Catalysis Communications 58 (2015) 53–58

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Catalysis Communications

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Short Communication

Iron(II) phenanthroline-resin hybrid as a visible light-drivenheterogeneous catalyst for green oxidative degradation of organic dye

Md. Rakibuddin, Sarifuddin Gazi, Rajakumar Ananthakrishnan ⁎Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India

⁎ Corresponding author. Fax: +91 3222 282252.E-mail address: raja.iitchem@yahoo.com (R. Ananthak

http://dx.doi.org/10.1016/j.catcom.2014.08.0351566-7367/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 July 2014Received in revised form 25 August 2014Accepted 28 August 2014Available online 6 September 2014

Keywords:PhotocatalysisHeterogeneous catalystVisible light1,10-Iron(II) phenanthrolineResinReactive oxygen species

A novel resin immobilized iron(II) phenanthroline [FePR] is prepared, characterized and exploited to understandC–C/C–O cleavage in Rhodamine B dye under visible light (λ≥ 420 nm). Studies infer that bothmolecular oxygenand visible light are playing essential roles in photocatalysis, and pH is influencing the degradation.Photocatalysis by FePR involves oxo-iron ([FeIV(phen)3 = O]) species as reactive intermediate. LC–MS studieson the aliquots of partially photodegraded RhB reveal benzoic acid as a main degradation product; however,complete degradation was achieved under optimum condition. Overall, the FePR is a stable heterogeneousphotocatalyst, which can be used for advanced oxidative process in greener route.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent decades, the degradation of persistent organic pollutants inwater by green photochemical processes has become a very active re-search topic [1]. In advanced oxidation processes (AOP), the recent di-rection is towards green chemical methods. It is trendy to developsuch a methodology with certain benign conditions such as, selectiveorganic oxidation reaction or treatment of organic pollutants, usage ofeco-friendly catalyst and suitable to work under visible light. Over-whelmingly, it should occur with molecular oxygen as the oxidant andwater as the solvent [2]. As the sunlight is an enormous energy reachingthe earth, the new strategies are emerging in utilizing the visible light[3–5]. Besides semiconductor photocatalysts (such as TiO2, ZnO) varioustypes of iron complexes such as, iron porphyrin [6], immobilized ferro-cene [7], iron ferrioxalate [8] and iron phthalocyanines [9,10] have beenused as photocatalysts for degradation of toxic organic pollutants. Re-cently, another iron complex, [Fe(III)-salen] Cl, has been employed ashomogeneous visible light photocatalyst for degradation of organicdyes by our group [11]. Now-a-days porous materials, such as resin,silica, clay, and zeolite have been proved to be a good solid support fordifferent metal complexes [6,9,12–16]. There are certain uniqueadvantages over homogeneous systems upon immobilization of metalcomplex onto solid support. It exhibits durability in life time, facilerecovery, and easy separation of the catalyst due to the stabilizationand isolation of the metal complex [17]. Jincai Zhao and his co-

rishnan).

workers have shown degradation and selective oxidations of organicpollutants by iron [II] bipyridine complex supporting on zeolite, clayand resin [2,17,18].

Previously, iron(II) phenanthroline has been used for photometricdetermination of Fe(II), cell permeable inhibitor for metalloproteases,hydroxylation of phenol and oxygen reduction catalyst for PEM fuelcell [19–22]. To the best of our knowledge, there is no other article inthe literature which discusses the use of [Fe(phen)3]2+ as photoactivematerial for any application. [Fe(phen)3]2+ itself does not have any sig-nificant photoactivity in homogeneous aqueous solution. Hence, in thiswork, we have tried to make [Fe(phen)3]2+ complex to be active undervisible light by grafting the complex on a cationic ion-exchange resin ina facile room temperaturemethod. The novel resin immobilized iron(II)phenanthroline (FePR) was prepared, characterized and applied for theactivation of O2 at room temperature to degrade Rhodamine B (RhB), amodel organic pollutant, under visible light irradiation (λ ≥ 420 nm).

2. Experimental

2.1. Preparation of the photocatalyst (FePR)

The FePR catalyst was prepared by ion-exchange method using anappropriate amount of the [FeII(phen)3]2+ solution in distilled waterstirred with 1 g of the dried resin at pH 2.0. Then the solid was filteredout and washed well with distilled water and methanol. And thus thesolid mass obtained had been dried in open air at room temperatureand was directly used as photocatalyst. Different percentage loadingsof [FeII(phen)3]2+ catalysts [25% (0.125 mmol/g), 50% (0.25 mmol/g),

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75% (0.375 mmol/g), and 100% (0.5 mmol/g) of the saturated exchangeamount of complex over 1 g resin]were prepared at pH2.0. The increasein loading percentage of metal complex was confirmed by the increasein absorbance value in the UV–Visible diffuse reflectance spectra(Fig. S1a).

