (a) (b) Figure 2. (a) XRD patterns of ZnO‐Ce2O3 (without calcined), (b). XRD patterns of calcined (400 oC) ZnO‐Ce2O3. Vuppala et al. / European Journal of Chemistry 3 (2) (2012) 191‐195 193 3.2. EDAX analysis EDAX spectrum of ZnO‐Ce2O3 (Figure 3) shows the peaks for zinc, cerium and oxygen elements indicating that the coupled catalyst is made up of only these elements. Peak indexing of the elements are oxygen 0.525 keV, zinc 8.63 keV, and cerium 5.01 keV. The compositions in mass percentage of the elements are zinc 58.90%, cerium 24.34 %, and oxygen 16.70%. The observed composition matches with the theoretically calculated composition. Figure 3. EDAX spectrum of calcined ZnO‐Ce2O3. 3.3. Photocatalytic activity 3.3.1. Effect of catalyst loading In order to study the photocatalytic property of the ZnO‐ Ce2O3, experiments were conducted using as‐synthesized and calcined catalysts. The effect of the photocatalyst loading, dye concentration and pH on degradation was studied. Pseudo first order kinetic model was used to determine the kinetics of the reaction. To determine the degradation, if any in the absence of the catalyst, test experiment was conducted and it was observed that no degradation occurred. To find the effect of adsorption on solid catalyst, the catalyst was dispersed in the solution and kept in the dark for 60 minutes with stirring, negligible adsorption of the dye was observed. To determine the optimal amount of the photocatalyst loading, the experiments were carried out by varying catalyst loadings for as‐prepared catalyst and calcined catalyst at pH = 7. Initial degradation experiments were conducted using as‐ prepared (uncalcined) catalyst for catalyst loading (1.0 to 5.0 g/L). The plot of C/CO as a function of irradiation time was represented in Figure 4. C is the concentration of dye in the reaction mixture (mg/L) and CO is the initial concentration of the dye (mg/L). Maximum degradation of 37% was achieved for catalyst loading of 5.0 g/L for 2 h. From Figure 2a we can observe that XRD of as‐prepared ZnO‐Ce2O3 is amorphous in nature and exhibits very low photocatalytic activity. The amorphous structure leads to the recombination of the photo‐ generated electrons and holes [26], which in turn reduces the photocatalytic activity. The other possible reason is the presence of trace amounts of nitrate in the catalyst, which may affect the generation of hydroxyl ions which in turn suppress degradation rates. For calcined ZnO‐Ce2O3, experiments were conducted using varying catalyst loading of 1.0 to 8.0 g/L. The plot of C/CO as a function of irradiation time is represented in Figure 5. It was found that after 2 h, 90% degradation was achieved for catalyst loading of 5.0 g/L at neutral pH. The degradation achieved by calcined ZnO‐Ce2O3 showed better photocatalytic activity when compared with the uncalcined catalyst. The catalytic degradation MB is better than degradation reported earlier for 10 mg/L dye concentration using different photocatalyst [7,13,23]. The decrease in the rate above 5.0 g/L for calcined catalyst loading is perhaps due to light scattering caused by the suspended catalyst [27‐29]. The phenomenon may be explained as follows: with increase in the catalyst loading the incident light penetration to the substrate catalyst interface becomes difficult. Increase in the catalyst concentration may decrease the photoabsorption, which in turn reduces the dye adsorption thus reducing MB degradation. The optimal catalyst loading depends on various factors such as the geometry of the reactor, type of the catalyst, the working condition and incident radiation reflux [27,30‐31]. In order to study the degradation kinetics a pseudo first order kinetic model was used .The general rate equation is given as: *. . OH dC k C C dt   (5) where C represents the methylene blue concentration and COH* the hydroxyl radical concentration. By the pseudo‐stationary hypothesis (i.e. the COH* can be considered to be constant because of the continuous hydroxyl ion generation by photocatalysis and the rate depends only on methylene blue concentration), hence the rate expression (5) can be written as (Equation 6) ln . O C k t C        (6) The linear plot of       C COln versus t gives the rate constant, k (slope). Figure 4. Effect of catalyst loading (without calcined) on the fractional degradation of MB at pH = 7. The degradation rate data obtained for different catalyst loading were plotted using pseudo first order kinetic model, the fairness of the fit is indicated by the fact that linear regression (r2) values are greater than 0.9. Therefore the model is in good agreement with the experimental data. The values of the first order rate constant and r2 are indicated in Table 1. 3.3.2. Effect of concentration Experiments were conducted to study the effect of dye concentration at constant loading of catalyst 5.0 g/L and pH = 7. The dye concentration was varied from 5.0 to 20 mg/L. An increase in the dye concentration leads to decrease in the rate of its degradation. Plot of C/CO versus time is represented in Figure 6. 