Photocatalytic Degradation of Nitrobenzene in an Aqueous System by Transition-Metal-Exchanged ETS-10 Zeolites

Photocatalytic Degradation of Nitrobenzene in an Aqueous System byTransition-Metal-Exchanged ETS-10 Zeolites

Praveen K. Surolia, Rajesh J. Tayade, and Raksh V. Jasra*,†

Discipline of Inorganic Materials and Catalysis, Central Salt & Marine Chemicals Research Institute, Councilof Scientific & Industrial Research (CSIR), G. B. Marg, BhaVnagar-364021, India

Engelhard titanosilicate ETS-10 was synthesized using Degussa P25 as the titanium source. Metal-exchangedmicroporous titanium silicate M-ETS-10 (M ) Fe, Co, Ni, Cu, and Ag) samples were prepared by ionexchange with respective metal salt solutions. The synthesized samples were characterized by powder X-raydiffraction, BET surface area, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), andUV-vis diffuse reflectance measurements (DRS). The photocatalytic activity of these samples was investigatedfor the decomposition of nitrobenzene (NB). The pristine ETS-10 and transition-metal-ion-exchanged ETS-10 samples were found to be active photocatalysts that can decompose nitrobenzene under UV irradiation.Results demonstrated that silver-ion-exchanged ETS-10 shows the highest photocatalytic activity for degradationand mineralization of nitrobenzene.

1. Introduction

The photocatalytic degradation of organic compounds presentinaqueousmediahasattractedtheattentionofmanyresearchers,1-9

and TiO2 both in powder and supported form is one of the mostwidely reported photocatalysts for this purpose. TiO2 hasbeen supported on silica,10,11 zeolites,12-14 fiber glass,15,16 andelectrodes,17 promising to avoid catalyst loss during use.However, the recovery of the catalysts after use is still achallenge even in case of supported photocatalysts. The titano-silicate ETS-10 has recently attracted attention as a photocatalystas it has built-in photocatalytically active TiO2 units. Thisstructural feature makes it an attractive material for photoca-talysis, as it will not have the leaching problem encounteredwith supported TiO2. In ETS-10, each Ti atom is octahedrallyconnected to four Si and two Ti atoms through oxygen bridges.This arrangement of Ti and Si atoms generates a three-dimensional structure which generally contains a considerabledegree of disorder and has small 7-membered and 12-memberedrings.18 ETS-10 contains chains of active -O-Ti(IV)-O- thatexhibit quantum confinement effects and behave as one-dimensional semiconductor nanowires in its framework.19-21

The TiO6 octahedra form linear chains running in two perpen-dicular directions of the crystal and are isolated from one anotherby the siliceous matrix. Thus generated one dimensionallyconfined atomic Ti-O-Ti-O-Ti wires give rise to the peculiaroptical and electronic properties of the material.22 This structuralfeature can help to overcome the problem of leaching encoun-tered in supported photocatalysts. Consequently, research effortsare in progress to enhance photocatalytic activity of ETS-10.ETS-10 and its ion-exchanged isomorphs have shown applica-tions in adsorption,17 photochemistry,23,24 acylation of alco-hols,25 and oxidative dehydrogenation of alcohols.26 Further-more, ETS-10 has been studied as a catalyst for photodegradationof alcohols1 and also for shape selective photocatalytic trans-formation of phenols in an aqueous medium.27 As ETS-10 hashigh cation exchange capacity, transition and alkali-metal-ion-exchanged ETS-10 has been studied for various catalytic

applications. Co-ETS-10 and Ag-ETS-10 has been reportedto show the photocatalytic activity to decompose acetaldehyde.28