2.2. Photodegradation experiment under the visible light irradiation

Exactly, 30mLof reactionmixture in an aqueousmediumcontainingRhB (6×10−5M) and catalyst (0.2 g/L)was taken in a Pyrex glass vesseland kept in the dark under stirring condition for 20min to complete ad-sorption–desorption equilibrium. Fig. S1b shows that the equilibriumexists only after 12min. After that, themixturewas irradiated using vis-ible light (500 W Philips halogen lamp with cut off filter; λ ≥ 420 nm)with an incident light intensity 2.98 × 101 Einstein L−1 s−1 (for detailssee Supporting information).

3. Results and discussion

3.1. Characterization of the FePR catalyst

The prepared FePR catalyst is characterized byUV–Visible diffuse re-flectance spectra to verify the binding of the FePR to the resin surface.The pure resin has no characteristic peak in the visible region (400–700 nm) but after binding with iron(II) phenanthroline the hybrid (i.e.FePR catalyst) displays a broad absorption band in 400–650 nm withmaxima at 540 nm (Fig. 1a). Owing to the enormous coverage of ab-sorption in visible region makes the hybrid suitable for utilizing energyfrom sunlight.

The prepared catalyst is again characterized by FTIR and UV–Visspectroscopy. From the IR spectra (Fig. 1b) of the Resin Amberlite IRA200 and the catalyst FePR, it is observed that the intensities of thepeaks at 1640.35 cm−1, 1180.5 cm−1 and 679.9 cm−1 due to the free

Fig. 1. (a) DRS and (b) FTIR spectra of i) FePR and ii) free resin; (c–d) UV–Vis spectra of thephenanthroline]= 5 × 10−6 g/mL and [FePR]= 1 × 10−3 g/mL. (For interpretation of the refer

resin are decreased and they are shifted to lower frequencies suchas 1634.25 cm−1, 1176.5 cm−1 and 673.8 cm−1, respectively in theFePR catalyst. Again, the peaks at 833.2 cm−1 and 1413.25 cm−1 ofthe free resin are shifted to higher frequencies (at 843.4 cm−1 and1423.75 cm−1, respectively) along with a new peak at 720.8 cm−1 inthe FePR catalyst. The latter peak is absent in the free resin. As per theliterature, the original phenanthroline moiety exhibits two intensebands at 725 cm−1, assigned to the out-of-plane motion of the six hy-drogen atoms on the heterocyclic rings and at 845 cm−1 assigned tothe two hydrogen atoms in the center ring [23]. Again, the UV–Vis spec-trum clearly shows that peaks at 265 nm and 511 nm are of ironphenanthroline moiety, which are shifted to 270 nm and 540 nm, re-spectively due to the interaction of complex on the resin (Fig. 1c–d).SEM images of the catalyst, before and after the photocatalysis are re-corded to understand the surface properties of the catalyst. In Fig. 2a,the SEM image shows rough surface structures of the amberlite resinand on the other hand, the Fig. 2b exhibits uniform smooth surface,which indicates that the [FeII(phen)3]2+ is immobilized uniformly onthe amberlite surface. Interestingly, the SEM image of the catalystafter photocatalytic treatment reveals (Fig. 2c) that the appearance ofthe surface of used catalyst FePR is nearly identical to the surface ofthe fresh FePR (small change is due to magnification). EDX analysis ofthe catalyst was also taken before and after the photoreaction, whichconfirms the presence of iron in the catalyst (Fig. S2).

In solution, [FeII(phen)3]2+ could not activate molecular oxygenas the low-lying ligand field (which originates from the lower energyeg⁎ orbitals) in the iron(II) phenanthroline complex provides non-radiative decay of the metal to ligand charge transfer (MLCT) state re-sult in shortening of the MLCT lifetimes [24]. Hence, the iron(II)phenanthroline complexes have been photochemically inactive or hav-ing unstable excited states. However, when hybrid formation occurs viaelectrostatic interactions of iron(II) phenanthrolinewith resinmatrix, itoffers enhancement to their excited lifetime [18]. Consequently, the

iron(II) phenanthroline (blue line) and catalyst FePR (red line) in water, where [iron(II)ences to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. SEM image of (a) free resin, (b) FePR before reaction and (c) after reaction; (d) fluorescent microscopic image and (e) fluorescence spectra of the FePR.