194 Vuppala et al. / European Journal of Chemistry 3 (2) (2012) 191‐195 Table 1. Pseudo‐first‐order kinetic parameters for degradation of MB at different catalyst loadings (10 mg/L dye concentration, pH = 7). Catalyst loading (g/L) k (1/min) r2 1 0.008 0.954 2 0.009 0.994 3 0.013 0.972 4 0.013 0.990 5 0.017 0.980 6 0.015 0.997 8 0.016 0.990 Figure 5. Effect of catalyst loading (calcined) on the photocatalytic degradation of MB at pH = 7. Figure 6. Effect of dye concentration on degradation of MB (5.0 g/L catalyst loading at pH =7). The degradation rate data obtained for different concentrations of dye was plotted using pseudo‐first order kinetic model. The data perfectly fits pseudo‐first order kinetics and rate constant data is tabulated in Table 2. k = 0.022 min‐1 was found to be maximum for dye concentration of 5.0 mg/L. MB degradation decreased as initial concentration increased. A possible reason should be the generation of hydroxyl radicals on the catalyst surface is reduced when the initial MB concentration is increased [32]. More the number of MB molecules adsorbed on the surface of ZnO‐Ce2O3 photocatalyst will mean fewer active sites are available for the hydroxyl radical adsorption. Once the MB concentration is increased, most of UV light is absorbed by the MB molecules [33], and photons do not reach the surface of photocatalyst to activate it to generate hydroxyl radicals. Table 2. Pseudo‐first‐order kinetic parameters for degradation of MB at different initial dye concentration (5.0 g/L catalyst loading, pH = 7). Concentration (mg/ L) k (1/min) r2 5 0.022 0.988 10 0.017 0.981 15 0.012 0.989 20 0.007 0.997 3.3.3. Effect of pH The effect of pH on the photocatalytic activity is considered an important parameter as most industrial waste streams are at different pH. In semiconductor mediated photocatalytic reactions, pH is an important parameter because the amphoteric behavior of the semiconducting particles influences the dispersion (suspension) of the catalyst in the substrate. In finely dispersed suspension, the catalytic activity was considered to be very high. To study the pH effect, experiments were carried out at three different pH, acidic (pH = 4), neutral (pH = 7) and basic (pH = 9.2). The MB concentration was maintained constant at 10 mg/L and catalyst loading was maintained at 5.0 g/L. The degradation of MB increased with increase in pH, as presented in Figure 7. The maximum degradation was observed at pH = 9.2. The efficiency at higher pH can be explained on the basis of zero point charge of ZnO‐ Ce2O3.The zero point charge is known as the pH value at which the concentration of protonated and deprotonated surface groups are equal. The zero point charge of zinc oxide is 9 and cerium oxide is around 7 to 9 [13]. When the pH is high, the catalyst surface is negatively charged by adsorbing hydroxyl ions, which favors the formation of hydroxyl radicals. Under acidic medium, the catalyst surface is preferentially covered by dye molecules. Therefore, increase in the pH value generates large hydroxyl ions, which increases the degradation rate. The degradation rate data obtained for different pH were plotted using pseudo first order kinetic model, the rate constants and r2 values were tabulated in Table 3. k = 0.021 min‐1 was found to be maximum at pH = 9.2. Table 3. Pseudo‐first‐order kinetic parameters for degradation of MB at different pH (10 mg/L dye concentration, 5 g/L catalyst loading). pH k (1/min) r2 4.0 0.000 0.934 7.0 0.017 0.981 9.2 0.021 0.992 Figure 7. Effect of pH on fractional degradation of MB (10 mg/L dye concentration at 5.0 g/L catalyst loading). 4. Conclusion Heterogeneous photocatalysis has proved to be very effective in the removal of toxic organic pollutants from the aqueous streams owing its ability to convert them into innocuous products such as carbon dioxide and water. For the first time, an attempt has been made to synthesize ZnO‐Ce2O3 coupled catalyst by gel combustion technique and its photocatalytic activity explored. Characterization was carried out using SEM, XRD and EDAX. From SEM analysis, it was clearly evident that cerium oxide was coupled (deposited) on zinc oxide. Vuppala et al. / European Journal of Chemistry 3 (2) (2012) 191‐195 195 The EDAX shows the purity of the coupled catalyst and from XRD, we can observe that the particles are crystalline and nano in size. The photocatalytic property of the calcined catalyst was found to be more effective when compared to as‐ prepared catalyst. The effect of catalyst loading, pH and dye concentration on photocatalytic activity was studied. A maximum degradation of 95% was achieved for a catalyst loading of 5.0 g/L at pH = 9.2. The degradation rate decreased, with increase in dye concentration. A pseudo first order kinetic model was fitted for all the experimental runs and they are in good agreement with the proposed model. Acknowledgements The authors thank the Department of Chemical Engineering, M.S. 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