The apparent semiconducting properties of ETS-10 and itsability to bring reactant molecules in close proximity to thesemiconductor chains in ETS-10 allow efficient interception ofphotoproduced holes and electrons by the adsorbed species.These interesting attributes of ETS-10 prompted us to investigateits potential as photocatalyst. It was further shown that the ionexchange may cause the changes in the electronic properties ofthe pristine ETS-10 material.29 The addition of transition metalions offers a way to trap the charge carrier and extend thelifetime of one or both of the charge carriers. It is known thatthe ions to be exchanged must have a work function higherthan that of the semiconductor providing a Schottky barrier.14

In view of this, we have investigated the effect of ion exchangeon the photoreactivity of ETS-10 zeolite. We chose the lowpercentage (0.5 wt %) of transition metal ion (Fe, Co, Ni, Cuand Ag) exchange, as this has shown promise for the improve-ment in photocatalytic activity.30,31

Photocatalytic activity of these photocatalysts for nitrobenzenedegradation under the UV-light irradiation was investigated. Themineralization of nitrobenzene in aqueous solution was con-firmed by chemical oxygen demand (COD) analysis.

2. Experimental Section

2.1. Chemicals and Materials. Titanium dioxide (P25) waspurchased from Degussa Corporation (Degussa AG, Frankfurt,Germany). Sodium silicate was procured from Aquagel, India.NaCl, KCl, nitrobenzene (NB), and metal nitrates (AR grade)were purchased from S.D. Fine-Chem Ltd., India. Deionizeddistilled water was used to make up the reaction mixture.

2.2. Synthesis of the Catalysts. The synthesis of ETS-10samples was as per the procedure reported by Yang et al.32

Typically, 20 g of sodium silicate solution (24 wt % SiO2, 8 wt% Na2O) was diluted with 70 mL of water and 13.8 g of NaCl;2.6 g of KCl was dissolved in this solution. In the resultanttranslucent thick gel, 2.6 g of Degussa P25 TiO2 was addedunder vigorous stirring. The slurry was stirred for 30 min atroom temperature and transferred to a Teflon-lined autoclaveand heated statically at 573 K for 42 h. The starting gel had thepH around 10.83. The solid product was isolated from mother

* To whom correspondence should be addressed. E-mail:[email protected]; [email protected]. Tel.: +91 265 6693935.Fax: +91 265 6693934.

† Present address: Reliance Technology Group, Vadodara-391346,Reliance Industries Limited, Gujarat, India.

Ind. Eng. Chem. Res. 2010, 49, 3961–3966 3961

10.1021/ie901603k 2010 American Chemical SocietyPublished on Web 03/19/2010

liquor by filtration, washed with distilled water, dried overnightat 333 K, and then calcined at 723 K for 5 h in air. The thusobtained ETS-10 had the stoichiometry of (Na,K)2TiSi5O13.

For preparation of the transition-metal-ion-exchanged ETS-10 samples, the weighed amount of ETS-10 (2 g) was stirredfor 24 h with the metal salt solution in 100 mL of water of theconcentration calculated for 0.5% (w/w) metal ion exchange atroom temperature. During the ion exchange, the alkali metalions are replaced by transition metal ion from ETS-10 givingrise to composition (Na,K)2sxMyTiSi5O13 (M ) transition metalion). After the ion exchange, the samples were washed withdistilled water and dried at 393 K for 12 h. The thus obtainedsamples were named as M-ETS-10, where M represents thetransition metal. Some of the catalyst samples were observedto develop color after ion exchange as mentioned in Table 1.