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irradiation of light on the stable iron(II) phenanthroline complex(in hybrid) can provide mobilize electron to the oxygen to aid thephotodegradation process. The fluorescentmicroscopic image offers di-rect evidence for the interaction between [FeII(phen)3]2+ and the resin(Fig. 2d–e). The FePR exhibits fluorescence in the red region (λem =633 nm) at excitation wavelength (λex

max) of 522 nm, while both thehomogeneous iron(II) phenanthroline solution and the resin itselfdoes not show any fluorescence. Moreover, the result indicates thatthe resin is not only acting as a support for the [FeII(phen)3]2+, butalso offers a distinctive skeleton that notably changes the photochemi-cal properties of [FeII(phen)3]2+.

3.2. Selection of the photocatalyst and degradation of RhB under differentparameters

The experimental results of optimization studies reveal that0.5 mmol of complex provide a saturation limit for 1 g of resin andtherefore, it is considered as 100% loading. Similarly, catalysts with

25%, 50% and 75% loading of complex on the 1 g resins are also prepared.As shown in Fig. 3a–b, among the different loadings, the 75% complexloaded catalyst exhibits the best photoactivity (70%), and is usedthroughout the study. The 100% loading exhibited lowest activity(33%) due to unavailable of free space on the resin surface to absorbthe dyemolecule. As shown in the control study (Fig. 3b), no significantdegradation is achieved in the absence of molecular oxygen andvisible light. Purging with nitrogen gas into the RhB solutions, thephotodegradation is significantly decreased due to the lack of molecularoxygen in the system, which demonstrates that both molecularoxygen and visible light irradiation are necessary components for thedegradation.

To investigate the effect of sensitizers on the photocatalytic degrada-tion of RhB, four organic dyes namely, Eosin Y (EO), Rose Bengal (RB), Ri-boflavin (RF) andMalachite Green (MG) are chosen. Organic dyeswith asubstituent on the xanthene skeleton increase the yield of intersystemcrossing into the triplet state of the dye [25]. Unlike EO, RF and RB, thedye MG does not possess those characteristic properties and has not

Fig. 3. (a) Absorption spectra of RhB during photodegradation in presence of FePR (75% loading); (b) control study: i) RhB solution with the catalyst in dark, ii) homogeneous aqueous[FeII(phen)3]2+ solution under visible light, iii) photodegradation of RhB by FePR under N2 atmosphere, iv) with the resin itself, under irradiation, v) RhB solution under light, withvi) 25%, vii) 75%, viii) 50% and ix) 100% loading of the FePR; c) effect of pH on the photodegradation of RhB; and d) reusability of the FePR.

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responded in enhancing the effect. The degradation ratewasmuch faster(Fig. S3) in the presence of the EO (rate constant, k= 0.731 h−1) follow-ed by RF (k = 0.641 h−1) and RB (k = 0.546 h−1), whereas the

Fig. 4. a)DRS of the FePR i) before and ii) after reaction; b) photodegradation of RhBwith andwi

degradation rate constant for only RhBwas 0.375 h−1. The higher degra-dation rate shows the superiority of our photocatalytic system than com-mercial Degussa P25 or some modified TiO2 [26].

thout benzoquinone (30%); and c) probablemechanistic pathway of thephotodegradation.

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The photodegradation of RhB in the presence of FePR catalyst ishighly depended on the pH of the solution. When the photoassisteddegradation of RhB is carried out under different pH conditions, it is ob-served that the percentage degradation increases with the decrease inthe pH of the solution (about 70% degradation for pH 2.0, and 30% forpH 8.0, respectively) (Fig. 3c). The decrease in the rate of degradationin basic medium has been attributed due to the lower stability of the[FeII(phen)3]2+ complex (Fig. S4).

Fig. 5. LC–MS spectra of the photodeg

3.3. Reusability of the catalyst

It is found that the catalyst can be reused formultiple times, and a setof four cycles is provided in the discussion (Fig. 3d). In our experiment,we had used the same catalyst for 10 times, and the average loss is about4% (for n = 10). In EDX, a very slight variation in the % of Fe wasobserved (in Fig. S2, 0.86% loss of Fe). However, EDX is not a preciseanalysis to signify any minute change or loss of element (i.e. Fe) on

radation products of RhB (A–G).

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the catalytic material. Hence, further studies are to be required toascertain the change in iron complex on the surface of the catalyticsurface.

Though the catalyst is stable enough, the diffuse reflectance spectra(Fig. 4a) show the decrease (~7%) in absorbance of the catalyst at520 nm. In addition, there was a meager increase in absorption valueat 265 nm in the UV spectra of the irradiated solution (Fig. 3a), whichdid not match with the λmax of any of the degradation products(Table S1). Hence, the qualitative information suggests that theremight be an extremely slow leaching from FePR, which could be dueto the mechanical disturbance of the surface upon usage. However,the leaching is quiet low and can be minimized by careful conditions.Hence, the decrease of photocatalytic activity of the hybrid material isnot very rapid upon reuse.

3.4. Possible mechanistic pathway of the photodegradation

To determine the reactive oxygen species formed in the photoreac-tion process, well known scavenger trapping reagents (isopropanol,benzoquinone) are used [27,28]. In the aqueous system the •OH radicalsformation could be expected, whereas •OOH/O2 radicals are unstable inwater but can appear in methanol [29]. The addition of 2-propanol, awell-known scavenger of •OH radicals, to the photoreaction systemcould not stop the degradation of RhB in either aqueous or methanolicsolutions. This indicates that the photoreaction catalyzed by FePR doesnot involve •OH radicals as the main active oxygen species. Hence,additional experiments are conducted with the presence of 30%benzoquinone (a well-known scavenger of superoxide radicals) inthe reaction medium which results a significant decrease of thephotodegradation of RhB (Fig. 4b). Therefore, the reactive oxidantspecies produced by FePR must be the oxo-iron ([FeIV(phen)3 = O])rather than •OH radicals. The hint for oxoferryl species was also noticedfrom the UV absorption spectrum of the catalyst under irradiation for30 min, where a weak absorbance at around λmax = 640 nm wasobtained (Fig. S5). This is a characteristic indication for the occurrenceof oxo-ferryl (IV) species under irradiation [13,30]. Such a change inabsorbance is not seen in the non-irradiated sample of aqueous solutionof the catalyst. Mechanisms involving oxo-iron as the crucial intermedi-ates in a reaction catalyzed by iron complexes for the oxidation oforganic species have been reported by some other groups [31–33].Based on the results and the literature, we have tried to give a probablemechanistic pathway of the photodegradation of RhB in the presence ofthe FePR catalyst (Fig. 4c). Firstly, the iron(II) phenanthroline getexcited in the presence of visible light and gives an electron to themolecular oxygen to make a superoxide radical and iron(II) convertsto iron(III). The superoxide radical forms complex with iron(III) andthen converts to iron(IV) oxo-iron intermediates. The active iron(IV)oxo-iron species then oxidize the dye by degradation, and it convertsto iron(II) species.

3.5. Possible degradation products of RhB

To determine the degradation end products, LC–MS studies in thesample have been performed. The TOC removal of RhB was 75% forour system, and the rest part are the eight organic degradation products.Among the eight products, benzoic acid is found to be the major degra-dation product (m/z = 122). The detailed mass spectra and theidentified products have been shown in Fig. 5. Besides benzoic acid(A) (m/z=122) andphthalic acid (B) (m/z= 167), the other identifiedproducts are, N,N-diethylaniline (E) (m/z = 149), N-methylmaleimide(F) (m/z = 111), N-ethylmaleimide (G) (m/z = 125), 6-(diethylamino)-3-hydroxy xanthylium (D) (m/z = 268) and 6-(ethylmethylamino)-3-hydroxy xanthylium (C) (m/z = 255). Onlyone product (m/z = 283) was not recognized. The photodegradationproducts, benzoic acid and phthalic acid, are found to be known withsome other reported literatures [34,35].

4. Conclusions

Resin supported iron(II) phenanthroline [FePR] has been successful-ly prepared and characterized and also it has been recognized as a stableheterogeneous photocatalyst. The prepared catalyst FePR was foundto be effective for the photo oxidative bond breaking (C–C/C–O bonds)of the model organic molecule (RhB dye) in aqueous system undervisible light irradiation. A basic understanding of the effect of differentparameters, mechanistic pathways of the photooxidation processand the degradation end products is achieved. In near future, the resinsupported iron(II) phenanthroline could be useful in many applicationsincluding photocatalytic products, new green synthetic routes,and also useful for secondary treatment of organic pollutants inwastewater.

Acknowledgments

Authors are thankful to Prof. S. K. Ghosh, IIT Kharagpur, for FM imag-ing. Author RA is grateful to SERB-DST, New Delhi (No. SR/FT/CS–146/2011) for funding, and MR thanks CSIR for fellowship. The authorsalso acknowledge the CRF-IITKGP for SEM and LC–MS analysis.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.catcom.2014.08.035. This data include MOL filesand InChiKeys of the most important compounds described in thisarticle.

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