2.3. UV Irradiation Experiments. The photocatalytic activ-ity of the catalysts was determined for nitrobenzene degradationunder UV irradiation using a reactor consisting of two parts asdescribed earlier.33 The reactor consists of inner quartz doublewall jacket with inlet and outlet facility for water circulation tomaintain the temperature of reaction mixture throughout thereaction time. The center of the jacket has an empty chamberfor immersion of a mercury vapor lamp. Photocatalytic reactiontakes place in the outer borosilicate glass container having avolume of 250 mL after insertion of the quartz inner part. Themercury vapor lamp of 125 W was used for UV irradiation(Crompton Greaves Ltd. India). The magnetic stirrer was keptbelow the reactor for continuous stirring. A 5 mL portion ofthe reaction mixture was withdrawn from the port by syringeat different time intervals. Photocatalytic activity was measuredat the pH of the initial reaction mixture, and temperature wasmaintained at 293 K for all the reactions. The photocatalyticactivity of the catalysts was evaluated by measuring the decreasein concentration of nitrobenzene in the reaction solution usingUV-vis spectroscopy. Standard samples of nitrobenzene of 5,10, 20, 30, 40, and 50 ppm were prepared, and the calibrationcurve was plotted for nitrobenzene solutions by measuringabsorption of these samples. The slope of the curve wascalculated and used to determine the nitrobenzene concentrationfrom absorbance values employing the Beer-Lambert’s lawfrom the absorption data.

Prior to commencing irradiation, a suspension containing 50mg of the catalyst and a 250 mL aqueous solution of ca. 50ppm of nitrobenzene was stirred continuously for 30 min inthe dark. An aliquot was then taken for analysis. Following this,the solution was irradiated, and samples were withdrawn foranalysis by syringe at intervals of 10 min for the first hour andthen every hour thereafter for 4 h. The catalyst was separatedfrom the samples by centrifugation prior to analysis. Theabsorbance was measured at λmax ) 267 nm for nitrobenzene.The amount of TiO2 in the ETS-10 catalyst calculated as perits stoichiometry was 17%. Thus the amount of photocatalyticactive part (-O-Ti(IV)-O-) of the prepared catalyst in 50 mgis 8.5 mg.

Adsorption studies in the dark were performed separatelyusing an aqueous 50 ppm solution of nitrobenzene for all the

catalysts for 1 h at the reaction conditions used for measuringphotocatalysis, to find out the decrease in the concentration dueto the adsorption. The decrease in concentration was observedby using UV-vis spectroscopy. The mineralization of nitroben-zene in aqueous solution was confirmed by COD analysis ofthe samples taken after different reaction time intervals by usinga HACH DR 2800 photometer.

2.4. Catalyst Characterization. The X-ray diffraction datawere collected on a Phillips X’pert MPD system using Cu KR1(λ ) 0.15405 nm) radiation at 295 K. Diffraction patterns weretaken in 10°- 80° 2θ range at the scan speed of 0.1 deg sec-1.The FT-IR spectroscopic measurements were carried out usingPerkin-Elmer GX spectrophotometer in the range 400-4000cm-1 with a resolution of 4 cm-1 as KBr pellets. TheBrunauer-Emmett-Teller (BET) surface area of the catalystswas obtained from nitrogen adsorption data at 77.4 K in therelative pressure range of 0.05-0.25 using Micrometrics ASAP2010 volumetric adsorption apparatus.

The band gap energy of the catalysts was determined usingdiffuse reflectance spectroscopy (DRS) at room temperature inthe wavelength range of 250-700 nm. The spectrophotometer(Shimadzu UV-3101PC) was equipped with an integratingsphere and BaSO4 was used as a reference. The band gap energyof the catalysts was calculated according to the equation,

where EG is the band gap energy (eV), h is Planck’s constant,c is the light velocity (m/s), and λ is the wavelength (nm).

Scanning electron microscope (Leo series 1430 VP) equippedwith INCA, energy dispersive system (EDX), Oxford Instru-ments, was used to confirm the presence of metal ions inmicroporous ETS-10 samples as well as to determine themorphology of catalysts. The sample powder was supported onaluminum stubs prior to measurement. An inductively coupledplasma-optical emission spectrophotometer (Optima 2000 DV,Perkin-Elmer, Eden Prarie, MN) was used to determine thepercentage of metal ion present in the catalyst. Thermogravim-etry and differential thermal analysis were performed on a TGA/SDTA851e, Mettler Toledo system in a temperature range of50-700 °C, with a heating rate of 5 °C min-1 under an airstream.

3. Results and Discussion

3.1. Structural, Textural, and Electronic Properties. Thepowder XRD patterns of pristine and ion-exchanged ETS-10samples are shown in Figure 1. The pattern of ion-exchangedsamples is similar to the parent ETS-10. This implies that thestructure of ETS-10 remains intact after metal ion exchange.The absence of any peak due to exchanged metal ion could bebecause of a very small amount of ion exchange. The XRDpattern exhibits a high degree of disorder in the ETS-10structure.34

The broad absorbance peak having values at around 3590and 3350 cm-1 seen in FT-IR spectra are due to stretchingvibrations, whereas absorbance at 1650 cm-1 is due to the

Table 1. Structural and Electronic Properties of the Catalysts

catalyst BET surface area (m2/g) pore volume (cm3/g) (Na+K)/Ti % metal (EDX) % metal (ICP) band gap (eV)

ETS-10 232 0.134 1.49 3.15Fe-ETS-10 191 0.125 1.30 0.53 0.55 3.13Co-ETS-10 203 0.125 1.39 0.63 0.68 3.12Ni-ETS-10 201 0.119 1.35 0.53 0.59 3.14Cu-ETS-10 216 0.124 1.30 0.61 0.65 3.15Ag-ETS-10 205 0.116 1.37 0.47 0.41 3.16

band gap (EG) ) hc/λ (1)

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bending vibration of water molecules and OH groups presentat the surface of the prepared catalysts.35,36 The absorbance at1136 cm-1 is attributed to asymmetric stretching vibrations ofSi-O-Si bridges, while the peak at 1020 cm-1 is assigned toSi-O-Ti stretching vibrations. The stretching and bendingvibrations of Ti-O-Ti give rise to the absorption peaks at 750and 661 cm-1, respectively. The comparison of the IR spectraof pristine ETS-10 and transition-metal-exchanged ETS-10samples showed that there is an additional peak at around 1386cm-1 in the IR spectra of transition-metal-exchanged ETS-10which is because of vibrations of H+ ion.37 The IR absorbancespectra (Figure 2) of the samples demonstrate that the exchangeof Na ions by different transition metal cations does not affectthe framework and the presence of water molecules at thesurface, probably due to the small amount, only 0.5 wt % ofmetal ion exchange, taken for the study.

The presence of transition metal ions in different M-ETS-10 samples could be seen from the UV-vis diffuse reflec-tance spectra (Figure 3). The comparison of diffuse reflec-tance spectra of pristine and ion-exchanged catalysts showeda change in the band gap due to ion exchange (Table 1).From the data, it is evident that all the ion-exchanged

catalysts have extended a red shift, except the silver-dopedETS-10 sample, where slight blue shift of 2 nm was observed.The highest red shift of 3.5 nm was observed with the cobaltmetal ion exchange.

The surface areas of ion-exchanged catalysts were found todecrease (Table 1) compared to pristine ETS-10. The decreasewas observed to vary from 16 m2/g for the Cu-ETS-10 catalystto a high of 41 m2/g for Fe-ETS-10. This could be due to thepore plugging of the starting material by ion exchange.

The SEM micrographs (Supporting Information) of thecatalysts show identical morphology of a truncated bipyramidalas reported earlier.18,32 The largest dimension of the crystal isalong the 4-fold axis.

The EDAX as well as ICP analysis confirms the presence oftransition metal ions in these samples. The ratio of Na and Kto Ti was observed to decrease after metal ion exchange (Table1), confirming the metal ion exchange of the Na and K ions.

The weight loss observed by TGA analyses varied from5-13%, with a high of 13% for Fe-ETS-10 and 5% forAg-ETS-10. Other samples showed similar weight loss valuesof 7-9%. This weight loss is attributed to loss of moisturepresent in the samples.18

3.2. Photocatalytic Activity. The spectral changes in absorp-tion spectra of nitrobenzene in Ag-ETS-10 under UV-lightirradiation were observed at 267 nm wavelength at whichnitrobenzene shows maximum absorbance. The absorbance fordifferent samples at this λmax was observed, and the concentra-tion of the nitrobenzene in each respective sample was calculatedfrom the absorbance using the Beer-Lambert’s law. Thepercentage degradation of nitrobenzene after 4 h reaction isshown in Figure 4. Comparison of the results from this figureshows that transition-metal-ion-exchanged ETS-10 samplesexhibit higher activity than pristine ETS-10. The percentagedegradation after 4 h reaction was 33, 41, 38, 39, 47, and 57%for pristine, Fe, Co, Ni, Cu, and Ag ion-exchanged ETS-10,respectively. Moreover, Ag-ETS-10 catalyst showed the highestphotocatalytic photodegradation (57%) compared to all othertransition-metal-impregnated ETS-10 catalysts. The photocata-lytic active sites are titanols located on the external surfaceswhere the -O-Ti-O-Ti-O- chain emerges, exposing asurface Ti-OH titanol group.38

Figure 1. The peak for TiO2 at 2θ ) 25.3 XRD patterns of ETS-10 andmetal-ion-exchanged ETS-10 catalysts.

Figure 2. FT-IR spectra of ETS-10 and metal-ion-exchanged ETS-10catalysts.

Figure 3. Diffuse reflectance UV-vis spectra of ETS-10 and metal-ion-exchanged ETS-10 catalysts.

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Figure 5 shows the ln Co/Ct vs time dependence. The straightline in Figure 5 confirms the pseudo-first-order kinetics whichfollows the equation

Here Co is the initial concentration of nitrobenzene, while Ct

is the concentration at time “t”. The apparent first rate constantkapp was calculated by the linear regression of the slope of ln Co/Ct vs time plot up to first 60 min. Thus the photocatalyticdegradation process follows the Langmuir-Hinshelwood (L-H)law. The initial rates and apparent rate constants of photodeg-radation calculated for all catalyzed reactions are shown in Table2. From initial rate, it can be concluded that M-ETS-10 arebetter catalysts than ETS-10 as such, and Ag-ETS-10 isobserved to be the best catalyst among studied metal-ion-exchanged ETS-10 samples. The percentage decrease in con-

centration due to adsorption in the dark was found in the rangeof 1.9-7.3%. This decrease was observed to be 7.3, 2.9, 5.8,1.9, 5.6 and 6.5% for pristine ETS-10, Fe, Co, Ni, Cu, and Agion-exchanged ETS-10 samples, respectively.

The mineralization of nitrobenzene was determined from CODvalues of the reaction mixture samples at different time intervals(Figure 6). The COD values also confirm that silver-metal-ion-exchanged ETS-10 shows the highest photocatalytic activity.

The transition metal incorporated in the pores of ETS-10could be reduced to M0 by photogenerated electrons in the-O-Ti(IV)-O- semiconductor nanowires under UV-irradiation.The reduction of transition metals by trapping photogeneratede- over TiO2-based photocatalysts is reported earlier. Thus, thetransition metal acts as a sink for photogenerated electrons. Thistrapping of e- by the transition metal ion minimizes the e-/h+

recombination and assists the photocatalytic oxidation ofnitrobenzene with the oxidizing species being free h+ or OH•

radicals. The contact of a metal with a semiconductor creates aheterojunction at the interface, with the difference in electro-chemical potential of both the surfaces set by their position ofwork function (φm) and Fermi level (EF), respectively. Thecontact between these two phases leads to the electron flow fromthe phase with more negative electrochemical potential to theother, until the electrochemical potentials of both phases are inequilibrium, in this case from the semiconductor to the metalion. As a result, a barrier height (φb) is formed at the interfacewhich stops the back transfer of electron, and the barrier heightenergy (qφb) is the difference between the equilibrium Fermilevel and the energy of the conduction band edge:

where φm is the work function of the metal (in eV) and �S isthe electron affinity of the semiconductor.

Figure 4. Percentage degradation of nitrobenzene after 4 h irradiation withdifferent ETS-10 catalysts.

Figure 5. First-order kinetic plot for the synthesized catalysts.

ln(Co/Ct) ) kappt (2)

Table 2. Kinetics of the Nitrobenzene Degradation from Its Aqueous Media during Photocatalysis with ETS-10-Based Catalysts

catalystinitial rate × 106

(mole liter-1 sec-1)rate constant

(k × 103) R2 valuestandard redox potential

E0 (V) of metal

ETS-10 2.0 4.9 0.98 -2.71Fe-ETS-10 2.7 5.3 0.96 +0.33Co-ETS-10 2.4 3.6 0.93 -0.28Ni-ETS-10 2.3 4.8 0.97 -0.25Cu-ETS-10 3.3 5.5 0.99 +0.68Ag-ETS-10 3.7 4.7 0.92 +0.80

Figure 6. Reduction in COD value with different synthesized ETS-10catalysts.

φb ) (φm - �S)/q (3)

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The conduction band energy level of TiO2 (Ec) is ap-proximately -0.3 eV,39 and thus the -O-Ti(IV)-O- semicon-ductor unit of ETS-10 is higher than that of the reductionpotential of the transition metal as shown in Table 2. This makesit thermodynamically feasible to transfer an e- from theconduction band to metallic particles. As the e-/h+ pairgeneration occurs in the semiconductor particle by the absorptionof UV light, the electron in the conduction band can be trappedby the exchanged transition metal particles due to their lowerreduction potential. This trapping of the electron frees theoxidizing species for the photocatalytic oxidation. The Ag-ETS-10 catalyst shows the blue shift in band gap energy. This ishelpful in our experimental condition, where we used the UVlight for e-/h+ pair generation.

On the other hand, Na+ and K+ ions in the original ETS-10are not reduced to Na and K metal by photogenerated e- becausethe reduction potentials of these ions are much higher (Table2); this causes the lower photoactivity of ETS-10 towardnitrobenzene degradation. Thus the presence of transition metalions enhances the photocatalytic activity of the ETS-10, whichis a special kind of photocatalyst having photocatalytically active-O-Ti(IV)-O- nanowires in the structure. The photocatalyti-cally active -O-Ti(IV)-O- unit of this material is only 17%.Considering this, the observed results are satisfactory, andfurther research for the modification in this existing material isneeded to enhance its photocatalytic activity.

4. Conclusions

Microporous ETS-10 exhibits photocatalytic activity under UVlight irradiation due to presence of the photocatalytically active-O-Ti(IV)-O- nanowires in the structure. Such material couldbe an alternative to the conventional photocatalysts to overcomethe problems of TiO2 leaching from their support. The presenceof transition metal ions as exchanged cations enhances ETS-10photocatalytic activity. This could be due to the trapping ofphotogenerated e- in -O-Ti(IV)-O- nanowires and the transitionmetal working as a sink for photogenerated electrons. Ag-ETS-10 catalyst shows the highest photocatalytic activity among thetransition-metal-ion-exchanged ETS-10 catalysts.

Acknowledgment

The authors are thankful to the Council of Scientific andIndustrial Research, New Delhi, for the financial assistance andanalytical science discipline of the institute for analytical support.

Supporting Information Available: SEM micrographs;TGA results; absorbance spectra; concentration decrease curve.This material is available free of charge via the Internet at http://pubs.acs.org.

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ReceiVed for reView October 14, 2009ReVised manuscript receiVed February 19, 2010

Accepted March 6, 2010

IE901603K

3966 Ind. Eng. Chem. Res., Vol. 49, No. 8, 2010